Nanocoatings: Size Effect in Nanostructured Films

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Engineering Materials

For further volumes: http://www.springer.com/series/4288

Mahmood Aliofkhazraei

Nanocoatings Size Effect in Nanostructured Films

123

Dr. Mahmood Aliofkhazraei Materials Engineering Department Tarbiat Modares University Jalal Ale Ahmad Highway 14115 Tehran Iran e-mail: [email protected]

ISSN 1612-1317

e-ISSN 1868-1212

ISBN 978-3-642-17965-5

e-ISBN 978-3-642-17966-2

DOI 10.1007/978-3-642-17966-2 Springer Heidelberg Dordrecht London New York Ó Springer-Verlag Berlin Heidelberg 2011 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: deblik, Berlin Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Preface

In recent years, nanostructured coatings are used on surface of different parts with the aim of increasing their resistance against erosion, corrosion etc. The scale of nanomaterials should be classified from 1–100 nm which means clusters or (atomic) nuclear pellet with no less than 100 nm, fibers less than 100 nm in diameter and films thickness less than 100 nm. In this range nanomaterials show specific properties which they did not show for bigger dimensions such as micrometers. This effects which have came from size of nanomaterials are known as ‘‘size effect’’. Today, with the development and expansion of nanotechnology, nanostructured coatings are widely used in military, aerospace, electronic and magnetic industries, etc. due to having appropriate and unique properties such as magnetic properties, corrosion resistance, high hardness and wear strength. They have been developed remarkably in recent two decades. These coatings improve properties of substrate. Hence, coatings resistant against erosion and corrosion, self-lubricant system and hard coatings can be produced. Increased attention to size effect in nanostructures during recent years can be seen in Fig. 1. In comparison with coatings with micrometer-ranged thickness, nanostructured coatings usually enjoy better and appropriate properties. Strength and resistance

Fig. 1 Comparison among number of journal articles about size effect of nanostructures identified on February 25, 2011 using the Scopus search engine. The key words used were (‘‘size effect’’ + nano) just in title, abstract and keywords

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Preface

against erosion are of the properties which have been taken into consideration. Transparency is of paramount importance in application of nanometer-ranged coatings. These coatings enjoy unique magnetic properties and are used with the aim of producing surfaces resistant against erosion, lubricant system, cutting tools, manufacturing hardened sporadic alloys, being resistant against oxidation and corrosion. This book reviews researches around fabrication and classification of nanostructured coatings with focus on size effect in nanometric scale. Size effect on electrochemical, mechanical and physical properties of nanocoatings discussed through different chapters of the book while different examples and figures were used for better discussion. February 2011

Mahmood Aliofkhazraei

Contents

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Synthesis, Processing and Application of Nanostructured Coatings . . . . . . . . . . . . . . . 1.1 What is Nanotechnology? . . . . . . . . . . . . . 1.2 The Role of Size Effect in Nanotechnology 1.3 Nanoparticles. . . . . . . . . . . . . . . . . . . . . . 1.4 Production of Nanoparticles . . . . . . . . . . . 1.5 Applications of Nanoparticles . . . . . . . . . . 1.6 Thin Films . . . . . . . . . . . . . . . . . . . . . . . 1.7 Production and Application of Thin Films . 1.7.1 Introduction to Sol–gel Method . . . 1.7.2 Sol–gel Chemical Reaction . . . . . . 1.7.3 The Effect of Catalyst Hydrolysis . 1.7.4 Electric Precipitation . . . . . . . . . . 1.7.5 The Rotate Coating . . . . . . . . . . . 1.7.6 Scattering Coating . . . . . . . . . . . . 1.7.7 Self Layout . . . . . . . . . . . . . . . . . 1.7.8 Plasma Polymerization . . . . . . . . . 1.7.9 Annealing . . . . . . . . . . . . . . . . . . 1.7.10 Heating Oxidation . . . . . . . . . . . . 1.8 Applications of Nanostructured Coatings . . 1.9 Coating and Surface Engineering . . . . . . . . 1.10 Coating Issues and Applications . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Size Dependency in Nanostructures . . . . . . . . . . . . 2.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Nanocomposites and Their Production Methods. 2.2.1 Thermal Spraying Nano-Composites . . 2.2.2 Transitional Metal Nitride Coatings . . .

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2.2.3

Super Rough and Super Hard Nanocrystalline Coatings. . . . . . . . . . . . . . . . 2.2.4 Nanocomposite Coatings. . . . . . . . . . . . . . . . 2.3 Electrochemistry Role in Production of Nano-Coatings. 2.3.1 Electro-Deposition Using Porous Templates . . 2.3.2 Nano-Coatings Properties . . . . . . . . . . . . . . . 2.4 Mechanical Properties. . . . . . . . . . . . . . . . . . . . . . . . 2.5 Corrosion Properties . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Hydrogen Transition and Sensitivity. . . . . . . . . . . . . . 2.7 Magnetic Characteristics and Ionic Conductivity . . . . . 2.8 Thermal Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 Nanocoatings Applications . . . . . . . . . . . . . . . . . . . . 2.9.1 Structural Applications . . . . . . . . . . . . . . . . . 2.9.2 Functional Applications . . . . . . . . . . . . . . . . 2.9.3 Classification of Applications . . . . . . . . . . . . 2.10 Key Points for Development . . . . . . . . . . . . . . . . . . . 2.10.1 Environment and Stability. . . . . . . . . . . . . . . 2.10.2 Weight and Volume Reduction . . . . . . . . . . . 2.10.3 Smart Layers and Structures . . . . . . . . . . . . . 2.10.4 Processes’ Understanding . . . . . . . . . . . . . . . 2.10.5 Training . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11 Surface Engineering Share in Key Industry Sections . . 2.12 Estimation of Corrosion and Erosion Costs . . . . . . . . . 2.13 Surface Engineering in Automobile Industry . . . . . . . . 2.14 Surface Engineering in Power Generation Industry . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

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Characterization of Nanostructured Coatings . . . . . . . . . . . . 3.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Definition of Nanostructured Materials . . . . . . . . . . . . . . 3.3 Thermodynamics of Nanostructured Materials . . . . . . . . . 3.4 Interfaces Thermodynamics . . . . . . . . . . . . . . . . . . . . . . 3.5 Interface Traction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Additional Free Energy on a Solid with Nanometric Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Interface Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Chemical Equilibrium in Curved Interface . . . . . . . . . . . 3.9 Influential Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Phase Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11 Measurement of Thermal and Electrochemical Properties . 3.12 Condensed and Compressed Metals . . . . . . . . . . . . . . . . 3.13 Methods for Production of Nanostructured Materials . . . . 3.14 Nano-Technological Compatibility in Coating . . . . . . . . . 3.15 Improvement of Coating Quality Using Nanotechnology .

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3.16

Abrasion, Scratch, and Corrosion Resistant Coatings . . . . . . 3.16.1 Coatings Resistant Against Scratch, Abrasion, Corrosion, and Environmental Agents . . . . . . . . . . 3.16.2 Nano-Metric Abrasion, Scratch, and Corrosion Resistant Coatings . . . . . . . . . . . . . . . . . . . . . . . . 3.16.3 Using Alumina as a Scratch and Abrasion Resistant Coating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.16.4 Designing Light Resistant Panels for Plane Body. . . 3.16.5 Ceramics Reinforced by Carbon Nano-Tubes . . . . . 3.17 Nano-Coating Resistant Against Corrosion . . . . . . . . . . . . . 3.18 Using Nano-Particles for Coating in Transportation Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.19 Coating Applications in Defense and Aerospace Industries . . 3.20 Using Ceramic Nano-Coatings in US Navy . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Size Effect in Electrochemical Properties of Nanostructured Coatings . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Thermodynamic Equilibrium . . . . . . . . . . . . . . . . . . 4.3 Classical Nucleation Theory . . . . . . . . . . . . . . . . . . 4.4 Atomic Nucleation Theory . . . . . . . . . . . . . . . . . . . 4.5 Kinetics of Formation of Nucleuses in Electro-Crystallization . . . . . . . . . . . . . . . . . . . . 4.6 Electrode Surface Energy State . . . . . . . . . . . . . . . . 4.7 Nucleus Situation on Electrode Surface . . . . . . . . . . 4.8 Growth of 3-D Nano-Nucleuses . . . . . . . . . . . . . . . . 4.9 Metal Ion Structure . . . . . . . . . . . . . . . . . . . . . . . . 4.10 Double-Layer Structure. . . . . . . . . . . . . . . . . . . . . . 4.10.1 Helmholtz-Perrin Model . . . . . . . . . . . . . . . 4.10.2 Gouy-Chapman Model . . . . . . . . . . . . . . . . 4.10.3 Stern-Graham Model . . . . . . . . . . . . . . . . . 4.11 Determining Stages for Rate of Electrode Reactions . 4.12 Concentration Over-Potential. . . . . . . . . . . . . . . . . . 4.13 Charge Transfer Over-Potential . . . . . . . . . . . . . . . . 4.14 Crystallization Over-Potential . . . . . . . . . . . . . . . . . 4.15 Ohm or Resistance Over-Potential . . . . . . . . . . . . . . 4.16 Pulse Electrochemical Deposition Method. . . . . . . . . 4.17 Charging and De-Charging of Double Layer in Pulse Electroplating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.18 Nanostructured Coating Properties . . . . . . . . . . . . . . 4.18.1 Mechanical Properties. . . . . . . . . . . . . . . . . 4.18.2 Catalyst Properties . . . . . . . . . . . . . . . . . . .

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4.18.3 Electrical Properties . . . . . . . . . . . . . . . . . . . . . . . . . 4.18.4 Anti-Corrosive Properties . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

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Size Effect in Mechanical Properties of Nanostructured Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Nanocomposite Coating Production Method . . . . . . . . . . . 5.3 Provision of Nanocomposite Coatings with Plating Method 5.4 Plating of Nickel-Alumina Nanocomposite Coating . . . . . . 5.5 Effects of Participation of Alumina Nanoparticle in Nickel Coating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Plating of Nickel-Alumina Nanocomposite Coating . . . . . . 5.7 Size Effect in Mechanical Properties of Two Dimensional Nano-Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Studying Effective Factors on Simultaneous Deposition of Alumina Nanoparticles with Nickel . . . . . . . . . . . . . . . 5.8.1 Effect of Density of Alumina Nanoparticles in Electrolyte Bath . . . . . . . . . . . . . . . . . . . . . . . 5.8.2 Effect of Electroplating Current Density . . . . . . . . 5.8.3 Distribution of Alumina Nanoparticles . . . . . . . . . 5.8.4 Effect of Density of Nickel Ions in Plating Bath . . 5.8.5 Effect of pH . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.6 Pulse Current Effect . . . . . . . . . . . . . . . . . . . . . . 5.9 Size Dependency of Tensile and Fatigue Strength in Ultra-Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 Application of Coating for Strength Enhancement . . . . . . . 5.11 Nano-Coating Use in Dressing Industry . . . . . . . . . . . . . . 5.11.1 Making WC/Co–Ni Nano-Coating Using Electrodeposition . . . . . . . . . . . . . . . . . . . 5.11.2 Using (Me-Ti1-xAlxN)/(a-Si3N4) Nanocomposite Coatings. . . . . . . . . . . . . . . . . . . 5.12 Size Dependency in Nanocomposite Layers . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Size Effect in Physical and Other Properties of Nanostructured Coatings . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Silicides Specifications . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 SALICIDE Process . . . . . . . . . . . . . . . . . . . . 6.2.2 Necessary Conditions for Formation of Silicides

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6.2.3 6.2.4

6.3 6.4

6.5 6.6 6.7 6.8 6.9 6.10

6.11

Transition from TiSi2 to NiSi . . . . . . . . . . . . . . . NiSi Salicide Technology (Self-Aligned Nickel Silicide) . . . . . . . . . . . . . . . . . . . . . . . . . Size Effect in Sensing Characterization . . . . . . . . . . . . . . NiSi Thermal Stability . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 NiSi Transition to NiSi2 in Dual Ni-Si System . . . 6.4.2 Pt Effect in Increasing Thermal Stability . . . . . . . 6.4.3 Pd Effect in Thermal Stability . . . . . . . . . . . . . . . 6.4.4 Ge Effect in Thermal Stability. . . . . . . . . . . . . . . 6.4.5 Co and Ir Effects in Thermal Stability . . . . . . . . . 6.4.6 Capsulation and NiSi Thermal Stability . . . . . . . . Size Effect in Optical Properties of Nanostructured Films. . Optical Coatings: Using Ultraviolet Light Block Layers . . . Surface Improvement for Making Fog and Vapor Resistant Layers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production of Pieces with Nano-Coating. . . . . . . . . . . . . . Self-Cleaning Glasses . . . . . . . . . . . . . . . . . . . . . . . . . . . Medical and Hygienic Applications . . . . . . . . . . . . . . . . . 6.10.1 Inorganic Materials Nano-Coating for Medical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.10.2 Using Nano-Particle Masks . . . . . . . . . . . . . . . . . 6.10.3 Application of Hydroxyapatite Nano-Coating to Design Prosthesis . . . . . . . . . . . . . . . . . . . . . . 6.10.4 Using Nanocomposite Coating for Food Packaging 6.10.5 Antipollution Materials in Shipping Industry . . . . . 6.10.6 Nano-Coating Use Against SARS Virus . . . . . . . . 6.10.7 Application of Ag Nanoparticles as Antibacterial Coating . . . . . . . . . . . . . . . . . . . 6.10.8 Using TiO2 Nano-Particles to Decrease Environmental Contaminations . . . . . . . . . . . . . . Electrical and Electronic Applications . . . . . . . . . . . . . . . 6.11.1 Production of Transparent Conductor Coatings by Carbon Nano-Tubes . . . . . . . . . . . . . . . . . . . . 6.11.2 Application of Nano-Coating on Solar Cells . . . . . 6.11.3 Nano-Coating of Nickel Particles by Oxides . . . . . 6.11.4 Using Polarizer Nano-Layers to Produce LCD Monitors . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.5 Produce of Electrically Conductive Transparence Nano-Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11.6 Increase of Data Storing Capacity with Magnetic Nano-Layers . . . . . . . . . . . . . . . . . . . . . . . . . . .

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6.12 Lubricating and the Other Applications . . . . . . . . . . . 6.13 Ag-Polluted SnO2 Particles . . . . . . . . . . . . . . . . . . . . 6.14 Development of Nano-Coating for Surface Lubrication. 6.15 Size Dependency in Nanocomposite Layers . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1

Synthesis, Processing and Application of Nanostructured Coatings

1.1 What is Nanotechnology? Nanotechnology is a general term identified by advanced technology in the nano field. The scale of nano is usually from 1 to 100 nm [1–3]. First idea of nanotechnology appeared in 1959 (where the technology was not identified by the specific name yet) Richard Feynman proposed the idea of ‘‘There’s plenty of room at the bottom’’ in his hypothesis. Feynman argued that in the near future the molecules and atoms can directly be manipulated. The word ‘‘nanotechnology’’ first came to existence by Professor Norio Taniguchi in 1974. He used the word for accurate materials with high tolerance in nanometer limits. In 1986 K. Eric Drexler redefined the word in his book ‘‘Engines of creation: The coming era of nanotechnology’’. He has profoundly discussed nanotechnology in his thesis and explored the issue in ‘‘Nanosystems Molecular Machinery and Computation’’ [4–14]. Application of nanotechnology rooted in basic elements. Each one has specific characteristics which creates remarkable attributes in various fields. Application of nanoparticle in producing simple medicine and bandages with no need to renewal, diagnosing early cancer cells and analyzing environment pollutants are the examples of nanotechnology [15–24].

1.2 The Role of Size Effect in Nanotechnology Nanomaterials are the new materials that their basic construction has formed by nanometric scale engineering. On such scale the specific or completely different material indicates the possibility to create more accurate new materials and devices with vast capacities. The scale of nanomaterials should be classified from 1 to 100 nm which means clusters or (atomic) nuclear pellet with no less than 100 nm, fibers less than 100 nm in diameter and films thickness less than 100 nm. In this range nanomaterials show specific properties which they did not show for

M. Aliofkhazraei, Nanocoatings, Engineering Materials, DOI: 10.1007/978-3-642-17966-2_1, Springer-Verlag Berlin Heidelberg 2011

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1 Synthesis, Processing and Application of Nanostructured Coatings

bigger dimensions such as micrometers and bigger. This effects which have came from size of nanomaterials are known as ‘‘size effect’’ [25–35]. Eleven items all identify in nanomaterials field are as follow: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Nanostructured materials Nanoparticle/nanocomposite Nano-capsule Nano-porous materials Nanofibers Fullerenes Nanowires Single wall/multi walled carbon nanotubes Molecular electricity Quantum dots Thin films

Ohno et al. [36] studied the size effect of TiO2–SiO2 nano-hybrid particles. They described the effect of drying process on the preparation of TiO2–SiO2 nanohybrid particles. The hybrid sol was prepared by sol–gel process with controlled chemical modification (CCM) to attain the TiO2 nano-coating on the SiO2 nanosphere. The hydrolysis reaction of titanium alkoxide was controlled by RA ([CH3COOH]/[Ti]), and RW ([H2O]/[Ti]). The hybrid particles were dried by under the super critical drying to suppress the hard agglomeration of the asprepared nano-hybrid sol. As a result, the hard agglomeration of the primary particles was successfully suppressed to obtain TiO2–SiO2 nano-hybrid particles with controlled chemical modification of titanium alkoxide. The quantum size effect of nano-hybrid particles was confirmed by the band gap energy shift, using ultraviolet–visible spectroscopy (UV–vis). Figure 1.1 illustrates the ultraviolet– visible spectrum (UV-S) for the obtained TiO2–SiO2 nano-hybrid particles and that Fig. 1.1 The ultraviolet– visible spectrum (UV-S) for the obtained TiO2–SiO2 nano-hybrid particles (the dotted line) and that of the anatase crystals without the quantum size effect (the black line), reprinted with kind permission from Tomoya Ohno [36]

1.2 The Role of Size Effect in Nanotechnology

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of the anatase crystals without the quantum size effect. Size effect can be seen in all of properties of materials.

1.3 Nanoparticles These materials are the grains with the scale less than 100 nm, comparing to bigger clusters. Nanoparticles have new qualities in them which it’s specified features are covers size, distribution, shape, phase and etc. Nanoparticles can be constructed from a wide range of materials generally metallic oxide ceramics, metals, silicates and non-oxide ceramics. The scientists hope to use nanoparticles in fabrication of ideal materials with desired mechanical, electrical, magnetic or optical characteristics and progress their capacities. In the case that this prospect occurs, it will affect positive impact on environment and low budget achievements such as the influence of nanotechnology on medical systems [25, 37–45]. Figure 1.2 is an example of scanning electron microscope (SEM) and transmission electron microscope (TEM) images of silicon nitride (Si3N4) nanoparticles [46].

Fig. 1.2 a Scanning electron microscope and b transmission electron microscope images of silicon nitride (Si3N4) nanoparticles with average size of 72 nm [46]

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Most of the possible capacities that can be found in nanoparticles are connected to the list below: • Specific high space area (high surface to volume ratio) • Electrical and magnetic attributes (advanced electromagnetic elements) • Optical properties (attract or repel some wavelengths that can be manipulated, interaction with ligands and external disturbances) • Chemical elements (increased chemical reaction)

1.4 Production of Nanoparticles Production of nanoparticles is achievable through following solutions: • Solid state method (grinding, milling, mechanical alloying methods) • Commercial methods [Chemical vapor deposition (CVD), Physical vapor deposition (PVD)] • Chemical synthesis/dissolution methods (sol–gel, chemical collide) • Methods of gas phase pyrolysis of flame, electro-explosion, plasma synthesis, laser abrasion According to previous researches chemical activities are currently considered as the best method to produce nanoparticles [47–56].

1.5 Applications of Nanoparticles High volumes of nanoparticles can be applied in detergents, cosmetics, pesticides, environmental modification, catalysts, lubricants, sealants, adhesives and coatings fields. The coatings obtained by nanoparticles are remarkable category in current and future applications. By these coatings, a profound level of material properties can be used such as scratch resistance, optical features (transparency or adjusted reflection) and also self cleaning properties [57–64].

1.6 Thin Films Scientists apply the thin films by using different materials with thicknesses less than 100 nm. The basic advantage of thin films as any other coating is that it can transfer material properties to the surface so it is possible to use materials with their basic characteristics. The substrate material and thin film create a system with different enhanced properties. Nanotechnology creates instrument for control of three key parameters for thin films: (a) Chemical composition (crystalline structure

1.6 Thin Films

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in nanometer range), (b) Thickness and (c) Surface geometry (including thin films patterns on nanoscale) [65–74]. The most prominent properties of these materials are as follow: Optical properties: Including optical entrapment, transparency, opaque, florescence, waveguides, light valves, anti reflection and etc. [75–88]. Mechanical properties: Including resistance to abrasion and wear, stiffness, anti scratch, strain-to-failure and etc. [89–95]. Electronic properties: Potential energy, binding energy, conduction, insulation and etc. [96–111]. Chemical properties: water repellence, anti fog, chemical barriers, being chemically neutral, oxygen and moisturizing shields on polymers, antimicrobial surface and etc. [88, 112–116]. Magnetic properties: saving data [117–126]. Thermal properties: application of thin film in multiple layers as an obstacle to prevent expansion of atomic vibration which produce thermal transmission but permits the flow of electrons so they can be use in thermoelectric tools [127–130]. It is not strange that many scientists performed their research on thin films. These materials have vast capacities. For example they can be used in electronic industry as a light barrier in producing semi-conductor wafers or as metallic dielectrics (with low dielectrics constant) in integrated circuits and also in organic light emitting indicators or as anti corrosive coating. Organic thin films can be created in low temperatures (comparing to ceramic/mineral materials) so less complicated processes are achievable and can be controlled like sol–gel. The list of production and precipitation methods of thin films and the coatings are very vast which includes following items: • • • • • • • •

Chemical vapor deposition (CVD) Psychical vapor deposition (PVD) Sol–gel Electronic precipitation/electronic coating Electrodeposition Rotating coating Spray coating Self assembling

These processes act completely different from each other. Some of them produce thin films and also raw materials in atmospheric pressure like sol–gel (rotating coating is the common method) while others processes like PVD and CVD can provide thin films with very high quality under low pressure conditions. According to researches, the preparation of substrate is not necessary for some materials like carbon and some of the metals. However such preparation is necessary for many substrates. Preparation of substrate includes extensive methods such as cleaning, surface chemical modification and also using thin films under main film for the required stickiness. The necessity of clean surface for coating should not be ignored. Cleaning for the thinner coatings is harder and also has significant importance.

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1.7 Production and Application of Thin Films Currently different processes are applied for production of thin nanostructured films. Chemical vapor deposition (CVD) includes heating material (converting it to gas phase) and its deposition on the surface. In this method the speed of precipitation is low and the process needs higher temperatures. In most cases vacuum chamber is required. According to researches, this method is expensive for coating of large surfaces. Physical vapor deposition (PVD) includes transforming raw material to gas phase (resistive heating, electron beam etc.) and then deposition on the surface. There are different methods for disposition of material as thin films. Thermal evaporation, magnetron sputtering and laser pulsed deposition which are probably the high consumed methods. In this method controlling on nanoscale is still under limitation and achieving required specifications needs expensive substrates (unusual single crystal) which cause growth of thin film in specific orientation and with special structure. The process requires huge investment and to cover large surfaces (like wafers in semi-conducting industry), the expensive instrument is needed for the process. In fact this method needs vacuum chamber which limits the coating surface. As the aim of this chapter is not complete discussion of all fabrication methods, one of the most usual methods for fabrication of nanostructured films, sol–gel method, will be discussed here [131–133].

1.7.1 Introduction to Sol–gel Method The aim of sol–gel method is to perform chemical process in low temperature for producing objects, films, fibers, particles or composites with suitable form and surface. Traditional production processes of ceramics leads to fabrication of materials which has micro-structures with dimensions in 1–100 lm. Sol–gel process can change this limitation to dimensions in 1–100 nm and in molecular level. These materials usually have uniform chemical and physical properties. Sol is constant suspension from rigid Colloid ingredient or polymer which placed in one liquid. These particles can be crystalline or amorphous. Sols are diffused chloride particle in ointment on 1–100 nm dimension which due to it’s miniscule size and constant moving they stay floating. Gel is rigid network connected with pores under micrometer dimension and polymer chains with approximately more than 1 lm length. In Colloid gel the network is created by amalgamation of Colloid particle while the polymer gel has a under structure in other words polymer unites creates gel. In most sol–gel systems creation of gel occurs by covalence combination and gel is not reversible which means it can not turn into sol position again. If the gel has combination other than covalence it can be reverse [65, 134–141].

1.7 Production and Application of Thin Films

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In fact sol–gel procedure is a mineral network synthesis by chemical reactions in lotion on low temperature which is placed in lotion due to its formless size (in first stages) against crystallization. It is necessary to explain some key words and regular process. Gel’s categories are as: 1. Hydro gel: these gels usually produce in water environment, the expression Hydro gel is a gel sols filled by water it is also called Equagel. 2. Alco gel: Is the gel which it’s sols filled by alcohol, gels which drays by alcogel has ample sols and maintains it’s water structure and would have less fraction when it gets dry. 3. Gasro gel: It is a gel which has lost all the liquids inside sols and has compact structure and it’s crinkles are more visible than in Hydro gel, it specific surface has also condensed. 4. Aero gel: It is a dry gel. The liquid in the gel has gone out and has left no place for compact or change in gel’s structure. This kind of gel mostly created by heating the gel in 0 h temperature so there will be no maintenance between liquid and gas and gel will complete by high quality surface and keeping the structure in Hydro gel form, the following picture explain different postures of gel. As an example, different types of silica gel are as: (A) Hydro gel (B) gasro gel (c) aero gel (d) gasro gel with medium density Different process of sol–gel: 1. The process of Elcho oxide orbit 2. The process of Colloid orbit The most common methods of sol–gel process in producing mineral materials like glasses, Catalyst bases and advanced ceramics are Elcho oxide orbit. This method was set according to Elcho oxides in chemistry where sol created by base material of Elcho oxide resolved in water and so the Hydrolysis and density of the base material has turned sol to gel which construct mineral network under dry condition. The following equation indicates the basic sub reactions that transform base material of Elcho oxide into gel and finally mineral network. In hydrolysis reaction Elcho oxide turns into mineral types solvable in alcohol. MðORÞm þ H2 O ! MðORÞm1 ðOHÞ þ ROH MðORÞm1 ðOHÞ þ H2 O ! MðORÞm2 ðOHÞ2 þ ROH In densing reaction the activated types are reacting together and polymer network include M–O–M combination (with one Elcho oxide) or M–O–M (with different types of Elcho oxide) start to generate in salve: M OH þ HO M ! M O M þH2 O M OH þ ROM ! M O M þR OH

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In this reaction M can be Si, Al, Ti, Zn, Sn, Pb, Ta, Cu, Ni, Co … etc. and R is a Alkyl group like Methyl, Ethyl, Isopropyl, Butyl and etc. The process of Colloid orbit follows the base of Colloid chemistry to produce Colloid particle in Ionic and non Ionic in non aqua environment. In this method sol created by scattered Colloid particle in a liquid environment usually water. The sol’s viscosity increases by losing the liquid turn into hard gel. Therefore the mineral network created by separate particle design and transformation of solution into gel. Currently the two pointed methods used in sol–gel process however the Elcho oxide method is more common and most of the works in this field performed by Elcho oxide as a pre material because Elcho oxide is a suitable source for mineral monomers and in the Elcho oxide method the speed of main reaction can be controlled by chemical patterns like controlling the speed of Hydrolysis reaction. Hence chemical process of sol–gel according to Elcho oxide can be easily controlled like onus on the surface or absorbed items on the surface of particle in addition the method of Elcho oxide has the possibility to provide specific production with high purity level in low temperature. Process of sol–gel used to produce various materials from Elcho oxide to pre material categorized by series of constant stages. Each stage is a bridge and functions between last and next one. The stages are as follows: 1. 2. 3. 4. 5. 6. 7.

Mixing Forming Gelation Aging Drying Dehydration or chemical stabilization Densification and sintering

The process of sol–gel first discovered in early 19th century and it began to widespread in early 1940s. Sol–gel is a moist chemical method in which it’s pre material with M(OR) structure is turned into a mineral network which includes one metal oxide. In process of sol–gel the primary material is dissolve in a solution (considering the conditions of reactive of a sol or a gel) it precipitate. Sol–gel process consists of 4 stages: hydrolysis, densification, particle polymerization, particle growth and accumulation and creating network. The result of this process depends on different factors that affect the speed of Hydrolysis and density. Some items are more effective: Catalyst nature and thickness toward molar ratio of primary material to water, temperature, pH, initial material to produce sol usually mineral metal salts or the organic metal compound like Alco oxides. The metal used in pre material production can be a middle metal like Ti, Zr, V, Zn, Al or even a metal network like Si. In one process of sol–gel the pre material is exposed in series of Hydrolysis reaction and polymerization to create a Colloid suspension or a sol. The sol process generates the possibility to construct different forms of ceramic material.


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Thin films can be produce by rotate coating or floating coating on a sub layer. When the sol is purred in a mold it forms a moist gel with drying and heating procedure the gel is turned into glass particle or compact ceramics. If gel’s liquid has moved out in a critical condition a high porosity material with extra density called Aero gel is formed. By maintaining the sol viscosity in ideal amplitude it can produce ceramic yarn sol. Infinitesimal and constant ceramic powders can also be produced by precipitate, scattering pyrolysis or immolation methods. Nano particle process into sol–gel has the ability to produce cheap pieces which is a complex of Nano silica particle and other additives put into a container, then it’s necessary that moist gel is being dried to control and prevent cracks so the result can be transform to transparent glass. By the help of sol–gel process the ceramic or glass material can be constructed in different forms, ultra small powder or spherical, thin film coating, ceramic yarn, mineral curtain micro holes, one piece ceramic and glasses. This method can also produce different nano constructions like nano particle, nano porous material or nano yarn. Mean while it can be used to produce Thin film with different methods (like coating, floating). Although the method is a solid process but there are some technical obstacles : cracks and wrinkle of main part of process which is usually hard to control (especially with low density material and thin films with high thickness) how ever using organic mineral complex formula can solve the problem. Also there are lots of parameters which have simultaneous effect on final product. When the solvent pours out of the cavity on the time of its dryness leads to forces and contraction of gel’s network. Xerogel is the result of dryness. Comparing to basic gel Xerogel has less volume due to stress in the dryness procedure integrated gel is broken and turns to powder. When the wet gel is dried up in a situation that network cavity has not changed the volume so the result of dried gel is equals to wet gel which is called Aero gel since there is no contraction in this gel it has very low density and porous yet it has no stress of contraction. Constancy or sol’s coalescence in process of sol–gel is very important and the construction of gel’s network is related to basic form of assembled particle [142–144]. The constancy of a Colloid sol depends on Zeta potential so whenever the Zeta potential is more sol will have more constancy. For a potential it has equal surface, barrier and defense force for larger particle. The difference between gravity and defensive force with measures cause the augmentation of particle and dominancy of gravity on the defense force and consistency of the sol. Basically the coalescence of sol particle is the result of following items: 1. Reduction of surface potential by change in pH 2. Increasing electro lit thickness in the solvent in some cases the coalescence colloid can be spread which called Peptide process. Overall the sols can be categorized in 2 groups 1. Mono depress colloid sols In this type of sols all particles have equal size and form due to huge desire to agglomeration the creation of this sol is very hard.

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2. Micro molecule or polymer sols In this system particle growth generates with creation of polymer, these polymers joined each other and entrap the solvent and finally create a semi solid body called gel. This kind of sols usually can be obtained by Argo no metallic in alcohol soluble. Figure 1.3 is an example of condensed nanostructured cerium oxide sol–gel thin film that illustrates presence of cracks [145].

1.7.2 Sol–gel Chemical Reaction 1. Hydrolysis: Generated by dissolving salt in water, metal cation shapes water complex. In this condition transfer of electron double from water molecule to empty medium metal cation orbital. Hydrogen onus increased and water molecule obtains acidic quality. According to water acidic capacity, it can have different types of hydrolysis reactions. Overall the increase of bridges of metallic ions by exeo and hydro exeo ligands and increase of hydrogen in the ligands simplifies the hydrolysis process. 2. Condensation: This reaction is generated by two different mechanism of friendly core which depends on metal coordination number. Condensation created by substitute reaction of friendly core which means by the time the coordination number is not available the possibility of adding the friendly core exist. 3. Olation: A condensation process that hydro oxide bridges created between two metals. 4. Oxolation: A condensation reaction that oxygen bridges created between two metals [146–149].

Fig. 1.3 SEM images of cerium oxide treated specimens after a 30 min and b 60 min of immersion in CeCl3 solution [145]


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1.7.3 The Effect of Catalyst Hydrolysis The use of acid catalysts or alkali can have impact on hydrolysis and condensation speed so it can alter the final product. By giving their H+ the acids increase the Alco oxide speed in the Hydrolysis reaction. But by reducing the pH the Condensation reaction reduces as well. Adding alkali can either increase or decrease Hydrolysis which depends on types of Alco oxide but the speed of Condensation increase without considering the kind of material and therefore the density of chains will enhance. Gels that created by Acid catalysts have longer time to transform into jelly condition and have less density that face much contraction while Gels that created by catalysts alkali will have less contraction in the process of drying due to it’s high density.

1.7.4 Electric Precipitation The electric precipitation is a coating process based on electro circuit mostly used to produce metal coating. The precipitation occurs by negative onus of sub layers of the coating and floating it in a metal salt solvable. The metal salt ions have positive onus and the absorption of sub layer. While these ions touch the sub layer it obtains electron and creates metal coating chemically consistence by revival of procedure. Although this method has usage in copper coating or some solvent, this is not compatible to environment and is usually it is a toxic that irritates the skin for example toxic material like chromate should have alternative like coating and thin film.

1.7.5 The Rotate Coating In this method while the sub layer is rotating liquid is pouring inside the centre and by using centrifuge force it will covers the sub layer surface. About 100 percent of liquid removes out from outside surface. With increase in sub layers like wafers the process of rotating face technical problem. Impracticality of sub layers/big pressure room s/braking the sub layer and corner defects in addition to De-wetting are the main problems of creating steady thin films. Although this process is on the production line but it requires more solvent and high purity materials, also these limitations would waste big amount of material.

1.7.6 Scattering Coating Two main methods exist: plasma scattering coating, thermal scattering coating. In thermal scattering coating (also called plasma arc plating, plasma arc spraying,

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plasma coating) the powder material enters to the gas circuit pit of a plasma gun. This powder material will spray on the coating surface after the melting. Thermal spray coating includes heating material (in form of powder or wire) and speeding up to high pace by gas circuit. The particles collide into sub layer surface and change the form and finally freeze. The pace of collision is main element which affects the coating features. Controlling three variants (material, heating and speed) is necessity for obtaining reliable and repeatable which match the expected features [150–156].

1.7.7 Self Layout Self lay out created under proper condition by molecule and atomic design which occurs in chemical and physical process. It leads to self organization of atoms and molecules in suitable location with proper construction. The layout of thin films (mostly in single layers) and on sub layer surfaces which simplify molecules growth and organization. According to scientists the method is one the rare down to up methods which pass its infancy and considering many coatings before entering the industrial phase the basic science should develop. Also there is a vast confinement in the materials that can be utilized in self layout construction still there is large area to be discovered. The idea of multiple layers with same action adds another aspect to the issue. One of the technical problems insisted by many specialist is molecule activity which becomes polymerize easily under effect of moisture and create blocks on the surface and reduce the film reaction. The fact that chemical formulation and surface pattern of self layout layers are suitable for specific sub layer and leads to progress in specific usage which also become very expensive. The patterned sub layers that direct the de coring process or effect growth of thin films is an obstacle which will be appear by start of industrialized process.

1.7.8 Plasma Polymerization Plasma polymerization use source of plasma to exit gas, the eviction has the necessary energy to activate or analysis gas and liquid monomer which usually include vinyl group so the polymerization process starts. The plasma polymerization method profits from AC/RF/MW/DC and pulse methods, the method leads to steady thin polymer films with extensive width connections and heat resistance. By choosing certain kind of monomer and energy density for each monomer the chemical form and structure of thin film can be alter in a wide range. The speed of plasma polymer precipitation is identified by the following parameters: geometry system, primary monomer reaction, the pace of monomer circuit, gas pressure, conjoined frequency, signal and finally the sub layer temperature.


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Most serious obstacles like temporary constancy (thermal, mechanical, optical and electricity) or chemical and physical unevenness prevent wider applications. Some process has the capacity to improve adhesiveness, coincide shine or precipitation by energetic ions or electronic ray or X-ray. Due to polymer precipitation coating larger 3D pieces will usually face problem in the reaction position although the issue leads to waste of material, the loss is less dangerous than material waste from moist chemical process that aims for evening the surface. So there should be better understating of precipitation concerning the structure of plasma polymer film and the effects of aging and possibility of removing. The process of polishing is necessary after fixing the film by wet method or sol– gel or rotate coating. In other occasions polishing after thin film function as an important phase in the production (like thermal oxidation for obtaining SiO2 thin film or glazing with UV for improving thin film adhesiveness by creating width union). Commonly, most after production processes that have been used many years in the industry includes following items: 1. Re-decoction 2. Thermal oxidation 3. UV production According to scientists the first method is the most common way for after production [157–169].

1.7.9 Annealing Annealing is a heating treatment which has an altered material in it’s structure which leads to change in it’s features like constancy and steadiness. This process includes two phases: slow heating and cooling. The heating treatment usually leads to alteration in crystal structures of atoms. This alteration includes crystal defects removal which ends by basic changes in primary features like electric aspect of material. The main processes are gas annealing and vacuum annealing. The vacuum annealing will cause adhesive improvement, solidity stretchiness and electric treatment and material pit. Although annealing is developed process but it still needs growth in mechanism and structure control and morphology. Understanding the effect of annealing on thin film features especially thin polymer films has significant importance. High temperature of the process and long period for (slow cooling) prevents any economical advantages of this process. Some scientists believe that researches should focus on removing the annealing procedure. They also underline lack of adjustment with specific material and lack of confidence on repeatability of annealing process. Figure 1.4 illustrates an example for cross-section of SEM images of the ceria films which were obtained by sol–gel methods and heat treated at different temperatures [170].

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Fig. 1.4 Cross-section of SEM images of the ceria films which were obtained by Sol–gel methods and heat treated at different temperatures a 150C b 250C c 300C and d 400C [170]

1.7.10 Heating Oxidation The thermal oxidation is method used under high temperature (about 700–1,300C) to increase growth rate of oxide layer. The high temperature is used to enhance the speed of oxidation process (the method used by many materials like silicon that have natural low rate on oxidation). In this process the raw material of sub layer has been put in oxidizing environment (oxygen needs dry environment for oxidation and water for moist oxidation). This action leads to replacement of raw material with exponential oxide. The oxide growth rate directly depends on pressure and temperature. There are two obstacles in this method lack of confidence in process repetition and the need to observe the process implementation. Ultraviolet–visible decoction can be used for drying coating (from solid to liquid) and creating joint width between sub layer and coating. Excessive areas of materials from metal papers to polymer films are suitable for UV decoction. There two parameters in this process: UV intensity and time of decoction. It is possible that we require different time lines or intensity for various decoction phases. By the time UV is used for joint width the material has two parts. The adhesive resin and optical debut (it has been molded by resin earlier). Optical debut only react to resin when the UV light absorbed suitable (intensity and wave length).


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According to scientists when we use UV to create joint width the key issue is the pressure on the length of decoction process in the film. The proper solvable, needs conjoining elements (generate joint width) with low wrinkling or no wrinkling and specific tests. The other issue is the problem of decoction of complex 3D pieces with UV, removing shadow regions (with no light) is very hard. According to scientists view this process is standardized and perfectly applies in its restricted area but the required instruments to solve problem of under 100 nm is very expensive. Observing the decoction process is a possible solution for the suggestive obstacles. The following items are the most important applications of thin films and coatings. • • • • • • • • •

Thin film transistor (TFT) Electronic instrument with extensive surface (like displayers) Solar pills (for example thin films under sub layer of polymer glass) Navigator of wave papers (for evenness of optical pieces) M-RAM Micro electronic memory systems (MEMS) Friction reduction surfaces Insulator windows Self cleansing surfaces

Other applications of thin films are anti scratch coating (in auto pieces and optical components), electro chromic glasses (tungsten oxide layers), anti reflex coating, anti fog mirrors (by using titanium oxide on the glass), environment friendly coatings, un-adhesive material and super conductors (micro wave filters or campers of wrong circuits) [171–182].

1.8 Applications of Nanostructured Coatings Nowadays, nanotechnology is rapidly developing and promoting the quality of many products. Industries are making efforts to use these created opportunity to enhance their products efficiency and quality, as well as decreasing their products’ price. With no doubt, the future belongs to those companies and industries which invest in this area and extensively apply it. As this is a fairly new emerging technology, it may enable many industries of our country to invest in this field and realization using this technology’s outcomes and advantages. Nano-structured coating is one of most effective broadly used applications of nano-technology. In most cases, nano-structured coatings characteristics have a significant improvement in comparison with traditional ones. Some of these characteristics are: increase of hardness, wear strength, abrasion, decadence, environmental pollutions, and etc. This section attempts to point out some of most important uses of nanostructures which are currently applied or will be in an upcoming future. Various sections mostly focus on applicable cases for indoor industry. Furthermore, in

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some parts the important applications of nano-structured coatings have been noticed. The section includes different parts. At first it deals with competence and applications of coating and surface engineering. The next, discuss the ways to enhance coating quality using nano-structured coatings and capabilities of nanostructured coatings—compared with traditional coatings. Other parts are dedicated to different applications of nano-structured coatings. The categorized applications include: scratch resistant coatings, abrasion and corrosion, optical coatings, medical and hygienic applications, electrical and electronic applications, and the other applications. It must also be added that at final part of this section some materials with extended applications, namely nano-particles, are introduced with in addition to their characteristics. Next chapters focus on scratch and wear resistant coatings. Here, some cases have been mentioned, including: manufacturing panels for plane body, using nano-structured coatings in transportation, military gears, and etc. Also, nano-composite coatings are briefly discussed at this section. At next part, optical layers have been studied. Most noticeable applications of nano-structured optical coatings are for making lightproof ultra violet layers, fog and vapor resistant layers, and self-cleaning glasses. Hygienic and medical applications are introduced in the next part. Throughout this part the following cases were studied: Application of nano-particles in microbial and viral anti-pollution masks, nano-structured coatings for making prosthesis, using nanostructured coatings in food packaging, anti-pollution materials in shipping industry, nano-particle silver coatings (as a protective agent against bacterial activities), and using TiO2 nano-particles for reducing environmental pollutions. It seems that materials such as TiO2 and Ag nano-particles are currently available for many indoor industries. These materials fairly low prices make them handy for mass uses. Also, TiO2 particles can be commercially provided in a commercial scale. Electrical and electronic applications are also a big share of nanoparticles uses. The use of carbon nano-tubes, nano-structured coatings in solar cells, coating with nano-structured oxides, polarizers, and transparent conductive magnetic layers have been previously investigated. Using some of these applications for indoor industry requires high quality applied technology. It seems that some of applications are not technically approved. Using nano-structured materials for coatings process must be available and economically justified. Hence, regarding vastness of transparent conductive coatings applications, this point is of great importance. Some applications of nano-structured coatings to use in lubricators are also separately studied. Currently, it seems to be a vital issue to concentrate on nano-structured coatings for investments and applying in some industries. Among most important industries which can easily harness nano-structured coatings, one can name automobile industry, glasswork industry, packaging industry, making toiletries, and transportation and military industry. Nano-particles of some materials are of great interests for nano-structured coatings. Todays, they are widely used in some practices. Some of most important practices of nano-particles are: TiO2, CuO2, ZnO2, and Al2O3. Metallic nano-particles such as Ag, also, will be vastly used in an upcoming future.

1.8 Applications of Nanostructured Coatings

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Some of applied nano-particles characteristics, compared with those of traditional ones, are also introduced in last section [183–194].

1.9 Coating and Surface Engineering During production, packaging, and finally preparing in markets, all applied products needed form require coating. Coatings are used to enhance resistance against different environmental agents such as various types of corrosion, create new compatibility in surfaces (e.g. in optical layers), increase hardness, and improve some physical features such as magnetic and electrical ones. Practice of coating to deal with corrosion, by its own, is enough to highlight importance of using coatings. In many developed countries damages induced from corrosion is from 3.5 to 5% of gross product. Regarding to importance and broad applications of coating, applied methods in this field are permanently developing and the latest technology is applied in this field. Among main objectives of this field, which are of great interest for a long time, one can name: • Promoting coating quality, including increase of lifetime and etc. • Lowering costs for production, repair, and maintenance • Adapting with environment Todays, considering development of nano-technology and its practice in different fields, coating can be regarded among most active industries. However, before advancements in nano-technology field, micrometric and nanometric layers were used for coating and then the present issues for application of nano-technology in the other fields, were rarely seen in coating area. With respect to importance of coating in various industries, particularly in key industries, first of all we will preview applicable cases and associated problems with coating and then we will pay to nano-coating applications.

1.10 Coating Issues and Applications Coating is among main parts of surface engineering. Surface engineering is an important technology and competition of various industries depends on sponsoring of this part for them. Besides, as it will be discussed, coating is an economically vital issue. Considering this, in many countries surface engineering share in key industry sectors is defined and some of future questions and different predictions are answered. Discussed topics are similar for many countries. Then, regarding available reports in this field, upcoming developments of surface engineering markets and plans in England are studied. In 1995, England market for surface engineering exceeded 10 billion pounds, where 4.5 billion pounds of these figure

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was expended on coating for improvement and enhancement of surface properties and resistance against corrosion. This figure has caused a 95.5 billion pounds value effect on amounts of different products. Mentioned values for 2005 are estimated 21.3 and 14.3 billion pounds, respectively. In a more clear fashion, most growth is for aerospace, agriculture, automobile, electronic consumers, and electronic sections. This concludes that surface engineering does define differences between products in terms of function quality and current costs. Surface engineering involves simultaneous designing and building of surface and substrate as a system, through promoting their efficiency in term of economic viewpoint. Main objective of surface engineering is proportional changing of different technologies to obtain considered surface features in designing to reach a particular application, with very convenient cost and quality. Surface engineering can serve in a way that conveys technology and specialty between final consumers.

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Chapter 2

Size Dependency in Nanostructures

2.1 Introduction Nanostructures are some 0, 1, 2 or 3 dimensional materials which are mostly composed of one dimensional and zero dimensional nanomaterials such as nanopowders, nano-particles, nano-wires, and etc. Nanostructures consist of twodimensional nano-materials arrangement or thin layers, called nano-coatings or nanostructured coatings. For instance, nano-powders can be served as raw materials to produce nano-coatings in processes such as thermal spraying (plasma spraying and high velocity oxygen fuel spraying) [1–8]. In this chapter, at first, various types of nanostructures (especially nanocoatings) and their producing methods, including thermal spraying coatings, transitional metal nitride coatings, super-hard coatings, multi-layers, nano-composite and environmental coatings will be analyzed. Then, the role of electrochemistry in production of nano-composites and also electrodeposited coatings characteristics will be explained, and finally nano-composites application will be examined. Finally the effect of size on the properties of nanostructures will be discussed.

2.2 Nanocomposites and Their Production Methods 2.2.1 Thermal Spraying Nano-Composites Thermal spraying involves particles quick surface melting and freezing. Thermal spraying nano-composites are of higher abrasive resistance in comparison with micro-coatings. For their high hardness, thermal stability, cosmetic appearance, and chemical neutrality, transitional metal nitride coatings are of a great interest among researchers. In normal circumstances, these coatings are produced through chemical vapor deposition (CVD) and physical vapor deposition (PVD), although their nano-structural coatings can be obtained using ion beam. Mentioned

M. Aliofkhazraei, Nanocoatings, Engineering Materials, DOI: 10.1007/978-3-642-17966-2_2, Ó Springer-Verlag Berlin Heidelberg 2011

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2 Size Dependency in Nanostructures

nano-coatings are of a great hardness. This increase in hardness of multi-layers and multi-grids (two-layers) are more intense. Spraying of transition metal nitride nano-particles in an amorphous nitride matrix gives a rise to development of grains with dimensions lower than one nanometer, which makes them efficient for uses such as enhancement of abrasive resistance in copper cutting tools. Thermal spraying method is a suitable method for production of hard coatings on selected matrixes. Coating material is heated in a gaseous environment and is sprayed toward matrix surface in melted drops form, in a high velocity. Due to hits, the drops are settled in a homogenous form on the surface and convey their initial heat to cold matrix and rapidly change into solid state. Applicable raw materials in these methods include powder, rod, and wire. Regarding these materials and efficiency of regarded coatings, there are different processes based on thermal spraying, such as plasma spraying, high velocity oxy fuel (HVOF), flame spraying, and etc. In traditional plasma spraying, there is a high-temperature plasma jet in the gun. Powder particles, with dimension of several microns are injected into plasma jet, which changes them into a melted state. Then this combination is sprayed toward matrix. For quick heating and accelerating to coating process, combustion process is fairly common in HVOF method. Gaseous fuels, such as acetylene, propane, propylene or hydrogen, are mixed with oxygen. Then this gaseous combination is combusted, and produces a flame with approximate velocity of 2,000 m/s. powder particles inter into a combustion container, which involves a noble gas such as Ar, and are heated. Then particles are accelerated within a fluid under supersonic velocity toward matrix. Micro-crystalline ceramic and metallic coatings are obtained through low pressure plasma and HVOF spraying. During last decades, availability of different processes for providing nanopowders, including aerosol process, sol–gel process, chemical production, alloying, and mechanical grinding have made some progresses in producing nano-coatings. Thermal spraying methods, using nano-powders, give rise to production of coatings with higher hardness, strength, and abrasive resistance, in comparison with traditional method. It is revealed that HVOF and metallic and ceramic nano-powders plasma spraying is a useful method for creating nano-structured coatings. Since its higher velocity, drops moving, and lower thermal energy quantities, HVOF, compared with plasma spraying, produces a more compacted structure and higher cohesion between coating and matrix [9–12]. Oxide ceramics such as alumina, chromia, titania, and zirconia, are widely used as surface coating materials for improvement of abrasive resistance, wearing, and cavity. Coatings made from zirconia are used for cylinder head and piston crown at internal combustion engines to improve thermal efficiency, output force, and fuel efficiency. These coatings involve cavities which are characteristics of plasmasprayed coatings. Nano-crystalline zirconia coatings show lower porosity (8%) in comparison with micro-crystalline coating (12%). TEM test exhibits fine structure of nano-crystalline coating’s at presence of co-axis grains (60–120 nm) and columnar grains (150–350 nm). Fine co-axis grains are cooled because of homogenous germination of mentioned melt, while columnar grains growth is due

2.2 Nanocomposites and Their Production Methods

31

to heterogeneous germination in boundaries, where there is a higher cooling gradient. For efficient melting of nano-zirconia source at plasma jet, boundaries are very thin and their interface is fairly narrow. This leads to an improvement of cohesion between coating and matrix, then nano-zirconia coatings indicate lower abrasion rate, compared with its micro micro-coatings (Fig. 2.1). Over the past few years, hydroxyapatite (HAP) has been introduced as a porous layer on metallic substrates to provide easier in-growth of bony tissues. Dey et al. [13] studied the size effect on these kinds of coating that were fabricated by microplasma spray. The excellent biocompatibility and bio-stability of HAP layers have become well established and the usages of this material for prosthetic applications have been rapidly popularized recently. Plasma spraying (PS) with a high power (e.g. 20–40 kW) is the most popular and commercially accepted method of coating. However, due to the high temperature of plasma jet, the degradation of HAP occurred during spraying, which involved the formation of unwanted tetracalcium phosphate (TTCP), tricalcium phosphate (TCP) and calcium oxide phases. In addition, due to the rapid cooling of sprayed particles, amorphous calcium phosphate also appears in the HAP coatings on Ti6Al4V substrates. The degree of crystallinity (Xc) of PS-HAP coatings usually lied less than 70%. To tackle these problems, recently the microplasma spraying (MPS) process with a low power (e.g. 1–4 kW) has been used because it can provide a higher degree of crystallization, e.g. Xc * 90% and phase purity than those provided by conventional plasma spraying method. Dey et al. [13] used the metallic substrate from a surgical grade, biocompatible austenitic stainless steel (SS316L). The choice was done in accordance to better corrosion resistance properties, mechanical properties and lower cost of SS316L than those of the conventional Ti6Al4V alloy. The stability and reliability of the coated implant in vivo depend mainly upon the local mechanical properties of the layer. Dey et al. [13] used a low plasmatron power (*1.5 kW), i.e. microplasma was used to coat HAP on SS316L and the local mechanical properties, e.g. nano-hardness (H) and Young’s modulus (E) of the MPS-HAP coating were examined by the well established nanoindentation technique. The local mechanical properties, e.g. H and F of HAP and/or HAP

Fig. 2.1 Changes of abrasion with applied load for plasmasprayed zirconia coating a micro-crystalline coating, b nano-crystalline coating

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composite coating as well as global mechanical properties, e.g. microhardness have not been discussed to a great detail in literature. Most of the researchers used nanoindentation data with a Berkovich indenter for plasma sprayed HAP coating on Ti6Al4V substrate. The reported values on H and E spanned a range of &4–5 and 83–123 GPa, respectively as one profiled from the coating-substrate interface to the free coating side across the coating cross-section. The nanoindentation data revealed further, that Young’s modulus value of amorphous zone was much lower than that of the crystalline zone of HAP coating. On the other hand, for HAP coating fabricated by using Nd-YAG laser on titanium, the nanoindentation measurements with a Vicker’s diamond pyramidal indenter along the coating cross-section showed that both H and E values were lower at the coating side than at the coating-substrate interface. Nano-hardness and Young’s moduli data have also been reported for functionally graded coating (FGC) of HAP/glass composite and HAP/a-TCP composite. Others have evaluated Vicker’s microhardness and nano-hardness of different composite coating systems, e.g. (a) plasma sprayed 50 vol.% HAP/50 vol.% Ti6Al4V composite coating on Ti6Al4V substrate, (b) plasma sprayed HAP/YSZ/Ti6Al4V composite coating, (c) HAP/carbon nanotube (CNT) composite coating and (d) biomimetic HAP coating deposited on Ti6Al4V and Ti13Nb11Zr alloy substrates. Most of these reports involve a Ti6Al4V or Ti or Ti alloy substrate and thus the amount of information on micro- or nanomechanical properties of microplasma sprayed HAP coating on SS316L substrate is almost insignificantly small. Dey et al. [13] prepared phase pure and flowable HAP granule from the conventional wet chemical route. HAP coatings of thickness near 200 lm were prepared by microplasma spraying on SS316L substrates. The degree of crystallization for MPS-HAP was found to be high (near 91%). The statistical validity of their data was established through the application of Weibull statistics, because of the porous and heterogeneous nature of the coating. For both H and E values of the coating, the values of the Weibull modulus (‘‘m’’) showed an overall increasing trend with respect to load although some occasional deviations were observed. Such deviations might have risen due to the presence of pores and cracks in different layers of the coating. It was assumed that higher scatter of data at lower load could be linked to stochastic nature of interaction between the indenter that penetrated a very shallow depth and the flaws that scale with the size/depth of the indentation and which possessed a highly statistical size distribution in the surface and in the close vicinity of sub-surface region. At higher load, it was suggested that due to a larger indentation zone of influence, an averaging out effect of indenter-flaw interaction predominated to affect a reduction in data scatter. At a low load of 10 mN, the coating demonstrated a hardness value of about 5 GPa at a depth of about 170 nm which dropped by 60%, e.g. near 2 GPa at a depth of about 3 microns for a higher load of 1,000 mN. These data recommended the presence of a strong indentation size effect in the nano-hardness behaviour of the coatings. Figure 2.2 illustrates the SEM images of the polished cross-section of the MPSHAP coating taken at progressively higher magnifications: (a) at 91 K; (b) 96 K; (c) 910 K.


33

Fig. 2.2 SEM images of the polished cross-section of the MPS-HAP coating taken at progressively higher magnifications: a at 91 K; b 96 K; c 910 K, reprinted with kind permission from Mukhopadhyay [13]

2.2.2 Transitional Metal Nitride Coatings Using hard coatings for protection of structure constituents against abrasion is of a great interest. Due to their high hardness, nitride coatings, such as titanium nitride, titanium carbo-nitride, titanium boro-nitride, and titanium aluminide nitride, are very suitable for cutting tools and drilling machines. In addition transitional metal nitrides are among important materials in decorative coating industry since they create beautiful colors within the range of visible wavelength. Hard coatings of titanium nitride, produced by PVD and CVD methods are used for a long time on industrial scale. For practical apply these hard coatings must be efficiently stuck with context. In spite of those mentioned above, PVD is a linear method and coating’s cohesion to matrix is less than CVD method. This is caused by diffusion of coating material during CVD thermal process. The most important drawback of CVD method is corrosive nature of applied gasses, such as SiCl4, and TiCl4 which may jeopardize health of operators. In addition, it is possible for matrix to be deformed due to imposing in high temperature of the environment. For these applications, drills, and gears it is required to a low deposition temperature to prevent deformation of coated constituents and loss of their mechanical properties. These objectives are difficult to obtain in thermal CVD. On the other hand, a lower deposition temperature (480–560°C) is needed to develop titanium nitride coatings. However, this technology is not very handy and

34


only a limited number of commercial and industrial equipment are produced through this method. Hence, there has been an effort to produce nano-crystalline coatings with better cohesion, using PVD method with ions contributions. Atomic bombardment of developed layer can delay grain growth and cause development of nano-crystalline layers. Through IBAD there it is a strongly expectation for development of metallic nitride coatings with a noticeable improvement in abrasion, corrosion, electrical strength, and optical properties with a change in deposition parameters, such as atomic flux, ionic energy, matrix temperature, and etc. IBAD is addressed for a process through that a thin layer is developed simultaneously using PVD method, using an independent ionic beam. Though IBAD method it is possible to control ionic flux and energy. IBAD method is mostly used because of a need for independent control of layer composition and better cohesion between matrix and coating. Through changing deposition parameters, such as atom flux, ion energy, matrix temperature, and etc. it is predicted to be a particular improvement in coatings’ characteristics. Production of hard coatings with transitional metal nitrides, through IBAD method is an extensive study area. These nitrides include titanium nitride, chromium nitride, vanadium nitride, zirconium nitride, and aluminum nitride. Also, their obtained coatings have different mechanical and chemical properties. For example, titanium nitride has a structure similar to that of NaCl, but titanium nitride have more hardness, higher chemical stability, and efficient cohesion to matrix, which makes it most famous coating for cutting tools. Titanium nitride is oxidized at temperatures higher than 500°C. This causes development of pure titanium oxide, attached to titanium nitride, which leads to reduce of abrasive resistance of titanium nitride coatings. Due to development of a passive and compacted oxide layer, chromium nitride indicates a higher resistance against oxidization in comparison with chromium oxide, which limits next oxidization. Aluminum nitride is among substances which can be applied at higher temperatures, where nitrogen and aluminum atoms are bonded with strong covalent bonds. Once, this coating is subjected to high temperatures, aluminum move to surface and compose aluminum oxide layer, which is an extremely efficient barrier to prevent later oxidization reactions. IBAD method is more applied in practical investigations. At thin layers, low rate of energy (less than 100 eV) for ionic fluid is applied at lower temperatures to control fine structures of the layers. When matrix temperature is lower than 15% of coatings material’s melting point, the layer includes co-axis fine grains, ranged 20–50 nm. This is caused by low mobility of deposited atoms at lower temperatures of the matrix. Next zone is fine transitional zone of the fine structure between columnar zones, where temperature varies between 3 and 15% of matrix melting point. Atoms can migrate at higher temperatures of matrix due to surface diffusion. In next zone one can observe columnar structure since deposited atoms have enough surface mobility to diffusion and increase of grain size. In final zone grain growth is controlled by volumetric diffusion and obtained when matrix temperature is higher than 50% of melting temperature [14–18].


35

2.2.3 Super Rough and Super Hard Nanocrystalline Coatings At industrial applications there is an increasing demand for coatings having higher resistance against oxidation, higher hardness, and longer life than those of single layer coatings. To supply industrial needs for development of improved coatings, there has been many efforts to design and produce super consolidated coatings. Some researchers proposed notion of designing solids with strong coatings, using two alternative layers with high and low elastic constants. Each layer’s thickness must be in nano range and there must be no dislocation source between layers. If dislocations could be created in the zone of materials with lower modulus, they must be overcome to the noticeable stress diffused from the phase with higher modulus, before creep phenomenon (along the layers). Thus they must prohibit the creep along the layers. Such multilayer coatings are called super-lattice and their two layers can be metallic, carbide, and nitride. A multilayer includes different piled materials on atomic scale. During multilayer coatings designing both related structural and constitutional factors must be considered. These factors are: Grain size, layers individual thickness, combination module, the number materials interfaces (assuming the last layer is resistant against abrasion) [19–21]. Physical and mechanical properties of some hard materials can be combined in multilayer coatings, leading to optimization of materials properties. Abrasion is one of most important factors for destruction of engineering equipment. For instance, cutting tools are subjected to great loads, high temperatures, and inefficient lubrication; hence during mechanisms such as scratching, cohesion, thermal softening, and chemical abrasion there will be an overall abrasion on them. Then, to improve their characteristics it is recommended to use some multi-constituents coatings such as titanium nitride, aluminum/titanium nitride and aluminum/chromium/nickel nitride. Succeeding progresses leads to bring on development of multi-layer coatings such as titanium-aluminum nitride, chromium nitride and aluminum-titanium nitride and vanadium nitride. It seems that these multi-layers are of a higher potential for improving cutting tools lifetime. According performed studies, multilayer coating of aluminum-titanium nitride/chromium nitride have highest abrasive resistance and hardness, in comparison to other coatings. Besides, multilayer film of titanium nitride/aluminum nitride also enjoys both high mechanical and anti-oxidation properties. When it comes to compare multilayer coatings with single layer one, there reveals to be some advantages and disadvantages including: 1. A multilayer film may have a better hardness and ductility, comparing with all layers one by one. 2. A multilayer film with limited thickness has equal or higher mechanical stability with each of single layers. 3. A multilayer with desirable constituents from different single layer films can be adapted with practical needs. 4. There is an increase in cohesion between multilayer film and matrix. 5. Remained stress in multilayer film decreases. 6. Multilayer films have a more compact structure.

36


There are different methods for producing these multilayers but the most common way is through evaporation, due to its highest efficiency among the other methods for controlled preparation of high quality structures on atomic scale. On the other hand, electrochemical methods are also very efficient, for their low costs and possibility for mass production. As well as abrasive properties, which are initial reasons for using multilayer coatings, reaching to suitable magnetic properties it is suggested to use such nano-multilayers. For multilayer coating—where growth conditions are decent—it is possible for magnetic stabilization at one direction (vertical to layer plain). Particularly, some multilayer films based on Co, such as Co/Pd, Co/Pt, and Co/Au, indicate a high magnetic anisotropy at vertical directions. Tri-layers of Co/Cu/Co have same situations [22–34]. It has been proved that this anisotropy of the properties is due to Co layer thickness. When its thickness decreases (up to 0.4 nm) its magnetic properties have an increase and magnetic direction of multilayer film changes from parallel to coating layer to vertical on Co layer thickness. Current advances in coating technology, using PVD and CVD plasma methods, lead to deposition of multilayer coatings with more preferable mechanical and chemical properties. As an example for these multilayer structures, one can name Al/Cu and Al/Ag. Once dual layer’s constant reaches to 5 nm, hardness of vanadium nitride/titanium nitride and niobium nitride/titanium nitride coatings reaches to 50 GPa. Super-lattice coatings enjoy higher hardness than that of single-layer coatings such as titanium nitride, vanadium nitride, and niobium nitride. Increasing hardness in super-lattice coating was investigated, based on examination of dislocations mixed movements within and into the layers. The model implies a maximum peak, where there is a difference in shear modulus between two materials and their sharp interface. Here, once super-lattice constant is more than 5 nm its hardness declines to 14 Gpa. Super-lattice’s physical properties have made them suitable to be used in Micro Electromechanical Systems (MEMS), as a small tool for protection against abrasion. Layers in super-lattice should be amorphous; as amorphous can connect the lattice more conveniently. Hard singlelayer nano-composite coatings were designed, using plasma CVD process. This is occurred at high frequency under direct current. Through this process a hard transitional metal nitride and a covalent nitride (e.g. silicon nitride or bore nitride) are simultaneously deposited to obtain immiscible phases with interfaces and high cohesion energy. In the other words, the coating includes transitional metal nitride, where nano-crystalline with 4–6 nm size is located in an amorphous matrix with thickness of less than 1 nm. Such a coating is called nano-composite layer [35–47]. As an interesting example of size dependency, plasma electrolysis has been used for fabrication hard nanocrystalline layers. The usage of nanocrystalline plasma electrolytic saturation by applying pulsed current in an organic electrolyte based on Glycerol has been studied. Response Surface Methodology was applied to optimize the operating conditions for small nanocrystallite sizes of coatings. The levels studied were peak of applied cathodic voltage range between 500 and 700 volts, peak of applied anodic voltage between 200 and 400 volts and the ratio


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Fig. 2.3 SEM nanostructure for treated samples by cathodic plasma electrolysis with average size of a 32.6 nm and b 95.1 nm [48]

of duty cycle of cathodic direction to duty cycle of anodic direction of 0.25–0.35. The usage of high applied cathodic voltages and low anodic voltages and also low ratio of duty cycle of cathodic direction to duty cycle of anodic direction is more suitable for achieving lower sizes of complex nanocrystallites. The samples with high height to width ratio of distribution curves of nanocrystallites have simultaneously, smaller average sizes and lower length to diameter ratio of nanocrystallites [48]. Response surface methodology proved to be fairly accurate in predictive modeling and optimization of conditions for minimizing the average sizes of nanocrystallites obtained in pulsed bipolar nanocrystalline plasma electrolytic carbo-boriding, and that the average sizes of nanocrystallites to be reasonably approximated by quadratic non-linearity. In this process, the samples with high height to width ratio of distribution curves of nanocrystallites have smaller average sizes of nanocrystallites and lower length to diameter ratio of nanocrystallites. Figure 2.3 illustrates SEM images of treated samples with different effective factors. These samples have different average size of nanocrystallites. Narrower distributions for lower average size of nanocrystallites were observed for these samples. Figure 2.4 illustrates the distribution curves of these samples [48].

2.2.4 Nanocomposite Coatings The first investigations on composite coatings were performed in 1962. In 1970 for the first time Ni-SiC composite coating was used to improve engines abrasive resistance. This composite is yet applied for some panels in automobile industry. Composite coatings are obtained through simultaneous deposition of tiny neutral particles in a metallic matrix. Due to its competence for producing films with excellent mechanical properties such as abrasive resistance, wear strength, hardness, and lubrication, this method is matter of great interest. Simultaneous deposition of non-metallic and metallic phases for development of composite layers has

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Fig. 2.4 Distribution curves of nanocrystallites for mentioned treated samples in a Fig. 2.3a and b Fig. 2.3b [48]

a significant improvement in most of mechanical and physical properties of the coating. Such properties depend on neutral particles morphology in composite coating. Furthermore, metallic matrix of nano-composite coatings exhibit unique optical and magnetic properties and are promising for production of materials for fine tools. Applied ceramic particles mostly include aluminum oxide, carbide, chromium oxide, titanium oxide, molybdenum oxide, tungsten carbide, and etc. Besides, polymeric particles such as polyethylene and polytetrafluoroethylene are used to decrease friction ratio and achieve a nonstick composite surface. According to performed studies, fine-grained Ni-SiC composite has a smoother surface and there


39

is stronger bond between SiC and Ni. Once SiC particles are bigger than 0.1 lm, usually there develops an oxide layer on SiC particles which have a weak bond with nickel matrix, which leads to development of cavities and cracks in grains boundary. On the other hand, interface of a very fine SiC and mixed Ni is free of any defect. In the same volumetric fraction very fine particles are more abundant, which prevent grains growth at higher temperatures. However, investigations show a decrease in particles size leads to decrease of simultaneous deposition of the particles. It was showed that concentration of SiC (with dimension of 0.1 lm), obtained from spinning wheel test, in a nickel sulphamate solution is less than 0.7 weight percentage, which is very close to thresholds scale obtained from EDS analysis. In contrary, concentration for carbide, where grain size is 0.2 and 2.8 lm, is 2 and 6 volumetric percentage, respectively. In general, concentration changes of polyethylene particles surrounded in the matrix on an electrode of a spinning plain is obtained basically from throw analysis. According this model, the required amount for simultaneous deposition of 5 lm particles is 10 time less than that of 20 lm particles. Although it is long time since hard metallic coatings application through plating deposition has a drastic advancement, but mechanisms of simultaneous deposition have not completely been solved, yet [49–61]. Guglielmi was the first who proposed successful two-staged absorption mechanism. Through this mechanism he suggests that the results depend on volumetric fraction of co-deposited particles with Langmuir absorption isotherm. The first step of this free absorption mechanism is where particles from metallic ion coating on the cathode have a considerable amount of free physical absorption. In this step there is a layer of absorbed ions and solvent molecules; and also there is a reaction between electrodes and particles. The first step is a strong absorption which seems to be with contributed to electrical field, as a strong electrochemical reaction causes strong absorption of the powder on the electrolyte. Absorbed particles progressively are surrounded by metallic layer. This mechanical model can be expressed as equation below: C Mi 1 expðA BÞg þ C ¼ a nFqm V0 k

ð2:1Þ

where: M: deposited metal’s atomic weight, io: exchanged current density, n: deposited metal capacity, F: Faraday constant, qm: density of deposited metal, g: extra voltage of electrode reaction, and k: Langmuir isotherm constant, which is determined by intensity of the reaction between particles and cathode. B and V0 parameters are dependent on particles deposition and both play the same role with A and i0, which are dependent on metallic deposition. Guglielmi model’s parameters changes with deposition system changes, such as SiC and titanium oxide particles with nickel in sulphate bath or alpha aluminum oxide particles with copper in CuSiO4 plating bath. The mechanism shows a simple effective method to analyze direct effect of basic parameters on composite plating [62–68].

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Recently, electrodeposition of tertiary Al2O3/Y2O3/CNT nanocomposite by using pulsed current has been studied. Coating process has been performed on nickel sulphate bath and nanostructure of obtained compound layer was examined with high precision figure analysis of SEM images. The effects of process variables, i.e. Y2O3 concentration, treatment time, current density and temperature of electrolyte have been experimentally studied. Statistical methods were used to achieve the minimum of corrosion rate and average size of nanoparticles. Finally the contribution percentage of different effective factors was revealed and confirmation run show the validity of obtained results. Also it has been revealed that by changing the size of nanoparticles, corrosion properties of coatings will change significantly in same trend. AFM and TEM analysis have confirmed smooth surface and average size of nanoparticles in the optimal coating. The Taguchi method for the design of experiment has been used for optimizing tertiary nanocomposite electrodeposited coating process parameters for the corrosion protection of treated samples. The contribution of Y2O3 concentration is more than the sum of the contributions of all the other three factors. It is evident that, among the selected factors, Y2O3 concentration has the major influence on the corrosion rate of performed coatings. It can be seen that the current density is second important factor that affects on corrosion rate of the treated substrates. Furthermore, it can be assumed that treatment time and temperature of electrolyte have almost the same effect on corrosion rates of coatings because of the minor difference in the contribution percentages among these two factors. By ranking their relative contributions, the sequence of the four factors affecting the corrosion rate is Y2O3 concentration, current density, treatment time and temperature of electrolyte. In the case of average size of nanoparticles ranking of effective factors by their relative contributions is as same as for corrosion rate which show strong relation among these two measured properties of coatings. AFM and TEM analysis have confirmed smooth surface and average size of nanoparticles in the optimal coating. Figures 2.5 and 2.6 illustrate the SEM and AFM images of optimal coating, respectively [69].

2.2.4.1 Nitride Nano-Composite Coatings These coatings have typical structure of nc-MnN/a-Si3N4, where c and n are, respectively, crystalline and amorphous phases and Mn stands for transitional metals such as Ti, W, V, and Zr. In nano-composite coatings, transitional metalnitride phase is hard enough to bear exerted load while, on the other hand, amorphous nitride provides flexibility of the structure. Based on computer simulations plastic deformation in nano-crystalline materials, where particle size is less than 10 nm, can be corresponded with particle boundary. Here, grains boundaries slip—which is controlled by diffusion of grain boundary—may be responsible for plastic deformation in nano-crystalline materials. Slip is caused by atomic movements and stress induced from 3D free migration; in the other words, once nano-crystalline materials are extremely tiny indicate soft behaviors.


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Fig. 2.5 SEM nanostructure of optimal nanocomposite coating [69]

Fig. 2.6 AFM surface profile of optimal nanocomposite coating [69]

Hence, an increase of hardness is required locking in grains slip boundaries. Indeed, this is the reason for increase of hardness in nc-MnN/a-Si3N4 system, for nanocomposite coatings of nc-TiN/a-Si3N4 and nc-W2N/a-Si3N4, where particles’ size decreases up to 4 nm. It was declared that these developed nano-composite coatings by CVD method, will reach to diamond hardness (70–80 MPa), where grain size is about 2 nm. Achieving a high hardness, nitride phase concentration must be around 17–23 molar percentage. The reason for hardness increase is progress of submerged nitride’s nano-structure. nc-MnN/a-Si3N4 system shows noticeable thermal stability until 1,000°C. CVD plasma process provides high chemical activity for gas and controlled surface mobility, as well as ionic bombardment. Other methods such as PVD can be used for preparation of other nano-crystalline/ amorphous coatings, such as titanium carbide in a carbon matrix or tungsten carbide

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in carbon matrix, which are of a unique combination of hardness and ductility. Carbon serves as a hard, ductile, and lubricating matrix; while nano-particles act as crystals which enhance hardness and other mechanical properties. As nc-MnN/aSi3N4 with high elasticity typically shows brittle behavior, some researchers designed nano-grain coatings (where grain size is 10–50 nm) with high ductility in an amorphous matrix. This state leads to development of dislocations; however they are too small for expansion of cracks. Segregation of larger grains leads to adjustment of non-apparent strains and development of nano-cracks between crystals, which finally results in plasticity behavior. According to this state, titanium carbide coatings in a carbon matrix include: hardness of 30 GPa, fraction coefficient: 0.15–0.2, and ductility: 4 times greater than nano-crystalline titanium carbide. According to above, super-lattice or multilayer coating is materials which can be applied in MEMS method. All in all, silicon and other electronic materials are used for production of mechanical miniature panels (micro-machines), such as membranes, cantilever, gears, engines, and valves, using standard process of concentrated circuit industry instead of surface machining. Surface machining is a process for creating surface structures from tiny deposited layers. Surface fine-structures’ thickness varies from 0.1 to several micrometer to final size of 10–500 micrometer. Currently, some researchers produced super-nano-crystalline coatings of diamond with CVD method, by short waves using unique chemical such as C–Ar or methane-Ar. Hence carbon couples are obtained from methane through following reactions. 2CH4 ! C2 H2 þ 3H2

ð2:2Þ

C2 H2 ! C2 þ H2

ð2:3Þ

There is a very small amount of hydrogen in atmosphere. Through traditional CVD method, developed diamond film constitutes: methane (1%) and hydrogen (99%), and an extra hydrogen contained gaseous mixture. This extra mixture solves diamond phase and develops columnar morphology with larger grain size and higher surface roughness. Final rough surface of diamond microstructure can cause extra scratches along slip plain. It was applied the term of super-nano-diamond coatings to make a distinction among these materials, micro-structures of diamond with grain size of 1–10 micrometer, and nanocrystalline diamond (50–100 nm). AFM studies for thin films of super-nano and micro diamond showed that super-nano diamond coating has s smoother surface. These coatings’ hardness is about 88GPa and their modulus is close to that of mono-crystalline diamond (70GPa). Besides, their fracture strength is too much more than that of silicon, silicon carbide, pseudo-diamond carbon, and mono-crystalline diamond. This film’s fracture coefficient is comparable with that of natural diamond and its abrasion against hard materials is a minimum amount, due to smooth appearance of the surface. Thus, in these layers with improved mechanical and tribological properties, are ideal materials for MEMS applications [30, 70–79].


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2.2.4.2 Nanocomposite Coatings of Ni/Al2O3 Nano-composites coatings of Ni/Al2O3 are used to enhance abrasive resistance of metal’s surface in micro-tools. Although micro-composite coating of Ni/Al2O3 has had a significant advancement, but there are some difficulties during their preparation. Volumetric amount of alumina particles in Ni/Al2O3 composite coating is not controllable in quantitative sense and particles in composite coating are persistent. Some researchers recorded that alumina particles can easily stick to each other in electrolyte. This causes weak mechanical properties in the coatings. Alumina weight in composite coating can be increased between 3.5 and 14.6%, using inverse pulse electrical deposition, which results in improvement of mechanical properties. In spite of that distribution of tiny alumina particles is yet a problem during coating preparation. Putting smaller neutral particles in sediment layer is more difficult, due to problem of neutral particles distribution. Volumetric amount of nano-particles within the composite coating under work circumstances is very few. Distributed particles in an electrolyte solution are persistently moving. Once one particle reach to another one, their energy content defines weather they are separated or connected. Particles connection occurs when their absorption energy is higher than detractive energy. The pure energy in a continuous structure rests upon nature and condition of the system. Information about structure of interface zone is an important factor to perceive stability of solid particles dispersion in an electrolyte. For creating decent dispersion for alumina particles in a nickel sulfamate bath chemical and physical methods, which change particles size in interface zone, are necessary. Chemical effect occurs once particles involve absorbed surfactants or macro molecules for development of electrostatic interference in internal particles. Under particular circumstances this interference results in increase of absorbed layer rejection and situational entropy release at internal particles. On the other hand, chemical effect occurs once particles absorb a destructive energy such as ultrasonic. Creation of ultrasonic waves in liquid environment results in an extraordinary pressure (100 atm), which induces huge stress and destruction of cohesive energy between internal particles. Through previous investigations, the average size of continuous alumina in deionized water, and nickel sulfamate bath were 183 and 1,109 nm, respectively. It seems that effect of solution’s ionic stability on particles accumulation in nickel bath is not negligible. Average dimensions of continuous alumina using physical dispersion by ultrasonic energy decreases up to 280 nm, while this reduction is 448 nm when it comes to use chemical dispersion released from surface factors in nickel bath. Although chemical and physical dispersion are considered at electrochemical preparation of nano-composite coating, these methods, to some extent, impede dispersion of neutral alumina particles in nickel sulfamate bath since electrolyte ionic concentration is an important factor in effective distribution of aluminum particles. Alumina particles distribution in a dilute nickel sulfamate bath, along using ultrasonic dispersion, is an effective method to prevent continuity of alumina particles [80–93].

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2.2.4.3 Al Based Composite Nano-Coatings Al based composites with aluminum borate whiskers—which are created using high pressure casting—indicate a comparable strength and modulus with those of aluminum composites with SiC or silicon nitride whiskers. However, they have a lower thermal expansion and higher abrasive resistance. Besides, another priority of these whiskers is their very low costs in comparison with those of SiC—1/20 of SiC whiskers. Hence, aluminum borate whisker is of great qualifications for expansion of aluminum based composite applications. Also, based on existed theoretical and empirical studies, it was revealed that aluminum borate whisker is unstable in Al alloys, and the reaction occurs in their interface. To control reaction in interface, nitriding process of these whiskers, based on thermodynamic calculations, was suggested. To reach a continuous and homogenous phase nitrided nano-coating must be used. Phase analysis implies presence of BN and alumina on nitrided surface. Nitrided nano-coating with thickness of 40–60 nm isolates the whisker from surrounding matrix and aluminum/coating interface will be free reaction productions [60, 94–104].

2.2.4.4 Al/TiO2 Nanocomposite Coatings Titanium oxide is of abundant usage in gas sensors and photo-catalysts. For example, it is used in gas sensors to detect explosion released gases such as natural gas and hydrogen. Due to their crystalline structure, surface area, their cavity types (in terms of opening and closure), and their size distribution, photo-catalysts are used for segregation of air pollutants and organic contaminator in waters. It has been currently shown that TiO2 nano-coatings are of a greater sensation compared with that of micro-structure ones. The easiest and simplest way to achieve a nano-coating with thermal spray method is using raw materials with nano-size. However, directly adding such nano-powders during spray process is difficult. Moreover, plasma or gas flame leads to melting and removing its initial structure. Therefore, it was achieved that better characteristics through simultaneous spray of the other substance which prevents development of Ti-O2 powder in the furnace. Thus particles of metallic Al, which are of a lower temperature and higher reactivity in comparison with TiO2, are added to Al/TiO2 composite powders to enhance spraying efficiency. Al particles have significant role to create homogenous sediment. They lead to reach to unique characteristics of nano-structures, maintaining nanometric structure during spraying process [105–115].

2.2.4.5 Al/Al2O3 Nanocomposite Coating Useful effect of alumina nano-particles was recorded in 1990’s. It is found that development of nano-size dual metallic phases in alumina can noticeably enhance its thermal and mechanical characteristics. Metallic phase exhibits higher thermal


45

conductivity and resistance against thermal shock in comparison with alumina ceramic. Also, metallic phase can increase ceramic’s ductility as metallic particles deform plastically. In performed operations on metallic/alumina nano-composites, metals such as Cr, Ni, Fe, W, titanium carbide were used, which leads to 2–3 times increase of ductility. Second phase has been added through mechanical combining of alumina and metallic powders, and their under-pressure sintering of graphite crucibles. The main problem of mechanical combination method is to find out how to reach to second phase’s fine dispersion and favorite thermal expansion difference between alumina and metal. Thus, a chemical coating method was used for preparation of ceramic/metallic nano-composites, which has variant advantages compared with mechanical combination method. The obtained powder in this method is more homogenous and of a higher cohesion between metal and ceramic. Preparing nano-composite coating of Al2O3/Al wet chemical coating method was applied. Aluminum nano-particles are solved in appropriate solution, then Al2O3 is added, and finally considered composite is deposited in the solution. Through occurred reactions, there develops a thick Al(OH)3 layer on aluminum particles’ surface which, after calcification, is converted to alpha alumina nano-particles (with grain sizes of 10–20 nm) and distributed Al particles. The advantage of Al2O3/Al composite is development of a thin transition layer between Al and Al2O3, which is able to improve their bond [110, 116–127].

2.2.4.6 Nanostructured Coatings of Tungsten Carbide/Ni-Co Although tri-valence chromium ions, and particularly hexa-valence ones, are very poisonous, chromium plated coatings are widely used to enhance surface abrasive resistance. Another problem of plated chromium coatings is their decrease in thermal mobility with increase of temperature, so hardness and abrasive resistance of plated layers reduces. Hereabout there have been many studies in surface engineering to find a suitable substitute for this coating, leading to promising results. First choice is tungsten carbide or tungsten-carbide/cobalt. As it previously mentioned nano-crystalline materials show unusual chemical, physical, and mechanical properties, in comparison with amorphous ones. This is caused due to nano-crystalline materials’ noticeable decrease in grain size and volumetric ratio of grain’s boundary, and triple connections. Here, a decrease in tungsten carbide grain size up to 70 nm in tungsten-carbide/Co composite leads to a two-time increase in abrasive resistant. Nano-crystalline nickel with grain sizes of 10–20 nm, created with electrical deposition method, has abrasive resistance of 100–170 times and friction ratio of 40–45% higher than that of multi-crystalline nickel, where grain size is 10–100 lm. it was found that nano-composite coating of diamond in nickel matrix under effect of distributed nano-diamond strength indicates less internal stress and higher fine-hardness. Mentioned nano-composite shows excellent abrasive

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characteristics at room and even higher temperatures. Anti-abrasive properties of this composite coating are 4 times more than that of pure nickel coating [128–130].

2.3 Electrochemistry Role in Production of Nano-Coatings Electrochemistry is an advanced technology in production of nano-particles. Before studying use of different electrochemical methods for nano-coatings production, first it should be defined that how colloidal chemical state leads to creation of nano-particles. This leads to better understanding of electrochemistry concept and its effect on nano-coatings. In colloid science, nano-particles mostly obtained from surfactant contained saturated solutions. The first rule of organic ligands is inactivation of surface and development in suspending state. This preparation technique of nano-particles is called engaged sedimentation. Similar methods for development of nano-particles on conductive matrix have dramatically advanced in electrochemistry. It has been proved that adding surface intermediates can lead to deposition of nano-particles during plating. Additives prevent particles growth and maintain particles’ size to be approximately constant. A more common method is creating changes in plating parameters, e.g. voltage or current. However, there is another two-step method including a high extra voltage in a short time for germination of metallic particles on surface and then slow growth of particles in a lower extra voltage. Low extra voltage results in minimum change (about 7%) in particle size. this stops diffusion of mixed layers and decrease in growth rate. particles shape produced by engaged electro-deposition depends on applied matrix and extra voltage. Metals such as Au, Ag, Ni, and polymeric nano-particles with spherical geometry on graphite matrixes, are created by this method. Palladium nano-wires with 55 nm diameter and length of several hundred meters were created through this method, which are used in a polymeric matrix as hydrogen sensor. It is worthy to say this wires strength decrease when they are subjected to hydrogen [80, 131–140].

2.3.1 Electro-Deposition Using Porous Templates Electro-deposition is one of the effective methods in nano-composite production. For its low costs and high production potential, it is of a great interest. The only way to produce nano-coatings through this method is changing parameters such as current, voltage, bath composition, pH, and etc. It is also found that in most cases created coatings properties with electro-chemical method is preferable, compared with the other methods; because most compacted coating without any pre-stress is produced through this method. Material development using porous templates to control size and shape is a common method to create nano-particles. Despite, there are many problems of using templates in sedimentation methods, due to

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heterogeneity and pores block; however grain growth in electro-deposition can only occur suing a template. Template electrodes are constituted from materials such as etched Mica and porous alumina membrane. Electro-deposition is applied using a template for preparation of nano-wire made of different materials. Through an advance initiative in production, using templates, nano-wires are created by periodic movements of wording electrode in a solution including Au ions and a solution including Ag ions. Difference in Au and Ag cross sections creates wires with nanobarcodes. Electro-deposition method with template, for preparation of materials with high surface area includes used nano-pores. Spherical poly-styrene nanoparticles are created on an Au matrix of a colloidal cell. In electro-deposition a metal develops on an electrode, a metal-polystyrene develops, and then polystyrene particles are solved and a metallic layer with nano-pores will create. Currently, so many researchers have had focus on common plating methods with direct current as deposition methods for creation of nano-crystalline materials. In most cases, electro-deposition is a product with no porosity on it and there are no integration processes, compared with other methods for producing nanocrystalline materials. Through this method one can either create coating on surface or make a definite shape (such as foil, sheet, or regular shapes). Using this method, some special metals (e.g. Ni, Co, Pd), dual alloys (such as Ni–P, Co-W, Ni-Zn, and Ni-Mo), and triple alloys (like Ni–Fe–Cr) can be produced. Basically, electrodeposition results in production of nano-structural material whenever process parameters (such as bath composition, pH, temperature, extra voltage, and etc.) are selected in a way that electro-crystallization induced by germination is in a high rate and grain growth has a low rate. Electro-crystallization occurs under effect of two competitive reactions: production of new crystals and growth of existed crystals, under effect of different factors. Two main steps determining the rate are: charge transition step on electrode surface and surface diffusion of extra ions on crystal surface. Grain growth occurs at low extra voltages and high surface diffusion rate. On the other hand, high extra voltages and low surface diffusion lead to development of new grains [141–153].

2.3.2 Nano-Coatings Properties Determined properties associated with crystalline nano-coatings reveals that these properties can be categorized in two groups: 1. Coating properties which are strongly depended on grain size including: abrasive resistance, strength, malleability, hardness, friction coefficient, electrical resistance, solid solubility, hydrogen solubility, permeability, local wear resistance, stress corrosion cracking, and thermal stability. 2. Properties which are weakly influenced by grain size, including: bulk density, thermal expansion, Young modulus, and coercivity.

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2.4 Mechanical Properties As it is expected plastic deformation behavior of nano-crystalline materials is strongly depended on grain size. Most performed tests are related to determination of fine-hardness at room temperature on samples with thickness of 0.1–0.5 lm; where first they are plated on Ti matrix and then Ti is used to determine finehardness. The results of hardness measurement for plated Ni–P whiskers at room temperature were reported. Same results were obtained for Pd and Cu produced from neutral gas evaporation method. An increase in grain size is accompanied with considerable decrease of hardness in range of lower than 20 nm. These observed reductions of hardness are not corresponded with Hall–Petch behavior. Recently, performed investigations on tensional strength of Ni nano-crystal at room temperature have shown a behavior similar to that of determined with hardness. It is found that grain boundary diffusion in creep phenomenon is not an efficient factor to determine mechanical behavior of Pd and nano-crystalline Cu at room temperature. Start point for hardness decrease, i.e. deviation from Hall–Petch behavior, occurs once triple lines occupy a high ratio of sample volume. This phenomenon is generally in accordance with softening effect of triple lines. Through electrochemical grinding of wires to sizes lower than grains average size, triple connections can be displaced in fine structures. At all cases this transition, increase of strength, and decrease of malleability is shown from co-axis state to columnar one. Modified theory of dislocation locking with fewer numbers of dislocations can be used to explain deviational behavior from Hall–Petch equation. It was shown that there is a considerable decrease in Hall–Petch gradient—obtained in critical circumstances—due to presence of a spread dislocation cycle. Some researchers state that dislocation mechanism is not used for nano-crystalline material with grain size lower than a critical limit, for example 10 nm, for FCC metals. A combined model, based on above geometric assumptions for matrix, volumetric ratio of intra-crystalline and crystalline constituents, were proposed to determine nano-crystalline materials strength. It has been proved that the model can be applied for interpretation of different approaches including deviation from Hall– Petch equation and negative gradient of Hall–Petch curve. This analysis includes quadric nodes where triple lines meet each other, as well as grain binderies and triple connections. Strength distribution for grain boundaries (rgb), triple links (rtl), and triple nodes (rqn), as: rqn [ rtl [ rgb Researchers also, reached to an analytical explanation to examine creep rate of nano-crystalline materials for a diffusion mechanism involving triple lines. General rate of the creep is sum of creep rate due to network diffusion, grain boundary diffusion, and triple line diffusion. It has been proved that, due to triple line diffusion, creep speed has stronger association with grain size compared with grain

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boundary diffusion. For instance, distribution of triple line diffusion in creep speed at stable stage seems to be inverse power of four, which is one order more than that of grain boundary diffusion and two orders more than that of grain-size-depended network diffusion in matrix. Besides, in comparison with dislocation mechanism, where applied stress power is more than 3, secondary creep rate, yet, is linearly related with tensional stress. Conclusion was that at higher stresses, grain boundary slip is main deformation mechanism at room temperature for pure electro-deposited nano-crystalline nickel. There was recorded a negative gradient of Hall–Petch, where grain size was less than 10 nm. One can claim that deviation from Hall–Petch equation might be for dynamic creep process induced from diffusion mechanisms [154–164]. Thanks to advancements in applications of electro-deposited nano-crystalline materials, recently a comprehensive investigation was carried out on their mechanical properties. Also yield strength and tensional strength increase with grain size decrease, well as considerable increase in hardness. It is interesting to know that work coefficient of hardness decrease to around zero where grain size reduces up to 10 nm. For typical materials, material malleability with reduction in grain size up to 50% of length to fracture in tension decreases to 15 and 1%, where grain size is 10 and 1 nm, respectively. It is found that in most of cases there is more malleability in buckling. For grain sizes of 10 nm, a slow return in malleability was recorded. Compared with multi-crystalline Ni, abrasion rate of electrodeposited nano-crystalline Ni is intensely decreased and its friction ratio is fairly low. In contrary with recent calculation about nano-crystalline materials, carried out with methods with homogenizing stage, electrodeposited nanocrystalline Ni shows no significant decrease in Young modulus. This approves the previous obtained results which mention a drop in Young modulus in nanoprocesses is due to high value of remained porosity. Numerous investigations have been performed on the formation of nanocomposite layers during recent years, such as papers about Ni–P-TiO2 as lubricious layer, Ni-SiC and Ni-Co-SiC as wear and corrosion-resistant coatings, Ni-TiO2 as photocatalytic layer, and Ni-SiO2 as corrosion-resistant layer. The most important features of a well-performed layer are constant concentration along the nanocomposite layer and uniform distribution of nanoparticulates in matrix. Some modifications in electrodeposition such as using pulsed current and ultrasonic bath were usually employed for better dispersion of nanoparticulates in obtained nanocomposite layer. Some papers reported usage of ultrasonic bath during nanocomposite electrodeposition process. Results about the effect of the ultrasonic condition outside of the cell during electrodeposition demonstrated that the ultrasonic condition increases uniform distribution of Al2O3 nanoparticulates but decreases their concentration in the metallic matrix. Also, some reports were published about the effect of pulsed current on electrochemical coating process. It has been revealed that usage of pulsed current will lead to fabrication of harder nanocomposite layers. Pulse generator has been utilized for fabricating nanocomposite layer in order to achieve more concentration of carbon nanotubes and to increase the uniform distribution of nanoparticulates in deposited layer [137].

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There is no wide study on specific nickel-tungsten/carbon nanotube (Ni-W/ CNT) nanocomposite layer formation by electrodeposition. Ni-W/CNT nanocomposite layer was performed by pulsed current and study the concentration of nanoparticulates and process effective parameters on the electrochemical and mechanical properties of coated samples. Distribution of nanoparticulates in nanocomposite layers has also been investigated. The effect of duty cycle on distribution of carbon nanotubes in nanocomposite layers shows strong attendance but does not change the W content in the metallic matrix. Microhardness increased for different nanocomposite layers with different amounts of carbon nanotubes. Microhardness of nanocomposite layers did not change significantly by changing the duty cycle. Figure 2.7 illustrates the nanostructures of nanocomposite layers formed by different (low, medium, and high) duty cycles of pulsed current. Comparison of nanostructures of obtained nanocomposite layers shows that increasing duty cycle significantly alters the distribution and content percentage of carbon nanotubes in nanocomposite layers. It has been revealed that carbon nanotube content will increase from 4.3 to 13.1 wt% by increasing duty cycle from 20 to 80%, respectively, and agglomeration of nanoparticulates will decrease in higher duty cycles. The first mentioned result was predictable since in higher duty cycles the

Fig. 2.7 Nanostructures of Ni-W/CNT nanocomposite layers formed by different duty cycles of pulsed current: a 20% (AFM); b 50% (AFM); c 50% (TEM); d 80% (AFM) [137]

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electrochemical reaction for deposition of the metallic matrix has longer times for its occurrence; hence, deposition of nanoparticulates in layer has longer times to occur (in each cycle of pulsed current). By considering ideal distributed nanoparticulates in electrolyte, it can be concluded that increasing duty cycle will lead to longer ‘‘on times’’ (of applied pulsed current in each cycle) and lower applied potential (for obtaining constant average current density), which means lower power for embedment of nanoparticulates into nanocomposite layer, so agglomeration is less than that in lower duty cycles that act in the opposite manner. Figure 2.8 shows that the W content in the metallic matrix did not change significantly by increasing duty cycle of pulsed current. Changing trend of the W content is the same as carbon nanotube content. W content increased from 10.8 to 12.1 wt%. It can be assumed that the interaction of nanoparticulates and pulsed current has an influence on the W content in the metallic matrix. It can easily be concluded that effect of carbon nanotubes is much more than duty cycle, and decreasing carbon nanotube content will also lead to a decrease in the W content of the metallic matrix [137]. Microhardness of Ni-W and nanocomposite layers with respect to different concentrations of carbon nanotubes as well as different applied duty cycles is reported in Table 2.1, which increases from 522 HV for Ni-W alloy to 779 HV for nanocomposite layer with 13.1 wt% of carbon nanotubes. Also, the W content in nanocomposite layer will not change by changing the duty cycle of pulsed current, so increasing microhardness of the obtained different nanocomposite layers with the applied different duty cycles should be concerned by the presence of carbon nanotubes. As mentioned before, there is less carbon nanotube in nanocomposite layers, which are formed by lower duty cycles, but the microhardness of nanocomposite layers will not change significantly by changing the applied duty cycles (Table 2.1). Thus, increasing duty cycle will lead to mutual effect of higher contents of carbon nanotubes in the metallic matrix with simultaneous less normal distribution, which in total will lead to approximately constant microhardness of the obtained layer. Figure 2.9 illustrates the distribution of carbon nanotubes in a 500 nm 9 500 nm area of analyzed SEM nanostructures. Changing trend of distribution in this figure confirms our conclusions.

Fig. 2.8 Influence of duty cycle of pulsed current on CNT nanoparticulate contents in obtained nanocomposite layers and W contents in the metallic matrix of nanocomposite layers [137]

52 Table 2.1 Microhardness of Ni-W/CNT nanocomposite electrodeposited layers [137]

2 Size Dependency in Nanostructures Sample

Duty cycle/%

HV

Ni-W Ni-W/CNT

– 20 35 50 65 80

522 725 739 754 767 779

2.5 Corrosion Properties In general, corrosion resistance of nano-crystalline materials in aqueous solutions is of great importance in an extensive area for future applications. There performed few studies in this area. Both improved and disadvantageous results for development of nano-crystalline in corrosion process have been recorded for corrosion behavior of nano-crystalline produced by amorphous materials crystallization. Obtained results are highly influenced by weak characteristics of crystallized amorphous materials. In the other words, during last few years, there have been considerable advancements in perception of fine structures effect on corrosion properties for materials produced by electrodeposition process. In previous studies, polarization at both constant and changing potentials, with deaerated 2 N sulfuric acid (pH = 0) were performed on bulk pure Ni nano-crystals with grain sizes of 50, 32, and 500 nm and compared with pure multi-crystalline Ni with grain size of 100 lm. Nano-crystalline specimens show the same active-inactive-trance-passive behavior; and differences are between passive current density and open circuit potential. Nano-crystalline specimens indicate more intense current in passive zone, implying higher corrosion rate. This current density is mostly because of more grain boundaries and triple connections in nano-crystalline samples, creating electrochemically active spots. This density difference in current density decreases for higher potentials (1,100 mV versus Calomel reference electrode). At this potential difference, final dissolve rate overcome to structurally control dissolve rate, existed at low potentials. Another obvious difference in polarization result, in varying potential of nano-crystalline and multi-crystalline samples, is open circuit potential. It seems that positive transition of open circuit potential for nanocrystalline samples is due to catalyst properties of hydrogen releasing reaction. Both samples show extensive corrosion; however this corrosion is more homogenous nano-crystalline Ni, while sample with grain size of 100 lm reveals more developed local corrosion across grain boundary and triple connections. XRD analysis of polarized samples for passive zone suggests that created passive layers on nano-samples are less developed. This porous layer allows nano-crystalline sample for a more homogenous destruction; while, on the other hand, for coarse Ni, destruction of passive layer first happens at grains boundary and triple connections rather than crystal surface, leading to preferred invasion in these zones. There has been obtained similar approaches for corrosion behavior of

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Fig. 2.9 Distributions of CNT nanoparticulates in the metallic matrix of nanocomposite layers for different applied duty cycles of pulsed current: a 20%; b 50%; c 80% [137]

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nano-crystalline stainless steel (304) with grain size of 25 nm, in HCl, obtained through spraying process. A decrease of sensitivity against local corrosion is due to fine-grained micro-structure, conducts in an even distribution of Cl ions. Recently, corrosive behavior of Ni nano-crystals, in 30 weight percentage KOH solution and a solution with normal pH of 3 weight percentage of NaCl, has been studied which produced results similar to those of sulfuric acid. Compared with Ni multi-crystal, overall corrosion has an increase; however, nano-crystalline materials are more protected against this local destruction which leads to catastrophic fracture. Using salt spraying test, it was found that under electrochemical conditions fie-structure of Ni has a few effect on final corrosive performance. Both micro-crystalline and nano-crystalline coatings reveal similar corrosive protection on steel samples. Another corrosion study was performed on nano-crystalline Ni according to existed conditions on steam generator alloy, as a part of electro-sleeve development program. Tests of sensitivity against intra-granular invasion and stressaccompanied sensitivity against corrosion were performed on polytonal acids and MgCl2, while alternative emerging test was carried out in NaCl. The results show that electrodeposited nano-crystalline Ni with grain size of 100 nm is resistant against intra-granular phenomena such as grain boundary invasion and corrosion with grain boundary stresses. This material is resistant against local pitting attacks and shows just a negligible sensitivity against crevice corrosion. Second group of tests are concentrated on particular environments, where steam generator materials are imposed. These environments include alkaline, acidic, and a compound of oxidizing and reducing particles ones. Tests have shown excellent strength of nano-crystals in base and reducing acidic environments. Resistance against corrosion is limited in acidic and oxidizing environments [165–174].

2.6 Hydrogen Transition and Sensitivity Hydrogen transition behavior in thin sheets of nano-crystalline Ni, with average size of 17 nm at temperature of 293°K, is determined using electrochemical dual storage. Based on determined permeability, permitted flux values, and surface fraction (i.e. given volume), these changes are due to hydrogen transition across distinct triple connections, grains boundary, and network paths. Permeability of triple connection is about 3 and 70 times quicker than grain boundary and network diffusion, respectively. This shows effect of triple connections defects. Moreover, diffusion from triple connection zones in nano-crystalline Ni implies importance of triple connection defect on bulk properties of nano-crystals. Nano-crystalline Ni with average size of 20 nm shows more electro-catalytic behavior, in comparison with cooled, fine grained, and completely annealed Ni. Another study on hydrogen transition behavior of Ni, using electrolytic charging method, shows that an essential increase in permeability of hydrogen and its capacity is obtained whenever Ni is in nano-crystalline form. Collecting

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hydrogen in dual electrodes of Ni with same thickness has this following order: nano-crystal, fine grain, mono-crystalline structures. Besides, apparent concentration of hydrogen in a 20 nm sample is around 60 times more than that of monocrystalline structure, based on allowed exchanges. Hydrogen permeability and capacity is due to its more amounts of intra-crystalline spaces, offering these following features: 1. High density from short circle diffusional paths 2. More free volumes, resulting in more segregation of hydrogen

2.7 Magnetic Characteristics and Ionic Conductivity Many experiments suggest that magnetic characteristics depend on material size. Although understanding magnetic structure of nano-structure materials is far away from its complete state, there is a clear imagination from saturated magnetism; as recent contradictory results about chemical and physical structure of nano-crystalline materials is justifiable. According first studies, nano-crystalline materials show a great deal of decrease in saturated magnetism with decrease in grain size. Approximately 40% of decrease in saturation magnetism was obtained in comparison with bulk alpha Fe for nano-crystalline Fe with grain size of 6 nm, developed by simultaneous deposition of nano-particles obtained from consolidation of pure gas. This behavior is due to differences in magnetism fine-structure of nano-crystal and common multi-crystalline Fe. In a same way, strong effects of particle size on saturation magnetism were obtained during study of super tiny unconsolidated particles produced through gas evaporation. For super tiny particles (10–50 nm) of Ni, Co, and Fe, an intense decrease was observed in saturated magnetism with grain size reduction, which was accompanied with nonmagnetic oxidized layers on particle. Another study on these super tiny particles has shown magnification is negatively associated with decrease of particles size. Decrease in saturation magnetism is accompanied with surface effects—which are more important than grain size. Also, decrease of saturation magnetism rate in Ni powder, due to evaporation of produced gas resulted from structural disorder in interface, was recorded. Measured magnetic momentum of interface atoms is about half of that of atoms in coarse grain material. Further, it was found that super tiny Ni particles saturation magnetism considerably reduces with grain size decrease. It was recorded that accidental magnification of nano-crystalline gallium (Ga) samples produced by gas consolidation and dimensional compaction is about 75% of its multicrystal. It must be added all mentioned samples are created using gas consolidation method resulting in production of materials with high internal porosity, which creates a big deal of surface area for oxide development after posing to free air. On the other hand, it was recorded that saturation magnetism is not significantly associated with grain size. Ni grains size has been declined from 10 to 100 nm;

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then for Ni samples with tiniest grain size observed magnetism is just 10% less than that of multi-crystalline Ni. These results were observed for bulk nanocrystalline Ni created with electro-deposition method and its creation mechanism was said to be unavoidable development of porous oxide. Obtained results are coordinated with recent calculations, implying effect of structural disorder. At these studies, grain boundary size is a source for different disorder states. Measurements show that magnetic momentum is not really sensitive to magnitude of structure disorder from grain boundaries. Once material structure is amorphous, average momentum is only 15% of decrease; hence, for nano-crystalline Ni with grain sizes of 10 nm, where grain boundary atoms occupy 30% space, final effect of structural disorder on medium momentum would be negligible. Other recent records prove these results. For instance, for nano-crystal created by rolling, there is no significant difference in saturation magnetism for material with grain sizes of 1 nm and 50 lm. similar results have recorded for Ni nano-crystals. Also, for nano-crystalline Ni created from gas consolidation method, before posing it to free air saturation magnetism is independent from grain size, but as soon as its pose to free air saturation magnetism declines to 80% of its original value. Recently Ishihara et al. [175] fabricated thin films of La1.61GeO5-d as a new oxide ionic conductor, on dense polycrystalline Al2O3 substrates by a pulsed laser deposition (PLD) method and studied the effect of the film thickness on the oxide ionic conductivity on the nanoscale. The effective deposition parameters were optimized to obtain La1.61GeO5-d thin films with stoichiometric composition. Annealing was found necessary to get crystalline La1.61GeO5-d thin films. It was also found that the annealed La1.61GeO5-d film exhibited extraordinarily high oxide ionic conductivity. Due to the nano-size effects, the oxide ion conductivity of La1.61GeO5-d thin films increased with the decreasing thickness as compared to that in bulk La1.61GeO5-d. In particular, the improvement in conductivity of the film at low temperature was significant.The electrical conductivity of the La1.61GeO5-d film with a thickness of 373 nm is as high as 0.05 S.cm-1 (log (r/S cm-1) = -1.3) at 573°K. The oxide ion conductor is an important functional material applied in different fields such as fuel cells, oxygen sensors, oxygen pumps, water electrolysis, and oxygen separating ceramic membrane. Among these application areas, the electrolyte of fuel cell is attracting much interest. Several numbers of new oxide ion conductors such as strontium and magnesium doped lanthanum gallate (LSGM) and La10Si6O27 apatite oxide and were reported recently. Among the new oxide ion conductors fabricated recently, La-deficient La2GeO5, is highly interesting, because of its high oxide ion conductivity over a wide range of oxygen partial pressure and unique structure. In La2GeO5 based oxides, La deficiency leads to the formation of oxygen vacancies and oxide ion conductivity in La1.61GeO5-d is the highest in La2GeO5 based oxides. The transport number of the oxide ion is nearly unity in the O2 partial pressure range 1–10-21 atm and the conductivity is comparable to that of well-known fast oxide ion conductors, e.g., La0.9Sr0.1Ga0.8Mg0.2O3-d and Gd-doped CeO2. Recently, nano-size effects on ionic

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conductivity have been attracting much interest because of improved ion conductivity. Some researchers reported that the fluoride ionic conductivity in CaF2 and BaF2 hetero-layered films, prepared by molecular-beam epitaxy, increases proportionally with increasing interface density, namely, decreasing thickness, when the interface spacing is larger than 50 nm, which is in agreement with the semi-infinite space-charge calculation. In contrast, due to the positive charge at grain boundary, negative nano-size effects were reported for the oxide ion conductivity in CeO2 based oxides. On the other hand, it is reported that the oxide ion conductivity in the laminated films consisting of ZrO2 and Gd doped CeO2 (GDC) thin film increases with decreasing number of lamination. The effects of nano grain size on the ionic conductivity on La2GeO5 based oxide film and it was found that the conductivity was improved by decreasing film thickness of La2GeO5. However, in the conventional study, nano-size effects are not studied systematically and so, the nano-size effects are still not clear. New oxide ion conductor of La1.61GeO5-d film of various thicknesses was fabricated as thin films of varying thickness on dense polycrystalline Al2O3 substrates by using pulsed laser deposition. The obtained La1.61GeO5-d film by Ishihara et al. [175] exhibited almost the pure oxide ionic conductivity and the oxide ion conductivity increased with the decrease of the film thickness. In particular, increase of conductivity at low temperature was more significant. Considering the stable valence number of La and variable valence of Ge (3+ and 4+), the amount of oxygen vacancies can be determined by the composition of the film. Since the composition of the prepared La1.61GeO5-d films is almost the same, it is generally considered that the increased conductivity may not be explained by the change in the amount of oxygen vacancy but by the improved mobility of oxide ion by the local stress caused by the mismatch in lattice parameter between the film and the substrate. Figure 2.10 illustrates arrhenius plots of La1.61GeO5-d thin films and

Fig. 2.10 Arrhenius plots of La1.61GeO5-d thin films and that of bulk La1.61GeO5-d sample, reprinted with kind permission from Tatsumi Ishihara [175]

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Fig. 2.11 PO2 dependence of the electrical conductivity in La1.61GeO5-d thin film with various thicknesses at 873°K, reprinted with kind permission from Tatsumi Ishihara [175]

that of bulk La1.61GeO5-d sample. PO2 dependence of the electrical conductivity in La1.61GeO5-d thin film with various thicknesses at 873°K can be seen in Fig. 2.11.

2.8 Thermal Stability Thermal stability of nano-crystals is of a great importance in high temperature applications. For electro-deposited nano-crystals thermal stability is examined through TEM and an indirect method, involving determination of thermal stability using harness measurements as a function of annealing time. For synthetic growth of grains there are some preventing factors for grain boundary movements leading to their thermal stability. There is a slowing dual force in nano-crystals due to triple connections. It can be easily shown that grain growth for fined multi-crystal materials is controlled by inherent movement of triple connections. For thermal stability of nano-structures, extra distributions of triple connections lead to preferred dissolve in these spots. Such a dissolve was observed in nano-crystals in triple connections using TEM method. Ni stability with grain sizes of 10 and 20 nm was investigated, using TEM. Degradation temperature for these materials is 353°K. This lack of stability is due to unusual germination after annealing.

2.9 Nanocoatings Applications Nano-crystalline structures made of electro-deposition have some commercial applications, due to these following reasons: 1. Electro-shaping and electroplating are recognized industries with extensive usage.

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2. Their low cost: Nano-crystals can be created using a simple modification in bath electrical parameters applied for electroplating and electro-shaping current. 3. High potential of producing materials, alloys, and composites with metallic matrix in different forms at one stage (i.e. coatings, complicated shapes, and etc.). 4. Capability of producing nano-structures with high density and no porosity.

2.9.1 Structural Applications As it is expected from Hall–Petch assumptions, there are different practical applications for nano-crystals based on existed criteria for development of resistant coatings. Preferential mechanical properties of electro-deposited nano-structures are among their most important industrial applications. Electroplating process is applied for in situ maintenance of nuclear steam generator tubes. This process is successfully applied in aqueous reactors in US and Canada and registered as a standard method for repairing pressure tube. Through this application, Ni with grain size of 100 nm, is created on interior walls of steam generator tubes to perform a complete structural maintenance in places where primary homogeneity of tube structure is mitigated. High strength and convenient malleability of these 100 nm grains result in application of a thin plate (0.5–1 mm) which minimizes fluid current and heat transition in steam generator. Recent geometrical models and empirical achievements have shown that nano-structural materials can have a high resistance against creep and inter-granular cracking. Different applications of nano-structural materials, where their inter-granular properties of resistance against cracking are used, include: positive plates of Acid-Pb batteries and load shaped lines (made of Cu, Pb, and Ni) for industrial applications.

2.9.2 Functional Applications One of the most successful applications of nano-structural materials is in soft magnetic materials for engines, transformators, and etc. Predicted decrease in anisotropy of magnetic crystal during grain size decrease, compared to its predefined thickness, has been investigated. Electro-deposited nano-crystals would have a low coordination without causing any damage to saturation magnetism. Hence, application of these ferromagnetic materials with high efficiency in engines, transformators, anti-attack applications, has been enhanced due to recent advancements in electroplating technology. Through this technology it is possible to economically mass production of plates, thin sheets, and wires. Another important application of electroplated nano-crystalline materials is for production of thin copper-made sheets for print circuit sheets.

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Etching rate increases when grain size declines and grain sizes of 50–100 nm provide optimum etching with maintaining convenient electrical conductivity. At it previously mentioned, high density of intra-crystalline defects is present in bulk state and cutting free surface of nano-structure materials offers a good chance for hydrogen and catalyst storage applications. There are many different applications for usage of these materials in both electrodeposited and electro-shaping methods for battery systems and alkaline fuel cell electrodes.

2.9.3 Classification of Applications Improved hardness, wear and corrosion resistances, as well as decrease of saturation magnetism, acceptable thermal range, elastic properties, and electrical resistance make nano-crystal coatings an ideal candidate for protecting and associated coatings (such as in contact of hard and soft surface, coatings with less abrasive resistance, electronic conductivity, and alternative coatings for Cr and Cd in aerospace applications). Once such thin coatings are used, sediment finestructural changes with coating thickness increase of a great importance. Most previous studies on electrodeposited metals, not necessarily on their nanocrystalline form, have shown that increase of coating thickness causes to increase of grain size. For electrodeposited nano-crystalline Ni, it is found that first the sediment was amorphous with transition to nano-crystalline state and then there is an increase in grain size. In contrary, electrodeposited nano-crystals of Ni suggest that in most cases nano-crystals are exactly settled on interface with matrix and grain size is basically dependent from coating thickness. For distinct electrochemical conditions there is a thin transition layer made of coarser grains. Finally it has been proven that at initial layer with thickness of 200 nm grain sizes is independent from thickness [155, 176–185]. Table 2.2 introduces some applications of nano-coatings.

2.10 Key Points for Development 2.10.1 Environment and Stability In most cases surface engineering leads to economically use of materials and, consequently, profitability in many applications. For instance, increasing service lifetime there will be a decrease in wastes and energy consumption, which caused to retrieve improvement. Many advanced surface engineering processes have negligible environmental effects. One of developing activities in this filed is recoating of high-cost panels. Environmental rules, limiting each one of these panels wear, have a big share in progress of these industries.

(continued)

Table 2.2 Applications of some size affected nano-coatings Industrial applications of Coatings resistant against scratch, wear, corrosion, and environmental Coatings resistant against scratch, abrasion, corrosion, nano-coatings factors and environmental factors Using alumina as a scratch, abrasion resistant coating Manufacturing light resistant panels for airplane structure Nano-metric corrosion protector coating Using nano-particles for coating in transportation industry Using nano-metric coating of ceramics in navy force Using nano-metric coating for strength increase Using nano-metric coating for clothes Manufacturing nano-metric coating of WC/Co–Ni through electrodepositing method Various types of optical coatings including antireflection, mist Using light-resistant layers of UV resistant coating and protective coatings Surface improvement for making mist and steam resistant layers Making panels with nano-metric coating Self-cleaner glasses Coatings with medical, biologic, and environmental applications Nano-metric coating of inorganic particles for medical applications Using nano-particles in masks Using nano-metric coating of hydroxyl-apatite for making prosthesis Using nano-composite coating for food packing Anti-pollution materials in shipping industry Nano-composite coating for dealing with viruses Using Ag nano-particles as an anti-bacterial coating Using TiO2 nanoparticles to decrease environmental pollutions

2.10 Key Points for Development 61

Table 2.2 Continued Coatings with electrical and electronic applications Making transparent conductor coatings using carbon nano-tubes Using nano-metric coating in solar cells Nano-metric coating of Ni particles with oxides Using nano-metric polarizer layers in production of LCD monitors Manufacturing transparent electrically conductor nano-metric coating Increasing storage capacity by magnetic nano-layers Development of nano-metric coating for lubrication of surfaces

62 2 Size Dependency in Nanostructures

2.10

Key Points for Development

63

2.10.2 Weight and Volume Reduction From this viewpoint, applied panels—especially ones used in vehicle—are considered. Al, Ti, and Mg alloys are required for improvement of surface corrosive and abrasive resistance. In these cases a mixture of two or several processes are needed in surface engineering. Also, polymers’ surface engineering has a great potential for development in structural applications.

2.10.3 Smart Layers and Structures Application of enhanced structures is of suitable accountability for increasing environmental conditions. This can conduct in more development in technological application of sensors which are able to create a revolution in applications such as intelligent anti-oxide layers in steam turbines, self-watching structures, and packing food products. At all of these cases surface engineering plays a key role.

2.10.4 Processes’ Understanding Surface engineering processes and relationship between processes’ features should be better understood. This adequate perception leads to improvement of process control quality and trustable quality and more insurance of the buyer. Modeling has a key role at these processes and generating convincing data is preferable. Some of important technologies of surface engineering must be improved through development of processes to increase rate of marked layer or change step by step processes to a continuous ones.

2.10.5 Training Engineers must mainly be aware of surface engineering potential and its exact role on designing; also mechanical engineering courses should involve these points. For instance, surface engineering must be created as a major in manufacturing engineering. Lack of harmony and compatibility between industrial needs and academic works will create some problems. This has been examined in various cases. Surface engineering programs will finally cause to a development in university and industry, which should be better emphasized.

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2.11 Surface Engineering Share in Key Industry Sections Building planes and vehicles comprehensively depend on surface-engineered panels. About 80% of these industries make use of coating. England has first position in field of steam turbines coating and annually obtains 1 billion pound bonus. Advanced coating of panels has considerably increased during last decades. In 1997, 23% of quick machining components for steel and 67% of monolithic carbide panels are coated by physical and chemical evaporation. Coating with Physical Vaporization Detector (PVD) method involves 5 billion pounds annual revenue. PVD coating in production of multi-layers for high efficiency cutting tools has made dry machining possible, which needs a cooler coating with no decrease in profitability. Surface engineering of dual compounds and light alloy panels are used in special vehicles and advance applications such as coast equipment, medicine, and sports. Continuous manufacturing and performance of packaging industry is widely associated with surface engineering. Thanks to surface engineering, 44 billion drink cans are annually produced in Europe. Coating role for improvement of productions and their packaging is vital; especially in preventing air and moisture diffusion on potato chips and prepared food, or as a protector against crashing or cracking for thin glasses. Multilayer sputtering magnetron of a coating with low diffusion on construction glasses decreases thermal waste to 60% during winters, which amounts to a significant annual figure of fuel/m2 of glass. It is expected that 50% of buildings use these glasses till 2005. Plasma improvement of polyethylene surface with high molecular weight causes its durability against slight shakes, which is applied in medicine. For tissue recovery with advanced bio-sensors, all attached plastic biomedical panels to substrates use engineered surfaces to create efficient and compatible performance with human body. All electronic panels and advance sensors, optical coatings, and etc. also are benefited from surface engineering technology during building process [186–188].

2.12 Estimation of Corrosion and Erosion Costs During last 10 years, due to educations for improvement of awareness from corrosion related issues, there was a 515 million pounds saving in England. Studies show similar values of total gross products for India, China, and United States. For United States decay costs exceeds 100 billion dollars annually. This is 1,300 million pounds for England, which is 3.5% of State gross product, where 3.0 million pounds is saved. This amount is 48–50 billion dollars in United States, which compose 4% of per capita gross production. As it previously remarked, this amounts to 10% of national gross production in developing countries.

2.13

Surface Engineering in Automobile Industry

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2.13 Surface Engineering in Automobile Industry There are nearly 27 million cars and 3.3 commercial vehicles in England roads. Car sale in Europe was about 12 million in 1997, where England share was about 1.7 million. It is estimated that car manufacturing will be 17% of England’s surface engineering. Surface engineering has a significant role in this industry; since 6% of engine building costs and power transfer is for coating technology. Total produced color for this industry exceeds 300 million pounds. Protective steel and body panels, as infrastructures, need to be repaired; so, many of them are galvanized and prepared for painting with another suitable coating. While metals mostly are coated by organic painting, plastics should be initially metalized. Here, application of wet processes competes with advancements in PVD and CVD methods. Market is not the only effective factor on technology progress, but there are some environmental and legislation concerns which are considered as a powerful encouragement in this field. Painting just involves about 2 kg of vehicles weight, while painting process emits about 5 kg Volatile Organic Compounds (VOC) to the atmosphere. Efforts for reduction of this effect led to development of powder technology advancement. Most of coatings changes involve using the CrVI, where some substitute are being made for that. Automobile industry is directed to reaching products with higher compatibility with environment; as in their producing method application of CrVI is avoided. Mentioned cases are only a small part of coating applications.

2.14 Surface Engineering in Power Generation Industry Here, power generation is addressed to high efficiency engines for aerospace industry, marine gas turbines, and electric generating turbines. Gas turbines are considered as a part of combinational cyclic developed machinery. Gas turbines have some advantages compared with equipment of power generating from fossil fuels, such as higher efficiency, lower constructing costs and using different fuels. This equipment work at higher temperatures and combustion and hot gas pass occurs within them. Here, creep, oxidization, and corrosion are regarded among important factors. Achieving ideal conditions, coating is of a great importance. Coating system is widely applied in gas turbines to protect gas pass route and increase system lifetime. The following objectives are supposed to be achieved during 5 coming years: 1. 2. 3. 4.

A 50°C increase in work temperature 10,000 h increase of panels lifetime One hundred percent Increase in system visitation time A wide increase of applied fuels, natural gas, heavy fuels originated from coal, biomass, and etc.

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Achieving these objectives, coating systems should be developed. There are so many researches in this field. Regarding various findings all studies are directed on intelligent corrosion resistant coatings. These coatings can be appropriate answer for corrosive environments, in order to avoiding panels’ oxidization. This state is achievable through plasma spray technology, CVD, development in platinum aluminate, and new insulating materials (layering and staining where both have low electrical conductivity). As well as above-mentioned applications, use of composite surfaces as a substrate for main layer of ceramics—which of a good thermal compatibility with the layer as well as stability against corrosion—is also progressing. Nowadays, there are comprehensive investigations at this area.

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167. Liu, L., Li, Y., Wang, F.H.: Pitting corrosion behavior of a sputtered nanocrystalline thin film of austenitic stainless steel in 3.5 mass% NaCl solution. Corros. Sci. Protect. Technol. 22, 283–288 (2010) 168. Luo, W., Xu, Y., Wang, Q., Shi, P., Yan, M.: Effect of grain size on corrosion of nanocrystalline copper in NaOH solution. Corros. Sci. 52, 3509–3513 (2010) 169. Chandrasekar, M.S., Shanmugasigamani, M.S., Malathy, P.: Synergetic effects of pulse constraints and additives in electrodeposition of nanocrystalline zinc: corrosion, structural and textural characterization. Mater. Chem. Phys. 124, 516–528 (2010) 170. Xu, J., Zhou, C., Jiang, S.: Investigation on corrosion behavior of sputter-deposited nanocrystalline (MoxCr1-x)5Si3 films by double cathode glow plasma. Intermetallics 18, 1669–1675 (2010) 171. Nie, F.L., Zheng, Y.F., Cheng, Y., Wei, S.C., Valiev, R.Z.: In vitro corrosion and cytotoxicity on microcrystalline, nanocrystalline and amorphous NiTi alloy fabricated by high pressure torsion. Mater. Lett. 64, 983–986 (2010) 172. Okouchi, H., Seki, Y., Sekigawa, T., Hira, H., Kawamura, Y.: Nanocrystalline LPSO Mg– Zn–Y–Al alloys with high mechanical strength and corrosion resistance. In: Materials Science Forum, pp. 1476–1481. (2010) 173. Lee, H.B., Wuu, D.S., Lee, C.Y., Lin, C.S.: Study of the corrosion behavior of nanocrystalline Ni–P electrodeposited coating. Metallurg. Mater. Trans. A Phys. Metallurg. Mater. Sci. 41, 450–459 (2010) 174. Sousa, C.C., Kiminami, C.S.: Corrosion of Fe-based nanocrystalline alloys with soft magnetic properties. J. ASTM Int. 7, 1–12 (2010) 175. Ishihara, T., Yan, J., Matsumoto, H.: Extraordinary fast oxide ion conductivity in La1.61GeO5-[delta] thin film consisting of nano-size grain. Solid State Ionics 177, 1733– 1736 (2006) 176. Ravi, S., Ganesh, K.V., Ramanathan, A., Annamalai, J., Jaiswal, P.K.: Development of nano crystalline nickel coating for engineering applications. In: Key Engineering Materials, pp. 487–492. (2010) 177. Koleva, D.A., Boshkov, N., Bachvarov, V., Zhan, H., de Wit, J.H.W., van Breugel, K.: Application of PEO113-b-PS218 nano-aggregates for improved protective characteristics of composite zinc coatings in chloride-containing environment. Surf. Coat. Technol. 204, 3760–3772 (2010) 178. Wang, R.M., Wang, B.Y., He, Y.F., Lv, W.H., Wang, J.F.: Preparation of composited nanoTiO2 and its application on antimicrobial and self-cleaning coatings. Polym. Adv. Technol. 21, 331–336 (2010) 179. Seong, M.R., Kwon, J., Lee, G.Y., Kim, D.K., Kim, Y.S., Lee, C.S.: Optimization of surface coating condition using vapor form of alkanethiol on Cu nano powders for the application of oxidation prevention. Appl. Surf. Sci. 256, 2332–2336 (2010) 180. Wang, J.E.: Application of nano-titatium polymer coating to the coolers in oil refine factory. Corros. Protect. 30, 933–935 (2009) 181. Izumi, T., Izumi, K., Kuroiwa, N., Senjuh, A., Fujimoto, A., Adachi, M., Yamamoto, T.: Preparation of electrically conductive nano-powder of zinc oxide and application to transparent film coating. J. Alloys Compd. 480, 123–125 (2009) 182. Tian, Z., Wang, X., Shu, L., Wang, T., Song, T.H., Gui, Z., Li, L.: Preparation of nano BaTiO3-based ceramics for multilayer ceramic capacitor application by chemical coating method. J. Am. Ceram. Soc. 92, 830–833 (2009) 183. Shibli, S.M.A., Jayalekshmi, A.C.: A novel nano hydroxyapatite-incorporated Ni–P coating as an effective inter layer for biological applications. J. Mater. Sci. Mater. Med. 20, 711–718 (2009) 184. Lewis, F., Mantovani, D.: Methods to investigate the adhesion of soft nano-coatings on metal substrates—application to polymer-coated stents. Macromol. Mater. Eng. 294, 11–19 (2009)

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185. Zhang, X., Cheng, X., Yin, H., Yuan, J., Xu, C.: Preparation of needle shaped nano-copper by microwave-assisted water system and study on its application of enhanced epoxy resin coating electrical conductivity. Appl. Surf. Sci. 254, 5757–5759 (2008) 186. Jacquot, P., Andreux, C., Stauder, B.: Effect of industrial surface finishes on heat treatment and surface engineering of steel. Int. Heat Treat. Surf. Eng. 4, 110–116 (2010) 187. Stevenson, P., Ata, S., Evans, G.M.: Erratum: the behavior of an oscillating particle attached to a gas–liquid surface (Industrial and Engineering Chemistry Research 48 (8025)). Ind. Eng. Chem. Res. 49, 3504 (2010) 188. Kaufmann, H.: Industrial applications of plasma and ion surface engineering. Surf. Coat. Technol. 74–75, 23–28 (1995)

Chapter 3

Characterization of Nanostructured Coatings

3.1 Introduction Materials with nano-crystalline structure are away from equilibrium state. Nanometric (nano-crystalline) structure is the term which is applied for each material provided that its microstructure comprises of extremely fine particles, measuring from 1 to 100 nm generally. Comparing materials with ordinary structure, nanostructured materials, due to having high density of interface levels and also high volume of crystal imperfections like vacancy and dislocations, show unique and unparallel properties. In inter-crystal areas, atomic density and the way of arrangement of atoms differ from crystal areas and consequently, physical and chemical properties of inter-crystal areas differ from crystal areas as well. The main reason behind this behavioral change can be defined as a result of superficial energy increase in nano-crystalline materials. The special high level of nanostructured materials and following increase of superficial free energy cause that sensitive properties to surface (like superficial reaction phenomena) is improved coupled with accelerating the processes which superficial energy is operated as progressive force. The inter-crystalline areas place in higher balance than crystalline areas in terms of energy level. Hence, existence of these inter-crystalline areas can provide necessary energy for overcoming thermo-dynamical and kinetic dams in nucleation of new phases and increase rate of reaction accomplishment and formation of new phases on the surface. In fact, inter-crystalline areas are considered as appropriate centers for nucleation of new phases. The results obtained from Moth– Shotky electrochemical tests indicated that passive film formed on iron electrode surface was a semiconductor in a way that electron donor density in passive film is reduced with the reduction of size of electrode surface grains. In the same direction, density of oxygen-free places, which is regarded as hub of adsorbing corrosive ions and causing eradication of passive film like chloride ion, is reduced with the reduction of electron donor density at passive film and with the aim of safeguarding balance of electrical load [1–15].


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This subject resulted in increase of sustainability of passive film formed on nano-crystalline iron surface. Also, results obtained from electrochemical impedance indicated increase of density of passive film formed on surfaces with smaller granule size. Since gradient of potential in passive film has been tasked with controlling kinetics of transfer of load components like ions and electron in passive film, increase of passive film density with reduction of sub-layer granule size can be related to spread of energy band in passive film. Sodium nitrate and sodium benzoate were used in partly neutral aqueous solutions respectively with the aim of studying the effect of reduction of size of superficial grains on kinetics of adsorbing corrosive inhibitors from mineral and organic inhibitors. For effective adsorption of inhibitors on metal surface, interaction forces between metal and inhibitor should be larger than interaction forces between metal and water molecules. The amount of inter-crystalline areas, including grain boundaries and triple junctions, is increased in tandem with fining size of granules. Existence of high amounts of inter-crystalline energy-rich areas at nano-crystalline materials causes to place them in higher level of superficial energy in comparison to microcrystal materials with the same chemical compound, and consequently, reactivity between their surfaces and the environment is increased. The results, obtained from electrochemical tests, indicated increase of overlapping percentage and/or increase of inhibition efficiency with the reduction of size of crystalline iron granules. Here, a definition of nanostructured materials has first been mentioned. Nanostructured materials create a new approach for improvement of properties, without imposing any change in chemical composition. The unique and single properties of nano-crystallites, in comparison with coarse multi-crystalline materials, originate from vast grain boundary and consequently, high inner-fuzzy energy in them. This issue causes that knowing resources and reasons behind increase of superficial energy of nano-crystalline materials is of paramount importance. For this reason, indexes, which are constructive and effective on increase of free superficial energy, have been discussed thermodynamically. The production methods of nanostructured materials have been explained briefly. With regard to using electrochemical deposition method for creation of nanostructured coating, mechanisms of growth and nucleation in electrical crystallization process has been explained thoroughly. Some of the properties resultant from nanostructured coatings have been reviewed in comparison with coarse-granular structure. Moreover, properties of coatings have also been reviewed [16–24].

3.2 Definition of Nanostructured Materials The nanostructured materials are defined to a group of materials which their structural dimensions are measured in nanometric (nm) scale. More inclination is on dimensions smaller than 100 nm. Hereunder are considered as salient specification of nano-crystalline metallic systems with regard to their efficacy: sizes of

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granules, areas of metal with direction of specified crystal screens, atomic density gradient and areas with different chemical composition. The various types of nanostructured materials enjoy two joint and common specifications: (1) Atomic areas (granules or phases) have been restricted in space smaller than micron scale. (2) High volume fraction of atoms exists in interface areas. Nanostructured materials can be observed in dimensionless, one dimensional, two dimensional and three dimensional forms. Atom nucleate or atom sprouts have been recognized as dimensionless nanostructures. Nanorods and similar nanostructures such as nano-whiskers can be considered as one dimensional nanostructures while separate or isolated layers or multi-layers, grown in one or two directions, are known as two-dimensional nanostructures. In addition, nano-crystalline or nano-phases are considered as three-dimensional (3D) nanostructures and regarded as highly-used nanostructures in metal systems. The significance of nanometer scale is originated from the fact that some of properties undertake remarkable amounts at this dimension range. Since properties of a solid material is first controlled by density and atom co-ordination number, nano feature of sizes of granules in crystalline materials, despite chemical composition in nanocrystalline and microcrystalline state, causes that properties like specific heat, thermal capacity, thermal extension coefficient, magnetic properties and mechanical properties in nano-crystalline state are considered completely different from microcrystalline state [25–31]. Birringer [32] has reported formation of nanostructure iron with average granule size of 10 nm by gas condensation method. The results, obtained from images of electron microscope, indicate fair correspondence with the model. It should be noted that the average atomic density in grain boundaries stands at approximately 7–85% of density of atoms inside granules. Its main reason is related to misfit of orientation and direction of atoms in interfaces. If density of a solid is decreased uniformly (for example, if solid is expanded uniformly), inter-atomic space will be increased and consequently, gravity forces between atoms are weakened to such an extent that if this expansion is reached up to final limit i.e., 30–40%, then it may result in separation of atoms. The results obtained from micrograph image with high decomposition of iron nanostructured metal shows that reduction of density at grain boundaries is not in uniform manner; rather, a distribution of atoms places in grain boundary and averagely, their density is less in comparison with the atoms placed inside granule. This subject has been reported for palladium metal by other researchers. The actual nature of boundaries of grain in a nano-crystalline structure has not yet been recognized completely. Some researchers believe that there is not any difference between grain boundaries in a nano-structure structure with large granule (coarse) structure. Some others believe that grain boundary in nano-crystalline structure is amorphous. Since fraction of atoms placed at grain boundary areas is comparable with fraction of atoms placed inside granules in a nano-crystalline structure, structure of grain boundary and contact areas of grain boundaries is instable and show tendency to lower energy level. Therefore, there exists strong driving force for growth

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of granule in nanostructured materials and these materials are instable thermodynamically. Dense, crack-free piezoelectric thick film integrated on silicon substrate has been the subject of considerable attention in recent years for potential application in micro-electro mechanical systems (MEMS). Wang et al. [33] were proposed a new spin-coating route for this purpose in which films are made from homogenous slurry of sol–gel precursor and nano-sized powders. Ceramic PZT powders are ball milled into different particle sizes by a higher energy planetary ball mill machine and uniformly dispersed into, with a selected dispersant, a PZT sol–gel solution identical with the powders in composition. The well-prepared resulting slurry looks like paint and was spin deposited onto a silicon wafer, fired, and annealed in the same manner as conventional sol–gel process. Thick films on platinum coated silicon can be fabricated with this route. The microstructure, crystallization process and the ferroelectric properties of the film have been investigated. It is revealed that the success of this route is due to the elimination of the agglomeration among the nano-sized particles. The smaller the particle, the more uniform the microstructure. The film derived from micron-sized particles has the porous structure with average pore size of sub-micron. For the films derived from nanosized particles, the nanocrystalline structure is obtained at sintering temperature of 600–700C and fully developed submicron-sized grains have been demonstrated at sintering temperature of 700–800C. This thick film has comparative ferroelectric properties with respect to bulk PZT ceramic. Dense, crack-free piezoelectric thick film integrated on silicon substrate has been the subject of considerable attention in recent years for potential application in MEMS. Combining surface of micro-machined silicon wafer with piezoelectric and ferroelectric films has resulted in novel devices such as micro fluidic devices, micro-pumps, acoustic transducers, cantilever or bimorph accelerometer, ultrasonic devices, ink-jet printer heads, micro-actuators for high-density hard-disk drives, and several others. These novel devices require the piezoelectric coating either to actuate a substrate or self-resonate at an ultrasonic frequency. Since resonant frequency, generative force and displacement of the devices are determined mainly by their dimensions and material properties such as density and Young’s modulus, these novel devices require the films of specific dimensions. The thickness of the piezoelectric films are typically in the range of 0.5–5 lm for MEMS applications, 2–10 lm for micro-actuators with relatively large generative force, 30–50 lm for ultrasonic transducers at frequency larger than 50 MHz and 50–200 mm for millimeter-sized devices. In addition, the density of the piezoelectric film also plays an important role in overall performance of the devices. Therefore, fabrication of crack-free and also dense thick films are of great significance. Wang et al. [33] concluded that high-energy ball-milling is demonstrated to be an effective approach to obtain a well-dispersed slurry of fine powders and a sol–gel solution slurry made this way can be used in spin-coating or screen-printing to make dense thick films with uniform microstructures. The use of nano-sized powder is of great benefit to the uniformity of the microstructure. The success of


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this process can be ascribed to the elimination of the agglomeration among the nano-sized particles. The film derived from micron-sized particle has the porous structure with average pore size of sub-micron level thus it has low permittivity, while the film derived from nano-sized particles has relatively poor ferroelectric performance due to the poor crystallinity of the nano-sized powders. Figure 3.1 illustrates a schematic flow chart of thick film deposition using nanocrystalline composite technique. Leakage current–voltage and resistivity characteristics of a 10-mm-thick PZT thick film prepared by using nanocomposite route can be seen in Fig. 3.2. As an example, Fig. 3.3 shows an as-sintered surface morphology of a screen-printing PZT thick films on Au/Al2O3 substrate annealed at 800C for 30 min. A totally new homogeneous polycrystalline matrix has been formed at the expense of originally added powder-phase and sol–gel derived phase.

Fig. 3.1 Flow chart of thick film deposition using nanocrystalline composite technique, reprinted with kind permission from Zhu [33]

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Fig. 3.2 Leakage current– voltage and resistivity characteristics of a 10-mmthick PZT thick film prepared by using nanocomposite route, reprinted with kind permission from Zhu [33]

Fig. 3.3 As as-sintered surface morphology of a screen-printing PZT thick film on Au/Al2O3 substrate annealed at 800C for 30 min. The fully developed grains are sub-micron in size, reprinted with kind permission from Zhu [33]

TiO2 nanoparticle coatings possess good thermal and electrical properties and they are resistant to oxidation, corrosion, erosion and wear in high temperature environments. This property is very important factor in the applications such as pipelines, castings and automotive industry. Shanaghi et al. [34] applied a uniform TiO2 nanoparticle coating on mild steel, using sol–gel method. The coating was deposited on mild steel substrate by dip coating technique. The morphology and structure of the coating were analyzed using SEM, AFM and X-ray diffraction. The anticorrosion performances of the coating have been evaluated by using electrochemical techniques. The film uniformity was retained in high temperatures and no crack and flaking off from the substrate was observed. The tafel polarization measurements provide an explanation to the increased resistance of TiO2 nanoparticle coated mild steel against corrosion and icorr was decreased from 18.621 to 0.174 (lA/cm2). AFM images from performed coatings indicate their homogeneity and roughness. Stainless steels due to Cr2O3 presence on their surface (and their low difference with TiO2 thermal expansion coefficient) are more


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suitable than mild steel for substrate of sol–gel method. Performing an interlayer on the surface of sample caused decreasing the difference of thermal expansion coefficients between substrate and coating. Final coating was studied with AFM and its roughness was in nanometric scale. Figure 3.4 illustrates AFM result of final coating. The TiO2 nanoparticle sizes were about 40–60 nm obtained from XRD and AFM.

3.3 Thermodynamics of Nanostructured Materials As mentioned in the beginning, materials with nanometer (nano-crystalline) structure are away from equilibrium state. The factors, which affect on additional free energy of these materials, have been recognized. High density of interface and sever changes of chemical composition and stress gradient are the cases which can be effective in abnormal thermodynamic changes of these materials. The origin of these impacts has been recognized theoretically and experimental tests have proven these cases as well. The nano-crystalline structure can be considered as compound form, comprised of small crystalline areas with long-range order and with various crystallography orientations and inter-connected lattice of inter-grain areas, lacking any full crystalline order. The degree of additional free energy of these materials is related to the following cases: • • • •

Interface free energy, Interface curvature and inter-level connections, Stresses extant at interface, Mixture of phases (including semi-sustainable phases),

Fig. 3.4 AFM nanograph of TiO2 nanoparticle coating on mild steel by sol–gel method a morphology of surface b cross section, c relationship between roughness of TiO2 nanoparticle coating and its thickness obtained by AFM [34]

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• • • •


Chemical composition changes, Stresses as a result of coherency, Stresses as a result of non-coordination in thermal expansion, Lattice imperfections.

The above mentioned cases may create changes in degree of free energy of other objects which have not nanometer (nano-crystalline) structure. Of course, the utmost effect of these parameters is related to nanostructured materials, because, length of layers, which causes stress and appearance of changes in chemical composition, is very short. Effect of each above-mentioned factors on nanostructured materials, is different. The degree of additional free energy changes strictly hinges on the aforementioned cases and neutralization of these cases to each other and balancing them for reaching the lowest free energy level is inevitable. For example, a semi-sustainable phase may be produced in structure of material for reducing interface energies and consequently, this issue will reduce total free energy of object. For better studying, the nanostructured materials can be classified into the following four groups: 1. 2. 3. 4.

Mono-phase multi-crystalline materials Multi-crystalline materials (Two or multiphase) Filamentary composites Filamentary thin films

For mono-phase multi-crystalline materials, utmost free energy increase is related to grain boundary and inter-granule boundary connections. These connections have the utmost effect on interface energy. Double-phase multi-crystalline materials have usually been formed from the phases which do not show any reaction with each other and/or do not penetrate into each other. When granules are measured very small, they are called as nano-phase. When two phases are comprised of metallic property, the term ‘‘nano-crystalline alloys’’ will be applied. If one phase is metallic and the other phase is non-metallic, it is called granular metal. Sample of these materials are solids which particles of a metal have placed inside non-metallic background (like oxides). At this state, matrix phase is not reacted with metal and metallic particles are placed away from each other in isolated form. With regard to double-phase materials, the degree of radius of curvature of phases is considered as the most significant issue. This issue can cause increase of free energy and also can be considered as a driving force for enlargement of granules. The other significant issue is increase of stresses as a result of interface and also disparity of thermal expansion coefficient of phases and granules in tandem with each other. The cases discussed up to now, may have nanometer size in each three dimensions. But other materials like composites with nanometer filaments have nanometer structure only in two dimensions. Also, multilayer materials, which are created through controlled deposition process, have nanometer structure only in one dimension. With regard to multilayer materials, free energy increase effects include effects of coherency stresses, interface stresses and also chemical composition

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changes. Although double-phase multi-crystalline materials in normal condition have been formed from phases which do not show reaction with each other, they can be studied along with multilayer materials. Also, for nanometric materials, high rate of imperfections like vacancies and dislocations, can be observed inside their crystalline structure [9, 35–47].

3.4 Interfaces Thermodynamics With regard to phase balance in microscopic structure, energy interface does not play a significant role and only Gibbs free energy of whole object is important. With regard to materials with microscopic structure, thermodynamics of interface can be ignored. Of course, some common cases are controlled and monitored through interface energy. For example, it can be referred to the phenomenon of growth of grain. For nanostructured materials, conducting thermodynamics studies merely on whole object is not sufficient due to very high density of interface. And following interface thermodynamics should be taken into consideration. Interface thermodynamics has been studied comprehensively by Gibbs which is briefly explained in following sections [39, 41, 46, 48–52]. Comparative study of reactivity of nano- and micro-sized alumina and nickel oxide, obtained by the electrical explosion of metal wires in oxidizing atmosphere, was carried out for the reactions NiO ? MoO3, NiO ? Al2O3, and Al2O3 ? Bi2O3 by coupled anneals of ceramics, measurements of the conductivity of individual oxides and raw oxide mixtures, X-ray diffraction and differential thermal analysis by Neiman et al. [53]. The total conductivity of nano-structured oxides was found lower than that of micro-structured ceramics. Mixing bismuth oxide with nano-structured alumina leads to stabilization of the low temperature polymorph a-Bi2O3 up to 780C. The diffusion permeability of NiMoO4 layer grown at the surface of NiO ceramics, having submicron grains, was found 2 times lower if compared to NiMoO4 grown at micro-sized NiO ceramics. NiO and Al2O3 nano-powders preserve the high reactivity even when heated up to 1,000C. Despite much attention and intensive investigation of the different physical properties in nano-sized systems, there is an obvious lack of information available on their reactivity and transport properties. Concerning the reactivity of such systems, one can see that the majority of research work has been dealing with surface reactions in supported catalytic systems, ion exchange and intercalation processes but not with solid state synthesis. The latter class of interactions is the most interesting for the clarification of reactivity peculiarities of substances in the nano-state, oxides in this case. During the synthesis reaction, surface force field of nano-sized grains in contact with reaction partner may influence the reactivity and transport properties of partner and reaction product. The subject of greatest interest is displaying the phenomena associated with specific features of the non-equilibrium state of nano-sized oxide in solid state reactions, namely trivial size factor versus true (mesoscopic) size effect. According to recent ideas, the manifestations

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of ‘‘trivial’’ size effect (increasing vapor pressure, reducing melting temperature, etc.) can be predicted on the basis of classical thermodynamics of surfaces (Gibbs equation) or small particles (Thomson–Kelvin equation) and are associated with gradual increasing of specific area and volume fraction occupied by grain boundaries. Contrary, the essence of ‘‘true’’ size effect relates to the change of local parameters of the single- or multi-component system: change of predominant charge carrier type, surface phase transitions, stabilization of non-equilibrium structures via so-called hetero-phase doping, etc. The synthesis reactions can be studied using powder mixtures or diffusion couples of initial reagents, separately compacted. This gives a valuable opportunity to obtain their complementary characteristics and to detect different details of the reaction mechanism. In particular, the study of diffusion zone development via the coupled anneals using nano-materials has not been carried out before. Another remarkable aspect is that the majority of commercially available nano-powders are produced by techniques where water is used as a solvent or happens to be one of the by-products, e.g., sol–gel, spray techniques and hydrothermal synthesis. This might lead to a specific ‘‘water memory’’ effect on the main reactivity characteristics. Therefore, Neiman et al. [53] used nano-powders of NiO and Al2O3 obtained by the electrical explosion of metal wires in an oxidizing atmosphere. They concluded that the results obtained show that manifestation of the ‘‘nano-factor’’ depends on the nature of the oxide-partner, in particular, on its solid state dispersal ability, surface mobility, temperature and type of experiment (coupled annealing of ceramic compacts or interaction in the powder mixtures). Some of the results can occur due to ‘‘trivial’’ size factor, i.e., increase in interaction contact area in powder (if at least one of the initial oxides is mixtures NiOn ? MoO3, NiOm,n ? Al2Om,n 3 nano-scaled) and low conductivities of nano-structured NiO and Al2O3. However, some data cannot be explained on a conventional basis. This assertion relates to the stabilization of a-Bi2O3 low temperature polymorph in contact with Al2On3 due to the action of strong surface force of the latter. Figure 3.5 illustrates bright-field TEM images of used nano-structured Al2O3 and NiO prepared by the electrical explosion of metal wires. Figure 3.6 shows DTA curves for equimolar NiO–MoO3 mixtures while temperature dependencies of the conductivity of equimolar NiO–Al2O3 as-pressed mixtures (heating–cooling) can be seen in Fig. 3.7.

3.5 Interface Traction Generally, the main discussion is on interface in solid objects (between two phases and/or between two granules inside one metal). But, for facilitation of modeling, interface between two fluids are first considered. Actual interfaces have usually limited width but for ease of analysis, interface between two parts, being completely uniform in terms of structure and properties, has been considered in line form (or one curve) with zero width and is called Gibbs separating surface. When this model is adapted with actual conditions, it is observed that interface will cause

3.5 Interface Traction Fig. 3.5 Bright-field TEM images of nano-structured Al2O3 (a) and NiO (b) prepared via the electrical explosion of metal wires, reprinted with kind permission from Neiman [53]

Fig. 3.6 DTA curves for equimolar NiO–MoO3 mixtures, reprinted with kind permission from Neiman [53]

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Fig. 3.7 Temperature dependencies of the conductivity of equimolar NiO–Al2O3 as-pressed mixtures (heating–cooling), reprinted with kind permission from Neiman [53]

creation of some additional terms. Specially, term of free energy increase per surface unit which is shown with r. Generally, these additional terms are dependant on the selected area for positioning of separating surface. Finding accurate and appropriate place of separating surface has been studied by a number of researchers [54–59]. Although situations can be selected for interface provided that additional terms to be independent of curvature of interface and amounts of additional terms can be set for that situation. For a fluid, term r is defined as degree of work which seems necessary for production of a surface unit of interface. Mechanically, interface is studied as a stretched part and r is numerical amount of main tensile stress. For fluids, r is an inter-surface force and is usually called surface stress and/or interface stress. For two fluid phases in equilibrium with each other, effect of r on interface will cause pressure on surface of phases which are not equal with each other necessarily, i.e., pressure on phase r (Pa) may differ from pressure on phase b (Pb): 1 1 Pa Pb ¼ r þ r1 r2 At this equation, r1 and r2 are main radiuses of interface curve which their centers are located in phase r. In a specific state, if phase r in spherical form with equal radius r is placed inside phase b, the above equation will be as follows: 2r Pa P b ¼ r This equation is called Gibbs approach which is used for studying effects of interface on solid surface.

3.5 Interface Traction

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3.6 Additional Free Energy on a Solid with Nanometric Structure Free energy of a phase in a multi-crystalline material has higher amount than materials with mono-crystalline state. Such additional amount is related to the existence of inner-structure imperfections. High amount of grain boundary constitutes major part of these imperfections. The amount of grain boundary can be measured with regard to a nanostructured material. The total area, which a grain boundary is occupied, is in compatible with d-1 in which d is diameter of granule. The contact area of grain boundary has separate structure that can affect on degree of free energy. Total length of these contact areas is proportional to d-2. Contact area of these connections with grain vertices affect on free energy. The population of these areas is proportional to d-3. Hence, additional free energy of whole object (Gexcess) for volume unit is as follows: Gexcess ¼ ad1 þ bd2 þ cd3 At this equation, a, b, and c are constants proportional to geometrical factors. Parameter a is proportional to the degree of free energy increase of grain boundary per surface area, b is proportional to connections energy per length unit and c is related to three-dimensional (3D) energy (vertex energy). For any other object, which does not have very small grain size, these terms can be ignored in above equation. The posed analysis is analysis of effect of grain boundary on basis of Gibbs Approach. In the same direction, additional terms related to imperfections have been considered zero in ideal state. Of course, research activities have indicated that properties of grain boundary are varied in its length. At this state, average of these properties has merely been considered. Method of manufacturing material is regarded as one of the most important and effective factors on the way of positioning and distributing grain boundary. Even if distribution of grain boundary can be considered as constant, when size of width of imperfections has a larger amount in a way that it can be compared with diameter of grain (d), Gibbs approach will not produce any response. Consequently, use of models like ‘‘penetration at grains boundary’’ seems necessary. With regard to multilayer materials, which have been comprised of two different phases, if interface of phases have properties independent from repetition distance of phases (k), then terms related to contact of grain boundary and vertices will be omitted and additional free energy per surface unit related to interface, is explained with the following simple equation [60–69]. Gexcess ¼ ak1

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3.7 Interface Stresses When a reversible work is carried out with the aim of increase of a solid surface, two separate states can be occurred. At first state, new surface may be produced by the process like fracture or plastic flow. Here, a is known as work done by free energy of surface (or interface). At second state, surface can be stretched in elastic form. The carried out work is equal to superficial stress (interface stress) which is shown with fij. At first process, new atoms are brought to the surface. Hence, properties of surface in surface level will not be changed. On the contrary of the first state, at the second state, new atoms do not proceed to the surface and consequently, properties of surface are changed. Superficial stress is related to surface free energy and also surface elastic strain. The equation related to it has been shown in below: fij ¼ rdij þ fij ¼ rdij þ

or oeij

At this equation, d is Kronecker Delta. For isotopic surfaces, isotopic superficial stress can be defined with the amount of f. Elastic stress and elastic stretch will not exist for interface between fluids. Therefore, eij will be equal to zero and f indicates surface free energy. More research activity is needed for interface between solid and liquid with regard to f. For sustainable surfaces, r is a positive amount and f will have the same sign as r. Many tests and calculations have been made with regard to the amount and sign of interface stress on free surfaces of metals. It seems that for free surface of a metal, interface stress will have similar amount with interface energy and will cause compressive stresses in micro-metallic particles. For a sphere with radius r and a solid phase a which is placed inside liquid b, pressure difference between two phases depends on f and at this state r does not play an effective role. Hence, the above equation can be corrected as follows: 2f Pa Pb ¼ r For state of interface between two solid objects, if properties of interface in all areas are considered equal, at least three inter-superficial forces will exist. Interface free energy r and two interface stresses, apart from each other, one of which is known as f, can be considered as a base for elastic tension in solid surface. Also, pressure in surface of two solids depends on elastic modulus in each two solid surfaces. Consequently, equation of pressure difference will be as follows: Pa Pb ¼

12lb Ba 2f 3Ba þ e r0 4lb þ 3Ba 4lb þ 3Ba

At this equation, r0 is radius of hole in b solid background, provided that this surface is stress free. Li and Bi are equal to shear modulus and bulk modulus of a and b phases and e is interface stress of a and b phases [70–77].

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3.8 Chemical Equilibrium in Curved Interface Curved interface plays a key role in solvability. A specific state is assumed that spherical-shaped solid phase a has been constituted merely from a constituent A and is placed in fluid phase b that can be formed from various and many constituents. Spherical phase has feature of equilibrium with surrounding area and pressure inside sphere at this state depends on its radius. According to the following equation, A this issue will cause changes in chemical potential of constituent A in phase a la . A A 2f la r la r¼1 ¼ VaA r At this equation, V A/a is molar volume of constituent A inside phase a. Transfer of atoms A between two phases of a and b will change interface surface. Therefore, free energy of system will be changed as well. Consequently, chemical potential of constituent A at surrounding fluid area will also be changed according to the following equation. A A A 2r lb lb ¼ Va r r¼1 r For equilibrium state in planar interface, pressures in two phases and also chemical potential will be equaled with each other. According to the above equations, it is observed that when interface is a curve, not only pressure but also chemical potential will also be changed. lAb lAa r¼1 ¼ 0 r¼1

2ðr f Þ lAb lAa r ¼ r r

If the two phases are fluids, then, f will be equal to r and chemical potential will remain unchanged even with regard to curved surfaces in equilibrium state. Changes of l A/B with radius obeys molar fraction changes of constituent A which is solved in phase b. The changes at this molar fraction x A/B is obtained according to the following equation: 0 1 xA A B b r C Va 2r ln@ A¼ RT r xAb r¼1

At this equation, R is gas constant. The effective solvability of constituent A in phase b, is increased in the vicinity of minute particles of c. Changes in x A/B can be considered as a driving force for Ostwald Ripening phenomenon, causing changes

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in distribution of a phase particles. If the phase which surrounds a particles is solid, chemical equilibrium analysis will be more complicated. At this state, phase b applies a non-hydrostatic stress. Hence, chemical potential inside it cannot be defined. However, inter-surface free energy r plays a key role once again. Above equations can be used in many of systems through such method. In previous discussions, chemical equilibrium was considered in interfaces which atom will move from one phase to another phase and/or from one grain to other grain. In multipartial systems, this issue can be important i.e., in equilibrium state, each constituent can have its unique interface in such a way that some constituents are preferentially adsorbed to the interface surface and other constituents are preferentially repelled. This property will affect on interface free energy. This affair depends on temperature as well. It can be concluded that for equilibrium between phases and between chemical constituents, interface free energy r is considered and for the state of stress within solid phases, interface stress f is considered.

3.9 Influential Interface In previous sections, interface was studied as a sharp specified separating surface. However, in materials with nanometer (nano-crystalline) structure, this possibility exists that this affair may not happen. Influential interface model is very important for comprehension of actual penetration in interface and effect of microstructure on it. This subject was studied by Hilliard et al. At this research, influential changes of each intensive and scalar quantity (like density and concentration of a specified part) in interface between two environments with different properties are discussed. For this reason, the desired property has been considered as mole fraction of constituent A (CA) in a double-constituent solid solution. It is assumed that these parameters are changed with distance change (X). Also, it has been shown that local Holmholtz free energy for f system not only depends on local chemical composition of object, but also depends on local density gradient and also higher differentials (derivatives) which is explained in the following formula: 2 ! oc f ðxÞ ¼ f0 ðcÞ þ k ox x

At this equation, f0(c) is free energy per volume unit for a homogenous system with molar fraction of c and in this formula k is energy gradient coefficient. In interface between two insolvable objects, width of interface is determined by competition between chemical composition gradient reduction and also material volume reduction in a non-equilibrium composition. It has been anticipated that interface width will depend on temperature, aimed at increasing penetration at higher temperatures and diverging toward infinity. Energy gradient coefficient can be obtained by models related to solid solution. This amount is positive for the solution which shows more tendency to phase isolation (in other words, showing tendency to

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interface). This amount is negative for the solution which shows tendency to chemical composition. Energy gradient in a phase, which shows tendency to isolation, causes that total free energy, be increased.

3.10 Phase Interface Another approach that can be studied is as follows: interface can be considered as an isolating phase along with homogenous properties which in some cases, it can contribute to studying behavior of material. Details of this hypothesis were studied by some researchers. Generally speaking, direct measurement of thermodynamic properties (enthalpy, entropy and Gibbs free energy) of nanostructured materials is followed with doubt. In continuation, the results obtained from measurement of components of nanostructure metals, produced by condensation processes, have been explained thoroughly [78–89].

3.11 Measurement of Thermal and Electrochemical Properties Knowing specific heat will give allowance of calculation of temperature changes of enthalpy and entropy with respect to system. The absolute amounts of the mentioned parameters, irrespective of this fact that entropy of materials follows 3rd rule cannot be calculated. For this reason, more interests are focused on amount of enthalpy change (DH), entropy change (DS) and inter-phases free energy change (DG). These amounts are related in below form with difference in specific heat of phases. DGðT1 Þ ¼ DHðT1 Þ T1 DSðT1 Þ ¼ DHðT0 Þ þ

ZT1

0

B DCP ðTÞdT T1 @DSðT0 Þ þ

T0

ZT1

1 DCP ðTÞ C dTA T

T0

More interest is on the difference of these amounts for nano-crystalline and monocrystal state. For example: DH = H(Nano crystal) - H (Single crystal). Here, (DH) can be calculated by measurement of exchanged heat but due to lack of equilibrium between nanostructure and mono-crystalline state, determination of (DS) is impossible. Since electrochemical measurements greatly contribute in determination of absolute amounts, they are of paramount significance. The potential difference (DE) between electrodes of two samples is directly related to Gibbs free energy: DE ¼ E1 E2 ¼

l1 l2 DG ¼ nF nF

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Where E1 and E2 are electrochemical potentials of two samples (measured with respect to reference electrode), l1 and l2 show chemical potentials of atoms in samples, n is amount of charge of metallic ions and F is Faraday constant.

3.12 Condensed and Compressed Metals Many well-grounded reports have been presented with regard to measurement of specific heat of nanostructured materials like copper, palladium and platinum which are produced based on inert-gas condensed method. The results obtained from studies indicate that specific heat of nanostructured materials is more than multi-crystalline materials with ordinary grains and even is more than amorphous metals. Such difference is more for palladium metal, which is apparently related to low density of this material. When measurements of additional specific heat are used in low temperatures, it is observed that additional specific heat with a fair approximate is related to absolute temperature in linear form within studied thermal range (DCP = hT). Enthalpy stored for nano-crystalline copper metal stands at 0.026 JK-2 mol-1 and it is equal to 0.077 JK-2 mol-1 for palladium metal. Since share of electrical and magnetic parameters in specific heat can be ignored, additional specific heat of nanostructured materials (in comparison with ordinary structure) is related to situation entropy and high shakes of grain boundary. Recent research activities have shown that additional specific heat for palladium belongs to presence of light elemental impurities. Calorimetric measurements on the materials, which have been compressed as much as 85%, show releasing of heat during annealing process. This process is made during two stages, firstly, along with releasing of unsustainable grain boundary and secondly, as a result of growth of grains. It should be noted that total enthalpy stored for a sample with 12–18 nm grain size is equal to 6.2 kJ mol-1 according to additional enthalpy as a result of grain boundaries before and after relaxation process which stand at *2 and *1 JM-2 respectively which is very similar to accepted value for coarse-grain enthalpy. Also, entropy increase is created with grain boundary in such a way that it can be said that additional free energy is mostly related to enthalpy. Generally, electrochemical potential between single and multi-crystalline samples as size as ordinary grains have slight disparity with each other but this difference is high between these materials and nanostructured materials. Since multi-crystalline materials or samples with nanostructure procedure are not in internal equilibrium state, measurement of their electrochemical potentials is very difficult. All results, which are obtained from above equation for determination of potential difference (DE), are related to difference in average chemical potential of atoms that have participated in electrochemical reaction. Amount of average chemical potential (l) for multi-crystalline samples with ordinary grain size and also mono-crystalline samples can be a good approximation of sample’s

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actual chemical potential. But, this subject doesn’t apply for nanostructured samples. Atoms extant in grain boundary may be more active than atoms inside grains. This subject may affect electrochemical measurements to some extent. Measurement of electrical driving force of palladium metal through the application of melted salt electrolyte by some researchers at the temperature range of 613–693 K showed that nano-crystalline materials are even sustainable at relatively high temperatures as compared with structures with normal grain size. Electrical driving force difference of palladium is lessened with time between nano-crystalline state and normal-sized grain state. In accordance with enthalpy measurements, change in nano-crystalline samples is occurred at two stages. First, it is followed with relaxation of grain boundary and then with increase of grain size. Gartner has estimated additional free energy of nanostructured palladium, which is equal to *1 kJ mol-1 for grain size near 20 nm and approximately *6.7 kJ mol-1 for grain diameter of 11 nm according to the free energy increase of grain boundary approximately 1 jm-2. These amounts are in good accordance with measurement carried out for additional enthalpy. It can be concluded that for small sized grains up to *10 nm, grain boundary in nanocrystalline pure metals doesn’t have properties different from multi-crystalline properties in ordinary grain size. The grain boundary areas in nanostructured samples result in increase of free energy which primarily is dependant to enthalpy [90–102].

3.13 Methods for Production of Nanostructured Materials Nano-crystalline materials can be produced by various methods. Provision of nano-crystalline bulk materials can be obtained both with bottom-up arrangement of atoms or molecules in nanometer clusters and through top-down methods which starts with bulk solid and nanostructure is obtained by decomposition of bulk structure. The bottom-up arrangement methods for atoms include inert-gas condensation and compression. This method has been comprised of evaporation of metals inside a chamber, containing a partial pressure (approximately some hundred Pascal) of a neutral gas like helium which evaporated atoms strike with the atoms inside chamber and lose their energy, condensing into separate small crystals as loose powders. Tiny condensed powders on the part cooled by accumulated liquid nitrogen are separated by Teflon ring and guided into the compressing device. Irrespective of this fact that this method provides materials for initial study of nanostructure properties, its disadvantages can be referred to the limitation of size and being a two-stage process which may not be created at the stage of full condensation and material contact. Generally speaking, methods of manufacturing based on preliminary phase classify at three categories of steam, liquid and solid. In Table 3.1, some samples of sub-branches methods of each of these categories and type of crystal materials (in dimensional term) have been specified. Selection

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Table 3.1 Various types of methods for manufacturing nano-crystalline materials Initial Method Product nanometer phase dimensions Steam

Liquid

Solid

Inert-gas-condensation Glazing physical steam, evaporating and sputtering Plasma processes Condensing chemical steam Chemical reactions Fast freezing Electrolyte glaze Chemical reactions Milling and alloying mechanically Crystallization of glassy phases Sparkling erosion Sliding erosion

3-D Mono-dimensional 3-D Two- and three dimensional 3-D 3-D Mono- and three dimensional 3-D 3-D 3-D 3-D 3-D

of process depends on the parameters like capability of control of microstructure components like size of grains, chemical composition and chemistry of surface or degree of interface pollution and also process expenses and costs. Some methods are very expensive and complicated while some others are cheap but do not create a clean surface. Inert gas-condensation processes, grinding and mechanical alloying, electrochemical deposition, crystallization of glassy phases and spray conversion processing has more application for manufacturing of nanostructured materials [25, 42, 103–110]. At all two-stage processes for formation of nanostructured materials, nanometer particles are produced at first stage or powdered particles with nanometric structure are produced in mechanical erosion process. These particles should be stabilized for formation of bulk. The problem of stabilization has been remained as an open-air space for progress and development and has not thus far been settled. The problem is related to the below issue: these processes are in dire need of combination of appropriate temperature and pressure for establishing atomic connection between particles, aimed at establishing theoretical density in the long run of all inter-particle contacts. This affair should be occurred without enlarging nanometer microstructure and/or without establishing any structural imperfections and also unwanted phases. Electrochemical deposition process is placed at the category of bottom-up methods for production of nanostructured materials and is a mono-stage process which does not need to be stabilized. Coarse and thick coatings as a result of electrochemical deposition may be interpreted as a bulk. After 1980s, electrical deposition was studied as a method for production of nano-crystalline materials and then was developed as industrial production method of these materials. Electrochemical deposition method occurs by nucleation of crystals on sub-layer surface and their consequent growth along with nucleation of new crystals. With the aim of obtaining nanometer grain size, nucleation process should witness preference to growth. Variables in electrochemical

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deposition process include composition of bath, pH of bath, temperature, overvoltage, bath additives and type of imposed cathodic current. Mirzamohammadi et al. [111] studied electrodeposition of tertiary Alumina/ Yitria/carbon nanotube (Al2O3/Y2O3/CNT) nano-composite by using pulsed current. Coating process has been performed in nickel sulphate bath and nanostructure of the obtained compound layer was examined with high precision figure analysis of SEM nano-images. The effects of process variables, i.e., Y2O3 concentration, treatment time, current density and temperature of electrolyte have been experimentally studied. Statistical methods were used to achieve the minimum wear rate and average size of nanoparticles. Finally the contribution percentage of different effective factors was revealed and confirmation run showed the validity of obtained results. Also it has been revealed that by changing the size of nanoparticles, wear properties of coatings will change significantly. AFM and TEM analysis have confirmed smooth surface and average size of nanoparticles in the optimal coating. Nickel and nickel-based alloys are used widely for numerous applications, which most of them require corrosion, wear and heat resistances, including different turbine plants, nuclear power systems, and chemical and oil industries. Ceramic or metal matrix nanocomposite coatings usually have special properties such as dispersion hardening, self-lubricity, high temperature inertness, good wear and corrosion resistance, chemical and biological compatibility. This accounts for the increased application of Ni-based nanocomposites in different industries. In order to meet the requirement for developing novel metal-based nanocomposites, many preparation techniques have been investigated. Considering a technique conducted at a normal pressure and ambient temperature and with low cost and high deposition rate, electrodeposition is considered to be one of the most important techniques for producing nanocomposites and nanocrystals. Mirzamohammadi et al. [111] used the Taguchi method for the design of experiment for optimizing tertiary (Al2O3/Y2O3/CNT) nanocomposite electrodeposited coating process parameters for wear protection of treated samples. The contribution of Y2O3 concentration is more than the sum of the contributions of all the other three factors. It is evident that, among the selected factors, Y2O3 concentration has the major influence on the wear rate of performed coatings. It can be seen that the current density is the second important factor that affects the wear rate of the treated substrates. Furthermore, it can be assumed that treatment time and temperature of electrolyte have almost the same effect on wear rates of coatings because of the minor difference in the contribution percentages between these two factors. By ranking their relative contributions, the sequence of the four factors affecting the wear rate is Y2O3 concentration, current density, treatment time and temperature of electrolyte. In the case of average size of nanoparticles ranking of effective factors by their relative contributions is the same as for wear rate which shows strong relation between these two measured properties of coatings. AFM and TEM analysis have confirmed smooth surface and average size of nanoparticles in the optimal coating and can be seen in Figs. 3.8 and 3.9 respectively.

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Fig. 3.8 AFM nanostructure of optimal nanocomposite electrodeposited coating [111]

Fig. 3.9 TEM (BFI) nanostructure of optimal nanocomposite electrodeposited coating [111]

3.14 Nano-Technological Compatibility in Coating As it previously mentioned, coating industry is determined to use nano-technology and nano-layers was even applied before advancements in nano-technology. First of all, a complete relevant definition of nano-coating needed be offered. It may be acclaimed that the best definition for nano-coating is a type of coating using nanostructured materials such as: • • • •

Nano-particles Nano-tubes Nano-wires Other available nano-structures such as nano-layers

Looking at this definition, one can state thickness of nano-coating may be more than nano scale, but nano-materials are used. Here, one may ask why industries are this much interested in using nano-coating. Due to occurrence of quantum phenomena at sizes lower than 100 nm, nanomaterials are of unique features. In most of cases combination of these materials improve many required characteristics, as well as keeping original materials properties. Hence, using nano-materials a novel revolution happened in technology

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of making new materials, particularly composite ones. Many research centers have started widespread movements and even produced some important materials such as nano-particles. Some of materials have extensive industrial uses or are used in key industries such heavy or military industries; then are of strategic applications such as scratch resistant nano-composites. Nano-metric coating is widely used and in some cases they even have commercial uses and are produced in mass quantities.

3.15 Improvement of Coating Quality Using Nanotechnology Coating has variant applications, such as protecting coatings against corrosion, scratch, stain, denting, bending, foliation, or making water proof and acid resistant coatings. Coating is used in every aspect of life from coating on glass, wood, plastic, paper, fibers, or similar foliations, to coating in heavy industries and special applications (e.g., military applications). Also, coatings have a key role as protector against climatic conditions such as rain, snow, solar ultraviolet beam, or chemical pollutions. Regarding abundant applications of coating in variant tools, researchers have always been trying to promote coating quality and lower its costs in academy and industry. Todays, advancements in technology have significantly increased coating quality. Recently, Coating through nano-materials has reached to practical scale and some famous supplier companies of industrial panels and automobiles use this coating in their products [112–116]. With respect to applied study in this field, main applications of nano-particles are categorized into following cases: • Coat resistant against scratch, abrasion, corrosion, and environmental agents • Variety of optical coatings, including coatings resistant against reflection and fog, or protecting ones • Coatings with medical, biological, and environmental applications • Coatings with electrical and electronic applications

3.16 Abrasion, Scratch, and Corrosion Resistant Coatings 3.16.1 Coatings Resistant Against Scratch, Abrasion, Corrosion, and Environmental Agents Coatings resistant against scratch, abrasion, corrosion, and environmental agents are mostly used to coat many panels. It is one of fields where nano-technology is capable of creating dramatic changes. However, this type of coating is a large market for applications of nano-products, due to its widespread usages. Then

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industrial researches and production of these coatings are economically valuable. Throughout following lines some of important applications of this coating will be studied. Needless to say, this type of coating will be used in so many similar cases and the following applications are ones which large industries are investing for them or about to produce or supply their products.

3.16.2 Nano-Metric Abrasion, Scratch, and Corrosion Resistant Coatings Nono-technology advancements had significant effects on methods and processes of coating. Nano-scale materials, including materials with particle size of lower than 100 nm, are studied and prepared for industrial scale applications. Metallic coating of stainless steel using crystalline nano-powder has higher hardness compared with traditional materials. Other materials such as tungsten carbide powder is also used for similar—but in smaller scale—applications. High Velocity Oxy Fuel (HVOF) is among methods applied for nano-composite coating, where a combination of polymer and ceramic is used. Advantage of this method is its lack of destruction in polymer. To apply these materials, polymer-ceramic joining mechanisms have to be promoted. Also, thin layers coating has been applied to improve surfaces and constructing new panels, although the objective of making nano-layers is achieving to interesting, practical and quantum features of these systems. Once these thin layers are made in such scales, they will be of particular properties, such as absorption of ultraviolet beam, antistatic, and electrical conductivity, in comparison with its mass state. Painting and coating industries are among pioneer industries which use these interesting properties. The industries found that thin layers obtained from nano-particles are of better cohesion to applied surface, more flexibility, and a very small difference in their final prices. Through this method one can achieve smoother layers, compared with traditional methods. These layers are used for self-cleaner surfaces and scratch resistant coating. Todays, many companies use different properties of nano-particles to improve coating quality. Advanced Nano Products Co. Ltd in Taejon, South Korea, is one of active companies in this field. At products of this company metallic oxide particles are used to prevent intrusion of infrared, ultraviolet, and thermal light. Glasses coated through this method can be used in windows of buildings, cars, and offices. One of noteworthy advantages of these particles is their transparency at visible wavelengths (400–700 nm) and, consequently, invisibility with naked eyes. While some particles are added to improve scratch resistance properties they diffract the light and cause its seeming opaque, due to their coarse grains. On the other hand, adding nano-particles maintains products transparency due to lack of light diffracting, as well as improving other given characteristics. Other important applied materials for this objective include Al2O3, antimony tin oxide (ATO), and ZnO, which is use

3.16

Abrasion, Scratch, and Corrosion Resistant Coatings

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as zinc oxide in sun block coatings. ATO is also used for making antistatic coatings. There are some other cases for practical applications of nano-particle, such as coating of fire retardant wood doors and scratch resistant coatings. Also, Mercedes Benz applied nano-coating as scratch resistant and antibacterial coating at interior part of SLK, CL, S, SL, and E automobile series. Another developing aspect is surface nano-coating with TiO2, which is used as a corrosion resistant coating with self-cleaning properties [117–128].

3.16.3 Using Alumina as a Scratch and Abrasion Resistant Coating Among important objective of coating one can name enhancement of surface resistance against scratch and abrasion, so material development and completion and abrasion and scratch resistant coatings methods is of great importance. In most industrial and military applications scratch and abrasion resistant coatings are very important. Initiating advancements in nano-technology is its uses in different sections for products quality promotion nano-coating is applied to construction and improvement of abrasion and scratch resistant coatings. One of important materials for this aim is alumina nano-powder, developed by Nanodur Company. To use this material it should be mixed in different colors, where due to its small particle sizes there will be no significant changes in color and apparent features of the block.

3.16.4 Designing Light Resistant Panels for Plane Body Thanks to nano-technology, Research and Development section of Boeing Company has accomplished designing new materials for plane body. Their aim for upcoming future is development of new materials, lighter and stronger than previous ones, for plane body. Their employ from nano-coating leads to creation of panels with no need for repainting, as well as producing stronger, lighter, and of longer life ones. This method is also used to produce long-lasting batteries for satellites.

3.16.5 Ceramics Reinforced by Carbon Nano-Tubes These ceramics were fabricated by researchers of materials science in Ve davice. They can conduct electrical current and can be served as conductor or insulator, depending to direction of applied materials. These materials are very hard and of a great resistance against chemicals, so they can be applied as a convenient coating for turbine fans. They are made of alumina powder plus 5–10% carbon nano-tubes, with a scanty amount of niobium. Compared with pure alumina, these fabricated

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materials are of grater electrical conductivity. They convey the heat along array of carbon nano-tubes, while in the vertical direction they act as thermal insulator layer. It is predicted that these ceramics be applied for thermal coating in coming future.

3.17 Nano-Coating Resistant Against Corrosion NANOMAG has invented a type of nano-composite materials resistant against corrosion, as a substitute for hazardous coating with Cr components. This coating can be applied for reinforcement of Mg alloys. These coated alloys are used in transportation, aerospace, and aviation industries. Weight reduction is one of most important objectives followed for making vehicles, results in decrease of fuel consumption and release of pollutants. For this aim Mg alloys are used for a long time. Different methods are used to coat these alloys. Among them one can name anodic coating and using CrVI components. First method is expensive, while the latter is environmentally dangerous due to poisonous nature of CrVI components. Regarding this point, the objective for development of nano-composite coating is making a low priced and environmentally compatible product. This coating needs to be of a high resistance against corrosion. Making such a product various research groups are cooperating. Also countries such as Austria and Israel are very active in this field. Among their objectives, one can name using these alloys in exterior body of vehicles. For this type of coating, it is supposed to use PlasmaEnhanced Chemical Vapor Deposition (PECVD), sol–gel, and ceramic coating methods. Making these alloys there has been accomplished a noticeable promotion in transportation industry and environmental pollution decreases. It is noteworthy that these alloys recycling costs is lower than the others.

3.18 Using Nano-Particles for Coating in Transportation Industry PPG is one of most important producers of coating, esp. those used in transportation industry. This company also produces flat glasses, especial coatings, and main coatings of aviation industry. The company mentions nano-materials of silicon oxide as a convenient coat for transportation industry. The coating is of a great resistance against foliation, scratch, dentation, and pollution stains. A thin layer with a very strong lateral bond causes to high hardness of the coating and increases its resistance against scratch. However, it must be noted this hardness leads to coatings’ brittleness against thermal changes, hence layers with weaker lateral bonds is recommended. Then, silicon colloidal coatings are used. PPG uses alumina-silicate nano-particles as an essential material for coating process. It has refractive index of 10 in its nano form. Particles are spherical with no color, and their surface has no hydroxyl groups; above all they do not be coagulated in their

3.18

Using Nano-Particles for Coating in Transportation Industry

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proximity. Surfaces coated with this material have a high resistance against foliation, crushing, or scratching. Through optimization of composition and applying nano-materials it is also possible to reach other useful features. This company researches suggest adding only 1 weight percent nano-particles to traditional coatings leads to a significant improvement in coating properties.

3.19 Coating Applications in Defense and Aerospace Industries Coating is widely used in defense and aerospace industries to improve products quality, capability, and safety. Among these coatings one can name ones resistant against corrosion, decay, and wear or enhancing of surface quality. Some of coating applications are: versatile nano-metric coatings, which now are in developing phase. These coatings are resistant against abrasion and corrosion; they can also detect cracks, chemical and physical damages, and etc. in plain body. Besides, they are of a high cohesion to coated block. Some of these coatings are also capable of changing damaged part color, which helps detect damaged zone. Using conventional coatings in defense industry is difficult and also hazardous for people who are dealing with that. It must be noted that most of applied coatings in defense industry should be touched by hand. This can cause many damages to used coatings and the operator. Reports show that annual expenses of Defense Ministry of United States for dealing with corrosion and abrasion problems exceed 10 billion dollars, where 2 billion dollar of this figure is allocated to coating. Currently, 20% of US armies’ vehicles are worn-out, due to damages in coating and need for repainting. Considering above discussions, it is planned to apply intelligent coatings for vehicles used in military industries. Scratch resistant coatings detect, repair, and moderate damages. Other important suggested coatings are those with color change compatibility in different operational conditions of battle arena for camouflage. These coatings are supposed to be applied in tanks or military vehicles body to decrease visionary and detection capabilities.

3.20 Using Ceramic Nano-Coatings in US Navy Strength and stability of nano-coatings has encouraged US navy to invest in this field and use nano-coatings in military gears. Once using nano-coatings, objectives such as increase of panels’ life and decrease of repair and maintenance expenses are considered. Achieving this aim, there have been financial aids to different centers to find a convenient material, and finally alumina and Titania were chosen. After a while, a product, Babi2613, was emerged. Using this substance is not recommended for coating applications due to its high costs. Every pound (453.6 g) of this substance costs about 30–50 dollars.

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Compared with traditional coating methods, nano-ceramic coatings show a noticeable improvement in their characteristics, as some of required properties, such as strength, has increased up to 4–6 times. Nowadays, US navy use nanocoating in its equipment. Researchers declare that nano-ceramic coating is currently used for different pieces such as fan axle, periscope axle, and more than 150 other applications, where corrosion resistant surface is needed. For example, air outlets and inlets in submarines are coated with nano-materials, which bring about 400,000 dollars in each military gear and 20 million dollars annual saving. Power transfer axles in submarines or mine sweepers have also leads to 10 million annual saving. These coatings can also be used for commercial aims. Also in 2003, Spire Corp has received 75,000 dollars financial support to develop nano-coating for orthopedic objectives. This group is determined to develop nano-coatings, which are harder and have more cohesive coating and minimum corrosion and abrasion.

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Chapter 4

Size Effect in Electrochemical Properties of Nanostructured Coatings

4.1 Introduction Electro-crystallization processes happen at solid and liquid electrochemical interface. Since driving force of this process is controlled by density of current and electrode potential, it has attracted the attention of many researchers to itself. Metal cathode deposition process on surfaces of other property and/or the same property by electrolyte, containing metallic ions, is regarded as major studied electrical crystallite process. Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) have brought about this possibility to study the phenomenon of electrical crystallization in in situ with the range of atomic level. Exertion of metallic coating with electrical plating is based on discharge of electrical load of metallic ions extant at electrolyte on cathode surface (sub-layer or desired component for coating). Existing metallic ions precipitate on surface as metallic atom with receiving electron from interface of solid-electrolyte conductor. The electrons required for reduction of metallic ions are provided through various methods. These electrons are either provided by a foreign potential source and/or are provided by reductant agents extant in solution (coating with electroless method). Source of providing metallic ions are the metallic salts added to solution or these metallic ions are supplied as a result of anodic solution (known as sacrificial anodes). Electrochemical deposition process includes a great number of intermediate stages, the most important of which is as follows: • Transfer of hydrated metallic ion or metallic complex from mass of solution to cathode. • Isolation of hydrated layer from metallic ion in metal-solution interface. • Transfer of load and formation of ad-atom on cathode surface. • Formation of crystalline nucleuses as a result of penetration of adsorbed atoms on cathode surface. • Connection of crystalline nuclei and formation of metallic layer.


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4 Size Effect in Electrochemical Properties of Nanostructured Coatings

Transfer of metallic ion from bulk solution to cathode surface is as a result of penetration and convection phenomena. Discharge of electrical load of adsorbed ions and converting them into adsorbed atoms are made in double layers which is instantaneously occurred at solution-metal interface. Although metallic ions lose most of their charges at this stage, the remaining charge amount causes that a part of hydration sheath to be remained. At this stage, these ions after passing through dual layer are adsorbed on cathode surface and form adsorbed atoms. With the aim of creation of a coherent metallic film on cathode surface, two stages or processes must be occurred. These stages include nucleation and growth which are known as electrical crystallization stages. In fact, nucleation stage includes migration under control of adsorbed atoms which differs from penetration process inside solution. The growth stage happens when created nuclei are reached to a critical size [1–14]. Nickel-tungsten/carbon nanotube nanocomposite layers with high content and uniform dispersion of carbon nanotubes were fabricated using pulsed electrodeposition technique [15]. Nanocomposite layers were analyzed by scanning electron microscopy, atomic force microscopy, microhardness, and Tafel polarization tests. Effect of duty cycle of pulsed current or concentration of carbon nanotubes in the metallic matrix has been investigated on electrochemical and mechanical properties of obtained layers. It has been shown that both electrochemical and mechanical properties of nanocomposite layers that formed by pulsed current were improved significantly with respect to un-composed Ni–W layer. Results were not only concerned by concentration of carbon nanotubes in the layer but also influenced by the distribution of nanoparticulates in the metallic matrix. Plots of Tafel polarization tests for Ni–W and nanocomposite layers formed by different duty cycles (or different concentrations of carbon nanotubes in the metallic matrix) are shown in Fig. 4.1. Changing trend of corrosion potentials (Ecorr) and corrosion current densities (icorr), which were analyzed through Tafel

Fig. 4.1 Tafel plots of (open square) Ni–W and Ni–W/CNT nanocomposite layers formed by different duty cycles of pulsed current ((open triangle) 20% of duty cycle (or 4.3 wt% of carbon nanotubes content), (times) 50% of duty cycle (or 9.1 wt% of carbon nanotubes content), and (open circle) 80% of duty cycle (or 13.1 wt% of carbon nanotubes content) [15]

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113

Fig. 4.2 Changing trends of Ecorr and icorr with respect to applied duty cycle (or concentration of CNT nanoparticulates in the metallic matrix) [15]

plots, are illustrated in Fig. 4.2. Addition of nanoparticulates will shift corrosion potentials of electroplated layers towards noble direction (positive values). Increasing the concentration of carbon nanotubes will have an effect in an opposite manner, but the changing amount is ignorable. Changing trend of corrosion current densities shows that increasing the amount of carbon nanotubes has an optimum level for decreasing them. It was found that 9.1 wt% of carbon nanotubes in the metallic matrix will show minimum corrosion current density of nanocomposite layer. Surfaces have approximately no porosity, so it can be concluded that differences in electrochemical properties are just related to the content of carbon nanotubes and their distribution and amount of agglomeration in the metallic matrix. When carbon nanotubes distribute uniformly, they will protect nanocomposites and substrates from corrosive agents and decrease the corrosion current densities. In an opposite manner, the agglomerated nanoparticulates will decrease the electrochemical properties of the obtained layer.

4.2 Thermodynamic Equilibrium In equilibrium state in an electrochemical cell, electrochemical potential of extant components should be equal. ~ c;1 ¼ l ~ ad;1 ~s;1 ¼ l l ~s;1 : electrochemical potential of electrolyte containing Mez metallic ions, l ~c;1 : l ~ad;1 : electrochemical electrochemical potential of metal electrode (anode), l potential of metallic ions of Mez adsorbed on cathode surface. Electrochemical potential of each of components is as follows: ~ 0s þ KT lnas;1 þ Ze /s;1 ~s;1 ¼ l l ~ c;1 ¼ l ~ 0c þ Ze /c;1 l ~0ad þ KT lnaad;1 þ Ze /c;1 ~ad;1 ¼ l l

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In above equations, l0ad ; l0c ; l0s are standard chemical potentials, and xad;1 and ac;1 ; as;1 ¼ 1 is ion activities and /c;1 ; /s;1 are galvanic potential of electrolyte solution and metallic electrode (anode) respectively. ~s;1 , ~c;1 ¼ l In equilibrium state, regarding above equation, and considering l Nernst Equation, which indicates equilibrium potential of metallic electrode immersed in solution containing metallic ions of the same material with activity as,?, is obtained. E1 ¼ E 0 þ

KT ln as;1 Ze

~ad;1 ¼ l ~ s;1 state, the following equation will be obtained: Also, in l ðl0 l0c Þ Kt as;1 E1 ¼ s þ ln aad;1 Ze Ze With equalizing above equations, activity of adatoms in equilibrium state will be calculated as follows: 0 ðl l0ad Þ aad;1 ¼ exp c KT With the aim of commencing crystalline growth or process of formation of nucleation on neutral surface of cathode, metal ions (Mez) in electrolyte should be reached to supersaturated state. That is to say that electrochemical potential of ions extant at solution should be more than electrochemical potential of metal electrode ~ c;1 Þ. (anode) ð~ ls [ l In fact, electrochemical supersaturation is defined as follows: ~c;1 [ 0 ~s l D~ l¼l As it is observed, the amount of D~ l is defined as driving force for transmission of electrochemical phase. Thanks to its significance, it seems necessary that controllable and measurable physical values should be expressed. General formula used for D~ l is as follows: D~ l ¼ Zeg Where g indicates cathodic over-voltage which is defined as follows: g ¼ E1 E g¼

KT as ln Ze as;1

Regarding the aforementioned equations, electrochemical super-saturation state for precipitation on neutral surface of cathode seems possible through two methods. At the first method, ionic activity of solution (as) is kept constant in a(1) s,?

4.2 Thermodynamic Equilibrium

115

amount and system state is changed from equilibrium point (1) to equilibrium point (P), located at super-saturation area. This affair is made by change of electrode potential from equilibrium state E(1) ? to a more negative amount E = E(2) . At second method, electrode potential is kept constant in E(2) ? ? amount and ionic activity of solution is increased from equilibrium state of situation (2) to super-saturation situation (P). The second method of making super-saturation state is made by pulse potentiostatic method. Also, this method is used for local precipitation of metal nucleations as size as nano. Formation of a n-atomic nucleation from new phase ~ requires overcoming thermodynamic obstacle of nucleation work DGðnÞ which is explained according to the following formula: ~ DGðnÞ ¼ nD~ l þ /ðnÞ Here, /(n) explains total energy increase as a result of creation of new interface (creation of nucleation on cathode surface) [16–29].

4.3 Classical Nucleation Theory At this theory, it has anticipated that nucleuses should be large enough. At this state, the number of atoms (n) can be considered as a dependant variable and the amount of /(n) is explained by energies related to specific free surface, interface and linear interface in system constituted from cathode electrode, electrolyte and ~ nucleuses. At this state, DGðnÞ is a differential function and under conditions of ~ ¼ 0 we will have: final limit ½dDGðnÞ=dn n¼nc

D~ l¼

~ dUðnÞ dn n¼nc

The above equation is a general expression of Gibbs–Thomson equation which explains relation between super-saturation D~ l and size of critical nucleuses nc in an unsustainable equilibrium state. Studying theoretical formulas for nucleation ~ shows that relation between DGðnÞ in comparison with nucleus size n will have an ~ c Þ; nc ; D~ utmost state in n = nc [30–39]. The amounts of DGðn l are related to each other for formation of a 3-D nucleus on sub-layer of external layer as follows: ~ c;3D Þ ¼ 1= nc;3D D~ ~ c;3D Þ ¼ 1= Uðn DGðn l 3 2 If nucleus is formed in 2-D form on the surface, we will have: ~ c;2D Þ ¼ nc;2D D~ ~ c;2D Þ ¼ 1= Uðn l DGðn 2

116


4.4 Atomic Nucleation Theory At this state, the formed nucleuses are of very small size and therefore, size of nucleus is considered as a separate variable and macroscopic classic theory is not applicable. Here, formation process of nucleuses is explained by considerations related to atom and through the application of general formula for nucleation work. ~ Relation between size of nucleus (n) in contrast with DGðnÞ is not as a smooth curve and has the minimums and maximums which are dependent on structure and energy state of nucleus. The highest amount in a super saturation state indicates size of critical nucleus. Non-continuous change in size of nucleuses in small-scale dimensions will affect relation between D~ l and size of nucleus (n). In this state, for each critical nucleus size, there is a supersaturation range instead of a constant value D~ l. This specific property of small nucleuses will severely affect process of formation of phase during electro-crystallization process and should be considered at the time of interpretation of experimental information for electrochemical nucleation on external surface. At this state, size of critical nucleuses does not exceed size of some atoms [40, 41]. Properties of Si3N4/Ni electroplated nanocomposite such as corrosion current density after long time immersion, roughness of obtained layer and distribution of nanometric particulates have been studied [42]. All of the other effective factors for fabrication of nanocomposite coatings have been fixed for better studying the effect of the average size of nanoparticulates. The effects of the different average size of nanometric particulates (ASNP) from submicron scale (less than 1 lm) to nanometric scale (less than 10 nm) have been studied. The nanostructures of surfaces were examined by a scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). Corrosion rates of the coatings were determined using the Tafel polarization test. It has been seen that decreasing the ASNP will lead to lower corrosion current densities however in some cases pitting phenomena has been observed. The roughness illustrated a minimum level while the distribution of nanometric particulates will be more uniform by decreasing the ASNP. The effects of pulsed current on electrodeposition (frequency, duty cycle) and concentration of nanoparticulates on electrodeposition bath on trend of obtained curves have been discussed. Response Surface Methodology was applied for optimizing the effective operating conditions of coatings. The levels studied were frequency range between 1,000 and 9,000 Hz, duty cycle between 10 and 90% and concentration of nanoparticulates among 10–90 g l-1. Figure 4.3 illustrates changing trend of corrosion current densities (CCD) after immersion in 3.5 wt% NaCl solution in room temperature. As it can be seen from this figure, however CCD will increase a little after long time immersion in corrosive solution but lowering the ASNP will lead to lower CCDs which at first indicate that decreasing ASNP is useful for decreasing CCD. However observations with unequipped eye did not indicate presence of pitting phenomena but AFM studies show that nanocomposites with very low ASNP will show some

4.4 Atomic Nucleation Theory

117

Fig. 4.3 Changing trend of corrosion current densities for different ASNPs after immersion in 3.5 wt% NaCl solution in room temperature [42] Fig. 4.4 AFM nanostructure of nanometric pits of the layer with ASNP equal to 9 nm [42]

nanometric pits. Figure 4.4 shows the observed pits for the nanocomposite layer with ASNP equal to 9 nm [42].

4.5 Kinetics of Formation of Nucleuses in Electro-Crystallization ~ Nucleation work DGðnÞ is regarded as criteria of thermodynamic obstacle that must be overcome in order to transfer (nc) ions from inside electrolyte solution for

118


formation of new atomic nucleuses on electrode surface. The rate of this process, which is a kinetic value, is defined by parameter J(t). If nucleation rate is assumed constant, and/or the amount of J(t) is reached constant amount of J0, dependency of nucleation rate to over-potential and density will be as follows: ~ c Þ DGðn J0 ¼ Z0 Wk1 exp kT where ðcm2 ÞZ0 is the number of density of active points on the surface, (s-1)W is joining frequency of isolated atoms to critical nucleuses and k-1 is a dimensionfree value which has been considered for disparity between the number of constant critical nucleuses and quasi-equilibrium ones. In microscopy classic nucleation theory, k-1 is defined as follows: k

1

~ 1 DGðnc Þ 2 ¼ 3pn2c kT

and is known as Zeldovich factor. When critical nucleus is very small and its size remains constant during supersaturation distances, its value is equal to one for supersaturation state or active sub-layer. At this specific state of electrochemical phase formation especially on external sub-layer surface, dependency of constant nucleation rate to over-potential and concentration should be explained in atomic theoretical terms of formation of electrochemical phase. At this state, the amount of W will be equal to: U azeE exp W ¼ kt exp kT kT where kt is factor of frequency, a is charge transfer constant and U is energy obstacle for transfer of ion from electrolyte solution to the critical nucleuses in electrode potential (E = 0). The above equation for W is used when supersaturation term D~ l is changed by change of concentration of metal ions in a constant electrode potential E. With substitution of equations, for equation of J0 in k1 ! 1 state, we will have: J0 ¼ Z0 XðEÞcnc þ1 In such a way that: XðEÞ ¼ kt

c c1 c1

nc

~ cÞ U þ azeE þ Uðn exp kT

Here, c? is equilibrium concentration of metal ions in temperature T. c?, c is a ; c ¼ aCs respectively. The above coefficients of activity in the form of c ¼ Cs;1 1 equation shows that critical nucleus size can be determined through experimental information of J0(c) which has been obtained in a constant potential.

4.5 Kinetics of Formation of Nucleuses in Electro-Crystallization

nc ¼

119

dlnJ0 1 dlnc

If supersaturation is considered with change of electrode potential (E) in constant concentration (C? = c?as,?), it is better to use cathode over-potential in above equation. At this case, constant nucleation rate in k1 ! 1 will be as follows: ðn þ aÞzeg J0 ¼ Z0 Xðc1 Þexp kT

XðcÞ ¼

kt ðc1 c01 Þ1x exp

~ cÞ U þ azeE0 þ Uðn kT

As it is observed, the size of critical nucleus size (ne) can be determined through test information of J0(l) obtained in C = C? according to the following formula: kT d ln J0 nc ¼ a ze dg The above equations have been extracted through assuming constant number of active areas for nucleation on surface of electrode which is regarded as the simplest electrochemical phase state. In practice, the number of active sites for nucleation on surface can depend on amounts of time, potential and pH [43–54].

4.6 Electrode Surface Energy State Active points are probably considered as the most significant obstacle in nucleation kinetics. Regarding the unique properties of a specific electrochemical system, active points are appeared or disappeared completely different on surface of electrode. The main reason of the issue is related to different chemical and electrochemical reactions like adsorption and separation of organic and non-organic ions or molecules, direct oxidation or reduction of electrode surface which is occurred previously or concurrent with process of formation of nucleation on electrode surface. That is to say that assumption of a neutral electrode is considered as a completely optimistic approach. A theoretical model has been proposed for formation of nucleus thanks to time-dependency of number of active points on electrode surface. If active points on electrode surface is appeared or disappeared concurrent with nucleus formation process, time dependency of number of nucleuses (N(t)) can be expressed by following second-order differential equation. d2 N dN þA þ BðN N0 Þ ¼ 0 dt2 dt

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Here A and B is dependent on following factors: frequency of nucleation, Kn ¼ a+ c Þ Wk1 expDGðn kT and appearing frequencies (K ) of active areas on electrode surface and disappearing frequencies of active areas (Ka-) from electrode surface. N0 = Z0a ? Z01 is equal to total active areas in t = 0 time. Here, Z01, Z0a is existing active places and number of places having activation potential at t = 0 time respectively. With analytical solution of above equation, nucleation rate JðtÞ ¼ dNðtÞ dðtÞ

and also the number of nucleuses N(t) can be expressed theoretically. It is observed that nucleation rate shows an initial non-zero amount 0 Jð0Þ ¼ dN ¼ K Z n a , a final amount of J(?) = 0 and also a maximum amount dt t¼0 in t = tm. Also, it is observed that amount of N(t) is changed from zero to a maximum limit of N0 = Z0a ? Z01 in which all active areas are developed and are occupied by new phases of nucleation. At this complicated state of formation of electrochemical phase, the constant state can be set up only under specific terms and conditions and after a certain time period. In fact, the amount, which is measured as constant nucleation rate, not only specifies experimental information with regard to kinetics of nucleation, but also clarifies appearance of active areas on electrode (provided that this process is determiner of start state rate of phase change). These cases should be considered while interpreting experimental information on kinetics of formation of nucleuses on sub-layer of the same material or another material [55–66].

4.7 Nucleus Situation on Electrode Surface Based on law of probability distribution, the distance between nth neighbor of a nucleus, which has been randomly distributed inside a v-dimensional space, can be expressed as follows: dPv;n ¼

v=2 v=2 v p N0;v rv;n v p N0;v mn1 rv;n drv;n exp Cð1 þ v=2Þ ðn 1Þ! Cð1 þ v=2Þ

In fact, in above equation, parameter dPv,n specifies probability of presence of nth neighbor of a nucleus at the distance between rv,n and rv,n ? drv,n where No,v is nucleus’s average density, T is gamma function while v is spatial dimensions in such a way that if nucleus is formed on a surface, v = 2 and if nucleus is formed inside a 3-D space, v = 3. The average distance between nucleuses can be expressed as follows: rv;n ¼

Z1 0

Cðn þ 1=vÞ Cðn þ v=2Þ 1=m rv;n dPv;n ðn 1Þ! pv=2 N0;v

4.7 Nucleus Situation on Electrode Surface

121

It is observed that probability of presence of small distances on results obtained from practical test is less in comparison with theoretical calculations. This subject vanishes for second and third neighbors of nucleus. That is to say areas with meager nucleation rate are and areas which their nucleation rate are decreased, surround sustainable nucleuses. This issue has been observed in other electrochemical systems. Electrodeposition of tertiary Al2O3/Y2O3/CNT nanocomposite by using pulsed current has been studied [67]. Coating process has been performed on nickel sulphate bath and nanostructure of obtained compound layer was examined with high precision figure analysis of SEM images. The effects of process variables, i.e. Y2O3 concentration, treatment time, current density and temperature of electrolyte have been experimentally studied. Statistical methods were used to achieve the minimum of corrosion rate and average size of nanoparticles. Finally the contribution percentage of different effective factors was revealed and confirmation run show the validity of obtained results. Also it has been revealed that by changing the size of nanoparticles, corrosion properties of coatings will change significantly in same trend. AFM and TEM analysis have confirmed smooth surface and average size of nanoparticles in the optimal coating [67]. Electrodeposition is a very important scientific based technology. Coating wide range of available base materials with electrodeposited layers of different metals/ alloys with different enhanced properties extends their use to industrial applications. Corrosion and wear are the effective factors in the failure of industrial components; hence, many attempts have been made to find different methods of reducing corrosion and wear costs. Recently, several techniques have been applied to produce protective electrodeposited coatings, such as different alloys electrodeposition, metal/oxide electrodeposition and nanocomposite electrodeposition. Different obtained results exhibited that nanocomposite electrodeposited coatings usually exhibit superior mechanical, electrochemical and oxidation properties as compared to pure metal coatings as well as microcomposite electrodeposited coatings. Improvement of these properties depended mainly on the size and the percentage of the particles electrodeposited and the distribution status of the nanoparticles. Due to high wear and corrosion rate, Ni-based nanocomposites with Al2O3 or Y2O3 nanoparticles or carbon nanotubes (CNT) have been investigated for a good protection of under friction parts. The Taguchi method for the design of experiment has been used for optimizing tertiary nanocomposite electrodeposited coating process parameters for the corrosion protection of treated samples. The contribution of Y2O3 concentration is more than the sum of the contributions of all the other three factors. It is evident that, among the selected factors, Y2O3 concentration has the major influence on the corrosion rate of performed coatings. It can be seen that the current density is second important factor that affects on corrosion rate of the treated substrates. Furthermore, it can be assumed that treatment time and temperature of electrolyte have almost the same effect on corrosion rates of coatings because of the minor difference in the contribution percentages among these two factors. By ranking their relative contributions, the sequence of the four factors affecting the corrosion

122


Fig. 4.5 Relation among average size of nanoparticles and corrosion rate of electrodeposited tertiary nanocomposite layer [67]

rate is Y2O3 concentration, current density, treatment time and temperature of electrolyte. In the case of average size of nanoparticles ranking of effective factors by their relative contributions is as same as for corrosion rate which show strong relation among these two measured properties of coatings. AFM and TEM images have confirmed smooth surface and average size of nanoparticles in the optimal coating. Figure 4.5 illustrates relation among average size of nanoparticles and corrosion rate.

4.8 Growth of 3-D Nano-Nucleuses In l constant over-potential, the current I1(t) of a separate semi-spherical nucleus, which is grown under combined conditions of charge transfer and limit current, is expressed as follows: " # 1 þ nt 1 I1 ðtÞ ¼ p ð1 þ 2ntÞ1=2 The amounts of n and p will be as follows:

4pðzFDc1 Þ2 azFg ð1 þ aÞzFg p¼ exp exp i0 RT RT

4.8 Growth of 3-D Nano-Nucleuses

n¼

VM i20 ðzFÞ2 Dc1

123

exp

2azFg ð1 2aÞzFg exp RT RT

where D is diffusion constant of metal ions and i0 is density of exchanged current at nucleus-solution interface. Growth of a separate nucleus is highly dependent on concentration and distribution of over-potential in the vicinity of nucleus. Over-potential and concentration in distance q from a nucleus can be expressed as follows: I1 ðtÞ cðqÞ ¼ c1 1 2pqFDc1 RT I1 ðtÞ gðqÞ ¼ g þ ln 1 zF 2pqFDc1 With substitution of these two equations inside above equation, the constant nucleation rate of J0(q) at the distance of q from nucleus will be as follows: nc þ1 I1 ðtÞ J0 ðqÞ ¼ J0 1 2pqzFDc1 Non-dimensional distribution of constant nucleation rate (critical nucleus) around a semi-spherical nucleus inside the area was observed which over-potential and concentration have been decreased. If various semi-spherical nucleuses are formed on electrode surface, the local areas around nucleuses, which nucleation rate has been decreased in there, are developed and are overlapped each other gradually. That is to say that a general theoretical model, indicating overall nucleation rate, should be considered with taking into account effects of interaction of nucleuses on each other. When growth of nucleuses is carried on under conditions of controlled diffusion, Total Current, provided that N(t) nucleuses are formed progressively on electrode surface at time interval (t, 0), will be equal to: 1=2

D 1 1=2 2 J 1 exp 0 pð8pcVM Þ Dt iN ðtÞ ¼ bzFc pt 2 If N(0) nucleuses are appeared instantaneously at time t = 0 on electrode surface, the following equation will be obtained: iN0 ðtÞ ¼ zFc

1=2 n h io D 1 exp N0 pð8pcVM Þ1=2 Dt pt

In the above mentioned equation, b is a numerical constant. Specific property of electrochemical nucleation is presence of a critical over-potential (lcrit) that below this amount, nucleation rate of Jo is practically zero and above this amount, it is

124


increased exponentially. Regarding this subject, dual-polarization pulse is known as an appropriate technique for electrochemical precipitation of metal nano-particles with a distribution of limited size and also compressed metal thin films. In initial short pulse of l1 lcrit, a great number of metal nucleuses is formed. Subsequently, only formed nucleuses are allowed to grow in second pulse of l2 lcrit.

4.9 Metal Ion Structure Existing ions in solution are primarily encircled by other species such as molecules, ions, especially water molecules. When these ions are encircled by water molecules, geometrical structure of water molecules plays a key role in subsequent processes. The 104.45 angle formed between two hydrogen–oxygen bonds, existing in water molecule, is as a result of strong bipolar forces inside it. Electrostatic gravity between metal cations with positive charge and water molecules results in hydration of existing ions or in other words, covering metal ions by water molecules. The hydration process metal ions are shown in which for comparison, display of an anion is also shown. Generally speaking, taking two major points into consideration is of paramount significance. Firstly, salt of most metals even in solid state is in hydrated form and therefore, in many cases, metal salts in solid state are written similarly with actual hydrated conditions. But, these two states differ from each other completely. Secondly, the number of water molecules in hydrated formula of a salt is not an accurate and clear-cut number in such a way that metal ions are generally encircled by numerous layers of water molecules. With regard to hydration of anions and cations, we should consider that cations, in comparison with anions, are hydrated more and are encircled by water molecules completely. The reason for this phenomenon is smaller ionic radius of cation in comparison with anion. The number of water molecules, which create the first hydrated layer around a metal ion, can be varied between 1 and 10 water molecules. Consequently, interaction of metal cations with water molecules, encircling a hydrated cation, forms an electrical field around the cation. As it is observed, regarding weaker electrostatic forces of external layers, balance of water molecules, encircling a metal ion, is connected weakly to ion in such a way that this cation loses external layers of water molecules and a metal cation encircled by a layer of water molecules is remained.

4.10 Double-Layer Structure Upon placing metal in aqueous solution, an electrical dual layer is created at interface of these two phases. Electrochemical double-layer structure severely affects kinetics of metal precipitation. Numerous models were presented for

4.10

Double-Layer Structure

125

showing electrochemical double-layer structure between 1950 and 1970, the most important classic models are explained in below [68–81].

4.10.1 Helmholtz-Perrin Model Based on Helmholtz-Perrin Model, double layer includes a row of charged species in metal-solution interface. According to this model, electrons are placed at cathode side while metal ions are placed at electrolyte side equally. The distance between these two layers (row) is at the level of ionic radius. At Helmholtz-Perrin model, double electrical layer, similar to a capacitor, includes two compressed parallel layers with constant capacity and potential gradient in Helmholtz layer (dH) is linear. du dlinear ¼ Const: dx Helmholtz-Perrin Model is a very simple structured model of reality. At this model, spatial distribution of metal ions against electrode surface, has not been considered as a result of dual effects of electrostatic gravity forces and accidental movements (as a result of thermal movements). This model is not appropriate for condensed electrolytes, but does not show actual spatial distribution of metal ions on cathode surface in diluted electrolytes.

4.10.2 Gouy-Chapman Model At this model, fluid flow, which is of paramount importance especially for diluted electrolytes, has been considered. Based on this model, concentration of metal ions with increase of distance from cathode surface is reduced exponentially and this density is reached zero level in whole solution. Therefore, a little number of ions is adsorbed by forces existing on cathode surface. Based on Gouy-Chapman Model, two potential distribution areas can be considered in cathode-electrolyte interface. At the first area, potential increases linearly with distance from cathode and after this area, penetration area is observed in which, potential is changed exponentially. Inner layer in Gouy-Chapman Model is known as Helmholtz layer. At this layer, potential gradient is approximately in linear form and in outer layer (penetration layer), potential gradient is changed exponentially and this layer is developed to whole solution. The way of potential changes at penetration area is expressed as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s du 2RT X zi Fu ddiff ¼ 1 ci exp dx e i¼1 RT

126


In the above equation, R is gas constant, Y is absolute temperature, e is dielectric constant, ci is concentration of metal ion in whole solution, zi is charge of metal ion, and F is Faraday constant. In diluted electrolytes, the thickness of penetration layer is measured approx. 10-4 cm while this thickness is measured approx. 10-7 to 10-8 cm in condensed electrolytes [82–87].

4.10.3 Stern-Graham Model Gouy-Chapman Model is not able to introduce actual specifications of double layer in the layer near to cathode and especially creates an adsorbent layer on cathode surface of water bipolar. In addition, various species existing in electrolyte, like superficial active anions or aqueous molecules existing in electrolyte, can also be adsorbed on cathode surface. This phenomenon is known as specific adsorption or contact adsorption. Hydration of anions, due to larger radius of anion compared to cation, is weaker than hydration of cations. Hence, anions can be adsorbed on cathode surface strongly and also can near cathode surface through losing hydrated layer of a hydrated cation. With regard to cations, due to keeping hydrate, cations are adsorbed by Coulomb forces on cathode surface. The amount of these Columbus forces is function of a cathodic potential. As it is observed, internal Helmholtz layer includes a screen of adsorbed anions. External Helmholtz layer is a plate which passes from charge centers of adsorbed metal ions and indicates external region of Helmholtz layer. After this layer, penetration layer is placed which is comprised of movable metal ions [88–94]. The concentration of ions with positive charge at this layer should be at such a level that total electrical charge would be zero. Potential gradient versus distance from metal-solution interface is as follows: Du ¼ ulinear þ uspec þ udiffuse

4.11 Determining Stages for Rate of Electrode Reactions There are numerous separate stages during transfer of metal ion from inside of whole solution to creation of metal lattice on cathode surface. Each of these stages can be a rate controller. The first stage at this route is transfer or movement of the desired ions from whole solution to the vicinity of cathode. Metallic ions, which exist either in hydrated or complex forms, first move towards cathode in solution. This stage is usually carried out by convection flows inside solution. Migration of ions under effect of potential gradient between anode and cathode can play a key role in ionic transfer, but comparatively play a partial and meager role in material transfer resulted from convection inside the solution. Transferred ions reach external part of penetration layer and pass from penetration layer as a result of existing concentration gradient. At this stage, solution convection does not play

4.11

Determining Stages for Rate of Electrode Reactions

127

a key role in transfer of ions. In fact, convectional flows do not affect exchange of ion in Nernst layer, but it can affect thickness of Nernst layer so that can reduce thickness of Nernst layer dN and increase concentration gradient. When these ions pass from diffusion layer and electrical dual layer (Helmholtz Layer), they reach to cathode surface and are turned into pure metal ions. Pure metal ions are turned into adatoms through combining with electrons on cathode surface, adatoms start moving on surface with the aim of being adsorbed in active areas as well as forming a very strong chemical bond. At this stage, metal atoms are operated and distributed appropriately, aimed at creating a crystal lattice. This stage includes nucleation and coating growth stages. Controlling stages of reactions rate should be reduced or omitted in order to increase rate of reactions on cathode surface. When rate controlling stage is under control of material transfer, increase of over-potential will accelerate reaction. The total over-potential is sum of concentration potential (gc), activation or charge transfer potential (gD), crystallization potential (gK) and resistance potential (gW). In metal electrical plating process, a great number of these over-potentials affect rate of reaction. gtot ¼ gc þ gD þ gK þ gW Electrolyte type, density of cathodic current and type of coating metal are considered as factors effective on determination of amount of over-potential. Overpotentials of charge transfer and concentration is of paramount significance, playing the highest constructive role in this regard. From among these two factors, metal ion transfer from double layers is a critical stage and over-potential of charge transfer can control rate [62, 95–107].

4.12 Concentration Over-Potential Concentration over-potential originates from this fact that in electrolysis process, metal ion concentration in whole solution cathode surface differs from each other. This concentration in cathode surface is less than whole solution in such a way that metal cations are consumed on cathodic surface with higher speed than providing them from whole solution and are precipitated as metal atom. Existence of such over-potential will cause precipitation of metal as powder on surface of cathode and will decrease quality of coating. Concentration over-potential includes reaction over-potential and penetration over-potential. Reaction over-potential is occurred when metal ions, in their route for reaching cathode, participate in chemical reactions and these reactions should be so slow to control speed. For example, hydration and dehydration or formation or decomposition of complex compounds is samples of these types of overpotential which can control speed before charge transfer reaction. Penetration overpotential is related to the penetration of metal ion before discharge of its electrical charge. In fact, existence of a concentration gradient will form a penetration layer.

128


This over-potential is reached to its maximum amount when concentration is reached zero on electrode (cathode) surface. Reduction of thickness of penetration layer through mixing of solution is one method for eliminating or decreasing this kind of over-potential.

4.13 Charge Transfer Over-Potential Charge transfer in cathode with metal ion hydrated in metal-solution interface is prerequisite for precipitation of metal with electrochemical method. The driving force of such process is linear potential difference between metal surface and solution-metal interface. In this process, hydrated layer at environs of metal ion will be eradicated or will be split enough in such a way that metal ion can pass from Helmholtz layer, aimed at performing charge transfer due to approaching the cathode surface. In cathode, electrons pass from Helmholtz layer and charge transfer process is made. Any delay in charge transfer process is known as activation over-potential or charge transfer over-potential. We face such overpotential type in all electrical plating reactions.

4.14 Crystallization Over-Potential Metal deposition reaction is carried out through electroplating method at two stages. The first stage includes formation of crystal nuclei in certain points of cathode which these nuclei are sustainable thermodynamically. The next stage is growth of established nuclei. These two stages are known as crystallization over-potential. We should consider that nucleation has usually more share at this type of over-potential than growth, because activation energy is for nucleation more than growth. It should be noted that conversion of metal atoms to crystalline lattice is carried out in specified growth points of cathode surface in such a way that after discharge of metal ions in specified points of cathode surface, these atoms should penetrate towards growth areas, aimed at setting up crystalline lattice. Crystallization over-potential would be important when nucleation and growth stages can control rate. For example, if penetration of metal atoms on cathode surface is slow, such circumstances will be generated. The difference in concentration of adsorbed atoms at growth points can cause emergence of difference in crystallization over-potential in these points.

4.15 Ohm or Resistance Over-Potential In fact, this type of over-potential is potential loss, beyond double layer and unlike other over-potentials, this type of potential follows Ohm Law in such a way that there exists a linear relation between this type of over-potential and current. For

4.15

Ohm or Resistance Over-Potential

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example, resistance of electrolyte at Nernst penetration layer is a result of existence of a film of reactants or reaction products on cathode surface. Such layer can be a solid layer or fluid with low electrical conductivity which covers all surfaces of cathode. Hence, accessing to metal ions on cathode surface is prevented [103, 108–123].

4.16 Pulse Electrochemical Deposition Method Pulse Electrochemical Deposition process has been paid more attention in recent years. Some researchers have reported reduction of porosity value of gold cover by pulse current. Precipitation of palladium with very minute and smooth granule and with less hydrogen has been reported by some researchers. Electrochemical depositions, free from chrome and rhodium crack have been produced by pulsed electrolysis. Pulsed electrochemical depositions of copper which resulted in more smaller and compressed granule have also been reported by some of researchers. Some researchers have obtained nickel electrochemical depositions from electrolytes free from additives by pulse currents. It should be noted that a wide spectrum of shapes of pulse fluctuation current waves can be produced by new electronic devices. Shapes of pulse current waves can be classified into two general categories. The first category is related to mono-polar pulse currents while the second category is related to bipolar pulse currents (anode and cathode). Variables of pulse electrochemical deposition include maximum current density (Jp), on-pulse time (Ton), off-pulse time (Toff). Parameter Jm in the figure indicates average current density. Regarding Jp, Ton and Toff parameters, all other amounts of pulse current parameters can be calculated as follows: Pulse frequency: f = 1/(Ton ? Toff) Work period: h ¼ Ton =ðTon þ Toff Þ Average current density: Jm ¼ Jp h The theory related to pulse plating (electroplating) is simple. The cathode layer is kept rich with metal ions and impurities are decreased as much as possible. During on-pulse time (Ton), when current is connected, metal ions are reduced on cathode. When current is disconnected (off-pulse time), any type of concentration gradient, appeared during on-pulse time (Ton), will be eradicated and there is possibility of separation of gas bubbles and impurities which have been adsorbed on cathode and this process is repeated once again. In pulsed electrochemical deposition method, related parameters can be changed in wide scale independently. In electrochemical deposition method with D.C. current, current density is the only changeable parameter. Consequently, creation of various situations faces more restriction. There are impurities which have been adsorbed on cathode and then this process is repeated once again. In electrochemical deposition method

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with pulse current, related parameters is changeable in a wide range independently. In electrochemical deposition method with direct current, current density is the sole changeable parameter and consequently, creation of various situations faces more restriction. In the beginning of on-pulse time of first pulse, nucleation of precipitation granules are happened as a result of penetration of metal ions through electrolyte. This nucleation process is carried on preferably on high-energy areas, which are mainly grain boundary and these nuclei grow with continuation of time. With the inception of second on-pulse time, penetration of metal ions and precipitation of them on grain boundary, formed from first pulse, is appeared. Hence, since current is turned on or turned off alternatively and new nucleation is appeared at fullenergy areas during on-pulse time of each cycle, hence, new granules grow continuously in line with new grain boundaries of electroplated metal. On the other hand, during off-pulse time of cycle, concentration of existing solved components in the vicinity of cathode surface is decreased as a result of cathodic reduction reactions of metal ions. Hence, each concentration gradient in the vicinity of cathode surface will be decreased during off-pulse time of cycle and even is omitted and consequently, metal concentration on surface returns to the normal concentration of solution’s volume [124–136].

4.17 Charging and De-Charging of Double Layer in Pulse Electroplating In an electrochemical process, a current, which is supplied by a power supply (It) is divided into two parts. One part is related to non-Faradic current or capacity current (Ic) which is spent for charge of dual layer while the other part is Faradic current (IF) which is used for reducing and precipitating of metal. The sum of these two currents is equal to total current. It ¼ IF þ Ic Dual layer charging requires spending time. The necessary time for reaching full potential from zero potential, according to imposed current (or time of reaching It = IF state) is known as double-layer charging time (tc). Decharge time (td) is necessary time in which electrode potential reach an amount according to zero current and/or a state in which equation IF = 0 holds. In electroplating with direct current, electrical double-layer charge and de-charge is carried out in the beginning and end of electroplating respectively. At this state, capacity current can be ignored compared to Faradic current. However, in pulsed electroplating, regarding existing time cut, electrical double-layer charge and de-charge is carried out alternatively at time intervals (based on Ton and Toff). Therefore, it seems necessary that rate and share of capacity current should be specified with regard to total current in each pulse. In pulsed electroplating, the ideal state is brought about at

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Ton tc condition. At this state, electrical double-layer has enough opportunity to be charged and de-charged and current is imposed fully. In practice, ideal state is not accessible and charging time occupies a portion of pulse length and meager change is observed in pulse shape. If charging time is larger than on-pulse time, (Ton \ tc), potential will never reach the desired amount and always the equation IF \ It will be hold during electroplating. The similar phenomenon also is occurred at the end of each pulse. If de-charging time is larger than off-pulse time (Toff \ Td), then double electrical layers will not be de-charged fully and the equation IF = 0 will not be hold. Under such circumstance, giving the name of pulsed electroplating to the system is not accurate, because current only shows fluctuation in a way that work conditions is much more similar to electroplating with direct current. Hence, in pulsed electroplating, observing two conditions for assuring connection of pulse seems necessary. 1. On-pulse time should be larger than charging time (Ton [ tc). 2. Off-pulse time should be larger than de-charging time (Toff [ td). In case of given charging and de-charging time, minimum allowable amount is determined for Ton and Toff (largest frequency) in which pulse state is completely observed. For calculation of charging and de-charging time in pulse electroplating, moreover high calculations, getting access to enough information, like capacity of double layer, over-potential of cathode reaction, electrolyte constituents, electrode geometrical shape, electroplating current density, and electrode crystalline structure seems necessary. The principles of calculation of charging and de-charging time of electrical double layer are based on Butler-Volmer equation.

anFg ð1 aÞnFg JF ¼ J0 exp exp RT RT where JF is density of Faradic current (A/cm2), J0 is exchange current density (A cm2), a is charge transfer constant, n is the number of electrons, F is Faraday constant (96,500 C), g is over-potential, R is gas constant (J/mol K) and T is absolute temperature (K). Calculation of charging or de-charging time based on numerical methods is time-consuming and complicated. Puippe has proposed the following equation as a fast and acceptable method for calculation of charging or de-charging time based on maximum current density. tc ¼

17 ; Jp

td ¼

120 Jp

In the above equation, tc and td are in terms of microsecond and Jp is in A/cm2. Studies made in this regard on many systems showed that charging time is within the range of tens to hundredth of microseconds while de-charging time is within the range of multi-thousand microseconds. We should bear in mind that consumption of a part of total current for charging double layer (capacity current) in the beginning of each pulse does not affect efficiency of total current, because this

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amount of current is compensated in double-layer de-charging mode and at the off-pulse time (Toff) [78, 137–139].

4.18 Nanostructured Coating Properties In terms of production methods, it can be stated that electrochemical coating method is regarded as one of the oldest methods for imposing nanostructured coatings. Of course, creation of nanostructure coating has been made without any knowledge in many cases. Before 1980s, not any subject has been mentioned in the field of creation of nanostructure coating by this method. Various research activities have been made in the field of creation of nanostructure coating as size as 10 nm by electrochemical deposition method. In the field of coating of pure metals, it can be referred to the coating of nickel, cobalt, palladium and copper. With regard to duplex alloys, it can be referred to creation of nickel–phosphor alloy coating, nickel–iron coating, zinc–nickel coating, palladium–iron coating and cobalt–tungsten coating [140–144]. Triple alloys of nickel–iron–chrome have been coated successfully with this method. Even, in various resources, it has been mentioned that multilayer, composite and ceramic coatings have been coated with granules smaller than 100 nm. With various studies made in this regard on nanostructure coating, it is observed that amount of hole and/or vacancy is very little in these materials. In some research activities made in this regard, it has been observed that converting into nanostructure mode is made when the ratio of triple junctions to grain boundary is moving towards infinity. Properties of nanostructure costing can be classified into two major categories. The first category is the properties which are dependent on size of crystal or cluster (grain size). These properties include strength, softness, stiffness, resistance against erosion, friction coefficient, electrical resistance, dissolving in solid state, hydrogen dissolving and penetration of it inside material and also resistant against local corrosions and stress corrosion. Unlike these properties, there are other material properties which do not depend on grain size such as volume density, thermal expansion, Young Modulus and resistance against environment of salt spray test. In continuation, some of these properties will be studied [145–157].

4.18.1 Mechanical Properties As a general principle, it should be said that plastic deformation of a nanostructure coating strictly depends on its grain size. Various tests in environment temperature have shown that with decreasing grain size beyond certain limit, Hall-Petch equation doesn’t apply and contrary to expectation, material will become softer. It has been observed that this affair occurs when amount of triple junctions shows

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remarkable growth. This subject accords with ‘‘Triple Line Softening Effect’’ theory. Some other mechanical properties of these coatings can be justified with studying behavior of this material and considering the triple junctions. Reduction of granule size has remarkable effect on improvement of resistance against erosion in nanostructure and composite materials. Of course, most research activities have been made on double-phase and/or composite nanostructures and not appropriate research activities have been made with relation to pure nano-crystalline metals. Research activities made on frictional behavior of multilayer nano-coats show that their behaviors have been more affected with chemical composition of materials and their nano-microstructure has not any effect on their frictional behavior. Erosion behavior, under high stresses, is severely affected with chemical composition of materials and their nano-microstructure has not any remarkable affect on their frictional behavior. Study of erosion behavior under low and average loading condition in nano-materials shows that because of higher stiffness, erosion resistance of these materials is more than that of materials with large size. With regard to creep in nanostructured materials, since grain boundary’s slip is regarded as one of the mechanisms of creep, creep rate in nanostructured materials is more than microcrystal materials [144, 145, 147, 148, 153–155, 158–163].

4.18.2 Catalyst Properties Superficial topography of solids plays a key role in distribution of energy in surface of materials, especially when irregularities are observed on atomic level. In fact, superficial activities are increased with the decrease of grain size. Nanostructure catalysis, similar to very minute particles, enjoy high surface/volume ratio and create unique catalytic properties.

4.18.3 Electrical Properties In nanostructured materials, due to presence of high fraction of grain boundaries, electrical resistance is very high which this property is considered as an advantage for manufacturing of soft magnetic materials.

4.18.4 Anti-Corrosive Properties In spite of this fact that corrosive properties of nanostructure coating is of paramount significance, very few research activities have been made in this regard. Nano-crystalline materials provide new approach for improvement of properties, without any change in chemical composition. Small size of grain and high

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volumetric fraction of grain boundary can cause different corrosion behavior with respect to multi-crystalline materials. Effect of reduction of grain size on increase of resistance against local corrosion of stainless steel has been studied. As it is observed, potential of passive film fracture is equal to (0.3 VSCE) for stainless steel with normal grain size of 30 micron and (1.15 VSCE) for stainless steel with the same chemical composition and surface grain size of 25 nm. In fact, fracture potential of passive film for nanostructure has turned more positive approx. 850 mV. It can be concluded that nanostructure has caused increase of resistance against local corrosion. This subject has been studied for aluminum metal and accords with the above mentioned results. Its main reason has been explained as uniform distribution of imperfections in passive film. Presence of a great number of these imperfections results in more distribution of chloride ion on metal surface [164–177]. Density of chloride ion in each imperfection, extant in nanostructure, due to its distribution in imperfections of grain boundary in comparison with coarse structure, is reduced tremendously which will cause that local condensation of chloride ion in imperfections extant in grain boundary require a stronger driving force and consequently, more anodic potential is required for growth of a stable hole. Potentiostatic and potentiodynamic tests, made on special nanostructure nickel and multi-crystalline nickel in bi-normal sulphuric acid, shows that sample with nano structure shows the same active, passive, transpassive behavior of normal metal. But, difference of these two materials is disclosed in open-circuit potential and also in passive currents. Nanostructure sample in passive part shows higher current density which can be related to higher corrosion rate than normal metal. This high corrosion rate can be related to the existence of high grain boundary and triple junctions. These regions are corrosive-prone due to having high energy. This difference in current density is observed less in high potentials, because in this situation, effect of structure is not longer important due to high corrosion rate. The other difference in diagrams is observed on open-circuit potential. Increase in degree of open-circuit potential is related to the catalytic role of nanometer structure in hydrogen-reduction reaction. In research activities it is observed that multi-crystalline nickel has been corroded locally but nanostructure nickel has been corroded uniformly. The main reason is related to eradication of passive layer formed on nanostructure metal surface due to existence of high imperfections inside it. Hence, passive layer formed on nanostructure nickel, has been eradicated fully but passive layer, existing on multi-crystalline nickel is eradicated locally and on grain boundary. Therefore, corrosion will be continued locally. Due to distribution of chloride ion, tendency to creation of local corrosion is very low. However, reduction of sensitivity with regard to local corrosion, for preventing from sudden destruction, can be beneficial. The salt spray tests, carried out on nickel, show that creation of nanostructure has not significant effect on corrosion resistance properties and multi-crystalline and nanostructure samples have protected steel equally in test conditions.

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Also, in another research, it has been observed that sensitivity to corrosion of grain boundary and stress corrosion of grain boundary has been lessened for coating of nanostructure nickel by electrochemical method. It has been observed that material is resistant against pitting corrosion and also its sensitivity has been reduced against crevice corrosion. Other tests have been shown that nanostructure nickel is resistant against alkaline environments and reduction acids. These reports indicate that more amounts of grain boundary and triple junction in nano-crystalline samples, will increase the amounts of atoms which participate in reaction and consequently, they create places for electrochemical activity [168, 171, 178–182]. To prepare high dielectric thin film of polymer-based materials, nanometer sized barium titanate (BaTiO3) particles, which should have high dielectric coefficients and low energy dissipation factors due to nano-size effects, were dispersed in polyvinylidene fluoride (PVDF) or siloxane-modified polyamideimide (SPAI) [183]. The BaTiO3 particles with crystal sizes of 10.5–34.6 nm were synthesized with a complex alkoxide method. Polymer/N-methyl-2-pyrrodinone solution suspending the BaTiO3 particles was spin-coated on ITO glass substrates to prepare polymer-based composite films with thickness of submicron meters. The BaTiO3 particles were dispersed more homogeneously in the PVDF film than in the SPAI film. The good dispersion of the particles in the PVDF film brought about a smooth surface of the film that had a root mean square roughness less than 20 nm at a particle volume fraction of 30%. The roughness was less than one-tenth of the roughness of the SPAI composite film. An increase in the BaTiO3 crystal size from 10.5 to 34.6 nm in the PVDF film at a particle volume fraction of 30% increased the dielectric constant of the film from 20.1 to 31.8. The BaTiO3–PVDF composite film attained high dielectric constant that had more than twice the dielectric constant of the BaTiO3–SPAI composite film. The dissipation factor of the PVDF composite film was as low as 0.05 at 104 Hz. Ferroelectric lead zirconate titanate and barium titanate (BaTiO3) are candidates for capacitor materials because of their high dielectric constants. The titanates, however, require high temperature processing, which is not compatible for embedding the capacitors in the printed circuit board of a resin substrate. To realize low temperature processing, a number of attempts have been devoted, one of which is the application of a nano-crystalline seeding technique. Previous work with this technique reported that high dielectric nano-composite films of titanates could be fabricated at a temperature as low as 350C. Fabrication of ceramic– polymer composites is another attempt, and is receiving attention since this technique combines the low temperature processing of polymers and the high dielectric constant of ceramics. Traditional approach for preparation of such composite films is to mix a dielectric polymer solution and submicron- or micronsized ferroelectric particles, and evaporate the solvent of the polymer solution. Since a composite film at high particle contents has a roughness of at least the ceramic particle size, the film thickness has to be much larger than the particle size to achieve uniform film thickness. Consequently, the thickness of composite films containing such large particles exceeded 1 lm, and it was difficult to achieve high capacitance densities. In addition, fabrication of the films in integrated circuits

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requires a surface roughness much less than a 100 nm. Therefore, to meet the demands of film thickness and smoothness, it is strongly required to prepare composite films that disperse ceramic nanoparticles. Ceramics such as BaTiO3 with particle sizes larger than several 10 nm have ferroelectric properties. However, when their sizes approach to nanometers, they tend to have para-electric properties that result in low dissipation factor. These properties are suitable for dielectric materials. Composite films of BaTiO3 nanoparticle/polymer were spin-coated on ITO substrates with NMP solvent [183]. The BaTiO3 particles that were synthesized via hydrolysis reaction of complex alkoxide were homogeneously dispersed in PVDF, whereas the particles aggregated in SPAI. A root mean square roughness of the BaTiO3–PVDF composite film attained less than 20 nm at a particle volume fraction of 30%. The BaTiO3–PVDF films had high dielectric constants compared to the BaTiO3–SPAI film, which might be related to difference in the dispersibility of the BaTiO3 particles in the

Fig. 4.6 TEM images of BaTiO3 particles prepared at a 0.12 kmol/m3 metal, 10 kmol/m3 H2O and 70C, b 0.06 kmol/m3 metal, 20 kmol/m3 H2O and 70C, c 0.12 kmol/m3 metal, 20 kmol/m3 H2O and 70C, and d 0.06 kmol/m3 metal, 20 kmol/m3 H2O and 50C. Pure ethanol solvent was used for the samples a, b and d, and 50% (v/v) benzene/ethanol solvent was used for the sample c, reprinted with kind permission from Konno [183]

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137

Fig. 4.7 Dielectric constant (open circle) and dissipation factor (filled circle) of BaTiO3–PVDF composite films measured at 104 Hz as a function of BaTiO3 crystal size. BaTiO3 volume fraction: 30%, spin speed: 3,000 rpm and drying temperature: 150C, reprinted with kind permission from Konno [183]

Fig. 4.8 Thickness of BaTiO3–PVDF (filled circle) and BaTiO3–SPAI composite films (open circle) as a function of volume fraction of particles. The BaTiO3 crystal size: 27.3 nm, spin speed: 3,000 rpm and drying temperature: 150C, reprinted with kind permission from Konno [183]

film. The dielectric constant of the composite films increased with an increase in the BaTiO3 crystal size and with BaTiO3 volume fraction in the composite film. The dielectric constant reached 31.8 for the 30 vol%–BaTiO3–PVDF film with a BaTiO3 crystal size of 27.3 nm, which was four times larger than the dielectric constant of the PVDF film without BaTiO3 and corresponded to a capacitance density as large as 0.63 nF/mm2. Figure 4.6 illustrates TEM images of BaTiO3 particles. Different solvents were used for the samples. Size effect can be seen in Fig. 4.7 that shows dielectric constant and dissipation factor of BaTiO3–PVDF composite films measured at 104 Hz as a function of BaTiO3 crystal size. Thickness of BaTiO3–PVDF and BaTiO3–SPAI composite films as a function of volume fraction of particles can also be seen in Fig. 4.8.

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Chapter 5

Size Effect in Mechanical Properties of Nanostructured Coatings

5.1 Introduction Research studies have shown that when particles’ size reaches to the dimensions of nanometer, remarkable improvement will be observed in strength of composite. For example, remarkable increase was observed in hardness of nickelalumina composite when size of improving particles was decreased from 10 lm to 10 nm. Shape, size and surface of nanoparticles play important role in properties of nanocomposite. In recent years, nanocomposites have been used widely due to their better magnetic, mechanical, optical and physical properties. Interface volume, layer thickness, superficial energy and interface are the parameters which have noticeable effects on nanostructure thin films [1–12]. Figure 5.1 shows schematics of particles used in nanocomposites and degree of proportion of surface area to their volume. As an example of particulate nanomaterials, Ohno et al. [13] studied the size effect of TiO2-SiO2 nano-hybrid particles. The well-dispersed primary TiO2-SiO2 nano-hybrid particles were successfully prepared by using the super critical drying of the moleculardesigned nano-hybrid precursor. The particle diameter of the resultant hybrid particles was about 140 nm. The crystal size of titania on the surface of the silica core particle was determined to be 7 nm from the result of TEM and XRD analysis. The crystal structure was anatase. The band gap energy was measured form the ultraviolet–visible spectrum. As a result, the band gap energy of the nano-hybrid particles were 0.13 eV blue shifted compared with that of the anatase crystal without the quantum size effect. Therefore, we concluded that nano-hybrid particles has the possibility to control the quantum size effect, if we can successfully develop the well handling method for nano-materials. Figure 5.2 illustrates the surface morphology of the obtained TiO2-SiO2 nano-hybrid particles with super critical drying process [13].


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Fig. 5.1 Various types of nanoparticles and degree of surface area to their volume

5.2 Nanocomposite Coating Production Method Extant physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes for the provision of nanocomposite coatings can be applied for the production of nanocomposite coatings through improvement of process parameters or through the application of initial powder with nanostructure. Application of various nanoparticles provided from steam, liquid and/or solid methods have accelerated development of nanocomposite coatings with resistant against friction and oxidation. Under various PVD accessible processes, deposition through ion ray is effective especially for making metallic nitride nano-crystallization coatings with higher stick and controlled microstructure. Process with less dependency parameters than PVD is salient advantage of deposition through ion ray. Energy and Flux can improve ion bombardment, size and direction of crystallography of grain as well. Ever increasing requirement for advanced materials with the aim of tolerating practical conditions has caused that many research works carried out in the field of very hard coatings. Recently, special research activities have been carried out on designing very hard coatings with superior strength, high stiffness and toughness. Formation of multilayer or super elasticity structures with various elasticity modules between layers is one of designing principles. Thickness of each layer should be in nano dimension, aimed at preventing from operation of unwanted supply source between layers. Production of films with pleasant stiffness amounts will be possible through layer to layer sit. Formation of nanocomposite of a nanocomposite layer with microstructures, including crystalline network, is the other method of design with grains in nano dimensions envisioned in an amorphous background. Plating method is the other methods of production of nanocomposite coatings. Since this method is economical, it enjoys

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Fig. 5.2 The surface morphology of the obtained TiO2-SiO2 nano-hybrid particle with super critical drying process: a agglomerate state, b primary TiO2-SiO2 nano-hybrid particle, c titania crystals on the surface of the silica core particle and d the first Fourier transform (FFT) image obtained from the titania crystals on the silica particle, reprinted with kind permission from Ohno [13]

many capabilities for production of nanocomposite coatings in industrial scale. This method is carried out based on accepted composite plating principles in a way that process parameters in this method are not much more complicate; rather, their controls are made easily [14–26].

5.3 Provision of Nanocomposite Coatings with Plating Method During recent years, successful coexistence of very minute particles, like metallic powders, silicon carbide, oxides, diamonds and polymers, has been reported with metallic or alloy field and their accordance structures and

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Table 5.1 Various types of nanocomposite coatings obtained through electro-deposition method Methods of Type of nanostructured materials electrodeposition Nanoparticles in a NanoNanotubes/ Nanocrystalline metallic matrix multilayer nanowires materials Direct current (DC) Ni/Alumina Pulsed direct current Ni-W/CNT (PDC) Pulsed reverse Multilayered current (PRC) composites Potentiostatic (P) Pulsed potetiostatic (PP)

Ni–Cr

Co nanowires

Ni-W alloy

properties have been studied by various researchers. Not only structure and properties of nanocomposite coatings depends on density, size, distribution and nature of improved particles nature, but also it depends on type of used solution, current density plating parameters, temperature, and degree of pH, etc. Nanocomposite plating includes revival of metallic ions from suspension electrolyte and insoluble powders like oxides (SiO2, TiO2, Al2O3), carbides (SiC), nitrides (Si3N4), polymers (PTFE, Polytetrafluoroethylene). This activity will result in entering very minute particle to the growing metallic or alloy substrate. High superficial energy and inclination of nanoparticle to agglomeration in high conductor metallic electrolyte will bar congruousness of distribution of particles. For this reason, many research activities have been done in the field of nanocomposite coatings entitled ‘‘Congruous Distribution of Very Minute Particles in Metallic Substrate and Avoiding their Agglomeration in Electrolyte’’, aimed at boosting volumetric percentage of nanoparticle in coating. Table 5.1 shows schematic of various types of nanocomposite coatings which have been prepared through plating.

5.4 Plating of Nickel-Alumina Nanocomposite Coating Due to the application of nickel as protective coating, nickel nanocomposite coatings, containing ceramic nanoparticle with high hardness and resistance to erosion, have been taken into consideration seriously. Hardness and resistance of deposited coatings of nickel nanocomposite strictly depends on degree of ceramic particles extant in nickel background. Al2O3, WC, MoS2, TiO2, Cr2O3, ZrO2 and diamond are of ceramic powders which have been used in manufacturing of nanocomposites with nickel background. Up to the present time, more research activities have been made on nanocomposite coatings of Ni/Al2O3 and Ni/SiC [27–37].

5.4 Plating of Nickel-Alumina Nanocomposite Coating

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5.5 Effects of Participation of Alumina Nanoparticle in Nickel Coating Participation of alumina nanoparticle will improve hardness and resistance of coating against friction remarkably. Final tension will boost traction and tension of submit in comparison with nickel, depending on degree of participation of particles in coating, partly twofold or more than two fold. Resistance against friction is boosted upon increase of density of alumina in coating. At any rate, ductility of nickel-alumina nanocomposite coatings is less than sole nickel. Annealing of nanocomposite in high temperature will increase ductility but will reduce their strength. Resistant against corrosion of nickel is improved with alumina nanoparticle. Increase of hardness is made based on preventing grains from growing and according to Hall-Petch Law. Alumina hard nanoparticle will generally improve trio biological properties and hardness of nickel composite layers by dispersant hardness mechanism. Alumina nanoparticle can prevent nickel grain boundary from movement and also can prevent grains from growing while conducting heat treatment operations. When material is exposed foreign (external) tension, alumina particles can prevent from unwanted movement in metal substrate. Consequently, plastic shape change will be more complicated. Hence, hardness of nanocomposite layers is increased while resistance against friction is improved. Also, resistance against friction is improved due to the reduction of grains size of metal substrate, containing alumina nanoparticle at grain boundary.

5.6 Plating of Nickel-Alumina Nanocomposite Coating In systems with simple counting ions, zeta potential is regarded as a criterion for gradient of electrical potential, when surface potential is fixed. The pH, which its zeta potential is equal to zero, is called Iso Electric Point. To enrich loading of hydrated surface by OH– and H3O+, increase or decrease of pH from Iso Electric Point will first boost absolute fraction of zeta potential. Iso electric point for alumina is pH = 9 i.e. alumina particles will have negative superficial load in the electrolyte with pH more than 9. In the same direction, alumina particles will have positive superficial load at the pH with less than 9. Consequently, alumina nanoparticle can be seeped simultaneously with nickel for formation of composite layers without needing to specific additives. Because, pH of all composite plating solutions of nickel is smaller than IEP for alumina and alumina particles at these baths have positive superficial load. But, alumina nanoparticles are agglomerated easily in electrochemical electrolyte due to their high superficial energy and this activity will cause weak mechanical properties in nanocomposite coatings, for, it prevents particles from being distributed equally. After carrying out operations, physical distribution of nanoparticles at electrolyte solution by mixing and

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ultrasonic operation and/or through distribution of chemical dispersants in electrolyte is a mandatory activity. The more volumetric percentage of alumina nanoparticles can be boosted in Ni/Al2O3 nanocomposite coating the more provided hardness of nanocomposite coat can be expected. Hence, this activity requires getting familiarity with effective parameters in simultaneous electrical deposition process of alumina and nickel [38–41]. For this reason, electroplating variables and their effects in amount of participating alumina nanoparticles in coat will be studied in next part.

5.7 Size Effect in Mechanical Properties of Two Dimensional Nano-Films He et al. [42] analyzed ultra-thin elastic films of nano-scale thickness with an arbitrary geometry and edge boundary conditions. An analytical model is proposed to study the size-dependent mechanical response of the film based on continuum surface elasticity. By using the transfer-matrix method along with an asymptotic expansion technique of small parameter, closed-form solutions for the mechanical field in the film is presented in terms of the displacements on the mid-plane. The asymptotic expansion terminates after a few terms and exact solutions are obtained. The mid-plane displacements are governed by three two-dimensional equations, and the associated edge boundary conditions can be prescribed on average. Solving the two-dimensional boundary value problem yields the threedimensional response of the film. The solution is exact throughout the interior of the film with the exception of a thin boundary layer having an order of thickness as the film in accordance with the Saint–Venant’s principle. The surface of a solid is a region with small thickness which has its own atom arrangement and property differing from the bulk. For a solid with a large size, the surface effects can be ignored because the volume ratio of the surface region to the bulk is very small. However, for small solids with large surface-to-bulk ratio the significance of surfaces is likely to be important. This is extremely true for nano-scale materials or structures. Recently, mechanical experiments of nanoscale bars and plates indicate that the effective elastic properties of these minute structural elements strongly depend on their size. The understanding and modeling of such a size-dependent phenomenon has become an active subject of much research. Classical elasticity lacks an intrinsic length scale, and thus cannot be used to model the size effect. Atomistic simulation, though very powerful in pursuing the details at microscopic level, seems too complex for practical applications as it needs tremendous computation. An efficient approach has been developed by upon the continuum concept of surface stress. They examined unidirectional tension and pure bending of nano-scale bars and plates, and found more remarkable size effect in bending than in tension. The results are in excellent agreement with their atomistic simulation by embedded atom method for facecentered cubic aluminum and the Stillinger–Weber model for silicon.

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From the continuum point of view, a surface is regarded as a negligibly thin object adhering to the underlying material without slipping, and the material constants for both are different. A generic and mathematical exposition on surface elasticity has been presented by some researchers. In their work, surface stress depends on deformation. The equilibrium and constitutive equations of the bulk solid are the same as those in the classical elasticity, but the boundary conditions must ensure the force balance of the surface object. This model has been applied by several authors. He et al. [42] concluded that a continuum model based on surface elasticity is proposed to analyze the size-dependent mechanical response of ultra-thin elastic films of nano-scale thickness. Being expressed in terms of displacements of the mid-plane, the governing equations are two-dimensional and the associated boundary conditions are specified at the edge of the film in an average manner as in the classical plate theory. Once the two-dimensional equations are solved, the three-dimensional mechanical field that is exact in Saint–Venant’s sense is generated directly. The asymptotic analysis developed for solving the twodimensional equations can be regarded to yield exact solutions because the expansion terminates after a few terms. The solution procedure is illustrated by analyzing a clamped circular film under a concentrated force. The result is consistent with the other existing studies and it approaches the classical plate solution without surface stress effects. It is concluded that the size-dependence is due to the dependence of surface stress on strain. Ignoring this strain-dependence of surface stress will lead to the disappearance of size effect. The presence of surface Lamé constants and residual surface tension under unconstrained conditions increases and decreases the film stiffness, respectively. Figure 5.3 shows

Fig. 5.3 Through-thethickness distribution of the dimensionless transverse shear stress at r = R/2, reprinted with kind permission from Lim [42]

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through-the-thickness distribution of the dimensionless transverse shear stress at r = R/2. The effect of the material micro-structural interfaces increases as the surfaceto-volume ratio increases. Abu Al-Rub [43] showed that interfacial effects have a profound impact on the scale-dependent yield strength and strain hardening of micro/nano-systems even under uniform stressing. This is achieved by adopting a higher-order gradient-dependent plasticity theory that enforces microscopic boundary conditions at interfaces and free surfaces. Those nonstandard boundary conditions relate a microtraction stress to the interfacial energy at the interface. In addition to the nonlocal yield condition for the material’s bulk, a microscopic yield condition for the interface is presented, which determines the stress at which the interface begins to deform plastically and harden. Hence, two material length scales are incorporated: one for the bulk and the other for the interface. Different expressions for the interfacial energy are investigated. The effect of the interfacial yield strength and interfacial hardening are studied by analytically solving a onedimensional Hall–Petch-type size effect problem. It is found that when assuming compliant interfaces the interface properties control both the material’s global yield strength and rates of strain hardening such that the interfacial strength controls the global yield strength whereas the interfacial hardening controls both the global yield strength and strain hardening rates. On the other hand, when assuming a stiff interface, the bulk length scale controls both the global yield strength and strain hardening rates. Moreover, it is found that in order to correctly predict the increase in the yield strength with decreasing size, the interfacial length scale should scale the magnitude of both the interfacial yield strength and interfacial hardening. The emerging areas of micro and nanotechnologies exhibit important strength differences that result from continuous modification of the material micro-structural characteristics with changing size, whereby the smaller is the size the stronger is the response. For example, experimental works have shown increase in strength by decreasing: (a) the particle size of particle-reinforced composites while keeping the volume fraction constant; (b) the diameter of micro-wires under torsion; (c) the thickness of thin films under bending or uniaxial tension; (d) the indentation depth in micro/nano-indentation tests; (e) the grain size of nanocrystalline materials (the well-known Hall–Petch effect); the void size in nanoporous media; and several others. Therefore, accurate identification of the mechanical properties of micro/nano-systems (e.g. micro/nano thin films, micro/ nano wires, micro/nano-composites) is essential for the design, performance, and development of, for example, micro/nano electronics and micro/nanoelectromechanical systems (MEMS/NEMS) to be used, for example, as actuators or sensors (e.g. pressure, inertial, thermal, and chemical sensors, position detectors, accelerometers, magnetometers, micromirrors, etc.). The mechanical properties of small-scale structures are different from those of the conventional or bulk counterparts because they are very sensitive to the micro-structural features of the material such as the grain size, the finite number of grains, the boundary layer thickness, texture, and dislocation structure. Therefore, when one or more of the


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Fig. 5.4 Illustration of strengthening in micro/nanosystems. a Stress–strain diagrams for various sizes, and b increase in the yield strength and/or the rate of strain hardening as size decreases, reprinted with kind permission from Abu Al-Rub [43]

dimensions of these systems begin to approach that of their microstructural features, the material mechanical properties (e.g. yield strength, strain hardening, fracture toughness) begin to exhibit a dependence on the structure size as schematically shown in Fig. 5.4. In metallic systems this translates to plastic yielding occurring at increased stresses over their bulk counterparts. The small sizes involved limit the conventional operation of dislocations and the application of classical continuum mechanics concepts; thus, a fundamental question arises: since the initial yield stress (i.e. onset of plasticity) in micro/nano-systems is sizedependent, a question that needs to be addressed, what yield strength should be used in the design of these systems? Size effects in micro/nano-systems could not be explained by the classical continuum mechanics since no length scale enters the constitutive description. A multiscale continuum theory, therefore, is needed to bridge the gap between the classical continuum theories and micromechanical theories. Since the increase in strength with decreasing scale can be related to proportional increase in the strain gradients, which accommodate the evolution of geometrically necessary dislocations (GNDs), the gradient plasticity theory has been successful in addressing the size effect problem. This success stems out from the incorporation of a microstructural length scale parameter through functional dependencies on the plastic strain gradient of nonlocal media. Furthermore, for mathematical consistency, in the gradient-dependent framework, additional boundary conditions have to be specified at interfaces and free surfaces allowing one to include interfacial effects. However, recently many researchers who are engaged in nano/micro characterization have questioned the ability of the gradient plasticity theory in predicting the Hall–Petch-like size effect; i.e. the increase in the yield strength with decreasing the grain size under macroscopically homogeneous stressing or straining (i.e. under uniaxial tension or compression). This is attributed to lack of the physical understanding of the nature of the non-classical boundary conditions that the gradient plasticity theory enforces at the material free surfaces and interfaces. Free surfaces and interfaces of a material confined in a small volume can strongly affect the mechanical properties of the material. Free surfaces in submicron and nano-systems can be sources for development of defects and its propagation towards the interior. Hard, soft, or intermediate interfaces between

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distinct phase regions can also be locations for dislocations’ blocking and pile-ups that give rise to strain gradients to accommodate the GNDs. The increase in the initial yield stress with decreasing thickness observed in tensile tests of various thin films in the size range of 100–500 nm may be taken as a hint in this direction. The free surfaces of the thin film and the interface between the film and substrate, therefore, can have a significant effect on the strength of the thin film. Lower-order strain gradient plasticity theories which neglect the application of the corresponding higher-order boundary conditions at interfaces and free surfaces indeed fail to predict boundary layer effects. Therefore, the focus of this paper is laid on the effect of dimensional constraints on the yield strength and plastic flow and to show that higher-order gradient plasticity theories (as opposed to lower-order theories) can be used successfully to interpret size effects under macroscopically homogeneous stressing or straining conditions. Dislocation pile-ups, which result in local plastic strain gradients, could be encountered at free surfaces and interface depending on the level of surface/ interfacial energy which increases as the surface-to-volume ratio increases. In other words, it is expected that as the characteristic size decreases, the higher is the surface/interfacial energy and the more significant is the effect of the boundary layer thickness on the strength of the system. Therefore, size effect can be explained by constrained plastic slipping due to grain boundaries and interfaces which result in a nonuniform straining, thereby setting up strong gradients of strain. Plastic deformation in small-scale structures, accommodated by dislocation nucleation and movement, is therefore strongly affected by interfaces. Until now, little attention is devoted to interfacial strengthening effects (e.g. filmsubstrate interface, phase or grain boundaries, inclusion’s interface, void free surface, nano-wires free surfaces, etc.) on the scale-dependent plasticity in smallscale systems. Interface and boundary conditions for higher-order variables are generally modeled as infinitely stiff or completely free; and the references quoted therein). These conditions are very difficult to be satisfied in reality, particularly, for systems with large surface-to-volume ratios. However, recently there have been few attempts to model intermediate (i.e. not free and not stiff) boundary conditions for higher-order variables within the higher-order strain gradient plasticity framework. Abu Al-Rub [43] studied the effect of interface properties (yield strength and hardening) on the scale-dependent behavior of small-scale systems within the framework of higher-order gradient plasticity theory. It is shown that the additional microscopic boundary conditions, which are supplemented by the gradient approach, allows one to predict size effects under uniform stressing. This is achieved by relating the microtraction stress at interfaces to an interfacial energy that depends on the plastic strain at the interface. Furthermore, by examining linear and nonlinear expressions for this interfacial energy, it is shown that an interfacial yield condition, besides the nonlocal yield condition for the bulk, can be formulated. This condition governs the emission/transmission of dislocations across the interface and is expressed in terms of the microtraction stress, the interfacial yield strength, the interfacial hardening, and the interfacial length scale. Therefore, two


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internal length scales are incorporated in the present formalism, one for the bulk, ‘, and the other for the interface, ‘I. It is shown that the higher-order gradient plasticity theory when supplemented by interfacial energy effects, at least for the one-dimensional example presented here, can qualitatively describe many features of the size effect due to GNDs, including the strengthening, the development of boundary layers, and the strain hardening. The qualitative modeling of the strengthening is explained by the interfacial yield strength, whereas the strain hardening is described by accounting for the interfacial hardening effect. Four different forms for the interfacial energy (or equivalently the interfacial yield condition) in terms of the plastic strain at the interface are examined: (a) a linear one which allows the interface to yield in a perfectly plastic manner without hardening; (b) a quadratic form which allows the interface to harden but yields at the same time as the bulk; (c) a combination of (a) and (b) such that the interfacial yield strength and interfacial hardening can be altered independently; and (d) a combination of (a) and (b) such that the interfacial yield strength and interfacial hardening are both scaled with the interfacial length scale. It is found through (a) that that interfacial yield strength controls the overall yield strength (i.e. onset of plasticity) of the specimen. Moreover, an analytical expression for the interfacial yield stress at which interface deforms plastically is derived. This is one of the most interesting features of the present formulation. From this expression, it is concluded that the yield strength of ultra-fine grained materials is controlled by the interfacial strength of the grain boundary. Moreover, it is found through (b) that interfacial hardening controls the increase in the plasticity tangent hardening modulus and in the flow stress with decreasing size. The expression in (c) shows that the interfacial hardening contributes to the global yield strength as well as to the strain hardening rates (i.e. flow stress). However, it is shown that the expression in (c) yields incorrect decrease in the yield strength when increasing the interface stiffness. This is corrected by adapting the expression in (d) which shows that by increasing the interfacial hardening, stiffer interfaces are formed that in turn increases the yield strength of the material due to dislocation networking at the interface which obstructing further emission/transmission of dislocations across the interface. Therefore, it is concluded that the interfacial length scale should scale the effect of both the interfacial yield strength and interfacial hardening. Moreover, one should be careful when choosing a proper form for the interfacial energy such that it should at least qualitatively confirms with the experimental observations of size effect behavior. It is concluded that the increase in the material’s yield strength and strain hardening rates with decreasing size is determined by the weakest link of bulk and interface. If the interface is compliant then the properties of the interface control the yield strength and hardening rates of the material (i.e. controlled by the interfacial length scale ‘I). On the other hand, if the interface is rigid, the yield strength and hardening rates are controlled by the bulk behavior (i.e. controlled by the bulk length scale ‘). Therefore, for intermediate interfaces, a competition between those two mechanisms exists. Interfacial effect is an important aspect for further development of gradient-dependent plasticity that is

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capable of modeling size effects in micro/nano-systems that are initially subjected to macroscopically uniform stresses or strains. It is shown that the existence of both gradients and interfacial energies contribute to the observed size effects. Moreover, it is emphasized that in the absence of the interfacial energy, the material would support uniform fields and hence the constitutive gradientdependence would have no influence. Therefore, strain gradients come into play if the boundaries are assumed to constrain the plastic flow. Therefore, if continuum theories are to be used to predict plastic behavior at the micron or submicron length scales, a higher-order theory with interfacial energies is likely to be required. Also, it would be interesting to compare the results provided by the present theory and its rate form counterpart obtained in two- or three-dimensional applications. In a forthcoming work, a detailed Finite Element implementation of the proposed model will be presented and used to simulate size effect in small-scale structures under various loading conditions (e.g. bending, torsion, cyclic loading). Moreover, it is interesting to validate the present conclusions by performing detailed discrete dislocation dynamics. It is noteworthy that several researchers have questioned the ability of strain gradient plasticity theory to explain the observed size effect in nano/micro pillars or columns when subjected to

Fig. 5.5 Size effects due to interfacial yield strength only without interfacial hardening. The interfacial yield strength is varying according to a and c, d1 = 0.1 and b and d, d1 = 0.45. a, 0 ¼ 2: c, d Normalized stress–strain b Normalized plastic strain distribution along d for r relations. Different sizes are represented by ‘/d = 0.1, 0.5, 1, 1.5, 2, reprinted with kind permission from Abu Al-Rub [43]


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macroscopically homogenous deformation. These debates are attributed to the absence of strain gradients in these systems when subjected to uniform straining or stressing. Moreover, it has been argued that this type of size effect is due to dislocation starvation; i.e. the rate at which dislocations multiply is less than that rate at which dislocations escape and annihilate from the pillar surface as the size decreasing to hundreds of nanometers. Finally, more than 50 years of research on grain boundaries has established their impact on the overall strength of materials, yet experimental studies on their yield strength or Young’s modulus are rare in the literature. This is because grain boundaries are random networks of interfaces that are only a few nanometers wide and cannot be isolated and characterized by conventional tensile, bending, indentation tools. Therefore, the fundamental understanding on the interfaces in materials will impact grain boundary engineering, an evolving research direction towards optimized materials design. Figure 5.5 shows size effects due to interfacial yield strength only without interfacial hardening while Fig. 5.6 illustrates size effects due to interfacial yield strength and interfacial hardening for d1 = d2 = d.

Fig. 5.6 Size effects due to interfacial yield strength and interfacial hardening for d1 = d2 = d. Both the interfacial yield strength and hardening is varying simultaneously according to a and c, 0 ¼ 2: c, d = 0.45 and b and d, d = 1. a, b Normalized plastic strain distribution along d for r d Normalized stress–strain relations. Different sizes are represented by ‘/d = 0.1, 0.5, 1, 1.5, 2, reprinted with kind permission from Abu Al-Rub [43]

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5.8 Studying Effective Factors on Simultaneous Deposition of Alumina Nanoparticles with Nickel 5.8.1 Effect of Density of Alumina Nanoparticles in Electrolyte Bath Guglielmi Model has specified that density of particles in bath affects on degree of participation of these particles in coat. With the increase of their density in electrolyte, the degree of their attraction will be increased on cathode surface and consequently, it will cause increase of participation of these nanoparticles in nickel-based coating. In Celice model, which has been posed based on possibility of passage of particle from penetrated layer, with the increase of density of alumina nanoparticles in electrolyte solution, possibility of passing them from penetrated layer is increased and consequently, degree of participation of nanoparticles will be increased in nanocomposite coat. Some researches have been made on effect of pulse current variables on hardness and resistance of friction of nickel-alumina composite coating he attained similar results. These results have been shown in Fig. 5.7. It is observed that alumina density increase at bath has boosted hardness of composite coating which is related to more participation of alumina particles in coating. In another research on electrical deposition of Ni/ Al2O3 with revolving multidimensional electrode, effect of density of alumina particles was studied on particles volumetric percentage in coating [39, 44–57].

Fig. 5.7 Effect of alumina concentration in bath on hardness of nickel-alumina composite coatings electroplated with a direct current (DC) with 5 A/dm2, b pulse current 5 A/dm2 and duty cycle 20% and frequency 75 Hz

5.8 Studying Effective Factors on Simultaneous Deposition

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5.8.2 Effect of Electroplating Current Density According to Guglielmi Model, it was specified that degree of participation of neutral particles in coating will be decreased in tandem with increase of density of plating current. This subject was studied by Guglielmi in simultaneous leakage of Ni/TiO2 and Ni/SiC particles. This proportion has been increased in tandem with increase of current density, i.e. degree of participation of particles has been reduced in composite coating. But, effect of current density has been studied on degree of participation of alumina particles in nickel-alumina composite coating. The obtained results accord with Guglielmi Model, indicating reduction of degree of alumina particles in coating due to the increase of current density. Reduction on participation of alumina particles with current density increase can be correlated to electrochemical potential which affects on attraction of alumina particles on substrate surface. Higher current will create higher cathodic deposition potential and cause double layer more negative coupled with repulsion of alumina particles. Consequently, less alumina will participate in nickel base. Figure 5.3 indicates results on effect of current on degree of participating alumina in Ni/Al2O3 nanocomposite coating. According to Fig. 5.3, it is specified that hardness of nickelalumina composite coatings is reduced with the increase of density of current. In fact, density of current can have two various impacts: density of current can affect on degree of alumina entered nickel base which will increase hardness of composite coatings and also can change in microstructure. Change of microstructure will cause hardness change. Figure 5.8 indicates that composite coatings deposited in density of lower currents contain more alumina particles than coatings deposited in density of higher currents. This issue accords with many of reports developed at different papers.

Fig. 5.8 Effect of density of direct current on hardness degree of electroplated nickel-alumina composite coating from bath containing 80 g/1 alumina

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5.8.3 Distribution of Alumina Nanoparticles Coating neutral nanoparticles in deposited layer is hard due to agglomeration of nanoparticles in electrolyte than particles with micro dimensions, because, when ceramic particles turn into fine particles, superficial forces will be increased and consequently they will increase agglomeration activities. The particles distributed in an electrolyte solution are based on Brownian motion. When two particles approach or near each other, the energies, which exist between two particles, will determine whether particles will be isolated or agglomerated from each other. Generally, agglomeration of particle is occurred due to the gravity energy larger than repulsion energy between particles. Size of net involved forces in creation of agglomerated structure depends on condition and nature of system clearly. Knowledge of structure of interface region is regarded as a very important factor in comprehension of sustainability of distribution of sold particles with electrolyte. For fair and appropriate distribution of alumina in nickel-sulfamate bath, a change in inner-particle interface region is required by physical or chemical methods.

5.8.3.1 Physical Distribution of Alumina Nanoparticle with Ultrasonic Operations Physical effect is occurred when absorbs destructive energy particles like ultrasonic waves. Disperse of ultrasonic waves in liquid environment produces very high pressure (over thousand fold of atmosphere pressure), causing exertion of a huge tension, which entangle bonding energy between particles. Air bubbles enter inter-particle grooves from holes, aimed at reducing diameter of agglomerated alumina particles.

5.8.3.2 Chemical Distribution of Alumina Nanoparticle with Dispersants Aid Chemical effect happens when electrolyte, containing surfactants or molecules, turns macro with the aim of forming electrostatic or Setric interface between particles. Under such specific condition, these interfaces will result in repulsion thanks to amalgamation of absorbed layer and reduction of entropy. When colloidal two-particle double layers start overlapping each other with the same load, repulsion forces between them will confront with Van der Waals force. If repulsive potential turns large enough for overcoming Van der Waals potential, it will result in sustainability of electrical double layer [58–71]. A. Setric sustainability Principally, Setric sustainability has been developed based on two mechanisms. When two particles near or approach each other with attracted polymers, the number of situations, which polymer creates, is reduced due to the participation of other particles. This activity will result in reduction of entropy. Secondly, density


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of polymer is increased at overlapping region which will result in Osmotic Repulsion between particles in suspension. B. Electrostatic sustainability Suspension sustainability can be obtained with combination of electrostatic and Setric forces. This activity will be obtained through the following three methods: 1. With the participation of particles with electric load and one neutral polymer molecule, 2. With the participation of neutral particles and one polymer molecule with electric load, 3. With the participation of particles with electric load and one loaded polymer molecule, Presence of particles with electric load creates repulsive potential with long range and presence of polymer will create repulsive potential with short range when particles near or approach each other. C. Getting familiarity with chemical dispersants In the processes followed with suspensions, a number of fundamental and basic interactions can be applied for affecting inter-particle forces. These forces include Van der Waals attraction forces and electrical double-layer repulsive forces. The two forces emerge when solution ions with loaded agent group attracted surface of particles and/or isolation of solution ions is made from surfaces of particles. Setric forces are created by large-size molecules which have been struck to surface of particles. In the same direction, large loaded molecules, which are polyelectrolyte, will create repulsive electro-setric forces. Combination of interaction of Van der Waals and double-layer repulsion is basic of renowned DLVO theory which will provide network interaction energy. In addition, attraction forces are affected by change of dispersant environment. The largest effect is made on Van der Waals attraction through change of solution for solvability with up-to-down dielectric coefficient. General explanation for mechanism of particles superficial load in polar solutions depends on congregation of ion species in interface. Dispersants can be organic chemicals with low molecular weight, carrying agent groups for stabilizing on surface and marking specific part. That part of specific section is better to be severely polar additional group especially the groups like acid carboxylic and those groups which increase negative load of surface in proton dispersant environment in pH above 3. Citric acid or Maleic acid is regarded as examples of the said group. Other organic dispersants include polymer and polyelectrolyte with molecular weight less than 2,000 for polyelectrolyte containing high load and 10,000 for polymers. The separable agent or typical polar groups for dispersants include as follows: Hydroxyl (–OH), carboxyl (–COOH), sulfanate (–SO3-), sulfate 9-OSO3-), ammonium (–NH4+), ammonia (–NH2), amino (–NH-) and poly oxy ethylene (–CH2CH2O-). Setric sustainability enjoys major share in non-ion large molecules in solid/solvable system. Better output can be observed or Setric sustainability in non-ion large molecules in comparison with the compounds having low molecular weight. Some of common dispersants have been shown in Table 5.2.

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Table 5.2 Common dispersants used in ceramic processing Low molecular weight Sodium borate Sodium carbonate Sodium pyrophosphate Sodium silicate Citric acid Ammonium citrate Sodium citrate Sodium tartrate Sodium succinate Glycerol trioleate

High molecular weight Poly(acrylic acid) PAA Poly(methacrylic acid) (PMAA) Ammonium polyacrylate Sodium polyacrylate Polyisobutene Menhaden fish oil Phosphate ester Sodium polysulfonate

5.8.4 Effect of Density of Nickel Ions in Plating Bath Nickel coating baths include as follows: (1) Watts bath, (2) sulfate bath, (3) chloride bath, (4) nickel-sulfamate, (5) nickel fluorobrate. Nickel is usually deposited in watts bath type. This method is yet used for most nickel plating whether as sublayer or thick geometrical sections. Hereunder are regarded as main components of formation of bath: nickel-sulfamate as noncomplex (Ni2+), chloral nickel for improvement of solvability of anode and increase of density of currents through fracturing penetrated layer, boric acid for fixation of pH approximately 4 and reducing inclination to hydrolysis due to reduction of establishing non-acid salts, various additives for increasing of leveling surface, reduction of tension and crystallization of coating. Full-chloride baths can be used for delicacy of grains and granules in hard coatings and full sulfate baths can be applied while using lead nonsolvable anodes. Fluorobrate and sulfamate solutions have specific applications. Using sulfamate baths in electroforming, which speed of plating is of paramount importance, has been developed more. Comparison of output of cathode current for various baths of nickel is as follows: • • • •

Watts nickel bath 98–90% Full chloride 98–99% Sulfamate 97–99% Fluorobrate 90–95%

The baths used for nickel-alumina nanocomposite coating are principally are as follows: Watts baths, sulfamate and rarely chloride bath. The degree of effect of nickel ion on agglomeration of alumina nanoparticles in electrolyte and also degree of participation of alumina nanoparticles in nanocomposite coating is the salient and the most significant point at these baths. Increase of metal ions in electrolyte will cause reduction of distance of alumina nanoparticle electrical double layers.


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Consequently, repulsion force is reduced between particles. This activity will cause facilitation of agglomeration of alumina nanoparticles in high-conductor metal electrolyte. Using chemical and physical distribution has limited effect in preventing from agglomeration of alumina nanoparticles, rather, using bath with low nickel ion density will cause reduction of degree of agglomeration of alumina nanoparticles. In some researches, it was specified that average diameter of agglomerated alumina has been de-ionized in water without application of using ultrasonic energy and nickel-sulfamate bath will be 183 and 1109 nm respectively. That is to say that effect of solution ion power on agglomeration of particles can not be ignored. They reduced the average diameter of agglomerated alumina particles through the application of physical distribution (with imposed ultrasonic energy) and chemical distribution (with adding surfactants to nickel bath) 280 and 448 nm respectively. The size of alumina nanoparticles used by them stood at 80 nm which electrical deposition was carried out in three nickel baths, (1) without surfactant and ultrasonic operation, (2) with ultrasonic operation with 5 W/l for a period of 40 min and (3) with adding Cetytrimethyl Ammonium Bromide (CTAB) as surfactant. The diameter of agglomerated particles of alumina nanoparticles in Ni bath without adding surfactant and conducting ultrasonic operation stands at 1109 nm. It will cause presence of very low degree of alumina 1.42% vol, with agglomerated structure in coating. The diameter of alumina agglomerated particles was reduced to 280 and 448 nm in order through the application of physical distribution with imposed ultrasonic energy and chemical distribution through surfactant added to nickel bath. They used four baths with various nickel densities for studying effect of nickel ion density on degree of agglomeration. It is observed that average diameter of agglomerated alumina is reduced in tandem with reduction of density of nickel ion in bath. Volumetric percentage of alumina at this coat has reached from 8.37%, in a bath with high nickel density, to maximum amount of 26.78%, in a bath with low nickel density and has reduced slowly to 24.65% through more reduction of nickel density in bath. Distribution of alumina particles in nickel-sulfamate bath will be improved when density of electrolyte is low. It is not unnatural that volumetric percentage of alumina in nanocomposite coating (Ni/Al2O3) is increased through reduction of nickel ion at this electrochemical reaction. But when nickel ion density is very low, output of density of low current will result in revival of hydrogen ion. Therefore, volumetric percentage of alumina in Ni/ Al2O3 coating has maximum amount i.e. approx. 26.78%. At this figure, distribution of particles in composite coating with low ion density is more congruous than its distribution in dense solution. The average diameter of alumina agglomerated in composite coating is reduced with the reduction of low nickel ion density. The degree of fine and minute particles in coating will be increased with dilution of solution. Figure 5.9 indicates effect of nickel electrolyte ion density on degree of nanoparticles entered in coating. It is observed that maximum of weight and volumetric percentage of alumina in coating happens in [Ni] = 3% M density.

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Fig. 5.9 Volumetric percentage of Al2O3 by image analysis and weight percentage by EDS Fig. 5.10 Schematic of effect of pH on viscosity of alumina suspension

5.8.5 Effect of pH Low pH is usually used in nickel baths. At the baths with low pH, nickel density is kept constant with solving anode. With regard to the baths with low pH (high sulfate), cathode has more inclination to be turned into a hole. Unlike baths with high pH type, baths with low pH does not need more care. pH used in papers for plating nickel-alumina nanocomposite stands at 3.5–4.5 output. The degree of pH has salient effect on viscosity of suspension. With regard to oxides, viscosity is changed with zeta potential. Alumina suspension viscosity is increased with the increase of pH up by specified amount. After pH, viscosity will be reduced with pH increase. An amount of pH, in which its viscosity is reached to maximum amount, nears iso-electric point for suspension. Figure 5.10 shows viscosity changes of alumina suspension based on pH.


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With due observance to Fig. 5.10, it is specified that the more pH rate is reduced, the more alumina suspension viscosity will be reduced. In nanocomposite plating, the more rate of bath viscosity is less, the more neutral particles can be transferred to surface of cathode. Hence, pH reduction will help greatly to more presence of alumina nanoparticles in nanocomposite coating (Ni/Al2O3). Since iso-electric point of alumina in pH is 9 (pH = 9), with more reduction of pH, alumina particles surface load in electrolyte will turn more positive. This activity will cause enlargement of electrical double layer. Consequently, double layer repulsion forces will be increased. Moreover boosting electrolyte sustainability, alumina particles inclination for agglomeration will be reduced. This subject will result in more presence of alumina nanoparticles in nickel-alumina nanocomposite coating. We should bear in mind that cathode has more inclination to be turned as hole at the baths with low pH degree. For this reason, neither pH degree should be lessened nor increased.

5.8.6 Pulse Current Effect Imposed current is regarded as one of the other parameters which has major effect on microstructure and morphology of deposited composite coatings. This subject has shown that more and better control can be imposed on properties of coatings by improvement of their microstructure through the application of pulse current for electrical deposition of metals and alloys. Consequently, metals and alloys’ pulse deposition are of paramount significance due to the possibility of change of their properties by accurate setting of pulse parameters. Nickel pulse deposition has attracted the attention of many to itself. Some researchers have recently studied effect of pulse plating on roughness of deposited nickel thin films surface. Pulse plating of metals and alloys has been also studied. Reports show that selection of imposed pulse parameters affects alloy deposition compound tremendously. It has been reported that remarkable reduction is appeared in internal tension of electrical depositions while using imposed current as compared with common direct current at the same density. Some other researchers studied pulse plating parameters on resistance against corrosion of nickel depositions. Their results show that nickel deposition by pulse current imposition can produce nickel coatings with less porosity and resistant against corrosion better in comparison with direct current plating. Using TEM has shown that distribution of nanoparticles size in deposited films under Direct Current (DC) condition is naturally the same distribution of size of nano powders used in deposition. This subject is of paramount significance with relation to obtaining composite films. When films deposited from the same bath under pulse direct current (PDC), large nanoparticles or part of particle which has been agglomerated, may not coat by growth of sprout during a pulse on a film. Therefore, it may exclude before any pulse is occurred which does not necessarily cause continuous growth of juvenile. The initial results on electric deposition

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(PDC) of nickel-alumina composite films show that smaller nanoparticles are coated as larger particles while PDC plating. Hence, a selection of size of nanoparticle is occurred. Some researchers studied effect of pulse deposition parameters on hardness and resistance against nickel-alumina composite friction. They indicate that degree of higher hardness is obtained at duty cycle and lower frequencies. This subject points this matter that reduction of duty cycle and frequency will cause presence of more particles of alumina particles, and consequently, hard coat is obtained. Low duty cycle is meant an off-time more when not current passes from electrolyte and such issue will create more chance for ceramic particles for attaining double layers. Hence, more alumina will be participated in coat. Frequency of pulse current also has similar effect. In low frequencies, there is low number of cycles, resulted in creation of a better situation for alumina particles. Finally, hard coat is deposited in low-frequency pulses. Effect of duty cycle and pulse frequency has been studied on degree of participating alumina particles [28, 72–85]. In study of reverse pulse plating of nanocomposite thin film, some researchers have studied effect of duty cycle on presence of alumina particles in copper nanocomposite thin films. Although they have not studied effect of pulse frequency on degree of participation of alumina, they showed that reduction of duty cycle will increase presence of alumina nanoparticle in nanocomposite coating. Metallographic test of nickel-alumina composite coating deposited in duty cycle and various frequencies specify that morphology of these coatings is severely affected with the selection of pulse parameters. In Table 5.3, effect of pulse parameters has been shown on rate of participating alumina particles and hardness of composite coatings. Deposition in low duty cycle will cause increase of proportion of large particles in coating, as the pulse off-time is more in low duty cycle. Consequently, large particles can reach cathode surface through effect of mass transfer in pulse off-time period. Increase of duty cycle of two various effects is on presence of particles in coating: 1. Remarkable reduction of degree of participation of particles in coating 2. Participation of smaller particles more than large-size particles Some researchers have recently explained that selection of particles size is occurred while PDC plating clearly. In deposited samples with DC, ceramic

Table 5.3 Laboratory data related to obtained results for nanocomposite Ni/alumina electroplating Coating thickness Duty cycle Frequency Vol.% of Hardness VHN (microns) (%) (Hz) alumina (kg/mm2) 15 20 10 12

20 80 60 60

50 50 20 80

35 13 15 11

364 250 280 238


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particles larger than 100 nm are observed clearly and simultaneous deposited particles have been agglomerated. For the sample provided under PDC condition (right hand), any particle larger than 100 nm and agglomerated particle similar to what is observed in DC, is not found. In addition, the average base grain size stands at 20 nm. The research activities on simultaneous deposition of alumina nano-wisckers in nanocomposite coating in pulse method specified that rate of alumina nano-wisckers coated in composite coating is increased with reduction of frequency and more congruity is obtained in distribution of nano-wisckers in coating with lower frequencies.

5.9 Size Dependency of Tensile and Fatigue Strength in Ultra-Thin Films Tensile and fatigue tests of ultra-thin copper films were conducted using a microforce testing system by Zhang et al. [86]. Fatigue strength as a function of film thickness was measured under the constant total strain range control at a frequency of 10 Hz. The experimental results exhibit that both yield strength and fatigue lifetime are dependent on film thickness. Fatigue damage behavior in the 100 nm thick Cu films with nanometer-sized grains is different from that in the micrometer-thick copper films with large grains observed before. A comparison of the present results with those reported in literatures is conducted. Possible fatigue strengthening mechanism in the ultra-thin copper films is discussed. Fatigue of thin metal films is a key issue for the long-term service of microdevices. Previous investigations of fatigue of thin metal foils show a tendency of the improved fatigue strength with decreasing foil thickness. Especially, several studies on thin metal films, such as thin Ag films and Cu films, have demonstrated that fatigue properties of these metal films are significantly different from those of the bulk materials. When the film thickness approaches 200 nm, interface-induced fatigue damage becomes more prevalent. In these studies, the film thickness and grain size are usually ranged from several micrometers to sub-micrometers. However, little is known about fatigue damage and strength of metal films with nanometer-scale thickness and grain size. Zhang et al. [86] present the evaluation of tensile yield strength and fatigue lifetime of ultrathin Cu films with a thickness of about 100 nm or less and nanometer-sized grains. Figure 5.11 shows tensile yield strength of the ultrathin Cu films as a function of film thickness and Fig. 5.12 shows the mechanical energy loss versus the number of cycles of the 60 nm thick Cu films and the number of cycles to fatigue damage (Nf) as a function of film thickness. Zhang et al. [86] concluded that the yield strength of the ultra-thin Cu films further increases with decreasing film thickness down to several tens of nanometers. A comparison of fatigue lifetimes between the ultra-thin films and those in the literature indicates that the fatigue resistance of the ultra-thin Cu films is

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Fig. 5.11 a Tensile yield strength of the ultrathin Cu films as a function of film thickness, reprinted with kind permission from Zhang [86]

Fig. 5.12 a The mechanical energy loss vs. the number of cycles of the 60 nm thick Cu films; b the number of cycles to fatigue damage (Nf) as a function of film thickness, reprinted with kind permission from Zhang [86]

5.9 Size Dependency of Tensile and Fatigue Strength in Ultra-Thin Films

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higher than those of the micrometer-thick films, but somewhat less than that of the sub-micrometer-thick films. It is suggested that the activated GB-related deformation mechanism is responsible for the potential decrease in the fatigue resistance in the nanometers thick films compared with the sub-micrometer-thick films.

5.10 Application of Coating for Strength Enhancement Researchers produced nano-composite materials made of steel alloy, with very few molecules in their particles which can be used in buildings to increase strength and other similar cases. There existed a common physical mechanism which contributes to control alloy hardness. Hardness increase causes malleability, foliating, and tabularization decrease. Using nano-composites in these alloys it is possible to decrease these shortages to a high extent. This is caused by an increase in controlling mechanisms for each material property in nano scales. This method involves creating an alloy in frozen glass structure. Grinding obtained product make it possible to produce a particular powder which make bonds with other materials and create a very dense coating during heating process. Under this conditions particles diameter is about 50 nm. The process can generate very strong bonds in substances. Available steels are of strength about 10% of those calculated through theoretical methods, once using this method enables us to reach strengths about 40–45% of calculated one. This method also contributes to obtain better corrosion resistant properties. Experiments show that steel nano-coating is harder than traditional steel. This coating can be performed either construction of main data or after that. The method is very cheap compared with other conventional ones. This material can also be used for aluminum coating which significantly enhances its strength, while adds no mentionable weight to that. Also, empirical observations show that this type of coatings makes bonds with aluminum, while conventional Fe–Al coating is not easily performed. It is expected this method be of frequent applications [87–101].

5.11 Nano-Coating Use in Dressing Industry Some clothes producers have used a nano-protector coating to coat clothes’ surface. These include coatings resistant against pollution, decay, abrasion, and fire; however, they do not bring a good feeling to customers while wearing them. LLC Nano-Tex uses these coatings in United States. Also, U-Right uses this kind of coating, developed by Sweden researchers, in its products. The coating is used in clothes fabric as well as its other parts.

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5.11.1 Making WC/Co–Ni Nano-Coating Using Electrodeposition WC/Co–Ni coating is broadly used for its high hardness and low friction ratio in coating process of variant pieces; while in conventional methods thermo-aerosol method is applied. Through this method a bid deal of WC is used which leads to increase of piece weight and decrease of its prices. Inframat Company, with contribution of National Science Foundation (NSF), has developed coating with WC/Co–Ni nano-particles. Cr–C/W coating is used in some similar methods. Cr compounds are convenient to create hardness but are environmentally hazardous due to CRVI release. New substitute material for mentioned materials should have their high hardness and the other coating properties. In new methods, WC nano-coatings are coated via electrolyte method with Ni-Co matrix. Coating with this method creates an equal thickness and does not involve any high cost mechanical methods for compellation of coating process. Through new method a lower ratio of WC is consumed. This considerably decreases pieces’ weight and cost, as well as friction ratio and surface hardness.

5.11.2 Using (Me-Ti1-xAlxN)/(a-Si3N4) Nanocomposite Coatings Aluminum and titanium alloys and (Me-Ti1-xAlxN)/(a-Si3N4) nano-composite coatings are used for cutting tools coating. These coatings are of unique features make them suitable for these tools, including: • High rate of hardness (25–38 GPa) • Hardness in high temperatures (in 800C about 30–40%) • Resistance against oxidization (15-20 lg/cm2), TiCN, and TiN, respectively, at 800, 400, and 550C • Thermal conductivity There are some other issues while using these alloys which must be dealt, including: • Optimization of fuel processes • Optimization of crystalline structure to prevent creation of columnar structure for improvement of pieces resistant against corrosion • Making multi-layers • Adding other materials such as – Cd and Y to enhancement of strength against oxidization – Zr, V, and B to enhancement of strength against wear and corrosion – Si for increase of hardness and resistance against chemical agents Among most important achievements of TiAlCN detecting layer one can name nano-coating and increase of Al ratio in coatings. Using nano-coating of this alloy, as well as multi-stage coating with nano thickness, decreases different features of

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the surface including hardness, scratch resistance, and oxidization. Through this new method, nano-composite coating is used for multi-stage coatings. During this method different materials such as Al, Si, and Ti, which cannot be mixed with each other, serve as detector layer. They mix one another in plasma state and placed in amorphous Si3N4 matrix. At high temperatures (up to 1100C), this obtained nanocomposite is of high hardness (40–50 GPa). This kind of nano-composite coating is required for nano-composite coating of highly efficient melting pieces. Regarding this coating’s high efficiency compared with other materials and methods, their application is persistently increasing.

5.12 Size Dependency in Nanocomposite Layers Properties of Si3N4/Ni electroplated nanocomposite layer such as roughness of obtained layer and distribution of nanometric particulates have been studied [102]. All of the other effective factors for fabrication of nanocomposite coatings have been fixed for better studying the effect of the average size of nanoparticulates. The effects of the different average size of nanometric particulates (ASNP) from submicron scale (less than 1 lm) to nanometric scale (less than 10 nm) have been studied. The roughness illustrated a minimum level while the distribution of nanometric particulates will be more uniform by decreasing the ASNP. The effects of pulsed current on electrodeposition (frequency, duty cycle) and concentration of nanoparticulates on electrodeposition bath on trend of obtained curves have been discussed. Response Surface Methodology was applied for optimizing the effective operating conditions of coatings. The levels studied were frequency range between 1,000 and 9,000 Hz, duty cycle between 10 and 90% and concentration of nanoparticulates among 10–90 g l-1. Figure 5.13 illustrates the effect of different ASNPs on the Ra of coatings. Interpolated equation shows that there is a quadratic relation among the roughness of obtained layer and ASNP. It can be concluded that the interaction among Fig. 5.13 Effect of different ASNPs on the Ra of electroplated nanocomposite Si3N4/Ni coatings [102]

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nanoparticulates with low ASNP (approximately less than 90 nm) will increase the roughness of obtained layer. The minimum roughness has been obtained for the nanocomposite layer with ASNP equal to 93 nm. The effect of ASPN on the distribution of nanometric particulates has been illustrated in Fig. 5.14. It can be easily concluded that the gaussian shape of distribution curves are narrower for lower amounts of ASPN. Also it can be seen that the distribution curves of obtained layer for higher amount of ASPN are wider which means that although the nanometric powders with narrow distribution of particulates around the specific ASPN have been used but the distribution of nanometric particulates in obtained nanocomposite layer is not as same as the distribution of used nanometric particulates for large amounts of ASPN. So, in this point of view, it is better to use the nanometric particulates for fabrication of nanocomposite layer, with lower amounts of ASPN. In another study, hard silica/epoxy nanocomposite coatings were prepared by spinning method on the surface of AA6082 aluminum alloy with addition of CdTe quantum dots as the second phase in hard nanocomposite coating with different ratios in respect to main phase (silica nanoparticulates). Wear tests have been done on the coatings for investigation of the possible enhanced or inverse effects of addition QDs on properties of hard nanocomposite. It has been shown that by adding QD nanoparticulates the electrical conductivity of layers is completely controllable without adverse effect on wear resistance. Figure 5.15 shows the effect of different SiO2/QD ratios on the wear rate of obtained layers. Wear rate illustrates an optimum level and increasing the SiO2/QD ratio after this level will decrease wear rate significantly. QD nanoparticulates are softer than SiO2nanoparticulates and this behavior in wear rate was predictable, somehow

Fig. 5.14 Distribution curve of electroplated nanocomposite Si3N4/Ni coatings with ASPN equal to a 9 nm, b 72 nm, c 168 nm, d 499 nm [102]

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determining the optimum level will affect considerations for possible industrial usage especially for achieving desirable wear rate and electrical conductivity together, in economical point of view [103]. In another study, ultra hard ceramic based matrix nanocomposite layers of tungsten carbide (WC) on matrix of titanium carbide were fabricated in an organic electrolyte. The dependence of WC amount in nanocomposite coatings was investigated in relation to the temperature of electrolyte, WC concentration in bath, current density and stirring rate. It was shown that volume percentage of WC in the layer can be affected by these parameters. Increasing of the WC nanoparticles concentration in the electrolyte in a constant stirring rate will lead to an increase in content of nanoparticles in the nanocomposite layers. Concentration of WC nanoparticles in the bath illustrated specific level for increasing of tungsten percentage in the nanocomposite layers [104]. Fig. 5.15 Effect of SiO2/QD ratio on wear rate of different silica/epoxy nanocomposite layers [103]

Fig. 5.16 X-ray diffraction pattern of WC/TiC nanocomposite layer fabricated by plasma electrolysis [104]

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The XRD and GAXRD pattern (Fig. 5.16) confirms the formation of a TiC/WC nanocomposite layer. The concentration of WC nanoparticles was increased slightly by decreasing GAXRD angle which means that the amount of WC nanoparticles was increased toward the top surface of nanocomposite layer. Average grain sizes was determined by Scherer equation around 51, 58, 72 and 89 nm for the layers by 1, 5 and 10 of glancing angle and also simple XRD, relatively. Tungsten carbide nanoparticles probably act as new sites for grain growth and hence decrease the final size of grains. Roughness values of the nanocomposite layers were calculated to be approximately between 1.6 and 4.9l. Figure 5.17 illustrates the changes of roughness with the change in the concentration of WC nanoparticles in the electrolyte. The increase of roughness is due to the agglomeration of WC nanoparticles on the surface of the treated sample. Concentration of WC nanoparticles in the electrolyte has an optimum level for achieving the minimum roughness on the surface of the nanocomposite layer at higher current densities. In fact, the increase of nanoparticles concentration in the electrolyte and the increase of the current densities have similar effects on surface roughness. Higher current densities will lead to big sparks with more damaging effects and their effects will show themselves on low concentrations of nanoparticles in the electrolyte. Electrodeposition of tertiary Alumina/Yitria/carbon nanotube (Al2O3/Y2O3/ CNT) nanocomposite layer by using pulsed current has been also studied. The Fig. 5.17 Relation among surface roughness of coating and WC nanoparticle concentration in electrolyte in different current densities [104]

Fig. 5.18 Relation between average size of nanoparticles and wear rates for electrodeposited tertiary nanocomposite layer [105]

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effects of some process variables have been experimentally studied and statistical methods were used to achieve the minimum wear rate and average size of nanoparticles. It has been revealed that by changing the size of nanoparticles, wear properties of coatings will change significantly. In the case of average size of nanoparticles ranking of effective factors by their relative contributions is the same as for wear rate which shows strong relation between these two measured properties of coatings [105]. This relation can be seen in Fig. 5.18.

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Chapter 6

Size Effect in Physical and Other Properties of Nanostructured Coatings

6.1 Introduction Rapid development of microelectronics in recent decades has been proven based on miniaturization and integration of electronic parts and according to the predictions of the ITRS1 institute, this fast development will also continue in the next decade. In this regard, MOS field effect transistors or MOSFET2 is the main and basic component of most of the electronic systems nowadays. Among the numerous parameters of these systems, MOSFET gate length, which is a critical criterion of integrated circuits, will decrease to about 10 nm in 2016 [1–6]. The predicted process of decrement of the gate length by ITRS, has been shown in the Figs. 6.1 and 6.2. The reasons for the dimensions decrease are as follow: improvement of the part’s quality and performance, improvement of reliability, decreasing the power loss, improving the output efficiency, and decreasing its price. Moreover, the speed of an electronic circuit is one of the most important factors. In order to increase the speed, parasitic capacity and serial resistances must be minimized so that RC delay time decreases and the Clock Frequency increase. Increment of the contact resistance in electronic parts has been one of the main limitations of size decrement in the recent decades. Hence, using new materials in joints has been considered in order to fix the circuit quality and increase the speed. In order to do so, metal silicides have been used in the process of metallization of joints and local systems. Selection of metal silicides has of great importance due to these reasons: their low special resistance, their contact resistances are low against both kinds of silicon, their high thermal stability, and their processes are compatible with silicon standard technology. Nowadays metal silicides are an important component of an electronic part. The SALIIDE3 process leads to the formation of a uniform type of metal silicides 1 2 3

International Technology Roadmap for Semiconductors. Metal Oxide Semiconductor Field Effect Transistor. Self-aligned Silicide.


185

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6 Size Effect in Physical and Other Properties of Nanostructured Coatings

Fig. 6.1 The approach of gate length decrease according to ITRS predictions

Fig. 6.2 Approach to decrease the thickness of silicide in joints according to ITRS predictions

formed simultaneously in the regions of gate, source, and drain; and it is so successful due to its reliability and simplicity. Therefore, application of metal silicides has been promoted in electronics industry. The most common silicides used in electronic parts are PtSi, TiSi2, and CoSi2, although using C54–TiSi2 (phase with less special resistance) and CoSi2 in smaller parts (less than 0.2 and 0.04 lm respectively) is so difficult. In future parts we must use silicides layers with very low thickness. The silicides layer’s thickness was about 20 nm in 2005, and it is expected that it will be reached to 5.5 nm in the year 2015 [7–14]. Recently NiSi has attracted much attention and the latest progresses reference to the vast efforts in order to the application of NiSi in MOS parts in future

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technologies. NiSi is of great importance because of its low special resistance, low contact resistance, potential to form in low thicknesses, and its consistency. Therefore, it is inevitable to study its specifications. Ohno et al. [15] described the control of the quantum size effect by controlling the coating layer thickness in TiO2–SiO2 core–shell hybrid particles obtained by the liquid phase deposition (LPD) method. The coating layer thickness of TiO2 on SiO2 nano-particles was controlled by changing the [Ti]/[Si] ratio. The titania coating thickness and crystallite size were estimated by transmission electron microscope (TEM) and X-ray diffraction (XRD), respectively. The quantum size effect of the obtained nano-hybrid particles was estimated by the band gap energy shift, using ultraviolet–visible spectroscopy (UV–vis). As a result, we successfully controlled the degree of the quantum size effect by controlling the coating layer thickness in core–shell TiO2–SiO2 hybrid particles. The nano-particles have attracted considerable attention because of their potential application such as electronic, catalytic materials. The studies of nano hybrid particles have provided important fundamental insights for new functional materials such as photonic catalyst, high performance electronic materials and so on. Therefore, a lot of studies have been carried out to prepare the organic/ inorganic hybrid particles. The quantum size effect has been widely studied for the microelectronic devices because of their recent trend of downsizing. In addition, several reports demonstrated that the quantum size effect resulted in the new properties and/or the improved properties. In general, the quantum size effect is expected for the nano-particles below 50 nm in many cases. However, there are difficulties in manipulation and handling of such nano-particles, mainly due to the agglomeration and the adhesion. Therefore, good handling method should be developed for the nano-materials with quantum size effect in many fields, such as electronics and so on. From this point of view, Ohno et al. [15] were proposed the nano-coating of functional materials on the nano-particles. They successfully prepared the TiO2–SiO2 hybrid particles by the modified sol–gel process. However, the obtained hybrid particles were not core–shell materials with homogeneous coating, and they couldn’t control the quantum size effect. Therefore, they attempt to prepare the core–shell TiO2–SiO2 hybrid particles with homogeneous coating by LPD. LPD is well known as film deposition process, and some researchers applied the LPD method to prepare the TiO2-Polystyrene (PS) Latex hybrid particles. Ohno et al. [15] concluded the successful preparation of core–shell type TiO2– SiO2 hybrid particles by LPD. The coating layer thickness and the crystallite size were controlled by controlling the [Ti]/[Si]. In the case of the [Ti]/[Si] ratio of over 0.1, silica particles were completely coated by titania. The degree of the blue shift of the band gap energy by the quantum size effect for the obtained particles was approximately 0.13 eV larger than that of the pure titania, because of the existence of the Ti–O–Si bond. If the Ti–O–Si bond effect was removed, the blue shift of the band gap energy for core–shell type TiO2–SiO2 particles was nearly the same value as that of the reported values for the pure titania. From these results, the quantum size effect was successfully controlled by controlling the coating layer thickness of

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core–shell type TiO2–SiO2 hybrid particles. Figure 6.3 illustrates the high resolution TEM (HR-TEM) image of surface morphology for the obtained TiO2–SiO2 hybrid particle with different [Ti]/[Si] ratio and the surface area of the obtained

Fig. 6.3 The surface morphology of the obtained TiO2–SiO2 hybrid particle with different [Ti]/ [Si]: a [Ti]/[Si] = 0.1, b 0.3, c 0.5, d 0.75, e 1.0, and f high resolution TEM (HR-TEM) image of the surface area of the obtained hybrid particle with [Ti]/[Si] = 0.1, reprinted with kind permission from Ohno [15]

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Fig. 6.4 Change in the crystallite size as a function of the coating layer thickness, reprinted with kind permission from Ohno [15]

hybrid particle with [Ti]/[Si] = 0.1. Also Fig. 6.4 shows change in the crystallite size as a function of the coating layer thickness.

6.2 Silicides Specifications Among different metal silicides, which are used in SALICIDE technology, TiSi2 has been vastly studied due to its widespread application in CMOS4 metallization. For the gate length 0.25 lm and for less than that TiSi2 was replaced by CoSi2. This substitution faces less resistance in low dimensions due to hard formation of C54–TiSi2 phase [16–23].

6.2.1 SALICIDE Process SALICIDE process leads to the simultaneous formation of a uniform type of metal silicide in gate, source, and drain regions; and is of great importance in advanced electronic parts technology nowadays. SALICIDE process, for example, using the NiSi is described below. When gate is defined as a MOSFET, a Ni layer is layered on top of the sub-layer. The first thermal process, usually RTP,5 at the low temperatures (often between 260 and 310C) leads to the formation of Ni2Si in gate, source/drain areas where Si, which is in direct contact with layered Ni, is formed.

4 5

Complementary Metal Oxide Semiconductor. Rapid Thermal Process.

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No interaction happens between Ni and the surrounding oxide environment. After formation of Ni2Si, the extra Ni layer is removed through selective wet etch method. The second thermal process usually forms NiSi in gate and source/drain areas between 450 and 500C. One of the advantages of Ni-SALICIDE is the application of single-stage thermal process (usually about 400–500C). Practically, the problems related to non-uniform formation of NiSi and formation on Si lines with more thickness (thinner than wider lines) have made it necessary to use two-stage aniline process. SALICIDE process is applicable for Co and Ti, although the main objective of two-stage aniline process is different. The used temperatures for two-stage aniline depend on the type of silicide. In Ti and Co mode, the first aniline typically forms in thermal range of 650–700C for C49–TiSi2 and in 400–600C for Co2Si or CaSo in order to prevent the formation of silicide on SiO2 which causes a short circuit between the gate and source/drain electrodes. The second aniline is important to form low-resistance silicide phases C54–TiSi2 (above 850C) and CoSi2 (above 700C).

6.2.2 Necessary Conditions for Formation of Silicides Decreasing serial resistance and common parasitic resistances in gate, and source/ drain areas is fundamental in order to improve the quality of the parts. Also, it is necessary that SALICIDE process do not influence the part and its alloying profile. The used metal in the formation of silicides must have the following basic conditions; • • • • • •

high conductivity Low contact resistance for both alloyed Si types Good chemical stability in contact with Si Suitable mechanical and thermal specifications Suitable thermal stability considering the morphology Compatibility with standard processes technology of Si including Etching and Cleaning • No need to extra thermal processes • Void of harmful pollutions which decrease the efficiency of the part Moreover, fundamental ideas to integrate the SALICIDE processes can be summarized in three ideas: 1. On the gate: formation of phase in low dimensions in thin lines of poly-silicon and small areas of single Si crystal in source/drain regions, morphologic and thermal stability of gate electrode-Silicide/Poly-Si. 2. On the source/drain: contact resistance between the silicide and source/drain and the integration of joint regions of source/drain.

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3. Between the gate and source/drain: the bridge between gate and source/drain regions.

6.2.3 Transition from TiSi2 to NiSi Except the application of silicide as a barrier layer in contact with the common side of silicon and metal, Silicide was firstly used in LSI6 as a poly-side (the electrode gate of poly silicon/silicide). The poly-side line MoSi2 was one of the first applications of poly-side in DRAM7 in early 1980. Then the poly-side WSi2 was used for electrode gate of logic circuits LSI in mid 1980, due to its low resistance compared with MoSi2. Application of TiSi2 due to its low resistance compared with WSi2 and its low contact resistance with both types of silicon, and also it’s high thermal stability goes back to early 1990. When the MOS parts’ dimensions are minimized to 0.2 lm or less, the formation of TiSi2 faces troubles. TiSi2 has two structural phases: C49 and C54. The phase C54 is more sought to be used in these parts due to its lower special resistance and higher stability. The resistance of C49 has been reported between 60–80 lXcm and that of C54 between 15–20 lXcm. When the dimensions of the part decrease, the transition from C49 to C54 gets harder. The cause of this difficult transition is connected with low density of C54 nucleuses inside the C49 network. Since the transition of C49 phase to C54 is a process controlled by nuclear stage. On the Si lines, if the line thickness is less than the average distance between two nucleuses of C54, the formation of C54 phase will be dominated in single-manner development. This single-dimension development causes noncomplete transformation of C49 to C54 and increases the resistance. Decreasing the dimensions of the part leads to the decrease of the temperature in which TiSi2 starts to form. One of the ways to prevent this is to develop C54–TiSi2 in thin lines, and also making the Si amorphous with the signal ion. In another different approach, the application of slow-fusible like Nb, Mo, Ta, and W has been suggested to form C54–TiSi2. The problems related to TiSi2 in late 1990, led to the replacement of TiSi2 by CoSi2 in using lines thinner than 0.2 lm. CoSi2 has a lower resistance compared to C54–TiSi2. Regarding the technology based on Co, it seems that Co-SALICIDE has specifications like high formation temperature, oxygen impurities, high consumption of Si in forming silicide, formation of pores and inter-facial irregularities. High consumption of Si creates thick layers and creates problems in forming thin layers (less than 100 nm), so consumption of a suitable silicide free of all these problems will be necessary in manufacturing future parts.

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Large Scale Integration. Dynamic Random Access Memory.

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6.2.4 NiSi Salicide Technology (Self-Aligned Nickel Silicide) It is expected that NiSi is a good alternative for TiSi2 and CoSi2 in technologies which have 100 nm scale and smaller. In fact, NiSi has some fundamental advantages for CMOS processes including: 1. 2. 3. 4. 5. 6. 7.

Formation of silicide layers in low temperatures Little consumption of silicon Void of undesirable bridging specifications Little mechanical tensions Having no effect of thin line on surface resistance Low contact resistance to both types of Si (n and P) Independence of resistance of silicide layer from decrease of thickness of connection lines

The formation of self-aligned silicide happens after the formation of source/ drain. Therefore, the temperature formation of silicide must be low enough to keep the joint thin in under 100 nm CMOS technologies. NiSi has the lowest formation temperature among all the silicides, and the thermal stability interval is 350–750C. For TiSi2, the thermal stability interval is very limited. Contact resistance of TiSi2 is high before 800C, due to the presence of C49 crystal phase. Between 850C and 950C it decreases due to the formation of phase with formation of C54. Above the 950C, due to the accumulation of surface resistance, it increases. In the other words, the contact resistance of NiSi gets stable and minimized between 350C and 750C. The increase of resistance above 750C is due to the fusion of phase from NiSi to NiSi2. Therefore, NiSi is thermally suitable for the technologies under 100 nm. In the other words, it must be kept in mind that the temperatures of processes after the formation of silicide must not exceed 750C [24–37].

6.3 Size Effect in Sensing Characterization Tan et al. [38] prepared non-equilibrium nanocrystalline xSnO2-(1-x)a-Fe2O3 powders by using the mechanical alloying technique. The thick film screen printing technology is then employed to fabricate these ethanol gas sensors. Their particle size and structural properties are systematically characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The gas sensing characteristics are also measured. Based on the experimental results, it was observed that particle size of the powders is drastically milled down to about 10 nm after 24 h of high-energy milling. A very high gas sensitivity value of 845 for 1000 ppm of ethanol gas in air has been obtained. New structural model for these non-equilibrium nanocrystalline xSnO2-(1-x)a-Fe2O3 materials explains both the lattice expansion of these high energy mechanically alloyed powders as

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well as the charge neutrality in terms of additional oxygen dangling bonds at the nano-sized particle surfaces. It is those enormous oxygen-dangling bonds at the particle surfaces that give rise to the high gas sensitivity. The sensors are found to be 32.5 times more selective to the ethanol gas compared to CO and H2 gases. The increase concern over safety in civilian homes and industrial activities has generated great interest for reliable gas detection. Many thick film metal oxide semiconductor gas sensors based on their resistive changes, such as SnO2 and Fe2O3, have been commercially designed to detect toxic gases (e.g., CH4, CO, and NO2). Nano-sized materials have been widely used to produce new semiconductor gas sensors, owing to the great surface activity provided by their enormous surface areas. Hence, they are expected to exhibit higher gas sensitivity. Being a promising gas sensing material, nano-sized a-Fe2O3 powders have been prepared by various methods, including chemical co-precipitation, sol–gel process, metallo-organic deposition (MOD), and plasma enhanced chemical vapor deposition (PECVD). These methods are basically chemical processing techniques to build homogeneous structure on an extremely fine scale of a few nanometers from the molecular level. Tan et al. [38] used a different technique called the high-energy ball milling technique to obtain nano-sized a-Fe2O3-based powders as the sensing materials. In this technique, the decrease of the particle size into fine powders of a few nanometers arises from the high-energy impacts during the collisions. This method, which is also known as mechanical alloying, has recently been used to prepare nano-sized SnO2-(a-Fe2O3)-based powders with the grain size down to 8 nm for gas sensing. Such initially immiscible, mechanically alloyed SnO2-(aFe2O3) materials are far from their equilibrium state. It is suggested that the content of Sn4+ ions may play an important role in the gas sensitivity. However, the sensing mechanism in this SnO2-(a-Fe2O3) system has not been well understood because of the lack of a complete understanding of the microstructure of the materials. Tan et al. [38] have illustrated a promising method of using mechanical alloying in the preparation of nano-sized a-Fe2O3 materials for gas sensing applications. In particular, the sensor has shown good ethanol gas sensitivity values of as high as 845 at 1000 ppm in air. The sensor is selective to ethanol gas

Fig. 6.5 Correlation between grain size and sensitivity for different milling times, reprinted with kind permission from Tan [38]

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over carbon monoxide and hydrogen gases. The gas sensitivity is also found to be very stable. These excellent experimental results can be explained by the fact that such mechanical alloying materials have nano-sized particle grains and exhibit enormous oxygen dangling bonds at their particle surfaces. Figure 6.5 shows correlation between grain size and sensitivity for different milling times while Fig. 6.6 illustrates TEM micrograph for powders after different milling times.

Fig. 6.6 TEM micrograph for powders after (a) 2 h and (b) 120 h of milling, reprinted with kind permission from Tan [38]

6.4 NiSi Thermal Stability

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6.4 NiSi Thermal Stability 6.4.1 NiSi Transition to NiSi2 in Dual Ni-Si System Thermodynamically, the formation of silicide is a result of decrement in reaction’s free energy. When a metal is heated by contacting the silicon, and silicide is formed, the silicide phase is dependent on the formation temperature. In Ni-Si system (Fig. 6.7) and in the temperatures around 200C, the NiSi2 is formed, in temperatures around 350C the NiSi is formed, and in temperatures around 750C NiSi2 is formed. Contrary to the Ni2Si and NiSi situations, NiSi2 development is non-uniform. Phase NiSi is stable to the temperature 750C, NiSi reacts with the silicon sublayer to form NiSi2 which looks like the development of islands in NiSi network. An irregularity is seen in the common line of Silicide-Si. This irregularity is caused by the development of branches in some parts of the layer. The concentric development of the particles, reminds us of dendrite development inside the pillar particles. By time, more NiSi aniline is transformed into NiSi2 and other NiSi2 particles develop in order to reach each other. These big particles seem to be composed of smaller pillar particles. Numerous factors like temperature and the structural condition of previous phase seem effective in defining the new phase. NiSi layer consists of particles with different crystal structures (cubic and orthorhombic). Adjacent particles are aligned optionally and cause the formation of some big particles. In temperature 700C and after annealing, 5 min of swelling effect of each particle is seen under the silicon layer, which makes the common line of Si-Silicide have waves. This is direct effect of penetration of Ni network from NiSi into Si sub layer in the process of formation of NiSi2. A large portion of Ni transfer is in the form of network penetration compared with penetration into the border of particles, and this can be due to the effects of swelling. The effects of swelling also depends on the structure, for instance for orthorhombic structure, the penetration ratio is low; therefore, the swelling ratio also decreases [24, 34, 35, 39–47].

6.4.2 Pt Effect in Increasing Thermal Stability Pt and Ni have the same metallurgic behavior. In a crystallographic view, NiSi and PtSi have orthorhombic MnP structure, which is belonging to Pnma spatial group. Therefore, it can be expected that NiSi and PtSi are well solved in each other, and Fig. 6.7 Stages of formation of different phases based on temperature (C)

196


so increase NiSi2 nucleus-formation temperature. Some researchers have studied the effect of little (about 5%) in thermal stability of NiSi layer on the surface of Si(111) and Si(100). XPS8 and XRD9 together show the increase of thermal stability until the temperature 850C. The constant negative section of NiSi for the second sample has of high stability to 850C. Moreover, by increasing the temperature, the difference between bonding energy decreases, which shows that Pt has a constant distribution in NiSi structure. This result is also confirmed by XRD spectrum and for the sample without platinum, NiSi2 is formed in the temperature 750C, but for other samples the formation of NiSi2 is delayed until the temperature 900C. XRD spectrum shows some of the high-pressure textures for NiSi, which largely decreases energy for the common line of NiSi/Si compared with poly crystal NiSi layer without the Pt. As it was mentioned, PtSi and NiSi have the same structures. Therefore, Gibbs free energy NiSi, G(NiSi) is largely decreased due to the formation of NiSi(PtSi). On the other hand, Pt is not solved properly in NiSi2, so it creates a little change in G(NiSi2). Hence, we can ascribe NiSi2 nucleus thermal rise to the decrease of G(NiSi), and the rise of common line energy can be attributed to the trend to the formation of NiSi with high-pressure texture on the Si. In fact, the formation of NiSi2 is done because of the following reaction. Ni1x Ptx Si þ ð1 xÞSi ! ð1 xÞNiSi2 þ xPtSi

ð6:1Þ

This shows that the presence of PtSi together with NiSi2 will increase Dr in comparison with the normal reaction NiSi þ Si ! NiSi2 . Besides, the formation of Ni1x Ptx Si largely decreases Gibbs energy: DG ðNi1x Ptx SiÞ ¼ ð1 xÞG ðNiSiÞ þ xGðP þ SiÞ TDSmix

ð6:2Þ

DSmix ¼ R½x ln x þ ð1 xÞ lnð1 xÞ

ð6:3Þ

Where:

So the change in Gibbs energy for the relation (6.1) will be as: DG1 ¼ ð1 xÞ½GðNiSi2 Þ GðNiSiÞ GðSiÞ þ TDSmix ¼ ð1 xÞDG0 þ TDSmin ð6:4Þ For x = 0.05, the TDSmin will be about 2 kg/mol in 1100 K, which is of the DG0 degree. Therefore, the driving force to transform the reaction NiSi ! NiSi2 has decreased. Some researchers studied the Pt inter-layer effect in stabilizing the NiSi layer on Si(111). The results show that by increasing the temperature, the direction of (200) NiSiII(111)Si will change into the direction (002) NiSi11(m)Si. Such a

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X-Ray Photoelectron Spectroscopy. X-Ray Diffraction.


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transformation can not be seen in the previous work. The texture of NiSi layer from (200) NiSi11(111) into (002)NiSi11Si(111) will be transformed in high temperature after aniline. NiSi2 nucleus will take place in NiSi texture after this transformation. This transition in the texture will consume some kinetic energy of atoms and the NiSi2 nucleus configuration will be delayed. The comparison of surface resistance in two samples of Ni/Si and Ni/Pt/Si according to aniline temperature in the Fig. 6.8 confirms the previous discussions on the delay on formation of NiSi2. As mentioned before, by decreasing the dimensions of micro electronic parts, the thickness of joints will also decrease. For instance for a line thickness of 130 nm, the silicide layer will be about 34 nm. So, studying the thermal stability will have great importance by decreasing the thickness of metal layers signaled on Si and aniline. This has been studied by evaluating the following systems which have been annealed in different temperatures. A. Ni (50 nm)/Pt(4 m)/Si(100) B. Ni(25 nm)/Pt(2 nm)/Si(100) C. Ni(12.5 nm)/Pt(nm)/Si(100) Studying NiSi(211) peak situation in the temperature 800C for one hundred samples shows that Pt percentage in all the samples is a fixed value, therefore the difference in transition from NiSi to NiSi2 in samples; is not only related to the layers’ thickness. The effect of thickness of the layer on thermal stability of NiSi in Ni/Pt/Si(100) system has been shown in Fig. 6.9. In the sample A in temperature X . Considering the XRD 840C, the surface resistance is increased from 1.7 to 2.3 Sq results, the increase in surface resistance is due to the formation of NiSi2. In the sample B, transition temperature happens at 80C and in the sample C, it remains

Fig. 6.8 Changes of surface resistance according to baking temperature for Ni/Si and Ni/Pt/Si systems

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Fig. 6.9 Surface resistance according to different baking temperatures

with the surface resistance even after 880C. Therefore, we can understand that by decreasing the thickness in Ni/Pt/Si(100) system, thermal stability is increased. As we said before, the increase of platinum, decreases jDgj and this leads to increase of activation energy for nucleus. Also decrease of jDgj will ultimately increase the critical radius for the nucleus r . As only the nucleuses with radiuses bigger than r are allowed to develop, when the layer’s thickness is low, the development will be limited in the direction of perpendicular with the layer’s surface, and the nucleus will be delayed. We can have a numerical estimation from the value of r . For the normal reaction kJ NiSi þ Si ! NiSi2 , the quantity equals 2:35 kcal mol or 9:83 mol. Considering the structure (cubic, CaF2) and constant network of NiSi2 (a = 5.4 Å), Gibbs free energy in the unit of volume will be Dg0 ¼ 413 cmJ 2 . Considering the critical radius: r ¼

2b Dri 3a Dg0

ð6:5Þ

And also that Dr, change in inter-layer energy is of 104 cmJ 2 degree, the approximate value of r for NiSi2 will be about some nanometers.

6.4.3 Pd Effect in Thermal Stability Palladium, Pd, increases thermal stability of NiSi like Pt do. It is interesting to compare how Pd and Pt increase thermal stability. Some researchers have dealt with this issue by studying Ni/Pt/Si and Ni/Pd/Si systems. In their study, Ni’s thickness has been chosen as 100 nm and Pt and Pd’s were equally picked as 8 nm.


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For the sample having Pt, a high pressure peak NiSi(200) exists and this means that there is a strong alignment in this structure. Such a texture is not seen in the sample having Pd. Aniline in temperature 900C shows that in samples containing Pd, transition is done fully, but in samples with pt, NiSi(002) phase exists next to the formation of NiSi2, which prevents the completion of the transition. That is in presence of Pt, the system shows more stability. These results are compatible with the results produced by Raman analysis. Pt and Pb both increase nucleus activation energy NiSi2, because PdSi and PtSi have a similar structure with NiSi and form a solid solution. But this increase is more for Pt, and it is because of justification direction NiSi(200)11Si(111). This direction decreases inter-layer energy between NiSi and Si(111) which rises Dr and DG , so that we have: DG ðNi=Pt=SiÞ [ DG ðNi=Pd=SiÞ [ DG ðNi=SiÞ

ð6:6Þ

Comparison of thermal stability of these three systems is there in Table 6.1 with considering inter-layer energy effect r between NiSi and Si, and the driving force jDGj for nucleus configuration. Increasing the inter-layer thickness Pd will increase the system’s thermal stability. For example, with Pd thicker, it shows more stability and when the thickness increases to 7 and 10 nm, nucleus configuration of NiSi2 will be delayed until 900C.

6.4.4 Ge Effect in Thermal Stability Ge also increases temperature in nucleus configuration NiSi2. For system Ni/Ge/Si the phase transition from NiSi to NiSi2, even at 900C will not complete in comparison with NiSi (800C), and it proves a considerable rise in nucleus configuration of NiSi2. A layer’s conductivity also depends on its morphology, since after aniline at 800C, the NiSi layer is developed on Ge/Si evenly, and it gets irregular and uneven by increasing temperature. The accumulation of atoms takes place by forming NiSi2. The temperature for nucleus configuration in the reaction of layer Ni with sub layer Si1-xGex increases too. In both cases, we could attribute temperature rise to the entropy effect of NiSi–NiGe mixture. NiGe has a Mnp structure, similar to that of NiSi, and so Ge is well solved in NiSi but not in NiSi2. So, the level of free energy decreases as TDSmin and the pure value decreases, consequently activation energy DG related to NiSi2 nucleus configuration also increases. Table 6.1 Comparison of thermal stability of NiSi with Pt and Pd barriers r decrease Thermal stability sequence jDGj decrease Ni/Pt/Si(111) Ni/Pd/Si(111) Ni/Si(111)

Yes No No

Yes Yes No

1 2 3

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6.4.5 Co and Ir Effects in Thermal Stability Some researchers have shown that using Ir and Co as penetration barriers, not only improves thermal stability, but considerably decreases leaking flow in thin joints with depth of 40 nm. In Ni/Si, the development of NiSi2 plates in the direction (111) will increase surface resistance and leaking flow. The rise in stability and little leak of the flow is probably due to the very even common line in samples containing Ir and Co.

6.4.6 Capsulation and NiSi Thermal Stability The contaminations caused by oxygen during silicide process might be resulted from following sources: (1) aniline environment, (2) Oxygen accumulated during metal’s layer signaling (3) inter-layer oxide in silicon/metal common line. Interlayer silicon oxide in the common line of Ni/Si decreases the reaction between Ni and Si too. So taking methods to absorb oxygen from Si surface is necessary. Ti Capsulation also decreases the inter-layer oxides. Also the Ti cap increases regularity and thermal stability, and also decreases the leak in flow. Before annealing, Ti surface is covered with a TiO2 oxide layer. During aniline at 500C, two important things happen: First, Ti cap has more reaction with oxygen. Second, Ti atoms in cap penetrate into Ni layer, and accumulate in the common line of Ni/ Si. Aniline at 600C not only forms NiSi, but also forms Ti–Ni–Si (TiNiSi and (TiNix)Siy) and a layer of TiOSi2. The formation of NiSi at 600C shows the restoration of SiO2 by Ti. SiO2 cap layer also delays the surface resistance. For the capped samples, the surfaces get more irregular with increasing temperature. The arrows show areas not covered by silicide. The density of these areas is more for capped samples. Using the cap layer also forms an even layer of silicide. Using the cap also decreases the development of furrows, and the morphologic behavior of NiSi layers’ surface, in cap case or without the cap, are bound with the furrows. In the other words, using cap layer delays the appearance of thermal furrows and so the accumulation of NiSi atoms, that is decreases the cap layer of surface energy NiSi. Finally, one of the most important challenges in miniaturizing the parts in electronic industry that is, increasing the resistance when the dimensions decrease was studied here. By decreasing the thickness, the resistance increase in joints. We also mentioned some specifications and capabilities in using TiSi2 (phase C54) and CoSi2. TiSi2 has two structural phases of C49 (high resistance) and phase C54 (low resistance). By decreasing the thickness of lines in joints; transition from high resistance phase to low resistance phase becomes more difficult. Therefore, using TiSi2 for dimensions lower than 200 nm faces the problem of resistance rise. CoSi2 as an alternative for TiSi2 has some basic problems despite the low resistance, including high consumption of silicon and high formation temperature


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(550C). High consumption of silicon makes it more difficult to develop this layer in nanometer dimensions. Silicide Nickel (NiSi) with low special resistance, even in dimensions lower than 100 nm, and with less silicon consumption, lower formation temperature (350C), etc. can be a good replacement for TiSi2 and CoSi2. The only problem relating the limited thermal stability is due to the fusion into a phase with the resistance more than NiSi2 at 750C, and the taking place of Agglomeration phenomenon on the surface. We mentioned some of the studies by researchers in recent years, like using different types of impurities or taking penetration barriers to improve thermal stability of NiSi, and we studied the reasons for thermal stability rise in the framework of classic theory of nucleus configuration and the effect of entropy mixture [48–57].

6.5 Size Effect in Optical Properties of Nanostructured Films Kale et al. [58] fabricated cadmium selenide nano-crystallites onto amorphous glass substrate from an aqueous alkaline medium, using chemical bath deposition method at room temperature. The samples are annealed in air for 4 h at various temperatures and characterized by structural, optical and electrical properties. The as-deposited CdSe layers grew in the nanocrystalline cubic phase, with optical band gap, ‘Eg’ 2.3 eV and electrical resistivity of the order of 106 X cm. After annealing metastable nanocrystalline cubic phase transformed into stable polycrystalline hexagonal phase. Depending upon temperature, decease up to 0.6 eV and 103 X cm were observed in the Eg, and electrical resistivity, respectively. These changes have been attributed to the increase in the grain size of the CdSe crystallites. Presently nanocrystalline materials have opened new chapter in the field of electronic applications, since material properties could be changed by changing the crystallite size and/or thickness of the film. New applications in various fields are also emerging. Development of such materials, whose structural, electrical and optical properties could be controlled, will be useful many ways. For example optoelectronic devices, particularly solar energy conversion devices could be modified accordingly. The synthesis of binary metal chalcogenide of groups II–VI semiconductors in a nanocrystalline form has been a rapidly growing area of research due to their important non-linear optical properties, luminescent properties, quantum-size effect and other important physical and chemical properties. The semiconductor nanocrystallites belong to state of matter in the transition region between molecules and solids. Their physical and chemical properties are found to be strongly size dependent. The properties of materials prepared by different methods are critically dependent on the nature of preparation technique and subsequent heat treatments like annealing in air, vacuum or different gaseous environments like H2, N2, Ar, etc. The micro-structural features of nanocrystallites are found to govern their electro-optical behavior. Cadmium selenide (Eg = 1.7 eV) is one of the

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promising semi-conducting material that has been studied for application in solar cells, c ray detectors, thin film transistors, etc. A number of workers have chemically prepared CdSe thin films. However, little attention has been paid to the various properties of chemically deposited CdSe thin films. Kale et al. [58] report on the room temperature chemical deposition of CdSe thin films from an aqueous alkaline medium. In order to get good quality CdSe thin films, the preparative parameters such as concentration of cadmium, deposition time and pH were optimized. Freshly deposited thin films may contain many defects such as voids, pinholes, etc. Annealing of thin films reduces the defects and increases crystallite size along with recrystallization process. Kale et al. [58] deposited the CdSe thin films by aqueous alkaline medium at room temperature grows with nanocrystalline phase with band gap 2.3 eV and electrical resistivity of the order of 106 X cm. Air annealing was found to increase crystallinity of the CdSe films along with recrystallization process that changed nanocrystalline to metastable cubic to stable hexagonal phase (673 K) at higher annealing temperature. The crystallite size of the particles was increases as a result of increasing the annealing temperature. Consequently, the electrical resistivity was decreased and CdSe films showed ‘red shift’ of 0.6 eV. Figure 6.10 shows plot of CdSe crystallite size versus annealing temperature of CdSe thin films. Tabulation of crystallite size, band gap ‘Eg’, electrical resistivity and activation energy of as-deposited and annealed CdSe thin films can be seen in Table 6.2. Fig. 6.10 Plot of CdSe crystallite size versus annealing temperature of CdSe thin films, reprinted with kind permission from Lokhande [58]

6.5 Size Effect in Optical Properties of Nanostructured Films

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Table 6.2 Tabulation of crystallite size, band gap ‘Eg’, electrical resistivity and activation energy of as-deposited and annealed CdSe thin films, reprinted with kind permission from Lokhande [58] Thin films Crystallite Band Electrical Activation energy (eV) size (Å) gap ‘Eg’ (eV) resistivity ðX:cmÞ HR LR As-deposited 373K 473K 573K 673K

40 60 80 120 180

2.3 2.0 1.8 1.8 1.7

3.25 9.58 5.38 8.23 1.17

9 9 9 9 9

105 104 104 103 103

0.86 0.79 0.72 0.69 0.65

0.34 0.31 0.27 0.18 0.16

HR high temperature region and LR low temperature region

Du et al. [59] prepared nano-copper films by DC magnetron sputtering. Their reflectivity and transmittivity to electromagnetic wave in infrared region were measured with Fourier Transformation Infrared Spectrometer (FTIR), by which their complex optical constant and permittivity were obtained. The results show that the complex optical constant and permittivity of nano-copper films depend upon the film thickness. This dependence is correlated with microstructure transition during the film growth. Nano-sized metal films have been of considerable interest both from fundamental point of view and for potential applications in photonics and electronics devices based on their unique properties which are generally very different from bulk materials. For example, it has been shown that electrical conductivity, r, of ultrathin metal film decreases apparently with its decreasing thickness. Strong interaction between metal films and electromagnetic wave occurs when the film thickness decreases to nanometer scale. Meanwhile, electromagnetic compatibility (EMC) of overall quality of nanostructured materials, devices and systems becomes a more and more serious question with decreasing size and increasing working frequency of electronic systems. In those cases, a precise knowledge on the complex optical constant and complex permittivity of ultra-thin metal films, for a thickness range from a few nanometers up to opaque layer, is very important. Copper as a kind of metal with high conductivity, electromagnetic performances of its films have been widely studied for the application in Integrated Circuit (IC) and microelectronics devices as interconnection parts, in semimirrors as UV radiation filters and as telescope mirror layers. Du et al. [59] deposited nano-copper films by magnetron sputtering method. Since the complex optical constant and the complex permittivity are not directly measurable quantities, they are calculated with reflectivity and transmittivity of copper films to electromagnetic wave. At the same time, the thickness dependence of nano-copper films on complex optical constant and permittivity are analyzed. Du et al. [59] measured the reflectivity and transmittivity of nano-copper films to electromagnetic wave in infrared region, by which their complex optical constant and permittivity, both the real and the imaginary parts are calculated. All these parameters are essentially dependent on the film thickness. This dependence

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should be considered in application of nano-copper film. The dependence of electromagnetic parameters on the characteristic size provides new possibilities for designing high performance electromagnetic functional materials and devices. The evolution of the film microstructure plays an important role in the size effect of nano-copper film on complex permittivity. Figure 6.11 shows dependence of reflectivity (a), transmittivity (b) of nano-copper films on thickness in infrared region. Figure 6.12 also shows dependence of complex optical constant real and imaginary parts of nano-copper films on thickness in infrared region. Tang et al. [60] reviewed studies on ultraviolet stimulated emission and lasing observed at room temperature from nano-structured ZnO thin films. The nanostructured ZnO thin films were grown on sapphire substrates using Laser-Molecular-Beam-Epitaxy (L-MBE). The thin film was consisted of regularly arrayed hexagonal nano-crystallite columns, whose facets form natural micro-cavities. These nano-crystallites confine the centre-of-mass motion of excitons. As a result of the quantum size effect, the oscillation strength of the excitons is largely

Fig. 6.11 Dependence of reflectivity (a), transmittivity (b) of nano-copper films on thickness in infrared region, reprinted with kind permission from Du [59]


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Fig. 6.12 Dependence of complex optical constant real part (a), imaginary part (b) of nano-copper films on thickness in infrared region, reprinted with kind permission from Du [59]

enhanced, which is favored to the radiate recombination of exciton at room temperature. Excited using the frequency-tripled output of a YAG laser, the nanostructured ZnO thin film showed strong ultraviolet lasing at room temperature with a threshold as low as 24 kW/cm2. At a moderate pumping intensity, the room temperature stimulated emission is associated with an exciton-exciton collision process. At higher pumping density, the excitons are dissociated, and the ultraviolet stimulated emission is dominated by an electron–hole plasma recombination process. Because of the large enhancement of oscillator strength of the excitons, the optical gain of the stimulated emission measured at room temperature reaches as high as 320 cm-1, which is an order higher than that observed in bulk ZnO crystals. In comparison with the electron–hole plasma stimulated emission in most of commercial semiconductor lasers, the excitonic stimulated emission can be realized at relatively low external pumping density. The observation of excitonic lasing effect at room temperature might be valuable in realization of practical ultraviolet semiconductor laser devices.

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Compact short-wavelength semiconductor laser diodes (SLDs) present vast possibilities in many high-technology applications. For example, in the area of optical storage, the storage density is limited by the size of the diffraction spot which is proportional to the square of the laser wavelength. Hence, the availability of short-wavelength SLD means much greater density of data can be stored on a disc. Towards this end, InGaAlP-based yellow SLDs had been developed with wavelength as short as 650 nm. Since this achievement, there was no report of shorter wavelength SLDs until 1990, when the first demonstration of green–blue lasing actions in ZnSe-based heterostructures was reported, followed soon by the successful operation of ZnSe-based laser diode in 1991. However, practical shortwavelength SLDs are still not available because the lifetime problem. Recently, room temperature (RT) ultraviolet (UV) stimulated emissions and laser emissions have been reported in metal nitride systems as a result of the breakthrough in the growth of high-quality GaN-based heterostructures and successful development of p-type doping. ZnO, as an oxide, is superior over nitrides and selenides in thermal stability as well as in resistance to chemical attack and oxidation. Its RT band gap is 3.37 eV which is suitable for fabricating UV optoelectronic devices. Its large exciton binding energy (60 meV) should in principle favour efficient RT excitonic emission. Because of the current difficulty in heavy p-type doping, research works have been carried out for study on the properties of stimulated emissions using electron beam pumping and optical pumping for bulk ZnO crystal at cryogenic temperatures. Few works have been reported on the UV stimulated emission at room temperature in bulk ZnO crystal, but no emission spectra were given. Tang et al. [60] reviewed their progress on ZnO nano-crystal ultraviolet laser research. The nano-crystallite thin films grown by the laser molecular beam epitaxy (L-MBE) consist of self-assembled, ordered arrays of hexagonal nanocrystallites. They described the structure and formation mechanism of the hexagonally shaped nano-crystals. The facets of the close-packed and ordered hexagonal nanocrystallites form natural lasing cavities. The optical gain is shown to be of excitonic nature and has a very large value that is dependent on the size of the nano-crystallites. The peak gain value is as high as 320 cm-1 for a 55 nm thick film, an order of magnitude larger than the largest known value for bulk ZnO. The large gain is attributed to the modification of the spontaneous emission created by the dielectric photonic structure of these films. The observations reported demonstrated that ZnO may be a viable material for short-wavelength optoelectronics application. Figure 6.13 shows lasing threshold as a function of film thickness. Bilotsky et al. [61] studied the size dependence of electron-lattice energy exchange in nanoparticles. Both surface and bulk energy exchange parameters are examined and it is demonstrated that the bulk energy exchange has non-monotonic oscillations versus size of the particles. It has been found that the amplitude of such oscillations increases with decreasing a particle size until the critical size reaches Lc. These bulk interaction related oscillations disappear for the particles less than Lc, and only the surface energy exchange remains as the energy flow


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Fig. 6.13 Lasing threshold as a function of film thickness. An excitation with a size of 500 lm 9 30 lm stripe was used, reprinted with kind permission from Tang [60]

between electrons and phonons subsystems. It has been shown that there exists an interval of particles sizes with total energy exchange of few orders less than in massive bulk metals. This condition is crucial for existence of hot electrons in stationary conditions in metal nanoparticles, metal island films and thin films as have been observed experimentally. Hot electrons in metal nanoparticles have been discovered experimentally, or more specifically, the lighting of gold nano-islands film on a dielectric was observed when a BIAS voltage was applied to the film. Electrical current flow resulted radiated light along the system of tunnel-connected island metal films (IMF). Later a new phenomenon has been observed—electron emission and nonlinear current–voltage characteristics in IMFs altogether with lighting. These observations were made with applied voltage bias of *10–30 V but the application of the same voltage to continuous (thick) films or bulk metallic samples did not produce above-mentioned phenomena. These phenomena were explained by a hot electrons concept. It should be noticed that hot electrons were observed in stationary condition in IMFs gold samples only but they did not appear in continuous films or bulk metal. However, hot electrons can be also obtained in IMFs by irradiation of a laser beam. Particularly, the irradiation with a pulsed CO2 laser (s * 0.2–1.0 ls) has been used. The duration of the pulse is much larger than all relaxation processes times in IMFs. The same phenomena (such as lighting and electrons emission in voltage applied experiments) have been observed in laser irradiation experiments. Therefore, the hot electrons appeared in quasi-stationary conditions in these experiments as well. Bilotsky et al. [61] studied the total electron–phonon energy exchange in small metal particles which size is less than free path of electron–phonon collisions. This expression contains bulk and surface terms. The bulk contribution oscillates as the function of the particle size. It is important that the long wave acoustics phonons generated by the hot electrons can be in non-equilibrium state with others phonons. Thus, the use of electron–phonon collisions integral

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Fig. 6.14 Electron-phonon energy exchange constants g(x), g0(x) versus x, reprinted with kind permission from Bilotsky [61]

approach with Plank distribution function of phonons could be incorrect. As it has been mentioned above, the hot electrons have been observed in stationary conditions only in IMFs. Nevertheless, there is one common electron–phonon energy exchange feature for both IMFs and thin metal films. Microelectronic devices electric conductivity with such as thin conductors may be quite sensitive to a film thickness. Figure 6.14 shows electron–phonon energy exchange constants g(x), g0(x) versus x.

6.6 Optical Coatings: Using Ultraviolet Light Block Layers For a long time, removing and destruction of different substructures and substrates, such as polymer wooden sub-layers under ultraviolet light, was an important problem for construction and application of these pieces in different poor climates. During recent years researchers were able to solve this problem using nanomaterials, particularly ones for nano-coating. It the past different materials were used, which did not enjoy decent quality and had defects during efficient absorption of ultraviolet light. Among disadvantages of traditional methods one can mention decrease of substance transparency and deactivation of ultraviolet resistant coating before end of piece life. Through new method, nano-coating of surface is performed with pure and impure ZnO2 and CuO nano-particles. Nanodur is one of important companies at this field. Using new coating leads to sufficient absorption of ultraviolet light. Compared with traditional ones, these coatings are of longer lifetime. Other advantages of these coatings are their higher resistance against cracking and abrasion, apparent transparency, and lack of deformation and color change. One of most important advantages of anti-ultraviolet nano-coating it their permanent

6.6 Optical Coatings: Using Ultraviolet Light Block Layers

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activity, in contrary with organic materials which deactivate after a while. These coatings can be applied to optimize most of pieces [62–74].

6.7 Surface Improvement for Making Fog and Vapor Resistant Layers Researchers succeed to produce a fog and vapor resistant nano-metric surface through improving plastic and poly-carbonate surfaces. At this method a new formulation (Clarity Fog Eliminator) was applied to prevent development of fog and vapor on surface. These products can be used in lightproof, face protector, and common or sport eyeglasses. As well as mentioned applications, this product is even applicable in refrigerator environments.

6.8 Production of Pieces with Nano-Coating Nanofilm Company is to produce products which use nano-coating technology. In most of these products coated part serves as main piece part and is considered among most important applications of nano-coating. One of applied coatings is one which repels water, snow, ice and other similar pollutions from the glass’s surface. It is expected to use these glasses in automobile industry. Among other applications of these coatings one can name other cases such as eyeglasses. Another fabricated coating is steam resistant nano-coating for sport and military utilities, optical pieces, and customary, safety, and laser glasses. These pieces can be applied in car windscreen, bath mirrors, and etc. Another type of fabricated nano-coat is anti-reflection one, which can be appropriately used in interior mirror of cars and cosmetic cases. The coat prevents glaring reflection of the light.

6.9 Self-Cleaning Glasses Self-cleaning glasses is addressed to glasses with photo-catalyst metallic oxide (especially titanium oxide) coatings, which its surfaces have hydrophobic properties. Due to sunlight or any source of ultraviolet light on activated surfaces these nano-coatings get activated and develop catalyst properties. Because of oxidizing feature of glass surface coating, any artifact is broken on the surface and converted to inorganic aqua-soluble material. Raining or water spraying on surface destructs surface of contaminator particles and causes to their collapse, for their hydrophobic property [75–84].

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6.10 Medical and Hygienic Applications 6.10.1 Inorganic Materials Nano-Coating for Medical Applications Chemists at UCLA University have invented a new unique nano-coating for inorganic materials, which is capable to produce them similar to proteins. Using this method one can apply these particles as a measuring tool for detecting intercellular activities. Using these products can be followed by significant achievements in pharmacy and detecting tools. During this method nano-metric coating particles and quantum wires by fiber amino-acids with short loops (peptides) are used. Through this kind of coating living cell authorize entering and exiting of particles, due to their similarity with proteins. These inorganic materials can even be poisonous. The method makes it possible to perform some investigations in living cells at molecular scale, so this is one of most important applications of nano-technology in biology and medicine. The method enables us to produce nano-coating on the surface of semi-conductor particles and import them as electronic measuring tools. Among their important applications one can name their fluorescence light reflection. During this method different particles can be imported to the living cells and be stimulated by blue light. Any of these particles have their own particular fluorescence response. Using different proteins coating with different fluorescence colors on nano-particles make it possible to use them as biological label. Researchers mention peptide nanocoating method as a link between organic and inorganic materials. Also, they consider their application as a method to produce intelligent drugs.

6.10.2 Using Nano-Particle Masks After onset of SARS, using anti-bacterium masks was of a great interest. Hence, efforts for fabrication of bacterium and tiny particles filter have been accelerated. One of interested fields of this part is nano-coating. Traditional coatings are able to stop bacterium to enter in living cells; in the other word the entrap bacteria on their coating. However, due to bacteria and other dangerous particles accumulation in masks they should be replaced as often as possible. To deal with this problem, using nano-coating was of a great interest among researchers. In these filters TiO2 nano-particles coating or polluted nano-particles of silver are used. The main advantage of these filters is that they eliminate dangerous bacteria and organic particles; then applied filter has no need to be replaced and can be used for a long time. Applying such filters has been focused to prevent bacterium and virus entrance. As a considerable share of these materials fabrication leads to using nano-coating with nano-materials, there predicted to be a satisfactory market.

6.10

Medical and Hygienic Applications

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6.10.3 Application of Hydroxyapatite Nano-Coating to Design Prosthesis IMC (Infromat Company) produces some prosthesis with nano-structured n-Hydroxyapatite HA coatings which serve as marker layer with electrophoretic method at common temperatures. In comparison with conventional pieces, newly made ones are of more compatibility with human body and are much cheaper. Coating according mentioned method and through nano-structures, compared with common coating methods, is of so many advantages such as enhancement of pieces strength, bonds stability, and resistance against corrosion. To examine features of this kind of coating there have been performed various tests to assess hardness of these pieces. One of these tests was for evaluation of coated Ti6Al4V pieces hardness, which reveals up to 3 times increase in cohesion and strength of surface coating. That mentioned coating is very dense and hard and exhibits a high resistance against corrosion in human body. One of existed problems at this method is lack of adjustment thermal for expansion coefficient of titanium/HAP. This inequality against thermal expansion coefficient leads to development of stresses in the piece. Another existed problem during application of these pieces is development of an oxide, with low resistance against corrosion, at interface of titanium and nano-structure coating. To solve the problem, IMC has used a glass coating, compatible with human body. Using this coating at interface of nano-structured coating and titanium the problem of incompatibility of thermal expansion coefficient can be easily removed. Besides, using similar coatings it is possible to prevent development of instable oxide at the interface. Using nono-structural coating of nano-hydroxyapatite lifetime of pieces has been dramatically increased. There are different methods for making abovementioned nano-structured coating, such as plasma aerosol, chemical deposition, and electrophoretic. It seems that electrophoretic coating method has higher compatibilities for different applications.

6.10.4 Using Nanocomposite Coating for Food Packaging The food is offered in different packages, in terms of the food kind and required time. In some food the product should maintain its original and natural shape. One of important points in this field is paying attention on purchasers needs. Consumer is interested to buy fresh food with least of necessary changes during extra processes for increase of food preservation. Since its initiation time, food packaging industry has been subjected to many changes, based on demands of consumers, such as improvement of preservation, food health, and food compatibility with environment. Responding to these needs, there have been various researches in field of packaging quality and use of new intelligent materials.

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Todays, technological achievements in food packaging industry have enabled us to apply intelligent packaging in order to change conditions and data delivery from food situation. This package is called active packaging. One applied method, is use of nano-composite coating to coat plastic layers. Over the time, food releases gasses and moisture in their packages. In some packaging gas and moisture absorbent are used, which leads to healthiness and preservation of the food. Some intelligent coatings show leakage or increase of temperature. Nowadays, there are some attempts to promote packaging quality through coating of food packages using antimicrobial coatings. Till now, use of these coatings has not been of much interest. Regarding available technologies use of these coatings is not economic yet. However, some active companies in this field hope to produces low cost surfaces with decent anti-bacterial property—using nano-coating technology—to coat food products. Another application of nano-coating is production of biosensors, attached to packages in coating form. Biosensors can offer information about quality of packaged materials, their state of healthiness, and etc. [85–91]

6.10.5 Antipollution Materials in Shipping Industry Recently, nano-materials are used in shipping industry as an antipollution and deodorant agent. Nano-particles are remained in network of antipollution coating and release their ions over the time, which causes antipollution traits. Todays, use of Sn-tributyl, once broadly used, is stopped and there is a need for other substitute materials. Lifetime of materials such as copper oxide and other similar materials is not efficient for this application, though nano-materials used for this application seems to be suitable. Polluted Copper oxide or zinc oxide nano-particles can be efficiently replaced at coating formulation of shipping industry. Researchers have shown that these materials exhibit a loner lifetime for antimicrobial applications.

6.10.6 Nano-Coating Use Against SARS Virus Recently, SARS virus is proposed as one of main dangers and there have been so many concerns about its epidemic issue. To deal with this virus, researchers of nano-material field have achieved nano-coating, which is a good weapon to stop this virus activity. These nano-materials include titanium oxide coated with Ag nano-particles. Researches have shown that release of Ag ions during a long time to stop activities of these viruses. These coatings with inappropriate ratio of constituents can cause serious damages to human body. There are some other researches performed by TN Nanovation. GmbH, which produce a nano-powder, called Nanozid, which is added as additive to the color, in order to coat diurnal appliances and tools such as beds and other staffs used in hospitals. These coatings, as well as the similar ones, can be used in food beverage industry.

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Medical and Hygienic Applications

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6.10.7 Application of Ag Nanoparticles as Antibacterial Coating German researchers at Institute for New Materials (INM) have created antibacterial surfaces with Ag nanoparticles. Adding a little amount of these materials to coating make it possible to obtain a big deal of these nano-materials. These materials can release a considerable deal of Ag ions, which is also able to eliminate other pollutions. Different nano-coatings for antipollution goals are available in the market; however produced compound in this research institute is of a unique application variety. The coating can be applied in all surfaces which can potentially be harmful for health. Among these materials one can name hospitals, offices, public places and even houses. These coatings are used on all touchable surfaces, even metallic, plastic, or glass ones. They can also be useful in appliances with public uses [92–100].

6.10.8 Using TiO2 Nano-Particles to Decrease Environmental Contaminations Contaminated materials, especially those caused by oil pollutions, due to transportation process or other similar accidents are among most important sources for environmental contaminations. Another case of pollution is one induced by uncontrolled release of industrial wastewater. There are different methods to decrease pollutions caused by organic materials, e.g. using catalysts for degradation of organic molecules to harmless ones. Among most important catalysts one can name TiO2 particles. These particles are coated on efficient substrates and exposed to ultraviolet light in particular pools. Energy gap of this semi-conductor is about 3 eV. Due to radiation of ultraviolet light, there develops an oxidizing property in electron, produced cavity, and the material. Due to oxidization, they are degraded to some harmless materials, such as H2O, CO2, and other inorganic materials. Some experiments suggest that a wastewater polluted with oil organic materials can be completely degraded after 7 days. To improve efficiency of these nano-particles there also is a use of TiO2 polluted with Fe and Er. Due to pollution energy gap significantly decreases and oxidization process happens under radiation with longer wavelength.

6.11 Electrical and Electronic Applications 6.11.1 Production of Transparent Conductor Coatings by Carbon Nano-Tubes With respect to broad applications of transparent conductor coatings there have been developed different materials and methods to produce these coatings.

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Among most commonplace methods one can name using ITO coatings. Regarding achievements in nano-technology, another method is application of carbonic nano-particles and nano-tubes. In case of broad using these materials for transparent conductor coating, there would be a dramatic growth in application of these coatings. One of famous companies in this field is Eikos which uses carbon nano-tubes to product conductor coatings. For industrial applications, produced layers must be of high transparency, suitable conductivity, low price, decent printability, and flexibility and resistance against environmental agents. Carbon nano-tubes have efficient electrical and thermal conductivity, equal to those of diamonds. Regarding their low weights (1/6 of steel’s) this tubes have considerable hardness. Todays, vaporization of polluted zinc oxide with InSnO2 in vacuum tube is applied as a standard industrial coating. The thin film of this material has efficient light transparency and excellent conductivity. Thin films of ITO are not suitable to coat polymeric surfaces; also conductivity of pieces considerably decreases through its bending and other types of deformation. Metallic nano-particles have fairly high price and very low transparency and then are not convenient for painting. According to mentioned points produced transparence conductive coating with carbon nano-tubes is completely efficient, compared with the other mentioned methods. They only setback of this method is its lower, but enough, conductivity. For different coatings, depending on type and application of the coating, different materials are used. Among these materials one can mention atomic nano-clusters such as quantum particles, inorganic and molecular particles, nano-tubes with quantum wires, and nano-composites. The other important used compounds for coating are: • • • •

Silicide, carbide, nitride, and oxide Boride, selenaride, fluoride, and various types of sulfides Halide, alloys, intermetallic materials, metals And organic polymers

6.11.2 Application of Nano-Coating on Solar Cells Since nano-coating can be applied to improve quality of existed products and produce of new materials, some companies, e.g. Nanogate have used these coatings to promote available systems. This company, which has previously developed sport facilities with nano-coating, attempts to improve some of its other products. One of its study fields is antipollution coating of solar cells. Coatings surface of solar cells with antipollution and snow resistant coatings, a bigger deal of solar energy can be absorbed and solar cells show a significant efficiency [101–107].

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6.11.3 Nano-Coating of Nickel Particles by Oxides Ceramic multilayer capacitor is a piece with broad use in electronic. Using these capacitors in smaller volumes and materials such as BaTiO3, it is possible to achieve higher capacities. For economizing these capacitors there was an effort to alternate Pd/Ag electrodes with those made of metals such as Ni. These particles problem is their oxidation with increase of temperature, happening in concentrated circuits with many capacitors. One solution for this phenomenon is using oxide coating with stable oxides. BaTiO3 is rather desirable for this goal. Unless coating with this material is performed in nano thickness, it is probable dielectric properties of the layer be changes. For this aim, method of TiCl hydrolyzing in butanone is applied. Using this method it is predicted to obtain low price high efficiency capacitor in small volumes.

6.11.4 Using Polarizer Nano-Layers to Produce LCD Monitors Todays, a big share of monitors is devoted to LCDs. For their many advantages, compared with CRT monitors, their application is dramatically increase. These monitors include two groups: Active Matrix and Passive Matrix monitors. About 30% of monitors’ price is for their polarizer part. Nowadays, a type of 200 lm layers is used to produce Active LCDs. Optiva Company is determined to use selforganizer nano-structures which are capable of being coated with (Thin Crystal Film) TCF method. These layers should be of equal cohesion and homogeneity in any surface. In case of using this method there will be monitors with high resolution, broader sight angle, and lower prices.

6.11.5 Produce of Electrically Conductive Transparence Nano-Coatings Eikos and FLEXcom Companies are determined to cooperatively produce electrically conductive transparence layers with carbon nano-tubes. Defense Ministry of the United State has devoted 1 million dollars to development of this technology. The layers can be used in solar cells, flat monitors, organic photonic diodes, intelligent windows, and etc. Compared with traditional ITO (Indium Tin Oxide) methods, this technology is cheaper and more trustable, with less volume and higher efficiency. Regarding present applications for ITO it seems there will be a good market for this new product.

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6.11.6 Increase of Data Storing Capacity with Magnetic Nano-Layers Thanks to nano-layers, researchers have achieved higher density of storing in thin layers. In this method thin magnetic contained nano-layers are used and storing capacity has reached to 12 Gb/in2; however the bigger figures are also achievable. Researchers believe that the nano-particle contained layers, as well as having an enhancement in their efficiency and homogeneity, will completely dominate present products.

6.12 Lubricating and the Other Applications Using nano-technology and Self-Organizing polymers and nano-particles, Nanogate Coating Systems Gmbh has achieved production of very high quality oils for sport applications such as ski. Using this lubricators is easier that previous ones; also they are more compatible with variant climates and increase speed and maneuverability in slopes. The layers are also compatible with different types of snow.

6.13 Ag-Polluted SnO2 Particles Recently, Nanophute Technology group announced produce of Ag-polluted SnO2 particles. These nano-particles diameter is about 30 nm and contain 3 weight percent of Ag. The nanoparticles are currently used in semi-industrial scale. Ag-polluted SnO2 nano-particles are used for many aims such as industrial antimicrobial applications of wood protection, additives for plastic, and for electrically conductive coating.

6.14 Development of Nano-Coating for Surface Lubrication Nano-coated surfaces have shown an acceptable potential for lubrication. For this aim, nano-metric coating of materials such as Al2O3/TiO2, WC/Co, and Cr2O3/ TiO2 have been widely focused. Another applied material is Yttria-stablizedZinconia. Today, many companies and institutes dependent to nano-particles coating and production, decided to apply available technologies for nano-coating. For example, one of widely used methods in coating industry is thermal spray. Use of this method for nano-coating is accompanied with some difficulties. It has been tried to eliminate the problems. Defense centers such as Navy Force of United States have extensively invested in this field of nano-coating research and

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development. For this aim there is a comprehensive cooperation between academic and industrial centers [108–121].

6.15 Size Dependency in Nanocomposite Layers Hard silica/epoxy nanocomposite layers were prepared by spinning method on the surface of AA6082 aluminum alloy with addition of CdTe quantum dots as the second phase in hard nanocomposite coating with different ratios in respect to main phase (silica nanoparticulates). Electrical conductivity tests have been done on the coatings for investigation of the possible enhanced or inverse effects of addition QDs on properties of hard nanocomposite. The effects of some effective factors have been investigated and it has been shown that by adding QD nanoparticulates the electrical conductivity of layers is completely controllable without adverse effect on wear resistance. Figure 6.15 shows the effect of different rotating speeds on the electrical conductivity of obtained layers. Density of different nanoparticulates will decrease significantly by increasing rotating speed. By increasing rotating speed, nanoparticulates will distribute far from each other. By increasing rotating speed, electrical resistivity completely obey a linear relation with respect to rotating speed. Electrical resistivity acts to some what different from previous relations however it remains also completely predictable. Its plot (Fig. 6.16) can be divided into three sections. First section is a completely slight linear increase follows by a severe linear increase in second section and the third section is like the first one. It is worthwhile mentioning here that due to difference in trends which observed in this plot these sets of experiments have been done three times and reproducibility of the results has been proved. It can be said that due to the effective share of QD nanoparticulates in electrical conductivity of obtained layers, the amount of them and bridging among them will cause the rapid increase in second section of the curve. After reaching a special level for presence of QD nanoparticulates and increasing their joining, the electrical conductivity will increase again with a slight slope, as it can be seen in third section of the curve. Fig. 6.15 Effect of rotating speed on electrical resistivity of different silica/epoxy nanocomposite layers [122]

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Fig. 6.16 Effect of SiO2/QD ratio on electrical resistivity of different obtained layers [122]

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Chapter 7

Conclusions

Based on the discussions in different chapters of the book, these conclusions and suggestions were extracted: 1. The scale of nano (in nanotechnology) is usually from 1 to 100 nm (usually less than 10 nm). Application of nanotechnology rooted in basic elements. Each one has specific characteristics which creates remarkable attributes in various fields. Application of Nanoparticle in producing simple medicine and bandages with no need to renewal, diagnosing early cancer cells and analyzing environment pollutants are the examples of nanotechnology. 2. Nanomaterials are the new materials that their basic construction has formed by nanometric scale engineering. On such scale the specific or completely different material indicates the possibility to create more accurate new materials and devices with vast capacities. The scale of nanomaterials should be classified from 1 to 100 nm which means clusters or (Atomic) nuclear pellet with no less than 100 nm, fibers less than 100 nm in diameter and films thickness less than 100 nm. In this range nanomaterials show specific properties which they did not show for bigger dimensions such as micrometers and bigger. This effects which have came from size of nanomaterials are known as ‘‘size effect’’. Size effect can be seen in different properties of nanomaterials. For example, the hard agglomeration of the primary particles was successfully suppressed to obtain TiO2–SiO2 nano-hybrid particles with controlled chemical modification of titanium alkoxide. The quantum size effect of nano-hybrid particles was confirmed by the band gap energy shift, using ultraviolet–visible spectroscopy (UV–vis). 3. Nanoparticles have new qualities in them which it’s specified features are covers size, distribution, shape, phase and etc. Nanoparticles can be constructed from a wide range of materials generally metallic oxide ceramics, metals, silicates and non-oxide ceramics. The scientists hope to use nanoparticles in fabrication of ideal materials with desired mechanical, electrical, magnetic or optical characteristics and progress their capacities. In the case that this prospect occurs, it will affect positive impact on environment and low


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budget achievements such as the influence of nanotechnology on medical systems. High volumes of nanoparticles can be applied in detergents, cosmetics, pesticides, environmental modification, catalysts, lubricants, sealants, adhesives and coatings fields. The coatings obtained by nanoparticles are remarkable category in current and future applications. By these coatings, a profound level of material properties can be used such as scratch resistance, optical features (transparency or adjusted reflection) and also self cleaning properties. Scientists apply the thin films by using different materials with thicknesses less than 100 nm to benefit the size effect in nanostructured films. The basic advantage of thin films as any other coating is that it can transfer material properties to the surface so it is possible to use materials with their basic characteristics. The substrate material and thin film create a system with different enhanced properties. Nanotechnology creates instrument for control of three key parameters for thin films: (a) Chemical composition (crystalline structure in nanometer range), (b) Thickness and (c) surface geometry (including thin films patterns on nanoscale). The most prominent size effects of nanostructured films are as (a) Optical properties (including optical entrapment, transparency, opaque, florescence …) (b) Mechanical properties (including resistance to abrasion and wear, stiffness …) (c) Electronic properties (including potential energy, binding energy …) (d) Chemical properties (including water repellence, anti fog …) (e) Magnetic properties (such as saving data ability) (f) Thermal properties (such as application of thin film in multiple layers as an obstacle to prevent expansion of atomic vibration.) Different processes applied for production of thin nanostructured films. Some of them are: chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal evaporation, magnetron sputtering and laser pulsed deposition. Controlling on nanoscale is an important issue to classify these methods. Sol–gel method is an effective method to produce nanomaterials. The aim of sol–gel method is to perform chemical process in low temperature for producing objects, films, fibers, particles or composites with suitable form and surface. Traditional production processes of ceramics leads to fabrication of materials which has micro-structures with dimensions in 1–100 micrometers. Sol–gel process can change this limitation to dimensions in 1–100 nm and in molecular level. These materials usually have uniform chemical and physical properties. Sol is constant suspension from rigid Colloid ingredient or polymer which placed in one liquid. These particles can be crystalline or amorphous. Sols are diffused chloride particle in ointment on 1–100 nm dimension which due to it’s miniscule size and constant moving they stay floating. Gel is rigid network connected with pores under micrometer dimension and polymer chains with approximately more than one micrometer length. In Colloid gel the network is created by amalgamation of Colloid particle while the polymer gel has a under structure in other words polymer unites creates gel. In most Sol–gel systems creation of gel occurs by covalence combination and gel is

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not reversible which means it can not turn into Sol position again. If the gel has combination other than covalence it can be reverse. The use of acid catalysts or alkali can have impact on hydrolysis and condensation speed so it can alter the final product. By giving their H+ the acids increase the Alco oxide speed in the Hydrolysis reaction. But by reducing the pH the Condensation reaction reduces as well. Adding alkali can either increase or decrease Hydrolysis which depends on types of Alco oxide but the speed of Condensation increase without considering the kind of material and therefore the density of chains will enhance. Gels that created by Acid catalysts have longer time to transform into jelly condition and have less density that face much contraction while Gels that created by catalysts alkali will have less contraction in the process of drying due to it’s high density. Self lay out created under proper condition by molecule and atomic design which occurs in chemical and physical process. It leads to self organization of atoms and molecules in suitable location with proper construction. The layout of thin films (mostly in single layers) and on sub layer surfaces which simplify molecules growth and organization. According to scientists the method is one the rare down to up methods which pass its infancy and considering many coatings before entering the industrial phase the basic science should develop. Also there is a vast confinement in the materials that can be utilized in self layout construction still there is large area to be discovered. The idea of multiple layers with same action adds another aspect to the issue. One of the technical problems insisted by many specialist is molecule activity which becomes polymerize easily under effect of moisture and create blocks on the surface and reduce the film reaction. The fact that chemical formulation and surface pattern of self layout layers are suitable for specific sub layer and leads to progress in specific usage which also become very expensive. The patterned sub layers that direct the de coring process or effect growth of thin films is an obstacle which will be appear by start of industrialized process. Plasma polymerization use source of plasma to exit gas, the eviction has the necessary energy to activate or analysis gas and liquid monomer which usually include vinyl group so the polymerization process starts. The plasma polymerization method profits from AC/RF/MW/DC and pulse methods, the method leads to steady thin polymer films with extensive width connections and heat resistance. By choosing certain kind of monomer and energy density for each monomer the chemical form and structure of thin film can be alter in a wide range. The speed of plasma polymer precipitation is identified by the following parameters: geometry system, primary monomer reaction, the pace of monomer circuit, gas pressure, conjoined frequency, signal and finally the sub layer temperature. Annealing is a heating treatment which has an altered material in it’s structure which leads to change in it’s features like constancy and steadiness. This process includes two phases: slow heating and cooling. The heating treatment usually leads to alteration in crystal structures of atoms. This alteration includes crystal defects removal which ends by basic changes in primary

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features like electric aspect of material. The main processes are gas annealing and vacuum annealing. The vacuum annealing will cause adhesive improvement, solidity stretchiness and electric treatment and material pit. Although annealing is developed process but it still needs growth in mechanism and structure control and morphology. Understanding the effect of annealing on thin film features especially thin polymer films has significant importance. Nowadays, nanotechnology is rapidly developing and promoting the quality of many products. Industries are making efforts to use these created opportunity to enhance their products efficiency and quality, as well as decreasing their products’ price. With no doubt, the future belongs to those companies and industries which invest in this area and extensively apply it. As this is a fairly new emerging technology, it may enable many industries of our country to invest in this field and realization using this technology’s outcomes and advantages. Nano-structured coating is one of most effective broadly used applications of nano-technology. In most cases, nano-structured coatings characteristics have a significant improvement in comparison with traditional ones. Some of these characteristics are: increase of hardness, wear strength, abrasion, decadence, environmental pollutions, and etc. During production, packaging, and finally preparing in markets, all applied products needed form require coating. Coatings are used to enhance resistance against different environmental agents such as various types of corrosion, create new compatibility in surfaces (e.g., in optical layers), increase hardness, and improve some physical features such as magnetic and electrical ones. Practice of coating to deal with corrosion, by its own, is enough to highlight importance of using coatings. In many developed countries damages induced from corrosion is from 3.5 to 5 percent of gross product. Regarding to importance and broad applications of coating, applied methods in this field are permanently developing and the latest technology is applied in this field. Coating is among main parts of surface engineering. Surface engineering is an important technology and competition of various industries depends on sponsoring of this part for them. Besides, as it will be discussed, coating is an economically vital issue. Considering this, in many countries surface engineering share in key industry sectors is defined and some of future questions and different predictions are answered. Discussed topics are similar for many countries. Then, regarding available reports in this field, upcoming developments of surface engineering markets and plans in England are studied. In 1995, England market for surface engineering exceeded 10 billion pounds, where 4.5 billion pounds of these figure was expended on coating for improvement and enhancement of surface properties and resistance against corrosion. This figure has caused a 95.5 billion pounds value effect on amounts of different products. Thermal spraying involves particles quick surface melting and freezing. Thermal spraying nano-composites are of higher abrasive resistance in comparison with micro-coatings. For their high hardness, thermal stability, cosmetic appearance, and chemical neutrality, transitional metal nitride coatings

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are of a great interest among researchers. In normal circumstances, these coatings are produced through chemical vapor deposition (CVD) and physical vapor deposition (PVD), although their nano-structural coatings can be obtained using ion beam. Mentioned nano-coatings are of a great hardness. This increase in hardness of multi-layers and multi-grids (two-layers) are more intense. Spraying of transition metal nitride nano-particles in an amorphous nitride matrix gives a rise to development of grains with dimensions lower than one nanometer, which makes them efficient for uses such as enhancement of abrasive resistance in copper cutting tools. 17. Oxide ceramics such as alumina, chromia, titania, and zirconia, are widely used as surface coating materials for improvement of abrasive resistance, wearing, and cavity. Coatings made from zirconia are used for cylinder head and piston crown at internal combustion engines to improve thermal efficiency, output force, and fuel efficiency. These coatings involve cavities which are characteristics of plasma-sprayed coatings. Nano-crystalline zirconia coatings show lower porosity (8%) in comparison with micro-crystalline coating (12%). Fine co-axis grains are cooled because of homogenous germination of mentioned melt, while columnar grains growth is due to heterogeneous germination in boundaries, where there is a higher cooling gradient. For efficient melting of nano-zirconia source at plasma jet, boundaries are very thin and their interface is fairly narrow. 18. In recent years, hydroxyapatite (HAP) has been introduced as a porous layer on metallic substrates to provide easier in-growth of bony tissues. The size effect on these kinds of coating that were fabricated by microplasma spray has been studied. The metallic substrate from a surgical grade, biocompatible austenitic stainless steel was used. The choice was done in accordance to better corrosion resistance properties, mechanical properties and lower cost of SS316L than those of the conventional Ti6Al4V alloy. Phase pure and flowable HAP granule from conventional wet chemical route were prepared. HAP coatings of thickness near 200 lm were prepared by microplasma spraying on SS316L substrates. It was assumed that higher scatter of data at lower load could be linked to stochastic nature of interaction between the indenter that penetrated a very shallow depth and the flaws that scale with the size/depth of the indentation and which possessed a highly statistical size distribution in the surface and in the close vicinity of sub-surface region. At higher load, it was suggested that due to a larger indentation zone of influence, an averaging out effect of indenter-flaw interaction predominated to affect a reduction in data scatter. At a low load of 10 mN, the coating demonstrated a hardness value of about 5 GPa at a depth of about 170 nm which dropped by 60%, e.g., near 2 GPa at a depth of about 3 microns for a higher load of 1,000 mN. These data recommended the presence of a strong indentation size effect in the nano-hardness behaviour of the coatings. 19. Production of hard coatings with transitional metal nitrides, through IBAD method is an extensive study area. These nitrides include titanium nitride, chromium nitride, vanadium nitride, zirconium nitride, and aluminum nitride.

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Also, their obtained coatings have different mechanical and chemical properties. For example, titanium nitride has a structure similar to that of NaCl, but titanium nitride have more hardness, higher chemical stability, and efficient cohesion to matrix, which makes it most famous coating for cutting tools. Titanium nitride is oxidized at temperatures higher than 500C. This causes development of pure titanium oxide, attached to titanium nitride, which leads to reduce of abrasive resistance of titanium nitride coatings. Due to development of a passive and compacted oxide layer, chromium nitride indicates a higher resistance against oxidization in comparison with chromium oxide, which limits next oxidization. Aluminum nitride is among substances which can be applied at higher temperatures, where nitrogen and aluminum atoms are bonded with strong covalent bonds. Once, this coating is subjected to high temperatures, aluminum move to surface and compose aluminum oxide layer, which is an extremely efficient barrier to prevent later oxidization reactions. 20. At industrial applications there is an increasing demand for coatings having higher resistance against oxidation, higher hardness, and longer life than those of single layer coatings. To supply industrial needs for development of improved coatings, there has been many efforts to design and produce super consolidated coatings. Some researchers proposed notion of designing solids with strong coatings, using two alternative layers with high and low elastic constants. Each layer’s thickness must be in nano range and there must be no dislocation source between layers. If dislocations could be created in the zone of materials with lower modulus, they must be overcome to the noticeable stress diffused from the phase with higher modulus, before creep phenomenon (along the layers). Thus they must prohibit the creep along the layers. Such multilayer coatings are called super-lattice and their two layers can be metallic, carbide, and nitride. A multilayer includes different piled materials on atomic scale. During multilayer coatings designing both related structural and constitutional factors must be considered. These factors are: Grain size, layers individual thickness, combination module, the number materials interfaces (assuming the last layer is resistant against abrasion). 21. Applied ceramic micro- and nanoparticles mostly include aluminum oxide, carbide, chromium oxide, titanium oxide, molybdenum oxide, tungsten carbide, and etc. Besides, polymeric particles such as polyethylene and polytetrafluoroethylene are used to decrease friction ratio and achieve a nonstick composite surface. According to performed studies, fine-grained Ni–SiC composite has a smoother surface and there is stronger bond between SiC and Ni. Once SiC particles are bigger than 0.1 lm, usually there develops an oxide layer on SiC particles which have a weak bond with nickel matrix, which leads to development of cavities and cracks in grains boundary. On the other hand, interface of a very fine SiC and mixed Ni is free of any defect. In the same volumetric fraction very fine particles are more abundant, which prevent grains growth at higher temperatures. However, investigations show a decrease in particles size leads to decrease of simultaneous deposition of the particles.

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22. These coatings have typical structure of nc–MnN/a–Si3N4, where c and n are, respectively, crystalline and amorphous phases and Mn stands for transitional metals such as Ti, W, V, and Zr. In nano-composite coatings, transitional metal-nitride phase is hard enough to bear exerted load while, on the other hand, amorphous nitride provides flexibility of the structure. Based on computer simulations plastic deformation in nano-crystalline materials, where particle size is less than 10 nm, can be corresponded with particle boundary. Here, grains boundaries slip—which is controlled by diffusion of grain boundary—may be responsible for plastic deformation in nano-crystalline materials. Slip is caused by atomic movements and stress induced from 3D free migration; in the other words, once nano-crystalline materials are extremely tiny indicate soft behaviors. Hence, an increase of hardness is required locking in grains slip boundaries. Indeed, this is the reason for increase of hardness in nc–MnN/a–Si3N4 system, for nano-composite coatings of nc–TiN/a–Si3N4 and nc–W2N/a–Si3N4, where particles’ size decreases up to 4 nm. It was declared that these developed nano-composite coatings by CVD method, will reach to diamond hardness (70–80 MPa), where grain size is about 2 nm. 23. Al based composites with aluminum borate whiskers—which are created using high pressure casting—indicate a comparable strength and modulus with those of aluminum composites with SiC or silicon nitride whiskers. However, they have a lower thermal expansion and higher abrasive resistance. Besides, another priority of these whiskers is their very low costs in comparison with those of SiC—1/20 of SiC whiskers. Hence, aluminum borate whisker is of great qualifications for expansion of aluminum based composite applications. Also, based on existed theoretical and empirical studies, it was revealed that aluminum borate whisker is unstable in Al alloys, and the reaction occurs in their interface. To control reaction in interface, nitriding process of these whiskers, based on thermodynamic calculations, was suggested. To reach a continuous and homogenous phase nitrided nano-coating must be used. Phase analysis implies presence of BN and alumina on nitrided surface. Nitrided nano-coating with thickness of 40–60 nm isolates the whisker from surrounding matrix and aluminum/coating interface will be free reaction productions. 24. Titanium oxide is of abundant usage in gas sensors and photo-catalysts. For example, it is used in gas sensors to detect explosion released gases such as natural gas and hydrogen. Due to their crystalline structure, surface area, their cavity types (in terms of opening and closure), and their size distribution, photo-catalysts are used for segregation of air pollutants and organic contaminator in waters. It has been currently shown that TiO2 nano-coatings are of a greater sensation compared with that of micro-structure ones. The easiest and simplest way to achieve a nano-coating with thermal spray method is using raw materials with nano-size. However, directly adding such nanopowders during spray process is difficult. Moreover, plasma or gas flame leads to melting and removing its initial structure. Therefore, it was achieved that better characteristics through simultaneous spray of the other substance which

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prevents development of TiO2 powder in the furnace. Thus particles of metallic Al, which are of a lower temperature and higher reactivity in comparison with TiO2, are added to Al/TiO2 composite powders to enhance spraying efficiency. Al particles have significant role to create homogenous sediment. They lead to reach to unique characteristics of nano-structures, maintaining nanometric structure during spraying process. 25. It is found that development of nano-size dual metallic phases in alumina can noticeably enhance its thermal and mechanical characteristics. Metallic phase exhibits higher thermal conductivity and resistance against thermal shock in comparison with alumina ceramic. Also, metallic phase can increase ceramic’s ductility as metallic particles deform plastically. In performed operations on metallic/alumina nano-composites, metals such as Cr, Ni, Fe, W, titanium carbide were used, which leads to 2–3 times increase of ductility. Second phase has been added through mechanical combining of alumina and metallic powders, and their under-pressure sintering of graphite crucibles. The main problem of mechanical combination method is to find out how to reach to second phase’s fine dispersion and favorite thermal expansion difference between alumina and metal. Thus, a chemical coating method was used for preparation of ceramic/ metallic nano-composites, which has variant advantages compared with mechanical combination method. The obtained powder in this method is more homogenous and of a higher cohesion between metal and ceramic. Preparing nano-composite coating of Al2O3/Al wet chemical coating method was applied. Aluminum nano-particles are solved in appropriate solution, then Al2O3 is added, and finally considered composite is deposited in the solution. Through occurred reactions, there develops a thick Al(OH)3 layer on aluminum particles’ surface which, after calcification, is converted to alpha alumina nano-particles (with grain sizes of 10–20 nm) and distributed Al particles. The advantage of Al2O3/Al composite is development of a thin transition layer between Al and Al2O3, which is able to improve their bond. 26. Although tri-valence chromium ions, and particularly hexa-valence ones, are very poisonous, chromium plated coatings are widely used to enhance surface abrasive resistance. Another problem of plated chromium coatings is their decrease in thermal mobility with increase of temperature, so hardness and abrasive resistance of plated layers reduces. Hereabout there have been many studies in surface engineering to find a suitable substitute for this coating, leading to promising results. First choice is tungsten carbide or tungstencarbide/cobalt. As it previously mentioned nano-crystalline materials show unusual chemical, physical, and mechanical properties, in comparison with amorphous ones. This is caused due to nano-crystalline materials’ noticeable decrease in grain size and volumetric ratio of grain’s boundary, and triple connections. Here, a decrease in tungsten carbide grain size up to 70 nm in tungsten-carbide/Co composite leads to a two-time increase in abrasive resistant. 27. Electrochemistry is an advanced technology in production of nano-particles. Before studying use of different electrochemical methods for nano-coatings

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production, first it should be defined that how colloidal chemical state leads to creation of nano-particles. This leads to better understanding of electrochemistry concept and its effect on nano-coatings. In colloid science, nanoparticles mostly obtained from surfactant contained saturated solutions. The first rule of organic ligands is inactivation of surface and development in suspending state. This preparation technique of nano-particles is called engaged sedimentation. Similar methods for development of nano-particles on conductive matrix have dramatically advanced in electrochemistry. It has been proved that adding surface intermediates can lead to deposition of nano-particles during plating. Additives prevent particles growth and maintain particles’ size to be approximately constant. A more common method is creating changes in plating parameters, e.g., voltage or current. However, there is another two-step method including a high extra voltage in a short time for germination of metallic particles on surface and then slow growth of particles in a lower extra voltage. Low extra voltage results in minimum change (about 7%) in particle size. this stops diffusion of mixed layers and decrease in growth rate. particles shape produced by engaged electro-deposition depends on applied matrix and extra voltage. Metals such as Au, Ag, Ni, and polymeric nano-particles with spherical geometry on graphite matrixes, are created by this method. 28. The results of hardness measurement for plated Ni–P whiskers at room temperature were reported. Same results were obtained for Pd and Cu produced from neutral gas evaporation method. An increase in grain size is accompanied with considerable decrease of hardness in range of lower than 20 nm. These observed reductions of hardness are not corresponded with Hall–Petch behavior. Recently, performed investigations on tensional strength of Ni nano-crystal at room temperature have shown a behavior similar to that of determined with hardness. It is found that grain boundary diffusion in creep phenomenon is not an efficient factor to determine mechanical behavior of Pd and nano-crystalline Cu at room temperature. Start point for hardness decrease, i.e., deviation from Hall–Petch behavior, occurs once triple lines occupy a high ratio of sample volume. This phenomenon is generally in accordance with softening effect of triple lines. Through electrochemical grinding of wires to sizes lower than grains average size, triple connections can be displaced in fine structures. At all cases this transition, increase of strength, and decrease of malleability is shown from co-axis state to columnar one. 29. In general, corrosion resistance of nano-crystalline materials in aqueous solutions is of great importance in an extensive area for future applications. There performed few studies in this area. Both improved and disadvantageous results for development of nano-crystalline in corrosion process have been recorded for corrosion behavior of nano-crystalline produced by amorphous materials crystallization. Obtained results are highly influenced by weak characteristics of crystallized amorphous materials. In the other words, during last few years, there have been considerable advancements in perception of fine structures effect on corrosion properties for materials produced by

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electrodeposition process. Nano-crystalline specimens show the same activeinactive-trance-passive behavior; and differences are between passive current density and open circuit potential. Nano-crystalline specimens indicate more intense current in passive zone, implying higher corrosion rate. This current density is mostly because of more grain boundaries and triple connections in nano-crystalline samples, creating electrochemically active spots. This density difference in current density decreases for higher potentials. At this potential difference, final dissolve rate overcome to structurally control dissolve rate, existed at low potentials. Another obvious difference in polarization result, in varying potential of nano-crystalline and multi-crystalline samples, is open circuit potential. It seems that positive transition of open circuit potential for nano-crystalline samples is due to catalyst properties of hydrogen releasing reaction. 30. Thermal stability of nano-crystals is of a great importance in high temperature applications. For electro-deposited nano-crystals thermal stability is examined through TEM and an indirect method, involving determination of thermal stability using harness measurements as a function of annealing time. For synthetic growth of grains there are some preventing factors for grain boundary movements leading to their thermal stability. There is a slowing dual force in nano-crystals due to triple connections. It can be easily shown that grain growth for fined multi-crystal materials is controlled by inherent movement of triple connections. For thermal stability of nano-structures, extra distributions of triple connections lead to preferred dissolve in these spots. Such a dissolve was observed in nano-crystals in triple connections using TEM method. Ni stability with grain sizes of 10 and 20 nm was investigated, using TEM. Degradation temperature for these materials is 353 K. This lack of stability is due to unusual germination after annealing. 31. As it is expected from Hall-Petch assumptions, there are different practical applications for nano-crystals based on existed criteria for development of resistant coatings. Preferential mechanical properties of electro-deposited nano-structures are among their most important industrial applications. Electroplating process is applied for in situ maintenance of nuclear steam generator tubes. This process is successfully applied in aqueous reactors in US and Canada and registered as a standard method for repairing pressure tube. Through this application, Ni with grain size of 100 nm, is created on interior walls of steam generator tubes to perform a complete structural maintenance in places where primary homogeneity of tube structure is mitigated. High strength and convenient malleability of these 100 nm grains result in application of a thin plate (0.5–1 mm) which minimizes fluid current and heat transition in steam generator. Recent geometrical models and empirical achievements have shown that nano-structural materials can have a high resistance against creep and inter-granular cracking. 32. The various types of nanostructured materials enjoy two joint and common specifications: (1) Atomic areas (granules or phases) have been restricted in space smaller than micron scale. (2) High volume fraction of atoms exists in

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interface areas. Nanostructured materials can be observed in dimensionless, one dimensional, two dimensional and three dimensional forms. Atom nucleate or atom sprouts have been recognized as dimensionless nanostructures. Nanorods and similar nanostructures such as nano-whiskers can be considered as one dimensional nanostructures while separate or isolated layers or multi-layers, grown in one or two directions, are known as two-dimensional nanostructures. In addition, nano-crystalline or nano-phases are considered as three-dimensional (3D) nanostructures and regarded as highly-used nanostructures in metal systems. The significance of nanometer scale is originated from the fact that some of properties undertake remarkable amounts at this dimension range. Since properties of a solid material is first controlled by density and atom co-ordination number, nano feature of sizes of granules in crystalline materials, despite chemical composition in nano-crystalline and microcrystalline state, causes that properties like specific heat, thermal capacity, thermal extension coefficient, magnetic properties and mechanical properties in nano-crystalline state are considered completely different from microcrystalline state. 33. Materials with nanometer (nano-crystalline) structure are away from equilibrium state. The factors, which affect on additional free energy of these materials, have been recognized. High density of interface and sever changes of chemical composition and stress gradient are the cases which can be effective in abnormal thermodynamic changes of these materials. The origin of these impacts has been recognized theoretically and experimental tests have proven these cases as well. The nano-crystalline structure can be considered as compound form, comprised of small crystalline areas with long-range order and with various crystallography orientations and inter-connected lattice of inter-grain areas, lacking any full crystalline order. 34. With regard to phase balance in microscopic structure, energy interface does not play a significant role and only Gibbs free energy of whole object is important. With regard to materials with microscopic structure, thermodynamics of interface can be ignored. Of course, some common cases are controlled and monitored through interface energy. For example, it can be referred to the phenomenon of growth of grain. For nanostructured materials, conducting thermodynamics studies merely on whole object is not sufficient due to very high density of interface. And following interface thermodynamics should be taken into consideration. 35. Free energy of a phase in a multi-crystalline material has higher amount than materials with mono-crystalline state. Such additional amount is related to the existence of inner-structure imperfections. High amount of grain boundary constitutes major part of these imperfections. The amount of grain boundary can be measured with regard to a nanostructured material. The total area, which a grain boundary is occupied, is in compatible with d-1 in which d is diameter of granule. The contact area of grain boundary has separate structure that can affect on degree of free energy. Total length of these contact areas is proportional to d-2. Contact area of these connections with grain vertices affect on free energy. The population of these areas is proportional to d-3.

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36. Coating has variant applications, such as protecting coatings against corrosion, scratch, stain, denting, bending, foliation, or making water proof and acid resistant coatings. Coating is used in every aspect of life from coating on glass, wood, plastic, paper, fibers, or similar foliations, to coating in heavy industries and special applications (e.g., military applications). Also, coatings have a key role as protector against climatic conditions such as rain, snow, solar ultraviolet beam, or chemical pollutions. Regarding abundant applications of coating in variant tools, researchers have always been trying to promote coating quality and lower its costs in academy and industry. Todays, advancements in technology have significantly increased coating quality. Recently, Coating through nano-materials has reached to practical scale and some famous supplier companies of industrial panels and automobiles use this coating in their products. 37. Active points are probably considered as the most significant obstacle in nucleation kinetics. Regarding the unique properties of a specific electrochemical system, active points are appeared or disappeared completely different on surface of electrode. The main reason of the issue is related to different chemical and electrochemical reactions like adsorption and separation of organic and non-organic ions or molecules, direct oxidation or reduction of electrode surface which is occurred previously or concurrent with process of formation of nucleation on electrode surface. That is to say that assumption of a neutral electrode is considered as a completely optimistic approach. A theoretical model has been proposed for formation of nucleus thanks to time-dependency of number of active points on electrode surface. 38. Non-dimensional distribution of constant nucleation rate (critical nucleus) around a semi-spherical nucleus inside the area was observed which overpotential and concentration have been decreased. If various semi-spherical nucleuses are formed on electrode surface, the local areas around nucleuses, which nucleation rate has been decreased in there, are developed and are overlapped each other gradually. That is to say that a general theoretical model, indicating overall nucleation rate, should be considered with taking into account effects of interaction of nucleuses on each other. 39. Existing ions in solution are primarily encircled by other species such as molecules, ions, especially water molecules. When these ions are encircled by water molecules, geometrical structure of water molecules plays a key role in subsequent processes. The 104.45 angle formed between two hydrogenoxygen bonds, existing in water molecule, is as a result of strong bipolar forces inside it. Electrostatic gravity between metal cations with positive charge and water molecules results in hydration of existing ions or in other words, covering metal ions by water molecules. The hydration process metal ions are shown in which for comparison, display of an anion is also shown. Generally speaking, taking two major points into consideration is of paramount significance. 40. There are numerous separate stages during transfer of metal ion from inside of whole solution to creation of metal lattice on cathode surface. Each of these

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stages can be a controller of velocity. The first stage at this route is transfer or movement of the desired ions from whole solution to the vicinity of cathode. Metallic ions, which exist either in hydrated or complex forms, first move towards cathode in solution. This stage is usually carried out by convection flows inside solution. Migration of ions under effect of potential gradient between anode and cathode can play a key role in ionic transfer, but comparatively play a partial and meager role in material transfer resulted from convection inside the solution. Transferred ions reach external part of penetration layer and pass from penetration layer as a result of existing concentration gradient. At this stage, solution convection does not play a key role in transfer of ions. When these ions pass from diffusion layer and electrical dual layer (Helmholtz Layer), they reach to cathode surface and are turned into pure metal ions. Pure metal ions are turned into adatoms through combining with electrons on cathode surface, adatoms start moving on surface with the aim of being adsorbed in active areas as well as forming a very strong chemical bond. At this stage, metal atoms are operated and distributed appropriately, aimed at creating a crystal lattice. This stage includes nucleation and coating growth stages. Controlling stages of reactions velocity should be reduced or omitted in order to increase velocity of reactions on cathode surface. When velocity controlling stage is under control of material transfer, increase of over-potential will accelerate reaction. 41. Pulse Electrochemical Deposition process has been paid more attention in recent years. Some researchers have reported reduction of porosity value of gold cover by pulse current. The theory related to pulse plating (electroplating) is simple. The cathode layer is kept rich with metal ions and impurities are decreased as much as possible. During on-pulse time (Ton), when current is connected, metal ions are reduced on cathode. When current is disconnected (off-pulse time), any type of concentration gradient, appeared during on-pulse time (Ton), will be eradicated and there is possibility of separation of gas bubbles and impurities which have been adsorbed on cathode and this process is repeated once again. In pulsed electrochemical deposition method, related parameters can be changed in wide scale independently. In electrochemical deposition method with DC current, current density is the only changeable parameter. Consequently, creation of various situations faces more restriction. There are impurities which have been adsorbed on cathode and then this process is repeated once again. In electrochemical deposition method with pulse current, related parameters is changeable in a wide range independently. In electrochemical deposition method with direct current, current density is the sole changeable parameter and consequently, creation of various situations faces more restriction. In terms of production methods, it can be stated that electrochemical coating method is regarded as one of the oldest methods for imposing nanostructured coatings. 42. As a general principle, it should be said that plastic deformation of a nanostructure coating strictly depends on its grain size. Various tests in environment temperature have shown that with decreasing grain size beyond certain

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limit, Hall–Petch equation doesn’t apply and contrary to expectation, material will become softer. It has been observed that this affair occurs when amount of triple junctions shows remarkable growth. Some other mechanical properties of these coatings can be justified with studying behavior of this material and considering the triple junctions. Reduction of granule size has remarkable effect on improvement of resistance against erosion in nanostructure and composite materials. Of course, most research activities have been made on double-phase and/or composite nanostructures and not appropriate research activities have been made with relation to pure nano-crystalline metals. Research activities made on frictional behavior of multilayer nano-coats show that their behaviors have been more affected with chemical composition of materials and their nano-microstructure has not any effect on their frictional behavior. Erosion behavior, under high stresses, is severely affected with chemical composition of materials and their nano-microstructure has not any remarkable affect on their frictional behavior. Study of erosion behavior under low and average loading condition in nano-materials shows that because of higher stiffness, erosion resistance of these materials is more than that of materials with large size. With regard to creep in nanostructured materials, since grain boundary’s slip is regarded as one of the mechanisms of creep, creep rate in nanostructured materials is more than microcrystal materials. 43. In spite of this fact that corrosive properties of nanostructured coating is of paramount significance, very few research activities have been made in this regard. Nano-crystalline materials provide new approach for improvement of properties, without any change in chemical composition. Small size of grain and high volumetric fraction of grain boundary can cause different corrosion behavior with respect to multi-crystalline materials. Effect of reduction of grain size on increase of resistance against local corrosion of stainless steel has been reported. 44. During recent years, successful coexistence of very minute particles, like metallic powders, silicon carbide, oxides, diamonds and polymers, has been reported with metallic or alloy field and their accordance structures and properties have been studied by various researchers. Not only structure and properties of nanocomposite coatings depends on density, size, distribution and nature of improved particles nature, but also it depends on type of used solution, current density plating parameters, temperature, and degree of pH, etc. Nanocomposite plating includes revival of metallic ions from suspension electrolyte and insoluble powders like oxides (SiO2, TiO2, Al2O3), carbides (SiC), nitrides (Si3N4), polymers (PTFE, Polytetrafluoroethylene). This activity will result in entering very minute particle to the growing metallic or alloy substrate. High superficial energy and inclination of nanoparticle to agglomeration in high conductor metallic electrolyte will bar congruousness of distribution of particles. 45. Due to the application of nickel as protective coating, nickel nanocomposite coatings, containing ceramic nanoparticle with high hardness and resistance to erosion, have been taken into consideration seriously. Hardness and resistance

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of deposited coatings of nickel nanocomposite strictly depends on degree of ceramic particles extant in nickel background. Al2O3, WC, MoS2, TiO2, Cr2O3, ZrO2 and diamond are of ceramic powders which have been used in manufacturing of nanocomposites with nickel background. Up to the present time, more research activities have been made on nanocomposite coatings of Ni/Al2O3 and Ni/SiC. 46. Participation of alumina nanoparticle will improve hardness and resistance of coating against friction remarkably. Final tension will boost traction and tension of submit in comparison with nickel, depending on degree of participation of particles in coating, partly twofold or more than two fold. Resistance against friction is boosted upon increase of density of alumina in coating. At any rate, ductility of nickel-alumina nanocomposite coatings is less than sole nickel. Annealing of nanocomposite in high temperature will increase ductility but will reduce their strength. Resistant against corrosion of nickel is improved with alumina nanoparticle. Increase of hardness is made based on preventing grains from growing and according to Hall-Petch law. 47. In systems with simple counting ions, zeta potential is regarded as a criterion for gradient of electrical potential, when surface potential is fixed. The pH, which its zeta potential is equal to zero, is called Iso Electric Point. To enrich loading of hydrated surface by OH– and H3O+, increase or decrease of pH from Iso Electric Point will first boost absolute fraction of zeta potential. Isoelectric point for alumina is pH = 9 i.e., alumina particles will have negative superficial load in the electrolyte with pH more than 9. In the same direction, alumina particles will have positive superficial load at the pH with less than 9. Consequently, alumina nanoparticle can be seeped simultaneously with nickel for formation of composite layers without needing to specific additives. Because, pH of all composite plating solutions of nickel is smaller than IEP for alumina and alumina particles at these baths have positive superficial load. But, alumina nanoparticles are agglomerated easily in electrochemical electrolyte due to their high superficial energy and this activity will cause weak mechanical properties in nanocomposite coatings, for, it prevents particles from being distributed equally. After carrying out operations, physical distribution of nanoparticles at electrolyte solution by mixing and ultrasonic operation and/or through distribution of chemical dispersants in electrolyte is a mandatory activity. The more volumetric percentage of alumina nanoparticles can be boosted in Ni/Al2O3 nanocomposite coating the more provided hardness of nanocomposite coat can be expected. Hence, this activity requires getting familiarity with effective parameters in simultaneous electrical deposition process of alumina and nickel. 48. Some researchers analyzed ultra-thin elastic films of nano-scale thickness with an arbitrary geometry and edge boundary conditions. An analytical model is proposed to study the size-dependent mechanical response of the film based on continuum surface elasticity. By using the transfer-matrix method along with an asymptotic expansion technique of small parameter, closed-form solutions for the mechanical field in the film is presented in terms of the displacements

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on the mid-plane. The asymptotic expansion terminates after a few terms and exact solutions are obtained. The mid-plane displacements are governed by three two-dimensional equations, and the associated edge boundary conditions can be prescribed on average. Solving the two-dimensional boundary value problem yields the three-dimensional response of the film. The solution is exact throughout the interior of the film with the exception of a thin boundary layer having an order of thickness as the film in accordance with the Saint– Venant’s principle. 49. The effect of the material micro-structural interfaces increases as the surfaceto-volume ratio increases. Researchers showed that interfacial effects have a profound impact on the scale-dependent yield strength and strain hardening of micro/nano-systems even under uniform stressing. This is achieved by adopting a higher-order gradient-dependent plasticity theory that enforces microscopic boundary conditions at interfaces and free surfaces. Those nonstandard boundary conditions relate a micro-traction stress to the interfacial energy at the interface. In addition to the non-local yield condition for the material’s bulk, a microscopic yield condition for the interface is presented, which determines the stress at which the interface begins to deform plastically and harden. Hence, two material length scales are incorporated: one for the bulk and the other for the interface. Different expressions for the interfacial energy are investigated. The effect of the interfacial yield strength and interfacial hardening are studied by analytically solving a one-dimensional Hall-Petch-type size effect problem. It is found that when assuming compliant interfaces the interface properties control both the material’s global yield strength and rates of strain hardening such that the interfacial strength controls the global yield strength whereas the interfacial hardening controls both the global yield strength and strain hardening rates. On the other hand, when assuming a stiff interface, the bulk length scale controls both the global yield strength and strain hardening rates. Moreover, it is found that in order to correctly predict the increase in the yield strength with decreasing size, the interfacial length scale should scale the magnitude of both the interfacial yield strength and interfacial hardening. 50. Tensile and fatigue tests of ultra-thin copper films were conducted using a micro-force testing system by some researchers. Fatigue strength as a function of film thickness was measured under the constant total strain range control at a frequency of 10 Hz. The experimental results exhibit that both yield strength and fatigue lifetime are dependent on film thickness. Fatigue damage behavior in the 100 nm thick Cu films with nanometer-sized grains is different from that in the micrometer-thick copper films with large grains observed before. Fatigue of thin metal films is a key issue for the long-term service of microdevices. Previous investigations of fatigue of thin metal foils show a tendency of the improved fatigue strength with decreasing foil thickness. Especially, several studies on thin metal films, such as thin Ag films and Cu films, have demonstrated that fatigue properties of these metal films are significantly different from those of the bulk materials. When the film thickness approaches

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200 nm, interface-induced fatigue damage becomes more prevalent. In these studies, the film thickness and grain size are usually ranged from several micrometers to sub-micrometers. However, little is known about fatigue damage and strength of metal films with nanometer-scale thickness and grain size. 51. Researchers produced nano-composite materials made of steel alloy, with very few molecules in their particles which can be used in buildings to increase strength and other similar cases. There existed a common physical mechanism which contributes to control alloy hardness. Hardness increase causes malleability, foliating, and tabularization decrease. Using nano-composites in these alloys it is possible to decrease these shortages to a high extent. This is caused by an increase in controlling mechanisms for each material property in nano scales. This method involves creating an alloy in frozen glass structure. Grinding obtained product make it possible to produce a particular powder which make bonds with other materials and create a very dense coating during heating process. Under this conditions particles diameter is about 50 nm. The process can generate very strong bonds in substances. Available steels are of strength about 10 percent of those calculated through theoretical methods, once using this method enables us to reach strengths about 40 to 45 percent of calculated one. This method also contributes to obtain better corrosion resistant properties. 52. Nowadays metal silicides are an important component of an electronic part. The Self-aligned Silicide (SALIIDE) process leads to the formation of a uniform type of metal silicides formed simultaneously in the regions of gate, source, and drain; and it is so successful due to its reliability and simplicity. Therefore, application of metal silicides has been promoted in electronics industry. The most common silicides used in electronic parts are PtSi, TiSi2, and CoSi2, although using C54–TiSi2 (phase with less special resistance) and CoSi2 in smaller parts (less than 0.2 and 0.04 lm respectively) is so difficult. In future parts we must use silicides layers with very low thickness. The silicides layer’s thickness was about 20 nm in 2005, and it is expected that it will be reached to 5.5 nm in the year 2015. Recently NiSi has attracted much attention and the latest progresses reference to the vast efforts in order to the application of NiSi in MOS parts in future technologies. NiSi is of great importance because of its low special resistance, low contact resistance, potential to form in low thicknesses, and its consistency. Therefore, it is inevitable to study its specifications. 53. Some researchers described the control of the quantum size effect by controlling the coating layer thickness in TiO2–SiO2 core-shell hybrid particles obtained by the liquid phase deposition (LPD) method. The quantum size effect of the obtained nano-hybrid particles was estimated by the band gap energy shift, using ultraviolet-visible spectroscopy (UV–vis). As a result, we successfully controlled the degree of the quantum size effect by controlling the coating layer thickness in core-shell TiO2–SiO2 hybrid particles. They concluded the successful preparation of core-shell type TiO2–SiO2 hybrid particles by LPD.

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The coating layer thickness and the crystallite size were controlled by controlling the [Ti]/[Si]. In the case of the [Ti]/[Si] ratio of over 0.1, silica particles were completely coated by titania. The degree of the blue shift of the band gap energy by the quantum size effect for the obtained particles was approximately 0.13 eV larger than that of the pure titania, because of the existence of the Ti– O–Si bond. If the Ti–O–Si bond effect was removed, the blue shift of the band gap energy for core-shell type TiO2–SiO2 particles was nearly the same value as that of the reported values for the pure titania. From these results, the quantum size effect was successfully controlled by controlling the coating layer thickness of core-shell type TiO2–SiO2 hybrid particles. 54. The formation of self-aligned silicide happens after the formation of source/ drain. Therefore, the temperature formation of silicide must be low enough to keep the joint thin in under 100 nm CMOS technologies. NiSi has the lowest formation temperature among all the silicides, and the thermal stability interval is 350–750C. For TiSi2, the thermal stability interval is very limited. Contact resistance of TiSi2 is high before 800C, due to the presence of C49 crystal phase. Between 850 and 950C it decreases due to the formation of phase with formation of C54. Above the 950C, due to the accumulation of surface resistance, it increases. In the other words, the contact resistance of NiSi gets stable and minimized between 350 and 750C. The increase of resistance above 750C is due to the fusion of phase from NiSi to NiSi2. Therefore, NiSi is thermally suitable for the technologies under 100 nm. In the other words, it must be kept in mind that the temperatures of processes after the formation of silicide must not exceed 750C. 55. Some researchers prepared non-equilibrium nanocrystalline xSnO2–(1 - x) a-Fe2O3 powders by using the mechanical alloying technique. The thick film screen printing technology is then employed to fabricate these ethanol gas sensors. The gas sensing characteristics are also measured. New structural model for these non-equilibrium nanocrystalline xSnO2–(1- x)a-Fe2O3 materials explains both the lattice expansion of these high energy mechanically alloyed powders as well as the charge neutrality in terms of additional oxygen dangling bonds at the nano-sized particle surfaces. It is those enormous oxygen-dangling bonds at the particle surfaces that give rise to the high gas sensitivity. The sensors are found to be 32.5 times more selective to the ethanol gas compared to CO and H2 gases. They have illustrated a promising method of using mechanical alloying in the preparation of nano-sized a-Fe2O3 materials for gas sensing applications. In particular, the sensor has shown good ethanol gas sensitivity values of as high as 845 at 1,000 ppm in air. The sensor is selective to ethanol gas over carbon monoxide and hydrogen gases. The gas sensitivity is also found to be very stable. These excellent experimental results can be explained by the fact that such mechanical alloying materials have nano-sized particle grains and exhibit enormous oxygen dangling bonds at their particle surfaces. 56. Some researchers fabricated cadmium selenide nano-crystallites onto amorphous glass substrate from an aqueous alkaline medium, using chemical bath

7 Conclusions

243

deposition method at room temperature. The samples are annealed in air for 4 h at various temperatures and characterized by structural, optical and electrical properties. After annealing metastable nanocrystalline cubic phase transformed into stable polycrystalline hexagonal phase. Depending upon temperature, decease upto 0.6 eV and 103 X cm were observed in the Eg, and electrical resistivity, respectively. These changes have been attributed to the increase in the grain size of the CdSe crystallites. They report on the room temperature chemical deposition of CdSe thin films from an aqueous alkaline medium. In order to get good quality CdSe thin films, the preparative parameters such as concentration of cadmium, deposition time and pH were optimized. Freshly deposited thin films may contain many defects such as voids, pinholes, etc. Annealing of thin films reduces the defects and increases crystallite size along with recrystallization process. 57. Some researchers prepared nano-copper films by DC magnetron sputtering. Their reflectivity and transmittivity to electromagnetic wave in infrared region were measured with Fourier Transformation Infrared Spectrometer (FTIR), by which their complex optical constant and permittivity were obtained. The results show that the complex optical constant and permittivity of nano-copper films depend upon the film thickness. This dependence is correlated with microstructure transition during the film growth. They measured the reflectivity and transmittivity of nano-copper films to electromagnetic wave in infrared region, by which their complex optical constant and permittivity, both the real and the imaginary parts are calculated. All these parameters are essentially dependent on the film thickness. This dependence should be considered in application of nano-copper film. The dependence of electromagnetic parameters on the characteristic size provides new possibilities for designing high performance electromagnetic functional materials and devices. The evolution of the film microstructure plays an important role in the size effect of nano-copper film on complex permittivity. 58. Studies on ultraviolet stimulated emission and lasing observed at room temperature from nano-structured ZnO thin films have been reviewed. The nano-structured ZnO thin films were grown on sapphire substrates using Laser-Molecular-Beam-Epitaxy (L-MBE). The thin film was consisted of regularly arrayed hexagonal nano-crystallite columns, whose facets form natural micro-cavities. These nano-crystallites confine the centre-of-mass motion of excitons. As a result of the quantum size effect, the oscillation strength of the excitons is largely enhanced, which is favored to the radiate recombination of exciton at room temperature. At a moderate pumping intensity, the room temperature stimulated emission is associated with an exciton-exciton collision process. At higher pumping density, the excitons are dissociated, and the ultraviolet stimulated emission is dominated by an electron-hole plasma recombination process. Because of the large enhancement of oscillator strength of the excitons, the optical gain of the stimulated emission measured at room temperature reaches as high as 320 cm-1, which is an order higher than that observed in bulk ZnO crystals. In comparison with the

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7 Conclusions

electron-hole plasma stimulated emission in most of commercial semiconductor lasers, the excitonic stimulated emission can be realized at relatively low external pumping density. The observation of excitonic lasing effect at room temperature might be valuable in realization of practical ultraviolet semiconductor laser devices. 59. Some researchers studied the size dependence of electron-lattice energy exchange in nanoparticles. Both surface and bulk energy exchange parameters are examined and it is demonstrated that the bulk energy exchange has nonmonotonic oscillations versus size of the particles. It has been found that the amplitude of such oscillations increases with decreasing a particle size until the critical size reaches Lc. These bulk interaction related oscillations disappear for the particles less than Lc, and only the surface energy exchange remains as the energy flow between electrons and phonons subsystems. It has been shown that there exists an interval of particles sizes with total energy exchange of few orders less than in massive bulk metals. This condition is crucial for existence of hot electrons in stationary conditions in metal nanoparticles, metal island films and thin films as have been observed experimentally. They studied the total electron-phonon energy exchange in small metal particles which size is less than free path of electron-phonon collisions. This expression contains bulk and surface terms. The bulk contribution oscillates as the function of the particle size. It is important that the long wave acoustics phonons generated by the hot electrons can be in non-equilibrium state with others phonons. Thus, the use of electron-phonon collisions integral approach with Plank distribution function of phonons could be incorrect. 60. Todays, technological achievements in food packaging industry have enabled us to apply intelligent packaging in order to change conditions and data delivery from food situation. This package is called active packaging. One applied method, is use of nano-composite coating to coat plastic layers. Over the time, food releases gasses and moisture in their packages. In some packaging gas and moisture absorbent are used, which leads to healthiness and preservation of the food. Some intelligent coatings show leakage or increase of temperature. Nowadays, there are some attempts to promote packaging quality through coating of food packages using anti-microbial coatings. Till now, use of these coatings has not been of much interest. Regarding available technologies use of these coatings is not economic yet. However, some active companies in this field hope to produces low cost surfaces with decent antibacterial property—using nano-coating technology—to coat food products. Another application of nano-coating is production of biosensors, attached to packages in coating form. Biosensors can offer information about quality of packaged materials, their state of healthiness, and etc.

Index

A Abrasion, 4–5, 15–16, 31, 33–36, 42, 49, 61, 99–101, 103–104, 173, 208, 226, 228, 230 Abrasion resistant coating, 61, 101 Abrasive resistance, 29–30, 34, 37, 43, 44–45, 47, 60, 63, 228–232 Accumulation, 8, 43, 192, 199–200, 242 Adatom, 114, 127, 237 Adhesives, 4, 236 Aero gel, 7, 9 Agglomeration, 2, 9, 50, 80, 113, 152, 164, 166–167, 169, 178, 201, 225, 235 Air pollutants, 44, 231 Alco gel, 7 Aluminum oxide, 21, 34, 38–39, 230 Amalgamation, 6, 164, 226 Amorphous, 6, 30–32, 36, 40–42, 52, 56, 60, 94, 150, 175, 191, 201, 226, 229, 231–233 Anatase, 2–3, 149 Anisotropy, 36, 59 Annealing, 13, 56, 58, 86, 94, 153, 195, 200–202, 227–228, 234, 239, 243 Anode, 111, 113–114, 126, 129 Anti corrosive coating, 5 Anti fog, 5, 15, 226 Anti scratch, 15 Antibacterial coating, 101, 213 Anti-bacterium masks, 210 Anti-corrosive properties, 133 Antipollution, 212–214 Antistatic, 92–93 Asymptotic analysis, 155 Atomic force microscopy (AFM), 166 Atomic nucleation theory, 116 Atomic vibration, 5, 226

Atomistic simulation, 154 Atoms, 1, 12–13, 15, 34, 55–56, 77, 79, 90–91, 94–95, 111–112, 114–116, 118, 127–128, 135, 197, 199, 200, 220, 227, 230, 237 Automobile industry, 37, 65, 209 Average current density, 47, 119 Average size of nanometric particulates (ASNP), 175

B Band gap energy, 2, 149, 187, 225, 241 Binding energy, 5, 188 Biocompatibility, 31 Bio-stability, 31 Butler-volmer equation, 131

C Cantilever, 38, 72 Carbon nanotubes, 2, 49–51, 112–113, 121 Catalyst, 4, 7–8, 11, 44, 52, 60, 133, 187, 209, 213, 226–227, 231, 234 Catalyst properties, 52, 133, 209, 234 Catalytic systems, 85 Cathode, 39, 111, 114–115, 119, 125–130, 162, 166, 168, 170, 236–237 Cathodic over-voltage, 114 Celice model, 162 Ceramics, 3, 6–7, 9, 30, 61, 66, 68, 85, 101–102, 135, 225–226, 229 Cerium oxide, 10 Characterization, 77, 180, 182, 192 Charge transfer, 118, 122, 127–128, 131 Chemical barriers, 5 Chemical collide, 4

245

246

C (cont.) Chemical composition, 4, 78–79, 83–84, 92–93, 96, 133–134, 226, 235, 238 Chemical elements, 4 Chemical potential, 91–92, 94–95, 113–114, 163 Chemical reaction, 4, 7, 10, 51, 94, 96, 119, 127, 167, 236 Chemical synthesis, 4 Chemical vapor deposition (CVD), 4–6, 29, 150, 226, 229 Chromium oxide, 34, 230 Clusters, 1, 3, 95, 214 CMOS, 189, 192, 242 Coercivity, 47 Coherency, 84 Cohesion, 30–31, 33–36, 45, 100, 103, 211, 215, 230, 232 Commercial methods, 4 Compact structure, 35 Compatibility, 17, 31, 63, 65–66, 97–98, 103, 190, 203, 211, 228 Complex compound, 127 Compressed metals, 94 Compression, 95, 157 Compressive stress, 90 Concentration over-potential, 127 Conduction, 5 Convection phenomena, 112 Corrosion current density, 112 Cosmetics, 4, 226 Creep, 35, 48, 49, 59, 65, 133, 230, 233, 234, 238 Creep rate, 48–49, 238 Crushing, 103 Crystalline, 4, 6, 21, 30–31, 34–36, 40–42, 44–45, 47–49, 52, 54–60, 71, 75, 77–81, 83–85, 89, 92–96, 100, 111, 114, 128, 131, 133–135, 139, 152, 156, 174, 183, 192, 201–202, 226, 229, 231–235, 138, 242–243 Crystalline nuclei, 111 Crystalline structure, 4, 44, 55, 58, 77, 79–80, 83–85, 92, 174, 231, 235 Crystallization of glassy phases, 96 Cyclic loading, 160

D Data storing capacity, 216 Decomposition, 79, 95, 127 Densification, 8 Density of deposited metal, 39

Index Detergents, 4, 226 Dielectric constant, 135–137 Dip coating, 82 Dislocation, 35–36, 42, 48–49, 77, 85, 156–161, 230 Dispersant, 80, 153–154, 164–166, 239 Dispersion hardening, 97 Dissolution methods, 4 Distribution, 3, 32, 37–38, 43–44, 48–51, 53–54, 58, 79, 89, 92, 112–113, 116, 120–121, 123–125, 133–134, 152–156, 164, 167, 169, 171, 175–176, 196, 208, 225, 229, 231, 234, 236, 238–239, 244 Double layer, 112, 124–128, 130–132, 163–166, 169 Double-layer structure, 124–125 Driving force, 79, 84, 91, 95, 111, 114, 134, 196, 199 Drying process, 2, 149, 151 Ductility, 35, 42, 45, 153, 232, 239 Duty cycle, 37, 50–53, 112–113, 116, 162, 170, 175

E Elastic strain, 90 Elastic stress, 90 Elastic tension, 90 Elcho oxide orbit, 7 Electrical, 3–4, 16–17, 34, 39, 43, 45, 56, 58–60, 62, 66, 77–78, 82, 85–87, 94–96, 99–102, 111–112, 124–128, 130–133, 152, 154, 162–163, 165–167, 169, 176–177, 201–203, 207, 212, 214–217, 224, 228, 237, 239, 243 Electrochemical deposition, 78, 96, 111, 129, 130, 132, 237 Electrochemical deposition process, 96, 111, 129, 237 Electrochemical nucleation, 123 Electrochemical phase state, 119 Electrochemical potential of electrolyte, 113 Electrochemical properties, 93, 111, 113, 138 Electrochemical supersaturation, 114 Electrochemistry, 29, 46, 141, 232, 233 Electro-crystallization, 47, 111, 116 Electrode surface energy state, 119 Electrodeposition, 5, 40, 49–50, 52, 69, 97, 121, 146, 152, 174–175, 178, 234 Electro-explosion, 4 Electromagnetic compatibility, 203

Index Electron-phonon collision, 206, 244 Electrostatic sustainability, 165 Entangle bonding energy, 164 Enthalpy, 93–95 Environmental modification, 4, 226 Equilibrium state, 77, 83, 91–92, 94, 113–115, 192, 206 Evaporation, 6, 36, 48, 55, 64, 95, 226, 233 Exchanged current density, 39

F Fabrication methods, 6 Faraday constant, 39, 94, 126 Fatigue strength, 171, 240 Finite element implementation, 160 Florescence, 226 Foliating, 173, 241 Foliation, 99, 102, 236 Food packaging, 16, 211–212, 244 Free energy, 77, 85, 88–95, 195–196, 199, 235 Friction, 15, 38, 45, 47, 49, 121, 132–133, 150, 153, 162, 170, 174, 230, 238–239 Fullerenes, 2 Functional application, 59 Functionally graded coating (FGC), 32

G Galvanic potential, 114 Gamma function, 120 Gas annealing, 13, 228 Gas phase pyrolysis of flame, 4 Gas-condensation process, 96 Gasro gel, 7 Gibbs free energy, 85, 93, 196, 235 Gibbs-thomson equation, 115 Glasses, 7, 9, 15–16, 61, 64, 100, 102, 209 Gouy-chapman model, 125–126 Grain boundary, 40, 48–49, 52, 54, 56–58, 78–79, 84, 94–95, 130, 132–135, 153, 159, 161, 232, 238 Grain size, 34, 39, 41–42, 45, 47–49, 52, 54–56, 58–60, 89, 94–96, 132–134, 156, 171, 178, 193–194, 201, 230–232, 234, 237–238, 241, 243 Granule size, 78, 133, 238 Grinding, 4, 30, 48, 96, 157, 233, 241 Guglielmi, 39, 60, 162–163, 222 Guglielmi model, 39, 162–163

247 H Hall-petch, 48–49, 59, 132, 156–157, 233–234, 238–240 Hardness, 15, 17, 29–36, 41–42, 45, 47–52, 60, 100, 102, 112, 149, 152–154, 162–163, 170, 173–175, 211, 214, 228–233, 238–239, 241 Helmholtz-perrin model, 125 High temperature inertness, 97 High velocity oxygen fuel spraying, 29 Holmholtz free energy, 92 Homogenous system, 92 Hybrid sol, 2 Hydro gel, 7 Hydrogen-reduction reaction, 134 Hydrogen solubility, 47 Hydrogen transition, 54 Hydrolysis, 2, 7–8, 10–11, 136, 227 Hydrolysis reaction, 2, 7–8, 10–11, 136, 227 Hydroxyapatite, 31, 144, 211, 229 Hydroxyl groups, 102 Hygienic, 16, 210

I Infrared, 25, 100, 203–205, 243 Inhibition efficiency, 78 Insulation, 5 Integrated circuits, 5, 135, 185 Interface atoms, 55 Interface curvature, 83 Interface free energy, 83, 90 Interface traction, 86 Interfaces thermodynamics, 85 Inter-level connections, 83 Intra-crystalline spaces, 55 Intra-granular, 54 Ion exchange, 85 Ionic activity, 114–115 Ionic conductivity, 55–57 Ionic radius, 124–125 Iso electric point, 153, 168–169, 239

K Kinetic, 77–78, 117–120, 124, 197, 236

L Langmuir absorption isotherm, 39 Laser abrasion, 4 Lattice imperfections, 84 LCD monitors, 62

248

L (cont.) Light resistant panels, 61, 101 Light valves, 5 Local corrosion, 52, 54, 132, 134, 238 Lubricants, 4, 226 Luminescent properties, 201

M Magnetic, 3–5, 16–17, 36, 38, 55–56, 59, 62, 72, 79, 94, 133, 149, 203–204, 216, 225–226, 228, 235, 243 Magnetic nano-layers, 62 Magnetron sputtering, 6, 27, 205, 226, 243 Malleability, 47–49, 59, 223–224 Mechanical, 3–5, 13, 30–39, 42–45, 48, 59, 63, 69, 70–71, 73, 79, 80, 88, 96, 112, 121, 132–133, 151–157, 171–172, 174, 190, 192–194, 225–226, 229–230, 232–235, 238–239 Mechanical alloying, 4, 192–194, 242 Mechanical properties, 5, 31–33, 35, 37, 42–43, 45, 48–50, 59, 71, 79, 112, 132, 133, 149, 153–154, 156–157, 226, 229, 232, 234–235, 238–239 Medical applications, 16, 61 Membranes, 42 Metal capacity, 39 Metal cation, 10, 124, 127, 236 Metal chalcogenide, 201 Metal ion structure, 124 Metal lattice, 126, 136 Metallic ions, 10, 94, 111–114, 126, 152, 237–238 Micro electromechanical systems (MEMS), 36 Micro/nano-systems, 156–157, 161, 240 Microelectronics, 185, 203 Milling, 4, 80, 96, 192–194 Mineral materials, 5, 7 Miniaturization, 185 Mole fraction, 92 Molecular electricity, 2 Molecular machinery, 1 Molecules, 1, 12, 25, 39 Monomer, 8, 12, 227 Mosfet, 185, 189 Multi-crystalline, 45, 49, 52, 55–56, 78, 84–85, 89, 94–95, 134, 234–235, 238 Multi-layers, 29–30, 35, 65, 79, 174, 229, 235

Index N Nano porous material, 2, 9 Nano yarn, 9 Nano-capsule, 2 Nanocoating, 29, 58 Nanocomposite Nanocomposite coatings, 37, 43–44, 69, 97, 116, 146, 150–152, 174–177, 179, 238–239 Nano-crystalline alloys, 84 Nanocrystalline coatings, 35 Nano-crystalline nickel, 41, 45 Nanofibers, 2 Nano-hybrid particles, 2, 149, 187, 225, 241 Nanometric scale, 1, 83, 116, 175, 225 Nano-nucleuses, 122 Nano-particle masks, 210 Nanoparticles, 3–4, 16, 40, 61, 97, 121–122, 136, 149–154, 162, 166–167, 169–170, 177–179, 182, 206–207, 213, 216, 225–226, 230, 239, 244 Nano-porous materials, 2 Nano-powders, 29, 30, 44, 85–86 Nanorod, 79, 235 Nano-scale thickness, 154–155, 225 Nanostructured coating, 1, 15, 27, 29, 45 Nanostructured films, 6, 201, 226 Nanostructured materials, 2, 78–79, 83–85, 93–96, 133, 152, 203, 234–235, 238 Nanosystems, 1, 157 Nanotechnology, 1, 3–4, 15, 99, 225–226, 228 Nano-whisker, 79, 235 Nano-wires, 2, 29, 46–47, 98, 152, 158 Nernst equation, 114 Neutral surface, 114 Neutralization, 84 Nickel-alumina nanocomposite coating, 152–153, 166, 239 Non-linear optical properties, 201 Non-oxide ceramics, 3, 225 Norio taniguchi, 1 Nuclear pellet, 1 Nucleation, 77–78, 96, 112, 114–121, 123, 127–128, 130, 158, 236 Nucleus situation, 120 Nucleuses, 111–124, 191, 198, 236

O Off-pulse time, 129–132, 237 Olation, 10 On-pulse time, 129–131, 237 Opaque, 5, 100, 203, 226

Index Optical, 3–5, 13–17, 34, 38, 61, 64, 99, 149, 201, 203–205, 207–208, 225–226, 228, 243 Optical entrapment, 5, 226 Optical features, 4 Organic contaminator, 44 Organic ligand, 233 Organic thin films, 5 Oxolation, 10

P Particle growth, 8, 10 Particle polymerization, 8 Permeability, 47, 54–55, 85 Pesticides, 4 Phase interface, 93 Phase isolation, 92 Phase transition, 86, 199 Photo-catalysts, 44, 231 Physical properties, 6, 36, 38 Physical vapor deposition (PVD), 6, 29, 150 Pitting corrosion, 135 Plasma arc plating, 11 Plasma arc spraying, 11 Plasma coating, 12 Plasma polymer precipitation, 12, 227 Plasma scattering coating, 11 Plasma spray technology, 66 Plasma synthesis, 4 Polarizer, 16, 62, 215 Polarizer nano-layers, 215 Polyethylene, 38, 64, 230 Polytetrafluoroethylene, 38, 152, 238 Porosity value, 129, 237 Porous templates, 46 Potential energy, 5, 226 Power generation industry, 65 Pulsed current, 36, 40, 49–51, 53, 112, 116, 175, 178

Q Quantum dots, 2, 176, 217 Quantum phenomena, 98 Quantum size effect, 2–3, 149, 187, 204, 225, 241–243

R Rate of electrode reactions, 126 Reductant agents, 111 Reduction, 9, 15

249 Reflection, 4–5, 61, 99, 209–210, 226 Resistance over-potential, 128 Response surface methodology, 36–37, 175 Reversible work, 90 Richard feynman, 1 Rotating coating, 5

S Saint-Venant’s principle, 154 Salicide, 189–192 Salt spray test, 132, 134 Scale of nano, 1, 225 Scanning electron microscope (SEM), 3 Scanning tunneling microscopy (STM), 111 Scratch resistance, 4, 100, 175, 226 Scratching, 35, 103 Sealants, 4, 226 Self assembling, 5 Self-cleaning glasses, 16, 209 Self cleaning properties, 4, 101, 226 Self-lubricity, 97 Semi-conductor wafers, 5 Semi-spherical nucleus, 122–123, 236 Sensing characterization, 192 Separation, 79, 119, 129, 237 Setric forces, 165 Setric sustainability, 164–165 Silicates, 3, 225 Silicide, 185–186, 189–193, 196, 198, 201, 203, 214, 241–242 Silicides specifications, 189 Silicon nitride (Si3N4), 3 Size dependency, 29, 36, 171, 175, 217 Size effect, 1–3, 31–32, 56–57, 85, 86, 111, 135, 137, 149, 154–161, 187, 189, 192, 201, 204, 225–226, 240–243 Size-dependent phenomenon, 154 Sodium benzoate, 78 Sodium nitrate, 78 Solar cells, 62, 214–215 Sol-gel, 2, 4–10, 13–14, 30, 80–83, 86, 102, 187, 193 Solid solubility, 47 Solid state method, 4 Specific heat, 79, 93–94, 235 Spin-coating, 80 Spray coating, 5, 17 Stern-graham model, 226 Stiffness, 5, 132–133, 150, 155, 159, 238 Stillinger-weber model, 155 Strain hardening rate, 156, 159, 240 Strain-to-failure, 5

250

S (cont.) Strength, 15, 30, 34, 37, 42, 44–49, 54, 59, 61, 104, 132, 149–150, 153, 156–161, 170, 172–173, 204–205, 210, 228, 231, 233–234, 239–241, 243 Strength enhancement, 173 Stress corrosion cracking, 47 Stress gradient, 83, 235 Structural applications, 59, 63 Structural disorder, 55–56 Super elasticity, 150 Super hard, 29, 35 Super-hard coatings, 29 Super-lattice coating, 36 Super rough, 35 Supersaturated state, 114 Surface elasticity, 154–155, 239 Surface engineering, 16–18, 60, 63–65, 228, 232 Surface geometry, 5, 226 Surface stress, 88, 154–155 Surface-to-volume ratio, 158 Surfactant, 43, 46, 164, 167, 233 Suspension, 6, 8, 152, 165, 168–169, 226, 238

T Tabularization, 173 Tafel polarization, 112, 116 Taguchi method, 40, 97, 121 Tensile stress, 88 Tertiary nanocomposite, 40, 121–122, 178 Thermal capacity, 79, 235 Thermal evaporation, 6, 226 Thermal expansion, 44–45, 47, 82–84, 132, 211, 231–232 Thermal extension coefficient, 79 Thermal light, 100 Thermal oxidation, 13–14 Thermal scattering coating, 11 Thermal stability, 29, 41, 47, 58, 187, 190–192, 195–202, 207, 228, 234, 242 Thermodynamic equilibrium, 113 Thermodynamics, 83, 85–86, 235 Thermoelectric tools, 5 Thickness, 1, 4–5, 8–9, 32, 35–36, 42, 44, 48, 55–60, 80, 83, 98, 126–128, 135–137, 149–150, 154–156, 158, 170–172, 174, 186–187, 189–192, 197–208, 225–226, 229–231, 239–243 Thin film transistor (TFT), 15

Index Thin films, 2, 4–6, 9, 11–12, 15, 42, 56–57, 84, 124, 149, 156, 158, 169–171, 202–204, 206, 208–209, 214, 226–227, 243–244 Titanium aluminide nitride, 33 Titanium boro-nitride, 33 Titanium carbo-nitride, 33 Titanium nitride, 33–36, 229–230 Torsion, 160 Transformator, 59 Transitional metal nitride coatings, 29, 33, 228 Transparency, 4–5, 226 Transparent conductor coatings, 62, 213 Transparent glass, 9 Transpassive behavior, 134 Tungsten carbide, 38, 41, 45, 100, 177–178, 232 Turbine fans, 101 Two dimensional nano-films, 154

U Ultrasonic bath, 49 Ultrasonic energy, 43, 167 Ultra-thin films, 171 Ultraviolet, 2, 14, 99–100, 149, 187, 204–206, 208–209, 213, 225, 236, 241–243 Ultraviolet beam, 99–100, 236 Ultraviolet-visible spectroscopy (UV-VIS), 2, 187, 241 Ultraviolet–visible spectrumn, 2, 149 Uniaxial tensio, 156–157

V Vacancy, 57, 77, 132 Vacuum annealing, 13, 228 Vacuum chamber, 6 Van der waals force, 164 Vapor resistant layers, 16 Vertex energy, 89 Viscosity, 8–9, 168–169 Volatile organic compounds, 65 Volumetric percentage, 39, 152, 154, 162, 167–168, 239

W Water memory, 86 Water repellence, 5, 226 Waveguides, 5 Wavelengths, 4

Index Wear, 5, 15–16, 30, 37, 47, 49, 60–61, 82, 97, 103, 121, 173, 174, 176–179, 217, 226, 228–229 Weibull statistics, 32

X Xerogel, 9 X-ray diffraction (XRD), 187

251 Y Yield strength, 49, 156–160, 171, 240 Young modulus, 47, 49, 132

Z Zeldovich factor, 118 Zero dimensional nanomaterials, 29 Zeta potential, 9, 153, 168, 239

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