Nanocoatings and ultra-thin films - Technologies and applications

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Author: Abdel Salam Hamdy Makhlouf and Ion Tiginyanu

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Nanocoatings and ultra-thin films

© Woodhead Publishing Limited, 2011

Related titles: Nanostructured metals and alloys (ISBN 978-1-84569-670-2) Nanostructured metals and alloys have enhanced tensile strength, fatigue strength and ductility and are suitable for use in applications where strength or strength-toweight ratios are important. Part I of this important book reviews processing techniques for bulk nanostructured metals and alloys. Parts II and III discuss microstructure and mechanical properties, whilst part IV outlines applications of this new class of material. Electroless copper and nickel-phosphorus plating (ISBN 978-1-84569-808-9) Compared with electroplating, electroless plating allows uniform deposits over different surfaces. Electroless copper and nickel-phosphorus deposits provide protective and functional coatings in industries as diverse as electronics, automotive, aerospace and chemical engineering. Written by leading experts in the field, this important book reviews the deposition process and the key properties of electroless copper and nickel-phosphorus deposits as well as their practical applications. Thermal barrier coatings (ISBN 978-1-84569-658-0) Thermal barrier coatings are used to counteract the effects of high temperature corrosion and degradation of materials exposed to environments with high operating temperatures. The book covers both ceramic and metallic thermal barrier coatings as well as the latest advances in physical vapour deposition and plasma spraying techniques. Advances in nanostructured thermal barrier coatings are also discussed. The book reviews potential failure mechanisms in thermal barrier coatings as well as ways of testing performance and predicting service life. A final chapter reviews emerging materials, processes and technologies in the field. Details of these and other Woodhead Publishing materials books can be obtained by: • • •

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Nanocoatings and ultra-thin films Technologies and applications

Edited by Abdel Salam Hamdy Makhlouf and Ion Tiginyanu

Oxford

Cambridge

Philadelphia

New Delhi


Published by Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK www.woodheadpublishing.com Woodhead Publishing, 1518 Walnut Street, Suite 1100, Philadelphia, PA 191023406, USA Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com First published 2011, Woodhead Publishing Limited © Woodhead Publishing Limited, 2011 The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Control Number: 2011934932 ISBN 978-1-84569-812-6 (print) ISBN 978-0-85709-490-2 (online) The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elemental chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Toppan Best-set Premedia Limited, Hong Kong Printed by TJI Digital, Padstow, Cornwall, UK


Contents

Contributor contact details Introduction Part I

Technologies

1

Current and advanced coating technologies for industrial applications A. S. H. Makhlouf, Max Planck Institute of Colloids and Interfaces, Germany Introduction Electro- and electroless chemical plating Conversion coatings Chemical and physical vapor deposition (CVD and PVD) Spray coating Other coating techniques New lightweight materials Trends in environmentally friendly coatings, self-assembling and self-cleaning coatings Trends in nanocoatings New composite and powder coatings Advanced polymers and fillers Developments in coating processes Acknowledgements References

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12 1.13 1.14 2

2.1 2.2 2.3

Nanostructured thin films from amphiphilic molecules J. Y. Park and R. C. Advincula, University of Houston, USA Langmuir monolayer Amphiphilic polymers Dendrons and dendrimers

xi xv 1 3

3 4 5 6 7 10 12 13 14 16 17 18 20 20

24

24 28 36 v


vi

Contents

2.4 2.5 2.6 2.7 2.8

Metal/semiconductor nanoparticles 2-D arrays of colloidal spheres Conclusions Acknowledgements References

3

Chemical and physical vapor deposition methods for nanocoatings I. V. Shishkovsky, P. N. Lebedev Physics Institute of the Russian Academy of Sciences, Russia Substrate preparation for ultra-thin films and functional graded nanocoatings Paradigm of functional graded layer-by-layer coating fabrication Nanocoating fabrication methods Physical vapor deposition-based technologies Chemical vapor deposition-based technologies Conclusion and future trends References

3.1 3.2 3.3 3.4 3.5 3.6 3.7 4

4.1 4.2 4.3 4.4 4.5 4.6 5

5.1 5.2 5.3 5.4

Surface-initiated polymerisation for nanocoatings V. Harabagiu, L. Sacarescu, A. Farcas, M. Pinteala and M. Butnaru, ‘Petru Poni’ Institute of Macromolecular Chemistry, Romania Introduction Physisorption and chemisorption, equilibrium and irreversible adsorption Preparation of surface-bound polymer layers Properties and applications Acknowledgement References Methods for analysing nanocoatings and ultra-thin films D. M. Bastidas, M. Criado and J.-M. Bastidas, National Centre for Metallurgical Research (CENIM), CSIC, Spain Introduction Electrochemical methods Surface-sensitive analytical methods for ultra-thin film coatings Spectroscopic, microscopic and acoustic techniques for ultra-thin film coatings


41 44 48 48 48

57

57 60 61 63 71 74 75 78

78 79 87 110 112 112

131

131 132 140 145

Contents 5.5 5.6 5.7

Conclusions Acknowledgements References

Part II Applications 6

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7

7.1 7.2 7.3 7.4 7.5 7.6 7.7 8

8.1 8.2 8.3 8.4 8.5 8.6 8.7

Conventional and advanced coatings for industrial applications: an overview A. S. H. Makhlouf, Max Planck Institute of Colloids and Interfaces, Germany Introduction Conventional coating technologies for the automotive and aerospace industries Advanced coating technologies for the automotive and aerospace industries Packaging applications Coatings for the electronics and sensors industry Paints and enamels industry Biomedical implants industry Acknowledgements References Nanocoatings for architectural glass J. Mohelnikova, Brno University of Technology, Czech Republic Introduction Spectrally selective glass Dynamic smart glazings Glass surface protections Conclusion Acknowledgements References Nanocoatings and ultra-thin films for packaging applications A. Sorrentino, University of Salerno, Italy Introduction Nanomaterials in packaging High barrier packaging Anti-microbial packaging Nanosensors in packaging Packaging as a drug carrier and for drug delivery Nanotechnology solutions for the packaging waste problem


vii 153 153 153 157 159

159 159 162 170 171 173 174 175 177 182

182 183 188 194 195 196 196 203 203 208 209 215 216 218 219

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Contents

8.8 8.9 8.10 8.11

Anti-static packaging applications Regulation and ethical issues in the new packaging industry Future trends References

9

Advanced protective coatings for aeronautical applications M. G. S. Ferreira, M. L. Zheludkevich and J. Tedim, University of Aveiro, Portugal Introduction: corrosion in aeronautical structures Types of corrosion in aircraft Factors influencing corrosion Corrosion of aluminum and its alloys Corrosion of magnesium alloys Protective coatings in aerospace engineering Pre-treatments Anodizing coatings Functional nanocoatings in aerospace engineering Nanocoatings for detection of corrosion and mechanical damage Self-healing coatings: nanostructured coatings with triggered responses for corrosion protection Application of nanomaterials for protection of aeronautical structures Conclusion and future trends References

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 10

10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11

Nanoimprint lithographic (NIL) techniques for electronics applications I. Tiginyanu, V. Ursaki and V. Popa, Academy of Sciences of Moldova, Republic of Moldova Lithography techniques and nanoimprint lithography (NIL) fundamentals Thermoplastic and laser-assisted NIL Photo-assisted nanoimprinting Soft NIL Extensions of soft NIL Scanning probe lithography (SPL) Edge lithography NIL for three-dimensional (3D) patterning Combined nanoimprint approaches Applications Conclusions


220 221 222 223

235

235 236 241 243 244 246 247 253 258 259 261 266 270 270

280

280 286 291 297 301 307 309 311 315 317 320

Contents

ix

10.12 10.13

Acknowledgement References

322 322

11

Ultra-thin membranes for sensor applications I. Tiginyanu, V. Ursaki and V. Popa, Academy of Sciences of Moldova, Republic of Moldova Introduction Graphene and related two-dimensional (2D) structures Nanometer-thick membranes of layered semiconductor compounds Ultra-thin membranes of gallium nitride Conclusion Acknowledgement References

330

Nanocoatings for tribological applications S. Achanta and D. Drees, Falex Tribology NV, Belgium and J.-P. Celis, Katholieke Universiteit Leuven, Belgium Introduction Use of nanostructured coatings in tribology Review of nanostructured coatings for friction and wear applications Advanced techniques for characterizing tribological properties of nanostructured coatings Conclusions and future trends Acknowledgements References

355

Self-cleaning smart nanocoatings J. O. Carneiro, V. Teixeira, P. Carvalho, S. Azevedo and N. Manninen, University of Minho, Portugal Introduction: TiO2 photocatalysis Photocatalysis processes The photocatalytic cleaning effect of TiO2-coated materials New and smart applications of TiO2 coatings Conclusions References

397

Index

414

11.1 11.2 11.3 11.4 11.5 11.6 11.7 12

12.1 12.2 12.3 12.4 12.5 12.6 12.7 13

13.1 13.2 13.3 13.4 13.5 13.6


330 331 341 344 351 351 351

355 356 367 382 391 391 392

397 399 402 406 410 411

Contributor contact details

(* = main contact)

Chapters 1 and 6

Editors

A. S. H. Makhlouf Max Planck Institute of Colloids and Interfaces Department of Interfaces, Am Mühlenberg 1 14476 Potsdam-Golm Germany E-mail: abdelsalam.makhlouf@ mpikg.mpg.de

A. S. H. Makhlouf Max Planck Institute of Colloids and Interfaces Department of Interfaces, Am Mühlenberg 1 14476 Potsdam-Golm Germany E-mail: abdelsalam.makhlouf@ mpikg.mpg.de I. Tiginyanu Academy of Sciences of Moldova Chisinau Republic of Moldova and Technical University of Moldova Chisinau Republic of Moldova E-mail: [email protected]

Chapter 2 J. Y. Park and R. C. Advincula* Department of Chemistry and Department of Chemical and Biomolecular Engineering University of Houston Houston Texas 77204-5003 USA E-mail: [email protected]

xi © Woodhead Publishing Limited, 2011

xii


Chapter 3

Chapter 7

I. V. Shishkovsky P. N. Lebedev Physics Institute Russian Academy of Sciences Samara branch Novo-Sadovaja st. 221 443011 Samara Russia E-mail: [email protected]

J. Mohelnikova Faculty of Civil Engineering Brno University of Technology Veveri 95 602 00 Brno Czech Republic E-mail: [email protected]

Chapter 8 Chapter 4 V. Harabagiu*, L. Sacarescu, A. Farcas, M. Pinteala and M. Butnaru ‘Petru Poni’ Institute of Macromolecular Chemistry 41A Aleea Grigore Ghica Voda 700487 Iasi Romania E-mail: [email protected]

Chapter 5 D. M. Bastidas, M. Criado and J.-M. Bastidas* Department of Surface Engineering, Corrosion and Durability National Centre for Metallurgical Research (CENIM), CSIC Avda. Gregorio del Amo, 8 28040 Madrid Spain E-mail: [email protected]

A. Sorrentino Department of Industrial Engineering University of Salerno via Ponte Don Melillo I84084 Fisciano - SA Italy E-mail: [email protected]

Chapter 9 M. G. S. Ferreira*, M. L. Zheludkevich and J. Tedim Department of Ceramics and Glass Engineering CICECO University of Aveiro Aveiro, 3810-193 Portugal E-mail: [email protected]



xiii

Chapters 10 and 11

Chapter 13

I. Tiginyanu*, V. Ursaki and V. Popa Academy of Sciences of Moldova Chisinau Republic of Moldova

J. O. Carneiro*, V. Teixeira, P. Carvalho, S. Azevedo and N. Manninen Department of Physics University of Minho 4800-058 Guimarães Portugal E-mail: [email protected]

and Technical University of Moldova Chisinau Republic of Moldova E-mail: [email protected]

Chapter 12 S. Achanta* and D. Drees Falex Tribology NV Wingepark 23B Rotselaar 3110 Belgium E-mail: [email protected] J.-P. Celis Katholieke Universiteit Leuven Dept. MTM 3001 Leuven Belgium


Introduction

Ultra-thin films and nanocoatings play a major role in many areas such as micro- and nanoelectronics, machine building, car and aircraft manufacturing, robotics, etc. Nanocoatings, in particular, represent the interface between the product and the environment and therefore determine not only aesthetic aspects of goods, but also important specific properties such as, for example, anti-corrosion, self-cleaning, chemical and scratch resistance, etc. The term ‘nanocoatings’ is usually used when the coating is nanostructured or its thickness is in the nanometer scale. Nanostructuring is usually applied because of its ability to increase hydrophobicity, radiation hardness, and corrosion resistance and because it makes materials much more flexible. Ultra-thin films and nanocoatings represent two-dimensional (2D) systems, i.e. free electrons in conductive systems can propagate only in the x–y plane. Confinement in the z-direction may add many specific characteristics, especially in the case of electronic materials. Properly designed ultra-thin films and nanocoatings are sometimes used to reduce stiction and light reflection, for surface modification in extreme conditions, and to enhance dirt release properties. Nowadays there is increasing interest in nanophase thermal barrier coatings that exhibit extremely low thermal conductivity. A great deal of attention is also paid to decorative nanocoatings based on special paints and inks. There are various methods of producing ultra-thin films and nanocoatings: vacuum deposition, thermal spray, electrochemical deposition, etc. Vacuum deposition can be based on evaporation, sputtering, thermal decomposition, etc. Among thermal spray methods, one can mention plasma spray and arc spray as well as the high velocity oxygen fuel thermal spray process that provides high density coatings with unique performances in aggressive wear and corrosive environments. The most widely used industrial coating processes are, in fact, based on electroplating and electroless plating. These approaches are relatively simple and cost-effective and are applicable for a wide variety of coatings. xv © Woodhead Publishing Limited, 2011

xvi

Introduction

Currently, nanotechnology enables the production of ultra-thin films and nanocoatings consisting of just one monolayer or a few atomic layers. Such ultra-thin films may functionalize the surface to support desired chemical interactions, or, in contrast, passivate the surface to make it chemically inert. Note that the formation of a few-atomic-layer thick native oxide on the surface of many semiconductor materials is an example of surface passivation. Nanotechnology revolutionizes the application of nanocoatings in many fields, especially taking into account the potential to fabricate nanocoatings with specially designed nanoarchitecture, e.g., nanocomposite polymerbased coatings comprising networks of metal nanodots, aligned metal nanorods, nanowires, or nanotubes, etc. The occurrence of phenomena related to surface plasmon-polariton excitation and negative refraction opens new opportunities for the development of novel focusing optical elements with super-resolution. The discovery of graphene (one-atom-thick sheet of carbon) can be considered as an important breakthrough in the development of nanocoatings. Graphene possesses excellent electrical conductivity and therefore is a unique material for anti-electrostatic applications. Over the last years, researchers have succeeded in fabricating a few-atoms-thick membranes of BN, MoS2, Bi2Te3, Bi2Se3 and GaN. Besides obvious applications in microand nanoelectronics, nanomembranes of GaN seem to be promising for spintronic and biomedical applications. When choosing the type of nanocoatings and the technological approaches for their fabrication, it is very important to take into account their possible impact on the environment. As a rule, increasing investment is made in technologies that are characterized by high efficiency-to-cost ratio and at the same time are environmentally friendly. There is no doubt that in the near future a new generation of multifunctional nanocoatings will be developed with flexible characteristics controlled, in particular, by the conditions of the environment (temperature, pressure, intensity of illumination, etc.). Nanocoatings and ultra-thin films is both a reference and a tutorial for understanding the most common thin-films and coating techniques. The book encompasses recent approaches and future trends in coating and thinfilms technology, looking at essential innovations in the development of industrial nanocoatings and ultra-thin films based on new findings resulting from basic and applied research in the fields of both physics and chemistry. The goal of this book is to discuss the basics of ultra-thin films and nanocoatings and their synthesis techniques, surface characterization, and performance for possible industrial applications. It addresses important questions frequently posed by end-user design engineers, coaters, and coatings suppliers in their quest for multifunctional and superior coating


Introduction

xvii

qualities for industrial applications. Therefore, contributions in this book emphasize thin films, self-healing coatings, self-cleaning coatings, super-hard nanocoatings, corrosion, tribological and nano-ceramic and nanocomposite coatings with respect to their mechanical and physical properties. Chapter 1 addresses the most common coating techniques. It includes recent developments and future trends in coatings technology and considers the essential innovations in the development of industrial coatings. The chapter highlights future improvements in coating processes based mainly on reduction of the number of coating layers; full automation of the coating process; controlling the end product color through a module method and automatic quality control. Chapter 2 discusses the nanostructuring of thin films of amphiphilic macromolecules and nanomaterials at the air–water interface. The chapter introduces several synthesized amphiphilic materials which have been recently used in the Langmuir–Blodgett (LB) technique. The surface chemistry and properties of the synthesized amphiphilic materials at the air– water interface are also described. Examples of thin film applications using LB film are discussed. Chapter 3 provides a comprehensive analysis of vacuum deposition methods for nanocoating and the production of functional graded (FG) multilayers. A general approach of FG layer-by-layer synthesis is based on a paradigm of the type of connectivity of the internal structure. The objective of the chapter is to demonstrate the particularities and versatility of PVD, CVD, laser-, electron-, and ion-assisted technologies in the engineering of FG nanocoatings with control microstructure. The chapter also provides a description of the nanoperspectives of FG thin films and surface structures with nanoelectromechanical systems (NEMS) properties. Chapter 4 discusses surface-initiated polymerization for nanocoatings. In this chapter, thin polymer layer–surface conjugates are proposed as appropriate materials for studying surface/interface physicochemical properties and material interactions with the environment, allowing performance control over the entire system. Recent advances in surface-attached polymer layers are presented, and thermodynamic and kinetic aspects of polymer physi- and chemisorption are discussed. The chapter also summarizes the preparation methods for polymer-grafted surfaces with the emphasis on controlled processes able to achieve polymer surfaces meeting well-defined criteria. A comparison between the unique properties of polymer brushes and the bulk characteristics or the physisorbed layers is highlighted. Chapter 5 reports the most common and advanced methods for characterization and surface-sensitive analysis of nanocoatings and ultra-thin films. A correlation between the linear potential sweep and impedance measurements for copper specimens under different tarnishing treatments is discussed. The changes in the dielectric constant caused by water


xviii

Introduction

absorption and the pigment/polymer proportions and porosity of the organic coatings are described with a reasonably good approximation using electrochemical methods. These coatings are characterized by different analytical techniques such as AFM, XPS, infrared, Raman and Mössbauer spectroscopies, x-ray diffraction, ion spectroscopy, glow discharge optical emission spectroscopy, electronic microscopy, scanning acoustic microscopy, and Kelvin probe force microscopy. Chapter 6 provides an overview of conventional and advanced coatings for industrial applications and describes the role of coating technologies in some important industrial applications. The chapter also presents a critical review of recent research and development work on advanced coatings such as smart coatings, ‘super’-hard coatings, and multifunctional coatings, ... etc. The most important aspects of coating technologies for the automotive industry and for sensing, packaging, and biocompatible applications are discussed. Chapter 7 provides a general overview of the main types of nanocoatings for architectural window glass. Glass plays an important role in building design because of its influence on thermal and visual comfort in buildings. Highly transparent coatings are deposited onto architectural windows to be employed in commercial and residential buildings for the purpose of saving energy for heating and air conditioning. They offer environmental benefit because they reduce heat loss and allow passive solar heat gain, reducing the energy consumption required to heat a building as well as energyrelated CO2 emissions from buildings. Chapter 8 discusses the challenges of nanocoatings and ultra-thin films for packaging applications. Packaging technology is of strategic importance as it can be a key to competitive advantage in the modern industry. An innovative pack design can open up new distribution channels, providing a better quality of presentation, enabling lower costs, increasing margins, enhancing brand differentiation product safety and integrity, and improving the logistics service. Thus, there is a persistent challenge to provide cost-effective pack performance, with health and safety being of paramount importance. At the same time, there is a continuous legislation and political pressure to reduce the amount of packaging used and packaging waste. The chapter reports a variety of polymers currently used in packaging and the most widely used plastics in flexible packaging. It also reports different designs and processing techniques used to produce packaging products. Chapter 9 deals with conventional coating technologies and smart nanocoatings for corrosion protection in aerospace engineering. The types and factors which influence corrosion are reviewed as well as the protective coatings that have been in use or which have shown potential for future applications. Moreover, particular attention is given to functional


Introduction

xix

nanocoatings for sensing corrosion, nanostructured coatings which self-heal when either corrosion starts or the corrosivity of the environment becomes critical, and other coating properties important in reducing maintenance costs. The chapter concludes that fundamental and applied research in the area of sensor-based, corrosion active and anti-icing/self-cleaning smart coatings is expected to grow in the near future, contributing to the generation of high performance, added-value products. Chapter 10 discusses nanoimprint lithographic (NIL) techniques for electronics applications. The potential of these techniques to surpass photolithography in resolution, and, at the same time, to allow mass fabrication at a lower cost is highlighted. Current and potential uses of NIL are discussed in such fields as data storage, optical components, image sensors, and phase change random access memory devices. Challenges faced by nanoimprint lithography in becoming a standard fabrication technique are also considered. Chapter 11 addresses some technological approaches for the fabrication of ultra-thin membranes for sensor applications and flexible, stretchable, foldable electronics. The discussion focuses on graphene and 2D sheets of layered compounds. The potential to build multifunctional threedimensional (3D) nanoarchitectures based on 2D graphene hybridized with one-dimensional (1D) semiconductor nanostructures is highlighted. The chapter also reviews the fabrication of ultra-thin GaN membranes of nanometer scale thickness by using the concept of surface charge lithography based on low energy ion treatment of the sample surface with subsequent photoelectrochemical etching. Chapter 12 discusses the use of nanostructured coatings as tribological surfaces for both friction and wear reduction with examples from state-ofthe-art research. The chapter gives a general overview of common friction and wear mechanisms encountered in engineering applications. Moreover, it provides a brief review of methods used to deposit nanostructured coatings on substrates. Different advanced techniques for friction and wear characterization of nanostructured coatings and the scale dependence of tribological properties are discussed. The challenges encountered in extrapolating laboratory experiments to field applications are discussed. Chapter 13 looks at the concept of smart materials/coatings – terms usually applied to materials able to change their properties in response to an external stimulus such as light or temperature. New insight is provided into self-cleaning smart coatings and the chapter expands to cover the major features of the photocatalytic materials developed to date. The chapter also gives a historical overview of TiO2 photocatalysis in order to clarify the fundamental characteristics of the photocatalysis processes which take place on TiO2 surfaces. The electronic processes are also discussed, highlighting the main factors controlling the intensity of light


xx

Introduction

absorption by the molecule or substrate. The chapter discusses actual and potential applications of TiO2 photocatalysis in industry and in the development of self-cleaning glass materials, giving some practical examples of the application of TiO2 nanoparticles in environment protection. Abdel Salam Hamdy Makhlouf Ion Tiginyanu


1 Current and advanced coating technologies for industrial applications A. S. H. MAKHLOUF, Max Planck Institute of Colloids and Interfaces, Germany

Abstract: This chapter addresses the most common coating techniques currently in use. Recent developments and future trends in coating technology are discussed, taking into account the essential innovations in the development of industrial coatings. These are based on new findings resulting from basic and applied research in the fields of both physics and chemistry. Key words: nanocoatings, coating processes, coating techniques, composite coatings, trends in coatings.

1.1

Introduction

Coatings have been used for centuries in numerous areas of society. The main function of coatings lies in the protection and decoration of materials, and the extent of their use has broadened with increasing social and industrial development. Gooch1 provided a review of the history of paints and the development of coatings. He claimed that the earliest reported paints originated in Europe and Australia approximately 20 millennia ago. During that period, paints based on iron oxide, chalk or charcoal were applied with the fingertips or with brushes made by chewing on the tips of soft twigs. In 9000 bc, the North American people used their primitive paints in the same manner as their European and Australian counterparts to paint the rock walls of their living quarters with pictures of animals and people. More advanced coating technology based on polymeric coatings and paints was developed in ancient Egypt, and later in Greece, Rome and China. Ancient Egyptians used natural resins and wax to form coatings, and artists employed lacquers based on dried oils to protect their paintings. Although polymeric coatings were traditionally mainly used for the protection of various surfaces, other important applications for this type of coating should also be mentioned. Ancient Egyptian scientists developed a very fine coating technology that showed similarities with nanotechnology. Several theories therefore treat nanotechnology as a re-innovated technology, with 3 © Woodhead Publishing Limited, 2011

4


the initial attempts at developing nanoscale coatings carried out by Egyptian and, later, Chinese artists. Nowadays, there are probably a few thousand coating systems, ranging from simple systems based on one or two coating steps to sophisticated systems based on multilayers and complicated instruments. However, most of these have an adverse effect on the environment and, in many cases, do not wholly fulfill the demands of the manufacturing industries or of society. The main driving forces behind the sharp increase in research and development in coatings science and surface technology are: • •

an increase in industry requirements for high performance coatings at relatively low cost; increasing regulatory pressure to reduce the hazardous waste (such as hexavalent chromate and volatile organic compounds (VOC)) produced by coating processes, which results in air and water pollution.

There are several techniques employed for the application of a coating onto a substrate. Coatings may be applied as liquids, gases or solids. The following section describes some of the most common coating technologies for metal and alloy substrates.

1.2

Electro- and electroless chemical plating

The modification of the surface properties of the materials to be coated is one of the most desirable methods of improving corrosion and wear resistance, electrical conductivity or decorative appearance. Historically, the chemical processes of electroplating and electroless plating have always constituted the most common, cost-effective and simple techniques for applying a metallic coating to a substrate. In both cases, a metal salt in solution is reduced to its metallic form on the surface of the material to be coated.

1.2.1 Electrochemical plating In electrochemical plating, the electrons for reduction are supplied from an external source. High reactivity materials such as magnesium alloys can quickly form an oxide layer when exposed to air; this oxide layer must be removed prior to plating. Therefore, finding the appropriate chemical surface treatment to prevent oxide formation during the plating process is one of the major challenges involved in plating processing.2–5 Another potential issue is that the quality of the final coating depends on the materials being plated. As a result, different chemical surface treatment processes must be developed for each material to be coated. Uneven


Current and advanced coating technologies

5

distribution of current density in the plating bath, resulting in non-uniform coatings, is a further problem with this technique. Electroplating also uses a large amount of electricity which can significantly increase the cost of the plating process.

1.2.2 Electroless chemical plating In electroless chemical plating, the reducing electrons are supplied by a chemical reducing agent in solution or from the material itself. This process does not suffer from the same disadvantages as those noted previously for electroplating and even allows complex shapes to be coated. Another advantage of electroless plating is that second-phase particle such as alumina, carbides or diamonds can be co-deposited during the plating process in order to improve some desirable properties such as wear resistance, hardness or abrasion.4,6–9

1.3

Conversion coatings

Conversion coatings are produced by a chemical or electrochemical reaction at a metal surface, which creates a layer of substrate metal oxides, vanadate, chromates, cerate, molybdate, phosphates or other compounds that are chemically bonded to the substrate surface. Conversion coatings are widely used as low-cost coating processes which are able to protect the metal substrate from corrosion by acting as an insulating protective barrier between the metal surface and the environment.

1.3.1 Chromate conversion coating Chromate conversion coating is the most common type of conversion coating applied to improve the corrosion protection performance of many metals and their alloys, including aluminum, zinc, copper and magnesium. Major reasons for the widespread use of chromating are the self-healing nature of the coating, the ease of application, the high electric conductivity and the high efficiency : cost ratio. These advantages have made them a standard method of corrosion protection. Moreover, they provide the greatest level of under-film corrosion resistance and facilitate the application of further finishing treatment. However, the Environment Protection Agency (EPA) ranks hexavalent chromate as one of the most toxic substances due to its carcinogenic effect and because it is environmentally hazardous as a waste product. As a result of current environmental legislation, along with increasing calls for a total ban on toxic hexavalent chromate in coating processes, many attempts have been made to develop less toxic or


6


eco-friendly alternatives. Trivalent chromate was proposed as a possible alternative but proved to be less effective than hexavalent chromate.

1.3.2 Chrome-free conversion coatings In the last few decades, chrome-free conversion coatings based on salts such as cerate, stannate, vanadate, molybdate, silicate and zirconate have been developed. These can provide covalent bonding for strong coating adhesion and can act as a barrier coating, limiting the transport of water to the surface of the material.10–23

1.3.3 Anodizing Anodizing is an electrolytic process which is used to produce a thick oxide layer on the surface of metals and alloys. These films are used to improve corrosion resistance and paint adhesion to the substrate.23 The anodizing process includes the following stages: (i) mechanical treatment; (ii) degreasing, cleaning and pickling; (iii) electropolishing; (iv) anodizing using AC or DC current; (v) dyeing or post-treatment; and (vi) sealing.24 The anodized films formed consist of a thin barrier layer at the metal–coating interface and a relatively thick layer of a cellular structure. Each cell contains a pore the size of which is determined by the type of electrolyte and the experimental conditions. The pore size and density in turn determine the quality of the anodized film.23 Electrochemical inhomogeneity due to phase separation in the material to be coated is one of the main challenges faced in the production of uniform anodic coatings. The presence of flaws, porosity and inclusions from mechanical treatment can also result in uneven deposition which, in turn, can enhance corrosion.25 Another disadvantage of anodizing is that the fatigue strength of the materials to be coated can be affected by localized heating at the surface during the treatment,25 especially in thicker films. Moreover, the anodized film formed is made of a brittle ceramic material that may not have the appropriate mechanical properties to fulfill the requirements of some industrial applications.

1.4

Chemical and physical vapor deposition (CVD and PVD)

1.4.1 Chemical vapor deposition Chemical vapor deposition (CVD) is one of the most common processes used to coat almost any metallic or ceramic compound, including elements,



7

metals and their alloys and intermetallic compounds. The CVD process involves depositing a solid material from a gaseous phase; this is achieved by means of a chemical reaction between volatile precursors and the surface of the materials to be coated. As the precursor gases pass over the surface of the heated substrate, the resulting chemical reaction forms a solid phase which is deposited onto the substrate. The substrate temperature is critical and can influence the occurrence of different reactions. There are several types of CVD process, including atmospheric pressure chemical vapor deposition, metal-organic chemical vapor deposition, low pressure chemical vapor deposition, laser chemical vapor deposition, photochemical vapor deposition, chemical vapor infiltration, chemical beam epitaxy, plasma-assisted chemical vapor deposition and plasma-enhanced chemical vapor deposition.

1.4.2 Physical vapor deposition Physical vapor deposition (PVD) is a vaporization coating technique, involving the transfer of material on an atomic level under vacuum conditions. The process is in some respects similar to CVD, except that in PVD the precursors, i.e. the material to be deposited, start out in solid form, whereas in CVD, the precursors are introduced to the reaction chamber in gaseous form. The process involves four steps: (i) evaporation of the material to be deposited by a high energy source such as an electron beam or ions–this evaporates atoms from the surface; (ii) transport of the vapor to the substrate to be coated; (iii) reaction between the metal atoms and the appropriate reactive gas (such as oxygen, nitrogen or methane) during the transport stage; (iv) deposition of the coating at the substrate surface. PVD has several advantages including: (i) coatings formed by PVD may have improved properties compared to the substrate material; (ii) all types of inorganic materials and some types of organic materials can be used; (iii) the process is environmentally friendly compared to many other processes such as electroplating. However, PVD has also some disadvantages including: (i) problems with coating complex shapes; (ii) high process cost and low output; (iii) complexity of the process.

1.5

Spray coating

Spray coating is a process in which molten or softened particles are applied by impact onto a substrate to produce a coating. Spray coating techniques are widely used in industry for organic lacquers and for coating irregularly shaped glass and metals.26 Examples of some common spray coating techniques are on the following pages.


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1.5.1 Thermal spraying In the thermal spraying process, melted coating materials are sprayed onto the substrate to be coated. Particles of 1–50 μm are partially melted and accelerated to high velocities by a flame or an arc. The particles deposit onto a surface forming a coating, the quality of which is determined by the oxide content, porosity and adhesion to the substrate. The coating materials are usually heated by electrical or chemical means, and the sprayed material can be metal, ceramic or polymer. One of the main advantages of the thermal spray technique is its ability to provide coatings ranging from 15 μm to a few mm thick for substrates with large surface areas, at a high deposition rate compared with other conventional coating processes such as electro- and electroless deposition, CVD and PVD. Another advantage is the possibility of feeding powders of different coating materials such as ceramics, plastics and composites, or pure metal, and spraying them over the substrate surface.27,28

1.5.2 High-velocity oxygen fuel spraying High-velocity oxy-fuel spraying (HVOF) is a modified version of the thermal spray technique, developed in 1980. In this technique, a mixture of liquid or gaseous fuel in addition to oxygen is fed into a combustion chamber, where these are ignited and react with each other. There are several types of fuels used in HVOF. Gaseous fuels such as hydrogen, natural gas, methane, propane, or liquid fuels such as kerosene are commonly used. The resultant hot gas at high pressure (about 1 MPa) passes through a jet of very high velocity (∼1000 m/s). A powder of the coating materials is injected into the hot gas stream, which accelerates the powder up to 700–800 m/s. The stream of hot gas and powder is directed towards the substrate to be coated. The powder partially melts in the stream, and is deposited over the substrate.26 The resultant coating has a thickness of about 10 mm and is commonly used to improve corrosion and wear resistance.

1.5.3 Plasma spraying Plasma spraying is a coating process in which powders of the coating materials are fed into the plasma jet at around 10 000 K, at which the coating materials melt and are sprayed over the substrate to be coated. Owing to the interaction between the plasma coating materials and the substrate to be coated, several factors affect the final properties of the coating, such as the nature of the coating powders, composition of the plasma gas, gas flow



9

rate, energy input, torch geometry, distance from the substrate and final coating/substrate cooling parameters.29

1.5.4 Vacuum plasma spraying Vacuum plasma spraying was developed on the basis of the plasma spray technique, and can operate at relatively low temperatures, ranging from 40–120 °C, thus avoiding thermal damage to some types of coating materials such as polymers, rubbers or plastics. Moreover, this process can induce non-thermally-activated surface reactions, causing surface changes which cannot occur with molecular chemistries at atmospheric pressure.

1.5.5 Cold spraying The cold spraying coating technique is broadly based on the same ideas as the HVOF spraying technique, in that high velocity is used in order to enhance the interaction between the coating materials and the substrate to be coated. However, in the cold spraying technique, particles are accelerated to very high speeds by the carrier gas, which is forced through a nozzle. Upon impact, solid particles deform plastically and bond mechanically to the substrate to form a coating. Selecting a high velocity range is an important issue in cold spraying, where the velocity must be sufficient to create bonds between the coating materials and coating substrate. The velocity depends on the properties of the material, powder size and temperature. The first attempt to use the cold spraying technique was carried out in 1990 by a Russian research team who were testing the particle erosion of a target exposed to a high velocity gas steam loaded with fine powder. One major advantage of this technique is the possibility of using soft metals such as copper or aluminum as well as elements with high melting points such as tungsten, titanium and tungsten carbide and cobalt.26 Another advantage is the possibility of using an inert carrier gas such as nitrogen or helium instead of oxygen. The disadvantages of this technique are the low deposition efficiency, and the need to use a very fine powder in order to allow higher velocities, which is industrially unattractive.26

1.5.6 Warm spraying The warm spray technique was recently introduced as a novel modification of HVOF spraying.26 In this technique, the temperature of the combustion gas is lowered by mixing it with nitrogen, which is similar to the principle behind the cold spraying technique.


10


The advantage of this technique is that the coating efficiency is higher than when cold spraying is employed. Moreover, the lower temperatures used for warm spraying result in reduced melting and fewer chemical reactions in the feed powder compared to HVOF. These advantages are especially important for coating materials such as titanium, plastics and metallic glasses, which rapidly oxidize or deteriorate at high temperatures.

1.6

Other coating techniques

1.6.1 Sol–gel coatings New developments in the chemical tailorability of mixed alkoxide sol–gel coatings have led to the creation of an environmentally friendly and longlasting conversion coating for many ferrous and nonferrous alloys. Two of the most common problems associated with applying the sol–gel technique to protect metals against corrosion are: (i) the poor adhesion performance of the coatings formed using sol–gel processing; and (ii) the absence of high-performance coating systems based on environmentally acceptable salts. Recent research has shown that coatings based on silica, ceria, vanadia and molybdate can be adapted using sol–gel technology to produce a functionally gradient coating. This coating is able to provide covalent bonding for strong coating adhesion and can act as a high performance surface treatment, limiting water transport-induced attack on the surface of the material.12,13,30–37 This technique was successfully applied with different aluminum alloys.

1.6.2 Spin coating Spin coating has been used for several decades for the application of thin films. In this process, a small drop of the coating material is loaded onto the centre of a substrate, which is then spun at a controlled high speed. In the spin coating process, the substrate spins around an axis which should be perpendicular to the coating area. As a result, the coating material spreads towards, and eventually off, the edge of the substrate leaving a thin film of coating on the surface. Final film thickness and other properties will depend on the nature of the coating (viscosity, drying rate, percent solids, surface tension, etc.) and the parameters chosen for the spin process such as the rotation speed.

1.6.3 Gravure coating The gravure coating process relies on an engraved roller running in a coating bath, which fills the engraved dots or lines of the roller with the



11

coating material. The excess coating on the roller is wiped off by the doctor blade and the coating is then deposited onto the substrate as it passes between the engraved roller and a pressure roller.38

1.6.4 Roll-to-roll coating Roll-to-roll coating is the process of applying a coating to a flat substrate by passing it between two (or more) rollers. In this technique, the coating material is applied by one or more auxiliary rolls onto an application roll after the gap between the upper roller and the second roller has been appropriately adjusted. The coating is wiped off the application roller by the substrate as it passes around the support roller at the bottom. After curing, the coated substrate is then shaped to the final form; this has no effect on the properties of the coating. Roll-to-roll coating is made up of two different techniques: direct roll coating and reverse roll coating. In the direct roll coating technique, the applicator roll rotates in the same direction as the substrate. In the reverse roll coating technique, the applicator roll rotates in the opposite direction to the substrate.38

1.6.5 Knife over roll coating The knife over roll coating process is one of the most suitable coating techniques for high viscosity coatings and rubbers. In this process, the coating material being applied to the substrate passes through a gap between the knife and the roller. The excess coating is scraped off using the knife.38

1.6.6 Air/knife coating The air/knife coating process is similar to the knife over roll coating process. However, a powerful air jet is used instead of the knife. This is an extremely simple process, in which the coating is applied to the substrate and the excess is ‘blown off’ by the air jet; however, the noise associated with the air jet makes the process industrially unattractive.38

1.6.7 Meyer rod coating In the Meyer rod coating process, the coating is applied onto the substrate as it passes over a roller partially immersed in the coating material. The quantity (and sometimes the shape) of the coating to be applied on the substrate is controlled by a wire-wound metering rod (known as the Meyer rod), with the quantity determined specifically by the dimensions of the wire used.38


12


1.6.8 Slot/die and slot/extrusion coating In the slot/die process, the coating is squeezed out by gravity or by externally-applied pressure through a slot and onto the substrate surface. If the coating is composed entirely of solids, the process is known as ‘extrusion’. The coating thickness can be controlled by adjusting the line speed compared with the speed of the extrusion.38

1.6.9 Dip coating In the dip coating process, the substrate is immersed into a bath of the coating material of determined viscosity, with withdrawal speed and immersion time carefully controlled. The substrate is then removed from the bath and allowed to drain. The coated substrate can then be dried by force-drying or baking. Dip coating is extremely dependent on the viscosity of the coating. The coating viscosity must remain constant during the coating process. The main use of this process is the application of primers prior to final coats.

1.6.10 Curtain coating In the curtain coating process, a bath with a slot of a determined dimension in its base allows a continuous curtain of the coating to fall into the gap between two conveyors. The coating substrate is passed along the conveyor at a controlled speed so the coating material can be applied at the substrate surface.38

1.7

New lightweight materials

Interest in lightweight materials is increasing both in industry and in research circles. Magnesium alloys are one example of these lightweight materials to replace heavy alloys in the automotive and aerospace industries, resulting in savings in fuel consumption and a reduction in CO2 emissions. Magnesium alloys have a variety of excellent properties, including a high strength-to-weight ratio, low density, dimensional stability and castability. However, despite their excellent mechanical properties, magnesium alloys remain very susceptible to corrosion. Several coating schemes have been proposed to improve the corrosion resistance of magnesium.10–22 However, the existing methods are frequently either expensive, such as PVD and nitrogen ion implantation, or unable to create the surface properties desired for many applications where magnesium alloys would otherwise be highly competitive. On the other hand, an increased demand for new plastics and composite materials will require the development of both new coating solutions and



13

new application processes. Innovative changes in manufacturing processes will also require efficient coatings to be developed, which meet the appropriate surface quality standards.39,40

1.8

Trends in environmentally friendly coatings, self-assembling and self-cleaning coatings

This section describes the future trends in environmentally friendly coatings, self-assembling and self-cleaning coating technologies, reviewing the essential innovations in (i) the materials to be coated, (ii) the structure and chemistry of the coatings and (iii) coating techniques, based on new findings from basic research in physics and chemistry.

1.8.1 Environmentally friendly coatings The demand for more environmentally friendly coatings will increase as a result of stricter environmental legislation and tightening regulation on the use of hexavalent chromate and VOCs. The industrial coatings of the future will always be chrome-free, and low in solvent or solvent-free, provided that the resulting properties, both in terms of protection and appearance, are still acceptable. The old days of trial and error formulating with standard binder systems will give way to new binder resins with tailor-made properties and welldefined molecular structures, especially in the paint industry where a broad molecular weight distribution increases the viscosity of the solution. More attention will be paid to the statistical design of functionality in polymers to ensure greater uniformity of crosslinking during curing, and to optimize film performance. A number of monomers will cease to be used due to their toxic potential.39

1.8.2 Self-assembling molecules Self-assembling molecules can arrange themselves regularly and closely on a metal surface, and can then polymerize as a second step. This very flexible, strongly-anchored layer could improve coating adhesion, corrosion protection and mechanical and chemical resistance. The use of natural materials such as oils and resins has declined in paint binders because their performance did not match the desired requirements for modern coatings, but biochemical gene modifications could make natural raw materials attractive as components for paint binders. Natural polymers such as cellulose or chitin can be modified to make them more acceptable in paint binders.39–41


14


1.8.3 Self-cleaning coatings Another interesting trend is the development of self-cleaning coatings which are resistant to dirt, water and oil when applied to a wide range of substrates. Micro-patterns on surfaces are known to resist water and dirt penetration, and this so-called ‘lotus effect’ is already used in self-cleaning roof tiles and sanitary ware.39 Bright surfaces on car bodies, however, do not seem to be a realistic goal: artificial lotus surfaces are not self-renewing, so their sensitivity to cleaning and other mechanical treatments presents a problem. Other surface effects such as shark skin and fur-like films might find outlets in the coating of plastics.39

1.9

Trends in nanocoatings

The evolution of nanotechnology is behind the recent dramatic changes in several areas of scientific research and technology. In the area of surface coatings, new approaches that make use of nanoscale effects can be used to create tailor-made coatings with significantly enhanced properties. The ultimate impact of nanotechnology in the area of coatings will depend on its ability to direct the assembly of hierarchical systems that include nanostructures. Future approaches will focus on tailor-made coatings for specific functions, either by incorporating existing identifiable components into the desired coating or by the formation of new structures during the coating process.42 Interest in nanocoatings has increased because of the potential to synthesize materials with unique physical, chemical and mechanical properties. There are several types of design models for nanocoatings, such as nanocomposite coatings, nanoscale multilayer coatings, super lattice coatings, nanograded coatings, etc.43 In the last decade, research interest in nanostructured coatings has increased due to the potential for enhancing the coating functionality for specific applications such as environmentally friendly anti-corrosion coatings for the automotive and aerospace industries.13,15,37 The data showed a significant improvement in the corrosion resistance of materials as compared to materials processed using conventional coating methods. The following section reviews the most common nano-based coating systems and their expected future impact.

1.9.1 Micro- and nanocapsule-based coatings Micro- and nanocapsules or containers are of great interest to both industry and the scientific research community, and have a wide range of applications such as molecular biology, electronic materials, medical imaging and



15

photonic crystals. Moreover, they have also been increasingly used as fillers, coatings, capsule agents, etc., because of their low density and optical properties. These shells are created either by in situ hydrolysis of the corresponding metallic salt in the presence of core materials,44–49 or by calcination of polymer particles coated with uniform inorganic shells.41, 50–56 Recently, several nanocontainers were synthesized using a two-step process. In the first step, charged polystyrene nanospheres were prepared using emulsion polymerization or polymerization in suspension. In the second step, the polystyrene lattices were coated using the sol–gel method to form a layer of inorganic oxide(s). The composites were treated in air to burn off the polystyrene latex. Using this approach, different nanocontainers such as cerium/molybdenum oxide, cerium/titanium oxide, iron/titanium oxide, silicon/calcium oxide, polypyrrole and polyaniline were produced.55 However, further research still needs to be carried out in order to optimize the experimental coating conditions such as the coating thickness and temperature. Moreover, a multistep process is industrially unattractive. To make self-repairing coatings, the researchers first encapsulated a catalyst into spheres less than 100 μm in diameter. They also encapsulated an inhibitor or a healing agent into similarly sized microcapsules. The microcapsules are then dispersed within the desired coating material and applied to the substrate.39 When the coating is subjected to corrosion or scratching, some of the capsules break open, spilling their inhibitor contents onto the damaged region. The healing agent reacts with the environment to form a protective oxide to repair the damage, depending upon environmental conditions. Another approach based on a dual-function tailor-made capsule containing a healing agent and a catalyst has also been proposed. The healing agent (inhibitor) offers a self-healing property which protects against scratches and corrosion and the catalysis provides extra functions such as antimicrobial effects or other desired functions.

1.9.2 Nanocomposite coatings A nanocomposite coating consists of a nanocrystalline phase and an amorphous phase. Several techniques have been proposed for the preparation of nanocomposite coatings. However, reactive magnetron sputtering is most commonly used. Nanocomposite coatings can be tailored to offer superior hardness above the maximum given by the rule of mixture (materials with hardness greater than 80 GPa are called ultra-hard materials). Therefore, detailed theories have been developed and extensive experiments carried out with the aim of optimizing the hardness of nanocomposite coatings for many industrial applications. However, while a great deal of attention is


16


given to increasing the hardness of a material, insufficient attention is paid to its toughness. Accordingly, further research should be carried out which considers both hardness and roughness while taking into account corrosion behavior. In designing nanocomposite coatings, several factors and application requirements must be considered. Research areas in the field should involve prevention of the formation of dislocations in the nanocrystalline phase; blocking of the grain-boundary sliding of nanograins; the effect of changes made to the lattice parameter; the role of the crystal size; the nature of the grain boundaries; and finally the effects of impurities and intermediate phases.56

1.10

New composite and powder coatings

1.10.1 Composite coatings Composite coating technology has been developed to fulfill the industrial demands for coatings whose specifications exceed the capabilities of conventional coating technologies, and that are capable of functioning in extreme environments and in the face of challenges posed by temperature, corrosion, abrasion, fatigue, friction and erosion.57 Tungsten carbide hard metals and their analogues are now a mature technology; however, recent research has focused on changing the design of the microstructure with the aim of producing alternatives to the conventional two-phase structure. Significant improvements in the performance of coatings have been achieved by changing the size, shape and distribution of the phases to produce ultra fine-grained materials.57 Some approaches involve the use of nanoscale powder to form nanocomposite coatings as discussed in Section 1.9.2.

1.10.2 Powder coatings Designing a high performance clear coating is a key target for the automotive industry. Powder coating offers a limitless choice of colors and finishes. Moreover, powder coating produces a high specification coating which is relatively hard, abrasion-resistant and tough. The thickness of the coating applied can be varied considerably according to requirements. The prospects for powder coatings will improve with the development of materials with lower curing temperatures, and the production of thinner films will become an achievable goal as powder particle morphology improves. The use of UV curable powder coatings seems to have great potential. Controlling the humidity content of the surface can also facilitate electrostatic coating. The size and shape of the powder particle will be important factors in achieving much smoother films. Ultrasonic waves seem



17

to be a promising means of adjusting the shape of the powder particle to make it more spherical.

1.11

Advanced polymers and fillers

Developments in the structure of coatings using advanced polymers represent one noteworthy future trend in coating technology.

1.11.1 Hyperbranched polymers Controlled radical polymerization will be used for the synthesis of hyperbranched polymers with low melt viscosities; this is ideal for coatings with a high solid content and for powder coatings. The addition polymerization of acrylic monomers in hypercritical fluids is one potential means of producing solvent-free binders with narrow melting ranges for use in powder coatings with low film thickness and low temperature curing characteristics.39,40

1.11.2 Organic–inorganic hybrid polymers Hybrid polymers have proved to be of great interest in the development of future coating systems. Combinations of organic polymers and silicates make it possible to improve the overall coating qualities, as the stability and scratch resistance of inorganic networks can be combined with the elasticity of organic polymers. The sol–gel method has been successfully used for the synthesis of hybrid polymers.39 Most of the current multiphase polymer systems do not produce translucent films, a fact that has limited their wider acceptance as coatings. A method for producing polymers with improved translucent qualities was reported by Brock.39 This method uses a construction of block and comb polymers with intramolecular incompatibility, followed by phase separation in the nanofield. Future developments in phase separation, along with a better understanding of the interface energies of polymer mixtures, will lead to improved adhesion to the surface with no negative effect on the coating qualities.

1.11.3 Conductive polymers Newly-developed conductive polymers based on polyanilines and polythiophens might offer improved corrosion resistance. These could also permit electrostatic and electrochemical coating of non-metallic substrates, as well as producing heat-conductive layers for electrically heatable surfaces.


18


Microporous coatings containing catalysts are currently the subject of extensive research in environmental science, due to their ability to remove toxic gases. Coatings that contain a small amount of active components (such as photochromic coatings, which are translucent or opaque according to light intensity) will be increasingly used as a means of identifying ownership of stolen items.39,40

1.11.4 Water-soluble paint An increase in the use of water-soluble latex paints is one of the biggest future trends in architectural paint industries. Water-soluble paints are less expensive, lower in odor than alkyd-based solvent-borne paints, and produce no toxic waste. In the coatings and paints industries, water-soluble paints have met with strong competition from recently-developed superior resins with unique characteristics.

1.11.5 Fillers Fillers are widely used in every coatings industry, mainly to reduce costs, although they can negatively affect the coating quality. Future developments in the chemistry and structure of the fillers will lead to an increase in their use in tailor-made coating systems; the fillers will then have a specific function such as strengthening the mechanical properties of the coatings or improving the coating quality for specific applications and decorative effects. Nanoscience and nanotechnology will have a significant impact on the design of ultra-thin films containing nanolayers of special fillers or additives embedded into the matrix of the polymer. It is expected that such fillers will improve the mechanical strength, coating transparency and corrosion resistance. Phyllo silicates with their unique leaf-like structure will increasingly be the subject of both industrial and basic research as a filler for tailor-made coatings. Some studies used phyllo silicates interlocked into the polymer matrix in nano form to improve the barrier effect of the coating. This approach will become more popular and will be used for thermal and electrical insulation, and especially for fire protection.39,40

1.12

Developments in coating processes

The processing of nanocoatings will be of interest to researchers due to the superior hardness and strength that these coatings can offer. However, developments in nanocoatings are determined by improvements in coating processing and the availability of nanopowders. The use of nanocoatings is still in its infancy because the process requires large-scale control during



19

the synthesis of nanoparticles. Moreover, the technology requires sophisticated instruments and multistep processes. Nanopowders are used as feedstock materials for thermal spray processes (plasma spraying or HVOF spraying). Thermal spraying offers the unique advantage of moderate to high rate of throughput and the ability to coat target materials with complex shapes using nanostructured feedstock powders prepared from vapor, liquid and solid routes. Thermal sprayed nanocoatings with moderate hardness showed better wear resistance than those fabricated by micro powders.58 HVOF will get the most research attention in the next generation of nanocoatings, due to its ability to deposit dense nanocrystalline ceramic coatings with wear properties superior to those produced by plasma spraying, thanks to the lower spraying temperature involved. Moreover, HVOF allows the development of nanocoatings with low porosity, high strength and increased wear resistance. Some modifications will be made to the electro-dip processes in order to allow lower curing temperatures; new curing mechanisms using radiation curing will also extend their field of application to low melting point materials such as plastics.39 Automation of the coating process in order to increase line speed, reduce labor dependency and save energy will be the main target for coating industries. Future improvements in coating processes will include reduction of the number of coating layers; full automation of the coating process; controlling the color of the end product using a module method; and automatic quality control. The future trends in coating technology are generally based on: 1. Advanced lightweight materials such as magnesium alloys and composite materials. Developments in magnesium alloys and composite materials will continue, focusing on rare-earth alloying elements such as cerium and yttrium with the aim of providing some desirable properties. 2. Tailor-made coating systems with unique chemistry and structures will become commonplace thanks to innovations in polymers, new binders and fillers. 3. Nanoscience and nanotechnology will play a distinct role in the next generation of coating technology. More attention will be paid to selfhealing coatings based on nanocontainers or nanocapsules that can be filled with inhibitor to protect the substrate from corrosion upon damage or scratching in the coating layer. However, more studies still need to be carried out to optimize the coating conditions since the technology currently involves multisteps (around 8–10 steps) and expensive raw materials, both of which are industrially unattractive. 4. Many future efforts will be devoted to the design of biocompatible nanocoatings systems for medical implant applications and to the


20


development of suitable techniques for preparing high performance hydroxyapatite coatings. 5. Developments in the coating process will be an important topic for industrial and basic research. New surface modification techniques and faster and cheaper curing methods using UV and electrostatic application will be the focus of the next decade of research.

1.13

Acknowledgements

Many thanks to all the authors of papers, books, and websites and to all published sources (listed below) that were used to prepare the materials for this chapter.

1.14

References

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coated and bare metals in aqueous solutions’, in Kalnin˛sˇ T and Gulbis V (eds), Corrosion Protection: Processes, Management and Technologies, New York: Nova Science Publishers, No. 7, 161–173. Hamdy AS (2010), High Performance Coatings for Automotive and Aerospace Industries, New York: Nova Science Publishers. Hamdy AS (2010), ‘An attempt for designing economically attractive chromefree conversion coatings for magnesium alloys’, in Hamdy AS (eds), High Performance Coatings for Automotive and Aerospace Industries, New York: Nova Science Publishers, 127–138. Hamdy AS (2008), ‘A novel approach in designing chrome-free chemical conversion coatings for automotive and aerospace materials’, Pitture e Vernici (European Coatings), 86(3), 43–50. Hamdy AS (2006), ‘Enhancing corrosion resistance of magnesium alloy AZ91D in 3.5% NaCl solution by cerate conversion coatings’, Anti-Corr. Meth. Mater., 53(6), 367–373. Hamdy AS (2006), ‘Green cerate based surface treatment for improving the corrosion resistance of magnesium alloy AZ91D in marine environments’, Proceedings Green Engineering for Materials Processing Symposium, Materials Science & Technology Conference and Exhibition (MS&T’06), 15–19 October, Cinergy Center, Cincinnati, OH, 141–150. Hamdy AS (2007), ‘Alkaline based surface modification prior to ceramic based cerate conversion coatings for magnesium AZ91D’, Electrochem. Solid-State Lett., 10(3), C21–C25. Hamdy AS and Butt D (2007), ‘Eco-friendly conversion coatings for automotive and aerospace materials’, invited talk, European Coatings Conference ‘New Concepts for Anti-Corrosive Coatings, 14–15 June, Berlin. Marx B, Hamdy AS, Butt DP and Thomsen D (2007), ‘Assessing the performance of stannate conversion coatings on Mg alloys’, Symposium ‘Corrosion and Coatings Challenges in Industry’, 88th Annual Meeting, 17–21 June, Boise, ID, co-located with the 62nd Annual Meeting of the American Chemical Society. Hamdy AS, Marx B, Butt DP and Thomsen D (2007), ‘Novel eco-friendly stannate-based conversion coatings for Mg alloys’, Surface Treatments and Processing Session, Symposium: Automotive and Ground Vehicles: Materials and Processes for Vehicles, MS&T ’07 Conference and Exhibition, 16–20 September, Detroit, MI. Hamdy AS, Marx B and Butt DP (2009), ‘A new approach in designing ecofriendly low cost chemical conversion coatings for magnesium alloys’, Keynote Speaker, 6th International Symposium on Surface Protective Coatings, 25–28 February, Goa, India. Mittal CK (1995), ‘Merbromin as a colouring agent for anodized surfaces of Al and its alloys’, Trans. Metal Finishers Association of India, 4, 227. Kasten LS, Grant JT, Grebasch N, Voevodin N, Arnold FE and Donley MS (2001), An XPS study of cerium dopants in sol–gel coatings for aluminum 2024T3, Surf. Coat. Technol., 140, 11–15. Sharma AK, Uma Rani R and Giri K (1997), Studies on anodization of magnesium alloy for thermal control applications, Metal Finishing, 95, 43–51.


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26. Kuroda S, Kawakita J, Watanabe M and Katanoda H (2008), Warm spraying – a novel coating process based on high-velocity impact of solid particles, Sci. Technol. Adv. Mater., 9, 17. 27. Paulussen S, Rego R, Goossens O, Vangeneugden D and Rose K (2005), Plasma polymerization of hybrid organic–inorganic monomers in an atmospheric pressure dielectric barrier discharge, Surf. Coat. Technol., 200(1–4), 672–675. 28. Leroux F, Campagne C, Perwuelz A and Gengembre L (2008), Fluorocarbon nano-coating of polyester fabrics by atmospheric air plasma with aerosol, Appl. Surf. Sci., 254(13), 3902–3908. 29. Anon. (2010), Plasma spraying process, available at: http://www.zircotec.com/ page/plasma-spray_processing/39 (accessed April 2011). 30. Hamdy AS (2006c), ‘Corrosion protection of aluminum composites by silicate/ cerate conversion coating’, Surf. Coat. Technol., 200(12–13), 3786. 31. Hamdy AS and Butt DP (2006), ‘Environmentally compliant silica conversion coatings prepared by sol–gel method for aluminum alloys’, Surf. Coat. Technol., 201(1–2), 401–407. 32. Hamdy AS (2006), ‘Advanced nano-particles anti-corrosion ceria based sol gel coatings for aluminum alloys’, Mater. Lett., 60(21–22), 2633–2637. 33. Hamdy AS and Butt DP (2006), ‘Corrosion protection performance of nanoparticles thin-films containing vanadium ions formed on aluminum alloys’, AntiCorr. Meth. Mater., 53(4), 240–245. 34. Hamdy AS and Butt DP (2007), ‘Novel anti-corrosion nano-sized vanadia-based thin films prepared by sol–gel method for aluminum alloys’, Mater. Proc. Technol., 181(1–3), 76–80. 35. Hamdy AS (2006), ‘A clean low cost anti-corrosion molybdate based nano-particles coating for aluminum alloys’, Prog. Org. Coat., 56(2–3), 146– 150. 36. Hamdy AS, Butt DP and Ismail AA (2007), ‘Electrochemical impedance studies of sol–gel based ceramic coatings systems in 3.5% NaCl solution’, Electrochim Acta, 52, 3310–3316. 37. Hamdy AS, Shoeib M and Butt DP (2009), ‘A novel approach in designing environmentally compliant sol–gel based ceramic coatings and nanocomposite coatings for industrial applications’, in Malik A and Rawat RJ (eds), New Nanotechniques, New York: Nova Science Publishers, 649–659. 38. Anon. (2010), Technical Coating International Inc., available at: http://www. tciinc.com/coating.html (accessed April 2011). 39. Brock T (2005), ‘Trends in coatings technology’, Pitture e Vernici (European Coatings), 81, 15–24. 40. Murata K (1996), ‘Trends in coatings technology’, J. Macromol. Sci., Part A, 33(12), 1837–1841. 41. Wang M, Jiang M, Ning F, Chen D, Shiyong L and Duan H (2002), ‘Blockcopolymer-free strategy for preparing micelles and hollow spheres: self-assembly of poly(4-vinylpyridine) and modified polystyrene’, J. Macromolecules, 35, 5980. 42. Baer DR, Burrows PE and El-Azab AA (2003), ‘Enhancing coating functionality using nanoscience and nanotechnology’, Prog. Org. Coat., 47, 342–356. 43. Zhang S, Sun D, Fu Y and Du H (2003), ‘Recent advances of superhard nanocomposite coatings: a review’, Surf. Coat. Technol., 167, 113–119.



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44. Hwang YJ, Oh C and Oh SG (2005), ‘Controlled release of retinol from silica particles prepared in O/W/O emulsion: The effects of surfactants and polymers’, J. Control Release, 106, 339–349. 45. Zhang Y, Hu Q, Fang Z, Cheng T, Han K and Yang X (2006), ‘Self-assemblage of single/multiwall hollow CeO2 microspheres through hydrothermal method’, Chem Lett., 35, 944–945. 46. Zhan L and Wan M (2003), ‘Self-assembly of polyaniline – from nanotubes to hollow microspheres’, Adv. Funct. Mater., 13, 815–820. 47. Ocana M, Hsu WP and Matijevic E (1991), ‘Preparation and properties of uniform-coated colloidal particles 6. Titania on zinc oxide’, Langmuir, 7, 2911–2916. 48. Kawahashi N and Matijevic E (1990), ‘Preparation and properties of uniformed coated colloidal particles V. Yttrium basic carbonate on polystyrene latex’, J. Colloid Interface Sci., 138, 534–542. 49. Tapeinos C, Kartsonakis IA, Liatsi P, Danilidis I and Kordas G (2008), ‘Synthesis and characterization of magnetic nanocontainers’, J. Am. Ceram. Soc., 91, 1052–1056. 50. Yang M, Niu Z, Dong X, Xu H, Zhaokai M, Zhaoguo J, Lu Y, Hu Z and Yang Z (2005), ‘Synthesis of spheres with complex structures using hollow latex cages as templates’, Adv. Funct. Mater., 15, 1523–1528. 51. Tartaj P, Teresita GC and Serna CJ (2001), ‘Single-step nanoengineering of silica coated maghemite hollow spheres with tunable magnetic properties’, Adv. Mater., 13, 1620–1628. 52. Wang D, Song C, Lin Y and Hu Z (2006), ‘Preparation and characterization of TiO2 hollow spheres’, Mater. Lett., 60, 77–80. 53. Eiden S and Maret G (2002), ‘Preparation and characterization of hollow spheres of rutile’, J. Colloid Interface Sci., 250, 281–284. 54. Song C, Wang D, Gu G, Lin Y, Yang J, Chen L, Fu X and Hu Z (2004), ‘Preparation and characterization of silver/TiO2 composite hollow spheres’, J. Colloid Interface Sci., 272, 340–344. 55. Anon. (2009), MULTIPROTECT Newsletter, No 2, p. 3, available from: http:// multiprotect.org. 56. Zhang G, Yu Y, Chen X, Han Y, Di Y, Yang B, Xiao F and Shen J (2003), Silica nanobottles templated from functional polymer spheres, J. Colloid Interface Sci., 223, 467–472. 57. Allcock BW and Lavin PA (2003), ‘Novel composite coating technology in primary and conversion industry applications’, Surf. Coat. Technol., 163–164, 62–66. 58. Tjong SC and Chen H (2004), ‘Nanocrystalline materials and coatings’, Mater. Sci. Eng. R 45, 1–88.


2 Nanostructured thin films from amphiphilic molecules J. Y. PARK and R. C. ADVINCULA, University of Houston, USA

Abstract: Over the past several decades, the Langmuir monolayer and Langmuir–Blodgett (LB) techniques have been widely utilized for the characterization of small amphiphilic molecules such as organic polymers (i.e. block copolymers, star block copolymers, and dendritic polymers), inorganic nanomaterials, and even carbon nanotubes and biomolecules. This chapter will introduce several amphiphilic and colloidal materials and describe their surface chemistry and properties at the air–water interface. Finally, some examples of thin film applications using LB films will be discussed. Key words: Langmuir monolayer, Langmuir–Blodgett (LB), Langmuir– Schaefer (LS), air–water interface, phase transition.

2.1

Langmuir monolayer

Thin insoluble or amphiphilic monolayers at the air–water interface have been studied for more than 200 years since Benjamin Franklin characteristically made the first observation in 1774. Later, in 1890, Lord Rayleigh1 reported a quantitative measurement on a monolayer of oil molecules resulting from spreading on water. In 1891, Agnes Pockels2 applied a simple equipment to observe general behavior of surface tension with varying surface concentrations of oil and to measure the thickness of films of various amphiphillic substances on the surface of water. Then, in 1917, Irving Langmuir further improved the technique that was already used by Pockels.3 With this new device he confined the film between a fixed barrier on one side and a floating one on the other side, and the monolayer was oriented on the water surface via the compressing floating barrier (Fig. 2.1). The surface pressure was recorded by measuring the actual force on the floating barrier. Since then, a ‘Langmuir monolayer’ has been defined as a molecularly thin layer trapped at the air–water interface.4 Typically, small insoluble, amphiphilic molecules possess a polar or charged hydrophilic head group, e.g. -COOH and -NH3, and a hydrophobic hydrocarbon tail (-CH3(CH2)n, with n > 4).5 24 © Woodhead Publishing Limited, 2011

Nanostructured thin films from amphiphilic molecules

25

2.1 A schematic representation of a Langmuir–Blodgett rough being compressed.

dc

I

Barrier

2.2 Isotropic liquid film in a trough with a movable barrier. The arrows show the direction of the barrier’s movement.

2.1.1 Surface pressure The surface pressure (π) of a film is obtained from the difference between the surface tension of the pure subphase, γ0 (pure water) and that of the subphase with monolayer, γ. That is: π = γ0 − γ

[2.1]

For a simple isotropic system as shown in Fig. 2.2, a thin film is on a pure water surface. If the barrier is moved back by dx (i.e. the film area is stretched by dA = ldx), the change of surface free energy, G, is ΔG = γldx

[2.2]

where the surface tension γ is the free energy per surface area (unit: mN/m) and l is the length of the barrier. If the barrier is moved slowly so that the temperature is not changed and the force acting on the barrier is invariant, the force should be γl. The work on the system while stretching the film is γldx. Also, the surface tension γ could be considered as the force per length.


26


2.1.2 Surface pressure isotherm A surface pressure isotherm is one of the major experimental techniques to examine the phase behavior of quasi-two-dimensional systems on air– liquid interface. Depending on the balance between hydrophilicity and hydrophobicity, the molecules may be more or less soluble in water. When amphiphilic molecules between the two barriers are spread on the water surface, the molecules are far apart and lie on the surface randomly. This is a two-dimensional (2-D) gaseous (dilute) phase, shown as ‘G’ phase in Fig. 2.3. Upon compression from a gaseous phase, the monolayer undergoes several phase transitions. As the barriers move towards each other, the molecules become closer, resulting in increasing interactions among them. Phase separation into a ‘gas’ and a ‘liquid’ occurs on the monolayer. After that, the monolayer behaves like a two-dimensional liquid. This disordered liquid, a ‘liquid expanded phase’ corresponds to ‘LE’ in Fig. 2.3. Further

Liquid expanded (LE) Untilted condensed

60

Gaseous (G)

Surface pressure (mN/m)

50 (LE+G) 40

200

Tilted condensed

400

(LC)

30

Liquid expanded 20

(LE) (LC+LE)

10

Phase coexistence: condensed+liquid expanded 20

25

30 35 Area per molecule (Å2)

40

2.3 A typical surface pressure isotherm for a Langmuir monolayer after reference 7. The insets in the figure show the molecular orientation at the air–water interface. The surface pressure is from reference 8 measured in pentadecanoic acid.



27

compression of the monolayer gives rise to a transition from liquid expanded to a condensed phase with a plateau indicating a first-order transition.6 The molecules become more compact and they also begin to orient with some order in the hydrocarbon chains (a ‘liquid condensed phase’, i.e. ‘LC’ phase in Fig. 2.3). With further compression, all the molecules go over into an untilted condensed phase without any tilt angle. This is a typical surface pressure isotherm of a fatty acid. If the layer is compressed further, the monolayer will break and the molecules will form three-dimensional (3-D) structures of various orientations (it is called ‘collapse’).4

2.1.3 Langmuir trough and other experimental techniques A Langmuir trough is the main instrument to spread amphiphiles from an organic solvent solution and generate thin monolayers at the air–water interface. Instead of polymer coated troughs used in the early stages of development, metal or glass troughs with a wax coating were used. These days, a Teflon trough is the most commonly used for troughs because it is hydrophobic and chemically inert. While compressing or expanding the molecules on the surface with Teflon barriers, the effect of the monolayer on the surface pressure of the liquid is measured through use of a Wilhelmy balance, or electronic wire probes. A Langmuir–Blodgett (LB) film can then be transferred to a solid substrate by vertical dipping the substrate through the monolayer or by ‘picking-up’ the monolayer through a flat substrate in a horizontal dipping method, the so-called Langmuir–Schaefer (LS) method. Various methods and devices can be used in conjunction with the trough in order to investigate in-situ the properties of thin films. These methods include Brewster angle microscopy (BAM) to observe reflectivity images in real time and x-ray reflectivity to measure the diffraction behavior of a deposited film, the Wilhelmy Plate to measure the surface pressure, and a surface potential meter to measure the surface potential. Usually, a water bath is used to control the temperature and a robotically controlled dipper is used to transfer the LB films onto a solid substrate, i.e. whilst maintaining a surface pressure or area while controlling the deposition of alternating LB or LS monolayers. The transfer of a monolayer to a substrate is a sophisticated process which is dependent on many factors, such as the direction and speed of a substrate, surface pressure, temperature, and pH of the subphase not to mention the stability of the insoluble monolayer or amphiphile for transfer. A dipping holder with the substrate can be programmed to pass through the interface from top to bottom or bottom to top at a set speed. Depending on the hydrophilic or hydrophobic substrate, the dipping process can start from below the liquid surface or above the liquid surface, respectively. Multilayers can be achieved by sequential dipping through alternating monolayers.


28


2.1.4 Recent research related to Langmuir monolayers and Langmuir–Blodgett (LB) While the Langmuir monolayer and the LB technique have been utilized mostly for characterization and deposition of small amphiphilic molecules, they have been highly useful tools to determine equilibrium and dynamic behavior in thin monolayers at the air–water interface, related to phase transition (from gas phase to liquid and solid phase) of all types of materials in general. Recently, these techniques have been extensively used for characterization and application of a variety of polymers (i.e. block copolymers, star block copolymers, and dendritic polymers), inorganic nanomaterials, and even carbon nanotubes and biomolecules. Based on these techniques, molecular interaction, molecular dynamics, and molecular orientation at the air–water interface, reflecting morphological changes and phase transition behaviors, can be more precisely analyzed with a combination of other instrumental techniques. The rest of the chapter introduces several synthesized amphiphilic macromolecules and nanomaterials which have been investigated with the LB technique. Their physico-chemical properties at the air–water interface will be described. Finally, some examples of applications using the LB film method will be discussed.

2.2

Amphiphilic polymers

2.2.1 Block copolymers Block copolymer self-assembly at the air–water interface is commonly regarded as a two-dimensional counterpart of equilibrium block copolymer self-assembly in solution. Thus, the interfacial behavior and surface morphology of aggregates of block copolymers on the water surface using the LB technique can be used to fundamentally understand the nature of polymeric materials at interfaces. Since year 2000, various synthesized polymers have been extensively introduced and investigated for their unique properties and organization at the air–water interface. Polystyrene (PS) is a very common anchoring block because it is nonpolar and forms tightly clustered aggregates when spread at moderate concentrations under ambient conditions on the water surface. Recently, poly(styrene)–b–poly(ethylene oxide) [PS–b–PEO] copolymers have been extensively studied. Predominantly hydrophobic PS–b–PEO LB films have three types of features: dots (circular aggregates), spaghetti (rod-like aggregates), and continents (planar aggregates), upon change in concentration. This parameter directly controls a competition between the film spreading and polymer entanglement during solvent evaporation (Fig. 2.4a).9 The other parameter is control of ratio of the PEO blocks. By increasing the



29 Air

Compression High conc. Low π

High π

Water

Compression Low conc. (a) Solvent evaporation Concentration increase

NPS >> NPEO PS

PEO

NPS Ph-(CH2CH2-O)1–2-Li -OH

-S-C6H4-C6H4-Li -O-Si-C6H4-C(C6H4)(C5H11)-Li -N+(Cl−)(C2H5)2-C12H24-O-C6H4-C(C6H4)(C5H11)-Li -O-Si(Me2)-C11H22-O-C6H4-C(C6H4)(C5H11)-Li -S-C11H22-O-C6H4-C(C6H4)(C5H11)-Li

Initiating species

Coordination-insertion polymerisation, monomer activation by enzymes Gold and silica surfaces -S-C11H22-(O-C2H4)3-OH + Sn(octyl)2 -O-Si-C3H6-NHCO-(O-C2H4)2-OH + Sn(octyl)2 Gold -S-C11H22-(OC2H4)3-OH + lipase Silicon wafer, quarz; -O-Si-C3H6-NH2 -O-Si-C3H6-NH2 + Sn(octyl)2 glass fibres Carbon nanotubes >Ph-CH2CH2-OH + Sn(octyl)2 Magnetite nanoparticles -OCO-CH(NH2)-CH2-CH2-OH + Sn(octyl)2 stabilised with L-serine -OCO-CH2-OH + Sn(octyl)2 or glycolic acid

Gold nanoparticles Silica nanoparticles

Cationic polymerisation Silicate

Hydroxyapatite

Carbon nanotubes

Silica particles

Ionic polymerisation Gold Silicon wafer Clay Flat silica and gold

Surface

Yoon et al., 2003a,b, 2004 Yoon et al., 2003c Wieringa et al., 2001; Jiang et al., 2005 Priftis et al., 2009 Nan et al., 2009a,b

CLO NCA CLO CLO, EO CLO, LA

Jordan et al., 2001 Kim et al., 2004

Ox IB

DiOx

Zhao and Brittain, 2000a

Wiegand et al., 2008

Sakellariou et al., 2008

Yang et al., 2007

Jordan et al., 1999 Quirk and Mathers, 2001 Zhou et al., 2001 Advincula et al., 2002

Reference(s)

St

St, CLO LA

St Is St St or 1. St; 2. Is or 1. Bd; 2. St CLA

Monomer*

Table 4.4 ‘Grafting from’ of unsaturated and cyclic monomers by using ionic, metathesis and other polymerisation mechanisms


-O-Si-(5-bicycloheptenyl or norbornenyl) Ru[(cyclohexyl)3P]2Cl2

1. -O-Si-C3H6NH2 2. O-Si-C3H6-NH=C(NH-C4H9)[Ni(CH=N-C4H9)3]2+2ClO4− PE + thioxantone + UV light PTFE-OOH through ion irradiation -CONH-C6H3(Me)-NH2 -S-alkyl-NH2 -O-Si-alkyl-NH2 1. -O-Si-C6H4-CMe2-OCD3/TiCl4 2. -O-Si-C6H4-CMe2-PSt-Cl/CuBr/anisole 1. H3N+(Cl−)-C6H12-OCO-C(Me2)-Br 2. H3N+(Cl−)-C6H12-OCO-C(Me2)-PSt-Br/AgPF6

Silicon wafers Quarz Low density PE PTFE Carbon nanotubes Gold Silica Silicate

Zhao and Brittain, 1999

1. 2. 3. 4.

St (cationic) MMA (ATRP) St (ATRP) THF (cationic)

Bai et al., 2009 Choi et al., 2009 Lafuente et al., 2009 Tuberquia et al., 2009

GMA AA FD DAM

Yenice et al., 2009

Ogawa et al., 2007 Lim et al., 2008

Vestberg et al., 2007

Feng et al., 2007; Harada et al., 2003; Jeon et al., 1999; Rutenberg et al., 2004

1. DeMPA-N3 2. DeMPA-C≡CH a.s.o DISPB + DEMTEB ICAME

NOR BCH COD

* AA = acrylic acid; BCH = 5-bicycloheptadiene; Bd = butadiene; CLA = ε-caprolactam; CLO = ε-caprolactone; COD = cyclooctadiene; DAM = diazomethane; DeMPA-C≡CH are alkyne functionalised dendrimers based on 2,2-bis(methylol)propionic acid; DeMPA-N3 = azide functionalised dendrimers based on 2,2-bis(methylol)propionic acid; DiOx = p-dioxanone; DEMTEB = 1,4-diethynyl-2,5bis(methoxytriethoxy)benzene; DISPB = 2,5-diiodo-1,4-bis(3-sulfonatopropyl)benzene; EO = ethylene oxide; FD = 1,10-bis[94-formyl3-hydroxyphenyl0oxy]decane; GMA = glycidyl methacrylate; IB = isobutene; ICAME = L-isocyanatoalanyl-L-alanine methyl ester; Is = isoprene; LA = lactides; MMA = methyl methacrylate; NCA = N-carboxyanhydrides of γ-benzyl L-glutamate and γ -methyl L-glutamate; NOR = norbornene; Ox = 2-oxazolines; St = styrene; THF=tetrahydrofuran

Clay

-O-Si-C3H6- NHCO-C6H4-I + Pd(PPh3)4 +CuI + NEt3

Silica microparticles

Other mechanisms and combined mechanisms Silicon wafer -O-Si-C3H6- NHCO-C2H4-C≡CH

Metathesis Inorganic surfaces

102


C(Me)(CN)-N=N-C(Me(CN)-R

CN

CN

hν or Δ

+

C

–N2

Me

C-R Me

Bulk polymerisation

(a)

Photoexcitation Ph-CO-Ph [Ph-CO-Ph]*

(b)

Ph

+ C Hydrogen Ph abstraction OH Low reactive Surface initiating species radical

4.4 Surface initiated conventional radical polymerisation: (a) scission of surface-bound azo labile groups and promoting the side bulk polymerisation; (b) polymerisation in the presence of photosensitisers.

attached to solid substrates. The peroxide initiators were also introduced directly, by physical methods. Table 4.2. shows such polymerisation systems. The shortcomings of this procedure arise from the fairly high polydispersity of the resultant polymers and the formation of bulk polymer, as the thermal or photochemical scission of the initiator gives raise to two active radical species, only one being bound to the surface (Figure 4.4a). The bulk polymer will entangle with the surface-linked polymer or will adsorb on the surfacebound layer, thus requiring a careful post-polymerisation cleaning. Moreover, crosslinking may occur because of the termination of the chain growth through the recombination. To minimise bulk polymerisation, photosensitisers such as benzophenone were used (Hu et al., 2006). Under a photo-excited state, benzophenone is able to abstract a hydrogen from the substrate, yielding a non-reactive ketyl radical and an initiating radical on the surface (Figure 4.4b). However, a more controlled radical polymerisation in the presence of surface-linked azo-initiators may be achieved when the surface is not flat but is porous (Ikeda et al., 2009), due to the reduced termination reaction determined by restriction of the movement of the polymer chains covalently bound to the inside walls of the porous membrane. Controlled radical polymerisation Living radical polymerisation mechanisms such as reversible addition– fragmentation chain transfer polymerisation (RAFT), nitroxide mediated radical polymerisation (NMP) and atom transfer radical polymerisation


Surface-initiated polymerisation for nanocoatings

103

(ATRP) of vinyl monomers, (meth)acrylates or macromonomers (Table 4.3), as well as ionic polymerisation of unsaturated and cyclic monomers and methatesis, are alternative applications for achieving a better control of the surface-bound polymer layers (Table 4.4). Owing to their living character, these approaches are also appropriate for the preparation of surface grafted block copolymers (Boyes et al., 2004; Minko, 2008). RAFT polymerisation requires chain transfer compounds bearing a leaving and re-initiating R group and a stabilising Z group [i.e., Z–C(= S) S–R]. The surface bounding of the chain transfer agent can be performed either from R side (R-group approach) or from Z side (Z-group approach) of RAFT molecule (Figure 4.5a). The polymerisation is initiated by thermal or chemical (AIBN) methods. The chain propagation involves the dynamic equilibrium between the propagating and dormant radicals (Figure 4.5b). RAFT polymerisation supposes fairly simple reaction conditions, similar to those of the conventional radical polymerisation performed in the presence of chain transfer agents. The properties of the resultant surface-bound polymer layers are strongly dependent on the manner in which the RAFT species are linked to the surface. While polymerisation in the presence of RAFT molecules which are surface-linked via the leaving and re-initiating R group is similar to the ‘grafting from’ approach, the other binding method provides similar conditions to the ‘grafting to’ approach. Thus the R-approach allows the preparation of polymer surface layers with high grafting densities of high molecular chains but of broader molecular weight distribution, as a consequence of the chain coupling. The differing Z-group approach gives rise to well-defined grafted polymers, with monomodal molecular weight distribution, while the grafting density is lower as a result of the shielding effect. To provide controlled surface-grafted polymers, a free RAFT agent should be added into the polymerisation system. The surface radical migration effect (Tsujii et al., 2001), which determines the recombination of the propagating chains, can be minimised by decreasing

S Z C

S S R

R S

C

Z

(a) S C Z (b)

S + Pn S Pm

S

Pn

C Z

Pn

C S Pm

Z

+

Pm

S M

M

4.5 RAFT polymerisation: (a) surface mediated Z and R approaches; (b) reversible addition-fragmentation chain transfer.


104


the surface density of the RAFT species and by using low initiator/chain transfer agent ratios. Barner-Kowollik and Perrier (2008) have recently highlighted the advantages and challenges of RAFT polymerisation. The preparation of polymer-coated surfaces by nitroxide-mediated polymerisation (NMP) has been recently reviewed (Ghannam et al., 2006). Chemically differing alkoxyamine functionalities were linked to planar and particle surfaces to further promote the controlled radical polymerisation of different vinyl and acrylic monomers (Table 4.3). Bimolecular and unimolecular initiating systems were described (Figure 4.6a). The propagation of the polymer chain takes place through activation–deactivation processes involving a reversible combination of the growing chains with nitroxide radicals (Figure 4.6b). As in RAFT polymerisation, obtaining high grafting densities with polymers of high molecular weight and controlled polydispersity requires some free alkoxyamine to be sacrificed. Practically all monomers can be polymerised by this method, including those which are not suitable for other mechanisms. However, not all surfaces can be grafted

(a) initiating systems Unimolecular initiating systems O R''' X R O C

R'

C

O

N

R1 R2

R'' Bimolecular initiating systems O X R

O C C2H4 N N C2H4 R' + O

X R'' O R''' O

N

R1 R2

R2 CN

CN + CH3 C

R1

N

N

N

CH3

C CH3 CH3

X = anchoring group R, R', R'' = hydrocarbonate radicals R''' = hydrocarbonate radical or succinimidyl ester R1 = hydrocarbonate radical R2 = phosphonate substituted radical (b) chain propagation polymer

O

N

R1

ka

R2

kd

polymer +

O

N

kp monomer

4.6 Nitroxide-mediated surface grafting.


R1 R2


105

due to the fairly high reaction temperature. Grafting densities as high as 0.9 chains/nm2 were obtained by using nitroxide SG1 bearing N-succinimidyl ester groups (Parvole et al., 2010). ATRP is by far the most widely used method of preparing grafted polymers and copolymers on either flat or curved surfaces (Table 4.3). It is based on the dynamic exchange between halogen-terminated growing chains/ Cu(I)XL2 (dormant species) and macro-radical/Cu(II)X2L2 complexes (active species) (Figure 4.7), where X and L are halogen and amino ligand, respectively. The chain propagation and termination are first-order and second-order reactions, respectively. The equilibrium in Figure 4.7 is usually strongly shifted to the left, the concentration of the free radicals is low (10−7, 10−8 mol/L) and the concentration of Cu(II) should be as high as 5% of that of Cu(I) to ensure good control of the process. Thus, the termination reaction is diminished and all the chains grow simultaneously, producing polymers of low polydispersity. The amount of surface-bound initiator is too low, especially in flat surface-initiated polymerisation, to provide the requested Cu(II) concentration. To overcome this shortcoming, the addition of free (‘sacrificial’) initiator and of the necessary amount of Cu(II) at the beginning of the polymerisation is imposed. The addition of the ‘sacrificial initiator’ provides sufficient concentration of the persistent radicals and control of the degree of polymerisation. It also permits an easy determination of the polymer molecular weight by using the free polymer formed in solution. The grafting density which affects the tethered polymer conformation and the thickness of the polymer layer also depends on the surface coverage by the initiator, which can be modulated by attaching active and inactive coupling agents to the surface. High grafting density and an easier control of the thickness of the polymer layer can be achieved in particle-initiated polymerisation due to the large curvature of the surface and to the ease with which the ratio between the bound initiating species and the monomer concentrations can be varied. The surface-initiated ionic polymerisation was performed in the presence of surface-attached cationic or anionic catalytic moieties. Both unsaturated and cyclic monomers were polymerised in the presence of different types of activated surfaces. As in other surface-initiating systems, the active

Pn X / Cu(I) X L2

k1 k2

Pn / Cu(II) X2L2 k3 M

4.7 Mechanism of surface ATRP propagation.


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species are attached to the surface mainly by the self-assembly monolayer technique. Ring opening methatesis, polycondensation, click chemistry or combined mechanisms were also used to prepare surface grafted polymers on different inorganic surfaces by surface-initiated polymerisation. Table 4.4 shows some recent results. The ‘grafting through’ method The ‘grafting through’ method was used less in preparation of surfaceattached polymer layers. However, due to the simplicity of the method, some trials have been performed recently, either to modify the properties of flat surfaces, or to increase the colloidal stability or compatibility of inorganic particles. This approach supposes the linking of a polymerisable moiety to the surface (usually through self-assembling monolayers), followed by the copolymerisation of the activated surface with an appropriate monomer (Fig. 4.3d). Polymers such as poly(meth)acrylates, poly(vinyl acetate/pivalate), water-soluble or photo-active polymers were bound to inorganic or polymer surfaces by this method using conventional free radical polymerisation, ATRP or polycondensation reactions (Table 4.5).

4.3.3 Nanocoating by ‘reverse grafting’ This approach is designed to provide solid inorganic particles of nanometric dimensions covered with thin layers of polymers (see Fig. 4.8). The most challenging case is concerned with the in situ preparation of a stable polymer–metal nanoparticle solution. This so-called bottom-up method employs chemical approaches to the assembly of nanoparticles from either mononuclear metal ions or structures with a lower inclination to nucleation. The synthesis and stabilisation of nanoparticles in a polymeric solution takes place in several stages, among which the main ones are: (i) nucleation and formation of the new phase; (ii) formation of the stable polymer–metal nanoparticle complexes. Nucleation and phase formation The formation of nanoparticles from a single metal atom and their conversion into a compact metal proceeds through the generation of intermediates such as clusters, complexes and aggregates. As the number of atoms increases, a fairly stable state is reached. At a certain point, the mean frequency of an atom attaching to the ensemble becomes equal to the mean frequency of separation and this makes unproductive further joining of



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Table 4.5 Surface-bound polymer layers obtained by ‘grafting through’ approach Monomer functionalised surface*

Polymerising monomer**

Initiator***

Reference(s)

Silicon wafer-O-Si-CH=CH2

VAc

AIBN

Poly(VUD-co-EGDMA)-OCOC(Me)=CH2 Maghemite(-O)2P(=O)-[OC(Me)-CH2]5-OCOC(Me)=CH2 -O-Si-C3H6-NHCO-OCH2-C≡CH

VP

AIBN

GMA

AIBN

Nguyen et al., 2003 Nguyen et al., 2007 Tocchio et al., 2009

5-decyne

Silicon wafer-(O-Si-C3H6)2 (9,9′-dibromofluorene) Mesoporous silica nanoparticles-O-Si-CH=CH2 Magnetite-O-Si-C3H6-OCOC(Me)=CH2 Magnetite-O-Si-C6H4-CH2-Cl

DBrDHeF

WCl6/Ph4Sn + microwave Ni(0)

Ethylene

Ni(COD)2 / EtPA AIBN CuBr/amine

NIPAAm OEGMA

Jhaveri et al., 2007b Jhaveri et al., 2009 Wei and Zhang, 2009 Frickel et al., 2010

* Poly(VUD-co-EGDMA) = poly(vinylundecanoate-co-ethylene glycol dimethacrylate) particles ** DBrDHeF = 2,7-dibromo-9,9-dihexylfluorene; GMA = glycidyl methacrylate; NIPAAm = N-isopropylacrylamide; OEGMA = oligo(ethyleneglycol) methylether methacrylate; VAc = vinyl acetate; VP = vinyl pivalate *** AIBN = 2,2′-azobis(isobutyronitrile); EtPA = ethyl-4,4,4-trifluoro-2(triphenylphosphoranylidene)-acetoacetate; Ni(COD)2 = bis(1,5-cyclooctadiene) nickel

Metal salt Nucleation Polymer Solid phase formation

Particle growth

Polymer–metal complex Nanocoated metal particle

4.8 Nanocoating by ‘reverse grafting’.


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metal atoms to the aggregate. Such structure is called the critical nucleus of the new phase and its dimensions are below 10 nm (Abraham, 1974; Morokhov et al., 1977; Villuendas and Bowles, 2007). The nucleation of a new phase is critical to the formation of an interface within the bulk of the polymeric phase. This interface binds the amount of critical nuclei which are capable of progressive spontaneous growth (Barret, 1973; Morokhov et al., 1977; Gibbs, 1993; Anisimov, 2003; Fokin et al., 2005). Homogeneous nucleation is a process characterised by the appearance of new phase nuclei within a metastable homogeneous system (Abraham, 1974). This can be referred to as transition of the substance into the thermodynamic stable state through a sequential reversible association. During nucleation, elements of the interface, and the interfaces themselves, first emerge and then disappear. Heterogeneous nucleation is a process during which the interactions with the formation of new phase nuclei are running in contact either with heterogeneities found in the generating phase, or with the surface. Heterogeneous nucleation occurs in multicomponent systems with spatial inhomogeneities at the interface (substrates, including polymer substrates, extraneous inclusions and surfaces, additives or crystalline particles already formed) (Fokin et al., 2005; Villuendas and Bowles, 2007). The nanoparticles formed on the polymer surface by heterogeneous nucleation have specific features related to the surface energy anisotropy at the crystalline nucleus– surface–medium interface. The general picture of nucleation and growth of the new phase during chemical reactions includes a set of numerous interrelated processes as follows. (i) Chemical reactions which can be thought of as the source of building material for the new phase. These may consist of one or more reactions with the participation of one or several reagents and may proceed on the surface or within the bulk. During chemical transformations, highly reactive particles capable of condensation reach a certain local concentration at which they associate and form a new phase. (ii) The mass transfer processes of reagents into the reaction zone (if few compounds take part in the reaction), the movement of products of chemical interaction capable of aggregation to the condensation zone, or the removal of some products of reaction which do not participate in crystallisation in the zone, in case new particles grow on the surface, particularly that of the polymer. (iii) Sorption processes which could make a contribution to nucleation and growth of the new phase particles. The processes are observed in adsorption of synthesised particles, of reagents or products of interaction on the surface of growing nascent clusters and in desorption from the surface. Chemisorption interactions on the growing particle surfaces are interrelated with the processes of stabilisation.



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Formation of polymer–nanoparticle complex The reduction of metal ions in polymer solutions results in formation of metal nanoparticles with a narrow size distribution and a mean size of less than 10 nm. It is known that shielding of the growing metal nanoparticle by macromolecules (the formation of polymer–metal complexes) will finally be followed by the growth step (Hirai and Toshima, 1986). Therefore, the size distribution and mean size of the particles in the resulting solution can be defined by the dependence of the probability of the complex formation on the size of the growing metal particles. This probability was assumed to be defined by thermodynamic stability of the corresponding complex (Litmanovich et al., 2010a, b). It was suggested (Papisov and Litmanovich, 1999; Litmanovich et al., 2010b) that the interactions between very small particles and long macromolecules are similar to those between oligomers and polymers and, for this reason, both can be treated using the model of adsorption of small species on a long polymer chain. The results of these interpretations have several important consequences (Papisov and Litmanovich, 1999): (i) at low concentrations of protective polymer and nanoparticles with diameters within 1–10 nm, a high stability of polymer– nanoparticle complexes can be provided for systems having ΔG values as high as a few percent of the specific surface energy values for common solids; (ii) the stability of polymer–nanoparticle complexes strongly depends on the size of the nanoparticles; (iii) the macromolecules–nanoparticles interactions are very selective in regard to both the macromolecular structure and the size of the particles. These conclusions explain and predict the features of the new phase formation processes which are taking place in the polymer solution. The thermodynamic stability of the polymer–metal nanoparticle complex depends on the particle’s surface area only when the polymer chain is able to shield the entire surface of the particle. If either the chain is too short or the particle is too big, only a part of the surface area can be shielded and the stability of the complex will depend on the length of the chain. In fact, a stable polymer–metal nanoparticle solution should be considered to be a fine dispersion of micelles which represent the polymer–metal complexes formed due to the cooperative non-covalent interaction between macromolecules and the surface of the metal nanoparticles. Polymer chains envelope the metal nanoparticles, thus lyophilising and protecting them from aggregation or oxidation. The nature of the interaction and the structure of the polymer–metal complexes are under continuous investigation. It is generally supposed that the hydrophobic interaction between the macromolecular chains and hydrophobic surfaces of metals plays an essential role in protection (Hirai and Toshima, 1986). Arguments supporting these assumptions have been presented in reports describing the interaction between


110


copper nanoparticles and poly(N-vinyl-caprolactame) (Hirai and Yakura, 2001) or the coulomb interactions between oppositely charged metal nanoparticles and polymer chains (Ostaeva et al., 2006). The interaction between the macromolecular chains and the nanoparticles is reversible. The probability of a complex formation (often described as the mutual recognition between a growing particle and a macromolecule) rapidly increases from zero to unity in a narrow interval of the particle’s diameter. The recognition is followed by the shadowing of the particles and the cessation of their growth. The mutual recognition term (the probability of the formation of polymer–metal nanoparticle complexes) indicates that the polymer–metal nanoparticle should be considered as a specific, and sometimes unique, dual system built with a polymer having a specific chemical structure and a metal nanoparticle which is precisely defined, both in its nature and size. Recent work has offered strong arguments concerning the specificity of the polymer–metal nanoparticle dual systems (Sacarescu et al., 2010). According to these reported results, the polyhydrosilanes develop specific interactions with the silver nanoparticles which are synthesised in situ, resulting in weak charge transfer complexes located along the macromolecular chains. These structures represent the nucleation centres for the growth of the silver nanoparticles. Therefore in the first stage of the process, the polymer represents an active template which binds the silver atoms and permits the formation of metal nanoparticles. In the second stage, the particle growth and increase in size result in the enveloping and protection of the surface, thus producing a highly stable polymer–nanoparticle system.

4.4

Properties and applications

4.4.1 Physicochemical properties of surface-bound polymer layers and comparison to bulk properties The surface-bound polymer layers showed significantly different physicochemical properties when compared to the bulk polymers of identical structure and similar molecular weight. The glass transition temperature (Tg), the elastic behaviour and miscibility are strongly influenced by the nature and characteristics of the polymer backbone, and also by the type of surface binding and grafting density. Tg of the surface-bound layers was found to depend on the film thickness and on the nature of the surface (Prucker et al., 1998; Fryer et al., 2001). Moreover, Tsujii et al. (2004) established significant differences in the variation of Tg for surface-tethered poly(methyl methacrylate) layers with low, medium and high grafting densities (brush regime) and cast films of the same molecular weight. The Tg of the brush layer decreases, while that of



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the cast film increases up to a thickness limit of about 60 nm, due to the surface effects. At higher thicknesses, both layers have an independent variation of Tg on the thickness, but the Tg of the brush film was 8 °C higher than that of the cast film. This differing behaviour of films which are adsorbed and highly grafted on the surface (not observed in low and medium grafting densities) is explained by the anisotropic structure and conformation of the brush layer. The elastic properties of the polymer layers on the surface also depend on the nature of the binding. Urayama et al. (2002) explained the lower compressibility of the polymer brushes when compared to the cast films, by the strain-hardened effect of the highly stretched and entangled chains. Polymer layers of low and medium grafting densities are miscible with polymers of similar structure, while the highly stretched polymer brushes are not (Tsujii et al., 2004). The surface-bound polymers also showed a different responsiveness to specific stimuli when compared to the unbound polymers. For example, pH-sensitive brushes, such as high density surface-tethered poly(methacrylic acid), present a significant increase of the pKa value (about 10) when compared to the bulk polymer (pKa about 4–5) (Tsujii et al., 2004). Poly(Nisopropylacrylamide) (PNIPAM), a thermosensitive polymer with a lower critical solution temperature (LCST) of about 32 °C, when tethered to a surface (a thickness of dry film of about 50 nm) does not present a sharp drop to the LCST, but a continuous transition of between 10 and 40 °C, suggesting the existence of partially collapsed PNIPAM chains at room temperature (Balamurugan et al., 2003). The same authors observed a sharp increase of the advancing contact angle at 32 °C, concluding that the outermost region of the brush remain highly solvated, while less solvated segments within the polymer brush undergo dehydration and collapse over a broad range of temperatures.

4.4.2 Properties and applications of modified surfaces Adhesion is a fundamental property of modified surfaces and affects both surface and interface properties. Raphaël and de Gennes (1992) and Ji and de Gennes (1993) were the first to investigate the adhesion properties of surface layers by modelling adhesion between two rubber surfaces having brush-type layers extending from one surface to another across their interface. The adhesion was found to be a function of the sum of the thermodynamic work of adhesion between elastomeric surfaces in the absence of the polymer brush and of the energy needed to pull out the connecting polymer chains from one layer. For surface-bound functional polymers, O’RourkeMuisener et al. (2003) used the Scheutjens–Fleer self-consistent mean-field theory to demonstrate that polymers with adjacent low-energy functional


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groups located at one end of a chain show an optimum low energy release surface, whereas an optimum high energy adhesion surface is obtained by the introduction of adjacent high energy functional groups in the middle of a polymer chain. Both low and high adhesive surfaces are of great interest for bio-applications such as artificial implants, cell cultures and tissue engineering (Caster, 2004; Ayres, 2010) as well as for nanotechnology (Luzinov et al., 2004; Ayres, 2010). Surfaces of switchable properties contain either stimuli-responsive homopolymers or copolymers composed of incompatible blocks or grafts. Remarkable switchable properties were also found in mixed polymer (Minko, 2008). The changing of surface structures and properties by the action of selective solvents, pH or temperature (Jiang and Li, 2009; Lenz et al., 2010) is of great interest for bio-applications. The surface properties of biomaterials and biosensors, etc. can be modulated on demand, providing dynamic control over surface–biomolecule interactions such as protein adsorption–desorption (Balamurugan et al., 2005; Cole et al., 2009; Bucatariu et al., 2010) or cell attachment–detachment (Hyun et al., 2004; Canavan et al., 2005a,b; Cheng et al., 2005; Tsuda et al., 2005; Smith et al., 2005). Stimuli-sensitive surfaces were also proposed as appropriate materials for the preparation of different catalytic systems (Li et al., 2008; Jiang et al., 2009; Marten et al., 2010).

4.5

Acknowledgement

One of the authors (Maria Butnaru) acknowledges the financial support of European Social Fund – ‘Cristofor I. Simionescu’ Postdoctoral Fellowship Programme (ID POSDRU/89/1.5/S/55216), Sectoral Operational Programme Human Resources Development 2007–2013.

4.6

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5 Methods for analysing nanocoatings and ultra-thin films D. M. BASTIDAS, M. CRIADO and J.-M. BASTIDAS, National Centre for Metallurgical Research (CENIM), CSIC, Spain

Abstract: Linear potential sweep and impedance measurements are methods frequently used for studying corrosion behaviour, mass transport processes and the protective properties of coatings applied on metal substrates. Surface-sensitive analytical techniques are useful for understanding the composition and structure of ultra-thin film coatings. Among these techniques, atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), specular reflectance infrared, Raman and Mössbauer spectroscopies supply excellent results. Ion spectroscopy and glow discharge optical emission spectroscopy provide information about the concentration and the depth of the elements on the surface of a coating. Finally, electron microscopy provides morphologic and topographic information about the surface of solids. Key words: cathodic stripping, electrochemical impedance spectroscopy, atomic force microscopy, x-ray photoelectron spectroscopy, ion spectroscopy, electronic microscopy.

5.1

Introduction

This chapter deals with methods used in the study of corrosion behaviour, mass transport processes and the protective properties of coatings applied to metal substrates. Linear potential sweep and impedance measurements have been used to study the relationship between current and potential peaks for copper specimens subjected to different tarnishing treatments. Electrochemical methods can be used to provide a reasonable approximation of changes in the dielectric constant caused by water absorption and the pigment/polymer proportions and porosity of organic coatings. Surfacesensitive analytical techniques are useful for understanding the composition and structure of ultra-thin film coatings. Techniques that supply excellent results in this area include atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), specular reflectance infrared, Raman and Mössbauer spectroscopies. Moreover, ion spectroscopy and glow discharge optical emission spectroscopy make it possible to obtain information about the concentration and the depth of 131 © Woodhead Publishing Limited, 2011

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the elements present on coating surfaces. The final technique discussed is electron microscopy, which provides morphologic and topographic information about the surface of solids.

5.2

Electrochemical methods

5.2.1 Potentiodynamic potential/current measurements Linear potential sweep is a method commonly used for studying the films formed on metallic surfaces. Using potentiostatic transients as a basis, Müller (1931) proposed that a number of metals can be passivated by the formation of an insoluble film on the metallic surface. This film can nucleate at several points and then spread laterally across the metallic surface. It is known to be useful to evaluate the potentiodynamic potential/current density relationships for a film formation process under ohmic resistance control (Bastidas et al., 1997). Copper and its alloys are widely used in many environments. On exposure to the atmosphere, clean copper transforms from salmon-pink to a progressively darker brown. The natural green film that forms on copper and its alloys after prolonged outdoor or indoor exposure is known as patina (Cano et al., 2005). Six copper surface treatments are considered here: mechanical polishing, indoor exposure for 7 days, chemical etching in 1.6 M nitric acid (HNO3), chemical etching and heating at 160 °C and chemical etching and dipping in 9 × 10−4 M or 0.9 M potassium sulphide (K2S) solution at 70 °C. As an example, Fig. 5.1 shows the reduction curves for mechanically polished copper specimens exposed for 7 days to the indoor atmosphere of the laboratory as a function of ν (potential scan rate, ν = dE/dt). Only one main broad ill-defined current density peak can be observed, which shifts to negative potential values as ν increases. Figure 5.2 shows the dependence of the current density peak (im) and the potential peak (Em) on ν for copper specimens subjected to the six tarnishing treatments. The expressions for im and Em describe, reasonably approximately, the linear relationship between im versus (ν)1/2 and Em versus (ν)1/2, derived from Müller’s model for film formation on electrodes (Müller, 1931). This can be described as follows: (i) im = (k1/k0)1/2(ν)1/2, where k0 is a constant (k0 = ρ/d), ρ is the resistivity and d the density of the tarnish film, k1 = nF/M, n is the charge on the metal cations, F is Faraday’s constant (96 500 C/mol) and M is the average molecular weight; and (ii) Em = (Rp + R0)(k1/k0)1/2(ν)1/2, where Rp is the resistance of the electrolyte solution inside the pores and R0 is the resistance of the external supporting test solution to the tarnish film. According to the equations for im and Em, there is a linear relationship between im, Em and (ν)1/2. The proportionality factor has the


Methods for analysing nanocoatings and ultra-thin films

133

Current density (μA cm–2)

0

–500

0.44 mV s–1 1 5 10 20

–1000

–1500

–2000 –1.2

–1.1

–1.0 –0.9 –0.8 Potential (VSCE)

–0.7

–0.6

5.1 Cathodic stripping for mechanically polished copper specimens exposed for 7 days to the indoor atmosphere of the laboratory. A 0.1 M sodium acetate (NaCH3COO) pH 8 was used as supporting test solution.

dimensions of a resistance, and the film dissolution process may be said to be controlled by the ohmic resistance. The parameters im and Em increase linearly with the square root of ν, and the slope depends on the properties and thickness of the tarnish film. It should be noted that the (k1/k2)1/2 parameter can be used to estimate a ρ value of the tarnish film (Cano et al., 2005).

5.2.2 Electrochemical impedance spectroscopy (EIS) EIS is probably the most important electrochemical method used for the characterisation of coatings applied or formed on metal substrates, providing information about chemical reactions, corrosion, mass transport, adsorption–desorption processes and capacitance of the interfacial region. Impedance measurements have also been used to characterise such aspects of materials as their dielectric properties. The EIS method allows quantification of the three parameters defining a corrosion process: (i) the corrosion rate, through the charge transfer resistance (Rct) (Ω cm2) where Faraday’s law can be used to estimate the penetration of the attack (μm/year); (ii) the mass transport processes (diffusion) defined by the parameter (σ) (Ω cm2 s−1/2); and (iii) the electrochemical double layer capacitance at the metal/solution interface (Cdl) (F cm−2). By applying a low amplitude sine-wave voltage (V) signal across a test system ΔV = Vmsin(ωt), where Vm is intermediate voltage, it is possible to measure the frequency (f), f = ω/2π where ω is the angular frequency and



Peak current density (mA cm–2)

(a)

0.0

0 –2

–0.5

–4 –1.0

–6 –8

–1.5

–10 –2.0 –12 –2.5

Peak potential (VSCE)

(b)

–0.05

Peak current density (mA cm–2)

134

–14 Polished 7 days lab. HNO3 160 °C 9×10–4 M K2S 0.9 M K2S

–0.10

–0.15

–0.20

1 2 3 4 5 Root of sweep rate (mV s–1)1/2

5.2 Dependence of (a) current density peak (im), and (b) potential peak (Em) on potential scan rate (ν) for copper specimens under different tarnishing treatments.

t the time. The phase shift or angle (φ) and the amplitude of the resulting sine wave current density (I) is given by ΔI = Imsin(ωt + φ), where Im is intermediate current density. Impedance (Z) is a vector defined as magnitude or modules (|Z| = Vm/Im and φ) containing both resistive (R) and reactive (C and/or L) components. Both of these types of components need to be determined in order to characterise the component. Z is represented in the complex plane as Z = Z′+jZ″ (Nyquist plot) where Z′ and Z″ are the real and imaginary parts respectively, and j = (−1)1/2. Another complex formalism is admittance (Y) which is defined as Y = 1/Z and, in the same way, Y = Y′ + jY″. The procedure for interpreting impedance measurements is to use a mathematical model or empirical equivalent circuit. The parame-


Methods for analysing nanocoatings and ultra-thin films

135

ters can be estimated and compared with the experimental data (Bastidas et al., 2010). For analysis of the impedance data, a complex non linear least squares (CNLLS) procedure is frequently used: n

{

min ∑ [Zexp ′ (ω i ) − Zsim ′ (ω i )] + [Zexp ′′ (ω i ) − Zsim ′′ (ω i )] i =1

2

2

}

where Z′exp and Z′sim are the real, experimental and simulated impedance data respectively, Z″exp and Z″sim represent the imaginary impedance, n is the number of data points and ωi is the i-th angular frequency data point. As an example, Fig. 5.3 presents the Nyquist plot for an American Iron and Steel Institute (AISI) 316L stainless steel (SS) immersed for 30 minutes in a 5% sodium chloride (NaCl) solution and polarised at the pitting potential region (at 0.5 V vs saturated calomel electrode, SCE). The shape of the Nyquist plot shows a capacitive behaviour including: (i) a depressed semicircle at high frequencies (from 10 kHz to ∼251.19 Hz) with the centre lying below the real axis, which is associated with the frequency dispersion of impedance data; (ii) a capacitive loop at intermediate frequencies (∼6.3 Hz), which resembles an inductive-type loop over the real axis; and (iii) a third capacitive loop drawing a straight line or a portion of a second semicircle at low frequencies (∼ 10 μm Al2O3

Wear depth (μm)

4 5N–500 μm–5 Hz–10 000 cycles 23 °C, 50 % RH

3

2 HVOF

APS

1

(0

Al

C

W

C

–C

o–

C

r– C

W

o– –C W

C

r % –C r–A C ) l( o– C 25 % r– C Al ) (5 0 % W C C –C ) o– C r

)

o–

C

no C

–C

ee –C

o(

na

71

st C W

St

ai

ni

es

s

lR

ti In

co

ne

as C

l

8

n ro

iC iS N

(L 8 71

W

In

co

ne

W

lR

C

–C

o(

co

C

)

n)

0

12.14 Comparison of wear resistance of thermally sprayed nanostructured WC–Co with industrial reference materials sliding against alumina counterbody in ambient conditions.74

Comparative test results (average values) 25 420 Dimension (mm)

25 410 25 400

Cr-coated

25 390

Nanosprayed

25 380 25 370 25 360 25 350 0

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Operation time (h)

12.15 Comparison of nanostructured HVOF WC–12Co–2Al-coated and chromium-plated crankshaft in terms of dimensional change in bench test for marine diesel engines.75


382


Table 12.4 Mechanical and tribological properties of some commonly reported nanostructured coatings Process and coating material

Coefficient of friction

Wear coefficient (mm3/N.m)

Countermaterial

Reference

35–45

0.45–0.60

1.1 × 10−4 against

steel

Ma et al.77

Hardness (GPa)

PVD nc-TiN/aSi3N4 PVD nc-TiN/a-BN CVD TiC/a-C:H ED Co/W

27–36

0.50–0.60

10−5

15 6.2–6.5

0.08–0.1 –

10−7–10−8 –

–

ED (TiAl)N/Mo

40–50

–

–

–

HVOF WC-Co

20–22

0.24

10−6

Alumina

12.4

Patscheider et al.63 Svensson et al.78 Tavares et al.79 Basak et al.74

Advanced techniques for characterizing tribological properties of nanostructured coatings

The tribological characterization of novel coatings requires special techniques that allow scientists to realistically simulate tribological contacts. A correct selection of tribological simulation should be based on a thorough material and mechanical analysis, complemented with a good insight into surface reactivity of materials. At a fundamental level, an understanding of the functional properties of the microstructural constituents requires advanced methods with sensitive force detection complemented by surface analytical tools. In this section, state-of-the-art tribological techniques for friction and wear characterization and major challenges in scaling up nanostructured coatings for industrial applications are discussed.

12.4.1 Techniques for friction and wear characterization Transfer mechanism and lubricious layer formation are major ways in which solid lubricants function. The key to assessing coating performance is an understanding of the formation and dynamics of the lubricating layers. There are two directions of tribological testing, namely: •

standardized test procedures like ASTM2625, AMS2488, etc. to estimate the endurance limit and lifetime of the coatings – these test procedures are used to screen the suitability of the developed materials against industrial benchmarks;


Nanocoatings for tribological applications •

383

scientific research aimed at understanding friction and wear mechanisms using advanced surface analytical techniques. In this section we limit ourselves to the scientific techniques.

With the range of tribometers that have become available since year 2000, it is now possible to quantify friction and wear from the nanoscale up to the macroscale. Such measurements over a broad range are necessary to understand the origins and scale dependence of friction and wear phenomena. Bidirectional tribological experiments at the nanoscale are done using an atomic force microscope, and the technique is called lateral or friction force microscopy (LFM/FFM). A sharp tip, typically between 1 nm and 100 nm, that simulates a single asperity is brought in direct contact with a surface and raster scanned across it over a fixed scan size (Fig. 12.16a). The cantilever deflection or torsion is recorded using laser deflection, capacitance, magnetic force, etc. For any reciprocating sliding test, a plot of friction force against sliding distance during a cycle is known as friction loop. The area of this friction loop gives a quantitative measure of the energy lost per cycle. Based on the fluctuations on the friction loop, information on topography or surface roughness and chemical/phase variations can be detected. For example, LFM is extremely sensitive to local topography and phase effects. The phase inhomogeneity causes a mirrored feature in the friction loop while the topographic effects appear as a parallel feature as shown in Fig. 12.17. A good example of phase contrasts in friction images was reported by Overney et al.80 on a silicon surface partially covered by a Langmuir–Blodgett film. In recent years, microtribometers were introduced with operating parameters that fill up the measurement gap between AFM and conventional tribometers. The measurement principle of such microtribometers is similar to the one on which LFM equipment is based but at a different normal force scale (Fig. 12.16b). Unlike in LFM, the contact sizes are larger and may range from a few μm2 up to hundreds of μm2. More information on such microtribometers can be found in Achanta et al.81 Using a microtribometer, Achanta et al.82 reported similar topography and phase effects on friction in dual-phase steel consisting of coarse austenitic grains (typical 40–60 μm) dispersed in a ferritic matrix (Fig. 12.18) similar to LFM (Fig. 12.17). Figure 12.18 shows a wear track crossing two austenitic grains in the sliding path (at locations 1 and 2) with the rest of the sliding on the ferritic matrix. Friction loops recorded at increasing sliding cycles up to 2000 cycles for the corresponding test are shown in Fig. 12.19. The friction loop corresponding to the 5th cycle exhibits some fluctuations (Fig. 12.19a). The one recorded at the 100th cycle shows at two locations in the trace and retrace direction variations in the tangential force that have mirror-like features (see locations 1 and 2 in Fig. 12.19b). On


384


5 μm (a)

FOS Bending elements

S

(b)

Sample, tool Sample

Cantilever

+Z

+Y

+X

12.16 (a) Silicon nitride tip 40 nm ø used as counterbody in LFM measurements. (b) Cantilever spring element used in microtribometer.81

comparing the distance separating the locations corresponding to those mirror-like fluctuations, they appear at a separation distance of 72 μm where austenitic grains are located. The topography-phase effects noticed at nanoscale are also visible at microscale. At cycle 2000 the loops appear smooth with no features, indicating attainment of surface homogeneity due to mechanical mixing of the phases (Fig. 12.19c). In a similar way, the lubricant phases in the material also smear in the wear track and the use of such microtribological techniques helps in unraveling friction mechanisms and the lubrication behavior of coatings.


Nanocoatings for tribological applications

385

Steps

Different phase

Topography effect Phase effect + Ft 0 – Ft

12.17 Schematic of a friction loops showing topography and phase effects.

nm

714 600 500 400

2

72 mm

300 1

Sliding path

200 100 0 –100 –200 –300 –400 –577

12.18 Wear track on etched dual phase steel after a reciprocating test performed at 20 mN normal force and displacement amplitude of 500 μm for 2000 cycles against a 2 mm ø corundum in ambient air at 23 °C and 50 % RH.82


25

Tangential force (mN)

20

5th cycle

15 10 5

1

2

1

2

0 –5 –10 –15 –20 –25 0

100

300 400 200 Displacement (μm)

500

(a) 25


20

100th cycle

15

2

1

10

72 μm

5 0 –5 1

–10

2

–15 –20 –25 0

100


500

(b) 25 2000th cycle


20 15 10 5 0 –5 –10 –15 –20 –25 0

100


500

(c)

12.19 Friction loops recorded on etched dual phase steel at 20 mN normal force and 500 μm displacement amplitude at increasing sliding cycles. Sliding was done against 2 mm ø corundum in ambient air at 23 °C and 50 % RH.82



387

In-situ tribometry In recent years, in-situ tribometry has opened new doors for researchers to observe the evolution of sliding contacts. A detailed explanation of in-situ methods was recently given by Sawyer and Wahl (2008).83 Raman and electron microscopy techniques are used to link real-time changes in the contact with friction and wear phenomena. In a different study, atomic-scale measurement of wear was carried out by combining a sliding test and TEM investigation to understand the role of defects and chemistry in the initiation of wear.84 In-situ tribometry measurements are valuable in understanding, the competition between different components of multifunctional nanostructured coatings made to function under varying environments. For example, in-situ and ex-situ Raman studies of the interfacial lubricating phases for MoS2/C/ Au/YSZ nanocomposites in dry and humid environments confirmed that the primary lubricant providing low friction at room temperature was MoS2 (Sawyer and Wahl, 2008).83 Similarly, the formation of MoS2 in self-mated Pb–Mo–S composite coatings was confirmed during sliding experiments. Combinatorial methods and multistation testing Modern commercial tribometers tend to be modular, so that the user could rebuild them from one configuration to another. This is done to reduce the cost of multiple equipments and speed up the analysis process. A modular tribometer of this type was recently introduced which has a pin-on-disk head, profilometer, chemical analysis, and microscopy option integrated on one machine (the DS4 tester, Tetra GmbH, Germany). In particular, the efficiency of analysis and screening is greatly improved with such modular experiments. On introducing new materials, thorough statistical information is required. This means that a number of coated samples must be subjected to testing in order to derive reliability data. New tribometers have from 50 to up to 100 stations where 100 different tribological tests can be done in one go (e.g., TE67, Pheonix tribology, UK). This is an extremely time-saving and economical way of deriving reliability information for a coating. These multistation tribometers are already used in biomaterials field85 and similar approaches are definitely of interest in screening novel nanosturctured coatings.

12.4.2 Scale dependence of tribological properties Friction and wear are highly scale dependent and the scale dependence of friction has been extensively studied. It is now accepted that the term


388


‘coefficient of friction’ is not a constant but varies depending on the friction mechanism acting at a particular measurement scale. This means that the friction force varies in a non-linear way with the applied normal load in opposition to the classical Amontons’ law. In the case of bidirectional sliding tests performed over a broad range of normal forces, the friction-determining mechanisms in the case of homogeneous surfaces are shown in Fig. 12.20. The surface roughness greatly influences the friction mechanisms, especially at low normal forces and low contact sizes, while at high normal forces and large contact sizes the surface roughness is quickly lowered by destroying the asperities which give rise to wear particles. The wear mechanisms and wear coefficients also depend on the measurement scale. Like the coefficient of friction, the wear coefficient is not a constant for a given material couple. This results from the fact that as the contact size decreases, say from macro to nanometer scale, the deformation mechanisms involving excessive plastic deformation, crack propagation, delamination, fatigue, etc. do not apply anymore. At atomic scales, wear occurs by a transfer of atoms from one surface to the other, a process that is referred to as adhesive wear.86 The abrasive wear at atomic scale is defined as the dragging of atoms from one position to another under high shear forces. Keeping in mind the scale dependence of wear mechanisms, different characterization methods must be used to achieve a meaningful quantification of wear rate.

32 Mesotester

Macrotester 105 Wear loss

24 20

101

16 12

103

10–1

Geometric effects

10–3

8 4 0 10–10

10–5

Adhesion Ad A dh d hesio ion on effect effects eff ef e ffffe fec ecctt

Hertzian contact area (μm2)

Average surface roughness (nm)

Nanotester 28

10–7 10–8

10–6 10–4 10–2 Normal force (N)

100

102

12.20 Schematic representation of various friction mechanisms operating at different ranges of normal force in the case of homogeneous surfaces like DLC and TiN coatings.



389

Consider a heterogeneous sample with two phases, namely matrix and solid lubricant (as in nanostructured composites with solid lubricants). Imagine a contact with area A sliding over the heterogeneous material giving rise to a friction force of Ff. If τ1 is the shear strength of the matrix and τ2 the shear strength of solid lubricant then, the friction force recorded can be expressed as Ff = τ1.α.A + τ2.(1 − α).A

[12.7]

where α is the area fraction of matrix. The equation holds under the assumption that the surfaces are very smooth: μ.FN = τ1.α.A + τ2.(1 − α).A

[12.8]

μ = μ1.α + μ2.(1 − α) = μ2 + α(μ1 − μ2)

[12.9]

where μ1 and μ2 are the coefficients of friction of the homogeneous matrix and solid lubricant, respectively, recorded over an area A under similar conditions. Based on Eq. 12.9, the overall friction depends on the contact size and the volume fraction of the solid lubricant phase. This illustrates that in inhomogeneous materials, friction and wear data can vary from location to location. Therefore, the selection of appropriate test parameters should minimize the scale dependence of tribological phenomena caused by microstructural effects.

12.4.3 Challenges to establish scale up The scale up of nanostructured coatings prepared at laboratory scale to the industry is a challenging task. The most important factors that must be addressed before commercializing the coatings are as follows.

Ability to coat different sizes of samples and retention of nanostructure The required thickness of a particular coating depends on the application. Sometimes a thin coating of a few μm is needed to protect the components, whereas other applications may require a larger thickness. Apart from this, the coating technology should be capable of coating complex shapes such as curved surfaces, etc. Conventional PVD, CVD coatings are typically limited to tens of μms before internal stresses affect the adhesion and mechanical integrity of the coated systems. Thermal spray and cold spray deposition methods allow a large thickness in the range of mm. The coating technology must be thoroughly optimized to achieve a homogeneous structure throughout the sprayed region.


390


The retention of nanoscale over large surface areas is a challenging task. The deposition parameters used for lab-scale deposition of nanostructured coatings can fail when it comes to large industrial surfaces. Perform relevant laboratory tests, reliability analysis, to convince the end users, e.g., good lab-scale simulations of tribology representative of the field component It is not always evident that the coatings which perform exceptionally in lab-scale simulations also perform well in field tests. There are reports in which super-hard coatings were found to fail when used on components.29 This is an unwanted scenario and shows how lab-scale simulations can be misleading if the simulation conditions don’t comply with the application. Many of the industrial sectors are rather conservative and the introduction of newer technology can meet with fierce resistance. The ‘fear of failure’ is natural because both reputation and economical factors come into the picture. This means that a great effort is needed to convince end users with reliable data and lab-scale simulations. The tribosystem usually consists of two materials in contact with each other and with a relative motion, together with the environment in which they operate. This environment can be as simple as a single fluid (lubricant, water) or can be complex and dynamic (changing through the lifetime of the components). Apart from the physical environment, other environmental parameters play a role in the tribological behavior, such as temperature, vibrations, acoustic waves, contamination (foreign particles or liquids), etc. In a tribosystem, the materials are interacting with each other under mechanically described parameters such as speed and contact load but can also have chemical and electrochemical interactions. An electrochemical reaction will happen and will influence the friction and wear behavior, because both mechanisms are surface related. As a result, just the description of a tribosystem can be very complex. As a tribological system is complex, it is a challenge to design a good laboratory test that takes into account all the system properties. This is often a meticulous exercise and requires thorough assessment based on the following factors: • • • • •

correct mechanical simulation based, e.g., on the use of TAN number;87 the required results in terms of tribometrics; correct contact pressures; feasibility of accelerated testing; wear evolution rather than absolute wear.

Apart from a good laboratory simulation, a reliability analysis and a mapping of wear mechanisms must be done on the coating systems against



391

industrial benchmarks. With such extensively generated data, the confidence of the users can be gained for marketing these novel coatings. Balance cost/performance ratio that would encourage integration of the technology in the production line A major hurdle in introducing nanostructured coatings is the cost factor. Because new technologies are in general expensive in terms of developmental costs, the developed coatings should give an adequate performance compared to contemporary materials. Therefore, the invented materials should have an optimum cost/performance ratio to be introduced into the production lines of companies. In some cases, the cost is not a factor as compared to the reliability or safety of the component (e.g., in space shuttles, airplanes, production lines, etc.). In recent years, thermal-sprayed coatings have been successfully introduced in the automotive industry for engine liners.88 Such examples are encouraging, and the same approach should be used for introducing new coatings into the market.

12.5

Conclusions and future trends

We live in a world where ‘saving energy’ and ‘cost cut down’ are two important notions. The tribological issue involves both, i.e., friction = energy loss and wear = increase in maintenance costs. Both friction and wear can only be mitigated, never eliminated. Nanostructured coatings serve as good alternatives to the conventional materials thanks to their superior mechanical and tribological properties. Industrial data on the performance of nanostrucutured coatings in the field are still scarce. More tribological data illustrating their superior reliability compared to current industrial benchmarks are needed to establish confidence in the technology among end users. The search for optimum materials with multifunctional properties will continue. Features such as self-healing, smart coatings capable of adjustment based on tribological needs, and compatible surfaces with an affinity towards lubricant additives are some promising research avenues.

12.6

Acknowledgements

Part of the information presented in this chapter has been obtained within the European FP6 research project ‘Nanospraying’ contract no. G5RD-CT2002-00862, the European FP7 research project ‘Supersonic’ CP-IP 228814-2 and the scientific community on Surface Modification of Materials funded by Science Foundation Flanders (WOG). Special thanks to Mr James Grebmeier (Schlumberger, France), Dr Marc van Drogen (SKF, The Netherlands), Dr Xiao Bo (SKF, The Netherlands), Mr Ravi Kiran (Smith


392


Bits, USA), and Mr Philippe Lambert (Medacta, Switzerland) for their valuable inputs.

12.7

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74 Basak A K, Achanta S, Matteazzi P, Vardavoulias M, Celis J P and DeBonte M (2007), ‘Effect of Al and Cr addition on tribological behaviour of HVOF and APS nanostructured WCCo coatings’, Trans Inst Met Finish, 85, 1–7. 75 Basak A K (2009), Tribology and tribocorrosion of thermally sprayed nanostructured cermet coatings, Ph.D Thesis, K.U.Leuven. 76 Erdemir A, Eryilmaz O, Orgen M and Kazmanli K (2008), Development of multifunctional nanocomposite coatings for advanced automotive applications, Proc. 16th International Colloquium Tribology, 15–18 January, Esslingen, Germany. 77 Ma S, Procházka J, Karvánková, Ma Q, Niu X, Wang X, Ma D, Xu K and Veprek S (2005), ‘Comparative study of the tribological behaviour of superhard nanocomposite coatings nc-TiN/a-Si3N4 with TiN’, Surf Coat Technol, 194, 143–148. 78 Svensson M, Wahlström U and Holmbom G (1998), ‘Compositionally modulated cobalt-tungsten alloys deposited from a single ammoniacal electrolyte’, Surf Coat Technol, 105, 218–223. 79 Tavares C J, Rebouta L, Rivière J P, Pacaud J, Garem H, Pischow K and Wang Z (2001), ‘Microstructure of superhard (Ti, Al) N/Mo multilayers’, Thin Solid Films, 398–399, 397–404. 80 Overney R M, Meyer E, Frommer J and Brodbeck D, et al. (1992) ‘Friction measurements on phase-separated thin films with a modified atomic force microscope’, Nature, 359, 133–135. 81 Achanta S, Drees D, Celis J P and Anderson M (2007), ‘Investigation of friction in the meso normal force range on DLC and TiN coatings’, J ASTM Int, 4, 1–12. 82 Achanta S, Liskiewicz T, Drees D and Celis J P (2009), ‘ Friction mechanisms at the microscale’, Tribol Int, 42, 1792–1799. 83 Sawyer G W and Wahl K (2008), ‘Observing interfacial sliding processes in solid-solid contacts’, MRS Bulletin, 33, 1–4. 84 Dickinson J T (2007), ‘Single asperity nanoscale studies of tribochemistry’, in Gnecco E, Meyer E (eds), Fundamentals of Friction and Wear on the Nanoscale, Berlin: Springer, 481–520. 85 Saikko V (2003), ‘Effect of lubricant protein concentration on the wear of ultrahigh molecular weight polyethylene sliding against a CoCr counterface’, J Tribol, 125, 638–643. 86 Colasco R (2007), Surface damage mechanisms: from nano- and microcontacts to wear of materials, in Gnecco E, Meyer E (eds), Fundamentals of Friction and Wear on the Nanoscale, Berlin: Springer, 453–480. 87 Drees D and Celis J P (2000), ‘Intelligent test selection using tribological aspect number’, Proc. 12th International Tribology Colloquium, 12–14 January, Esslingen, Germany, 409–411. 88 G Barbezat (2005), ‘Advanced thermal spray technology and coating for lightweight engine blocks for the automotive industry’, Surf Coat Technol, 200, 1990–1993.


13 Self-cleaning smart nanocoatings J. O. CARNEIRO, V. TEIXEIRA, P. CARVALHO, S. AZEVEDO and N. MANNINEN, University of Minho, Portugal

Abstract: This chapter describes the major features of current smart, self-cleaning photocatalytic materials. These materials include semiconductor materials such as titanium dioxide (TiO2). This chapter focuses on TiO2-based materials because they are most widely studied. Characteristics such as low toxicity, high chemical stability, availability and low cost make TiO2 the ideal candidate for industrial applications. Key words: titanium dioxide (TiO2), photocatalysis, superhydrophilicity, self-cleaning, smart applications.

13.1

Introduction: TiO2 photocatalysis

Since the discovery of the photocatalytic properties of some semiconductor materials, different products have been gradually introduced in the market. This development dates back to the 1990s when the number of research studies resulting in patent applications increased considerably (Paz, 2010; Carp et al., 2004). According to the report ‘Photocatalyst: Technologies and Global Markets’, the global market of photocatalytic products was $848 million in 2009 and was predicted to grow to $1.7 billion by 2014 (Gagliardi, 2010). The field of photocatalysis emerged about 80 years ago through research into the chalking and degradation of outdoor TiO2-based paints (Mills and Hunte, 1997; Fujishima et al., 2008). However up until 1960, scientific studies did not produce any product of commercial interest using TiO2 as a photoactive material (Paz, 2010). Nevertheless, these research studies provided the foundation for its eventual commercial applications. Among different photocatalytic semiconductors based on oxides and sulphides, such as titanium dioxide (TiO2), zinc oxide (ZnO), tungsten oxide (WO3) and cadmium selenide (CdSe), TiO2 has received the greatest attention, due to its higher photocatalytic activity, chemical stability, availability and low cost (Hoffmann et al., 1995). During the 1960s A. Fujishimabegan to study the photo-electrolysis of water using a TiO2 semiconductor electrode to oxidize water to oxygen. In 1969 he and his co-workers demonstrated, for the first time, the electrochemical photolysis of water using TiO2 (Hashimoto et al., 2005). This work 397 © Woodhead Publishing Limited, 2011

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was reported in 1972 in Nature (Fujishima and Honda, 1972), and started a revolutionary change in photo-electrochemistry (Paz, 2010). In response to the oil crisis of the 1970s, research on TiO2 photocatalysis for hydrogen production was undertaken to the mid-1980s. Despite the high photocatalytic efficiency of TiO2, it was found to function best irradiated by UV light, making it unattractive for H2 production. Academic and industrial research shifted to the application of TiO2 to photodegradation of pollutants (Hashimoto et al., 2005). This new use was first proposed in 1977 for water purification (Lia et al., 2006). In subsequent years, the detoxication of dissimilar compounds in water and air was achieved by using powdered TiO2. During the 1990s, researchers noticed that TiO2 was not efficient in the treatment of large quantities of water and air, since UV radiation only corresponds to a small fraction of solar light and/or artificial light sources. Fujishima and his coworkers began to study different applications which only required a small amount of UV light-promoting photon-induced reactions on a TiO2 surface. Research on TiO2 photocatalysis now focused on the material’s selfcleaning properties. The first reports on photocatalytic cleaning materials date to 1992, with a self-cleaning ceramic tile coated with TiO2 developed by Fujishima et al. Fujishima and his co-workers found huge differences in TiO2 water wettability before and after UV light exposure. They reported that UV radiation exposure promoted a considerable reduction in water contact angle, which resulted in a non-water-repellent surface. This became known as superhydrophilic behavior. They also reported that hydrophilicity was kept for 1–2 days without the presence of UV radiation. After this period of time, without a UV source, the water contact angle slightly increases, reducing hydrophilic properties. These results made TiO2 even more attractive for self-cleaning applications, once it is possible to achieve a cleaning effect without UV light for a small period of time (Fujishima et al., 2000). Currently, the self-cleaning behavior based on TiO2 surfaces is explained by two main concepts: super-hydrophilic surfaces (water contact angle of about 0°) and super-hydrophobic surfaces, showing contact angles nearly of 150°. These surface states have been mainly achieved through the tailoring of their roughness and surface energy (adhesion work) (Feng et al., 2002). At present, self-cleaning surfaces based on the photocatalytic activity of TiO2 are applied in many areas such as buildings, road paving, vehicle sideview mirrors, lamps and even in textiles (Agrios and Pichat, 2005). The collaboration of Fujishima (and co-workers) with TOTO Co. has significantly advanced the photocatalytic TiO2 market (Mills and Lee, 2002). In 1998 TOTO Co. developed HydrotectTM, which has been used for photocatalytic surfaces on tiles, glass and aluminium panels in more than 5000 buildings in Japan. The coatings keep surfaces clean for more than 20 years,


Self-cleaning smart nanocoatings

399

while uncoated surfaces need to be washed every five years, representing a considerable maintenance cost (TOTO, 2011). Since year 2000, many more companies have emerged in this sector.

13.2

Photocatalysis processes

Photocatalysis is generally defined as the catalysis of a photochemical reaction at a solid surface, usually a semiconductor (Heller, 1995; Mills and Hunte, 1997; Fujishima and Zhang, 2006; Fujishima et al., 2008). Owing to their electronic structure, which is characterized by a filled valence band and an empty conduction band, semiconductors can act as sensitizers for light-induced redox processes (Banerjee et al., 2006). When a photon impinges a semiconductor with energy equal to or higher than the respective energy band-gap (Eg), an electron is promoted from the valence to the conduction band, creating a hole in the valence band. In semiconductors, a portion of these photo-excited electron/hole (e−/h+) pairs diffuse to the surface of the catalytic particle and electron/hole pairs are trapped at the surface. These (e−/h+) pairs take part in chemical reactions with the adsorbed donor or acceptor molecules. The holes can oxidize donor molecules whereas the conduction band electrons can reduce appropriate electron acceptor molecules. Figure 13.1 provides a schematic representation of these photo-excitation mechanisms. For a specific semiconductor to undergo photoinduced electron transfer to adsorbed particles, the potential redox level of the acceptor species must, because of thermodynamic requirements, be below the conduction band of the semiconductor. In the same way, the potential level of the donor species needs to be above the valence band position of the semiconductor in order to be oxidized by the holes (Stamate and Lazar, 2007). This means that the energy level at the bottom of conduction band determines the reduction

hν

Energy

Conduction band

O2–→O2–

Eg H2O/OH–→OH• Valence band

13.1 Operation of a photochemical excited TiO2 particle. (Adapted from Benedix et al., 2005).


400


ability of photo-electrons and the energy level at the top of valence band determines the oxidizing ability of the photoholes (Banerjee et al., 2006). Metal oxides are ideal catalysts for solar-driven photocatalytic applications due to their low cost and high stability in aqueous solution (Alexander et al., 2008; Carneiro et al., 2007; 2008; Osterloh, 2008). The band-edge positions of several metal oxides are shown in Fig. 13.2. The positions are derived from the flat band potentials in a contact solution of an aqueous electrolyte at pH = 0. The pH of the electrolyte solution influences the band edge positions of the different semiconductors compared to the redox potentials for the adsorbate (Linsebigler et al., 1995). Heterogeneous photocatalytic oxidation by TiO2-based materials makes these materials particularly attractive in comparison to other oxidizing contaminant processes (Stamate and Lazar, 2007). TiO2 heterogeneous photocatalysis involves two different reactions that occur simultaneously. The first one is the initial oxidation promoted by the photogenerated holes and the second is the reduction by photogenerated electrons. These two supracited processes mean that the photocatalyst itself does not undergo considerable change (Fujishima et al., 2008). A heterogeneous photocatalytic system is based on the presence of semiconductor particles that are in close contact with a liquid or a reactive gaseous medium. The photo-induced molecular transformations or chemical reactions that take place at the catalyst’s surface depend on where the initial excitation occurs; they can thus be divided into two classes (Linsebigler et al., 1995). The so-called sensitized Photoreaction happens when the initial photo-excitation occurs at the photocatalyst’s surface and energy or an electron is transferred into the molecule’s ground state. However, if the photo-excitation process occurs in molecules adsorbed at the catalyst’s surface (interacting with it the ground state), the process is termed catalyzed photoexcitation.

V –2 –1 0

TiO2 Nb2O5 ZnO Fe2O3 SnO2 H2/ H2O Lower edge position

1 O2/ H2O

of conduction band

2 3 4

Upper edge position of valence band

13.2 Band-edge energies of typical semiconductors metal oxides. (Adapted from Benedix et al., 2005).



401

The general mechanism of photochemical catalysis using TiO2 under UV light irradiation (wavelength less than 385 nm) follows several stages (Hoffmann et al., 1995; Stamate and Lazar, 2007). The basic standard photochemical reactions can be written as follows (Yu et al., 2000; Schrank et al., 2002): •

photo generation electron/hole pairs υ − + TiO2 ⎯h⎯ → ecb + hvb

•

[13.1]

formation of super-oxide radicals − • − ecb , surf + O2 ( ads ) → O2

•

[13. 2]

formation of hydroxyl radicals + • + hvb , surf + H 2 O( ads ) → HO( ads ) + H

[13. 3]

TiO2 can also photodegrade organic compounds through dissimilar oxidation reactions that lead to the formation of innocuous substances such as carbon dioxide and water products. The above chemical reactions can be extended to organic materials and/or biomicroorganisms (Banerjee et al., 2006): HO(•ads) + organic compounds → → x CO2 + y H 2 O •

[13.4]

− 2

Hydroxyl radicals (HO ) and super-oxide ions (O ) are highly reactive species that will oxidize organic compounds adsorbed on the semiconductor surface. The number and lifetime of (e−/h+) pairs are particle size dependent (Shah et al., 2002). For large particles, the (e−/h+) pair’s volume recombination is the dominant process. For small sized particles, the distance covered by (e−/h+) pairs (during their trajectory from crystal interface to the surface) is short, increasing the migration rate to the surface in order to take part in the chemical reaction. Besides the effect of particle size on photocatalytic activity, the role of a metal ion dopant is also very important because it can act as an electron trap in the semiconductor interface. The trap of charge carriers can decrease the volume recombination rate of (e−/h+) pairs and thus increase the lifetime of charge carriers. The process of charge trapping can be described as follows (Shah et al., 2002): − M n + + ecb → M ( n − 1)+

[13.5]

+ M n + + hvb → M ( n + 1)+

[13.6]

+ OH − + hvb → OH•

[13.7]

where Mn+ is the metal ion dopant. The energy level of Mn+/M(n−1)+ lies below the conduction band edge. The energy level of transition metal ions thus affects trapping efficiency. Electron trapping makes it easy for holes to


402


transfer onto the TiO2 surface, reacting with OH− to form active hydroxyl radicals (HO•) that will participate in the overall degradation of organic compounds (Carneiro et al., 2005).

13.3

The photocatalytic cleaning effect of TiO2-coated materials

The development of self-cleaning surfaces has followed several directions. In general, surfaces have been modified through the application of surfactants to induce hydrophilic properties so that the use of a water stream would be sufficient to remove stains and soiling caused by organic compounds. The aim was to develop permanently active surfaces. However, the lack of durability, hardness and weather resistance have been major restrictions to the large-scale use of these customized surfaces. Heterogeneous photocatalysis, on the other hand, is a promising potential technology for self-cleaning (Ramirez et al., 2010). Indeed, one of the first commercial products using a photocatalytic mechanism was coated selfcleaning glasses for tunnel lighting in Japan. Uncoated lamps tend to lose brightness due to contaminants from vehicle exhaust gases that are adsorbed onto the lamp exterior. Sodium lamp glasses coated with TiO2 semiconductor emit enough UV light (~3 mW/cm2) to allow catalytic reactions and photodecompose adsorbed contaminants (Fujishima et al., 2000). The automotive industry (glasses), chemical industry (paints), textile industry (TiO2 nanoparticles to treat effluents from processing), ceramic industry (tiles with bactericidial effect), construction industry (facade tile materials, glass and pavements) and the photovoltaic industry (application of self-cleaning glasses to optimize optical transmission) are some examples (Agrios and Pichat, 2005). Japan has been the pioneer in the development and commercialization of photocatalytic products, particularly in the collaboration between Dr A. Fujishima and co-workers in collaboration with TOTO Co. (Mills and Lee, 2002; TOTO, 2011). The wetting of a solid with water (Barthlott and Neinhuis,1997) is dependent on the relation between the interfacial tensions (water/air, water/solid and solid/air). The ratio between these tensions determines the contact angle q between a water droplet on a given surface. A contact angle of 0° means complete wetting, and a contact angle of 180° corresponds to complete non-wetting. For contact angles above 120°, the surface’s wettability state is considered to be super-hydrophobic (an example can be seen in Fig. 13.3). In general, the higher the angle the lower is the value of the adhesion work (surface energy). Hydrophobic surfaces with low wettability and contact angles of about 90° ≤ q ≤ 120° have been known for a long time (Barthlott and Neinhuis, 1997). In contrast, decreasing the contact angle should lead to enlarged values of adhesion (hydrophilic surfaces).



403

65°

=1

CA

13.3 Contact angle of a water droplet on a superhydrophobic surface.

The contact angle of TiO2-coated materials also depends considerably on surface morphology, especially on average roughness. The deposition of photocatalytic thin films with controlled porosity can significantly improve photocatalytic efficiency (Hashimoto et al., 2005). In 1995 it was found that, besides the photocatalytic properties of TiO2, it demonstrates an intrinsic photo-induced surface super-hydrophilicity. This is one of the unique properties of TiO2 materials. Depending on processing techniques and chemical composition, a given TiO2 surface can have a more super-hydrophilic and less photocatalytic character, or vice versa (Fujishima et al., 2000). When irradiated by UV light, the water adsorbed on this semiconductor material spreads forming a thin film instead of a water droplet. Other materials do not possess these properties. Strontium titanate surfaces, which have a photocatalytic oxidation power comparable to TiO2, are not superhydrophilic under UV irradiation. A WO3 surface possesses photo-induced surface super-hydrophilicity conversion but does not show photocatalytic activity (Fujishima and Zhang, 2006). It has been proposed that the mechanism behind this property of TiO2 is related to the reconstruction of hydroxyl groups under UV irradiation (Fujishima et al., 2000). Photo-excited electrons are captured by molecular oxygen and the holes diffuse to the TiO2 surface where they are trapped by lattice oxygen atoms. The energy between the Ti atoms and lattice oxygen is weakened by hole trapping. Another adsorbed water molecule breaks this bond, forming a new hydroxyl group (Fujishima et al., 2000). The successive dissociative adsorption of water induces the trapping of these


404


hydroxyl species leading to the photogeneration of a hydrophilic domain (size of about 10 nm) (Sakai et al., 2003). Given the unstable state of this surface, the photogenerated bound hydroxyl groups desorb gradually and the surface returns to the initial state demonstrating less hydrophilic behavior. In the case of photo-inducedsuper-hydrophilicity, the photo-electrons are trapped by tetravalent titanium ions Ti4+ through the formation of Ti3+ ions that are then oxidized by oxygen (Shapovalov, 2010). The following reaction equation describes the entire process (Fujishima et al., 2000): hυ ≥ E , k

bg 2 ⎯⎯⎯⎯ ⎯⎯⎯ → ≡ Ti − OH HO − Ti ≡ ≡ Ti − O − Ti ≡ + H 2 O ← ⎯

dark , Δ , k−2

[13.8]

where k2 and k−2 are the rate constants of the hydrophilic transformation for direct (under UV radiation) and inverse processes (without UV radiation), respectively. In this sense, TiO2-based surfaces can maintain their hydrophilic properties indefinitely as long as they are irradiated by UV light (Fujishima et al., 2000). If one recalls the cleaning action of a stream of water, TiO2-based surfaces could be, in theory, anti-staining and anti-soiling without the need of any chemical detergents. Furthermore, the application of TiO2-based materials can lead to the creation of anti-fogging surfaces which are particularly important when applied to glasses and mirrors such as vehicle side-view mirrors. The fogging of mirrors and glass surfaces appears due to the cooling of water steams that lead to the formation of many small water droplets which scatter light (Augugliaro et al., 2010). Super-hydrophilic surfaces prevent the formation of such droplets through the creation of a uniform water film which prevents the surface becoming unclear (Hashimoto et al., 2005). Moreover, super-hydrophilic surfaces are averse to absorbing organic liquids and can easily be cleaned with water. This feature is highly desirable in kitchens and toilets as well as in surfaces exposed to more polluted environments, e.g. in urban areas. Additionally, multilayer thin films can be used to combine the exceptional TiO2 properties with other technologies and/or materials (Augugliaro et al., 2010). As an example, the Leonardo Fioravanti (former Ferrari designer), presented novel car windows that do not require widescreen wipers at the Geneva Motor Show. The Hidra’s front and rear panels are composed of a four-layer film that imparts self-cleaning properties to the glass under any environmental conditions. In the layered structure, a super-hydrophilic coating of TiO2 was applied to avoid water droplets formation and to photodegrade volatile pollutants. Since TiO2 absorb UV light, the panels will also behave as a shield against this fraction of the solar spectrum (Augugliaro



405

et al., 2010). In fact, discovering the photo-induced-super-hydrophilicity of TiO2 has markedly broadened the application range of titanium-based materials. There is a wide range of approaches used to deposit TiO2 layers on glass and polymer substrates, such as physical and chemical vapor deposition, sol–gel and dip-coating techniques, among others (Sabate et al., 1992; Chen et al., 1999; Zheng et al., 2003; Carneiro et al., 2005). TiO2 layers have generally required further heat treatments at relatively high temperature (Al-Jufairi, 2006). The production of crystalline layers at room temperature is preferable. TiO2 occurs in three crystalline phases (rutile, anatase and brookite), among which anatase is believed to be the most efficient photocatalyst during chemical reactions (Carneiro et al., 2005). Liquidphase deposition techniques based on sol–gel methods can be used to prepare high quality crystalline TiO2 nanoparticles-based layers at low production costs. The large-scale application of TiO2-based materials is limited by the need for an external UV excitation source (Turchi and Ollis 1990; Tavares et al., 2007). TiO2 photocatalytic efficiency is low under outdoor solar light irradiation. Most research has focused on nanosized TiO2 to improve light absorption through the high surface-to-volume ratio of nanograins (Shannon, 1976). In addition, increasing the generation rate of charge carriers is one strategy to enhance photocatalytic activity. Nanoparticles, with their increased surface area, can provide surface states within the band-gap to effectively reduce it (Zheng et al., 2003). Moreover, the addition (doping) of foreign ions (Fe+3, Cr+3or Pd+4) has been reported as a promising strategy to increase the visible light absorption of TiO2 materials (Carneiro et al., 2005). Figure 13.4 shows the optical transmittance spectra of TiO2 coatings deposited on glass substrates by DC reactive magnetron sputtering. It can be seen that, for Fe-doped TiO2 coatings, the absorption edge shifts to long wavelengths. This red-shift has been attributed to the excitation of 3d electrons of Fe3+ to the conduction band (Asiltürk et al., 2009). In fact, the main purpose of Fe doping is to extend the light absorption edge in order to make use of the majority of the ambient light spectrum. Another important technological application of TiO2-based materials (applied as nanoparticles or thin films) is its use as an anti-microbial agent across larger surface areas. The presence of dust, soil and spilled fluids in association with suitable temperature and humidity provides good conditions for a rapid multiplication of microorganisms. The application of TiO2-based coatings to tiles placed in surgical facilities could substantial decrease the amount of microorganisms present in materials in close contact to patients. The anti-bacterial mechanism of TiO2 is based upon the following:


406


Transmittance (%)

90

60

30 Undoped TiO2 Fe-doped TiO2 0 300

400

500 600 700 Wavelength (nm)

800

900

13.4 UV-vis specta of undoped and Fe-doped TiO2 coatings produced by DC reactive magnetron sputtering.

• •

the TiO2 photocatalyst generates highly reactive species that attack the outer membrane of cells; an intrusion of copper ions into cytoplasmatic membrane kills microorganism’s cells.

In medical textiles, for instance, anti-microbial agents can be used to prevent staining and rotting of fibers, decrease unpleasant odors and the health risks associated with microbial growth. This mechanism of photocatalytic deactivation of microorganisms requires more time in indoor conditions than in outdoor conditions. However, the anti-bacterial properties of TiO2 induced by its photocatalytic capacity are strongly enhanced by fluorescent lamps (even with weak UV light) using either silver or copper metal dopants, both of which are harmless to human beings. An anti-bacterial effect can be observed on TiO2 doped with silver (Ag) (Hashimoto et al., 2005).

13.4

New and smart applications of TiO2 coatings

TiO2-based materials have been used to provide self-cleaning, anti-bacterial and anti-fogging functions based on photo-induced hydrophilicity and decomposition photoreactions. It is important to note that these functions do not require chemical compounds: theoccurrence of sunlight and rainwater are sufficient. In this sense, TiO2-based materials are environmentally friendly.



407

13.4.1 Photocatalytic oxidation of NOx gases applied to constructive materials Industrial activities and road traffic are the main causes of the emission of pollutant gases such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). The health costs related only with road traffic air pollution represent 0.9–2.7% of the gross domestic product (GDP) in France, for example (Sommer et al., 1999). This pollution also reduces the appearance and durability of building materials. Inorganic photocatalysts, such as TiO2, have proven to be a relatively cheap and effective way to remove toxic organic compounds and pollutant gases from air and aqueous environments (Chen et al., 1997; Bilms et al., 2000; Hashimoto et al., 2000; Nakamura et al., 2000). An important and smart application of TiO2 photocatalysis is its use in construction and building materials. Given their large surface areas, buildings can even be used to promote the removal of pollutant gases from the surrounding air. Under UV irradiation, a TiO2 electron located in the valence band is promoted to the conduction band, creating an electron-hole pair (see Eq. 13.1) which can participate in a variety of redox reactions. Harmful NOx gases can be oxidized to nitrates on TiO2 surfaces activated by UV radiation (Oosawa and Graetzel, 1988). Since hydroxyl radicals (OH•) and super-oxide ions (O2−) are highly reactive species, they can react with nitrogen oxides to form nitrates. These nitrates can then be easily consumed and recycled by plants. Dalton et al. (2002) suggested that the NOx removal process involves the following steps: 1. Oxidation using hydroxyl radicals: OH• NO( g ) + 2OH(•ads) → NO2(ads) + H 2 O(ads) or NO2(ads,g ) + OH(•ads) → NO3−(ads) + H(+ads) 2. Oxidation using active oxygen: O2− O−

2 ( ads ) NOx (ads) ⎯⎯⎯ → NO−3(ads)

3. Reaction with Ti–OH via disproportion 3NO2 + 2OH − → 2 NO−3 + NO + H 2 O 4. Removal of [HNO3] complex from a material surface by water [HNO3](ads)→HNO3(aq) Figure 13.5 summarizes the overall reactions. Currently, the photocatalysis process requires a large amount of both light energy and photocatalyst material to remove pollutants from the


408

Nanocoatings and ultra-thin films Sunlight (UV radiation)

NO

NO

NOx

NOx

NO2

NO2 OH•

•

O2–

OH

•

•

O2–

TiO2

NO3–

TiO2 NO3–

13.5 Schematic illustration showing NOx photodegradation process.

Activation of TiO2 Inserted glass cullets

TiO2-modified bituminous formulation

13.6 Pathways of light and activation of TiO2 in road pavement materials using glass as aggregates. (Adapted from Chen, and Poon, 2009).

surrounding environment. This has not yet made practical applications feasible. A suggested application is the inclusion of TiO2 micro/nanoparticles and recycled glass cullets in traditional road pavement material formulations. Since photocatalytic activity depends on the available electron/hole photo-induced pairs on surface of TiO2 micro/nanoparticles, the option of adding recycled glass cullets onto road pavement formulations should promote an in-depth conduction and entrapment of light, increasing NOx photodegradation efficiency. In theory, solar light would be carried to a greater depth, activating the TiO2 within the inner part of the surface layers as well as on the surface. This both meets recycling objectives and reduces pollution. The proposed mechanism for photocatalytic activity enhancment by inclusion of glass cullets is illustrated in Fig. 13.6.



409

13.4.2 Treatment of soils polluted by volatile organic compounds Environment pollution of water, air and soil is becoming an increasingly serious problem. Hashimoto et al. (2005) have suggested a system for purification of soils polluted by VOCs based on the application of TiO2 photocatalysis activated by solar light. This purifying system employs a photocatalytic sheet, made of grooved paper, which has a TiO2 powder adsorbed onto activated carbon powder, as shown in Fig. 13.7. This sheet covers the polluted soil. The covered soil needs to be heated to volatilize the pollutant gases, that are subsequently captured by adsorption on activated carbon incorporated in the sheet material. Meanwhile, the TiO2 photocatalyst completely decomposes the pollutants by a photo-activated redox reaction. This purification method promises a sustainable form soil de-pollution.

13.4.3 Efficient energy-saving technology: water evaporation using hydrophilic surfaces The level of energy consumption in high density urban environments causes an inevitable temperature increase in the urban environment, the so-called heat island phenomenon. This can increase the heat absorbed by individual buildings which need then to be cooled by air-conditioning to maintain an acceptable living environment. Hashimoto et al. (2005) have suggested a system (illustrated in Fig. 13.8) which involve sprinkling water continuously onto building facades previously coated with a TiO2 photocatalyst.

VOCs photocatalytic degradation VOCs

TiO2 corrugated sheet Polluted soil

13.7 Purification system for polluted soils using solar energy and photocatalytic sheets. (Adapted from Hashimoto et al., 2005)


410

Nanocoatings and ultra-thin films TiO2-coated surfaces water sprinkling

Hydrophilic surface Latent heat flux by water evaporation

Rain water reservoir

13.8 Energy-saving system using solar light and stored rainwater.

Under the action of solar light irradiation the TiO2-coated building surface becomes super-hydrophilic. This surface state means that even a small amount of water sprinkled on the building would be enough to form a water thin film that would cover the entire building facade. It is important to highlight that the surfaces of buildings are cooled via a thermodynamic process, by releasing a certain amount of heat that results from water evaporation. This smart process for cooling down the buildings would decrease the electricity consumed by conventional air conditioning.

13.5

Conclusions

Although photocatalysis research emerged about 80 years ago, it was not until 1960 that scientific research resulted in any commercial product using TiO2 as a photo-active material. However, heterogeneous photocatalytic oxidation by TiO2-based materials makes them an attractive option in comparison to other oxidizing contaminant processes. Currently, this semiconductor has been widely applied to produce surfaces with self-cleaning, de-polluting, anti-fogging and anti-microbial abilities. TiO2-based materials have been deposited, using different technologies, to tailor surface properties of dissimilar surfaces such as textiles, vehicle side-view mirrors, lamps, buildings and road paving. Among them, liquid phase deposition techniques based on sol–gel methods are particularly promising since they can be used to prepare high quality crystalline TiO2 nanoparticles-based layers at low production costs and on an industrial scale.



411

Since their self-cleaning and de-pollution abilities arise from the intrinsic properties of TiO2-based materials (without the need of any chemical compound), they are environmentally friendly. Given their large surface areas, buildings and pavements can be used to promote the removal of pollutant gases (NOx, for instance) from the air, thus improving air quality and the population’s health.

13.6

References

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Index

adhesion, 59 aerospace engineering conventional coating technologies and smart nanocoatings for corrosion protection, 235–70 aluminium and its alloys, 243–4 anodising coatings, 253–7 applications of nanomaterials, 266–70 corrosion in aeronautical structures, 235–6 corrosion of magnesium alloys, 244–5 detection of corrosion and mechanical damage, 259–61 factors influencing corrosion, 241–3 functional nanocoatings, 258–9 future trends, 270 pretreatments, 247–53 protective coatings in aerospace engineering, 246–7 self-healing coatings, 261–6 types of corrosion in aircraft, 236–41 air/knife coating, 11 Alclad, 244 aluminium see also Alclad corrosion, 243–4 Amontons-Coulomb law, 357 Amontons’ law, 388 AMS2488, 382

analytical methods electrochemical methods, 132–40 cathodic stripping for mechanically polished copper specimens, 133 dependence of current density peak and potential peak on potential scan rate for copper specimens, 134 electrical equivalent circuit used to model the impedance data, 136 electrochemical impedance spectroscopy, 133–9 experimental data vs impedance calculated using KK relationships, 137 Nyquist plot for AISI 316L stainless steel, 135 organic coatings porosity, 140 parameters used in the simulation of impedance data, 136 potentiodynamic potential/current measurements, 132–3 water absorption in organic coatings, 139–40 nanocoatings and ultra-thin films, 131–53 spectroscopic, microscopic and acoustic techniques, 145–53 glow discharge optical emission spectroscopy, 150–1

414 © Woodhead Publishing Limited, 2011

Index infrared, Raman and Mössbauer spectroscopies, 145–7 ion scattering, Rutherford backscattering and secondaryion mass spectroscopy, 148–50 scanning acoustic microscopy and Kevin probe force microscopy, 152–3 scanning electron microscopy and transmission electron microscopy, 151–2 x-ray diffraction, 147–8 surface-sensitive analytical methods, 140–5 characterisation by AFM, 141 specular reflectance infrared spectroscopy characterisation, 143–5 XPS study of corrosion protection, 142–3 anodic aluminum oxide, 292 anodisation, 63 anodising, 6, 253–7 electrochemical chracterisation, 255–6 fatigue properties, 256–7 S-N plots for Al 2024-T3, 257 film structures, 254–5 micrograph of anodised Al 2024, 256 TEM micrographs of aluminium, 255 anti-microbial packaging, 215–16 anti-Stokes scattering, 146 antistatic packaging, 220–1 architectural glass, 182–95 dynamic smart glazings, 188–94 activated photoelectrochromic device, 192 composition of electrochromic glazing, 189 composition of glass laminate with liquid crystals or suspended particles, 192 electrochromic glazings, 188–9

415

gasochromic electrochromic device composition, 191 gasochromic-electrochromic glazings, 190–1 glazing composition with two complementary electrochromic layers, 189 light control and thermal imaging glazings, 193–4 liquid crystal and suspended particle glazings, 192–3 photochromic-electrochromic glazing devices, 191–2 photovoltaic-powered electrochromic device, 190 photovoltaic-powered electrochromic glazings, 190 thermally-activated glazings, 193 glass surface protections, 194–5 water droplet contact angle on hydrophilic and hydrophobic surface, 195 spectral transmittance and reflectance clear float and low iron and low-e glass, 186 low-e glass with single and double Ag layer, 187 low-e glass with single and triple Ag layer, 187 spectrally selective glass, 183–8 glass with low-emissivity coatings, 184–8 low-e coating layer, 185 optical properties and emissivity of glass, 183–4 ASTM 2625, 382 ASTM B449-93, 247 atmospheric pressure CVD, 72–3 atom transfer radical polymerisation, 102–3, 105 atomic force microscopy, 32, 308–9 atomic layer deposition, 71 automotive, 363–4 piston skirt damage due to severe scuffing, 364


416

Index

Bayresit, 210 benzophenone, 102 biomaterials, 366 biomedical implants industry, 174–5 most common techniques for hydroxyapatite coatings formation, 176 bismuth telluride, 344 bit patterned media, 318 Boegel, 252 ‘bond coat,’ 138 bottom-up method, 106 Bragg’s law, 147 Brasher–Kingsbury empirical relationship, 140 Brewster angle microscopy (BAM), 27 capillary force lithography, 301 carbon nanotubes, 217, 221 catalysed photoexcitation, 400 cationic polymerisation, 295 cerium, 248–9 chalcogenide, 343 chemical conversion coatings, 159–61 chromate and chromate-free conversion coatings, 160–1 conversion coatings by hydrothermal treatment, 160 chemical liquid deposition, 63 chemical shift, 142, 147 chemical solid growth, 63 chemical vapour deposition, 6–7, 71–4, 336, 373–5 chemisorption, 82 chromate conversion coating, 5–6, 152–3, 247–8 chrome-free conversion coatings, 6 chromic acid anodising, 254 chromium-free inhibitors, 248–50 chromogenic glazings, 183 cluster-beam PVD, 68 CMOS image sensors, 318–19 coating capacitance, 139 coating technologies advanced polymers and fillers, 17–18 conductive polymers, 17–18 fillers, 18

hyperbranched polymers, 17 organic–inorganic hybrid polymers, 17 water-soluble paint, 18 chemical and physical vapour deposition, 6–7 chemical vapour deposition, 6–7 physical vapour deposition, 7 coating processes developments, 18–20 conversion coatings, 5–6 anodising, 6 chromate conversion coating, 5–6 chrome-free conversion coatings, 6 current and advanced for industrial applications, 3–20 electro- and electroless chemical plating, 4–5 electrochemical plating, 4–5 electroless chemical plating, 5 new composite and powder coatings, 16–17 composite coatings, 16 powder coatings, 16–17 new lightweight materials, 12–13 other coating techniques, 10–12 air/knife coating, 11 curtain coating, 12 dip coating, 12 gravure coating, 10–11 knife over roll coating, 11 Meyer rod coating, 11 roll-to-roll coating, 11 slot/die and slot/extrusion coating, 12 sol–gel coatings, 10 spin coating, 10 spray coating, 7–10 cold spraying, 9 high-velocity oxygen fuel spraying, 8 plasma spraying, 8–9 thermal spraying, 8 vacuum plasma spraying, 9 warm spraying, 9–10


Index trends in coatings, 13–16 environmentally friendly coatings, 13 micro- and nanocapsule-based coatings, 14–15 nanocomposite coatings, 15–16 self-assembling molecules, 13 self-cleaning coatings, 14 coatings see also nanocoatings advanced technologies for automotive and aerospace industries, 162–70 modelling and computer simulations, 169–70 powder coating, 168 self-cleaning coatings, 167–8 ‘smart’ multifunctional coatings, 163 ‘super’-hard coatings, 164–7 thermal barrier coatings, 169 transparent coatings, 168 biomedical implants industry, 174–5, 176 conventional and advanced for industrial applications, 159–76 conventional technologies for automotive and aerospace industries, 159–62 chemical conversion coatings, 159–61 organic and inorganic coatings, 161–2 thermal-sprayed coatings, 162 electronics and sensor industry, 171–3 electronic nanodevices, 172–3 photovoltaic surfaces, 173 sensors, 171–1 most common techniques for hydroxyapatite coatings formation, 176 packaging applications, 170–1 coatings for food and pharmaceutical industries, 170 coatings in the paper industry, 170–1

417

paints and enamels industry, 173–4 coefficient of friction, 388 cold spraying, 9 ‘colloid probe,’ 141 combustion chemical vapour deposition, 73 complex non linear least squares (CNLLS) procedure, 135 compliance packaging, 206 composite coatings, 16 conductive polymers, 17–18 conductors, 58 controlled radical polymerisation, 102–6 conventional radical polymerisation, 92, 93–4, 102 conversion coatings, 5–6 anodising, 6 chromate conversion coating, 5–6 chrome-free conversion coatings, 6 conversion electron Mössbauer spectra, 147 corrosion, 236 aeronautical structures, 235–6 aluminium and its alloys, 243–4 anodising coatings, 253–7 electrochemical chracterisation, 255–6 fatigue properties, 256–7 film structures, 254–5 applications of nanomaterials, 266–70 deicing process, 268 XTEM bright field images of TaSi2–Si3N4 nanocomposite coating, 269 coating technologies and smart nanocoatings, 235–70 future trends, 270 factors, 241–3 susceptibility of metallic materials, 243 functional nanocoatings, 258–9 self-healing effect, 258 magnesium alloys, 244–5 nanocoatings for detection and mechanical damage, 259–61


418

Index

pH sensing coatings on aluminium alloys, 260 pretreatments, 247–53 chromate conversion coatings, 247–8 chromium-free inhibitors, 248–50 magnesium-rich primers, 252–3 sol–gel coatings, 250–2 protective coatings in aerospace engineering, 246–7 schematic of aerospace coating system, 246 self-healing coatings, 261–6 electrochemical impedance spectra of AA2024substrates, 262 scanning vibrating electrochemical technique, 265 types of corrosion in aircraft, 236–41 CFRP in Boeing 777 design, 240 fastener joint, 238 optical micrograph of scribe and filament, 237 corrosion fatigue, 241 crevice corrosion, 238 ‘critical brush density,’ 30 critical nucleus, 108 curtain coating, 12 deep drilling tools, 365–6 rock drill with cemented carbide buttons, 366 deformation, 360 Derjaguin–Muller–Toporov theory, 358 diamond-like carbon, 293–4 1,2-dichloroethane, 333 dielectric layers, 58 diffuse reflectance technique, 144 dip coating, 12 dip-pen nanolithography, 308 discrete track recording, 318 doping, 342 ‘double-pass transmission,’ 144 DS4 tester, 387 Durethan, 214 dynamic smart glazings, 188–94

edge lithography, 309–11 edible packaging, 220 electret, 303 electrochemical impedance spectroscopy, 133–9, 255–6 Bode diagrams for aluminium, 257 electrochemical liquid growth (ECLG) method, 63 electrochemical plating, 4–5 electrochromic glazings, 188–9 electroless chemical plating, 5 electron acoustic images, 152 electron-beam evaporation, 65 electron-beam evaporation PVD, 65–6 electron beam lithography, 283 electron spectroscopy for chemical analysis, 161 electronic packaging, 206–8 electronics nanoimprint lithography, 280–321 applications, 317–20 combined nanoimprint approaches, 315–17 edge lithography, 309–11 extension of soft NIL, 301–7 lithography techniques and fundamentals, 280–6 NIL for 3D patterning, 311–14 photo-assisted nanoimprinting, 291–6 scanning probe lithography, 307–9 soft NIL, 297–301 thermoplastic and laser-assisted nanoimprint lithography, 286–91 electroplating, 375 Elektron, 245 encapsulation, 218–19 ‘end-grafted polymers’ see ‘polymer brush’ energy dispersive x-ray spectroscopy (EDS), 148 environmentally friendly coatings, 13 equilibrium adsorption, 80 ethylene tetrafluoroethylene, 287 EVG, 318 exfoliation, 239


Index extreme UV lithography, 282–3 extrusion, 12 F-NAD, 161–2 fatigue tests, 256–7 field-effect transistor, 311, 332 filiform corrosion, 237–8 fillers, 18 fluorine-modified polymers, 161 focused ion beam, 283 food packaging, 205–6 fourier transform infrared Raman spectroscopy, 146 Fresnel equations, 145 fretting, 365 fretting corrosion, 241 friction, 356–8 functional graded nanocoatings, 57–60 gallium nitride, 344–51 AFM images, 347 CL spectra from dislocation cluster, 350 SEM images, 346, 348 SEM vs monochromatic μ-CL images, 349 galvanic corrosion, 239 gasochromic-electrochromic glazings, 190–1 GENOCURE, 296 geometric hindered factor, 213 glass surface protections, 194–5 glass transition temperature, 290 glow discharge optical emission spectroscopy (GDOES), 150–1 grafting density, 83–4 ‘grafting from’ approach, 92 ‘grafting through’ method, 106 ‘grafting to’ approach, 89–91 graphene 2D structures, 331–41 graphene FETs in the electrolyte solution, 341 HRTEM images of graphene membrane, 335 microfabrication steps, 333

419

morphological and structural analyses of the ZnO–G HAs, 338 optoelectronics, 339 graphene nanoribbons, 333 graphene oxide, 336 graphene sheets, 221 graphene transparent conducting films, 339–40 graphite fibres, 239–40 graphite particles, 221 gravure coating, 10–11 Greenwood–Williamson theory, 357–8 Hall–Petch relationship, 367–8 heterogenous nucleation, 108 hexagonal boron nitride phase, 342 high barrier packaging, 209–15 models of aligned mono-disperse flakes in periodic arrangement, 212 oxygen permeability as function of water vapour barrier properties, 210 oxygen scavenging materials, 214–15 samples of oxygen scavenging materials, 215 high ordered pyrolytic graphite, 47 high velocity oxy-fuel spraying, 8, 148, 162 holographic patterning, 312 homogenous nucleation, 108 hot embossing see thermal nanoimprint lithography Hydrotect, 398–9 8-hydroxiquinoline, 266 hydroxyapatite, 174, 175 hyperbranched polymers, 17 hyperfine splitting, 147 immersion lithography, 282 Imprio, 318 ‘in situ polymerisation,’ 209 in-situ tribometry, 387 indium tin oxide, 339 infrared spectroscopy, 145–6 InMat Nanocoatings, 211


420

Index

inorganic coatings, 161–2 interfacial shear strength, 358–9 intergranular corrosion, 239 International Technology Roadmap for Semiconductors, 282 intimate contact area, 359–60 ion beam milling, 194 ion implantation process, 68 ion-plating PVD, 68 ion scattering spectroscopy, 148–9 irreversible adsorption, 80 isomer shift see chemical shift Johnson–Kendall–Roberts contact theory, 358 Kevin probe force microscopy (KPFM), 152–3 Kirchhoff’s law, 184 knife over roll coating, 11 Kramers–Kronig test, 137, 145 Langmuir monolayer, 24 Langmuir–Blodgett assembly, 334 Langmuir–Blodgett film, 383 Langmuir–Blodgett technique, 87, 92 Langmuir–Blodgett–Kuhn multilayers, 32 Langmuir–Schaefer method, 27 Langmuir–Schaefer monolayers, 32 laser ablation PVD, 66 laser-assisted direct imprint, 290–1 laser-induced chemical vapour deposition, 308 lasercarb coating, 380 lateral force microscopy, 383 laws of friction, 357 layer-by-layer deposition, 87, 264 layered double hydroxides, 211, 262–4 light control and thermal imaging glazings, 193–4 light-emitting diodes, 193 linear potential sweep, 132 ‘liquid condensed phase,’ 27 liquid crystal and suspended particle glazings, 192–3 ‘liquid expanded phase,’ 26

‘liquid glass,’ 194 lithography techniques, 280–6 nomenclature of methods, 281 process steps, 285 lotus effect, 14 low energy ion scattering, 149 low pressure CVD, 72–3 magnesium alloys, 244–5 magnesium-rich primers, 252–3 matrix-assisted laser desorption/ ionisation, 66 matrix-assisted pulsed-laser evaporation, 66 mean surface roughness, 141 melt mixing process, 209 2-mercaptobenzothiazole, 266 metal corrosion, 235 metal-organic chemical vapour deposition, 365 Meyer rod, 11 Meyer rod coating, 11 micro-based coatings, 14–15 microcontact printing, 297, 298 microelectromechanical systems, 359 micromachining, 307 micromolding in capillaries, 300 microtribometers, 363 sample, 364 MIL-DTL-5541, 247 molecular beam epitaxy, 311 molecular-beam epitaxy PVD, 67–8 molybdates, 249 montmorillonite, 211 Mössbauer absorption spectrometry, 147 Mott–Schottky plots, 139 nanocapsule-based coatings, 14–15 nanoclay particles, 220 nanocoatings and ultra-thin films analytical methods, 131–53 electrochemical methods, 132–40 spectroscopic, microscopic and acoustic techniques, 145–53


Index surface-sensitive analytical methods, 140–5 architectural glass, 182–95 dynamic smart glazings, 188–94 glass surface protections, 194–5 spectrally selective glass, 183–8 chemical and physical vapour deposition methods, 57–75 chemical vapour deposition based technologies, 71–4 D-gun spray deposition of functional gradient coating from titanium and hydroxyapatite, 70 future trends, 74–5 paradigm of functional graded layer-by-layer coating fabrication, 60–1 physical vapour deposition based technologies, 63–71 substrate preparation for ultrathin films and functional graded nanocoatings, 57–60 typical installation diagram for PVD and CVD coating, 64 corrosion protection in aerospace engineering, 235–70 aluminium and its alloys, 243–4 anodising coatings, 253–7 applications of nanomaterials, 266–70 corrosion in aeronautical structures, 235–6 corrosion of magnesium alloys, 244–5 detection of corrosion and mechanical damage, 259–61 factors influencing corrosion, 241–3 functional nanocoatings, 258–9 future trends, 270 pretreatments, 247–53 protective coatings in aerospace engineering, 246–7 self-healing coatings, 261–6 types of corrosion in aircraft, 236–41 fabrication methods, 61–3

421

approximate classification scheme of nanocoating technologies, 62 packaging applications, 203–22 anti-microbial packaging, 215–16 antistatic packaging applications, 220–1 future trends, 222 high barrier packaging, 209–15 nanomaterials in packaging, 208–9 nanosensors in packaging, 216–18 nanotechnology solutions for packaging waste problem, 219–20 packaging as a drug carrier and for drug delivery, 218–19 regulation and ethical issues in new packaging industry, 221–2 surface-initiated polymerisation, 78–112 physisorption and chemisorption, equilibrium and irreversible adsorption, 79–87 properties and applications, 110–12 surface-bound polymer layers preparation, 87–110 nanocomposite coatings, 15–16 nanoimprint lithography, 284 3D patterning, 311–14 gold pattern fabrication, 314 electronics, 280–321 applications, 317–20 combined nanoimprint approaches, 315–17 edge lithography, 309–11 extension of soft NIL, 301–7 lithography techniques and fundamentals, 280–6 photo-assisted nanoimprinting, 291–6 scanning probe lithography, 307–9 soft NIL, 297–301 thermoplastic and laser-assisted nanoimprint lithography, 286–91 nanoimprinting in metal/polymer bilayer, 298 nanomaterials, 208–9


422

Index

Nanomiser device atomisation, 73 nanomolding in capillaries, 300 nanoreservoirs, 163 nanosensors, 216–18 nanoshaving, 308 nanostructure coatings challenges to establish scale up, 389–91 balance cost/performance ratio, 391 coat different sample sizes and nanostructure retention, 389–90 performance of laboratory tests and reliability analysis, 390–1 friction and wear applications, 367–82 deposition methods, 373–9 engineering materials, 379–82 structure-property relationships, 367–73 tribology, 355–91 friction and wear applications, 367–82 future trends, 391 tribological properties characterisation, 382–91 use of nanostructure coatings, 356–66 nanostructured composite, 209 nanostructured thin films 2-D arrays of colloidal spheres, 44–7 A2-D array fabrication methods at air–liquid interface, 45 amphiphilic molecules, 24–48 amphiphilic polymers, 28–36 block copolymers, 28–32 interfacial behaviour of ionic block copolymers, 31 π-conjugated polymers, 32–6 poly(p-phenylene) derivative chemical structure and π-A isotherm, 33 poly(phenylenevinylene) derivatives possible arrangement at the air–water interface, 34 polythiophene derivatives, 35

PS–b–PEO diblock copolymer surface morphological behaviour, 29 dendrons and dendrimers, 36–41 chemical structure of fourth generation poly(ethyl ether) with hexa(ethylene glycol) tail, 38 poly(amidoamine), 37–40 poly(benzyl ether), 36–7 poly(propylene imine), 40–1 PPI dendrimers, 41 tetra-dendronpoly(amidoamine) dendrimers, 39 Langmuir monolayer, 24–8 isotropic liquid film in trough with movable barrier, 25 Langmuir trough and other experimental techniques, 27 Langmuir–Blodgett rough being compressed, 25 recent researches, 28 surface pressure, 25 surface pressure isotherm, 26–7 surface pressure isotherm for Langmuir monolayer after reference 7, 26 metal/semiconductor nanoparticles, 41–4 TOPO-capped CdSe quantum dot, 43 nanotechnology, 208 solutions for packaging waste problem, 219–20 nanotransfer printing, 298 Nielson equation, 213 nitroxide mediated radical polymerisation, 102, 104–5 1-octadecanethiol, 43 optical proximity correction, 282 organic coatings, 161–2 organic electrochemical transistor, 34 organic thin film transistors, 296 organic–inorganic hybrid polymers, 17 Orowan mechanism, 369 oxalic acid, 254


Index oxide film, 254–5 oxygen scavenging materials, 214–15 packaging anti-microbial packaging, 215–16 antistatic packaging applications, 220–1 multilayer structure and chemical antistatic agent, 221 as a drug carrier and for drug delivery, 218–19 definition, 206 future trends, 222 high barrier packaging, 209–15 models of aligned mono-disperse flakes in periodic arrangement, 212 oxygen permeability as function of water vapour barrier properties, 210 oxygen scavenging materials, 214–15 samples of oxygen scavenging materials, 215 nanocoatings and ultra-thin films applications, 203–22 electronic packaging, 206–8 electronic packaging products, 207 examples of food packaging, 205 food packaging, 205–6 pharmaceutical packaging, 206 pharmaceutical packaging samples, 207 nanomaterials, 208–9 nanosensors, 216–18 radio-frequency identification, 218 nanotechnology solutions for packaging waste problem, 219–20 biodegradable food packaging, 219 principal objectives, 204 regulation and ethical issues in new packaging industry, 221–2 packaging applications, 170–1 food and pharmaceutical industries, 170 paper industry, 170–1

423

barrier-coated papers, 170–1 curtain coating, 171 patina, 132 3-pentadecylphenol, 30 pharmaceutical packaging, 206 phase-change random access memory, 318–19 phase changed materials, 267 photo-assisted nanoimprinting, 291–6 photocatalysis photocatalytic cleaning effect of TiO2-coated materials, 402–6 contact angle of water droplet, 403 UV-vis specta of undoped and Fe-doped TiO2 coatings, 406 processes, 399–402 band-edge energies of semiconductors, 400 photochemical excited TiO2 particle, 399 titanium dioxide, 397–9 photochromic-electrochromic glazing devices, 191–2 photoelectrochromic glazing, 191 photolithography, 282 photoresist patterns, 310 photovoltaic-powered electrochromic device, 190 photovoltaic-powered electrochromic glazings, 190 phyllo silicates, 18 physical liquid deposition, 63 physical solid deposition, 64 physical vapour deposition, 7, 63–71, 373–5 physisorption, 82 pitting corrosion, 238–9 plasma-assisted chemical vapour deposition, 372 plasma energy CVD, 73, 74 plasma spraying, 8–9, 162 poly(1,1-diethylsilacyclobutane)–b– poly(methacrylic acid), 30 poly(3-hexyl-thiophene), 149 polyamide 6, 210 poly(amidoamine), 37–40 poly(benzyl ether), 36–7


424

Index

polycarbonate, 288 polyetherimide, 288 polyethylene, 210 poly(ethyleneimine), 317 poly(hydrogenated–isoprene)–b–poly(styrenesulphonate), 30 polylactic acid, 210 polymer-assisted deposition (PAD) method, 69 ‘polymer brush,’ 87 polymer multilayer process, 60 ‘polymerisable complex (PC) route,’ 151 polymethyl glutarimide, 332 poly(methyl methacrylate), 288–9, 332 poly(N-dodecylacrylamide), 34 poly(N-isopropylacrylamide), 111 polypropylene, 210 poly(propylene imine), 40–1 polystyrene, 28 poly(styrene)-b-poly(ethylene oxide) (PS–b–PEO) copolymers, 28 poly(styrene–b–ferrocenyl silane), 30 polystyrene–block–poly(Nisopropylacryamide) (PS–b– PNIPAM) diblock copolymer, 29 polystyrene–b–poly(methyl methacrylate) (PS–b–PMMA) diblock copolymer, 29 poly(styrene)–b– poly(styrenesulphonate), 30 polystyrene–b–poly(vinylpyridine) (PS–b–PVP) copolymers, 30 polystyrene–graft–poly(ethylene oxide) (PS–g–PEO) copolymers, 29 powder coating, 16–17, 168 primary packaging, 204 projection lithography, 282–3 propyltrimethoxysilane, 210 quadruple splitting, 147 quantum dots, 43 quintuple layer, 343–4 radio-frequency identification, 218 Raman spectroscopy, 147

raster, 151 Rayleigh scattering, 146 reactive ion etching, 286–7 regioregular poly(3-hexylthiophene), 34 resistance-heated source evaporation, 66 reverse contact UV-NIL, 314 reversible addition–fragmentation chain transfer polymerisation, 102, 103–4 roll-to-roll coating, 11 Rutherford backscattering spectrometry, 149 ‘sacrificial initiator,’ 105 ‘sacrificial layer,’ 185 scanning acoustic microscopy, 152 scanning beam lithography, 282–3 scanning electron acoustic microscopy, 152 scanning electron microscopy, 151 scanning probe lithography, 307–9 Scheutjens–Fleer self-consistent mean-field theory, 111 secondary-ion mass spectroscopy, 149–50 secondary packaging, 204 self-assembled monolayers, 89 self-assembling molecules, 13 self-cleaning coatings, 14, 167–8 self-healing coatings, 261–6 electrochemical impedance spectra of AA2024substrates, 262 LDHs dual role in corrosion protection, 263 scanning vibrating electrochemical technique, 265 self-switching liquid crystal glazing technology, 193 semiconductor compounds, 341–4 semiconductors, 58 sensitized photoreaction, 400 sensor ultra-thin membranes, 330–51 gallium nitride, 344–51


Index graphene and 2D structures, 331–41 nanometer-thick membranes of semiconductor compounds, 341–4 silicone-modified polymers, 161 silver particles, 215–16 sliding test, 383 slot/die coating, 12 slot/extrusion coating, 12 ‘smart’ multifunctional coatings, 163 smart nanocoatings self-cleaning, 397–411 new and smart applications of TiO2-coated materials, 406–10 photocatalysis processes, 399–402 photocatalytic cleaning effect of TiO2-coated materials, 402–6 TiO2 photocatalysis, 397–9 smart packages, 222 Snell’s law, 144 soft lithography, 286 soft nanoimprint lithography, 297–301 conventional nanoimprint vs electrochemical nanoimprint vs surface charge lithography, 302 GaN-based mesostructures, 305 GaN nanowire merging triangular mesostructures, 306 SEM images from GaN layers subjected to PEC etching, 304 sol–gel coatings, 10, 250–2 sample structure, 252 silane deposition on metallic substrate, 250 solvent-assisted micromolding, 299–300 spectrally selective glass, 183–8 specular component, 144 spin coating, 10 spray coating, 7–10 cold spraying, 9 high-velocity oxygen fuel spraying, 8 plasma spraying, 8–9 thermal spraying, 8 vacuum plasma spraying, 9 warm spraying, 9–10 sputtering PVD, 67

425

Stefan–Boltzmann’s law, 184 step and flash imprint see UV nanoimprint lithography Stokes scattering, 146 stress-corrosion cracking, 241 structure-property relationships, 367–73 graded coatings, 370 historical trends in tribological coatings development, 368 nanocomposites, 370–2 TEM image of nc-TiN/a-C:H, 372 TEM image of WC–12Co coating, 372 smart adaptive coatings, 373 Au/MoS2/DLC/YSZ chameleon coating, 373 strengthening mechanisms, 367–9 hardness as function of grain size, 369 superlattice multilayer, 370 nanomultiayers of TiN/(Ti,Al)N, 371 sulphuric acid anodising, 254 ‘super’-hard coatings, 164–7 conventional coatings, 164–5 composite coating, 165 nitride/carbide.boride coatings, 164–5 nanocoatings, 165–6 nanocomposite coatings, 167 nitride nanocoatings, 166–7 oxide nanocoatings, 166 supersonic free-jet PVD, 69 surface corrosion, 236–7 surface-initiated polymerisation, 78–112 adsorption parameters, 82–5 free energy barrier and the strength of binding interaction, 82–3 grafting density and solvent nature, 83–5 polymer concentration in bulk, 83 nanocoating by ‘reverse grafting,’ 106–10 formation of polymer– nanoparticle complex, 109–10


426

Index

nucleation and phase formation, 106 schematic representation, 107 physisorption and chemisorption, equilibrium and irreversible adsorption, 79–87 adsorbed layers topologies, 86 comparative kinetic curves for amino mono-functionalised polystyrene chemisorption, 81 thermodynamics and kinetics, 79–82 topology of surface-bound layers, 86–7 polymer chemisorption processes, 88–106 controlled radical polymerisation, 102–6 conventional radical polymerisation, 92, 102 ‘grafting from’ approach, 92 ‘grafting through’ method, 106, 107 ‘grafting to’ approach, 89–91 properties and applications, 110–12 modified surfaces properties and applications, 111–12 surface-bound polymer layers physicochemical properties and comparison to bulk properties, 110–11 surface-bound polymer layers preparation, 87–110 ‘grafting from’ of unsaturated and cyclic monomers, 100–1 ‘grafting from’ systems based on controlled radical polymerisation, 95–9 ‘grafting from’ systems based on conventional radical polymerisation, 93–4 methods for preparation of surface linked polymer layers, 88 nitroxide-mediated surface grafting, 104 polymer physisorption techniques, 87–8

RAFT polymerisation, 103 surface ATRP propagation mechanism, 105 surface-bound polymer layers obtained by ‘grafting through’ approach, 107 surface-bound polymer layers obtained through ‘grafting to’ approach, 90–1 surface initiated conventional radical polymerisation, 102 surface polymerisation by ion-assisted deposition, 69 TE67, 387 tertiary packaging, 204 tethered density, 84 ‘tethered polymer layers’ see ‘polymer brush’ tetrabutylammonium hydroxide, 332 tetraisopropyl orthotitanate, 146 thermal barrier coatings, 138, 169 thermal evaporation PVD, 65 thermal expansion coefficient, 286 thermal nanoimprint lithography, 284–5 SEM images, 287 thermal-sprayed coatings, 162 thermal spraying, 8, 375, 377–8 thermally-activated glazings, 193 thermally grown oxide, 138 thermopolymerisation reaction, 290 thermotropic glazing, 193 ‘third body’ friction, 359 titanium, 174–5 titanium diboride, 269 titanium dioxide, 216 efficient energy-saving technology, 409–10 energy-saving system using solar light and stored rainwater, 410 new and smart applications, 406–10 NOx gases applied to construction materials, 407–8 pathways of light and activation of titanium dioxide, 408 schematic illustration, 408 photocatalysis, 397–9


Index photocatalytic cleaning effect, 402–6 soil treatment, 409 purification systems, 409 transmission electron microscopy, 151–2 transparent coatings, 168 tribology, 356 applications, 363–6 aerospace, 363 automotive, 363–4 biomaterials, 366 deep drilling tools, 365–6 mechanical components, 364 repair coatings, 366 turbines, 365 deposition methods, 373–9 coating deposition classification, 374 contact AFM images of WC–12Co, 378 growth pattern in PVD-deposited nc-TiN/a-Si3N4, 376 TEM image of cobalt-hardened gold electrodeposited nanostructured coating, 377 thermal spraying techniques, 378 engineering materials, 379–82 abrasion vs wear resistance of TiN/TaN multilayer coatings, 380 hardness of electrolytic Ni coating versus grain size and abrasion resistance, 379 mechanical and tribological properties of nanostructured coatings, 382 nanostructured HVOF WC–12Co– 2Al-coated vs chromium-plated crankshaft, 381 wear resistance comparison, 381 friction and wear applications, 367–82 structure-property relationships, 367–73 nanostructure coatings, 355–91 future trends, 391 tribological properties characterisation, 382–91

427

challenges in scale up establishment, 389–91 friction loops on etched dual phase steel, 386 friction loops showing topography and phase effects, 385 scale dependence, 387–9 schematic representation of friction mechanisms, 388 techniques for friction and wear characterisation, 382–7 wear track, 385 use of nanostructure coatings, 356–66 friction and wear mechanisms, 356–63 trioctylphosphineoxide, 43 Tupolev TU-134, 245 turbines, 365 ultra-hard materials, 15 ultra-thin films, 57–60 and nanocoatings analytical methods, 131–53 electrochemical methods, 132–40 spectroscopic, microscopic and acoustic techniques, 145–53 surface-sensitive analytical methods, 140–5 packaging applications, 203–22 anti-microbial packaging, 215–16 antistatic packaging applications, 220–1 future trends, 222 high barrier packaging, 209–15 nanomaterials in packaging, 208–9 nanosensors in packaging, 216–18 nanotechnology solutions for packaging waste problem, 219–20 packaging as a drug carrier and for drug delivery, 218–19 regulation and ethical issues in new packaging industry, 221–2 ultra-thin membranes sensor, 330–51 gallium nitride, 344–51


428

Index

graphene and 2D structures, 331–41 nanometer-thick membranes of semiconductor compounds, 341–4 ultrananocrystalline diamond , 167 UV nanoimprint lithography, 283–4, 285, 291–6 vacuum deposition methods nanocoatings and functionally graded multilayers, 57–75 vacuum monomer technique, 60 vacuum plasma spraying, 9 vanadates, 249 vapour phase epitaxy (VPE) process, 73 ‘velocity accommodation mode,’ 359 very low-temperature pyrolysis LPCVD, 73

Volta potential map, 153 vortical surface method, 47 warm spraying, 9–10 water-soluble paint, 18 wear, 360–3 mode types and sample case studies, 361 white graphite see hexagonal boron nitride phase Wien’s law, 184 Wilhelmy Plate, 27 X-ray lithography, 316 x-ray Mössbauer spectra, 147 x-ray powder diffraction method, 148 Zenner pinning, 369 zinc dialkyl dithiophosphate, 364 zirconium oxide coatings, 150


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