CERAMICS AND CERAMIC COMPOSITES: MATERIALOGRAPHIC PREPARATION
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Ceramics and Ceramic Composites: Materialographic Preparation Gerhard Elssner, Helmut Hoven, Gonde Kiessler, and Peter Wellner
Translated by Randall Wert
ELSEVIER AMSTERDAM - LAUSANNE - NEW YORK - OXFORD - SHANNON - SINGAPORE - TOKYO 1999
ELSEVIER SCIENCE Inc. 655 Avenue of the Americas New York, NY 10010, USA © 1999 Elsevier Science Inc. All rights reserved. This work is protected under copyright by Elsevier Science, and the following terms and conditions apply to its use: Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all forms of document deUvery. Special rates are available for educational institutions that wish to make photocopies for nonprofit educational classroom use. Permissions may be sought directly from Elsevier Science Rights & Permissions Department, PO Box 800, Oxford OX5 IDX, UK; phone: ( + 44) 1865 843830, fax: ( + 44) 1865 853333, e-mail:
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Table of Contents Abbreviations
ix
1 Introduction
1
2 Fundamentals of preparing polished sections
2
2.1 Selection and sampling
2
2.2 Sectioning
2
2.3 Mounting and impregnation
8
2.3.1 Hot (compression) mounting
9
2.3.2 Cold mounting
9
2.3.3 Recovery of embedded samples
12
2.3.4 Impregnation
12
2.4 Grinding, lapping, and polishing
17
2.4.1 Preparation equipment
23
2.4.2 Grinding
25
2.4.3 Materialographic lapping
27
2.4.4 Polishing
28
3 Revealing the microstructure 3.1 Microstructural imaging in the optical microscope (OM)
33 33
3.1.1 Imaging methods of optical microscopy
33
3.1.2 Electronic image processing and contrast enhancement
34
3.2 Tips on contrast enhancement and etching
39
3.2.1 ReUef poHshing
39
3.2.2 Contrast enhancement with interference layers
39
3.2.3 Chemical dip etching
40
3.2.4 Thermal etching
43
VI
TABLE OF CONTENTS 3.2.5 Plasma etching
43
3.2.6 Ion etching
45
3.2.7 Electrolytic etching
46
3.3 Microstructural imaging in the scanning electron microscope
48
3.4 Ultrasonic scanning microscopy
54
4 Material-specific preparation of polished sections
59
4.1 Properties of ceramic materials
61
4.2 Pores in ceramic materials
67
4.3 Examples of preparing ceramic materials
74
a-Al203 (corundum) in single-crystal form
74
Aluminum oxide ceramic: 99.5% a-Al203
74
Aluminum oxide ceramic: 99.7% a-Al203
76
Aluminum oxide ceramic: a-Al203 with Si02 and MgO additives
78
Aluminum oxide ceramic: a-Al203 with glass phase
79
Aluminum nitride AIN, sintered
80
Boron carbide B4C, high density
81
Boron nitride BN, sintered
83
Calcium carbonate CaC03, white marble
84
Cerium oxide Ce02
85
Chromite
86
Refractory ceramics
87
Glass
88
Graphite
89
Coal
90
Lanthanum strontium manganese oxide
91
Silicon carbide SiC, pressureless-sintered
92
SiUcon carbide varistor ceramic
95
Silicon nitride Si3N4, hot-pressed
96
Si-SiC-C ceramic
97
VII
TABLE OF CONTENTS Zinc oxide ZnO
97
Zirconium oxide Zr02
100
Zirconium oxide with aluminum oxide inclusions
103
Zirconium oxide, partially stabilized with MgO and Si02
105
4.4 Preparation of ceramic composites
105
4.5 Examples of preparing ceramic composites
107
Active-brazed joint between graphite and a TZM molybdenum alloy Aluminum alloy reinforced with boron
107 fibers
110
Flame-sprayed aluminum oxide coating on steel
Ill
Glass fiber reinforced plastic
113
Solder glass/stainless steel joint
114
Ceramic/cermet composite
115
Carbon fiber reinforced carbons
116
Carbon fiber reinforced plastic
117
Spherical nuclear fuel
120
Copper coating on an aluminum nitride ceramic
122
Plasma-sprayed aluminum oxide coating on steel
124
Plasma-sprayed chromium oxide coating with Ni-20%Cr interlayer on steel
125
Plasma-sprayed zirconium oxide coating on a nickel super alloy
126
SiC/C fibers in an aluminum alloy
127
Titanium carbide coating on graphite
129
Titanium nitride coating on an inconel alloy
130
WC-Co carbide metal
131
5 Preparing polished sections for examination 5.1 Polished sections
134 135
5.1.1 Oblique sections
135
5.1.2 Controlled removal
136
VIII
TABLE OF CONTENTS
5.2 Thin sections
140
5.2.1 Preparation of thin sections
140
5.2.2 Microscopic examination of thin sections
143
6 Analysis of hardness testing indentations
144
6.1 Hardness testing of ceramic materials
144
6.2 Determining fracture toughness by indentation hardness testing
151
6.3 Application of hardness testing to composite layers and surface layers
154
6.3.1 Indentations in the layer surface
156
6.3.2 Measuring the layer hardness in the section
157
7 Literature
159
Appendix
174
Abbreviations BF
bright field
D
diamond
DF
dark field
Die
differential interference contrast
Fe/02
reactive sputtering with iron cathode and oxygen gas
OM
optical micrograph
POL
polarized fight
SEM
scanning electron micrograph
TEM
transmission electron micrograph
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Chapter 1
Introduction Ceramics and ceramic composites are now used in almost all areas of technology. Consequently, it is becoming increasingly important to study polished sections of these materials in industrial laboratories by means of optical microscopes and scanning electron microscopes. In some cases, the experience required for the preparation of pohshed sections is lacking, or preparation steps typically used with metals are unsuccessfully apphed to these materials. Chapter 2 therefore presents fundamental information on preparing pohshed sections of ceramics and ceramic composites. This information encompasses the main steps of samphng, sectioning, mounting and impregnation, and mechanical grinding, lapping and poUshing. In past experience, the use of diamond products has been preferred for the sectioning and abrasive processing of ceramics and ceramic composites. The use of automated grinding, lapping, and poUshing machines makes it possible to obtain a uniformly high degree of section quaUty with reproducible results. In cases of lowvolume sample throughput, manual preparation of polished sections can also produce quite useful results. Chapter 3 discusses microstructural imaging in the optical microscope (OM) and the use of the scanning electron microscope (SEM). After a section of a ceramic or a ceramic composite has undergone final poUshing, it must be subjected to etching or contrast enhancement in order to reveal its microstructure. This practice is similar to the study of metal specimens. Chapter 4 presents material-specific preparation procedures for polished sections. These procedures take into account the properties of the ceramic or composite being examined and the purpose of the examination. These procedures have proven to be very suitable for the respective materials and are effective for reveaUng the pores. The examples presented here for ceramics and ceramic composites provide polished sections of good to exceUent quaUty for routine examination under the optical microscope. They include tips for etching and contrast enhancement, as weU as microstructural images. Chapter 5 discusses the preparation of polished sections for purposes of examination and contains information on producing obUque sections and controlled removal of material. It also addresses the production of thin sections. As a complement to the examination of the microstructure, chapter 6 provides insight into the evaluation of hardness testing indentations. Chapter 7 concludes this work with an overview of the technical Uterature.
Chapter 2
Fundamentals of preparing polished sections 2.1 Selection and sampling After a region of a structural member or test material has been selected for examination, the samples are taken. Here it is important to consider microstructural differences between the longitudinal and transverse directions, as well as special conditions arising from the manufacture of ceramic workpieces. Sintered products may exhibit microstructural differences between the core and surface regions, for example. Examples of microstructural inhomogeneities may include the "firing skin" found on ceramics or the variations in porosity between different regions of a structural member, as caused by uneven compression. The selection of the specimen size is critically important in preparing poUshed sections. Relatively small specimens with a section surface area of 100 mm^ are preferable. These are considerably easier and quicker to prepare than very small or very large specimens.
2.2 Sectioning The sectioning of ceramic materials is generally performed with the help of powerdriven diamond cut-off wheels. Water, emulsions, aqueous solutions or low-viscosity mineral oils may be used as coolants and lubricants. Alcohol or mineral oils are used with water-sensitive materials, such as P-AI2O3, calcium oxide, magnesium oxide, sintered magnesite, or cement cUnker. It is not advisable to obtain samples by dry sectioning or by chopping them off. Compared to dry sectioning, the use of cutting fluid in wet sectioning not only reduces the sectioning time, but also considerably improves surface quality and extends the service life of the wheels. Wet sectioning usually involves the use of machines and diamond cut-off wheels operating by the plunge cutting method. The abrasive layer is found along the circumference of the disk (see Fig. 1). The feed motion proceeds in a radial direction. A sufficient quantity of cutting fluid must be supplied to the proper location during
2.2 SECTIONING
p>(z%/^specimen
t
feed
Figure 1. Plunge-cutting method for sectioning ceramic specimens.
sectioning, because this fluid helps to eject from the kerf that material which has been removed from the cut surface and the cut-off" wheel. This fluid also provides the necessary lubrication and cooHng of the wheel and sample material. Conventional sectioning machines operate either in the low-speed range at rotational speeds up to 1000 rpm or in the high-speed range at 1000-7600 rpm. Low-speed sectioning machines are available as table models. These allow sectioning of samples up to approximately 30 mm in diameter. When suitable wheels and cuttingfluidsare used, it takes about 5-10 min to section a round specimen of an AI2O3 ceramic with a diameter of 10 mm. Low-speed sectioning is especially well suited for small structural members - even those with comphcated shapes, for which special clamping devices are suppUed by equipment manufacturers. It should also be noted that the use of thin cut-off* wheels in low-speed sectioning greatly reduces the cutting loss, and that the quaUty of the sample surface is relatively high after sectioning. High-speed sectioning machines of robust design are preferred for sectioning large pieces of material. The sectioning time is shorter than in low-speed sectioning. However, the wider kerf of the cut-off* wheels results in a greater loss of material. There is also a greater risk that grains will break loose from the cut surface of the ceramic, creating flaws known as "pull-outs". The removal of these flaws requires a longer grinding time in the subsequent stages of preparing the polished section. The diamond cut-off" wheel consists of a supporting disk of steel or bronze. On its circumference is the abrasive layer, consisting of diamond particles and the binder. Cut-off* wheels can be described by characteristics such as the type of bond, diamond concentration, and diamond grain size (see Appendix A). Wheels with continuous or segmented rims are available. Wheels with continuous rims are used for preparing poUshed sections. They produce smooth cut surfaces with a minimum of material damage. Wheels with segmented rims are most commonly used for sectioning large pieces of material. The diamond grain size and type of bond must be matched to one another so that blunted grains are pulled out during sectioning, while new, sharp-edged grains of the
4 CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED SECTIONS abrasive layer are exposed. The term "binder hardness" indicates the degree to which the bond resists the detachment of diamond grains. Cut-off wheels with a soft bond will ordinarily wear more quickly than wheels with a hard bond. The following rule of thumb is useful in practical appHcation: A harder bond is preferred for softer materials, while wheels with a softer bond are used with harder materials. Manufacturers offer diamond cut-off wheels with metal bonding (bronze or nickel) and resin bonding. Cut-off wheels with resin bonds have proven effective for hard ceramics, e.g., hot-pressed sihcon nitride. Aside from their relatively high rate of wear, they are sensitive to tilting of the sample during sectioning. Cut-off wheels with metal bonding are predominantly used with oxide ceramics. Diamond grain sizes are estabhshed by a standard issued by the (FEPA) Federation Europeenne des Fabricants des Produits Abrasifs (see Appendix A). The grain size and grain size distribution of the diamond particles affect the cutting performance and service life of the wheel. As a rule, a finer diamond grain size will also produce better surface quahty in sectioning. In addition to the specifications pertaining to the abrasive layer, the diameter and thickness of the wheels are also critically important to the sectioning operation. The rotational speed, which affects cutting performance, is determined by the wheel diameter and the arbor speed of the machine. For purposes of reUabiUty and directional stabiUty, one third of the cut-off wheel diameter should be enclosed by theflange.The width of the kerf and the cutting loss are determined by the thickness of the cut-off wheel. Wheels with a thickness of approximately 0.8-2 mm are used to ensure the stabiUty of machines operating at high speeds. Thinner wheels with a thickness of about 0.2-0.6 mm can be used for low-speed machines. An ordinary dressing stone can be used to help restore the cutting effectiveness of used, metal-bonded wheels. Lubricants are used in sectioning to protect against corrosion and to improve cutting quaUty. When sectioning ceramic coatings, it is extremely important to ensure that both the rotation and feed motion of the wheel proceed from the coating into the base material. When the wheel enters a stationary, round specimen which has not been embedded - for example, a plasma-sprayed WC-Co coating on a steel substrate - the coating can be severely damaged (Fig. 2(c)). When the wheel exits the sample, the coating can be damaged even more severely (Fig. 2(b)). Good results are exhibited by the specimen region indicated in Fig. 2(a), while satisfactory results are exhibited by the region indicated in Fig. 2(d). Sensitive samples should be embedded before sectioning, in order to prevent pull-outs in the surface region of the workpiece. Fig. 3(a) shows another non-embedded sample of a plasma-sprayed WC-Co coating on a steel substrate, in which pull-outs are visible after sectioning. Theseflawsare eliminated by embedding the sample prior to sectioning (Fig. 3(a)). When selecting the sectioning blades, it is important to consider the specifications pertaining to the abrasive layer and the dimensions of the cut-off wheel. The type of
2.3
SECTIONING
Figure 2. Sectioning of a fixed, non-embedded round specimen. The positions of the cut surfaces are labeled (a)-(d). The scanning electron micrographs indicate the sectioning quahty of the round steel specimen, which is plasma-sprayed with tungsten carbide-cobalt.
6
CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED
SECTIONS
Figure 3. Scanning electron micrographs of the cut surfaces of a plasma-sprayed WC-Co coating on steel, (a) Sample not embedded prior to sectioning, (b) Sample embedded prior to sectioning. ceramic material being sectioned exerts a strong influence on the surface quality of the sectioned specimen, as well as the cutting performance and service Hfe of the wheel. It is advisable to conduct preUminary tests in which the quality of the cut surface is studied, in order to establish the optimum conditions for the sectioning operation. Fig. 4(a) and (b) shows optical micrographs of the cut surfaces of a sintered SiC ceramic, as produced with diamond blades on low-speed and high-speed machines, respectively. Fig. 5 contains scanning electron micrographs of surfaces of ceramics consisting of silicon carbide, silicon nitride, and aluminum oxide. These surfaces were produced by sectioning with low-speed and high-speed machines, respectively. Lowspeed sectioning at 200 rpm with a wheel of thickness 0.35 mm generally produces a better surface quality than high-speed sectioning at 3400 rpm with a diamond wheel of thickness 1.5 mm. Although both types of cut surfaces display a large number of pull-outs, low-speed sectioning produces less subsurface damage. This is evidenced by the larger unbroken regions of material with a relatively level surface.
Figure 4. Optical micrographs of the cut surfaces of a sintered SiC ceramic, (a) Using a lowspeed machine (200 rpm) and a diamond wheel of thickness 0.35 mm. (b) Using a high-speed machine (3400 rpm) and a diamond wheel of thickness 1.5 nmi.
2.3 SECTIONING
SiC
Si3N4
A1203
low-speed (200 rpm)
high-speed (3400 rpm)
Figure 5. Scanning electron micrographs of ceramic surfaces consisting of SiC, Si3N4, and AI2O3, produced by sectioning with a diamond wheel. Comparison of results obtained with low-speed and high-speed sectioning machines.
It is absolutely essential to heed the following rule: After sectioning, the sample must be cleaned in an ultrasonic bath and then dried with a hot-air blower or in a drying cabinet.
8 CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED SECTIONS In porous ceramics, cutting fluid and swarf penetrate the pores and pull-outs and are practically impossible to remove without cleaning and drying. Specimens intended for examination by scanning electronic microscope must be subjected to especially intense cleaning.
2.3 Mounting and impregnation Mounting in plastic - also known as embedding or encapsulation - has proven to be an advantageous method of preparing a ceramic sample for grinding and poUshing, because of the easier manipulation and improved edge retention it makes possible. Specimens are rarely left unmounted or merely glued to a metal plate or clamped into a specimen holder for further processing. Fig. 6 shows a Tribaloy coating on a steel substrate, in which cracks have formed after the specimen was clamped into a steel holder. Both hot-setting and cold-setting plastics can be used for mounting. These plastics consist of small molecules - so-called "monomers" - which combine to form tangled or cross-Unked macromolecules as they cure. One important distinguishing characteristic is their respective plastic behavior. There are thermoplastics (which become plastic when heated), thermosets (which harden when initially subjected to heat and pressure), and elastomers. Thermoplastics are plastics which exhibit plastic flow when subjected to heat and pressure or the action of a catalyst (hardener) and then harden upon cooling. These consist of individual molecule chains. The state changes from sohd to plastic and back to solid can be repeated, i.e., they are reversible. Thermoplastics can be shaped without cutting. Examples of thermoplastics include polymethyl methacrylate and PVC. Thermosets undergo changes in molecular structure when subjected to heat and pressure or the action of catalysts. This results in spatially cross-linked molecule
IOOM^I
Figure 6. Cracking after a Tribaloy coating on a steel substrate has been clamped into a steel holder.
2,3 MOUNTING AND IMPREGNATION
9
chains. In other words, the molecules enter into new chemical bonds, that is, they polymerize. This state is irreversible. Thermosets can only be shaped by machining. Examples of thermosets include polyester resins, phenolic resins, Bakelite, diallyl phthalate, and epoxy resins. The molecules contained in elastomers - e.g., rubber are only partially cross-linked. The degree of cross-linking determines the hardness and elasticity of an elastomer. The selection of a suitable mounting compound is determined by the properties of the plastic, such as viscosity, shrinkage, adhesion to the sample, abrasion resistance, chemical resistance, and thermal resistance, as exhibited in the processes used to prepare the section and render the microstructure more visible. Another determining property is hardness, which serves as an indicator of possible edge rounding. For the most part, the requirements for a mounting medium are fulfilled by the plastics available on the market. Electrically conductive mounting media containing iron, copper, or graphite are used for the examination of specimens in scanning electron microscopes. Mounting processes are ordinarily categorized as hot mounting (also known as compression mounting) or cold mounting. In hot mounting, curing is accomplished by means of pressure and temperature. In cold mounting, a hardener serves as a catalyst. Even the cold mounting process can involve heating up to approximately 100°C. Tables 1 and 2 provide an overview of the properties and possible apphcations of hot and cold mounting media.
2.3.1 Hot (compression) mounting Thermosets with or withoutfillersare suitable for hot mounting in a mounting press. Thermoplastics are also suitable for this purpose. Thermoplastics do not need to be heated under pressure, but they do need to be subjected to pressure while cooling. Thermosets, which are heated under pressure, can be removed from the press immediately after polymerization has occurred. For a mold diameter of 25 mm, the mounting time lies within the approximate range of 12-15 min. Thermoplastics are not as hard as thermosets. Unlike thermosets, thermoplastics are transparent. The temperatures required for hot mounting (15O-190°C) do not alter the microstructure of ceramics. However, cracks in the specimen can result from the molding pressure and the rapid temperature drop in cooUng. In such cases, it is best to resort to cold mounting. Fig. 7 shows a chromium oxide coating on steel, in which cracks developed after hot mounting in epoxy resin at a load of 20-30 kN and a diameter of 25 mm, or a pressure of 0.4-0.6 kbar.
2.3.2 Cold mounting The basic substances used in cold mounting media include epoxy resins and polyester resins (thermosets), as well as acrylates (thermoplastics) that cure exothermically
10 CHAPTER 2
FUNDAMENTALS OF PREPARING POLISHED SECTIONS
Table 1. Hot mounting media Plastic
Hard- Rate of Adhesion ness abrasion*' (HV) (^m/min)
Thennosets Phenolic resins^ (Bakelite)
46
560
Epoxy resins*
71
130
Diallyl phthalate*
60
440
Thermoplastics Acrylates (transparent)
20
520
Slight to good; dependent on pressure
Resistant to
Dilute acids and lyes, organic solvents, oils, benzene, alcohol Very good Dilute acids and lyes, organic solvents, oils, benzene, alcohol Very good, Dilute acids no and lyes, shrinkage organic solvents, oils benzene, alcohol Aqueous Moderate, solutions dependent on pressure and lyes. greases, and alcohol up to 30%
Not resistant to
Thermal resistance
Strong acids and lyes
120-150°C flameresistant
Concentrated acids and lyes, ketones, esters, acetone
13a-180°C flameresistant continues to bum
Concentrated acids and lyes, organic solvents, hydrochlorocarbons Concentrated acids, solvents. hydrochlorocarbons. gasoUne, benzene, nitro[?], and plasticizers
120-160°C briefly up to 200°C
Max 65-95°C bums in a sizzling manner. without residue
* Plastic with hardening filler. ^ Determination of rate of abrasion: diameter of mounting medium 25 mm, new SiC abrasive paper of grain P600; pressure 35 kbar, rotational speed of wheel 150 rpm, specimen holder counterrotating at 66 rpm. These data were obtained from: Werkstoff*-Fuhrer Kunststoffe, Carl Hanser Verlag and B. Bousfield, Surface Preparation and Microscopy of Materials, Wiley, 1992.
when mixed with hardeners and catalysts. The ratio of curing time to heat of reaction is constant, i.e., the curing time is short at a high heat of reaction. The heat of reaction is influenced by the mixing ratio of the substances, the method of stirring, the quantity of mounting compound, and the external temperature (as affected by the drying cabinet, hot-air blower, or water cooling system). High reaction temperatures pro-
2.3 MOUNTING AND
II
IMPREGNATION
Table 2. Cold mounting media Plastic
Hardness Rate of Adhesion (HV) Abrasion^ (^im/min)
Resistant to
Not resistant to
Thermal resistance
Epoxy resins
25
Concentrated acids and lyes. ketones, esters. acetone, boiling water Concentrated acids and lyes, hydrochlorocarbons, organic solvents, benzene, boiling water Concentrated acids. alcohol. hydrochlorocarbons, nitro thinner[?]
80°C
870
Very good, no shrinkage, suitable for impregnation
Dilute acids and lyes, hydrochlorocarbons, oils, benzene, toluene
Polyester 24 resins
1100
Moderate
Dilute acids and lyes. oils. benzene, alcohol
Acrylates 23
1040
Shght
Aqueous acids and lyes. alcohol up to 30%
80-160°C dependent on structure of polyester
max 65-95°C
^Determination of rate of abrasion: diameter of mounting medium 25 mm, new SiC abrasive paper of grain P600, pressure 35 kbar, rotational speed of wheel 150 rpm, specimen holder counterrotating at 66 rpm. These data were obtained from: Werkstoff-Ftihrer Kunststoffe, Carl Hanser Verlag and B. Bousfield, Surface Preparation and Microscopy of Materials, Wiley, 1992.
mote shrinkage, cracking, and brittleness. Cold mounting media can be used to prepare poUshed ceramic sections without problems. The addition of fillers - e.g., in the form of spongelike AI2O3 particles - increases the hardness and abrasion resistance of epoxy resins and acrylic resins and reduces both shrinkage and edge rounding in the ceramic specimen. However, detachment of filler material can lead to troublesome scratches on the section surface. Skin contact should be avoided during cold mounting by the use of protective gloves and safety glasses, and by working under a fume hood. This appUes especially to work involving epoxy resins.
12 CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED SECTIONS
100 Mm Figure 7. Development of cracks in a chromium oxide coating on steel after hot mounting in epoxy resin, BF.
2.3.3 Recovery of embedded samples If the specimen is going be examined in a scanning electron microscope, or even etched thermally or in molten salts, the mounting medium must first be removed. It is generally possible to do this by sawing or breaking along the surfaces of the embedded specimen. Any residual resin clinging to the specimen can be decomposed in a laboratory furnace at about 500-600°C in an atmosphere of air or in boiling A^, A^dimethylformamide (at approximately 150°C). Heat can be used to extract specimens from thermoplastic mounting media. Please note: Toxic gases are released when residual resin is burned out or decomposed. It is absolutely essential that such work be performed under a fume hood. The resin must be carefully and completely removed before the impregnated specimen is subjected to thermal etching, because the infiltrate will expand as it decomposes under these temperatures, possibly causing the specimen to break apart.
2.3.4 Impregnation When imaging the microstructure of a ceramic or ceramic composite material, it is desirable to obtain the most accurate possible image of the open and closed porosity in the material, because the overall porosity and the size and distribution of the pores provide indications of the probable material properties. (The term "open porosity" indicates that there are open channels between pores.) During grinding and polishing of a specimen, however, exposed pores represent possible starting points for surface damage caused by pull-outs. These exposed pores can also become clogged with swarf. This creates a risk that the finished poHshed section will not accurately rep-
2,3 MOUNTING
AND
13
IMPREGNATION
resent the porosity of the ceramic material under investigation. Fig. 8 shows these effects in the preparation of impregnated and non-impregnated poUshed sections of a refractory ceramic. In order to make the poUshed section considerably easier to prepare, and to ensure a credible representation of its porosity, the sectioned sample should be impregnated with a synthetic resin of low viscosity. This should ordinarily occur after sectioning, but in the case of very brittle and highly porous samples, it should be performed even before sectioning. Epoxy resins with a low viscosity of approximately 250 cP are especially well suited for use as impregnating media. Aided by capillary forces, these
Non-impregnated specimen showing pull-puts and pores Schematic representation
Microstmctural image
Impregnated with epoxy resin Schematic representation
Microstmctural image. Epoxy resin is gray.
Figure 8. Effect of impregnation in the preparation of polished sections. Left: schematic representation. Right: microstructure of a refractory ceramic (OM, BF).
14 CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED SECTIONS resins will completely fill the open pores to a depth that is sufficient for purposes of sample preparation. Closed pores, i.e., pores which are mostly closed-off from one another, are only impregnated by the resin in the surface region of the specimen. In these cases, it is advisable to impregnate the specimen once again after planar grinding has been performed. The presence of thefillerin the pores greatly reduces the risk that material will break out of the pore edges. It also protects the pores from clogging with swarf and prevents contamination of the specimen by polish residues or cleaning fluids in the pores. For purposes of impregnation with epoxy resin, porous ceramic materials can divided into three groups: • Specimens with porosity between 0 and approximately 5% do not require impregnation. • Specimens with porosity between approximately 5% and 15% can be prepared more effectively by impregnating them with epoxy resin. It is often impossible to thoroughly impregnate the specimen, however, due to the presence of closed pores, the lack of pore channels, or the narrowness of these channels. In such cases, it may be helpful to perform a second impregnation close to the surface after embedding and planar grinding have been performed. To this end, the specimens being prepared by a semiautomatic process are left in the specimen holder, cleaned thoroughly, and dried. They are subsequently placed in a vacuum desiccator, which is then evacuated. A few drops of epoxy resin are appHed to the specimen surfaces. This is followed by a repeated cycle of aeration and evacuation. • Specimens with porosity greater than 15% can usually be impregnated quite effectively. However, there are a few highly porous materials with suchfinepore channels that impregnation is impossible. Vacuum impregnation represents a simple and effective method for impregnating a sample with epoxy resin. The apparatus used for this process consists of a vacuum vessel, a backing pump (also known as a "fore pump" or "rough vacuum pump") or a water jet pump, and a dehydration vessel supphed with a drying agent. The samples are placed in embedding molds or other small containers and then transferred to the vacuum vessel. Together with the resin, which is placed in a separate container, the samples are evacuated to a pressure of approximately 1000-1500 Pa (10-15 mbar). The vacuum is maintained as impregnation is performed by pouring the resin over the surface of the sample. It is important for the samples to be dry before they are impregnated. It may be necessary to dry them in a drying cabinet for approximately 30-60 min (depending on their size) at approximately 110°C. It is important to ensure that the epoxy resin is free of bubbles after being mixed. During the impregnation process itself it is also important to ensure that the negative pressure does not become excessive. Excessive negative pressure could allow the development of gas bubbles, which would disrupt the impregnation process and impair its effectiveness. It is therefore advisable to evacuate the samples for a few minutes before impregnation.
2.3 MOUNTING AND IMPREGNATION
15
The resin is then poured in, and the vacuum is maintained for about 10-30 min. The vacuum chamber is then aerated; in this operation, the atmospheric pressure forces the liquid resin into any open pores that remain. The thoroughness of the impregnation is strongly affected by the sample thickness, the pore size, and the number of channels connecting the pores. The samples are subsequently removed from the vacuum chamber and the resin cures upon exposure to air. Each type of epoxy resin used for infiltration and mounting will cure at its own rate. This curing time (or "pot life") may vary between 1 and 12 h. It can be shortened by heating the sample to a temperature of 30-60°C. It is important to note that while the increase in temperature will shorten the curing time, it will also promote the decomposition of the hardener and thus the formation of gas within the impregnating medium. The equipment shown in Fig. 9 is an example of a suitable vacuum impregnation apparatus. A motor-driven turntable inside the vacuum vessel allows multiple samples to be impregnated simultaneously. A simple pouring mechanism makes it possible to pour the premixed epoxy resin components (cresin and hardener) over the samples or into the embedding molds. Fig. 10 is an image of a porous, fine-grained graphite specimen which has been impregnated with epoxy resin. The cavities that have been filled with resin appear as dark gray areas. Some of the epoxy resins on the market are mixed, or can be mixed, with special dyes that cause the mounting materials to glow as the specimen is examined in an optical microscope under a bright field or polarized Ught. The same effect can be achieved by means of fluorescent substances and a filter set intended for fluorescence applications. The illumination of these components has the advantage of making impregnated features of the microstructure (e.g., pores and cracks) immediately visible. It also causes them to stand out clearly from other microstructural features
Figure 9. Vacuum impregnation apparatus.
16 CHAPTER 2
FUNDAMENTALS
OF PREPARING POLISHED
SECTIONS
Figure 10. Porous, fine-grained graphite specimen impregnated with epoxy resin, OM, BF.
Figure 11. Ceramic specimen embedded in fluorescent epoxy resin. The regions of the poUshed section that are filled with the fluorescent mounting medium appear to glow brightly.
17
2.4 GRINDING, LAPPING, AND POLISHING
and any undesirable pull-outs or cracks introduced in the preparation process (see Fig. 11.)
2.4
Grinding, lapping, and polishing
Material is damaged in the processing of ceramic surfaces, just as in the preparation of metal samples. In contrast to metals, ceramic materials are particularly likely to exhibit pull-outs, fragmentation of grains, and cracking. This comparison is shown in Fig. 12. Deformation twins and dislocations can also develop during grinding. These features are usually arranged along scratches caused by grinding and poHshing. Fig. 13 is a transmission electron micrograph of the microstructure of a scratch on the surface of an AI2O3 ceramic that was poUshed with diamond (grain size: 6 |im). This shows the formation of dislocation arrays, in which parallel dislocation hues run roughly perpendicular to the direction of the scratch. In preparing polished sections, damages that distort the image of the microstructure should be eliminated as thoroughly as possible while maintaining a plane section surface. This is accomplished by reducing these damages in a stepwise manner in the individual processing steps. This can be monitored by means of an optical microscope. Fig. 4 shows the grooves that result from sectioning a sample of a sintered SiC ceramic. As shown in Fig. 14(a), these grooves are removed by semiautomatic grinding with a diamond disk of grain size 20 jim. Fig. 14(b) shows the appearance of the surface after subsequent grinding with a 10 ^m diamond film, and Fig. 14(c) shows the results of pohshing with a 6 jim diamond suspension on a synthetic fiber cloth. Fig. 14(d) and 14(e) shows the satisfactory quahty of the specimen after polishing with diamond of grain size 3 ^m on a synthetic fiber cloth, and then with diamond of grain size 1 jiun on a nylon cloth, respectively.
—
roughened surface induced detects disturbed layer — with — internal stresses
tnie _ microstaicture
METAL plastic deformation
abrasive preparation materials parameter
CERAMIC cracldng pull-outs porosity
Figure 12. Schematic representation of surface damage in the preparation of sohd section.
18 CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED SECTIONS
Figure 13. Transmission electron micrograph of a surface scratch resulting from the polishing of an aluminum oxide ceramic with diamond paste (grain size: 6 jim).
Fig. 15 shows scanning electron micrographs of the surface quality of a semiautomatically prepared section of an AI2O3 ceramic (99.7% AI2O3 by weight) after sectioning, grinding, and polishing. Fig. 15(a) shows the surface created by sectioning with a diamond wheel of grain size 64 )im on a high-speed sectioning machine. This surface is fragmented by pull-outs. The surface structure remains essentially unchanged after subsequent semiautomatic planar grinding (Fig. 15(b)). This was followed by grinding with a disk of grain size 20 |xm, which smoothed the coarser surface hard synthetic fiber cloth, using an alcohol-based lubricant, large portions of the surface are already smoothed (Fig. 15(d)). The depth of the pull-outs has been reduced considerably. After polishing on a hard synthetic fiber cloth with a diamond grain size of 3 jim, the pores are still deceptively enlarged by pull-outs (Fig. 15(e)) regions, giving them a scaly appearance (Fig. 15(c)). At this point, however, these smoothed regions still alternate with areas affected by pull-outs. After poUshing with diamond of grain size 6 |im on a hard synthetic fiber cloth, using an alcohol-based or water-based lubricant, large portions of the surface are already smoothed (Fig. 15(d)). The depth of the pull-outs has been reduced considerably. After poHshing on a hard synthetic fiber cloth with a diamond grain size of 3 ^im, the pores are still deceptively enlarged by pull-outs (Fig. 15(e)). The true pore structure is not exposed until final polishing has been performed with a diamond of grain size 1 ^im on a cotton cloth (Fig. 15(f)). Fine scratches have also been eliminated at this point. The small, dark areas on the scanning electron micrograph are caused by subsurface pores. Fig. 15(g) shows the structure after etching the poUshed section in boiUng phosphoric acid. This exposes additional pores. Before taking up the description of the individual preparation steps farther below, some information on material removal, abrasives, and preparation parameters may be helpful to the reader's understanding of the processes occurring here. The various modes of utihzing the abrasive and the underlying material serve as the basis for distinguishing between grinding, lapping, materialographic lapping, and poUshing (Fig. 16).
2.4 GRINDING, LAPPING, AND POLISHING
19
Figure 14. Optical micrographs of a sintered SiC ceramic after grinding with a diamond disk of grain size 20 pm (a), grinding with a resin-bonded diamond fihn of grain size 10 ^mi (b), polishing with diamond of grain size 6 ^ml on a syntheticfibercloth (c), polishing with diamond of grain size 3 jim (d), and poUshing with diamond of grain size 1 ^mi on a nylon cloth (e). •
Grinding involves working with a firmly bonded abrasive for the purpose of removing material rapidly and flattening the sample surface. The force Ri acting on the sample lies at an angle a to the horizontal force F. Although the grinding process reduces the damage caused by sectioning the sample, it also introduces damage to the surface regions. This is especially true of planar grinding. The sample displays a uniform pattern of scratches after grinding.
•
Lapping involves working with tumbling particles on a lapping disk. It removes less material than grinding. The force R2 acts upon the sample at a greater angle e, which is suitable for brittle materials. The sample surface plainly exhibits a dull appearance.
20 CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED SECTIONS
21
2.4 GRINDING, LAPPING, AND POLISHING
Figure 15. Scanning electron micrographs of an AI2O3 ceramic: (a) after sectioning, (b) and (c) after grinding with diamond disks of grain sizes 64 and 20 ^m, respectively, (d)-(0 after poUshing with diamond of grain sizes 6, 3, and 1 ^m, respectively, (g) after etching with boiUng phosphoric acid. • Composite disk are used for materialographic lapping. This operation represents a combination of grinding and lapping. The variety of component materials in the composite disk yields the following possible modes of material removal: Abrasive particles tumble across hard regions of the disk surface, as in normal lapping. Abrasive particles become embedded in softer regions of the disk surface, as in grinding disks. Abrasive particles are stopped at the interfaces between different surface layers of the disk and function as they would in a grinding operation. Abrasive particles are held firmly on the disk surface as a result of the controlled porosity of the surface layer. One mode of material removal may predominate, or there may be a combination of all of the above-mentioned possibiUties, as is often the case in actual practice. This is determined by the structure of the composite disk and the types of surface layers present. • In a poHshing operation, the poUshing grains move up and down within the short-napped or long-napped surface layer of the cloth. Polishing is performed
ccmposite tapping disk Materialographic Lapfiing
Polishing
Figure 16. Schematic representation of grinding, lapping, materialographic lapping, and polishing (from B. Bousfield).
22 CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED SECTIONS with various polishes on different cloths. The surface of a ceramic sample will have a dull finish with Uttle or no scratching. PoUshes with fine grain sizes remove only a small amount of material. The effect of a poUsh is also influenced by its concentration and the lubricants being used. Aside from diamond, cubic boron nitride B4N (CBN), boron carbide B4C, siUcon carbide SiC, aluminum oxide AI2O3, and alimiinum oxide/zirconium oxide mixtures are used as abrasives for ceramics and composite materials. • Diamond is the abrasive used most often, due to its great hardness and removal performance. Both monocrystaUine (single-crystal) and polycrystalUne diamonds are used. The polycrystalUne form provides the best removal performance, due to its great number of cutting edges. Both resin-bonded diamond films and resin-bonded and metal-bonded diamond grinding disks contain bonded grains. Loose grains are contained in diamond suspensions and sprays based on water, oil, and alcohol, as well as diamond pastes. More detailed information on grain size, diamond concentration, grain shape, and the use of natural or synthetic diamonds can be found in manufacturers' specifications. The grain size distribution should be as narrow as possible, because the maximum scratch depth is determined by the coarsest diamond particles present. • Boron nitride is used in metal-bonded boron nitride grinding disks. It is also used in the form of loose grains. • Boron carbide is used in slurries of loose grains. • Silica (silicon dioxide) in the form of a colloidal, basic suspension is an abrasive that is used effectively for final poUshing in a broad range of applications. • SiUcon carbide is used in a bonded form in SiC wet abrasive papers. It is also used in a loose form. SiC grains exhibit hard, weU-formed cutting edges, but they are relatively brittle and thus break easily. Fig. 17 is a scanning electron micrograph of the surfaces of an SiC wet abrasive paper (grain size P1200). • Aluminum oxide is used as a slurry and as a suspension. Aluminum oxide grains are not as hard as siUcon carbide, but they do not break as easily, either. Some AI2O3 wet abrasive papers still contain a certain amount of zirconium oxide. Water is used as the abrasive medium (lubricant) for aU bonded abrasives in grinding disks and films. In addition to water, fluids and carriers containing oil and alcohol are used most commonly for all loose abrasives in pastes and suspensions. The greatest amount of material removal is achieved with bonded diamond grinding disks. Progressively lower rates of removal are associated with polycrystalUne diamond suspensions, monocrystaUine diamond suspensions, diamond sprays, diamond pastes, SiC papers, and AI2O3 slurries (in that order). However, as the rate of material removal increases, so does the amount of damage caused to the sample surface. In the case of relatively soft or porous ceramics, it can be practical to use an
2A GRINDING, LAPPING, AND POLISHING
23
Figure 17. Scanning electron micrograph of a silicon carbide wet abrasive paper (grain size P1200). oil-based abrasive medium, rather than one based on alcohol or water. This makes it possible to reduce the damage to the material. Diamond suspensions are especially well suited to the automatic preparation of polished sections. It is easy to meter these suspensions and to distribute the diamond particles uniformly across the platen. As a rule, the use of these suspensions also eliminates the need for any additional abrasive mediimi. These properties of diamond suspensions make it possible to plan and automate the preparation stages of materialographic lapping and polishing with relatively httle trouble and a high degree of reproducibiUty.
2.4.1 Preparation equipment Table 3 provides an overview of the advantages and disadvantages of manual and automatic devices for sample preparation. The automatic preparation devices make it possible to vary the rotational speed of the disk, the sample pressure, the preparation time, and the rotational speed and direction of rotation of the sample holder. As the rotational speed of the disk increases up to approximately 250 rpm and the pressure on the sample is held constant, the removal rate also increases. As the speed continues to increase beyond this point, however, the removal rate remains essentially constant. The grinding speed will assume values between 1 and 18 m/s, as determined by the disk diameter, the rotational speed, the position of the sample, and the motion of the sample induced by the sample holder. While grinding, the samples should go sUghtly beyond the edge. Incorrect positioning of the sample holder and excessive rotational speeds can lead to uneven removal of material from the sample. This is called the "half-moon effect".
24 CHAPTER 2
FUNDAMENTALS
OF PREPARING POLISHED
SECTIONS
Table 3. Equipment for mechanical grinding, lapping, and polishing Equipment
Advantages
Disadvantages
©Manual preparation. Flat plate, manual grinding equipment
Economical, universally appHcable, portable; for large and small samples. Individual preparation. Quick preparation of standard materials Individual preparation. Removal is quicker than in 1. Sample is flatter Individual loading. Preparation time is controlled. For individual preparation steps
Long preparation times for hard materials. Great risk of edge rounding. No lapping
©Manually on rotating platen, belt, or rotating wheel ® Automated, with samples inserted individually
©Automated, with samples clampedfirmlyin place
©Fully automated, with samples clamped firmly in place and automatic changing of samples and abrasive supply
Samples are very flat. High edge definition. Faster cleaning and faster changing of specimen holder than in 3. Reproducible preparation Best reproducibility. Optimum results. High-volume sample throughput
Equipment is more expensive than in 1. Greater risk of sample deformation Equipment is more expensive than in 2. Samples are not sufficiently flat. Edge definition is less than optimum Longer set-up time. Must start with planar grinding step. High removal rate, thus a risk of losing sample Expensive equipment. Large space requirement. Not as suitable for individual samples
On some devices, it is possible for the sample holder to rotate in either the same direction as the disk or in the opposite direction. These modes are referred to as complementary rotation and counterrotation, respectively. Counterrotation yields a higher removal rate, which can be especially advantageous in planar grinding. With complementary rotation, a medium disk rotational speed (150-200 rpm) will allow the surface to be processed gently and uniformly. For this reason, the use of complementary rotation predominates. As the contact pressure (load divided by total surface area) increases, the removal rate also increases. But the roughness (peak-to-valley height) and depth of damage increase by a similar proportion. For example, in the case of six clamped samples, each with a diameter of 25 mm, under a total load of 200 N, the contact pressure measures 0.07 N/nmi^. In the technical literature, however it is customary to specify only the rotational speed of the disk in rpm and the load in N instead of the removal rate and contact pressure. When testing recommendations for the preparation of polished sections, the rotational speed and load must be converted accordingly.
2A GRINDING, LAPPING, AND POLISHING
25
Conventional semiautomatic equipment can function with either central pressure or single sample pressure. In the case of central pressure, the loaded sample holder is pressed against the disk by centralized pressure apphed by means of a pressure cylinder. On equipment that employs single sample pressure, the pressure can be appUed to each individual sample by means of a corresponding pushrod. The advantage of single sample pressure consists in the fact that it allows for either a single sample to be processed, or for the sample holder to be loaded with the desired number of samples. Another advantage is that all samples acquire an optimum degree of flatness. This is not possible in the central pressure method, and unsatisfactory preparation results are sometimes obtained. Another advantage of the single sample pressure method is that it allows the sample to be removed from the sample holder as often as desired. This offers great benefits when preparation is directed at a limited objective - specifically, preparation in one particular plane. Furthermore, individual samples that have been reinserted can be supplied to other preparation processes. When working with central pressure, it is especially important to ensure that the samples are securely clamped in the sample holder and lie in a common plane. They may not be removed during the individual preparation steps. This means that they are inserted into the holder after sectioning and mounting, and that the entire set of samples must be processed completely from planar grinding through the end of the preparation process. Semiautomatic rotating wheel grinding machines with specimen holders are equipped with grinding disks with a diameter of 200-300 mm. They are designed for disk rotational speeds between 50 and 1200 rpm. They allow either a choice between one or twofixedspeeds or continuously variable speed settings. Loads between 0 and approximately 300 N can be appUed to the specimen holder by means of a hydraulic system or a compression spring. The specimen holder, which is driven by an additional motor, rotates during the grinding operation. This rotation occurs at a relatively low speed, either in the same direction as the grinding disk, or opposite to it. Semiautomatic rotating wheel grinding machines allow multiple samples to be processed simultaneously. They are also used for the subsequent pohshing of the sections. In comparison to manual grinding methods, these devices offer improved surface quaUty, greater edge definition, and an improvement in the flatness of the polished sections. They also ensure greater reproducibility of the preparation results.
2.4.2 Grinding Mechanical grinding is performed either with automated equipment or by hand. Manual grinding is generally performed on a rotating wheel. When manual grinding is performed with diamond particles, a three-step process is advisable. The process parameters will be determined by the surface quality of the samples after sectioning. The samples are generally subjected to planar grinding at a wheel rotational speed of 120 rpm and with a coarse diamond grain size within a range of 70-120 jxm. They are then subjected to coarse and fine grinding with smaller grain sizes. Semiautomatic grinding often requires only two processing steps. Planar grinding can be performed
26 CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED SECTIONS under a load^ of 120-150 N and at rotational speeds of 120-300 rpm. In fine grinding, the load is decreased to a range of 60-90 N, in order to reduce pull-outs and improve the surface quality of the samples. Planar grinding is intended to create a flat sample surface after sectioning. It is a very important preparation step and must be performed carefully. Depending on the surface quality of the samples after sectioning, planar grinding of hard ceramics and composites containing hard ceramics may be performed with diamond grinding disks. Softer and less dense ceramics may be planar-ground with composite lapping disks and diamond suspensions, or with silicon carbide paper. Planar grinding is complete when the samples have been flattened to a common plane. The time required for planar grinding is determined by the degree of surface damage caused by sectioning, and by the sample material's resistance to removal. Of course, this also applies to the subsequent grinding, lapping, and polishing steps. In planar grinding, it is also important to ensure that the selected grain size of the abrasive is not too coarse in relation to the grain size of the cut-off* wheel. Disks with an abrasive layer of resin-bonded or metal-bonded diamond grains are used to process ceramic surfaces because of their excellent cutting quaUty, their low rates of wear, and the abihty of diamond to hold its edge. Disks with a synthetic resin bond are suitable for hard materials, such as siUcon nitride ceramics and sialon ceramics. Metal-bonded grinding disks are used for oxide ceramics and siHcate ceramics. The type of bond on the disk helps determine the surface quaUty of the polished section. Two or three grinding steps are used after planar grounding. These steps are carefully planned to form an overall grinding process. Chapter 4 describes standard methods for grinding ceramics and composite materials with semiautomatic equipment and by hand. Abrasive layers with diamond grain sizes D15-D90 and with nonstandard grains up to medium grain sizes of 10 ^m are used. As the size of the grains used in grinding decreases, the roughness and depth of damage of the sample surfaces are reduced. Large proportions of excessively coarse abrasive grains create scratches that will be diflScult to eliminate in subsequent preparation steps. On the other hand, if abrasive grains of insuflScient size are present in excessive proportions, the rate of removal will be reduced. Optimum results are therefore achieved by maintaining a narrow grain size distribution for the diamond abrasive. For grinding disks, the preferred diamond concentration is C50, which corresponds to a diamond proportion of 12.5% of the total volume of the abrasive layer. The right-hand side of Fig. 18 shows a metalbonded diamond grinding disk. When grinding is performed on a rotating wheel instead of diamond disks, it is possible to use pastes, suspensions, or sprays with diamond grain sizes of 9-45 |Am. Hard synthetic cloths, which are sometimes perforated, are used as the un^ All loads specified in N apply to a set of six clamped samples of diameter 25 mm.
2,4 GRINDING, LAPPING, AND POLISHING
11
Figure 18. Disks for preparing polished sections. Four composite lapping disks (top and bottom), two diamond grinding disks (center and right), and one pohshing platen (left).
derlying material. Grain sizes of 45-25 |im are suitable for a two-step grinding process. Although diamond is the abrasive that is used most frequently for ceramics, other abrasives are also used. The basic rule is that their hardness must be greater than the hardness of the material being processed. It is generally best to process aluminum oxide ceramics with diamond abrasives. SUicon carbide wet abrasive papers can be used for graphite, zinc oxide, and silicate ceramics. SiC wet abrasive papers are especially well suited to the grinding of highly porous samples and samples that tend toward pull-outs. Non-impregnated samples with a porosity greater than 8% can be ground with SiC abrasive papers of grain sizes P320, P500, and PIOOO, provided that the SiC abrasive is harder than the samples. Due to the weak bonding of the abrasive grains, wet abrasive papers work quite gently, because they wear down within a few minutes and thus continuously change to the next finer grain size. Running water is generally used as a medium when grinding on diamond disks or silicon carbide papers. The water serves to carry away the swarf and cool the sample. Oils, kerosene, and glycerol-alcohol mixtures are used as media for grinding watersensitive ceramics. If loose diamond particles are used in place of diamond grinding disks or siUcon carbide papers, then oil-soluble pastes or the conventional diamond suspensions based on water, alcohol, or oil are used in connection with suitable lubricants.
2.4.3 Materialographic lapping Like grinding with bonded grains, lapping is intended to eUminate surface irregularities and damages in the ceramic sample. It can be used for the preparation steps
28 CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED SECTIONS that follow planar grinding, replacing fine grinding and coarse polishing. In practical appUcation, lapping disks with diamond abrasives of grain size 6-30 ^m are generally used. After planar grinding has been completed, a single lapping step and subsequent poUshing are often sufficient to produce samples of high quality. Lapping is especially effective in maintaining the flatness and edge definition of samples consisting of composite materials with hard and soft components. Lapping disks can be classified as: hard disks, in which the abrasive grain does not become embedded; soft disks, in which loose grains become embedded and held fast; and composite disks. Composite disks may consist of plastic and metal, or of metal, ceramic, and plastic on metal supporting disks. Diamond sprays or suspensions may be used as lapping abrasives with composite disks. Composite disks prove to be very effective in quick, economical preparation routines for ceramics and ceramic composites. The sections created with composite disks display excellent edge definition. Examples include smooth disks with a layer of gray cast iron powder in plastic, into which softer, circular regions of copper powder and plastic are incorporated. Some disks feature a grooved plastic coating containing finely distributed metal particles of varying hardness. Many composite disks feature a segmented, concentrically shaped surface, or a surface with a spiral groove. These features help prevent the samples from "seizing" and allow the swarf to be carried away more effectively. Hard lapping disks can only be operated at low speeds. At high speeds, the grains enveloped in the lapping medium would be removed from the disk by the powerful radial forces. Four different composite lapping disks are shown at the top and bottom of Fig. 18. In the lapping process, samples move in constantly alternating directions on a flat disk (lapping disk) while abrasive grains are suppUed to the process. The samples are separated from the disk by a film that contains the loose lapping grains, the lapping fluid, and the swarf. Conventional suspensions with various diamond grain sizes are preferred as lapping abrasives. Lapping fluids include water, low-viscosity mineral oils, paraffin oils, alcohol, kerosene, and aqueous solutions with additives. These fluids improve lubrication and cooling while preventing the agglomeration of the lapping grains and swarf. The thickness of the lubricant film plays an important role in this process, as shown in Fig. 19. A glass plate provided with a uniform layer of silicon carbide grains is well suited to the lapping of titanium boride and zirconium boride. These samples can then be pohshed effectively in 20 min or less with diamond paste or a diamond suspension.
2.4.4 Polishing After grinding or materialographic lapping, the preparation process for ceramic materials continues with mechanical polishing. Electropolishing is only possible with electrically conductive ceramics, and ion poUshing is uneconomical because of its low removal rates.
29
2.4 GRINDING, LAPPING, AND POLISHING lapping grain
The lapping grains remove the material surface, breaking down in the process and thus adapting as the surface becomes progressively finer.
thin lubricant film high removal rate
thick lubricant film low removal rate
excessively thick lubricant film, grains slip without removing material
The viiBJOoslty of the lubricant film determines the film thickness and thus the aggressiveness of the grains.
Figure 19. Lapping and viscosity of the lubricant film.
Mechanical polishing involves the use of loose grains. The transition between grinding and poUshing cannot be clearly defined in terms of the surface quaUty achieved. Surface processing with abrasive grain sizes less than 10 ^im is generally described as poUshing. Polishing can be performed either by an automated or manual method. Diamond is now used almost exclusively in polishing. The poHshing of sample sections with diamonds requires more time than grinding, regardless of whether manual or automated methods are used. Standard times for automatic polishing are always shorter than the standard times for manual poHshing. Diamond polishes are available in the form of pastes, sprays, and suspensions in grain sizes of 0.25, 1, 3, 6, and 9 jim. The grain size and grain size distribution affect the surface quality of the sample. When working with diamond suspensions, the rotating polishing platen is continuously supplied with lubricant by means of a drip dispenser, and the suspension is replaced at predetermined time intervals. When using diamond paste, a "string" of paste approximately 6 cm in length may be evenly distributed on a polishing cloth with a diameter of 300 mm, for example. Polishing is then performed as lubricant is supplied through the drip dispenser. This removes the swarf and enhances the poUshing effect of the diamond grains. The supply of Uquid rinses the swarf from the poUshing platen in an outward direction. Carrier media based on alcohol, glycerol, or oil must be used very sparingly. An excessive amount of such media causes a "hydroplaning" effect in which the diamond grains no longer abrade the sample. The amount of lubricant can be tested by rubbing a finger over the polishing cloth. If this leaves a sUghtly glossy film on the finger, the amount of lubricant is correct.
30 CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED SECTIONS A sequence of diamond polishing stages - for example, with grain sizes of 6, 3, and 1 ^im, respectively - is often followed by a polishing operation using colloidal silica (silicon dioxide) or aluminum oxide in an aqueous solution. The size of the Si02 or AI2O3 particles would be 0.05 ^im in this case. This extra poUshing step removes any very fine scratches that remain and produces a slight degree of surface relief. It also faciUtates phase identification and prepares the sample for chemical etching. The diamond or other polishing abrasive is applied to a cloth, which in turn is either slipped over, or glued onto, a metal or plastic platen. Metal platens are preferred for their hardness and thermal conductivity. The left side of Fig. 18 shows an example of this. Synthetic cloths intended for coarse polishing can be either hard or soft and are sometimes perforated. Hard cloths yield very Uttle to pressure. They remove material at a high rate and create a flat section surface. Hard cloths are suitable for all ceramics. Perforated, hard cloths may be used for pohshing steps requiring grain sizes of 9 ^im and then 3 ^im, for example, while non-perforated and short-napped synthetic cloths are predominantly used for polishing at 3 |im or even finer grain sizes. These cloths remove material less aggressively, i.e., more slowly. In this case, greater contact pressure must be applied in order to maintain a flat poUshed surface. Fine polishing involves the use of natural and synthetic silk cloths, as well as napped synthetic cloths. Satisfactory results have been obtained by using napped synthetic cloths with long fibers for final polishing. A diamond grain size of 1 ^im, or even 0.25 yun under special conditions, is used for this purpose. Natural silk cloths slipped over aluminum platens are suitable for final polishing of AI2O3, Zr02, and other oxide ceramics. Extended fine polishing on a soft cloth can lead to the development of surface relief. Final polishing with colloidal silica or aluminum oxide is performed with chemically resistant cloths. A napless nylon cloth is shown in Fig. 20. It is also capable of preventing the development of rehef. Fig. 21 shows the scanning electron micrograph of a hard, woven silk cloth with 50 threads per centimeter. This cloth is well suited to the polishing of extremely brittle materials. Fig. 22 shows a short-napped synthetic fiber cloth that can be used to produce surfaces with a low degree of rehef and edge rounding. Table 4 hsts a sequence of poUshing cloths from hard to soft. Information on selecting appropriate polishing cloths can be obtained from any suppUer of consumables or equipment to the ceramographic and metallographic industries. The same applies to the selection of lubricants and carrier media and the special considerations pertaining to them. The platen speeds used in polishing usually he within a range of 120-300 rpm. Typical poUshing rates for ceramics Ue between 1 and 5 m/s. PoUshing pressures of 0.02-0.07 N/nrni^ have proven effective in diamond poUshing. A high initial poUshing pressure is generally used to eUminate puU-outs that result from the grinding of hard ceramics (see Figs. 14 and 15). For softer or more porous materials, or when using finer diamond grain sizes, this pressure is reduced by two thirds. High pressure can
2.4 GRINDING, LAPPING, AND
31
POLISHING
Table 4. Polishing cloths Hard, perforated synthetic fiber cloth Hard synthetic fiber cloth Hard, woven synthetic fiber cloth, nylon cloth (Fig. 20) Woven silk cloth (Fig. 21) Chemically resistant synthetic fiber cloth Short-napped fiber cloth (Fig. 22) Long-napped fiber cloth
hard
soft
Figure 20. Scanning electron micrograph of a hard, woven nylon cloth.
Figure 21. Scanning electron micrograph of a hard, woven silk cloth.
32 CHAPTER 2 FUNDAMENTALS OF PREPARING POLISHED SECTIONS
Figure 22. Scanning electron micrograph of a short-napped syntheticfibercloth. shorten preparation times, but it can also impair the quality obtained in polishing the section. As a rule, the poUshing time spent on a particular processing step must be sufficient to eliminate all surface damage caused in the preceding preparation step. For routine examinations requiring a polished section of average quality, a two-step poUshing process using diamond grain sizes of 6 and 3 |im is often suflBcient. Annotation In Figure captions: BF DF
Die
OM POL SEM
bright field dark field differential interference contrast optical microscope polarized hght scanning electron microscope
Chapter 3
Revealing the microstructure 3.1 Microstructural imaging in the optical microscope (OM) Polished and etched sections of composite materials and ceramics are examined by conventional methods of optical microscopic imaging at magnifications up to approximately 1000:1. These methods are described in Section 3.1.1 and illustrated by microstructural images. Methods of electronic image processing are addressed in Section 3.1.2. Section 3.2 contains tips on contrast enhancement and etching of the poHshed section.
3.1.1 Imaging methods of optical microscopy • The bright field (BF) method is used most often in optical microscopy. In this method, details of the sample appear either hght or dark, as determined by their respective positions in relation to the incident hght ray, and by their optical properties. In order for adjacent regions of the sample to be distinguished from one another, the difference between their reflectivity values must be at least 10%. •
In dark field (DF) imaging, only those object regions that he at an obhque angle to the incident Ught ray are reflected. Diff'used hght rays are captured in the objective. This method makes it possible to detect fine cracks and scratches and distinguish pores from inclusions.
•
The polarization contrast method (POL) is suitable for surfaces with structures that alter the polarization state of the Ught when it is reflected. When the analyzer and polarizer are crossed, optically anisotropic phases can be distinguished from optically isotropic phases. By eUminating troublesome lens reflections - which impair the clarity of the image - it is possible to increase the image contrast, even when examining poUshed sections of low reflectivity.
•
In the differential interference contrast (DIC) method, the polarized Ught ray is spUt into two coherent component rays. Interference between these component rays is produced after they pass through the system separately. The DIC method makes it possible to distinguish even fine increments by representing 33
34
CHAPTER 3 REVEALING THE MICROSTRUCTURE them as relief. It creates textured images of slight surface irregularities caused by the structure and/or the preparation process. This method is also used in evaluating the quality of the section, because it makes any remaining scratches clearly visible.
Fig. 23 shows the effect of using different imaging methods of optical microscopy to create microstructural images, using a mounting medium with included aluminum oxide particles as an example. The sample surface is shown in sharp focus in the bright field image in Fig. 23(a). The porous AI2O3 particles have a bright appearance, while surface holes caused by pull-outs (detached particles) are dark in appearance. Because the mounting medium is transparent, regions below the sample surface can be shown in the bright field. The ceramic particles are blurred, while the edges of the deeper, darker holes are sharply defined (Fig. 23(b)). When bright field imaging (Fig. 23(c)) is used, coating a specimen with a thin gold film suppresses reflections and scattered light that originate in subsurface regions and also enhances the contrast when imaging AI2O3 particles. In dark field imaging (Fig. 23(d)) of the gold-coated poUshed section, the AI2O3 particles have a flattened appearance and stand out sharply, exhibiting light edges. Holes in the dark plastic background can be easily recognized by their rehef-Uke structure. In the polarization contrast method (Fig. 23(e)), the image of the specimen is still somewhat transparent, despite the presence of the thin gold coating. Even relatively deep subsurface regions are visible. AI2O3 particles which have been cut in the section plane have a bright appearance, and holes appear as transparent bubbles. In the differential interference contrast method (Fig. 23(0), the roughened AI2O3 particles stand out clearly. The mounting compound displays rehef and the pores are dark. Because of the high transmittance exhibited by the phases of ceramic materials, the Ught that is present during microscope examination also penetrates samples of these materials. This light is then scattered and reflected below the surface by pores, inclusions, grain boundaries, and phase boundaries. The image quality of a ceramic microstructure can also be improved by applying a highly reflective gold coating with a thickness of approximately 5 nm. With gold coatings of this thickness, even phases of a ceramic material that have different degrees of reflectivity can be distinguished from one another through the gold coating. Fig. 24 shows an example of how a prepared section can be examined by the DIC method for purposes of quality control. The quality obtained by the preparation of the aluminum oxide specimen shown here is unsatisfactory, as indicated clearly by the grinding scratches and pull-outs at the pores.
3.1.2 Electronic image processing and contrast enhancement Electronic image processing is a method for purposefully altering a microscopic image. Modem electronic microstructural analyzers and contrast enhancement systems
3.1 MICROSTRUCTURAL
IMAGING IN THE OM
35
Figure 23. Imaging methods from optical microscopy, using as an example a mounting medium with spherical, porous AI2O3 particles to adjust hardness, (a) BF, focused on section plane, (b) BF, focused beneath the section plane, (c) BF, specimen coated with thin gold fihn. (d) DF, specunen coated with thin gold fihn. (e) POL, specimen coated with thin gold film, (f) Die, specimen coated with thin gold film. are capable of electronicaUy enhancing or otherwise modifying the contrast of images for which physical, chemical, and optical contrast enhancement methods have not produced adequate results. The normal contrast range of an image can be altered to intensify interesting components or structures while partially or completely suppressing insignificant information contained in the image. Microstructural components that display low
36
CHAPTER 3 REVEALING THE MICROSTRUCTURE
Figure 24. Image of scratches and pull-outs in an AI2O3 ceramic using the DIC method. contrast because of their similar gray values can be converted by electronically assigning them to values that he farther apart. For example, Ught gray can be converted to white, and medium gray can be converted to black. It is also possible to represent the gray values by colors. This color assignment method offers even better recognition and identification of the structural details. Electronic contrast enhancement systems can process the image of a normal blackand-white or color TV camera or a digital camera that has been adapted for use with the microscope. The preferred steps in manipulating the image include: • Enhancement of contours, grain boundaries, and phases of polished sections of low contrast. • Anisotropic enhancements, e.g., the representation of fibers, band structures, and preferred orientations. • Pseudo-3D-effect lending the image the appearance of three dimensions, comparable to the differential interference contrast method. • Image storage. Changes in microstructure (transformations, growth) resulting from heat treatment can be directly detected by switching from the stored image to the live image and comparing them. The creation of difference images also makes it possible to show surfaces that are not plane. • Image editing eliminates disturbing objects and flaws from the image. • Pseudocolor. Incremental gray values can be distinguished and displayed more clearly by electronically assigning colors to them.
3.1 MICROSTRUCTURAL IMAGING IN THE OM
37
The application of electronic image processing and contrast enhancement is documented in the two examples that follow. Fig. 25 shows the electronic creation of a difference image from two imaging planes of an uneven titanium carbide surface, followed by detection of grain boundaries for the purpose of grain size analysis. Fig. 26 shows the effect of electronic phase contrast enhancement in the example of a porous AI2O3 sample with approximately 3% glass phase. When examined before or after etching, the normal ceramographic section gives no clear indication of the amount, position, or distribution of the glass phase. After mild gas ion etching, it is possible to electronically enhance the contrast of the glass phase and eUminate the pores by means of the 3D-effect, gray value assignments, and color assignments.
imaging plane 1
imaging plane 2
difference image
detected grain boundaries
Figure 25. Difference image and detection of grain boundaries on an uneven titanium carbide surface. Instrument: IBAS, manufacturer: Kontron.
3D effect
assignment of gray values
assignment of colors (matrix: blue) and elimination of pores Figure 26. Electronic image processing for enhancing the contrast of the glass phase in a poUshed section of aluminum oxide. Instrument: Multicon, Leica.
5.2 TIPS ON CONTRAST ENHANCEMENT AND ETCHING
3.2
39
Tips on contrast enhancement and etching
Contrast enhancement with interference layers and rehef poUshing are used to reveal the microstructure in some routine examinations in which efficiency is desired. Polished sections of ceramic materials are generally etched by either chemical or thermal methods for the purpose of evaluating the microstructure. Grain boundary etching is usually used, but grain face etching and color-specific etching methods are also used to a lesser extent. Twin boundaries and phase boundaries are also rendered visible by grain boundary etching. Chemical etching recipes exist for many ceramic materials. Thermal etching can only be used for ceramics with little or no silicate content. In both chemical and thermal methods of etching, there is a risk that the section surface will not be attacked evenly. It is therefore advisable to etch in small increments until the most suitable etching conditions are determined. 3.2.1
Relief polishing
For ceramic materials that are not extremely hard (e.g., stabilized zirconium oxide or multiphase materials), surface relief can be created within a few minutes by means of final poUshing with colloidal silica on a chemically resistant, short-napped fiber cloth. Because removal is dependent on the grain orientation and the type of phase, it is possible to distinguish between grains of a single phase and between diiferent phases during microscopic examination. AppUcation of the DIC method makes it possible to use even poorly defined rehef to display microstructure under the optical microscope. A brief final poUshing step with very fine alumina (0.05 ^im) wiU help reveal the spinel phase in aluminum oxide materials, for example.
3.2.2 Contrast enhancement with interference layers Interference layer microscopy is used to increase the contrast between two or more phases of a material that have been cut in the section plane. This involves the application of thin interference layers to the poUshed specimen surface. These interference layers can consist of suitable dielectric substances appUed by means of vapor deposition, or metal oxides appUed by reactive sputtering. During subsequent examination under the microscope, the Ught waves that strike the coated section are attenuated by multiple reflections at the substrate/coating interface and the coating/ air interface. This causes an increase in contrast between adjacent components of the microstructure. The magnitude of this increase is dependent on the respective indices of refraction and absorption coefficients of the two adjacent phases. Increase in both the brightness contrast and the color contrast can be observed. Because of the low index of refraction and the minimal Ught absorption exhibited by the components of ceramic materials, interference layer microscopy can only be used on a Umited scale in ceramography. Although the use of absorption-free coating materials with low indices of refraction (e.g., NasAlF^, PbF2, and MgF2) leads to optimum contrast conditions with ceramics, it has not yet become a well-estabUshed practice.
40
CHAPTER 3 REVEALING THE MICROSTRUCTURE
In ceramic-metal composites, the application of nonmetallic interference layers to polished or etched sections can also be used for purposes of brightness matching. This involves reducing the reflectivity of the phases of the metal component while increasing the reflectivity of the ceramic material. Fig. 27 shows an example of this, using an image of a transverse section through the material transition zone (interface) in a composite of zirconium (top) and a siUcon nitride ceramic (bottom). The polished transverse section is covered with a thin iron oxide coating that has been created by reactive sputtering with an iron cathode in oxygen gas. If the coating were not present, the ceramic part would appear dark or even black. Here it appears in a Hght gray tone. Moving from top to bottom, the regions shown here in polarized hght include: the thin intermediate reaction layer, consisting of siUcides and nitrides; the individual grains of the hexagonal a-Zr phase; and the a-P-Zr basket-weave microstructure in various bright and dark shades. Fig. 28 shows the pohshed section of an ore specimen after it has been coated with lead fluoride PbF2. The coating thickness has been adjusted to correspond to firstorder blue. The phases hematite (H), magnetite (M), calcium ferrite (C) exhibit the maximum amount of contrast when shown in the monochromatic hght produced by a green filter.
3.2J
Chemical dip etching
Material-specific etching recipes exist for a variety of ceramics based on oxides, nitrides, and carbides. Highly concentrated acids or molten salts are often used as etchants, although basic etchants are also used in some cases. Hydrofluoric acid etching is usually performed on aluminum oxide materials containing Si02. Etching
r^^^
^5,
mMiTt Figure 27. Material interface between zirconium (top) and silicon nitride. The sample was coated with Fe/02 by reactive sputtering, POL.
3.2 TIPS ON CONTRAST ENHANCEMENT AND ETCHING
41
Figure 28. Polished section of ore, with contrast enhanced by lead fluoride coating. BF. H: hematite, M: magnetite, C: calcium ferrite.
conditions for several materials are presented in Table 5. Detailed information on etching recipes and instructions on etching can be found in the book Metallographic, ceramographic, plastographic etching by G. Petzow as well as in individual publications from the technical literature (see Chapter 7). Tables 6-9 present a variety of etchants and corresponding etching conditions for aluminum oxide materials with AI2O3 contents between 94% and 99.8% by weight, as well as Zr02 ceramics, silicon carbide materials, and siUcon nitride materials. Hydrofluoric acid etching is reconmiended only when there is a substantial glass phase content. Table 5. Conditions for chemical dip etching of several oxide ceramics and nitride ceramics Material
Etchant
Barium titanate
1 part cone. HF + 3 parts cone. HCl, 75 ml H2O, 15 ml HCl, 10 ml HF 90 ml lactic acid, 15 ml HNO3, 5mlHF Concentrated hydrochloric acid
20°C
2 bis 5% ige FluBsaure 5% glacial acetic acid Murakami etching solution: 100 ml H2O, 10 g NaOH, 10 g K3[Fe(CN)6] 10 ml distilled water, 10 ml glacial acetic acid, 10 ml HNO3 (65%) 30 ml glycerol, 10 ml nitric acid, 10 ml hydrofluoric acid
BeO ceramics CaO and MgO ceramics Silicate ceramics ZnO ceramics AIN and TiNceramics ZrN ceramics
Etching conditions
65°C
7 min-2 h 4 min 10 min-2 h
20°C
3 s-6 min
20°C 20°C 20°C
1-20 min 30 s-5 min 10 s-10 min
100°C
10 s-2 min or 50 min fur AIN 10 s-3 min
42
CHAPTER 3 REVEALING THE
MICROSTRUCTURE
Table 6. Chemical etching recipes for aluminum oxide materials with AI2O3 contents between 94% and 99.8% by weight Etchant
Etching conditions
10% hydrofluoric acid Concentrated sulfuric acid Concentrated phosphoric acid
20°C 230°C 250°C 650°C 300°C 900°C 900°C
Molten K2S2O4
Molten KHSO4 Molten V2O5 Molten borax
15 min 2-10 min 1-10 min 1-1.5 min 15-20 s 1 min 15-45 s
Table 7. Chemical etching recipes for zirconium oxide materials Etchant
Etching conditions
Hydrofluoric acid (40%) Phosphoric acid (85%) 90 ml HNO3, 18 ml HP, 90 ml H2O 50 ml H2O2, 50 ml distilled H2O
2-5 min, room temperature 20-30 min, 140°C 20 min, room temperature 1-5 min, boiUng
Table 8. Chemical etching recipes for siUcon carbide materials Etchant
Etching conditions
Molten NazCOa or molten K2CO.J 30 g K3[Fe(CN)6], 3 g NaOH, 60 ml distilled H2O
10 min 5-30 min, 110°C
Table 9. Chemical etching recipes for sihcon nitride materials Etchant
Etching conditions
Molten NaOH Hydrofluoric acid 40% Phosphoric acid 85%
10 s-1 min, 350^00°C 10-15 min, room temperature 5-30 min, 250°C
Listed below are some general undesirable effects that may occur in chemical etching. If high quality is required, these problems can be minimized by modifying the etching conditions or choosing a different etchant. •
Etching may be uneven.
•
Grain boundaries may vary in width.
•
Etching pits and other undesirable structures may develop on the grain faces.
•
Very small grains may fall out or be etched away.
3.2 TIPS ON CONTRAST ENHANCEMENT AND ETCHING
43
• Coatings consisting of foreign substances may conceal the microstructure. • All scratches may be widened. • Pores and cavities may become enlarged. • Results may be difficult to reproduce, especially when molten salts are used.
3.2.4 Thermal etching Thermal etching involves anneaUng polished, unmounted specimens in a furnace. At about 100°C below the sintering temperature, grooves that are detectable by optical and scanning electron microscopy develop at the grain boundaries and phase boundaries. The development of these grooves is determined by the equihbrium conditions for the interfacial tensions between the soUd phases that are in contact with the surrounding atmosphere. This process does not make scratches any more noticeable. AppUcation of thermal etching requires knowledge of the sintering temperature or hot pressing temperature of the material under examination, as well as knowledge of its behavior when subjected to heat treatment. For example, the heating, anneaUng, and cooHng of a Zr02 ceramic may cause precipitation reactions that alter the microstructure of the sample. When working with an aluminum oxide ceramic with an AI2O3 content less than 96% by weight, there is a risk of causing reactions between that corundum phases and glassy phases, which may also create false impressions regarding the original microstructure. Annealing at an excessively high temperature may lead to a coarsening of the grain. Thermal etching is performed in a furnace prepared especially for this purpose, in order to avoid undesirable residual impurities from previous anneahng operations with unknown sample materials. The sample is removed from any mounting material and thoroughly cleaned. The furnace is heated to the required temperature, and the sample is then slowly inserted into the hot region. When etching recipes are first tested, the annealing temperature is varied. Thermal etching conditions for several ceramic materials are presented in Table 10. The example shown in Fig. 29 is an optical micrograph of a section through the joint between two aliuninum oxide materials that had been bonded together. This shows the appearance of the section after thermal etching at 1400°C for 1 h. The micrograph demonstrates that the effects of etching were uniform in both the hotpressed, fine-grained material (99.9% AI2O3 by weight) and the coarse-grained, sintered material (99.7% AI2O3 by weight). The dark areas represent pores and cavities.
3.2.5 Plasma etching Plasma etching is a quick and effective etching method for revealing the microstructure of ceramics based on silicon nitride (Chatfield 1983; O'meara (1986); Taffner 1990). It makes possible a clear representation of grain boundaries, grain boundary
44
CHAPTER 3 REVEALING THE MICRO STRUCTURE
Table 10. Conditions for thermal etching of several ceramic materials Material
Atmosphere
Time
Temperature
a-Al203(>96% AI2O3) P-AI2O3
Air Air
0.5-4.5 h 1-5 min
Sr and Ba ferrites Ti02 Silicon nitride
Air Air Vacuum N2 Vacuum
1h 1h 15 min 5h 1 h bis 3 h
1250-1500°C 1470°C (200°C below the sintering temperature) 1050-1150°C 1350°C 1250°C 1600°C 1300-1500°C
Hot-pressed silicon carbide
yk W^
|^^^^^^^y.j4^^
Figure 29. Joint between two bonded aliuninum oxide materials, thermally etched at 1400°C for 1 h, BF. phases (such as Si02, AI2O3, AIN, and Y2O3), and intergranular inhomogeneities. It also makes it possible to distinguish between a and P-Si3N4 and show fine cracks and pores. Specimens etched by this method are usually fine-grained and are generally examined by means of a scanning electron microscope. Etching is performed in plasma etching systems, which are also known as "tunnel reactors" or "cold incinerators". Fig. 30 shows the schematic structure of such a system. A CF4-O2 gas mixture is used for the etching of the unmounted specimens. This gas mixture is introduced into the discharge chamber at a regulated rate via two flowmeters. The discharge chamber is arranged between the plates of a high-frequency discharge capacitor (13 MHz). This discharge capacitor transmits energy-rich electromagnetic oscillations, thus producing a plasma. The etching gas is excited, ionized, or spUt by
45
3.2 TIPS ON CONTRAST ENHANCEMENT AND ETCHING throttle valve flowmeter
j=3
**i-0
^''''-inlet
I—Intake manifold — O M
M
M j J
M I ; I I
( V r ^ ' " * * ' ^ chamber-V j ; ; L discharge I ; r capacitor
high frequency signal generator
11
U i-j U L-HL.
j f-reaction tut)e-[-( counter -4^^ ^ electrode ' 1 1 1 exhaust manlfbld+O
o g Lj LJjtzn|_ feed opening vacuum gauge ^
-^ to the vacuum pump
Figure 30. Schematic structure of a plasma etching system. atomicfission,and ions and radicals of ozone and fluorine are formed by recombination in this process. A chemical gas-surface reaction and/or ion bombardment of the specimen surface may be caused, depending on the energy state of the plasma. Here it is most desirable for thefluorineradicals to react with the Si3N4 matrix to form gaseous SiF4, which is then drawn off" with the rest of the combustion gases. The following conditions represent optimum settings: Reaction gas Gas pressure High-frequency energy consumption of capacitor Reflected power Etching time
CF4-O2 in a ratio of 2:1 60 Pa 200-300 W 20-30 W 1-5 min
Fig. 31 shows the results of plasma etching. This example is a scanning electron micrograph of an Si3N4 specimen. 3.2.6
Ion etching
In ion etching, the microstructure is revealed by the purposeful removal of atoms from poUshed specimen surfaces. This is accomplished by means of sputtering with an ion beam or ion bombardment from a gas discharge. Fig. 32 shows the structure of an ion beam etching system. An apparatus of this type can even be used to etch specimens that are embedded in synthetic resin. The operating conditions can be described by the following data: voltage 1-7.5 kV; ion current 60-400 ^lA; argon or krypton as the working gas; beam angle 5-60°; and etching time 20-90 min. Thus far, there is relatively Uttle available information on the ion etching of ceramic materials (Bierlein
46
CHAPTER 3 REVEALING THE MICRO STRUCTURE
Figure 31. Scanning electron micrograph of a plasma-etched Si3N4 specimen with 12.5% Y2O3. The Y2O3 grain boundary phase is lighter in appearance. 3-
1 ion gun 2 power supply and operating unit 3 specimen 4 vertically adjustable and rotating specimen holder 5 gask)ottle 6 viewing window 7 connection for vacuum gauge 8 vacuum system
Figure 32. Schematic structure of an ion beam etching system. et al., 1958; Politis and Ohtani et al., 1978, 1980, 1981; Schluter 1967). Good results are obtained with porcelain, silicon-bonded siUcon carbide, and zirconium oxide ceramics. Fig. 33 shows the surface of a Zr02 ceramic which has been partially stabilized with MgO and CaO and subjected to ion beam etching. In addition to the grain boundaries and pores (dark), fine precipitates are also barely noticeable. The precipitates grow from the grain boundaries inward toward the center of the grain. They consist of monoclinic Zr02, while the matrix consists of cubic Zr02 with dissolved CaO and MgO.
3.2.7 Electrolytic etching Electrolytic etching can only be used with electrically conductive ceramic materials. Etching guidelines have been provided for materials based on NiO, TiC, TaC, SiC,
47
3.3 TIPS ON CONTRAST ENHANCEMENT AND ETCHING
Figure 33. Zr02 ceramic after ion beam etching. BF. B4C, TaB2, and LaB4 (Petzow 1964, 1994). The practicability of the electrolytic etching process is specifically determined not only by the composition of the base material and its impurities, but also by the type, quantity, and distribution of other phases, including the glass phase. Electrolytic etching often produces good results with coarse-grained siUcon carbide ceramics, while very fine-grained silicon carbide ceramics are more effectively etched by chemical means, according to the modified Murakami method (Table 11). The conditions for color etching and subsequent grain boundary etching of siUcon carbide and grain boundary etching of boron carbide are given below as examples. Fig. 34 shows the electrolytically etched section of a porous SiC ceramic. Before the section was etched and mechanically ground and polished, the pores of the material were impregnated with epoxy resin. Electrolytic etching causes surface layers to form on the individual grains. The thickness and interference colors of these surface layers are determined by the grain orientation. The grains therefore appear in various gray tones in the black-and-white image. Twin bands are also visible. After the specimen surface has been electrolytically etched, it is brushed off with an aqueous Table 11. Electrolytic etching of SiC and B4C Material
Etching conditions
Time
SiC
Color etching, dependent on grain orientation with 10% oxalic acid at 15 V Grain boundary etching after removal of colored surface layer with 10-20% hydrofluoric acid Grain boundary etching with 1% KOH solution at 4-10 V Grain boundary etching with 50% sulfuric acid at 4^10 V
30-40 s
B4C
1 s-5 min 1 s-5 min
48
CHAPTER 3 REVEALING THE MICROSTRUCTURE
Figure 34. Porous SiC ceramic. Surface layers result from electrolytic etching with 10% oxaUc acid, BF.
Figure 35. Porous SiC ceramic. Surface layers removed by brushing off with aqueous 10% hydrofluoric acid, BF.
10% hydrofluoric acid solution. This removes the surface layers and clearly accentuates the grain boundaries (Fig. 35). The result is a high degree of brightness contrast between the SiC grains and the pores filled with epoxy resin.
3.3 Microstructural imaging in the scanning electron microscope A scanning electron microscope (SEM) is used instead of an optical microscope to examine microstructural regions whenever the following features are required: • magnification greater than can be obtained with the optical microscope; • greater depth of focus for representing spatial structures; • optimum contrast adjustments; • quick adjustment of magnification; • localization of details; • local X-ray microanalysis. These features have become routine in the examination of both fracture surfaces and polished sections of ceramic materials. The essential elements of a scanning electron microscope are shown in Fig. 36:
49
3.3 MICROSTRUCTURAL IMAGING microscope column 6V
-100 K 5-50KV
electron gun
• electromagnetic lenses
r-*•
• specimen holder • electron detectors • electronic imaging system Figure 36. Basic structure of scanning electron microscope (from G. Pfefferkom).
The electrons emitted by the electron gun are focused on the specimen by the Wehnelt cylinder and two to three electromagnetic lenses. As the narrowly focused electron beam strikes the specimen surface, it has a focal diameter of 2-10 nm. A sweep generator is controlled in such a way that the electron beam scans the specimen surface line-by-line and point-by-point. The electron beam also sweeps across a cathode-ray tube. These two motions of the electron beam are synchronized so that each point on the specimen surface is depicted on the screen. The image on a second screen is recorded by a camera. When the electron beam (consisting of primary electrons) strikes a point on the specimen, the specimen emits secondary electrons (SE), backscattered electrons (BSE), and X-rays (Fig. 37). The secondary electrons and/or backscattered electrons emitted by the specimen are registered by detectors. The signals emitted by these detectors pass through a video ampUfier and are then used - individually or in an electronically mixed form - to perform intensity modulation on the cathode-ray tube. The intensity of a given point on the screen is determined by the electron emission from the corresponding point on the specimen. High electron emission from a point on the sample produces a bright point on the screen, while low electron emission produces a dark point. Intermediate values produce fine gradations of gray tones.
50
CHAPTER 3 REVEALING THE MICROSTRUCTURE primary electron beam backscattered fc electrons X-rays ^
\
W W
secondary ^^^^^^
M
.>^^
Figure 37. Signals resulting from interactions between electrons and the specimen.
Incident eledronbeam
specimen surfece Interaction - ' ^ J ^ ^ ^ S . volume of y l l ^ ^ ^ s s s r secondary / \ electron / \ emission/
Interaction volume of l)adcscatlered electrons
region penetrated by primary electrons volume of x-ray excitation
Figure 38. Volume of specimen excited by primary electron beam. Regions of electron emission. Secondary electrons are used to create images that depict surface topography (Fig. 38). Secondary electrons (SE) are electrons of low energy {E = 1-100 eV) from regions close to the surface, i.e., at penetration depths of 5-50 nm. Secondary electron emission is determined by the slope of the area struck by the primary electron beam. Areas that he perpendicular to the beam emit very few secondary electrons and thus appear dark. Areas lying almost parallel to the beam emit the most secondary electrons and have a bright appearance in the image. This gives rise to the following types of contrast: Topographic contrast. Areas are shown in various degrees of brightness, depending on the angle of incidence of the primary electron beam.
51
3,3 MICROSTRUCTURAL IMAGING
Shadow contrast. Areas that face the electron deflector appear more brightly than areas turned away from the detector, despite having an equal angle in relation to the primary electron beam. Edge effects. In images produced by secondary electrons, protruding edges, points, and pore edges are especially bright, because of the high emission of secondary electrons at these locations. Texture contrast. As a result of the edge effect, rough surfaces are brighter than smooth surfaces of the same material. When images of poHshed sections are created by means of backscattered electrons, it becomes possible to make quaUtative distinctions between phases of different composition. Backscattered electrons come from deeper zones, extending halfway to the range of the primary electrons. These are regions in which the beam has already begun to expand. The backscattering coefficient is determined by the atomic number Z (Fig. 39). Low-Z materials appear darkly on the screen, while high-Z materials are bright. Material contrast thus makes it possible for phases with different mean atomic numbers to be distinguished from one another as a result of their different brightness levels. Grains of different orientations on highly poUshed surfaces of a single-phase polycrystalline material can also be distinguished from one another; this is known as crystal orientation contrast. The magnification setting depends on the size of the scanned specimen surface. It is calculated from the ratio of the screen size to the size of the scanned specimen surface. The magnification ranges of conunercial scanning electron microscopes he between 10:1 and 100,000:1. The resolution is determined not only by the smallest diameter of the electron beam, but also by the size of the emitting object region. The most outstanding characteristic of the scanning electron microscope is its great depth of focus. It can essentially be attributed to the fact that the image is produced
secondary electrons 20
40 60 atomic number Z
BO
Figure 39. Backscattering coefficient and secondary electron emission {EQ = 30 keV) as a function of atomic number Z (from Seidel, Wittry).
52
CHAPTER 3 REVEALING THE MICROSTRUCTURE
without lenses, and that secondary electrons from various depths contribute to the creation of the image. Fig. 40 is a comparison of the resolution, magnification, and depth of focus for imaging with the scanning electron microscope and the optical microscope. The range of appUcation of the scanning electron microscope (SEM) can be made even considerably larger by the use of accessories. Aside from the imaging of surface topography, the SEM can be used to evaluate the chemical composition of small regions of the microstructure. In addition to secondary and backscattered electrons, the specimen struck by the electron beam emits characteristic X-rays from a depth of approximately 1 ^im. The X-ray spectrum can be analyzed by either energy-dispersive (EDA) or wavelength-dispersive (WDA) methods, in order to provide information on the chemical composition of a material. When the SEM is supplemented with an X-ray spectrometer, it becomes possible to perform the following analytical studies: • point measurements on microprecipitates (particle diameter > 1 ^m); • measurement of surface area; • linear concentration profiles; • local element distribution images of the specimen surface. Sample preparation for purposes of examination in the SEM is very simple. In order to avoid disruptive charges, electrically nonconductive ceramic materials must be coated with a film of gold, carbon, or platinum about 20 nm thick by a sputtering process. Etched and unetched sections, fracture surfaces, glazes, firing skins, powder compacts, and prefired and finished sintered products can then be examined directly in the scanning electron microscope. paint resoiirtion lOpm 10 mm
^ \im
\O0nm lOnmSnm
S EM § 100 pm
\ light
\
tical 1nil Mt>SC ^ Npm
0,1 pm 20
100
1000
10000
40000
efl8Ctiv» magnification
Figure 40. Point resolution, magnification, and depth of focus in imaging with the scanning electron microscope (SEM) and the optical microscope (from Pfefferkom).
3.4 MICROSTRUCTURAL IMAGING
53
There are many ways in which the scanning electron microscope can be used for purposes of examination and evaluation; only a few examples are Usted below. When the mean grain size is less than 10 ^m, the optical micrograph of an etched specimen at a magnification of 500:1 can provide only a general impression of the uniformity of the microstructure (Fig. 41). However, a scanning electron micrograph at a magnification of 3000:1 (Fig. 42) makes it possible to easily recognize and determine grain shapes, grain size, and grain size distribution. An example of material contrast is provided by Fig. 43, which shows a scanning electron micrograph of a portion of a weld between two aluminum oxide bodies. The weld was doped with yttrium oxide. In the polished and thermally etched section, the yttrium oxide grains have a relatively bright appearance. This is due to the fact that Y2O3 has a higher mean atomic number than the coarser AI2O3 grains on both sides of the weld, which appear considerably darker in the image. By examining the fracture surfaces of a ceramic material with the scanning electron microscope, it is possible to obtain initial information on the microstructure without preparing a poUshed section. This can be shown by comparing the image of the fracture surface of an aluminum oxide ceramic (Fig. 44) with a scanning electron micrograph at the same magnification (Fig. 45), which was prepared from an etched section of the aluminum oxide ceramic.
Figure 41. Hot-pressed SiC ceramic, etched with Murakami's solution, BF.
Figure 42. Hot-pressed SiC ceramic, etched with Murakami's solution, SEM.
54
CHAPTER 3 REVEALING THE
-/
MICROSTRUCTURE
0%
§^^^m^
to pm Figure 43. Weld doped with yttrium oxide, located between two aluminum oxide bodies, SEM.
mr^^^ Figure 44. Scanning electron micrograph of the fracture surface of an aluminum oxide ceramic.
Figure 45. Scanning electron micrograph of the chemically etched section of an aluminum oxide ceramic.
Fig. 46 provides an example of a high depth of focus combined with high resolution. This is an image of the very fine-grained microstructure of a siUcon nitride ceramic after etching in molten NaOH. The scanning electron microscope can also be used to monitor the preparation of polished sections. This can be accompUshed by producing images of specimen surfaces after various preparation stages (Fig. 15).
3.4 Ultrasonic scanning microscopy Ultrasonic scanning microscopy is one of the newer and less familiar methods of examination. It represents a meaningful contribution to the field of ceramography and could create new possibilities for this science. It is used predominantly in the nondestructive characterization and imaging of microcracks, crack paths and pores,
3.4 ULTRASONIC SCANNING
55
MICROSCOPY
Figure 46. Scanning electron micrograph of a silicon nitride ceramic after etching in molten NaOH at 550°C for 30 s. the testing of adhesion between contacting surfaces, and the detection of density differences, doping differences, states of stress, and textures. The ultrasonic scanning microscope operates according to the pulse-echo method to produce an image of the surface and immediate subsurface volume of a flat ground or polished specimen. A sapphire cylinder combines the functions of a transducer and an acoustic lens. It features a thin ZnO coating on its top end and a cup-shaped indentation (formed by polishing) on its front side. It produces, transmits, and receives brief sound pulses (Fig. 47).
Changeover swHch
0.05..2GHZ
K-C
matching j^ network scanner
lens coating
receiver
ZnO piezoelectric transducer .^sapphire coupling medium: water
Figure 47. Functional principle of ultrasonic scanning microscope.
56
CHAPTER 3 REVEALING THE MICROSTRUCTURE
High-frequency electromagnetic oscillations are converted to sound waves by a piezoelectric ceramic in the form of a ZnO coating. The cup-shaped indentation and the coupling medium (water) combine to function as an acoustic lens, which focuses the sound field on the specimen. The sound pulses reflected from the specimen are received by the sapphire cylinder during the breaks between transmissions. The acoustic transducer converts these sound pulses back to electromagnetic pulses, which are then subjected to signal processing and displayed on the monitor as predefined gray values. The image is constructed Une-by-line by scanning the specimen with the transducer/acoustic lens combination. It is possible to examine specimens with a maximum area of 160 x 160 mm^ and a height of 40 nmi. Fig. 48 shows an ultrasonic microscope. The resolution of the ultrasonic microscope is determined by the aperture and sound wavelength. The sound wavelength, in turn, is determined by the ultrasonic frequency. Using water as the coupUng medium, the maximum resolution is 0.4 |j,m at 2 GHz, 15 jjm at 100 MHz, and 500 ^im at 10 MHz. The penetration depth depends on the frequency, the type of material, and the surface quality of the specimen. The scattering of the sound on the specimen surface is influenced by its roughness. The penetration depth can reach several millimeters at low sound frequencies. One example of the application of this method of examination is the analysis of defects and weak points in surface layers. In one case involving a Zr02 electrolyte layer on the anode substrate of a fuel cell, an undesirably high permeability to gas was found. This could be attributable to pores, bubbles, or cracks in the surface layer. The image shown in Fig. 49 was created at a frequency of 100 MHz. It clearly shows defects in the surface volume of the electrolyte layer. These defects can be rendered
Figure 48. View of the SAM 100 ultrasonic microscope from Kramer Scientific Instruments, Herbom, Germany.
57
3.4 ULTRASONIC SCANNING MICROSCOPY
8 mm
defects
6 mm
2 mm
Figure 49. Top view of Zr02 electrolyte layer with defects; image created by acoustic microscopy.
-
7
^
6
_
5
.
4
-
3
^
2
1
1
A
Omm
Gate pos: 700 ns Gate width: 57 ns
Figure 50. 3D reconstruction of a region of an electrolyte layer, showing defect.
Figure 51. Ceramographic transverse section through the region examined by acoustic microscopy. Damage appears as pores, bubbles, and cracks.
58
CHAPTER 3 REVEALING THE MICROSTRUCTURE
more clearly visible by a computer-aided 3D reconstruction. This reconstruction of the defective Zr02 surface layer is shown in Fig. 50. The material region under examination by acoustic microscopy was also studied by preparing a ceramographic transverse section. In the resulting image (Fig. 51), the detected defects are clearly recognizable as pores, bubbles, and crack formations in the Zr02 layer.
Chapter 4
4 Material-specific preparation of polished sections Based on experience gained in preparing polished sections of ceramic materials and composites, information on grinding and polishing certain materials can be summarized into procedures that yield favorable results. These procedures account for semiautomatic grinding and poHshing with a specimen clamping and loading device, as well as manual sample preparation. The standard times apply to mounted specimens with a surface area of approximately 1 cm^ and a mounted specimen diameter of 25-30 nmi. Larger specimens require longer preparation times. After the individual grinding and pohshing steps, the specimens must be thoroughly cleaned in an ultrasonic bath. This removes abrasive particles, poHsh particles, and swarf residue from the surface and pores of the specimen. These materials cannot be removed from the section surface by simple cleaning methods. The cleaned specimens should be examined by optical microscope after each preparation step, in order to monitor the quality of the section. This method makes it possible to quickly detect any defects resulting from processing and to optimize the sample preparation process with respect to time. The preparation process for ceramics can be modified by the introduction of materialographic lapping steps. Pohshing times must be kept short in order to avoid the formation of rehef. Diamond polishing produces a lower reUef than pohshing with substances of lesser hardness. The suitabiUty of the section for mechanical processing is affected by the chemical composition, hardness, and phase distribution of the ceramic material, as well as the porosity and the presence of cracks. For materials of moderate hardness, such as aluminum oxide materials containing SiOi and siUcate ceramics, wet grinding with silicon carbide papers can be used instead of grinding with diamond grains. In these cases, three processing steps are generally sufficient as a preparation for the subsequent diamond pohshing step. Because silicon carbide papers wear quickly, it is extremely important to replace them in a timely manner. A reduced level of grinding pressure must be used with ceramic materials of low hardness, nonimpregnated specimens of high porosity and specimens with cracks. After grinding on a diamond 59
60
CHAPTER 4
MATERIAL-SPECIHC PREPARATION
disk, it may be helpful to process the specimen with SiC wet abrasive paper with a grain size of 1000 before polishing with diamond. This may help prevent the occurrence of pull-outs in the section. Hygroscopic or water-soluble substances such as magnesium oxide, calcium oxide, and ot-aluminum oxide must be prepared by the use of grinding and polishing fluids in a base of glycerol and oil and then cleaned in anhydrous liquids. Ceramic composites and metallized ceramics are also prepared by semiautomatic methods with diamond grinding disks and diamond polishes, in accordance with the standard procedure. In this case, too, it may be advantageous to use materialographic lapping instead of fine grinding and coarse poUshing, provided that the metal is not too soft. The advantages of this method consist in a lower degree of edge rounding at the material transitions and higher edge definition. Excessive contact pressure will cause deformation of the metal. When final poUshing is performed with alumina for the purpose of removing scratches from the metal, the poUshing time must be kept short to prevent the development of great height differences between the metal and the ceramic. To illustrate this point. Fig. 52 shows two interferograms of transverse sections through a composite of a hard single-crystal aluminum oxide and a soft metal (single-crystal Nb). After the transverse section has been poUshed with diamond, it is subjected to final poUshing with alumina. After a half-hour, this final polishing step produces a height difference of 1.3 |im between the ceramic and the metal. After one hour of final poUshing with alumina, the height difference is 4.6 jam. 100 Mm ^^
=: 2c17
sapphire
phase niobium boundary after final polishing with alumina for 0.5 h Ah = 1.3 Mm
sapphire
phase niobium boundary after final polishing with alumina for 1 h
Ah=^
Ah = 4.6 Mm
Figure 52. Interferograms of poUshed sections of a sapphire/niobium composite. A/i = height difference between sapphire and niobium; z = number of bands; X = wavelength of Ught.
4.1 PROPERTIES OF CERAMIC MATERIALS
61
4.1 Properties of ceramic materials In addition to size, shape, and distribution and etchability of the phases, Ught reflectivity is a criterion for distinguishing and identifying the phases in a ceramic material. The reflectivity of ceramics is considerably lower than the reflectivity of metals. As an aid to microstructural examination, Figs. 53 and 54 plot the reflectivity
rvfractive index n
Figure 53. Reflectivity R of phases of ceramic materials and minerals as a function of the index of refraction n for the range « = 1 to « = 2.2 (from G. Hiibner).
62
CHAPTER
4
MATERIALrSPECmC
PREPARATION
iO Z.5 refractive index n Figure 54. Reflectivity R of phases of ceramic materials and minerals as a function of the index of refraction n for the range « = 1 to w = 3.5 (from G. Hiibner).
R (as a percentage) of the phases of ceramic materials and refractory construction materials over the index of refraction n. The index of refraction n was calculated from the reflectivity R, disregarding the low amount of Ught absorbed by ceramics, and using the Fresnel formula:
4.1 PROPERTIES OF CERAMIC MATERIALS
63
The scatter range plotted on the graph occurs because the optical properties are anisotropic, reflectivity is dependent on wavelength, and the chemical composition of the phases varies. The brightness contrast K of two phases 1 and 2 occurring next to one another in the polished section is represented by the value: i^ = ^ ^ i ^ where y?i>/?2. R\ Assuming that a minimum contrast value of 0.2 is necessary for distinguishing between two adjacent phases with certainty, then the contrast value of ^ = 0.21 between muUite 3AI2O3 • 2Si02 (i^2 = 6%) and corundum AI2O3 (i?i = 7.6%) barely allows them to be distinguished from one another. Considerably more favorable conditions are found when examining a hot-pressed ceramic consisting of aluminum oxide and silicon carbide. SiC has a reflectivity Rx of approximately 21%. With R2 = 76%, K assumes a value of 0.64. As shown in Fig. 55, AI2O3 appears black in the image produced under these conditions, while the greater reflectivity of SiC lends it a brighter appearance. Tables 12-14 present standard values for other properties of oxides, nitrides, and carbides. In addition to the crystal system and melting point, these tables Ust density, hardness, modulus of elasticity, compressive strength, bending strength, fracture toughness, thermal conductivity, and thermal expansion.
: •• f i i l f "^Bliiv
' '
x:-.-.;-:^- S: :• braze '^x -^.^t/'^^•::>^' %:.^ ' -^-^^
:^ #^*^
H--f Figure 101. Laminar composite of graphite/AgCuTi braze/TZM, unetched, BF. Active braze with 70% silver, 27% copper, and 3% titanium. The braze has penetrated deeply into the porous graphite.
4.5 EXAMPLES
OF PREPARING
Unetched, POL
CERAMIC
COMPOSITES
109
Etched with Murakami's solution, HF
Etched with Murakami's solution and subjected to reactive sputtering with Fe cathode in oxygen gas, BE. Figure 102. Laminar composite of graphite/AgCuTi braze/TZM molybdenum alloy. Microstructural imaging by various methods of contrast enhancement.
no
CHAPTER 4 MATERIALrSPECinC
PREPARATION
Aluminum alloy reinforced with boron fibers Table 45. Recommended preparation of an aluminum alloy reinforced with boron fibers Sectioning Diamond wheel low speed
Mounting
In epoxy resin
Step
Working surface
Load RPM Time Lubricant and coolant N^ min
Disk or film Composite lapping disk Perforated synthetic fiber cloth Short-napped fiber cloth Short-napped fiber cloth
Water 20 Suspension 20 water-based Suspension 20
150 150 150
5
Suspension
20
150
5
Suspension
20
150
5
Abrasive Grain size (^tm)
Grinding Lapping
Diamond 20 Diamond 6
Polishing
Diamond
6
Diamond
3 1
Until flat 60
^For a specimen with a diameter of 31.8 mm. Etching: (1) Contrast enhancement by vapor deposition of ZnTe coatings or by reactive sputtering with a Fe cathode and oxygen gas. (2) Electrolytic: with 10% oxahc acid at 8 V for 20 s. Surface layer is then removed with 10% HF-solution at room temperature for up to 10 s.
Figure 103. Boron fibers in an aluminum alloy, electrolytically etched, BF. The boron fibers have a bright tungsten core.
4.5 EXAMPLES
OF PREPARING
CERAMIC
111
COMPOSITES
Flame-sprayed aluminuin oxide coating on steel Table 46. Recommended preparation of aflame-sprayedaluminum oxide coating on steel Mounting 1 With cold mounting medium before sectioning, with impregnation of porous ceramic layers Trennen
Diamond wheel (0, 6 mm), low speed, sectioning proceeds from the coating into the substrate
Mounting 2
Step
Abrasive
Grain size Working surface
Lubricant and coolant
Load RPM Time N^ min
Grinding
SiC
P220
Water
120
300
Until flat
SiC
P500
Water
120
300
2
Lapping
Diamond
6 jim
Water-based
80
300
Polishing
Diamond
6 ^im
Suspension
120
150
Colloidal Si02
0.05 ^im
Water-based
30
150
Wet abrasive paper Wet abrasive paper Composite lapping disk Hard synthetic fiber cloth Chemically resistant synthetic fiber cloth
In epoxy resin
*For a specimen with a diameter of 31.8 mm. Atzen: Contrast enhancement by reactive sputtering with Pt electrode and oxygen gas.
20 pm Figure 104. Flame-sprayed aluminum oxide coating on steel, DIC. The sample was mounted in epoxy resin before sectioning. Arrows indicate cavities, which have a gray tone that differs only sUghtly from the aluminum oxide.
112
CHAPTER 4 MATERIAL-SPECmC
PREPARATION
20|JIT1
Figure 105. Flame-sprayed aluminum oxide coating on steel with Ti02 inclusions, BF. The sample was not mounted before sectioning. Improper sectioning with a high-speed machine has caused the coating to detach from the substrate (as indicated by arrows). The Ti02 and the steel substrate have a bright appearance.
2Qym Figure 106. Flame-sprayed aluminum oxide coating on steel, with contrast enhanced by Pt/02, BF. The sample was impregnated with epoxy resin before sectioning and then mounted in epoxy resin. The gap at the boundary between the ceramic and the metal (see arrows) is filled with resin, which indicates that this defect was already present before sectioning. The impregnated cavities are dark, the aluminum oxide is gray, and the steel substrate is bright.
113
4.5 EXAMPLES OF PREPARING CERAMIC COMPOSITES Glassfiberreinforced plastic Table 47. Recommended preparation of glass fiber reinforced plastic Sectioning Diamond wheel low speed
Mounting
Step
Abrasive Grain size Working surface
Grinding
SiC
P320
SiC
P600
Diamond Diamond Diamond Colloidal Si02
9 |im 3 |im 1 iim 0.06 ^mi
Polishing
Wet abrasive paper Wet abrasive paper Hard nylon cloth Hard nylon cloth Hard nylon cloth Chemically resistant synthetic fiber cloth
Cold mounting with epoxy resin Lubricant and coolant
Load RPM Time min N^
Water
20
200
Until flat
Water
20
200
1
Water-based Water-based Water-based Water-based
20 20 20 20
150 100 100 80
4 4 4 3
^For a specimen with a diameter of 31.8 mm. Etching: Contrast enhanced by reactive sputtering with iron cathode and oxygen gas.
Figure 107. Glass fiber reinforced plastic, BF. The glass fibers do not show clearly in the plastic.
114
CHAPTER
4
MATERIALrSPECIHC
PREPARATION
Figure 108. Glass fiber reinforced plastic, contrast enhanced, BF. The glass fibers are bright, while the plastic is dark. The glass fibers are arranged in bundles at right angles to one another.
Solder glass/stainless steel joint Table 48. Recommended preparation of a solder glass/stainless steel joint Trennen
Diamond disk low speed
Step
Abrasive
Grinding
SiC
PoUshing
Mounting
Grain size Working surface
P400 P800 Diamond 6 pm Diamond
1 ^mi
Colloidal Si02
0.05 ^m
Wet abrasive paper Wet abrasive paper Hard synthetic fiber cloth Perforated synthetic fiber cloth Chemically resistant short-napped synthetic fiber cloth
In bakehte or polyester resin Lubricant Load RPM Time and coolant N^ min Water Water Suspension
20 20 20
200 200 150
Until flat 8
Suspension
20
150
6
Suspension
18
150
4
*For a specimen with a diameter of 31.8 mm. Etching: (1) Solder glass: contrast enhanced with Fe cathode and oxygen gas. (2) Chromium steel: etching by Vilella method, 45 ml glycerol (87%), 30 ml hydrochloric acid (32%), 15 ml nitric acid (65%).
4.5 EXAMPLES
OF PREPARING
CERAMIC
115
COMPOSITES
Figure 109. Joint between chromium steel/solder glass/chromium steel, etched by the Vilella method, BF. The solder glass (dark) consists of Si02, MgO, AI2O3, and B2O3.
Ceramic/cermet composite Table 49. Recommended preparation of a ceramic/cermet composite Sectioning Diamond disk low speed
Mounting
Cold mounting in epoxy resin
Step
Abrasive Grain size
Working surface
Lubricant Load RPM Time and coolant N^ min
Grinding
SiC
Wet abrasive paper Wet abrasive paper Wet abrasive paper Grooved lapping disk Perforated synthetic fiber cloth Chemically resistant short-napped synthetic fiber cloth
Water Water Water Suspension
Pohshing
P320 P400 P800 Diamond 6 ima Diamond 3 ^m Colloidal 0.05 pm Si02
16 16 16 16
150 150 150 150
Until flat 1 1 10
Suspension 16
150
20
Suspension 14
150
15
^For a specimen with a diameter of 31.8 mm. Etching: Contrast enhanced by reactive sputtering with Fe cathode and oxygen gas.
116
CHAPTER 4
MATERIAL^SPECMC
P+YSZ perovskite , | , YSZ
cathode
electrolyte
PREPARATION
YSZ+Ni
anode
Figure 110. Ceramic/cermet composite from a high-temperature fuel cell. The section has been subjected to contrast enhancement, BF. The composite was produced by shp casting and a coat mix process. P = perovskite Lao.84Sro.i6Mn03; YSZ = Y203-stabihzed zirconium oxide. Carbon fiber reinforced carbons Table 50. Recommended preparation of carbon fiber reinforced carbons Sectioning Diamond disk low speed Step
Abrasive
Grinding Pohshing
SiC P800 Diamond 3 |xm
Mounting
Grain size Working surface
Diamond 1 ^m Colloidal 0.05 ^im Si02
Wet abrasive paper Perforated synthetic fiber cloth Hard nylon cloth Short-napped chemically resistant synthetic fiber cloth
Cold mounting epoxy resin Lubricant Load RPM Time min and coolant N^ 20 20
250 250
Until flat 5
Suspension 15 Water-based 15
250 150
5 3
Water Suspension
*For a specimen with a diameter of 31.8 mm. Etching: (1) Optical: with polarized hght and a coatue TiX^aie. (2) Electrolytic: with 20 g K2Cr207, 500 ml phosphoric acid at 8 V for 15 s. Remove reaction products with 10% HF solution. (3) Plasma etching at an oxygen flow rate of 50 cm^/cm^/min, O2 pressure: 50 Pa, power: 200 watts, etching time: 20 min.
4.5 EXAMPLES
OF PREPARING
CERAMIC
117
COMPOSITES
Figure 111(a) and (b). PAN (polyacrylonitrile) fibers in an anisotropic pyrocarbon matrix, POL. (a) fibers sectioned longitudinally, (b) fibers sectioned transversely. Cavities are dark.
Carbon fiber reinforced plastic Table 51. Recommended preparation of carbon fiber reinforced plastic Sectioning Diamond wheel. low speed
Mounting
In cold mounting medium
Step
Abrasive
Grain size
Working surface
Lubricant and coolant
Load RPM Time N^ min
Grinding
SiC SiC Diamond Diamond Diamond Colloidal Si02
P320 P600 9 |im 3 ^m 1 |im 0.06 |im
Wet abrasive paper Wet abrasive paper Hard nylon cloth Hard nylon cloth Hard nylon cloth Short-napped chemically resistant synthetic fiber cloth
Water Water Water-based Water-based Water-based Water-based
20 20 20 20 20 20
Polishing
*For a specimen with a diameter of 31.8 nun. Etching: Optical: with polarized fight and a wave plate.
200 200 150 100 100 80
Until flat 1 4 4 4 3
118
CHAPTER 4
MATERIAL-SPECIFJC PREPARATION
Electrolytically etched, DIC. fibers sectioned longitudinally fibers sectioned transversely
Plasma-etched, SEM. fibers sectioned transversely higher maginification
4.5 EXAMPLES
OF PREPARING
CERAMIC
COMPOSITES
119
Figure 112(a)-(d). Pyrocarbon matrix with laminar structure surrounding the PAN fibers. The core of each PAN fiber is a thread-Hke carbon fiber of high crystallinity.
Figure 113. Carbon fiber reinforced plastic, BF. The carbon fibers have a bright appearance.
Figure 114. Carbon fiber reinforced plastic, POL. Even the subsurface carbon fibers appear in the image. Some fibers are bright, while others are dark, depending on their orientation.
CHAPTER 4
120
MATERIAL-SPECIFIC PREPARATION
Spherical Nuclear Fuel Table 52. Recommended preparation of spherical nuclear fuels (coated particles). Nuclear fuel: UO2 - pyrocarbon - silicon carbide SiC Sectioning Not applicable
Mounting
Cold mounting in epoxy resin
Step
Abrasive
Grain size
Working surface
Lubricant Load RPM Time min and coolant N^
Grinding
SiC
P800
water
18
150
^
Lapping
Diamond
9 |im
Suspension
18
120
10
Polishing
Diamond and 3 |im colloidal 0.05 ^m Si02 Colloidal 0.05 ^m Si02
Wet abrasive paper Metall/KunstsoffScheibe gerillt Perforated synthetic fiber cloth Chemically resistant short-napped fiber cloth
Suspension 18 water-based
120
8
Water-based 18
120
8
^For a specimen with a diameter of 31.8 mm. ^ Grind with grain size P800 down to the equatorial plane of the fuel. Etching: (1) SiC: grain face etching and electrolytic etching to reveal the grain orientation, using 4 g K2Cr207, 100 ml H2O, 100 ml H3PO4 at 30 V, for 30 s. (2) SiC: electrolytic etching to reveal grain boundaries and growth processes, using 10% oxalic acid at 10-15 V for 30-40 s. Remove surface layers with 10% HF solution.
Figure 115. Cut-away view of coated particle sphere, SEM. The layered structure and inner UO2 sphere are revealed here.
4.5 EXAMPLES OF PREPARING CERAMIC COMPOSITES
121
Figure 116. Polished section of a coated particle sphere, BF. Shown here are the UO2 core and the sequence of layers of pyrocarbon - sihcon carbide (bright) - pyrocarbon.
dense pyrocarbon
sihcon carbide dense pyrocarbon porous pyrocarbon Figure 117. Detailed image of the pyrocarbon/SiC composite, BF.
deposition temperature 1500°C
deposition temperature 1700°C
Figure 118. Electrolytic grain face etching with K2Cr207 reveals the grain orientation of the SiC layers, BF. Layers with coarser grains at a higher deposition temperature.
122
CHAPTER 4 MATERIAL-SPECIFIC PREPARATION
deposition temperature 1300°C
deposition temperature 1500°C
deposition temperature 1700°C
Figure 119. After removing the etch coatings with 10% hydrofluoric acid solution, growth phenomena in the sihcon carbide layers are revealed by electrolytic grain boundary etching with 10% oxalic acid, BF.
Copper coating on an aluminum nitride ceramic Table 53. Reconmiended preparation of a copper coating on an aluminum nitride ceramic Coating: Nickel applied by electroless method, or TiN apphed by PVD method Sectioning: Diamond wheel Mounting: In a cold mounting medium (0.6 mm), low speed RPM Time min
Step
Abrasive
Grain size
Working surface
Lubricant Load and coolant N^
Grinding
SiC
P220
Water
40
300
Until flat
SiC
P500
Water
40
300
2
SiC
P800
Water
40
300
2
SiC
PIOOO
Water
40
300
2
Diamond
3 ^m
Suspension
50
300
3
Colloidal Si02
0.05 ^m
Wet abrasive paper Wet abrasive paper Wet abrasive paper Wet abrasive paper Hard synthetic fiber cloth Chemically resistant synthetic fiber cloth
Suspension
10
150
3
PoUshing
^For a specimen with a diameter of 31.8 mm.
4.5 EXAMPLES
OF PREPARING
CERAMIC
COMPOSITES
123
Figure 120. Copper coating on an AIN ceramic containing Y2O3, BF. The sample was coated with titanium nitride before sectioning and mounting, in order to provide accurate imaging of the copper coating. The preparation of the copper coating has resulted in a high degree of edge definition in the image. For coatings with poor adhesion, see the recommendations for flamesprayed coatings on Table 46.
^^^^tt^.
*:^fPfi^^if
50 |jm
Figure 121. Plasma-sprayed aluminum oxide coating on steel. Contrast enhanced with Fe/02, BF. The mounting medium and pores in the coating have a dark appearance. The coating is gray, while the steel substrate is bright.
124
CHAPTER 4 MATERIAL^SPECIHC PREPARATION
Plasma-sprayed aluminum oxide coating on steel Table 54. Recommended preparation of a plasma-sprayed aluminum oxide coating on steel Sectioning Diamond wheel Mounting (0.6 mm), low speed, sectioning proceeds from the coating into the substrate
In cold mounting medium
Step
Abrasive
Grain size Working surface
Lubricant and coolant
Grinding
Lapping
SiC SiC SiC Diamond
P220 P400 P600 6 ^m
Water 90 Water 120 Water 80 Water-based 80
300 300 300 300
Until flat 3 3 5
Polishing
Diamond 6 ^m
Suspension
120
150
8
Suspension
120
150
5
Suspension
120
150
3
Water-based 120
150
1
Diamond 3 ^im Diamond
1 ^m
Colloidal Si02
0.05 ^m
Wet abrasive paper Wet abrasive paper Wet abrasive paper Composite lapping disk Hard synthetic fiber cloth Short-napped fiber cloth Short-napped fiber cloth Chemically resistant synthetic fiber cloth
Load RPM Time min
^ For 6 specimens, each with a diameter of 25 nmi. Etching: Contrast enhancement by reactive sputtering with Fe electrode and oxygen gas.
Figure 122. Plasma-sprayed aluminum oxide coating on steel, DIC. The AI2O3 grains and pores in the coating are more apparent.
4.5 EXAMPLES
OF PREPARING
CERAMIC
125
COMPOSITES
Figure 123. Plasma-sprayed Al203/Ti02 coating on steel, BF. The aluminum oxide is gray. The titanium dioxide inclusions and steel substrate have a bright appearance, while the pores are dark.
Plasma-sprayed chromium oxide coating with Ni-20%Cr interlayer on steel Table 55. Recommended preparation of a plasma-sprayed chromium oxide coating with Ni20%Cr interlayer on steel Sectioning Diamond wheel
Mounting
Hot method, epoxy
Step
Abrasive Grain size Working surface (Mm)
Lubricant Load RPM Time and coolant N^ min
Grinding Lapping
Diamond 20 Diamond 9
Water 20 Water-based 20
150 150
Until flat 5
Pohshing
Diamond
6
Suspension
20
150
10
Diamond
3
Suspension
20
150
10
Diamond
1
Suspension
20
150
5
Disk Composite lapping disk Hard synthetic fiber cloth Woven nylon cloth Woven nylon cloth
^For a specimen with a diameter of 31.8 mm. Etching: (1) Contrast enhanced by reactive sputtering with iron cathode and oxygen gas. (2) For steel: 1 % alcohohc nitric acid.
Figure 124. Plasma-sprayed chromium oxide coating with Ni-20%Cr interlayer on steel, etched, BF. The steel displays ferrite and pearUte. A thin, dark oxide layer lies between the Ni20%Cr and the steel.
Plasma-sprayed zirconium oxide coating on a niclcel super alloy Table 56. Recommended preparation of a plasma-sprayed Zr02 coating Sectioning Diamond wheel, low speed sectioning or feed motion proceed toward the coating Step
Abrasive
Einbetten Cold method with epoxy resin of low shrinkage; sample may also be impregnated before sectioning in cases of high porosity
Grain size Working surface
Wet abrasive paper P500 Wet abrasive paper P800 Wet abrasive paper Pohshing Diamond 6 ^im Hard perforated synthetic 3 \im fiber cloth Diamond and 1 ^m and Chemically colloidal 0.05 ^im resistant synthetic Si02 fiber cloth
Grinding SiC
P320
Lubricant Load RPM Time min and coolant N^ Until flat
Water
20
250
Water
20
250
1
Water
20
250
1
Suspension
20
200
20
Suspension 20 Water-based 15
200 150
10 10
^For a specimen with a diameter of 31.8 mm. Atzen: (1) Vapor deposition of a ZnSe coating or reactive sputtering with Fe/02. (2) Contrast increased by coating with a thin film of gold. (3) Optical: by DIC.
Figure 125(a) and (b). Plasma-sprayed zirconium oxide coating on a nickel super alloy, (a) Direct bonding without bond coat, low porosity, BF. (b) With bond coat and intentionally high porosity, no pull-outs, BF.
SiC/C fibers in an aluminium alloy Table 57. Recommended preparation of SiC/C fibers in an aluminum alloy Sectioning Diamond wheel low speed
Mounting
In Bakehte or polyester resin
Step
Abrasive Grain size Working surface
Lubricant Load RPM Time and coolant N^ min
Grinding
SiC
Water Water Water Suspension waterbased Suspension water-based Suspension
Lapping
P320 P500 P800 Diamond 6 \nn
PoUshing
Diamond 3 ^un Colloidal 0.05 ^im Si02
Wet abrasive paper Wet abrasive paper Wet abrasive paper Grooved metal/ plastic composite lapping disk Perforated synthetic fiber cloth Chemically resistant synthetic fiber cloth
20 20 20 20
150 150 150 150
Until flat
20
150
15
20
150
30
10
^For a specimen with a diameter of 31.8 nun. Etching: (1) Contrast enhanced by vapor deposition of ZnTe coatings or by reactive sputtering with an Fe cathode and oxygen gas. (2) Electrolytic: with 10% oxaUc acid at 8 V und 20 s. Then remove surface layer with 10% HF solution at room temperature for up to 10 s.
128
CHAPTER 4 MATERIAI^SPECMC
PREPARATION
Figure 126. SiC fibers in an aluminum alloy, unetched, BF. Each fiber displays a carbon core, which appears black in the image. The aluminum alloy is white. The fibers are arranged with fine-grained SiC (gray) on the inside and coarse-grained SiC on the outside.
Figure 127. SiC fibers in an aluminum matrix. Contrast enhanced with Fe/02, BF.
4.5 EXAMPLES
OF PREPARING
CERAMIC
129
COMPOSITES
Figure 128. SiC fibers in an aluminum matrix. Electrolytically etched, surface layer removed, BF. The layer of fibers consisting of substoichiometric SiC and carbon protects the matrix.
Titanium carbide coating on graphite Table 58. Recommended preparation of a titanium carbide coating on graphite Sectioning
Diamond disk, low speed, sectioning or feed motion proceed toward the coating
Step
Abrasive
Grain size Working surface
Lubricant Load RPM Time min and coolant
Grinding Lapping
Diamond Diamond
20 |im 9 pm
Water 20 Suspension 20
150 150
Until flat 12
Suspension 20
150
30
Water-based 15
150
45
Polishing
Mounting
Disk Composite disk with grooves Diamond 3 |im Hard perforated synthetic fiber cloth Diamond and 1 ^im and Chemically colloidal 0.05 iim resistant Si02 synthetic fiber cloth
Cold method with an epoxy resin
^For a specimen with a diameter of 31.8 mm. Etching: 30 ml HF, 15 ml HNO3, 15 ml glacial acetic acid, 15 ml H2O, 5-20 min.
130
CHAPTER 4
MATERIALrSPECIFIC PREPARATION
Figure 129(a) and (b). Titanium carbide coating onfine-grainedgraphite, POL. (a) Unetched sample. The titanium carbide has a bright appearance, but its structure is not apparent, (b) Etched sample. Grain boundaries are visible in the titanium carbide coating.
Titanium nitride coating on an Inconel alloy Table 59. Recommended preparation of a titanium nitride coating on an inconel alloy Trennen Diamond wheel (0.6 nmi), low speed, sectioning proceeds from the coating into the substrate Step
Abrasive
Grinding Diamond Lapping Diamond Pohshing Diamond Diamond and colloidal Si02 (0.05 nm)
Mounting
In a cold mounting medium, epoxy resin
Grain size Working (|im) surface
Load RPM Time Lubricant min and coolant N^
30 9 3
Water 22 Water-based 20 Water-based 20
200 120 120
Until flat 30 40
Water-based 20
120
20
1
Disk or film Composite disk Hard synthetic fiber cloth, perforated Short-napped fiber cloth
* For 6 specimens, each with a diameter of 25 mm. Etching: (1) Nitric acid, hydrofluoric acid, and water in a ratio of 1:1:1. Etching time: from seconds to minutes. (2) Glycerol, nitric acid, hydrofluoric acid in a ratio of 1:1:1. Etching time: from seconds to minutes.
131
4.5 EXAMPLES OF PREPARING CERAMIC COMPOSITES
lnconeli^2S
«st:i
tSdym^. Figure 130. Titanium nitride coating on Inconel 625, etched, BF. The grain structure of the TiN coating is rendered visible by etching in a solution of glycerol, nitric acid, and hydrofluoric acid in a ratio of 1:1:1 for an etching time of 25 min.
WC-Co carbide metal Table 60. Recommended preparation of WC-Co carbide metal Sectioning Diamond wheel
Mounting
Hot mounting in Bakehte
Step
Abrasive Grain size Working surface jxm
Lubricant Load RPM Time and coolant N* min
Grinding Lapping
Diamond 20 Diamond 9
Water 20 Suspension 20
120 150
3 until flat 8
Suspension 20
150
8
Suspension 15
150
4
PoHshing Diamond 3 Diamond 1 + colloidal + 0.05 SiOs
Film Grooved composite disk Perforated synthetic fiber cloth Chemically resistant synthetic fiber cloth Chemically resistant synthetic fiber cloth
^For a specimen with a diameter of 31.8 mm. Etching: (1) Temper etching at 400°C for 30-60 min. Phases can be identified. (2) Color contrast enhanced by vapor deposition of ZnSe or ZnTe-coatings or by reactive sputtering with an Fe-cathode and oxygen gas. (3) Elektrolytic: 100 ml H2O, 10 g KOH, 2 g NasCOs at 3 V for 10 s. (4) Chemical: 100 ml H2O, 10 g KOH, 10 g K3[Fe(CN)6] at room temperature for 20 s.
132
CHAPTER 4 MATERIAI^SPECIHC PREPARATION
Figure 131. WC-Co carbide metal, BF. Reactive sputtering with Fe/02. Monochromatic light X = 520 ^m. The WC phase has a bright appearance, while the Co is dark gray and the Ti(Ta)C is dark.
Figure 132. WC-Co carbide metal, BF. Etching method was the same as in Fig. 131. Monochromatic hght X = 480 ^im. The Ti(Ta)C phase has a bright appearance.
4.5 EXAMPLES
OF PREPARING
CERAMIC
COMPOSITES
133
Figure 133. WC-Co carbide metal, POL. The Ti(Ta)C and Co are dark. Grains of the WC phase with different orientations have become visible in the polarized light.
Figure 134. WC-Co carbide metal, BF. Etched by chemical method (see Table 60). Grain boundaries have been rendered visible
Chapter 5
Preparing polished sections for examination Materials being examined for purposes of quantitative microstructural analysis must be prepared with semiautomatic equipment, in order to ensure that the results will be reproducible. Pull-outs, enlarged pores, scratches, uneven polishing, and high relief must be prevented by checking the section surface after each individual processing step and then modifying the preparation parameters and times accordingly, taking into account the unique properties of the given material. Information on the quahty of the section can be obtained by comparing the porosity determined from density measurements to the proportion of pores determined from test measurements. For precise determinations of porosity and pore size distribution, unetched sections are preferable to etched sections. This also applies to the determination of phase proportions in cases where the contrast between the phases in the optical microscope or scanning electron microscope is adequate. To avoid measurement errors when determining microhardness, sections of ceramic materials must be of high quality and free of relief. The quality of the section can be monitored by the use of an interference microscope or an optical microscope with D i e optics. Sections of ceramic samples are often subjected to microprobe analyses or SEM examinations with an accessory device for energy-dispersive X-ray analysis. It is essential to avoid the introduction of any elements which are intended to be measured in these examinations or which may have a disruptive influence on them. It is therefore essential to pay special attention to swarf from metal-bonded diamond wheels, composite disks, or disks made of cast iron, lead, copper, or tin, as this swarf may accumulate in pores or in the form of thin surface films. This also applies to swarf from the sample and polish residue, which may remain on the sample after final polishing with alumina, for example.
134
5.7 POLISHED SECTIONS
135
5.1 Polished sections 5.1.1 Oblique sections Some coating parameters and microstructural parameters vary along the length of the specimen. When preparing the sample surface or a thin near-surface region of a composite or a coating-substrate system for examination and measurement of such parameters, the use of an obUque section can be very practical. In addition to coatings applied by electrochemical deposition (ECD), physical vapor deposition (PVD), and chemical vapor deposition (CVD), this method is also appHed to many surface-treated material samples and samples that have been produced by joining techniques and exhibit film growth caused by diffusion, reaction, or corrosion. In the case of an obUque section (Fig. 135), the sample is mounted at an angle a to the section plane. As a result, the coating thickness being examined (symbolized by d) is increased to: sma Table 61 presents the factor for the increase in the apparent thickness of the coating as a function of the section angle. It is preferable for the section angle a to be less than 8° in order to achieve the highest possible ratio of similitude. However, even slight deviations of ±2° resulting from the preparation process lead to considerable differences in the ratio of simiUtude. Therefore, even the use of a wedge with a predefined angle a does not always ensure a precise section angle on the sample. Fig. 136 shows a 30° oblique section through the coating of a Ni-Zr02 anode on a soHd Zr02 electrolyte serving as the substrate. In this example, the coating is enlarged by a factor
mounting compcHincI
d »D-sina
Figure 135. Schematic representation of the oblique section technique. D is the apparent coating thickness; d is the true coating thickness, and a is the section angle.
136 CHAPTER 5 PREPARING
POLISHED
SECTIONS FOR
EXAMINATION
Table 61. Apparent widening of coating as a function of section angle Section angle a 2° 40
6° 8° 10° 15° 30° 45°
sin a
Ratio of similitude
0.0349 0.0698 0.1045 0.1392 0.1736 0.2588 0.5000 0.7071
28.6 14.3 9.6 7.2 5.8 3.9 2.0 1.4
mounting medium • ^^ * • :JI ^
^m
i f
j ^ N I (white) 1 anode coating : consisting of \ nici(el and i zirconium 1 dioxide 1 — pores (black) L ^ zirconium 1 dioxide (gray) '
0
- ^^ : * %
•;rt--^::::ilj
.*"•%:*
iSlWi;:,.:.:^^:^^^^^ liiliiill^
:••: * ,;;|f
1 zirconium 1 dioxide doped with 1 yttrium oxide
Figure 136. 30° oblique section of a joint between the anode coating and the soHd electrolyte of a high-temperature fuel cell. of 2 from top to bottom. An additional protective layer - e.g., nickel deposited by galvanic or wet chemical methods - should always be appHed to especially sensitive samples with very thin coatings. This will help prevent edge rounding and other artifacts.
5.1.2 Controlled removal In some cases, it is desirable to perform a "targeted" preparation procedure, i.e., to grind the sample in a precisely defined plane. For example, this may be essential for
137
5.1 POLISHED SECTIONS
the microscopic examination of material transitions, phase boundaries, or diffusion zones, which in turn can be indicative of the quality of a component. It may also be desirable to examine coatings or composites at predefined depths below the sample surface. At the present time, predefined amounts of material can only be removed by the use of semiautomatic grinding, lapping, and poUshing systems with the following methods: • Working with stops made of diamond or boron carbide: in this method, the maximum limit for the removal of material from the sample is the reference plane formed by mechanically adjustable diamond (or boron carbide) stops. These stops are built into a sample holder that has been manufactured to a high degree of precision. •
In some semiautomatic preparation systems, an accessory device (measuring device with digital readout) is coupled directly to the cyUnder stroke of the machine head. This accessory makes it possible to remove a predefined amount of material from the sample as it is held securely in the sample holder.
These two methods offer only a limited degree of precision in the controlled removal of material. It may be possible to remove between 10 and 50 ^im, depending on the stabiHty of the machine. The determination of removal depth is fraught with an error of 25%. "Ball cratering" represents an entirely different method of removing a controlled amount of material for purposes of determining the thickness of thin ceramic coatings (0.1-30 |jm). This method has proven extremely useful in practical application, especially in quaUty control of hard coatings apphed by PVD and CVD (see VDI Guideline 3198). Ball cratering can also be apphed successfully to other thin, extremely hard coatings, e.g., nitride coatings on steel, anodic coatings, or hard galvanic coatings of nickel or gold on ceramic substrates. The ball cratering method offers a genuine alternative to transverse and oblique sectioning techniques, because it can be performed with great simphcity, speed, and accuracy. In this method, a concave depression is ground into the sample surface by means of a steel ball rotating on a precisely centered spindle. An abrasive is introduced to the process in the form of a diamond suspension. When the appropriate spindle speed, abrasive, and grinding ball (usually made of hardened steel) are selected, it is possible for the ball to penetrate the hard coating to the required depth within 20-30 s in practical apphcations. A concave depression with a circular or elUptical shape and a depth of 5-20 jim is produced in the test surface. The functional principle is represented schematically in Fig. 137. The following formula for the coating thickness s at an accuracy of
