Preparation and Microstructural Analysis of High-Performance Ceramics

Ulrike Ta¨ffner, Veronika Carle, and Ute Scha¨fer, Max-Planck-Institut fu¨r Metallforschung, Stuttgart, Germany Michael J. Hoffmann, Institut fu¨r Keramik im Maschinenbau, Universita¨t Karlsruhe, Germany

IN CONTRAST TO METALS, high-perfor- and impurities. These microstructural variables cubic ZrO2 lattice). Cubic stabilized zirconia is mance ceramics have higher hardness, lower have a strong influence on the method selected also used in as k-sensors for automobile catalytic ductility, and a basically brittle nature. Other for preparation. An example for two different converters and for p(O2) measurement in liquid general properties to note are: excellent high- ZrO2 ceramic materials is illustrated in Fig. 1 and metals. temperature performance, good wear resistance 2. Figure 1 shows the microstructure of tetrag- Because of these differences in mechanical and thermal insulation (low thermal conductiv- onal ZrO2 (TZP, or tetragonal zirconia polycrys- properties and microstructure, the ceramo- ity), as well as high resistance to corrosion and tals). This is a high-strength structural ceramic graphic preparation of TZP and CSZ is quite dif- oxidation. However, the full advantage that these used for room-temperature applications (e.g., ferent. The tough, fine-grained TZP requires materials can provide is strongly dependent on knives and scissors). Tetragonal zirconia poly- longer polishing times for the fine-polishing step composition and microstructure. crystals have a grain size less than 1 lm, an ex- with 1 and 0.25 lm diamond, while CSZ needs Most high-performance ceramics are based on tremely high bending strength ranging from 800 longer polishing times for the coarser polishing high-purity oxides, nitrides, carbides, and bo- to 2400 MPa (115 to 350 ksi), and fracture with 6 and 3 lm diamond compounds. rides with carefully controlled compositions. Ce- toughness (KIc) between 6 and 15 MPaΊm (5.5 Depending on the type of ceramic or ceramic ramic engineering components are usually pro- and 15.5 ksiΊin .), which renders this material component, the mechanical properties (e.g., frac- duced by powder metallurgical methods. The resistant to pullout during preparation. ture toughness and strength) may vary consid- required properties of a specific part are opti- The microstructure of cubic ZrO2 (CSZ, or cu- erably, and therefore the ceramographic prepa- mized by selecting parameters associated with bic stabilized zirconia) is shown in Fig. 2. The ration procedures have to be adjusted the powder mixture and the pressing and sinter- mechanical properties of this material are con- accordingly. ing operations to obtain the desired microstruc- siderably poorer than TZP, with a bending ture. strength of 200 MPa (29 ksi) and a fracture High-performance ceramics can be divided toughness of 2 to 3 MPaΊΊmin (1.8 to 2.7 ksi ). Specimen Preparation into two main categories; structural and func- The microstructure is characterized by a high in- tional ceramics. While optimization of structural tragranular porosity and a grain size of approx- Similarly to metallographic preparation, se- ceramics is directed toward improved mechani- imately 30 to 50 lm. These materials are very quential steps have to be performed to prepare cal properties, the performance of components sensitive to mechanical shock. Applications of ceramics for microstructural investigations (Ref produced from functional ceramics is controlled TZP and CSZ are focused on their high ion con- 1–3). Careful selection of sectioning, mounting, by electrical, magnetic, dielectric, or optical ductivity (e.g., mobility of O2 ions across the grinding, polishing, and etching procedures is re- properties. Therefore, restrictions with respect to mechanical properties can be tolerated. Typical structural ceramics are aluminum oxide (Al2O3), zirconium dioxide (ZrO2), silicon nitride (Si3N4), and silicon carbide (SiC). However, Al2O3-, ZrO2- and SiC-based ceramics are also often used as functional ceramics. Other functional ceramics of technological interest are bar- ium titanate (BaTiO3) and lead zirconate titanate (Pb(Ti,Zr)O3). Due to the large variations in microstructure, different ceramographic preparation techniques are applied to achieve the surface quality desired so structural details are revealed under the microscope. For ceramographic preparation, it is not sufficient to know that a sample is Al2O3. The manufacturing conditions must also be Fig. 1 Tetragonal zirconia polycrystals (TZP) with 2 Fig. 2 Light micrograph of a cubic stabilized zirconia mol% Y2O3, thermally etched in air at 1300 C (CSZ) with 12 mol% Y2O3, thermally etched in known because they provide important infor- (2730 F). The scanning electron micrograph shows a fine- air at 1300 C (2730 F). The large cubic grains show inter- mation regarding expected porosity, grain size, grained microstructure. Pores appear black. and intragranular porosity. © 2004 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook Volume 9: Metallography and Microstructures (#06044G) 1058 / Metallography and Microstructures of Ceramics, Composite-Metal Forms, and Special-Purpose Alloys quired, and each step must be optimized for each Mechanical Preparation (Grinding and Microscopic Examination type of ceramic. However, due to the brittleness, Polishing). It is preferable to perform the grind- porosity, and chemical resistance of ceramics it ing and polishing procedures with an automatic For the investigation of ceramic microstruc- is quite often difficult to polish them in the same or semiautomatic machine. The structure of each tures and the identification of flaws and defects, and every ceramic product has been specifically way as metals. Automated sample preparation is the use of light optical microscopy (LOM) or adjusted to exhibit required properties, and thus recommended. The capability to adjust polishing scanning electron microscopy (SEM) are most each material will exhibit a unique behavior dur- pressure and the use of special grinding disks common. Since most of the ceramics are electri- ing preparation. Table 1 contains preparation with diamond as the abrasive material is also cal insulators, samples for SEM investigations standards for structural ceramics (e.g., Si N and preferred. With this equipment, a flat surface that 3 4 have to be coated by an electrical conductive Al O ) as well as AlN, and Table 2 provides the displays an undistorted “true” microstructure 2 3 layer such as carbon, gold, or gold-palladium al- preparation standards for functional ceramics may be prepared in a reasonable time. loys. Metals are used for simple microstructural (e.g., BaTiO and PZT). These tables should be Sectioning. Generally, ceramics are cut with 3 analysis, while carbon is used for simultaneous used as a rough guide only; the parameters will a lubricated (water or a special cutting fluid), ro- chemical analysis, for example, energy-disper- need to be adjusted according to the preparation tating diamond cutting wheel on a bench-type sive x-ray (EDX) analysis. Standard scanning requirements of specific ceramics. lab machine or on a precision cutting machine. electron microscopes are normally equipped The cutting speed (low-speed cutting machine: In general, resin-bonded diamond disks are employed for grinding. In individual cases, sili- with different detectors. The backscatter detector 25 to 500 rpm; high-speed cutting machine: 500 is useful for multiphase materials, when the dif- to 5000 rpm) and the cutting pressure should be con carbide paper is used. For example, this type of paper would be selected for the functional ce- ferent phases reveal a strong mass contrast. In optimized for the properties of a given material. this case, no etching is required. Secondary elec- A slow cutting speed and low pressure produce ramics. The surface damage generated during section- tron detectors are sensitive to small differences less cutting and surface damage for most ceram- in height of a polished and etched surface. The ics, although some ceramic materials require the ing and grinding has to be removed during fine grinding, or lapping and polishing. Fine grinding microscopic examination of ceramic specimens opposite (e.g., TZP-ZrO2). Diamond cutting in the as-polished state has proved useful. An wheels are either metal bonded or resin bonded, and/or lapping retains the plane of the specimen surface, and no further damage is introduced. evaluation of the number of pores, their distri- and normally metal-bonded cutting wheels are bution, and possible pullouts can only be as- selected. However, for very brittle and sensitive Complete removal of the damaged surface must therefore be achieved by subsequent polishing sessed in this state. Evaluation of inclusions, ceramics, resin-bonded cutting wheels are rec- contamination, and cracks should also be made ommended. These wheels are softer and will steps. Polishing should be performed on hard cloths. The highest removal rates will occur dur- before etching. generally produce a better cut-surface finish than In order to reveal grain boundaries, phases, a metal-bonded diamond wheel, but their weaker ing steps with the application of 6 and 3 lm diamond grain size. Polishing with 1 lm diamond and other microstructural details, ceramic spec- bond shortens service life. Additional criteria for imens have to be etched. Ceramic microstruc- selection of cutting wheels also include: the con- removes a minimal amount of material. All will (ןbreakouts and scratches should be removed dur- tures, examined under the LOM (200 centration of abrasive, the grain size of abrasive, ing these steps. In general, fine polishing with a show a low contrast and a milky image. Ceram- and the wheel thickness. Generally, low abrasive SiO suspension is only applied if small and fine ics permit light rays to penetrate the surface concentration blades are used, because the lower 2 scratches have to be removed. In some cases, this where scattering and internal reflection occurs. the abrasive loading, the higher the contact stress final step can also produce a slight relief on the To eliminate light scattering and to improve the on each abrasive particle and therefore the higher sample surface, which may be beneficial for mi- reflectivity, coating the surface with a reflective the cutting rate. The grain size of a diamond croscopy. layer is recommended. Such a layer should be blade is usually 94 lm, but for delicate samples, it may be advisable to cut with a diamond blade of a finer grit to avoid unnecessary damage to the material. The thickness of the diamond cut- Table 1 Standard preparation conditions for structural ceramics (e.g., Si3N4, AlN, SiC and ting wheel is dependent on the thickness of the Al2O3), semiautomatic preparation sample. For samples with limited dimensions it Diamond Speed, Pressure(a) Time, is advisable to use a thinner wheel. Process Base grade, lm Lubricant rpm N lbf min Mounting. For automatic sample preparation, Grinding Diamond disk 65 Water 300 180 40 Until flat specimens can be mounted or may be glued di- Diamond disk 20 Water 300 180 40 5–10 rectly onto a sample holder. The two possibilities Fine grinding Composite disk 6 Alcohol-based 300 100 23 5–15 Polishing Pellon cloth 6 Alcohol-based 150 150 34 15–120 for metallographic mounting are hot mounting, Hard synthetic cloth 3 Alcohol-based 150 120 27 15–120 with compression and heat, and cold mounting. Hard silk cloth 1 Alcohol-based 150 90 20 5–10 It should be noted that sensitive, small, and very Fine polishing Short napped, fiber cloth Silicon dioxide suspension 150 50 11 0.5–5 brittle ceramics specimen are susceptible to dam- This table should be used as a rough guide only, and parameters will need to be adjusted according to the preparation requirements of specific ceramic age and cracking when using hot mounting, be- samples. (a) Pressure specification for a sample holder with six 25 mm (1 in.) diam specimens cause of the high pressure and temperature that is needed for this process. The mounting material should be either very hard or have good abrasion resistance. Additionally, before the sample Table 2 Standard preparation conditions for functional ceramics (e.g., BaTiO3, PZT, and is mounted consideration must be given to the ZnO), semiautomatic preparation etching technique that is to be used. This will Diamond Speed, Pressure(a) Time, guide the selection of a mounting material most Process Base grade, lm Lubricant rpm N lbf min suited to the complete sample preparation. For Grinding Silicon carbide paper 320 Water 300 150 34 Until flat example, when using thermal etching techniques Lapping Hard synthetic cloth 6 Lubricant 150 100 23 2–15 or molten-salt etching techniques the sample Polishing Hard synthetic cloth 3 Lubricant 150 120 27 5–30 should be removed from the mount before etch- Hard silk cloth 1 Lubricant 150 90 20 5–15 Fine polishing Short napped, fiber cloth Silicon dioxide suspension 150 50 11 0.5–1 ing. When etching in a boiling chemical solution, the mount should be of a material that will This table should be used as a rough guide only, and parameters will need to be adjusted according to the preparation requirements of specific ceramic not be attacked by the acid mixture. samples. (a) Pressure specification for a sample holder with six 25 mm (1 in) diam specimens © 2004 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook Volume 9: Metallography and Microstructures (#06044G) Preparation and Microstructural Analysis of High-Performance Ceramics / 1059

between 5 and 10 nm thick and can be applied ally takes about two days, the specimen is sliced aggressive chemicals at elevated temperatures, by sputter coating with either gold or aluminum. off to a thickness of about 500 lm. Thereafter, or the application of molten-salt reagents. For Thin-Section Polarized Light Microscopy. the specimen is ground down to a thickness of the etchants given in Table 3, only general details In general, all nonmetallic materials are suitable about 80 to 100 lm using a diamond grinding can be given in regard to the concentration and for examination by transmission polarized light disk. Because there is always a danger of deep etching time required for a given sample. The microscopy, including the high-performance ce- pullouts and cracking, which then have to be desired level of etching on a sample must be de- ramics (Ref 4). The only limitations are the grain eliminated by the following stages of prepara- termined by trial and error. size of the material and its mechanical behavior. tion, the specimen should not be ground any Oxide Ceramics. Apart from etching with Many ceramics have a very small grain size (1 thinner than 80 lm. Depending on the material very aggressive chemical solutions, thermal lm), which makes LOM examination impossi- being prepared, the specimen is either manually etching in air is also an important method for ble. Because resolution depends on the wave- or semiautomatically fine ground and then pol- developing the microstructures of Al2O3 and length of light, the smallest particle size that can ished. As the thin section nears the end of its ZrO2 ceramics (Ref 6, 7). The polished and “de- be observed is 1 lm. In order for a thin section preparation, it must be handled extremely care- mounted” samples are placed in an air furnace. to be mechanically stable it needs to be about 5 fully and must be repeatedly checked with the Exposure temperature and time are selected so to 30 lm thick. At this thickness, however, the microscope under polarized light. When the tar- that the heat treatment does not alter the micro- grains in fine-grained materials lie over one an- get thickness is achieved—that is, when all the structure. The temperature should be approxi- other, and this multioverlayering results in for- grains, depending on their double refraction, ex- mately 100 to 150 C (180 to 270 F) below the mation of a diffuse image. This can cause diffi- hibit the colors gray, yellow-brown, or blue- sintering temperature. Temperatures that are too culty in differentiating between certain optical green—a thin cover glass is bonded to the sur- high or etching times that are too long may result effects. It is for this reason that a grain size of at face of the specimen for protection, using in grain growth. least 5 lm is preferred. thinned epoxy resin. Generally, chemical solutions are used to de- The preparation of thin sections requires the velop microstructures, grain boundaries, and the use of precision cutting, grinding, and polishing domain structure of other oxide ceramics such as machines. It should be remembered that several Ceramographic Etching BaTiO3 and PZT (see Table 3). Thermal etching square centimeters of specimen must be removed in air can also be used to show the grain bound- by cutting, grinding, and polishing to achieve a Figure 4 presents a review of ceramographic aries, but it will not develop the domain struc- specimen of uniform 5 to 30 lm thickness with etching methods (Ref 1, 5). Three etching meth- ture. If the grain size exceeds 5 lm, the micro- a smooth, polished surface on either side, free of ods can be distinguished: optical, electrochemi- structure of the ceramic material can also be surface breakouts. cal, and physical contrasting. The most fre- revealed with a thin section under polarized In the first stage of preparation, the specimens quently used etching methods for light. An example is shown in Fig. 19 for are treated in the same way as polished sections high-performance ceramics are solution etching, BaTiO3. for incident light microscopy. The ceramic is cut thermal etching, and plasma etching. Plasma Nitride Ceramics. When Si3N4 is prepared and mounted and then ground and polished using etching works only in case of silicon-base ce- with solutions containing concentrated phos- a semiautomatic grinding and polishing ma- ramics (SiC, Si3N4). Details of the etching pro- phoric acid or molten NaOH, the intergranular chine. A roughened glass slide is then bonded to cedure are described below. Thermal etching is grain-boundary phase between the Si3N4 grains the polished surface of the specimen with a drop mostly used for oxide ceramics. Typical etching is dissolved away by chemical attack (Fig. 5–7) of epoxy resin (Fig. 3). To prevent the formation temperatures are 150 C (270 F) below sintering (Ref 8). One disadvantage of these two etchants of pores and air bubbles between the specimen temperature in air. Etching time can vary be- is their poor reproducibility. This is because the and the glass slide, this operation should be car- tween 15 min and some hours, depending on the optimal etching time is extremely dependent on ried out in a vacuum infiltration chamber. Once structure and composition of the grain boundary both the microstructure and the composition of the specimen has fully hardened, which gener- that evaporates during the heat treatment. Ther- the glass phase, which changes with the com- mal etching of nonoxide ceramics requires an position of the additives used. inert atmosphere or an encapsulation of the sam- In contrast to solution etching for Si3N4 ma- ple in a quartz tube. Due to the chemical resis- terials, plasma etching attacks the ceramic-ma- tance of many ceramics, solution etching of pol- trix grains and not the grain-boundary phase ished specimens requires the application of very (Fig. 8 and 9) (Ref 9). Plasma etching is carried

Fig. 3 Ceramographic specimen preparation sequence from top to bottom for a thin section Fig. 4 Etching techniques used in ceramography © 2004 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook Volume 9: Metallography and Microstructures (#06044G) 1060 / Metallography and Microstructures of Ceramics, Composite-Metal Forms, and Special-Purpose Alloys out inside a special chamber, which provides a Table 3 Etchants for ceramics gaseous mixture of CF and O . A radio fre- 4 2 Etchant composition Conditions Comments Uses quency generator emits high-frequency electro- Oxide Ceramics magnetic oscillations and thus produces fluorine radicals inside the reaction chamber. These rad- Thermal etch in air 15–20 min at 1300–1400 C Grain-boundary etchant, Al2O3, pure and with (2370–2550 F) (Etch also small grains additives icals then react with the silicon contained in the temperature is approximately matrix of the ceramic (Si3N4). The intergranular 150 C (300 F) below grain-boundary phase remains unattacked by the sintering temperature.) plasma etching process. The etchants for AlN, H3PO4 (85%) 5 s to 3 min at 250 C (480 F) Grain boundaries are not Al2O3, pure and with uniformly revealed additives given in Table 3, are proven for chemical solu- Spinel and glass phases tion etching and give reproducible results (Ref are dissolved. 10). 100 mL alumina 10–15 min etch polishing on a Identification of the spinal Al2O3 with additives Carbide Ceramics. Several suitable etchants 10 mL Murakami’s hard fiber synthetic cloth phase, glass phase and for different SiC materials exist (Table 3). Mol- solution secondary phases Thermal etch in air 10 min at 1460 C (2660 F) Grain-boundary etchant Na b-Al2O3 ten salt, thermal etching, hot chemical etchants, HBF4 (35%) 1–2 min at 110 C (230 F) Grain-boundary etchant Na b-Al2O3 and a number of Murakami solutions with dif- 10 mL glycerol (87%) 1 min Grain-boundary etchant Al2TiO5 ferent modifications reveal the microstructure by 10 mL HNO3 (65%) attacking the grain-boundary phases (Ref 11). 10 mL HF (40%) 95 mL distilled H2O 5 s to 2 min Grain-boundary etchant BaTiO3 For SiC with 5 to 15% oxide additions, the 3 mL HCl (32) plasma etching technique (see above for Si N ) 2 mL HF (40%) 3 4 can be successfully applied. In this case the SiC Thermal etch in air 30–60 min at 1200 C (2190 F) Grain-boundary etchant BaTiO3 95 mL distilled H2O 5 s to 2 min Grain-boundary etchant PZT (PbZrTiO) grains are attacked, leaving the grain-boundary 4 mL HCl (32%) phase untouched. 1 mL HF (40%) 100 mL distilled H2O 10 s to 5 min Grain-boundary etchant SrTiO3 25 mL HF (40%) 20 mL distilled H2O 5 s to 5 min Grain-boundary etchant SrTiO3, MgTiO3 Classification and Microstructure 20 mL HNO3 (65%) 10 mL HF (40%) of Frequently Used Ceramics 20 mL distilled H2O Seconds to minutes Reveals twins ZnO 1 mL glacial acetic acid 100 mL distilled H2O Seconds to minutes Grain-boundary etchant ZnO Details of the microstructure of common 10 g NaOH structural and functional ceramics along with 10gK3Fe(CN)6 typical applications follows. Thermal etch in air Minutes to 1 h at 1300–1400 C Grain-boundary etchant ZrO2-TZP, -PSZ, -CSZ, (2370–2550 F) (according to -ZTC Aluminum oxide (Al2O3) is the most common ceramic and, depending on the adjusted sintering temperature) H3PO4 (85%) 3 s to 2 min at 250 C (480 F) Grain-boundary etchant ZrO2-TZP, -PSZ, -CSZ, properties, is either used as a structural or a func- -ZTC tional ceramic material (Ref 12). Applications of Nitride Ceramics high-temperature-resistant alumina are cruci- 50 mL distilled H2O 1–3 min at 100 C (212 F) Secondary phase remain AlN-La2O3 bles, tubes, and rods for furnaces. Sealing disks, 50 mL saturated aqueous pumps, thread guides, and other specialized NaOH items are made of alumina, due to its excellent 50 mL cold saturated wear and corrosion resistance. It is used as an aqueous picric acid 10 mL distilled H2O 40–60 min at 100 C (212 F) Grain-boundary etchant AlN-Y2O3 electrical insulator in spark plugs and, taking ad- 10 mL glacial acetic acid vantage also of its high thermal conductivity, as 10 mL HNO3 substrate material for integrated circuits (Ref 50 mL distilled H2O 30 min at 100 C (212 F) Grain-boundary etchant AlN-Al2O3 13). 10 mL 10% aqueous NaOH solution Fully dense and coarse-grained alumina be- 10 mL 10% aqueous comes optically transparent and is used as the potassium ferricyanide lamp tube for high-temperature sodium vapor solution street lamps. It is also biocompatible; therefore, (Modified Murakami’s many prosthetic devices are either made from etchant) Plasma etching 1–5 min, 60–80 W Secondary phase remain Si3N4 alumina or contain alumina components. CF4 gas and O2 gas (2:1) The densification of alumina powder com- H3PO4 (85%) 5–30 min at 250 C (480 F) Grain-boundary etchant Si3N4 pacts is achieved by a sintering process, which Molten NaOH (free of water) 20 s to 3 min at 300–350 C Grain-boundary etchant Si3N4 (570–660 F), 2–3 samples per controls density and grain size (Ref 12). These melt parameters determine the strength characteristics Carbide Ceramics of the final product. Figure 10 shows a cross section of a fluorescent tube with isotropically Plasma etching 3–5 min, 60–80 W Secondary phase remain SiC with 5–15% oxide grown grains with 15 lm average grain size and CF4 gas and O2 gas (1:1) Additions 60 mL distilled H2O 5–30 min at 110 C (230 F) Alpha/alpha (␣/␣) grain SSiC a porosity of less than 0.5%. Figure 11 illustrates boundaries. Alpha/beta a microstructure characterized by anisotropically (␣/b) phase boundaries grown grains with an average size of 2 to 4 lm. 30gK3Fe(CN)6 The resulting texture is caused by a preferred 3 g NaOH 20 mL distilled H2O 5–10 min Grain-boundary etchant SSiC doped with B orientation of alumina seed crystals during in- 10 g NaOH jection molding. The microstructure of a ceramic 10gK3Fe(CN)6 sealing disk with a typical residual porosity of 3 (continued) to 4%, a bimodal grain size distribution, and a glassy phase at the grain boundaries (liquid- © 2004 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook Volume 9: Metallography and Microstructures (#06044G) Preparation and Microstructural Analysis of High-Performance Ceramics / 1061

Table 3 (continued) F). The spinel phase is dissolved, and the grain boundaries look broader. Etchant composition Conditions Comments Uses Zirconium dioxide (ZrO2) based ceramics Carbide Ceramics (continued) belong to those materials that are characterized 80 mL distilled H2O 10–20 min Grain-boundary etchant SSiC doped with Al by having both good mechanical properties and 10 g NaOH exceptional electrical properties (Ref 14, 15). 10gK3Fe(CN)6 90 g KOH 5–10 min at 480C Beta/beta (b/b) grain SSiC The thermomechanical and electrical properties boundaries have led to a wide range of applications. Tough, 10 g KNO3 Melt Preheat the samples. wear-resistant, and refractory ZrO is being de- 2 100 mL distilled H2O 30–40 min at 60 C Grain-boundary etchant SiC with B4C veloped for applications as extrusion dies, ma- 10 mL HNO3 (65%) 10gK3Fe(CN)6 chinery wear parts, and piston heads. Zirconium 100 mL distilled H2O Seconds to minutes, boiling Grain-boundary etchant B doped SiC dioxide is a good ion conductor and, as a solid 10 mL HNO3 (65%) electrolyte, is used in oxygen sensors and in 10gK3Fe(CN)6 solid oxide fuel cells (SOFC). Zirconium dioxide 60 mL distilled H2O 8–15 min Use boiling Grain-boundary etchant SiSiC 3 g NaOH is also used for furnace elements. The low ther- 30gK3Fe(CN)6 mal conductivity of ZrO2 makes it important as a thermal barrier coating in aerospace engine components and land-based gas turbines. The phase sintering) is given in Fig. 12. Figure 13 equiaxed grains with black gaps along the grain different kinds of application require different shows the microstructure of a spark plug with boundaries can be seen. The gaps are pores and properties that are realized by four basic types of anisotropic grains, a glassy grain-boundary voids formed by an evaporated spinel phase microstructure: cubic stabilized zirconia (CSZ), phase, and large pores. The effect of different (MgAl O ) (Fig. 14). The microstructure shown 2 4 partially stabilized zirconia (PSZ), tetragonal zir- etching techniques is demonstrated in Fig. 14 in Fig. 15 is from the same material, but chem- conia polycrystals (TZP), and zirconia tough- and 15. After thermal etching of the magnesium- ically etched in phosphoric acid at 270 C (520 ened ceramics (ZTC). doped sample at 1500 C (2730 F) in air, The CSZ microstructure is a solid solution of ZrO2 and a stabilizing oxide such as Y2O3, MgO, or CaO. These additives maintain the cubic structure down to room temperature. Cubic sta-

Fig. 7 Scanning electron micrograph of hot isostatically Fig. 5 Scanning electron micrograph of a sintered Si3N4 pressed Si3N4 (HIPSN) etched in molten NaOH ceramic (SSN) after etching in phosphoric acid at 320 C (610 F) for 2 min. The intergranular glassy phase at 250 C (480 F) for 30 min. The intergranular glassy between the Si3N4 grains has been attacked by the etchant. Fig. 9 Scanning electron micrograph of hot isostatically phase between the Si3N4 grains has been dissolved by the It is not possible to identify intergranular porosity using this pressed Si3N4 (HIPSN) after plasma etching. etchant. etching technique. HIPSN ceramics show more equiaxed grains and less grain- boundary phase compared to GPSN (see Fig. 8).

Fig. 6 Scanning electron micrograph of a hot-pressed Fig. 8 Scanning electron micrograph of a plasma Si3N4 ceramic (HPSN) etched for 30 min. in etched gas-pressure sintered Si3N4 (GPSN) ce- Fig. 10 Light micrograph of translucent alumina. Iso- phosphoric acid at 250 C (480 F). The volume fraction of ramic with typical acicular Si3N4 crystallites. The micro- tropically grown grains with 15 lm average the intergranular grain boundary phase is lower in com- structure shows gray Si3N4 matrix grains and a lighter grain- grain size and a porosity of less than 0.5%. Thermally parison to SSN (Fig. 5). boundary phase. etched at 1500 C (2730 F) © 2004 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook Volume 9: Metallography and Microstructures (#06044G) 1062 / Metallography and Microstructures of Ceramics, Composite-Metal Forms, and Special-Purpose Alloys

bilized zirconia is manufactured from powder nal and cubic), small lenticular particles of the ficients of the matrix and the ZrO2 particles mixtures or prealloyed powders and homoge- tetragonal phase are precipitated. These particles causes stresses during cooling, which is essential nized completely during sintering in the homo- are not resolved in the LOM. They are coherent to the stability of the dispersoid. The increase in geneous cubic region at temperature between with the cubic {100} planes. The precipitates are toughness results from the strain-induced trans- 1600 and 1800 C (2910 and 3270 F). The mi- shown in the electron microscope image insert formation and microcrack formation (Ref 16). crostructure of stabilized ZrO2 consists of cubic in Fig. 16. Piezoelectric Ceramics. BaTiO3 and PZT grains together with an intergranular glassy Tetragonal zirconia polycrystals contains only (Pb(Ti,Zr)O3) are the most widely used piezo- phase (Fig. 2). As a result of the high processing 2 to 3 mol% Y2O3 (or 10 to 20 mol% CeO2)as electric materials, in electronic devices. The temperatures, a comparatively large grain size in stabilizer and crystallizes in a metastable tetrag- properties can be varied over a wide range by the range 10 to 150 lm is formed. The inter- onal form. However, due to the high activation donor or acceptor dopants to achieve the re- granular glassy phase contains various impuri- energy of the transformation, the tetragonal quired properties for a given application. BaTiO3 ties originating from the manufacturing process structure is retained down to room temperature is mainly used in capacitors because of its high and the raw materials, but these are mainly SiO2. and can be activated for a strain-induced trans- dielectric strength as well as the ability to have Partially stabilized zirconia is a ceramic with formation to the stable monoclinic structure. The a positive or negative temperature coefficient of 5 to 15 mol% of stabilizer additives (Y2O3, tetragonal and monoclinic crystallites (grains) the dielectric by dopant addition and process MgO, CaO). It is sintered in the cubic homoge- both have the same morphology and cannot be procedures. Pb(Ti,Zr)O3 exhibits a high piezo- neity range. The PSZ microstructural develop- distinguished by LOM or SEM, but require x- electric constant and is used for sensors and ac- ment requires a special program of sintering. Ini- ray diffraction. An example of the TZP micro- tuators and for ultrasonic wave generation (Ref tially, the ceramic is compacted at high structure, taken in the SEM, is shown in Fig. 1. 17, 18). temperatures (1700 C) in the cubic solid-so- Zirconia toughened ceramics consist of a ce- The starting powders with the corresponding lution region. A coarse microstructure with cubic ramic matrix (e.g. Al2O3) in which 3 to 15 vol% dopants are usually prepared by a mixed-oxide grains and an intergranular glassy phase forms. of ZrO2 particles are embedded (Fig. 17). The process. The required oxides and/or carbonates By heat treating in the two-phase field (tetrago- difference between the thermal expansion coef-

Fig. 11 Light micrograph of a dental prosthesis. Tex- Fig. 13 Spark plug alumina insulator. The light micro- Fig. 15 Light micrograph of the same sample shown in tured alumina with anisotropically grown graph shows the microstructure with aniso- Fig. 14, but chemically etched in phosphoric grains, with an average size of 2–4 lm and a porosity of tropic grains, a glassy grain-boundary phase, and large acid at 270 C (520 F) for 3 min. The spinel phase is dis- 0.5%. Thermally etched at 1500 C (2730 F) pores. Etch polished with alumina and Murakami’s solution solved, and grain boundaries look broader.

Fig. 16 Partially stabilized zirconia (PSZ) with 7 mol% Fig. 12 Sealing disk of alumina. The light micrograph Fig. 14 Light micrograph of magnesium-doped alu- Y2O3, thermally etched in air at 1300 C (2730 shows typical residual porosity of 3–4%, a bi- mina after thermal etching at 1500 C (2730 F) F). The light micrograph shows a coarse-grained micro- modal grain size distribution, and a glassy phase at the in air for 40 min. Equiaxed grains with black gaps along structure with pores (black dots). The transmission electron grain boundaries (liquid-phase sintering). Thermallyetched the grain boundaries can be seen. The gaps are pores and microscope image insert shows tetragonal precipitates at 1400 C (2550 F) voids formed by an evaporated spinel phase (MgAl2O4). within the cubic grains. © 2004 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook Volume 9: Metallography and Microstructures (#06044G) Preparation and Microstructural Analysis of High-Performance Ceramics / 1063

are first milled and subsequently calcined to observed by chemical etching of polished BaTiO3 mains. A typical microstructure of a PZT ce- tain a single-phase powder. Consolidation of the ceramics samples (Fig. 18) or by using the thin- ramic is shown in Fig. 20. After chemical etch- prereacted powders is achieved by either press- section technique (Fig. 19). ing, the grain boundaries, as well as the domain ing or tape casting. Sintering is performed in air Paraelectric PZT is a solid solution between structures within the grains, are revealed in this or in an oxygen atmosphere at a temperature lead zirconate and lead titanate with a cubic crys- SEM image. range of 1000 to 1350 C (1830 to 2460 F). Both tal structure. Below the Curie temperature it Aluminum Nitride (AlN). High-purity AlN BaTiO3 and Pb(Ti,Zr)O3 crystallize above the transforms into a ferroelectric tetragonal phase has a high thermal conductivity of more than 200 Curie temperature in the cubic perovskite struc- for titanium-rich PZT and ferroelectric rhom- W/m • K together with a low thermal expansion ture and transform into a lower symmetry struc- bohedral phase for zirconium-rich PZT. The Cu- coefficient. These, and other useful properties, ture during cooling. The phase transformation is rie temperature is not well defined and depends make AlN a very suitable material for applica- accompanied by lattice distortion, typically on the zirconium-to-titanium ratio and the tion as a substrate in the electronics industry. about 1% for BaTiO3 and 1.0 to 2.5% for amount of additives. The maximum piezoelec- Aluminum nitride has an hexagonal crystal Pb(Ti,Zr)O3, depending on the zirconium-to-ti- tric constants and coupling factors are achieved structure. Pressureless sintering is impossible for tanium ratio. The lattice distortion generates infor compositions with approximately equal vol- pure AlN powder, because of its covalent bond- ternal stresses, which are reduced by the for- ume fractions of tetragonal and rhombohedral ing. Densification is achieved by liquid-phase mation of domains. PZT (morphotropic PZT). The domain structures sintering with additives. Commonly used addi- The phase transformation from the paraelec- of these materials are more complex than in tives are Y2O3 (Fig. 21), rare earth oxides (Fig. tric cubic phase to the tetragonal ferroelectric BaTiO3 because both crystallographic modifi- 22), or fluorides such as CaF2 and YF3. The ther- phase occurs for BaTiO3 at 130 C (265 F). Be- cations can coexist within one grain, and they mal conductivity of AlN is affected by the pres- low 0 C (32 F) other phase transformations oc- have different possible directions for the spon- ence of both secondary phases and impurities at cur, but they have no technological interest. The taneous polarization vector. Tetragonal PZT the grain boundaries. domain configuration consists of 90 and 180 forms have only 90 and 180 domains, but domains based on the different possibilities for rhombohedral PZT also has 71 and 109 do- the orientation of the spontaneous polarization vector. The domain structures can be readily ob-

Fig. 21 Scanning electron micrograph of an AlN ce- Transmission optical micrograph of a thin sec- ramic, densiﬁed with 10 wt% Y2O3. The AlN Fig. 19 grains appear dark gray. Yttrium-aluminum-garnet parti- tion of a BaTiO3 ceramic with crossed polars Fig. 17 Scanning electron micrograph of a zirconia and a 530 nm ﬁlter. The parallel stripes represent 90 do- cles, formed by reaction between Y2O3 and Al2O3 in the toughened ceramic (ZTC), thermally etched in main walls. starting powder mixture, appear as the light gray phase. air at 1300 C (2730 F). Lighter, tetragonal ZrO2 grains are dispersed in the Al2O3 matrix. The parallel arrays of facets within the grains are related to the ZTC crystallography.

Fig. 22 Scanning electron micrograph of an AlN ce- Fig. 20 Scanning electron micrograph of a lanthanum- ramic, densiﬁed with 2.5 wt% La2O3. Etching Fig. 18 Light micrograph of a chemically etched sur- doped PZT ceramic. The grain boundaries as attack of AlN grains depends on crystallographic orienta- face of a BaTiO3 ceramic showing the grains well as the domain structure are visible after chemical etch- tion. A secondary phase (LaAlO3) is retained in the micro- and domain patterns within the grains ing. structure and appears bright. © 2004 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook Volume 9: Metallography and Microstructures (#06044G) 1064 / Metallography and Microstructures of Ceramics, Composite-Metal Forms, and Special-Purpose Alloys

Silicon nitride (Si3N4) is a structural ceramic ramic by nitriding at temperatures between 1200 aries remain intact and the evidence of residual with excellent mechanical properties, good oxi- and 1400 C (2190 and 2550 F) in the presence porosity is not lost. dation resistance, and good thermal shock be- of H2/N2 gas. Because iron is required as a cat- Silicon carbide (SiC) is the most widely used havior at ambient and high temperatures (Ref 19, alyst, iron silicides can also be formed. They are nonoxide ceramic material with superior high- 20). Si3N4 ceramics can be processed by two dif- visible in the LOM as metallic inclusions. Al- temperature abrasion and corrosion resistance ferent methods: reaction-bonded silicon nitride though the chemical reaction is combined with (Ref 21). Dense SiC ceramics exhibit a high ther- (RBSN) and (pressureless) sintered silicon ni- a volume expansion, fully dense ceramics are not mal conductivity. This is due to its closely tride (SSN). Other variants of the latter are obtained. The porosity can vary between 5 and packed wurtzite structure. The electrical prop- known by the following abbreviations, accord- 15%. Figure 23 shows a typical micrograph of a erties depend on the impurities and vary from ing to their method of manufacture: gas-pressure RBSN ceramic in the unetched condition. the high electrical resistivity of pure silicon car- sintered, GPSN; hot-pressed, HPSN; and hot All high-performance silicon nitride ceramic bide to an electrically conductive material when isostatically pressed silicon nitride, HIPSN. components are densified by liquid-phase sinter- the impurity content increases. Silicon carbide Porous Si3N4 ceramics produced from inex- ing. The most common sintering additives are ceramics can be used under high mechanical and pensive raw materials are used as refractories. MgO, Al2O3,Y2O3, and RE2O3 (rare earth ox- thermal stresses in corrosive atmospheres and in Dense materials with high strength in combina- ides) or mixtures of these materials. A typical aggressive media. Important silicon carbide tion with a high wear resistance are developed polished surface of a sintered silicon nitride with products in the area of chemical and mechanical for cutting tools to machine cast iron. The high- some residual porosity and a few silicide inclu- engineering are wear parts, such as sealing disks, temperature-resistant Si3N4-based ceramics are sions is shown in Fig. 24. The end use of the and components for pumps. Other types of ap- being studied for applications in gas turbines en- Si3N4 and the selected sintering conditions de- plication include kiln supports, heat exchangers, gines and for components in motor vehicles termine the amount and type of sintering addi- or burner nozzles. Electrically conductive SiC (e.g., valves). tives. After densification, the sintering additives ceramics are used as heating elements for fur- RBSN ceramics are made from a silicon pow- form an intergranular phase that significantly af- naces for operation up to 1400 C (2550 F) in der compact, which is converted into a Si3N4 ce- fects the properties of the material. Depending air. on the sintering parameters, the material pro- Four types of SiC ceramics can be produced: duced is SSN, HPSN, GPSN, or HIPSN. Figures recrystallized SiC (RSiC), silicon-infiltrated SiC 5 and 6 compare the microstructure of an SSN (SiSiC), sintered SiC (SSiC), and liquid-phase material with that of an HPSN material after sintered SiC (LPS-SiC). They have significantly etching in phosphoric acid. The different amount different manufacturing routes and therefore dif- of sintering additives in the SSN and HPSN ma- ferent microstructures and properties. Recrystal- terials becomes clearly visible when the grain lized SiC is produced by a powder compact with boundaries are dissolved by chemical attack a bimodal grain size distribution. During heat (large black areas between the grains). In con- treatment, the fine-grained powder evaporates trast, Fig. 8 and 9 compare the microstructure of and condenses at the points of contact with the a GPSN material with that of an HIPSN material large SiC particles. Because no densification oc- after plasma etching, where the Si3N4 matrix curs during heat treatment, a higher volume frac- grains are attacked, not the grain-boundary tion of porosity remains, and this results in a phase. At higher magnification, the HIPSN mi- relatively low strength. Figure 26 shows a typi- crostructure (Fig. 25) shows that the etching at- cal microstructure of an RSiC ceramic. tack is also sensitive to the crystallographic ori- Silicon-infiltrated SiC is produced by an infil- entation. Some Si3N4 grains are deeper etched tration of a SiC/C powder compact with liquid than others. The same sample etched in molten silicon. Part of the silicon reacts with the carbon NaOH at 320 C (610 F) for 2 min is shown in to form fine-grained secondary silicon carbide. Fig. 7. A comparison of both etching techniques A larger volume fraction of silicon remains un- Fig. 23 Light micrograph of a reaction-bonded Si3N4 (RBSN). The unetched specimen contains iron reveals that molten NaOH attacks the grain reacted and reduces the oxidation and corrosion silicides (arrow) and characteristic high porosity (black). boundary, whereas plasma etching attacks the resistance of SiSiC, but increases the strength in matrix. After plasma etching, the grain bound- comparison to RSiC. A typical microstructure of

Fig. 24 Light micrograph of an unetched sintered Si3N4 Fig. 25 Scanning electron micrograph of hot isostati- Fig. 26 Light micrograph of an RSiC ceramic. Gray ar- (GPSN) specimen. The white inclusions are cally pressed Si3N4 (HIPSN) after plasma etch- eas are ␣-grains on which b-SiC has recrystal- iron silicide, and porosity is black. The porosity is much ing, shown at a magnification sufficient to reveal micro- lized, dark areas are pores, and white areas are silicon. lower compared to Fig. 23 (RBSN). porosity. Etchant: modified Murakami’s solution © 2004 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook Volume 9: Metallography and Microstructures (#06044G) Preparation and Microstructural Analysis of High-Performance Ceramics / 1065

the multiphase ceramic SiSiC with the primary minum as sintering additives. The microstruc- ing conditions and so improve the material prop- and secondary SiC and the unreacted silicon is tures can be designed over a wide range, which erties under abnormal stress conditions. A pre- shown Fig. 27. enables components to withstand a variety of dif- determined porosity level can be introduced Densification of SiC ceramics can be achieved ferent conditions. This is demonstrated for during sintering, as shown in Fig. 29. The ma- by solid-state sintering with boron, B4C, or alu- highly loaded wear parts. The grain shape is in- terial behavior under abnormal stress can be fur- fluenced by sintering aids; equiaxed grains can ther improved by introducing graphite as a sec- be generated by the presence of boron (Fig. 28) ondary phase (Fig. 30). or as plates in the presence of aluminum (Fig. Silicon carbide powder compacts can also be 29). Anisotropic grain growth results in a strong densified by liquid-phase sintering (LPS) in the interlocking of the grains and reduces the risk of presence of oxides and nitrides (LPS-SiC). The breakout compared to a microstructure with most common additives are similar to the silicon globular grains. Small grains increase strength, nitride ceramics: Y2O3, rare earth oxides, Al2O3, but large grains have proportionally fewer grain and AlN. After complete densification, most of boundaries and the material becomes less sus- the liquid phase forms an oxide-rich grain- ceptible to grain-boundary corrosion. Small boundary phase, which may be either amorphous grains can be pulled out and ground into the wear or crystalline. This grain-boundary phase leads surface, which can lead to hydrodynamic grain- to a weaker bonding between the grains com- boundary corrosion. The presence of pores re- pared to SSiC ceramics. The weaker interface duces the strength and reliability of the compo- results in an increase in fracture toughness as nent. However, these pores can also serve as well as strength, but the strength at higher tem- lubricant reservoirs under certain wear and slid- perature as well as the creep resistance is reduced. Figure 31 shows a microstructure of LPS- Fig. 27 Light micrograph of an SiSiC microstructure SiC after plasma etching. Two phases are visible: with large ␣-SiC grains and a fine-grained fraction of the b modification. The bright phases represent in- the bright, unattacked grain-boundary phase and filtrated silicon, which densifies the material but also im- the SiC grains. Some of the SiC grains reveal a pairs its corrosion resistance. Etchant: Murakami’s solution darker region within them. This is the remnant of the original SiC seed. The different etching attack between the seed material and the rest of the grain is attributed to a small difference in chemical composition (i.e., solid-solution differences between SiC and AlN).

ACKNOWLEDGMENT The authors would like to thank Chris Bagnall for the helpful discussions and the critical review of the manuscript.

Fig. 30 Light micrograph of a pressureless-sintered REFERENCES SSiC ceramic with a pronounced bimodal platelet microstructure consisting of ␣-SiC grains. The large 1. G. Petzow, Metallographic Etching, 2nd ed., dark spherical particles, approximately 50 lm in diameter, ASM International, 1999 are graphite (arrow). The smaller dark spots within the SiC grains are intragranular pores. Etchant: modified Murak- 2. G. Elssner, H. Hoven, G. Kiessler, and P. Fig. 28 Light micrograph of an SSiC ceramic, pressure- ami’s solution Wellner, Ceramic and Ceramic Composites: less sintered with boron as additive. The mi- Materialographic Preparation, Elsevier, crostructure shows fine equiaxed grains and pores (dark). 1999 Etchant: modified Murakami’s solution 3. W.E. Lee and W.M. Rainforth, Ceramic Mi- crostructures: Property Control by Process- ing, Chapman & Hall, London, 1994 4. U. Ta¨ffner, Transmission Polarised Light Microscopy of High Performance Ceramics, Pract. Metallogr., Vol 38, 1990; p 385–390 5. V. Carle, U. Scha¨fer,U.Ta¨ffner, F. Predel, R. Telle, and G. Petzow, Ceramography of High Performance Ceramics: Description of the Materials, Preparation, Etching Tech- niques, and Description of the Microstruc- ture—Part I, Ceramographic Etching, Pract. Metallogr., Vol 28, 1991, p 359–377 6. V. Carle, B. Trippel, U. Ta¨ffner, U. Scha¨fer, F. Predel, R. Telle, and G. Petzow, Cera- Fig. 31 Scanning electron micrograph of a liquid- mography of High Performance Ceramics: phase sintered SiC ceramic (LPS-SiC) after Description of the Materials, Preparation, plasma etching. The central and edge zones of the gray SiC Etching Techniques, and Description of the Fig. 29 Light micrograph of a pressureless-sintered matrix grains differ in their chemical composition, which Microstructure—Part VIII, Aluminum Ox- SSiC ceramic with a bimodal grain size distri- causes a different etching attack. The light constituent is bution and 3.5% porosity. Etchant: modified Murakami’s the grain-boundary phase, formed by the sintering addi- ide, Pract. Metallogr., Vol 32, 1995, p 54– solution tives. 76 © 2004 ASM International. All Rights Reserved. www.asminternational.org ASM Handbook Volume 9: Metallography and Microstructures (#06044G) 1066 / Metallography and Microstructures of Ceramics, Composite-Metal Forms, and Special-Purpose Alloys

7. U. Scha¨fer, H. Schubert, V. Carle, U. Ta¨ff- mography of High Performance Ceramics: 15. N. Claussen and M. Ru¨hle, Ed., Science and ner, F. Predel, and G. Petzow, Ceramogra- Description of the Materials, Preparation, Technology of Zirconia II, Vol 12, Advances phy of High Performance Ceramics: De- Etching Techniques, and Description of the in Ceramics, American Ceramic Society, scription of the Materials, Preparation, Microstructure—Part IV, Aluminum Ni- 1983 Etching Techniques, and Description of the tride, Pract. Metallogr., Vol 28, 1991, p 16. N. Claussen, Strengthening Strategies for Microstructure—Part III, Zirconium Oxide, 542–552 ZrO2-Toughened Ceramics (ZTC) at High Pract. Metallogr., Vol 28, 1991, p 468– 11. V. Carle, U. Scha¨fer,U.Ta¨ffner, F. Predel, Temperatures, Mater. Sci. Eng., Vol 71, 483 R. Telle, and G. Petzow, Ceramography of 1995, p 23 8. U. Ta¨ffner, V. Carle, U. Scha¨fer, F. Predel, High Performance Ceramics: Description of 17. B. Jaffe, W.R. Cooke, and H. Jaffe, Piezo- and G. Petzow, Ceramography of High Per- the Materials, Preparation, Etching Tech- electric Ceramics, Academic Press, 1971 formance Ceramics: Description of the Ma- niques, and Description of the Microstruc- 18. Y. Xu, Ferroelectric Materials and Their terials, Preparation, Etching Techniques, ture—Part II, Silicon Carbide, Pract. Me- Applications, North Holland, 1991 and Description of the Microstructure—Part tallogr., Vol 28, 1991, p 420–434 19. M.J. Hoffmann, Si3N4 Ceramics, Structure V, Silicon Nitride, Pract. Metallogr., Vol 28, 12. E. Do¨rre and H. Hu¨bner, Alumina, Springer and Properties of, Encyclopedia of Materi- 1991, p 592–610 Verlag, 1984 als Science and Technology, Elsevier Sci- 9. U. Ta¨ffner, M.J. Hoffmann, and M. Kra¨mer, 13. A.J. Moulson and J.M. Herbert, Electrocer- ence Ltd, 2001, p 8469–8471 A Comparison of Different Physical-Chem- amics: Materials, Properties, Applications, 20. F.L. Riley, Silicon Nitride and Related Ma- ical Methods of Etching for Silicon Nitride Chapman and Hall, London, 1990 terials, J. Am. Ceram. Soc., Vol 83 (No. 2), Ceramics, Pract. Metallogr., Vol 27, 1990; 14. A.H. Heuer and L.W. Hobbs, Ed., Science 2000, p 245–265 p 385–390 and Technology of Zirconia, Vol 3, Ad- 21. S. Somiya and Y. Inomata, Silicon Carbide 10. F. Predel, J.P. Bazin, V. Carle, U. Scha¨fer, vances in Ceramics, American Ceramic So- Ceramics—1, Fundamental and Solid Re- U. Ta¨ffner, R. Telle, and G. Petzow, Cera- ciety, 1981 action, Elsevier Applied Science, 1991 ASM International is the society for materials engineers and scientists, a worldwide network dedicated to advancing industry, technology, and applications of metals and materials.

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