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Preparation and Microstructural Analysis of High-Performance Ceramics © 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 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 func- tional 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 mi- croscope. 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 ЊF). The scanning electron micrograph shows a fine- air at 1300 ЊC (2730 ЊF). 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 di- amond grain size. Polishing with 1 lm diamond and other microstructural details, ceramic spec- bond shortens service life. Additional criteria
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