quirement is either to eliminate defects—the voids, agglomerations, and chemical impurities from which cracks originate—or to toughen ceramics by devising ways to prevent cracks from spreading.
Several approaches yield pure, minute particles, the starting materials needed to achieve a void-free microstructure during synthesis. Attrition, melt atomization, and physical vapor deposition produce such particles without chemical change. Chemical precursors are involved in sol-gel and thermal degradation processes.
Dense ceramic bodies are usually obtained by pressing and sintering of fine particles. Modern processes include hot isostatic pressing and forging, which apply pressure during the sintering process. Dense bodies are also formed by direct casting of the melt and by processes that involve oxidation of a melt to obtain a ceramic or metal-ceramic composite. Infiltration of a ceramic precursor with a melt or vapor is another method used, and vapor forming is yet another. In self-propagating, high-temperature synthesis, reactive species are heated in a mold under pressure and are allowed to heat to a temperature at which they react and densify.
Developing tough, strong ceramics is another important, major goal of much ceramics research today. Transformation toughening is one important method, a practical example of which is seen in ceramics whose structure is composed of at least partly tetragonal zirconia. As a crack begins to grow in such ceramics, the tetragonal structure becomes monoclinic, with a resulting volume change of 3 to 5 percent that arrests crack growth. Fifteenfold increases in toughness have been achieved in such materials, and important structural ceramics and abrasive materials are built on these compositions. Other ways of improving the toughness of ceramics involve achieving and maintaining very small grain sizes within the ceramic body, incorporating controlled and dispersed voids, and synthesizing ceramic-ceramic composites. Toughening mechanisms in a ceramic are illustrated in Figure 3.2. An example of a ceramic composite (silicon carbide in alumina) is shown in Figure 3.3.
Research on new toughening mechanisms, as well as on new processing technologies, offers much promise for the future. Concomitant advances in nondestructive testing will also be needed. X-ray tomography, ultrasound, and other existing nondestructive testing methods must be refined to improve flaw detection, and new methods should be developed for ceramics systems. Probabilistic design methodology can play an important role in predicting the performance of ceramic parts. Modeling efforts, however, will require statistically valid data on crack propagation and the stress behavior of ceramics.
One exciting new development is the growth of diamond or diamondlike materials on the surface of various substrates. Whereas previously diamonds were grown at high pressures and temperatures, it is now possible to grow