of complex interactions, but mathematical models of failure mechanisms are limited to simple two-dimensional systems or are based on averages that mask many important details. These models must be extended, and new ones developed, to account for the interaction of atoms in three dimensions. The use of the whole arsenal of characterization tools and a complete understanding of synthesis-processing-structure-property relationships will also be needed. It is anticipated that the latter, in particular, will be aided by artificial intelligence and expert systems and by the development of reliable data bases.

Ceramics

Not so many years ago, ceramics processing comprised predominantly separation and crushing of naturally occurring minerals, followed by sintering. Today, many advanced and even conventional ceramics are produced chemically from pure materials with vapor processing, aqueous precipitation, or sol-gel techniques. Melt processing, melt alloying, and melt refinement—followed sometimes by rapid solidification—are used in producing a wide range of ceramic materials, including abrasives. Hot isostatic pressing, forging, and extrusion of ceramics are now, or will be, playing an important role in forming ceramics.

New processes developed specifically for advanced structure control include many of the above processes. Ultrafine grain sizes, useful in a wide range of materials, are obtained (depending on the part or material) by condensation from the vapor, controlled rolling, heterogeneous nucleation, and electromagnetic stirring. Amorphous or glassy structures are obtained by rapid solidification processing, vapor deposition, electrodeposition, and other solidification processes that operate far from equilibrium. Oriented grains or crystals are obtained by a wide range of processes to control crystallization or recrystallization behavior. Processes have also been developed to control structure at the level of the lattice spacing. Low-dislocation or dislocation-free single crystals are now commonly grown from the melt or are obtained through subsequent processing; much improvement in the quality of such crystals is needed and can be expected in the years ahead.

The appeal of ceramics as structural materials is easy to understand. Ceramics are light, but their compressive strength matches or exceeds that of metals; they can withstand extremely high temperatures; they are exceptionally hard, are resistant to abrasion, and are chemically inert; and they excel as electrical and thermal insulators. If research can add two other properties to this list—tensile fracture toughness and ease of processing—ceramics are likely to become ubiquitous in structural applications.

How rapidly ceramics will achieve their potential will be determined largely by research on processing techniques, which first must overcome the problem of brittleness and then must prove to be economical. One processing re-



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