Fine-structure superplastic ceramics

Abstract

Despite extensive studies of superplastic behavior in metallic systems since the 1960s, work on superplasticity in ceramics and ceramic composites is of very recent origin. This is primarily because ceramics normally fracture intergranularly at low strain values, as a result of a weak grain-boundary cohesive strength. The low grain-boundary cohesive strength is a result of inherent high grain-boundary energy. Research in superplastic ceramics began actively only in the late 1980s but has expanded very rapidly since then. The ceramics and ceramic composites made superplastic to date are essentially based on the principles developed for metallic alloys. Existing data indicate that for polycrystalline ceramics, however, a grain size of less than 1 μm is necessary for superplastic behavior. This is in contrast to superplastic metals, in which grain sizes are typically only required to be less than 10 μm. To highlight the dominant effect of grain size on the deformation behavior of ceramics, Figure 6.1 shows the modulus-compensated flow stresses measured from a number of studies on tetragonal zirconia as a function of diffusivity-compensated strain rate. It is evident that for a given stress, the strain rate increases dramatically as grain size decreases. (Or, conversely, that for a given imposed strain rate the stress required decreases dramatically as grain size decreases.) Figure 6.1 illustrates the importance of grain-boundary-sliding (GBS) mechanisms in the deformation of fine-grained ceramics.