Ceramic deformability

Elastic Behavior. Elastic deformation is defined as the reversible deformation that occurs when a load is appHed. Most ceramics deform in a linear elastic fashion, ie, the amount of reversible deformation is a linear function of the appHed stress up to a certain stress level. If the appHed stress is increased any further the ceramic fractures catastrophically. This is in contrast to most metals which initially deform elastically and then begin to deform plastically. Plastic deformation allows stresses to be dissipated rather than building to the point where bonds break irreversibly. 

Ceramics deform plastically more readily at higher temperatures and therefore hardness decreases with increasing temperature according to... 

At room temperature, most metals and ceramics deform in a way which depends on stress but which, for practical purposes, is independent of time ... 

Most nonmetallic materials, such as salts, oxides, and ceramics deform also in such a linear fashion, although in a very small range if the applied force is further increased the compound fractures in a catastrophic manner. 

Superplastic ceramics deform without necking and fail by intergranular cracks that propagate perpendicular to the applied tensile axis. Grain size has been considered as a critical factor in superplasticity. In Fig. 8.88, fracture strain is indicated as a function of flow stress in FG ceramics, while the strain-rate sensitivity, m, remains high. In Fig. 8.88, various ceramics are plotted, (the reader is referred to the original works) ... 

Creep. The phenomenon of creep refers to time-dependent deformation. In practice, at least for most metals and ceramics, the creep behavior becomes important at high temperatures and thus sets a limit on the maximum appHcation temperature. In general, this limit increases with the melting point of a material. An approximate limit can be estimated to He at about half of the Kelvin melting temperature. The basic governing equation of steady-state creep can be written as foUows ... 

Boltzmann s constant, and T is tempeiatuie in kelvin. In general, the creep resistance of metal is improved by the incorporation of ceramic reinforcements. The steady-state creep rate as a function of appHed stress for silver matrix and tungsten fiber—silver matrix composites at 600°C is an example (Fig. 18) (52). The modeling of creep behavior of MMCs is compHcated because in the temperature regime where the metal matrix may be creeping, the ceramic reinforcement is likely to be deforming elastically. 

R. C. Bradt and R. E. Tressler, eds.. Deformation of Ceramic Materials, Proceedings of the 1974 Symposium, Plenum Press, New York, 1975. 

Plastic Forming. A plastic ceramic body deforms iaelastically without mpture under a compressive load that produces a shear stress ia excess of the shear strength of the body. Plastic forming processes (38,40—42,54—57) iavolve elastic—plastic behavior, whereby measurable elastic respoase occurs before and after plastic yielding. At pressures above the shear strength, the body deforms plastically by shear flow. 

The overriding concern with regard to the mechanical performance of ceramics is their brittieness and hence sensitivity to flaws. There is usually htfle or no warning that failure is imminent because deformation strain prior to failure is usually less than 0.1%. As a result, a primary thmst of stmctural ceramics research has been the development of tougher ceramics. Ceramics now exist that have toughness values of 20 and strengths that... 

This mechanism of crack inhibition is almost unique among ceramic systems, which do not undergo the plastic deformation under stress which is found in metallic systems (Figure 7.3). 

This competition between mechanisms is conveniently summarised on Deformation Mechanism Diagrams (Figs. 19.5 and 19.6). They show the range of stress and temperature (Fig. 19.5) or of strain-rate and stress (Fig. 19.6) in which we expect to find each sort of creep (they also show where plastic yielding occurs, and where deformation is simply elastic). Diagrams like these are available for many metals and ceramics, and are a useful summary of creep behaviour, helpful in selecting a material for high-temperature applications. 

Ceramics, on the other hand, often deform predominantly by diffusional flow (because their grains are small, and the high lattice resistance already suppresses power-law creep). Special heat treatments to increase the grain size can make them more creep-resistant. 

In compression, of course, the strength is greater. Most ceramics are about fifteen times stronger in compression than in tension, for the reasons given in Chapter 17. For ice the factor is smaller, typically six, probably because the coefficient of friction across the crack faces (which rub together when the ceramic is loaded in compression) is exceptionally low. At stresses below 6 MPa, ice loaded in compression deforms by creep at 6 MPa it crushes, and this is the maximum stress it can carry. 

Frost, H.J. and Ashby, M.F. (1982) Deformation-Mechanism Maps The Plasticity and Creep of Metals and Ceramics (Pergamon Press, Oxford). 

In 1986, Wakai et al. (1986) in Japan discovered that ultra-fine-grained ceramics can also be superplastically deformed they may be brittle with respect to dislocation... 

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