Effect of Grain Size on Hertzian Contact in Alumina Ceramics

The role of microstructural scale on deformation-micro-fracture damage induced by contact with spheres is investi-gated in monophase alumina ceramics over a range 3-48 pm in grain size. Measurement of a universal indentation stress-strain curve indicates a critical contact pressure-5 GPa, above which irreversible deformation occurs in alu-mina. A novel sectioning technique identifies the deforma-tion elements as intragrain shear faults, predominantly crystallographic twins, within a confining subsurface zone of intense compression-shear stress. The twins concentrate the shear stresses at the grain boundaries and, above a threshold grain size, initiate tensile intergranular micro-cracks. Below this threshold size, classical Hertzian cone fractures initiate outside the contact circle. Above the threshold, the density and scale of subsurface-zone micro-cracks increase dramatically with increasing grain size, ulti-mately dominating the cone fractures. The damage process is stochastic, highlighting the microstructural discreteness of the initial deformation field; those grains which lie in the upper tail of the grain-size distribution and which have favorable crystallographic orientation relative to local shear stresses in the contact field are preferentially acti-vated. Initial flaw state is not an important factor, because the contact process creates its own flaw population. These and other generic features of the damage process will be dis-cussed in relation to microstructural design of polycrystal-line ceramics. I