Effect of Grain Size on Hertzian Contact Damage in Alumina

The role of microstructural scale on deformation‐micro fracture damage induced by contact with spheres is investigated in monophase alumina ceramics over a range 3–48 μ in grain size. Measurement of a universal indentation stress–strain curve indicates a critical contact pressure ≅5 GPa, above which irreversible deformation occurs in alumina. A novel sectioning technique identifies the deformation 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 microcracks. Below this threshold size, classical Hertzian cone fractures initiate outside the contact circle. Above the threshold, the density and scale of subsurface‐zone microcracks increase dramatically with increasing grain size, ultimately 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 activated. 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 discussed in relation to microstructural design of polycrystalline ceramics.