EFFECT OF GRAIN-SIZE ON HERTZIAN CONTACT DAMAGE IN ALUMINA

The role of microstructural scale on deformation-microfracture damage induced by contact with spheres is investigated in monophase alumina c eramics over a range 3-48 mum in grain size. Measurement of a universa l indentation stress-strain curve indicates a critical contact pressur e almost-equal-to 5 GPa, above which irreversible deformation occurs i n alumina. A novel sectioning technique identifies the deformation ele ments as intragrain shear faults, predominantly crystallographic twins , within a confining subsurface zone of intense compression-shear stre ss. The twins concentrate the shear stresses at the grain boundaries a nd, above a threshold grain size, initiate tensile intergranular micro cracks. Below this threshold size, classical Hertzian cone fractures i nitiate outside the contact circle. Above the threshold, the density a nd scale of subsurface-zone microcracks increase dramatically with inc reasing grain size, ultimately dominating the cone fractures. The dama ge process is stochastic, highlighting the microstructural discretenes s of the initial deformation field; those grains which lie in the uppe r tail of the grain-size distribution and which have favorable crystal lographic orientation relative to local shear stresses in the contact field are preferentially activated. Initial flaw state is not an impor tant factor, because the contact process creates its own flaw populati on. These and other generic features of the damage process will be dis cussed in relation to microstructural design of polycrystalline cerami cs.