Quasi-ductile mechanisms in porous liquid-phase sintered alumina induced by Hertzian contact.

Hertzian indentation has been effective in evaluating material response and deformation behavior through single and repeated contacts of a hard sphere into a representative bulk sample in laboratory conditions. Using this technique, the macroscopic and microscopic deformation characteristics of a commercial alumina substrate were evaluated. Significant 'quasi-ductile' behavior was observed, not unlike that observed for other advanced ceramic systems with heterogeneous microstructures. In pure dense alumina, quasi-ductility is controlled by twin fault formation where a transition from a fine grained to a coarse grained microstructure corresponds to a change from classical cone-crack behavior to a purely quasi-ductile indentation response. The quasi-ductility in the commercial alumina was unexpected because the average grain size was very small---well below the size where one should expect any contribution from a twin faulting mechanism.;Subsequent work focused on reproducing the commercial microstructures and then altering the grain size, porosity, and presence of the glassy (liquid) phase. Macroscopic indentation revealed a quasi-ductile residual impression formed prior to the observation of ring crack formation in the porous liquid phase sintered materials. Furthermore, the glass containing samples produced a deeper residual impression for an equivalent load and porosity level. Fully dense samples with or without a glass phase remained completely brittle. Subsurface images corresponded to the macroscopic observations; porous liquid phase materials with a 5 mum grain size revealed greater microstructural damage with increasing loads over that of the pure material.;A 2D theoretical treatment of the problem used finite element modeling and periodic boundary conditions to understand the magnifying effect of multiple pores on the stress around a given pore in a biaxial compressive stress state linked to the Hertzian stress at yield. A periodic pore structure was assumed to simplify the modeling. Peak stress was a function of the pore network orientation. The results were in agreement with previously published analytical work at the same orientations. When the pore network was tilted at 45° to the applied stress directions an increase of the peak stress around a pore was observed with increasing porosity. An attempt to randomize the pore network revealed only small changes in the peak stress. Therefore, a linear stress magnification function determined from the FEM results was incorporated into a previously published stress intensity factor solution for an individual pore/flaw arrangement. The stress concentrating effect of the subsurface pores were related to the macroscopic indentation driving force of the Hertzian indenter. Predictions of the critical mean indentation load to initiate subsurface damage were in reasonable agreement with published results. We conclude that porosity acted as a stress concentrator and was the controlling mechanism for quasi-ductility under Hertzian contact in the liquid phase sintered alumina.