Indentation-induced crack interaction in alumina ceramics

Polycrystalline aluminas of tailored composition and grain size, prepared using liquid-phase sintering aids based on the A1 2 O 3 ‒CaO‒SiO 2 and Al 2 O 3 ‒MgO‒SiO 2 systems, have been indented at loads close to their critical fracture limit P*, using closely spaced arrays of low-load Vickers indentations. The propagation of cracks caused by indentation is influenced by internal residual stresses at alumina grain boundaries developed during cooling from the processing temperature, and primarily the result of thermal expansion mismatches between the α-Al 2 O 3 crystals of the matrix and an intergranular glass of composition related to that of the sintering aid. The Al 2 O 3 ‒MgO‒SiO 2 system gives a glass of significantly lower expansivity than α-Al 2 O 3 hoop tensile stresses are generated at the alumina-alumina boundaries which assist microcrack propagation and the intexlinking of lateral cracks from the closely spaced indentations. The converse is the case for the A1 2 O 3 ‒CaO‒SiO 2 system, giving a glass of slightly higher expansivity than alumina, and which generates compressive stresses suppressing crack interlinking. Indentation arrays provide a useful technique for modelling the processes occurring during erosive wear caused by hard particle impact.