INDENTATION-INDUCED CRACK INTERACTION IN ALUMINA CERAMICS

Polycrystalline aluminas of tailored composition and grain size, prepa red using liquid-phase sintering aids based on the Al2O3-CaO-SiO2 and Al2O3-MgO-SiO2 systems, have been indented at loads close to their cri tical fracture limit P, using closely spaced arrays of low-load Vicke rs indentations. The propagation of cracks caused by indentation is in fluenced by internal residual stresses at alumina grain boundaries dev eloped during cooling from the processing temperature, and primarily t he result of thermal expansion mismatches between the alpha-Al2O3 crys tals of the matrix and an intergranular glass of composition related t o that of the sintering aid. The Al2O3-MgO-SiO2 system gives a glass o f significantly lower expansivity than alpha-Al2O3; hoop tensile stres ses are generated at the alumina-alumina boundaries which assist micro crack propagation and the interlinking of lateral cracks from the clos ely spaced indentations. The converse is the case for the Al2O3-CaO-Si O2 system, giving a glass of slightly higher expansivity than alumina, and which generates compressive stresses suppressing crack interlinki ng. Indentation arrays provide a useful technique for modelling the pr ocesses occurring during erosive wear caused by hard particle impact.