The influence of sintering additives on the indentation response of liquid-phase-sintered polycrystalline aluminas

Abstract A range of dense polycrystalline liquid-phase-sintered aluminas was prepared by hot pressing submicrometre alumina powder (D 50 = 0.4 μm) with additions of 5 wt% magnesium or calcium silicate. The SiO2-to-MgO or SiO2-to-CaO ratio varied between 10 to 1 and 1 to 10. A systematic study was carried out using a depth-sensing indentation, in order to establish the response of liquid-phase-sintered alumina to low-load point loading, and to measure the microscale hardness and elastic modulus. For comparison, pure undoped alumina was also tested. Repeated loading at 50 and 100 mN with a 90° (cube corner) indenter was applied in order to evaluate the materials' resistance to microcrack initiation and propagation. The residual indentation imprints were examined by scanning electron microscopy. All liquid-phase-sintered specimens were found to be softer and less stiff than the pure alumina. For magnesium-silicate-sintered specimens the hardness and elastic modulus appeared to reach the maximum at a SiO2-to-MgO ratio of 0.5, and then decreased with increasing SiO2-to-MgO ratio. For calcium-silicate-sintered aluminas both the hardness and the modulus increased with increasing SiO2-to-CaO ratio. Materials with high SiO2-to-CaO ratios showed the highest resistance to repeated loading. The pure alumina, despite its high hardness and stiffness, showed low resistance to repeated loading, which is attributed to weaker grain boundaries and relatively easy initiation and propagation of microcracks.