Wet erosion of liquid phase sintered alumina

Abstract

A study has been made of the mechanism of wet erosive wear of polycrystalline alumina. The aluminas were prepared with controlled grain size, and contained up to 10% by weight of magnesium silicate sintering aid. For materials of grain size <1 μm the dominant wear mechanism appeared to be tribochemical, giving polishing with a very low material removal rate. For coarser grain size materials the wear mechanism appeared to involve microfracture initiation and propagation, leading to partial or complete grain removal. For pure alumina materials fracture was predominantly intergranular with crack interlinking; for the liquid phase sintered materials fracture was mainly transgranular. The presence of the magnesium silicate sintering additive decreased the wear rates considerably, compared to pure alumina materials of the same mean grain size. No correlation was found between wear rates and Vickers indentation hardness and fracture toughness values. However, use of a depth sensing nanoindentation technique revealed differences between the pure and the magnesium silicate doped aluminas, with the pure alumina being stiffer and harder. Grain facetting was also a strong feature of the nanoindentation damage zones in the case of pure alumina, providing supporting evidence that crack development predominantly followed the grain boundaries. Magnesium silicate densified materials, in contrast, showed mainly intragranular fracture around the indentation crater. It is concluded that the wear process in alumina materials of mean grain size >1 μm is at least partly dependent on the residual grain boundary stresses arising from the thermal expansion anisotropy of the alumina grains. The intergranular silicate film has two functions: it effectively strengthens the grain boundaries, and it increases the compliance of the material, so as to improve its ability to absorb and dissipate impact energy.