Wet erosion of liquid phase sintered alumina

A study has been made of the mechanism of wet erosive wear of polycrystalli ne alumina, The aluminas were prepared with controlled grain size, and cont ained up to 10% by weight of magnesium silicate sintering aid. For material s of grain size < 1 mu m the dominant wear mechanism appeared to be triboch emical, giving polishing with a very tow 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. F or pure alumina materials fracture was predominantly intergranular with cra ck interlinking; for the liquid phase sintered materials fracture was mainl y transgranular. The presence of the magnesium silicate sintering additive decreased the wear rates considerably, compared to pure alumina materials o f the same mean grain size. No correlation was found between wear rates and Vickers indentation hardness and fracture toughness values. However, use o f a depth sensing nanoindentation technique revealed differences between th e pure and the magnesium silicate doped aluminas, with the pure alumina bei ng stiffer and harder. Grain facetting was also a strong feature of the nan oindentation 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 intrag ranular fracture around the indentation crater. It is concluded that the we ar process in alumina materials of mean grain size > 1 mu m is at least par tly dependent on the residual grain boundary stresses arising from the ther mal expansion anisotropy of the alumina grains. The intergranular silicate film has two functions: it effectively strengthens the grain boundaries, an d it increases the compliance of the material, so as to improve its ability to absorb and dissipate impact energy. (C) 1999 Elsevier Science S.A. All rights reserved.