Grain-boundary structure/viscosity in high-purity SiAlONs

The changes of ceramics properties are influenced by a microstructure, name ly grain size and grain boundary. Available silicon nitride ceramic powder is usually covered with very thin silicon oxide layer. This silicon oxide i s the origin of a glass film at grain boundaries. The purpose of this paper is to produce the new SiAlON materials (Si6-ZAlZOZN8-Z) which were control led glass film. SiAlONs (with Z = 1, 2, 3) were hot isostatically pressed ( HIPed) at 2273 K under an argon-gas pressure of 180 MPa using high-purity S iAlON powders (Ube Industries). We can obtain SiAlON ceramics without glass film at grain boundary. Microscopy studies revealed the presence of a 15R SiAlON phase precipitated at the grain boundaries or triple-grain junctions of Z = 2, 3 SiAlONs. Z = 1 SiAlON revealed presence of glass-SiO2 grain bo undary structure in which a conspicuous amount of Al was found. The viscous behavior of grain boundary at high temperature was studied by the internal friction method. Internal friction data collected up to very high temperat ures showed no grain boundary relaxation peak in the Z = 2, 3 SiAlON materi als, and a relaxation peak centered at a rather low temperature in the Z = 1 SiAlON.