Fabrication and microstructure of sialon-bonded silicon carbide

Abstract

The sintering behaviour of ceramic-matrix composites consisting of a sialon matrix and silicon carbide particulate reinforcement has been studied in order to elucidate the densification behaviour and the formation of the microstructure. The materials are produced from a mixture of clay, silicon nitride and yttria sintering additive mixed with 60% of silicon carbide grit. The results lead to a description of the mechanism of phase formation which consists of the following steps: 1. Decomposition of china clay and formation of mullite and silica on heating. 2. Formation of liquid phase from these reaction products, together with excess silica from silicon carbide and yttria sintering additive. 3. Dissolution of silicon nitride in the liquid and precipitation of sialon X and O′ phases. 4. Formation of a high-yttrium, nitrogen glass on cooling. Composites of this kind with porosity of about 14% and zero linear shrinkage on firing, due to the rigid skeleton of carbide particles, are promising candidate materials for high-temperature applications where the microstructure offers the prospect of superior high-temperature strength through the continuous interlocking texture of the X-phase and O′-silicon oxynitride crystalline phases, the carbide skeleton and the high viscosity yttrium-nitrogen glass matrix. ©