Thermal Stability of Calcium α-sialon Ceramics

The lack of thermal stability of some rare earth α-SiAlON phases has recently raised serious doubts concerning the high temperature behaviour of such materials. As information on the Ca-SiAlON system is minimal this work investigates the thermal stability behaviour of Ca α-SiAlON (α′). Compositions near the α′ plane in the Ca-Si-Al-O-N system were prepared by hot-pressing. These materials were subsequently heat treated at temperatures between 1100 and 1500°C for 24 h and for an extended duration, 200h, at 1450°C. The addition of glass prior to hot-pressing to various compositions enabled the investigation of its effect on the initial phase assemblage and the thermal stability behaviour. It was found that neither the excess glass nor the heat treatment devitrification products, destabilised the Ca α′ phase. The presence of β′ grains appeared to facilitate the transformation behaviour of the Ca α′, however a single crystalline phase Ca α′ with a high value of m was thermally stable under extended heat treatment at 1450°C.