THERMAL-STABILITY OF CALCIUM ALPHA-SIALON CERAMICS

The lack of thermal stability of some rare earth alpha-SiAlON phases h as recently raised serious doubts concerning the high temperature beha viour of such materials. As information on the Ca-SiAlON system is min imal this work investigates the thermal stability behaviour of Ca alph a-SiAlON (alpha'). Compositions near the alpha' plane in the Ca-Si-Al- O-N system were prepared by hot-pressing. These materials were subsequ ently, hear treated at temperatures between 1100 and 150O degrees C fo r 24h and for an extended duration, 200 h, at 1450 degrees C. The addi tion of glass prior to hot-pressing to various compositions enabled th e investigation of its effect on the initial phase assemblage and the thermal stability behaviour. It was found that neither the excess glas s nov the heat treatment devitrification products, destabilised the Ca alpha' phase. The presence of beta' grains appeared to facilitate the transformation behaviour of the Ca alpha', however a single crystalli ne phase Ca alpha' with a high value of m was thermally stable tinder extended heat treatment at 1450 degrees C. (C) 1998 Elsevier Science L imited.