α→β sialon transformation in calcium-containing α-SiAlON ceramics

Recent studies on rare earth densified a-sialon ceramics have shown that the resulting alpha-sialon product, present either as a single phase or in conjunction with beta-sialon, is unstable when heat treated at lower (1350-1600 degrees C) temperatures, and transforms to a mixture of p-sialon plus other crystalline or liquid metal sialon phases. The present paper describes similar studies carried out on calcium-densified alpha-sialon compositions, and shows that for a wide range of starting compositions, with calcium as the sole sintering additive or present with other (Nd, Sr) cations, the resulting calcium stabilised alpha-sialon products are fully resistant towards alpha-->beta transformation when heat treated in the temperature range 1450-1550 degrees C. Whereas alpha-->beta transformation in rare earth stabilised alpha-sialons is influenced by the nature of the rare earth cation, the alpha-sialon composition, the composition and melting behaviour of the liquid phase and the presence or absence of beta-sialon nuclei, transformation in calcium alpha-sialons appears to be influenced by none of these parameters. Clearly if alpha-->beta sialon transformation occurs in this system, the transformation temperature for calcium alpha-sialons must be below 1450 degrees C, the heat-treatment temperature which has been most frequently used in current research on rare earth densified alpha-sialons. (C) 1999 Published by Elsevier Science Limited. All rights reserved.