α→β Sialon Transformation in Calcium-containing α -SiAlON Ceramics

Recent studies on rare earth densified α-sialon ceramics have shown that the resulting α-sialon product, present either as a single phase or in conjunction with β-sialon, is unstable when heat treated at lower (1350-1600°C) temperatures, and transforms to a mixture of β-sialon plus other crystalline or liquid metal sialon phases. The present paper describes similar studies carried out on calcium-densified α-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 a-sialon products are fully resistant towards α→β transformation when heat treated in the temperature range 1450-1550°C. Whereas α→β transformation in rare earth stabilised α-sialons is influenced by the nature of the rare earth cation, the α-sialon composition, the composition and melting behaviour of the liquid phase and the presence or absence of β-sialon nuclei, transformation in calcium a-sialons appears to be influenced by none of these parameters. Clearly if α→β sialon transformation occurs in this system, the transformation temperature for calcium α-sialons must be below 1450°C, the heat-treatment temperature which has been most frequently used in current research on rare earth densified α-sialons.