New post-sintering treatments for improved high-temperature performance Si3N4-based ceramics

Previous attempts to minimise the deleterious effects of grain-boundary glass on the high-temperature mechanical performance of nitrogen ceramics have focused mainly on post-sintering devitrification treatments, which leave a minimum of high-viscosity, uncrystallised glass in grain-boundaries. The maximum useful operating temperature of the resulting ceramics is typically in the range 1300 - 1400 degrees C, determined principally by the relevant matrix - grain-boundary phase eutectic temperature. The present paper focuses on recent attempts to totally remove grain-boundary phase in volatile form, by carrying out post-sintering heat-treatments in either vacuum (VHT) or hydrogen (HHT), at temperatures where the matrix is stable, but the liquid grain boundary phase is carbothermally reduced to volatile species. >99% dense final products were obtained when silicon nitride hot-pressed with 0.5-2% MgO was heat-treated in either vacuum or hydrogen; lower final densities were obtained for pressureless sintered materials. The process is not as successful when applied to sialon ceramics, because aluminium cannot be as readily removed in volatile form at temperatures below the decomposition temperature of the sialon matrix.