The effect of additives and impurities on the mechanical properties of silicon oxynitride ceramics was investigated. The toughening of the ceramics was affected by three factors: (1) the thermal tensile stress in the intergranular glassy phase and the compressive stress in Si 2 N 2 O grains, developed by a thermal expansion mismatch between Si 2 N 2 O grains and the intergranular glassy phase; (2) the large grain size of Si 2 N 2 O; and (3) the concentration of impurities in the intergranular glassy phase. The degradation of high-temperature strength was primarily dependent on the basic chemical composition (the kind of additives (MeO x ) and Me/Si ratio) of the intergranular phase and impurities.
