Effect of selected impurities on the high temperature mechanical properties of hot-pressed silicon nitride

Abstract

The effect of impurities on the high temperature mechanical properties of hot-pressed silicon nitride has been determined. Selected impurity additions were made to both relatively pure α-phase and Β-phase silicon nitride starting powders. These powders were hot-pressed to full density using 5 wt % MgO as the pressive additive. The silicon nitride hot-pressed from the α-phase powder exhibited higher strength at both 25 and 1400‡ C than that fabricated from the Β-phase powder. The impurity additions had no effect on the room temperature mechanical properties. The CaO additions had the most significant effect on the high temperature mechanical properties. In both the material hot-pressed from the α-phase and Β-phase powders, increasing CaO additions severely reduced the high temperature strength and increased the amount of non-elastic deformation observed prior to fracture. Although alkali additions (Na2CO3, Li2CO3, K2CO3) also tended to have the same effects as the CaO, the high volatility of these compounds resulted in a much reduced concentration in the hot-pressed material, thus minimizing somewhat their tendency to enhance the high temperature strength degradation. The Fe2O3 and Al2O3 had no apparent effect on the high temperature mechanical properties.