Impurity-Enhanced Intergranular Cavity Formation in Silicon Nitride at High Temperatures

The effect of trace impurities on high‐temperature strength is examined in Si3N4 sintered without additives. Strength degradation above 1000°C occurs only in the low‐purity material, which also exhibits an intergranular slow crack growth (SCG) rate 2 orders of magnitude faster than that of the high‐purity material at 1400°C. A relaxation peak of internal friction is observed only in the low‐purity material, at ≅1200°C, and the origin of this peak is ascribed to the initial stage of SCG—that is, the cavity‐nucleation stage, enhanced by impurities. Based on the present results, a model for impurity‐enhanced SCG is proposed.