Effect of Y2O3 and Yb2O3 on the microstructure and mechanical properties of silicon nitride

The effect of Y2O3 and Yb2O3 on the microstructure and mechanical properties of silicon nitride was investigated. The effect of heat-treatment on the crystallization of grain boundary phases and on the high-temperature strength was also studied. Sinterability and α to β phase transformation of silicon nitride with Y2O3 were more pronounced than that with Yb2O3, for the same additive content. A wider grain size distribution and a larger average grain size were observed in the specimens with Y2O3. The specimens with Yb2O3 showed higher flexure strength, hardness and fracture toughness owing to a more frequent pullout of grains. Strength retention of silicon nitride at elevated temperatures was attributed to the crystallization and refractoriness of the grain boundary phases.