Abstract
The effect of Y 2O 3 and Yb 2O 3 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 Y 2O 3 were more pronounced than that with Yb 2O 3, for the same additive content. A wider grain size distribution and a larger average grain size were observed in the specimens with Y 2O 3. The specimens with Yb 2O 3 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.
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