Effect of Selected Impurities on the High Temperature Mechanical Properties of Hot-Pressed Silicon Nitride
WESTINGHOUSE RESEARCH AND DEVELOPMENT CENTER PITTSBURGH PA
Pagination or Media Count:
The strength of commercial hot-pressed silicon nitride has been shown to degrade severely at elevated temperatures. As the temperature is increased the amount of non-elastic deformation occurring prior to fracture has been found to increase significantly. This reduction in high temperature strength and creep resistance is believed to be associated with impurities. Selected impurities were therefore added as oxides or carbonates Al2O3, CaO, Fe2O3, Li2CO3 and Na2CO3 to both a relatively pure alpha-phase silicon nitride powder and a relatively pure beta-phase silicon nitride starting powder. These powders were then hot-pressed to full density using 5 wt MgO as the pressing additive. The silicon nitride hot-pressed from the alpha-phase powder exhibits higher strength at both 25C and 1400C than that fabricated from the beta-phase powder. The impurity additions had no effect on the room temperature mechanical properties.
- Ceramics, Refractories and Glass