Stability of Phases in the Si‐C‐N‐O System

Abstract

The stability of the phases in equilibrium is calculated for the Si‐C‐N‐O system in order to analyze and predict the reactions in ceramic whisker formation and sintering of silicon nitride composites. Equilibria among SiC, Si3N4, Si2N2O, SiO2, Si, and the gas phase are evaluated at different carbon activities, nitrogen pressures, and temperatures. Phase stability diagrams are constructed as a function of nitrogen and oxygen pressures for two levels of carbon activity. Silicon nitride becomes a stable phase with increasing nitrogen pressure or decreasing carbon activity and temperature, whereas silicon carbide becomes a dominant phase at lower nitrogen pressures or at higher temperatures when carbon activity is unity. The maximum sintering temperature of the SiC/Si3N4 composite is higher with an elevated nitrogen pressure or a reduced carbon activity.