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

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.