Thermodynamic approach to the vaporization and growth phenomena of SiC ceramics. I. SiC and SiC–SiO2 mixtures under neutral conditions

Matter transport by vaporization and condensation processes during sintering or consolidation of SiC components at high temperature is analysed using thermodynamics of the binary Si–C and ternary Si–C–O systems. The erosion flows due to vaporization and the potential growth flow of SiC are calculated in order to determine the conditions prevailing at the surface of SiC powder grains. Pure SiC vaporization leads to rapid precipitation of carbon at the SiC surface. Vaporization of SiC–SiO2 mixtures under neutral atmospheric conditions or absolute vacuum contributes to the rapid departure of any Si or C impurities first of all, and then silica according to congruent vaporization in the SiC–SiO2 pseudo-binary system. The calculated SiC growth rate by vapour transport is always less than the erosion rate and further subsequent growth of pure SiC cannot be obtained as long as silica co-exists with SiC.