Thermodynamic analysis for phase equilibrium of MC(B)-C-O system in ultra-high temperature conditions

ΔG⁰-T diagrams of the reactions between some typical transition metal carbides (or borides) and oxygen were drawn using Matlab software. On the basis of these ΔG⁰-T diagrams, equilibrium partial oxygen pressures were obtained accordingly. The volatility diagrams for ZrC-ZrO² system and SiC-SiO² system at 2500 K were drawn and the function of the partial press of various related products in the ZrC-ZrO² system with temperature was evaluated. The calculation results indicated that the oxygen activity and equilibrium partial oxygen pressure of these carbides changed with temperature. According to the volatility diagrams of ZrC-ZrO² and SiC-SiO² systems, the vapour pressure of related oxides may be very high at the interfaces, even higher than 10₅ Pa, which is detrimental to the formation of entire oxide scales on carbides during ultra-high temperature oxidation. This work provided significant insight into the behaviour of ultra-high temperature oxidation of C/C composites and ultra-high temperature ceramics. It especially provided theoretical considerations for choosing oxidation-resistance dopants in C or C/C composites.