THERMODYNAMIC ANALYSIS FOR PHASE EQUILIBRIUM OF MC(B)-C-O SYSTEM IN ULTRA-HIGH TEMPERATURE CONDITIONS

△G0 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△G~0-T diagrams,equilibrium partial oxygen pressures were obtained accordingly.The volatility diagrams for ZrC-ZrO_2 system and SiC-SiO_2 system at 2500 K were drawn and the function of the partial press of various related products in the ZrC-ZrO_2 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-ZrO_2 and SiC-SiO_2 systems,the vapour pressure of related oxides may be very high at the interfaces,even higher than 10~5 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.