Calculation of heterogeneous phase equilibria in the system Si-Mg-N-O

Abstract

Abstract The thermodynamic calculations of phase equilibria especially refer to the pseudoternary system Si3N4-Si2N2O-Mg2SiO4. With the model, the liquid phase is treated as a mixture of the quasicomponents 1 7 Si3N4, 1 3 SiO2 and 1 2 MgO, for which an eutectic point was calculated at 1510°C with the composition 0·153 Si3N4+0·194 Si2N2O+0·653 Mg2SiO4. For the equilibria Si3N4-liquid phasegaseous phase the partial pressures of Mg and SiO were calculated in dependence on temperature, nitrogen partial pressure and composition of the liquid phase. The results of thermodynamic calculations and experimental studies of sintered Si3N4 materials with MgO additives are used to interpret the development of microstructure and the mechanical behaviour at high temperatures.