Phase Relations in the System Si3N4-SiO2-MgO and Their Interrelation with Strength and Oxidation

Phase relation studies of Si₃N₁, SiO₂, and MgO have established three important subsolidus tie lines, viz. Si₃N₄-MgO, Si₃N₄-Mg₂SiO₄, and Si₂N₂O-Mg₂SiO₄ for nonoxidizing fabrication conditions. Strength measurements at 1400°C show that optimum strengths are obtained for compositions approaching the Si₃N₄-MgO and Si₃N₄-Si₂N₂O tie lines and that inferior strengths are obtained for compositions approaching the Si₃N₄-Mg₂SiO₄ tie line. Oxidation measurements at 1375°C show that the oxidation kinetics depend on the content of MgO and Mg₂SiO₄ phases. Optimum oxidation resistance is observed for compositions approaching the Si₃N₄-Si₂N₂O tie line. Strength and oxidation results are discussed with regard to phase equilibrium considerations.