Abstract
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.

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