Phase Relations in the System Si ₃ N ₄ ‐SiO ₂ ‐MgO and Their Interrelation with Strength and Oxidation

Abstract

Phase relation studies of Si 3 N 1 , SiO 2 , and MgO have established three important subsolidus tie lines, viz. Si 3 N 4 ‐MgO, Si 3 N 4 ‐Mg 2 SiO 4 , and Si 2 N 2 O‐Mg 2 SiO 4 for nonoxidizing fabrication conditions. Strength measurements at 1400°C show that optimum strengths are obtained for compositions approaching the Si 3 N 4 ‐MgO and Si 3 N 4 ‐Si 2 N 2 O tie lines and that inferior strengths are obtained for compositions approaching the Si 3 N 4 ‐Mg 2 SiO 4 tie line. Oxidation measurements at 1375°C show that the oxidation kinetics depend on the content of MgO and Mg 2 SiO 4 phases. Optimum oxidation resistance is observed for compositions approaching the Si 3 N 4 ‐Si 2 N 2 O tie line. Strength and oxidation results are discussed with regard to phase equilibrium considerations.