Effect of Oxygen Impurities on the Phase Composition of Self-Propagating High-Temperature Synthesis Products in the α-Si3N4–MgO System

Abstract

It has been shown that raising the oxygen impurity concentration in starting-mixture components from 0.5 to 2.7 wt % reduces the content of the α-phase form 98 to 83 wt % at synthesis temperatures between 1400 and 1550°C. The role of magnesium oxide as a catalyst of the α → β phase transition becomes more important as the oxygen impurity concentration rises to 2.5 wt % and above. The use of starting-mixture components containing less than 0.3–0.6 wt % oxygen impurities has enabled the synthesis of Si₃N₄–MgO composites containing up to 95 wt % α-Si₃N₄ at temperatures in the range 1600–1700°C.