Phase Relations and Stability Studies in the Si 3 N 4 ‐SiO 2 ‐Y 2 O 3 Pseudoternary System

Composite powders were hot‐pressed to determine the phase relations in the Si 3 N 4 ‐SiO 2 ‐Y 2 O 3 pseudoternary system. Four quaternary compounds, Si 3 Y 2 O 3 N 4 , YSiO 2 N, Y 10 Si 7 O 23 N 4 , and Y 4 Si 2 O 7 N 2 , were identified. Studies of polyphase and single‐phase materials in this system showed that these 4 compounds are unstable under oxidizing conditions. Materials within the Si 3 N 4 ‐Si 2 N 2 O‐Y 2 Si 2 O 7 compatibility triangle precluded the unstable compounds, and are extremely resistant to oxidation.