Mechanical Behavior of MoSi ₂ Reinforced–Si ₃ N ₄ Matrix Composites

Abstract

The mechanical behavior of MoSi 2 reinforced–Si 3 N 4 matrix composites was investigated as a function of MoSi 2 phase content, MoSi 2 phase size, and amount of MgO densification aid for the Si 3 N 4 phase. Coarse‐phase MoSi 2 ‐Si 3 N 4 composites exhibited higher room‐temperature fracture toughness than fine‐phase composites, reaching values >8 MP·am 1/2 . Composite fracture toughness levels increased at elevated temperature. Fine‐phase composites were stronger and more creep resistant than coarse phase composites. Room‐temperature strengths >1000 MPa and impression creep rates of ∼10 −8 s −1 at 1200°C were observed. Increased MgO levels generally were deleterious to MoSi 2 ‐Si 3 N 4 mechanical properties. Internal stresses due to MoSi 2 and Si 3 N 4 thermal expansion coefficient mismatch appeared to contribute to fracture toughening in MoSi 2 ‐Si 3 N 4 composites.