MECHANICAL-BEHAVIOR OF MOSI2 REINFORCED-SI3N4 MATRIX COMPOSITES

The mechanical behavior of MoSi2 reinforced-Si3N4 matrix composites wa s investigated as a function of MoSi2 phase content, MoSi2 phase size, and amount of MgO densification aid for the Si3N4 phase. Coarse-phase MoSi2Si3N4 composites exhibited higher room-temperature fracture toug hness than fine-phase composites, reaching values >8 MPa.m(1/2). Compo site fracture toughness levels increased at elevated temperature, Fine -phase composites were stronger and more creep resistant than coarse p hase composites. Room-temperature strengths >1000 MPa and impression c reep rates of similar to 10(-8) s(-1) at 1200 degrees C were observed, Increased MgO levels generally were deleterious to MoSi2-Si3N4 mechan ical properties, Internal stresses due to MoSi2 and Si3N4 thermal expa nsion coefficient mismatch appeared to contribute to fracture tougheni ng in MoSi2-Si3N4 composites.