Development and characterization of SiC(f)/MoSi2-Si3N4(p) hybrid composites

Intermetallic compound MoSi2 has long been known as a high temperature mate rial that has excellent oxidation resistance and electrical/thermal conduct ivity. Also its low cost, high melting point (2023 degrees C), relatively l ow density (6.2 g cm(-3) versus 9 g cm(-3) for current engine materials), a nd ease of machining, make it an attractive structural material. However, t he use of MoSi2 has been hindered due to its poor toughness at low temperat ures, poor creep resistance at high temperatures? and accelerated oxidation (also known as 'pest' oxidation) at temperatures between approximately 450 and 550 degrees C. Continuous fiber reinforcing is very effective means of improving both toughness and strength. Unfortunately, MoSi2 has a relative ly high coefficient of thermal expansion (CTE) compared to potential reinfo rcing fibers such as SiC. The large CTE mismatch between the fiber and the matrix resulted in severe matrix cracking during thermal cycling. Addition of about 30-50 vol.% of Si3N4 particulate to MoSi2 improved resistance to l ow temperature accelerated oxidation by forming a Si2ON2 protective scale a nd thereby eliminating catastrophic 'pest failure'. The Si3N4 addition also improved the high temperature creep strength by nearly five orders of magn itude, doubled the room temperature toughness and significantly lowered the CTE of the MoSi2 and eliminated matrix cracking ill SCS-6 reinforced compo sites even after thermal cycling. The SCS-6 fiber reinforcement improved th e room temperature fracture toughness by seven times and impact resistance by five times. The composite exhibited excellent strength and toughness imp rovement up to 1400 degrees C. More recently, tape casting was adopted as t he preferred processing of MoSi2-base composites for improved fiber spacing , ability to use small diameter fibers, and for lower cost. Good strength a nd toughness values were also obtained with fine diameter Hi-Nicalon tow fi bers. This hybrid composite remains competitive with ceramic matrix composi tes as a replacement for Ni-base superalloys in aircraft engine application s. (C) 1999 Elsevier Science S.A. All rights reserved.