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

Abstract Intermetallic compound MoSi 2 has long been known as a high temperature material that has excellent oxidation resistance and electrical/thermal conductivity. Also its low cost, high melting point (2023°C), relatively low density (6.2 g cm −3 versus 9 g cm −3 for current engine materials), and ease of machining, make it an attractive structural material. However, the use of MoSi 2 has been hindered due to its poor toughness at low temperatures, poor creep resistance at high temperatures, and accelerated oxidation (also known as ‘pest’ oxidation) at temperatures between approximately 450 and 550°C. Continuous fiber reinforcing is very effective means of improving both toughness and strength. Unfortunately, MoSi 2 has a relatively high coefficient of thermal expansion (CTE) compared to potential reinforcing 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 Si 3 N 4 particulate to MoSi 2 improved resistance to low temperature accelerated oxidation by forming a Si 2 ON 2 protective scale and thereby eliminating catastrophic ‘pest failure’. The Si 3 N 4 addition also improved the high temperature creep strength by nearly five orders of magnitude, doubled the room temperature toughness and significantly lowered the CTE of the MoSi 2 and eliminated matrix cracking in SCS-6 reinforced composites even after thermal cycling. The SCS-6 fiber reinforcement improved the room temperature fracture toughness by seven times and impact resistance by five times. The composite exhibited excellent strength and toughness improvement up to 1400°C. More recently, tape casting was adopted as the preferred processing of MoSi 2 -base composites for improved fiber spacing, ability to use small diameter fibers, and for lower cost. Good strength and toughness values were also obtained with fine diameter Hi-Nicalon tow fibers. This hybrid composite remains competitive with ceramic matrix composites as a replacement for Ni-base superalloys in aircraft engine applications.