Synthesis of a MoSi2SiC composite in situ using a solid state displacement reaction

Abstract A high strength in-situ composite of MoSi2SiC was synthesized using a solid state displacement reaction between Mo2C and silicon. Diffusion couples between Mo2C and silicon processed at 1200°C revealed the formation of aligned SiC platelets in an MoSi2 matrix. The reaction zone of this couple had a Vickers microhardness of 12.8 GPa (HV 1000). In-situ composites were also synthesized by blending Mo2C and silicon powders and vacuum hot pressing for 2 h at 1350 °C followed by 1 h at 1700 °C. The resulting microstructure consisted of 30 vol.% SiC particles 1 μm in diameter uniformly dispersed in a fine-grained MoSi2 matrix. Densities of 5.53 g cm−3 were obtained together with a microhardness of 14.2 GPa (HV 1000). Bend bars and chevron-notched bars cut from large-diameter, and slightly less dense, hot-pressed disks revealed a strength of 475 MPa had a fracture toughness of 6.7 MPa m 1 2 at room temperature. Bend strengths increased to 515 MPa at 1000 °C and then decreased to 112 MPa at 1200 °C. Measured fracture toughness increased to 10.5 MPa m 1 2 at 1050 °C. Fractography revealed that the MoSi2 grain size was on the order of 1–2 μm, and it was suggested that the observed SiC particle size and aspect ratio could result in ineffective dislocation pinning and relatively rapid recovery at temperatures above the ductile-to-brittle transition temperature of MoSi2. This was substantiated by comparing these results with those obtained for SiC-whisker-reinforced MoSi2 composites.