Molybdenum disilicide matrix composites reinforced with Mo2C

Molybdenum disilicide matrix composites containing 5, 15 and 20 vol.% of Mo 2 C were prepared by isostatic pressing and pressure sintering. The addition of Mo 2 C increased the room temperature hardness of MoSi 2 from 8.87 GPa to 10.6-1 1.3 GPa and the indentation fracture toughness from 2.11 MPa√m to 6.56-7.50 MPa√m. Small weight losses observed when the composites were heated for 24 h at temperatures between 800-1300°C were associated with the generation of the volatile oxide MoO 3 . The 5 vol.% Mo 2 C composite exhibited a weight gain of 0.4 % after heating for 24 h at 1500°C, which was associated with the development of a vitreous silica coating. Smaller weight gains observed in 15 and 20 vol.% Mo 2 C samples indicated thinner silica coatings, with a thickness comparable to the coating developed on pure MoSi 2 . A sample of the 15 vol.% composite sintered in argon showed a small amount of low temperature surface degradation (the pest effect) after heating for 247 h at 450°C, but no evidence of the pest effect was observed in a sample of this composition that was previously heated for 24 h at 1500°C, to develop a protective coating of vitreous silica. The heat treatment at 1500°C was also shown to be effective in reducing residual stresses generated during fabrication. An optimum reinforcement of 15 vol.% Mo 2 C gave the maximum increase in room temperature hardness combined with a significant improvement in fracture toughness, and the silica coating develped at 1500°C protected the MoSi 2 matrix from both high temperature oxidation and the low temperature pest effect.