The influence of molybdenum silicide additions on high-temperature oxidation resistance of silicon nitride materials

The influence of additions of MoSi2 on the microstructure and the mechanical properties of a silicon nitride material with Nd2O3 and AlN as sintering aids was studied. The composites containing 5, 10 and 17.6 wt per cent MoSi2 were fabricated by hot pressing. All materials exhibit a similar phase composition detected by X-ray diffractometry. Up to 10 wt percent additions of MoSi2, the mechanical properties as like strength, fracture toughness or creep at 1400 deg C were not affected significantly in comparison to the monolithic silicon nitride. The oxidation resistance of the composites in terms of the weight gain was found degradated. After 1000 h oxidation at 1400 and 1450 deg C in air, an about three times higher weight gain was obtained in comparison with the material without MoSi2. Nevertheless, after 1000 h oxidation the degradation in strength of the composites was found considerable less severe than that of the material without MoSi2. Caused by processes at the surface of the S i3N4-material an additional layer was formed preventing the formation of pores, cracks or glassy phase rich areas which are common features of oxidation damage in silicon nitride materials. This surface layer containing Mo5Si3 and Si2ON2 was the result of reactions between MoSi2, Si3N4 and the oxygen penetrating by diffusion into the material during the high-temperature treatment.