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

The influence of additions of molybdenum disilicide (MoSi2) on the microstructure and the mechanical properties of a silicon nitride (Si3N4) material, with neodymium oxide (Nd2O3) and aluminum nitride (AIN) as sintering aids, was studied. The composites, containing 5, 10, and 17.6 wt% MoSi,, were fabricated by hot pressing. All materials exhibited a similar phase composition, detected by X-ray diffractometry. Up to MoSi2 additions of 10 wt%, mechanical properties such as strength, fracture toughness, or creep at 1400 degrees C were not affected significantly, in comparison to that of monolithic Si3N4. The oxidation resistance of the composites, in terms of weight gain, degraded. After 1000 h of oxidation at 1400 degrees and 1450 degrees C in air, a greater weight gain (by a factor of approximately three)was obtained, in comparison to that of the material without MoSi2. Nevertheless, after 1000 h of oxidation, the degradation in strength of the composites was considerably less severe than that of the material without MoSi2. An additional layer was formed, caused by processes at the surface of the Si3N4 material, preventing the formation of pores, cracks, or glassy-phase-rich areas, which are common features of oxidation damage in Si3N4 materials. This surface layer, containing Mo5Si3 and silicon oxynitride (Si2ON2), was the result of reactions between MoSi2, Si3N4, and the oxygen penetrating by diffusion into the material during the high-temperature treatment.