Influence of Molybdenum Silicide Additions on High‐Temperature Oxidation Resistance of Silicon Nitride Materials

The influence of additions of molybdenum disilicide (MoSi 2 ) on the microstructure and the mechanical properties of a silicon nitride (Si 3 N 4 ) material, with neodymium oxide (Nd 2 O 3 ) and aluminum nitride (AIN) as sintering aids, was studied. The composites, containing 5, 10, and 17.6 wt% MoSi 2 , were fabricated by hot pressing. All materials exhibited a similar phase composition, detected by X‐ray diffractometry. Up to MoSi 2 additions of 10 wt%, mechanical properties such as strength, fracture toughness, or creep at 1400°C were not affected significantly, in comparison to that of monolithic Si 3 N 4 . The oxidation resistance of the composites, in terms of weight gain, degraded. After 1000 h of oxidation at 1400° and 1450°C in air, a greater weight gain (by a factor of approximately three) was obtained, in comparison to that of the material without MoSi 2 . Nevertheless, after 1000 h of oxidation, the degradation in strength of the composites was considerably less severe than that of the material without MoSi 2 . An additional layer was formed, caused by processes at the surface of the Si 3 N 4 material, preventing the formation of pores, cracks, or glassy‐phase‐rich areas, which are common features of oxidation damage in Si 3 N 4 materials. This surface layer, containing Mo 5 Si 3 and silicon oxynitride (Si 2 ON 2 ), was the result of reactions between MoSi 2 , Si 3 N 4 , and the oxygen penetrating by diffusion into the material during the hightemperature treatment.