OXIDATION OF MOSI2 AND COMPARISON WITH OTHER SILICIDE MATERIALS

The oxidation behavior of MoSi2 in three fabrication conditions has been studied in oxygen and in air. The cast material was studied over the temperature range 500-1400-degrees-C, the hot isostatically pressed (HIP) material was studied at 500 and 1000-degrees-C and the single crystals were studied at 500-degrees-C. The cast material exhibited three regimes of behavior. Above 1000-degrees-C a continuous protective silica scale formed. Between 600 and 1000-degrees-C a silica scale formed, but formation of silica within grain boundaries, which are believed to be cracked, was observed. At temperatures near 500-degrees-C accelerated linear oxidation, involving the formation of both Mo and Si oxides was observed and the specimen fragmented into powder ("pested"). The HIP material exhibited two regimes. At 1000-degrees-C a protective silica film formed. At temperatures around 500-degrees-C the HIP material underwent accelerated oxidation but did not fragment. The oxidation of the single crystal was qualitatively the same as that for the HIP material at 500-degrees-C. It was concluded, therefore, that accelerated oxidation is a necessary, but not sufficient, condition for pesting to occur. The pesting of the cast material was concluded to occur by oxidation along pre-existing microcracks in the MoSi2. Preoxidation at 1000-degrees-C was found to be only partially successful in limiting accelerated oxidation during subsequent exposure at 500-degrees-C. The oxidation of TaSi2 was observed to be qualitatively the same as that for MoSi2. However, the high thermodynamic stability and low volatility of Ta2O5 result in much higher temperatures being required for the occurrence of selective oxidation of Si.