Abstract
ABSTRACTAn oxidation study of pure, dense MoSi 2 at 400-600°C and MoSi2/20 vol. % (TiB2, Al2O3, Nb) mixtures at 400–1200°C has been performed. Dense MoSi2 does not fail catastrophically (“pest”) in dry air, wet air, or oxygen, and oxidizes heavily only near 500°C. Heavy oxidation is related to the slow growth of a protective, amorphous SiO2 layer; a low rate of Mo removal as vapor species; and cracking of an initially protective Mo9O26-SiO2 layer. Pre-oxidation of MoSi2 at 1200°C prevents subsequent heavy oxidation at 500°C. MoSi2/TiB2 and MoSi2/A12O3 mixtures oxidize more slowly than MoSi2 for T < 600°C and more rapidly for T > 600°C. Protective oxides are formed on MoSi2/reinforcement mixtures except at 500°C, for which high-temperature pre-oxidation is again effective. MoSi2/Nb mixtures suffer severe mass losses upon oxidation at both 500 and 1200°C owing to rapid oxidation of the Nb and spalling of the product Nb2O5 Only the latter case poses problems for the application of MoSi2-matrix composites as high-temperature materials.
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