LOW-TEMPERATURE OXIDATION OF MOLYBDENUM DISILICIDE

Cyclic oxidation rates of 95 to 97 pct dense, powder-source molybdenum disilicide (MoSi2) in dry air, wet air, and oxygen have been measured between 400-degrees-C and 600-degrees-C. Dense MoSi2 does not disintegrate catastrophically (pest) in these atmospheres for exposure times up to 688 hours. Between 400-degrees-C and 500-degrees-C, Mo and Si oxidize simultaneously to form amorphous SiO2, monoclinic Mo9O26, and vapor-deposited MoO3 plates, and the oxidation rate of MoSi2 in air is influenced by its microstructure, composition, and surface defects. Rapid oxidation obeying a linear rate law occurs over a narrow temperature range near 500-degrees-C, where Mo vapor transport by (MoO3)n species is sufficiently rapid to produce large numbers of surface MoO3 plates but simultaneously is slow enough to allow nucleation and growth of solid Mo oxides in conjunction with SiO2. Addition of water vapor to the oxidant stream at 500-degrees-C retards nucleation and growth of solid Mo oxides by formation of MoO3 . H2O (g), which has a high vapor pressure relative to those of (MoO3)n species. The transition from nonselective oxidation to high-temperature selective oxidation of Si to form a protective SiO2 layer occurs between 500-degrees-C and 550-degrees-C. Preoxidation of MoSi2 at 1200-degrees-C creates a SiO2 barrier layer which prevents further oxidation upon subsequent exposure at 500-degrees-C. The oxidation kinetics and microstructural observations support the model of MoSi2 pest in which oxidation in pores and cracks is required for disintegration. Based on these results, low-temperature oxidation phenomena are not expected to restrict the use of MoSi2 as a high-temperature material.