A gravimetric and microscopical study of some factors affecting the oxidation behaviour of molybdenum disilicide.

The oxidation behaviour of Molybdenum Disilicide, MoSi[2], has been studied at 1300° and 1400°C, in free flowing atmospheres containing 10%, 20%, 50% and 100% oxygen. The effect of iron impurity was also investigated. The oxidation behaviour was followed using gravimetric and microscopical techniques and additional information derived by electron probe microanalysis. It has been shown that, on oxidation of MoSi[2], a state of non-equilibrium exists in the system at 1400°C but not at 1300°C. This showed very clearly on the gravimetric oxidation curves and was found to be caused by the accumulation of molybdenum atoms at the base material/silica layer interface. The system became closer to equilibrium by the loss of the molybdenum atoms, as molybdenum trioxide, after diffusion to the outer surface of the silica layer. Oxygen pressure affected the time over which the state of non-equilibrium existed and also the magnitude of the effect. The presence of iron caused the state of non-equilibrium to occur at 1300°C as well as at 1400°C. Microscopical changes were related to gravimetric changes in all cases