OXIDATION OF TISI2 AND MOSI2

Isothermal oxidation experiments were carried out with TiSi2 and MoSi2 in air between 1000 and 1400-degrees-C. The oxidation rate of TiSi2 can be approximated at 1100-degrees-C by a linear rate law and at 1200 and 1300-degrees-C by a parabolic one. The scale formed at 1100-degrees-C consists of an outer layer of TiO2 and an inner layer of a mixture of SiO2 and TiO2. Increasing temperature increases the SiO2 content in the outer layer and at T > 1200-degrees-C a SiO2 scale with precipitates of Ti-Si oxides is found. TiSi2 possesses optimum oxidation resistance between 1200 and 1300-degrees-C. The oxidation Of MoSi2 leads to the formation of the volatile oxide MoO3 which evaporates at T > 1200-degrees-C. This process favours the formation of a rather pure and very protective SiO2 layer on the surface. Obviously the evaporation occurs after solid-state diffusion through the SiO2 at the SiO2/gas interface.