Oxidation of stoichiometric poly- and single-crystalline MoSi2 at 773 K

Pest oxidation and oxidation kinetics of MoSi2 at 773 K were investigated and compared using as-cast, spark plasma sintered polycrystalline MoSi2 and MoSi2 single crystal. Specimens (either poly- or single-crystalline) containing cracks are liable to pest disintegration after a certain period of exposure. High temperature in situ observation shows that new cracks tend to nucleate at the pre-existing cracks' tip but not at grain boundary or porosity. The MoSi2 single crystal shows a much lower weight gain compared to the dense poly-crystal during a 300 h exposure. The oxidation kinetics changes from a linear law for the poly-crystalline specimen to a parabolic-like law for the single crystal. A protective compact oxide scale is formed on the MoSi2 single crystal. However, the oxide scale formed on the poly-crystalline specimen is porous and full of cracks. This difference is discussed in the light of different diffusion rates in the bulk matrix and at the grain boundary.