Reaction diffusion of MoSi2 and Mo5SiB2

To establish quantitative basis for oxidation-protective coating of Mo-Si-B ternary alloys by MoSi2, phase transformations in MoSi2 vs. Mo5SiB2 diffusion couples have been studied. Two layers are formed on reaction diffusion at temperatures between 1400 and 1600 °C: a single-phase layer of Mo5Si3 and a two-phase layer consisting of Mo5Si3 and MoB. The growth obeys the parabolic low for both the layers, and the rate constants of the two layers are found to be approximately equal. The interdiffusion coefficient in the T1 layer has also been evaluated. The microstructural evolution in the diffusion zone is modeled in terms of mass conservation, as well as that of a Mo-9Si-18B two-phase alloy coated with MoSi2 reported previously [Intermetallics 12 (2004) 407].