Oxidation resistance comparison of MoSi₂ and B-modified MoSi₂ coatings on pure Mo prepared through pack cementation

A MoSi₂ coating and a B-modified MoSi₂ coating were prepared on pure Mo by halide activated pack cementation (HAPC) method. The B-modified MoSi₂ coating consisted of a thick MoSi₂ outer layer with MoB precipitates distributed in its inner part, a Mo₅Si₃ second layer, a MoB third layer, and a Mo₂B inner layer. Both coatings were oxidized isothermally at 873, 1523, and 1773-K. It was found that pest oxidation did not occur in both coatings at 873-K for full cover scales could form on their surfaces after 100-h oxidation. During oxidation, B atoms in the B-modified MoSi₂ coating diffused outward to the coating surface and oxidized to low viscose B₂O₃, which promoted the formation of full cover scale and significantly reduced the volatilization of MoO₃ at 1523-K, while that effect was negligible at 1773-K for silica had good fluidity at such a high temperature. It was also found that the MoB layer of the B-modified MoSi₂ coating retarded the inward diffusion of Si from MoSi₂ to the substrate, which could increase the coating oxidation-resistant lifetime.