Fabrication and Oxidation Resistance of a Novel MoSi₂-ZrB₂-Based Coating on Mo-Based Alloy

To enhance the oxidation resistance of Mo-based TZM alloy (Mo-0.5Ti-0.1Zr-0.02C, wt%), a novel MoSi₂-ZrB₂ composite coating was applied on the TZM substrate by a two-step process comprising the in situ reaction of Mo, Zr, and B₄C to form a ZrB₂-MoB pre-layer followed by pack siliconizing. The as-packed coating was composed of a multi-layer structure, consisting of a MoB diffusion layer, an MoSi₂-ZrB₂ inner layer, and an outer layer of mixture of MoSi₂ and Al₂O₃. The composite coating could provide excellent oxidation-resistant protection for the TZM alloy at 1600 °C. The oxidation kinetic curve of the composite coating followed the parabolic rule, and the weight gain of the coated sample after 20 h of oxidation at 1600 °C was only 5.24 mg/cm². During oxidation, a dense and continuous SiO₂-baed oxide scale embedded with ZrO₂ and ZrSiO₄ particles showing high thermal stability and low oxygen permeability could be formed on the surface of the coating by oxidation of MoSi₂ and ZrB₂, which could hinder the inward diffusion of oxygen at high temperatures. Concurrently, the MoB inner diffusion layer played an important role in hindering the diffusion of Si inward with regard to the TZM alloy and could retard the degradation of MoSi₂, which could also improve the long life of the coating.