Diffusion barrier effect of Al₂O₃ layer at the interface between Mo-Si-B coating and Nb-Si based alloy

An oxidation-resistant composite coating, including Mo-Si-B layer and Al₂O₃ diffusion barrier, was prepared on Nb-Si-based alloy by a two-step method. The Al₂O₃ film was fabricated by firstly deposition of Al on the alloy through pack cementation, and then by in situ oxidation of the aluminide coating. Mo-Si-B coating was then prepared on the Al₂O₃ film by spark plasma sintering. The Mo-Si-B coating is mainly composed of MoSi₂ and a small amount of MoB and SiO₂ particles. After long-term exposure at 1250 °C, the growth of the interdiffusion zone (IDZ) was suppressed effectively at the interface with the Al₂O₃ diffusion barrier. The results indicate that the diffusion barrier can significantly inhibit the outward diffusion of Nb, Ti and Cr alloying elements as well as the sluggish inward consumption of Si element. The Mo-Si-B coating with Al₂O₃ diffusion barrier exhibited a good oxidation resistance with a mass gain of about 0.97 mg/cm² after 500 h of oxidation.