Oxidation Behavior of (Mo,Hf)Si₂-Al₂O₃ Coating on Mo-Based Alloy at Elevated Temperature

To improve the oxidation resistance of Mo-based alloys, a novel (Mo,Hf)Si₂-Al₂O₃ composite coating was fabricated on a Mo-based alloy by the method of slurry sintering. The isothermal oxidation behavior of the coating was evaluated at 1400 °C. The microstructure evolution and phase composition of the coating before and after oxidation exposure were characterized. The anti-oxidant mechanism for the good performance of the composite coating during high-temperature oxidation was discussed. The coating had a double-layer structure consisting of a MoSi₂ inner layer and a (Mo,Hf)Si₂-Al₂O₃ outer composite layer. The composite coating could offer more than 40 h of oxidation-resistant protection at 1400 °C for the Mo-based alloy, and the final weight gain rate was only 6.03 mg/cm² after oxidation. A SiO₂-based oxide scale embedded with Al₂O₃, HfO₂, mullite, and HfSiO₄ was formed on the surface of the composite coating during oxidation. The composite oxide scale exhibited high thermal stability, low oxygen permeability, and enhanced thermal mismatch between oxide and coating layers, thus improving the oxidation resistance of the coating.