Microstructure and oxidation resistance of a multiphase Mo-Si-B ceramic coating on Mo substrates deposited by a plasma transferred arc process

To improve the oxidation resistance of a Mo substrate, a multiphase Mo-Si-B ceramic coating was deposited using a plasma transferred arc (PTA) process. The phase constituents, microstructure, and oxidation resistance of the coating were investigated. The results show that the microstructure of the Mo-Si-B coating is characterised by rod-like Mo₅SiB₂ dendrites, a lamellar structure of Mo₃Si/Mo₅SiB₂ binary eutectics, and cellular Mo₅Si₃ dendrites. Compared with the Mo substrate, the Mo-Si-B coating exhibits a significant improvement in the oxidation resistance at 1300?°C in an air atmosphere. This is mainly owing to a dense and continuous borosilicate layer formed on the surface of the Mo-Si-B coating, which acts as an oxygen diffusion barrier. Moreover, it is observed that the Mo₅SiB₂ dendrites exhibit a higher oxidation resistance compared to Mo₃Si/Mo₅SiB₂ eutectics during high-temperature oxidation exposure. The oxidation behaviour is discussed based on the oxidation kinetics and a cross-sectional microstructure analysis of the oxidised Mo-Si-B specimen.