Effects of Al on Si-Al-Y₂O₃ co-deposition coatings on Nb-Ti-Si base ultrahigh temperature alloy

Si-Al-Y₂O₃ co-deposition coatings on an Nb-Ti-Si base ultrahigh temperature alloy were prepared by pack cementation processes at 1050°C. The effects of Al content (mass fraction) in pack mixtures on the microstructure and constituent phases of the co-deposition coatings were studied, and the oxidation-resistant performance of the coating was investigated. The results show that all coatings prepared at 1050°C with pack mixtures containing different Al contents have a multiple layer structure, and the inner layers of all coatings are composed of Al₃(Nb, X) (X represents Ti, Cr and Hf) and (Cr, Al)₂(Nb, Ti) phases. The outer layer of the coating prepared at 1050°C for 10 h with the pack mixture containing 10% Al is composed of (Nb, X)Si₂, and the middle layer is composed of (Nb, X)₅Si₃. However, the constituent phases change into Al₃(Nb, X) and (Nb, X)₅Si₃ in the middle layer of the coating prepared with the pack mixture containing 15%Al. (Nb, X)₃Si₅Al₂ and (Nb, X)Si₂ phases have been detected in the outer layer of the coating prepared with the pack mixture containing 20%Al, and Al₃(Nb, X) is the main phase constituent in its middle layer with dispersed (Nb, X)₅Si₃ particles in it. EDS analysis reveals that the distribution of Y in the coatings is not uniform, with its content increasing with the rising of Al content in the pack mixtures. Co-deposition of Al and Si occurs in a sequential manner during the deposition process and the growth of the coatings obeys a parabolic kinetics. After oxidation at 1250°C for 5 h, dense Al₂O₃ scales have formed on the coatings, which prevent the oxygen from diffusion into the substrate effectively.