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 degrees 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 degrees C with pack mixtures containing different Al contents have a multiple layer structure, and the inner layers of all coatings are composed of Al-3(Nb, X) (X represents Ti, Cr and Hf) and (Cr, Al)(2)(Nb, Ti) phases. The outer layer of the coating prepared at 1050 degrees C for 10 h with the pack mixture containing 10% Al is composed of (Nb, X)Si-2, and the middle layer is composed of (Nb, X)(5)Si-3. However, the constituent phases change into Al-3(Nb, X) and (Nb, X)5Si3 in the middle layer of the coating prepared with the pack mixture containing 15%Al. (Nb, X)(3)Si5Al2 and (Nb, X)Si2 phases have been detected in the outer layer of the coating prepared with the pack mixture containing 20%Al, and Al-3(Nb, X) is the main phase constituent in its middle layer with dispersed (Nb, X)(5)Si-3 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 degrees C for 5 h, dense Al2O3 scales have formed on the coatings, which prevent the oxygen from diffusion into the substrate effectively.