Microstructure of Si-Al-Y co-deposition coatings on an Nb-Ti-Si based ultrahigh temperature alloy

The oxidation-resistant Si-Al-Y co-deposition coatings on a Nb-Ti-Si based ultrahigh temperature alloy were prepared by halide activated pack cementation processes. SEM, EDS and XRD analyses were used to study the influences of the holding temperature and the content of Al in pack mixtures on the microstructural formation of the coatings. The results show that the coatings prepared with the pack mixture of 10Si-10Al-3Y-5NaF-72Al₂O₃ (wt%) at different temperatures (1050°C, 1080°C and 1150°C, respectively) have a similar structure, consisting of a (Nb, X)Si₂(X represents Ti, Cr and Hf elements) outer layer, a (Nb, X)₅Si₃ mid layer, a sub-inner layer composed of (Cr, Al)₂(Nb, X) and (Nb, X)Al₃ phases, and a very thin inner layer consisting of (Nb, X)₂Al phase. The microstructure of the coatings prepared at 1050°C for 10 h changes evidently with the content of Al in the pack mixtures (10Si-xAl-3Y-5NaF-(82-x)Al₂O₃ (wt%) (x=10, 15, 20, respectively)). Increasing the content of Al in the pack mixture to 15wt%, the constituent phases of the mid layer of the coating change into (Nb, X)Al₃ and (Nb, X)₅Si₃, but the constituent phases remain unchanged in the outer layer, sub-inner layer and inner layer in the coating. The main constituent phases of the outer layer of the coating prepared with the pack mixture containing 20wt% Al are (Nb, X)Si₂ and (Nb, X)₃Si₅Al₂, while the constituents phases of the other layers in this coating are the same as those in the coating prepared with the pack mixture containing 15wt% Al. The scale developed on Si-Al-Y co-deposition coating which was prepared with 10Si-15Al-3Y-5NaF-67Al₂O₃ (wt%) pack mixture at 1050°C for 10 h, upon oxidation at 1250°C for 0.5 h, is about 10 μm thick, and composed of Al₂O₃, TiO₂ and SiO₂.