Structure formation of Si-Y₂O₃ co-deposition coatings on Nb-silicide-based alloy

Si-Y₂O₃ co-deposition coatings on an Nb-silicide-based ultrahigh temperature alloy were prepared by pack cementation processes at 1050, 1150 and 1250°C for 5, 10, 15 and 20 h, respectively. The coating structure, phase constituents and compositional distribution were investigated by SEM, EDS and XRD. Both the formation mechanism and growth kinetics of coatings and the catalysis mechanism of Y₂O₃ were discussed. The results show that all Si-Y₂O₃ co-deposition coatings are composed of a (Nb, X)Si₂ (X represents Ti, Hf and Cr elements) outer layer and a (Nb, X)₅Si₃ transitional layer. Some thin discontinuous (Cr, Al)₂(Nb, Ti) Laves precipitates form between the substrate and (Nb, X)₅Si₃ transitional layer. The distribution of Y in the coatings is not uniform. The content of Y in (Nb, X)Si₂ and (Nb, X)₅Si₃ phases is about 0.46%-0.57% (molar fraction) while that in (Cr, Al)₂(Nb, Ti) is about 0.94%. With increasing co-deposition temperature, the content of Y increases obviously, while it increases slightly with the prolonging co-deposition time. The addition of Y₂O₃ in the pack mixtures not only refines the microstructure of the coatings, but also obviously catalyzes the coating growth.