Microstructure and high temperature oxidation resistance of Si-Y₂O₃ co-deposition coatings prepared on Nb-silicide-based ultrahigh temperature alloy by pack cementation process

Si-Y₂O₃ co-deposition coatings on Nb-silicide-based ultrahigh temperature alloy were prepared by pack cementation processes. The co-deposition holding temperatures were 1050°C, 1150°C and 1250°C, and the holding times were 5, 10, 15 and 20h, respectively. The microstructure and isothermal oxidation resistance of the coatings were studied. 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 phase precipitates exist between the substrate and (Nb,X)₅Si₃ transitional layer. EDS analyses reveal that the distribution of Y on the coatings is not uniform. The content of Y in (Nb,X)Si₂ and (Nb,X)₅Si₃ phases is about 0.46at%-0.57at% while that in (Cr,Al)₂(Nb,Ti) is about 0.94at%. After oxidation at 1250°C for 5, 10, 20, 50 and 100h respectively, the retained Si-Y₂O₃ co-deposition coatings still possess double layers structure with their original constituent phases and combine tightly with both scale and substrate. The dense scale developed on Si-Y₂O₃ co-deposition coatings during oxidation at 1250°C is composed of TiO₂, SiO₂ and Cr₂O₃. Si-Y₂O₃ co-deposition coatings possess better oxidation resistance than simple Si deposition coatings.