Microstructure and High Temperature Oxidation Resistance of Si-Y_2O_3 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℃,1150℃ and 1250℃,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℃ 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℃ is composed of TiO2,SiO2 and Cr2O3.Si-Y2O3 co-deposition coatings possess better oxidation resistance than simple Si deposition coatings.