Oxidation Behavior Of An Advanced Niobium-Silicide Based Ultrahigh-Temperature Alloy And Preparation Of Its Oxidation-Resistant Coatings

An advanced niobium-silicide based ultrahigh temperature alloy was prepared by vacuum consumable arc-melting method. Oxidation experiments for the arc-melted specimens at high temperatures were carried out with the use of self-made high temperature oxidation furnace with tensile stress. The oxidized microstructure and high temperature oxidation kinetics of the specimens were analyzed. Silicide coatings were prepared on the arc-melted specimens by pack cementation process in the self-made furnace with vacuum or controllable atmosphere. Microstructure of the coatings and their oxidation behavior at 1250℃ were also studied.With the increase of the oxidizing time, the scales of the specimens were thickened at 850℃, 1150℃ and 1250℃. It has been found that completely different inner and outer oxidation layers form for the advanced niobium-silicide based ultrahigh temperature alloy: the former has compact microstructure with some small strip-like holes in it, and the latter has coarse microstructure with massive circular or elliptical holes.Combined with the analysis results of EPMA and XRD, it can be determined that the microstructure of the outer layers consisted of oxides of titanium and niobium, SiO₂ and Cr₂O₃ after oxidizing for different hours at the temperatures mentioned above. The content of oxygen was low and there were still some Nb₅Si₃ not be oxidized in the inner oxidation zones. The oxidation kinetics of this alloy at 850℃and 1150℃ accorded with parabolic equations.Uniform and compact (Nb, X)Si₂ (X=Ti, Cr, Hf and Al) coatings were prepared by pack cementation process. Boundaries between coatings and the substrates were evident, and the microstructure of transition layer mainly consisted of Nb₅Si₃. The coated specimens were underwent oxidation at 1250℃, the cross-section of the specimens oxidized for a time shorter than 50 hours was composed of four layers including outermost oxide layer, coating layer, diffusion layer and substrate. Its oxidation kinetics was in agreement with the parabolic law...