Formation and oxidation behavior of SiO2/NbSi2 multilayer coating fabricated by one-step method

SiO 2 /NbSi 2 multilayer coatings were prepared on Nb alloy to enhance its high-temperature oxidation resistance by utilising nano-Al 2 O 3 as the inert filler in halide-activated pack cementation (HAPC). In this study, the effect of nano-Al 2 O 3 on the morphology and formation of the silicide coating was studied. Moreover, the oxidation resistance of the SiO 2 /NbSi 2 multilayer coating and single NbSi 2 coating were investigated. The results revealed that nano-Al 2 O 3 promotes the formation of the SiO 2 layer during HAPC. After oxidation at 1250 °C for 100 h, the weight gain of the SiO 2 /NbSi 2 multilayer coating was only 7.20 mg/cm 2 , which was significantly lower than that of the single NbSi 2 coating. The SiO 2 outer layer effectively prevents the inward diffusion of oxygen at 1250 °C; however, it quickly transformed into a porous layer of mixed oxides at 1350 °C owing to the outward diffusion of Nb and crystallisation of SiO 2 .