Oxidation resistance of MoSi2/NbSi2 coatings with or without an Al-Si-O interlayer on Nb-based alloy

Abstract

A MoSi2/NbSi2 coating with an Al-Si-O interlayer as diffusion barrier was deposited on Nb-based alloy by firstly halide activated pack cementation (HAPC) and then hot press sintering (HPS). The multilayer structure of MoSi2/Al-Si-O/NbSi2/Nb5Si3 or MoSi2/(Nb, Mo)5Si3/NbSi2/Nb5Si3 was formed in the coating with or without the diffusion barrier, respectively. With the addition of diffusion barrier, elemental diffusion was suppressed markedly during formation of the MoSi2 overlayer and during thermal exposure of the coating sample. Exposed at 1350 °C for 100 h in air, the MoSi2/Al-Si-O/NbSi2 coating exhibited a weight gain about 0.10 mg/cm2 with a slow degeneration in the NbSi2 transition layer and the thickening rate of surface oxide scale was 0.048 μm2/h, which was increased to 0.15 mg/cm2 and 0.050 μm2/h respectively for the MoSi2/NbSi2 coating without the diffusion barrier. Combining with analyses of the coating formation and the microstructural evolution, mechanism of oxidation resistance and of diffusion for the two coatings were compared and built.