Compound silicide coatings are commonly fabricated to protect refractory alloys against oxidation at elevated temperatures with the hope of better oxidation resistance and longer service life. In this paper, oxidation resistance and failure mechanism of MoSi2/WSi2/NbSi2 compound coating on Nb-Si based alloy upon oxidation at 1250oC was studied, and the effect of NbSi2 layer has also been discussed. Owning to NbSi2 layer, oxidation resistance of this compound coating could be improved by accelerated formation process of a dense protective scale. The scale is about 5.5 μm thick after oxidation at 1250℃ for 100 h and its parabolic growth rate constant is only 0.3841μm2/h within 100 h, which is obviously lower than it in MoSi2/WSi2 case (0.9216 μm2/h). Also, degradation mechanism of WSi2 barrier layer could be changed and the degradation could be slowed down greatly by NbSi2 layer. In MoSi2/WSi2/NbSi2 case, diffusion and losing of W atoms resulted in formation of micro pores in WSi2 layer while diffusion and losing of Si atoms caused obvious formation of W5Si3 phase and pores in MoSi2/WSi2 case. Nevertheless, due to mismatch of thermal expansion coefficient, vertical cracks could be generated in NbSi2 layer during the coating fabrication process and oxidation at high temperature. Oxidation of Hf, Al and NbSi2 phase cause accumulation of their oxides and internal stress, and then result in growth and expansion of the cracks. Fracture and exfoliation of MoSi2/WSi2 layer took place and the compound coating failed ultimately after a long-time oxidation about 280 h. 

Compound silicide coating; Oxidation behavior; Internal oxidation; Failure mechanism