Oxidation Behavior and Failure Mechanism of MoSi2/WSi2/NbSi2 Compound Coating on Nb-Si-Based Alloy at 1250 °C

The oxidation behavior of a MoSi2/WSi2/NbSi2 compound coating prepared on a Nb-Si-based alloy through a two-step method upon oxidation at 1250 °C was investigated, and its possible failure mechanism after long-term oxidation was also discussed. In this study, the MoSi2, WSi2 and NbSi2 layers were assigned as the outermost anti-oxidation layer, the middle diffusion barrier layer and the innermost buffering layer, respectively. The oxidation results indicated that this coating had admirable oxidation resistance at 1250 °C. After oxidation for 100 h, a dense SiO2-based scale with thickness of about 6.5 μm formed on the coating surface, and the parabolic rate constant of scale growth is about 0.42 μm2/h within 100 h. The presence of the NbSi2 layer could suppress the fast degradation of WSi2 into W5Si3 by providing sufficient Si, while migration of W could cause the formation of Kirkendall voids in the WSi2 layer and then impair its diffusion performance. Due to the diffusion of Si and internal oxidation, there was massive formation of Nb5Si3 phases in the NbSi2 layer. During the oxidation process, oxidation of NbSi2 and reactive elements like Hf and Al also occurred inside the vertical cracks, and this process might promote formation and expansion of penetrable cracks through the compound coating. The persistent internal oxidation inside the penetrable cracks and at the WSi2/NbSi2 layer interface was ultimately likely to induce fracture and spallation of the MoSi2/WSi2 layer after long-term oxidation at 1250 °C.