Inhibition of Interdiffusion From MCrAlY Overlay Coatings by Application of a Ni-Re Interlayer

The durability of protective coatings on combustion turbine blades and vanes is a critical issue in the power generation industry. Coating life usually dictates the refurbishment intervals for these components, and these intervals have generally been of shorter duration than desired by the operators of the equipment. Both MCrAlY and aluminide type coatings protect against oxidation and hot corrosion by forming a protective Al₂O₃ surface layer. Degradation of the coatings occurs by depletion of the aluminum content of the coating through interdiffusion with the substrate and through the formation and spallation of an external Al₂O₃ scale. The results obtained in this study clearly show that the application of a thin interlayer of Ni-Re beneath the MCrAlY coating can significantly decrease the growth rate of the inner β-NiAl depletion zone. Order of magnitude reductions in the inner depletion zone thickness formed at 1000 h were obtained with both the Ni-32 wt.% Re and the Ni-47 wt.% Re interlayer coatings. Since formation of the inner depletion zone is believed to result from interdiffusion with the substrate, these results suggest that the Ni-Re interlayer provided a significant impediment to the inward diffusion of Al into the substrate.