Effect of Re-base diffusion barrier on the oxidation performance of a gradient NiCoCrAlY coating

A gradient NiCoCrAlY coating with a Re-base diffusion barrier (RG-NiCoCrAlY) was prepared by sequential treatments of NiRe coelectrodeposition, arc ion plating and chemical vapor aluminization. For comparison, normal NiCoCrAlY and simply aluminized NiCoCrAlY coatings (G-NiCoCrAlY) were also prepared to assess their cyclic oxidation behavior at 1050 °C. The results indicated that the oxide scale formed on the normal NiCoCrAlY coating peeled off after 400 cycles, while the G-NiCoCrAlY and RG-NiCoCrAlY coatings developed and maintained continuously dense alumina scale after oxidation for 600 cycles, confirming the benefits of increasing the Al content in NiCoCrAlY. In contrast to G-NiCoCrAlY, the presence of Re-base diffusion barrier mitigated element interdiffusion, which not only reserved Al in the coating but also reduced the outward diffusion of refractory elements from the substrate, thus was capable of minimizing precipitation of topologically close-packed phase (TCP) in single-crystal superalloy. Instead of outward diffusion, refractory elements were dissolved in the diffusion barrier and became fixed. Undoubtedly, the best anti-cycling performance was achieved by the RG-NiCoCrAlY coating. The mechanisms of the effect of the Re-base diffusion barrier on oxide scale growth and elemental interdiffusion, which influence the overall oxidation behavior of the NiCoCrAlY coating, are discussed.