Effective Hf-Pd Co-doped β-NiAl(Cr) coatings for single-crystal superalloys

Cyclic oxidation behavior of β NiAlCr coatings was investigated at 1150 °C with emphasis on effective Pd + Hf and Pt + Hf co-doping composition and the concomitant phase transformations and interdiffusion of Pd and Pt into the substrate. Coatings with compositions in the range of Ni-(33-39)Al-5Cr-(2-5)Pd/Pt were deposited on René N5 single-crystal substrates and subjected to long-term cyclic oxidation. The Pt + Hf- and Pd + Hf-modified overlay coatings exhibited comparable oxidation kinetics and matched the range of oxidation behavior exhibited by the standard Pt-aluminide coating. In particular, additions of about (2-5)Pd+(0.15-0.3)Hf and (2-5)Pt+(0.15-0.3)Hf among the studied composition range exhibited long lifetimes of >2000 oxidation cycles. A positive synergistic effect between Pd-Hf and Pt-Hf was clearly observed when comparing the lifetimes of β NiAlCr-Pd and β NiAlCr-Pt quaternary coatings with the quinary coatings. Transformations from β → 3R + 7R-martensites at low temperature and β → γ′ → γ + γ′ at high temperature occurred in both Pd- and Pt-modified coatings due to Al depletion to substrate and surface oxide. A greater depletion of Pd to the substrate compared to Pt was observed at 1150 °C compared to 1100 °C; the interdiffusion coefficients were consistent with those measured in NiAl-NiAl + Pt/Pd diffusion couples. These results are very encouraging in supporting the exploration of Pd-modified coating supplanting the currently used, expensive Pt-based coatings.