Effects of targeted γ-Ni + γ′-Ni3Al-based coating compositions on oxidation behavior

The 1150 °C oxidation behavior of γ + γ′-based alloys containing up to 20 at.% Pt and 15–22 at.% Al was investigated in an effort to further develop and optimize alternative bond coating compositions for advanced thermal barrier coating systems. It was found that the co-addition of Pt and Hf was extremely beneficial to oxidation resistance, to the extent that Ni–20Al–20Pt–Hf and Ni–20Al–10Pt–Hf alloys oxidized at significantly slower rates than a Ni–50Al–15Pt β-NiAl alloy. Moreover it was found that there is an optimum Hf content that depends on Pt content in the alloy. A feasible method to deposit Pt + Hf-modified γ + γ′ coatings having overall compositions closely approaching those targeted (Ni–Al15–20–Pt15–20–Hf) on the basis of the bulk-alloy study was subsequently developed. Cyclic oxidation testing of a representative γ + γ′ coating at 1150 °C in air revealed the absence of surface rumpling and excellent compatibility with the superalloy substrate as major advantages compared to traditional Pt-modified β-NiAl coatings.