Pt effect on early stage oxidation behaviour of Pt-diffused γ-Ni/γ’-Ni3Al coatings

The early stage oxidation behaviours of γ/γ’-based NiAl coatings with different Pt additions are investigated. It is found that Pt addition can slow down the θ-Al2O3 to α-Al2O3 transformation. High resolution phase mapping by scanning diffraction analysis shows that α-Al2O3 nucleation in the θ-Al2O3 scale is inhomogeneous along the coating/scale interface. Spatially resolved PLPS results show a clear correlation between the θ-Al2O3 to α-Al2O3 transition and the γ or γ’ microstructure in the underlying alloy: where Pt stabilises the γ’ structure, the suppression of θ-Al2O3 to α-Al2O3 transition is observed. The slower θ-Al2O3 to α-Al2O3 transition rate due to Pt addition leads to a lower compressive stress of the stable oxide scale, which contributes to the long term stability of the scale as a result of Pt addition.