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The oxidation behavior of two-phase, γ+γ’ Pt diffusion coatings and model alloys has been evaluated to better understand the potential of this class of coatings for power generation applications. Coatings were deposited on both directionallysolidified alloy 142 and single crystal N5 superalloys. Fabrication issues such as initial Pt thickness and annealing temperature were evaluated. Oxidation testing was performed in both dry O 2 and with the addition of H 2 O to study the performance of γ+γ’ and conventional coating materials. After exposure for 1000, 1-h cycles at 1150°C in dry O 2 , a γ+γ’ coating on N5 had evolved into a thicker, almost continuous γ’ layer with the Pt content dropping to ~7at.% in this layer and diffusing ~250 µm into the substrate, disrupting the γ+γ’ substrate microstructure. Characterization of the oxide formed on a γ+γ’ composition showed that even 30 at.%Pt did not prevent the formation of a Ni-rich spinel layer at 1200°C. In order to assist the coating development process, model alloys also were evaluated. Other Pt group metals (Pd and Ir) were evaluated as well as the addition of Cr and various Hf levels. As expected, Cr and Hf were beneficial for oxidation resistance. However, Pd was not an adequate replacement for Pt because of the multi-phase microstructure that formed. A γ+γ’ composition had a 1000°C yield stress in compression that was significantly higher than γ’-Ni 3 Al or β-NiAl. Also, the thermal expansion coefficient was very similar to other Ni-Al alumina-formers. The performance of γ+γ’ compositions are very promising, however, sensitivity to superalloy substrate composition, Pt interdiffusio n during long-term service, spinel formation and the potential need for a secondary processing step are areas of concern for this class of coatings.

The Performance of Pt-Modified Alumina-Forming Coatings and Model Alloys