Effect of Hf, Y and C in the underlying superalloy on the rumpling of diffusion aluminide coatings

One form of degradation of platinum-modified nickel-aluminide (NiPtAl) coatings on single-crystal superalloys at high temperatures involves a surface instability known as rumpling. In this work, the evolution of rumpling during isothermal and cyclic oxidation at 1150 °C of a NiPtAl aluminide coating deposited on five different modifications of a commercial second generation CMSX-4 superalloy is characterized as a function of oxidation time. The rumpling is sensitive to the Hf and C content of the underlying superalloy, being largest for the alloy containing low hafnium and high carbon concentrations and smallest for the low-carbon, high-hafnium alloy. It is argued that hafnium decreases the propensity for rumpling when it diffuses into the coating and to the growing aluminum oxide increasing their creep resistance. An increase of the carbon content in the superalloy has the opposite effect due to the formation of tantalum-rich mixed carbides, which tie up hafnium, thereby decreasing its amount available to diffuse into the coating and the growing oxide. The effect of yttrium additions on rumpling is minor.