Influence of Sulfur,Platinum, and Hafnium on the Oxidation Behavior of CVD NiAl Bond Coatings

The influences of S, Pt, and Hf on the oxidation behavior of chemical vapor deposition (CVD) NiAl bond coatings on high-S and low-S superalloys were investigated. Cyclic and isothermal-oxidation testing at 1150°C revealed that alumina-scale adherence to NiAl coatings was very sensitive to substrate S impurities. Scale spallation, as well as the growth of voids at the oxide–metal interface, increased as S increased. However, Pt-modified coatings were not sensitive to S, and did not form voids at the oxide–metal interface. Transmission-electron microscopy (TEM) revealed that alumina scales formed on (Ni,Pt)Al had Hf ions (from the superalloy) segregated to grain boundaries, whereas Hf was not detected in the alumina scale formed on NiAl coatings. Results suggested that the detrimental influence of S on scale adherence to NiAl is primarily related to void growth, which is eliminated or significantly reduced in Pt-modified coatings. A simple model relating void growth to excess vacancies and sulfur segregation is proposed.