Superalloy Substrate Influences on the Oxidation Behavior of Aluminide Bond Coatings

摘要

The influence of substrate composition on the oxidation behavior of aluminide bond coatings was investigated. Low-activity NiAl and (Ni,Pt)Al bond coatings were deposited by chemical vapor deposition (CVD) onto Y-free single-crystal superalloys and onto high-S and H_2 de-sulfurized NiCrAl alloys designed to model the Cr and Al content of the superalloy. Isothermal and cyclic oxidation testing of aluminized specimens at 1150 deg C revealed a significant influence of substrate trace element composition (Hf; S and C) on oxidation rate, oxide microstructure, resistance to void growth and scale adherence. Substrate Hf systematically reduced scale growth rates on both simple and platinum aluminide coatings by diffusing through the coatings to dope the Al_2O_3 scales, regardless of the type of substrate. There was a strong detrimental influence of S, and possibly C, on scale adherence to simple aluminide coatings, especially on alloys that did not contain Hf. Although (Ni,Pt)Al coatings exhibited excellent scale adherence and resistance to sulfur effects, at the higher S levels of the NiCrAl substrates even the Pt-modified coatings formed voids at the oxide-metal interface and experienced premature scale spallation. Furthermore, variations in superalloy substrate trace element levels appeared to significantly influence (Ni,Pt)Al coating surface deformation during thermal cycling. It is recommended that the potential influences of substrate composition (in some cases at trace levels) be carefully considered in the future design of bond coating systems for higher-temperature applications.