To produce power more efficiently and cleanly, the next generation of power turbines will have to operate at extreme temperatures and pressures. Currently, single-crystal, nickel-based superalloys are used in such extreme environments. MCrAlY coatings (where M = Co, Ni, or Co/Ni) are widely applied to first- and second-stage turbine blades and nozzle guide vanes, where they may be used as corrosion-resistant overlays or as bond coats for use with thermal barrier coatings.
During exposure to high temperatures, an oxide scale forms on the MCrAlY surface, becoming part of the original system. This thermally grown oxide is largely comprised of alumina and often represents the starting point for failure. Correspondingly, the growth rate and adhesion of the oxide scale, and the aluminum depletion in the bond coat, are among the determining factors for the useful lifetime of a MCrAlY bond-coated article.
Even though these components are coated with a bond coat followed by a thermal barrier coating, substrate oxidation and corrosion is a concern. Under these operating conditions, alloys are exposed to an aggressive corrosive setting that either shortens component functional lifespan, or requires the use of thicker components or more costly alloys, resulting in increased cost and reduced efficiency.
While the use of protective thermal barrier and bond coatings has resulted in significant improvements in superalloy performance including thermal, oxidative, and mechanical protection, these coatings still allow for oxygen diffusion and subsequent reaction with the underlying substrate.
A surface treatment process was developed in which reactive metal elements are applied directly to the alloy substrate prior to bond-coating, resulting in reduced oxidation. Further, the process significantly increases corrosive resistance of nickel-based superalloys, reducing premature component failure, and resulting in increased system efficiency and cost savings.