A comparative study of the cyclic thermal oxidation of PVD nickel aluminide coatings

Static components used in the hot sections of gas turbines increasingly have a thermal barrier coating (TBC) of partially yttria-stabilized zirconia (PYSZ) routinely applied to improve their high temperature properties. Developments are also in progress to make TBCs better suited for use on rotating components such as blades. To help protect against high temperature oxidation of such components, an intermediate bond coat is applied, typically of an MCrAlY-type metal alloy. An alternative bond coat material is nickel-aluminum intermetallic alloy. Various processing routes have been studied for both bond coat and TBC deposition. A potentially attractive processing route is to deposit both bond coat and TBC by a physical vapor deposition (PVD) method; this would have the advantage of permitting sequential deposition in the same coating cycle. Whilst much research has been carried out on PVD MCrAlY and PYSZ coatings, relatively little work has been carried out on PVD NiAl, which represents a simpler intermetallic alloy with less critical composition control requirements. Thus, we have investigated NiAl deposition by three ion-assisted PVD coating routes: arc, electron-beam, and sputter ion plating. Coatings were deposited on a nickel-based alloy (Inconel 600) and an AISI 304 stainless steel. The differences in microstructure and phase composition from each deposition method are reported, together with data on the cyclic oxidation performance. The influence of process parameters on coating characteristics and degradation mechanisms is discussed.