High temperature oxidation of conventional and nanocrystalline HVOF thermal-sprayed NiCrAlY coatings

摘要

Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion. They consist of a metal bond coating (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO_2/Y_2O_3). In this work the oxidation behavior of conventional and nanostructured HVOF NiCrAlY coatings have been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a Nickel alloy foil to form a nanocrystalline coating. Free-standing bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at 1273 k for different periods in order to form the thermally grown oxide layer (TGO). The oxidation experiments show that the oxidation kinetics of the nanostructured coating is lower than the conventional coating. In the present work, this observation is explained by the formation of a continuous α-Al_2O_3 layer on the top of the nanostructured HVOF NiCrAlY coating that protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating. The as-sprayed coatings as well as the morphology of the oxide scale presents on the oxidized coatings were characterized by XRD and SEM.