The thermal cyclic behavior of the VPS and HVOF-sprayed MCrAlY coatings

Rotating components used in the hot sections of land-based gas turbine are exposed to severe environment of several ten thousands operating hours above 1100°C, To protect such components against high temperature oxidation an intermediate bond coat is applied, typical of a MCrAlY-type metal alloy. Various processing methods have been studied for bond coat deposition. This study is concerned with the cyclic oxidation behavior of CoNiCrAlY coatings. Coatings were deposited by a vacuum plasma spray and high-velocity oxygen fuel method on a nickel-based superalloy (GTD-111). Cyclic thermal oxidation test condition is at 1100°C in ambient air for various periods of time. Tests were used to evaluate the oxidation resistance of the spray-coated specimens. The microstructure and morphology of as-sprayed and of tested specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The oxide phases formed on the coating surface are NiO, CoCr₂O ₄, and Al₂O₃. The nickel oxide of them was to be dominant with increasing cycles. The differences in microstructure and phase composition in the interface with coating layer are reported. The influence of coating process methods on coating characteristics and degradation mechanisms is discussed. The HVOF coating with the splats was more resistant on the high temperature oxidation than the VPS coating.