High temperature oxidation studies on HVOF sprayed NiCrAl coatings on superalloys

Abstract

High velocity oxyfuel (HVOF) spray coating technology is widely used for depositing wide range of powders on the surface of the materials which require protection in the high temperature environment, without significantly heating the substrate materials. HVOF spray technique has been used to deposit NiCrAl coating on the Ni and Fe based superalloys in the present work. The coating is expected to form protective oxidation resistant scales on the superalloys when it is exposed to corrosive environment as manifested in aero and land based turbine engines. Oxidation kinetics was established for both bare and NiCrAl coated superalloys at 900°C for 100 cycles by thermogravimetric analysis to assess the protective nature of the latter. The scales formed on the coated and bare superalloys, upon subjected to air oxidation at high temperature, were analysed using X-ray diffraction (XRD) and field emission scanning electron microscope attached with energy dispersive spectrometer (FESEM/EDS) to elucidate the oxidation mechanisms. The coating was found to be effective in enhancing the oxidation resistance of all the superalloys at 900°C under cyclic conditions, due to the formation of protective oxide scales, as observed in the present work. The EDS analysis of the oxide scales revealed the presence of NiO, Cr₂O₃ and Al₂O_3.