High Temperature Oxidation of Laser Treated Air Plasma Sprayed Ni-20Cr Coating

Abstract

Ni-20Cr coating offers superior resistance against high temperature oxidation and corrosive salts. Therefore it is widely used in several industries for the protection of metallic components. In present study, Ni-20Cr coating was deposited on stainless steel (AISI-304) substrate by using air plasma spraying system. The inherent features of air plasma spray coating, for example, porosity, oxides and splat boundaries, act as channels for the diffusion of oxygen during high temperature exposure. The coating was re-melted using CO2 laser varying laser power and defocus distance while keeping the remaining parameters constant. The laser tracks applied on the coating at different powers and defocus distances were characterized with the help of optical microscope, SEM and XRD. After optimizing the CO2 laser parameters the coated surface was completely cladded. Considerable reduction in the defects was observed after cladding the plasma sprayed surface. The (1) uncoated, (2) air plasma sprayed and (3) laser re-melted specimens, were exposed to cyclic oxidation at 900°C for a 100 cycles run. The oxidation products were characterized using XRD and SEM. Weight changes were determined after every 4th cycle; uncoated samples showed severe oxidation indicated by substantial weight loss, whereas air plasma coated samples demonstrated noticeable weight gain. However, oxidation resistance of laser-cladded samples was found to be significantly improved as the samples showed negligible weight change; porosity within the coating was minimized with an improvement in interface quality causing reduction in delamination damage