Degradation Studies of Micro and Nanocrystalline NiCrAlY Coatings for High Temperature Corrosion Protection.

Abstract

NiCrAlY coatings are widely used in gas turbines for the protection against the high temperature oxidation and corrosion. Micro and Nanocrystalline NiCrAlY coatings on Superalloy were fabricated by Sputtering and High Velocity Oxide Fuel Spray (HVOF), respectively, in the present work. The coatings were subjected to high temperature cyclic oxidation at 900°C in presence of air to investigate its oxidation kinetics. The weight change of the microcrystalline (mc) and nanocrystalline (nc) coatings was monitored after each cycle to establish its degradation rate in the corrosive environment. The parabolic rate constant was evaluated using the weight change data obtained for both mc and nc coatings by least square fitting method. It was observed that the nanocrystalline coatings showed lower value of parabolic rate constant as compared to that of its bulk coatings. The protective oxide scales were identified by the characterisation techniques such as XRD, FESEM, EDS and X-ray Mapping. The nanocrystalline NiCrAlY coatings exhibit superior high temperature corrosion resistance in air due to the formation of adherent, non-porous, and uniform protective scales such as NiCr2O4, Cr2O3 etc when compared to mc coatings. On the other hand, the protective scales formed over the mc coatings are non-uniform, discontinuous which provides relatively less protection in the corrosive environment. The mechanisms of nanosize effect on the rapid formation of protective scale, through high diffusivity of alloying elements, in nc coatings are proposed.