Microstructural features and oxidation behavior of nano and conventional NiCrAlY coatings developed by LVOF process

Abstract

The present investigation deals with difference in the microstructure and high temperature oxidation performance of the thermally sprayed NiCrAlY microcrystalline (MC) and nanocrystalline (NC) coatings on superalloy substrates. Low Velocity Oxy Fuel (LVOF) technique has been used to deposit commercially available NiCrAlY and mechanical milled NiCrAlY nanopowder. As sprayed NC coatings showed lesser intersplat oxidation as compared to as sprayed MC coatings due to use of Ar gas to deposit the nanopowder through an LVOF gun. The deposited coatings were oxidized at 900°C up to 100cycles (each cycle consisted of 1hour heating at 900°C and 20minute cooling in air). The as sprayed and oxidized coatings were characterized by X-ray Diffraction Analysis (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Spectroscopy (EDS) and X-ray Mapping techniques. The NC coatings showed overall lesser weight gain as compared to MC coatings during oxidation. Very fine (200–300nm) Al2O3, Cr2O3 and NiCr2O4 oxides were formed on the oxidized surface of NC coatings whereas the coarser (1–2μm) NiO and NiCr2O4 oxides were formed on the unoxidized regions of the as sprayed MC coatings after oxidation tests.