Thermal stability and oxidation behavior of nanostructured NiCoCrAlY coatings

In this work, commercially available NiCoCrAlY powder was cryomilled to produce a nanostructured feedstock for HVOF coating. The nanostructured powders were heat treated for 1 h at different temperatures to evaluate their thermal stability. The grain size increased to 63 nm after 1 h heat treatment at 700 °C, while it increased to 251 nm after the same heat treatment at 900 °C. Oxidation experiments were performed for 48 h at 1000 °C on a stand-alone coating obtained by thermal spraying of the nanostructured feedstock using the HVOF process. The resulting oxide scale was composed of an adherent and dense α-Al2O3 inner layer with a top layer of fast growing oxides such as NiO, Cr2O3, CoAl2O4 and NiAl2O4. The formation of a two-layer scale could be prevented through surface grinding prior to oxidation.

Research highlights
► Systematic work isolating the contribution of grain size and spraying on oxidation.
► Spraying has higher influence over grain size of the powder for oxide selectivity.
► Oxidation of powders showed that ultra-fine grains delay the mixed oxide formation.