Abstract
Improving the efficiency and lifetime of high temperature coatings in a cost-effective way has always been receiving more attention. In this study, it was focused to investigate the high temperature oxidation behavior of thermally sprayed Ni-5wt.% Al coating as a foundation of protective Ni-based alloys under the influence of in-situ formation and dispersion of oxide nanoparticles and grain size refinement. Therefore, high-energy ball-milling process and High Velocity Oxy-fuel (HVOF) system were employed to produce nanocrystalline powders and coatings. Isothermal oxidation evaluation of free-standing coatings was carried out at 950°C. Also the microhardness of the nanostructured coatings was examined and compared with conventional Ni–5Al coatings. The oxidation rate constant (kP) of the modified coating was considerably lower than that of the conventional coating. This phenomenon happened because the continuous protective alumina layer was formed as a thermally grown oxide (TGO) on the nanostructured dilute NiAl from the beginning of the oxidation. The presence of the in-situ alumina nanodispersoids, formed during the milling process, as nucleation sites and numerous grain boundaries as easy diffusion paths facilitates the α-Al2O3 protective layer. The microhardness test results also confirmed the higher hardness of nanostructured coating.
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