HVOF Thermal Sprayed MCrAlY-Alumina Composite Coatings for High Temperature Applications

A novel composite MCrAlY coating with a fine dispersion of sub-micron Al₂O₃ particles were sprayed by high velocity oxygen fuel (HVOF) thermal spraying for high temperature oxidation protections. The presence of Al₂O₃ could reinforce the metallic MCrAlY coatings, in a similar way to ODS alloys (oxide dispersion strengthening). It could also enhance the oxidation resistance owing to the dispersion of Al₂O₃ on the surface as nucleation sites, which promotes the early formation of a coherent α-Al₂O₃ scale. This is essential for an effective protection against oxidation attack, especially for these applications at relatively lower temperatures (<900 °C) with slower growing Al₂O₃. In this study, a suspension route was employed to achieve a uniform dispersion of 0-10 wt.% Al₂O₃ particles with commercial MCrAlY powders. A liquid fueled HVOF spray gun (MetJet IV) was used to deposit the composite MCrAlY-Al₂O₃ coatings onto 304 stainless steels substrates. The composite coatings were examined thoroughly by field emission gun scanning electron microscopy (FEGSEM) with energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis. Isothermal oxidation at 900 °C was carried out to study the effect of Al₂O₃ on the growth of oxide on the coatings surface. The composite coatings exhibited superior oxidation behavior against the conventional metallic coatings with the formation of nearly exclusively Al₂O₃ on the coatings surface and inhibited NiO growth.