Effects of surface oxidation during HVOF processing on the primary stage oxidation of a CoNiCrAlY coating

Nanostructured CoNiCrAlY coating specimens, prepared from cryomilled powder, were deposited using HVOF thermal spraying. During the thermal spraying process some of the particles oxidized, resulting in a surface oxide on the coatings. In order to investigate the effects of this surface oxide on the subsequent oxidation behavior of the CoNiCrAlY coating, the surface oxide on some of the coating specimens was removed by polishing the surface with fine sandpaper. Both as-sprayed and polished specimens were isothermally oxidized at 1000 °C. It was found that the oxide scales that formed on the as-sprayed and polished coatings after 1 and 24 h oxidation were different, i.e. while a θ- and α-alumina duplex scale was formed on the polished coating, an oxide scale, containing spinel-type oxides, was formed on the as-sprayed coating. These findings indicate that the surface oxide that forms on a CoNiCrAlY coating during the HVOF thermal spray process can significantly affect its subsequent oxidation behavior.