Effects of β-NiAl powder addition on the high temperature oxidation behaviour of thermally sprayed CoNiCrAlY coatings

This paper investigated the effect of β-NiAl addition on the microstructure and high temperature oxidation behaviour of thermally sprayed CoNiCrAlY coatings. Three powders, namely CoNiCrAlY, NiAl and CoNiCrAlY + NiAl blended powders, were deposited onto Ni substrates by atmospheric plasma spraying. The spraying parameters were selected on purpose to consume the β-NiAl phase in the as-received CoNiCrAlY powder, in attempt to reveal the addition of β-NiAl powder on the oxidation behaviour of the CoNiCrAlY + NiAl coating. The coatings were subjected to isothermal oxidation at 1100 °C for periods up to 100 h. The microstructures of coatings before and after oxidation were characterised by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was shown that the CoNiCrAlY coating experienced significant oxidation of in-flight powder particles and exhibited a nearly γ phase structure due to that Al was largely consumed during the spraying process. The NiAl coating was a single β phase structure and the CoNiCrAlY + NiAl coating showed a blended γ+β structure. During oxidation, Cr2O3 and spinel oxides were found at the CoNiCrAlY coating surface, which was due to that the Al content was insufficient to form the alumina scale. A dual oxide layer that consisted of an inner layer of alumina and an outer layer of spinel oxides was developed on the NiAl coating and CoNiCrAlY + NiAl coating. It was demonstrated that the addition of β-NiAl powder was able to facilitate the growth of alumina in CoNiCrAlY coatings.