Microstructure and high temperature oxidation behaviour of thermally sprayed CoNiCrAlY, NiAl and CoNiCrAlY+NiAl coatings

This thesis 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 (Co,Ni)Cr₂O₄ 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 was formed, comprising an inner alumina layer and an outer layer of NiAl₂O₄ spinel on the NiAl coating, while a (Co,Ni)(Al,Cr)₂O₄ spinel formed as the outer layer on the CoNiCrAlY+NiAl coating. It was demonstrated that the addition of β-NiAl powder was able to facilitate the growth of alumina in CoNiCrAlY coatings. Furthermore, it was found that interdiffusion zones (IDZs) formed at the coating/substrate interface after oxidation at 1100  °C for 100 h: the CoNiCrAlY coating exhibited the thickest IDZ (~58.2 μm) with Cr-rich oxides above the interface, while thinner Ni-rich IDZs formed in the NiAl (~22.3 μm) and CoNiCrAlY+NiAl (~37.6 μm) coatings.

Keywords: CoNiCrAlY coating; NiAl coating; Microstructure; Atmospheric Plasma Spraying (APS); Oxidation;