Experimental and modelling study on the β-phase depletion behaviour under the growth of spinel oxides and alumina in a CoNiCrAlY coating

This paper presents an experimental and modelling study on the β-phase depletion behaviour in a vacuum plasma sprayed (VPS) CoNiCrAlY coating during isothermal oxidation. The microstructure of the as-deposited CoNiCrAlY coating before and after oxidation exposure was characterised by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was shown that the as-deposited CoNiCrAlY coating retained the γ + β two phase structure from the feedstock powders. A dual oxide layer, consisting of an outer spinel oxides and an inner alumina scale, was formed in the as-deposited VPS CoNiCrAlY coating during isothermal oxidation. The β-phase depletion behaviour under the growth of spinel oxide and alumina scale was modelled using Thermo-Calc and DICTRA software. Both Ni and Al fluxes were considered at the boundary to reflect the loss of Ni and Al to form the spinel oxides and alumina scale at the coating surface. Good agreement between the experimental and modelled β-phase depletion was obtained. The small discrepancy between the experimental measurements and calculated results is likely due to the infinite boundary flux at the beginning of the diffusion calculation and the overestimated Al flux from the experimentally determined oxide growth kinetics.