Characterisation of transient oxide formation on NiCrAlY after heat treatment in vacuum

Abstract

To better understand transient oxide formation on the surface of standalone NiCrAlY in a vacuum, plasma sprayed NiCrAlY samples were subjected to a series of heat treatments at temperatures ranging from 1000 to 1100°C and for two different holding times. The morphology, composition and type of transient oxide(s) formed after heat treatment were characterised using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction (XRD). All samples exhibited the formation of a top layer of discrete or island like Ni rich oxide in cellular shaped alumina. The alumina layer, although difficult to be detected in cross-section, was very dense and covered most of the coating surface. The transisent oxide formation on the heat treated surfaces was further analysed using XRD and α-Al₂O₃ was detected on all heat treated samples in addition to NiO and possible spinel. Cr₂O₃ seemed to be present on the samples heat treated at 1000 and 1050°C but not on the sample heat treated at 1100°C. Increased Al surface content, in comparison to the as sprayed sample, was found on all heat treated samples. High Al content, corresponding to the extent of alumina formation on heat treated samples, was observed for samples heat treated for a longer time as more oxygen diffusion took place. The coating surface Al content increased with heat treatment from 1000 to 1050°C and reduced from 1050 to 1100°C.