Effects of Surface Chemistry on Splat Formation during Plasma Spraying

Abstract

Abstract In this study, aluminium 5005 and stainless steel 304L substrate surfaces were modified by thermal and hydrothermal treatments. Ni-Cr single splats were deposited onto these substrates at room temperature using plasma spraying. The collected splats were characterised qualitatively and quantitatively by Scanning Electron Microscopy (SEM) and ImageJ software. A splat classification scheme was developed based around splash and disk-type splats. The proportions of the different splat types were found to vary significantly as a function of substrate pretreatment, especially when the pretreatment involved heating. The effect of surface roughness to splat formation and splat shape was investigated. It was observed that surface roughness was not a critical factor in splat morphology. The above substrate surfaces were also characterised by X-ray photoelectron spectroscopy (XPS) using in-situ heating in vacuum to determine the effect of thermal pretreatment on substrate surface chemistry and the oxide thickness. It was found that heat treatment promoted a change in the chemical structure of the oxide surface layer which was consistent with the dehydration of oxyhydroxide to oxide. Dehydration of the substrate surface layer improved the physical contact between the splat and substrate which enhanced the formation of disk-splats, decreased the number of pores evident in the splats and increased number of splats and their diameter.