NiCr single splats were plasma-sprayed on aluminum substrates which were modified by thermal treatments over a range of specific temperatures. It was observed that the splat morphologies changed from highly fingered splats to disk splats with increasing thermal treatment temperature. In addition, the density of the bubble pores which formed at the underside of NiCr splats significantly reduced with an increase of substrate temperature. X-ray photoelectron spectroscopy was used to examine the variation of surface chemistry with thermal treatment. The release of water from the dehydration of an oxyhydroxide layer, which was found to form on the substrate surface, was strongly correlated with the splat morphology and bubble formation at the splat underside. It is proposed that the water desorption from the substrate surface causes turbulence in the spreading process of the splat and the bubble formation at the splat underside, which leads to the splashing of the splat. Preheating or heating the substrate during spraying can partly remove the water prior to spraying, thereby greatly improving the contact between the splats and the substrate which enhanced the formation of a disk splat and significantly reduced the bubble formation at the splat underside.