Study of the Splat Formation for HVOF Sprayed NiCr on Stainless Steel Substrates and the Effects of Heating and Boiling Pre-Treatments

The HVOF process is a widely applied thermal spray technique used to form dense coatings with high bond strength. However, little is known about the mechanisms by which the coating forms and adheres to the substrate. The present study investigates the splat formation process by examining the morphology and microstructure of NiCr single splats sprayed on to stainless steel, using a range of electron microscopy techniques. Notable features include evidence of the deformation of the steel substrate by the impacting particle, the presence of porosity towards the centre of the splat, and under the rim due to the curling-up of the splat, and the identification of several oxide phases, including Cr2O3, FeO and NiO. From these observations, a description of the splat formation process is proposed. Effects of the substrate surface chemistry were studied by comparing the morphology of the splats on several substrates having undergone various pre-treatments.