The various thermal spraying methods available include the plasma process, which uses a plasma flame to melt a fine powder before it is sprayed onto a substrate, and the High Velocity Oxy-Fuel (HVOF) spray process, in which the flame is made from the combustion of oxygen. These methods differ both in the temperature and velocity with which the molten particles impact the substrate, leading to different coating characteristics. This includes differences in splat morphology and the nature of microstructural interactions at the splat-substrate interface. That is, features such as local melting of the substrate, the existence of porosity and the presence of oxides. For this study a nickel-chromium powder was sprayed onto mirror-polished stainless steel substrates using both plasma spray and HVOF to form single splats. These splats, and their interface with the substrate, were characterized using a range of microstructural characterization techniques and the observed differences were correlated to the spray conditions used.

A study of plasma-sprayed NiCr splats shows that the morphology and characteristics of the splat-substrate interface depends on the substrate material. Splats on stainless steel tend to be regular and disc-shaped and localized melting of the substrate and intermixing and diffusion were observed at the interface. Splats on aluminum, on the other hand, were slightly more irregular and the contact at the splat-substrate interface was found to be porous and poor. For both types of splats, several oxide phases were observed. Their formation and the formation of the splats are discussed, although further study is required to fully identify the origins of the differences observed.