Microstructural studies of effect of substrate pre-heating temperature on the splat formation for Ni-5Al particles plasma sprayed onto stainless-steel substrates

In this study, the Ni-5Al powder was plasma sprayed onto polished 304 stainless-steel substrates heated to a different temperature (room temperature, 150^oC and 300^oC). Detailed investigations of the splat microstructure and splat-substrate interface were carried out by using scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). The substrate pre-heating resulted in a change in the surface roughness, which significantly reduced the splashing and promoted the formation of disk-shaped splats. More interestingly, energy dispersive spectroscopy (EDS) mapping, performed on the TEM cross-sections, showed inter-diffusion and inter-mixing between the splat and the substrate on both non-heated and heated substrates, although there was no evidence of substrate melting. Of particular note a ‘jet’ of iron/chromium-based material, presumably from the substrate, was observed to be intimately intermixed into the interior of the nickel splat. Moreover, oxidation of Al occurred at various location around the splat on the heated substrates, which resulted in the formation of alumina and formation mechanism of alumina is discussed.