Microstructural evolution and bonding of HVOF sprayed Ni particles on both mild and stainless-steel substrates

Abstract

In the present study, Ni powder was sprayed onto both mild and stainless-steel substrates using high-velocity oxy-fuel (HVOF) deposition to comparatively analyse the effect of substrate material properties and surface condition on splat formation. A range of microscopy techniques including scanning electron microscopy (SEM), focused ion beam (FIB) microscopy, transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) were employed to characterize both the splat morphologies, including their cross-sectional structure, as well as the nature of the splat-substrate interface. It was shown that the majority of the particles reached the substrate surface in a partially melted form owing to the high velocity typical in the HVOF process. Despite some splat splashing observed on the stainless-steel sample, the diffusion profiles, determined by STEM-EDX line scans, revealed evidence of elemental interdiffusion at the splat-substrate interface, suggesting metallurgical bonding in this sample. It was observed that splat morphologies, their frequency of occurrence and splat-substrate bond quality are all greatly affected by the surface condition of the substrate. These microstructural observations were correlated with the thermo-mechanical characteristics of the substrates to explain the mechanisms driving splat formation. Differences in the degree of plastic deformation of the substrates due to particle impact are also discussed.