Microstructural study of HVOF sprayed Ni particles on a grit-blasted stainless-steel substrate

Abstract

Ni feedstock particles were sprayed onto a grit-blasted stainless-steel substrate using the high-velocity oxy-fuel (HVOF) deposition process to analyse the flattening behaviour and bonding features of single splats on a roughly textured substrate surface. Both substrate surface features and splat morphologies were analysed through scanning electron microscopy (SEM), while cross-sectional features, including the nature of the splat-substrate interface, were characterised through focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). The chemistry of both the splat and substrate were analysed by energy-dispersive X-ray spectroscopy (EDS) interfaced to both the SEM and TEM. It was observed that despite the surface roughness, fully melted splats demonstrated good flattening performance, as well as efficient bonding, with the substrate surface. Such bonding was revealed through elemental interdiffusion across the splat-substrate interface. The particle size and the location of impact on the rough surface played important roles in defining the melting state, flattening degree and, ultimately, adhesion of the particles. Fully melted splats derived from smaller feedstock particles demonstrated good flattening behaviour, comparable to that observed on polished surfaces. Such particles exhibited better bonding efficiency than the partially melted splats, which originated from relatively larger feedstock particles. The presence of residual alumina contaminants, retained after grit blasting, is also analysed and discussed.