Microstructural investigation of bonding and melting-induced rebound of HVOF sprayed Ni particles on an aluminum substrate

In this study, nickel powder was sprayed on a polished aluminum substrate using the high-velocity oxy-fuel spraying process. Detailed microstructural characterization was conducted to analyse the mechanims of splat formation, with particular attention to the changes occurring at the splat-substrate interface and the subsequent bonding behaviour due to the impact of high thermal and kinetic energy particles with a soft, low melting point substrate. Characterization was performed using scanning electron microscopy, focused ion beam microscopy, transmission electron microscopy, together with the scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. It was observed that a large number of particles had rebounded upon impact, which was correlated with the degree of substrate melting and the associated time for re-solidification. The remnant splats that adhered to the substrate surface were found to exhibit both mechanical and metallurgical bonding with the substrate. The experimental results were supported by transient finite element based thermal simulation.