Effect of electroless Ni plating on the properties of cold-sprayed Ni–Al₂O₃ coatings

This study investigated the effect of electroless nickel (Ni) plating (N_p) on the properties of cold-sprayed nickel–alumina (Al₂O₃) coatings. To develop this two-layered coating system, nickel–alumina powder coating was first deposited by the cold spraying method on a mild steel substrate. Then, electroless N_p was deposited on the as-sprayed nickel–alumina coating. The coating properties of the as-sprayed nickel–alumina coating and of the two-layered N_p/nickel–alumina system were critically examined. The microstructural evolution was analysed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. In order to study possible changes in the mechanical properties of the system, Vickers microhardness experiments on the coatings and on the cross-sections were conducted. A possible improvement in the microhardness (14%) of the system was found after N_p. Further, the corrosion behavior of the coating system was also studied critically. It was observed that the corrosion resistance of the N_p/nickel–alumina coating system significantly improved and no corrosion effects and crack generation were observed at the substrate–coating interface.