Microstructure and Mechanical–Tribological Properties of HVOF-Sprayed (WC–Co+Ni ) Coatings on Ductile Cast Iron

High Velocity Oxy-Fuel (HVOF) thermal spraying is widely used for the deposition of dense coatings with low porosity, high hardness, and superior fracture resistance. Tungsten carbide–cobalt (WC–Co) coatings are extensively employed in industrial and aerospace applications due to their excellent wear resistance and mechanical performance; however, further improvement in crack resistance and adhesion remains a key challenge. In this study, WC–Co+Ni composite coatings were deposited on ductile cast iron by HVOF, with particular emphasis on the role of Ni particle addition in tailoring coating microstructure and performance. Microstructural characterization was carried out using light, scanning, and transmission electron microscopy (LM, SEM, TEM), while phase composition and chemical analysis were determined by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The coatings exhibited a dense, low-porosity microstructure composed of partially molten Ni particles and fine WC and W₂C carbides embedded in a cobalt-based matrix, with locally nanocrystalline features. XRD analysis confirmed WC and W₂C as the dominant phases, with weak reflections indicating the possible formation of the η-phase (Co₆W₆C). Mechanical and tribological performance, evaluated by instrumented indentation and scratch testing, showed that Ni addition significantly enhances crack resistance, wear resistance, and coating–substrate adhesion. The results demonstrate that Ni-modified WC–Co coatings deposited by HVOF enable effective microstructural design, leading to improved durability and performance, which makes them promising candidates for advanced coating applications.