Microstructure, Hardness, and Tribological Behavior of Electrodeposited Ni–Mo, Ni–W, and Ni–Mo–W Alloy Coatings

Ni–Mo, Ni–W, and Ni–Mo–W alloy coatings were electrodeposited from citrate-based electrolytes, and their microstructure, hardness, and tribological behavior were systematically investigated. X-ray diffraction results showed that all coatings possessed a nanocrystalline face-centered cubic (fcc) Ni structure without detectable secondary crystalline phases. Scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) analyses revealed dense coatings with distinct surface morphologies and uniform incorporation of alloying elements. The average hardness values of the Ni–Mo, Ni–W, and Ni–Mo–W coatings were 620 ± 6 HV0.2, 685 ± 6 HV0.2, and 1274 ± 4 HV0.2, respectively. Despite exhibiting the highest hardness, the Ni–Mo–W coating showed the poorest wear resistance, whereas the Ni–Mo coating exhibited the lowest wear rate and the most stable friction behavior. Detailed surface observations indicate that the wear behavior is governed by debris generation, microstructural stability, and tribo-film evolution during sliding. The inferior wear resistance of the Ni–Mo–W coating is associated with microcracking, fragmentation, and unstable tribo-oxide layers, while the Ni–Mo coating exhibits mild abrasive–oxidative wear. The results demonstrate that hardness alone cannot determine the wear resistance of Ni-based coatings, and the tribological performance depends on the combined effects of alloy composition, microstructural stability, and interfacial wear processes.