Effect of Modified Graphene on the Tribotechnical Characteristics of Electrolytic Nickel–Molybdenum Coatings

Abstract— The effect of co-deposited graphene oxide (GrO) modified with COOH groups on the tribological characteristics of electrolytic nickel–molybdenum coatings has been studied. The coatings were formed on the surface of polished high-carbon steel by electrodeposition from an aqueous electrolyte based on nickel sulfate (NiSO4) and sodium molybdate (Na2MoO4) with dispersed graphene oxide. The coatings were obtained at current densities of 2.5 A/dm2 and GrO concentrations from 0.05 to 0.2 g/L. Tribotechnical tests have shown that introduction of GrO at a concentration of 0.1 g/L makes for the formation of a coating with a uniform structure ensuring a minimum friction coefficient (0.38–0.45) and a wear track width (25–110 µm) under a load of 0.25–1.0 N. A dependency of tribological characteristics on the concentration of graphene particles in the working solution was revealed: at 0.1 g/L, a uniform distribution of GrO particles is achieved, which leads to a 30–40% reduction in the friction coefficient and a 40–55% decrease in wear track width compared to the base Ni–Mo coating. When the particle concentration exceeds 0.1 g/L, an increase in coating roughness is observed, which is likely caused by agglomeration and excessive inclusion of filler particles into the coating structure; all this results in the formation of a rougher and more heterogeneous surface, thereby degrading the tribological properties of the coating. The results confirm the promising use of GrO in the composition of electrolytic Ni–Mo coatings to reduce friction and increase wear resistance.