Improvement of microstructural and tribological properties of multi-layer functional Nİ-W based composite coatings

In this study, single-layer (monolithic) and multilayer Ni–W-based composite coatings were developed on St37 steel substrates via the electrodeposition method, and their structural, mechanical, tribological, and electrochemical properties were comprehensively investigated. The primary objective of this work was to enhance the limited wear and corrosion resistance of conventional Ni–W alloy coatings by employing suitable ceramic and carbon-based reinforcements within a multilayer architectural design. The ultimate goal was to obtain functionally engineered surfaces that exhibit both high hardness and low friction coefficient, suitable for industrial tribological applications operating under simultaneous mechanical and corrosive loads. In the first stage, the Ni–W plating bath was optimized using the Taguchi experimental design approach. Parameters such as current density, temperature, pH, and bath composition were systematically varied, and the optimal conditions yielded dense, crack-free coatings with uniform morphology and high hardness. The optimized parameters were subsequently used as the baseline for composite and multilayer coating fabrication. Following this optimization, graphene and Al2O3 reinforcements were added to the bath at different concentrations to produce single-layer composite coatings. In the final stage, three systems—Ni–W, Ni–W/graphene, and Ni–W/Al2O3—were deposited sequentially in different stacking sequences to construct multilayer architectures. In the naming scheme, G, A, and N denote graphene-reinforced, alumina-reinforced, and pure Ni–W layers, respectively. Throughout the experimental process, bath conditions were kept constant to isolate the effects of the reinforcement phases on the coating properties. The characterization of the coatings was carried out using multiple complementary techniques. SEM, FESEM, and EDS analyses were performed to determine the surface morphology, particle dispersion, and elemental composition, while XRD analyses were employed to identify phase formation, crystallographic orientation, and possible microstrain variations. The XRD results revealed that the amorphous-to-nanocrystalline structural transition strongly depended on the type of reinforcement and that the addition of Al2O3 promoted grain refinement. Micromechanical behavior was assessed through nano- and micro-hardness measurements, while elastic modulus values were determined to evaluate the stiffness of the coatings. Tribological tests were performed under dry sliding conditions using a pin-on-disk setup (applied load: 5 N, sliding speed: 0.1 m/s, sliding distance: 500 m). Electrochemical characterization was conducted in 3.5 wt.% NaCl solution using potentiodynamic polarization (Tafel), open-circuit potential (OCP), and electrochemical impedance spectroscopy (EIS) measurements. For the single-layer coatings, the hardness and elastic modulus of the plain Ni–W coating were measured as 864 HV and 24 GPa, respectively. The optimum graphene concentration was determined as 150 mg/L, at which the coating exhibited a nano-hardness of ~914 HV and an elastic modulus of ~234 GPa. When the graphene concentration was increased to 200 mg/L, particle agglomeration occurred, leading to a reduction in both hardness and modulus. In the Al2O3 reinforced system, the optimum concentration was identified as 15 g/L, corresponding to a hardness of ~1068 HV and an elastic modulus of ~252 GPa. Increasing the Al2O3 content to 20 g/L caused particle clustering and discontinuities, resulting in a deterioration of the mechanical properties. These findings quantitatively confirm that both reinforcements improve mechanical performance up to their optimal concentrations, beyond which excessive loading induces agglomeration and a loss of homogeneity. Tribological analysis demonstrated that the unreinforced Ni–W coating exhibited the poorest performance, with a friction coefficient of μ = 0.49 and a wear rate of 2.08×10−6 mm3/Nm. The incorporation of graphene and Al2O3 significantly enhanced tribological behavior. For the Ni–W/graphene coating, the friction coefficient decreased to μ ≈ 0.35 and the wear rate to 1.76×10-6 mm3/Nm, whereas for the Ni–W/Al2O3 coating, these values were μ ≈ 0.41 and 1.71×10-6 mm3/Nm, respectively. These results indicate an approximate 28% reduction in friction for graphene reinforcement and a 17% reduction for Al2O3 reinforcement, along with a ~20% decrease in wear rate compared to the pure Ni–W coating. When multilayer architectures were evaluated in terms of mechanical, tribological, and electrochemical performance, a pronounced synergistic effect was observed. Among all configurations, the lowest friction coefficient was obtained for the GAN sequence (top layer: graphene), with μ ≈ 0.27, corresponding to a ~45% decrease relative to the single-layer Ni–W coating. The highest wear resistance was recorded for the AGN configuration, with a wear rate of 1.39×10-6 mm3/Nm, representing a ~33% improvement over the monolithic Ni–W coating. These numerical findings confirm that different reinforcement phases, when deposited sequentially in a multilayer structure, interact synergistically to optimize tribological performance. Mechanical testing further revealed that the multilayer configuration containing an Al2O3 middle layer and a graphene-rich top layer exhibited the highest hardness, exceeding 1100 HV. The enhancement in hardness was attributed to the refined grain structure and the formation of dense microstructures induced by heterogeneous nucleation at interlayer interfaces. The lower layers were found to influence the nucleation behavior of subsequent layers, promoting oriented grain growth and improving coating uniformity. The formation of heterogeneous nucleation zones at interlayer boundaries facilitated better adhesion and load transfer, resulting in a uniform hardness distribution and a denser surface. Thus, the multilayer architecture not only represented the simple stacking of dissimilar phases but also functioned as a dynamic system where interlayer interactions continuously refined the overall microstructure. In summary, the results demonstrate that the multilayer Ni–W-based composite coatings provide simultaneous improvements in mechanical, tribological, and corrosion performance compared with their monolithic counterparts. Quantitatively, the multilayer systems achieved a ~45% reduction in friction coefficient, a >30% reduction in wear rate, and an approximately 40% increase in corrosion resistance relative to the single-layer Ni–W coating. The observed synergy originates from the complementary roles of the reinforcements: graphene acts as a solid lubricating film at the surface, reducing shear stresses, while Al2O3 serves as a hard load-bearing phase that strengthens the coating and enhances its chemical stability. Furthermore, interlayer interfaces act as barriers against dislocation motion and crack propagation, providing enhanced mechanical integrity. Overall, this study clearly demonstrates that the integration of graphene and Al2O3 within multilayer Ni–W matrices result in a synergistic enhancement of coating performance. The optimized multilayer architectures—particularly GAN and AGN configurations—exhibit excellent durability, superior tribological stability, and improved electrochemical resistance, positioning these coatings as promising candidates for advanced surface engineering applications in energy, defense, aerospace, and automotive industries, where simultaneous wear and corrosion resistance are critically required.