Influence of compositional and microstructural gradient on the wear performance of nanocrystalline Ni–W coatings

Although the nanocrystalline Ni–W coatings offer superior wear resistance, under sufficiently higher loads and strain rates they often suffer from the strain localization which leads to a severe wear damage. Further, the presence of tensile residual stresses exaggerates the situation. Herein, we report a facile approach of microstructural gradation to circumvent this problem. Present study investigates the sliding wear performance of gradient nanocrystalline Ni–W coatings prepared by pulse electrodeposition. By tailoring the W-content along the deposition direction, a concomitant modulation in the microstructure, hardness and elastic modulus was obtained as revealed by the depth-wise glancing incidence XRD and nanoindentation, respectively. The gradient Ni–W coatings have demonstrated a low coefficient of friction (CoF) and superior wear resistance over their homogeneous counterparts. The micro-Raman spectroscopy has indicated the formation of tribo-oxide layer composed majorly of CoWO4, WO3 and NiO on the surface of the wear tracks. Based on the crystal-chemical model, the lower CoF of gradient Ni–W is attributed to the presence of a stable and adherent tribo-oxide. Furthermore, the enhanced wear resistance of the gradient Ni–W coating is ascribed to the lower tensile residual stresses, strain delocalization and its higher thermal diffusivity.