Development of continuous compositional gradient Ni-W coatings utilizing electrodeposition for superior wear resistance under sliding contact

In the present work, the widely used ammonia-citrate bath for electrodeposition of homogeneous Ni-W coatings has been judiciously modified and a strategy has been developed to deposit dense nanocrystalline Ni-W coatings with continuous compositional gradient across the coating thickness. It was observed that the ammonium chloride, which is typically used to control the pH and increase the Faradaic efficiency of the bath, affects the compositional gradient profile in Ni-W coatings. The concentration of ammonium chloride has been optimised to obtain gradient Ni-W coatings with linear compositional variation in the W-content. Utilizing chemical stability diagrams and the existing theories, a plausible deposition mechanism for the influence of ammonium chloride has been proposed by correlating the W-content with the concentration of mixed metal complexes. The compositional gradient Ni-W coatings have demonstrated significantly lower residual stresses and superior scratch resistance compared to their homogeneous counter parts.