Investigation of tribological and corrosion performance of duplex electroless Ni-P/Ni-Cu-P coatings

To improve the hardness, wear, and corrosion resistance of substrate materials, the duplex electroless Ni-P/Ni-Cu-P coatings are developed on mild steel substrates. Two types of coatings are deposited with Ni-Cu-P and Ni-P layers once with Ni-Cu-P as the outer layer and subsequently with Ni-P as the outer layer respectively. Heat treatment is carried out at temperatures between 200 and 800°C. Heat treatment is found to significantly affect the coatings. The hardness is increased by about 43 % after heat treatment whereas friction and wear rate are reduced by about 50 % and 40 % respectively. Heat treatment at 400°C is found to be the best for tribological performance of the coatings. The microstructural tests show the formation of nickel phosphide which are responsible for improved hardness of the coatings after heat treatment. However, heat treatment at higher temperature lead to grain coarsening and oxide formation which affect the tribological performance of the coatings negatively. The coating with Ni-Cu-P as the outer layer shows better corrosion resistance ability due to the presence of copper which promotes the formation of passive layer.