Effect of Ni sublayer thickness on sliding wear characteristics of electrodeposited Ni/Cu multilayer coatings

The electrodeposition of Ni/Cu multilayer coatings with varied Ni sublayer thickness (tNi) ranging 1.8–11 nm, keeping Cu sublayer thickness (tCu) fixed at 1.97 nm, has been carried out from sulphate based electrolyte. Residual stress and crystal structure of the electrodeposited multilayers were characterized by X-ray diffraction (XRD) method. It was found that the stress level within the deposited multilayer decreases with increase in tNi. Coefficient of friction (COF) and sliding wear test of the deposited multilayer coatings have been carried against standard WC ball in ball-on-plate geometry under dry condition. After wear test, the wear scar morphology was examined under laser surface profilometry and scanning electron microscopy (SEM) for calculation of wear volume and evaluation of wear mechanism respectively. The COF was found to be dependent on load and tNi. A typical sinusoidal variation of COF with sliding distance was noticed indicating stick–slip behaviour under adhesive–abrasive wear process. The hardness of the multilayers was found to be dependent on wavelength of the modulation. Among multilayers, sample with tNi = 7.2 nm has shown the lowest wear rate owing to its high (H/E) ratio, high hardness and low residual stress. The calculated wear rate order was found to be quite well matching with the order of elastic strain to failure, i.e. (H/E) ratio of the multilayers. The multilayers with very high and low tNi showed poor wear behaviour due to very weak multilayer effect.