Wear characteristics of nickel electrodeposits in ultrasonically agitated bath

Wear rates of nickel electrodeposits obtained from a conventional Watts bath were studied. A comparison was made between the wear rates of nickel electrodeposits obtained with an ultrasonically agitated bath and those obtained with a still bath. Deposition was done on mild steel, and wear rates were measured for unlubricated rubbing against a rotating hardened steel disc. The wear loss of the deposits obtained with the ultrasonic bath was lower than that with the still bath. When ultrasonic waves are passed through the electrolyte solution, bubbles are formed which grow and collapse due to continuous absorption of energy from alternating compression and expansion cycles of the sound waves. This results in work-hardening of the surface, which leads to higher microhardness and higher wear resistance. Nickel plating is widely used for decorative purposes. It also finds application to minimize abrasive wear in cases such as sliding contacts on 2.0 hydraulic rams. The use of ultrasound in a plating bath has been found to improve the surface quality of the deposit. The application of ultrasonic irradiation during plating affects the deposition rate and internal stress [1]. Ultrasound has decreased whisker growth, giving smoother surfaces [2], The fatigue strength of mild steel deposited with nickel is found to be improved when deposition is done in an ultrasonic field [3]. This work was aimed at evaluating the extent to which ultrasonic irradiation influences the wear resistance of the deposit. The composition of the plating bath was 250 g 1-1 nickel sulphate (NiSO4.6H20), 60g1-1 nickel chloride (NiC12.7H20) and 30g l -~ boric acid (H3BO3) in distilled water. Circular specimens, 30 mm in diameter and 5 mm thick, were plated at a current density of 3.5 A dm -2 both in an ultrasonic bath and in a still bath. The ultrasonic generator had a frequency of 22 kHz and a maximum rated capacity of 500 W. The thickness of the deposit was maintained at 40/zm for each specimen. The wear-testing rig used was as described elsewhere [4]. The mating disc was hardened steel of hardness 700 kgmm -2. Both the counter disc and the specimen were of the same diameter and the counter disc rubbed the specimen coaxially. The test Speed duration in all experiments was 30 min. The speci(r.p.m.) mens were weighed before and after the wear test. 700 The wear losses of the deposits were evaluated at 8oo different speeds. The spring load on the specimen 9oo against the counter disc was varied from 7.8 to 1000 11 kgf. 1100 Fig. 1 shows the wear rate of the specimens tested at different sliding speeds. The sliding speed is given by 27rrn/60, where n is the shaft speed in r,p.m. Wear rates were calculated using W = w/dLP, where w is the weight loss of the deposit during testing, d is the density of nickel, L is the sliding distance and P is the normal load. The wear rates of the coatings at different speeds and sliding speeds are listed in Table I. Specimens coated in an ultrasonic field showed a higher wear resistance than nickel coated under still conditions. Fig. 2 shows the wear loss per unit area at different loads, and corresponding data are given in Table II. Optical micrographs were taken after wear testing. Figs 3 and 4 show wear tracks of the specimens plated in a still bath and an ultrasonically