Electrodeposition of nickel-carbon nanotube nanocomposite coatings for enhanced wear resistance

Composite electrodeposited coatings consisting of hard or lubricous micron sized reinforcement particles within a metal matrix have been used commercially in a number of high wear resistance and low friction applications over the last 40 years. In recent years, it has been demonstrated that this process may be extended into the nanoscale regime, using high performance nanoscale reinforcement particles which offer a number of advantages that justify their higher cost. In this study, nickel-carbon nanotube (Ni-CNT) composite coatings were prepared by electrodeposition from a Watts electrolyte containing both unmodified and acid treated (functionalised) CNTs. The Ni-CNT coatings fabricated using functionalised CNTs exhibited significantly improved homogeneity compared to the coatings fabricated from unmodified CNTs, as well as higher and more consistent hardness. Both Ni-CNT coatings (fabricated from functionalised CNTs) and pure nickel coatings were then subjected to wear testing in reciprocating dry conditions over a range of loads and CNT volume fractions. The influence of CNTs on the wear properties of the nickel matrix was assessed using profilometry and SEM. It was found that CNTs substantially improved the wear resistance of the electrodeposited nickel, reducing wear rate to as little as a third of that of the unreinforced electrodeposit. The pure nickel coatings exhibited no transfer film formation, whereas the Ni-CNT coatings exhibited an oxide layer which was well adhered; it is proposed that the CNTs themselves promote the development of the adhered oxide transfer layer.