Robust and self-lubricating composite coating by electrodeposition for tribological applications

Wang, Xingyu and Wang, Shuncai (2021) Robust and self-lubricating composite coating by electrodeposition for tribological applications. In Leeds-Lyon Symposium on Tribology. (In Press)

Abstract

Solid lubrication shows promising potential for advanced applications in automotive and aerospace industries where the challenging environment of high vacuum and extreme temperature have rendered it unpractical for traditional liquid lubrication. Solid lubricants such as graphite, molybdenum disulphide (MoS2) and tungsten disulphide (WS2) exhibit excellent friction reduction due to their unique lamellar structure. Incorporating these solid lubricants into metal matrix (e.g. nickel, cobalt, copper) via electrodeposition has been a facile and versatile method for self-lubricating composite coating synthesis [1]. Recently our group has successfully deposited Ni-MoS2 coatings with superior low friction of coefficient of 0.08 [2]. However, during electroplating, the conductive nature of co-deposited MoS2 nanoparticles resulted in enhanced electric field in the vicinity of particle absorbed sites. In consequence, more metal ions would be attracted towards these sites due to greater Coulomb force, which accelerated the reduction of ions at these locations and created composite coatings with non-compact and porous structures. These porous structures become a major challenge to achieve long-time durability as they can be easily sheared off during the sliding contact motion.

The current research aims to develop a robust coating achieved by densifying structure of these existing coatings. Our previous study showed that a combination of conductive / nonconductive particles (MoS2 / PTFE) helped to form a compact Ni composite coating. In this research, the effect of mixing different particles on formation of coatings has been systematically investigated. The results revealed that the mixture of conductive and semiconductive particles (MoS2+WS2) could provide a synergetic effect to develop a dense and compact coating surface. The line contact friction test (maximum contact pressure = 0.15 GPa) also testified that the Ni-MoS2/WS2 coating with dense structure was more durable than Ni-MoS2 coating with porous structure. For Ni-MoS2/WS2 coating, friction coefficient remained below 0.1 after 1 h friction test, while Ni-MoS2 coating showed a continuously increasing friction coefficient (from 0.12 to 0.14) during the 1 h test period.

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