Synthesis and properties of electrodeposited Ni-Co/WS2 nanocomposite coatings

Abstract

Ni-Co coatings have gained widespread attention due to their potential in replacing hard chromium deposits (which have traditionally utilized toxic and corrosive chromic acid baths). A major challenge is to lower the high coefficient of friction of coated surfaces against steel, under dry sliding conditions. In this research, low friction Ni-Co/WS2 nanocomposite coatings have been prepared by a convenient, one-pot electrodeposition from aqueous Ni-Co plating baths containing WS2 particles. The embedment of the WS2 lubricants is found to reduce the friction coefficient of coating significantly, and an ultra-low friction coefficient of 0.16 is obtained for the coating having a WS2 content of 7.1 wt.%. Morphology and composition characterization of wear tracks reveal that the formation of a WS2-rich lubricating tribofilm on the contact surfaces is beneficial to a low friction coefficient and good oxidation resistance. The wettability of electrodeposited coatings was also investigated. Compared to pure Ni-Co coating, the Ni-Co/7.1 wt.% WS2 coating has an excellent hydrophobicity with a high water contact angle (WCA) of 157°, due to a rough surface with dual scale protrusions and the low surface energy of WS2.