Carbon nanophases in ordered nanofluid lubricants

Protic ammonium carboxylate tri-[bis(2-hydroxyethylammonium)] citrate (DCi) ionic liquid has been used as neat lubricant of the stainless steel-sapphire pair. In order to improve its tribological performance, few-layers graphene nanoplatelets (G) or 4 wt.% nanodiamonds aqueous suspension (ND) have been dispersed (in a nanophase proportion of 0.1 wt.%) in DCi. Both carbon nanophases reduce the friction coefficient of neat DCi. The new graphene-containing (DCi+G) dispersion covers the sliding path with a protecting layer that prevents wear. This anti-wear effect is not observed for (DCi+ND). Water-free thin-film boundary lubricant layers have been generated on the steel surface under static conditions. Thin films reduce the amount of lubricant and the friction coefficients with respect to the corresponding full-fluid lubricants. Wear mechanisms and surface analysis results obtained by optical profilometry, SEM microscopy, TEM microscopy, XPS and Raman spectroscopy, are discussed. The results reported here show the possibility of reducing both the toxicity and the cost of ionic liquid lubricants and nanophases.