Branched polyethyleneimine modified carbon nanoparticles as the effective additives of water lubrication

Branched polyethyleneimine (PEI) modified carbon nanoparticles (CNPs-PEI) with an average particle size of 20.1 nm were facile synthesized by hydrothermal treatment of citric acid and PEI. The PEI and oxygen-containing groups on CNPs-PEI endowed them with excellent dispersion stability in water. The tribological properties of CNPs-PEI as the water-based additives were evaluated under the four-ball mode, steel/steel contact and fixed load of 40 N by varying additive concentration. Amazingly, adding only 0.15 wt% of CNPs-PEI made the mean friction coefficient and wear volume of base liquid reduced by 53.4% and 94.9%, respectively, reflecting that the CNPs-PEI could be an effective additive of water lubrication. The wear scar surface analyses indicated that CNPs-PEI as additives promoted the formation of boundary lubrication films on the rubbing surfaces by mending and depositing effects, directly accounting for the distinguished friction-reducing, antiwear and oxidation resistance properties of CNPs-PEI.