Renewable Carbon Nano-onions Induced Friction and Wear Reduction in Ca-Grease

The increasing electrification of mechanical systems has intensified the need for lubricants that can operate reliably under combined mechanical and electrical loading. In this study, the effects of recycled-plastic-derived carbon nano-onions (CNOs) on the tribological performance of the aluminum-steel interface lubricated by calcium sulfonate grease under applied direct current were investigated. CNOs with a defect-rich graphitic structure were incorporated into Ca-based grease at multiple concentrations under various DC current conditions. Rheological measurements showed that CNOs increase grease viscosity in the absence of an electric field, whereas the application of an electric field decreases viscosity through field-induced structural changes. Electrochemical impedance spectroscopy demonstrated a monotonic decrease in impedance with increasing CNOs content, indicating enhanced electrical conductivity. Tribological results revealed that a low CNOs loading (0.1 wt %) significantly reduced wear by up to ∼35% relative to the base grease and rendered wear largely insensitive to applied current. In contrast, higher CNOs loadings (0.5 and 3.0 wt %) led to pronounced current-induced wear amplification. Surface profilometry and SEM analyses showed a transition from mild wear to electrically assisted pitting and severe abrasive wear at high CNOs concentrations and high current. These results establish a clear conductivity-stability trade-off governing electro-tribological performance and identify an optimal CNOs loading that maximizes wear resistance while mitigating electrically assisted damage. The findings provide practical design guidelines for sustainable grease formulations in electric vehicles and other electrified mechanical systems.