Carbon quantum dots as lubrication-enhancing additives for DLC/PAO composite systems

Abstract

Lubricant additives are an effective strategy to address the key bottlenecks of diamond-like carbon (DLC)/poly-α-olefin (PAO) solid-liquid composite (DLC/PAO) systems, yet conventional options rely on sulfur/fluorine-containing components, raising environmental concerns. Herein, citric acid-derived carbon quantum dots (CQDs) with an average size of 2.3 nm, graphitic structure, and surface -OH/-COOH/Cdouble bondO groups were hydrothermally synthesized as eco-friendly nano-additives for DLC/PAO systems. Systematic tribological tests reveal that CQDs significantly improve the tribological properties of the DLC/PAO system, with the enhancement effect strongly dependent on CQD concentration. The optimal performance is achieved at 0.50 wt.% CQDs, leading to a reduction in friction coefficient by 34.8% (4 N) and 20.8% (12 N), and a decrease in wear rate by 45.9% (4 N) and 42.7% (12 N) compared to the DLC/PAO system. The lubrication improvement originates from the synergy of DLC surface graphitization and CQDs’ nano-bearing effect. At the optimal concentration, CQDs avoid excessive agglomeration, ensuring timely interfacial repair and stable nano-bearing function to synergistically enhance boundary and hydrodynamic lubrication. High load (12 N) induces oil film failure, intensified interfacial contact, weakened hydrodynamic lubrication, and severe plastic deformation, thereby deteriorating tribological performance. These results provide valuable insights for the optimal design of DLC/PAO systems.