Zero-Dimensional Carbon Quantum Dots and Alkylphosphonate Ionic Liquids as Synergistic Water-Based Lubricant Additives for Ultralow Friction at Steel Interfaces

Abstract

Water-based lubricants offer clear sustainability advantages but remain difficult to apply on steel surfaces because of their inherently weak load-carrying capacity and the challenge of forming a stable low-friction interface. To address this problem, a noncovalent compounding strategy was designed that pairs long-chain alkylphosphonate ionic liquids (ILs) with carbon quantum dots (CQDs) derived from citric acid (CA). A short-chain, medium-chain, and long-chain IL were synthesized and screened, and the best-performing IL was physically compounded with the CQDs in a water-polyethylene glycol base fluid. When the IL and CQDs were added at 3 and 1 wt %, respectively, the coefficient of friction (COF) reached 0.012 under the optimum test condition (196 N and 1450 r/min), corresponding to an approximately 97% reduction relative to the neat base fluid, and the wear volume decreased to approximately 0.68% of that of the neat base fluid. The zero-dimensional nanostructure and abundant surface functional groups of the carbon quantum dots contribute to enhanced dispersion stability, nanoball-bearing lubrication, and tribofilm formation, leading to the outstanding synergistic tribological performance of the hybrid lubricant. Worn surface analysis showed that the phosphonate anions formed a protective tribofilm rich in iron phosphate species, the quaternary ammonium cations assembled into an ordered adsorbed layer, and the CQDs acted as nanoscale ball bearings that converted sliding friction into rolling friction while filling and repairing surface defects. The hydrophilic groups on the CQDs and the PEG chains further contributed to boundary lubrication by forming a hydration layer through hydrogen bonding with water molecules. This hierarchical interface, constructed entirely through noncovalent synergy, provides a practical route toward high-performance and environmentally friendly water-based lubricant additives for engineering steel contacts.