Carbon Dots as Ligand Operons to Expand Cluster Size Distribution for High Load‐bearing Liquid Superlubricity

Abstract

Adding additives (e.g., carbon dots, CDs) to liquid lubricants is an effective method for enhancing their load‐bearing capacity and friction reduction. However, it is challenging to simultaneously impart high load‐bearing capacity and superlubricity for them (e.g., ionic liquid analogs, ILAs) through this strategy due to strong Coulombic interactions. With CDs as a competing ligand operon, the cluster size distribution can be expanded, conferring superlubricity with ultrahigh load‐bearing capacity (>705 MPa, the highest value of CDs‐based superlubricious materials thus far) for CDs/ILAs. In particular, methodologies such as Raman, 1H‐NMR, fourier transform infrared spectroscopy, and laser particle size analysis are employed to characterize the evolution of their H‐bonding interactions and particle size distribution. CDs can act as competing ligands and enable CDs/ILAs to form systems with ultra‐low coefficient of friction and wear consisting of a mixture of large and small clusters. The mixed and expanded size clusters facilitate the reduction of the viscosity for CDs/ILAs under shear, thereby reducing the interfacial shear resistance during superlubricity, while the CDs within them efficiently transfer the normal bearing loads. The progress is made from the perspective of cluster particle size distribution rather than individual additive properties, providing a new avenue for designing ILAs‐based superlubricious materials.

Electric Potential Controlled Ionic Lubrication

“…Therefore, the electric potential may promote the electrochemical reaction between the lubricant and surfaces to enhance the lubrication performance but whether this enhancement is reversible still remains unknown. However, due to the difference between micro-and macro-scale experiments, especially under higher loads and speeds at macro-scale conditions, the nano-confined lubricating film may change at macroscopic conditions [45,46]. How to establish the relationship between macro-and micro-scale research already faces a challenge.…”

“…In other words, the normal force is measured by moving the surface at the base of the double-cantilever force spring using the differential micrometer, motor-driven fine micrometer, and piezo tube [61,62]. Experiments show that the normal force versus separation distance curves can be fitted by the Derjaguin-Landau-Vervey-Overbeek (DLVO) theory, which shows the nano-confined interaction between the surfaces clearly [44,45]. The surfaces can also be sheared past each other using a motor-driven micrometer to move the upper surface in the lateral direction, or by using a piezoelectric bimorph slider [61,62].…”

“…The ions in water-based lubricants can form an adsorption layer through the electrostatic interaction, which can bear a large applied load and reduce friction at the interface. Ionic aqueous lubricants even provide favorable conditions for the realization of superlubricity under all lubrication regimes, including boundary lubrication (BL) [69,82,[110][111][112][113][114], mixed lubrication (ML) [14,45,55,107,115,116], and elastohydrodynamic lubrication (EHL) [46,[117][118][119][120]. In a BL regime, the repulsive force (like hydration repulsion) generated by the adsorption of ions on the friction surfaces is conducive to bearing the applied load and reducing the friction.…”