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

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.