Carbon dots in tribology

The development of high-performance lubricants to address friction and wear issues in mechanical equipment is critical to achieving the goal of carbon emission peak and carbon neutrality. Carbon dots (CDs), owing to their ultra-small dimensions, tunable surface properties, exceptional biocompatibility, superior dispersion and stability, have emerged as promising candidates in tribology. Based on relevant reports, this review first summarizes the synthesis routes of CDs and their composites, providing a detailed introduction to the synthesis of pure CDs as well as surface modification, heteroatom doping, constructing nanofluids and composite nanoparticles for regulating the tribological properties. Subsequently, it then reviews the applications in tribology, including as lubricants, lubricant additives, lubricant additive promoters, lubricant coatings, and joint lubrication. Importantly, the mechanisms are comprehensively reviewed, following the micro-bearing theory, polishing and repair effects of carbon cores, and the formation of tribofilms on contact area. In actual process, the excellent tribological properties stem from the synergistic effects of two or more mechanisms. Finally, a summary and outlook for this significant and promising field is provided. We hope this review will provide valuable reference for researchers developing novel green high-performance lubricant materials, and promote the application of CDs in tribology.