Effect of carbon quantum dots’ surface functional groups on tribological performance of polyethylene glycol

Nano additives play an important role in improving the lubricating properties and enhancing the bearing capacity of lubricants. However, traditional nano-additives contain harmful elements such as heavy metals, sulfur, phosphorus, which have a non-negligible impact on the green production of modern industry. In this study, four kinds of carbon quantum dots (CQDs) were prepared by hydrothermal synthesis method, namely CQDs, amino modified CQDs (CQDs-NH 2 ), alkyl modified CQDs (CQDs-CH 3 ), and hydroxyl modified CQDs (CQDs-OH). The microscopic morphology, chemical structure and luminescence of CQDs were characterized. The effects of four kinds of CQDs additives on the tribological properties of polyethylene glycol (PEG) were studied. The results showed that compared with PEG, the average friction coefficient and wear volume of PEG containing 3% CQDs-OH decreased by 60.50% and 95.05%, respectively. A composite protective film containing carbon, nitrogen, FeO, Fe 2 O 3 forms on the worn surface, due to the end hydroxyl group of CQDs-OH has good adsorption. In addition, after 3 months of dispersion experiment, the PEG containing 3% CQDs-OH showed the best dispersion stability, and the antifriction effect was still better than that of pure PEG. Therefore, CQDs-OH is a new type of environmentally friendly lubricating additive with great potential.