Hydrophobic Nitrogen and Sulfur-Codoped Carbon Dots Synthesized via a Solvent-Involved Approach as Lubricating Additives for Enhanced Tribological Performance

Carbon dots (CDs) are promising nanobased lubricating additives due to their rigid structure, wide availability of raw materials, and exceptional potential to enhance antiwear and friction reduction performance, but most of them are severely hindered by their hydrophilicity. In this work, hydrophobic nitrogen and sulfur-codoped CDs (N,S-CDs) were synthesized using glucose and thiourea as precursors and dodecylamine as both solvent and surface passivating agent. The successful attachments of long-chain alkyl groups onto CDs demonstrate the involvement of solvent in the reaction. Four components with distinct surface functional groups and polarity were isolated using silica column chromatography, and a formation sequence proceeding from 4-CD to 3-CD to 2-CD and finally to 1-CD was achieved. Although the four N,S-CDs have different dispersibilities in polyalphaolefin 6 (PAO6), they exhibited significant enhancement in the tribological performance. Under a load of 392 N, the friction coefficient of PAO6 with 0.5 wt % of 4-CD was 27.7% lower than that of pure PAO6. The best antiwear property was achieved with PAO6 containing 0.75 wt % of 2-CD, in which the wear scar diameter and wear volume were significantly reduced by 36.9 and 84.2%, respectively, relative to pure PAO6. The formation of tribofilm and the mechanism of enhancing lubrication by N,S-CDs are proposed based on the analysis results of the worn surface by scanning electron microscopy, X-ray photoelectron spectroscopy, and Raman mapping.