Transforming water-soluble carbon dots with high particle size regularity as the high-performance oil-based lubricant additives by one-step surface modification

Developing high-performance carbon dot (CD)-based lubricant additives towards non-polar base oils is always challenging because it is an arduous task to gain hydrophobic CDs with high quality. In this work, with the aid of the chemical reaction between hydroxyl groups and lauroyl chloride molecules, the water-soluble CDs-OH derived from the solvothermal method with particularly high particle size regularity were transformed as the oil-soluble CDs-LA by the one-step surface modification process. As expected, the CDs-LA capped by abundant long alkyl chain groups exhibited excellent long-term dispersion stability in non-polar base oil of polyalphaolefin (PAO4) and hence could be served as the additives of PAO4 directly. Under the same test conditions, the tribological performance of CDs-LA was obviously superior to the hydrophobic CDs obtained by the pyrolysis method, implying that the high particle size regularity was beneficial for making full use of the lubrication functions of CDs. Loading 1.7 wt% of CDs-LA made the friction and wear of PAO4 reduced by 32.2 % and 76.4 %, respectively. When the duration prolonged from 20 min to 200 min, the friction reduction of base oil triggered by CDs-LA further enlarged to 46.9 %. The friction-reducing and anti-wear performance of CDs-LA under high loads was also satisfactory. The worn surface analysis results demonstrated that the robust and homogenous boundary lubrication films composed of FexOy and CDs have generated on the rubbing surfaces lubricated by CDs-LA/PAO4 dispersion, well accounting for the outstanding friction reduction and wear resistance functions of CDs-LA.