Synthesis of polyethylene glycol modified carbon dots as a kind of excellent water-based lubricant additives

The polyethylene glycol (PEG) modified carbon dots (CDs-PEG) were facile synthesized by one-pot pyrolysis of the mixture of gluconic acid and PEG. When the average molecular weight of PEG was 200, 600, 1000 and 4000 g/mol, the CDs-PEG were denoted as CDs-PEG200-4000, respectively. As the water-based lubricant additives, the tribological properties of CDs-PEG200 under four-ball mode and steel/steel contact were far superior to PEG200, CDs derived from gluconic acid only, and CDs-PEG600-4000, reflecting the carbon cores and surface groups of CDs-PEG200 should play a synergetic lubrication effect. Specifically, at load of 40 N, adding 0.20 wt% of CDs-PEG200 into base liquid resulted in the largest friction coefficient and wear volume reductions up to 83.5% and 90.9%. Meanwhile, the load carrying capacity of base liquid increased from 50 N to at least 200 N. Amazingly, the lubrication ability of base liquid containing 0.20 wt% of CDs-PEG200 sometimes was even comparable to some lubricant oils. The results of worn surface analyses revealed that the distinguished friction-reducing and antiwear performance of CDs-PEG200 as additives could be attributed to the formed composite lubrication film composed of CDs-PEG200 and Fe3O4 on the rubbing surfaces.