Carbon quantum dots doped with silver as lubricating oil additive for enhancing tribological performance at various temperatures

Carbon quantum dots (CQDs) doped with silver (Ag-CQDs) was prepared using a facile hydrothermal method. With the combination of ultrasound vibration and ionic liquid (IL), the dispersion stability of the Ag-CQDs additive in poly alpha olefin (PAO) was improved. The tribological performance of the CQDs and Ag-CQDs as additives over a wide temperature range were evaluated through four-ball tests and a ball-on-disk reciprocating configuration. The four-ball test results revealed that the CQDs and Ag-CQDs additives could effectively reduce the coefficient of friction (COF) of the base oil and wear scar diameter (WSD) of lower balls. When the concentration was 0.05 wt%, the Ag-CQDs additive could lead to 42.2% lower COF and 35.3% lower WSD than the base oil. Meanwhile, the tribological performance at elevated temperatures results revealed that the CQDs and Ag-CQDs additives could substantially decrease friction and wear when the ambient temperature was above 200 °C. The Ag-CQDs exhibited better lubricating performance than the CQDs. The lubricating mechanism was attributed to the formation of a tribofilm composed of Ag-CQDs, multicomponent oxides, carbonates and nitrides induced by tribochemical reactions, which provided effective protection for the rubbing pair and maintained low friction and wear.