Interaction mechanism for ionic nitrogen-doped carbon dots and ZDDP combinations as lubricant additives

The advent of low-viscosity lubricants has enhanced the economic viability of fuel utilization. As a concomitant effect, machine workpieces are more prone to operate in the context of boundary or mixed lubrication regime with thin oil film, which in turn elevates the probability of damage to parts. Consequently, there is an increased requirement for the tribological properties of the conventional anti-wear additive zinc dialkyl dithiophosphate (ZDDP) to be more robust. In this study, we have selected the combination of a low amount of ionic nitrogen-doped carbon dots (NCDs) and ZDDP as an additive, the interaction of combined additive serves to mitigate the high friction characteristics of ZDDP while simultaneously enhancing its low wear performance, offering a highly effective solution for a common industry challenge. The wear surface analyses demonstrate that the combination of NCD cations, phosphate ester anions and ZDDP adsorbed on the tribo-pair forms a phosphate hybrid tribofilm with carbon-rich on the surface through tribochemical reaction, which works a synergistic lubrication effect in the friction process, demonstrating the excellent tribological properties of the combined additive. •Ionic NCDs can produce good synergistic lubrication effect with ZDDP.•High friction of ZDDP under mixed lubrication regime is reduced by the self-mending property of ionic NCDs.•The interaction mechanism of ionic NCDs and ZDDP as combined additives has been discussed.