N,S,P-Modified Carbon Dot Nanoadditives for High-Load Antiwear Protection for Steel-to-Steel Contact

Carbon dots are attractive lubricant nanoadditives because their nanoscale size and tunable surface chemistry allow them to interact with confined friction interfaces. However, achieving effective antiwear protection under high contact loads requires surface structures that can couple physical filling with interfacial film formation. Here, N, S, and P-modified carbon dots (NSP-CDs) were prepared from glucose-derived carbon using taurine and phytic acid as heteroatom sources. The resulting NSP-CDs had an average size of about 2.5 nm and exhibited abundant oxygen-, nitrogen-, sulfur-, and phosphorus-containing functional groups. When used as nanoadditives, 0.3 wt % NSP-CDs markedly improved the antiwear performance of steel-to-steel contacts. At 392 N and 1400 rpm, the average wear scar diameter decreased by 47.5% compared with the base oil, while the wear volume and apparent wear rate both decreased by about 92.4%. Wear scar morphology, elemental mapping, and X-ray photoelectron spectroscopy showed that NSP-CDs formed a composite protective film containing a carbonaceous adsorption layer, iron oxides/hydroxides, P–O/Fe–O–P-related phosphates, and oxidized sulfur species. The antiwear effect is attributed to the combined action of nanoparticle filling, rolling-polishing, and N, S, and P-assisted interfacial film formation. These results suggest that multielement surface modification is an effective strategy for designing carbon dot nanoadditives for high-load antiwear lubrication.