Covalently Functionalized Graphene Oxide/Nitrogen-Doped Carbon Dot Composites for Enhanced Tribological Performance

The present investigation demonstrates a green, sustainable approach to synthesizing nitrogen-doped carbon quantum dots (NCDs) as nanoadditives via hydrothermal carbonization (HTC) using Phyllanthus emblica fruit extract. Due to high surface energy, NCDs are highly susceptible to agglomeration and sedimentation over time, which can significantly reduce their dispersion stability and lubrication efficiency in the base oil. To overcome these demerits, the hydrophobic coupling agent (3-aminopropyl) triethoxysilane (APTES) was used as a bifunctional linker to covalently functionalize NCDs via its silane end, forming NCDs-APTES. To achieve superior tribological performance, the amino-terminated functional group of APTES was further utilized for the covalent grafting of NCDs-APTES onto graphene oxide (GO) nanosheets, yielding NCDs-APTES-rGO nanocomposites. The prepared nanomaterials─GO, CDs, NCDs, NCDs-APTES, and NCDs-APTES-rGO─were systematically characterized using p-XRD, FTIR, TGA, Raman, HR-SEM, TEM/HR-TEM, EDX, and XPS, confirming their structural integrity, thermal stability, and successful functionalization. Tribological evaluations conducted on a four-ball tester according to ASTM D4172 standards demonstrate that the optimized concentration (0.005% w/v) of NCDs-APTES-rGO nanoadditives exhibits an 88% reduction in the average coefficient of friction (COF) and a 58% diminution in the mean wear scar diameter (MWD) value. The SEM, AFM, EDX, and XPS analyses of worn surfaces confirmed the formation of a robust, uniform tribofilm composed of SiO 2, Fe 2 O 3, and graphitic compounds, synergistically improving antiwear, antifriction, and load-bearing properties. The order of lubrication performance followed: PEG < GO < CDs < NCDs < NCDs-APTES < NCDs-APTES-rGO, highlighting the synergistic effect of covalent grafting and the formation of an in situ tribofilm. The as-prepared nanomaterials underwent hemolysis assays to check their environmental sustainability and biocompatibility. All samples exhibited nonhemolytic activity, remaining well below the 5% cytotoxicity threshold. This outcome demonstrates their excellent biocompatibility and confirms their nontoxic nature. This sustainable approach offers a green pathway for developing eco-friendly, highly efficient, oil-dispersible NCDs-APTES-rGO additives with enhanced tribological properties for advanced industrial lubrication applications.