Tribological properties and molecular dynamics analysis of citric acid modified 3D porous graphene/carbon black as lubricant additives

Abstract

Graphene holds significant potential as a green additive for lubricating oils, but its poor dispersion stability limits its applications. In this study, citric acid-modified graphene (CAG) was prepared by modifying 3D porous graphene with citric acid, achieving stable dispersion in lubricating oil for up to six months without the need for dispersants. The lubricating properties were investigated using a four-ball friction tester, and wear scars were characterized using the depth-of-field microscope, Scanning electron microscopy (SEM), and Raman spectroscopy. The experimental results demonstrated that adding a composite (CC) of CAG and carbon black to castor oil reduced the coefficient of friction (COF) and anti-wear by 24.57 % and 70.27 %, respectively, under a load of 392 N. Under a load of 588 N, the COF and anti-wear decreased by 32.59 % and 56.54 %, respectively, while the last non-seizure load (P_B) increased by 125.26 %. The sheet-like CAG adsorbed onto the friction surface, enhancing the oil film's load-carrying capacity, while the spherical carbon black acted as microbearings, converting part of the sliding friction into rolling lubrication. A synergistic effect between CAG and carbon black was observed, with the composite exhibiting better lubricating performance than either material alone. Moreover, molecular dynamics simulation results revealed that adding CAG alone enhanced the lubricant's adsorption but reduced its mobility. The combined addition of CAG and carbon black retained the improved adsorption while enhancing the lubricant's mobility, further confirming the synergistic action between CAG and carbon black.