Improved tribological behavior of lubricating oil dispersed with hybrid nanoparticles of functionalized carbon spheres and graphene nano platelets

Highlights: • A novel synthesis of FCS/GNP hybrid has been reported. • The formation of hybrid was confirmed using SEM, TEM and Raman spectroscopy. • The tribological performance of the hybrid was investigated using a four ball tester. • An 18% reduction in coefficient of friction was obtained for the FCS/GNP hybrid-2 (10:1) in neat oil. A novel synthesis method of Functionalized Carbon Spheres (FCS) coated with Graphene nanoplatelets (GNP) using a simple sonication technique is reported. The FCS/GNP hybrid nanoparticles were found to be effectively improving the tribological characteristics of lubricating oils. For the synthesis of FCS, Carbon Spheres (CS) synthesized via non-catalytic chemical vapor deposition was functionalized using nitric acid. The hybrid nanoparticles were characterized using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Raman spectroscopy. The tribological behavior of CASTROL-20 W 40 engine oil dispersed with different concentrations of FCS and GNP using ultrasonication was investigated using a four-ball tester. The tribological behavior of FCS and GNP were investigated separately to compare the tribological performance of the FCS/GNP hybrid nanoparticles. The neat oil dispersed with 0.275 wt% of FCS/GNP hybrid nanoparticles in 10:1 ratio shows excellent dispersion and stability for 30 days without the addition of any surfactant. The results indicate that the addition of these hybrid nanoparticles causes a remarkable reduction (~18%) in the Coefficient of Friction (COF) of the neat oil. The optical image of wear scar on the ball immersed in neat oil shows a significant reduction in the Wear Scar Diameter from 425.6 µm to 390.0 µm with the addition of hybrid nanoparticles. The Raman mapping of the wear scar on chromium steel ball reveals that the FCS/GNP hybrid nanoparticles remain unseparated after the tribology test.