Onion-like carbon nanoparticles as additives in lubricant oil deform inelastically during the wear testing for improved lubrication performances

Nano-carbon has been extensively investigated as an additive to lubricant oil for improved performance. It is still far from a final verdict as to how the physicochemical features of the carbon nanomaterials affect their lubrication efficacy. Here we report the use of onion-like carbon (OLC) particles with the size of 5-10 nm as additives to lubricant oil, and their role in regulating the lubrication is elucidated through characterizing the wear-induced changes to their physiochemistry. Multiple loads are chosen for the wear testing using the four-ball friction tester. Stable dispersion of the OLC particles in lubricant oil was attained by pre-coating the particles with oleic acid molecules. Results showed that the oil with the treated OLC particles of 0.125 wt% reduced the wear scar by 30 % in diameter and the wear volume by 77 %. It was surprisingly noted that incorporation of the OLC nanoparticles particularly alleviated the heavy loading wear by remarkably reducing the friction coefficient. Raman spectroscopy characterization suggested intact chemistry of the particles after the wear testing. However, curvature changes of the particles were seen as revealed by high resolution transmission electron microscopy after the wear testing, indicating inelastic deformation of the particles triggered by the wear-squeezing.