Tribological behavior of nanocarbon materials with different dimensions in aqueous systems

Due to the widespread use of nanocarbon materials (NCMs), more researchers are studying their tribological performances. In this work, the tribological behaviors of the following five types of NCMs with different geometric shapes were evaluated in a novel oil-in-water system: spherical fullerenes (C60, 0D), tubular multi-walled carbon nanotubes (MWCNT, 1D), sheet graphene oxide (GO, 2D), sheet graphene oxide derivative (Oct-O-GO, 2D), and lamellar graphite (G, 3D). Among these, GO with two types of oxidation degrees, i.e., GO(1), GO(2), and Oct-O-GO(1) were synthesized and characterized using Fourier-transform infrared spectroscopy, Raman spectroscopy, x-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and contact angle measurements. The load-carrying capacity of the NCM emulsions were evaluated using a four-ball test machine, and the lubrication performances were investigated using a high-frequency reciprocating friction and wear tester with a sliding distance of 1,800 mm under different loads (50 N and 100 N) at 0.5 Hz. The results revealed that the Oct-O-GO(1) emulsion exhibited the best load-carrying capacity, and the best friction-reducing and anti-wear properties compared to other emulsions. Moreover, the anti-wear advantage was more prominent under high load conditions, whereas the other emulsions exhibited a certain degree of abrasive or adhesive wear. The lubrication mechanism was determined through the analysis of worn surfaces using scanning electron microscopy/energy-dispersive x-ray spectroscopy, micro-Raman spectroscopy, and x-ray photoelectron spectroscopy. The results revealed that during frictional sliding, the ingredients in the emulsion can absorb and react with the freshly exposed metal surface to form surface-active films to protect the surfaces from abrasion. Moreover, it was found that the higher the amount of ingredients that contain alkyl and O-H/C=O, the better was the lubrication performance in addition to an increase in the carbon residue in the tribofilm generated on the meal surface.