Anti-wear and friction reducing mechanisms of carbon nano-onions as lubricant additives

Carbon nano-onions have better tribological properties than graphite powder when used as additives dispersed in a poly-alpha-olefin base oil. Carbon nano-onions give a better dispersion in the liquid base oil due to their nanometre-scale size. In particular, the anti-wear efficiency of carbon onions under boundary lubrication and mild wear regime is much better than that of graphite powder. This effect can be attributed to the different structure of the carbon layers in the two species. High-resolution transmission electron microscopy and electron energy-loss spectroscopy were used to characterize the carbon samples, and significant differences in the carbon layer spacing and the density were shown. Wear debris were also observed by the same techniques. In the tribofilm material we depicted new chemical and crystal nano-structures species similar to some microstructures observed in chondrite meteorites, the most interesting one being maghemite iron oxide. The origin of the low friction and wear is still largely unknown.