Influence of surface roughness and running-in on the lubrication of steel surfaces with oil containing MoS2 nanotubes in all lubrication regimes

Despite several studies that have confirmed the beneficial effect of MoS2 and WS2 nanoparticle-assisted lubrication, an understanding of how the nanoparticles behave in different, even very common contact conditions, such as roughness, is still missing. As a result we have focused on a comparison of the lubrication behaviour of MoS2 nanotubes mixed with PAO oil using steel surfaces with different roughnesses. Moreover, we have investigated the MoS2-nanotubes-assisted lubrication of steel/steel contacts in all lubrication regimes and also the effect of the running-in of these contacts. It was realized that the friction with the nanotubes-containing oil was 40–65% lower compared to the base oil, depending on the different contact conditions used. Furthermore, we showed that by using MoS2 nanotubes in the oil the friction is the same for rough and smooth steel surfaces, meaning that the nanotubes completely govern the lubrication behaviour in self-mated steel contacts in the boundary- and mixed-lubrication regimes, irrespective of the surface roughness or the running-in.