Mechanisms and improvements in the friction and wear behavior using MoS2 nanotubes as potential oil additives

Several types of nanoparticles have been experimentally investigated as possible lubrication agents and the results suggest that this is a promising idea. However, so far, MoS2 nanotubes have not been experimentally evaluated in this respect. Accordingly, this study is focused on the tribological behavior of MoS2 multi-wall nanotubes (MWNTs) as a potential additive in lubricating oils. The experiments were performed in the boundary-lubrication regime under a contact pressure of 1 GPa (Hertz, max) and a sliding velocity of 0.005 m/s using a ball-on-disc tribotester. The results were compared to a reference base oil, and it was found that MoS2 nanotubes significantly decreased the friction and wear compared to the base lubricant. The friction was more than 2-times lower and the wear as much as 5–9-times lower. Several tribological mechanisms and effects due to the MoS2 nanotubes are presented. It was established that a wear-protective and low-shear film was forming on the surface. This tribofilm was formed either by (i) the adhesion of thin MoS2 nano-sheets on the surface, where these sheets were deposited on the surface by one of the four possible sub-mechanisms proposed in this work; or by (ii) the compaction and deformation of nanotube aggregates, which resulted in a thicker boundary film. Thus, in contrast to the rolling of the nanotubes, which we do not consider to be plausible, the exfoliation and deformation of the nanotubes were found to be the prevalent effects for the nanotubes in the boundary-lubrication regime.

► Investigation of boundary-lubrication with MoS2 multi-wall nanotubes in PAO oil.
► Mechanisms for the significant friction and wear reduction are discussed.
► Two mechanisms for different tribofilm formations were proposed.
► Four mechanistic routes for the adhesion of MoS2 nano-sheets to the surface were proposed.