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.
COBISS.SI-ID: 12245787
Diamond-like carbon (DLC) coatings provide low friction and wear in the most demanding tribological contacts. However, their chemical reactivity with oil additives is poor and difficult to optimise. Moreover, even the partially effective, but high-SAPS (sulphuric ash, phosphor, sulphur) additives, will be phased out in the near future for environmental reasons. Based on recent advancements in the nanotechnology of inorganic MoS2 and WS2 nanoparticles, which lubricate through the low shear of basal planes, we propose a potential replacement of the current chemical-based lubrication with this novel, physical-based additive lubrication technology for poorly reactive DLC coatings. In our work, 30 % less friction compared to steel surfaces and 50 % less friction compared to the base oil was achieved by employing MoS2 nanotubes in the base oil in self-mated DLC contacts. This physical-based lubrication technology represents an innovative solution for highly effective but non-reactive surfaces and simultaneously provides green-lubrication performance.
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.
COBISS.SI-ID: 12730139
DLC coatings are low friction and low wear coatings and thus inherently possess some of the characteristics that may be beneficial or even enable green lubrication requirements. In particular, their low wear behavior may represent a potential for different, less harmful and more environmentally adapted lubrication compared to conventional materials, which may become an attractive approach for novel green technologies in various mechanical systems. In this work, we present some of several possible strategies for designing green lubrication, focusing only on concepts that are well-suited and enabled in a combination with specific properties of DLC coatings. These may include approaches through predominantly chemical-based interactions, physical-based lubrication technologies, and a combination of both, i.e. physical-chemical properties.
COBISS.SI-ID: 12491035
MoS2 and WS2 nanoparticles, on the one hand, and DLC coatings, on the other, are receiving increasing attention for tribological applications. However, investigations that combine DLC coatings and lubrication using nanoparticles are very scarce and the understanding of such tribological systems remains limited. In this work we looked at such a lubrication system by studying the effects of surface roughness and running-in on the behaviour of DLC-coated contacts in all lubrication regimes. We also present a model of how the surface roughness influences the mechanism of lubrication for the MoS2 nanotubes. This model is based on a 2D 1:1 projection scale of dimensions of the surface profile, including its asperities, nanoparticles and film thicknesses, and is thus independent of the contacting materials. It was realized that the addition of the nanotubes to the base oil lowered the coefficient of friction of the DLC by more than 50 % for the smooth, DLC-coated surfaces and up to 40 % for the rough, DLC-coated surfaces. The nanoparticles were the most effective under boundary-lubrication conditions and had a negligible effect in the EHL regime. The surface roughness has a notable and two-fold effect: while the friction was lower on the smooth, compared to the rough, DLC-coated surfaces, the rough surfaces were better able to retain the nanoparticles within the contact during the running-in.