Microtribology and Direct Force Measurement of WS2 Nested Fullerene-Like Nanostructures

Abstract

While experiments did not confirm earlier speculations on exceptional lubrication behavior of carbon-based fullerenes, inorganic fullerene-like nested nanostructures (IFs) were recently reported to show improved tribological behavior when added to lubricant fluids. [1] In this report, we confirm the tribological advantages of WS 2 IFs as additives to tetradecane between two shearing mica surfaces, and show that the lower friction in this system is associated with friction-induced material transfer of WS 2 from the IFs to the mica surfaces. While the material transfer occurs via delamination and structural degradation of the IFs, their addition to conventional lubricant fluids provides an effective route for in situ deposition of an ultrathin solid lubricant coating on the shearing surfaces.Spherical molecules such as C 60 and C 70 were speculated to have tribological advantages due to their high frequency rotation in the solid lattice, which would allow them to act as molecular ball bearings at shearing interfaces. Nevertheless, despite the large number of experimental studies, this issue remains controversial. Addition of C 60 to fluid lubricants has been shown to dramatically reduce the viscous drag at the solid±liquid interface, [2±5] and other earlier studies reported encouraging results. [6,7] On the other hand, organic fullerene-based solid lubricant films were reported to show disappointing friction and wear properties, [8±10] failed to produce coatings of commercial value, [11] and fullerene additives were reported to offer no advantage over conventional additives. [12] Coatings of Mo and W chalcogenides were reported to be effective solid lubricants [13] which exhibit a prolonged wear life. [14] The tribological properties were shown to be highly sensitive to the atmosphere due to chemical reactions with oxygen and water under shearing conditions. [15±18] While the low friction of metal chalcogenide solid lubricants is often explained by the facile shear of the c-planes of the 2H lattice, this mechanism cannot explain their unusually long

Friction mechanism of individual multilayered nanoparticles

“…For the sliding mechanism, which also provides low friction, the NP should also be mechanically stable and exhibit low adhesion and chemical affinity to the underlying surfaces. In the exfoliation mechanism, the multilayered IF NP act as reservoir of low friction layers which cover the mating surfaces with low friction thin layers, a few monolayers thick (15).…”

“…This structural deformation gives rise to dislocation and dislodging of WS 2 nanosheets from the NP surface. The external nanosheets of the IF NP, a few monolayers thick each (15), are gradually transferred onto the substrate, thus reducing the friction between the two mating surfaces. Furthermore, this process reduces the local heating, and hence the plastic deformation of the underlying metal surface and its oxidation, thereby slowing down the wear (16 of the tribological behavior.…”

Tribological performance of novel nickel-based composite coatings with lubricant particles

“…Again, ploughing-like abrasive wear would be the wear mechanism in this case. The causes for this beneficial presence of the WS 2 particles in lubricated tribosystems are diverse: (i) rolling friction [20], (ii) WS 2 particles acting as spacers, preventing the contact between the asperities in both metal surfaces [20], (iii) third body material transfer (i.e., gradual transference of layers of WS 2 onto the metal surfaces during repeated sliding) [21], and (iv) the gradual formation of a thin WS 2 film after continuous and repeated sliding [22]. In the latter referenced study, a thin black WS 2 film gradually formed on the metal surface, very similar to the black film observed on the contact surface of the test ball used for the scratch test performed on the Ni/WS 2 composite coating (Fig.…”

“…Nevertheless, the potential presence and contribution of these WS 2 -rich regions on the surface of the wear track must not be discarded, and more detailed studies are on their way in order to fully understand the friction mechanisms that may take place when WS 2 particles are incorporated into Ni coatings used as tribologically active layers in non-lubricated tribological systems. The latter is of particular importance, as all of the more fundamental research dealing with the friction mechanisms in systems with the presence of WS 2 particles is mainly focused on the use of WS 2 particles in lubricated systems [20][21][22][23][24][25][26].…”

Mechanical, Adhesive and Thermodynamic Properties of Hollow Nanoparticles

“…Inorganic fullerenes have also been shown to offer favorable tribological properties [5]. Detailed investigations with the Surface Force Apparatus demonstrated that low friction and wear is caused by material transfer onto the sliding surfaces [6]. For these applications, a good theoretical understanding of the physical properties of hollow nanoparticles is important.…”