Thin Friction The rubbing motion between two surfaces is always hindered by friction, which is caused by continuous contacting and attraction between the surfaces. These interactions may only occur over a distance of a few nanometers, but what happens when the interacting materials are only that thick? Lee et al. (p. 76; see the Perspective by Müser and Shakhvorostov) explored the frictional properties of a silicon tip in contact with four atomically thin quasi–two dimensional materials with different electrical properties. For all the materials, the friction was seen to increase as the thickness of the film decreased, both for flakes supported by substrates and for regions placed above holes that formed freely suspended membranes. Placing graphene on mica, to which it strongly adheres, suppressed this trend. For these thin, weakly adhered films, out-of-plane buckling is likely to dominate the frictional response, which leads to this universal behavior. A universal trend is observed for the friction properties of thin films on weakly adhering substrates. Using friction force microscopy, we compared the nanoscale frictional characteristics of atomically thin sheets of graphene, molybdenum disulfide (MoS2), niobium diselenide, and hexagonal boron nitride exfoliated onto a weakly adherent substrate (silicon oxide) to those of their bulk counterparts. Measurements down to single atomic sheets revealed that friction monotonically increased as the number of layers decreased for all four materials. Suspended graphene membranes showed the same trend, but binding the graphene strongly to a mica surface suppressed the trend. Tip-sample adhesion forces were indistinguishable for all thicknesses and substrate arrangements. Both graphene and MoS2 exhibited atomic lattice stick-slip friction, with the thinnest sheets possessing a sliding-length–dependent increase in static friction. These observations, coupled with finite element modeling, suggest that the trend arises from the thinner sheets’ increased susceptibility to out-of-plane elastic deformation. The generality of the results indicates that this may be a universal characteristic of nanoscale friction for atomically thin materials weakly bound to substrates.

Frictional Characteristics of Atomically-Thin Sheets

This paper describes the synthesis, structure, and tribological behavior of nanocomposite tungsten disulphide (WS2) solid lubricant films grown by atomic layer deposition. A new catalytic route, incorporating a diethyl zinc catalyst, was established to promote the adsorption and growth of WS2. The films were grown down to 8 nm in thickness by sequential exposures of WF6 and H2S gases in a viscous flow reactor on Si, SiO2, stainless steel, and polycrystalline Si and electroplated Ni microelectromechanical systems structures. Films were studied by cross-sectional transmission electron microscopy (XTEM) with Automated eXpert Spectral Image Analysis (AXSIA) software for X-ray spectral images and X-ray diffraction to determine the coating conformality and crystallinity. The coatings exhibited a hexagonal layered structure with predominant preferentially orientated (0 0 2) basal planes. Regardless of orientation to the substrate surface, these basal planes when sheared imparted low friction with a steady-state friction coefficient as low as 0.008 to 50,000 cycles in a dry nitrogen environment. The formation of smooth transfer films during wear provided low interfacial shear stresses during sliding thus achieving low friction and wear. The XTEM combined with AXSIA of the wear tracks identified this mechanism and the effects of vapor phase reaction by-product etching on insulating and native polycrystalline Si and Ni surfaces.

Growth, structure, and tribological behavior of atomic layer deposited tungten disulphide solid lubricant coatings with applications to MEMS.

Continuously increasing global population and, therefore, energy consumption as well as diminishing resources combined with environmental aspects such as global warming ask for more efficient, sustainable and reliable processes/applications of mechanically moving parts. Especially under harsh conditions, such as high temperatures, vacuum or dry contacts, 2D layered nano-materials used as solid lubricants have demonstrated to be promising candidates to ensure low friction and wear over the entire component’s lifetime. Therefore, this review article aims at summarizing the existing state-of-art regarding solid lubricants with a special emphasis on 2D layered nano-material beyond graphene including graphene oxide, reduced graphene oxide, MoS2, WS2 as well as Ti3C2Tx MXene nanosheets. Initially, experimental approaches allowing for a large-scale and layer-dependent synthesis are reviewed for each nano-material. Subsequently, their friction and wear mechanisms at the nano-scale are discussed. Afterwards, the ability to improve friction and wear are reviewed when using the aforementioned 2D nano-materials either as a solid lubricant, lubricant additive under lubricated conditions or reinforcement phase in composite materials. Finally, the existing challenges and shortcomings of each 2D nano-material are discussed before deriving the general conclusions and giving some future research directions.

2D nano-materials beyond graphene: from synthesis to tribological studies

Few-layer WS2 modified BiOBr nanosheets with enhanced broad-spectrum photocatalytic activity towards various pollutants removal

Nanoparticles and two-dimensional (2D) nanosheets are well-investigated as lubricant additives, which can significantly reduce frictional energy consumption. However, the tribological properties of the additives will deteriorate because of the occurrence of aggregation in the lubricant and the difficulty in entering the frictional contact area. In the present work, the new sandwichlike nanostructure of Mn3O4 nanoparticles and graphene nanosheets (Mn3O4@G) has been developed by an in situ green synthesis method; i.e., the impurities of Mn2+ ions in crude graphite oxide as the precursor are directly transferred into Mn3O4 precipitate between the graphene sheets. The graphene has a lamellar structure without folds and wrinkles, and the Mn3O4 nanoparticles are not only uniformly anchored on the graphene surfaces but also intercalated in the layers of the graphene nanosheets. The Mn3O4@G exhibits excellent tribological properties and high stability because of a synergistic lubrication effect between the graphene nanosheets and the Mn3O4 nanoparticles. Even at an ultralow concentration (0.075 wt %) and a high temperature of 125 °C, the friction coefficient and the wear depth have been reduced by 75% and 97% compared with base oil, respectively. The synthesis method and the Mn3O4@G nanocomposite have significant potential in various tribological applications for saving energy.

In Situ Green Synthesis of the New Sandwichlike Nanostructure of Mn3O4/Graphene as Lubricant Additives.

Molybdenum disulfide (MoS2) is a promising candidate for use as a supercapacitor electrode material and non-noble-metal electrocatalyst owing to its relatively high theoretical specific capacitance, Pt-like electronic feature, and graphene-like structure. However, insufficient electrochemically active sites along with poor conductivity significantly hinder its practical application. Heteroatom doping and phase engineering have been regarded as effective ways to overcome the inherent limitations of MoS2 and enhance its ion storage and electrocatalytic performance. In this study, a plasma-assisted nitrogen-doped 1T/2H MoS2 heterostructure has been proposed for the first time, resulting in excellent supercapacitor performance and hydrogen evolution reaction activity. XPS, Raman, and TEM analysis results indicate that N atoms have been successfully doped into MoS2 nanosheets via room-temperature low-power N2 plasma, and the 1T/2H hybrid phase is maintained. As expected, the 1T/2H MoS2 heterostructure after a 10 min plasma treatment displayed a much boosted supercapacitive performance with a high specific capacitance of 410 F g-1 at 1 A g-1 and an excellent hydrogen evolution property with a low overpotential of 131 mV vs RHE at 10 mA cm-2 for hydrogen evolution reaction. The excellent performance is superior to most of the recently reported outstanding MoS2-based electrode and electrocatalytic materials. Moreover, the as-assembled flexible symmetric supercapacitor shows a high specific capacitance of 84.8 F g-1 and superior mechanical robustness with 84.5% capacity retention after 2000 bending cycles.

Controllably Doping Nitrogen into 1T/2H MoS2 Heterostructure Nanosheets for Enhanced Supercapacitive and Electrocatalytic Performance by Low-Power N2 Plasma.

Selective release of less defective graphene during sliding of an incompletely reduced graphene oxide coating on steel

Understanding of the lubrication mechanism of reduced graphene oxide coating via dual in-situ monitoring of the chemical and topographic structural evolution

Yuzhen Liu

Papers

Controllably Doping Nitrogen into 1T/2H MoS2 Heterostructure Nanosheets for Enhanced Supercapacitive and Electrocatalytic Performance by Low-Power N2 Plasma.

Selective release of less defective graphene during sliding of an incompletely reduced graphene oxide coating on steel

Understanding of the lubrication mechanism of reduced graphene oxide coating via dual in-situ monitoring of the chemical and topographic structural evolution

Superior lubrication of dense/porous-coupled nanoscale C/WS2 multilayer coating on ductile substrate

Tribochemical reaction and wear mechanism of MoDTC based friction modifier