Haojie Lang

Controllable Nanotribological Properties of Graphene Nanosheets

Graphene as one type of well-known solid lubricants possesses different nanotribological properties, due to the varied surface and structural characteristics caused by different preparation methods or post-processes. Graphene nanosheets with controllable surface wettability and structural defects were achieved by plasma treatment and thermal reduction. The nanotribological properties of graphene nanosheets were investigated using the calibrated atomic force microscopy. The friction force increases faster and faster with plasma treatment time, which results from the increase of surface wettability and the introduction of structural defects. Short-time plasma treatment increasing friction force is due to the enhancement of surface hydrophilicity. Longer-time plasma treatment increasing friction force can attribute to the combined effects of the enhanced surface hydrophilicity and the generated structural defects. The structural defects as a single factor also increase the friction force when the surface properties are unified by thermal reduction. The surface wettability and the nanotribological properties of plasma-treated graphene nanosheets can recover to its initial level over time. An improved spring model was proposed to elaborate the effects of surface wettability and structural defects on nanotribological properties at the atomic-scale.

Nanotribological Behavior of the Single Silver Nanowire on Graphite

The nanotribological characteristics of silver nanowires (Ag NWs) are of great importance for the reliability of their applications in flexible nanodevices involving mechanical interactions. The frictional behaviors of Ag NWs on graphite substrate were directly investigated by atomic force microscopy (AFM) nanomanipulation. The relatively short NWs demonstrate three forms of motion-rotation, translation and a combination of the two-whose frictional behaviors behave like rigid rods. The relatively long Ag NW shows characteristics of a flexible beam, whose friction increases with an increase in the bending angle of the NW. The friction between the NW and substrate increases linearly with an increase in the length of the NW. The long Ag NW displays extraordinary flexibility that can be folded to different shapes, and the friction of the folded NW becomes smaller due to the decreased bending deformation. The critical aspect ratio of the Ag NW on graphite substrate for two different frictional behaviors between the relatively long and short NWs is found to be 12-15. These findings can deepen the understanding of the frictional characteristics of Ag NWs and contribute to their quantitative interface design.

Enhanced tribological properties of composite films based on ionic liquids with MoS2 nanosheets as additives

High-performance composite films based on ionic liquids (ILs) and two-dimensional MoS2 have attracted considerable attention because of their unique structure and properties, especially in the field of lubrication. Ultrathin MoS2 nanosheets obtained via a liquid exfoliation process exhibited good dispersion in the organic solution of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) without the addition of any other additives. A series of uniform composite films with different amounts of MoS2 were spin-coated onto the hydroxylated Si/SiO2 wafers, and the tribological performance of the composite films was studied. The load-carrying capacity and anti-wear properties of the IL films were greatly improved with the addition of MoS2 nanosheets, and the enhanced tribological properties of the composite films were attributed to the synergistic effect of the IL and MoS2. The low-friction composite films may provide a kind of reference and be useful for the design of ultralow friction films for practical lubrication applications.

Probing the difference in friction performance between graphene and MoS 2 by manipulating the silver nanowires

Graphene and molybdenum disulfide (MoS2) are promising solid lubricants to deal with the interfacial friction and adhesion concerns in silver nanowires (Ag NWs)-based nanodevices, but their difference in friction performance is seldom considered. Here, the difference in friction performance between graphene and MoS2 has been comparatively studied by manipulating the Ag NWs on graphene and MoS2 surfaces through atomic force microscopy (AFM) tip-on-side and tip-on-top manipulations. The tip-on-side and tip-on-top manipulations demonstrate the atomically thin MoS2 has better friction performance than graphene. The tip-on-top manipulation further shows the shear strength in NW–MoS2 interface is smaller than in NW–graphene interface. The underlying mechanism for the friction difference between the two interfaces is attributed to the different interfacial adhesion interactions. The relatively small adhesion interaction in NW–MoS2 interface leads to a small interfacial shear strength, resulting in a small friction when the NW slides on MoS2 surface. The measured water contact angles, calculated work of adhesion and the adhesion force directly measured by AFM tip confirm the relatively small adhesion interaction in NW–MoS2 interface. These findings suggest that MoS2 may be more appropriate as lubricants for application in the NW-based nanodevices.

Dynamic Sliding Enhancement on the Friction and Adhesion of Graphene, Graphene Oxide, and Fluorinated Graphene

Graphene and functionalized graphene are promising candidates as ultrathin solid lubricants for dealing with the adhesion and friction in micro- and nanoelectromechanical systems (MEMS and NEMS). Here, the dynamic friction and adhesion characteristics of pristine graphene (PG), graphene oxide (GO), and fluorinated graphene (FG) were comparatively studied using atomic force microscopy (AFM). The friction as a function of load shows nonlinear characteristic on GO with strong adhesion and linear characteristic on PG and FG with relatively weak adhesions. An adhesion enhancement phenomenon that the slide-off force after dynamic friction sliding is larger than the pull-off force is observed. The degree of adhesion enhancement increases with the increasing surface energy, accompanied by a corresponding increase in transient friction strengthening effect. The dynamic adhesion and friction enhancements are attributed to the coupling of dynamic tip sliding and surface hydrophilic properties. The atomic-scale stick-slip behaviors confirm that the interfacial interaction is enhanced during dynamic sliding, and the enhancing degree depends on the surface hydrophilic properties. These findings demonstrate the adhesive strength between the contact surfaces can be enhanced in the dynamic friction process, which needs careful attention in the interface design of MEMS and NEMS.