Robust Superlubric Interface across Nano- and Micro-Scales Enabled by Fluoroalkylsilane Self-Assembled Monolayers and Atomically Thin Graphene.

Robust superlubrication across nano- and microscales is highly desirable at the interface with asperities of different sizes in durable micro/nanoelectromechanical systems under a harsh environment. A novel method to fabricate superlubric interfaces across nano- and microscales is developed by combining a batch of surface modification with atomically thin graphene. The robust superlubric interface across nano- and microscales between hydrophobic 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) self-assembly monolayers (SAMs) and graphene was achieved under high relative humidity, sliding speed, and contact pressure. The superlubric mechanisms at the interface of FDTS/graphene could be attributed to the following at different scales: the hydrophobicity of FDTS SAMs and graphene preventing the capillary interaction of the interfacial friction under high relative humidity; the high elastic modulus of graphene leading to small interfacial contact area; the compressing and orientating of FDTS SAMs decreasing interfacial shear strength under high contact pressure; the surface modification of FDTS molecules reducing the interfacial potential barriers when sliding on the atomically thin graphene. The robust superlubric interface across nano- and microscales reducing the friction at the complicated interfaces with asperities at different scales and improving the performance and durability have great potentials in the field of micro/nano mechanical systems.

Impact of the Surface and Microstructure on the Lubricative Properties of MoS2 Aging under Different Environments.

Molybdenum disulfide (MoS2) with a hydrophobic property and layered structure possesses an excellent lubricative property and has been widely used as a lubricant in various areas, including satellites, aircraft, and new energy vehicles. Aging is a ubiquitous phenomenon in MoS2 and plays a key role in its tribological application for shortening its service life. The effect of the surface and microstructure on the lubricative properties of MoS2 aging under different environments, including deionized water (DI water), ultraviolet/ozone (UV/Ozone), and high-temperature, was investigated. First, the lubrication of MoS2 transiently degrades because of physical adsorption and recovers after mechanical removal. The lubrication of MoS2 also degrades slightly when its surface becomes hydrophilic, thereby enhancing the adhesion energy due to atomic oxygen interaction under UV/Ozone exposure. Second, the lubrication of MoS2 degrades irreversibly because of the formation of stripes with the destroyed structures under accelerated aging. The lubrication of MoS2 further degrades with the formation of small triangular pits under high-temperature annealing. Finally, the lubrication of MoS2 deteriorates due to the destroyed structure and complete oxidation. The severe aging of MoS2 is accompanied with large triangular pits due to anisotropic oxidation etching of MoS2. The lubrication failure of MoS2 was determined on the basis of structural defect formation and surface property degradation induced by the extent of oxygen diffusion. The enhanced out-of-plane deformation due to the reduced out-of-plane stiffness and the increased energy barriers of defects are fundamentally responsible for the lubrication degradation of MoS2 at the atomic scale. These findings can provide new insights into the atomic-scale mechanism underlying the lubrication failure of MoS2 and pave the way for the realization of MoS2-based lubrication application under various environments.

Atomic-Scale Friction Characteristics of Graphene under Conductive AFM with Applied Voltages.

The current-carrying nanofriction characteristics play an important role in the performance, reliability, and lifetime of graphene-based micro/nanoelectromechanical systems and nanoelectronic devices. The atomic-scale friction characteristics of graphene were investigated using conductive atomic force microscopy by applying positive-bias and negative-bias voltages. The atomic-scale friction increased with applied voltages. Also, the friction under positive-bias voltages was lower than under negative-bias voltages, and the friction difference increased with the voltages. The different frictional behaviors resulted from the inherent work function difference and the water molecules between the tip and graphene. The applied voltages amplified the effect of the work function difference on the friction, and the water molecules played different roles under negative-bias and positive-bias voltages. The friction increased rapidly with the continuous increase of negative-bias voltages due to the electrochemical oxidation of graphene. Nevertheless, the friction under positive-bias voltages remained low and the structure of graphene was unchanged. These experimental observations were further explained by modeling the atomic-scale friction with a modified Prandtl-Tomlinson model. The model allowed the determination of the basic potential barrier and the voltage-induced potential barrier between the tip and graphene. The calculation based on the model indicated that the negative-bias voltages induced a larger potential barrier than the positive-bias voltages. The studies suggest that graphene can show a better lubricant performance by working as a lubricant coating for the cathodes of the sliding electrical contact interfaces.

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.

Dependence of the friction strengthening of graphene on velocity.

Graphene shows great potential applications as a solid lubricant in micro- and nanoelectromechanical systems (MEMS/NEMS). An atomic-scale friction strengthening effect in a few initial atomic friction periods usually occurred on few-layer graphene. Here, velocity dependent friction strengthening was observed in atomic-scale frictional behavior of graphene by atomic force microscopy (AFM). The degree of the friction strengthening decreases with the increase of velocity first and then reaches a plateau. This could be attributed to the interaction potential between the tip and graphene at high velocity which is weaker than that at low velocity, because the strong tip-graphene contact interface needs a longer time to evolve. The subatomic-scale stick-slip behavior in the conventional stick-slip motion supports the weak interaction between the tip and graphene at high velocity. These findings can provide a deeper understanding of the atomic-scale friction mechanism of graphene and other two-dimensional materials.

Nanotribological behavior of a 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.

An ultra-low frictional interface combining FDTS SAMs with molybdenum disulfide.

Interfacial friction is of crucial importance to ensure the friction-reducing and anti-wear properties of mechanical microstructures in micro/nanoelectromechanical systems (MEMS/NEMS). An ultra-low frictional interface combining hydrophobic 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) self-assembled monolayers (SAMs) coated on an AFM tip with mechanically exfoliated molybdenum disulfide (MoS2) nanosheets deposited on a planar Si/SiO2 substrate was achieved. The FDTS SAMs/MoS2 interface between the FDTS SAMs and the MoS2 nanosheets exhibits an ultra-low friction force that is independent of the relative humidity. The incommensurate contact with ultra-low energy dissipation between FDTS and MoS2 nanosheets and hydrophobic surface properties lead to this ultra-low frictional FDTS SAMs/MoS2 interface. Also, the MoS2 nanosheets have a high elastic modulus, which gives them a smaller contact area than the FDTS SAMs and contributes to the low friction. The excellent hydrophobic properties of both the FDTS SAMs and MoS2 enable them to be unaffected by the relative humidity by preventing the capillary interaction. This study paves the way for extensive applications in reducing the friction of nanoscale contact interfaces.