A novel approach to decrease friction of graphene

Abstract

Graphene as a well-known solid lubricant is widely used in micro- and nano-scale mechanical devices, decreasing the friction of graphene as far as possible is a perpetual task to improve the performance of these devices. A novel approach was proposed to decrease the friction of graphene against atomic force microscopy (AFM) tip by plasma treatment of the substrate. The plasma treatment of the substrate enhances the adhesive attraction between graphene and SiO₂ substrate by generating stronger van der Waals attraction. Enhancing the adhesive attraction can indeed decrease the friction of graphene, regardless of the thickness of graphene and the kinds of AFM tip. Longer time of plasma treatment results in stronger adhesive attraction, leads to smaller friction. The decreased friction is mainly due to the combined action of the suppressed puckering of graphene and the reduced ability of graphene to adjust its atomic configuration. This novel approach will promote the engineering application of graphene and other related 2D materials as lubricants in MEMS/NEMS.

Introduction

Micro- and nanoelectromechanical systems (MEMS/NEMS), such as accelerometers in air bags, ink pumps in inkjet printers and micro-mirror arrays in computer projectors, have a major impact on the manufacturing industry [1]. However, the effects of adhesion and friction are challenging the development and commercialization of MEMS/NEMS devices [2]. For example, a MEMS actuator (electrostatic lateral output motor) operated in vacuum fails very quickly due to the catastrophic wear of device components (micro-dimples) [3]. Graphene, as a building unit for graphite, is an ideal candidate for solid lubricants that can be used to reduce the adhesion and friction between contact surfaces in MEMS/NEMS devices while protecting the coated surface [4], [5], [6]. Considering that anti-friction and wear-resistance is an eternal subject for promoting the engineering application of graphene in MEMS/NEMS devices, more new and practical approaches are needed to decrease the friction of graphene.

In previous studies, Lee et al. found the reduced friction can be achieved by increasing the number of graphene layers on SiO₂ substrates [7]. Meanwhile, this good frictional performance was also obtained on graphene deposited on mica, where the graphene was strongly bound to the substrate [7]. Wang et al. studied the tribology of reduced graphene oxide sheets covalently assembled onto silicon wafers, and obtained the good friction reduction and antiwear ability, which was ascribed to the covalent bonding to the substrate [8]. Analogously, the covalent bonding between graphene and substrate interface decreasing the friction of graphene was also discovered by Paolicelli et al. They found the shear strength and work of adhesion on graphene-Ni(111) interface were always smaller than that on graphene-SiO₂ interface, which resulted form the covalent bonding in graphene-Ni(111) interface [9]. However, by increasing the number of graphene layers to decrease the friction can not meet the requirements of ultrathin lubricating layer in nano-scale mechanical devices. Forming the covalent bonding is not applicable to most of the interfaces in engineering application. But these previous studies imply that the stronger interaction between graphene and underlying substrates enables superior tribological performance.

Since graphene was discovered on SiO₂ surfaces by optical measurement, SiO₂ substrates became the first choice to design and conduct various experiments on graphene, including the measurement of fundamental properties and device fabrication [10]. Meanwhile, NEMS/MEMS were usually fabricated from single- and multilayer graphene sheets by mechanically exfoliating thin sheets from graphite over trenches in silicon oxide [11]. Silicon and silicon oxide are extremely important substrate materials for NEMS/MEMS applications [12]. But it has been experimentally established that graphene is weakly bound to SiO₂ substrates through van der Waals and/or capillary forces, which suggests the interfacial adhesion between graphene and SiO₂ is much weak compared with covalent bonds [13], [14]. This weak interfacial adhesion will increase friction force and prevent the practical applications of graphene in NEMS/MEMS devices. How to enhance the adhesive attraction between graphene and underlying SiO₂ substrate in a practical and universal way is of great significance.

Plasma treatment has proven to be a relatively simple but effective technique for introducing some certain of chemical groups on the surface so as to improve the surface adhesion [15], [16]. Furthermore, this technique can offer a wide range of surface modifications by means of altering the discharge parameters. Given this, the interfacial adhesion between graphene and SiO₂ substrate can be enhanced by plasma treating the SiO₂ surface.

Here, a novel and universal approach was proposed to decrease the friction of mechanical exfoliated graphene by plasma treatment of the substrate. The interfacial adhesion between graphene and SiO₂ substrate was enhanced to varying extents with different time of plasma treatment. The friction of graphene was investigated by atomic force microscopy (AFM). Plasma treatment of the substrate enhances the adhesive attraction between graphene and SiO₂ substrate. The enhanced adhesive attraction between graphene and SiO₂ substrate decreases the friction force due to the combined actions of two aspects.