truefalse2016-03-16T23:02:09

To determine the friction coefficient of graphene, micro-scale scratch tests are conducted on exfoliated and epitaxial graphene at ambient conditions. The experimental results show that the monolayer, bilayer, and trilayer graphene all yield friction coefficients of approximately 0.03. The friction coefficient of pristine graphene is less than that of disordered graphene, which is treated by oxygen plasma. Ramping force scratch tests are performed on graphene with various numbers of layers to determine the normal load required for the probe to penetrate graphene. A very low friction coefficient and also its high pressure resistance make graphene a promising material for antiwear coatings.Graphene, a one-atom-thick planar sheet of carbon atoms, has been studied intensively in the last few years due to its unique characteristics. However, only a few studies investigate its mechanical properties [1, 2]. In particular, micro-scale friction coefficient of graphene has never been investigated despite its promising potential for low-friction antiwear coatings. In this report, scratch tests are conducted to determine the friction coefficient of mechanically exfoliated graphene on SiO 2 and epitaxial graphene on SiC under ambient conditions. Although the electrical properties of graphene are sensitive to the number of layers, no thickness dependence of friction coefficient is observed. The friction coefficient measurements on disordered graphene treated by oxygen plasma show that disorder in graphene increases the friction coefficient. Ramping force scratch tests are performed on graphene samples with different numbers of layers to determine the normal load required for the probe to penetrate through the graphene, inducing failure of the film. This load is referred to here the critical load. Single-, bi-, and tri-layer exfoliated graphene samples are identified by Raman spectra as shown in Fig (d) show error signal and topographical in-situ scanning probe microscopy (SPM) images after scratch tests, respectively. Although the magnitude of the error signal depends on the feedback parameters of the scan, the error signal image, the difference between the actual force and the set point at any given moment, is useful because it often shows more contrast than the accompanying topography image. Deformed parts of graphene after scratch tests are marked by red circles. . The difference from our data can be attributed to measurement environments [6]. First of all, our experiment is conducted at ambient conditions and their experiment is under ultra high vacuum conditions. It is well known that the existence of water can influence the friction coefficient, therefore it is reasonable to have a different 3 friction coefficient depending on the measurement conditions [6]. The different value could be also attributed to the difference in the size of probes; Filleter used an atomic force microscopy (AFM) based system, whereas we used a larger diamond probe with a 1 µm radius. This is in line with a previous study, where a lar...

Although a variety of fundamental mechanical properties of graphene have been investigated, the nature of interactions between graphene and other materials is not yet fully understood. Here, we report on adhesive interactions of between diamond indenters and monolayer, bilayer and trilayer graphene on silicon oxide as well as bare silicon oxide and graphite over relatively small spatial domains. Displacement-controlled nanoindentation with an ultralow noise force sensor allowed the complete adhesive responses to be observed without the usual instabilities associated with nanoindenters that operate in force control. It was found that the approach and withdrawal force profiles between diamond and graphene depended on the number of layers of graphene. The unloading response contained very characteristic features, which were attributed to separation between graphene and silicon oxide in subsequent stress analyses of the experiments. The numerical stress analyses accounted for the interactions between the probe and the graphene as well as between graphene and silicon oxide via traction-separation relations which included attractive and repulsive interactions. As a result, it was possible to extract the energy, strength and range of the interactions for all cases, thereby providing a much richer measure of the interactions than relying solely on force profiles.

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For the first time, a new method of scanning wear has been developed to expose and observe microstructural features of heterogeneous materials, and is presented in this paper. Experimental results of scanning wear of two computer hard disks with DLC protecting layers are also reported. These two applications have proven that scanning wear, as a unique format of wear caused by raster scanning a probe against its counterpart surface under controlled light load, is a very useful and powerful tool for tribological and microstructual investigations of materials at micro/nano scales. The further exploration and application of scanning wear is recommended.