Wear 300 (2013) 78–81
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Friction and atomic-layer-scale wear of graphitic lubricants on SiC(0001) in dry sliding a ¨ Felix Wahlisch , Judith Hoth a, Christian Held a, Thomas Seyller b, Roland Bennewitz a,n a b
INM—Leibniz-Institute for New Materials and Experimental Physics, Saarland University, Campus D2 2, 66123 Saarbr¨ ucken, Germany Lehrstuhl f¨ ur Technische Physik, Universit¨ at Erlangen-N¨ urnberg, Erwin-Rommel-Straße 1, 91058 Erlangen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 August 2012 Received in revised form 15 January 2013 Accepted 22 January 2013 Available online 8 February 2013
Sliding friction experiments on graphene grown on SiC(0001) have been performed using a combination of a microtribometer with an atomic force microscope (AFM) allowing for the investigation of atomic-scale wear. The graphene layer delaminates within 10 sliding cycles starting from substrate step edges. After run in, friction is dominated by the interaction between a changing conﬁguration of asperities at the probe sphere and a graphitic interface layer terminating the SiC substrate. Friction varies unpredictably due to changes in the contact conﬁguration. However, the linear relation between friction and contact area can be conﬁrmed and a shear strength as low as a few MPa is found for the contact between ruby and the graphitic layer on SiC, which remains intact under continuous sliding. & 2013 Elsevier B.V. All rights reserved.
Keywords: Sliding wear Sliding friction Tribophysics Surface topography AFM Nanotribology
2. Materials and methods
Since the ﬁrst isolation and electrical characterization of graphene in 2004 an impressive amount of research into its properties and applications has been carried out, not only because of its fascinating electronic and mechanic properties  but also because of the relative ease of sample preparation . As the building block of one of the most common dry lubricants, graphite, it is also of distinct interest to tribologists: the well deﬁned monolayer system reduces the complexity of experiments and thus can lead to a better general understanding of tribosystems. Recent studies have addressed fundamental questions like friction forces depending on the number of graphene layers and on the substrate conformation utilizing atomic force microscopy (AFM) [3–5]. The stability of graphene against scratching has been analyzed by atomistic simulations  More applied studies have evaluated the potential use of graphene as a lubricant on larger scales [7,8]. This study investigates the tribology of a hard sphere sliding on a hard substrate which is covered by a monolayer of graphene. The combination of hard sample and hard probe results in contact mechanics which are difﬁcult to predict and to reproduce. We demonstrate that a microscopic analysis of the wear patterns by means of AFM helps to understand the low reproducibility in such reciprocal sliding experiments.
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Linear reciprocal sliding experiments (similar to ASTM G133-05) have been performed using a home-built microtribometer. Lateral and normal forces are measured by means of a dual-double leaf spring (kN ¼ 32.55 N/m and kL ¼36.53 N/m are the normal and lateral spring constants, respectively) with two perpendicular mirrors attached, which allow measurement of normal and lateral deﬂection by means of two ﬁber optical sensors [3,9]. The detection sensitivity for both normal and lateral deﬂections was at least 20 nm. A ruby sphere (surface quality grade 10) with a diameter of 500 mm was glued to the bottom side of the cantilever as probe. The ruby sphere was worn in by sliding it over the sample along a track of 3.2 m length. For each experiment, the microtribometer was approached toward the sample, until a normal force of FN ¼0.1 mN of the sphere on the sample was reached. Then, the support position of the spring was kept constant. For reciprocal sliding experiments, the sample was moved by means of a motorized linear axis using a trapezoidal velocity proﬁle (acceleration 2 m/s2 and maximum velocity 50 mm/s) over a distance of 400 mm. Due to a slight tilt of the sample, the normal load varied along the sliding track. Thus, a point-by-point analysis was applied. Along each half cycle, 1000 time-equidistant data points were sampled comprising the displacement of the spring as well as the
F. W¨ ahlisch et al. / Wear 300 (2013) 78–81
The data presented in this study were recorded on a sample with an average coverage of 1.24 monolayers of graphene as conﬁrmed by quantitative X-ray photoelectron spectroscopy. The structure of this sample is as follows: atomically ﬂat terraces of micrometer width are separated by steps of nanometer height. The whole substrate is terminated by the interfacial layer and covered with the ﬁrst layer of graphene. Only some substrate steps are decorated with stripes of the second layer of graphene . 2.4. Measurement procedure
Fig. 1. A typical set of results for microtribological measurements with a normal load of 0.1 mN. Although the same sample, the same counter body, and the same parameters are used, the results exhibit signiﬁcant scatter. Numbers indicate the order of experiments. The amount of reciprocal sliding cycles was varied in order to compare the progress of surface damage.
All measurements were carried out in ambient conditions, i.e. a relative humidity of 40%–60% and a temperature of about 22 1C. Before each experiment, the sphere was cleaned with clean-room compatible gaze sticks to remove possible residues of previous experiments. A part of the area to be tested was imaged by AFM in order to allow a comparison of the surface structure before and after reciprocal sliding. Then, the microtribological experiment was carried out and ﬁnally the initial area was imaged by AFM again.
3. Results 3.1. Microtribology
axis position. The actual sphere position on the sample was calculated by adding the spring’s lateral displacement and the axis position. These data points were then mapped to a positionequidistant grid with lower resolution (512 points) for further evaluation. This procedure ensures that neither oversampling nor stick-slip events distort the data. Friction forces were calculated as the difference of the lateral forces measured during back and forth movement of one cycle, divided by two. 2.2. Atomic force microscopy An Agilent 5400 AFM controlled by Nanonis electronics is integrated into the setup. The motorized linear axis allows for positioning of the sample with a precision of 5 mm between tribometer and AFM. With this setup, even atomic-scale modiﬁcations of the sample surface in sliding experiments can be located and analyzed by the AFM. For imaging, contact mode with constant force is employed. The friction signal was computed from the difference between horizontal deﬂections of trace and retrace, divided by two. The NanosensorsTM PPP-CONT cantilever was calibrated using the geometrical method for rectangular cantilevers described in Ref. . 2.3. Sample preparation The substrate was a SiC–6H (0001) single crystal wafer. Graphene was grown by thermal decomposition at 1650 1C in 900 mPa argon atmosphere as described in Ref. . The nomenclature for the description of the surface layers follows previous studies of this sample [12,13]. There are three surface layers, the interfacial layer terminating the substrate, the ﬁrst layer of graphene, and the second layer of graphene. During thermal decomposition, the Si evaporates predominantly from step edges. The surplus carbon saturates on the surface and forms the so-called carbon-rich ‘‘interfacial layer’’ of graphitic structure that is covalently bound to the substrate . For longer preparation times a new interfacial layer is formed and the graphitic layer is no more covalently bound—it forms a monolayer layer of graphene on top of the interfacial layer, the so called ﬁrst layer. The growth takes place sub-surface  and for even longer decomposition times an additional layer is formed below the ﬁrst one called second layer.
A typical set of results for consecutive microtribological experiments is shown in Fig. 1. The coefﬁcient of friction—the ratio of friction force and normal force—is initially at a very low value below 0.05. It increases during a run-in period of about 10 cycles and then saturates at values in the range of 0.06–0.25. Given that all measurement parameters were the same for this series of experiments, the reproducibility is quite poor. The variation of friction between experiments was found to be independent of the position on the substrate: after a signiﬁcant change in friction occurred, a control experiment at a position previously measured was inconsistent with the previous local measurements, but usually in agreement with the results after the change. We conclude that the variation of friction is mostly due to a change in the state of the sphere rather than the sample. As the friction did not change monotonously but rather varied step-wise and unpredictably, a typical long term ﬂattening of the sphere can be excluded. 3.2. Atomic force microscopy Inspection by AFM of the surface before a microtribological measurement (Fig. 2a) reveals two levels of friction. The areas of high friction (bright color) correspond to coverage by the ﬁrst layer of graphene, while low friction (about 0.4 of the ﬁrst layer’s value, dark color) indicates the growth of a second layer underneath the ﬁrst one [5,12]. Unlike topography images, where the number of graphene layers cannot be identiﬁed easily, the thickness and quality of graphene layers can be identiﬁed well in the friction signal, thus it will be used to analyze the damage mechanisms. Furthermore, topography maps do not contribute additional information, as no change in the substrate step edges was found when comparing AFM images before and after a microtribological experiment. Depending on the amount of cycles of the microtribological experiment, the damage of the graphene layers advances. A typical wear scar is shown in Fig. 2b, featuring the same area as Fig. 2a. Interestingly, the width of wear scars varied signiﬁcantly for the different experiments. During the ﬁrst phase of run-in, small areas of substantially higher friction develop along the track of the ruby sphere. Cracklike structures are observed along the whole track, yielding the same high coefﬁcient of friction as the newly formed patches.
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Fig. 2. AFM friction maps of the graphene-covered SiC surface before (a) and after (b) a microtribological experiment. Dark areas indicate low and bright areas high friction. The images have a width of 20 mm, the color scale ranges from 1 nN to 12 nN (see Fig. 3 for quantitative analysis). The dashed lines in (b) indicate the estimate of the contact diameter.
Fig. 3. Distribution of friction forces for the AFM friction maps shown in Fig. 2 and of a blank interfacial layer (no friction map shown). In order to increase statistics of ﬁrst and second layer graphene, an image with an offset of about 5 mm to the right of Fig. 2b is used. The distributions show that ﬁrst and second layer of graphene are worn and the interfacial layer is exposed.
After run-in, a mostly continuous area of high friction is exposed, surrounded by cracks and debris as shown in Fig. 2b. As the graphene obviously delaminated, the question was whether the exposed high-friction layer can be identiﬁed as the graphitic interfacial layer. Therefore, a substrate of SiC–6H (0001) was prepared to expose only the carbon-rich interfacial layer. When repeating the experiment on the interfacial layer with the same parameters and using the same sphere and AFM-cantilever, no wear scar was observed. Friction forces on the graphenized sample, the worn sample, and the interfacial layer sample are compared in Fig. 3. Friction on the interfacial layer is two times higher than on monolayer graphene. The histograms in Fig. 3 reveal the expected bimodal distribution of friction between monolayer graphene and bilayer graphene on the graphenized sample. After several cycles of sliding, a third peak corresponding to the wear scar dominates. The friction peak recorded on the blank interfacial layer matches the one for the wear scar.
4. Discussion The AFM images indicate that the graphene is sheared off the substrate at the points of highest pressure. These are the step
edges, where most of the cracks originate. Starting from the cracks, ﬂakes of graphene delaminate from the sample, as they are only bound to the substrate by Van-der-Waals forces. The graphene patches remaining within the wear track are located near step edges which seem to topographically protect the patches of graphene. By comparison to a blank interfacial layer sample we can conclude that the graphene decays rapidly within about 10 cycles, leaving exposed the stable interfacial layer that is covalently bound to the substrate. The substrate surface itself and the interfacial layer are found not to be damaged. Debris on the sides of the wear track probably originates either from loosened graphene, as for example found in atomistic simulations  or more likely from the ruby sphere, which indeed undergoes abrasive changes as discussed above. A veriﬁcation of Amontons’ law was not possible for the ruby sphere sliding on graphenized SiC(0001), as the measurements resulted in coefﬁcients of friction varying by a factor of four for consecutive experiments with the same normal force. Since the width of the wear track was observed to vary signiﬁcantly between experiments, it is reasonable to assume that the contact area between sphere and sample did also change. These observations lead us to an analysis following the model of Bowden and Tabor , stating that the frictional force is determined by the shear strength and the area of microscopic contacts. Similarly, the DMT model  as well as atomistic simulations for multiasperity contacts  predict a linear relation between contact area and friction force. An analysis of the contact area by Hertzian contact mechanics is not possible, because SiC and ruby both are non-compliant materials and thus do not form a smooth contact. To estimate the contact area it is assumed to be circular and is calculated based on the width of the central wear scar. Although some scissure formation in the graphene outside the central wear track indicates weaker side contacts, we used this width as a representative quantity. Also, this assumption does not account for non-circular contacts like elliptic or complex multi-asperity contact geometries, since the wear scar is a projection of the contact area along the displacement axis. The resulting plot of friction force versus contact area is presented in Fig. 4. The linear relation between friction and contact area indicates that the friction is indeed determined by the actual contact area between the two hard materials SiC and ruby. The slope of the curve (1.2 MPa) provides us with an estimate for the shear strength of the contact between the ruby sphere and the graphitic interfacial layer. It is orders of magnitude lower than the shear strength of the contacting materials, which often serve as reference values in studies of metal friction.
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asperity conﬁguration, which prevent a meaningful application of the analytical laws of contact mechanics. However, the linear relationship between contact area and friction is still preserved and allows for an estimate of the contact shear strength. While the excellent lubrication of SiC(0001) by the graphene layer does not withstand the sliding of the hard counter-body, the graphitic interface layer is still a very stable low friction surface with a shear strength against ruby of only a few MPa. Our results demonstrate that the use of graphene as a lubricant in SiC-based MEMS and NEMS is possible but requires low loads and very smooth surfaces of counter bodies in order to not break the graphene layer. For larger shear stresses, softer substrates like metals seem favorable.
Fig. 4. Friction forces from the experiments with identical parameters shown in Fig. 1 plotted vs. the contact area as observed in AFM images (e.g. Fig. 2b). Numbers indicate the order of experiments. The slope of the linear ﬁt is 1.2 MPa and the intercept with the friction force axis is 4.9 mN. The last ﬁve cycles of reciprocal sliding were averaged for the friction force value. The error bars are calculated as the standard deviation.
A ﬁnite friction force results for an extrapolation of the linear curve to zero contact area. This could be an artefact of a nonlinear scaling between the approximated and the real contact area. In a multi-asperity contact, the real contact area is smaller than a disk ﬁtted into the wear track. The smaller the contact area, the better the approximation until the real contact area agrees with the disk approximation for a single asperity. Therefore, in the case of a multi-asperity contact the shear strength of 1.2 MPa would be a lower bound estimate; an adjusted curve would be steeper. However, the slope correction necessary to arrive at the expected zero friction for zero contact area would be not more than a factor of three. Alternatively, the friction signal could be the sum of contributions from a smooth contact area sliding nondestructively on the graphene layers and sharp asperities penetrating to the interfacial layer. The unexpectedly high friction for very narrow wear scars would then result from a large smooth contact sliding on graphene with only very few and small sharp asperities. The observation of cracks starting outside the wear track supports the idea that part of the load is carried by contacts outside the wear track.
5. Conclusions Combining the observations discussed above, we conclude that the friction between a ruby sphere and the graphenized SiC(0 0 0 1) surface is determined by the changing conﬁguration of contacting asperities at the ruby sphere. While the stepped substrate surface and its terminating graphitic interfacial layer are not altered in the sliding experiments, the graphene layer on top is delaminated by the action of contact asperities, starting from step edges of the substrate. Friction is then dominated by the interaction between the contact asperities of the ruby sphere and the interfacial layer. Details of the tribological processes can be revealed by a combination of tribometer and AFM with an offset known to micrometer precision, such that atomic-scale wear can be located and imaged. The notorious irreproducibility of tribological experiments in hard-on-hard contacts can be attributed to random changes in the
The authors acknowledge ﬁnancial support from the the Leibniz Association (SAW) and from the BMBF (WING) project TiGeR. We thank Prof. E. Arzt for his continuous support of the project.
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