Macroscale Superlubricity Enabled by Graphene‐Coated Surfaces

Abstract Friction and wear remain the primary modes for energy dissipation in moving mechanical components. Superlubricity is highly desirable for energy saving and environmental benefits. Macroscale superlubricity was previously performed under special environments or on curved nanoscale surfaces. Nevertheless, macroscale superlubricity has not yet been demonstrated under ambient conditions on macroscale surfaces, except in humid air produced by purging water vapor into a tribometer chamber. In this study, a tribological system is fabricated using a graphene‐coated plate (GCP), graphene‐coated microsphere (GCS), and graphene‐coated ball (GCB). The friction coefficient of 0.006 is achieved in air under 35 mN at a sliding speed of 0.2 mm s−1 for 1200 s in the developed GCB/GCS/GCP system. To the best of the knowledge, for the first time, macroscale superlubricity on macroscale surfaces under ambient conditions is reported. The mechanism of macroscale superlubricity is due to the combination of exfoliated graphene flakes and the swinging and sliding of the GCS, which is demonstrated by the experimental measurements, ab initio, and molecular dynamics simulations. These findings help to bridge macroscale superlubricity to real world applications, potentially dramatically contributing to energy savings and reducing the emission of carbon dioxide to the environment.

dry inert nitrogen (N 2 ), argon (Ar), reactive hydrogen (H 2 ), and humid air. [9] Until 2017, microscale superlubricity was demonstrated in ambient air under normal load of 1 µN with a scan size of 1 µm and a scan speed of 2 µm s −1 . [10] Lately, macroscale superlubricity has been realized in a nitrogen (N 2 ) environment between the diamond-like carbon and graphene films, through the formation of nanoscrolls of graphene flakes wrapping the nanodiamond particles during the sliding. [2] It is reported that gas environments have a significant influence on the friction coefficient and wear rate. [11][12][13][14] In addition, macroscale superlubricity was demonstrated between the inner and outer shells of centimeter-long double-walled carbon nanotubes. [1] Nevertheless, currently macroscale superlubricity has only been demonstrated under special environments or curved nanoscale surfaces. As a result, macroscale superlubricity has not yet been demonstrated on large surfaces under ambient conditions. It was believed that macroscale superlubricity did not exist due to the structural deformation of materials at large scale, except in special environments or curved nanoscale surfaces. [1,15] Therefore, it is a great challenge to conduct macroscale superlubricity under ambient conditions on macroscale surfaces.
Graphene was found in 2004 with an atomic layer of graphite. [16] It has extraordinary electronic transport properties, exceptionally thermal conductivity, mechanical stiffness and fracture strength. [16][17][18][19][20] Graphene is a basic building block for graphitic materials of all other dimensionalities, which can be wrapped up into 0D fullerenes, rolled into 1D nanotubes and stacked into 3D graphite. [21] Due to the unique physical and mechanical properties, graphene is a promising solid lubricant with an atomically smooth surface and high chemical stability. [22][23][24] Chemical vapor deposition (CVD) is an effective method to deposit graphene films on insulating materials. Nonetheless, traditional graphene CVD often demands metallic catalysts, such as nickel (Ni) or copper (Cu), and post-transfer or additional catalyst removal techniques. [10,25] These complicated processes induce wrinkles, holes, damage and contamination on the as deposited graphene films. It is a big challenge to fabricate a tribological system for macroscale superlubricity with coated graphene on insulating materials.
In this study, macroscale superlubricity was performed under ambient conditions on macroscale surfaces, which was conducted on a newly developed tribological system coated by multilayer graphene (MLG). The MLG were deposited by plasma enhanced CVD (PECVD) powered by radiofrequency at 900 °C on quartz and silica (SiO 2 ) surfaces, absent from the catalysts and post-transfer techniques. PECVD dissociates methane (CH 4 ) at 900 °C by radiofrequency power, forming the carbon (C) source of graphene. This does not need the metallic catalysts or conductive substrates as those used in traditional CVD. The fundamental mechanisms of macroscale superlubricity were elucidated by ab initio and molecular dynamics (MD) simulations. Figure 1 illustrates the schematic diagram of the fabrication processes for the macroscale superlubric system under ambient conditions. A ball and plate are coated with MLG by PECVD ( Figure 1a,b). [24,25] PECVD was performed by radiofrequency power at 900 °C, using CH 4 as the C source and hydrogen (H 2 ) as the carrier and protective gas. MLG coated microspheres (GCSs) conducted by PECVD are dispersed between the graphene-coated ball (GCB) and graphene-coated plate (GCP) (Figure 1c,d). The microsphere has an average diameter of 8 µm. The GCB is fixed on a developed ball-on-plate tribological tester, which is used to perform macroscale superlubricity under ambient conditions (Figure 1d).

Results and Discussion
After graphene deposition, all the surfaces turn dark (Figure 2d-f), compared with the bright pristine surfaces (Figure 2a-c). The Ball and plate are made of quartz, and powder or microsphere (MS) are prepared by SiO 2 . GCP, GCS, and GCB were coated with MLG by PECVD simultaneously in a tube furnace, without metal catalysts and post-transfer treatment, reducing effectively the fabrication processes and contaminations induced by traditional CVD techniques. [10,26] Eight layers of graphene were coated on the GCP (Figure 2g with diameters at around 8 µm. The surface of GCS is rough, due to the adhered SiO 2 or dust particles induced during preparation processes by the vendor. Raman spectra of GCP, GCS and GCB are drawn in Figure 2j-l, respectively. Peaks located around 1350, 1580, and 2700 cm −1 correspond to D, G, and 2D peaks, respectively. [2] The D peak is derived from the breathing modes of six-atom rings and requires a defect to activate. The G peak originates from the E 2g phonon at the Brillouin zone center. The 2D peak is the second order of the D peak, which stems from a process where momentum conservation is satisfied by two phonons with opposite wave vectors. It is always present without the requirement of defects for its activation. [27,28]  The ratio between the relative intensity of the G and 2D peaks (I G /I 2D ), indicates a MLG structure on the coated surfaces, [10,29] which is in good agreement with the TEM results in Figure 2g-i. The relative intensity of the D peak in Figure 2j-l is the maximum, revealing the structural disorder and effects of grain boundaries. Figure 2j-l are typical Raman spectra of defective graphene, which is consistent with those of previous reports. [2,10] For the high-quality single layer graphene, defects Adv. Sci. 2020, 7, 1903239 Figure 2. Photographs of pristine a) quartz plate, b) SiO 2 powder, and c) quartz ball, and after graphene deposition for d) GCP, e) GCS, and f) GCB, g-i) corresponding TEM images, respectively, and j-l) corresponding Raman spectra, respectively. Inset in (e) is the corresponding SEM image. Inset in (j) shows the Raman spectrum of standard high-quality single layer graphene.
are basically absent, leading the absence of D peak (Inset of Figure 2j). Due to the single layer of graphene, 2D peak is the highest compared with G peak. [19] It is reported that the grain size is ≈160 nm in the MLG films, grown by CVD on flat SiO 2 substrates. Under the same growth processes, the gain size is less than 160 nm on curved and rough surfaces. [10] Figure 3 pictures the friction coefficient as a function of time under normal load for different tribological systems. Displacement of the cantilever was calibrated and measured by a dual-frequency laser interferometer with the nonlinearity error less than 4.2 nm (10705A, 5517C, Keysight Technologies, USA). Lateral force was measured by the double-leaf cantilevers. After calibration, the cantilevers had a lateral spring constant of 1000 N m −1 . The resolution and accuracy of the developed tribometer was 0.01 mN, and the maximum load was 1 N. In Figure 3a, the average friction coefficients of Ball/Plate, GCB/GCP, Ball/MS/Plate, and GCB/GCS/GCP are 0.2, 0.11, 0.04, and 0.006 respectively under a normal load of 35 mN at a sliding frequency of 0.1 Hz. Macroscale superlubricity is realized by the GCB/GCS/GCP tribological system under ambient conditions. For a comparison, highly oriented pyrolytic graphite (HOPG) (Nanjing XFNANO Materials Tech Co., Ltd., China) was used as the tribo-pair with ball and GCB, as presented in Figure 3b Figure S1 in the Supporting Information. Under 30 mN at 0.1 Hz, the average friction coefficient of GCP/MS/ GCP is 0.017, and the friction coefficient decreases first from 0.04 to 0.016 under 50 mN at 0.1 Hz, then increasing sharply to 0.7. The experiments demonstrate that the MLG on curved and flat surfaces plays the decisive role for the robust macroscale superlubricity under ambient conditions. Figure 4 illustrates the optical images of the wear track of the GCP after friction tests for a) GCB/GCP (measured at 0.1 Hz), b) GCB/GCS/GCP (measured at 0.1 Hz), and c) GCB/GCS/GCP (measured at 0.5 Hz), corresponding Raman spectra d-f), respectively, and g-i) corresponding Raman mapping (, respectively. The wear width and depth of GCB/GCP are about 100 µm and 8 nm (Figure 4a), respectively. In Figure 4a, there are small scratches that are absent in Figure 4b,c, which might be induced by the rough surface of the pristine ball ( Figure S1c in the Supporting Information). Adding the GCS as shown in Figure 4b,c, the small scratches disappear due to the diameter of GCS at around 8 µm. There is no signature of graphene on the wear track of GCB/GCP (Figure 4d-g). This is consistent with the friction coefficient of 0.11 in Figure 3a. In Figure 4b, the width is ≈280 µm, and it is difficult to identify the depth of wear. The width and depth in Figure 4c are about 230 µm and 5 nm, respectively. The higher position in Figure 4b,c, might be attributed to the accumulation of exfoliated flakes of graphene during sliding. Adding GCS dramatically Adv. Sci. 2020, 7,1903239  reduces the wear of the GCP, and graphene is found after sliding, which is confirmed by Raman spectra and mapping (Figure 4e-i). The wear resistance of GCB/GCS/GCP at 0.1 Hz is better than that at 0.5 Hz, which is confirmed by their Raman spectra (Figure 4e,f) and mapping (Figure 4h,i). The relative intensities of D peaks in Figure 4e,f decrease, and the 2D peaks exhibit amorphous characteristics after sliding, compared to those in Figure 2j prior to sliding. This means that the defects of MLG in the wear track are improved. The decrease of intensity on D peaks is different from previous reports, in which the intensity of D peaks usually increases after sliding. [30,31] In this study, eight layers of graphene, i.e., MLG were deposited by PECVD. However, in previous work, single layer graphene was deposited by traditional CVD, [30] and two or three layers of graphene were deposited through evaporation of ethanol, resulting in the coverage area of graphene at less than 25% on the substrate. [31] In traditional CVD, only one layer graphene was deposited, which is easy to be worn out during sliding, increasing the defects of graphene, as well as the intensity of D peak. Through the evaporation of ethanol, two or three layers were deposited, while the coverage area was less than 25%. The exfoliated flakes of graphene formed debris during Adv. Sci. 2020, 7,1903239  sliding, which is difficult to cover the bare area left over 75%. The transferred film generated by debris induced new defects of graphene, leading to the increase of intensity of D peak. However, in this study, there are eight graphene layers deposited by PECVD, inducing the transferred films on the wear track made by debris derived from the exfoliated flakes during sliding. This improves the defects of graphene, resulting in the decrease of intensity for D peak. Debris is observed on wear tracks in Figures S4 and S5 Figure S3b, Supporting Information). Therefore, debris contributes immensely to the realization of macroscale superlubricity. Figure 5 shows the optical images of the wear areas on the GCB after friction tests under 35 mN for GCB/GCP (measured at 0.1 Hz), GCB/GCS/GCP (measured at 0.1 Hz), and GCB/ GCS/GCP (measured at 0.5 Hz), and their Raman spectra. In Figure 5a, the graphene was worn out. The wear diameter is 131 µm, the wear depth is 1.1 µm, and the wear volume is 7230.7 µm 3 . Wear rate, W s is calculated [12] = W V NS s (2) where V, N, and S are the wear volume, normal load and sliding distance, respectively. The wear rate calculated is It is difficult to identify the wear area without the marking shown with a red dotted circle in Figure 5b, demonstrating the enormous reduction of wear for materials under macroscale superlubricity. In Figure 5b, the diameter of the grey color is 91 µm. If this was the wear diameter, and the wear depth was 0.5 µm. This would lead to the worn out of graphene, due to the coated graphene with thickness <3 nm in Figure 2g-i. This means the coated graphene has no wear with adding the GCS, i.e., the GCB has no wear in macroscale superlubricity. In Figure 5d, the 2D peak is absent without the GCS taken from the black dot in Figure 5a meaning the worn out of graphene. With the GCS, D peaks decrease, and D + D′ peaks appear at around 2940 cm −1 in Figure 5d. D + D′ peak is the combination of phonons with different momenta, requiring a defect for its activation. [27] In Figure 5b,c, the graphene is present, the D peak decreases, and D + D′ peak appears compared with the pristine Raman spectra in Figure 2l. Due to a transfer film of exfoliated graphene flakes, D peak decreases and new defects are generated, activating the presence of D + D′ peak. During the sliding, a few layers of graphene at the top of GCB were worn out, leading to the generation of defects in the wear area and activating the D + D′ peaks. To investigate the wear conditions of the GCS, the SEM images prior to and after sliding under 35 mN at 0.5 Hz are depicted in Figure S4 in the Supporting Information. The GCS has no observable wear after sliding ( Figure S4b, Supporting Information). Exfoliated graphene flakes were transferred on the GCS ( Figure S4b,c, Supporting Information), which is beneficial for the macroscale superlubricity.
To elucidate the origin of macroscale superlubricity, ab initio calculations were performed, as illustrated in Figure 6. The rotation angles of the supercell from 8 to 10° and 30-32° are depicted in Figure 6b,c, respectively. The graphene is a symmetrical honeycomb structure, and therefore 30° is a period for rotation angles. In this regard, the rotation angle calculated is from 0 to 40°. The variation of energy per atom, ∆E 0 per atom increases monotonically from 0 to 8° (Figure 6a). When the rotation angle reaches 8°, the ∆E 0 per atom is 52.1 kJ, reaching the maximum value during the rotation from 0 to 40°. However, the ∆E 0 per atom is −35.4 kJ, when the rotation angle arrives at 9°, meaning the decreasing of ∆E 0 per atom from 8 to 13°. This results in the spontaneous rotation from 8 to 13° for the top layer graphene, unnecessary for the input of external energy and force. It is in good agreement with previous reports on microscale and nanoscale superlubricity for incommensurate contact of graphene and graphite. [32,33] For the rotation of graphene, 30° is a period, and hence spontaneous rotation happens from 30 to 34° with the ∆E 0 per atom values varying from positive to negative.
To illuminate the mechanisms of macroscale superlubricity, MD simulations are depicted in Figure 7. The friction coefficient of the GCP/GNC/GCP is 0.003, which is consistent with the experimental results of macroscale superlubricity ( Figure 3). GNC represents MLG coated nanocylinder (NC). In the inset of Figure 7a, the green color denotes the fixed C atoms of graphene, blue color means the graphene, yellow color represents the silicon (Si) atoms of the NC, and red color signifies the Si atoms of plates. In Figure 7b-h, contact atoms between the GCP and GNC are marked by green and red colors respectively, to recognize the movement of GNC during the sliding. The dynamic sliding video is displayed in Movie S3 in the Supporting Information. During the sliding of the top GCP, the upper contact atoms of GNC adhere on the sliding atoms, without basically any movement. However, the lower contact atoms of the GNC slide backward for 1 atom relative to the lower GCP at a distance of 1.4 nm (Figure 7c), and then slide forward quickly for 5 atoms at a distance of 1.5 nm (Figure 7d). At a distance of 2.1 nm (Figure 7e), the lower contact atoms of GNC slide backward for 1 atom, and then slide forward for 5 atoms at 2.2 nm (Figure 7f). At 2.4 nm (Figure 7g), the lower contact atoms of GNC slide backward for 1 atom, and then slide forward for 6 atoms at 3.1 nm (Figure 7h). From Movie S3 in the Supporting Information and Figure 7, the GNC waggles and slides to facilitates the superlubricity, which is different from the previous findings which showed rolling, sliding and transfer of exfoliated materials. [34] Nevertheless, the mechanism suggested in this study agrees well with the ab initio simulations in Figure 6. The rotation of graphene between 8-13° and 30-34° happens spontaneously, due to the reduction of energy. This contributes to the swinging and sliding of the GNC, due to the incommensurate contact between two layers of graphene during sliding. The error bars for the friction coefficient in Figure 3d are attributed to the asperity contact surface and spontaneous rotation, between 8-13° and 30-34° in a rotation period for the incommensurability contact during sliding. Figure 8 shows the friction coefficient of the GCB/GCP and GCB/GNS/GCP as a function of sliding distance and typical atomic configurations at different sliding distances with and without a MLG coated nanosphere (GNS). During scratching, the normal load was applied on the tip for 1500 nN, scratching Adv. Sci. 2020, 7,1903239  speed was 0.3 m s −1 along the length direction of the plate, and scratching distance was 6 and 7 nm for the GCB/GCP and GCB/GNS/GCP systems, respectively. Without GNS, the friction coefficient increases linearly when the sliding distance is less than 3.5 nm (Figure 8a). After 3.5 nm, the friction coefficient reaches a saturated value around ∼0.8, which is close to 0.11 measured in experiments (Figure 3a). After adding the GNS, the friction coefficient decreases dramatically. When x tip < 3.5 nm, the friction coefficient slightly increases to a peak value of about 0.015, and then decreases to 0.01. After x tip > 3.5 nm, the friction coefficient stabilizes at ≈0.01, approaching the value of macroscale superlubricity in experiments ( Figure 3). [35] The variation of friction coefficient is attributed to the relatively high normal load of 1500 nN and high sliding speeds. The hydrostatic stress state of the topmost graphene sheet attached on the plate is shown in Figure 8b at different sliding distances. The dynamic sliding without GNS of the topmost graphene sheet is displayed in Movie S4 in the Supporting Information, and their typical atomic configurations are shown in Figure S5a in the Supporting Information. Prior to scratching, the topmost graphene sheet is ruptured under the maximum compressive stress of −190 GPa, when exerted to the normal load of 1500 nN (Figure 8b). When scratching to 3.0 and 6.0 nm, the tensile stress attains the maximum of 300 GPa, breaking the CC bonds and rupturing the topmost graphene sheet (Figure 8b). After scratching, a wear track is left behind the scratching tip. Through adding GNS, the hydrostatic stress is greatly decreased, without either rupture or formation of a wear track during sliding, and the wear resistance of the topmost graphene sheet is remarkably improved (Figure 8c). The dynamic sliding process and typical atomic configurations of the topmost graphene sheet with GNS, is displayed and shown in Movie S5 in the Supporting Information and Figure S5b in the Supporting Information, respectively.
To analyze the mechanism of superlubricity with GNS, a front view of Movie S5 in the Supporting Information is displayed in Movie S6 in the Supporting Information to observe clearly the movement of the GNS. From Movies S5 and S6 in the Supporting Information, the mechanism of superlubricity with GNS is seen to be due to swinging and sliding, which is consistent with the MD simulated results with different models in Figure 7. The adding of GNS contributes two aspects towards the superlubricity: one is the buffer function, and the other is stress dispersion through swinging. GNS is responsible for the buffer regime between the scratching tip and graphene sheet. The addition of GNS reduces the contact area that slides against the topmost graphene sheet. [2] The swinging of GNS disperses the contact point continuously, avoiding the concentration of stress on the topmost graphene sheet.
Adv. Sci. 2020, 7,1903239  Lateral force, F l of microscale superlubricity in graphite is presented [36] where γ g is the surface energy of the graphite basal plane, and L is the contact width. The calculated results using Equation (3) is in good agreement with experimental results previously reported. [37,38] The average number of MS is 54.4 on the contact areas, which was measured from the SEM images on 78 contact areas after sliding of macroscale superlubricity, as depicted in Figure S6a in the Supporting Information. The number of MS is selected as 55 on contact area, and the average normal force, F n on each MS is 0.636 mN. γ g is 0.227 J m −2 . [39] From Hertz contact model, the contact radius, r is addressed between the MS and a rigid surface [40] 3 where R is the radius of the MS, and E* is the effective elastic modulus. E* is described [40] where E is the elastic modulus, and v is the Poisson's ratio of the MS (s) and the plate (p). The elastic modulus and Poisson's ratio are 73.3 GPa [41] and 0.17, [42] 25,30,35,40,45, and 50 mN, respectively. These pressures are higher than 1 GPa calculated in microscale superlubricity. [10] It is reported that when the contact area exceeds a critical value, the superlubricity is destroyed. [5] From SEM images, 639 scratching widths were measured on the contact areas, and the average

Conclusion
In summary, the GCB/GCS/GCP tribological system was fabricated simultaneously in a tube furnace by PECVD, in which there are no metal catalysts and post-transfer process.
Macroscale superlubricity was realized on a flat quartz plate coated by MLG under ambient conditions, in which the normal load varies from 25 to 50 mN at a sliding speed of 0.2 mm s −1 for 1200 s. After sliding, the defects on the wear areas of GCB and wear tracks of GCP are improved by adding GMS, as confirmed by Raman spectra. The wear on GCB and GCP in the macroscale superlubricity system is the lowest, which is difficult to discern compared with other systems under the same sliding conditions. Ab initio and MD simulations were employed to elucidate the mechanism of superlubricity, according to the experimental conditions observed for macroscale superlubricity. It was found that the swinging and sliding of GCS plays a crucial role for the realization of superlubricity. Our results pave a way for the design and fabrication of high-performance devices with macroscale superlubricity, as well as for energy savings and reduction of emissions to the environment.

Experimental Section
Quartz plates had a length of 15 mm, a width of 15 mm, and a thickness of 2 mm. Prior to deposition, all the quartz plates were machined in a polisher (UNIPOL-1200S, Shenyang Kejing Auto-instrument Co., Ltd., China) by lapping, mechanical polishing (MP), and chemical mechanical polishing (CMP) sequentially. During lapping and polishing, the polishing pressure and rotation speed were 40.6 kPa and 80 rpm, respectively. The machining time was 3, 10, and 25 min for lapping, MP and CMP, respectively. Abrasive papers with a mesh size of 3000, polyurethane, and nubuck were used as the lapping, MP and CMP pads, respectively. Deionized water was the lapping solution. MP slurry consisted of 2 wt% alumina, 0.1 wt% sorbitol, and deionized water. CMP slurry was made from the MP slurry through adding citric acid to a pH value of 5.4. The developed machining processes and slurries for the quartz plates were efficient and environment-friendly, respectively. Surface roughness R a , root mean square (rms), and peak-to-valley (PV) values were 0.87 ± 0.03, 1.09 ± 0.04, and 9.13 ± 0.65 respectively on the polished surface, as illustrated in Figure S9a in the Supporting Information. After deposition with MLG, the surface roughness was improved for R a , rms and PV to 0.76 ± 0.04, 0.96 ± 0.05, and 8.90 ± 0.59, correspondingly ( Figure S7b, Supporting Information). The error bars for surface roughness were due to the five measurements on the polished surfaces prior to and after growth of graphene. The balls (Donghai County Zhenke Quartz Product of Co., Ltd., China) and SiO 2 powder or MS (Hunyuan County Fuhong Mineral Product of Co., Ltd., China) were made of quartz, and the diameter of balls was 4 mm. Balls and polished plates were ultrasonically cleaned in alcohol for 10 min, and then dried by compressed air. Balls, plates and powder were put in a tube furnace of PECVD (Anhui BEQ Equipment Technology Co., Ltd., China) simultaneously to deposit graphene films. Prior to deposition, the tube furnace was evacuated for 8 min to a vacuum of 5 Pa. Then, the tube furnace was heated, and the temperature was increased to 900 °C within 50 min with a flux of H 2 at 20 standard cubic centimeter per minute (sccm). When the tube furnace was at 900 °C, the temperature was kept constant for 10 min with a flux of H 2 at 20 sccm. During the deposition of graphene films, the power of the plasma was 250 W, and the flux of CH 4 and H 2 was 16 and 20 sccm, respectively. The deposition time was 30 min. To ensure the uniformity of graphene on the SiO 2 powders, the temperature and pressure of CH 4 and H 2 were kept constant at 900 °C, 16 and 20 sccm during the deposition, which was significant for the uniformity of deposited graphene. In addition, prior to deposition, the SiO 2 powders were ultrasonically dispersed in alcohol and sprayed on the quartz plate. The dispersed SiO 2 powders were heated at 60 °C for 5 min, and then they were put in the tube furnace of PECVD for deposition. After deposition, the PECVD system was turned off, and the temperature was allowed to naturally reduce to room temperature. Prior to and after deposition, photographs of balls, plates and powder were taken by a camera (DSC-RX100, SONY, Japan). Wear tracks and coated MLG were characterized and measured by a field emission SEM (Verios G4 UC, FEI, USA), confocal Raman microscope (inVia Reflex, Renishaw, UK), TEM (Tecnai F20, FEI, USA) and laser scanning confocal microscope (LSM 700, ZEISS, Germany). TEM samples of the MLGCB were prepared by focused ion beam technique (Helios G4 CX, FEI, USA). MLGCMS were ultrasonically dispersed in alcohol for 1 min, and then picked up by a Cu grid used for preparation of TEM specimens. TEM samples of the GCP were prepared manually. First, two GCP specimens were glued face-to-face, cut by an ultrasonic cutter to a disc of 3 mm in diameter (Gatan 601, USA), lapped by a disc grinder system (Gatan 623, USA), polished by a waterproof abrasive paper with a mesh size of 2000, thinned at the central area by a dimple grinder to a thickness of ranging from 10 to 30 µm (Gatan 656, USA), and finally thinned by a precision ion polishing system (Gatan 691, USA). Surface roughness was measured by a non-contact surface profilometer (NewView, 5022, ZYGO, USA).
Tribological tests were performed on a developed homemade tribometer. [22] The tribometer had a reciprocating ball-on-plate configuration. It was driven by a bending actuator with high displacement (PL140, PI, Germany). The normal load was applied by a precision highload linear stage (M-414, PI, Germany) and measured by a precision force sensor. Frictional force sensor was designed and measured by the double-leaf cantilevers. Lateral force was measured by the doubleleaf cantilevers. The cantilevers were calibrated by stiffness according to the method used for those in atomic force microscopy (AFM). During the tribological tests, the normal load varied from 25 to 50 mN, sliding length was 2 mm, and sliding frequencies changed from 0.1 to 0.5 Hz. For each test, the experiments were repeated for five times, and the average value was used for friction coefficient.
Ab initio calculations were carried out in the Vienna ab initio simulation package (VASP) using the projector augmented wave (PAW) method. [43,44] All energies were calculated by Perdew-Burke-Ernzerhof exchange-correlation potentials, based on generalized gradient approximation of density functional theory. [45][46][47] A supercell consisting of 64 C atoms was used to perform ab initio calculations. The supercell included two-layer graphene, and each layer contained 32 C atoms. Prior to calculation, the supercell was relaxed to optimize the total energy and force. The length of CC bonds was 1.42 Å, and the interlayer distance was 24 Å. The criterion of energy convergence was set as 10 −5 eV, and a k-point grid of 21 × 21 × 1 was employed to calculate the electronic structure. The cutoff energy of a plane wave was 500 eV. During the calculation of energy, the top layer of graphene was rotated for 40° from the initial equilibrium position, and the atoms were migrated stepwise for 40 steps until reaching the final positions.
The GCP/GNC/GCP tribological performance was conducted by MD simulations. Both plate and NC were set as rigid bodies composed of a silicon single crystal. The two plates and NC were wrapped with four-layer graphene to perform the superlubricity analysis, which was consistent with the macroscale superlubricity from the experiments. One-layer graphene was fixed on the plate to simulate the coated MLG on the quartz plate during the experiments. Two-layer atoms were fixed on two edges of the four-layer graphene wrapped on two plates.