Macroscale Superlubricity Enabled by Synergy Effect of Graphene

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Macroscale Superlubricity Enabled by Synergy Effect of Graphene-oxide Nanoflakes and Ethanediol Xiangyu Ge, Jinjin Li, Rui Luo, Chenhui Zhang, and Jianbin Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14791 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 4, 2018

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ACS Applied Materials & Interfaces

Macroscale Superlubricity Enabled by Synergy Effect of Graphene-oxide Nanoflakes and Ethanediol Xiangyu Ge, Jinjin Li*, Rui Luo, Chenhui Zhang, Jianbin Luo State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China.

Corresponding author: Telephone: 8610-62789482 E-mail: [email protected]

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ABSTRACT: Graphene has been recognized as an excellent lubrication material owing to its two-dimensional structure and weak interlayer interactions. However, most extant works concerning superlubricity involving graphene oxide have been limited to nanoscale or microscale dimensions (of the order of 1–10 μm). In present work, realization of a robust macroscale superlubricity state (μ = 0.0037), by taking advantage of the synergy effect of graphene-oxide nanoflakes (GONFs) and ethanediol (EDO) at Si3N4–SiO2 interfaces is reported. GONFs have been observed as being adsorbed on friction surfaces, thereby preventing direct contact between surface asperities. The extremely low shear stresses developed between these asperities contribute towards enhanced superlubricity and the resulting super-low wear. Meanwhile, the formation of partial-slip hydrodynamic boundary condition at the GONFs–EDO interface along with the formation of hydrated GONFs–EDO networks through hydrogen-bond interactions contribute to the generation of extremely low shear stresses of the liquid lubricating film. Such macroscale superlubricity provides a new approach towards realization of extremely low friction in GONFs through the synergy effect with liquids.

Keywords: graphene-oxide nanoflakes; superlubricity; interface; slip.

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INTRODUCTION Friction between and wear of surfaces of machine parts have been estimated to cost approximately 6% of the gross national product in developed countries.1 Therefore, improvements in the development of lubricating materials are being ceaselessly pursued by researchers,2,3 and great efforts have been directed towards improving the energy efﬁciency and durability of lubricants. Superlubricity refers to the phenomenon wherein the sliding coefficient of friction (COF) assumes a value of the order of 0.001.4-6 Realization of superlubricity, therefore, forms one of the most efficient methods to reduce frictional-energy dissipation and provide near wear-free operating conditions. Superlubricity can be realized by using various two-dimensional (2D) solid materials, including MoS2,7 nanoscale8 and microscale9 graphite flakes, and graphene maintained at a temperature of 5 K.10,11 Superlubricity has also been previously realized in dissimilar friction pairs, such as amorphous antimony and crystalline gold nanoparticles with graphite,12,13 graphene nanoribbons with gold,14 and silica probes with graphite, owing to formation of multiple transferred graphene nanoflakes.15 However, superlubricity of these materials is limited to only the nanoscale and microscale levels. Macroscale superlubricity, at present, only be realized between DLC and graphene surfaces via formation of graphene-nanoscrolls surrounding nanodiamond particles.16 Unlike solid materials, macroscale superlubricity can be realized by liquid materials with or without additives, such as boron nitride-containing poly-α-olefin (PAO),17 nanodiamondcontaining glycerol,18 ionic liquids with or without carbon quantum dots-containing,19,20 and acids.21,22 However, there are also some limitations for these liquids while realize superlubricity, such as long wearing-in period,17 very small applied load,20 and acidic corrosion.21 One of the 3 / 27



common characteristics for liquid lubrication is that a wearing-in period is necessary before the realization of macroscale superlubricity, which generally results in severe wear. It has been demonstrated that many nano-additives could provide wear protection, such as MoS2, black phosphorus, hexagonal boron nitride, and so on.23-25 Among them, graphene oxide (GO) materials, including GO and functionalized GO, possess much wider applications. The antiwear mechanism of GO materials mainly accounts for the formation of a GO-adsorption film and a tribochemical film, that prevents friction pairs from direct contact and leads to alleviation of wear.26,27 In the case of friction, GO can achieve a COF of 0.08 as an additive in base oil.26 Ionic liquid-modified GO as an additive can help epoxy to acquire a COF of 0.02.27 However, it is found that macroscale COFs of GO materials usually lie in the range of 0.02–0.1,26-31 which is much higher than superlubricity state. Therefore, this leads to the hypothesis that by using liquid lubricants in combination with GO, characteristics of macroscale superlubricity and super-low wear can be simultaneously realized. In this study, a robust macroscale superlubricity state was realized by merging together graphene-oxide nanoflakes (GONFs) and ethanediol (EDO), and the corresponding wear volume upon completion of the wearing-in period was observed to be super low. In addition, the establishment of the lubrication model reveals the superlubricity mechanism of GONFs at macroscale. MATERIALS AND METHODS Materials. Four GONF aqueous solutions with different GONF concentrations—0.5, 1.0, 1.5, and 2 mg/mL—were provided by Nanjing XFNANO Materials Tech Co., Ltd. The preparation of GONFs was based on the improved Hummers method (using graphite flakes via

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a liquid-oxidation process) (Figure 1a). During a typical preparation, 120-mL H2SO4 (98%) was mixed with 1-g graphite in a beaker maintained at 0 ℃ after cooling. Subsequently, 6-g of KMnO4 was tardily added to the mixture amidst constant stirring, with the temperature being continuously monitored and maintained below 20 ℃. Subsequently, the mixture was then placed into a sonication bath, wherein it was irradiated for 25 min at a stable temperature of 35 ℃. At the end of this process, the mixture attained a dark-brown appearance. Next, 133-mL deionized water along with 1-mL H2O2 (30%) was tardily dripped into the mixture being stirred, thereby causing the mixture to attain a dark-yellow appearance. The residual permanganate, after addition of H2O2 (30%), can be transformed into soluble manganese ions. To remove metal ions, the mixture was first filtered and subsequently washed using 260-mL aqueous HCl solution (7.7%). To remove residual acid, distilled water was used to clean the filtered material. The acquired solid was subsequently dried in air for 20 h, thereby obtaining pure GONFs. At last, the GONFs were dispersed in pure water via sonication to get GONF dispersion. Three types of polyhydroxy alcohols (PAs)—EDO, 1, 3-propanediol (PDO) and 1, 4butanediol (BDO)—with purity values exceeding 99%, were introduced, and all of them were purchased from Aladdin Industrial Corporation. GONFs and PAs were both directly used without any pretreatment. Aqueous EDO solution were prepared by diluting EDO with pure water (electrical resistivity > 18 MΩ·cm) in a 1:5 weight ratio, and denoted as EDO. Likewise, aqueous GONFs–PA solutions were prepared by diluting PAs with aqueous GONF solutions in the 1:5 weight ratio, and the three solutions obtained were denoted as GONFs–EDO, GONFs–PDO, and GONFs–BDO. GONF Characterization. To obtain GONF topography, 20-μL aqueous GONF solution 5 / 27



was dripped onto mica substrate and dried in an air oven to remove water. Subsequently, dryGONF dimensions were measured using an atomic-force microscope (AFM; Icon, Bruker, Germany) set to operate in the ScanAsyst mode. To obtain the layer structure of GONFs, 10 μL of aqueous GONF solution was dripped onto a copper grid and subsequently dried using a laminar-flow cabinet. Dry GONFs, thus obtained, were analyzed under a high-resolution transmission electron microscope (HRTEM; 2100F, JEM, Japan). The chemical groups of GONF powders were characterized by means of a Raman spectroscope (HR-800, Horiba, France) and a Fourier-transform infrared spectroscope (FTIR; Vertex 70V, Bruker, USA). The C/O ratio and concentrations of chemical groups within GONFs were characterized using an X-ray photoelectron spectroscope (XPS; PHI Quantera II, Ulvac-Phi Inc, Japan). Likewise, the crystal structure and thermal behavior of GONFs were characterized using an X-ray diffraction (XRD; D8/Advance, Bruker, Germany) within the 2θ range of 5–50° and a thermo-gravimetric analysis (TGA; Q5000, TA Instruments, USA), respectively. The viscosity of each solution was measured using a standard rheometer (Physica MCR301, Anton Paar) at 25 °C. Friction Experiments. Friction experiments were performed in the rotation mode using a universal micro-tribotester (UMT-5; Bruker, USA). The friction pair comprised a Si3N4 ball measuring 4-mm in diameter (Ra = 10 nm) and a SiO2 disc (Ra = 5 nm). The ball and disc were cleansed using sonication treatment in ethanol and acetone for 10 min, followed by washing by pure water, and lastly dried under airflow. During each experiment, 50 μL of the prepared solution was injected into the contact zone. Magnitude of the applied load lied in the range of 2–4 N, incremented in steps of 0.5 N, thereby corresponding to a maximum initial contact pressure of 750 MPa (4 N). The relative sliding speed varied from 0.025 to 0.25 m/s. To obtain 6 / 27


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accurate friction results, the level of the tribotester platform was adjusted to acquire identical values of COF in the clockwise and counterclockwise orientations. Each solution was triple tested, and COFs were recorded with an accuracy of ±0.001. All experiments were performed under atmospheric conditions with room temperature and a relative humidity of 10–25%. Surface Characterization. Wear volume of each worn-out zone was measured using a 3D white-light interferometer (Nexview, ZYGO Lamda, USA), and the topography of each worn-out zone was detected using a scanning electron microscope (SEM; Quanta 200 FEG, FEI, Netherlands) under high vacuum conditions. The ball and disc were platinum-coated prior to SEM observations in order to enhance their electrical conductivity and image contrast. Further, XPS inspection was performed to explore elements that exist within the worn-out zone lubricated using various solutions. HRTEM was also used to capture images of cross-sectional areas within the worn-out zone. The HRTEM samples were prepared using a focused ion beam (FIB; LYRA3, TESCAN. Q.S., Czech Republic). The selected parts from the contact zone on both the ball and disc measured 10 μm in length. A chromium film (20-nm thick) was first sputtered onto the ball and disc; subsequently, a platinum film (100-nm thick) was deposited as a protective layer prior to performing the micro-process employing FIB. RESULTS AND DISCUSSION Materials Characterization. The original GONF structure was detected using an AFM. As observed, the GONFs measured approximately 0.8-nm thick, 450-nm long, and 250-nm wide (Figure 1b). The histogram of the GONFs size (wide and length) is provided in Figure S1, and it indicates that the size of GONFs is in the range of 0.2−1 μm. The XPS result shows

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the C/O atomic ratio of the used GONFs is 1.9 (66/34), indicating the oxygen concentration is around 34% (Figure S2a). Moreover, the peaks observed in Figures S2b, and S2c indicate the existence of C−C, C−O, and C=O bonds.2,28,29 Similar, the FTIR spectrum of GONFs (Figure S2d) shows the existence of O–H bond, C=O in carboxyl group, and C–O–C in epoxy group.30 The XRD pattern of GONFs clearly shows the typical peak of GONFs at around 11.5°, indicating the existence of oxygen-containing groups (Figure S2e) and an interlayer spacing of around 0.7 nm.29,30 TGA result (Figure S2f) shows two steps of weight loss caused by adsorbed water evaporation and oxygen-containing groups decomposition, respectively.29,30 This TGA result is well in agreement with the results in previous works on GONFs,29,30 indicating the purity of the used GONFs in our work is high enough to satisfy the study requirements. The GONF layer structure were detected via HRTEM, wherein GONFs demonstrated a clear 5-layered structure, characterized by a thickness of 1.8 nm, and a local interlayer spacing measuring 0.45 nm (Figure 1c), which was larger compared to that of graphite (0.335 nm).32 The observed increase in the interlayer spacing of GONFs can be attributed to presence of oxygen-containing groups intercalated between graphene interlayers.32 Interactions between GONFs and EDO were explored via Raman spectroscopy (Figure 1d). The observed D band corresponding to the Raman shift of 1,377 cm-1 indicates existence of defects, distortions, and vacancies within GONFs. Correspondingly, the G band corresponding to 1,606 cm-1 represents stretching of C–C bonds.33,34 Similarly, the 2D and D + G bands correspond to Raman-shift values of 2,758 cm-1 and 2,972 cm-1, respectively,35,36 thereby indicating that GONFs were well distributed within the solution. As observed, the spectrum of dry GONFs, once again, comprised the D, G, 2D, D + G bands, although their corresponding Raman-shift values now 8 / 27


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equaled 1,349 cm-1, 1,593 cm-1, 2,716 cm-1, and 2,929 cm-1, respectively. Therefore, a slight shift in the Raman spectrum of GONFs–EDO solution can be observed after drying. An extant study has demonstrated that in a water-rich solution (original GONFs–EDO solution), GONFs could be covered by water molecules via action of oxygen-containing groups.37 Whereas the solution tends to become EDO-rich during the drying, EDO tends to substitute water, thereby forming GONF solvate on the first layer. Therefore, the observed shift in Raman spectrum can be attributed to variations in the solvate formed via hydrogen-bond interactions.38

Figure 1. (a) Preparation of aqueous GONFs–EDO solution; (b) AFM image of GONFs dried on mica substrate; inset depicts structural profile of GONFs demonstrating their thickness measuring approximately 0.8 nm; (c) HRTEM images of dry GONFs demonstrating a clear layered structure with thickness and interlayer spacing of 1.8 and 0.45 nm, respectively; inset depicts an HRTEM image at low magnification; (d) Raman spectra of dry GONFs and aqueous GONFs–EDO solution depicting clear peaks corresponding to the D, G, 2D, and D + G bands. Synergy Effect of GONFs and EDO. To investigate the synergy effect of GONFs and EDO, aqueous solutions of GONFs, EDO, and GONFs–EDO were prepared (denoted as

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GONFs, EDO, and GONFs–EDO, respectively), and their friction-reduction and anti-wear properties were tested (Figure 2). The observed high COF of water demonstrates its inability to form an effective lubricating film to reduce friction. COF of GONFs was observed to reduce from 0.35 to 0.013, thereby indicating that the presence of GONFs serves to greatly reduce friction upon completion of the wearing-in period; however, the observed reduction in friction does not lead to realization of superlubricity. COF of EDO demonstrated continuous reduction during the wearing-in period followed by an increase in value to 0.012 at the end of the test. This trend in COF variation indicates possible formation of an EDO lubricating film; this film, however, may not be stable and efficient in demonstrating superlubricity. In accordance with these results, it is clear that neither GONFs nor EDO can facilitate realization of robust superlubricity. However, the COF value of GONFs–EDO was observed to gradually reduce to a value less than 0.01 upon completion of the wearing-in period of 600 s (Figure 2a); subsequently, COF demonstrated further reduction and attained a minimum value of 0.0037, before finally attaining a stable value in the range of 0.0037–0.0052 for 2 h (Figure 2b). It is evident that the robust superlubricity state can clear be achieved via the synergy effect of GONFs and EDO. Moreover, results (Figure 2c) obtained via 3D white-light interferometry (Figure S3) demonstrate that wear volumes corresponding to the Si3N4 ball lubricated by GONFs–EDO (5.1 × 104 μm3) account for 12.5%, 5%, and 0.5% of those corresponding to the Si3N4 ball lubricated by GONFs (4.1 × 105 μm3), EDO (1.3 × 106 μm3), and water (9.3 × 106 μm3), respectively. These results concerning wear volumes demonstrate the excellent anti-wear property of GONFs–EDO—via realization of the synergy effect—along with its outstanding friction-reduction characteristics. Additionally, after completion of the wearing-in period, the 10 / 27


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contact pressure (as listed in Table S1) of GONFs–EDO (96 MPa) was observed to be 3, 5, and 16 times that of GONFs (29 MPa), EDO (18 MPa), and water (6 MPa), respectively. Usually, high contact pressures tend to inhibit the realization of superlubricity; the observed result, however, demonstrates that superlubricity can be realized even under relatively high contact pressures owing to the synergy effect between GONFs and EDO. Superlubricity Conditions. The influence of GONF concentration on friction is depicted in Figure 2d. Except at concentrations below 0.05%, superlubricity through use of GONFs– EDO can be realized at the other three concentrations considered in this study. With increase in concentration, COF values demonstrate a slight reduction from 0.005 to 0.0037. These results confirm the existence of a threshold GONF concentration (0.08 wt.%) for attainment of superlubricity. When the GONF concentration is less than this threshold, the synergy effect of GONFs–EDO is not obvious; consequently, superlubricity cannot be realized. With GONF concentrations exceeding the said threshold, the corresponding synergy effect of GONFs–EDO is sufficient to facilitate realization of robust superlubricity under given testing conditions. Figure 2e depicts the influence of sliding speed on COF. With sliding speed increasing from 0.0125 to 0.1 m/s, average COF values during superlubricity period were observed to reduce from 0.021 (at 0.0125 m/s) to 0.008 (at 0.05 m/s)—at which point superlubricity is first realized—followed by further reduction to 0.0037 (at 0.1 m/s). With further increase in sliding speed, observed COF values equal those corresponding to the superlubricity regime. At 0.25 m/s, the corresponding COF value equals 0.007. It is, therefore, clear that there exists a proper velocity range (0.05–0.25 m/s), over which robust superlubricity can be realized using GONFs– EDO. Figure 2f depicts average COF values for GONFs–EDO during the superlubricity period 11 / 27



under various applied loads. The observed COF values demonstrate slight fluctuations around 0.004 for applied load values of less than 3 N. Under an applied load of 4 N, COF demonstrates a slight increase to 0.006. Observed values of wear-scar diameters (178, 201, and 214 μm) under different applied loads corresponded to contact pressures measuring 81, 96, and 111 MPa, respectively, after completion of the wearing-in process. This implies that an increase in contact pressure within this range does not lead to nonrealization of superlubricity.

Figure 2. (a) COF evolution of water and aqueous solutions of GONFs, EDO, and GONFs– EDO; (b) COF evolution of GONFs–EDO over 2-h duration; (c) wear-volume comparison between water, GONFs, EDO, and GONFs–EDO used as lubricants; (d–f) average COF values for GONFs–EDO during the superlubricity period—(d) under different GONF concentrations; (e) at different sliding speeds between 0.0125 and 0.25 m/s under an applied load of 3 N; (f) under different applied loads at a sliding speed of 0.1 m/s. Note: for cases (a–d), the sliding speed and applied load equaled 0.1 m/s and 3 N, respectively. Discussion. Usually, superlubricity of GO materials is easy to achieve at nanoscale or

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microscale levels in dry or liquid conditions.8-11 However, at macroscale, the superlubricity of GO materials is very difficult to achieve in dry condition or used as lubrication additives. The COFs were usually in the range of 0.02–0.1, although the wear could be reduced significantly during the wearing-in period. Compared with these previous studies, the main finding in our present work is the achievement of the robust superlubricity and extremely low wear at macroscale by using an aqueous GONFs–EDO solution as lubricant under a high contact pressure of 111 MPa. Therefore, the corresponding mechanism underlying superlubricity can be clearly revealed thereby facilitating its use in practical applications. As a first consideration, topographies of worn-out zones lubricated by GONFs–EDO were detected and analyzed via SEM, wherein the said worn-out zones were observed to be rather rough with some pits on the ball (Figure 3a) along with grooves and abrasions on the disc (Figure 3b). These observations were made during the wearing-in process. When compared against worn-out zones lubricated by EDO (Figures S4e and S4g), those lubricated by GONFs–EDO (Figures S4a and S4c) were observed to be much smaller, narrower, and rougher owing supposedly to interactions between GONFs and friction surfaces. The GONFs–EDO in the worn-out zones are analyzed by XPS both before and after friction testing. The results (Figure S5) show that carbon of aliphatic chains (C–C, C=C, C–H), C–O bond, and C=O bond are observed both before and after testing.2,28,29 Since C=O bond can only be derived from GONFs, it is indicated GONFs were adsorbed in the worn-out zone. Moreover, an interesting phenomenon was observed that the concentration of C–O greatly reduced from 46.4% (before testing) to 25.7% (after testing), while that of C–C increased from 45.6% to 67.8%. Therefore, the density of oxygen groups are changed under sliding. To further confirm this adsorption film, corresponding worn-out zones 13 / 27



were, therefore, explored via Raman spectroscopy, results of which demonstrate that GONFs are adsorbed in worn-out regions of the ball and disc, both of which demonstrated nearly identical Raman-spectrum bands for dry GONFs (Figures 3a and 3b insets), including the D band (1347 cm-1), G band (1593 cm-1), 2D band (2713 cm-1), and D + G band (2925 cm-1). Layer thickness and structure of adsorbed GONFs on the ball and disc surfaces were subsequently detected via cross-sectional HRTEM, results of which are depicted in Figures 3c– 3f. It is clear that adsorbed GONFs on both the ball and disc exhibit similar multilayered structures. Thicknesses of adsorbed GONF layers on both the ball and disc attain a value of 10 nm (20 layers) with an interlayer spacing of 0.45 nm (Figures 3d and 3f). When compared against Figure 1c, it can be observed that the interlayer spacing of GONFs remains unchanged after the prior rubbing motion. However, thicknesses of adsorbed GONFs after rubbing were observed to be larger compared to those of original GONFs, thereby indicating that GONFs in EDO solution might gather to form a thicker, adsorbed layer on friction surfaces subjected to rubbing motion.37 This provides better protection to surfaces against direct contact.

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Figure 3. SEM images of worn-out zone lubricated by aqueous GONFs–EDO solution—(a) Si3N4 ball and (b) SiO2 disc; insets depict Raman spectra of adsorbed GONFs in worn-out zone. Cross-sectional structures of GONF in worn-out zone, as obtained by HRTEM—(c) and (e) represent low-magnification images; (d) and (f) represent high-magnification images that clearly depict layer structure; corresponding film thickness and interlayer spacing measure 10 and 0.45 nm, respectively. A major benefit of GONF adsorption on friction surfaces corresponds to the improvement in anti-wear property of a lubricant. It is well established that wear rates are extremely low when sliding occurs at the graphene–graphene interface.39-41 In this study, wear was mostly observed to occur in the asperity contact region during the wearing-in process. However, owing

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to GONF adsorption onto asperities, the contact interface would shift from being of the Si3N4– SiO2 type to the GONF–GONF type. The adsorbed GONF layer, therefore, serves to greatly reduce the wear rate of friction pairs. Another benefit of GONF adsorption corresponds to the observed reduction in friction in boundary lubrication (BL). COF values for the Si3N4–SiO2 interface usually exceed 0.6; however, corresponding values for the graphene–graphene interface generally lie in the range of 0.02–0.1 at macroscale.26-31 As regards the case considered in this study, sliding motion was observed to occur at the graphene–graphene interface, thereby resulting in significant friction reduction. To further demonstrate that the adsorbed GONF layers greatly contribute towards lowering friction, an experiment was designed, wherein residual EDO was removed from the friction surfaces after the attainment of the superlubricity state. Thus, only the adsorbed GONF layer remained on the friction surfaces. Subsequently, a dry sliding test was performed on the same wear track. The result of this experiment (depicted in Figure S6) demonstrates an initial COF value of approximately 0.02, which increased gradually. The COF value remained below 0.1 for up to 80 s, after which, it demonstrated a sudden jump in its value to 0.64, thereby indicating the occurrence of dry friction at the Si3N4–SiO2 interface. This confirms that the presence of adsorbed GONFs in the worn-out zone greatly contributes towards reducing friction, which is in good agreement with observations reported in the authors' previous studies.15,42 However, the presence of the adsorbed GONF layer alone does not lead to the realization of superlubricity, and therefore, GONF must be used in combination with EDO, to effectively utilize the synergy effect of them, which in turn, leads to attainment of the final superlubricity state. Because superlubricity can only be achieved via lubrication by an aqueous solution, there 16 / 27


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usually exists a liquid lubricating film confined within the contact zone. Thickness of this liquid lubricating film could be estimated using the Hamrock–Dowson equation43 (Supporting Information). In this study, use of the above equation yielded an average film thickness of 14.33 nm under an applied load of 3 N and a sliding velocity of 0.1 m/s. The ratio of film thickness to the combined surface roughness was observed to lie in the range of 1–3, thereby confirming realization of superlubricity in the mixed lubrication (ML) regime, which comprises hydrodynamic lubrication (HDL) and BL.22 A rational superlubricity model for GONFs–EDO has been established as depicted in Figure 4. Two types of contact states—solid asperity and liquid contact—exist at the Si3N4– SiO2 interface. The wearing-in process occurs within the solid asperity contact region (Figure 4b), and contributes towards contact-pressure reduction. Meanwhile, GONF adsorption leads to separation of the friction pair and transformation of the Si3N4–SiO2 interface to a GONF– GONF interface. The low shear stress between adjacent GONF layers results in significant friction reduction and super-low wear during the wearing-in period. After completion of the wearing-in process, the adsorbed GONF layer can easily slip between asperities owing to its low shear stress, thereby contributing significantly to the extremely low friction in the BL regime during the superlubricity period. In the liquid-contact region, the liquid lubricant fills in gaps between the friction pair, owing to which no direct contact occurs between friction surfaces, as depicted in Figure 4c. In the contact region where the surfaces are covered by adsorbed GONFs, friction at the EDO– GONF interface is necessary to be discussed. The friction of liquids on solid surfaces is usually discussed in terms of the partial slip hydrodynamic boundary condition (BC),44 which relates 17 / 27



the fluid slip velocity uliq on the solid surface to its gradient ∂nu along the direction normal to it; i.e., b∂nu = uliq, wherein b denotes the slip length, as depicted in Figure 4c.45 Indeed, the partial slip BC stems from identification of the "bulk" viscous stress σ = η∂nu (η denotes viscosity) corresponding to a surface fluid–solid friction force given by F/A = -τuliq = -σ, wherein τ refers to the fluid–solid COF; F denoted the drawing force; and A denotes the contact area.46 The slip length can accordingly be deduced using the relation b = η/τ. COF, therefore, can be identified as the physically relevant property that characterizes interfacial dynamics, wherein a large slip length corresponds to low friction. Water has been proven to possess a large slip length at graphitic surfaces owing to the smooth nature of graphene surfaces.46 Moreover, it has been proven in our most recently work that the shear would occur at water molecules/graphene interface due to the low shear stress, which contributes to extremely low friction between dissimilar friction pairs.47 Therefore, driven by the small amount of water remaining in the dried GONFs–EDO solution during the superlubricity period (Table S2), it is considered that the EDO–water mixture may also result in a large slip length on the GONF surface. This, in turn, would correspond to the low friction at the EDO–GONF interface. In the contact region where surfaces are not covered by adsorbed GONFs, friction is mainly reduced by hydrated GONFs–EDO networks. EDO and water can associate with GONFs via hydrogen-bond interactions occurring within the first GONF layer as previously mentioned.37,38 Because our previous work demonstrates that hydrated networks are able to provide a super-low frictional forces even between dissimilar friction pairs,47 it is reasonable that hydrated GONFs–EDO networks play an important role in the reduction of liquid shear stress in this study. Therefore, the low friction in the liquid-contact region mainly stems from 18 / 27


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the relative low shear stress at the EDO–GONF interface and between hydrated GONF and EDO networks. In accordance with this superlubricity model, when GONF concentration within the lubricating solution is too low, the lack of GONFs results in incomplete coverage on the friction surfaces, and the resulting direct contact between asperities results in the failure of superlubricity. When GONF concentration exceeds a given threshold value, sufficient GONFs are available to completely cover friction surfaces, thereby preventing direct contact between surfaces and realization of robust superlubricity. In terms of sliding speed, when it reduces to below 0.025 m/s, BL occurs owing to the rather small thickness of the liquid lubricating film (5.66 nm). Consequently, only the adsorbed GONF layer functions to reduce friction within the BL regime, as already discussed, thereby resulting in the failure of superlubricity.

Figure 4. Proposed mechanism underlying realization of superlubricity through use of GONFs–EDO—(a) overall view; (b) asperity contact region; and (c) liquid-contact region. Inset in (c) depicts schematic of hydrated GONFs–EDO networks; the GONF planar is depicted in stick model, wherein red sticks correspond to oxygen-containing group; hydrogen-bond

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interactions are marked by red dotted circles. Finally, to explore the potential of GONFs–EDO, several friction tests were performed. COF values of GONFs–EDO at two relative humidity (Figure S7a) indicate that humidity has no great effect on the realization of superlubricity. COF values of fresh GONFs–EDO and old GONFs–EDO (stand for 1 week) are nearly the same, indicating the good stability of GONFs– EDO (Figure S7b). It is rational to believe that GONFs would function with other PAs to facilitate realization of macroscale superlubricity. To confirm this hypothesis, two PAs—PDO and BDO—were used, and corresponding aqueous solutions of GONFs–PDO and GONFs– BDO were both observed to exhibit superlubricity, although their COF has a slight increase relative to that of GONFs–EDO (Figure S7c). Meanwhile, two ceramic friction pairs (Si3N4– Si3N4 and Si3N4–Sapphire) were also considered, and it was observed that GONFs–EDO could realize superlubricity with ceramic friction pairs under a maximum contact pressure of 200 MPa at a sliding velocity of 0.1 m/s (Figure S7d). Considering the Si3N4–Si3N4 friction pair as an example, the COF value was observed to reduce to as low as 0.0015 upon completion of a wearing-in period lasting 1000 s followed by subsequent stable maintenance for at least an hour. This verifies that GONFs–EDO is an excellent lubricant for use with ceramic surfaces. Although there exist other aspects that should be considered with regard to its use in engineering applications, results observed in this study demonstrate the potential of such a solution in the realization of robust superlubricity in practical applications. CONCLUSIONS This study qualitatively explains the mechanism underlying attainment of a robust macroscale

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superlubricity state upon completion of the wearing-in period. The said superlubricity was achieved via realization of the synergy effect of GONFs in combination with EDO at the Si3N4– SiO2 friction pair. Adsorbed GONFs were detected in the worn-out zone, thereby demonstrating improvement in the anti-wear property of lubricants and generating super-low wear volumes. The low shear stress between adjacent GONF layers, low friction at the EDO– GONFs interface, and formation of hydrated GONFs–EDO networks all play an important role in the significant friction reduction and contribute towards effective realization of macroscale superlubricity. This study presents a new means for realizing macroscale superlubricity and super-low wear volume by merging 2D materials with special liquid molecules. The authors believe that results of this research have great potential implications in the design of novel macroscale superlubricity mechanical systems. SUPPORTING INFORMATION Supporting Information is available free of charge on the ACS Publications website. Complementary experimental studies; AFM studies showing the histogram of the GONFs size (Section S1); XPS, FTIR, XRD, and TGA results characterizing the GONFs (Section S2); optical microscopy images of worn-out zones (Section S3); contact pressures for various lubricants (Section S4); SEM images of worn-out zones (Section S5); XPS spectra of worn-out zones (Section S6); experimental study showing the low interface friction of GONFs (Section S7); Film-thickness estimation (Section S8); the effect of relative humidity, stability, carbon chain length of PAs, and friction pairs on the friction values of GONFs−EDO (Section S9) (PDF)

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ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (Grant numbers: 51775295, 51405256, and 51527901). REFERENCES (1) Holmberg, K.; Erdemir, A. Influence of Tribology on Global Energy Consumption, Costs and Emissions. Friction 2017, 5, 263−284. (2) Zeng, X.; Peng, Y.; Yu, M.; Lang, H.; Cao, X.; Zou, K. Dynamic Sliding Enhancement on the Friction and Adhesion of Graphene, Graphene Oxide, and Fluorinated Graphene. ACS Appl. Mater. Interfaces 2018, 10, 8214−8224. (3) Arif, T.; Colas, G.; Filleter, T. Effect of Humidity and Water Intercalation on the Tribological Behavior of Graphene and Graphene Oxide. ACS Appl. Mater. Interfaces 2018, 10, 22537−22544. (4) Matta, C.; Joly-Pottuz, L.; Bouchet, M. I. D. B.; Martin, J. M.; Kano, M.; Zhang, Q.; Goddard III, W. A. Superlubricity and Tribochemistry of Polyhydric Alcohols. Phys. Rev. B. 2008, 78, 085436. (5) Saravanan, P.; Selyanchyn, R.; Tanaka, H.; Darekar, D.; Staykov, A.; Fujikawa, S.; Lyth, S. M.; Sugimura, J. Macroscale Superlubricity of Multilayer Polyethylenimine/Graphene Oxide Coatings in Different Gas Environments. ACS Appl. Mater. Interfaces 2016, 8, 27179−27187. (6) Chen, X.; Kato, T.; Nosaka, M. Origin of Superlubricity in a-C:H:Si Films: A relation to Film Bonding Structure and Environmental Molecular Characteristic. ACS Appl. Mater.

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Figure 1. (a) Preparation of aqueous GONFs–EDO solution; (b) AFM image of GONFs dried on mica substrate; inset depicts structural profile of GONFs demonstrating their thickness measuring approximately 0.8 nm; (c) HRTEM images of dry GONFs demonstrating a clear layered structure with thickness and interlayer spacing measuring 1.8 and 0.45 nm, respectively; inset depicts an HRTEM image at low magnification; (d) Raman spectra of dry GONFs and aqueous GONFs–EDO solution depicting clear peaks corresponding to the D, G, 2D, and D + G bands. 582x271mm (300 x 300 DPI)


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Figure 2. (a) COF evolution of water and aqueous solutions of GONFs, EDO, and GONFs–EDO; (b) COF evolution of GONFs–EDO over 2-h duration; (c) wear-volume comparison between water, GONFs, EDO, and GONFs–EDO used as lubricants; (d–f) average COF values for GONFs–EDO during the superlubricity period— (d) under different GONF concentrations; (e) at different sliding speeds between 0.0125 and 0.25 m/s under an applied load of 3 N; (f) under different applied loads at a sliding speed of 0.1 m/s. Note: for cases (a–d), the sliding speed and applied load equaled 0.1 m/s and 3 N, respectively. 562x305mm (300 x 300 DPI)



Figure 3. SEM images of worn-out zone lubricated by aqueous GONFs–EDO solution—(a) Si3N4 ball and (b) SiO2 disc; insets depict Raman spectra of adsorbed GONFs in worn-out zone. Cross-sectional structures of GONF in worn-out zone, as obtained by HRTEM—(c) and (e) represent low-magnification images; (d) and (f) represent high-magnification images that clearly depict layer structure; corresponding film thickness and interlayer spacing measure 10 and 0.45 nm, respectively. 440x492mm (300 x 300 DPI)


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Figure 4. Proposed mechanism underlying realization of superlubricity through use of GONFs–EDO—(a) overall view; (b) asperity contact region; and (c) liquid-contact region. Inset in (c) depicts a schematic of hydrated GONFs–EDO networks; the GONF planar is depicted in the stick model, wherein the red sticks correspond to the oxygen-containing group; hydrogen-bond interactions are marked by red dotted circles. 414x202mm (300 x 300 DPI)


Macroscale Superlubricity Enabled by Synergy Effect of Graphene

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