In-situ exfoliated graphene for high-performance water-based lubricants

In-situ exfoliated graphene for high-performance water-based lubricants

Carbon 96 (2016) 1181e1190 Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon In-situ exfoliated gra...

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Carbon 96 (2016) 1181e1190

Contents lists available at ScienceDirect

Carbon journal homepage: www.elsevier.com/locate/carbon

In-situ exfoliated graphene for high-performance water-based lubricants Shuaishuai Liang a, Zhigang Shen a, b, *, Min Yi a, Lei Liu a, b, Xiaojing Zhang a, Shulin Ma a a b

Beijing Key Laboratory for Powder Technology Research and Development, Beijing University of Aeronautics and Astronautics, Beijing 100191, China School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 January 2015 Received in revised form 22 October 2015 Accepted 23 October 2015 Available online 27 October 2015

In this study, the tribological behavior of in-situ exfoliated graphene for water-based lubricants is investigated. The aqueous graphene dispersions show high stability due to the assistance of a non-ionic surfactant, Triton X-100. The steelesteel ball-plate tribotest results reveal that the in-situ exfoliated graphene possesses brilliant frictional and anti-wear properties. Compared with pure deionized water, the graphene-enhanced lubricants can offer 81.3% and 61.8% reduction in friction coefficient and wear scar diameter (i.e. a reduction in wear volume by two orders of magnitude), respectively, owing to the formation of a protective graphene layer at the sliding contact zone. The performance of in-situ exfoliated graphene is also better than that of graphene oxide at the same additive concentration. Furthermore, the as-prepared lubricants exhibit enhanced lubrication effect even in high load and diverse velocity cases. It is proposed that the surfactant, Triton X-100, eases the deposition procedure of the graphene layer by adjusting the dispersion wettability of the sliding surfaces, thus leading to the superior tribological properties. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Many manufacturing operations, such as turning, milling and drilling processes, require not only proper lubrication but also adequate cooling [1,2]. Therein, the water-based lubricants serve as perfect candidates, because water can offer many advantages like good environmental compatibility, high fluidity and superb thermal conductivity [3]. However, water itself is a poor lubricant, and extra additives are necessary for enhancing its lubricating properties [4]. Usually, water-miscible oils are added into water to make emulsions [5]. The lubricating capacity can be tuned by adjusting the oil content in the mixture. Nevertheless, such scheme leads to the following disadvantages. Firstly, the advanced properties of water-based lubricants can be severely compromised as the oil content is increased. Secondly, the addition of oil causes negative effects on the environment during the disposal of waste fluids [6]. In order to solve these problems, nanofluids may provide a promising route. According to the reported works, a small amount of

* Corresponding author. Beijing Key Laboratory for Powder Technology Research and Development, Beijing University of Aeronautics and Astronautics, Beijing 100191, China. E-mail address: [email protected] (Z. Shen). http://dx.doi.org/10.1016/j.carbon.2015.10.077 0008-6223/© 2015 Elsevier Ltd. All rights reserved.

nanoparticle additives can noticeably improve the lubricating and cooling properties of the base fluids [7e12]. Meanwhile, due to the tiny quantity usage of nanoparticle additive, the negative influences on the environment are greatly suppressed. Therefore, it is of great significance and interest to develop novel and highperformance nanofiller additives for water-based lubricants. Graphene, a 2D flaked nanomaterial, has been prevailing in various fields nowadays, owing to its unprecedented electrical [13], mechanical [14] and thermal [15] properties. Besides these wellestablished applications, graphene has been recently labeled as an emerging solid lubricant with excellent tribological properties [16]. This makes graphene a potential candidate as lubricating additive for nanofluids. In fact, several studies have already been conducted in this field [5,17e20]. Zhang et al. [17] have modified chemically reduced graphene oxide (GO) with oleic acid and used it as additive for the base oil PAO9, as a result the friction coefficient (FC) and wear scar diameter (WSD) were respectively reduced by 17% and 14% at optimized concentration. In Eswaraiah's study [18], solar exfoliated graphene was added into commercial engine oil, enhancing its frictional, anti-wear, and extreme pressure properties by 80, 33, and 40%, respectively. Samuel et al. [5] have added thermal reduced GO in semisynthetic metal-working fluids, and the microturning experiments suggested that the additive had

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significantly improved the lubrication and cooling performances of the fluids, which are better than that for the carbon nanotubes. Additionally, GO was also directly added into water as a nanofiller additive, which can offer extremely low FC (around 0.1) and marked reduction in wear extent [19,20]. According to the published literature, graphene outperforms other graphitic materials, including graphite powder, GO and modified GO, etc. For example, when the deposited coatings undergo sliding tests in both ambient air and nitrogen, graphene achieves the lowest FC and wear rate in contrast with graphite powder and GO [16]. Ko et al. [21] have explored the tribological properties of pristine and functionalized graphene by using friction force microscopy. They revealed that hydrogenated, fluorinated and oxidized graphene generate 2-, 6- and 7-fold enhanced friction compared with pristine graphene, respectively. Meanwhile, scalable production of pristine graphene has been possible due to the liquid phase exfoliation methods [22e24], thus laying a solid foundation for its practical applications in lubrication industry. However, in despite of its great advantages, pristine graphene is intrinsically hydrophobic, and cannot be directly used as an additive for water-based lubricants. Consequently, most of the previous studies have focused on the functionalized graphene materials, as mentioned above. Herein, we introduce a one-step preparation method of water-based lubricant with in-situ exfoliated graphene as nanofiller additives, in which a non-ionic surfactant, Triton X100, was used as a stabilizer. The chemical formula of Triton X-100 is C34H62O11 and its chemical structure is shown in Fig. 1. The surfactant not only helps exfoliation and stabilization of graphene in water, but also contributes to the formation of graphene protective films in the tribological tests. Importantly, remarkable enhancement in lubricating and anti-wear properties was achieved by adding a small amount of graphene (0.0024e0.011 wt%) and surfactant (0.1 wt%). Therefore, the present research corroborates the great potential of directly exfoliated graphene as nanofiller additives for high-performance water-based lubricants.

2. Experimental 2.1. Preparation of graphene and GO The graphite powder used in our work, as the precursor for preparing graphene and GO, was purchased from Alfa Aesar (Product Number 43209). The solvent for graphene exfoliation was prepared by dissolving the non-ionic surfactant (Triton X-100) in deionized (DI) water at 1 mg/ml followed by 12 h magnetic stirring. Then 1.5, 3 and 6 g pristine graphite powder were respectively added to 300 ml of the storing solvent to form graphite dispersions at diverse initial concentrations (Ci): Ci1 ¼ 5 mg/ml, Ci2 ¼ 10 mg/ml and Ci3 ¼ 20 mg/ml. For each concentration, the graphite dispersion was transferred into 2 stoppered conical flasks (capacity 150 ml) and processed in bath sonication (KX-1730T, 120 W, 40 KHz) for 8 h. During this procedure, the temperature of the bath water was kept constant by using a cycle cooling system. After sonication, the dispersions were allowed 24 h free sedimentation. Subsequently, the upper dispersions were dispensed into the centrifugal tubes and centrifuged at 1500 r/min (317 g) for 1 h. Afterward the supernatant of the tubes was carefully assembled and stored as the final production, i.e. graphene enhanced lubricants (G-Ls). In Fig. 1, a flow scheme of the fabrication process of G-L dispersions is shown. According to the LamberteBeer law, A ¼ kbC, where A is the optical absorbance measured at the wavelength of 660 nm, b ¼ 0.01 m is the path length, and C is the concentration of the dispersion, the resultant graphene concentrations of the G-L

Table 1 Resultant graphene concentration for each initial graphite concentration. Initial graphite concentration

Resultant graphene concentration

Ci1 ¼ 5 mg/ml Ci2 ¼ 10 mg/ml Ci3 ¼ 20 mg/ml

23.8 mg/mL 69.9 mg/mL 110 mg/mL

Fig. 1. The schematic process flow for fabrication of the G-L dispersions.

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samples were determined (taking k ~ 2460 l/g/m [25]) to be 23.8, 69.9 and 110 mg/mL, as shown in Table 1. For comparison, GO was prepared by ultrasonic treatment of graphite oxide for 30 min in bath water. The graphite oxide was prepared from the pristine graphite powder by a modified Hummers method [26]. The GO was dispersed in pure DI water and the solvent and diluted to 110 mg/ mL, named as GO-L and GOT-L, respectively. 2.2. Tribological tests The tribological tests were carried out in the ambient environment by using a UMT-3 tribometer with a ball-plate configuration (rotary pattern). Highly polished ball (diameter in 9.53 mm) and plate samples are both made of GCr15 bearing steel. Before each test, the ball and plate counterparts were initially cleaned through an ultrasonic bath in acetone for 15 min and dried. For all the tribotests, the lubricant sample was first dropped onto the surface of the steel plate, and when the tests began, 5 drops of the lubricant were added every 200 cycles. The constant experimental conditions were a track diameter of 10 mm. The rotary speed was varied from 60 to 240 rpm, with a detailed research at 120 rpm. The normal test load was varied from 2 N to 15 N, with a detailed research at 2 N. The specific experimental conditions are shown in Table 2. Each experiment was repeated at least 3 times. After the tests, the specimens were cleaned by bath sonication in acetone and then in distilled water prior to the characterization procedures. 2.3. Characterization techniques The dimensions of the as-prepared graphene and GO were detected by atomic force microscopy (AFM). Samples were prepared by pipetting several drops of the diluted G-L or GO-L dispersions onto freshly cleaved mica wafers. Fig. 2a and b shows typical AFM images of the graphene and GO sheets, respectively. The height profiles show that the thickness of both graphene and GO were measured as around 1 nm. According to the reported interlayer distances for graphene and GO layers (0.34 nm and 0.7 nm [27]), the as-prepared graphene and GO sheets were thus verified to be few-layer. The lateral sizes for both graphene and GO were distributed from several hundred nanometers to several microns. In regard to morphology, the graphene sheets were lying smoothly against the mica surface (Fig. 2a), while the GO sheets presented some wrinkles across the plane as shown in Fig. 2b. These observations are in good agreement with those reported in the published literature [28,29]. The wear scars and tracks were investigated with optical microscopy (OM), white light interference (WLI), scanning electron microscopy (SEM), energy dispersive X-Ray spectroscopy (EDX) and Raman spectroscopy. The OM images were taken on the optical microscope (Carl Zeiss, Axio Imager A2W). The WLI observation

Table 2 Experimental conditions of the tribological tests. Sample

Concentration (mg/mL)

Normal load (N)

Rotary speed (rpm)

G-L G-L G-L GO-L GOT-L G-L G-L G-L G-L G-L G-L

23.8 69.9 110 110 110 110 110 110 110 110 110

2 2 2 2 2 5 10 15 2 2 2

120 120 120 120 120 120 120 120 60 180 240

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was carried out with a NanoMap-D 3D profiler. SEM imaging and EDX analysis were performed on a LEO 1530 VP equipped with an EDAX Phoenix DX 60s. Raman spectra were acquired on a Renishaw Rm2000 using a 514 nm laser. As for the G-L and GO-L/GOT-L dispersions, the following characterizations were carried out. The optical absorbance was measured at a wavelength of 660 nm using a Purkinje General TU1901 UVevis spectrometer and recorded periodically for over a month. The kinematic viscosity was estimated with a capillary type viscometer by measuring the efflux time under constant temperature conditions. The dispersion wettability of the sliding counterparts was evaluated by determining the contact angle of G-L and GO-L/GOT-L droplets on the steel surfaces. 3. Results and discussion 3.1. Tribological tests under constant load Three G-L samples of different graphene concentrations were directly used as lubricating fluids in the ball-plate tribological tests. DI water, the solvent, and the 110 mg/ml GO-L/GOT-L sample were also tested for comparison. Typically, the performance of lubricants can be evaluated by their friction and wear behaviors. Herein, the real-time FC curves were recorded automatically by the UMT-3 tribometer, as shown in Fig. 3a. The wear behavior was expressed by the WSDs on the ball side measured after the tests. Moreover, in order to specify the wear rates for each experiment case, we have calculated the wear volumes (WVs) for the corresponding WSDs through the following equations [30]:



   2 ph 3d þ h2 ; 6 4

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d2 h ¼ r  r2  4

(1)

(2)

where d is WSD and r is the radius of the steel ball. Both the WSD and WV data were presented in Fig. 3b. For all the experiments described in Fig. 3, the normal load during the tribology test remains constant as 2 N. As shown in Fig. 3a, the anti-friction ability for the samples follows this law: G-L series > GO-L/GOT-L > solvent > DI water. The 110 mg/ml G-L sample has provided a sharp reduction up to 81.3% in FC compared with DI water (from 0.563 to 0.105 in mean value). In contrast, the 110 mg/ml GO-L and GOT-L samples are less effective by offering 37.8% and 44.2% FC reduction (from 0.563 to 0.35 and 0.314 in mean value), respectively. Besides, the FC curves for G-L samples are very smooth, indicating stable friction throughout the test. In contrast, FC curves for other samples display fluctuation in a range of 0.05. Specifically, the mean values of FC for the 23.8, 69.9, and 110 mg/ml G-L samples are 0.14, 0.124 and 0.105, respectively. Therefore, the lubricating performance of G-L is gradually improved as the graphene concentration is increased. Additionally, the shape of the FC curves for GO-L and GOT-L is qualitatively different than the other curves. The reason for this behavior is better understood by examining the wear tracks and the worn surfaces, the results of which are presented later in Figs. 4e6. Fig. 3b shows the wear results for all the tested lubricants with both the measured WSD and the calculated WV. In this respect, the G-L series also outperform the rest samples. In comparison with DI water, lubrication by 110 mg/ml G-L leads to a noticeable reduction of WSD at 61.8% (from 513.4 to 196 mm in mean value), i.e. a reduction of WV by as much as two orders of magnitude (from 35.7 to 0.76  105 mm3 in mean value). The evolution of WSD or WV

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Fig. 2. AFM images of the prepared (a) graphene and (b) GO sheets. Insets are the height profiles corresponding to the lines below.

Fig. 3. (a) FC curves, (b) WSDs and WVs of tribological tests lubricated by different samples. All tests were conducted at 2 N normal load.

with increasing graphene concentration in Fig. 3b suggests that GLs with higher graphene concentration possess superior anti-wear ability. When using the 110 mg/ml GO-L/GOT-L, the wear damage is respectively reduced by 27.8%/26.1% in WSD and 72.7%/71.9% in WV in contrast with DI water, making their lubricating effect in

between those of the G-L series and the control experiment cases. In order to get better observations of the wear extent, we further characterized the wear tracks on the steel plates by using WLI, as shown in Fig. 4. The scan profiles across the white lines are shown under the corresponding WLI images. In Fig. 4a, a wide wear track with a depth of over 200 nm is formed under the lubrication of DI water, indicating a poor protecting effect. When GO-L (110 mg/ml) was present at the sliding interface, the wear damage on the steel plate was clearly reduced. Only a few thin furrows with depths around 100 nm were presented as shown in Fig. 4c. As for the 110 mg/ml G-L lubrication (Fig. 4b), the anti-wear performance has been remarkably improved and only slight scratched lines can be seen on the WLI image. Moreover, no obvious fluctuations are found in the line scan profile. Therefore, the WLI results prove the excellent anti-wear properties of the G-L samples. Fig. 5 shows typical SEM images of the rubbed surfaces under different lubrications. Gradually, the furrows become more and more unapparent from Fig. 5aee. This establishes the similar order as the anti-friction abilities: G-L > GO-L/GOT-L > solvent > DI water. As shown in the left part of Fig. 5c, some region of the GO-L lubricated surface is clean with several thin furrows. Meanwhile, deposited graphene sheets are observed to be attached to the worn surface in other areas as shown in the right part of Fig. 5c. Hence, only a partial coverage of protection film is formed during GO-L lubrication. In contrast, as for the GOT-L and G-L lubricated surface shown in Fig. 5d and e, the deposited graphene sheets can be found throughout the wear track, indicating the formation of a complete protection film. This phenomenon can be attributed to the different wettability of the steel counterparts for the lubricant samples, as will be discussed in the following. In Fig. 5e, it can be seen that the sizes of the deposited sheets are in the range of several hundred nanometers to several microns, which is in good agreement with the AFM measurement in Fig. 2a. Furthermore, an EDX carbon element mapping in Fig. 5f also gives a good match between the high carbon concentration areas and the dark deposition areas in the red square of Fig. 5e. Therefore, it is highly suggested that the graphene sheets in G-L are attached to the worn surface to form a discrete film, thus protecting the mating surfaces from severe wear and simultaneously reducing the friction force. The existence of graphene sheets on the worn surface is also evidenced by Raman spectroscopy. Fig. 6aej shows the OM images of the wear scars and tracks on the steel balls and plates, while Fig. 6k exhibits the Raman spectra of the pristine graphite powder, the in-situ exfoliated graphene, the Solvent lubricated surface, the G-L lubricated surface, the GO flakes and the GOT-L lubricated surface, which are labeled as curve ① to ⑥, respectively. All the

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Fig. 4. WLI images for wear tracks on the steel plates lubricated by (a) DI water, (b) 110 mg/ml G-L and (c) 110 mg/ml GO-L. The height profiles of the corresponding white lines are presented under each image.

Fig. 5. SEM images for the worn surfaces under lubrication of (a) DI water, (b) the solvent, (c) 110 mg/ml GO-L, (d) 110 mg/ml GOT-L, (e) 110 mg/ml G-L. (f) Carbon element mapping of the red square area in (e).

spectra were normalized to the intensity of the G-band. The spectrum of pristine graphite shows its characteristic bands: a G-band at ~1580 cm1 and a 2D-band at ~2700 cm1, while no D-band is observed. For the in-situ exfoliated graphene, its 2D-band is intrinsically different from that of the pristine graphite powder, indicating that the in-situ prepared graphene is highly exfoliated [31,32]. Besides, two disorder-induced Raman peaks, a low intensity D-band at ~1350 cm1 and a small D0 -band on the shoulder of the G-band at ~1620 cm1 are observed, indicating the

appearance of defects in the as-prepared graphene. However, the ratio of intensity of D-band to G-band (ID/IG), which is known as an estimation of the defect level, is calculated as 0.15, suggesting a low defect level for the as-prepared graphene [32]. Additionally, as revealed by the AFM results, the sizes of the exfoliated graphene are mostly smaller than that of the Raman laser spot (1e2 mm), thus the appearance of the D-band and D0 -band is likely derived from the edge effects instead of severe basal plane defects. Therefore, it can be concluded that the in-situ prepared graphene is highly

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Fig. 6. (aej) the OM images of the wear scars and tracks on the steel balls and plates. (k) Raman spectra for ① the pristine graphite powder, ② the in-situ exfoliated graphene, ③ the Solvent lubricated surface, ④ the G-L lubricated surface, ⑤ the GO flakes and ⑥ the GOT-L lubricated surface.

exfoliated and of high quality. The Raman spectrum of the prepared GO flakes exhibits an obvious widening of the D- and G-band, as well as the distortion of the 2D-band, indicating a high concentration of structural defects. After the tribotests, for the G-L lubricated surface, graphitic Raman signal can also be detected, suggesting a deposition layer of graphene at the sliding interface. However, the Raman spectrum for the deposited graphene is entirely different. The intensity of D0 band has increased to almost the same level as G-band. The value of ID/IG has grown drastically to 1.91. Besides, a new Raman peak, the D þ D0 -band at about 2940 cm1 emerged [33]. All these changes declare that substantial defects have been introduced in the deposited graphene. However, the Raman spectrum for the deposited GO shows almost the same as that of GO flakes. This may because that the GO flakes itself have a lot of basal plane defects, and undergoing the sliding test cannot exert further obvious damage on the GO flakes. From the Raman spectra, it can also be concluded that graphene or GO contributes to the anti-corrosion performance of the waterbased lubricant. Without graphene or GO, the Raman spectrum of

the solvent lubricated surface shows the characteristic peaks of iron oxides at ~700 cm1. The Raman signatures of the wear tracks lubricated by G-L and GOT-L show none and relatively low peak for iron oxides, indicating that graphene and GO are both able to reduce the oxidation process on the steel surface. From the results in Figs. 4e6, it is possible to explain the different behaviors of FC curves between GO-L/GOT-L and other lubricants in Fig. 3a. As suggested by Figs. 5 and 6, both GO-L and GOT-L can form GO lubricant layer at the sliding interface. Hence, at the early stage of the tribotests, the FC values of GO-L/GOT-L were relatively low. According to Fig. 4, the anti-wear ability of GO layers are inferior to that of the pristine graphene layers, thus the roughness of the surfaces lubricated by GO-L/GOT-L gradually increased as the sliding process went on. As a result, the steelesteel contact area of the furrows relatively increased compared with the area covered by GO flakes, leading to gradual increase of the FC value. Afterward, the roughness reached the level between those of pure DI water/solvent and G-L series (Fig. 4), and the corresponding FC D curves also evolved to a steady stage, as shown in Fig. 3.

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3.2. Tribological test under varied loads Water exhibits poor lubricating abilities in steelesteel sliding counterparts [4], especially in the case of high load. Hence in practical applications, water-based lubricants are often used in low load working conditions. It is highly expected that the addition of in-situ exfoliated graphene can enhance the lubricating properties of water-based lubricants even under higher normal loads. With this in mind, we have examined the effect of the 110 mg/ml G-L dispersions under higher normal loads (5, 10 and 15 N), with other experimental conditions remains the same. For comparison, DI water and the solvent were also tested under 15 N normal load. The FC, WSD and WV results are presented in Fig. 7. Fig. 7a shows the FC curves under different normal loads. Generally, higher load leads to higher FC level for the 110 mg/ml G-L lubrication. When the applied load is low, the lubrication at the sliding interface belongs to the mixed lubrication regime, which contains dry contact and boundary lubrication [17]. In this case, the friction area can be divided into three parts: the friction area of solvent between the contact interface (AS), the dry contact area of graphene (AG) and the metalemetal dry contact area (AM). As the applied load rises, the lubrication regime gradually converts to the dry contact regime. This will leads to a reduction in AS and an increase in AM, resulting in more and more adhesion and thus higher friction. Nevertheless, the addition of in-situ exfoliated graphene

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can still markedly reduce the FC value even under 15 N normal load, as shown in Fig. 7a. Compared with DI water lubrication, the G-L sample offers 58.7% reduction in FC (from 0.46 to 0.19 in mean value) under 15 N normal load. The rise of applied load also causes more severe wear extent. As shown in Fig. 7b, WSDs and WVs gradually increase for higher normal loads. Although the protection effect of graphene becomes less effective than that in the low load cases, the improvement is still obvious. According to the OM images of wear scars and tracks for the cases of solvent and 110 mg/ml G-L under 15 N in Fig. 7, the addition of in-situ exfoliated graphene has reduced the WSD from 682.1 mm to 556.3 mm, i.e. 18.4% reduction in WSD and 55.8% reduction in WV. The lubricating and anti-wear effects of the 110 mg/ml G-L under different rotary velocities were also tested, as shown in Fig. 8. From the evolution of the FC curves for 60, 120, 180, 240 rpm experiment case, it can be seen that the fluctuation of the FC curve became more and more severe as the rotary speed increased, while all the FC mean values are around 0.12, indicating that the increase of velocity will result in unsteady friction status. As for the wear scars and tracks for diverse velocities, the difference turned out to be insignificant. Therefore, it can be concluded that the G-L sample still can effectively reduce the friction force and wear extent under different rotary speed, but the FC would become unsteady as the velocity increases.

Fig. 7. (a) FC curves, (b) WSDs and WVs of tribological tests lubricated by DI-water, the solvent and 110 mg/ml G-L under different normal loads. OM images of the wear scars and tracks on the steel balls and plates for the experimental cases of solvent (c and d) and the 110 mg/ml G-L (e and f), respectively.

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Fig. 8. FC curves, wear scars and tracks of tribological tests lubricated by 110 mg/ml G-L under 2 N with rotate speed of 60 (aec), 120 (def), 180 (gei), 240 (jel) rpm.

3.3. The role of surfactant (Triton X-100) Although the directly exfoliated graphene is well known as a hydrophobic material, we have experimentally proved its high performance as a nano-additive for water-based lubricants. It is interesting to probe the mechanisms underlying the exciting results. Herein, we propose that the key lies in the addition of surfactant, which plays a critical role both in the graphene preparation and lubrication processes. The detailed analyses are presented as follows. The preparation method of graphene by exfoliation in aqueous surfactant solutions has been reported a lot in previous studies [22,24,34,35]. As for the non-ionic surfactant (e.g. Triton X-100 used here), the graphene dispersion is stabilized by the steric repulsion force [36]. As shown in Fig. 1, the Triton X-100 molecule has a hydrophobic tail and a long hydrophilic chain. When few- or single-layer graphene is exfoliated from bulk graphite material in

solution, the hydrophobic tail adsorbs on the graphene sheet, while the hydrophilic ends into the water. The graphene flakes will be coated with the surfactant molecule by adhesion. Thus, the interaction between the hydrophilic ends of the adhered molecules from adjacent graphene can generate the steric repulsion force, and prevents the exfoliated graphene sheets from reaggregation with each other. In Fig. 9, the sedimentation curves are established by periodically recording the concentration of the dispersions, which are determined by measuring their optical absorbance. According to the curves, the dispersions exhibit good stability. Over 90% of the exfoliated graphene remains stable after deposition for over a month (92.3%, 95.6% and 93.7% remaining for 23.8, 69.9, and 110 mg/ ml G-Ls, respectively). The inset photograph in Fig. 9 presents the dispersions after a month's deposition. It can be seen that the dispersions appeared dark, indicating that a significant part of the exfoliated graphene remains stable. Meanwhile, the sedimentation of the GO-L and GOT-L samples are less than 2%, due to the

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Fig. 9. Sedimentation curves for the G-L series and GO-L/GOT-L dispersions. Inset photograph shows the dispersions after deposition for over a month.

hydrophilic property of GO. The wettability of the sliding counterparts is an essential issue in lubrication systems, especially for water-based lubricants. In Fig. 10, the typical distribution models of lubricant in the ball-plate sliding configuration for hydrophilic and hydrophobic cases are presented [37]. For the hydrophilic case, as shown in Fig. 10a, the same affinity for lubricant of the ball and the plate makes the layer very thin. So it is not capable of hydrostatic lift, but still can lubricate as an adhesive layer. For G-L dispersions, such a thin lubricating layer is favorable for the exfoliated graphene to lie at the contact sliding interface and subsequently be embedded in the wear scars and tracks as the ball slides by. This will form a protective film on the worn surface. In contrast, Fig. 10b shows a drawing of the hydrophobic case, where there is no formation of any layer of lubricant between the surfaces. When the ball slides over the plate, the lubricant is inclined to flee to both sides. Consequently, it is hard to form a protective layer of lubricant or graphene film in this case. Fig. 11a shows the evolution of wettability of the sliding surfaces for the prepared dispersions, measured by contact angle. The addition of Triton X-100 in DI water has markedly reduced the contact angle from 80.1 to 44.7. Moreover, the contact angle further grows smaller as the graphene concentration gradually increases (37 for 110 mg/ml). The contact angle of the GOT-L sample was also as low as 43.5 . In contrast, the GO-L dispersion

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did not show obvious change in contact angle in contrast with DI water (79.7 versus 80.1 ). Therefore, the contact sliding interfaces are more wettable for the solvent and the G-L series and GOT-L in comparison with DI water and GO-L. Given the distribution models of lubricant in Fig. 10, it can be concluded that the solvent, the G-L series and GOT-L can tribologically perform better than DI water and GO-L. This is consistent with our test results. Furthermore, according to the models in Fig. 10, we can also deduce that it is more facile for the G-L series and GOT-L than the GO-L dispersion to form the protective layer due to their different wettability of the steel surfaces, and this may explain the SEM observations (Fig. 5) of total versus partial deposition layers for G-L/GOT-L and GO-L dispersions, respectively. It should be noted that all these high tribological performances are a result of the addition of surfactant and in-situ exfoliated graphene at low concentrations of 1 mg/ml and 23.8e110 mg/ml, respectively. Such a small amount of additives will not incur great change in the properties of water-based lubricants. For example, we have measured the kinematic viscosity of the dispersions, which is a critical parameter in the applications of atomization lubricating systems [5]. As presented in Fig. 11b, the addition of surfactant and in-situ exfoliated graphene caused a rise in viscosity by no more than 2.5%, while the addition of GO in water increased the viscosity by 9.3% and 9.1% for GO-L and GOT-L, respectively. Hence, it is expected that G-L series also outperform GO-L and GOT-L in keeping the natural advantages of water-based lubricants. 4. Conclusions In summary, in-situ exfoliated graphene has been successfully prepared in aqueous dispersion with the assistance of a nonionic surfactant, Triton X-100. The dispersions are tested as water-based lubricants on a steelesteel ball-plate tribometer. Results indicate that a small amount addition of graphene can substantially reduce the friction coefficient by 81.3% and the wear volume by two orders of magnitude, compared with DI water. Besides, the graphene dispersions also outperform GO dispersions at the same additive concentration. Moreover, the high lubricating performance of the as-prepared dispersions can maintain even when the applied load and the rotary velocity are varied. In regard to the lubrication mechanism, it is demonstrated that the surfactant plays a critical role in the formation of fluid adhesive layer and graphene protective film at the contact zone, by adjusting the dispersion wettability of the mating surfaces. This study has proved the great potential of pristine graphene as a nano-additive for water-based lubricants, and may provide a brand new application field for the liquid-phase

Fig. 10. The distribution models of lubricant in the ball-plate sliding configuration for (a) hydrophilic and (b) hydrophobic cases.

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Fig. 11. (a) Contact angles and (b) kinematic viscosities of DI water, the solvent, G-L dispersion series and GO-L/GOT-L dispersions.

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