Towards Excellent Tribological Performance as Oil-Based Lubricant

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Functional Nanostructured Materials (including low-D carbon)

Towards Excellent Tribological Performance as Oil-Based Lubricant Additive: Particular Tribological Behavior of Fluorinated Graphene Kun Fan, Xinyu Chen, Xu Wang, Xikui Liu, Yang Liu, Wenchuan Lai, and Xiangyang Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07635 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Materials & Interfaces

Towards Excellent Tribological Performance as Oil-Based Lubricant Additive: Particular Tribological Behavior of Fluorinated Graphene Kun Fan,a Xinyu Chen,a Xu Wang,a Xikui Liu,a Yang Liu,a Wenchuan Laia Xiangyang Liua* a

College of Polymer Science and Engineering, State Key Laboratory of Polymer Material and

Engineering, Sichuan University, Chengdu 610065, People's Republic of China.

ABSTRACT: The poor dispersibility, strong interlayer interaction and inferior crack resistance ability restrict the employment of grephene as lubricant additive. Herein, we prepared fluorinated graphene with different F/C ratios by direct fluorination of multilayer graphene utilizing F2. Among them, highly fluorinated graphene (HFG) with F/C ratio of about 1.0 presented prominent thermal stability and excellent tribological performance as oil-based lubricant additive, whose friction coefficient and wear rate had a 51.4% and 90.9% decrease compared with that of pristine graphene, respectively. It was confirmed that C-F bonds perpendicular to graphene plane contributed to increasing the interlayer distance and tribological performance of fluorinated graphene, while the randomly oriented CF2 and CF3 groups did not count as influential, as demonstrated via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and polarized ATR-FTIR. Meanwhile, Raman measurements traced the formation process of integrated and stable HFG tribofilm during friction process, and the corresponding stability was attributed to the physical and chemical interactions between HFG and friction pairs. More interestingly, the outstanding crack resistance ability of HFG preserved the sheet structure from destruction due to decreased in-plane stiffness and out-plane stress, thus constructing the tough tribofilm. 1

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The simple and feasible preparation makes HFG a promising candidate as advanced lubricant in industrial fabrication. KEYWORDS: fluorinated graphene, perpendicular C-F bonds, stable tribofilm, tough tribofilm, tribological performance 1. INTRODUCTION Due to the unnecessary energy consumption, environment pollution and machine failure, manipulating friction and reducing wear even achieving superlubricity are always crucial concerns around the world. Accordingly, multitudinous endeavors have been dedicated to design materials and surfaces for effectively controlling friction and wear, across scales from nano- up to macroscale. The unparalleledly-high mechanical strength and atomically-lubricating performance have made the two-dimensional (2D) materials promising candidates as advanced lubricants, including such as graphene, boron nitride, molybdenum disulfide and so on. Recent advances in 2D materials also opened a horizon to achieve superlubricity, e.g., the superlubricity was clearly observed when an incommensurate stacking transformed into a commensurate stacking between double-layers graphene.1 Subsequently, a series of superlubricity were observed with other 2D materials in different systems, as well.2-4 Unfortunately, in the face of applying at engineering scales or implementing in practical systems, it is still difficult to achieve superlubricity.5 In terms of graphene as lubricant additive, the strong interlayer interaction derived from π-π stacking effect brings about poor dispersibility in matrixes such as lubricating oil and polymer, and that also restricts interlayer slipping of graphene. Graphene sheet would readily crack 2


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even at lower than 5% strain, since it possesses large in-plane stiffness and Young’s modulus and the rigid carbon-carbon skeleton provides limited energy dissipation pathway for applied strain.6 The inertness of graphene makes it difficult to construct stable tribofilm on the surface of friction pairs through certain interaction to avoid materials wear. These aspects severely deteriorate the tribological performance of graphene in practical application, especially as oil-based lubricant additive. In order to solve the above-mentioned problems of graphene, the corresponding modification has received significant interests. On the one hand, non-covalent modification was developed, such as depositing zirconia7 and Au8 onto geaphene sheet, while the uniformity and stability of structure and tribological performance still need to improve. On the other hand, covalent modification has been considered a more effective means to design advanced graphene-based lubricants, such as graphene oxide9, acid-grafting graphene10 and polydopamine-functionalized graphene oxide11. Nevertheless, the corresponding thermal stability (lower than 250 ℃) and interlayer slipping ability can’t meet the need in industrial fabrication, as well. Fluorinated graphene (FG) or fluorographene, one important 2D material in graphene derivatives, has given rise to ever-increasing attention of the world since its first report in 2010.12 It should be emphasized that achieving high functionalization density, inheriting superior mechanical strength of graphene, presenting excellent thermal stability and owning large interlayer distance endow FG with bright application prospect in lubrication field.13-15 Notably, it has been demonstrated that the monolayer FG presented worse tribological performance than that of pristine 3



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graphene, from nano- up to macro-friction.16-18 The partially fluorinated graphene by liquid-phase exfoliation preparation presented long duration during friction process,19-20 while the relatively low F/C ratio restricted the improvement of tribological performance and thermal stability compared with highly fluorinated graphene.21 The particular tribological behavior (or mechanism) of FG during friction process is still indistinct, e.g., the intrinsic structure such as fluorine distribution and mechanical property of FG seldom comes into contact with the tribological performance. These aspects dramatically impede to explore more potential lubricating property of FG. Therefore, the tribological behavior of multilayer and highly fluorinated graphene is still deserved to investigate. In this work, we prepared FG with different F/C ratios (from 0 to 1.0) by directly heating fluorination of multilayer graphene using F2. It was demonstrated that the higher F/C ratio and more C-F bonds (perpendicular to graphene plane) were in favor of improving the tribological performance of FG, while the randomly oriented CF2 and CF3 groups did not count as influential. As a result, HFG presented excellent tribological performance as oil-based lubricant additive, and it possessed good dispersibility in oil and prominent thermal stability (over 400 ℃). To shed light on the in-depth tribological behavior of HFG, versatile investigations were performed as the following three aspects describe: a. HFG developed the self-lubricating behavior due to the increased interlayer distance and weak interlayer interaction. b. Raman measurements traced the formation process of integrated HFG tribofilm during friction process. Eventually, the stable tribofilm was formed due to the strong 4


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interaction between HFG and friction pairs. c. Raman measurements and density functional theory (DFT) calculations proved that the excellent crack resistance ability of HFG was attributed to the decreased in-plane stiffness and out-plane stress, thus constructing the tough tribofilm. Such tribological behavior of FG is systematically investigated for the first time, and it provides a guidance to get close to superlubricity in practical application. 2. EXPERIMENT 2.1. Materials. Graphene was purchased from The Sixth Elementary (Changzhou) Materials Technology Co., Ltd. The F2/N2 mixture (10 vol% of F2) with purity of 99.99% was purchased from Chengdu Kemeite Fluorine Industry Plastic Co., Ltd. The base lubricating oil of liquid paraffin was obtained from Sichuan Kelong Chemical Reagent Co, Ltd. with commercially analytical grade and was used without further purification. 2.2. Preparation of different F/C ratios of FG FG was prepared by direct fluorination using F2/N2 as fluorinating reagent, as reported in previous work.22 An amount of 500 mg graphene was placed in a closed stainless steel (SUS316) chamber (20 L), then oxygen gas and moisture were eliminated by exchanging with nitrogen gas three times. Subsequently, quantitative F2/N2 was introduced into the chamber and the chamber was heated up to 180 ℃ from room temperature at a heating rate of 5 ℃ min-1, then persistently keeping 1 h for fluorination reaction. After completing the fluorination, the unreacted F2 and 5



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produced gases like HF were removed by exchanged with nitrogen three times and absorbed by alkali aqueous solution. The quantitative F2/N2 was 10, 50 and 80 kPa, and the corresponding fluorinated products were denoted as LFG, MFG and HFG (The F/C ratios of LFG, MFG and HFG are about 0.1, 0.58 and 1.0, respectively). 2.3. Preparation of pristine graphene and different F/C ratios of FG oil dispersions In order to investigate the dispersibility of pristine graphene and HFG in oil and the corresponding tribological performance, the same concentration of 0.3 mg/ml were prepared by respectively adding 9 mg pristine graphene and HFG into 30 ml oil, accompanied by ultrasonication (300 W) of 30 min to form homogeneous dispersions. Similarly, the same concentrations of LFG and MFG with 0.3 mg/ml were prepared to test tribological performance. 2.4. Characterizations. Wide angle X-ray powder diffraction data (XRD) was implemented at room temperature using Cu Kα radiation (λ= 0.154 nm, U = 40 kV, I =40 mA) over the 。

。

angular range 3.5–50 (2θ) with a step size of 0.02 using an Ultima IV powder diffractometer (Rigaku Corporation). The X-ray photoelectron spectroscopy (XPS) was applied to analyze the chemical composition on a Kratos ASAM 800 spectrometer (Kratos Analytical Ltd, UK), with a nonmonochromatic Al Kα (1486.6 eV) X-ray source (a voltage of 15 kV, a wattage of 250 W) radiation, and the vacuum pressure was 10-6-10-7 Pa. Polarized ATR-FTIR spectra were performed using a 。

Nicolet 560 Fourier transform spectrometer. The polarized angle was changed from 0 。

。

to 90 , and a series of spectra were collected every 10 .The incidence angle of 6


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。

infrared light was 45 , and it would be split into two polarized lights, as shown in Figure S1. The IR peak intensity of chemical bonds perpendicular to graphene plane 。

was the strongest with the polarized light of 90 , and that parallel to graphene plane 。

was the strongest with the polarized light of 0 . Thermal property was measured by Thermal gravimetric analysis (TGA) (TA Instrument TG-209F1), with a heating rate of 10℃ min-1 from 30 to 800℃ under nitrogen and air atmospheres. High magnification TEM (Tecnai G2 F20 S-TWIN), aberration-corrected transmission electron microscope (AC-TEM, FEI Titan themis 200 TEM) and AFM (Nano-ScopeMultiMole & Explore from Vecco Instruments) were performed to observe the morphology and thickness. The UV-vis spectrum was characterized by a MAPADA UV-1800 PC spectrometer with a scanning wavelength ranging from 200 to 800 nm. Specific surface area (SBET) was measured by nitrogen sorption isotherms in a Micromeritics Tristar 3020 analyzer (USA). Raman spectra were measured before and after tribological tests at room temperature using LabRAM HR Raman spectrometer with an excitation wavelength of 532 ± 1nm, and the numerical aperture of the lens was 50 µm and the laser beam diameter was 1 mm. The corresponding spectra were collected ranging from 0 to 3500 cm-1 with the exposure time of 20 s and a laser power of 1 mW. The free radicals signals were captured using Electron paramagnetic resonance (EPR) (Bruker Beijing Science and Technology Ltd, USA) on Bruker EPR EMX Plus, with a frequency of about 9.8 GHz and a standard microwave power of 1 MW. Zeta potential (Zetasizer Nano-ZS, Malvern, UK) was applied to measure the surface charges with a dispersions concentration of 0.1 mg/ml 7



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using ethanol as solvent. DFT calculations for model molecules were carried out using the DMol3 module and implemented in Materials Studio 8.0, in order to investigate the elastic performances of pristine graphene and HFG. For all supercell models, vacuum separation of 20 Å was applied to guarantee no sheet-sheet interaction in the periodic boundary condition calculations. Geometry optimizations were performed by using with the generalized gradient approximation (GGA) of Becke-Lee-Yang-Parr (BLYP) functional, and the basis was set as DND with basis file 3.5. The self-consistent-field calculation had convergence criteria of 10-5 Hartree. The ball-on-plate pattern of tribological tests were carried out on a UTM-2 tribometer in reciprocating wear mode at room temperature (room humidity was 30%), accompanied by applied load of 10 N, stroke length of 8 mm, speed of 5 mm s-1 and total test time of 3600 s. This test conditions guarantee the boundary lubrication regime. The ball was made of GCr15 steel with a diameter of 9.525 mm and hardness of 766 HV, and the plate was RTCr2 alloy cast iron with a size of 25×12×6 mm and a hardness of 220 HV. Subsequently, the plate was polished so as to obtain a surface roughness of Ra≤0.03 µm. Meanwhile, both of them were ultrasonically washed with acetone and ethanol successively. The different oil dispersions of about 10ml were added onto the surface of plate prior to the tribological tests. The lower plate sample was soaked in a container in which the lubricant oil was stored, and the upper ball was moving. After completing each tribological test, both of friction interfaces on the ball and plate were ultrasonically cleaned (300 W, 10 min) using ethyl alcohol. Then, the 8


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morphology, elemental mapping and energy dispersive X-ray spectrometry (EDS) of worn surfaces were characterized by SEM (FEI Co, Hillsboro, OR, USA). Dual Mode 3D noncontact surface mapping microscope (RTEC MFT-5000) examined the morphology, 3D surface reconstruction, depth variation and wear rate (supporting information) of worn surfaces. 3. RESULTS AND DISCUSSION 3.1 Influence of fluorine content and distribution Figure 1a displays the tribological performance of FG samples with different F/C ratios (The F/C ratios of LFG, MFG and HFG are about 0.1, 0.58 and 1.0, respectively). It is found that the wear rate and friction coefficient of FG samples decrease with the increased fluorine content. For XRD measurements, the decreased value of 2 theta (2 θ) means larger interlayer distance for graphene derivatives, which is based on the Bragg equation of 2 d × sin θ = n × λ. As shown in Figure 1b, it should be noted that the increased fluorine content brings about larger interlayer distance according to XRD patterns (dLFG = 3.91 Å, dMFG = 6.32 Å and dHFG = 7.35 Å ), which thus means larger self-lubricating degree and better tribological performance. Furthermore, the thermal stability of FG is also highly dependent on the fluorine content, as shown in Figure 1d-f. HFG (or highly fluorinated graphene) presents the best thermal stability（over 400 ℃）under both nitrogen and air atmospheres compared with that of LFG and MFG, and the corresponding C-F bonds also possess the highest bond energy of around 689 eV. In previous work, we have demonstrated the clustering of fluorine atoms could increase the thermal stability of C-F bonds by 9



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experiments and DFT calculations.21 Here, HFG with the highest F/C ratio of 1.0 means more clustering of fluorine atoms, which contributes to the excellent thermal stability. Before and after friction, the color of HFG/oil dispersion has little change due to the little change of F/C ratio, which reflects the good thermal stability of HFG during friction process, as shown in Figure S2. However, the decomposition temperature of functional groups for most graphene derivatives is lower than 250 ℃, which restricts their practical applications. These aspects endow HFG with greater superiority in potential lubrication field.

Figure 1. (a) Histogram of wear rate and friction coefficient of pristine graphene, LFG, MFG and HFG with same concentration of 0.3 mg/ml and pure oil (b) XRD patterns of pristine graphene, LFG, MFG and HFG (c) Change in proportion of different fluorine-related groups to all carbon atoms of LFG, MFG and HFG (d) (e) The DTG curves obtained from LFG, MFG and HFG at a heating rate of 10 ℃ min-1 under N2 and air atmospheres respectively. Insets of Tp1 and Tp2 represent the decomposition of fluorine-related groups and carbon framework, respectively. (f) High-resolution XPS spectra of F1s for LFG, MFG and HFG

More importantly, the tribological performance of FG is highly dependent on the 10


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fluorine distribution, as well. As shown in Figure 1a, it can be found that the tribological performance of LFG presents little change compared with that of pristine graphene. Figure 1b reveals the interlayer distance of LFG also has little change 。

compared with that of pristine graphene（diffraction peak at around 2θ =23 ）, which implies the introduced fluorine provides limited contribution. While the introduced 。

fluorine of LFG may contribute to the new peak（diffraction peak at 2θ =8 ）, representing a relatively ordered structure but not interlayer distance. Furthermore, Figure 1c and Table S1 reveal that, besides of C-F bonds, LFG also contains larger proportion of CF2 and CF3 groups compared with that of MFG and HFG, which has been considered that fluorination occurred at the edge (or defect) of graphene.23-24 Figure 2a displays that the IR peak intensity of fluorine-related groups (C-F at 1150 cm-1, CF2 and CF3 at 1240 cm-1) from LFG hardly increases with the increase in the 。

。

polarized angle (from 0 to 90 ), and the corresponding dichroic ratios (I90/I0 ratio) are 1.07 (Figure 2b) and 1.17 (Figure S3) respectively, which implies that the fluorine-related groups tend to be randomly oriented. The optimized geometric model of LFG also proves that the fluorine-related groups (especially for CF2 and CF3 groups) existed on the edge are randomly oriented, as shown in Figure 2c. Therefore, the randomly oriented fluorine-related groups of LFG do not contribute to increasing the interlayer distance and improving the tribological performance compared with that of pristine graphene. Taking HFG into account, Figure 1c and Table S1 indicate it mainly contains the C-F bonds, which has been considered that fluorination mainly occurred at the basal 11



plane of graphene.

23-24

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Herein, from LFG to HFG, the fluorination process was

confirmed that fluorination occurred at edge (or defect) of graphene at first, then spread to basal plane. The dichroic ratio (I90/I0 ratio) of 1.87 (Figure 2e) and optimized geometric model of HFG (Figure 2f) also imply that the fluorine-related groups tend to be perpendicular to graphene plane, which well contributes to increasing the interlayer distance and improving the tribological performance. In a word, the C-F bonds perpendicular to graphene plane play a leading role in contributing to the increased interlayer distance and excellent tribological performance of FG, while the randomly oriented CF2 and CF3 groups do not count as influential.

。

。

Figure 2. (a) (d) Polarized ATR-FTIR spectra of LFG and HFG from 0 to 90 . The peak of 1150 cm-1 is chosen to represent fluorine-related groups of FG (b) (e) Polar diagrams of absorbance at 1150 cm-1 for LFG and HFG as a function of the polarization angle of IR beam (c) (f) Structure schematic of optimized geometric model for LFG and HFG

3.2 Structure and tribological performance of HFG 12


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HFG with the best thermal stability and tribological performance gives rise to our infinite interest to explore the intrinsic structure and tribological behavior (or mechanism). Figure 3a-b displays the high magnification TEM images of pristine graphene and HFG, respectively. The thickness of pristine graphene is about 3 nm, which possesses 8-9 layers according to the theoretical thickness of a signal-layer graphene sheet reported to be about 0.35±0.01 nm25. The thickness of HFG increases to about 6 nm, consistent with AFM measurement in Figure S4. The increased thickness of HFG is derived from the increased interlayer distance of 7.35 Å compared with 3.91 Å of pristine graphene, caused by repulsive force among introduced fluorine atoms, as shown in Figure 1b. The XPS C1s spectrum (Figure 3d) and aberration-corrected transmission electron microscope (AC-TEM) (Figure S5) illustrate that the fluorinated region on HFG sheet is dominated, not the aromatic region. Figure S6a displays that HFG presents better dispersibility in oil compared with pristine graphene, which may be attributed to the larger interlayer distance (Figure 1b), bigger specific surface area (BET) (Figure S6b) and better lipophilicity (Table S2). Good dispersibility in oil is the foundation for HFG to be applied as an effective oil-based lubricant additive. Furthermore, Figure 3c displays that the friction coefficient of pure oil always maintains a high level of 0.21 during friction process. After addition of pristine graphene, the friction coefficient begins to decrease and ultimate value arrives at about 0.19. Beyond expectation, the friction coefficient after addition of HFG rapidly decreases during friction process, eventually arriving at a 13



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stable value of 0.09. By further increasing the load, the corresponding friction coefficient still maintains a low value, as shown in Figure S7. Besides, the wear rate of HFG presents a 90.9% and 92.3% decrease compared with that of pristine graphene and pure oil, which indicates HFG immensely enhances the tribological performance of pure oil (Figure1a). The width, depth and 3D surface reconstruction of worn surfaces on the plate vividly reflect the excellent tribological performance of HFG (Figure S8), as well.

Figure 3. (a) (b) High magnification TEM images of pristine graphene and HFG (c) Friction coefficient curves of pure oil, 0.3 mg/ml pristine graphene and 0.3 mg/ml HFG. (d) Curve fitted XPS C1s spectrum recorded from 281 to 297 eV and the F/C ratio of HFG

3.3 Self-lubricating behavior 14


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During friction process, HFG developed self-lubricating behavior due to the increased interlayer distance and weak interlayer interaction compared with pristine graphene. While this self-lubricating behavior still requires some other proofs to validate the rationality. Owing to the experimental condition of air atmosphere, self-lubricating behavior of HFG also contributes to preventing the friction pairs from oxidation, namely low oxygen content on the friction pairs. Figure 4b displays that the region with aggregation of oxygen is always accompanied by rarefaction of fluorine, as shown in the white circles. Meanwhile, the oxygen content of worn surface on the ball and plate from HFG are lower than that of pristine graphene, as shown in Table S3. These results reconfirm the existence of self-lubricating behavior for HFG.

Figure 4．．Element mappings of worn surface on the plate (a) pristine graphene (b) HFG

3.4 Constructing integrated and stable tribofilm 15



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Tribofilm on the surface of friction pairs plays a crucial role in reducing materials wear.26-28 As shown in Figure 5b, optical micrograph of worn surface from HFG displays the existence of clear and integrated tribofilm; meanwhile, the Raman shift intensity at 1600 cm-1 assigned to the G band of graphene derivatives is always strong on the whole worn surface, which also verifies the formation of integrated tribofilm, as shown in the Raman mapping (Figure 5b). However, optical micrograph of worn surface from graphene displays that no obvious region is covered by tribofilm, which keeps space with corresponding Raman mapping that most regions present weak intensity at 1600 cm-1 (Figure 5a). In addition, Figure 5e illustrates that no obvious D and G bands (characteristic Raman shifts of graphene derivatives) appear on the Raman spectrum of severely damaged region on the worn surface of pristine graphene (Figure 5c), indicating the corresponding region is not covered by tribofilm. The worn surface of HFG has little damage (Figure 5d) with the existence of HFG tribofilm, which is demonstrated by the Raman spectrum with obvious D and G bands. In other words, reducing wear is highly dependent on manipulating the coverage of tribofilm on materials surface.

16


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Figure 5．．(a) (b) Optical micrographs and corresponding Raman mappings of the worn surface on the plate from pristine graphene and HFG. The G band at1600 cm-1 is chosen as the characteristic Raman shift of graphene derivatives. (c) (e) Severely damaged region existed on the worn surface of pristine graphene and the corresponding Raman spectrum at this region. (d) (f) Severely damaged region existed on the worn surface of HFG and the corresponding Raman spectrum at this region

Hence, Raman measurements were performed to trace the formation process of HFG tribofilm on the friction pairs. As shown in Figure 6a, insets in friction coefficient curve of HFG are the evolution of self-lubricating state, which is governed by the evolution of tribofilm. During which, Figure 6b reveals no obvious D and G bands appear on Raman spectrum of worn surface at the friction time of 100 s, which

17



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indicates the nonexistence of HFG tribofilm. Single self-lubricating at initial stage of friction is only assigned to the interlayer self-lubricating of HFG, which contributes to the rapid decrease of friction coefficient. Subsequently, HFG tribofilm gradually formed on the surface of friction pairs at transition state during friction process, leading to persistently decreasing the friction coefficient. Figure 6c reflects that some regions (region 1) of worn surface contain HFG tribofilm with obvious Raman signal (D and G bands), while the other regions (region 2) are not covered by HFG tribofilm by the lacking of obvious Raman signal (D and G bands) at the friction time of 600 s, which well demonstrates the incomplete HFG tribofilm, namely the transition state. In the end, Figure 6d illustrates the whole of worn surface contains HFG tribofilm with obvious Raman signal, which implies that the multiple self-lubricating interfaces including “ball-HFG sheet”, “HFG sheet-HFG sheet”, “HFG sheet-plate” and “ ball-plate” arrive to a stable state, accompanied by a stable friction coefficient of 0.09. In addition, by performing prolonged friction tests, the lifetime of tribofilm formed on the worn surface was investigated and the corresponding value came up to about 4600s, as shown in Figure S9. Besides of Raman measurements, element mapping of worn surface from HFG also reflects the existence of fluorine with uniform distribution (Figure 4b), which indicates the formation of HFG tribofilm during friction process. Similarly, HFG tribofilm was formed on the ball with the existence of fluorine, as shown in Table S2.

18


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Figure 6. (a) Friction evolution of HFG during friction process (b) Raman spectrum of worn surface on the plate of HFG: the total friction time is 100 s. Random five regions are chosen and five Raman spectra are similar (c) Raman spectra of worn surface on the plate of HFG: the total friction time is 600 s. Random five regions are chosen: three Raman spectra are similar to region 1 and two Raman spectra are similar to region 2 (d) Raman spectrum of worn surface on the plate of HFG: the total friction time is 3600 s. Random five regions are chosen and five Raman spectra are similar.

Having confirmed the formation process of HFG tribofilm, the corresponding stability deserves more attentions, because the easily removed tribofilm during friction process does not work efficiently on reducing materials wear. Notably, the friction pairs have been treated by ultrasonic cleaning (300 W, 10 min) before characterization, while Figure 5b still displays the existence of relatively integrated HFG tribofilm compared with that of pristine graphene, which demonstrates HFG 19



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tribofilm is more stable during friction process. It should be suggested that the physical and chemical interactions between HFG and friction pairs contribute to constructing the stable tribofilm. Recently, quantitatively investigating real-time charge transfer in triboelectrification has revealed that electron transfer was the leading process during friction between two inorganic solids.29 This means iron ions such as Fe2+ and Fe3+ would persistently produce on friction interface during friction process, which is also consistent with the experimental results in the other literatures.25,

30-31

Figure 7a reveals HFG is in

possession of larger negative zeta potential of -29.3 mV than -7.1 mV of pristine graphene due to the largest electronegativity of fluorine atom, which indicates the existence of larger physical interaction between HFG and friction pairs by electric charge effect (Figure 7c). On the other hand, under harsh friction conditions, lubricant medium inevitably reacts with the friction pairs on the interface due to the metal catalytic actions,32 and above physical interaction further promotes the reaction between HFG and friction pairs. As shown in Figure 7b, a new peak at about 707.3 eV appears on the high resolution XPS Fe2p spectrum on the worn surface of HFG compared with that of pristine graphene, which can be assigned to Fe(C5H5)2 according to the Fe2p electron binding energy table.30 In addition, taking the preparation of HFG into account, the fluorination for graphene is always accompanied by the generation of free radicals and paramagnetic structural defects,

22

so typical signals appear in the EPR spectrum

of HFG, as shown in Figure 7d. The stable existence of paramagnetic centers on the 20


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HFG nanosheet is dependent on the rigid two-dimensional (2D) structure of the carbon-carbon skeleton and -conjugation effect. Generated Fe3+ tends to get close to HFG and terminates the free radicals as they mix together,33 as shown in Figure 7e. Hence, it is easier for HFG to react with Fe3+ during friction process (Figure 7f).

Figure 7．．(a) Zeta potentials of pristine graphene and HFG as prepared in ethanol (b) High resolution XPS Fe 2p spectra on the worn surface of pristine graphene and HFG (c) Schematic of physical interaction between HFG and friction pairs (d) EPR spectra of pristine graphene and HFG (e) EPR spectra of HFG and HFG mixed with FeCl3 (f) Schematic of HFG reacting Fe3+ by radical mechanism

3.5 Building tough tribofilm Graphene with inferior crack resistance ability34 inevitably cracks during friction process due to the applied friction force and generated friction heat. The variation of ID/IG ratio in Raman spectra reflects crack degree of graphene sheet. 24, 35-36 As shown in Figure 8a, the ID/IG ratio of pristine graphene is 0.90 while that of graphene after friction increases up to 1.15, which indicates graphene sheet severely cracks during 21



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friction process (Figure 8c). As a result, the friction coefficient of pristine graphene persistently increases with continuous crack of graphene sheet as friction time goes on (Figure 3e). During this process, it adopts the form of either graphene sheet deformation and fatigue or crack triggering and diffusing, which results in the formation of loose even half-baked tribofilm and aggravating wear of friction pairs due to the direct contact between the ball and plate. Figure 8b reveals that the little change of ID/IG ratio (from 0.98 to 0.97) for HFG during friction process manifests the minimum sheet crack (Figure 8d), leading to forming the tough tribofilm with the excellent tribological performance.

Figure 8．．Raman spectra of pristine samples and samples after friction on the worn surface of plate (a) pristine graphene (b) HFG; Schematic diagrams of the ball tip sliding over the sheets surface (c) pristine graphene (d) HFG

22


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Here, the excellent crack resistance ability of HFG is in association with the mechanical property. DFT calculations were performed to calculate elastic performances of pristine graphene and HFG. In order to guarantee its rationality, supercell models were built in a periodic form and fractional unit cells were presented in Figure 9a-b. As a result, Figure 9c indicates the elastic constants of graphene on the X-axis and Y-axis present the same calculated value of 176.5 N/m. The elastic constant of HFG presents the decreased trend that corresponding calculated values on the X-axis and Y-axis arrive at 103.1 N/m and 120.9 N/m, respectively, as shown in Figure 9d. The different elastic constants in plane of HFG may be attributed to that, from graphene to HFG, transformation from sp2 carbon skeleton to sp3 carbon skeleton brings about the formation of boat and chair conformations,37-38 thus appearing anisotropy in plane. The calculated results indicate HFG presents the decreased in-plane bulk stiffness compared with that of pristine graphene, which is capable of tolerating larger in-plane strain without crack. Furthermore, the crack readily diffusing on graphene sheet can be changed by fluorination; that is, the disordered and highly corrugated fluorinated region provides additional energy dissipation pathways,39 resulting in terminating the crack diffusing. Taking the Z-axis (out-plane) into account, in terms of nanoscale friction of monolayer FG, when the tip slips over the surface, the increased energy dissipation ability of FG can’t give back the tip’s kinetic energy. This way provides limited help for the tip to overcome the energy barrier to slip over another position,

2

which

increases the nanoscale friction compared with pristine graphene. Meanwhile, the 23



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increased out-plane stiffness and corrugation of FG also result in the increased nanoscale friction.13-14Among them, it is indeed disadvantageous for monolayer FG to reduce friction and wear. Here multilayer FG at macroscale friction effectively solves this problem. As shown in Figure 9e-f, when the tip of ball contacts then slips over the surface, the corresponding stress (F) and friction force (f) appear on the contact point of top layer. Subsequently, the F and f transfer onto the next layer with the decreased values, namely F1 and f1, and the transitive model would continue to the next layer one by one, as well. During this process, the F1 and f1 of pristine graphene would still present a large value due to the small interlayer distance of 3.91 Å and strong interlayer interaction. On the contrary, HFG with the negative zeta potential of -29.3 mV (Figure 7a) endows it with the larger interlayer distance of 7.35 Å and weak interlayer interaction, immensely decreasing the F1 and f1. The transitive model of interlayer friction has been demonstrated in the heterostructure of fluorinated graphene/boron nitride stacking by DFT calculations.40 As a result, HFG presents low friction coefficient and excellent crack resistance ability.

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Figure 9. Atomic models and the corresponding primitive unit cells: (a) graphene (b) HFG (one of light-blue F is on the top of one gray C atom, and the other F is under the other C atom). (c) Calculated in-plane elastic constants on the X-axis of pristine graphene and HFG (d)Calculated in-plane elastic constants on the Y-axis of pristine graphene and HFG (e) (f) Stress (F) and friction force (f) evolution on the Z-axis for multilayer granphene and corresponding HFG

More deeply, it gives rise to our thought whether other graphene derivatives such as graphene oxide (GO) also present the same crack resistance ability. As shown in Figure S10, there is still a large increase of ID/IG ratio for GO (from 0.86 to 1.05) during friction process, implying the inferior crack resistance ability. Previous study also demonstrated this point in different friction systems.

24

The friction process is

always accompanied by the generation of friction heat, which inevitably damages the 25



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structure of lubricant medium. From the thermal gravimetric analysis (TGA) and relevant bond lengths and energies of GO and HFG (Figure S11 and Table S4), it is found that HFG presents better thermal stability. Meanwhile, the self-lubricating behavior of HFG also reduces the friction force and friction heat. But the friction heat would not obviously decrease for GO, which results from the fact that interlayer slipping of GO is difficult due to the attraction among oxygen functional groups.

41

Therefore, it is speculated that oxygen functional groups of GO easily fracture during friction process compared with C-F bonds of HFG, leading to the inferior crack resistance ability. The stable C-F bonds on graphene sheet, a superiority of FG in graphene derivatives, provide the foundation for HFG with the excellent crack resistance ability and potentially longer duration as advanced lubricant.

4. CONCLUSION In this paper, we systematically investigated the tribological behavior of fluorinated graphene for the first time. It has been demonstrated that the higher F/C ratio and more C-F bonds perpendicular to graphene plane contributed to improving the tribological performance, while the CF2 and CF3 groups did not count as influential. During friction process, HFG developed the self-lubricating behavior and constructed the stable and tough tribofilm. Such tribological behavior contributed to greatly reducing friction and wear. Therefore, HFG with F/C ratio of about 1.0 presented excellent tribological performance, and the good dispersibility in oil and prominent thermal stability make it a promising candidate as advanced lubricant in practical application. 26


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■ASSOCIATED CONTENT Supporting Information Computational formulas of wear rate; Scheme of polarized ATR-FTIR; Optical micrographs of HFG/oil dispersion before and after friction; Chemical composition of different FG samples; Polar diagrams of absorbance at 1240 cm-1 for LFG; AFM and AC-TEM images of HFG; UV-vis spectrophotometry and BET of graphene and HFG; lipophilicity measurements; Friction coefficient of HFG/oil dispersion at different loads; Optical micrographs, 3D surface reconstructions and position-depth curves of worn surface; Chemical composition of worn surface; Friction coefficient of HFG at prolonged friction tests; Relevant bonds energy of GO and HFG; Raman spectra of pristine GO and GO after friction; TGA of GO and HFG. This material is available free of charge via the Internet at http://pubs.acs.org.

■AUTHOR INFORMATION Corresponding author *Tel.:+86 28 85403948.

Fax: +86 28 85405138.

E-mail address: [email protected] (Xiangyang Liu)

Notes The authors declare no competing financial interest.

■ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 51573105 and Grant No. 51633004) and State Key Laboratory of Polymer Materials Engineering (Grant No.sklpme2017-2-03). 27



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Table of content

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Towards Excellent Tribological Performance as Oil-Based Lubricant

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