Excellent Lubricating Ability of Functionalization Graphene Dispersed

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Excellent Lubricating Ability of Functionalization Graphene Dispersed in Perfluoropolyether for Titanium Alloy Jianfang Sun, AoSong Li, and Fenghua Su ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02282 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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ACS Applied Nano Materials

Excellent Lubricating Ability of Functionalization Graphene Dispersed in Perfluoropolyether for Titanium Alloy Jianfang Sun, Aosong Li, Fenghua Su* School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China.

ABSTRACT：The dispersibility of graphene family nanomaterials in base oil is a crucial factor for their application as lubricating additive in oil-based fluids. Here, the 1H,1Hperfluorooctylamine (FOA) was covalently grafted onto the GO surfaces and then the product was reduced by hydrazine monohydrate for synthesizing a modified graphene nanomaterial of FOArGO. Various techniques including transmission electron microscopy (TEM), atomic force microscopy (AFM), Fourier transfer infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD) and Thermogravimetric analysis (TGA) were used to characterize the as-prepared FOA-rGO nanomaterial. The test results demonstrate that the long fluorocarbon chains of FOA are successfully linked to the edges of GO surfaces by covalently functionalizing, which corresponds to the excellent dispersibility of the resulting FOA-rGO in a space lubricating oil of perfluoropolyethers (PFPE). The tribological properties of FOA-rGO as nanoadditive in PFPE for lubricating Ti6Al4V/steel friction pairs were systemically evaluated by a UMT tribometer. The results show that the FOA-rGO dispersed in PFPE displays remarkable anti-wear and friction-reducing ability for Ti6Al4V/steel friction pairs. The friction coefficient is reduced by 52.7% and the wear rate is reduced by almost five orders of

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magnitude, when compared to the pure PFPE. The superior lubricating ability of the FOA-rGO additive is closely related to its excellent dispersibility in PFPE that favors the formation of a thin, durable, and stable boundary tribofilm between the contact surfaces.

KEYWORDS: graphene, functionalization, lubricating additive, titanium alloy, friction and wear


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1. INTRODUCTION Titanium (Ti) alloys are widely employed in the fields of aerospace, chemical industry and medical science owing to their high strength, low density and excellent corrosion resistance. However, the low wear resistance and high friction coefficient limit their usages in some applications.1-4 Selecting appropriate lubricant is an important method to broaden their utilizations in the field of tribology. Due to the low volatility, good temperature-viscosity characteristic, inflammability, good thermal and chemical stabilities, perfluoropolyethers (PFPE) has been extensively applied as lubricant in many fields, such as high-vacuum pump oil, hard disk drives, rigid magnetic media, as well as space mechanical components.5-7 However, the boundary lubrication capability of PFPE is relatively poor because of its special molecular structure and the poor bonding capacity on substrate. To address this issue, many methods including modification of the molecular structure and introduction of additives have been used for modifying PFPE in the past few decades.8-11 Kondo

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synthesized a new type PFPE, whose terminal group was an ammonium salt with a carboxylic acid. He confirmed that the new PFPE has better friction-reducing and anti-corrosion properties than the conventional one. Zhu et al.9 reported that the phosphazene-type additives were effective in enhancing the thermal stability and lubricating performance of a PFPE-type lubricant of Z-DOL. As mentioned above, the lubricating performances of a lubricant can also be improved by adding nanomaterial additive. However, the solid additive is always difficult to dissolve in PFPE because of its extreme inertness and insolubility in chemicals. Accordingly, numerous approaches have been developed to improve the dispersibility of the layered nanomaterial in PFPE. Wu et al.12 reported that nanosized MoS2 on graphene (MoS2/Gr) was successfully applied as additives in the base oil of PFPE for reducing the friction and wear of steel/steel contact. Graphene sheets used as substrate effectively improved the dispersibility of the nanosized MoS2 in PFPE. Huang et al.13 found that 3 ACS Paragon Plus Environment


candle soot particles (CSP) modified by 1H,1H,2H,2H-perfluorooctanol exhibited good dispersibility in PFPE, and the as-prepared CSP dispersed oil possessed better lubricating performance than the neat PFPE under different test conditions. Graphene, a fascinating 2D carbon nanomaterial, has attracted significant attentions in different fields, thanks to its excellent electrical, mechanical, physical and thermal properties. Furthermore, its large surface area and layered structure also allow it to readily enter the contact interface of a tribo-pair, preventing the direct contact of rough surfaces and reducing friction and wear. The potential of graphene to increase the durability and reliability of mechanical assemblies has been demonstrated when even a very small amount of graphene is supplied as lubricant and lubricating additive between rubbing surfaces.14-22 However, most compounds in the graphene family have strong π-π interaction and are innately difficult to dissolve in most solvents. Surface functionalization and modification have been widely applied to improve the dispersibility and solubility of the graphene family nanomaterials in the dispersed oils.23-28 Mungse et al.23 developed a chemical approach for grafting long alkyl chains through the amide linkage on GO surfaces, and confirmed that the modified GO exhibited excellent dispersibility in lube oil. Wu et al. 24 found that the modification of GO with myristyltrimethylammonium bromide greatly improved its stability and decreased its droplet size in the base emulsion. Friction coefficient and wear rate of the steel ball lubricated with the modified GO dispersed emulsion were reduced by about 18% and 48% respectively, when compared to the base emulsion. In this work, GO was covalently functionalized by 1H,1H-perfluorooctylamine (FOA) to introduce long fluorocarbon chains on its surface and then reduced by hydrazine monohydrate to prepare a modified graphene nanomaterial of FOA-rGO. Subsequently, the as-prepared FOA-rGO as lubricating additives was added in PFPE for lubricating Ti6Al4V/steel contact. The results showed


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that the GO was successfully grafted by FOA, and the modified nanomaterial of FOA-rGO could uniformly disperse in PFPE and employed as efficient lubricant for reducing the friction and wear of Ti6Al4V/steel friction pair. The findings here readily provide a valuable strategy for preparing a space liquid lubricant with remarkable anti-wear and friction-reducing abilities for lubricating Ti alloy space mechanical components.

2. EXPERIMENTAL SECTION 2.1. Materials. Graphene oxide (GO) with average particle size of 0.5 ~ 5 μm, thickness of 0.8 ~ 1.2 nm, and C/O ratio of 1.8 (64/36) was purchased from Nanjing XFNANO Materials Tech Co., Ltd., of China. 1H,1H-perfluorooctylamine (FOA) with 98% purity was purchased from Jiangsu Boke Chemical Co., Ltd., of China. PFPE (CF3O(OF2CF3O)mO(CF2O)nCF3, m/n=40/1, MW=3700) with the commercial name of Fomblin Y25 was purchased from Solvay Solexis Co., Ltd., Italy. Other chemical reagents including dimethylformamide (DMF), dichloromethane, triethylamine, ammonia water and hydrazine monohydrate were analytical grade and produced by Tianjin Fuyu Fine Chemical Co., Ltd., of China. 2.2. Synthesis of rGO and FOA-rGO. For synthesizing FOA-rGO, 50 mg GO was first added into 50 mL DMF with ultrasonic vibration for 20 min for obtaining GO/DMF solution. Then, 500 mg FOA was dissolved in a mixture of 100 mL dichloromethane and 5 mL triethylamine with the help of ultrasonic vibration for 10 min for preparing FOA solution. After that, the two solutions were transferred into a round-bottom flask and treated with magnetic stirring for 48 h at 90°C for producing the FOA-GO reaction suspension. As the reaction suspension was slowly cooled to room temperature, 1 mL ammonia water and 50 5 ACS Paragon Plus Environment


mL hydrazine monohydrate were subsequently introduced into the suspension and treated with magnetic stirring for 12 h at 50 °C for producing FOA-rGO suspension. Finally, the FOA-rGO products were washed with ethanol and distilled water several times and dried in a vacuum oven for 18 h at 60 °C to obtain the dried FOA-rGO powders. The synthetic schematic for producing FOArGO is illustrated in Fig. 1a. For preparing rGO, 1 mL ammonia water and 50 mL hydrazine monohydrate were directly added in the GO/DMF solution and treated with magnetic stirring for 12 h at 50 °C. After that, the products were washed with ethanol and distilled water and dried in the vacuum oven for 18 h at 60 °C to obtain the rGO powders. Fig. 1b shows the synthetic schematic for synthesizing rGO.

Figure 1. Synthetic schematic for producing (a) FOA-rGO and (b) rGO. 2.3. Characterizations. The chemical compositions of the nanomaterials including GO, rGO and FOA-rGO were characterized by Fourier transformation infrared spectroscopy (FTIR, Nicolet Nexus 670 FT-IR Spectrometer) in the range of 400−4000 cm−1 and X-ray diffractometer (XRD, Bruker D8 advance) with a Cu Ka radiation (1.5405 Å) over the 2θ range 5–70°. Thermogravimetric analysis (TGA) 6 ACS Paragon Plus Environment

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was performed from 30 to 1000 °C at a heating rate of 10 °C per minute in protective atmosphere of N2 to evaluate the thermal stability and chemical composition of the nanomaterials. The morphologies and microstructures of the GO and FOA-rGO were observed using field-transmission electron microscopy (TEM, JEOL JEM-2010F) and atomic force microscopy (AFM, Bruker Dimension Edge). X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) and Raman spectroscopy (Dilor Labram-1B multichannel confocal microspectrometer) were also introduced to analyze the structures and compositions of the nanomaterials. 2.4. Tribological tests. The nanomaterials of GO, rGO and FOA-rGO with a mass fraction of 0.5 wt.% were respectively dispersed in the PFPE oil and treated with ultrasonic vibration for 15 min for producing the nanooils (GO/PFPE, rGO/PFPE, and FOA-rGO/PFPE). The tribological performances of the neat PFPE oil and the nano-oils were investigated using a UMT-Tribolab tribometer (Bruker-CETR, USA) with a ball-on-plate reciprocating friction mode under ambient environment. Commercial GCr15 bearing steel ball with a diameter of 6.0 mm and Ti6Al4V plate after polishing with surface roughness approximately 0.02 μm were used as friction pairs. All tribo-tests were carried out at a loading of 3.0 N, a reciprocating speed of 10 mm·s-1, and a sliding amplitude of 2.5 mm. Before tribo-tests, the steel ball and Ti6Al4V plate were cleaned by ultrasonication in alcohol and dried in air. The Ti6Al4V plate was completely submerged into the as-prepared lubricant during the rubbing process. The instantaneous friction coefficients were automatically recorded by the UMT tribometer. After tribo-test, the friction pair was washed with ethanol via ultrasonication. The 3D wear track profile and the wear volume of the wear scar on the Ti-6Al-4V plate were measured by a RTEC Universal 3D Profilometer. Each tribo-test and wear track measurement was carried out at least three times to ensure standard deviations less than 5%. The formula W=V/S·F was adopted to 7 ACS Paragon Plus Environment


calculated the wear rate of the plate samples, in which W, V, S and F represented the wear rate, the wear volume, the sliding distance and the load value, respectively. The morphologies and chemical compositions of the wear surfaces on the Ti-6Al-4V plates and the counterpart balls were examined by optical microscope (OM, Leica DMI3000), fieldemission scanning electron microscopy (FESEM, JEOL JSM 6700F) and XPS.

3. RESULTS AND DISCUSSION 3.1. Morphology and Microstructure. FTIR spectra of the GO, rGO and FOA-rGO are shown in Figure 2a. In the spectrum of GO, the characteristic peaks at 3430, 1725 and 1630 cm-1 are respectively attributed to the stretch of O-H, carboxy C=O and aromatic C=C, and the peaks at 1425 and 1055 cm-1 are respectively ascribed to the C-OH stretch and the C-O stretch in hydroxyl groups. These strong peaks confirm the presence of a lot of oxygen functional groups on the GO surface.29 The characteristic peaks at 1725, 1425 and 1055 cm-1 become weaker in the spectrum of the rGO as compared to the GO, which results from the reduction reaction during the synthetic process for producing rGO. As expected, the peaks at 1725, 1425 and 1055 cm-1 almost disappear while the peak at 1630 cm-1 becomes stronger for the sample of FOA-rGO, which is due to the grafting reaction occurred between FOA and GO. The typical peaks at 1210 and 1012 cm-1 respectively assigning to C-F and C-N in-plane stretches30 can further confirm the FOA molecules have been successfully covalently grafted onto the GO surface during the synthetic process. Figure 2b shows the XRD patterns of the GO, rGO and FOA-rGO. In the pattern of the GO sample, the carbon (001) characteristic peak at 10° reveals an interlayer spacing of around 0.9 nm and the existences of hydrophilic oxygen functional groups. The carbon (002) diffraction peak at 24.3o 8 ACS Paragon Plus Environment

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observed in the pattern of rGO suggests that the layer spacing decreases to around 0.37 nm, which results from that the reduction reaction partially removes the hydrophilic oxygen functional groups of GO and reduces the layer distance between the graphene sheets.31 As for the pattern of FOA-rGO, the carbon (002) diffraction peak decays to 16.4° and becomes weaker in comparison with rGO, which suggests that the interlayer spacing of FOA-rGO is around 0.54 nm. This result confirms that the introducing of long fluorocarbon chains increases the layer distances in the FOA-rGO.

Figure 2. (a) FT-IR spectra, (b) XRD patterns, (c) Raman spectra, and (d) TGA curves of the GO, rGO, FOA-rGO. Raman spectroscopy is always employed to elucidate of the microstructures of carbon nanomaterial. Figure 2c presents the Raman spectra of the GO, rGO, and FOA-rGO. The D band (1354 cm-1) and 9 ACS Paragon Plus Environment


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G band (1596 cm-1) observed in the GO spectrum represents the in-plane bonding stretching motion of sp2 carbon atoms and the vibration of disordered sp3 hybridized carbon because of in-plane defects, respectively. The rGO spectrum also presents the typical D and G bands but is featured with the increased intensity ratio of D band to G band (ID/IG). The increased ID/IG ratio in the rGO corresponds to a decrease in the average size of the sp2 domains upon reduction of the exfoliated GO.32 After grafting with FOA, the ID/IG ratio in the FOA-rGO increases to 1.07 from 1.03 of the rGO. The increase of ID/IG ratio is mainly due to the increased defects on the FOA-rGO surface after modification, agreeing well with the results of XRD patterns.33 To sum up, the grafting modification with FOA don’t greatly change the lamellar structure of rGO but increase the defects on its surface, as confirmed by XRD and Raman spectra. TGA curves of the GO, rGO, and FOA-rGO are exhibited in Figure 2d. For the GO sample, 11% mass loss up to 100 °C is primarily due to the volatilization of water molecules adsorbed on the nanosheets, and the main 33% mass loss within 100−300 °C is ascribed to the decomposition of labile oxygen functional groups. A steady mass loss of 17% between 400 and 1000 °C is thanks to the pyrolysis of stable oxygen functionalities.34 In contrast, the weight loss of rGO drop slowly in the entire temperature range. And it presents the total mass loss of 33% that is much lower than GO of 61%, suggesting that a significant amount of labile oxygen groups have been removed by the hydrazine reduction. Similar with rGO, FOA-rGO exhibits low mass loss below 100oC, because lots of oxygen functional groups have been removed by the reduction reaction. The accelerated mass loss from 200oC for the sample of FOA-rGO may derive from to the pyrolysis of the covalently bonded FOA and residual labile oxygen functional groups.35 Therefore, the mass loss of FOA-rGO is higher than that of rGO till the end of the test.


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The atomic valence states and the chemical compositions of the FOA-rGO were characterized by XPS analysis. As illustrated in Figure 3a, the C1s peak in FOA-rGO can be de-convoluted into five strong sub-peaks at 284.5 eV (C=C), 285.4 eV (C–C), 286.4 eV (C–O), 287.6 eV (C=O) and 291.7 eV (CF2) and two weak sub-peaks at 286.0 eV (C–N) and 293.8 eV (CF3). The peak of F1s spectra appearing at 688.9 eV (Figure 3b) is ascribed to C–F.23,32,36 The result further demonstrates that the FOA molecules have covalently linked to the GO surface by the grafting reaction and form FOArGO product after the subsequent reduction process. Meanwhile, the XPS full spectra of rGO and FOA-rGO are shown in Figure S1 to verify the variation of the C/O atomic ratio by the reduction reaction. The C/O atomic ratio has increased to 91/9 and 64/6 for the rGO and FOA-rGO sample respectively, from 64/36 of the GO sample. It is clear that the reduction reaction has eliminated most of oxygen functional groups on the GO surface.

Figure 3. XPS spectra of (a) C1s, and (b) F1s of the FOA-rGO.



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Figure 4. TEM images of (a, b) GO and (c, d) FOA-rGO; and AFM images of (e) GO and (f) FOArGO.


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The morphologies of GO and FOA-rGO sheets are examined by TEM and AFM measurements and shown in Figure 4. Both the GO (Figure 4a) and the FOA-rGO (Figure 4c) present typical twodimensional sheet-like structures, which reconfirms that the grafting reaction doesn’t change the lamellar structure of the sample. Meanwhile, Figures 4a and 4c demonstrate that the nanosheets of GO and FOA-rGO are transparent and exhibit a very stable nature under the electron beam. As carefully observed, it can be found that the irregular atomic microstructure of the GO nanosheet (Figure 4b) has changed to be regular atomic arrangement for the FOA-rGO nanosheet (Figure 4d). This result means that the transformation from amorphous GO to highly crystalline FOA-rGO has occurred due to the grafting and reduction reactions. Meanwhile, the edge micrographs in Figure 4b and 4d suggest that the GO and FOA-rGO are of few layers, which can be further demonstrated by the AFM images (Figure 4e and 4f). The thickness of 4.5 nm for the GO sample suggests that it contains 4-5 layers, and the thickness of 3.8 nm for FOA-rGO corresponds to its layer numbers of 3-4. The AFM image of rGO shown in Figure S2 present that the thickness of rGO is around 5.7 nm, which suggests its layer numbers of 5-6. 3.2. Tribology Properties. The dispersibility of nanoadditive in base oil is a key factor to affect the lubricating ability of the resulting nano-oils. Figure S3 exhibits the photographs of the nano-oils dispersed with the same concentration of 0.5 wt % GO, rGO, FOA-GO and FOA-rGO after 30 min sonication and resting for two weeks. It is clear that the few GO nanosheets can dissolve in the PFPE oil with the help of sonication treatment. The rGO and FOA-GO nanosheets can dissolve in the PFPE oil with the help of sonication, however after resting for 2 weeks a few solid aggregates are clearly observed on the top of the bottles. In addition, it seems that the FOA-GO/PFPE contains more solid aggregates than rGO/PFPE, which indicates that the dispersion level of FOA-GO is worse than rGO in PFPE. As 13 ACS Paragon Plus Environment


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expected, the FOA-rGO nanosheets disperse uniformly in PFPE oil without any aggregates observed on the top or bottom of the bottle after resting for 2 weeks, which indicates the grafting modification of FOA has strongly changed the surface lipophilic of GO and rGO. The FOArGO/PFPE mixture is a uniform and stable solution suitable for employment as lubricant. Figure 5a shows the variations of friction coefficients for the pure PFPE and the nano-oils dispersed with GO, rGO and FOA-rGO with the increasing of rubbing time until the end of 9000 s. The friction coefficient of the pure PFPE is very high and wildly fluctuates between 0.21 and 0.35. Interestingly, the GO/PFPE oil exhibits low friction coefficient of 0.153 at the initial sliding stage, and then suddenly increase to 0.241 with great fluctuations like the pure PFPE after sliding for 1992 s. It is interesting to note that the rGO/PFPE oil presents similar trend of the friction coefficient as the GO/PFPE oil, except that the time when the friction coefficient suddenly increase has changed to be 5134 s. Surprisingly, the FOA-rGO/PFPE oil presents long steady and very low friction coefficient approximately 0.11 during the whole rubbing process. Even if the rubbing time prolongs to 18000 s, this sample still maintains very low and stable friction coefficient, as shown in Figure S4. The interesting variations of the friction coefficient with the increasing of rubbing time are closely related to the change of the lubrication state for these oils. Namely, the addition of GO, rGO and FOA-rGO in PFPE greatly affect the lubrication state of the dispersed oils for lubricating Ti6Al4V/steel friction pairs, which might be attributed to the formed different tribofilms between the rubbing surfaces induced by GO, rGO and FOA-rGO. Comparisons of average friction coefficients and wear rates of the Ti6Al4V plates lubricated with these oils are shown in Figure 5b. The friction coefficients lubricated with the GO/PFPE, rGO/PFPE and FOA-rGO/PFPE are respectively reduced by 9.6, 21.9 and 52.7% as compared with the pure PFPE. The wear rates lubricated with the GO/PFPE and rGO/PFPE were respectively 14 ACS Paragon Plus Environment

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reduced by 10.2 and 48.5% when compared to the pure PFPE. Amazingly, the wear rate has been reduced from 211×10-6 mm3(N·m)-1 for the Ti6Al4V plate lubricated with the pure PFPE to 0.031×10-6 mm3(N·m)-1 for the plate lubricated with the FOA-rGO/PFPE. It means that the wear rate lubricated with the nano-oil of FOA-rGO/PFPE is almost reduced by five orders of magnitude compared to the pure PFPE. The superior lubricating ability of the FOA-rGO additive is closely related to its excellent dispersibility in PFPE that favors the formation of a thin, durable, and stable boundary tribofilm between the contact surfaces.

Figure 5. (a) Typical friction coefficient curves of the pure PFPE and the nano-oils with GO, rGO and FOA-rGO with the increasing of sliding time; (b) Average friction coefficients and wear rates of the Ti6Al4V plates lubricated with these oils (3 N, 9000 s). Figure 6 compares the friction coefficients and wear rates of the Ti6Al4V lubricated by FOArGO/PFPE with other reported Ti6Al4V in recent literatures. The main techniques for reducing the friction and wear of Ti6Al4V including various coating technologies and special lubricants or lubricating additives were summarized in this figure. The special lubricants and lubricating additives employed for comparing with FOA-rGO/PFPE are ionic liquid multialkylated cyclopentanes (MACs) greases, nano-Cu/rapeseed oil, etc. Comparing the tribological properties of 15 ACS Paragon Plus Environment


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FOA-rGO/PFPE with other lubricants and coating technologies for Ti6Al4V can highlight the superiority of the modified graphene nanomaterial in this work. As shown in Figure 6, the friction coefficient around 0.10 and the wear rate of ~10-8 mm3(N·m)-1 for the Ti6Al4V lubricated with the FOA-rGO/PFPE are much lower than all other tested Ti6Al4V samples, even if some tested samples were coated with self-lubricating DLC or MoS2 film. It is indeed that the FOA-rGO as lubricant additive in PFPE outstandingly performs for lubricating Ti6Al4V in this work.

Figure 6. Friction coefficients and specific or volumetric wear rates of Ti6Al4V in this work and other reported literatures 3.3. Analyses of Wear Interfaces. Figure 7 shows the optical micrographs, line-scan profiles and three-dimensional images of the wear scars on the Ti6Al4V plates and the counterpart steel balls. Interestingly, the wear spots on the wear surfaces of the GCr15 balls are elliptical, as shown in Figure 7a3-d3. The counterpart GCr15 balls have higher hardness and better wear resistance than the Ti6Al4V plates. The Ti6Al4V surfaces are easier to be worn and destroyed by the friction force as compared to the counterpart GCr15 steel balls, resulting in the uneven distribution of the friction stress on the contact friction 16 ACS Paragon Plus Environment

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interfaces. Accordingly, the wear spots on the counterpart GCr15 balls gradually become elliptical. The wear tracks on the Ti6Al4V plate (Figure 7a1−a2) and the counterpart ball (Figure 7a3) lubricated with the pure PFPE are very rough and show large wear depth and width. As the friction pairs were lubricated with the GO/PFPE and rGO/PFPE, the wear surfaces on the plates and the steel balls present shallower wear scratches and narrower wear furrows as compared to the ones lubricated with the pure PFPE. But, these wear surfaces also have deep plowing grooves, as shown in Figure 7b1−b3 and 7c1−c3. The results confirm that the addition of GO and rGO can improve the lubricating ability of PFPE to a certain level. As expectedly, only slight rubbing signs are observed on the Ti6Al4V plate lubricated with the FOA-rGO/PFPE, as shown in Figure 7d1, d2. Accordingly, the counterpart ball is slightly scratched and shows almost undetectable wear scar, as shown in Figure 7d3. It is true that the FOA-rGO work well as nanoadditives to dramatically improve the lubricating ability of PFPE for Ti6Al4V/steel friction pairs.



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Figure 7. Optical images (a1-d1) and three-dimensional images (a2-d2) of wear tracks on Ti6Al4V plates (insert in a2-d2 being height profile measurements), and optical images of the wear scars on the counterpart steel balls (a3-d3). (a1-a3: pure PFPE; b1-b3: GO/PFPE; c1-c3:rGO/PFPE ; d1-d3: FOA-rGO/PFPE) Figure 8 shows the SEM images of the wear scars on the Ti6Al4V plates lubricated with the pure PFPE and the FOA-rGO/PFPE. As shown in Figure 8a,b, the wear surface lubricated with the pure PFPE exhibit wide furrows and deep scratches, which suggests that severe adhesion and abrasive wear have occurred for this sample. Adding FOA-rGO in PFPE greatly reduce the wear degree and 18 ACS Paragon Plus Environment

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the wear scar exhibits fine furrows and grooves, as shown in Figure 8c,d. Figure S5 shows EDS elemental mapping images of the wear tracks lubricated with the pure PFPE and the FOArGO/PFPE, and the relative mass fractions of typical elements from Figure S5a, S5b are listed as Table S1. As shown in Figure S5 and Table 1, the wear surface lubricated with the FOA-rGO/PFPE exhibits much lower mass fractions of F and Fe elements and higher mass fraction of C element than the surface lubricated with the pure PFPE. The result might derive from the fact that a uniform tribofilm composed of carbon-material has formed on the wear surface lubricated with FOArGO/PFPE. The formed uniform tribofilm reduces the damage to the counterpart steel ball and decreases the degradation of PFPE molecules during the rubbing process, 12 which corresponds to the decreased mass fraction of Fe and F elements detected on the wear surface.

Figure 8. SEM images of the wear surfaces on Ti6Al4V plates lubricated with the pure PFPE (a and b) and the FOA-rGO/PFPE (c and d).



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Figure 9 shows the Raman spectra of the wear surfaces lubricated with the different oils after rubbing for 1000s, 3000s and 9000s. Undoubtedly, the D band and G band are not observed on the wear surface lubricated with the pure PFPE, owing to no graphene materials in this oil. The D and G bands are observed on the wear surfaces lubricated with these nano-oils after rubbing for different time, which confirms the deposition behavior of graphene on the wear surfaces. However, the intensities of the D band and G band vary with the type of the employed nano-additives and the sliding time. Figure 9a shows strong signals of the D and G bands on the wear surface lubricated with the GO/PFPE, rGO/PFPE and FOA-rGO/PFPE after sliding time for 1000 s. The result indicates that the nano-additives of GO, rGO and FOA-rGO are absorbed and form a tribofilm on the wear surfaces in the initial stage of rubbing process. As a result, the low friction coefficients of 0.11-0.15 are achieved as lubricated with these nano-oils in the initial rubbing stage (Figure 5a). As the rubbing time increase to 3000 s, the signals of the D and G bands become very weak for the sample lubricated with the GO/PFPE, as shown in Figure 9b. Interestingly, the friction coefficient suddenly increases to 0.241 for this sample after sliding for 1992 s (Figure 5a). The dispersibility of GO in PFPE is the worst among all samples (Figure S3), so the formed boundary tribofilm deriving from the deposition of GO is very frail and there is few GO remaining on the wear surface after rubbing for 3000 s. The breakdown of the formed boundary tribofilm on the wear surface induce high friction coefficient (Figure 5a). In turn, the high friction will further destroy the structural integrity of the deposited graphene and then the detected signals of the D and G bands are very weak on the wear surface lubricated with the GO/PFPE.


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Figure 9. Raman spectra of the wear surfaces on Ti6Al4V plates lubricated with different oils after rubbing for varying time (a: 1000s; b: 3000s; c: 9000s). The improved dispersibility of rGO in PFPE leads to more graphene deposited on the wear surface, as a result the formed tribofilm is more uniform and the signals of the D and G bands are stronger even after rubbing for 3000 s (Figure 9b). With the increasing of the rubbing time to 9000 s, it can be seen that the signals of the D and G bands are very weak for the wear surface lubricated with the rGO/PFPE (Figure 9c). The result demonstrates that the integrity of the formed tribofilm on wear surface lubricated with the rGO/PFPE has been destroyed, which agrees well with the increased friction coefficient for this sample after rubbing for 5134 s (Figure 5a). Surprisingly, the wear surface lubricated with the FOA-rGO/PFPE still exhibits strong D and G bands after rubbing for 21 ACS Paragon Plus Environment


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9000 s, as shown in Figure 9c. Owing to the excellent dispersibility of FOA-rGO in PFPE oil (Figure S3), the FOA-rGO nanosheets are easily adsorbed on the interfaces and form a uniform and homogeneous tribofilm during the rubbing process. The uniform and strong tribofilm coming from the deposition of graphene can remarkably reduce the friction between the contact pairs. Therefore, this sample obtains very low and stable friction coefficient even after rubbing for 18000 s (Figure S4). Meanwhile, the low friction will not destroy the structure of the deposited graphene, which corresponds to the strong signals of the D and G bands detected on this surface after rubbing for 9000 s. XPS analysis of the wear surface on the Ti6Al4V plate lubricated with the FOA-rGO/PFPE was performed to further explore the lubrication mechanism of the FOA-rGO in PFPE. Figure 10 shows the XPS full spectrum (Figure 10a) and the deconvolutions of C1s, O1s, F1s, Fe2p and Ti2p (Figure 10b–f) spectra of the wear surface. As shown in Figure 10b, the C1s spectra with five subpeaks at 284.5, 285.4, 286.4, 288.3 and 291.7 eV belong to C=C, C–C, C–O, C=O and –CF2–, respectively, which results from the deposited FOA-rGO nanosheets from the nano-oil. Therefore, it can be speculated that the FOA-rGO nanosheets are adsorbed and deposit on the wear surface during the rubbing process. Meanwhile, it can be seen that the CF3 peak disappear and the CF2 peak become weak in Figure 10b as compared with in Figure 3a, which might result from the partial destruction of the chemical structure of the deposited FOA-rGO by friction force during the rubbing process. The sub-peaks in O1s spectra (Figure 10c) at 530.0, 531.6, and 532.8 eV respectively correspond to O2−, C=O, and C–O, indicating that oxidation reaction has occurred on the friction surface. As shown in Figure 10d, the XPS spectra of F1s can be de-convoluted into two sub-peaks corresponding to FeF2 (684.8 eV) and C–F (688.5 eV), which demonstrates that Fe and F elements have reacted with each other during the sliding process. XPS spectra of Fe2p (Figure 10e)


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and Ti2p (Figure 10f) confirm the existence of FeF2 and TiO2 on the wear surface,9,43 further verifying the tribochemical reaction occurred during the rubbing process. Figure S6 exhibits the XPS spectrum of F1s on the wear surface lubricated with the pure PFPE. The sub-peak at 689.6 eV in F1s spectra originates from the products of organic oxyfluoride or carbon fluoride species,12 which suggests the decomposition of PFPE occurred as lubricated with the pure PFPE. This disappeared sub-peak at 689.6 eV in F1s spectra for the wear surface lubricated with the FOArGO/PFPE (Figure 10d) demonstrates that the adsorption and deposition of FOA-rGO on the friction interfaces can prevent the decomposition of the PFPE molecules during the rubbing process. In general, the results form XPS analyses confirm that the tribochemical reaction and the deposition behavior of FOA-rGO have occurred on the wear surfaces, resulting in the uniform tribofilm formed on the wear surface. The formed uniform tribofilm can prevent the friction pairs from direct contact and reduce the decomposition of the PFPE molecules during the rubbing process.



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Figure 10. (a) XPS full spectrum and the deconvolutions of (b) C1s, (c) O1s, (d) F1s, (e) Fe2p and (f)Ti2p spectra on the wear surface of Ti6Al4V plate lubricated with the FOA-rGO/PFPE.


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3.4. Friction and wear mechanism of FOA-rGO Additives. According to the above analyses, the mechanism for the excellent lubricating abilities of FOArGO/PFPE is proposed. Figure 11 illustrates the effects of rGO and FOA-rGO as additives in PFPE on the friction and wear behaviours of the Ti6Al4V/steel friction pairs. In this work, the applied slow speed of 10 mm/s corresponds to boundary lubrication state. The graphene nanosheets with PFPE can penetrate into the interface of the contact pairs and gradually deposit on the wear surface for forming a transfer tribofilm. However, the durability and stability of the tribofilm are remarkably influenced by the dispersibility of graphene in PFPE. The tribofilm formed by FOArGO deposition is strong and homogenous, as confirmed by Raman and XPS spectra (Figures 9 and 10), which is closely related to the excellent dispersibility of the FOA-rGO in PFPE. The homogenous tribofilm together with the PFPE oil can form a stable solid-liquid lubrication film in the contact area and hence prevent the friction pairs from directly contacting during rubbing process, as illustrated in Figure 11b. Additionally, the excellent lubricating abilities of FOA-rGO/PFPE can greatly reduce the friction force and the produced friction heat, which can retard the decomposition of the PFPE molecules in base oil during the rubbing. However, the tribofilm formed by the deposition of GO and rGO is uneven and its integrity can be easily destroyed by the friction force during the rubbing process, as confirmed by the Raman spectra (Figure 9). Undoubtedly, the formed tribofilm on the wear surface lubricated with the GO/PFPE or rGO/PFPE is frail and unhomogeneous (Figure 11c), because of the worse dispersibility of the two nanomaterials in PFPE. As a result, the friction coefficient rapidly increases at a certain time during the rubbing process, which is the result of the failure of the formed uniform tribofilm by friction force.



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Figure 11. Schematic presentation of tribological tests (a) and evolution of the formed tribofilm on the interfaces lubricated with FOA-rGO/PFPE (b) and rGO/PFPE (c) during rubbing process.

4. CONCLUSIONS In this paper, a novel modified graphene nanomaterial of FOA-rGO is successfully synthesized by the first-step of covalent functionalization of GO with 1H,1H-perfluorooctylamine and the secondstep of reduction with hydrazine monohydrate. The successful grafting of long fluorocarbon chains in the as-prepared FOA-rGO nanosheets significantly facilitates their dispersions in the space lubricating oil of PFPE. The friction and wear properties of the FOA-rGO as nanoadditives in PFPE for lubricating Ti6Al4V/steel contact were investigated by a UMT reciprocation friction tester, and a control experiment by respectively adding GO and rGO in PFPE was performed as well. It is found that FOA-rGO as nanoadditives in PFPE exhibits the best lubricating performance among all samples tested. The nano-oil dispersed with FOA-rGO shows that the average friction coefficient and the wear rate are reduced by 52.7% and almost five orders of magnitude respectively, when 26 ACS Paragon Plus Environment

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compared to the pure PFPE. The outstanding lubricating performances of FOA-rGO additives can be explained by the fact that the FOA-rGO nanosheets can enter the contact surfaces and form a uniform and strong boundary tribofilm during rubbing process. The excellent dispersibility of FOArGO in PFPE favors the improvement in the durability and stability for the formed tribofilm. The findings here readily provide a valuable strategy for preparing a space liquid lubricant with remarkable tribological performances for lubricating Ti alloy space mechanical components. 

ASSOCIATED CONTENT

Supporting Information: XPS full spectrum of as-prepared nanomaterials; AFM image of rGO; Photographs of the dispersed nano-oils; Typical friction coefficient curves; EDS elemental maps of the wear surfaces; Relative mass fractions of typical elements on the wear surfaces; XPS spectra on the wear surfaces. The Supporting Information is available free of charge on the ACS Publications website.

 AUTHOR INFORMATION *Corresponding Author: E-mail: [email protected]; Tel: +86-20-82313996;

ORCID Fenghua Su: 0000-0002-6953-4663 Notes The authors declare no competing financial interest.

 ACKNOWLEDGMENTS



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The authors are grateful to the financial support of the National Natural Science Foundation of China (51775191), the Guangdong Natural Science Funds for Distinguished Young Scholar (2015A030306026), the Science and Technology Planning Project of Guangzhou City (20707010055) and the Fundamental Research Funds for the Central Universities (2018ZD29).

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Excellent Lubricating Ability of Functionalization Graphene Dispersed

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