Boundary and Elastohydrodynamic Lubrication Behaviors of Nano

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Boundary and Elastohydrodynamic Lubrication Behaviors of Nano-CuO/ Reduced Graphene Oxide Nanocomposite as an Efficient Oil-Based Additive Yuan Meng, Fenghua Su, and Zhujun Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01244 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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Boundary and Elastohydrodynamic Lubrication Behaviors of Nano-CuO/Reduced Graphene Oxide Nanocomposite as an Efficient Oil-Based Additive Yuan Meng1, Fenghua Su1 *, Zhujun Li2 1 School

of Mechanical and Automotive Engineering, South China University of Technology,

Guangzhou, China 2 School

of Mechanical and Electronic Engineering, Guangzhou Railway Polytechnic, Guangzhou,

China

[1*] Fenghua Su, Ph. D, Prof. School of Mechanical and Automotive Engineering, South China University of Technology, Wushan Road No. 381, Tianhe District, Guangzhou 510640, P. R. China Fax/Tel: +86 2082313996; E-mail: [email protected]

[1] Yuan Meng, Ph. D. School of Mechanical and Automotive Engineering, South China University of Technology, Wushan Road, Room 381, Tianhe District, Guangzhou 510640, P. R. China E-mail:[email protected]

[2] Zhujun Li, Ph. D, A/Prof. School of Mechanical and Electronic Engineering, Guangzhou Railway Polytechnic, Shijing Road, Room 100, Baiyun District, Guangzhou 510430, P. R. China E-mail: [email protected]

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ACS Paragon Plus Environment

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ABSTRACT Copper oxide/reduced graphene oxide (CuO/rGO) nanocomposite was synthesized by an effective approach combining a supercritical hydrolysis and a facile thermolysis. The as-synthesized CuO/rGO exhibited a unique microstructure that ultrafine CuO nanoparticles were encapsulated by several stacked rGO nanosheets. The highly dispersed CuO nanoparticles on the rGO nanosheets were in spherical shape with diameters around 6 nm and had a base-centered monoclinic crystal structure. Tribological performances of CuO/rGO nanocomposite as lubricant additive of 10w40 engine oil were evaluated by a ball-on-disc tribotester under boundary lubrication. As compared to the bare 10w40 engine oil, the engine oil with 0.06-0.18 wt% CuO/rGO exhibits the reductions of 46.62% and 77.05% for friction coefficient and wear rate, respectively. In addition, the CuO/rGO as nanoadditive displays superior lubricating abilities than the alone used rGO, CuO, as well as their mechanical mixture. This synergistic lubricating effect of CuO/rGO nanocomposite was mainly contributed to its special nanostructure rather than only the coexistence of CuO and rGO. The anti-wear and friction reducing mechanism of CuO/rGO nanoadditive mainly result from its synergistic lubricating effect and the formed tribofilms on the wear surface due to the spontaneous deposition behavior of the nanocomposite. In addition, influences of the nanoadditives including rGO, CuO, the mixture, and CuO/rGO, on the rheological behaviors and the elastohydrodynamic lubrication (EHL) oil film of the modified engine oil were also investigated. It results show that these nanoadditives can increase the viscosity of the engine oil to a certain extent throughout the test temperature range of 0–60 oC. The 2


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remarkable increase of viscosity by the addition of CuO/rGO nanoadditive corresponds to the thickening of EHL film and the strengthening of EHL state for the engine oil with CuO/rGO as compared to the bare one.

INTRODUCTION In the past few decades, various inorganic nanomaterials as oil or water-based lubricant additives were intensely studied for reducing friction and wear.1 As a kind of typical inorganic nanoparticle, copper oxide (CuO) nanoparticles received great attentions in tribological applications due to low cost and easy preparation.2−9 Previous studies showed that CuO nanoparticles can promote the tribological performances of raw oils and reduce the friction and wear efficiently. For instance, Battez et al.2 reported that friction shearing and heat could induce the tribo-sintering actions of CuO particles and thus resulted in forming a protective tribo-film for enhancing friction and wear resistance. Besides nano-metal oxide particles, carbonous nanomaterials, especially neotype carbon quantum dots,10-12 carbon nanotube (CNT),13, 14 and graphene15-20, were also found having great potentials in the field of lubrication. As for graphene and its derivatives, they were regarded as excellent lubricant additives due to their unique two-dimensional layer structure and extremely high mechanical strength.15−20 The application forms of graphene and its derivatives were quite varied, mainly including thin films, reinforcement phases, and lubricant additives. By employing appropriate modification approaches, graphene and its 3


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derivatives can well disperse in varying media and used as effective nanoadditives.21−24 For example, Gupta’s group obtained highly dispersed rGO into PEG oil by hydrogen bonding, and found that the friction coefficient and wear rate were reduced by 70% and 50% at optimized rGO concentration of 0.2 mg·L-1.21 As reported in recent studies, graphene-based nanocomposites prepared by anchoring inorganic nanoparticles onto graphene nanosheets displayed interesting synergistic effect in many fields, such as catalysis, sensor, electrochemistry and tribology, etc.25−28 In the term of the synergistic effect in the field of tribology, the graphene-based nanocomposites tend to be more efficient than graphene sheets and nanoparticles used alone for improving tribological performances. According to the reported lubricating potentials of graphene nanosheets and CuO nanoparticles, the nanocomposite including the two components is expected to be a novel promising nanomaterial to achieve the synergistic effect in reducing friction and wear. Up until now, tribological applications involving CuO/graphene nanocomposite as lubricant additive were not yet reported. Moreover, few studies referred to the mechanism of the synergistic effect of graphene-based nanocomposites as lubricant additives. The factors affecting the properties of graphene-based nanocomposites were always complex, and the most important one was generally the microstructure and morphology of the nanocomposites. Currently, it was difficult for the graphene-based nanocomposite synthesized by the major methods (hydrolysis or pyrolysis) to avoid the common microstructure or morphology defects (i.e., irregular shapes, coarse particle size and 4


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aggregation of anchored naoparticles),29−32 which severely limited the practical performances. As a result, optimizing the microstructure and morphology of the graphene-based nanocomposite (i.e., fine particle-size and high dispersion of anchored nanoparticles on graphene nanosheets) became a challenge as well as a key to achieve high performances. Fortunately, it seemed that this problem could be effectively solved by introducing supercritical fluids (SCF) due to their unique physicochemical properties.33−37 The nanocomposites synthesized with the aid of SCF usually presented a uniform microstructure and relatively high loading percentage of nanoparticles. Moreover, the synergistic effect of the synthesized nanocomposites in some fields was confirmed to be strengthened by optimizing the microstructure with the help of SCF. Herein, we reported an effective approach with two steps for synthesizing CuO/rGO nanocomposite. By introducing supercritical CO2 during the synthetic process, the ultra-fine spherical CuO nanoparticles were highly dispersed and anchored onto rGO nanosheets. The tribological performances of CuO/rGO nanocomposite as lubricant additive of 10w40 engine oil were investigated by a ball-on-disc tribotester under boundary lubrication (BL). After the tribo-tests, the wear surfaces of the ball and the disc were examined by optical microscopy, 3D-topography measurements, and X-ray photoelectron spectroscopy (XPS). The friction-reducing and anti-wear mechanism of CuO/rGO nanoadditive was reasonably proposed according to the characterization results of wear surfaces. The rheological behaviors and the elastohydrodynamic lubrication (EHL) oil film thickness of the engine oil with CuO/rGO nanoadditive were examined to evaluate the 5


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influences of this additive on the viscosity and the EHL oil film.

EXPERIMENTAL SECTION Materials Natural graphite powders (purity 99.99%, size ~44 μm) were purchased from Tianjin Fuchen Chemical Reagent Factory. Copper acetate, ethanol, dimethyformamide (DMF), Sodium dodecyl sulfate (SDS), and aqueous ammonia (NH3·H2O) were purchased from Tianjin Damao Chemical Reagent Factory and used as received. Carbon dioxide (CO2) gas of high purity (99.9%) and deionized water were used in the experiments. Synthesis and characterizations In this work, graphene oxide (GO) was synthesized from natural graphite powders by an improved Hummers method38, 39. The synthesis route of CuO/rGO was described below. Firstly, suitable amounts of copper acetate (300 mg), GO (100 mg) and SDS (15 mg) were dispersed into a mixed solution of water (40 mL) and ethanol (22.5 mL) using ultrasonic treatment of 30 min. Next, aqueous ammonia (2.5 mL) was dropwise added into the above solution with continuous stirring. Afterwards, the resulting mixed solution was loaded into a stainless autoclave (100 mL), which was then sealed away from the atmosphere after brief flushing with CO2. The autoclave chamber was rapidly pressurized to 12.5 MPa by CO2 and heated to 125 oC with electric heating. Then, this pressure and temperature was maintained for 3 h to ensure the supercritical hydrolysis of the precursors in the solution. The as-synthesized blue products were separated by centrifugation, and then repeatedly 6


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washed with copious ethanol and deionized water. Finally, the blue products were annealed at 250 oC for 2 h in air atmosphere, and then black CuO/rGO powders were obtained. For comparison, CuO nanoparticles were synthesized by the same approach as above excluding the addition of GO, and rGO powders were easily prepared without the participation of copper acetate. X-ray diffraction (XRD) analyses were characterized with a Philips X’pert X-ray diffractometer (Cu-κα radiation) operating at 40 kV and 40 mA over 5o–90o. Raman spectra were recorded by a Dilor Labram-1B microspectrometer with an excitation laser of 20 mW and 514 nm. Thermal gravimetric analysis (TGA) was conducted with a Netzsch Sta449F3 in air atmosphere at a heating rate of 10 oC·min-1 from room temperature to 800 oC. Morphology and microstructure of the as-synthesized products were further observed with a field-emission transmission electron microscope (TEM, JEOL JEM-2010F). Tribo-test under boundary lubrication All synthesized nanoparticles (rGO, CuO and CuO/rGO) were treated with a surface modification to improve the dispersity and stability in the 10w40 engine oil. Typically, a suitable amount of the nanoparticles (200 mg) were dispersed into a DMF solution of octadecylamine (8 mg·ml-1, 50 mL), and then the mixed solution was refluxed at 150 oC for approximately 24 h. After that, the solution was filtered immediately and the precipitates were repeatedly washed by hot ethanol. After the modification process, the as-prepared nanoparticles as lubricant additives can be facilely dispersed into the commercial engine oil (SN 10w40; Great Wall JUSTAR, Sinopec) with a short-time ultrasonic treatment. In this 7


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work, five different oil samples (i.e., 10w40 engine oil, engine oil with rGO, engine oil with CuO, engine oil with CuO/rGO, and engine oil with the mechanical mixture of rGO and CuO) were employed for comparing their tribological performances. Please note that the mass ratio of rGO and CuO in the mixture is equal to the CuO loading percentage of CuO/rGO. Particle-size distribution for these oil samples were characterized by a Horiba SZ-100Z nanoparticle analyzer with a green laser of 532 nm at temperature of 25 oC. Tribological properties of all the oil samples were run on a ball-on-disc tribotester (UMT Tribo-Lab, Bruker). During tribotest, a bottom disc (C1045 steel, Ø 36 mm×2.5 mm, and Ra 20 nm) slid under rotating motion against a stationary ball (GCr15 steel, Ø 9 mm, and Ra 20 nm). The sliding contact radius was fixed at 10 mm on the disc, and the sliding speed was 0.25 m·s-1. Each tribotest was conducted under a normal load of 15 N (maximum Hertzian stress 1.24 GPa) and room temperature (circa 25 oC) for 30 min. Prior to tribo-test, the ball and the disc were cleaned ultrasonically in ligarine. The friction coefficient curves were recorded automatically by a data acquiring system. The wear scar and the wear track were measured with an optical microscopy (107JA, Shanghai). The wear volume and wear rate of wear tracks on bottom discs were determined with a tactile profilometer (Talysurf CLI 1000, Taylor Hobson). The components and chemical states of the wear track surface were analyzed by X-ray photoelectron spectroscopy (XPS; AXIS-ULTRA DLD, Kratos). Evaluations of rheology and elastohydrodynamic lubrication oil film Rheological behaviors of the oil samples were investigated by a rotational rheometer (HAAKE MARS III; Thermo Fisher Scientific). The rotational disc (No. P35-Ti) with 8


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diameter of 36 mm was used in the rheological experiments. The gap between the rotational disc and the specimen stage was set at 1 mm and completely filled with the oil sample. Viscosity-temperature curve was tested in the temperature range of 0-60 oC; shear viscosity-shear rate curve, oscillating viscosity-frequency curve, and curves of storage modulus and loss modulus as a function of shear frequency were measured at the temperature of 20 oC. A model optics EHL test bench was used to measure the EHL oil film thickness.40 During each measurement, a super-polished steel ball with surface roughness of Ra=20 nm was loaded against a glass disc coated with a chromium semi-reflecting layer. The glass disc was driven by an electric motor, and thus the ball was forced to roll by friction force. The ball was partially submerged into the oil sample, and then the oil sample could be dragged into the contact interface by the rolling ball to form continuous oil film. The elastic modulus and the Poisson’s ratio of the steel ball are 207GPa and 0.28, and those of the glass disc are 81GPa and 0.209, respectively. The sphere diameter of the steel ball is 25.4 mm and the diameter of the glass disc is 210 mm. The ball and the disc worked in pure rolling contact, and the entrainment speeds were set at 0, 0.2 and 0.4 m/s. All the measurements were carried out under a normal load of 5 N at room temperature (circa 25 °C). The contact interface between the ball and the disc was illuminated by a halogen light with wave length of 600 nm, and thus the interference pattern of the oil film could be captured by an optical microscope and a CCD camera. Based on the interference pattern, the EHL oil film thickness can be calculated according to an improved optical interference 9


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intensity method.40, 41 The calculation equations are as follow: (1)

(2) Where λ is the wavelength of the halogen light, n is the refractive index of oil samples, k is interference sequence (round numbers), and

and

are the relative light intensity at the

position of Ik and I0, respectively. The refractive indexes (n) of the oil samples were measured with an Abbe refractometer.

RESULTS AND DISCUSSIONS Structure and morphology Figure 1(a) presents the powder X-ray diffraction (XRD) patterns of rGO, CuO and CuO/rGO. As shown in the XRD pattern of rGO, the characteristic diffraction peaks at 24.2o and 43.2 o are attributed to the graphitic (002) and (102) lattice plane of hexagonal graphite structure, respectively. In the case of the CuO pattern, the peaks at 32.50o, 35.42o, 38.70o, 48.70o, 53.50o, 58.26o, 61.50o, 66.22o, 67.9o, 72.37o, 74.98o and 83.06o are respectively attributed to the diffractions of (110), (002), (111), (20 ), (020), (202), (11 ), (31 ), (113), (311), (004) and (222) crystal planes of base-centered monoclinic CuO nanocrystals, which corresponds well to the reference crystallographic data (JCPDS No. 48-1548). These peaks in CuO pattern are all observed in the pattern of CuO/rGO, suggesting that the CuO nanoparticles are successfully decorated on rGO nanosheets. As 10


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comparing the XRD pattern of rGO with that of CuO/rGO, it can be found that the (002) carbon peak in the pattern of CuO/rGO shows a significant decrease in intensity and a slight shift in location, and the (102) carbon peak disappears completely. As well known, the diffraction peaks of metal-oxide particle always present much stronger diffraction intensity than the peaks from nonmetal carbon material. Accordingly, the graphitic peaks are nearly covered up by typical peaks of CuO in the XRD pattern of CuO/rGO. Raman spectra of rGO, CuO and CuO/rGO are presented in Figure 1(b).As for the sample of rGO, the characteristic peaks at around 1350 cm-1, 1580 cm-1, 2670 cm-1 and 2935 cm-1 are respectively ascribed to D, G, 2D and 2G bands. The intensity ratio of D to G band is rather high and approaches to 1.21, indicating that rGO have a few structure defects. The Raman spectrum of CuO shows three characteristic peaks at 295 cm-1, 620 cm-1 and 1120 cm-1, respectively. In the sample of CuO/rGO, the D and G bands show no clear changes when compared to the spectrum of rGO, indicating that the structure defects of rGO don’t obviously increase after anchoring CuO. And the three characteristic peaks of CuO are clearly observed in the spectrum of CuO/rGO sample, which suggests that the CuO nanoparticles are successfully anchored on the rGO nanosheets. Meanwhile, the corresponding peak intensities in the spectrum of CuO/rGO are much weaker than those of CuO, which might be due to the encapsulation structure of CuO/rGO. TGA curves of rGO, CuO and CuO/rGO are shown Figure 1(c). One can clearly see that the curve of rGO can be roughly divided into three stages of 25-110 oC, 110-350 oC and 350-520 oC, where the corresponding mass loss rates are 8%, 2% and 83%, respectively. 11


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The first stage is ascribed to the evaporation of adsorbed water; the second is attributed to the thermolysis of the oxygen-containing groups remaining in rGO, and the third is due to the pyrolysis of graphene structures. The TGA curve of CuO shows a slow and stable decrease with increasing the temperature and can be roughly divided into two stages. The first stage in the temperature range of 25-110 oC is attributed to thermal desorption of water molecules, and the mass loss is nearly 2.5%. The second between 110-400 oC is ascribed to the thermolysis of residual surfactants, and the corresponding mass loss is approximately 4%. The TGA curve of CuO/rGO shows similar trend with that of rGO. It can also be divided into three different stages of 25-110 oC, 110-300 oC, and 300-450 oC. The ingredients lost in each stage are exactly the same as that of rGO, and the mass losses are 2.5%, 2.5% and 28%, successively. However, the graphene structures in CuO/rGO show much lower thermolysis temperature than rGO, indicating that the anchored CuO particles have catalytic activity for the thermolysis of graphene. The final residual mass of rGO is around 6.82% due to the impurities introduced in the synthetic process, and the final residual mass of CuO is 92.76%. As for the sample of CuO/rGO, its CuO component has good chemical stability but the rGO component is burned out. As a result, the final residual mass of CuO/rGO can be considered as the loading percentage of 65.4% for the anchored CuO particles.

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Figure 1. XRD patterns (a), Raman spectra (b), and TGA curves (c) of rGO, CuO and CuO/rGO.

Figure 2 gives the typical TEM images of rGO, CuO and CuO/rGO. As shown in Figure 2(a), small fragments and numerous wrinkles are clearly observed on rGO nanosheets, suggesting that the rGO nanosheets have the structure defects, which corresponds to the Raman spectra. As seen in Figure 2(b), the CuO nanoparticles exhibit a grain-like shape and a narrow size distribution. Figure 2(c) shows that the rGO nanosheets of CuO/rGO are crumpled with a few wide wrinkles, and the spherical CuO nanoparticles are highly dispersed on the rGO nanosheets. Figure 2(d) presents the high magnification image of 13


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partial Figure 2(c) (the red dotted box region). It can be seen that numerous CuO nanoparticles are encapsulated by the rGO nanosheets, agreeing well with the analysis of Raman spectra. Actually, the anchored CuO particles have a high degree of crystallinity according to the XRD results. Nevertheless, it is still difficult to distinguish the lattice planes of the CuO particles from TEM image due to the unique encapsulation microstructure. Only the (002) lattice plane with interplanar spacing of 0.253 nm can be faintly seen in the HR-TEM image of a single nano-CuO particle, as shown in the top right corner of Figure 2(d). The other inset at the bottom right corner shows the particle-size distribution of the anchored CuO nanoparticles in Figure 2(d). It can be seen that these anchored CuO particles have a narrow particle-size distribution and their mean sizes are around 6 nm.

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Figure 2. TEM images of rGO (a), CuO (b) and CuO/rGO (c, d). (Inserts in (d): HR-TEM of single CuO particle and the size distribution of the anchored CuO particles in it)

3.2. Tribological performances and anti-wear mechanism under boundary lubrication Figure 3(a) shows the digital photographs of engine oil (10w40) and the oil samples dispersed with different nanoadditives after aging for 7 days. No obvious precipitates can be observed at the bottom or on the inner wall of each glass bottle, and there are no signs of stratification and color change for all the oil samples. The results indicate that these nanoadditives exhibit good dispersibility and stability in the engine oil. Figures 3 (b-e) present the laser particle-size distributions for the engine oil with different nanoadditives. It can be observed that the mean particle sizes of rGO (Figure 3(b)), CuO (Figure 3(c)), the mixture of rGO and CuO (Figure 3(d)), and CuO/rGO (Figure 3(e)) in the engine oil are 204.8, 175.2, 213.5, 205.0 nm, respectively. This result further proves that these nanoparticles can maintain at nanoscale sizes in the engine oil, and show good dispersion. Compared with the TEM image of Figure 2(b), Figure 3(c) shows a much wider laser particle-size distribution of CuO nanoparticles, indicating that CuO nanoparticles show a state of particle aggregation rather than a monodisperse state in the engine oil. As shown in Figure 3(d), when CuO and rGO are simultaneously dispersed into the engine oil, the resulting particle-size distribution becomes much narrower than that in Figure 3(b), which signifies that introducing CuO nanoparticles can reduce the aggregation of rGO nanosheets in the mechanical mixture. Figure 3(e) exhibits that the laser particle-size distribution for 15


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the engine oil with CuO/rGO is narrowest among all oil samples and mainly located in the range of 150-500 nm.

Figure 3. Digital photographs of different oil samples after aging for 7 days (a) and laser particle-size distributions of different oil samples ((b) engine oil with rGO, (c) engine oil with CuO, (d) engine oil with the mixture of rGO and CuO, (e) engine oil with CuO/rGO).

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Figure 4(a) shows the typical friction coefficient curves as a function of sliding distance under lubrication of different oil samples. Comparisons of the average friction coefficients and the corresponding wear rates of the bottom discs are shown in Figure 4(b). Because trace octadecylamine almost has no effect on the tribological performance for the 10w40 engine oil, the corresponding friction coefficient and wear rate were not included in this figure. As shown in Figure 4 (a), the friction coefficients decrease rapidly at the initial stage, and then remain relatively stable with increasing the sliding distance for all oil samples. The 10w40 engine oil shows the highest friction coefficient. The addition of different nanoadditives can reduce the friction coefficient to a certain degree, which indicates that the nanoparticle including rGO, CuO, the mixture, and CuO/rGO are effective lubricant additive in reducing friction for the 10w40 engine oil. Among all samples, the CuO/rGO additive presents the lowest friction coefficient, signifying that this nanoadditive has the best friction-reducing ability. Figure 4 (b) shows that adding different nanoadditives into the engine oil can reduce the average friction coefficients and the wear rates, but the friction-reducing and anti-wear efficiency vary with the type of the used nanoadditive. The average friction coefficient of the rGO is much lower than that of the CuO, but their wear rates present the opposite trend. This indicates that rGO has better friction-reducing but poorer antiwear abilities than CuO as lubricant additive. For the mechanical mixture of rGO and CuO, the average friction coefficient is slightly higher than that of the rGO and the wear rate is lower than that of the CuO, suggesting that the lubricating ability of the mixture is located between that of the used rGO and CuO alone. It seems that the 17


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synergistic lubricating effect cannot be achieved only by simply mechanical mixing of rGO and CuO. As expectedly, CuO/rGO nanoadditive has the lowest average friction coefficient and wear rate among all the samples, proving the more effective friction-reducing and antiwear ability of CuO/rGO than rGO, CuO and their mechanical mixture. Accordingly, it can be concluded that CuO/rGO nanoadditive has a synergistic effect in reducing friction and wear. This synergistic effect may be attributed to the compound effect caused by the unique microstructure of CuO/rGO.

Figure 4. Typical friction coefficient curves as a function of sliding distance (a) and comparisons of average friction coefficients and wear rates (b) under lubrication of different oil samples.

Variations of the friction coefficient and wear rate as a function of the CuO/rGO concentration in engine oil are shown in Figure 5. Both friction coefficient and wear rate rapidly decrease at first and then increase slowly with the increasing of CuO/rGO in oil. As observing the two variation curves, we can find that there is a concentration range where 18


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the friction coefficient and the wear rate both remain stable and relatively low. The optimum concentration of CuO/rGO is in the range of 0.06-0.18 wt% in the engine oil. As compared to the bare 10w40 engine oil, the engine oil with 0.06-0.18 wt% CuO/rGO exhibits the reductions of 46.62% and 77.05% for friction coefficient and wear rate, respectively.

Figure 5. Variations of friction coefficient and wear rate as a function of the CuO/rGO concentration in engine oil.

The wear surfaces were examined by optical microscopy, and the resulting images are shown in Figure 6. Under lubrication of 10w40 engine oil, the wear scar on the steel ball shows an approximately elliptical shape and the scar surface has many wide and deep ploughs (see Figure 6(a1)), which indicates that severe abrasive wear occurred during the 19


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rubbing process. Compared with Figure 6(a1), the wear scar lubricated by the engine oil with rGO additive shows a smaller wear scar and a lower surface roughness (see Figure 6(b1)), indicating that the abrasive wear lightly reduced. As shown in Figure 6(c1), the wear scar lubricated with CuO is much smaller than that lubricated with rGO, proving the better antiwear performances of CuO than rGO. As shown in Figure 6(d1), the wear scar lubricated with the mixture of rGO and CuO is quite smooth in the middle area and rather rough with deep grooves in other areas, indicating the inhomogeneous abrasive wear occurred during rubbing process. Apparently, the wear scar lubricated with CuO/rGO is the smallest one and featured with the lowest roughness (see Figure 6(e1)), further proving the excellent antiwear performance of CuO/rGO nanoadditive. Figures 6(a2-e2) present the wear track images corresponding to the wear scars in Figures 6(a1-e1). As shown in Figure 6(a2), the wear track lubricated by 10w40 engine oil has the biggest width with many deep ploughs. As for the wear track lubricated by engine oil with rGO (Figure 6(b2)), its width becomes smaller than that in Figure 6(a2). By comparing Figure 6(c2) with Figure 6(b2), one can find that the wear track lubricated with CuO is much narrower and smoother, indicating that CuO can effectively reduce the abrasive wear. For the wear track lubricated with the mixture of rGO and CuO (Figure 6(d2), its wearing degree is located between that in Figure 6(b2) and that in Figure 6(c2). This proves that the mixture of rGO and CuO exhibit better antiwear ability than rGO but poorer than CuO. Among all the wear tracks, the wear track lubricated with CuO/rGO (see Figure 6(e2)) shows the smallest width and the smoothest wear surface, indicating the best anti-wear 20


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performance of CuO/rGO nanoadditive.

Figure 6. Optical microscope images of wear scar on steel ball (a1-e1) and corresponding wear track on steel disc (a2-e2) lubricated by different oil samples ((a1, a2) 10w40 engine oil; (b1, b2) engine oil with 0.09wt% rGO; (c1, c2) engine oil with 0.09wt% CuO; (d1, d2) engine oil with 0.09wt% the mixture of rGO and CuO; (e1, e2) engine oil with 0.09wt% CuO/rGO).

In order to calculate the wear rate and further observe the wear topography, the wear tracks lubricated by different oil samples are examined by a Taylor profilometer, and the resulting topography images are presented in Figure 7. The wear track lubricated by 10w40 engine oil is rather rough with many deep ploughs, and its cross-sectional width is around 261 μm (Figure 7(a)). Clearly, Figure 7(a) has the largest wear loss among all images, and the corresponding wear rate is about 2.24×10-6 mm3/Nm. For the wear track lubricated by engine oil with rGO (Figure 7(b)), the cross-sectional width is reduced to 206 μm and the wear rate is approximately 1.75×10-6 mm3/Nm. The wear track lubricated with CuO has the much lower cross-sectional width and depth than that in Figure 7(a, b), as shown in Figure 7(c). Moreover, the surface roughness remarkably decrease and the resulting wear rate 21


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reduce to 7.26×10-7 mm3/Nm. The wear track lubricated with the mixture of rGO and CuO (see Figure 7(d)) has a cross-sectional width of 167 μm, and the wear rate is nearly 8.48×10-7 mm3/Nm. As expected, the wear track lubricated with CuO/rGO (Figure 7(e)) shows a quite smooth wear surface and has the lowest wear loss among all the wear tracks, and the corresponding wear rate is only 5.26×10-7 mm3/Nm.

Figure 7. Taylor 3D topography images of the wear tracks on steel discs lubricated by different oil samples ((a) 10w40 engine oil, (b) engine oil with 0.09wt% rGO, (c) engine oil with 0.09wt% CuO, (d) engine oil with 0.09wt% the mixture of rGO and CuO, (e) engine oil with 0.09wt% CuO/rGO).

Figure 8 presents the curve-fitted XPS spectra of typical electrons (C1s, O1s, Fe2p and Cu2p) on the bare steel disc surface and on the wear track lubricated by engine oil with CuO/rGO. The relative atomic concentrations of the typical elements determined by XPS 22


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are listed in Table 1. As shown in Figure 8(a, d), the two XPS spectra of C1s electron are de-convoluted into four subpeaks situated at 284.60, 285.15, 286.15 and 288.4 eV. The subpeaks in Figure 8(a) can be attributed to contaminated carbon on the surface and carbon from the steel substrate. The intensity of the subpeak at 284.60 eV in Figure 8(d) is remarkably enhanced compared with that in Figure 8(a). This enhancement is attributed to the sp2 hybridized carbon structure of the rGO component from CuO/rGO, because the CuO/rGO additive is the only possible carbon source. This indicates that the CuO/rGO additives have deposited on the wear track surface during rubbing process. As shown in Figure 8(b, e), the two XPS spectra of O1s electron have three same subpeaks located at 530.00, 531.45 and 533.35 eV, which are ascribed to iron oxides (Fe2O3), C=O bond and C-O bond, respectively. The subpeak signal of CuO at 530.30 eV appears in Figure 8(e), further proving the deposition behavior of the CuO/rGO additive. The XPS spectrum of Fe2p on the bare steel disc is almost same as that on the wear track, as shown in Figure 8(c, f). The subpeak situated at 707.1 eV is ascribed to Fe0, and that at 710.8 eV is attributed to the characteristic peak of Fe2O32p3/2 whose satellite peak is marked with “*”. This indicates that a thin oxidation layer has formed on the steel disc surface during the polishing process. It is undisputed that the XPS spectrum of Cu2p in Figure 8(g) doesn’t show any subpeaks because of the nonexistence of Cu element on the bare steel disc surface. In contrast, the XPS spectrum of Cu2p in Figure 8(h) shows four subpeaks. The characteristic peak of CuO2p3/2 is located at 933.9eV and that of CuO2p1/2 at 953.9 eV. The two subpeaks marked with “*” belong to the satellite peaks of the two characteristic peaks, respectively. 23


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Figure 8. Curve-fitted XPS spectra of C1s, O1s, Fe2p and Cu2p electrons on the bare steel disc surface (a-c, g) and on the wear track lubricated by engine oil with 0.09wt% CuO/rGO (d-f, h).

As seen in Table 1, the wear track surface lubricated by engine oil with CuO/rGO contains 2.7at% Cu element. Moreover, the wear track surface has 58.4at% C element, which is nearly 11at% higher than the bare steel disc surface. The comparative results of Cu and C elements can prove the remnants of the CuO/rGO additive existing on the wear track 24


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surface. According to the XPS analysis, we can reasonably conclude that the CuO/rGO additive can spontaneously deposit on wear surfaces and thus forms a protective film which can effectively weaken the abrasive wear and reduce the friction force. Table 1. Relative atomic concentration of typical elements on the bare steel surface and on the wear track surface lubricated by engine oil with 0.09wt% CuO/rGO Specimen

Atomic concentration (at%) C

O

Fe

Cu

Bare steel disc surface

47.5

42.6

9.9

0.0

Wear track surface

58.4

30.7

8.2

2.7

Based on the analysis above, the friction-reducing and antiwear mechanism under lubrication of the engine oil with CuO/rGO has been proposed, as schematically illustrated in Figure 9.When the tribopair (ball-on-disc) runs in boundary lubrication, oil molecules are adsorbed onto the contact surfaces and thus form an extremely thin oil film. However, the oil film thickness is lower than the mean roughness of the mating surfaces, and thus the asperities directly interact with each other resulting in many abrasive particles. The abrasive particles are forced to scratch the mating surfaces due to the friction force, and thus bring severe abrasive wear. As a result, the lubrication state further deteriorates, and the plowing action occurs on the mating surfaces (see Figures 6(a1, a2) and Figure 7a). When the tribopair runs in engine oil with CuO/rGO under boundary lubrication, the nanosized CuO/rGO additives can penetrate into the friction interface along with oil 25


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molecules, and then spontaneously deposit in the concaves of the mating surfaces (XPS analysis in Figure 8). On the one hand, the deposited CuO/rGO nanocomposite can repair the surface defects, and thus reduce the surface roughness and the numbers of surface asperities. The lower surface roughness is beneficial for reducing the friction resistance. On the other hand, the deposited CuO/rGO nanocomposite gradually accumulates and forms a protective film on the mating surfaces. The formed film can separate the mating surfaces and reduce the direct contact, thus effectively reduce the wear loss. Note that the CuO/rGO nanocomposite has shown the synergistic lubricating effect (see Figure 4). This effect is considered relevant to the unique multilayer encapsulation structure of the nanocomposite (Figure 2). Under continuous stress and shear, the multilayer structure of rGO in the nanocomposite can easily produce interlayer sliding behaviors, which can reduce the friction resistance. The rGO nanosheets endow the nanocomposite with strong mechanical strength to bear the load and resist the wear. With the evolution of the interlayer sliding, the rGO nanosheets are exfoliated from the CuO/rGO nanocomposite, and thus the encapsulated CuO nanoparticles are exposed. The exposed CuO nanoparticles are released from the rGO nanosheet surfaces because of friction and shear. The released CuO nanoparticles with spherical shape may work as “micro rolling balls” on the mating surfaces, thus have protective effect for the surfaces.

26


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Figure 9. Schematical illustration of friction-reducing and antiwear mechanism under lubrication of oil with CuO/rGO

Rheological behaviors and elastohydrodynamic lubrication oil film The viscosity-temperature curves for different oil samples are shown in Figure 10(a). The salient feature is that the viscosity declines with the increasing of temperature and the decline rate decreases gradually. At the same temperature, the oil samples dispersed with nanoadditives show higher viscosities than 10w40 engine oil, indicating that adding the nanoadditives can increase the viscosity of the engine oil.42-44 Moreover, the increase of the viscosity are presented throughout the temperature range of 0-60 oC. Furthermore, the increase rate changes a little in the temperature range of 0-60 oC for each oil sample. For instance, the increase rates of viscosity for engine oil with CuO/rGO at 0, 20, 40 and 60 oC are 20.2%, 20.1%, 19.8%, 19.6%, and the mean value is nearly 20%. The average increase rates of viscosity for engine oil with CuO, engine oil with the mixture of CuO and rGO, and engine oil with rGO are 22.4%, 16.2%, and 11.6%, respectively.The shear viscosity-shear

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rate curves for the different oil samples are presented in Figure 10(b). All the curves increase parabolically with the increasing of shear rate in the range less than 4s-1. When the shear rate is higher than 4s-1, all curves are nearly linear, suggesting that all tested oil samples behave as Newtonian fluids.45, 46 At a certain shear rate range (4~1000s-1), it can be found that the shear viscosity of 10w40 engine oil is enhanced to some extent by adding these nanoadditives.

Figure 10. Viscosity-temperature curves (a) and shear viscosity-shear rate curves for different oil samples (b). Figure 11(a) gives the oscillating viscosity-shear frequency curves for different oil samples. The complex viscosities are almost constant with the shear frequency increasing over the frequency range of 0.0~0.4 Hz for all oil samples. As the shear frequency is over 0.4 Hz, the complex viscosities of all oil samples dramatically increase with the increasing shear frequency. Among all the oil samples, the engine oil with CuO has the highest complex viscosity, followed by the engine oil with CuO/rGO. Variations of the storage modulus (G') and the loss modulus (G'') as a function of shear frequency for the 10w40 engine oil and the 28


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oil with CuO/rGO are shown in Figure 11(b). Apparently, G'' is nearly 1~2 orders of magnitude higher than G', indicating that the viscosity is far stronger than the elasticity for the two oil samples. Both G'' and G' show exponential increase with the increasing of the shear frequency, and the corresponding increase indexes are 1.61 and 1.88 respectively, which are in accord with typical rheological feature of lubrication oil. The G'' of the engine oil with CuO/rGO is a little higher than that of the unfilled one, further proving that the addition of CuO/rGO additive can improve the viscosity of 10w40 engine oil. The G'-shear frequency curves of the two oil samples show sudden drops with the increasing shear frequency, indicating the extremely poor elasticity.

Figure 11. Oscillating viscosity-shear frequency curves for different oil samples (a) and variations of storage modulus (G') and loss modulus (G'') as a function of shear frequency for 10w40 engine oil and for engine oil with CuO/rGO (b).

Figure 12 shows the interference patterns of elastohydrodynamic lubrication (EHL) oil 29


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films under lubrication of 10w40 engine oil and the oil with CuO/rGO at different entrainment speeds. The entrainment direction is horizontal from left to right, and the center lines of the patterns are marked with the dotted yellow lines. As shown in Figure 12(a, d), the two interference patterns both show a series of concentric circles at the entrainment speed of 0 m/s and the middle circular region is exactly the contact area of the steel ball and the glass disc. The contact areas for the two oil samples are almost the same and the diameter of the contact areas is around 183 μm. At the entrainment speeds of 0.2 and 0.4 m/s, the interference patterns show a horseshoe shape with a crescent stripe in the left exit-zone, as shown in Figures 12 (b, c, e, and f). At the same entrainment speeds, the crescent stripe of the interference pattern for engine oil with CuO/rGO is a little smaller than that for 10w40 engine oil, which indicates the differences in the EHL oil film thicknesses of the two oil samples.

Figure 12. Interference patterns of EHL oil films under lubrication of 10w40 engine oil (a-c) and engine oil with CuO/rGO (d-f) at different entrainment speeds ((a, d) 0 m/s; (b, e) 0.2 m/s; (c, f) 0.4 m/s). 30


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Table 2 shows the centre-point oil film thickness (hcen) and the center line minimum oil film thickness (hmin) of the interference patterns in Figure 12. It is clear that hcen and hmin of image (c) are both bigger than those of image (b), and both hcen and hmin of image (f) are also larger than those of image (e). This result indicates that the oil film thickness increases with the increasing of the entrainment speed. Meanwhile, it can be found that at a fixed entrainment speed (0.2 or 0.4 m/s), hcen and hmin of image (e) are higher than those of image (b), and hcen and hmin of image (f) are also higher than those of image (c). It is obvious that the oil sample of engine oil with CuO/rGO can produce a thicker and stronger oil film than 10w40 engine oil under the same entrainment speed. Namely, the CuO/rGO nanocomposite as additive has an enhancement effect on the state of EHL for the 10w40 engine oil. Table 2. hcen and hmin of different interference patterns in Figure 12 Interference patterns

(b)

(c)

(e)

(f)

hmin (nm)

184.8

287.6

211.8

327.8

hcen (nm)

273.7

397.4

307.9

443.8

CONCLUSIONS Here, we developed an effective strategy to synthesize the nanocomposite of reduced graphene oxide decorated with CuO nanoparticles. Introducing supercritical CO2 fluid during synthesis process greatly improves the microstructure and morphology of the as-synthesized CuO/rGO nanocomposite. The spherical CuO nanoparticles with size of around 6 nm are homogeneously anchored on rGO nanosheets. Moreover, the anchored 31


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CuO nanoparticles are encapsulated between rGO nanosheets and form a unique encapsulation microstructure. Tribo-tests confirmed that the CuO/rGO additive is proven to be effective for improving the friction-reducing and antiwear abilities of 10w40 engine oil under boundary lubrication. The friction coefficient and the wear rate of the engine oil loaded with low concentration of CuO/rGO (0.06-0.18 wt%) are reduced by 46.62% and 77.05%, respectively, as compared to the unloaded engine oil. In addition, the lubricating ability of CuO/rGO is better than CuO and rGO used alone as lubricant additive, confirming the nanocomposite having synergistic lubricating effect. The characterizations of wear surfaces prove that the CuO/rGO additive presents spontaneous deposition behavior and can form a protective film on the wear surface. The excellent tribological properties of the CuO/rGO additive can be attributed to its spontaneous deposition behavior and the synergistic lubricating effect. Tests of the rheological behaviors exhibited that adding these nanoadditives can increase the viscosity of the engine oil to different extent throughout the test temperature range of 0–60 oC. The remarkable increase of viscosity by the addition of CuO/rGO nanoadditive corresponds to the thickening of EHL film and the strengthening of EHL state for the engine oil with CuO/rGO as compared to the bare one.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID 32


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

ACKNOWLEDGMENTS The authors are grateful to the financial support of the National Natural Science Foundation of China (No.51775191), the Guangdong Natural Science Funds for Distinguished Young Scholar (grant: 2015A030306026), and the Fundamental Research Funds for the Central Universities (2018ZD29).

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Boundary and Elastohydrodynamic Lubrication Behaviors of Nano

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