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Fe3O4
Tribology International 141 (2020) 105952
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Tribology International journal homepage: http://www.elsevier.com/locate/triboint
Preparation, characterization and tribological properties of polyalphaolefin with magnetic reduced graphene oxide/Fe3O4 Qiangqiang Zhang, Bo Wu, Ruhong Song, Hui Song, Jun Zhang, Xianguo Hu * Institute of Tribology, Hefei University of Technology, Hefei, 230009, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnetic reduced graphene oxide/Fe3O4 Anti-wear additive Tribological properties Polyalphaolefin
Magnetic reduced graphene oxide/Fe3O4 (RGO/Fe3O4) composites were synthesized via chemical coprecipitation method. The RGO/Fe3O4 were characterized by field-emission transmission electron microscopy (FETEM), X-ray diffraction (XRD) and Raman spectrometry and magnetic property measurement system (MPMS), respectively. Polyalphaolefin (PAO) 6 with different contents of RGO/Fe3O4 was prepared by a dispersion process. The tribological properties of PAO 6 containing RGO/Fe3O4 were evaluated using a trib ometer. Friction results indicated that RGO/Fe3O4 can significantly improve the tribological properties, espe cially anti-wear properties, of PAO 6. From the analysis results, RGO/Fe3O4 possibly repairs the worn surface by forming an effective tribofilm, which benefits from the synergistic effect of RGO and Fe3O4 from the RGO/Fe3O4 nanocomposites.
1. Introduction Graphene, with its unique 2D lamellar structure and excellent physical and electrical properties, has become a hot research topic [1–4]. Graphene oxide (GO), as an important derivative of graphene, also possesses a 2D layered structure. Given its extreme properties, GO has been widely applied in sensors, field effect transistors, clean energy devices, and transparent conductors, etc. [5,6]. GO has also been used as a lubricant because of its weak interlaminar shear force. Kinoshita et al. [7] reported that monolayer GO sheets as water-based lubricant addi tives can reduce friction and surface wear. Zhe et al. [8] confirmed that few-layer GO sheets can also be oil-based lubricant additives to improve the tribological properties of lubricant. But laminar GO usually has trend toward overlap, impelling GO to form large agglomerates that cannot easily penetrate the rubbing area. In recent years, Fe3O4 nanoparticles have been widely applied in many fields, such as biomedicine, adsorbent, magnetic recording ma terials, due to their high magnetic property and large specific surface area [9–11]. Fe3O4 nanoparticles less than 10 nm in diameter are also good lubricant additives [12,13]. However, because of magnetic prop erty and large surface energy, Fe3O4 nanoparticle tend to agglomerate together, which will make against to improve friction in the sliding process. Consequently, it is also a hard problem to solve its dispersion. Wang et al. [14] reported that decorating graphene nanosheets with
nanoparticles can avoid the aggregation or multilayer of graphene nanosheets and reduce specific the surface area of composites. GO-based nanocomposites have been hot a spot for various applications, including electrocatalysts, adsorbents, supercapacitors, photocatalytic active agents and microwave absorbing materials. [14–18]. The combination of GO or graphene with magnetic nanoparticles to obtain magnetically functionalized GO nanocomposites has attracted considerable attention among many scholars. [19–23]. However, few studies have applied magnetic nanocomposites based on GO or graphene were applied in lubricating oil additives. Depositing Fe3O4 nanoparticles on the surface of a graphene-like material can prevent graphene-like material from lapping, and avoid Fe3O4 nanoparticles agglomeration, which is propi tious to help the composites enter into the rubbing interfaces. In addi tion, Fe3O4 nanoparticles deposited on the surface of GO may act as a power source under an external field to drive the composite to target the friction interface, which may be an important study in the future. Today, low-viscosity engine oil is becoming a preferable choice to promote energy conservation and improve energy utilization. Lowviscosity PAO, as a high-performance category IV synthetic base oil, has greater viscosity index, vapor pressure, and thermal stability than the currently used mineral oils. Thus, low-viscosity PAO has wide ap plications, including automotive engine oils, transmission fluids, and even space lubricants [24,25]. However, low-viscosity oil usually has low oil film strength and poor anti-wear properties. Therefore, a special
* Corresponding author. E-mail address: [email protected] (X. Hu). https://doi.org/10.1016/j.triboint.2019.105952 Received 19 June 2019; Received in revised form 23 August 2019; Accepted 4 September 2019 Available online 5 September 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic diagram of preparation process of PAO 6 with RGO/Fe3O4.
additive to improve tribological properties of low-viscosity base oil must be developed. In our work, reduced graphene oxide/Fe3O4 (RGO/Fe3O4) compos ites were fabricated by depositing Fe3O4 nanoparticles onto the surface of RGO. After modifying, RGO/Fe3O4 was dispersed in low-viscosity PAO 6. The tribological properties of RGO/Fe3O4 in low-viscosity PAO 6 were evaluated with CETR-UMT tribometer. The wear mechanism of RGO/Fe3O4 was also discussed.
2.2. Synthesis and characterization of RGO/Fe3O4 GO (15 mg) was dispersed in 45 mL of deionized water, ultrasoni cally treated for 1 h, and then poured into a 250 mL three-mouth flask. The solution was purged with nitrogen for 60 min. Then, 18 mg of FeCl3⋅6H2O and 28.5 mg of FeCl2⋅4H2O were dissolved in 7.5 mL of deionized water, and the iron ion solution was added into the flask while the mixed solution was vigorously stirred under nitrogen atmosphere for 7 h. An alkaline solution prepared by adding 0.795 g of NH3⋅H2O (25–28%) into 3.6 mL of deionized water was dripped into the flask. Afterward, the solution was heated to 65 � C and then continuously stirred for 2.5 h. After the reaction finished, RGO/Fe3O4 was acquired. A strong permanent magnet was used to collect the black products. The product was washed seven times with 15–20 mL deionized water to remove surplus reactants. Finally, the product was washed three times with ethanol to remove water. Fe3O4 nanoparticles were prepared in the same method as the RGO/Fe3O4 composites without GO. The morphological features of RGO/Fe3O4 were characterized by FETEM (JEM-2100F). Raman spectrometer (HR Evolution, HORIBA Jobin Yvon) and XRD (D/MAX2500V) were used to analyze the chem ical components of RGO/Fe3O4. The magnetization curve of RGO/Fe3O4
2. Experimental 2.1. Materials Graphene oxide (GO; lateral dimension about 5–10 μm), PAO 6, ashfree polyisobutylene succinimide dispersant (PIBSI), oleylamine (80–90%), ammonia solution (25–28%), FeCl2⋅4H2O (analytical re agent), FeCl3⋅6H2O (analytical reagent), deionized water and ethanol (analytical reagent) were purchased from commercially available reli able sources.
Fig. 2. Digital images of dispersion of the as-prepared oil samples with different mass fraction RGO/Fe3O4 within 24 h. 2
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Fig. 3. The UMT-2 tribometer, schematics of the motion mode of friction pairs, and record of the analysis conducted.
was measured as a function of the applied magnetic field H by MPMS (MPMS XL-7, Quantum Design). The hysteresis of the magnetization was obtained by varying H between þ10000 and 10000 Oe at 300 K.
lubricants containing five mass fraction additives (0.05, 0.1, 0.3, and 0.5 wt%) for friction experiments. Fig. 2 shows digital images of the as-prepared oil samples as a function of time. The dispersion results indicated that the stratification was not observed in all samples within 24 h. However, few particles could be observed at the bottom of the bottle with prolonged observation time because of gravity and the interaction between particles. Higher mass fraction indicated earlier precipitation. The samples with the mass fraction of 0.1, 0.3 and 0.5 wt % presented sedimentary particles at the bottom of the bottle after 24, 18, and 12 h, respectively. The sample with the mass fraction of 0.05 wt % remained stable after 24 h.
2.3. Preparation of PAO 6 with different contents of RGO/Fe3O4 In this paper, PAO 6 was selected in our initial experiments. Although the combination of RGO and Fe3O4 improved the dispersion in PAO6 to a certain extent than when GO or Fe3O4 alone was added into PAO6, the oil samples still lacked uniform dispersion and presented few precipitations within few hours. It is necessary to add some dispersants into PAO6 to further improve the dispersion of particles. PIBSI as a common dispersant has been widely used to solve the agglomeration of soot particles in the formulated lubricating oil. Therefore, refer to our previous work experience [26], we introduced moderate poly isobutylene succinimide to further solve the dispersion of RGO/Fe3O4 in PAO 6. Liquid transfer method was used to prepare PAO 6 with RGO/ Fe3O4. A schematic of the preparation of PAO6 with RGO/Fe3O4 is shown in Fig. 1. We modified the surface of RGO/Fe3O4 by using oleylamine to improve its dispersion stability in PAO6. The above pre pared RGO/Fe3O4 were dispersed into the surface modifier mixture with 50 mL of ethanol and 1.2 g of oleylamine by ultrasonic mixing for 10 min. The suspension was heated to 75 � C for 1 h. The final RGO/ Fe3O4 was acquired and collected using a strong permanent magnet and washed three times with ethanol to remove surplus oleylamine. Then the as-prepared modified RGO/Fe3O4 slurry was dispersed into 15 mL of ethanol and ultrasonically treated for 10 min. Before RGO/Fe3O4 sus pension were poured into PAO 6 containing PIBSI, 5 wt% of PIBSI was added into PAO 6 as a secondary surfactant. Then the mixture was stirred to evaporate ethanol at 85 � C. Until the ethanol was completely evaporated, RGO/Fe3O4 was dispersed stably into PAO 6. We prepared
2.4. Tribological tests and analysis Tribological tests were carried out on a CETR-UMT ball-disk trib ometer (UMT-2, Bruker). The upper ball (diameter ϕ6 mm, HV 647-861, Ra 0.016 μm) and the lower disk (ϕ28 � 2 mm, HRC 55-60, Ra 0.04 μm) were made of GCr15 steel. A schematic of the rubbing pairs is shown in Fig. 3. The upper ball was fixed, and the lower disk was reciprocated. The prepared lubricants were dripped in the contact interface at the beginning of the test. A 10 N load was applied to the upper ball. The reciprocating speed and stroke of the lower disk were 10 mm/s and 1 mm, respectively. The duration time was 30 min for each test. The testing temperature was kept at about 25 � C. In this friction test, the base oil was PAO 6 þ 5 wt% PIBSI. The testing oil samples include base oil, base oil þ (0.05, 0.1, 0.3, and 0.5 wt%) Fe3O4, base oil þ (0.05, 0.1, 0.3, and 0.5 wt%) GO, and base oil þ (0.05, 0.1, 0.3, and 0.5 wt%) RGO/ Fe3O4, where GO and Fe3O4 were selected alone as contrastive experi ment. Each experiment was repeated at least three times. The friction coefficient was recorded in real time automatically by the device, and the average friction coefficient was calculated after experiment. 3
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Fig. 4. TEM images of (a) GO, (b)Fe3O4, (c) RGO/Fe3O4, and (d)HRTEM image of RGO/Fe3O4.
In this paper, the lubrication regime under the selected testing con ditions could be estimated by calculating the parameter lambda λ, i.e. the ratio of estimated lubricant film thickness to composite surface roughness. The common classification of lubrication regime includes thin film/EHD, mixed and boundary lubrication, and their correspond ing lambda ratio are λ > 3, 0.5<λ < 3 and λ < 0.5 [27]. The lambda could be calculated according to the classical theory of Hamrock and Dowson. The calculated value of λ was 0.068 under the selected testing conditions, which is well below the limiting value of 0.5, indicating that the lubrication regime was mainly boundary lubrication during the wear tests. After friction, the disk was washed with ethanol for the following analyses. The wear volume of the disk was measured by 3D laser scan ning microscopy (LSM; VK-X100, Keyence). The morphology and elemental content of the worn area were analyzed by SEM (SU8010, Hitachi), LSM, Raman spectrometry (HR Evolution, HORIBA Jobin Yvon), and XPS (ESCALAB250, Thermo).
3. Results and discussion 3.1. Characterization of RGO/Fe3O4 The morphology and components of RGO/Fe3O4 were characterized by FETEM, as shown in Fig. 4. Fig. 4(a) shows the as-purchased GO. Fig. 4(b) shows the Fe3O4 nanoparticles prepared without adding GO into the reaction system. The diameter of Fe3O4 nanoparticles ranges from 5 nm to 10 nm. Fig. 4(c)–(d) show the RGO/Fe3O4 composites. Wrinkles of GO still existed after combining Fe3O4 nanoparticles. The HRTEM image of the RGO/Fe3O4 nanocomposites in Fig. 4(d) clearly shows that Fe3O4 nanoparticles uniformly were deposited on the surface of RGO sheets. Fig. 5(a) shows the XRD pattern of RGO/Fe3O4. Diffraction peaks at 2θ ¼ 30� , 35� , 43� , 53� , 57� , and 63� could be indexed to the (220), (311), (222), (400), (422), (511), and (440) planes of the magnetite structure of Fe3O4 nanoparticles, respectively [28]. The weak peak at
Fig. 5. Characterization of RGO/Fe3O4 component. (a) X-ray diffraction pattern; (b) Raman spectra with the laser wave length was 533 nm. 4
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23� could be attributed to the stacking of RGO [28,29]. The XRD results of RGO/Fe3O4 are consistent with previous reports [30]. Fig. 5(b) shows the Raman spectra of RGO/Fe3O4. The peaks at 510 and 670 cm 1 could be assigned to Fe3O4 nanoparticles [31]. Due to the existence of RGO, the characteristic peaks of Fe3O4 may slightly shift. Two Other peaks at 1350 and 1583 cm 1 corresponded to the D and G peaks of graphene, respectively, respectively [32]. The magnetic properties of RGO/Fe3O4 were also measured by MPMS. Fig. 6(a) shows the magnetic hysteresis loops of RGO/Fe3O4 as Sshaped curves. The magnetic induction intensity of the samples gradu ally increased with increasing external magnetic field strength until it reached the saturation magnetization (Ms). The Ms of RGO/Fe3O4 was about 20 emu/g. From insert map Fig. 6(b), As shown in the insert map of Fig. 6(b), the Mr was 0.72 emu/g, which is near 0 emu/g, when the external magnetic field strength decreased to 0 Oe, revealing super paramagnetic behavior. [33]. 3.2. Tribological properties of RGO/Fe3O4
Fig. 6. The magnetic hysteresis loops of RGO/Fe3O4. (a) The magnetization hysteresis loops of the RGO/Fe3O4; (b) Magnified image at a dotted line frame in Fig. 4(a). Saturation magnetization (Ms) was 20 emu/g, and the sample exhibits low remanence, Mr was 0.72 emu/g.
Fig. 7(a) shows the average friction coefficient of base oil with different concentration additives during the entire experiment. When pure Fe3O4 nanoparticles were added into base oil, they exhibited increased friction. A similar phenomenon has been reported in other
Fig. 7. (a) The average friction coefficient, (b) average wear volume of the lower steel disk and (c) average wear scar diameter of the upper steel ball lubricated by the base oil with different concentration additives. All tests were conducted with load of 10 N, reciprocating speed of 10 mm/s, reciprocating stroke of 1 mm, and testing time of 30 min. 5
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Fig. 8. Laser images of the upper ball, 3D images and wear profiles of the lower disk lubricated with the base oil ((a), (a)’ and (a)”), base oil with 0.1 wt% Fe3O4 ((b), (b)’ and (b)”), base oil with 0.1% wt% GO ((c), (c)’ and (c)”), and base oil with 0.1% wt% RGO/Fe3O4 ((d), (d)’ and (d)”).
papers [34,35]. This finding may be attributed to the fact that Fe3O4 nanoparticles tend to agglomerate, blocking the motion of friction pairs in the friction contact interface. However, both RGO/Fe3O4 and GO as additives can reduce the friction coefficient. This may be ascribed to the presence of sheet structure from GO and RGO, and the fact that friction pairs are relatively easy to slide [36]. The average friction coefficient of 0.1 wt% RGO/Fe3O4 was the lowest among the samples, and it was 7% lower than that of the base oil and has a very small error bar. Fig. 7(b) shows the average wear volume of steel disk lubricated with
base oil having different concentration additives during the experiment. When pure Fe3O4 nanoparticles were added into the base oil, the average wear volume increased with increasing content of Fe3O4 nanoparticles. The agglomerate Fe3O4 nanoparticles possibly acted as abrasive grains, aggravating the wear of friction pairs during friction. However, RGO/Fe3O4 showed better anti-wear properties than Fe3O4 and GO. The average wear volume of the steel disk lubricated using the base oil with 0.1 wt% RGO/Fe3O4 was 52.27% and 25.56% lower than those of the steel disks lubricated with the base oil and the base oil with 6
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Fig. 9. SEM images of wear scars of steel disk lubricated by the base oil (a), base oil with 0.1 wt% Fe3O4 (b), base oil with 0.1 wt% GO (c), and base oil with 0.1 wt% RGO/Fe3O4 (d).
0.1 wt% GO, respectively. The average wear volume of the steel disk initially decreased with increasing content of RGO/Fe3O4 and then increased when the content of RGO/Fe3O4 exceeded 0.1 wt%. This result indicates that the proper content can effectively improve the anti-wear properties of base oil, and this trend is consistent with other reports [36,37]. It may be ascribed to the fact that excessive RGO/Fe3O4 par ticles may cause agglomeration of particles and prevent the lubricant from entering the contact pair [37,38]. The optimized concentration of RGO/Fe3O4 is not a fixed value; it largely depends on the physical and chemical properties of the solid–body contact and contact stress [39]. Fig. 7(c) shows the average wear scar diameter of the upper steel ball lubricated by the base oil with different concentration additives. The trend of the wear scar diameter is similar to that of the wear volume. Adding pure Fe3O4 nanoparticles led to larger wear scar diameter. However, adding an appropriate mass fraction of RGO/Fe3O4 particles could effectively reduce the wear scar diameter. The average wear scar diameter of the steel ball lubricated using the base oil with 0.1 wt% RGO/Fe3O4 was 5.45% lower than that of the steel disk lubricated using the base oil. The addition of RGO/Fe3O4 particles possibly hindered the direct contact of friction pairs and then reduced the wear scar diameter.
For further analysis, SEM images of the wear scars on the steel disk lubricated with different oil samples were observed (Fig. 9). As shown in Fig. 9(a), furrows were dense and deep, and some peels presented on the worn surface lubricated by the base oil. The friction pairs were subjected to severe wear. Similarly, as shown in Fig. 9(b), when Fe3O4 nano particles were added in the base oil, the worn surface presented a large number of wide furrows. It may be ascribed to the agglomeration of Fe3O4 nanoparticles, leading to abrasive wear [34]. Although GO can partly improve the worn surface, a few wide furrows still existed (Fig. 9 (c)). Nevertheless, the number of furrows decreased markedly due to the existence of RGO/Fe3O4 in the base oil, and the worn surface became smoother, which indicated that the friction pairs were well protected (Fig. 9(d)). It may be attributed to the formation of tribo-film, which can separate friction pairs to avoid directly touching and then improve wear. 3.4. Friction and wear mechanism analysis The chemical compositions and elemental state of the worn surface were analyzed using Raman spectroscopy and XPS to investigate the reduce-wear mechanism of RGO/Fe3O4 as a lubricant additive on the worn surface. Raman spectra of wear scars of steel disk are shown in Fig. 10. As illustrated in Fig. 10(a), the Raman spectra of the wear scar showed peaks at 292, 305, 534, and 660 cm 1. The peaks at 305, 534, and 660 cm 1 can be assigned to Fe3O4, and the peak at 292 cm 1 is assigned to α-Fe2O3 [31]. This result indicates that the composition of the worn surface is mainly iron oxides, which can be attributed to some of the debris originated from the substrate during the friction process were oxidized [42]. As shown in Fig. 10(b), when the Fe3O4 nanoparticles were added into the base oil, the Raman spectra of worn surface shows more peaks of iron oxides, which possibly originated from the Fe3O4 nanoparticles. Fig. 10(c) shows the Raman spectra of the wear scars lubricated with the base oil containing GO. The Raman spectra showed the same peaks of iron oxides as lubricated by the base oil. In addition to the peaks of iron oxides, the D-peak at 1390 cm 1 and G-peak at 1583 cm 1 were observed, indicating that GO participated in the
3.3. Morphology of worn surface Fig. 8 shows the laser images of the upper ball, 3D images, and wear profiles of the lower disk lubricated with different oil samples. With the aid of Multifile Analyzer software matched with LSM, the profile of the wear mark can be obtained [40,41] the average wear area was obtained by calculating the average area of hundreds of cross sections starting from the middle of the wear scar. As shown in Fig. 8 (a)”, the wear profiles of the steel disk lubricated with base oil were very deep, and the surface roughness (Ra) of the wear scars reached 0.408 μm. As shown in Fig. 8 (b)”, the addition of Fe3O4 did not reduce the wear of friction pairs. However, when the as-prepared RGO/Fe3O4 nanoparticles were added in the base oil, the wear area of the wear scars evidently decreased, and the surface of the wear scars bacame smoother than that of the steel disk lubricated with base oil according to the value of Ra. 7
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Fig. 10. Raman spectra of wear scars lubricated by the base oil (a), base oil with 0.1 wt% Fe3O4 (b), base oil with 0.1 wt% GO (c), and base oil with 0.1 wt% RGO/ Fe3O4 (d).
formation of tribofilm on the worn zone [43,44], which may be reason for reducing wear. Fig. 10(d) shows the Raman spectra of the wear scars lubricated with the base oil containing RGO/Fe3O4. Compared with Fig. 10(b), the peaks of iron oxides were greater in the Raman spectra of wear scars lubricated with the base oil containing RGO/Fe3O4 than in those of the wear scars lubricated with the base oil containing Fe3O4 in Fig. 10(d). It may be ascribed to Fe3O4 nanoparticles tend to agglom erate, leading to severe abrasion, as shown in the SEM images from Fig. 9(b), and preventing the iron oxide films to exist stably. Compared to Fig. 10(c), the D-peak and G-peak also appeared in Fig. 10(d). Interestingly the G-peak became weaker and the D-peak became stron ger than the Raman spectra of wear scars lubricated with the base oil containing GO. It may be ascribed to the fact that the existence of Fe3O4 on the GO surface accelerated the transformation from G-peak to D-peak during the testing process, which is consistent with the result reported by Huang [45]. Besides, the stronger and more peaks of iron oxides in the range of 200–700 cm 1 may be ascribed to the presence of Fe3O4 nanoparticles on RGO, which promoted the formation of more iron oxide films. The Raman spectra confirmed that the GO particles could form graphene–structure tribofilm, while the RGO/Fe3O4 composites could promote the formation of iron-oxide and graphene-structure tri bofilms on the worn surface because of the existence of Fe3O4 nano particles. The tribological results showed that the friction and wear
markedly decreased with the RGO-Fe3O4 synergistic tribofilm on the worn surface. We further investigated the differences in lubrication mechanism between nonmagnetic GO and magnetic RGO/Fe3O4. The worn surfaces lubricated by the base oil with 0.1 wt% GO and RGO/Fe3O4 were analyzed by XPS, respectively, and the worn surface lubricated by the base oil was also measured by XPS as a control. Fig. 11(a) and (e) and 11 (i) show the survey spectra of the wear scars, where the peaks at 711, 285, and 532 eV corresponded to Fe2p, C1s, and O1s [45]. Comparison of element content shows that when GO was added into the base oil, C content of the worn surface markedly increased from 47.42% to 55.6% compared with that of the base oil. Interestingly, when RGO/Fe3O4 was added into the base oil, the contents of C and Fe were higher than those of the base oil, and the content of Fe was higher than that of the base oil with GO. This is the result of a good synergy between Fe3O4 nano particles and RGO. The C1s spectra in Fig. 11(b), (f), and 11(j) showed three peaks at 284.7, 285.5, and 288.5 eV, which corresponded to C–C or – O or O–C–O, respectively [46]. Fig. 11(c), (g) and 11 C–H, C–O, and C– (k) show the O1s spectra. The peaks at 529.8, 531.2, and 532.5 eV may – O, respectively [44,45]. The Fe2p be belonged to Fe–O, C–O, and C– spectra in Fig. 11(d), (h), and 11(l) showed five peaks at 706.9, 710.3, 711.7, and 724.5 eV, which belonged to Fe (0) simpler substance, Fe2þ(2p3/2), Fe3þ(2p3/2), and Fe(2p1/2), respectively. [45–48]. By 8
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Fig. 11. XPS analysis of the worn surfaces lubricated by the base oil (a–d), base oil with 0.1 wt% GO (e–h), and base oil with 0.1 wt% RGO/Fe3O4 (i–l).
comparing Fig. 11(h) with 11(l), the calculation of the area of the peak at 710.3(A1) and 711.7(A2) eV showed that the area ratio is 0.68 when the worn surface was lubricated by the base oil with GO. This value is close to the area ratio (0.56) when the worn surface was lubricated by the base oil. However, when RGO/Fe3O4 was added into the base oil, the area ratio significantly increased to 1.02 compared with that when the base oil with GO was used, which indicates that the proportion of Fe2þ increases. This result may be attributed to the presence of Fe3O4 nano particles on RGO. This is consistent with Zhao’ work [35]. The results of XPS confirmed that Fe3O4 nanoparticles and RGO jointly participate in the formation of tribofilm. Due to the presence of Fe3O4 nanoparticles on RGO, the RGO-Fe3O4 synergetic tribofilm formed to protect the friction pairs, and shown better tribological properties than that of without Fe3O4 nanoparticles. These results are consistent with the Raman spectra of wear scars. The above analysis results indicated that the anti-wear mechanism of RGO/Fe3O4 may be ascribed to the formation of the RGO–Fe3O4 syn ergistic tribofilm on the worn surface during the friction process, which separates friction pairs like a protective film to reduce wear. The spec ulation of the formation of the synergistic tribofilm is as follows. On the one hand, during the friction process, the applied load produced me chanical shear stress at the friction interface, causing a part of Fe3O4 nanoparticles to exfoliate from the RGO/Fe3O4 composite. The exfoli ated Fe3O4 nanoparticles can partly enter into furrows to repair the worn surface. Thus, the synergistic tribofilm formed on the worn surface because of the synergistic effect of RGO and Fe3O4 originating from the RGO/Fe3O4 composites. On the other hand, the RGO/Fe3O4 composites were capable of magnetic response under an external magnetic field, as indicated by the measurement of RGO/Fe3O4 by MPMS. However, when
the external magnetic field strength decreased to 0 Oe, the Mr was only 0.72 emu/g, which is near 0 emu/g, showing superparamagnetic behavior. This characteristic redered the particles themselves not magnetically attractive in the absence of an external magnetic field. Mishina [49,50] reported that weak magnetism could be generated between friction pairs because of the reciprocating movement of steel-material friction pairs under dry friction condition. Frictional magnetism is a common phenomenon. Considering the magnetic response of RGO/Fe3O4 composites, the formation of the RGO–Fe3O4 synergistic film may benefit from the interaction between the magnetic response of the RGO/Fe3O4 composites and the magnetism from friction pairs. Although Fe3O4 is also magnetic, nanoparticles tend to agglom erate, leading to abrasive wear, and then preventing the iron oxides films to exist stably. 4. Conclusion RGO and Fe3O4 were synthesized into the magnetic RGO/Fe3O4 composites successfully by chemical co-precipitation method. RGO/ Fe3O4 was dispersed into PAO 6. Comparison of the tribological prop erties of PAO 6 with Fe3O4, GO, and RGO/Fe3O4 revealed that PAO 6 with Fe3O4 nanoparticles showed worse tribological behavior than pure PAO 6. However, PAO 6 with the RGO/Fe3O4 composites showed better tribological properties than those with GO and Fe3O4 nanoparticles. The average wear volume of steel disk decreased by 52.27% after adding 0.1 wt% RGO/Fe3O4. The anti-wear mechanism was ascribed to the formation of the tribofilm originating from the synergistic effect of RGO and Fe3O4 in the RGO/Fe3O4 nanocomposites.
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Acknowledgments
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Enhanced tribological properties of bismaleimides filled with aligned graphene nanosheets coated with Fe3O4 nanorods. J Mater Chem A 2015;3:10559–65. [43] Kumar P, Wani MF. Tribological characterisation of graphene oxide as lubricant additive on hypereutectic Al-25Si/steel tribopair. Tribol Trans 2018;61:335–46. [44] Fan XQ, Wang LP. High-performance lubricant additives based on modified graphene oxide by ionic liquids. J Colloid Interface Sci 2015;452:98–108. [45] Huang J, Li Y, Jia XH, Song HJ. Preparation and tribological properties of coreshell Fe3O4@C microspheres. Tribol Int 2019;129:427–35. [46] Wang BB, Hu EZ, Tu ZQ, David KD, Hu KH, Hu XG, et al. Characterization and tribological properties of rice husk carbon nanoparticles Co-doped with sulfur and nitrogen. Appl Surf Sci 2018;462:944–54. [47] Bonnet F, Ropital F, Lecour P, Espinat D, Huiban Y, Gengembre L, et al. Study of the oxide/carbide transition on iron surfaces during catalytic coke formation. 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This research was supported by the National Natural Science Foun dation of China (Grant No. 51675153) and Major Science and Tech nology Special Project in Anhui (Grant No.17030901084), which are gratefully acknowledged. The authors thank Professor Kunhong Hu and Dr. Enzhu Hu of Hefei University and Dr. Yufu Xu of Hefei University of technology for their assistance in the experimental analyses and discussion. References [1] Shen JF, Hu YZ, Shi M, Li N, Ma HW, Ye MX. One step synthesis of graphene oxidemagnetic nanoparticle composite. J Phys Chem C 2010;14:1498–503. [2] Du X, Skachko I, Barker A, Andrei EY. Approaching ballistic transport in suspended graphene. Nat Nanotechnol 2008;3:491–5. [3] Palacios T, Hsu A, Wang H. Applications of graphene devices in RF communications. IEEE Commun Mag 2010;48:122–8. [4] Pasanen P, Voutilainen M, Helle M, Song XF, Hakonen PJ. Graphene for future electronics. Appl Phys Lett 2012;95:061101. [5] Zhu YW, Murali S, Cai WW, Li XS, Suk JW, Potts JR, et al. Graphene and graphene oxide: synthesis, properties, and applications. Adv Mater 2010;22:3906–24. [6] Becerril HA, Mao J, Liu ZF, Stoltenberg RM, Bao ZN, Chen YS. Evaluation of solution-processed reduced graphene oxide films as transparent conductors. ACS Nano 2008;2:463–70. [7] Kinoshita H, Nishina Y, Alias AA, Fujii M. Tribological properties of monolayer graphene oxide sheets as water-based lubricant additives. Carbon 2014;66:720–3. [8] Chen Z, Liu YH, Luo JB. Tribological properties of few-layer graphene oxide sheets as oil-based lubricant additives. Chin J Mech Eng 2015;29:439–44. [9] Tartaj P, Morales MP, Veintemillas-Verdaguer S, Gonz� alez-Carreno T, Serna CJ. The preparation of magnetic nanoparticles for applications in biomedicine. J Phys D Appl Phys 2003;36:182–97. [10] Kumar ASK, Jiang SJ. Synthesis of magnetically separable and recyclable magnetic nanoparticles decorated with β-cyclodextrin functionalized graphene oxide an excellent adsorption of As(V)/(III). J Mol Liq 2017;237:387–401. [11] Langlet M, Labeau M, Bochu B, Joubert JC. Preparation of thin films in the system γFe2O3-Fe3O4 for recording media by spray pyrolysis of organometallic solutions using an ultrasonic pump. IEEE Trans Magn 2003;22:151–6. [12] Huang W, Wang WL, Ma GL, Shen C. Study on the synthesis and tribological property of Fe3O4 based magnetic fluids. Tribol Lett 2009;33:187–92. [13] Zhou GH, Zhu YF, Wang XM, Xia MJ, Zhang Y, Ding HY. Sliding tribological properties of 0.45% carbon steel lubricated with Fe3O4 magnetic nano-particle additives in base oil. Wear 2013;2:753–7. [14] Wang Y, Zhang S, Chen H, Li HX, Zhang P, Zhang ZY, Liang GH, Kong JL. One-pot facile decoration of graphene nanosheets with Ag nanoparticles for electrochemical oxidation of methanol in alkaline solution. Electrochem Commun 2012;17:63–6. [15] Li BJ, Cao HQ, Yin G, Lu YX, Yin JF. Cu2O@reduced graphene oxide composite for removal of contaminants from water and supercapacitors. J Mater Chem 2011;21: 10645–8. [16] Zheng L, Zhang GN, Zhang M, Guo SH, Liu ZH. Preparation and capacitance performance of Ag–graphene based nanocomposite. J Power Sources 2012;201: 376–81. [17] Gao ZY, Liu JL, Xu F, Wu DP, Wu ZL, Jiang K. One-pot synthesis of graphene–cuprous oxide composite with enhanced photocatalytic activity. Solid State Sci 2012;14:276–80. [18] Zhu LY, Zeng XJ, Li XP, Yang B, Yu RH. Hydrothermal synthesis of magnetic Fe3O4/graphene composites with good electromagnetic microwave absorbing performances. J Magn Magn Mater 2017;426:114–20. [19] Liu MC, Chen CL, Hu J, Wu XL, Wang XK. Synthesis of magnetite/graphene oxide composite and application for cobalt (II) removal. J Mater Chem C 2011;115: 25234–40. [20] Li J, Zhang SW, Chen CL, Zhao GX, Yang X, Li JX, Wang XK. Removal of Cu (II) and fulvic acid by graphene oxide nanosheets decorated with Fe3O4 nanoparticles. ACS Appl Mater Interfaces 2012;4:4991–5000. [21] Wang JM, Fang JJ, Fang P, Li X, Wu SJ, Zhang WJ, Li SF. Preparation of hollow core/shell Fe3O4@graphene oxide composites as magnetic targeting drug nanocarriers. J Biomater Sci Polym Ed 2017;28:337–49 0,punct]" content-markup (child::sb:comment)>. [22] Yang XY, Zhang XY, Ma YF, Huang Y, Wang YS, Chen YS. Superparamagnetic graphene oxide–Fe3O4 nanoparticles hybrid for controlled targeted drug carriers. J Mater Chem 2009;19:2710–4.
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Preparation, characterization and tribological properties of polyalphaolefin with magnetic reduced graphene oxide/Fe3O4 Qiangqiang Zhang, Bo Wu, Ruhong Song, Hui Song, Jun Zhang, Xianguo Hu * Institute of Tribology, Hefei University of Technology, Hefei, 230009, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnetic reduced graphene oxide/Fe3O4 Anti-wear additive Tribological properties Polyalphaolefin
Magnetic reduced graphene oxide/Fe3O4 (RGO/Fe3O4) composites were synthesized via chemical coprecipitation method. The RGO/Fe3O4 were characterized by field-emission transmission electron microscopy (FETEM), X-ray diffraction (XRD) and Raman spectrometry and magnetic property measurement system (MPMS), respectively. Polyalphaolefin (PAO) 6 with different contents of RGO/Fe3O4 was prepared by a dispersion process. The tribological properties of PAO 6 containing RGO/Fe3O4 were evaluated using a trib ometer. Friction results indicated that RGO/Fe3O4 can significantly improve the tribological properties, espe cially anti-wear properties, of PAO 6. From the analysis results, RGO/Fe3O4 possibly repairs the worn surface by forming an effective tribofilm, which benefits from the synergistic effect of RGO and Fe3O4 from the RGO/Fe3O4 nanocomposites.
1. Introduction Graphene, with its unique 2D lamellar structure and excellent physical and electrical properties, has become a hot research topic [1–4]. Graphene oxide (GO), as an important derivative of graphene, also possesses a 2D layered structure. Given its extreme properties, GO has been widely applied in sensors, field effect transistors, clean energy devices, and transparent conductors, etc. [5,6]. GO has also been used as a lubricant because of its weak interlaminar shear force. Kinoshita et al. [7] reported that monolayer GO sheets as water-based lubricant addi tives can reduce friction and surface wear. Zhe et al. [8] confirmed that few-layer GO sheets can also be oil-based lubricant additives to improve the tribological properties of lubricant. But laminar GO usually has trend toward overlap, impelling GO to form large agglomerates that cannot easily penetrate the rubbing area. In recent years, Fe3O4 nanoparticles have been widely applied in many fields, such as biomedicine, adsorbent, magnetic recording ma terials, due to their high magnetic property and large specific surface area [9–11]. Fe3O4 nanoparticles less than 10 nm in diameter are also good lubricant additives [12,13]. However, because of magnetic prop erty and large surface energy, Fe3O4 nanoparticle tend to agglomerate together, which will make against to improve friction in the sliding process. Consequently, it is also a hard problem to solve its dispersion. Wang et al. [14] reported that decorating graphene nanosheets with
nanoparticles can avoid the aggregation or multilayer of graphene nanosheets and reduce specific the surface area of composites. GO-based nanocomposites have been hot a spot for various applications, including electrocatalysts, adsorbents, supercapacitors, photocatalytic active agents and microwave absorbing materials. [14–18]. The combination of GO or graphene with magnetic nanoparticles to obtain magnetically functionalized GO nanocomposites has attracted considerable attention among many scholars. [19–23]. However, few studies have applied magnetic nanocomposites based on GO or graphene were applied in lubricating oil additives. Depositing Fe3O4 nanoparticles on the surface of a graphene-like material can prevent graphene-like material from lapping, and avoid Fe3O4 nanoparticles agglomeration, which is propi tious to help the composites enter into the rubbing interfaces. In addi tion, Fe3O4 nanoparticles deposited on the surface of GO may act as a power source under an external field to drive the composite to target the friction interface, which may be an important study in the future. Today, low-viscosity engine oil is becoming a preferable choice to promote energy conservation and improve energy utilization. Lowviscosity PAO, as a high-performance category IV synthetic base oil, has greater viscosity index, vapor pressure, and thermal stability than the currently used mineral oils. Thus, low-viscosity PAO has wide ap plications, including automotive engine oils, transmission fluids, and even space lubricants [24,25]. However, low-viscosity oil usually has low oil film strength and poor anti-wear properties. Therefore, a special
* Corresponding author. E-mail address: [email protected] (X. Hu). https://doi.org/10.1016/j.triboint.2019.105952 Received 19 June 2019; Received in revised form 23 August 2019; Accepted 4 September 2019 Available online 5 September 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic diagram of preparation process of PAO 6 with RGO/Fe3O4.
additive to improve tribological properties of low-viscosity base oil must be developed. In our work, reduced graphene oxide/Fe3O4 (RGO/Fe3O4) compos ites were fabricated by depositing Fe3O4 nanoparticles onto the surface of RGO. After modifying, RGO/Fe3O4 was dispersed in low-viscosity PAO 6. The tribological properties of RGO/Fe3O4 in low-viscosity PAO 6 were evaluated with CETR-UMT tribometer. The wear mechanism of RGO/Fe3O4 was also discussed.
2.2. Synthesis and characterization of RGO/Fe3O4 GO (15 mg) was dispersed in 45 mL of deionized water, ultrasoni cally treated for 1 h, and then poured into a 250 mL three-mouth flask. The solution was purged with nitrogen for 60 min. Then, 18 mg of FeCl3⋅6H2O and 28.5 mg of FeCl2⋅4H2O were dissolved in 7.5 mL of deionized water, and the iron ion solution was added into the flask while the mixed solution was vigorously stirred under nitrogen atmosphere for 7 h. An alkaline solution prepared by adding 0.795 g of NH3⋅H2O (25–28%) into 3.6 mL of deionized water was dripped into the flask. Afterward, the solution was heated to 65 � C and then continuously stirred for 2.5 h. After the reaction finished, RGO/Fe3O4 was acquired. A strong permanent magnet was used to collect the black products. The product was washed seven times with 15–20 mL deionized water to remove surplus reactants. Finally, the product was washed three times with ethanol to remove water. Fe3O4 nanoparticles were prepared in the same method as the RGO/Fe3O4 composites without GO. The morphological features of RGO/Fe3O4 were characterized by FETEM (JEM-2100F). Raman spectrometer (HR Evolution, HORIBA Jobin Yvon) and XRD (D/MAX2500V) were used to analyze the chem ical components of RGO/Fe3O4. The magnetization curve of RGO/Fe3O4
2. Experimental 2.1. Materials Graphene oxide (GO; lateral dimension about 5–10 μm), PAO 6, ashfree polyisobutylene succinimide dispersant (PIBSI), oleylamine (80–90%), ammonia solution (25–28%), FeCl2⋅4H2O (analytical re agent), FeCl3⋅6H2O (analytical reagent), deionized water and ethanol (analytical reagent) were purchased from commercially available reli able sources.
Fig. 2. Digital images of dispersion of the as-prepared oil samples with different mass fraction RGO/Fe3O4 within 24 h. 2
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Fig. 3. The UMT-2 tribometer, schematics of the motion mode of friction pairs, and record of the analysis conducted.
was measured as a function of the applied magnetic field H by MPMS (MPMS XL-7, Quantum Design). The hysteresis of the magnetization was obtained by varying H between þ10000 and 10000 Oe at 300 K.
lubricants containing five mass fraction additives (0.05, 0.1, 0.3, and 0.5 wt%) for friction experiments. Fig. 2 shows digital images of the as-prepared oil samples as a function of time. The dispersion results indicated that the stratification was not observed in all samples within 24 h. However, few particles could be observed at the bottom of the bottle with prolonged observation time because of gravity and the interaction between particles. Higher mass fraction indicated earlier precipitation. The samples with the mass fraction of 0.1, 0.3 and 0.5 wt % presented sedimentary particles at the bottom of the bottle after 24, 18, and 12 h, respectively. The sample with the mass fraction of 0.05 wt % remained stable after 24 h.
2.3. Preparation of PAO 6 with different contents of RGO/Fe3O4 In this paper, PAO 6 was selected in our initial experiments. Although the combination of RGO and Fe3O4 improved the dispersion in PAO6 to a certain extent than when GO or Fe3O4 alone was added into PAO6, the oil samples still lacked uniform dispersion and presented few precipitations within few hours. It is necessary to add some dispersants into PAO6 to further improve the dispersion of particles. PIBSI as a common dispersant has been widely used to solve the agglomeration of soot particles in the formulated lubricating oil. Therefore, refer to our previous work experience [26], we introduced moderate poly isobutylene succinimide to further solve the dispersion of RGO/Fe3O4 in PAO 6. Liquid transfer method was used to prepare PAO 6 with RGO/ Fe3O4. A schematic of the preparation of PAO6 with RGO/Fe3O4 is shown in Fig. 1. We modified the surface of RGO/Fe3O4 by using oleylamine to improve its dispersion stability in PAO6. The above pre pared RGO/Fe3O4 were dispersed into the surface modifier mixture with 50 mL of ethanol and 1.2 g of oleylamine by ultrasonic mixing for 10 min. The suspension was heated to 75 � C for 1 h. The final RGO/ Fe3O4 was acquired and collected using a strong permanent magnet and washed three times with ethanol to remove surplus oleylamine. Then the as-prepared modified RGO/Fe3O4 slurry was dispersed into 15 mL of ethanol and ultrasonically treated for 10 min. Before RGO/Fe3O4 sus pension were poured into PAO 6 containing PIBSI, 5 wt% of PIBSI was added into PAO 6 as a secondary surfactant. Then the mixture was stirred to evaporate ethanol at 85 � C. Until the ethanol was completely evaporated, RGO/Fe3O4 was dispersed stably into PAO 6. We prepared
2.4. Tribological tests and analysis Tribological tests were carried out on a CETR-UMT ball-disk trib ometer (UMT-2, Bruker). The upper ball (diameter ϕ6 mm, HV 647-861, Ra 0.016 μm) and the lower disk (ϕ28 � 2 mm, HRC 55-60, Ra 0.04 μm) were made of GCr15 steel. A schematic of the rubbing pairs is shown in Fig. 3. The upper ball was fixed, and the lower disk was reciprocated. The prepared lubricants were dripped in the contact interface at the beginning of the test. A 10 N load was applied to the upper ball. The reciprocating speed and stroke of the lower disk were 10 mm/s and 1 mm, respectively. The duration time was 30 min for each test. The testing temperature was kept at about 25 � C. In this friction test, the base oil was PAO 6 þ 5 wt% PIBSI. The testing oil samples include base oil, base oil þ (0.05, 0.1, 0.3, and 0.5 wt%) Fe3O4, base oil þ (0.05, 0.1, 0.3, and 0.5 wt%) GO, and base oil þ (0.05, 0.1, 0.3, and 0.5 wt%) RGO/ Fe3O4, where GO and Fe3O4 were selected alone as contrastive experi ment. Each experiment was repeated at least three times. The friction coefficient was recorded in real time automatically by the device, and the average friction coefficient was calculated after experiment. 3
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Fig. 4. TEM images of (a) GO, (b)Fe3O4, (c) RGO/Fe3O4, and (d)HRTEM image of RGO/Fe3O4.
In this paper, the lubrication regime under the selected testing con ditions could be estimated by calculating the parameter lambda λ, i.e. the ratio of estimated lubricant film thickness to composite surface roughness. The common classification of lubrication regime includes thin film/EHD, mixed and boundary lubrication, and their correspond ing lambda ratio are λ > 3, 0.5<λ < 3 and λ < 0.5 [27]. The lambda could be calculated according to the classical theory of Hamrock and Dowson. The calculated value of λ was 0.068 under the selected testing conditions, which is well below the limiting value of 0.5, indicating that the lubrication regime was mainly boundary lubrication during the wear tests. After friction, the disk was washed with ethanol for the following analyses. The wear volume of the disk was measured by 3D laser scan ning microscopy (LSM; VK-X100, Keyence). The morphology and elemental content of the worn area were analyzed by SEM (SU8010, Hitachi), LSM, Raman spectrometry (HR Evolution, HORIBA Jobin Yvon), and XPS (ESCALAB250, Thermo).
3. Results and discussion 3.1. Characterization of RGO/Fe3O4 The morphology and components of RGO/Fe3O4 were characterized by FETEM, as shown in Fig. 4. Fig. 4(a) shows the as-purchased GO. Fig. 4(b) shows the Fe3O4 nanoparticles prepared without adding GO into the reaction system. The diameter of Fe3O4 nanoparticles ranges from 5 nm to 10 nm. Fig. 4(c)–(d) show the RGO/Fe3O4 composites. Wrinkles of GO still existed after combining Fe3O4 nanoparticles. The HRTEM image of the RGO/Fe3O4 nanocomposites in Fig. 4(d) clearly shows that Fe3O4 nanoparticles uniformly were deposited on the surface of RGO sheets. Fig. 5(a) shows the XRD pattern of RGO/Fe3O4. Diffraction peaks at 2θ ¼ 30� , 35� , 43� , 53� , 57� , and 63� could be indexed to the (220), (311), (222), (400), (422), (511), and (440) planes of the magnetite structure of Fe3O4 nanoparticles, respectively [28]. The weak peak at
Fig. 5. Characterization of RGO/Fe3O4 component. (a) X-ray diffraction pattern; (b) Raman spectra with the laser wave length was 533 nm. 4
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23� could be attributed to the stacking of RGO [28,29]. The XRD results of RGO/Fe3O4 are consistent with previous reports [30]. Fig. 5(b) shows the Raman spectra of RGO/Fe3O4. The peaks at 510 and 670 cm 1 could be assigned to Fe3O4 nanoparticles [31]. Due to the existence of RGO, the characteristic peaks of Fe3O4 may slightly shift. Two Other peaks at 1350 and 1583 cm 1 corresponded to the D and G peaks of graphene, respectively, respectively [32]. The magnetic properties of RGO/Fe3O4 were also measured by MPMS. Fig. 6(a) shows the magnetic hysteresis loops of RGO/Fe3O4 as Sshaped curves. The magnetic induction intensity of the samples gradu ally increased with increasing external magnetic field strength until it reached the saturation magnetization (Ms). The Ms of RGO/Fe3O4 was about 20 emu/g. From insert map Fig. 6(b), As shown in the insert map of Fig. 6(b), the Mr was 0.72 emu/g, which is near 0 emu/g, when the external magnetic field strength decreased to 0 Oe, revealing super paramagnetic behavior. [33]. 3.2. Tribological properties of RGO/Fe3O4
Fig. 6. The magnetic hysteresis loops of RGO/Fe3O4. (a) The magnetization hysteresis loops of the RGO/Fe3O4; (b) Magnified image at a dotted line frame in Fig. 4(a). Saturation magnetization (Ms) was 20 emu/g, and the sample exhibits low remanence, Mr was 0.72 emu/g.
Fig. 7(a) shows the average friction coefficient of base oil with different concentration additives during the entire experiment. When pure Fe3O4 nanoparticles were added into base oil, they exhibited increased friction. A similar phenomenon has been reported in other
Fig. 7. (a) The average friction coefficient, (b) average wear volume of the lower steel disk and (c) average wear scar diameter of the upper steel ball lubricated by the base oil with different concentration additives. All tests were conducted with load of 10 N, reciprocating speed of 10 mm/s, reciprocating stroke of 1 mm, and testing time of 30 min. 5
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Fig. 8. Laser images of the upper ball, 3D images and wear profiles of the lower disk lubricated with the base oil ((a), (a)’ and (a)”), base oil with 0.1 wt% Fe3O4 ((b), (b)’ and (b)”), base oil with 0.1% wt% GO ((c), (c)’ and (c)”), and base oil with 0.1% wt% RGO/Fe3O4 ((d), (d)’ and (d)”).
papers [34,35]. This finding may be attributed to the fact that Fe3O4 nanoparticles tend to agglomerate, blocking the motion of friction pairs in the friction contact interface. However, both RGO/Fe3O4 and GO as additives can reduce the friction coefficient. This may be ascribed to the presence of sheet structure from GO and RGO, and the fact that friction pairs are relatively easy to slide [36]. The average friction coefficient of 0.1 wt% RGO/Fe3O4 was the lowest among the samples, and it was 7% lower than that of the base oil and has a very small error bar. Fig. 7(b) shows the average wear volume of steel disk lubricated with
base oil having different concentration additives during the experiment. When pure Fe3O4 nanoparticles were added into the base oil, the average wear volume increased with increasing content of Fe3O4 nanoparticles. The agglomerate Fe3O4 nanoparticles possibly acted as abrasive grains, aggravating the wear of friction pairs during friction. However, RGO/Fe3O4 showed better anti-wear properties than Fe3O4 and GO. The average wear volume of the steel disk lubricated using the base oil with 0.1 wt% RGO/Fe3O4 was 52.27% and 25.56% lower than those of the steel disks lubricated with the base oil and the base oil with 6
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Fig. 9. SEM images of wear scars of steel disk lubricated by the base oil (a), base oil with 0.1 wt% Fe3O4 (b), base oil with 0.1 wt% GO (c), and base oil with 0.1 wt% RGO/Fe3O4 (d).
0.1 wt% GO, respectively. The average wear volume of the steel disk initially decreased with increasing content of RGO/Fe3O4 and then increased when the content of RGO/Fe3O4 exceeded 0.1 wt%. This result indicates that the proper content can effectively improve the anti-wear properties of base oil, and this trend is consistent with other reports [36,37]. It may be ascribed to the fact that excessive RGO/Fe3O4 par ticles may cause agglomeration of particles and prevent the lubricant from entering the contact pair [37,38]. The optimized concentration of RGO/Fe3O4 is not a fixed value; it largely depends on the physical and chemical properties of the solid–body contact and contact stress [39]. Fig. 7(c) shows the average wear scar diameter of the upper steel ball lubricated by the base oil with different concentration additives. The trend of the wear scar diameter is similar to that of the wear volume. Adding pure Fe3O4 nanoparticles led to larger wear scar diameter. However, adding an appropriate mass fraction of RGO/Fe3O4 particles could effectively reduce the wear scar diameter. The average wear scar diameter of the steel ball lubricated using the base oil with 0.1 wt% RGO/Fe3O4 was 5.45% lower than that of the steel disk lubricated using the base oil. The addition of RGO/Fe3O4 particles possibly hindered the direct contact of friction pairs and then reduced the wear scar diameter.
For further analysis, SEM images of the wear scars on the steel disk lubricated with different oil samples were observed (Fig. 9). As shown in Fig. 9(a), furrows were dense and deep, and some peels presented on the worn surface lubricated by the base oil. The friction pairs were subjected to severe wear. Similarly, as shown in Fig. 9(b), when Fe3O4 nano particles were added in the base oil, the worn surface presented a large number of wide furrows. It may be ascribed to the agglomeration of Fe3O4 nanoparticles, leading to abrasive wear [34]. Although GO can partly improve the worn surface, a few wide furrows still existed (Fig. 9 (c)). Nevertheless, the number of furrows decreased markedly due to the existence of RGO/Fe3O4 in the base oil, and the worn surface became smoother, which indicated that the friction pairs were well protected (Fig. 9(d)). It may be attributed to the formation of tribo-film, which can separate friction pairs to avoid directly touching and then improve wear. 3.4. Friction and wear mechanism analysis The chemical compositions and elemental state of the worn surface were analyzed using Raman spectroscopy and XPS to investigate the reduce-wear mechanism of RGO/Fe3O4 as a lubricant additive on the worn surface. Raman spectra of wear scars of steel disk are shown in Fig. 10. As illustrated in Fig. 10(a), the Raman spectra of the wear scar showed peaks at 292, 305, 534, and 660 cm 1. The peaks at 305, 534, and 660 cm 1 can be assigned to Fe3O4, and the peak at 292 cm 1 is assigned to α-Fe2O3 [31]. This result indicates that the composition of the worn surface is mainly iron oxides, which can be attributed to some of the debris originated from the substrate during the friction process were oxidized [42]. As shown in Fig. 10(b), when the Fe3O4 nanoparticles were added into the base oil, the Raman spectra of worn surface shows more peaks of iron oxides, which possibly originated from the Fe3O4 nanoparticles. Fig. 10(c) shows the Raman spectra of the wear scars lubricated with the base oil containing GO. The Raman spectra showed the same peaks of iron oxides as lubricated by the base oil. In addition to the peaks of iron oxides, the D-peak at 1390 cm 1 and G-peak at 1583 cm 1 were observed, indicating that GO participated in the
3.3. Morphology of worn surface Fig. 8 shows the laser images of the upper ball, 3D images, and wear profiles of the lower disk lubricated with different oil samples. With the aid of Multifile Analyzer software matched with LSM, the profile of the wear mark can be obtained [40,41] the average wear area was obtained by calculating the average area of hundreds of cross sections starting from the middle of the wear scar. As shown in Fig. 8 (a)”, the wear profiles of the steel disk lubricated with base oil were very deep, and the surface roughness (Ra) of the wear scars reached 0.408 μm. As shown in Fig. 8 (b)”, the addition of Fe3O4 did not reduce the wear of friction pairs. However, when the as-prepared RGO/Fe3O4 nanoparticles were added in the base oil, the wear area of the wear scars evidently decreased, and the surface of the wear scars bacame smoother than that of the steel disk lubricated with base oil according to the value of Ra. 7
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Fig. 10. Raman spectra of wear scars lubricated by the base oil (a), base oil with 0.1 wt% Fe3O4 (b), base oil with 0.1 wt% GO (c), and base oil with 0.1 wt% RGO/ Fe3O4 (d).
formation of tribofilm on the worn zone [43,44], which may be reason for reducing wear. Fig. 10(d) shows the Raman spectra of the wear scars lubricated with the base oil containing RGO/Fe3O4. Compared with Fig. 10(b), the peaks of iron oxides were greater in the Raman spectra of wear scars lubricated with the base oil containing RGO/Fe3O4 than in those of the wear scars lubricated with the base oil containing Fe3O4 in Fig. 10(d). It may be ascribed to Fe3O4 nanoparticles tend to agglom erate, leading to severe abrasion, as shown in the SEM images from Fig. 9(b), and preventing the iron oxide films to exist stably. Compared to Fig. 10(c), the D-peak and G-peak also appeared in Fig. 10(d). Interestingly the G-peak became weaker and the D-peak became stron ger than the Raman spectra of wear scars lubricated with the base oil containing GO. It may be ascribed to the fact that the existence of Fe3O4 on the GO surface accelerated the transformation from G-peak to D-peak during the testing process, which is consistent with the result reported by Huang [45]. Besides, the stronger and more peaks of iron oxides in the range of 200–700 cm 1 may be ascribed to the presence of Fe3O4 nanoparticles on RGO, which promoted the formation of more iron oxide films. The Raman spectra confirmed that the GO particles could form graphene–structure tribofilm, while the RGO/Fe3O4 composites could promote the formation of iron-oxide and graphene-structure tri bofilms on the worn surface because of the existence of Fe3O4 nano particles. The tribological results showed that the friction and wear
markedly decreased with the RGO-Fe3O4 synergistic tribofilm on the worn surface. We further investigated the differences in lubrication mechanism between nonmagnetic GO and magnetic RGO/Fe3O4. The worn surfaces lubricated by the base oil with 0.1 wt% GO and RGO/Fe3O4 were analyzed by XPS, respectively, and the worn surface lubricated by the base oil was also measured by XPS as a control. Fig. 11(a) and (e) and 11 (i) show the survey spectra of the wear scars, where the peaks at 711, 285, and 532 eV corresponded to Fe2p, C1s, and O1s [45]. Comparison of element content shows that when GO was added into the base oil, C content of the worn surface markedly increased from 47.42% to 55.6% compared with that of the base oil. Interestingly, when RGO/Fe3O4 was added into the base oil, the contents of C and Fe were higher than those of the base oil, and the content of Fe was higher than that of the base oil with GO. This is the result of a good synergy between Fe3O4 nano particles and RGO. The C1s spectra in Fig. 11(b), (f), and 11(j) showed three peaks at 284.7, 285.5, and 288.5 eV, which corresponded to C–C or – O or O–C–O, respectively [46]. Fig. 11(c), (g) and 11 C–H, C–O, and C– (k) show the O1s spectra. The peaks at 529.8, 531.2, and 532.5 eV may – O, respectively [44,45]. The Fe2p be belonged to Fe–O, C–O, and C– spectra in Fig. 11(d), (h), and 11(l) showed five peaks at 706.9, 710.3, 711.7, and 724.5 eV, which belonged to Fe (0) simpler substance, Fe2þ(2p3/2), Fe3þ(2p3/2), and Fe(2p1/2), respectively. [45–48]. By 8
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Fig. 11. XPS analysis of the worn surfaces lubricated by the base oil (a–d), base oil with 0.1 wt% GO (e–h), and base oil with 0.1 wt% RGO/Fe3O4 (i–l).
comparing Fig. 11(h) with 11(l), the calculation of the area of the peak at 710.3(A1) and 711.7(A2) eV showed that the area ratio is 0.68 when the worn surface was lubricated by the base oil with GO. This value is close to the area ratio (0.56) when the worn surface was lubricated by the base oil. However, when RGO/Fe3O4 was added into the base oil, the area ratio significantly increased to 1.02 compared with that when the base oil with GO was used, which indicates that the proportion of Fe2þ increases. This result may be attributed to the presence of Fe3O4 nano particles on RGO. This is consistent with Zhao’ work [35]. The results of XPS confirmed that Fe3O4 nanoparticles and RGO jointly participate in the formation of tribofilm. Due to the presence of Fe3O4 nanoparticles on RGO, the RGO-Fe3O4 synergetic tribofilm formed to protect the friction pairs, and shown better tribological properties than that of without Fe3O4 nanoparticles. These results are consistent with the Raman spectra of wear scars. The above analysis results indicated that the anti-wear mechanism of RGO/Fe3O4 may be ascribed to the formation of the RGO–Fe3O4 syn ergistic tribofilm on the worn surface during the friction process, which separates friction pairs like a protective film to reduce wear. The spec ulation of the formation of the synergistic tribofilm is as follows. On the one hand, during the friction process, the applied load produced me chanical shear stress at the friction interface, causing a part of Fe3O4 nanoparticles to exfoliate from the RGO/Fe3O4 composite. The exfoli ated Fe3O4 nanoparticles can partly enter into furrows to repair the worn surface. Thus, the synergistic tribofilm formed on the worn surface because of the synergistic effect of RGO and Fe3O4 originating from the RGO/Fe3O4 composites. On the other hand, the RGO/Fe3O4 composites were capable of magnetic response under an external magnetic field, as indicated by the measurement of RGO/Fe3O4 by MPMS. However, when
the external magnetic field strength decreased to 0 Oe, the Mr was only 0.72 emu/g, which is near 0 emu/g, showing superparamagnetic behavior. This characteristic redered the particles themselves not magnetically attractive in the absence of an external magnetic field. Mishina [49,50] reported that weak magnetism could be generated between friction pairs because of the reciprocating movement of steel-material friction pairs under dry friction condition. Frictional magnetism is a common phenomenon. Considering the magnetic response of RGO/Fe3O4 composites, the formation of the RGO–Fe3O4 synergistic film may benefit from the interaction between the magnetic response of the RGO/Fe3O4 composites and the magnetism from friction pairs. Although Fe3O4 is also magnetic, nanoparticles tend to agglom erate, leading to abrasive wear, and then preventing the iron oxides films to exist stably. 4. Conclusion RGO and Fe3O4 were synthesized into the magnetic RGO/Fe3O4 composites successfully by chemical co-precipitation method. RGO/ Fe3O4 was dispersed into PAO 6. Comparison of the tribological prop erties of PAO 6 with Fe3O4, GO, and RGO/Fe3O4 revealed that PAO 6 with Fe3O4 nanoparticles showed worse tribological behavior than pure PAO 6. However, PAO 6 with the RGO/Fe3O4 composites showed better tribological properties than those with GO and Fe3O4 nanoparticles. The average wear volume of steel disk decreased by 52.27% after adding 0.1 wt% RGO/Fe3O4. The anti-wear mechanism was ascribed to the formation of the tribofilm originating from the synergistic effect of RGO and Fe3O4 in the RGO/Fe3O4 nanocomposites.
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Acknowledgments
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