Probing the Function of Solid Nanoparticle Structure under Boundary

Research Article pubs.acs.org/journal/ascecg

Probing the Function of Solid Nanoparticle Structure under Boundary Lubrication Xiaoqiang Fan,*,† Wen Li,† Hanmin Fu,† Minhao Zhu,*,†,‡ Liping Wang,§ Zhenbing Cai,‡ Jianhua Liu,‡ and Hao Li† †

Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, 111#, Section of the Northbound 1, Second Ring Road, Chengdu 610031, China ‡ Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China § Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China ABSTRACT: Understanding the fundamental function of solid nanoparticle structure on boundary lubrication is of great significance. Here we prepared a series of solid naoparticles including lamellar carbon and molybdenum disulfide (MoS2), spherical MoS2 and carbon, graphene-like C-MoS2 composite, and graphene quantum dots (GQDs), and investigated their tribological properties and mechanism under boundary lubrication in detail. The experimental characterization and analysis found that the spherical nanoparticles can reduce friction and wear by 40% and 80%, depending on the “third body” composed of these nanoparticles and the frictioninduced nano-onion debris in the contact area and an easily shearing film formed by the exfoliated nanoslices on the sliding surfaces. Smaller nanosize GQDs allow the friction and wear to be reduced by up to 60% and 91%, which is attributed to the synergistic effect of a densely protective film on the sliding surfaces and the graphene-like debris in the contact area. KEYWORDS: Solid nanoparticles, Size and shape, Lubricant additive, Tribology property

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INTRODUCTION The rapid development of nanoscience and technology has stimulated an unprecedented effort to prepare nanomaterials with a controlled size and shape at affordable prices.1 Numerous possible applications have been implemented for these nanomaterials to obtain the desired performance. Various nanoparticles as potential lubricant additives have been experimentally evaluated through comparison analysis of their tribological behaviors. Some nanoparticles have been successfully employed in mechanical equipment to reach the required performance of high reliability and long service life. More importantly, increasing transportation and intensive industrial activities have consumed large amounts of energy resources and generated a large amount of CO2 and other exhaust gas, which friction accounts for a surprising proportion in energy consumption and waste emissions.2−4 So far, the most popular approach to control or reduce friction is to use the lubricant with excellent tribological behaviors. Solid nanoparticles as lubricant additives are very important to reduce friction and wear between the sliding surfaces. Many researchers have been committed to exploring the tribological performance of solid nanoparticles with customized size and shape, while understanding the friction mechanisms. For © 2017 American Chemical Society

example, metal nanoparticles (Cu, Ag, Fe, Co) and metal compounds (CuO, ZrO2, ZnO, La2O3, MgB2, ZnB2, LaF3, calcium sulfonate with different structures, and so forth) as lubricating additives provide excellent tribological behaviors because a protective film has been formed through deposition and/or tribo-sintering of nanoparticles on the worn surfaces, which enhances the tribological performance and load-carrying capacity.5−13 Molybdenum disulfide (MoS2), tungsten disulfide (WS2), and their fullerene-like structures have been regarded as potential solid lubricants because these materials are readily shearing between the sliding surfaces for reducing friction and wear.1,14−20 SiO2/MoS2 hybrid nanoparticles as lubricating additives give the excellent tribological properties due to the synergistic effect of nano-SiO2 and nano-MoS2.21 Furthermore, particles with a thin-layer structure including graphene and its derivatives (fullerene, carbon nanotubes, and nano-onions) can offer the excellent tribological properties due to high mechanical strength, easy-shearing capability, and good chemical inertness.4,22−29 Received: January 19, 2017 Revised: March 31, 2017 Published: April 10, 2017 4223

DOI: 10.1021/acssuschemeng.7b00213 ACS Sustainable Chem. Eng. 2017, 5, 4223−4233

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ACS Sustainable Chemistry & Engineering

Friction under a High Applied Load. The tribological performance of the lubricants was evaluated by an Optimal−SRV−IV reciprocation friction tester. An AISI 52100 steel ball with diameter of 10 mm, hardness of 710 HV, and surface roughness of 50 nm as the fixed upper specimens slides reciprocally against the stationary AISI 52100 steel substrates with Φ24 × 7.9 mm, hardness of 620 HV, and surface roughness of 80 nm at the applied load of 100 N (contact pressure of 1.405 GPa), frequency of 25 Hz with an amplitude of 1 mm, and room temperature (25 °C) for 60 min. About 0.02 mL of lubricant was added to the ball−disc contact area by microsyringe. During the friction process, a computer connected to the Optimal−SRV−IV tester recorded the friction coefficient as a function of time. The loadcarrying capacity of the lubricants was also investigated on the same friction tester with a load ramp test from 50 to 600 N stepped by 50 N under room temperature (25 °C), and the test duration for each load was 5 min. After the friction ends, the lubricants on the contact area were collected to analyze the structure of nanoparticles. The substrates were washed ultrasonically using petroleum ether for several times and then were dried with pure nitrogen. All friction experiments were conducted three times under the same conditions for ensuring the repeatability and reliability. Characterization and Analysis. Morphologies of the nanoparticles before and after friction were analyzed by an FEI Tecnai F300 transmission electron microscope (TEM) with an accelerating voltage of 300 kV. Raman spectra of these nanoparticles and the wear tracks were obtained by a Renishaw inVia Raman microscope with 532 nm laser excitation. Morphologies of worn surfaces and the related wear volume on the substrates were measured by a MicroXAM-3D surface mapping microscope profilometer. PHI-5702 multifunctional X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical states of the typical elements on the wear tracks. A pass energy of 29.3 eV determined the binding energies of the target elements, with the resolution of around ±0.3 eV, and the binding energy of oxygen (O 1s: 531.0 eV) as the reference.

Exploring the role of nanoparticles in lubricant and the friction mechanism is of great significance. The friction mechanism of nanoparticles in lubricant has been proposed according to a large number of experimental characterization and analysis: (a) ball bearing effect, playing a role of ball bearing between the friction pairs; (b) mending effect, depositing on the worn surface and compensating for the worn loss; (c) polishing effect, reducing the roughness of the friction surface through nanoparticle abrasion; (d) protective film as the key mechanism, serving as spacers or third body material for preventing the friction surfaces from straight asperity contact.30−32 In view of these friction mechanisms, Kalin et al.18 evaluated the tribological properties of seven different size particles including MoS2 platelets (2 and 10 μm), WS2 fullerene-like nanoparticles, MoS2 and WS2 nanotubes, graphite platelets (20 μm), and multiwalled carbon nanotubes as additives in polyalphaolefin (PAO) oil. They concluded that the tribological performance mainly depends on the nature of nanoparticles rather than their size and morphology. Alazemi et al.33 demonstrated that an ultrasmooth submicrometer carbon sphere as lubricating oil additive significantly improves the tribological performance because carbon spheres as lubricating additives filled in the gap between the sliding surfaces and did well as a nanoball bearing. However, the information on the tribological properties of nanoparticles with smaller size and other morphology is rare, and the comparative analysis of nanoparticles before and after friction is still missing. So, of primary interest in exploring the friction mechanism of nanoparticles is to observe their morphology and structure before and after friction. In this paper, a series of MoS2- and carbon-based nanoparticles with different sizes and shapes were prepared including lamellar MoS2, spherical MoS2 with the average particles size of 305.3 nm and carbon with the average particles size of 279.9 nm, graphene-like C-MoS2 composite, graphene, and graphene quantum dots. The tribological properties of these nanoparticles added into multialkylated cyclopentanes (MACs) were evaluated in detail under low and high applied loads, and friction mechanisms were also analyzed by X-ray photoelectron spectra and Raman spectra on the worn surfaces as well as transmission electron microscopy images of nanoparticles before and after friction.

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RESULTS AND DISCUSSION Characterization Results of Solid Nanoparticles. TEM was performed to reveal the structure of the nanoparticles. Figure 1a clearly shows the thin lamellar structure of MoS2; its TEM image with the selected area electron diffraction (SEAD) in Figure 1b displays that the periodic atom arrangement of MoS2 is a highly crystalline.38 The TEM images in Figure 1c and 1d present spherical structures of MoS2 (327 nm) and carbon (306 nm), respectively. HRTEM of the carbon sphere in Figure 1e illustrates that it is amorphous. Figure 1f shows a transparent graphene with some wrinkles and curvature on the edges; the inset displays the SEAD, suggesting that it is multilayer graphene because the electron diffraction pattern of single layer graphene with 6-fold symmetry is hexagonal.24,26 Figure 1g shows the TEM image of GQDs with an average size of 3 nm; the TEM inset illustrates that GQDs with a lattice parameter of 0.242 nm have the degree of high crystallinity.37 The TEM image with the EDS and SEAD inset in Figure 1h reveals that the C-MoS2 composite is comprised of lamellar MoS2 dispersed in amorphous carbon and is a graphene-like structure.36 Raman spectroscopy as an important analysis technique has been employed to probe the Raman mode of MoS2. Four firstorder Raman active modes have been reported in the previous studies, namely E1g (286 cm−1), E2g (383 cm−1), A1g (408 cm−1), and E22g (32 cm−1).38,39 Figure 2a provides Raman spectra of MoS2 samples; two characteristic Raman peaks are present at 383 and 408 cm−1 due to the S−Mo−S layer vibrational modes. Graphene-based materials have characteristic bands including the G band at 1580 cm−1 due to in-plane vibration of sp2 carbon atoms, the 2D band at 2670 cm−1 from

MATERIALS AND METHODS

Materials. Lamellar MoS2 and graphene were purchased from Beijing DK Nano Technology Co. Ltd. (Beijing, China). Spherical MoS2 and carbon, graphene-like C-MoS2 composite sheets, and graphene quantum dots (GQDs) were prepared by the previously reported procedure.34−37 MACs were commercially obtained by the Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS, Lanzhou, China). In this study, 2 wt % solid nanoparticle was dispersed homogeneously in MACs by sonication for 20 min. All chemical reagents were of analytical grade and used without further purification. Tribological Tests. Friction under a Low Applied Load. The tribological behaviors of the lubricants were evaluated by the reciprocating UMT−2MT sliding tester. AISI steel balls with a diameter of 4 mm and surface roughness of 50 nm as the stationary upper counterparts were pressed against the lower specimens with a traveling distance of 5 mm. The friction tests were carried out at a sliding frequency of 5 Hz, an applied load of 20 N (contact pressure of 1.513 GPa), a test duration of 60 min, and room temperature. After the friction test, the friction pairs were washed several times using petroleum ether and then were dried with pure nitrogen. 4224


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Two Mo 3d peaks appear at 288.8 and 231.9 eV, while the S 2p peak appears at 162.0 eV, illustrating that the MoS2 compound was generated.34 Combined with TEM images and Raman and XPS spectra of these nanoparticles, it illustrates that the nanoparticles with different structures have been prepared including spherical MoS2 and carbon, graphene-like C-MoS2 composite, and smaller GQDs. Tribological Properties under Low/High Applied Loads. To explore the effect of nanoparticles’ structure on the tribological performance of lubricant and the relationship between the tribological performance and the magnitude of the applied load, the tribological behaviors of as-prepared nanoparticles as MACs additives were evaluated for steel/steel contact under low and high applied loads. Figure 4 gives the average friction coefficient values and wear volume of MACs with 2.0 wt % nanoparticles, with contrast to neat MACs at 20 N and a sliding speed of 300 r/min for 60 min. The average friction coefficient values of MACs with additives and pure MACs are essentially the same (Figure 4a) because their realtime friction curves nearly overlap, whereas the addition of nanoparticles reduces the wear volume by 10% (Figure 4b) because the nanoparticles can play a positive role in mending and protecting sliding surfaces. Figure 5a and 5b present the friction curves as a function of time and wear volume of the substrates under lubrication of MACs with nanoparticles. By contrast, the addition of nanoparticles improves the tribological behaviors at a high applied load. Graphene-like C-MoS2 composite and spherical nanoparticles like MoS2 and C spheres provide much smaller and more smooth friction curves (∼0.15 to 0.18) as well as lower wear volume (∼8.8 × 105 μm3) than MoS2 and graphene. In particular, GQDs display the smallest and the most stable friction coefficient (0.1) as well as the lowest wear volume (∼3.0 × 105 μm3) than other additives. Figure 5c gives the extreme pressure performance of MACs with additives. The load carrying capacity of MACs was enhanced up to 400 N by adding nanoparticles, and GQDs provide higher load carrying capacity up to 500 N. The 3D topography and SEM images of the worn surfaces in the lower disks at a high applied load are observed in Figure 6 and Figure 7. Compared with these SEM images, the worn surfaces lubricated by nanoparticles hybrid oils have smaller wear bars and narrower wear tracks with relatively obvious bulges in the sliding direction, and the wear surfaces have formed the protective layers derived from nanoparticle additives. In particular, a dense protective film was formed from GQDs, which significantly enhances the tribological performance. These images gain an insight into the distribution of “third body” and wear debris. Combined with the results of friction tests under low and high applied loads, it can be confirmed that the structure of nanoparticles has a significant impact on the tribological performance of the lubricant under a high applied load. In general, lubricating oil can reduce friction and wear in the running-in stage because a thin oil-film was formed on the sliding surfaces, whereas the thin oil-film could be damaged with the increase of applied load and friction heat. In this case, the lubricating additives will play an effective role in improving the tribological behaviors through the synergy of physical adsorbed film and chemical reaction film. For example, nanoscale MoS2 and C spheres can act as third body particles filling the gap between contact surfaces and may possibly play a role of ball bearing, which significantly improves the tribological performance of MACs with spherical nanoparticles. MACs with

Figure 1. TEM and HRTEM micrographs of the solid nanoparticles: (a, b) MoS2, (c) MoS2 spheres, (d, e) carbon spheres, (f) graphene, (h) graphene-like C-MoS2 composite, and (g) graphene quantum dots (GQDs).

the two-phonon double-resonance Raman scattering process, and the D band from the introduction of defects.40,41 Figure 2b shows the Raman spectra of graphene, graphene-like C-MoS2, and spherical carbon. The D band locates at 1332 cm−1, which is ascribed to the disordered nature of carbon-based materials and the existence of some structure defects. The sharp G band at 1580 cm−1 is attributed to the sp2 carbon atoms in hexagonal lattice with E2g symmetry.35 For graphene-like C-MoS2 composite, the Raman features of MoS2 are affected by those of carbon because MoS2 dispersed uniformly in carbon. Figure 2c presents the Raman spectrum of GQDs; the peaks appear at 1363 and 1604 cm−1 due to the D and G bands, respectively. The D and G peaks show an obvious blue shift compared with graphene, suggesting that some oxygen-containing groups have been formed on the edges and the basal plane. As seen in Figure 2d, the XPS spectrum of GQDs demonstrates the existence of CC, C−O, CO, and COOH bonds. Mo 3d and S 2p XPS peaks of the as-prepared MoS2 spheres and graphene-like C-MoS2 composite are shown in Figure 3. 4225


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Figure 2. Raman spectra of nanoparticles: (a) lamellar MoS2 and MoS2 spheres, (b) C-MoS2 composite, carbon spheres, and graphene, (c) GQDs, and (d) XPS spectrum of C 1s on GQDs.

Figure 3. X-ray photoelectron spectra of the elements (Mo 3d and S 2p) on MoS2 spheres and C-MoS2 composite.

GQDs show the best tribological performance because GQDs can more readily penetrate into the sliding surfaces and form a dense protective film and graphene-like wear debris for enhancing the tribological ability. Analysis of Friction Mechanism. Understanding the interaction between the worn surfaces and nanoparticles and the role of nanoparticles in the process of friction testing is of fundamental importance. To explore the surface enhancement effect of nanoparticles, XPS spectra of typical elements on the wear tracks were detected to confirm their chemical states (Figure 8). The O 1s peak appears at 529.4 eV, indicating that oxidation products have been generated on the worn surface. XPS spectra of Fe 2p display the peaks at 709.9 eV, so

illustrating that Fe2O3 was generated. Observing the XPS spectra of C 1s, they give a symmetrical peak with high intensity at 284.2 eV, which is assigned to carbon derived from C-ball, CMoS2 composite, graphene, and GQDs nanoparticles.42 Compared with XPS spectra of original MoS2-based nanoparticles, two Mo 3d peaks locate at 228.1 and 231.0 eV, and the S 2p peak is at 162.0 eV, which is related to the presence of MoS2 on the worn surface from MoS2-ball.43 Surprisingly, the weak O 1s peak on the wear track lubricated MACs with GQDs locates at 531.4 eV, and the Fe 2p peak has not been detected, which may be attributed to a dense carbon layer derived from GQDs on the friction pairs. To further confirm the composition of the wear track lubricated by MACs with 4226


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Figure 4. Average friction coefficient values (a) and wear volume (b) of the lower disks lubricated by MACs and MACs with solid nanoparticles at a low applied load of 20 N (1.513 GPa) and room temperature (RT: 25 °C).

Figure 5. Friction curves (a) and wear volume (b) of the lower disks lubricated by MACs and MACs with solid nanoparticles at a high applied load of 100 N (1.405 GPa) with room temperature (25 °C) as well as extreme pressure properties (c) of MACs with solid nanoparticles at room temperature (25 °C).

the mating surfaces from straight asperity contact for improving the tribological performance. To further explore the friction mechanism of nanoparticles, the structural changes of nanoparticles debris from the worn surfaces under the high applied load were observed by TEM. TEM micrographs of the wear debris from the nanoparticles are shown in Figure 9. Compared with TEM images of the original MoS2 platelet (Figure 1a and 1b), a relatively thick wear fragment from the platelet is displayed (Figure 9a), and its HRTEM image with the SEAD inset (Figure 9b) illustrates that the MoS2 flakelike wear debris is a microcrystalline structure with the Fe nanocrystals on the basal plane or in the interlayer. Several typical features are observed on the TEM micrographs of spherical MoS2 debris (Figure 9c and 9d): (a) spherical structure transforming into fragment and (b) these fragments with onionlike crystal structure. Thus, structural change of spherical MoS2 participating in the friction process is attributed

GQDs, its wide and strong Raman spectrum was observed at 1516 cm−1, indicating that a high-density and easily shearing protective layer from GQDs has been formed on the sliding surfaces. Taking into account the XPS and Raman results of the worn surface lubricated by MACs with nanoparticles, we confirm that the solid nanoparticles can readily adsorb and pile up along the sliding direction to mend wear and enhance the surface, thereby significantly improving the tribological properties. Solid particles additives fill a niche for reducing friction and wear in situations where fluid lubricants are insufficient to realize the desired performance for mechanical elements. Liquid lubricants afford a good lubrication effect on the running-in process and mild friction conditions because of the formation of a thin oilfilm on the friction pairs.9 Under harsh conditions, the additives as third body are directly active in the contact area to prevent 4227


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Figure 6. 3D topography of the worn surfaces lubricated by MACs with nanoparticles at a high applied load of 100 N.

to the friction-induced effect. Figure 9e and 9f display TEM images of C-ball debris, illustrating that the amorphous carbon spheres have been exfoliated along the sliding direction and transformed into an onionlike crystal structure with a multilayer due to the friction. These onionlike nanoslices have ordered structure on the edge and disordered structure in their concentric shells because the upward and downward parts of nanospheres were increasingly uncovered and contacted

between the friction pairs with layer upon layer peeling off in the friction process. The good tribological performance of the MoS2 and carbon spheres depends on their chemical inertness, ball bearing effect, deformation, and exfoliation as well as transfer of fragments on the contact area. In the friction process, the friction-induced nano-onion structures with curved edge can readily slide and even roll in the contact area, and the exfoliated nanoslices from nanospheres or the onionlike 4228


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Figure 7. SEM images of the worn surfaces in the lower disks lubricated by MACs and MACs with MoS2 sheets, spherical MoS2, and carbon as well as GQDs.

structures are flexible and can easily adhere on the friction pairs, which function as effective lubrication.28,44,45 Meanwhile, the friction-induced onionlike wear debris with a highly ordered curved structure can enhance their stability and preserve the lamellar structure longer, which is possible to enhance the tribological properties.46 TEM images in Figure 9g and 9h give the MoS2 short-range structure with amorphous carbon. It is observed from the wear debris of graphene-like C-MoS2 composite that high-dispersed and twisted MoS2 sheets with variable length in amorphous carbon have been generated by friction.47 Figure 9i and 9j present the TEM and HRTEM micrographs of the wear debris from graphene; it is found that friction led to the remarkable increase of the thickness and the appearance of a sandwich structure comprised of Fe nanocrystals embedded in the graphene basal plane or interlayer.48 Figure 9m and 9n display the TEM images with the HRTEM inset of wear debris from GQDs. These images give a rare insight into “ganoderma lucidum cloud” morphology, illustrating that a large number of GQDs have participated in the friction process due to smaller nanosize and were stacked together to form the so-called “ganoderma lucidum cloud” due to friction. In order to further confirm the structure of “ganoderma lucidum cloud”, its Raman spectrum is shown in Figure 10, observing that its Raman features appear at 1354 cm−1 (D band), 1581 cm−1 (G band), and 2720 cm−1 (2D band). These Raman features are consistent with those of graphene, illustrating that the wear debris with “ganoderma lucidum cloud” morphology is a graphene-like structure.49 The excellent tribological performance of GQDs mainly depends on its significant advantage: (a) more easy and better dispersion in lubricant; (b) intensive involvement in friction by readily absorbing on the sliding surfaces; and (c) formation of a

graphene-like structure by friction-induced structural transformation. Combined with friction results of solid nanoparticles, we conclude that the tribological performance of solid nanoparticles with similar chemical composition is determined by the structure and size (The spherical nanoparticles possess better tribological ability than the lamellar ones, and GQDs possess the best tribological performance.). On the one hand, taking into account the surface/interface analysis results including 3D topography and XPS and Raman spectra of the worn surfaces, the good tribological behaviors of the solid nanoparticles mainly depend on the surface enhancement effect because they or their exfoliated nanoslices by friction can easily adsorb and deposit on the rough mating surfaces to compensate for wear and form an easily shearing film. On the other hand, compared with the structural changes of nanoparticles before and after friction, the improvement of tribological performance is attributed to the existence of third body composed of the nanoparticles and their wear debris between contact area. For example, spherical nanoparticles as third body fill between surfaces asperities to play a role of ball bearing, increasing real contact area for reducing contact pressure.33 In the friction process, the friction-induced nano-onion debris with highly ordered curved edge and the exfoliated nanoslices from solid nanoparticles can readily slide and even roll in the contact area and can easily adhere on the friction pairs to form an easily shearing film, which functions as effective lubrication. In particular, GQDs provide excellent tribological behaviors depending on the significant advantage of smaller nanosize and the formation of graphene-like wear debris. In short, solid nanoparticles with obviously different size and morphology provide substantially different tribological properties, depend4229


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Figure 8. X-ray photoelectron spectra of the elements (O 1s, Fe 2p, C 1s, Mo 3d, and S 2p) on the worn surfaces under lubrication by MACs with solid nanoparticles and Raman spectra of the worn surface lubricated by MACs with GQDs.

worn surfaces as well as TEM of wear debris. The conclusions are drawn as follows: (a) Solid nanoparticles with the same chemical composition exhibit different tribological behaviors at high applied loads, arising from the differences in morphology and size. Spherical nanoparticles show better tribological properties than the lamellar ones, and GQDs provide the best tribological behaviors. (b) The spherical nanoparticles improve the tribological performance of lubricant, depending on the synergies of an easily shearing film on the sliding surfaces and the third body in the contact area. The spherical nanoparticles and the frictioninduced derivatives including nano-onion debris with highly ordered curved edge and the exfoliated nanoslices can act as a

ing on the nature of nanoparticles and the friction-induced third body to form an easily shearing protective film for preventing the mating surfaces from straight asperity contact, thereby improving the tribological performance.

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CONCLUSIONS

To understand the effect of solid nanoparticles with different size and morphology on the tribological performance under boundary lubrication conditions, the tribological properties of various carbon and MoS2 nanoparticles including lamellar MoS2 and graphene, spherical MoS2 and carbon, graphene-like CMoS2 composite, and GQDs as additives in MACs were investigated in detail under low and high applied loads. Friction mechanisms were explored by XPS and Raman spectra of the 4230


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Figure 9. TEM and HRTEM micrographs of wear debris derived from solid nanoparticles after friction: (a, b) MoS2, (c, d) MoS2 spheres, (e, f) carbon spheres, (g, h) graphene-like C-MoS2 composite, (i, j) graphene, and (m, n) graphene quantum dots (GQDs).

(d) The significantly improved tribological performance of GQDs is ascribed to the synergy of a dense protective film formed on the sliding surfaces and the friction-induced graphene-like debris in the contact area, thereby significantly reducing the friction and wear.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-028-87601282, +86-0574-86325713. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaoqiang Fan: 0000-0001-7101-6577

Figure 10. Raman spectrum of the wear debris derived from GQDs after friction.

Notes

The authors declare no competing financial interest.

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third body and easily adhere on the friction pairs, which functions as effective lubrication. (c) The friction-induced transformation of amorphous carbon spheres into a highly ordered onionlike structure is beneficial to enhance the tribological performance because the presence of highly ordered curved planes could enhance the stability of the wear debris and preserve the structure longer in the contact area.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support provided by China National Funds for Distinguished Young Scientists (No. 51025519), the Changjiang Scholarships and Innovation Team Development Plan (No. IRT1178), and the National Natural Science Foundation of China (No. 51627806). 4231


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(22) Berman, D.; Erdemir, A.; Sumant, A. V. Few Layer Graphene to Reduce Wear and Friction on Sliding Steel Surfaces. Carbon 2013, 54, 454−459. (23) Eswaraiah, V.; Sankaranarayanan, V.; Ramaprabhu, S. Graphenebased Engine Oil Nanofluids for Tribological Applications. ACS Appl. Mater. Interfaces 2011, 3, 4221−4227. (24) Liu, X.; Pu, J.; Wang, L.; Xue, Q. Novel DLC/Ionic Liquid/ Graphene Nanocomposite Coatings towards High-vacuum Related Space Applications. J. Mater. Chem. A 2013, 1, 3797−3809. (25) Fan, X.; Wang, L.; Li, W. In Situ Fabrication of Low-Friction Sandwich Sheets Through Functionalized Graphene Crosslinked by Ionic Liquids. Tribol. Lett. 2015, 58, 12. (26) Fan, X.; Wang, L. High-performance Lubricant Additives Based on Modified Graphene Oxide by Ionic Liquids. J. Colloid Interface Sci. 2015, 452, 98−108. (27) Tenne, R. Inorganic Nanotubes and Fullerene-like Nanoparticles. J. Mater. Res. 2006, 21, 2726−2743. (28) Joly-Pottuz, L.; Vacher, B.; Ohmae, N.; Martin, J. M.; Epicier, T. Anti-wear and Friction Reducing Mechanisms of Carbon Nano-onions as Lubricant Additives. Tribol. Lett. 2008, 30, 69−80. (29) Peng, Y.; Hu, Y.; Wang, H. Tribological Behaviors of Surfactantfunctionalized Carbon Nanotubes as Lubricant Additive in Water. Tribol. Lett. 2007, 25, 247−253. (30) Lee, K.; Hwang, Y.; Cheong, S.; Choi, Y.; Kwon, L.; Lee, J.; Kim, S. H. Understanding the Role of Nanoparticles in Nano-oil Lubrication. Tribol. Lett. 2009, 35, 127−131. (31) Bakunin, V. N.; Suslov, A. Y.; Kuzmina, G. N.; Parenago, O. P.; Topchiev, A. V. Synthesis and Application of Inorganic Nanoparticles as Lubricant Components−a Review. J. Nanopart. Res. 2004, 6, 273− 284. (32) Rapoport, L.; Leshchinsky, V.; Lvovsky, M.; Lapsker, I.; Volovik, Y.; Feldman, Y.; Popovitz-Biro, R.; Tenne, R. Superior Tribological Properties of Powder Materials with Solid Lubricant Nanoparticles. Wear 2003, 255, 794−800. (33) Alazemi, A.A.; Etacheri, V.; Dysart, A. D.; Stacke, L. E.; Pol, V. G.; Sadeghi, F. Ultrasmooth Submicrometer Carbon Spheres as Lubricant Additives for Friction and Wear Reduction. ACS Appl. Mater. Interfaces 2015, 7, 5514−5521. (34) Peng, Y.; Meng, Z.; Zhong, C.; Lu, J.; Yang, Z.; Qian, Y. Tubeand Ball-like Amorphous MoS2 Prepared by a Solvothermal Method. Mater. Chem. Phys. 2002, 73, 327−329. (35) Pol, V. G.; Shrestha, L. K.; Ariga, K. Tunable, Functional Carbon Spheres Derived from Rapid Synthesis of Resorcinolformaldehyde Resins. ACS Appl. Mater. Interfaces 2014, 6, 10649− 10655. (36) Chang, K.; Chen, W.; Ma, L.; Li, H.; Li, H.; Huang, F.; Xu, Z.; Zhang, Q.; Lee, J. Y. Graphene-like MoS2/Amorphous Carbon Composites with High Capacity and Excellent Stability as Anode Materials for Lithium Ion Batteries. J. Mater. Chem. 2011, 21, 6251− 6257. (37) Peng, J.; Gao, W.; Gupta, B. K.; Liu, Z.; Romero-Aburto, R.; Ge, L.; Song, L.; Alemany, L. B.; Zhan, X.; Gao, G.; Vithayathil, S. A.; Kaipparettu, B. A.; Marti, A. A.; Hayashi, T.; Zhu, J. J. Graphene Quantum Dots Derived from Carbon Fibers. Nano Lett. 2012, 12, 844−849. (38) Liu, K.; Zhang, W.; Lee, Y. H.; Lin, Y. C.; Chang, M. T.; Su, C. Y.; Chang, C. S.; Li, H.; Shi, Y.; Zhang, H.; Lai, C. S.; Li, L. J. Growth of Large-area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates. Nano Lett. 2012, 12, 1538−1544. (39) Windom, B. C.; Sawyer, W. G.; Hahn, D. W. A Raman Spectroscopic Study of MoS2 and MoO3: Applications to Tribological Systems. Tribol. Lett. 2011, 42, 301−310. (40) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (41) Park, J. S.; Reina, A.; Saito, R.; Kong, J.; Dresselhaus, G.; Dresselhaus, M. S. G’ Band Raman Spectra of Single, Double and Triple Layer Graphene. Carbon 2009, 47, 1303−1310.

REFERENCES

(1) Rapoport, L.; Fleischer, N.; Tenne, R. Fullerene-like WS2 Nanoparticles: Superior Lubricants for Harsh Conditions. Adv. Mater. 2003, 15, 651−655. (2) Holmberg, K.; Andersson, P.; Erdemir, A. Global Energy Consumption due to Friction in Passenger Cars. Tribol. Int. 2012, 47, 221−234. (3) Erdemir, A.; Martin, J. M. New Materials and Coatings for Superlubricity and Near-wearless Sliding. ASME/STLE 2007 International Joint Tribology Conference, American Society of Mechanical Engineers: 2007; pp 1069−1071, DOI: 10.1115/IJTC2007-44402. (4) Berman, D.; Erdemir, A.; Sumant, A. V. Graphene: A New Emerging Lubricant. Mater. Today 2014, 17, 31−42. (5) Battez, A. H.; González, R.; Viesca, J. L.; Fernández, J. E.; Fernández, J. M.; Machado, A.; Chou, R.; Riba, J. CuO, ZrO2 and ZnO Nanoparticles as Antiwear Additive in Oil Lubricants. Wear 2008, 265, 422−428. (6) Padgurskas, J.; Rukuiza, R.; Prosyčevas, I.; Kreivaitis, R. Tribological Properties of Lubricant Additives of Fe, Cu and Co Nanoparticles. Tribol. Int. 2013, 60, 224−232. (7) Kalyani; Rastogi, R. B.; Kumar, D. Synthesis, Characterization, and Tribological Evaluation of SDS-Stabilized Magnesium-Doped Zinc Oxide (Zn0.88Mg0.12O) Nanoparticles as Efficient Antiwear Lubricant Additives. ACS Sustainable Chem. Eng. 2016, 4, 3420−3428. (8) Zhou, J.; Wu, Z.; Zhang, Z.; Liu, W.; Xue, Q. Tribological Behavior and Lubricating Mechanism of Cu Nanoparticles in Oil. Tribol. Lett. 2000, 8, 213−218. (9) Wu, Y.; Tsui, W. C.; Liu, T. C. Experimental Analysis of Tribological Properties of Lubricating Oils with Nanoparticle Additives. Wear 2007, 262, 819−825. (10) Liu, D.; Zhang, M.; Zhao, G.; Wang, X. Tribological Behavior of Amorphous and Crystalline Overbased Calcium Sulfonate as Additives in Lithium Complex Grease. Tribol. Lett. 2012, 45, 265−273. (11) Hu, Z.; Lai, R.; Lou, F.; Wang, L.; Chen, Z.; Chen, G.; Dong, J. Preparation and Tribological Properties of Nanometer Magnesium Borate as Lubricating Oil Additive. Wear 2002, 252, 370−374. (12) Dong, J.; Hu, Z. A study of the Anti-wear and Friction-reducing Properties of the Lubricant Additive, Nanometer Zinc Borate. Tribol. Int. 1998, 31, 219−223. (13) Gusain, R.; Khatri, O. P. Ultrasound Assisted Shape Regulation of CuO Nanorods in Ionic Liquids and Their Use as Energy Efficient Lubricant Additives. J. Mater. Chem. A 2013, 1, 5612−5619. (14) Xie, H.; Jiang, B.; He, J.; Xia, X.; Pan, F. Lubrication Performance of MoS2 and SiO2 Nanoparticles as Lubricant Additives in Magnesium Alloy-Steel Contacts. Tribol. Int. 2016, 93, 63−70. (15) Higgs, C. F.; Heshmat, C. A.; Heshmat, H. Comparative Evaluation of MoS2 and WS2 as Powder Lubricants in High Speed, Multi-pad Journal Bearings. J. Tribol. 1999, 121, 625−630. (16) Rapoport, L.; Bilik, Y.; Feldman, Y.; Homyonfer, M.; Cohen, S. R.; Tenne, R. Hollow Nanoparticles of WS2 as Potential Solid-state Lubricants. Nature 1997, 387, 791−793. (17) Tannous, J.; Dassenoy, F.; Lahouij, I.; Le Mogne, T.; Vacher, B.; Bruhács, A.; Tremel, W. Understanding the Tribochemical Mechanisms of IF-MoS2 Nanoparticles under Boundary Lubrication. Tribol. Lett. 2011, 41, 55−64. (18) Kogovšek, J.; Kalin, M. Various MoS2-, WS2- and C-based Micro-and Nanoparticles in Boundary Lubrication. Tribol. Lett. 2014, 53, 585−597. (19) Greenberg, R.; Halperin, G.; Etsion, I.; Tenne, R. The Effect of WS2 Nanoparticles on Friction Reduction in Various Lubrication Regimes. Tribol. Lett. 2004, 17, 179−186. (20) Hu, K.; Hu, X.; Xu, Y.; Huang, F.; Liu, J. The Effect of Morphology on the Tribological Properties of MoS2 in Liquid Paraffin. Tribol. Lett. 2010, 40, 155−165. (21) Xie, H.; Jiang, B.; Liu, B.; Wang, Q.; Xu, J.; Pan, F. An Investigation on the Tribological Performances of the SiO2/MoS2 Hybrid Nanofluids for Magnesium Alloy-Steel Contacts. Nanoscale Res. Lett. 2016, 11, 329. 4232


Research Article

ACS Sustainable Chemistry & Engineering (42) NIST X-ray Photoelectron Spectroscopy Database, version 4.1; National Institute of Standards and Technology: Gaithersburg, MD, 2012. http://srdata.nist.gov/xps/ (accessed March 26, 2013). (43) Grossiord, C.; Varlot, K.; Martin, J. M.; Le Mogne, T.; Esnouf, C.; Inoue, K. MoS2 Single Sheet Lubrication by Molybdenum Dithiocarbamate. Tribol. Int. 1998, 31, 737−743. (44) Hu, K. H.; Wang, J.; Schraube, S.; Xu, Y.; Hu, X.; Stengler, R. Tribological Properties of MoS2 Nano-balls as Filler in Polyoxymethylene-based Composite Layer of Three-layer Self-lubrication Bearing Materials. Wear 2009, 266, 1198−1207. (45) Drummond, C.; Alcantar, N.; Israelachvili, J.; Tenne, R.; Golan, Y. Microtribology and Friction-induced Material Transfer in WS2 Nanoparticle Additives. Adv. Funct. Mater. 2001, 11, 348−354. (46) Chhowalla, M.; Amaratunga, G. A. J. Thin Films of Fullerenelike MoS2 Nanoparticles with Ultra-low Friction and Wear. Nature 2000, 407, 164−167. (47) Martin, J. M.; Donnet, C.; Le Mogne, T.; Epicier, T. Superlubricity of Molybdenum Disulphide. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 10583. (48) Fan, X.; Wang, L. Graphene with Outstanding Anti-irradiation Capacity as Multialkylated Cyclopentanes Additive Toward Space Application. Sci. Rep. 2015, 5, 12734. (49) Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Perspectives on Carbon Nanotubes and Graphene Raman Spectroscopy. Nano Lett. 2010, 10, 751−758.

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