Tribological Study of SOCNTs@MoS2 Composite as a Lubricant

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Materials and Interfaces

Tribological Study of SOCNTs@MoS2 Composite as a Lubricant Additive: Synergistic Effect Wei Song, jincan Yan, and Hongbing Ji Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00740 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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Industrial & Engineering Chemistry Research

Tribological Study of SOCNTs@MoS2 Composite as a Lubricant Additive: Synergistic Effect Wei Song 1, Jincan Yan1,2*, Hongbing Ji1* 1 School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China 2 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China KEYWORDS: Carbon nanotubes, Molybdenum disulfide, tribological performance, Synergistic effect

ABSTRACT: Different layers of MoS2 were grown on the surface of carbon nanotubes (CNTs) successfully with a facile and effective chemical vapor deposition method. The pristine CNTs (PCNTs) were pre-treated with 5mol·L-1 HNO3, 10mol·L-1 HNO3 and mixed acid (sulfuric acid: nitric acid=3:1) to obtain weakly oxidized CNTs (WOCNTs), moderately oxidized CNTs (MOCNTs) and strongly oxidized CNTs (SOCNTs). According to the results of SEM, EDS, TEM, Raman, XRD and XPS, more layers of MoS2 were grown on the surface of SOCNTs uniformly due to the abundant defects on its surface. The friction-reducing and anti-wear performance of dibutyl phthalate (DBP) containing SOCNTs, MoS2, SOCNTs/MoS2 mixture and SOCNTs@MoS2 composite were investigated under 392N, 1200rpm at room temperature by four-ball tribotester. The DBP containing 0.02 wt % SOCNTs@MoS2 composite exhibited the

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best tribological performance, and the friction coefficient and wear scar diameter (WSD) were reduced by 57.93% and 19.08% respectively. The synergistic effect between SOCNTs and MoS2 was further explored by comparing the lubricant mechanism of the different additives. A possible mechanism was proposed, and the SOCNTs@MoS2 composite can be deposited on the rubbing surface due to the microstructure and translated into tribo-film which can protect the friction pair.

1 Introduction Friction and wear are the main causes of energy consumption and material loss. It’s reported that approximately two percent energy consumed in transportation, turbomachinery, power generation and industrial process can be saved with the function of lubricant.1 Hence, it’s necessary to improve the tribology performance of lubricating oil to reduce energy consumption. New lubricating oil additives were searched to improve the anti-wear and friction-reducing properties of base oil. Recently, nano-additive made great progress in tribology performance due to their stability and convenience.2-7 Carbon nanotubes (CNTs) have attracted the attention due to its excellent mechanical, thermal conductivity, electrical properties, and high specific surface area.8-10 Relied on its superior mechanical property, CNTs were used in many fields, for example, the hardness and mechanical strength of polymer matrix, metal matrix containing CNTs got obviously improvement. Meanwhile, the performance of friction-reducing and anti-wear were significantly improved when the CNTs were used as solid lubricated additive.11-15 But CNTs were easy to accumulate because of its high-surface-energy, it’s seldom used as lubricating oil additive. CNTs functioned with oleic acid or surfactants could disperse better in base oil, but the structure of


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CNTs could be damaged and new impurities could be introduced.16 One possible approach to solve the problem would be synthetic CNTs-based composite.17 The tribology performance and dispersion of carbon nanomaterial in lubricant was improved significantly when it was decorated with extraneous nanomaterial. 18-20 Molybdenum disulfide (MoS2), as a typical transition metal sulfide, was sandwich structure consisting of covalent bound S-Mo-S tri-layers similar to graphene.21 The layers could slide under the shear force due to the weak van der Waals force between the layers. MoS2 was excellent lubricating materials used in many aspects such as lubricating fluid additives,22-23 selflubricating coatings and solid lubricants.24-25 In previous studies, MoS2 nanolayers could effectively enhanced the tribology performance of nano carbon materials and formed tribofilm on the surface of friction pair.26-27 The CNTs coated MoS2 could improve the stability in base oil, and the mechanical property of MoS2 could also be further improved effectively. Both CNTs and MoS2 have the effect of friction-reducing and anti-wear, the CNTs/MoS2 composite could have the synergistic effect due to the microstructure. We described the synthesis process of the SOCNTs@MoS2 composite and explored the tribological performance of SOCNTs@MoS2 composite as additive in dibutyl phthalate (DBP). The microstructure of composite was characterized and it was found that 5~7 layers MoS2 were grown on the surface of the SOCNTs. The performance of anti-wear and friction-reducing were compared to individual SOCNTs, MoS2 and the mixture of them. The mechanism of the friction process was further discussed with the analysis of the wear scar. 2 Experiment 2.1 Materials


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Carbon nanotubes were purchased from Chengdu Organic Chemicals Co., Ltd., (NH4)6Mo7O24 (AR, 99%) and (NH4)2S (AR, 20% in H2O), concentrated sulfuric acid (98.3%) and nitric acid (68%) were purchased from Tianjin Damao Chemicals, China. All the chemicals were used without any purification operation. 2.2 Synthesis of SOCNTs@MoS2 composite The ammonium tetrathiomolybdate (ATTM) was synthesized according to the procedure reported.28 The pristine CNTs (PCNTs) were modified with 5mol·L-1 HNO3, 10mol·L-1 HNO3, and mixed acid (sulfuric acid: nitric acid=3:1) at 110oC for 5h followed by centrifugal washing with deionized water until the pH of the suspension reaching to 7.0. The modified CNTs were dried in a vacuum oven at 80oC overnight. The CNTs modified with 5mol·L-1 HNO3, 10mol·L-1 HNO3 and mixed acid were represented with weakly oxidized CNTs (WOCNTs), moderately oxidized CNTs (MOCNTs), strongly oxidized CNTs (SOCNTs). 100mg ATTM was dissolved in 50ml deionized water, and then 50mg modified CNTs was added. The mixed suspension was sonicated for 2h followed by stirring for 16h at 600 rpm. Then the suspension was dried at 80oC in an oven until the solvent was evaporated. The solid obtained was heated in a tube furnace at 300oC for 1 h and 900oC for 2 h with N2 flow. The heating rate and gas flow rate were 10 oC/min and 200 mL/min, respectively. The final CNTs@MoS2 composite was obtained after cooling to room temperature. For comparison, pure MoS2 sample was prepared from ATTM with the same procedure. 2.3 Characterization of CNTs@MoS2 composite The thermogravimetric analysis (TGA) was performed on a TG-209 thermo analyzer instrument (TA Instruments Inc., New Castle, DE). The condition was from room temperature to 900 °C at a


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heating rate of 10 °C min−1 under an N2 flow of 60 mL min−1. The morphology and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of nanomaterial was analyzed by Field-Emission scanning electric microscopy (FESEM, Quanta 400F) and transmission electric microscopy (TEM, FEI Tecnai G2 F30). Before observation with FESEM, the sample was dispersed in ethanol, and then dropped on the aluminum foil matrix. The element composition of the SOCNTs@MoS2 composite was analyzed by Energy Dispersive X-ray spectroscopy (EDX). The X-ray diffraction (XRD) was performed on D-MAX 2200 VPC at 40kV and 26mA. Raman spectroscopy was collected by using an excitation at 514.5nm from Laser Micro-Raman Spectrometer (Renishaw in VIA). X-ray photoelectron spectroscopy (XPS) spectrum was carried out with an Axis Ultra Instrument (ESCALab250). 2.4 Tribological test CNTs@MoS2 composite was dispersed ultrasonically for 30 min in 100 ml dibutyl phthalate (DBP). The tribological properties were performed on a MS-10A four-ball tribotester (Xiamen Tenkey Automation Co., China.) under 392 N with a rotary velocity of 1200 rpm at room temperature for 30 min which is similar to ASTM D-2783. The friction pair consists of four identical balls, one rotating on the top while the other three fixed beneath. The balls were composed of GCr15 bearing steel with a diameter of 12.7 mm and the surface roughness was not more than 10 nm. After the test, the balls were rinsed in an ultrasonic bath with petroleum for 10 min and then dried in the air before surface analysis. The surface morphology and composition of the steel balls after tribological test were analyzed with SEM (Hitachi SU1510) and XPS (ESCALab250). 3 Results and discussion


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3.1 Characterization of materials

Scheme 1. The preparation process for SOCNTs@MoS2 composite. The preparation process for the SOCNTs@MoS2 composite was depicted in Scheme 1. Pristine carbon nanotubes (PCNTs) were selected as starting template. The SOCNTs with abundant oxygen functional group containing hydroxyl, carboxyl group was obtained from the PCNTs modified with mixed acid. Afterward, the precursor of MoS2 can be easily deposited on the surface due to the interaction between oxygen functional group and ATTM. Then, the SOCNTs@ATTM could be further translated into SOCNTs@MoS2 in nitrogen atmosphere at 900oC. To verify the preparation method, the thermal stability of ATTM was analyzed by the thermogravimetric test (Figure 1). The thermal decomposition process can be divided into two steps. The first step began at 170oC which is close to the thermal decomposed temperature of ATTM.29 The residual mass fraction remained 58.55% when the temperature reaches to 210oC, which indicating the formation of MoS3 decomposed by ATTM.28-29 As the temperature increase, the TGA curves present a second slope at 351 oC, which means the MoS3 gradually convert to MoS2+X. Eventually, MoS3 completely turned into MoS2 when temperature reaches about 500 oC.


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100 90 Mass/%

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Figure 1. The TGA curves of ATTM. The surface morphology and element composition of PCNTs and SOCNTs@MoS2 composite were characterized by SEM and EDX. The typical tubular structure of PCNTs and SOCNTs@MoS2 composite were presented in Figure 2a and Figure 2b respectively. Although there was no significant difference in morphology between them, strong elemental peaks of S and Mo can be observed in the EDX results of SOCNTs@MoS2. More significantly, the molar ratio of S and Mo was 2.17:1 which was very close to 2:1. It’s clearly indicated the existence of MoS2 on the surface of the SOCNTs. The Al and O signals can also be detected, which mainly come from the aluminum matrix foil and oxygen-containing functional group on the SOCNTs surface.

(a)


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Figure 2. The SEM images of (a) PCNTs, (b) SOCNTs@MoS2 composite, (c) EDX results of SOCNTs@MoS2 composite. The XRD patterns of CNTs@MoS2 composite and MoS2 decomposed by ATTM were shown in Figure 3. The results of pure MoS2 agreed well with standard XRD pattern, and the typical diffraction peak of MoS2 can also be found in CNTs@MoS2 composite. The main diffraction peak at 14.3o, 32.6o and 38.8o can be well ascribed to (002), (100) and (103) miller plane of hexagonal MoS2. The peak at 26.0o and 53.2o can be assigned to (002) and (102) miller plane of CNTs. Interestingly, the relative intensity of diffraction peak of MoS2 at 14.3o, 32.6o and 38.8o became stronger when the PCNTs were treated with 5mol·L-1 HNO3, 10mol·L-1 HNO3 and mixed acid respectively. Besides, the diffraction peak of CNTs at 26.0o and 53.2o became weaker, which could be the results of the more layers of MoS2 on the surface of CNTs.30-31 It can


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be inferred that the acid with stronger oxidizing property can produce more defects on the out

(112)(110)

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surface of CNTs, and the defects were in favor of the growth of MoS2.32

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Figure 3. The XRD patterns of SOCNTs@MoS2, MOCNTs@MoS2, WOCNTs@MoS2, PCNTs@MoS2, and MoS2 decomposed by ATTM, drop lines corresponding to standard XRD patterns in JCPDS card No. #37-1492.

D SOCNTs

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The Raman spectroscopy of (a) PCNTs, WOCNTs, MOCNTs, SOCNTs; (b)

PCNTs@MoS2, WOCNTs@MoS2, MOCNTs@MoS2, SOCNTs@MoS2 composite. The Raman spectroscopy of the PCNTs treated with different acid was presented in the Figure 4a. The main peak at 1350cm-1 and 1582cm-1 was assigned to the D band and G band of CNTs. The D band corresponded to the scattering from local defects or disorders in the CNTs, and the G band originated from the in-plane tangential stretching of the C-C bonds in the graphitic structure.33 The defects on the surface of CNTs modified with different acid were investigated according to the calculation results of ID/IG. The ID/IG of SOCNTs was 0.856, with significant increase comparing to PCNTs, WOCNTs and MOCNTs, and it can be concluded that the acid with stronger oxidizing property can produce more defects on the surface of CNTs.34 Figure 4b showed the Raman spectroscopy of CNTs@MoS2 composite. Besides the D band and G band of CNTs, another two peaks at 377 cm-1 and 403 cm-1 were observed which corresponding to the E2g and A1g of MoS2.35 It’s remarkable that the characteristic peak of MoS2 became stronger and the characteristic peak of CNTs became weaker when the PCNTs were respectively treated with 5mol·L-1 HNO3, 10mol·L-1 HNO3 and mixed acid. The results of Raman spectroscopy agreed well with the XRD patterns. It’s further proved that the acid with more oxidizing property can produce more defects on the surface of CNTs which were beneficial to the growth of MoS2.


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Figure 5. The TEM images of (a)MOCNTs@MoS2, (b)SOCNTs@MoS2, (c) high resolution TEM images of SOCNTs@MoS2, (d) SAED pattern of SOCNTs@MoS2 composite, (e) HAADF-STEM image of SOCNTs@MoS2 composite, (f)-(i) STEM-EDS mapping images taken from the area marked with a red-dot in (e) showing the corresponding distribution of C, O, Mo and S. The microstructure of MOCNTs@MoS2 and SOCNTs@MoS2 were presented in Figure 5 (a)-(c). The layer structure of MoS2 coated on SOCNTs can be observed clearly in high resolution TEM images (Figure 5c), and the layer spacing can be measured about 0.64nm which was close to the calculation results of Bragg equation. Simultaneously, It’s found that 1~4 layers of MoS2 were grown on the surface of MOCNTs unevenly (Figure 5a), and 5~7 layers of MoS2 were decorated on the surface of SOCNTs uniformly (Figure 5b). Considering the SOCNTs@MoS2 composite also displayed the strongest characteristic peak of MoS2 in XRD patterns and Raman spectroscopy (Figure 3, Figure 4b), and it can be inferred that the more defects produced by


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mixed acid were in favor of the growth of MoS2. The selected area electron diffraction (SAED) in Figure 5d presented random diffraction spots indicating that MoS2 was in polycrystalline state.36-37 The mapping images of composite showed that the S and Mo were distributed uniformly on the surface of SOCNTs (Figure 5f-Figure 5i), which indicated that the MoS2 was coated on the surface of SOCNTs. (a)

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S 2p1/2

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158

Figure 6. The XPS spectrum of (a) SOCNTs@MoS2, (b) C1s, (c) Mo3d and (d) S2p of SOCNTs@MoS2. The existence of MoS2 was confirmed by the XPS spectrum in Figure 6a, and it revealed the presence of C, O, Mo, S. The C1s peak at 284.8eV (Figure 6b) could be attributed to graphite-like


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sp2 hybridized carbon.38 The spectrum of Mo (Figure 6c) can be deconvoluted into two peaks at 229.6 and 232.7 eV, which corresponding to the Mo3d5/2 and Mo3d3/2.39-40 The two peaks of Mo revealed the existence of MoIV oxidization state. The spectrum of S (Figure 6d) showed the typical peak at 162.3 and 163.5 eV, which were related to S2p3/2 and S2p1/2 species, respectively.41 The atomic molar ratio of S/Mo was 2.11 according to the results of semi-quantitative, which was close to the results of EDX spectrum, and it can be further confirmed the existence of MoS2. 3.2 Friction and Wear Performance

Figure 7. The images of 0.04 wt % SOCNTs, 0.04 wt MoS2 %, 0.04 wt % SOCNTs@MoS2 mixture, 0.04 wt % SOCNTs@MoS2 composite dispersed in DBP after 10 days. The images of 0.04 wt % SOCNTs, 0.04 wt % MoS2, 0.04 wt % SOCNTs/MoS2 mixture and 0.04 wt % SOCNTs@MoS2 composite dispersed in DBP after 10 days were presented in Figure 7. Sediment can be found in the base oil containing 0.04 wt % MoS2 and 0.04 wt %


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SOCNTs/MoS2 mixture. It’s obviously that the DBP containing 0.04 wt % SOCNTs and 0.04 wt % SOCNTs@MoS2 composite can be well-dispersed without any sediment. The friction coefficient (FC) and wear scar diameter (WSD) with the concentration of SOCNTs@MoS2 composite in DBP at 392 N and 1200 rpm for 30 min were shown in Figure 8. The curves of friction coefficient and WSD with concentration were concave and both of them reach the lowest value at 0.02 wt%. The friction coefficient and WSD reduced 57.93% and 19.08% respectively at 0.02 wt% comparing to the base oil. Because both SOCNTs and MoS2 had excellent mechanical property, they showed good performance in friction-reducing and antiwear. The friction coefficient and WSD increased when 0.04 wt %-0.10 wt % composite were added into DBP, indicating the higher concentration of the composite was not beneficial to the improvement of the performance. It could be caused by the agglomerate of the composite at high

FC WSD

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Friction Coefficient m

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0.10

Figure 8. The friction coefficient and WSD with the concentration of SOCNTs@MoS2 composite in DBP lubricated at 392 N and1200 rpm for 30 min.


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0.35 Friction Coefficient

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Base oil 0.04 wt % SOCNTs 0.04 wt % MoS2

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Time(s) Figure 9. The friction coefficient curves of base oil, 0.04 wt % SOCNTs, 0.04 wt % MoS2, 0.04 wt % SOCNTs/MoS2 mixture, and 0.04 wt % SOCNTs@MoS2 composite lubricated at 392 N and 1200 rpm for 30 min. The friction coefficient curves with time were shown in Figure 9. The friction coefficient of base oil was about 0.13~0.15 with violent fluctuations. It suggested that the base oil cannot reduce the friction and maintain a stable state during tribological test. For the initial 200 s of tribological test, the DBP containing additives appeared a short-term running-in period and displayed a high value of friction coefficient, which decreased obviously later. It’s well known that a proper running-in is important to yield a steady and long-life.43 The friction coefficient can be reduced and maintain a steady level when 0.04 wt % MoS2 was added into DBP. The oil containing 0.04 wt % SOCNTs exhibited excellent friction-reducing property in the initial stage of friction for 950 s. The friction coefficient rose rapidly at 950 s with a steep slope, and then began to fluctuate violently from 950 s to 1800 s. It was indicated that the SOCNTs can improve the friction-


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reducing property in a short time and lost effectiveness after a long time of friction. For comparison, the performance of the mixture containing SOCNTs and MoS2 was explored. The mixture of SOCNTs and MoS2 displayed a lower coefficient than individual component, especially at the last 250s. Significantly, it was obviously the 0.04 wt % SOCNTs@MoS2 composite in DBP possessed the most stable performance and the lowest friction coefficient. It can be inferred that the SOCNTs and MoS2 had a synergistic effect due to the microstructure of SOCNTs@MoS2 composite.36 The SOCNTs@MoS2 synthesized with different mass ratio precursor was prepared, and its friction-reducing and anti-wear performance was explored in Figure 10a. Generally, the friction coefficient and WSD increased evidently when the relative content of the precursor of MoS2 was decreased. To further explore the mechanism, the microstructure of SOCNTs@MoS2 synthesized with different mass ratio of SOCNTs/MoS2 was observed in Figure 10c-Figure 10e. About 4~8 layers of MoS2 were uniformly grown on the surface of the SOCNTs with the mass ratio of 1:3, and 2~4 layers of MoS2 were grown when the mass ratio was decreased to 1:1. The SOCNTs was only coated with 1~2 layers of MoS2 unevenly when the mass ratio is 3:1. Therefor it can be concluded that the more layers of MoS2 on the surface of the SOCNTs, the lower coefficient of the SOCNTs/MoS2 composite was, and the synergistic effect between SOCNTs and MoS2 could be further improved when the relative content of MoS2 was increased. Figure 10b showed the variation of friction coefficient and WSD with the diameter of SOCNTs prepared for the composite. The friction coefficient and WSD increased slightly with the increase of the diameter of SOCNTs under the condition that the diameter was more than 10 nm. It’s easy to deduce that the SOCNTs with larger diameter can be covered with less layers of MoS2 when the SOCNTs@MoS2 composite was synthesized at the same mass ratio of SOCNTs/ATTM. As a


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result, it’s detrimental to the improvement of tribology performance. In addition, the properties remain poor when the diameter of SOCNTs was less than 8 nm. Considering the surface roughness of the steel ball was 10 nm, the SOCNTs could be filled in the micro-gully and cannot roll in rubbing surface to decrease the friction.44-47 Besides, the SOCNTs@MoS2 composite was easy to agglomerate due to the small diameter and high surface energy, and it was not advantageous to the improvement of the performance.

Figure 10. Variation of the friction coefficient and WSD lubricated with DBP containing 0.04 wt % SOCNTs@MoS2 composite prepared with (a) the different mass ratio of SOCNTs/ATTM and (b) the different diameter of SOCNTs at 392 N and1200 rpm for 30 min. The TEM images of SOCNTs@MoS2 composite synthesized with different mass ratio of SOCNTs/ATTM (c) 1:3; (d) 1:1 (e) 3:1.


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3.3 SEM and XPS analysis of wear appearance The SEM images of surface morphology lubricated with five different additives were shown in Figure 11. The base oil without any additive presented the biggest wear scar diameter. The edge area of the wear scar was uneven and the central area was full of deep scratches and adhesion. It could be concluded that the wear mechanism was adhesive wear when lubricated with base oil.4849

Pure DBP without additives cannot function well during the friction process. The wear scar

diameter of base oil containing 0.04 wt % SOCNTs was nearly the same comparing to base oil. Some areas displayed a serious uneven appearance which can be caused by the malfunction of SOCNTs. The feature can be in accordance with fatigue wear, and the friction pair cannot be protected well under this condition. The WSDs reduced obviously when the 0.04 wt % MoS2 or the 0.04 wt % SOCNTs/MoS2 mixture were added into DBP, but there were many furrows and scratches on the surface of the wear scar. It was indicated that abrasive wear was the main wear mechanism, accompanied by the fatigue wear.18, 50 The surface of balls lubricated with 0.04 wt % SOCNTs@MoS2 composite exhibited the smallest WSD and the smoothest morphology than individual additive and the mixture of SOCNTs and MoS2. The effective protect film could be formed on the surface and prevent the direct contact of friction pair when 0.04 wt% SOCNTs@MoS2 composite was dispersed in DBP. It suggested that the SOCNTs and MoS2 have a synergistic effect due to the microstructure of SOCNTs@MoS2 composite.


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Figure 11 The SEM images under different magnification of the wear scar on the balls lubricated with (a)(b) base oil; (c)(d) 0.04 wt % SOCNTs; (e)(f)0.04 wt % MoS2; (g)(h) 0.04 wt % SOCNTs/MoS2 mixture; (i)(j) 0.04 wt % SOCNTs@MoS2 composite at 392N and1200rpm for 30min.

Figure 12 The XPS spectrum of (a)C1s, (b)O1s, (c)Fe2p, (d)Cr2p, (e) S2p, (f) Mo3d on the surface of the wear scar lubricated with 0.04 wt % SOCNTs@MoS2 composite at 392 N and1200 rpm for 30 min. (Ⅰ)0% ; (Ⅱ)0.04 wt %SOCNTs@MoS2 composite.


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The XPS spectrum in Figure 12 was used to explore the surface element composition of the wear scar. The main two peaks of C1s which appear at 284.8 eV and 288.7 eV belong to sp3C(C-C/CH) and sp2C(C=C/C) respectively. It suggested that the protect film formed in the rubbing process contains carbon nanotubes and organics.19, 51-52 The Isp3/Isp2 was calculated according to the area ratio of the peak at 284.8 eV and 288.5 eV. It’s worthwhile to observe that the Isp3/Isp2 lubricated with SOCNTs@MoS2 composite was 20.98 which was higher than the wear scar lubricated with base oil. It’s suggested that more adsorption film was generated on the surface of steel ball, and the adsorption film was converted to chemical reaction film which protected the friction pair. The peak of O1s at 532 eV indicated the formation of hydroxides and sulfates. The peak at 530.3 eV can be attributed to -Fe(Ⅲ)-O-, indicating the formation of Fe2O3 during the friction process.51, 53 Fe2O3 was the main component of tribo-film lubricated with composite, while the hydroxides and sulfates were the main product during the friction lubricated with base oil. It can be concluded that the formation of tribo-film containing Fe2O3 could be beneficial to the improvement of the performance. The Fe2p3/2 and Fe2p1/2 peak appeared at 710.7 eV and 724.6 eV were matched with Fe2p of Fe2O3 and the little peak at 706.8 eV and 720.1 eV can be caused by the existence of little Fe (0).18,

51-52

Less signal of Fe (0) can be detected on the

rubbing surface lubricated with DBP containing SOCNTs@MoS2 composite, which indicated more tribo-oxide film was deposited on the surface. The peak at 576.9 eV and 587.6 eV of Cr2p originated from Cr2O3, a product of tribo-reaction between friction pair and air.52 The two peaks positioned at 232.6 eV and 235.8 eV were ascribed to -Mo(Ⅵ) -O- of MoO3 which was formed from MoS2 in the process of friction oxidation.54 The peak of S2p at 169 eV was indexed to S(Ⅵ)-O- of SO42- oxidized from MoS2. In general, the SOCNTs@MoS2 can be deposited on the rubbing surface, and then the physical adsorption film was formed. During the tribo-reaction, the


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physical adsorption film was gradually translated into tribo-film including Fe2O3, Cr2O3 and MoO3, which can protect the surface of friction pair.55-56 3.4 Synergistic lubricating mechanism The lubrication models of SOCNTs/MoS2 mixture and SOCNTs@MoS2 composite dispersed in DBP were illustrated in Figure 13. In case of the lubrication with base oil containing the mixture of SOCNTs and MoS2, MoS2 can be absorbed on the surface of friction pair while the SOCNTs was gradually detached from the surface because of the high surface energy. The tribo-film was mainly generated from MoS2 but the excellent mechanical property of SOCNTs cannot be used. Hence, some furrows can be found on the wear surface without the protection of SOCNTs, as confirmed by Figure 11c, d. Nevertheless, as that of DBP containing SOCNTs@MoS2 composite, both of SOCNTs and MoS2 can be simultaneously penetrated into the valleys and grooves with the traction of machine. A physical adsorption film was firstly formed with the deposition of SOCNTs@MoS2 composite on the rubbing interface.18-19, 57 Because the adhesion between SOCNTs and steel surface can be enhanced with the introduction of MoS2 on the surface of SOCNTs, both of SOCNTs and MoS2 can play a role during the lubrication. With the increase of temperature and pressure, a new tribo-film composed of metallic oxide was further formed due to the complex tribo-chemical reaction.18-19 The tribo-film formed on the interface can develop the synergistic effect of SOCNTs@MoS2 composite and protect the contact area effectively as shown in Figure 11i, j.56 Therefore, the friction-reducing and anti-wear performance can be significantly improved with the addition of SOCNTs@MoS2 composite.


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Figure 13. The Schematic diagram of lubrication mechanism of DBP containing 0.04 wt % SOCNTs/MoS2 mixture and 0.04 wt % SOCNTs@MoS2 composite. 4 Conclusions The SOCNTs@MoS2 composite was synthesized by a facile and effective chemical vapor deposition method. The pristine carbon nanotubes prepared for the composite should be pretreated with mixed acid (sulfuric acid: nitric acid=3:1), and more defects were produced on the surface of the SOCNTs during the modification. The defects containing oxygen functional group were in favor of the growth of MoS2, thereby more layers of MoS2 were grown on the surface of SOCNTs successfully. The SOCNTs@MoS2 composite dispersed in DBP exhibited superior friction-reducing and anti-wear performance which was expressed as follows: (1) The friction coefficient was reduced by 57.93% when 0.02 wt % SOCNTs@MoS2 composite was added into DBP. The friction coefficient was the lowest comparing to individual SOCNTs, MoS2, and the mixture of them. The improvement of friction-reducing performance was attributed to the synergistic effect of SOCNTs and MoS2.


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(2) The wear scar diameter lubricated with DBP containing 0.02 wt % SOCNTs@MoS2 composite was reduced by19.08% comparing to base oil. Adhesive wear, fatigue wear, and abrasive wear were the main wear mechanism of base oil, base oil containing SOCNTs and SOCNTs/MoS2 mixture respectively. The surface of steel balls lubricated with individual component or the mixture was severely worn, but the surface of steel balls lubricated with SOCNTs@MoS2 composite was the smoothest. (3) The more layers of MoS2 on the surface of SOCNTs and the smaller diameter of SOCNTs prepared for composite could improve the friction-reducing and anti-wear performance better. But if the diameter of SOCNTs was less than 8 nm, the composite cannot function effectively. (4) The synergistic effect between SOCNTs and MoS2 was attributed to the special microstructure of SOCNTs@MoS2 composite. The SOCNTs@MoS2 composite could be gradually absorbed on the rubbing surface during the friction, but the SOCNTs were detached from the rubbing surface lubricated with SOCNTs/MoS2 mixture. The physical adsorption film composed of SOCNTs@MoS2 composite gradually translated into tribo-film containing Fe2O3, Cr2O3 and MoO3, which could protect the friction pair effectively. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]. ORCID


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Hongbing Ji: 0000-0003-1684-9925 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are grateful to National Natural Science Foundation of China (Grant no. NSFC 21646002), Guangdong Science and Technology Foundation 2017, the Visiting Fellowship Program of State Key Laboratory of Chemical Resource Engineering, 3rd Huizhou TianE Project, and Huizhou Science and Technology Foundation 2017 for the financial support. REFERENCES 1.

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Tribological Study of SOCNTs@MoS2 Composite as a Lubricant

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