Study of composite nanoparticles based on hydrotalcite as high

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Applied Chemistry

Study of composite nanoparticles based on hydrotalcite as high performance lubricant additives Zhaowen Ba, Yunyan Han, Dan Qiao, Dapeng Feng, and Guowei Huang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02831 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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Industrial & Engineering Chemistry Research

Study of composite nanoparticles based on hydrotalcite as high performance lubricant additives

Zhaowen Baa,b, Yunyan Hana, Dan Qiaoa*, Dapeng Fenga*, Guowei Huangc* (a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b

c

University of Chinese Academy of Sciences, Beijing 100049, China

State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China)

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract:

Composite

double-hydroxide/graphene

nanoparticle nanosheet

additives

of

(LDH/GO)

double-hydroxide/molybdenum disulfide (LDH/MoS2)

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hybrid

layered

and

layered

have been fabricated by

electrostatic self-assembly. The morphologies and compositions were confirmed by FESEM, EDS, XRD, Raman and FT-IR. Zeta potential test confirms that the composite nanoparticles have good dispersion stability in PAO-4. Meanwhile, the nanocomposites as additives can dramatically enhance the lubricating property of PAO-4. Research results indicate the good friction and anti-wear properties of LDH/GO and LDH/MoS2 composites can be attributed to the synergistic lubricating effects between LDH and GO or MoS2. Keyword: composite; nanoparticle; additives; lubricating effect; synergistic

1. Introduction


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Nano-particles as lubricant additives have attracted extensive interest in advanced engineering field attributing to their unique properties, such as high temperature stability, chemical inertness, high strength and good wear performance, et al

1-4

.

Thereinto, two-dimensional materials (such as MoS2, graphene, HBN and hydrotalcite, and so forth) are one of the most attractive particles owing to their high hardness, compressive strength, good bio-compatibility and oxidation resistance5-11. Meanwhile, many research results have manifested that nanometer lubricant additives perform excellent lubricating properties even at low concentrations12. As a typical two-dimensional material, layered double hydroxides (LDH) are composed

of

charged

brucite-like

hydroxide

layers

and

charge-balancing

exchangeable anions in the interlayer13-15. The special structural composition and physicochemical properties make LDH nanoparticles possess good tribological properties as lubricant additives and LDH is promising for next-generation nano-additives16-18. While, the lubricating property of single LDH nanoparticles is insufficient in practical working conditions. Therefore, the study of composite materials based on LDH have become the new research field. Owing to the rich positive charge on the surface, the LDH nanoparticles are prone to absorb the nanoparticles with rich negative charge (such as graphene, graphene oxide, MoS2 and so on) to form corresponding composite materials with better lubricating performance 19

. Meanwhile, it has been confirmed that MoS2 and graphene as two dimensional

materials exhibit good lubricating performance and load-carrying capacity and they



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are employed to modify the surfaces of nanometer lubricant additives

20-22

.

Researchers have discovered the oleylamine/graphene-modified hydrotalcite-based film behaved good hydrophobic and lubricating performance23. The strong interlayer covalent bonding and weak interlayer interaction endow MoS2

and graphene with

promising lubrication prospect24. Therefore, it can be predictable that the self-assembly process can greatly decrease the stack of LDH and avoid aggregation of GO and MoS2 nano-sheets, generating the larger interlayer spacing25. Above synthetic action could dramatically enhance the dispersibility of composite particles in based oil. But to our knowledge, the composite nanoparticles of LDH with GO or MoS2 as nanometer lubricant additives haven’t reported yet. In this paper, the hybrid layered double-hydroxide/graphene (LDH/GO) and layered double-hydroxide/molybdenum disulfide (LDH/MoS2) are prepared by electrostatic self-assembly. The tribological properties of composite nanoparticles as additives of poly-α-olefins-4 (PAO-4) were evaluated in detail. Furthermore, a number of surface analysis test techniques were employed to investigate the lubricating mechanism of the two kinds of composite nanoparticles.

2. Experiment 2.1 Materials Nickel nitrate, aluminum nitrate and urea were purchased from Sinopharm Chemical Reagent Co. Ltd. Ethanol (absolute) and acetone ( 99.5%) were AR grade and bought from Chem-Supply Pty. Ltd.. Poly-olefins-4 (PAO-4) was purchased from ExxonMobil. The nanoparticles MoS2 were got from Shanghai Shen Yu Industry &


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Trade Co., Ltd. 2.2 The preparation of LDH, LDH/GO and LDH/MoS2 The preparation of layered double-hydroxide (LDH): The LDH was prepared though the urea hydrolysis method. Firstly, Ni(NO3)2·6H2O (8.6 g), Al(NO3)3·9H2O (4.3 g) and urea (30 g) were mixed with deionized water as the solvent, and vigorous stirring at 92 °C for 24 h. Then, the obtained suspended solid was filtered and washed with deionized water. Finally, the green powders were obtained by vacuum drying at 100 °C for 12 h. The preparation of LDH/GO and LDH/MoS2: Graphene oxide (GO) nanosheets were prepared by a modified Hummers and Offeman method

26-27

. The mixtures of

Ni(NO3)2·6H2O (8.6 g) and Al(NO3)3·9H2O (4.3 g) in GO dispersion liquid (50 ml, 0.5 g/ml) were precipitated using the urea (30 g) hydrolysis method at 92 °C for 24 h. Finally, the black powder of LDH/GO was obtained. The powder of LDH/MoS2 was prepared by the similar method. The preparation schematic diagram of composite nanoparticles was illustrated in Figure 1. During the preparation process, the electrostatic assembly method was used to obtain the composites of LDH/GO and LDH/MoS2 because of the electrostatic attraction between the LDH with rich positive charge and GO (MoS2) with negative charge. 2.3 Characterization The microstructures of LDH/GO and LDH/MoS2 composite nanoparticles were characterized by field emission scanning electron microscope (FESEM, JSM-6701F).



The crystal structure was analyzed by powder X-ray diffraction (XRD, Rigaku D/Max-2400). Energy dispersive spectrometer (EDS) is used to investigate the compositions of LDH/GO and LDH/MoS2. The nanoparticles were also characterized by Raman spectroscopy (LabRAM HR Evolution) and Fourier Transform infrared spectroscopy (FT-IR). Thermo-gravimetric analysis (TGA, STA 449F3) was employed to study the thermodynamic performance of composite particles from room temperature (RT) to 700 °C at a heating rate of 10 °C /min under air flow. 2.4 Dispersion stability test To study the dispersibility of the different nanoparticles in PAO-4, the dispersion test of the different mixtures were carried out and the optical photographs were recorded every day through the static test for 72 h (Figure S1 in Supporting information). Moreover, the Zeta potential of the different dispersions were measured to conduct the stability determinations by Laser dynamic scatterer (Zetasizer Nano ZS ZEN 3600, Malvern). 2.5 Tribological property tests The lubricating properties of PAO-4 with LDH/GO or LDH/MoS2 were evaluated with the ball-on-disk machine SRV-IV, in comparison with the pure PAO-4 and PAO-4 with GO, MoS2 and LDH. The upper balls (diameter: 10 mm) were made of GCr15 bearing steel (AISI 52100) with the hardness of 700±10 HV. The lower discs (ø 24.0 mm × 7.9 mm, GCr15) were polished with abrasive paper (from 400-grit to 1500-grit) and the hardness is 650-700 HV. The friction coefficients were automatically recorded by the computer and corresponding wear volumes were


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measured with the non-contact 3D surface profiler. After the tribological tests, the wear scars were characterized by Raman spectroscopy (LabRAM HR Evolution). The wear debris were collected and measured by transmission electron microscopy (TEM, TF20). The morphologies of worn surfaces were observed by SEM equipped with energy dispersive spectrometer (EDS). The chemical compositions and elements distribution of the worn surfaces were analyzed by an X-ray photoelectron spectrometer (XPS, PHI-5702). And prior to all the characterizations, the steel discs were treated by ultrasonic cleaning three times in petroleum ether for 5min.

3. Result and discussion 3.1 Morphology and composition analysis Morphologies of LDH/GO, LDH/MoS2 and LDH were characterized by FESEM as described in Figure 2. Testing results indicate that the nanoparticles possess typical hexagonal laminate structure. Meanwhile, no remarkable agglomeration of GO nano-sheets is observed as shown in Figure 2a. For LDH/MoS2 (Figure 2b), MoS2 is also uniformly distributed on the surface of LDH sheets. Moreover, a higher magnification picture which is about LDH/MoS2 has been incorporated in Figure 2b. So, the composited MoS2 could be observed easily (The position of the red arrow). In addition, the elements (Al, Ni, O, C, Mo, S) distribution of LDH/GO, LDH/MoS2 and LDH are tested to investigate their compositions and the result is shown in Figure 2. It can be seen that the uniform distributions of Al, Ni and O elements over the marked position are ascribed to the main ingredient of LDH for the three kinds of



nanoparticles. And the element of C is evenly distributed on the surface of LDH/GO which is came from GO. Meanwhile Mo and S elements are detected on the surface of LDH/MoS2, meaning that MoS2 is compounded into the LDH/MoS2. Above results reveal that MoS2 and GO are bonded with the sheets of LDH by electrostatic self-assembly. Raman spectroscopy and XRD as helpful analysis techniques have been extensively employed to probe molecular structures and phase compositions. The molecular spectrum of LDH and GO are shown in Figure 3a. The peaks of 490 cm-1 and 550 cm-1 can be attributed to the Al-O-Al and Al-O-Ni bonds25. The peak of 1050 cm-1 is assigned to the vibration peak  of CO32-

17, 25

. For LDH/GO, the D and G

characteristic bands of carbon are observed at around 1350 cm-1 and 1585 cm-1, respectively. Thereinto, the D band is associated with disordered carbon or graphene edges, and the G band is the result of the first-order scattering of the E2g mode of sp2 carbon domains. For LDH/MoS2 powder, there are two groups of characteristic peaks including E12g (~377 cm-1) and A1g (~404 cm-1) modes for MoS2. To be precise, the in-plane E12g mode is associated with the opposite vibration of Mo and S atoms, while the A1g mode arises from the out-of-plane vibration of two S atoms 28, 29. The XRD patterns of LDH/GO and LDH/MoS2 are shown in Figure 3b. The diffraction pattern of the pristine LDH corresponds well with the characteristic peaks of (003), (006), (012) and (015) planes. Specifically, the peaks at 11.5° and 23.4° correspond to (003) and (006) diffraction planes of LDH, respectively 17, 25. However, the peak of 11° belonging to the (002) diffraction planes for graphene oxide is not


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detected from XRD pattern of LDH/GO. Two possible reasons are proposed: (i) the LDH aggregated in the layers of graphene and the graphene sheets return to the graphite are restrained; (ii) the peak of GO located at about 11° is overlapped with the (003) crystalline plane of LDH 25, 30. The peaks at 14°, 33° and 57° correspond well with (002), (100) and (110) diffraction planes of MoS2, indicating that MoS2exist on the surface of LDH. The crystal plane spacing

of particles was also calculated by

Scherrer’s equation 31 and the result is shown in Table 1. Data indicate that GO and MoS2 has no significant effect on the interlamellar spacing of LDH. D(hkl)= Kcos D(hkl) = md



Where is scherrer constant (0.89), is the wavelength of the diffraction light, is Bragg diffraction Angle, is the half peak width of the diffraction peak, m is the number of lattice plane and d is the crystal plane spacing.

Furthermore, the FT-IR of the three composites are detected and the results are shown in Figure 3c. The peaks at 3500-3400 cm-1 and 1380 cm-1 are attributed to O-H and intercalary carbonate ion for three kinds of material. The peaks at ~1700 cm–1 and ~1580 cm–1 reveal the existence of C=O and C=C, meaning that GO is compounded into the LDH/GO 32, 33. Meanwhile, the infrared peak at about ~600 cm−1 is assigned to Mo-S vibration in the LDH/MoS2, indicating that MoS2 is contained in the composite of LDH/MoS2 34, 35. In the light of the above results, GO and MoS2 are successfully compounded onto the



surface of LDH by the electrostatic attraction with each other. 3.2 Thermal stability The approximate proportions of element contents in LDH/GO, LDH/MoS2 and LDH nanoparticles were investigated by TG analysis under air atmosphere as depicted in Figure 4. The TG curve of LDH/GO composite in Figure 4 shows a major mass loss of 30% in the temperature range of 30-350 °C, which can be ascribed to the decomposition of LDH. And the exothermic peak at 450 °C can be assigned to the oxidation of GO. The LDH/MoS2 presents two mass loss processes: firstly, between 30 °C and 350 °C, resulting from the decomposition of LDH; the mass loss on the second stage is 13 %, from 390 °C to 550 °C, attributing to the oxidation of MoS2. 3.3 Dispersion stability As shown in Figure 5, the Zeta potentials of LDH, GO and MoS2 in PAO-4 are 2.40 mV, -17.90 mV and -3.9 mV respectively, meaning that the particles of LDH, GO and MoS2 in PAO-4 are strongly aggregated and sedimentary. Nevertheless, the Zeta potentials of LDH/GO and LDH/MoS2 are -26.2 mV and -38.2 mV, respectively. It signifies that the colloidal dispersion of LDH/GO and LDH/MoS2 in PAO-4 are relatively stable compared to LDH, GO and MoS2 in PAO-4. The reason is that GO and MoS2 are anchored onto the surface of LDH, and the special lamellar structure prevents the GO and MoS2 from agglomerating23’

25

. Thereby a good dispersion

stability is exhibited for the modified LDH. 3.4 Tribological properties of LDH/GO and LDH/MoS2 3.4.1 Effect of content


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To explore the effect of content of nanoparticles on the tribological performance, the lubricating performance of nanoparticles as PAO-4 additives were evaluated for steel/steel contacts under the conditions of RT, 200 N and 25 Hz, as described in Figure 6. It is clearly that the friction coefficient (COF) of blank PAO-4 sharply increases to 0.62 when the running time comes to 1 min. And then, the COF rapidly decreases to about 0.26 when the time comes to about 3 min. When 0.2 wt% LDH/GO is added into PAO-4, the COF of the mixture soars up to 0.22, suggesting the rupture of boundary lubricating film and the direct contact between sliding pairs. Fortunately, the COFs of the mixtures with 0.5 wt%, 1 wt% and 2 wt% LDH/GO are steady and they are about 0.11. Figure 6b shows the effect of content of LDH/MoS2 on the friction coefficient at room temperature under the load of 200 N. It can be seen that LDH/MoS2 as the additive also can dramatically enhance the lubricating property of PAO-4. The COFs (0.11) are smooth and steady even at low mass concentration (0.2 wt%), and they have no obvious change with the increasing of LDH/MoS2. The corresponding wear volumes are detected as shown in Figure 6c. When the concentration of LDH/GO is 0.2 wt%, the wear volume of mixture decreases two orders of magnitude than that of blank PAO-4. At the concentrations of 0.5 wt%, 1 wt% and 2 wt%, the mixtures exhibit excellent anti-wear properties. And the mixtures using LDH/MoS2 as the additive exhibit the similar trend and better anti-wear performance. Aforementioned results demonstrate that LDH/MoS2 and LDH/GO as additives can significantly improve the lubricating performance of PAO-4. 3.4.2 Control tests



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To further confirm aforementioned viewpoint, the lubricating performance of mixtures with 0.5 wt% MoS2 and 0.5 wt% GO were also detected as depicted in Figure 7. MoS2, GO and LDH were also selected as reference additives. For the mixture with MoS2, the COF becomes unstable and sharply rises to 0.4 when the running time is 1 min. After 3 min, the COF quickly decreases to about 0.16 and becomes stable. The additive of LDH exhibits similar tribological property to that of MoS2. When using GO as the additive, the COF increases slightly as the testing proceeds and the final value is about 0.15. However, there is no running-in time for the composite nanoparticles and the COFs are about 0.12. As illustrated in Figure 7b, the mixture containing composite nanoparticles also possess better anti-wear performance. Therefore, the above results confirmed that the good lubricating properties of mixtures with composite nanoparticles is arised from the synergy between LDH and modifier (GO and MoS2)24. 3.4.3 Load ramp tests Load-carrying property of lubricants makes them have potential application for mechanical equipment operated in harsh conditions to avoid seizure or jam

21

. To

investigate the load-carrying property of LDH/GO and LDH/MoS2, the test load increases from 50 N to 600 N along with the increase of 50 N every 2 minutes and the result is described in Figure 8. The tests will be stopped when the COF increases abruptly over 0.3, indicating that the lubrication has failed and the friction pairs have directly contacted with each other. The most exciting results are that the mixtures containing composite nanoparticles exhibit better load-carrying property, and the


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corresponding maximum failure loads for the mixtures containing the LDH/GO or LDH/MoS2 are 500 N and 550 N or higher, respectively. The COF of the lubricant containing LDH increases abruptly when the load reaches to 300 N, indicating that the highest load with no seizure is 250 N. While for the mixtures containing GO or MoS2, they have become failed when the load respectively comes to 350 N and 150 N. For blank PAO-4, when the load reaches to 150 N, the curve of COF shows an unstable status. In summary, the composite nanoparticles of LDH/GO and LDH/MoS2 as additives possess excellent load-carrying capacity in comparison with MoS2, GO and LDH. 3.4.4 Temperature ramp tests Lubricant technology has now developed to the stage where the work conditions experienced under high loads, high speeds and high temperatures, therefore, lubricants with better thermal stability are needed

22

. To investigate the tribological

properties of LDH/GO and LDH/MoS2 as lubricant additives in different temperatures, the friction and wear tests were conducted at 200 N and different temperatures as illustrated in Figure 9. It can be clearly seen that the average COF lubricated by blank PAO-4 increases from about 0.17 to 0.32 when the test temperature changes from 25 °C to 100 °C. While for the mixtures containing different additives, the average COFs are steadier and lower, especially for the ones with LDH/GO or LDH/MoS2. Aforementioned results indicate that the composite additives can form stable boundary films on the rubbing surfaces when the temperature changes. The corresponding wear volume lubricated by blank PAO-4 also increases



sharply (from 2.2×106 μm3 to 8.0×106 μm3) when the test temperature rises from 25 °C to 100 °C as depicted in Figure 9b. And for the mixtures containing single additives (LDH, GO and MoS2), the wear volumes also have substantial increases with the increases of temperature. But for the ones with LDH/GO or LDH/MoS2, they perform excellent anti-wear performance and the wear volumes always maintain at a lower level at the different temperature. Above results indicate that LDH/MoS2 and LDH/GO could serve as high temperature lubrication additives. From the friction test results, it can be seen that the composite nanoparticle LDH/MoS2 shows better lubricating performance than LDH/GO, while the tribological performance of the nanoparticle GO is better than MoS2. The reasons are as follows: firstly, more particles of MoS2 are evenly distributed on the surface of LDH/MoS2 compared to LDH/GO from the FESEM image (Figure 2); meanwhile, LDH/MoS2 possess slightly larger interlamellar spacing than LDH/GO in Table1, leading to more easier to slip between layers; More importantly, the colloidal dispersion of LDH/MoS2 in PAO-4 is relatively stable compared to LDH/GO in PAO-4, nevertheless, GO shows better dispersion stability than MoS2 in PAO-4. 3.5 Probing the lubricating mechanism To explore the lubricating mechanism of composite nanoparticles, the structure of the wear debris collected from the worn surfaces were observed by TEM as shown in Figure 10. Comparing with the structures of LDH/GO and LDH/MoS2 (Figure 2), a relatively thick and fragmented wear debris is observed, indicating LDH/GO and LDH/MoS2 are destroyed. Thus, structural change of LDH/GO is attributed to friction


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and wear during the friction process. In other word, the additives of LDH/GO and LDH/MoS2 are broken under high load and more small nanoparticles are formed, leading to the improvement of the antifriction and anti-wear performances. Raman is used to explore the differences of the worn surfaces lubricated by blank PAO-4 and the mixtures as described in Figure 11. Strong Raman signals are observed at 690 cm-1 and 850 cm-1 for the different lubricants, indicating that a high-density protective layer of iron oxides has been formed on the sliding surfaces

36

. The peaks

of 565 cm-1 and 1043 cm-1 are detected in the curve of Raman spectra for the additives of LDH/GO and LDH/MoS2, which belong to LDH. Based on the analysis,

an

absorption layer exists on the worn surface under the lubrication of composite additives, as well as a high-density iron oxides layer is formed during the friction process, so that the friction-reducing and anti-wear properties of the base oil are enhanced. To gain more insight into the lubricating mechanism, SEM (Figure S2 in Supporting Information) and EDS are employed to analyze the worn surfaces of steel disks as shown in Figure 12. Compared to worn surface lubricated by blank PAO-4, the worn surface lubricated by the mixture with 0.5 % LDH/GO contains more carbondue to the GO existing in the boundary lubricating film. The worn surface lubricated by the mixture with 0.5 wt% LDH/MoS2 shows apparent S and Al signals, indicating LDH and MoS2 are both contributed to the boundary lubricating film . Meanwhile, the O signals of LDH/GO and LDH/MoS2 are stronger than that of blank PAO-4, indicating that a high-density protective layer of inorganic oxide has been



formed on the sliding surfaces. XPS was also used to investigate the elemental composition of worn surface (Figure 13). For worn surface lubricated by the mixture with 0.5 wt% LDH/GO, O1s signal is composed of three peaks corresponding to FeO, Fe2O3, Fe(OOH) (529.5-530.8 eV), C-O bond (532.5 eV), which is consistent with the Raman and EDS results

37, 38

. The peaks at 711 eV and 725 eV of Fe2p spectrum can be attributed to

iron oxides such as FeO, Fe3O4, Fe2O3, FeOOH 22, 39. Above XPS results reveal that a stable protection film has been formed on the rubbing surface lubricated by the mixture with LDH/GO nanocomposites. The O1s and Fe2p spectra of worn surfaces lubricated by LDH/MoS2 are similar to the one lubricated by LDH/GO. However, the peaks of S2p and Mo3d spectra are obtained under the lubrication of LDH/MoS2. The S existing in MoS2 appears at 164 eV. When the S atoms are oxidized, a new peak at 168-170 eV will appear. The peak of Mo3d (236 eV) on the worn surface is also changed

compared to the Mo3d peak in MoS2 (232 eV), suggesting part of the MoS2

is oxidized into MoO3 during the friction process22, 24. According to the above analysis results, LDH/GO and LDH/MoS2 as lubricating additives have better anti-wear, friction-reducing performance and load-carrying capacity comparing with the single additive of LDH, GO and MoS2. The lubricating mechanism can be attributed to the synergistic effect between LDH and modifier. Meanwhile,

the nano-composites and

iron oxide exist on the worn surface. It’s speculated that the film is formed by adsorption and tribo-chemical reaction.

In conclusion


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Two kinds composite nanoparticles of LDH/GO and LDH/MoS2 were prepared by electrostatic self-assembly. The surface morphologies, structures and compositions were investigated by different analysis techniques. Because of the special lamellar structure of LDH, the dispersion stability of composite nanoparticles in PAO-4 was dramatically improved. Tribological tests reveal that LDH/GO and LDH/MoS2 as additives can remarkably enhance the friction-reducing and anti-wear properties of PAO-4. Furthermore, the lubrication mechanism of modified hydrotalcite was also probed and the schematic was illustrated in Figure 14. The outstanding tribological properties of LDH/GO and LDH/MoS2 are mainly attributed to the synergistic lubricating effects between LDH and GO or MoS2.

Supporting Information Dispersing properties of LDH/GO, LDH/MoS2, GO, MoS2 and LDH in PAO-4 at the concentration of 2.0 wt% (Figure S1) and SEM images of the wear surfaces lubricated by (a, d) blank PAO-4, the mixtures with (b, e) 0.5 wt% LDH/GO and (c, f) 0.5 wt% LDH/MoS2 at RT, 200 N and 25 Hz (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

Author information Corresponding Author 1

2

e-mail: [email protected]; Tel: +86-931-4968170. e-mail: [email protected]; Tel: +86-931-4968075.



3

e-mail: [email protected];Tel.: +86-931-2976688.

Notes The authors declare no competing financial interest

Acknowledgements The authors are grateful for the financial support from National Natural Science Foundation of China (51705506) and Tribology Science Fund of State Key Laboratory of Solid Lubrication (LSL-1708).

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Table captions Table 1 Lattice parameters of LDH/GO, LDH/MoS2, LDH, MoS2 and GO dLDH(003)/ dLDH(006)/ dLDH(009)/ dMoS2(002)/ dGO(002)/ Sample nm nm nm nm nm LDH

0.91

0.41

0.26

--

--

LDH/MoS2

0.90

0.39

0.27

0.54

--

LDH-GO

0.85

0.40

0.27

--

--

MoS2

--

--

--

0.45

--

GO

--

--

--

--

0.11


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Figure captions Figure 1 The schematic diagram of preparation of LDH/GO and LDH/MoS2

Figure 2 FESEM images and EDS analysis of (a) LDH/GO, (b) LDH/MoS2 and (c) LDH Figure 3 (a) Raman spectra, (b) XRD patterns and (c) TGA FT-IR of LDH/GO, LDH/MoS2, LDH, MoS2 and GO Figure 4 TGA of LDH/GO, LDH/MoS2 and LDH Figure 5 The Zeta potential of LDH/GO, LDH/MoS2 and LDH with PAO-4 at the concentration of 2.0 wt% Figure 6 (a, b) Friction coefficients and (c) wear volumes lubricated by the mixtures with different mass concentrations of LDH/GO and LDH/MoS2 under the conditions of RT, 200 N and 25 Hz Figure 7 (a) Friction coefficient and (b) wear volumes lubricated by blank PAO-4 and the mixtures containing the same amount (0.5 wt%) of GO, MoS2, LDH/GO and LDH/MoS2 under the conditions of RT, 200N and 25Hz Figure 8 COFs for steel/steel contacts lubricated the mixtures containing by blank PAO-4 and the mixtures containing the same mass concentration (0.5 wt%) of GO, MoS2, LDH/GO and LDH/MoS2 under the condition of load ramp, RT and 25 Hz Figure 9 Friction coefficients and wear volumes of the discs lubricated by blank PAO-4 and the mixtures containing 0.5 wt% GO, 0.5 wt% MoS2, 0.5 wt% LDH/GO and 0.5 wt% LDH/MoS2 at 200 N, 25 Hz and different temperatures Figure 10 Micrographs of wear debris collected from the worn surfaces after friction (condition: RT, 25 Hz and 200 N): (a) LDH/GO (b) LDH/MoS2 Figure 11 The Raman spectrum of substrate and wear scars lubricated by blank PAO-4 and the mixtures with 0.5 wt% LDH/GO or 0.5 wt% LDH/MoS2



Figure 12 EDS images of the wear surfaces lubricated by PAO-4 and the base oil plus 0.5 wt% LDH/GO and LDH/MoS2 Figure 13 XPS spectras of (a) C1s, (b) O1s, (c) Fe2p, (d) S2p, (e) Mo3d of worn scars lubricated by LDH/GO and LDH/MoS2 Figure 14 Lubrication mechanism of modified hydrotalcite as oil additives


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Graphic for manuscript：

Two kinds composite nanoparticles of LDH/GO and LDH/MoS2 were selected as lubricating additives for steel/steel contact. They show excellent dispersion performance and lubricating property. The outstanding tribological property of LDH/GO and LDH/MoS2 are mainly attributed to the synergistic lubricating effects between LDH and GO or MoS2.



Figure 1 The schematic diagram of preparation of LDH/GO and LDH/MoS2


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Figure 2 FESEM images and EDS analysis of (a) LDH/GO, (b) LDH/MoS2 and (c) LDH



Figure 3 (a) Raman spectra, (b) XRD patterns and (c) FT-IR of LDH/GO, LDH/MoS2, LDH, MoS2 and GO 453x119mm (72 x 72 DPI)


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Figure 4 TGA of LDH/GO, LDH/MoS2 and LDH



Figure 5 The Zeta potential of LDH/GO, LDH/MoS2 and LDH with PAO-4 at the concentration of 2.0 wt% 217x170mm (150 x 150 DPI)


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Figure 6 (a, b) Friction coefficients and (c) wear volumes lubricated by the mixtures with different mass concentrations of LDH/GO and LDH/MoS2 under the conditions of RT, 200 N and 25 Hz



Figure 7 (a) Friction coefficient and (b) wear volumes lubricated by blank PAO-4 and the mixtures containing the same amount (0.5 wt%) of GO, MoS2, LDH/GO and LDH/MoS2 under the conditions of RT, 200N and 25Hz 290x110mm (72 x 72 DPI)


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Figure 8 COFs for steel/steel contacts lubricated the mixtures containing by blank PAO-4 and the mixtures containing the same mass concentration (0.5 wt%) of GO, MoS2, LDH/GO and LDH/MoS2 under the condition of load ramp, RT and 25 Hz



Figure 9 Friction coefficients and wear volumes of the discs lubricated by blank PAO-4 and the mixtures containing 0.5 wt% GO, 0.5 wt% MoS2, 0.5 wt% LDH/GO and 0.5 wt% LDH/MoS2 at 200 N, 25 Hz and different temperatures


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Figure 10 Micrographs of wear debris collected from the worn surfaces after friction (condition: RT, 25 Hz and 200 N): (a) LDH/GO (b) LDH/MoS2



Figure 11 The Raman spectrum of substrate and wear scars lubricated by blank PAO-4 and the mixtures with 0.5 wt% LDH/GO or 0.5 wt% LDH/MoS2


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Figure 12 EDS images of the wear surfaces lubricated by PAO-4 and the base oil plus 0.5 wt% LDH/GO and LDH/MoS2



Figure 13 XPS spectras of (a) C1s, (b) O1s, (c) Fe2p, (d) S2p, (e) Mo3d of worn scars lubricated by LDH/GO and LDH/MoS2


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Figure 14 Lubrication mechanism of modified hydrotalcite as oil additives


Study of composite nanoparticles based on hydrotalcite as high

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