Investigating the tribological performance of nanosized MoS₂ on graphene dispersion in perfluoropolyether under high vacuum

Abstract

We report for the first time that nanosized MoS₂ on graphene (MoS₂/Gr) was used as an additive in perfluoropolyether (PFPE) base oil. The nanocomposite can not only form a stable dispersion in PFPE for two weeks but also dramatically improve the friction-reduction and antiwear properties of the base oil for steel/steel contact under high vacuum.

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Perfluoropolyether (PFPE) lubricant, because of its low vapor pressure, high thermal and chemical stabilities, wide liquid range and good temperature viscosity characteristics, is subject to various severe operating conditions, such as in the vacuum of outer space or in vacuum pump oil, at elevated temperatures in gas turbines and so on. However, there is room to improve the friction-reduction, antiwear (AW), anticorrosion, and thermal-oxidative stability of PFPE.^1,2 As we all know, addition of performance improvers is one of the methods. Despite conventional hydrocarbon-based additives not dissolving in PFPE due to its extreme inertness and insolubility in chemicals, a number of soluble wear-reducing and anticorrosion additives have been developed in the past few decades to address the deficiencies of the oil,^1–5 mainly including phosphazene-type and PFPE derivatives. However, much less is known with respect to investigating the tribological behavior of solid additives served as PFPE oil. Previous reports suggested that performance of lubricating oils could be improved by adding layered nano-material and nanoparticle additives.^6–13 For instance, multialkylated cyclopentanes (MACs), another space-based lubricants, in combination with Ag, WS₂, WO₃, La₂O₃ and LaF₃ nanoparticles,^9–11 graphene,¹² and modified graphene oxide by ionic liquids,¹³ shown better lubricity than the base oil under vacuum and other harsh conditions.

Molybdenum disulfide (MoS₂) and graphene, the two-dimensional (2D) layered materials with many intriguing properties,¹⁴ have attracted substantial attention as oil additives to improve tribological properties.^15–17 The lubrication mechanisms of these materials are thought to be associated with the weak van der Waals interaction between layers, and the formation of a tribofilm on the interface.¹⁵ In addition, Xu et al.¹⁸ have studied the effect of graphene, MoS₂ and graphene/MoS₂ blends with different mass ratios as additives in esterified bio-oil (EBO) for steel/steel contact. They claim that the graphene/MoS₂ hybrid with contents of 0.5 wt% dispersed in EBO exhibited better tribological performances than pure graphene and MoS₂. However, the typical size of MoS₂ mentioned in the literature is between 1 and 200 μm (the median size is ∼30 μm), which might influence the lubricity of graphene/MoS₂ hybrid application as oil additive. Recently developed advanced nanomaterials such as fullerene-like MoS₂ (IF-MoS₂) seem to be promising for the friction reduction and enhancing protection against wear,^19–23 so it is significant to investigate the effect of nanosized MoS₂ on graphene as lubricant additives.

In spite of the extraordinary tribological behavior of IF-MoS₂ nanomaterials,^19–22 there are several concerns about the complexity of synthesis, high cost of obtained materials, and their toxicity.^24,25 Thus, it is becoming harder to gain the pure MoS₂ nanoparticles in combination with graphene. Fortunately, intensive research is carried out at preparation of the nanosized MoS₂ on graphene for electronic and optoelectronic,^14,26 catalytic,^27,28 and Li-ion batteries applications.²⁹ In particular, Koroteev et al.²⁷ have prepared the nanocomposites of MoS₂/graphene sample for catalytic decomposition of formic acid vapor by decomposition of MoS₃ on graphene in vacuum at different temperatures, without involving the use of complicated steps or sophisticated techniques. Furthermore, to our best knowledge, the hybrids of nanosized MoS₂ and graphene have not been previously reported in the field of lubrication.

In this paper, we prepare the composite of nanosized MoS₂ on graphene (MoS₂/Gr) by a simple, environmental friendly method, and investigate the structural properties of MoS₂/Gr using several spectroscopic and microscopy techniques. The tribological performances of MoS₂/Gr nanocomposites used as additives in perfluoropolyether (PFPE) for steel/steel contacts under high vacuum were studied. Experimental details about the nanocomposites preparation, characterization and tribology testing can be found in the ESI.†

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the MoS₂ nanoparticles are homogeneously dispersed on the graphene sheets and the nanoparticles are ranging in size from 10–50 nm (Fig. 1a–c). In the selected area electron diffraction (SAED) pattern (inset in Fig. 1c), two sets of diffraction signals are observed: the three separated diffraction rings are assigned to the (002), (100) and (110) planes of MoS₂, respectively.³⁰ And the isolated diffraction spots belong to graphene sheets.²⁹ High-resolution TEM (HRTEM) revealed the hexagonal lattice structure with the lattice spacing of 0.27 nm corresponded to the (100) plane of MoS₂ (Fig. 1d, the area designated by the red contour in Fig. 1c). The inset displays the HRTEM image for the folded edge of the MoS₂ film with interlayer spacing of 0.63 nm at the selected location, and the MoS₂ nanoparticles mainly comprise less than 8 layers.

Fig. 1 (a and b) SEM images of the MoS₂/Gr nanocomposite. (c) TEM image of MoS₂/Gr. Inset in (c) is the corresponding SAED pattern. (d) HRTEM of MoS₂/Gr (zoomed from the red contour in (c)). The inset shows a magnified image of the folded edge of a MoS₂ nanoparticle. (e) XRD patterns of graphene and MoS₂/Gr; (f) Raman spectra of microsized MoS₂ (MoS₂) and MoS₂/Gr nanocomposite in the range of 300–500 cm⁻¹.

The XRD patterns of graphene and MoS₂/Gr nanocomposites are shown in Fig. 1e. As for MoS₂/Gr sample, three broad diffraction peaks centered at around 2θ = 14.5°, 32.7°, and 58.4° correspond respectively to the (002), (100), and (110) planes of the hexagonal MoS₂ (JCPDS: 00-037-1492), and the two new broad peaks of centered at around 2θ = 16.1° and 26.3° ascribable to stacked graphene sheets. It has been reported that the difference between the Raman peak frequencies of E¹_2g (associated with in-plane vibration) and A_1g (associated with out-of-plane vibration) exhibits a stepwise decrease with decreasing number of MoS₂ layers.^31,32 Therefore the decreased frequency difference for the MoS₂/Gr nanocomposites compared with the pure MoS₂ indicated that the few-layer MoS₂ had been successfully produced (Fig. 1f). Moreover, it is clear that the intensity of E¹_2g mode is significantly lower than that of A_1g mode,³¹ suggesting that we have formed the edge-terminated MoS₂ nanoparticles with few layers on the graphene sheets, which are in good accordance with the TEM observations.

The atomic valence states and the composition of MoS₂/Gr were characterized by XPS. High resolution Mo 3d XPS spectrum is shown in Fig. 2a. It can be deconvoluted into four peaks, and the high peak around 226.3 eV corresponding to the S 2s binding energy of MoS₂.²⁷ The two main intense Mo 3d_5/2 (229.1 eV) and Mo 3d_3/2 (232.2 eV) components are also characteristic of MoS₂.^15,27 A weak peak with negligible intensity centered at 234.8 eV is due to the Mo⁶⁺ contribution,¹⁵ indicating that little oxidized molybdenum is present in the MoS₂/Gr composites. This is in good accordance with the XRD result. Additionally, the main doublet located at binding energies of 162.0 and 163.1 eV corresponds to the S 2p_3/2 and S 2p_1/2 lines of MoS₂ (Fig. 2b). Meanwhile, the binding energy at 164.2 eV suggests the existence of disulfides (S₂²⁻) or elemental sulfur.²⁷ In contrast, no high-energy component at 168.6 eV corresponding to the S⁶⁺ state realized in SO₄²⁻ groups is observed in the S 2p XPS spectrum,²⁷ indicative of the complete removal of the raw material during the preparation process. Combining the results of SEM, TEM, XRD, Raman, and XPS presented above, nanosized MoS₂ was deposited on graphene layers can be established.

Fig. 2 XPS spectra of (A) Mo 3d, and (B) S 2p of MoS₂/Gr nanocomposite. Dispersions of (a) 1.0 wt% Gr, (b) 1.0 wt% MoS₂, (c) 0.5 wt% Gr + 0.5 wt% MoS₂ and (d) 1.0 wt% MoS₂/Gr. The hybrid lubricants were equilibrated for (C) 1 day and (D) 2 weeks after preparation.

Dispersions of different additives in PFPE, obtained one day and two weeks after preparation, are shown in Fig. 2C and D. It is remarkable that 1.0 wt% Gr, a mixture of 0.5 wt% Gr + 0.5 wt% MoS₂, and 1.0% MoS₂/Gr dispersion in PFPE are stable and resist sedimentation for 2 weeks after a magnetic stirrer for half an hour and then sonication for one hour (Fig. 2Da, c, d), while the MoS₂ fails to disperse (Fig. 2Db). This result indicated that the graphene sheets could be used as an efficient substrate for improving dispersibility of microsized or nanosized MoS₂ in PFPE base oil, which might be attributed to graphene's super-thin, lightweight, flexible structure.

Fig. 3 shows the friction curves and wear rate of PFPE and PFPE plus different additives under high vacuum. It is evident from Fig. 3a that the base oil alone produced a friction coefficient averaging 0.14, while the addition of 1.0 wt% Gr slightly decreased the friction coefficient values (averaging 0.133). As for 1.0 wt% MoS₂ and 0.5 wt% Gr + 0.5 wt% MoS₂, both types of additives showed similar behaviors for the initial 750 s, with low friction coefficients averaging 0.09. The test results then differed, as the friction coefficient remained stable throughout the approximately 1800 s for 0.5 wt% Gr + 0.5 wt% MoS₂, but fluctuated in a wide range of 0.086–0.115 for 1.0 wt% MoS₂, which can be explained as follows: the low friction tribofilm formed on the contact surfaces depends mainly on the concentration of MoS₂ sheets. In spite of the introduction of 1.0% MoS₂ additive to PFPE oil, most of the MoS₂ sheets might be pushed away due to their relatively big size,²⁰ with some transferring to the steel/steel contact surfaces and forming a thin tribofilm. In order to maintain this low-friction tribofilm, supply of transferred MoS₂ sheets is required. Once the tribofilm is worn off and can't complement rapidly, the friction suddenly increased, with the friction coefficient severely fluctuating. Xu et al.¹⁸ have demonstrated that graphene could improve the retention of MoS₂ on the frictional surfaces and prevent oxidation during rubbing, so the tribofilm might be thicker and consist of more MoS₂ sheets, providing conditions for long-lasting period, low-friction behaviour of the contact. Notably, the addition of 1.0% MoS₂/Gr to PFPE oil showed great friction reduction abilities, with a friction coefficient quickly dropping to 0.06 (Fig. 3a), indicative of excellent friction reduction property of nanosized MoS₂ on graphene dispersed in PFPE.

Fig. 3 (a) Friction coefficient and (b) wear volumes of the discs lubricated by neat PFPE oil, and the base oil plus 1.0 wt% Gr, 1.0 wt% MoS₂, 0.5 wt% Gr + 0.5 wt% MoS₂ and 1.0 wt% MoS₂/Gr under high vacuum.

Meanwhile, it is seen that all of the additives can improve the AW performance of the neat oil in high vacuum (Fig. 3b). Although the ability of graphene to reduce friction is not that prominent, the enhancement of the AW property is. The hybrid lubricants composition 1.0 wt% MoS₂, 0.5 wt% Gr + 0.5 wt% MoS₂ exhibited wear reduction of 67% and 71% to the pure base oil, respectively. In contrast, the addition of 1.0% MoS₂/Gr reduce the wear rate by 91% under the same conditions. Our results thus clearly show that the MoS₂/Gr nanocomposites dispersed in PFPE exhibited much better lubricating and wear-protective effect than the pure PFPE and PFPE with graphene, microsized MoS₂, and the hybrids of graphene with microsized MoS₂. The excellent tribological characteristics of MoS₂/Gr composites can be explained by the fact that during operation under high vacuum, nanosized MoS₂ on graphene can enter the contact area of the opposite sliding surfaces and form thin, durable, and stable surface boundary layers that maintain low friction and wear.

In order to explore the lubrication mechanism of MoS₂/Gr nanocomposites as additive in PFPE under high vacuum, the tribochemical products on the rubbed surfaces was analyzed by XPS. As shown in Fig. 4A, F 1s signal of the worn surfaces lubricated with 1.0% MoS₂/Gr is composed of three peaks corresponding to FeF₂ (685.0 eV), organic oxyfluoride or carbon fluoride species (689.1 eV),^4,33 and the neat PFPE oil (692.1 eV),⁴ respectively. Additionally, inset in Fig. 4A shows the XPS spectra of F 1s of the worn surfaces lubricated with PFPE (a), and PFPE added 1.0 wt% Gr (b), 1.0 wt% MoS₂ (c), 0.5 wt% Gr + 0.5 wt% MoS₂ (d) and 1.0 wt% MoS₂/Gr (e). It can be seen that F 1s signals at 685.1 eV of the worn surfaces lubricated with PFPE incorporation of 1.0 wt% Gr and 1.0 wt% MoS₂/Gr (b and e) show significantly lower intensity than those on the worn surfaces lubricated with pure PFPE (a), indicating that both graphene and MoS₂/Gr nanocomposites suppress the formation of iron fluoride on the worn surface during the sliding process and hence retard the degradation and decomposition of PFPE in high vacuum condition.⁴ The addition of microsized MoS₂ and the hybrid of MoS₂ with graphene can also restrain the formation of FeF₂ (c and d). However, their ability to prevent catalytic decomposition of PFPE are inferior to graphene and nanosized MoS₂ on graphene. The XPS spectra of Mo 3d shown in Fig. 4B is composed of four peaks corresponding to MoS₂ (229.1 and 232.2 eV), and MoO₃ (233.2 and 235.6 eV).²⁷ Inset in Fig. 4B shows that the Mo 3d peaks presenting at those binding energies on the worn surface lubricated with PFPE added 1.0 wt% MoS₂/Gr (e) have higher intensities than that on the worn surfaces lubricated with PFPE added 1.0 wt% MoS₂ (c) and 0.5 wt% Gr + 0.5 wt% MoS₂ (d), suggesting that during the sliding process in high vacuum, it is easier for nanosized MoS₂ on graphene to enter the contact surfaces with PFPE oil and then form boundary lubrication film containing more MoS₂ nanoparticles. Almost no difference is observed in XPS spectra of S 2p (inset in Fig. 4C) on the worn surfaces lubricated with 1.0 wt% MoS₂ (c), 0.5 wt% Gr + 0.5 wt% MoS₂ (d) and 1.0 wt% MoS₂/Gr (e), and the peaks of S 2p appear at the binding energy of 161.8, 163.1, 168.6 and 169.5 eV (Fig. 4C), which corresponds to the S²⁻ and S⁶⁺ species attributed to MoS₂ and FeSO₄/Fe₂(SO₄)₃,²⁷ respectively. The above XPS analysis reveals that a stable protection film has been formed on the rubbed surface lubricated by MoS₂/Gr nanocomposites with PFPE oil. The protection film composed of MoS₂, FeSO₄/Fe₂(SO₄)₃ and compound containing the organic oxyfluoride or carbon fluoride species on the lubricated metal surface, which contributes to the extraordinary tribological properties of MoS₂/Gr nanocomposites in PFPE oil under high vacuum.

Fig. 4 Deconvolution of (A) F 1s, (B) Mo 3d and (C) S 2p XPS spectra of the worn surfaces lubricated by 1.0 wt% MoS₂/Gr under high vacuum. Inset in (A)–(C) show the XPS spectra of F 1s, Mo 3d and S 2p of the worn surfaces lubricated by (a) the neat PFPE oil, (b) 1.0 wt% Gr, (c) 1.0 wt% MoS₂, (d) 0.5 wt% MoS₂ + 0.5 wt% Gr, and (e) 1.0 wt% MoS₂/Gr under high vacuum.

In summary, nanosized MoS₂ was deposited on graphene sheets (MoS₂/Gr) via decomposition of MoS₃ in flowing Ar at 800 °C. The graphene sheets play an important role not only in improving dispersibility of nanosized MoS₂ in PFPE base oil, but also in suppressing the formation of iron fluoride for steel/steel contacts when lubricated with PFPE oil. Tribological results indicated that the MoS₂/Gr nanocomposites as additives in PFPE oil exhibited much better friction-reduction and antiwear performances than pure PFPE oil and the base oil in combination with graphene, microsized MoS₂ and the hybrid of graphene with microsized MoS₂ under high vacuum. The excellent tribological characteristics of MoS₂/Gr composites are attributed to the formation of stable surface boundary films that maintain low friction and wear. XPS analytical results further proved that the tribofilm composed of MoS₂, FeSO₄ or Fe₂(SO₄)₃, and compound containing the organic oxyfluoride or carbon fluoride species was formed on the worn surfaces.

Acknowledgements

The authors are thankful for financial support of this work by “973” Program (2013CB632301) and national natural science foundation of china (NSFC 51475445).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: General experimental details, and supplementary figures. See DOI: 10.1039/c6ra22863a

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