Investigation on tribological behaviors of MoS2 and WS2 quantum dots as lubricant additives in ionic liquids under severe conditions

Despite excellent tribological behaviors of ionic liquids (ILs) as lubricating oils, their friction-reducing and anti-wear properties must be improved when they are used under severe conditions. There are only a few reports exploring additives for ILs. Here, MoS2 and WS2 quantum dots (QDs, with particle size less than 10 nm) are prepared via a facile green technique, and they are dispersed in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm]PF6), forming homogeneous dispersions exhibiting long-term stabilities. Tribological test results indicate that the addition of MoS2 and WS2 QDs in the IL can significantly enhance the friction-reducing and anti-wear abilities of the neat IL under a constant load of 500 N and a temperature of 150 °C The exceptional tribological properties of these additives in the IL are ascribed to the formation of protective films, which are produced not only by the physical absorption of MoS2 and WS2 QDs at the steel/steel contact surfaces, but also by the tribochemical reaction between MoS2 or WS2 and the iron atoms/iron oxide species.


Introduction
Ionic liquids (ILs), known for their extremely low volatilities, wide liquid temperature ranges, high thermal and chemical stabilities, and exceptional tribological properties, have been intensively studied as lubricants and lubricant additives in various applications [1][2][3]. For instance, ILs in space technology applications have attracted considerable interest over the past decade [4], and the usage of ILs as hightemperature lubricants under the conditions of high load, high speed, and elevated temperature has gained increasing attention [1,[5][6][7][8]. Numerous reports have been published on the synthesis of ILs, particular oil-miscible ILs for improving the friction reduction and anti-wear (AW) properties of lubricating oils in recent years [3,9,10]. Despite the excellent tribological behaviors of ILs as lubricating oils, their friction reduction and AW properties must be improved when they are used under severe conditions. There are only a few reports exploring additives for ILs. For example, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF 6 ]), a commercially available IL, has been observed to exhibit superior friction reduction and AW performances compared with conventional lubricants [1,8,11,12]; however, pure [BMIm][PF 6 ] offers poor lubricating properties when it is used under a high load condition [13,14]. Previous reports have demonstrated that covalent modification of multiwalled carbon nanotubes (MWCNTs) with imidazolium cation-based ILs and brush-like poly (ionic liquids) (PILs) can remarkably improve the dispersibility of MWCNTs in [BMIm][PF 6 ] base oil and the tribological behaviors of this IL at high loads [13,14], but surface functionalizations of MWCNTs with ILs and PILs are restricted by high-cost and complex synthesis processes. The development of a new type of additive with excellent tribological performances under severe conditions and low cost is required to fully exploit the advantages of ILs.
Studies have been extensively conducted for fabricating MoS 2 and WS 2 nanoparticles (NPs) for use as effective oil additives owing to their fascinating characteristics, such as high thermal and chemical stabilities, nanometric size, and exceptional lubricating properties. However, one of the major drawbacks of NPs as friction-reducing and AW additives is their poor dispersibility in lubricating oils, which limits their use in lubrication applications. To address this problem, various methods have been employed to disperse MoS 2 and WS 2 NPs in base oils. For instance, surface modification is the most promising method to promote MoS 2 dispersion [15][16][17]. Mixed MoS 2 / graphene dispersions in base oils resist sedimentation for two weeks [18]. MoS 2 with different morphologies and sizes was fabricated and could be dispersed in different oils for a few weeks [19][20][21]. In particular, MoS 2 and WS 2 quantum dots (QDs, with particle size less than 10 nm) can form a homogeneous and stable dispersion in polyalkylene glycol (PAG) base oil, and can significantly enhance the friction-reducing and AW properties of neat PAG base oil at elevated temperatures [22]. Notably, the small size effect, high surface effect, and quantum size effect of MoS 2 and WS 2 QDs might play an important role in the formation of a stable dispersion of PAG base oil additized with solid NPs. These exciting physical properties have prompted us to investigate the application of MoS 2 and WS 2 QDs as additives in ILs.
Herein, MoS 2 and WS 2 QDs are fabricated by using sonication combined with solvothermal processing of bulk MoS 2 and WS 2 powder in N,N-dimethylformamide (DMF) [23], and their dispersibility in [BMIm]PF 6 is evaluated. The tribological behaviors of MoS 2 and WS 2 QDs added to the IL are investigated under high loads and high temperatures. The friction-reducing and AW mechanisms of these additives are explored using scanning electron microscopy with energydispersive X-ray spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS).

Preparation of MoS 2 and WS 2 QDs and the dispersion of IL additized with MoS 2 and WS 2 QDs
MoS 2 and WS 2 QDs were synthesized using previously reported methods [23], as shown in Scheme 1. Briefly, 1g of commercial MoS 2 and WS 2 powder with an average grain size of 500 nm (Shanghai Research Institute of Rare-Metal) was dispersed in 100 mL of DMF. The mixture was sonicated for 3 h using a sonicator (SCIENTZSB-5200D, output power 250 W), and subsequently, the resulting dispersion was transferred to a 100-mL round-bottomed flask and heated for 6 h at 140 °C under vigorous stirring. Thereafter, the black mixture was centrifuged for 10 min at 3,000 rpm to remove the solid phase. The light-yellow dispersion of MoS 2 and WS 2 QDs was evaporated. The as-synthesized MoS 2 and WS 2 QDs were added to [BMIm]PF 6 (J&K Scientific Ltd., purity≥ 99%), and thereafter, the suspension was thoroughly mixed using a magnetic stirrer for 30 min and ultrasonic mixing for 30 min. The obtained dispersion of MoS 2 and WS 2 QDs added to the IL is homogeneous and resists sedimentation for several months after preparation.

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Friction 8(4): 674-683 (2020) | https://mc03.manuscriptcentral.com/friction X-ray powder diffraction (XRD, Bruker D8 DISCOVER: Cu Kα radiation, λ = 1.54 Å), and UV-vis spectroscopy (Hitachi U-4100). The tribological measurements were conducted using a reciprocating friction tester (Optimal-SRV-IV) with an upper ball (Ø 10 mm, AISI 52100 steel, hardness 59-60 HRC) running against a lower stationary disk (Ø 24 mm × 7.9 mm, AISI 52100 steel, hardness 58-60 HRC) at a frequency of 25 Hz, amplitude of 1 mm, and duration of 30 min. The friction curve was recorded automatically using a computer connected to the SRV tester. The corresponding wear volume of the lower disks was obtained using a noncontact three-dimensional surface mapping profilometer (MicroXAM-3D). Three repetitive measurements were performed for each sample, and the averaged values of the friction coefficient curve and wear volume are reported in this paper. The morphology of the wear scars was measured using SEM-EDS (JSM-5600LV). The typical elemental distribution of the worn surfaces was investigated using XPS (PHI-5702).

Physical properties of MoS 2 and WS 2 QDs
Transmission electron microscopy (TEM) was used to investigate the particle size and morphology of the MoS 2 and WS 2 QDs. As shown in Figs. 1(a) and 1(c), the median size of the MoS 2 and WS 2 QDs is approximately 3.0 nm and 3.2 nm, respectively, and the HR-TEM results (shown in Figs. 1(b) and 1(d)) indicated that the lattice spacing is 0.23 nm and 0.27 nm for MoS 2 and WS 2 QDs, respectively, corresponding to the (103) and (101) planes of MoS 2 and WS 2 crystals.
XPS established that the main compounds in the as-synthesized products were MoS 2 and WS 2 , along with some MoO 3 and WO 3 . The Mo 3d XPS spectrum ( Fig. 2(a)) collected for the MoS 2 QDs contained peaks at 227.0 eV, 229.6 eV, and 232.8 eV, assigned to the 2s electrons of S 2− , Mo 4+ 3d5/2, and Mo 4+ 3d3/2, respectively [22]. The S 2p signal in the XPS spectrum ( Fig. 2(b)) was centered at 162.2 eV and 163.4 eV, which are attributed to S 2− 2p3/2 and S 2− 2p1/2, respectively [23]. In addition, a small peak in the Mo 3d spectra at 236.3 eV is ascribed to Mo 6+ 3d3/2 and a peak in the S 2p spectra at 168.6 eV is assigned to S 6+ 2p3/2. The two species realized in MoO 3 and SO 4 2− might be generated during the synthesis process [23]. Similarly, the W 4f and S 2p XPS spectra of WS 2 QDs (Figs. 2(c) and 2(d)) demonstrated that WS 2 QDs were successfully synthesized with a relatively low content of W 6+ and S 6+ species (realized in WO 3 and SO 4 2− ) in our product.
Raman and XRD spectra of the MoS 2 and WS 2 QDs are also obtained, and no signal or peak is observed (data not shown), which might be due to the fact that both MoS 2 and WS 2 QDs are monolayer and have no interaction with each other [24]. The UV-vis spectra of MoS 2 and WS 2 QDs in DMF are shown in Fig. 3(a). The characteristic peaks of MoS 2 and WS 2 QDs are almost the same and are located at the near-UV region (λ < 300 nm), which are attributed to the excitonic features of MoS 2 and WS 2 QDs [25]. The long-term stability of the 1% MoS 2 and WS 2 QDs dispersion in the IL was evaluated using UV-vis spectroscopy [26]. It can be observed from Fig. 3(b) that the concentrations of these dispersion systems only decreased slightly within one month, indicating excellent dispersion stability of the MoS 2 and WS 2 QDs in the IL.

Tribological performances of MoS 2 and WS 2 QDs
The friction-reducing and AW properties of the MoS 2 and WS 2 QDs added to the IL are investigated using Optimal-SRV-IV at high load and high temperature. It    Fig. 4(a) shows the friction curve of pure perfluoropolyether (PFPE) base oil under the same conditions, and the friction coefficient of PFPE oil is significantly larger than those of the IL base oil and the oil additized with MoS 2 and WS 2 QDs. These results indicate that MoS 2 and WS 2 QDs have good friction-reducing property at high load and high temperature. Figure 4(b) displays the corresponding wear volume of steel disks lubricated by the IL and the IL with MoS 2 and WS 2 QDs. It shows that MoS 2 and WS 2 QDs reduce the wear volume of the base oil by approximately 91% and 89%, respectively. 3D optical microscopic images of the worn surfaces inset in Fig. 4(b) further indicate that MoS 2 and WS 2 QDs can significantly improve the AW property of ILs under high load and high temperature. The excellent tribological behaviors of these additives are probably because MoS 2 and WS 2 with particle size below 10 nm could easily enter the ball-disk contact interface and form an effective protective film. MoS 2 and WS 2 QDs will "fill" the asperity valleys and establish a smooth boundary film between the contacting surfaces. Both the films showed friction reduction and wear resistance at elevated temperatures. The tribological performances of the MoS 2 and WS 2 QDs dispersion in the IL were also evaluated at different temperatures and a constant load of 500 N. As shown in Fig. 5(a), the addition of MoS 2 and WS 2 QDs has no effect on the friction reduction and AW behaviors of the IL at a temperature below 50 °C , whereas the two NPs can significantly reduce the friction coefficients and wear volumes of the base oil when the temperature increases from 100 to 250 °C . This conclusion is in accordance with the result of addition of MoS 2 and WS 2 QDs to PAG base oil at different temperatures [22], and the possible mechanism for the lubrication action has been reported previously. In brief, high temperature plays an active role in the viscosity of nanofluids, the free movement of NPs in ILs, and the tribochemical reaction of the lubricant during friction and wear processes, which benefits the formation of a boundary protective film. In the case of low temperatures, MoS 2 and WS 2 QDs in the contact interface cannot be complemented timely owing to the increase in the viscosity of base oil and the restriction of motion of NPs in ILs as the lubrication film is worn away. Figure 6 shows the friction coefficient and wear volume of the IL additized with 1% MoS 2 and WS 2 QDs at various applied loads and a temperature of 150 °C . It can be observed that the addition of 1% MoS 2 and WS 2 QDs can improve the tribological behaviors of the IL with an increase in the load from 50 to 600 N at an elevated temperature. In particular, the friction coefficient and wear volume of the base oil are dramatically reduced when the load is 100 N, 200 N, 300 N, 400 N, and 500 N. This could be explained by the fact that the surface temperature is largely dependent on the load, and additives that might be effective at high loads may be ineffective at low loads (and vice versa) [27]. In addition, MoS 2 QDs and WS 2 QDs show similar friction-reducing and AW capacities for the ILs at different temperatures and loads.

Surface analysis
The wear surfaces of steel disks lubricated by the IL and the dispersions of the IL with 1% MoS 2 and WS 2 QDs at 150 °C and 500 N were investigated using SEM, and the tribofilms on these worn scars were analyzed using SEM-EDS. As shown in Figs. 7(a)-7(c), the wear surface under the lubrication of the pure IL shows a much wider worn scar, indicating that severe scuffing occurred in this case. However, the wear scars of the steel disks lubricated by the IL with 1% MoS 2 and WS 2 QDs evidently became narrow, suggesting that MoS 2 and WS 2 QDs can significantly improve the AW property of the IL base oil. This is consistent with the wear volume result in Fig. 3(b). The detailed views of the corresponding wear surfaces are shown in Figs. 7(a'), 7(b'), and 7(c') (the areas designated by the red contour in Figs. 7(a), 7(b) and 7(c), respectively). The result indicates that the tribofilm on the worn surface lubricated by the pure IL contains no Mo and S (inset in Fig. 7(a')), whereas the boundary lubrication films on the wear surfaces lubricated by the IL with   The friction-reducing and AW mechanisms of the IL with MoS 2 and WS 2 QDs are further explored using XPS, and the results are shown in Fig. 8. The XPS spectra of Fe 2p (Fig. 8(a)) can be deconvoluted into six peaks corresponding to FeS 2 (708.9 eV), FeO (709.7 eV), Fe 3 O 4 (710.7 eV), FeOOH (711.8 eV), FePO 4 (712.8 eV), and FeSO 4 (713.6 eV) [22,28], and the Fe 2p signals of the worn surfaces lubricated with the neat IL (a) and the IL additized with 1% MoS 2 QDs (b) and 1% WS 2 QDs (c) are similar to each other at 500 N and 150 °C , which might be attributed to the similar tribochemical reactions of the IL with the steel/steel contact surfaces. Similarly, the peaks of P 2p and F 1s of the worn surfaces under lubrication of the three types of lubricants appear at 133.7 eV and 684.9 eV (Figs. 8(b) and 8(c)), corresponding to FePO 4 and FeF 2 [28], respectively. The signals of S 2p of the worn scars lubricated by the dispersions of the IL with 1% MoS 2 and WS 2 QDs are located at 168.6 eV, which are assigned to FeSO 4 [22,28]. The XPS spectra of Mo 3d shown in Fig. 8(e) contain three peaks corresponding to Mo 5+ (231.4 eV), MoS 2 (232.4 eV), and Mo 6+ (233.2 eV) [22,23], and the spectra of W 4f shown in Fig. 8(f) are composed of three peaks corresponding to WS 2 (32.4 eV and 34.6 eV) and 37.5 eV (WO 3 ) [22,23]. These results indicate that the friction-reducing and AW behaviors of the IL are attributed to the formation of a boundary lubrication film containing FeO, Fe 3 O 4 , FeOOH, FeF 2 , and FePO 4 , and the addition of MoS 2 or WS 2 QDs can significantly improve the tribological properties of the IL because the dispersion of the IL with MoS 2 or WS 2 QDs could form a stable protective film composed of FeSO 4 , MoS 2 or WS 2 , and the compounds generated from the tribochemical reactions of the IL with the steel/steel contacts surfaces.

Conclusions
In summary, MoS 2 and WS 2 QDs were investigated as friction-reducing and AW additives in an IL ([BMIm]PF 6 ) for the first time. They could form a homogenous and stable dispersion in the IL for several months, and significantly reduce the friction coefficient and wear volume of the IL at 500 N and 150 °C . The excellent tribological behaviors of these two additives could be explained by the fact that MoS 2 and WS 2 QDs not only formed a boundary lubrication film via physical absorption but also generated protective films during tribochemical reactions. The film is composed of MoS 2 or WS 2 , FeSO 4 , FeS 2 , FeO, Fe 3 O 4 , FeOOH, FeF 2 , and FePO 4 , resulting in friction reduction and wear resistance at elevated temperatures. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Kuiliang GONG. He got his master degree (2010) in materials sicence from Qingdao University in China.
He is currently a Ph.D. candidate at Lanzhou Institute of Chemical Physics. His research is focused on nanoadditives for lubricating oil.