Effects of magnetic ionic liquid as a lubricant on the friction and wear behavior of a steel-steel sliding contact under elevated temperatures

A magnetic ionic liquid (abridged as MIL) [C6mim]5[Dy(SCN)8] was prepared and used as the magnetic lubricant of a steel-steel sliding pair. The tribological properties of the as-prepared MIL were evaluated with a commercially obtained magnetic fluid lubricant (abridged as MF; the mixture of dioctyl sebacate and Fe3O4, denoted as DIOS-Fe3O4) as a control. The lubrication mechanisms of the two types of magnetic lubricants were discussed in relation to worn surface analyses by SEM-EDS, XPS, and profilometry, as well as measurement of the electric contact resistance of the rubbed steel surfaces. The results revealed that the MIL exhibits better friction-reducing and antiwear performances than the as-received MF under varying test temperatures and loads. This is because the MIL participates in tribochemical reactions during the sliding process, and forms a boundary lubrication film composed of Dy2O3, FeS, FeSO4, nitrogen-containing organics, and thioether on the rubbed disk surface, thereby reducing the friction and wear of the frictional pair. However, the MF is unable to form a lubricating film on the surface of the rubbed steel at 25 °C, though it can form a boundary film consisting of Fe3O4 and a small amount of organics under high temperature. Furthermore, the excessive Fe3O4 particulates that accumulate in the sliding zone may lead to enhanced abrasive wear of the sliding pair.


Introduction
With the continuous progress of industrial technology, newly developed lubricants need to meet more and more stringent requirements for mechanical properties and lubricating performance [1][2][3]. Besides excellent load-bearing capacities, good friction-reducing, and antiwear abilities, they also need to exhibit self-repairing capabilities and environmental acceptance.
In this respect, magnetic lubricants as novel lubricants could be of special significance, because they can fill up the scratches and grooves on rubbed surfaces under an external magnetic field to achieve continuous lubrication, and they can counteract the effect of gravity and the centripetal force during the lubrication process, thereby preventing leakage and external pollution [4][5][6]. Currently, magnetic lubricants are often prepared by dispersing magnetic nanoparticles in conventional lubricants [7][8][9]. In recent years, magnetic fluids (MFs) as lubricants have been successfully applied to rolling bearings, sliding bearings, high-speed grinders, astronomical observation devices, and other mechanical equipments [10][11][12]. However, nanoparticles are thermodynamically unstable because of their high surface energy. In other words, they are susceptible to agglomeration and precipitation during long-term use or under varying environments (e.g., high temperature, high pressure), which hinders their practical application in engineering. Therefore, there is a need to develop new magnetic sealing and lubricating materials [13,14].
Magnetic ionic liquids (MILs) composed of organic cations as well as inorganic or organic anions could be promising magnetic fluids, as the anion of MILs contains a magnetic center (with single electron organic free radical or metal ion complex) that can respond to an external magnetic field, thereby becoming magnetized [15][16][17][18]. However, MILs are essentially different from conventional magnetic lubricants used in tribology. On one hand, traditional magnetic lubricants are usually mixtures (lubricating base liquid + magnetic particles), but MILs are pure compounds rather than mixtures with multi-components, which means that MILs are homogeneous systems with thermodynamic stability and are free of aggregation, sedimentation, and phase separation. On the other hand, traditional MFs require a large amount of surfactant to achieve dispersion in the base fluid, which means that they often exhibit poor stability as compared with MILs. Moreover, traditional MFs exhibit undesired volatility and phase separation, but MILs often possess excellent high temperature stability and low (no) vapor pressure [19,20]. Therefore, MILs as functionalized ionic liquids could be of special significance in tribology [21][22][23]. Driven by these perspectives, some researchers have been studying the synthesis of new MILs as well as investigating their physicochemical properties [24][25][26]. Now, the main problem with MILs as magnetic lubricants is that their magnetic properties are much poorer than those of conventional MFs. In other words, compared with traditional MFs, MILs often rely on a much stronger magnetic field to achieve magnetic manipulation [27]. Furthermore, few studies comparing the tribological properties of MILs and traditional MFs have been conducted.
Bombard et al. [28] prepared and investigated the tribological properties of ionic liquids, ionic liquidbased MFs (ionic liquids + carbonynyl iron particles), and PAO-based MF (PAO + carbonyl iron particles; PAO refers to poly-alpha olefin). They found that ionic liquids exhibit optimal antiwear effects for the steelsteel tribopair and polyformaldehyde-polyformaldehyde tribopair, and all ionic liquid-based MFs exhibit a much better friction-reducing ability than traditional PAO-based MFs. Shi et al. [29] synthesized an ionic liquid-based MF by modifying ferric oxide particles with a carboxyl functionalized ionic liquid, and suggested that the composite system could be applicable to extreme operating conditions requiring high temperature and low vapor pressure.
This research highlights the preparation of an MIL ([C 6 mim] 5 [Dy(SCN) 8 ]), where C 6 mim refers to 1-hexyl-3-methylimidazolium. We also compare the antiwear mechanism and tribological properties of the as-prepared MIL with those of a traditional MF composed of DIOS and Fe 3 O 4 (DIOS refers to dioctyl sebacate). Here, DIOS is used as the base stock for formulating the MFs, because, as an ideal carrier for preparing MFs widely used in mechanical sealing, lubrication, and other fields, it has excellent chemical stability and a good viscosity-temperature characteristic.

Preparation and characterization of MIL ([C 6 mim] 5 [Dy(SCN) 8 ])
The MIL ([C 6 mim] 5 [Dy(SCN) 8 ]) was prepared according to Ref. [27]. Fourier transform infrared (FTIR) spectrum ( Fig. S1) and element analysis results of the as-prepared MIL are shown in supporting information. The density of the MIL is 1.22 g/cm 3 . The molecular structure of the as-prepared MIL is shown in Fig. 1 Fig. 3, and it can be seen that the MIL is a homogeneous and transparent orange liquid. Figure 4 shows the variations in the shear stress and viscosity of the MIL and MF with shear rate at  25 °C . It can be seen that the as-prepared MIL has a higher viscosity than the MF in the selected shear rate range. Furthermore, the viscosity of the MF remarkably decreases with increasing shear rate, and it exhibits obvious shear-thinning characteristic in the early stage of shearing.

Friction and wear test
A high speed reciprocating friction and wear tester made by Huahui Instrument Technology Company Limited (Lanzhou, China) was employed to investigate the tribological properties of the as-prepared MIL and the as-received MF. The 316 steel ball (4 mm in diameter) and 304 non-magnetic steel disk (40 mm in diameter, 10 mm in thickness) comprised the ball-ondisk sliding pair. The schematic diagram of the tribopair is shown in Fig. 5. Prior to the sliding tests under various temperatures (25, 50, 100, and 150 °C ), the lubricant was dropped into the contact zone. The test results were automatically logged to a computer linked to the test rig. Table 2 shows the sliding friction and wear test conditions at different temperatures.

Analysis of worn steel surfaces
A surface mapping profiler (Bruker Contour GT-K   Contour-K1) was employed to obtain the twodimensional (2D) depth profiles and three-dimensional (3D) maps of wear scars. The morphology and element composition of the upper and lower worn surfaces were analyzed by field emission scanning electron microscopy (Gemini SEM 500). The chemical compositions of the worn surfaces were analyzed by X-ray photoelectron spectroscopy (XPS; Kratos AXIS ULTRA).  Fig. 7. The wear volume of a steel-steel sliding contact lubricated by the MIL under different loads rises slowly with increasing temperature, but the value for the steel-steel sliding contact lubricated by the MF under the same conditions tends to rise more quickly therewith.

Tribological behavior of a steel-steel contact lubricated by the MIL and MF
This implies that the as-prepared MIL is advantageous over the as-received MF in reducing the wear of the steel-steel contact at different temperatures. In particular, the wear volume of the lower steel disk lubricated by the as-received MF tends to rise sharply when the temperature rises from 100 to 150 °C , regardless of the varying load. In other words, it  Figure 8 shows the variation curves of friction over time at different temperatures. It can be seen that, under MIL lubrication at 25 °C , the initial coefficient of friction increases slightly from 0.17 to 0.18 when the normal load increases from 100 to 200 N. Under MF lubrication, however, the coefficient of friction is large and tends to fluctuate significantly, especially at 100 N ( Fig. 8(a)). At other temperatures (Figs. 8(b)-8(d)), the coefficient of friction varies in a manner similar to that in Fig. 5(a); in general, the friction curve under MIL lubrication at different temperatures is lower and more stable than that under MF lubrication. Namely, there is a small amount of scratches and deep grooves on the worn surface under MF lubrication (Figs. 9(e) and 9(f)), and the depth and amount of the grooves on the wear scar tend to increase with increasing temperature (Figs. 10(e) and 10(f)). However, the wear scars of the steel disk and ball lubricated by the MIL, are much smaller than those lubricated by the MF and contain fewer scratches and grooves, as evidenced by relevant 2D depth profiles of the wear scars (Figs. 9(g), 9(h), 10(g), and 10(h)).

Analysis of the worn surface of a lower steel disk and upper steel ball
In particular, under high temperature conditions, a transfer film higher than the worn surface is formed on the worn surface of the upper steel ball under MF lubrication at elevated temperatures, as evidenced by the relevant 3D topography (Fig. 10(f)). Figure 11 shows the SEM micrographs of worn disk surfaces under MF and MIL lubrication at 25 °C and 100 N. Under the MF lubrication, deep grooves and a large amount of wear debris are present on the worn surface of the steel disk (Figs. 11(a) and 11(b)). This indicates that the steel disk mainly undergoes abrasive damage under MF lubrication, which corresponds well with the 3D maps of the wear scars. This is because, during the sliding process, Fe 3 O 4 particles enter the frictional contact area and participate in the friction and wear process, resulting in three body abrasions. Under MIL lubrication, the worn surface    of the steel disk is smooth and shows almost no sign of friction-induced damage (Figs. 11(c) and 11(d)). This implies that the MIL exhibits favorable antiwear ability for the steel-steel sliding contact. Figure 12 shows the SEM micrographs of worn disk surfaces under MF and MIL lubrication at 100 N and 150 °C . At a high temperature of 150 °C , grooves are formed on the worn surface lubricated with MIL (Figs. 12(c) and 12(d)), which indicates that the wear of the sliding contact is exacerbated at elevated temperatures. Under MF lubrication, the steel disk surface seems to undergo more serious abrasion damage at 150 than at 25 °C (Figs. 12(a) and 12(b)). As reported elsewhere, the MF particle tends to agglomerate during the friction process, thereby resulting in abrasive wear in association with scratches and grooves on the rubbed surfaces [30][31][32]. As the temperature rises, abrasive wear is exacerbated to result in increases in the depth and amount of the scratches and grooves. Furthermore, the phase equilibrium of the MF may be destroyed under elevated temperature, and the deposition of the nanoparticle on the friction contact area could be enhanced, thereby causing an increase in the wear volume.
The SEM micrographs and EDS analysis of worn surfaces under MIL lubrication at 100 N and 150 °C are shown in Fig. 13. Dy, S, and N are detected on the rubbed surfaces of the steel ball and disk; in particular, S is significantly enriched in the contact area of the sliding pair. This means that MIL as a lubricant can form a boundary lubrication film via adsorption and a chemical reaction. Besides, the worn steel surface under MIL lubrication seems to be quite smooth and contains no obvious grooves compared with the one lubricated with MF, which conforms well with the observation that the MIL exhibits better frictionreducing and antiwear performances for the steel-steel contact than the MF.
The SEM micrographs and EDS analysis of the worn surfaces under MF lubrication at 100 and 150 N are shown in Fig. 14. Together with the 3D topography of the upper steel ball (Fig. 10(f)), they indicate that a deposit layer is formed on the worn surface of the lower steel disk, while a transfer layer is formed on the worn surface of the upper steel ball (Figs. 14(a) and 14(b)). Corresponding EDS analysis indicates that the tribofilm contains a large amount of Fe and O, which implies that a thick tribofilm is formed on the rubbed surface of the steel disk after sliding at 150 °C . Along with destruction of the asperities under a normal load and shear force, the large wear debris is formed, thereby causing scratches and grooves on the rubbed steel surface; furthermore, an increase and fluctuation in the coefficient of friction is observed [33].
Under the ideal fluid lubrication condition, the electric contact resistance (abridged as ECR) between the sliding pair should tend to be infinite, as the frictional pair would be separated by a layer of lubricant. When the steel-steel sliding pair comes into direct contact after extended sliding, the electric resistance would become very small, owing to the metal-metal contact. Figure 15 shows the variation in the electric contact resistance of the tribopair lubricated by MIL and MF at different temperatures with sliding | https://mc03.manuscriptcentral.com/friction  time. Under MIL lubrication at 25 °C , the electric contact resistance stays at a low level in the initial stage of rubbing (tens of second), and then it gradually increases with extending sliding time ( Fig. 15(a)). After sliding for 250 s, the resistance value tends to be stable. This is because MIL can form an adsorption film on the rubbed surfaces of the frictional pair (supporting information, Fig. S2) to hinder its direct contact.
As the sliding time rises, a boundary lubrication film composed of an adsorption film and tribochemical reaction film is formed on the rubbed steel surface, thereby leading to a gradual increase in the electric contact resistance therewith. However, the MF can hardly form an effective boundary lubrication film at 25 °C , and in this case, the electric contact resistance is almost zero owing to the direct contact between the steel-steel sliding pairs.
When the MIL is used as the lubricant at elevated temperature, the electric contact resistance of the steelsteel sliding pair reaches a relatively stable value after sliding for 100 s, but it stays at a lower level compared with the one at 25 °C (Fig. 15(b)). This could because it is more difficult for the adsorption film to form on the rubbed steel surface under an elevated temperature, and the tribochemical reaction mainly accounts for the formation of a boundary lubrication film thereon. As a result, the steel-steel sliding pair would have a greater opportunity to achieve direct contact under elevated temperatures, which results in significantly reduced electric contact resistance thereat. When the MF acts as the lubricant under a high temperature, the Fe 3 O 4 particles can form a deposited layer on the rubbed surface of the frictional pair. As the friction process progresses, the thickness of the deposited layer increases, and the contact resistance also increases therewith. As Fe 3 O 4 itself has a certain conductivity, the contact resistance value finally remains constant at approximately 10 mΩ.
To further explore the lubrication mechanism of MILs and MFs, we analyzed the chemical state of the worn surface elements by XPS (before performing XPS analysis, the steel disks were ultrasonically cleaned with ethanol and petroleum ether to remove any residual lubricant on the worn surfaces). Figure 16 shows the curve-fitted XPS spectra of the worn surface under MF at 150 °C and 100 N. The    [34]; in particular, the Fe2p 3/2 peak can be assigned to Fe 2+ and Fe 3+ species with a deconvoluted area ratio of 1:2, which is consistent with the stoichiometry of Fe 3 O 4 [35,36]. The O1s spectrum can be deconvoluted into three peaks at 529.6, 531.1, and 532.3 eV, and they are assigned to Fe 3 O 4 , C=O bond, and C-O bond, respectively [37,38].
The curve-fitted XPS spectra of the worn surface under MIL lubrication at 150 °C and 100 N are shown in Fig. 17. The Dy4d 3/2 and Dy4d 5/2 peaks emerge at 153.2 and 156.7 eV, respectively, which is in agreement with the oxidation state of Dy 2 O 3 [39,40]. Three S2p peaks emerge at 161.9, 163.1, and 168.5 eV, respectively. The peak at 161.9 eV is combined with the Fe2p peak at 710.1 eV, corresponding to FeS [41,42]. The S2p peak at 163.1 eV is attributed to the C-S-C bond [43]. The S2p peak at 168.5 eV, in association with the Fe2p peak at 713.2 eV and O1s peak at 532.4 eV, reveals the existence of FeSO 4 [41,44]. The Fe2p peak at 711.2 eV and the O1s peak at 530.2 eV can be attributed to Fe 2 O 3 [34,41]. The N1s spectrum can be deconvoluted into three peaks centered at 398.4, 400.0, and 402.1 eV, and they can be assigned to the C-N bond, amide group, and N-O or N-N bond, respectively [45][46][47]. The O1s peak at 533.3 eV corresponds to ether-type oxygen produced under complex friction conditions [48].
The results of the XPS analysis of the worn surfaces indicate that, when an MF acts as a lubricant, a boundary film mainly composed of a Fe 3 O 4 deposited layer (Fe2p 710.6 eV and O1s 529.7 eV) and a small amount of organics are formed on the worn steel disk surface. When an MIL is used as the lubricant, a tribochemical reaction film composed of Dy 2 O 3 (Dy4d 153.2 and 156.7 eV, O1s 530.1 eV), FeS (Fe2p 710.1 eV, S2p 161.9 eV), FeSO 4 (Fe2p 713.2 eV, S2p 168.5 eV and O1s 523.4 eV), nitrogen containing organics, and a thioether compound is formed on the rubbed steel surface. In other words, under MIL lubrication, the frictional sliding process involves tribochemical reactions, which help to reduce friction and wear.

Conclusions
A novel magnetic ionic liquid [C 6 mim] 5 [Dy(SCN) 8 ] is prepared and used as the magnetic lubricant of a steel-steel sliding tribo-pair. Comparative studies with a commercial mixture of dioctyl sebacate and Fe 3 O 4 as a control indicate that the as-prepared MIL is superior to the MF in reducing the friction and wear of the ball-on-disk sliding contact, especially under an elevated temperature of 150 °C . This is because, as evidenced by SEM-EDS, XPS, profilometry analyses of the worn steel surfaces, and measurement of the electric contact resistance, the as-prepared MIL participates in tribochemical reactions in the sliding process to form a desired boundary lubrication film on the worn steel surfaces. In other words, the tribochemical reactions give rise to a boundary lubrication film composed of Dy 2 O 3 , FeS, FeSO 4 , nitrogen-containing organics, and thioether on the worn area, which significantly reduces the friction and wear of the steel-steel tribo-pair. However, the commercial MF is unable to form a lubricating film on the rubbed steel surface under 25 °C  Open Access: This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.
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To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Jiajia JIA. She received her bachelor degree in 2016 in Nanyang Normal University, China. After then, she is a master graduate student in the Engineering Research Center for Nanomaterials, Henan University, China. Her interests include the design and preparation of novel ionic liquids as lubricant and lubricating additives.
Guangbin YANG. He received his Ph.D. degree in condensed matter physics in 2011 from Henan University, China. Now he is an associate professor at Henan Univer-sity. His research areas cover the nanotribology, the lubricants, and nano-additives. He has published more than 50 journal papers and possessed two ministerial and provincial-level science and technology awards.
Chunli ZHANG. She received her Ph.D. degree in polymer chemistry and physics in 2013 from Henan University, China. Now she is an associate professor at Henan Univer-sity, China. Her research interests include the design and preparation of functionalized ionic liquids as lubricant and lubricating additives, nanotribology and nano-additives.