Superlubricity and Antiwear Properties of In Situ-Formed Ionic Liquids

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Applications of Polymer, Composite, and Coating Materials

Superlubricity and Antiwear Properties of In Situ Formed Ionic Liquids at Ceramic Interfaces Induced by Tribochemical Reactions Xiangyu Ge, Jinjin Li, Chenhui Zhang, Yuhong Liu, and Jianbin Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21059 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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ACS Applied Materials & Interfaces

Superlubricity and Antiwear Properties of In Situ Formed Ionic Liquids at Ceramic Interfaces Induced by Tribochemical Reactions Xiangyu Ge, Jinjin Li*, Chenhui Zhang, Yuhong Liu, Jianbin Luo State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

Corresponding author to whom all correspondence should be addressed: Jinjin Li Telephone: +86-10-62789482 E-mail: [email protected]

Keywords: superlubricity; antiwear; ionic liquids; tribochemical reactions

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ABSTRACT: Several ionic liquids (ILs) are formed in situ with monovalent metal salts and ethylene glycol (EG). The macroscale superlubricity and antiwear properties of the ILs were studied between ceramic materials. Superlow coefficients of friction of less than 0.01 could be realized for all ILs at silicon nitride (Si3N4) interfaces induced by tribochemical reactions. Notably, the IL ([Li(EG)]PF6) formed with LiPF6 and EG exhibited the greatest superlubricity and antiwear properties. The results of film thickness calculations and surface analysis showed that the lubrication regime during superlubricity period was the mixed lubrication, and a composite tribochemical layer (comprised of phosphates, fluorides, silica (SiO2), and ammonia-containing compounds), hydration layer, and fluid film contributed to the superlubricity and wear protection. It was found that the small size of metal cation was beneficial for alleviating wear, and PF6- anion exhibited the smallest friction and best antiwear performance at Si3N4 interfaces. This work studied the lubricity and antiwear properties of ILs with different cations and anions, enriching the range of alternative ILs for macroscale superlubricity and low wear, and is of importance to engineering applications.

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INTRODUCTION Ionic liquids (ILs) possess multiple advantageous characteristics, for instance superlow volatility, a wide electrochemical window, non-flammability, and good chemical and thermal stability,1,2 which makes them a promising class of materials in many fields. In particular, ILs play a leading role in energy devices, such as fuel cells, batteries, capacitors, and solar cells.3−7 For instance, some ILs with particular physicochemical properties (such as containing −OCH3 groups) are widely used as interfacial layers in solar cells.8,9 In surface science and engineering, ILs are known as excellent neat lubricants and lubricant additives,10−13 which play indispensable roles in industry because they can enhance the lifetime of mechanical systems via reducing friction and wear and improving energy efficiency.14,15 The tribological properties of ILs have been explored in recent years, including the friction reducing and wear protection of light alloys and polymers16,17 and anti-corrosion properties.18,19 The coefficients of friction (COFs) of ILs at the macroscale are generally larger than 0.01. Reducing their COFs to less than 0.01 is still a challenge, and the possibility and mechanism for such a low friction of ILs have rarely been studied. Superlubricity is the lubrication state when the COF is less than 0.01 due to lubrication by either solid or liquid lubricants; wear nearly vanishes during a superlubricity period.20−22 Currently, superlubricity is a popular research subject, and researchers have achieved superlubricity with a variety of materials at both the microscale and macroscale.23,24 However, the superlubricity behaviors of ILs have only been studied under very small load in combination with carbon quantum dots25 or at the microscale. For example, under the lubrication of 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, the 3 / 25



microscale superlubricity was realized between a SiO2/graphite tribopair when the ion layer was dominated by anions at a positive potential of graphite.26 Most recently, we proved that one

kind

of

imidazolium-based

IL—1-ethyl-3-methylimidazolium

trifluoromethanesulfonate—can realize macroscale superlubricity via forming a tribochemical layer and electric double layer at Si3N4/SiO2 interfaces.27 However, the wear during the wearing-in period is severe when lubricated with this IL. It is clear that the current researches into ILs superlubricity are focused on single one IL, and only three ILs are found to possess superlubricity property for now. Therefore, the quantities and species of ILs possessing superlubricity property need to be extended. In the present work, Si3N4 was chosen as the tribopair, because it is widely used in many fields, including electronics, automobile industry, metal cutting tools, bearings, orthopedic applications, high-temperature materials, and so on,28 and the macroscale superlubricity was observed at Si3N4 interfaces under the lubrication of a series of in situ formed ILs, and one of them exhibited the greatest antiwear property. The roles of cations and anions in ILs in realizing superlubricity are studied and analyzed to understand the superlubricity and antiwear mechanisms.

MATERIALS AND METHODS Chemicals. The salts were provided by the Shanghai Macklin Biochemical Co., Ltd; they had a purity of over 98% and included lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiNTf2), sodium hexafluorophosphate (NaPF6), and potassium hexafluorophosphate (KPF6). Ethylene glycol

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(EG) was provided by the Aladdin Industrial Corporation and had a purity of 99.5%. No further treatment was applied to these chemicals. The ILs were prepared by dissolving salts in EG via sonication at 50 ℃ followed by magnetic stirring at a ratio of 1:20 by weight. ILs(aq) were immediately prepared by mixing the ILs with pure water at a ratio of 1:5 by weight, and were labeled

as

[Li(EG)]PF6(aq),

[Li(EG)]BF4(aq),

[Li(EG)]NTf2(aq),

[Na(EG)]PF6(aq),

and

[K(EG)]PF6(aq), respectively. Lubricity Experiments. The friction and wear experiment was performed on a universal micro-tribotester (UMT-5, Bruker) with a COF accuracy of ± 0.001. The operating mode was a ball-on-disk configuration (rotation). Both the ball (φ = 4 mm, Ra = 10 nm) and disk (Ra = 5 nm) were made of Si3N4, and they were cleaned in ethanol via sonication for 10 min followed by washing in pure water and drying in air. For each experiment, 50 μL of each IL(aq) were injected into the contact region. The normal loads applied to the tribopair varied from 2 N to 4 N (corresponding contact pressure of 1.4−1.8 GPa) and the sliding velocity ranged from 12.5 mm/s to 250 mm/s. Adjustment of the UMT-5 levelness was applied to eliminate the measurement error until the same COFs in both the clockwise and counter-clockwise were achieved. Each IL(aq) was triple tested at temperatures of 25−27 °C and at 49−65% relative humidity. The viscosity and structural fingerprint of ILs were characterized using a standard rheometer (Physica MCR301, Anton Paar) and a Fourier-transform infrared spectroscope (FTIR) at 25 °C, respectively. The roughness (Ra) and topography of the worn surface was determined using a 3D white light interferometry microscope (Nexview, ZYGO Lamda) and a scanning electron microscope (SEM, Quanta 200 FEG), respectively. Because Si3N4 is 5 / 25



insulated, the worn surfaces were platinum coated before SEM observations to improve the conductivity and image contrast. Moreover, the chemical elements on the worn surfaces after lubrication with ILs(aq) were detected using an X-ray photoelectron spectroscope (XPS, PHI Quantera II) to analyze the tribofilm.

Figure 1. (a) Formulation process of ILs and schematic diagrams of their chemical structures; (b) FTIR results of EG and [Li(EG)]PF6.

RESULTS AND DISCUSSION Characterization of ILs. It is well known that ILs can be synthesized with Li salts via in situ formation in oligoethers, such as ethers, tetraglyme, and polyethylene glycol.29−31 In this work, using LiPF6 and EG as an example (Figure 1a), a weakly Lewis-acidic cation [Li(EG)]+ could be formed by the donation of lone pairs on the oxygen of an EG molecule to Li+. Thus, this weakly Lewis-acidic cation could form an IL ([Li(EG)]PF6) with the weakly Lewis-basic anion (PF6-) of LiPF6. FTIR bands (Figure 1b) confirmed the coordination between LiPF6 and EG. The peaks at 3295 cm-1 showed the presence of O−H groups in EG and IL.32 The peaks at 2875 cm-1 and 2938 cm-1 showed the existence of C−H stretching bands in EG and IL.33

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Meanwhile, the P−F band overlapped with C−C skeletal vibrational bands at 840 cm-1.34 Another peak of a P−F band at around 740 cm-1 could be observed, thereby indicating the combination of a P−F band with EG molecules.35 Lubricity of ILs(aq). The friction reducing and antiwear performances of pure water and ILs(aq) were first tested and compared. Due to water lacking the ability to form a valid lubricating film in the wearing-in process, its COF maintained larger than 0.6 for the whole test and its wear scar diameter (WSD, 700 μm) was very large. Meanwhile, the COFs under the lubrication of ILs(aq) all started at a large value around 0.3. Thereafter, the COFs gradually decreased during the wearing-in process. After 1200 s of wearing-in, their COFs decreased to less than 0.01 (Figure 2a) and the superlubricity states were stable for over 1 h, during which the average COFs under the lubrication of ILs(aq) (Figure 2b) were 0.003 for [Li(EG)]PF6, 0.006 for [Li(EG)]BF4, and 0.008 for [Li(EG)]NTf2. The WSDs of the balls lubricated with ILs were determined using the 3D white light interferometry microscope (Figure 2c), and were 170 μm ([Li(EG)]PF6), 300 μm ([Li(EG)]BF4), and 410 μm ([Li(EG)]NTf2), which corresponded to the contact pressures of 132, 42, and 23 MPa, respectively. Given the similar viscosities of the ILs (Table 1), generally, the COFs should be small if the contact pressure was low. However, the highest contact pressure corresponded to the smallest COF in the present case, thereby indicating the unique lubricity properties of [Li(EG)]PF6. Due to these ILs possessing the same cation, it was inferred that the PF6- anion played the dominant role in the greatest superlubricity and antiwear properties among these anions.

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Figure 2. Friction experiments were conducted (a−c) with various anions and the fixed Li(EG)+ cation, (d−f) with various cations and the fixed PF6- anion under the load of 3 N and velocity of 100 mm/s. (a,d) COFs under the lubrication of water and ILs(aq); (b,e) average COFs of 3 independent measurements under the lubrication of ILs(aq) during superlubricity period, and the error bars are the upper and lower bound for these measured COFs; (c,f) average WSDs on 3 balls after 3 independent measurements under the lubrication of ILs(aq), and the error bars are the upper and lower bound for these measured WSDs. For (g−i), experiments were conducted with [Li(EG)]PF6(aq). (g) COFs under the lubrication of [Li(EG)]PF6(aq) with 12 h of suspension; (h) relationship between COFs and sliding velocities under the load of 3 N; (i) relationship between COFs and normal load under the sliding velocity of 100 mm/s. The error bars are the upper and lower bound for these measured COFs.

Meanwhile, to probe the effect of cations on friction and wear, the anion PF6- was fixed 8 / 25


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and three different kinds of monovalent metal cations (Li+, Na+, and K+) with different hydration abilities were selected.36,37 The lubricities of [Li(EG)]PF6(aq), [Na(EG)]PF6(aq), and [K(EG)]PF6(aq) were tested and compared, as shown in Figure 2d. The evolution of the COFs under the lubrication of these ILs(aq) was the same, and comprised a wearing-in period and a superlubricity period. The COFs during the superlubricity period were similar and followed the sequence of COF(Li+) < COF(Na+) < COF(K+) (Figure 2e). WSDs of the balls at the end of the superlubricity test are shown in Figure 2f. They were 175 μm for Li+, 425 μm for Na+, and 480 μm for K+, which corresponded to the contact pressures of 125, 21, and 17 MPa, respectively. Similar to the case of the anions, the highest contact pressure corresponded to the smallest COF, which indicated that the Li+ cation possessed the greatest antiwear property among these cations. Integrating the results of anions and cations from the lubricity experiments, it was summarized that [Li(EG)]PF6 exhibited the most effective lubricity and antiwear properties. Table 1. Properties of the ILs and ILs(aq). Code Viscosity IL (mPa·s) IL(aq)

[Li(EG)]PF6 20.1 1.3

[Li(EG)]BF4 [Li(EG)]NTf2 [Na(EG)]PF6 19.4 18.3 17.2 1.4 1.5 1.3

[K(EG)]PF6 17.4 1.3

The robustness of the superlubricity state achieved with [Li(EG)]PF6 was studied. After the superlubricity state was achieved with [Li(EG)]PF6(aq), the experiment was paused for 12 h in the absence of any change. Then, on the same wear track, the experiment was resumed. As depicted in Figure 2g, the superlubricity state could be restored and lasted for at least 30 min, which was indicative of the robustness of the superlubricity state achieved with [Li(EG)]PF6.

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In addition, the superlubricity property of [Li(EG)]PF6 at different sliding velocities was determined, as depicted in Figure 2h. The COF under the lubrication of [Li(EG)]PF6(aq) could only decrease to around 0.024 when the sliding velocity was 12.5 mm/s, indicative of the failure of superlubricity. Meanwhile, its COF was at the boundary of the superlubricity regime (COF of approximately 0.01) when the sliding velocity was 25 mm/s. When the sliding velocities were 50, 100, 150, 200, and 250 mm/s, the COFs were all less than 0.005, and the minimum COF (0.0033) was acquired when the speed was 100 mm/s. These outcomes implied that there is a critical sliding velocity for the realization of superlubricity with [Li(EG)]PF6, which is in the range of 50−250 mm/s. Moreover, the superlubricity state of [Li(EG)]PF6(aq) could be retained with a slight growth in the COF while the normal load increased from 2 N to 4 N, as shown in Figure 2i. Discussion. To explore the peculiarity of [Li(EG)]PF6, the topographies of the worn surfaces lubricated with [Li(EG)]PF6(aq) were determined using a 3D white light interferometry microscope. As depicted in Figure 3a and 3b, both the ball and disk exhibited a Ra of 10 nm after lubrication with [Li(EG)]PF6(aq). The SEM details of the worn surfaces on the ILs(aq)lubricated balls are shown in Figure 3c–3h. The worn surfaces lubricated with [Li(EG)]PF6(aq) (Figure 3c), [Li(EG)]BF4(aq) (Figure 3d), and [Li(EG)]NTf2(aq) (Figure 3e) were much smoother than those of H2O (Figure 3h), but contained a few small wear pits (< 2 μm), thereby indicating that the anions did not have a large influence on the surface topography. Unlike with anions, when the surfaces were lubricated with larger metal cations (Na+ (Figure 3f) and K+ (Figure 3g)), the worn surfaces were rougher than those lubricated with small metal cation (Li+ (Figure 3c)). The surface lubricated with Na+ showed clear grooves in addition to wear pits, and the 10 / 25


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surface lubricated with K+ had many abrasive particles in addition to the grooves. These results indicated that the cations rather than anions could affect the worn surface topography; with the increase in cation size (Li+ < Na+ < K+), the topography became rougher.

Figure 3. Images of the worn surfaces of (a) the ball and (b) the disk after the lubrication with [Li(EG)]PF6(aq) acquired by 3D white light interferometry microscopy; the inset is the surface profile of the worn region. Magnified SEM images of the worn surfaces on the ball after the lubrication with (c) [Li(EG)]PF6(aq), (d) [Li(EG)]BF4(aq), (e) [Li(EG)]NTf2(aq), (f) [Na(EG)]PF6(aq), (g) [K(EG)]PF6(aq), and (h) H2O.

In our previous study, we found that the tribochemical reactions are important for the realization of superlubricity.27 Therefore, XPS was used to detect the variation in elements on the worn surfaces of the balls after lubrication with ILs(aq), as shown in Figure 4. For all cases,

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two individual peaks were observed in the spectra of C 1s and proved the presence of an aliphatic C−C chain (284.8 eV) and C−O bond (286.2 eV) derived from EG.24 The peaks of Si 2p at 102.7 eV corresponded to the Si−O bond of SiO2, which was formed via the well-known reactions between Si3N4 and water, as shown in Equation (1).27 The appearance of a N–H bond (399.9 eV) in N 1s spectra further proved the occurrence of reaction (1) and the formation of ammonia-containing compounds and SiO2.38 Additionally, colloidal SiO2, which is known as a good boundary lubricant, may be generated as shown in Equations (2) and (3).39,40 Si3N4 + 6H2O = 3SiO2 + 4NH3

(1)

SiO2 + H2O → Si(OH)4

(2)

Si(OH)4 → H+ + H3SiO4-

(3)

Figure 4. XPS for the basic chemical elements on the worn surface of the ball lubricated with [Li(EG)]PF6(aq), [Li(EG)]BF4(aq), and [Li(EG)]NTf2(aq), respectively.

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Moreover, the elements from ILs are detected on the worn surfaces as well, as depicted in Figure 5. In the F 1s spectra of all ILs, the peaks corresponding to the original anions before friction testing (687.2 eV for PF6- and BF4-, 688.7 eV for NTf2-) shifted to around 685.3 eV after testing, which can be assigned to the production of fluorides.41−45 This result proved the existence of fluorides on the worn surfaces and indicated the full transformation of PF6-, BF4-, and NTf2- into F-, which may have occurred through the reactions shown in Equation (4) or (5).41−45 For the [Li(EG)]PF6-lubricated surface, the peak of P 2p corresponding to PF6- (135.3 eV) shifted to 134.4 eV after testing, thereby further proving the transformation of PF6- and existence of phosphates on the worn surface.41,46 For the [Li(EG)]BF4-lubricated surface, the original peak of B 1s corresponding to BF4- (195.5 eV) shifted to 191.9 eV after testing, indicating the fully transformation of BF4- into borides.41,45 For the [Li(EG)]NTf2-lubricated surface, the original peak of S 2p corresponding to NTf2- (169.5 eV) shifted to 160.3 eV after testing, indicating the fully transformation of NTf2- into sulfides.45,47 These results indicated that some of the anions (those adsorbed on the surfaces) transformed into other states induced by the rubbing motion. For [Li(EG)]PF6, it was concluded that a composite tribochemical layer formed between the tribopair via tribochemical reactions and contained SiO2, fluorides, phosphates, and ammonia-containing compounds. This tribochemical layer is formed during the wearing-in period, protects the surfaces from direct contact, and contributes to wear alleviation and superlubricity realization. Notably, phosphates have been well documented as good extreme pressure and antiwear materials;48,49 thus, for [Li(EG)]PF6, the existence of phosphates in the tribochemical layer may have resulted in the lower friction and wear compared to the other ILs. 13 / 25



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LiPF6 + H2O → LiF + 2HF + POF3

(4)

LiPF6 → LiF + PF5

(5)

Figure 5. XPS for the chemical elements (from ILs) on the worn surface of the ball lubricated with [Li(EG)]PF6(aq), [Li(EG)]BF4(aq), and [Li(EG)]NTf2(aq), respectively. The dash line accounts for the spectra of the original ILs before friction experiments.

According to the friction results, the type of cation has a great effect on wear. Therefore, the role of cations in superlubricity and antiwear was also discussed. The properties of these cations are listed in Table 2, including the ion radius, hydrated radius, hydration energy, and hydration number of the ion in water (hydration number is the number of water molecules in the primary shell).36 It was clear that the hydration ability of cations followed the sequence of Li+ > Na+ > K+ according to the hydration number of water molecules.37 Moreover, according to the hydrated radius and hydration energy of these cations, their hydration intensity followed the same sequence of Li+ > Na+ > K+. It was clear that superlubricity realization and wear protection were highly accordant with the hydration ability of these cations. As for Li+, it can be surrounded by water molecules due to its hydration ability, and thus a hydration network 14 / 25


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could be formed.36 Since Si3N4 surfaces are negatively charged, Li+ in the solution could be adsorbed onto both surfaces through electric attraction, thereby leading to the formation of a hydration layer and generation of hydration repulsion.50 Because Li+ has a relatively stronger hydration ability, its hydration layer and hydration repulsion may be more dense and stronger than those of Na+ and K+, thereby allowing it to protect the surface better and sustain larger contact pressure, as shown in Figure 2. Therefore, the antiwear property of Li+ was better than that of Na+ and K+. Table 2. Properties of the cations. Code Li+ Na+ K+

Ion radius (nm) 0.068 0.095 0.133

Hydrated radius (nm) 0.38 0.36 0.33

Hydration energy (kJ/mol) -510 -410 -337

Hydration number (± 1) 5 4 3

To determine the lubrication regime of [Li(EG)]PF6(aq) during the superlubricity period, the film thickness (h) between two Si3N4 surfaces was estimated using the same method as that in our previous works with the Hamrock-Dowson (H-D) formula.24,27 The film thickness (h = 24.86 nm) between the tribopair was determined. The ratio of the film thickness to the equivalent Ra was used to classify the lubrication regime during the superlubricity period as follows: λ=

ℎ 𝜎21

+ 𝜎22

(6)

where σ is the Ra for the worn surfaces (10 nm for both the ball and disk) and λ is 1.8 for [Li(EG)]PF6(aq), thereby indicating that during the superlubricity period, the lubrication regime for [Li(EG)]PF6(aq) was located at mixed lubrication (ML) regime that comprised boundary

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lubrication (BL) regime and elastohydrodynamic lubrication (EHL) regime. Based on these analyses, a feasible superlubricity and antiwear model of [Li(EG)]PF6(aq) was proposed. Two types of lubrication modes exist between the tribopair, namely BL and EHL, as shown in Figure 6a. BL mainly occurs at the contact region of solid asperities during the initial wearing-in period. The solid asperities collide against each other under lubrication of liquid molecules to perform the wearing-in process, which provides sufficient energy to complete tribochemical reactions and results in surface smoothening and contact pressure diminution.27 During this period, colloidal SiO2, fluorides, phosphates, and ammoniacontaining compounds were generated via tribochemical reactions between [Li(EG)]PF6 and the Si3N4 surface. In this way, the tribochemical layer is deposited on the worn surfaces, prevents direct asperities contact between the surfaces, and results in low friction and wear (Figure 6b). Notably, only the tribochemical layer formed by [Li(EG)]PF6 contains phosphates, which demonstrate excellent extreme pressure and antiwear properties.48,49 Therefore, the production of phosphates by tribochemical reaction may have been the key factor that made [Li(EG)]PF6 the best lubricating IL and PF6- the best anion at Si3N4 interfaces in this study. On the other hand, the hydration layer of Li+ on the surfaces can provide extremely low shearing resistance and hydration repulsion, by which it reduces friction and alleviates wear (Figure 6c).36 During the superlubricity period, EHL also occurs at Si3N4 interfaces in addition to BL because of the film thickness growth and the contact pressure reduction. In the EHL regime, a fluid film can exist between the contact surfaces (Figure 6d). This fluid film has low viscosity and shearing resistance, thereby preventing direct contact of the tribopair and further lowering the friction. Therefore, the conclusion was that the realization of superlubricity was attributed 16 / 25


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to the formation of a composite tribochemical layer, hydration layer of Li+, and fluid film.

Figure 6. Proposed superlubricity and antiwear model for [Li(EG)]PF6(aq): (a) macroscale view, (b) tribochemical layer forming at BL regime, (c) hydration layer forming at BL regime, and (d) fluid film forming at EHL regime.

These analyses indicated that there were three critical factors for the realization of superlubricity by in situ formed ILs. One factor was the formation of the composite tribochemical layer, which is the production of the tribochemical reactions between the anions of ILs and surfaces during the wearing-in period. Moreover, PF6-, which was transformed into phosphates, could provide the greatest superlubricity and antiwear performance among the selected anions. Another factor was the hydration layer formed by the adsorption of the monovalent metal cations, which was surrounded by water molecules due to their hydration ability. It has been found that the hydration layer can help lower friction, and the stronger hydration ability of a monovalent metal cation leads to better antiwear performance. The third factor was the fluid film formed by the lubricant, whose low viscosity and shearing resistance further lowered the friction and wear. Because different cations and anions of ILs were studied,

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our result enriched the range of alternative ILs for macroscale superlubricity. Among them, [Li(EG)]PF6 possessed the greatest superlubricity and antiwear properties at Si3N4 interfaces (commonly used in industry), thereby making it a potential promising lubricating material in engineering applications. Moreover, in situ formed ILs could be synthesized by combining a variety of cations and anions, which may result in many ILs with excellent lubricity. Anyway, the study of ILs with superlubricity and antiwear properties is still in the initial state, and more research on the lubricity of ILs would be performed in the near future.

CONCLUSIONS In conclusion, macroscale superlubricity can be realized under the lubrication of several in situ formed ILs(aq) at Si3N4 interfaces. The results of the film thickness calculations and surface analysis showed that superlubricity was realized in ML regime, and a composite tribochemical layer, hydration layer, and fluid film contributed to the superlubricity. It was found that the small size of monovalent metal cations is beneficial for alleviating wear. Moreover, PF6- possessed better superlubricity and antiwear properties than BF4- and NTf2- at Si3N4 interfaces. This was caused by the transformation of PF6- into phosphates via tribochemical reactions, as phosphates are excellent lubricating materials at Si3N4 interfaces. This work demonstrated that robust superlubricity can be achieved by ILs at the macroscale, which may be applied in industrial lubrication in the future.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (grant numbers 51775295, 51527901, and 51335005). 18 / 25


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TOC

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Figure 1 330x160mm (300 x 300 DPI)


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Figure 2 551x456mm (300 x 300 DPI)



Figure 3 214x218mm (300 x 300 DPI)


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Figure 4 473x396mm (300 x 300 DPI)



Figure 5 479x282mm (300 x 300 DPI)


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Figure 6 263x144mm (300 x 300 DPI)



TOC 383x167mm (300 x 300 DPI)


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Superlubricity and Antiwear Properties of In Situ-Formed Ionic Liquids

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