Liquid Superlubricity of Polyethylene Glycol Aqueous Solution

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Liquid Superlubricity of Polyethylene Glycol Aqueous Solution Achieved with Boric Acid Additive Xiangyu Ge, Jinjin Li, Chenhui Zhang, and Jianbin Luo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04113 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018

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Liquid Superlubricity of Polyethylene Glycol Aqueous Solution Achieved with Boric Acid Additive Xiangyu Ge, Jinjin Li*, Chenhui Zhang, Jianbin Luo

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

Corresponding authors: *To whom all correspondence should be addressed. Jinjin Li Telephone: 8610-62789482 E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Boric acid is a weak acid and has been used as a lubrication additive due to its special structure. In this study, we report that boric acid could achieve robust superlubricity (µ< 0.01) as an additive in polyethylene glycol (PEG) aqueous solution at Si3N4/SiO2 interfaces. The superlow and steady friction coefficient of approximately 0.004–0.006 could be achieved with boric acid in neutral conditions (pH of approximately 6.4), which is different from the acidic conditions leading to superlubricity. The influence of various factors, including boric acid concentration, sliding speed, applied load, PEG molecular weight, and the volume of lubricant on the superlubricity were investigated. The results reveal that the PEG aqueous solution with boric acid additive could achieve superlubricity under a wide range of conditions. Surface composition analysis shows that the synergy effect between boric acid and PEG provides sufficient H+ ions to realize running-in process. Moreover, a composite tribochemical film composed of silica and ammonia-containing compounds (ACC) were formed on the ball surface, contributing to the superlubricity. The film thickness calculation shows that the superlubricity was achieved in mixed lubrication region and therefore, the superlubricity state was dominated by both the composite tribochemical film formed via the tribochemical reaction on the contact surfaces and the hydrodynamic lubricating film between the contact surfaces. Such liquid superlubricity achieved in neutral conditions is of importance for both scientific understanding and engineering applications.

Keywords: superlubricity; boric acid; polyethylene glycol; friction; synergy effect; mixed lubrication


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INTRODUCTION

Superlubricity is first proposed by Hirano and Shinjo to illustrate a theoretic sliding regime in which sliding friction entirely vanishes,1 and recently it is defined as the sliding friction coefficient less than 0.01 (in the 10-3 range or even less) due to the limit of measurement precision.2 Nowadays, superlubricity has attracted immense attention among researchers in many fields. In the initial stage, numerous studies devoted to “solid superlubricity” have been performed on a specific structure or in a certain atmosphere. The superlubricity phenomenon is demonstrated via both computer simulations and experiments with lamellar compounds such as MoS2,3,4 and a series of carbon materials, such as graphite,5,6 carbon nanotubes,7 diamond-like carbon (DLC) film,8-10 and graphene,11-13 with incommensurate structure or weak interlayer tribochemical reaction.14,15

In contrast to solid superlubricity, “liquid superlubricity” is obtained with common friction pairs in the presence of various liquids under air atmosphere condition. Researchers have achieved liquid superlubricity with viscous oils such as ester-containing poly-α-olefin (PAO) when applied to DLC film16 and castor oil when applied to Nitinol 60 alloy/steel interface17 under boundary lubrication. Polyols such as glycerol are known to achieve superlubricity when applied to DLC films2 in various humidity environments between steel interfaces18 and with nanodiamond additives.19 Researchers have also achieved liquid superlubricity

with

water

applied

to

DLC

films,20

and

ionic

liquid

(1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate) applied to silica–


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graphite interface by controlling the electrical potential,21 or by ionic liquid-modified carbon quantum dots.22 All the aforementioned studies can achieve a coefficient of friction (COF) of less than 0.01. Previously, our group reports that phosphoric acid and sulfuric acid can achieve superlubricity in aqueous solutions or when mixed with a polyol solution.23-25 The mechanism of acid-based superlubricity, considering phosphoric acid as an example, is due to the synergy effect of hydrogen bond network and H+ ions, which indicates that the superlubricity is achieved in an extremely acidic condition (pH of approximately 1.5).26 The production of H+ ions and extreme acidity limit the application of these kinds of lubricants in engineering, owing to the corrosion of metals.

In contrast to these strong acids, boric acid is a weak acid and has been used as a lubricant additive to reduce friction and exhibited good performance.27,28 Considering its weakly acidic, boric acid was chosen as the additive in this study, and polyethylene glycol (PEG) was selected as the base lubricant because it is a common lubricant with good hydration and lubricity.29 The research into the tribological properties of PEG aqueous solutions with boric acid as additive was carried out, and the superlubricity could be achieved under a wide range of conditions. Its superlubricity mechanism was studied and analyzed in detail, and a possible superlubricity model was proposed.

MATERIALS AND METHODS

Materials. PEG (H(OCH2CH2)nOH) with four diverse levels of average molecular weight (MW) (200, 400, 600, and 1000 g/mol) was purchased from Sinopharm Chemical


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Reagent Co., Ltd with a purity of 99.5%. Five kinds of weak and mediate acids (Figure 1) were chosen for comparison: boric acid (H3BO3), acetic acid (C2H4O2), lactic acid (C3H6O3), citric acid (C6H8O7), and tartaric acid (C4H6O6), and all of them were commercial products obtained from Sinopharm Chemical Reagent Co., Ltd with a purity of 99.8%. All the chemicals were used as received without further treatment. PEG aqueous solutions (PEG(aq)) were prepared by diluting PEG with pure water (resistivity >18 MΩ·cm) at a concentration of 32 wt%, and denoted as PEG200(aq), PEG400(aq), PEG600(aq), or PEG1000(aq) corresponding to the diverse MWs of PEG. By dissolving 4 mM of each acid in 15 mL of as-prepared PEG(aq) with sonication, acid-containing PEG(aq) were prepared. Especially, the PEG(aq) containing boric acid was marked as B-PEG(aq). The mixture of pure PEG200 and boric acid was prepared by dissolving 4 mM of boric acid in 25 mM of PEG200 without pure water and denoted as B-PEG200; further, boric acid aqueous solution (H3BO3(aq)) was prepared by dissolving 4 mM of boric acid in 0.56 M of pure water.

Tribology Tests. Friction test was conducted on a Universal Micro-Tribotester (UMT-5, Bruker, Billerica, MA) in the rotation mode of ball-on-disk. The contact pairs were a Si3N4 ball (Ø4 mm, Ra=10 nm, purchased from Shanghai Research Institute of Materials) and a SiO2 disk (Ra=5 nm). Both the ball and disk were washed via sonication in acetone and ethanol for 15 min, and subsequently, they were flushed with pure water and dried with compressed air. Before each test, 30 µL of lubricant was introduced to the contact area between the ball and disk. The applied load ranged from 2 to 4 N, corresponding to a maximum contact pressure of approximately 750 MPa, and the disk rotation speed varied


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from 60 to 600 rpm with a track radius of 4 mm, corresponding to a sliding speed ranging from 0.02512 to 0.2512 m/s. Each test was repeated three times and the COFs were measured with an accuracy of ±0.001. In order to achieve more accurate friction results, the measuring errors were eradicated by adapting the tribotester levelness to obtain the same COFs in both clockwise and counterclockwise rotations. All the tests were performed at room temperature and relative humidity of 30–60%.

The pH and viscosity of each lubricant were measured using a pH probe (InLab Routine pro, Mettler Toledo) and a standard rheometer (Physica MCR301, Anton Paar) at 25 °C, respectively. The topographies of the worn surfaces were investigated using 3D white light interferometry and scanning electron microscopy (SEM, QUANTA 200 FEG) under high vacuum. Both the Si3N4 balls and SiO2 disks were platinum-coated before SEM observation in order to enhance their electrical conductivity, resulting in the reduction of noise level and the improvement of image contrast. Fourier transform infrared spectroscopy (FTIR) was performed to determine the state of PEG200 molecules and boric acid molecules. In order to determine the chemical composition of the adsorption film, virgin B-PEG200(aq) and the film formed on the SiO2 disk after the test were investigated using Raman spectroscopy (Jobin Yvon HR800). Moreover, to further explore the chemical characteristics of the tribofilm, we obtained the X-ray photoelectron spectroscopy (XPS) spectra of the elements typically present on the worn surface lubricated with B-PEG200(aq).


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Figure 1. Chemical structure of PEG and various kinds of acids (boric acid, acetic acid, lactic acid, citric acid, and tartaric acid)

RESULTS AND DISCUSSIONS

Superlubricity Behavior of B-PEG200(aq). First, the friction behavior of various liquid lubricants, including pure water, H3BO3(aq), PEG200, B-PEG200, and PEG200(aq) were tested between the Si3N4/SiO2 interfaces as shown in Figure 2a. The COF of pure water increased to 0.7 quickly and remained at a high level until the end of the test, which indicates that it is difficult for pure water to form an efficient lubricating film with short running-in period. Notably, the COF of H3BO3(aq) decreased from 0.5 to 0.03 after a running-in period of approximately 800 s, indicating boric acid could greatly reduce the friction of pure water after short running-in period. However, when there was little water left (approximately 800 s), solid boric acid precipitated out between the contact surfaces, making the contact no longer be lubricated, and resulting in the sharply increase in COF. This result infers that the solid boric acid does not have lubricity between Si3N4/SiO2 contact surfaces. Both pure PEG200


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and B-PEG200 exhibited a stable COF of approximately 0.04 throughout the test with no running-in period. Moreover, the friction behavior of PEG200(aq) in the absence of boric acid was also tested for comparison. Its COF was much larger than pure PEG200 at the beginning, as it was difficult to form a hydrodynamic film under high applied load and low sliding speed,30 due to its low viscosity (close to water). After a running-in period of 1500 s, its COF decreased to approximately 0.02 (lower than pure PEG) and remained stable. The lower friction of PEG(aq) than that of pure PEG is attributed to the presence of water, which leads to the formation of an ultra-smooth surface after the running-in period, which is beneficial for the hydrodynamic lubrication,31,32 and repulsive forces due to the formation of an electric double layer (boundary silica layer) during the rubbing of contact surfaces.33,34 Moreover, the reduction in contact pressure due to the increase in contact area by the running-in process also contributes to the lower friction.

Figure 2. COFs of Si3N4 ball against SiO2 disk lubricated with (a) diverse lubricants, including pure water, pure PEG200, PEG200(aq), B-PEG200, and H3BO3(aq); (b) B-PEG200(aq) for 6000 s; the inset shows the partial enlarged COF from 350 s to 3600 s; and (c) B-PEG200(aq) with 30 min of suspension. The load and sliding speed were 3 N and 0.075 m/s, respectively.


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It is obvious that all the aforementioned lubricants could not achieve superlubricity as µ is larger than 0.01 for all cases. However, it is observed from Figure 2b, that B-PEG200(aq) could achieve superlubricity (µ2, sufficient running-in process cannot be realized owing to insufficient H+ ions, resulting in nonsuperlubricity. However, although boric acid is a weak acid, it can react with PEG and produce H+ ions continuously to realize the running-in process, resulting in superlubricity.

In the case of Si3N4 ball, a reaction between Si3N4 and water can occur on the surface of the Si3N4 ball during the running-in period via tribochemical reaction,45,46 Si3N4+6H2O→3SiO2+4NH3

(2)

Subsequently, ammonia-containing compounds (ACC) such as ammonium boron salt, might be produced by BCC reacting with ammonia on the surface of the Si3N4 ball,36

(3) According to the above analysis, it is considered that during friction process, H+ ions are produced continuously as in eq. (1), participating in the running-in period, and resulting in similar friction curve to strong acids. Subsequently, the produced H+ ions might be neutralized by NH3 (derived from eq. (2)) as in eq. (3). Therefore, the concentration of H+ ions are in a dynamic equilibrium state, leading to the B-PEG(aq) maintains at neutral level during the running-in period.


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In order to further explore the chemical characteristics of the tribofilm and to prove our assumption, XPS spectra were used to investigate the elements typically present on the worn region of the surface of the Si3N4 ball. Figure 9 shows the XPS spectra of B1s, C1s, N1s, and Si2p on the worn region after washing with pure water. As shown in Figure 9a, two chemical states of C1s can be detected at 284.8 eV and 286.2 eV, which are characteristic of the presence of carbon in aliphatic chains (C–C and C–H) and oxidized carbon chemical groups (C–O), respectively.17,47 These chemical groups are considered to be derived from PEG or ACC. The peak of B1s (Figure 9b) located at 192.2 eV is assigned to the B–O bond of borides,48 which is considered to be deposited on the surface of the Si3N4 ball. The N1s (Figure 9c) peak appearing at 400.1 eV indicates that the worn surfaces have chemical substances containing the N–H bond,49,50 which is considered as further evidence of ACC deposition on the surface of the ball. The peak of Si2p (Figure 9d) at 102.6 eV is consistent with the Si–O bond of SiO2, which is regarded to be generated by Si3N4 and water on the surface of the ball.51


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Figure 9. XPS spectra of the elements on the worn area of the Si3N4 ball after lubrication with B-PEG200(aq): (a) C1s, (b) B1s, (c) N1s, and (d) Si2p. The load and sliding speed were 3 N and 0.075 m/s, respectively.

In order to further confirm the film deposition on the worn surfaces, an experiment was designed and conducted as shown in Figure 10a. First, B-PEG200(aq) was introduced to achieve superlubricity (the COF of B-PEG200(aq) was approximately 0.0052 after the running-in period). Subsequently, the test was suspended, and the residual solutions on the ball and disk were cleaned with dry paper to ensure that they could not be detected. Subsequently, the test was continued but in a dry sliding condition on the same wear track. The results demonstrate that, during the dry friction test, the initial COF was approximately 0.017 and it gradually increased. The relatively low friction remained less than 0.1 for approximately 140 s and subsequently increased to a very high value of approximately 0.7 (dry friction between Si3N4 and SiO2) sharply. These results confirm that there is a tribofilm deposited on the worn surface after the running-in period of superlubricity, which is believed to contribute to the relatively low friction for approximately 140 s. After a long sliding time, the film is destroyed by shear, resulting in a sudden increase of COF.


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Figure 10. COF of Si3N4 ball against SiO2 disk lubricated with B-PEG200(aq) to achieve superlubricity first; subsequently, the ball and disk were cleaned with dry paper; thereafter, the friction test was continued on the same wear track in (a) dry sliding condition and (b) lubrication of FS, which is defined as the solution prepared based on the formulation of the post-evaporation solution. The load and sliding speed were 3 N and 0.075 m/s, respectively.

It can also be observed from Figure 10a that the tribofilm deposited on the surfaces could not lead to superlubricity, and hence, there must be other factors contributing to superlubricity. According to the results mentioned above, the residual water concentration after the running-in period on the worn surface remains constant, which dominates the solution viscosity; therefore, it is concluded that the hydrodynamic lubricating films are also important for superlubricity. The results obtained with various MWs of PEG (Figure 7a) also indicate that the hydrodynamic lubricating film is a factor influencing superlubricity. Table 2. Different lubricants with their component concentration, viscosity, and pH Solution Solution Component (wt%) Solution Code Viscosity pH H2 O PEG H3BO3 (mPa·s) B-PEG200(aq) 66.7 31.7 1.6 3.1 6±0.1 FS 17.7 78.3 4 37 6.4±0.1


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B-PEG200

0

95.2

4.8

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89

6.1±0.1

Therefore, an experiment was designed to further investigate the effect of the hydrodynamic lubricating film on friction during superlubricity. First, we calculated the percentage of each component in virgin B-PEG200(aq) by weight. Subsequently, B-PEG200(aq) was placed on a laminar flow cabinet to make water evaporate for 40 min. Subsequently, the solution was weighed after evaporation to measure the mass loss, and all the mass loss was due to water evaporation. The percentage of each component in the solution after evaporation was calculated. Finally, a solution was prepared based on the formulation of this post-evaporation solution and denoted as FS. In addition, B-PEG200(aq) and B-PEG200 were also introduced for comparison. The mass fraction, viscosity, and pH of these solutions are measured and listed in Table 2. It can be observed that, with the reduction of water concentration in the solution, the viscosity increased from approximately 3.1 mPa·s (B-PEG200(aq)) to 89 mPa·s (B-PEG200).

A friction test was conducted to study the tribological behavior of these solutions. First, B-PEG200(aq) was introduced to obtain superlubricity. Subsequently, the test was suspended, and the residual solutions on the ball and disk were cleaned with dry paper. Finally, FS and B-PEG200 were introduced on the wear track respectively and the test was continued. The results in Figure 10b show that the COF of FS (approximately 0.0065) was very close to the COF before the test was suspended (approximately 0.006), indicating that FS has a similar state as the superlubricity liquid, and the pH of FS (approximately 6.4) was even neutral compared with virgin B-PEG200(aq) (approximately 6). In the case of B-PEG200, as there is


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no water existing in the solution, its viscosity (approximately 89 mPa·s) is larger than that of FS (approximately 37 mPa·s), leading to a higher COF (approximately 0.009) than FS. These results indicate that, in the superlubricity region, when lubricated with B-PEG(aq), the property of the hydrodynamic lubricating film formed between the Si3N4/SiO2 interfaces could be a factor influencing the friction.

In order to determine the lubrication region of B-PEG200(aq) in superlubricity state, the film thickness hc between the two contact surfaces was calculated using the H–D equation (4),30

ℎ = 2.69 where = ,

. . .

1 − 0.61 .

(4)

= !"/′% , & = '/ %( , and α refers to the pressure-viscosity

coefficient of the lubricant, which is usually in the range of 4–8 GPa-1 for water-based lubricants.35 Since PEG is a polymer of ethanediol, and it was diluted by water in this test, the pressure-viscosity coefficient of ethanediol (≈ 4 GPa-1)35 is used to estimate the film thickness. Further, η is the lubricant viscosity; w and u are the applied load (3 N) and average velocity of the ball and disk (approximately 0.0375 m/s), respectively; k is a coefficient (approximately 1); R presents the equivalent ball radius calculated using equation (5),

%=

) *+ ,-

(5)

where d represents the diameter of the worn area on the ball (d=0.2 mm). E’ is the reduced Young’s modulus of the two contact materials calculated as

= 2⁄.1 − /0( ⁄0 + 1 − /(( ⁄( 3


(6)

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where v1, v2 and E1, and E2 represent the Poisson’s ratio and elasticity modulus of the materials, respectively. The film thickness (hc=27.43 nm) between the two contact surfaces is calculated at the contact pressure of 96 MPa. The lubrication region can be classified by the ratio of film thickness to the combined surface roughness given in equation (7),

λ=

56

789: ;8::

(7)

where < 1 and < 2 represent the surface roughness of the worn area on the ball (10.3 nm) and disk (13.8 nm), respectively. The calculation shows that λ is approximately 1.6 for the lubricant in our test at the contact pressure of 96 MPa, indicating that the lubrication states of superlubricity for B-PEG200(aq) are in the mixed region.

Based on these analyses, a possible model of superlubricity is proposed, as shown in Figure 11. There are two types of contact states between the two surfaces: solid asperities and liquid contact. At the contact region of solid asperities, BCC is produced by the synergy effect between boric acid and PEG200, and meanwhile provides sufficient H+ ions to realize the running-in process, which contributes to the surface smoothening and contact pressure reduction.31,32 At the same time, as shown in eq. (8) and (9), the colloidal silica, which is a good boundary lubricant, with electric double layer may be formed.33,34 And since the superlubricity is achieved in neutral conditions (pH≈6), in which the colloidal silica system can maintain stable, thus the sufficient electrostatic support can provide repulsive force to increase the surface separation and avoid some direct contact.52 Moreover, BCC may react with ammonia (from Si3N4 ball) on the ball surface generating ACC,36 and form a composite


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tribochemical film, which may prevent the surfaces from contacting directly and provide a low shear strength. Therefore, all these factors contribute to the friction reduction and superlubricity at the contact region of solid asperities (Figure 11a).

SiO2+H2O→Si(OH)4

(8)

Si(OH)4→H++H3SiO4-

(9)

At the liquid contact region (Figure 11b), an adsorption film may be formed by PEG on the surfaces of the Si3N4 ball and SiO2 disk, owing to its hydroxyl groups on both ends of the carbon chain. Moreover, the existence of water makes it possible to form hydrated PEG network, which might be cross-linked by hydrogen-bonding interactions mediated by boric acid and ordered due to shear (like shear-thinning of branched polymer solutions). Thus, the viscosity and shearing force of such solution are very low, leading to superlow friction, similar to the superlubricity achieved with glycerol.25 On the basis of the above analyses, it can be concluded that the steady composite tribochemical film, the adsorption film, and the hydrated PEG network, contribute to the achievement of superlubricity in neutral conditions.

Figure 11. Proposed superlubricity mechanism for B-PEG200(aq): (a) contact of solid


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asperities, and (b) liquid contact between the contact surfaces; BCC is short for boron chelate complex, and ACC is short for ammonia-containing compounds.

Owing to the neutral condition for achieving the superlubricity of B-PEG200(aq), it is indicated that B-PEG200(aq) has great potential in steel/steel lubrication. In order to confirm this conclusion, AISI52100 steel ball was first immersed in B-EG200(aq) for more than 72 h and no apparent corrosion could be observed. Subsequently, B-PEG200(aq) was applied to lubricate steel/steel contact pairs to verify whether superlubricity could be achieved, as shown in Figure 12. It can be observed that B-PEG200(aq) can also achieve superlubricity for 500 s when applied to steel/steel pairs under a contact pressure of up to 1.2 GPa at 0.2 m/s. The COF decreased to as low as approximately 0.005 after a running-in period of 240 s, and finally increased to approximately 0.011 after testing for 1500 s. This indicates that the superlubricity behavior is not as stable as that achieved between Si3N4/SiO2 contact surfaces. Despite there are other factors that should be met for engineering application, this result demonstrates the potential of such neutral superlubricity liquid for lubrication engineering application. The mechanism of such superlubricity achieved between steel interfaces will be investigated in our future work in order to achieve a robust superlubricity state for engineering application.


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Figure 12. COF of steel/steel contact pairs lubricated with B-PEG200(aq); the load and sliding speed were 3 N and 0.2 m/s, respectively. The inset is the corrosion images of steel balls immersed in B-PEG200(aq) for 72 h and in H3PO4(aq) for 12 h, respectively; the H3PO4(aq) was prepared by diluting phosphoric acid with pure water to a pH=1.5 level.

CONCLUSIONS

In conclusion, boric acid can be neutralized with PEG200 in water and superlubricity can be achieved between the Si3N4/SiO2 interfaces under a wide range of conditions. The synergy effect between PEG and boric acid generates BCC, which can provide H+ ions continuously to realize the running-in period, and the composite tribochemical film composed of a boundary silica layer formed by Si3N4 and water and ACC formed by the consumption of H+ ions, the adsorption film formed by PEG, and the formation of hydrated PEG network contribute to the low friction during superlubricity stage. The result of film thickness calculation shows that during the superlubricity state, the lubrication region of B-PEG200(aq) belongs to the mixed lubrication, containing solid asperities and liquid contact. The


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composite tribochemical film contributes to the low friction at solid asperities contact, and the adsorption film of PEG and hydrated PEG network contribute to the low friction at liquid contact. The robust superlubricity of B-PEG200(aq) in neutral conditions is of significance for the scientific understanding of superlubricity with acids, by putting forwards a new viewpoint of the supplying methods of H+ ions, and also has great potential implications for designing efficient lubrication systems in mechanical systems.

ACKNOWLEDGMENTS

The work is financially supported by NSFC of China (51775295, 51405256, and 51527901).

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TOC


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Figure 1. Chemical structure of PEG and various kinds of acids (boric acid, acetic acid, lactic acid, citric acid, and tartaric acid) 165x176mm (300 x 300 DPI)


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Figure 2. COFs of Si3N4 ball against SiO2 disk lubricated with (a) diverse lubricants, including pure water, pure PEG200, PEG200(aq), B-PEG200, and H3BO3(aq); (b) B-PEG200(aq) for 6000 s; the inset shows the partial enlarged COF from 350 s to 3600 s; and (c) B-PEG200(aq) with 30 min of suspension. The load and sliding speed were 3 N and 0.075 m/s, respectively. 541x160mm (300 x 300 DPI)


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Figure 3. (a) Real time COFs and (b) the minimum COF under the lubrication of PEG200(aq) containing diverse acids as additives; the pH values represent the virgin solution acidity. The load and sliding speed were 3 N and 0.075 m/s, respectively. 374x160mm (300 x 300 DPI)


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Figure 4. SEM images of worn surfaces lubricated with PEG200(aq) containing 4 mM of various acids: (a,a’) boric acid, (b,b’) acetic acid, (c,c’) lactic acid, (d,d’) citric acid, and (e,e’) tartaric acid. The load and sliding speed were 3 N and 0.075 m/s, respectively. 282x243mm (300 x 300 DPI)


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Figure 5. Relationship between the pH/minimum COFs and boric acid concentration in B-PEG200(aq). The load and sliding speed were 3 N was 0.075 m/s, respectively. 254x203mm (300 x 300 DPI)


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Figure 6. Relationship between (a) the COFs and sliding speed when lubricated with B-PEG200(aq); the inset shows the minimum COFs at various speeds; the load was 3 N; and (b) the COFs and applied load when lubricated with B-PEG200(aq); the inset shows the worn area diameter on the ball at the end of the test under various loads, the sliding speed was 0.075 m/s. 313x129mm (300 x 300 DPI)


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Figure 7. COFs of (a) B-PEG(aq) prepared with various PEG MWs (200, 400, 600, and 1000); the inset shows the relationship between the MW of PEG and solution viscosity/pH; (b) various volumes of BPEG200(aq) (from 5 to 30 µL); in the inset, Tr is defined as the time elapsed from the beginning of the test to the onset of superlubricity. The load and sliding speed were 3 N and 0.075 m/s, respectively. 370x160mm (300 x 300 DPI)


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Figure 8. Spectra of various lubricants: (a) FTIR spectra of pure PEG200 and B-PEG200; and (b) Raman spectra of B-PEG200(aq) and the residual solution left on the disk after the test; the load and sliding speed were 3 N and 0.075 m/s, respectively. 374x161mm (300 x 300 DPI)


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Figure 9. XPS spectra of the elements on the worn area of the Si3N4 ball after lubrication with BPEG200(aq): (a) C1s, (b) B1s, (c) N1s, and (d) Si2p. The load and sliding speed were 3 N and 0.075 m/s, respectively. 305x250mm (300 x 300 DPI)


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Figure 10. COF of Si3N4 ball against SiO2 disk lubricated with B-PEG200(aq) to achieve superlubricity first; subsequently, the ball and disk were cleaned with dry paper; thereafter, the friction test was continued on the same wear track in (a) dry sliding condition and (b) lubrication of FS, which is defined as the solution prepared based on the formulation of the post-evaporation solution. The load and sliding speed were 3 N and 0.075 m/s, respectively. 365x160mm (300 x 300 DPI)


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Figure 11. Proposed superlubricity mechanism for B-PEG200(aq): (a) contact of solid asperities, and (b) liquid contact between the contact surfaces; BCC is short for boron chelate complex, and ACC is short for ammonia-containing compounds. 200x119mm (300 x 300 DPI)


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Figure 12. COF of steel/steel contact pairs lubricated with B-PEG200(aq); the load and sliding speed were 3 N and 0.2 m/s, respectively. The inset is the corrosion images of steel balls immersed in B-PEG200(aq) for 72 h and in H3PO4(aq) for 12 h, respectively; the H3PO4(aq) was prepared by diluting phosphoric acid with pure water to a pH=1.5 level. 254x203mm (300 x 300 DPI)


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TOC 400x320mm (300 x 300 DPI)


Liquid Superlubricity of Polyethylene Glycol Aqueous Solution

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