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The corrosion and lubrication properties of 2-Mercaptobenzothiazole functionalized ionic liquids for bronze
Tribology International 114 (2017) 121–131
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The corrosion and lubrication properties of 2-Mercaptobenzothiazole functionalized ionic liquids for bronze ⁎
Yi Lia,b, Songwei Zhanga,c, , Qi Dinga,c, Dapeng Fenga, Baofeng Qina, Litian Hua, a b c
MARK
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State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China Qingdao Center of Resource Chemistry & New Material, Qingdao 266071, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Functionalized ionic liquids 2-Mercaptobenzothiazole Low corrosivity Lubrication performances
Aiming at a better understanding of the structure-function relationship of the molecular structure, corrosion and lubrication properties of ionic liquids (ILs), three 2-mercaptobenzothiazole functionalized ILs were synthesized and their corrosion and lubrication properties were investigated. Results of electrochemical and tribological experiments indicated that the functionalized ILs showed much lower corrosivity and better lubricity for bronze than conventional ILs [BMIM][BF4]. And the investigation revealed that ILs with lower corrosivity promised the better lubrication properties via reducing the severe corrosive wear. Results from the surface analysis by XPS and SEM-EDS suggested that the anti-wear and friction-reducing mechanisms of the functionalized ILs were ascribed to synergistic effect of adsorbed ILs film and tribochemical products formed on the worn surfaces.
1. Introduction Bronze materials were widely used as worm, gear, bearing, and other machinery parts, because of their outstanding anti-scuffing capacity, low melting point, and corrosion resistance, etc. While these devices face with the harsh conditions (high contact stress, high temperature, corrosive environment, etc.), the lubricating oil loses its efficacy, causing high friction and severe wear to Cu alloy frictional components [1,2]. Thus, the issue on lubrication of bronze materials under the rigorous service conditions was becoming a hot area of research in recent years. Ionic liquids (ILs) exhibit remarkable physicochemical characteristics, such as negligible vapor pressure, nonflammability, high thermal stability, broad liquid range, wide electrochemical window and excellent conductivity. Thus, these properties give ILs potential for a large range of applications like batteries, cellulose processing, solar cells, chemical reaction medium, lubricants [3–8]. ILs were first reported as versatile lubricants for various engineering materials in 2001 by Prof. Liu [9]. Since then, considerable researches have been implemented on different types of ILs, applying them as lubricants and lubricant additives. ILs exhibited good friction-reducing and anti-wear performances on different frictional pairs, which make it possible for ILs to be used under some extreme conditions like high vacuum and high temperature [6,10]. Unfortunately, machine devices would be corroded
⁎
or damaged by the corrosive acids generated from hydrolysis reaction of some conventional ILs [11], which restricts their application as lubricants or additives. In order to solve this problem, some low corrosive ILs were developed, e.g., benzotriazole ILs [12,13], dioctyl sulfosuccinate based ILs [14], phosphonate ILs [10,15], and so on. These ILs showed excellent tribological properties and low corrosive to steel based materials at room temperature [10,12–15]. Unfortunately, they cannot offer good feasibility to the application of IL as lubricants or additives for copper alloy devices under rigorous service conditions. It is a good choice to improve the corrosion resistance of frictional pairs effectively by adding corrosion inhibitor into the base stocks. Liu et al. [16] added benzotriazole (BTA) into ILs and found that BTA can greatly improve the tribological properties of ILs, because of the alleviation of corrosion. BTA would sublimate while used this kind of mixture at high temperature, which reduced the tribological performance [12]. 2-Mercaptobenzothiazole (MBT) was widely used as corrosion inhibitor for copper alloys, as a protective film could be formed through the strong binding of MBT molecules on metal surface, which produced the high corrosion inhibition efficiency to the copper alloys [17,18]. Inspired by MBT's corrosion inhibition properties, it is a possible route to design functionalized ILs with excellent lubrication performances and low corrosivity by introducing the MBT group into structure of the conventional ILs. In this study, three MBT functionalized ILs with promising low
Corresponding authors at: State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China. E-mail addresses: [email protected] (S. Zhang), [email protected] (L. Hu).
http://dx.doi.org/10.1016/j.triboint.2017.04.022 Received 3 January 2017; Received in revised form 8 April 2017; Accepted 15 April 2017 Available online 17 April 2017 0301-679X/ © 2017 Elsevier Ltd. All rights reserved.
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corrosivity were synthesized and evaluated by electrochemical measurements. Their lubrication performances as lubricants or lubricant additives in PEG base oil were assessed at elevated temperature. The structure-function relationship of the molecular structure, corrosion and lubrication properties of ILs discussed sufficiently. This work may make an obvious progress towards low corrosive ILs lubricants in the application of lubrication engineering for copper alloys. 2. Experimental section 2.1. Materials and specimens preparation The tested metal is bronze, the chemical composition of which was (wt%): Sn (5.44), Zn (4.45), Pb (3.94) and Cu for balance. The specimens used for corrosion tests and tribological tests were cut into Φ24.0×7.9 mm sizes. Before measurements, all the specimens were mechanically abraded and polished to gain a smooth surface with the roughness (Ra) about 0.025 µm. Then they were rinsed with distilled water and cleaned in acetone ultrasonically. After drying at room temperature, they were stored under N2 atmosphere. The IL reagents, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1-ethyl-3-methylimidazolium bromide and 1-butyl-3methylimidazolium bromide with the purity of ≥98% were obtained from Lanzhou Institute of Chemical Physics, CAS. The following chemicals were commercially available: 2-mercaptobenzothiazole (Aladdin), choline chloride (J & K), polyethylene glycol 200, short as PEG (Guangdong Xilong Chemical Regent Company), potassium hydroxide (Tianjin Kaitong Chemicals Regent Company), formulated polyether-based worm gear oil-220, short as POE 220 (Anshan Haihua Oil Chemical Company). These functionalized ILs were synthesized following previous methods in literatures [19,20]. The specific synthesis route is presented as follows: 16.725 g (0.1 mol) 2-mercaptobenzothiazole was mixed with 5.611 g (0.1 mol) potassium hydroxide in methanol firstly. Afterwards, a solution of methanol with 0.1 mol of 1-ethyl-3-methylimidazolium bromide (respectively, 1-butyl-3-methylimidazolium bromide, Choline chloride) was added into the mixtures. The reaction system was stirred magnetically at ambient temperature over 1 h. Then the insoluble solids were separated. The solution was evaporated under reduced pressure to remove methanol and the byproduct water. In the process of removing water, some insoluble solids precipitated. Thus, anhydrous acetone was used to extract the ILs. And then the acetone was evaporated to get the products. The obtained ILs were dried overnight at 80 °C under vacuum. The structures of synthesized functionalized ILs were characterized by 1H NMR, 13C NMR, HRMS and the data were listed in Appendix. The molecular structures of tested ILs were presented in Fig. 1. Their thermal properties were measured on STA449C Jupiter simultaneous TGA from 25 °C to 600 °C with a heating rate of 10 °C/min in N2 atmosphere.
Fig. 1. The molecular structures of tested ILs.
amplitude of 5 mV. Nyquist and Bode plots were obtained. The EIS parameters were collected by fitting the experimental results to an appropriate equivalent circuit using Gamry Echem. Analyst software. The potentiodynamic polarization results were automatically recorded by increasing the electrode potential from −350 mV to 350 mV with a scan rate of 0.5 mV/s, the data were also analyzed by Gamry Echem. Analyst software.
2.3. Tribological tests The lubrication properties of MBT functionalized ILs were assessed using an Optimol SRV-IV oscillating reciprocating friction and wear tester at an elevated temperature of 100 ℃. The lower stationary disc was bronze. And the upper ball was AISI 52100 steel with 10 mm diameter. The elastic properties of steel and bronze were listed in Table 1. The stress and deformation parameters of the ball-on-disc contact were also calculated according to the reference [21]. Before starting the tribological tests, 0.2 ml lubricants were dropped onto the steel/bronze contact area. Tribological tests were performed under normal conditions (load: 100 N, test duration: 30 min, amplitude: 1 mm, frequency: 25 Hz, relative humidity: 10–30%). The friction coefficient was monitored continuously by the SRV instrument. The average friction coefficient, which indicates the average value of friction coefficients during the whole friction process, was presented in the Results and Discussion section. Some oil samples exhibited fluctuant friction coefficient curves in the whole process without any steady period. Therefore, the average friction coefficient would be a better choice than the steady friction coefficient, while we made the comparison of friction-reducing properties of these oil samples. The wear volume of bronze disc was calculated using a noncontact surface mapping microscope profilometer MicroXAM-3D. The results for each test were presented as an average of three replicates.
2.2. Electrochemical tests Electrochemical experiments were carried out by using Gamry Reference 3000 advanced electrochemical workstation in a traditional three-electrode system at 298 K ± 2 K. A saturated calomel electrode (SCE) and a platinum electrode were utilized as reference and auxiliary electrode respectively. The electric potential values listed in this article were all referred to SCE. Before the electrochemical tests were made, the bronze electrode was dipped in tested electrolyte at open circult potential (OCP) for 1 h to get a steady state. The electrolytes are 2.0 wt % ILs in ethanol/water (40 wt% ethanol) solutions. The ethanol/water solutions are to imitate the similar environments in working conditions and to give ILs a better solubility. The exposed surface area of electrochemical specimen was 1.0 cm2. The electrochemical impedance spectroscopy (EIS) measurements were performed in frequency range of 104 Hz to 10−2 Hz using a sinusoidal AC perturbation with an
Table 1 Characteristics of steel ball and bronze disk.
122
Parameter
Steel
Bronze
Young's modulus (E)/GPa Poisson's ratio (ν) Contact radius (a)/mm Normal approach (δ)/mm The Maximum Hertzian Contact Pressure (P0)/GPa
219 0.300 0.21 4.41×10−3 1.08
110 0.300
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Fig. 2. TGA curves of tested ILs in N2 atmosphere.
2.4. Surface analysis A scanning electron microscope (SEM, JSM-5600LV) equipped with an energy-dispersive X-ray Spectrometer (EDS, KEVEX) attachment was utilized to analyze the morphology and chemical composition of worn surfaces. The X-ray photoelectron spectrometer (XPS) was carried out to test the chemical changes of elements on the worn surfaces. The exciting source of XPS was Al Kα radiation. The binding energy of contaminated carbon at 284.8 eV was used as the reference. Before scanning, the specimens were cleaned in ethanol ultrasonically. 3. Results and discussion 3.1. Properties of functionalized ILs The pyrolysis properties of ILs were examined systematically and the TGA curves are shown in Fig. 2. As shown in Table 2, the onset decomposition temperatures are 369 °C, 228 °C, 205 °C and 180 °C for [BMIM][BF4], [EMIM][MBT], [BMIM][MBT] and [CHO][MBT] respectively. Results illustrate that the conventional ILs [BMIM][BF4] exhibited much higher thermal stability than MBT functionalized ILs. Unfortunately, it always causes serious corrosion to many metallic materials, which is an obvious deficiency about [BMIM][BF4] [11,12]. For MBT functionalized ILs, Fig. 2 and Table 2 explicitly demonstrate that the ammonium ILs showed lower stability than imidazolium ILs. The high thermal stability of imidazolium ILs was ascribed to the higher intermolecular interactions, smaller free volume and compact structure of imidazolium ring [22]. In the alkylimidazolium ILs, the length of alkyl chain connecting to N, has a significant effect on the thermal stability. Increasing the substituted alkyl chain length from two to four decreased the thermal stability for imidazolium ILs. This can be ascribed to the large free volume and steric hindrance [23].
Fig. 3. EIS for bronze in ILs solution, a: Nyquist plots; b and c: Bode plots. (scatter plot: tested results; full line: fitted results).
temperature were obtained and shown in Fig. 3. To determine the impedance parameters, the equivalent circuit models given in Fig. 4 were employed to fit the obtained EIS data, and the fitted EIS parameters are presented in Table 3. In equivalent circuit models, constant phase element (CPE) is often used instead of double layer capacitance to give a satisfactory fit to the EIS data. The impedance of the CPE can be given as follows:
3.2. Corrosion properties 3.2.1. EIS measurements Nyquist and Bode plots of bronze in ILs solutions at room Table 2 Comparison of thermal stability of tested ILs. ILs
[BMIM][BF4] [EMIM][MBT] [BMIM][MBT] [CHO][MBT]
ZCPE = [Y0 ( jω)n ]−1
TGA temperature for % weight loss,°C Onset
20%
50%
80%
369 228 205 180
376 230 219 185
407 247 235 201
424 265 252 222
(1)
where Y0 is the CPE constant, n is the phase shift, j is the imaginary unit, and ω is the angular frequency. According to the values of n, CPE can describe an ideal capacitor (n=1), resistance (n=0), inductance (n=−1) and Warburg impedance (n=0.5). The equivalent circuit shown in Fig. 4a was used for fitting electrochemical responses of the [BMIM][BF4] solution on bronze, and model in Fig. 4b was suitable for the MBT functionalized IL 123
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Fig. 5. Potentiodynamic polarization curves for bronze in ILs solutions. Fig. 4. Equivalent circuit models used to fit the obtained impedance spectra. Table 4 Corrosion parameters obtained from potentiodynamic polarization curves of bronze in ILs solutions.
solutions. In the equivalent circuit models, CPE1 is representing the double layer capacitance (the value of n1 is close to unity), which is associated with the relaxation process at high frequency. The CPE2 and R2 at low frequency region are related to the adsorption/desorption process of the ILs on working electrode surface. Rs represents solution resistance, R1 is ascribed to the charge transfer resistance of the corrosion process at the solution/metal interface. The W in Fig. 4a is Warburg impedance representing the diffusion of metal ions through surface generated from corrosion process. The polarization resistance (Rp) is often used to assess the corrosivity of the selected solution and the inhibition efficiency of corrosion inhibitors in corrosion science [24,25]. The working electrode shows a better corrosion resistance when Rp has a higher value in ILs solution. In both models in Fig. 4, Rp values are the sum of all Rfitted parameters (Rp=R1+R2) except solution resistance Rs [26]. As can be seen from Table 3, [BMIM][BF4] exhibits more serious corrosivity than functionalized ILs, due to its smallest Rp value. The order of corrosivity for functionalized ILs is as follows: [EMIM][MBT] > [BMIM][MBT] > [CHO][MBT]. The low corrosivity of MBT functionalized ILs may be ascribed to the strong adsorbability of ILs on bronze surface, which formed an adsorbed layer to protect the working electrode from these corrosive factors.
ILs
βa/mV dec−1
-βc/mV dec−1
Ecorr/mVSCE
icorr/μA cm−2
[BMIM][BF4] [EMIM][MBT] [BMIM][MBT] [CHO][MBT]
251.3 299.9 223.1 299.3
94.08 478.3 228.2 304.3
−664.5 −593.0 −512.2 −385.2
8.364 2.732 1.325 0.4458
is oxygen reduction reaction, while anodic reaction is the dissolution reaction of active metal on the surface. The icorr values represent the material corrosion rates at Ecorr [27]. The low icorr values indicate the chemical benignity of ILs solutions towards metals at Ecorr. [BMIM][BF4] showed the most serious corrosivity, speculated from highest icorr values in Table 4. The values of icorr followed the order as listed: [EMIM][MBT] > [BMIM][MBT] > [CHO][MBT]. Moreover, [CHO][MBT] has the lowest corrosivity in all the tested ILs. Results of electrochemical tests indicate that MBT functionalized ILs showed extremely low corrosivity to bronze, which suggest that they may be suitable for using as lubricants or lubricant additives for the industrial applications in lubrication engineering. 3.3. Lubrication performances
3.2.2. Potentiodynamic polarization curves measurements Potentiodynamic polarization curves tests have been performed to gain further knowledge about the corrosivity of the functionalized ILs. Polarization curves of the tested solutions are presented in Fig. 5. The electrochemical parameters derived from the Tafel extrapolation of the data, such as cathodic Tafel slope (βc), anodic Tafel slope (βa), corrosion potential (Ecorr) and corrosion current density (icorr) are given in Table 4. As can be seen from Fig. 5, the polarization curves of MBT functionalized ILs are below [BMIM][BF4] ILs. Furthermore, the curves of MBT functionalized ILs are parallel with each other, which indicates that similar cathodic and anodic reactions occurred though they have different cations. In these neutral solutions, cathodic reaction
3.3.1. ILs as lubricants The MBT functionalized ILs as lubricants were researched in steel/ bronze contact at 100 ℃. Fig. 6a shows the evolution of friction coefficients with time. The friction coefficient of PEG base oil fluctuated sharply during the whole friction process, while four tested ILs showed relatively steady friction coefficient curves, which indicates PEG base oil had a poor lubrication effect for steel/bronze frictional pairs at elevated temperature. For [BMIM][BF4], it took a long period about 1000 s before the friction coefficient curve coming to a steady state. For MBT functionalized ILs, they all have short running-in time about
Table 3 Fitting results of EIS for bronze in ILs solutions. ILs
[BMIM][BF4] [EMIM] [MBT] [BMIM] [MBT] [CHO][MBT]
Yo1/10−6 S sn cm−2
n1
355.9 574.7
62.6 12.3
0.701 0.836
0.0286 1.01
32.2 43.1
0.0443 0.223
705.8
4.45
0.790
1.35
2.23
540.3
4.60
0.740
5.26
2.56
Rs/Ω cm2
R1/104 Ω cm2
Yo2/10−5 S sn cm−2
124
B3/10−2 s1/ cm−2
Rp/104 Ω cm2
2
2
1.57 7.36
9.43 –
3.54 –
0.185 1.75
0.555
40.4
–
–
5.39
0.749
44.0
–
–
9.67
n2
R2/103 Ω cm2
Yo3/10−4 S s1/ cm−2
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Fig. 6. (a) Evolution of friction coefficient with time at 100 N for PEG base oil and tested ILs at 100 °C; (b) Average friction coefficients and wear volumes of bronze discs lubricated by PEG base oil and tested ILs at 100 °C.
[BMIM][BF4] fluctuated sharply during the whole friction process, which resembles the friction coefficient curve of PEG base oil. It revealed that [BMIM][BF4] could not improve the friction-reducing properties effectively when PEG base oil suffered a lubrication failure in steel/bronze frictional pairs at 100 °C. In contrast, the MBT functionalized ILs exhibited lower friction coefficients and relatively stable curves, which indicated that they could remarkably reduce the friction at the elevated temperature while using as additives in PEG base oil. Fig. 7b shows the comparison of average friction coefficients and wear volumes of bronze discs lubricated with PEG base oil with and without ILs. The wear volume and average friction coefficient of PEG base oil were significantly reduced after addition of four ILs respectively. Furthermore, a 2.0 wt% dose of functionalized ILs in PEG base oil gave better anti-wear and friction-reducing properties than that of [BMIM][BF4]. Thus, the MBT functionalized ILs would obviously be a better choice than the conventional ILs [BMIM][BF4] using as lubricant additives for steel/bronze contact at elevated temperature. In a word, the MBT functionalized ILs exhibit lower friction coefficient and stable friction process when used as lubricants. Smaller wear volumes also presented after tribological tests than [BMIM][BF4] and PEG base oil. When used as lubricant additives in PEG base oil, MBT functionalized ILs could significantly improve the anti-wear and friction-reducing properties, and [CHO][MBT] performed the best. Owing to the severe tribocorrosion process, [BMIM] [BF4] exhibited higher friction coefficient and wear volumes than MBT
100 s, and the friction coefficient curves are more smooth and steady during the whole friction process, comparing with [BMIM][BF4]. Results from friction tests indicate that MBT functionalized ILs give more steady friction coefficients and show better lubrication properties than the conventional ILs [BMIM][BF4] and PEG base oil. Fig. 6b shows the wear volumes and the average friction coefficients of the bronze discs lubricated by ILs and PEG base oil respectively. All the tested ILs showed lower average friction coefficient than PEG base oil, which proved the better friction-reducing property of ILs than that of PEG base oil. Moreover, three MBT functionalized ILs give lower average friction coefficient than [BMIM][BF4], and the lowest friction coefficient is gained by [CHO][MBT]. It can also be seen in Fig. 6b that ILs exhibit lower wear volumes than PEG base oil, and the best antiwear property is obtained by [CHO][MBT]. The above results discussed confirmed the excellent lubrication properties of the MBT functionalized ILs as lubricants for steel/bronze contact at 100 °C. Furthermore, because of the low corrosivity and good lubrication, MBT functionalized ILs are more qualified lubricants than [BMIM][BF4]. 3.3.2. ILs as lubricant additives The lubrication performances of MBT functionalized ILs as additives of PEG base oil were explored for steel/bronze contact at 100 °C. Fig. 7a displays the evolution of friction coefficient as a function of time for PEG base oil with and without ILs. The friction coefficient of
Fig. 7. (a) Evolution of friction coefficient with time at 100 N for PEG base oil and PEG plus 2.0 wt% tested ILs at 100 °C; (b) Average friction coefficients and wear volumes of bronze discs lubricated by PEG base oil and PEG plus 2.0 wt% tested ILs at 100 °C.
125
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Fig. 8. SEM and 3D morphologies of worn surfaces lubricated by PEG base oil and different ILs: (a, b, c) PEG; (d, e, f) [BMIM][BF4]; (g, h, i) [EMIM][MBT]; (j, k, l) [BMIM][MBT]; (m, n, o) [CHO][MBT].
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Fig. 9. SEM and 3D morphologies of worn surfaces lubricated by PEG base oil and different ILs as additives in PEG base oil: (a, b, c) PEG base oil; (d, e, f) 2.0 wt% [BMIM][BF4]; (g, h, i) 2.0 wt% [EMIM][MBT]; (j, k, l) 2.0 wt% [BMIM][MBT]; (m, n, o) 2.0 wt% [CHO][MBT].
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Fig. 10. SEM micrographs and corresponding element mapping on the worn surface of bronze lubricated by PEG base oil and different ILs.
The worn surface lubricated by PEG base oil showed severe plastic deformation with adhesive wear on the bronze surface, implying the direct contact between the bronze disc and steel ball [23]. The wear scar of bronze disc lubricated by [BMIM][BF4] was also very rough with adhesive wear on the surface. However, the wear scars of bronze surface lubricated by MBT functionalized ILs were much smoother with shallow friction scratches. The [CHO][MBT] ILs produced the smoothest wear scars compared with PEG and other ILs. Fig. 9 shows the SEM images and 3D optical microscopic images of worn surfaces of bronze discs lubricated by PEG base oil with and without ILs. When adding 2.0 wt% [BMIM][BF4] into the PEG base oil, it showed no effect on reducing severe scuffing damages. However, scuffing was greatly alleviated, and relatively smooth surfaces with few some plastic deformation signs were obtained when lubricated by PEG with functionalized ILs. Results of surface analysis furtherly proved that the MBT functionalized ILs possess excellent anti-wear performances when used as lubricant additives in PEG base oil for steel/ bronze contact at elevated temperature. Take a comprehensive consideration of the corrosion behaviors, lubrication properties and worn surface features of ILs, it is reasonable to conclude that ILs with lower corrosivity might always promise the better lubrication properties via reducing the severe corrosive wear. EDS was utilized to explore the elements distribution and content on the worn surface of bronze discs lubricated by PEG base oil and four tested ILs respectively. In Fig. 10, we can see that the characteristic elements distribute uniformly on the worn surface. The elements
Table 5 The element content on worn surfaces lubricated by PEG and ILs. ILs
PEG [BMIM][BF4] [EMIM][MBT] [BMIM][MBT] [CHO][MBT]
Elements Content (wt%)* C
O
B
F
S
15.36 8.52 9.09 8.24 8.14
2.51 1.64 1.96 1.39 0.11
0 3.15 0 0 0
0 0.3 0 0 0
0 0 2.94 1.52 0.46
*Cu and alloying elements for balanced.
functionalized ILs using as lubricants or lubricant additives. It can be discovered from the corrosion and lubrication properties that the higher corrosive ILs [BMIM][BF4] showed poorer lubrication properties, while the MBT functionalized ILs with lower corrosivity exhibited better anti-wear and friction-reducing performances. Considering the predominant lubricating performances and extremely low corrosivity, MBT functionalized ILs as lubricants or lubricant additives can be excellent candidates for the industrial application of bronze materials at elevated temperature. 3.4. Surface analysis The SEM and 3D optical microscopic images of worn surfaces of bronze discs lubricated by PEG base oil and ILs are displayed in Fig. 8. 128
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Fig. 11. XPS spectra of Cu2p (a), O1s and S2p on the worn surfaces lubricated by different ILs: [EMIM][MBT] (b, e); [BMIM][MBT] (c, f); [CHO][MBT] (d, g).
elements was analyzed. Fig. 11 presents the XPS spectra of Cu2p, O1s and S2p. The binding energy of O1s peaks were at 531.7 eV and 530.6 eV, which are belong to Cu2O [18] and C–O bonds [1], respectively. The organic compounds containing C–O bonds may generate from the oxidation reaction of ILs fragments during friction process. The exocyclic S (exo-S) and endocyclic S (endo-S) have different binding energy in the MBT anion due to the distinct chemical environment [28]. The S2p binding energy differences for three functionalized ILs were shown in Table 6. The binding energy of exoS and endo-S are about 161.5 eV and 164.0 eV. We can see a tinge of difference on the binding energy of S atoms, which was caused by cations in the ILs. In the Fig. 11, exo-S and endo-S atoms could be found on the worn surfaces, which revealed that the functionalized ILs might adsorb on the worn surface by means of the combination between MBT anion and Cu element (elementary copper or oxide of copper) [18,28]. Furthermore, the S2p peak appears at approximately 162.4 eV may correspond to CuS [29].
Table 6 Comparison of binding energy of S2p in ILs and on the worn surfaces. S atoms
exo-S endo-S CuS
Ionic liquids (eV)
Worn surfaces (eV)
[EMIM] [MBT]
[BMIM] [MBT]
[CHO] [MBT]
[EMIM] [MBT]
[BMIM] [MBT]
[CHO] [MBT]
161.46 163.99 –
161.67 164.02 –
161.46 164.00 –
161.46 163.99 162.40
161.67 164.02 162.40
161.46 164.00 162.40
content on the worn surfaces of bronze discs lubricated by PEG base oil and ILs listed in Table 5. 3.15 wt% of elements B and 0.3 wt% of F were detected on the worn surface lubricated by [BMIM][BF4], which indicates that the protective films may be mainly generated from the degradation or corrosion of BF4− during the friction process. Sulphur is the characteristic element of MBT functionalized ILs and it was observed in the wear scars, which reveals the formation of tribochemical thin film composed of functionalized ILs or/and tribochemical products of ILs. S elements observed on all the worn surfaces lubricated by MBT functionalized ILs implies the different lubricating mechanism with [BMIM][BF4]. The different tribochemical reactions occurred with the bronze surface between MBT functionalized ILs and conventional ILs should be further investigated. To gain further insight into the role of functionalized ILs on bronze surface during the friction process, the chemical states of typical
3.5. Mechanism discussion During the electrochemical corrosion process, MBT functionalized ILs exhibited higher Rp and lower icorr values than those of [BMIM] [BF4]. Results indicated that for [BMIM][BF4] solution, a great number of Cu2+ ions were dissolved out from the exposed work electrode. As a result, serious corrosion was caused to bronze by [BMIM][BF4] 129
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Fig. 12. Schematic representation of MBT functionalized ILs adsorbed layer on bronze.
Fig. 13. (a) Evolution of friction coefficient with time at 100 N for PEG plus 2.0 wt% tested ILs, POE 220 oil and tested ILs at 100 °C; (b) Average friction coefficients and wear volumes of bronze discs lubricated by PEG plus 2.0 wt% tested ILs, POE 220 oil and tested ILs at 100 °C.
steel/bronze tribopairs. Therefore, we investigated the lubrication performance of the commercial formulated worm gear oil POE 220 and made a comparison with MBT functionalized ILs. As it can be seen in Fig. 13a, when used as additives, the friction curves of three oil samples were above that of the POE 220 oil, while the curves of pure ILs were comparable with that of POE 220 oil. The friction curve of [CHO][MBT] with the lowest corrosivity was at the bottom. As regards to wear volumes and average friction coefficients in Fig. 13b, POE 220 oil exhibited lower wear volume and average friction coefficient than PEG plus ILs oil samples. For pure ILs, [EMIM][MBT] had comparable wear volume value with POE 220 oil. [BMIM][MBT] and [CHO][MBT] showed lower wear volumes. What's more, [CHO][MBT] had the lowest average friction coefficient. Based on the comparisons above, MBT functionalized ILs used as base oil or as additives in PEG base oil both showed comparable properties with commercial formulated worm gear oil POE 220. Especially for [CHO][MBT], it showed better performances both on friction and wear comparisons. While when used in PEG base oil, the three ILs additives exhibited inferior behaviors, which could be ascribed to the additive package in formulated POE 220 oil. As a conclusion, the MBT functionalized ILs have the potential to be lubricants or additives in worm gear industries, but more investigation and improvement still need to be done in the future.
solution. In the MBT functionalized ILs solutions, dissolution rate of bronze was slowed down through decreasing the numbers of active sites on exposed bronze electrode. Furthermore, the occupation of active sites was attributed to the adsorption of MBT functionalized ILs, and the specific process was that MBT anion firstly adsorbed on the surface through a S-N bridge connection [17,18]; the counter cations of ILs may then adsorb on the surface by electrostatic attractions and generate the physicochemical adsorbed film. That's why the MBT functionalized ILs solutions exhibited lower corrosivity for bronze. Surface analysis after tribological tests reveals that not only the CuS and organic compounds containing C-O bonds formed on the worn surfaces during friction process, but also the formation of adsorption film composed of ILs existed on the worn surface. Combined with the corrosivity discussion, the lubricating mechanism was proposed in Fig. 12. During the friction process, CuS generated from ILs and bronze due to the tribochemical reaction. The organic compounds containing C–O bonds also generated. Cu2O, CuS, organic compounds containing C–O bonds and ILs adsorption film could protect bronze discs synergistically during friction process. That is why the MBT functionalized ILs with low corrosivity exhibit excellent anti-wear and frictionreducing abilities for steel/bronze contact at elevated temperature.
3.6. Potential applications as worm gear oils
4. Conclusion
The worm gear and worm were widely used as transmission system of machine, which were mostly consisted of steel worm and bronze worm gear [30]. This kind of contact was consistent with the tested
The results and discussion above can be concluded to the following 130
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points: (1) Three 2-Mercaptobenzothiazole functionalized ILs were synthesized and their structures were well characterized. (2) Results of EIS and potentiodynamic polarization curves measurements revealed that the MBT functionalized ILs showed extremely low corrosivity to bronze, which is much lower than conventional ILs [BMIM][BF4]. (3) No matter using as lubricants or lubricant additives in PEG base oil, MBT functionalized ILs showed excellent lubrication performances for steel/bronze contact at elevated temperature. Furtherly, MBT functionalized ILs exhibited better lubrication abilities than [BMIM] [BF4], and [CHO][MBT] performed the best. (4) Results of electrochemical and tribological tests revealed that ILs with lower corrosivity might always promise the better lubrication properties via reducing the severe corrosive wear. (5) The extremely low corrosivity and excellent lubrication performances were attributed to the formation of adsorbed ILs film and also the tribochemical products CuS, organic compounds containing C-O bonds and Cu2O generated during the friction process. (6) When used as lubricants or additives in PEG oil, MBT functionalized ILs exhibited comparable tribological performances with worm gear oil POE 220, having the potential application in worm gear industries.
[3]
[4] [5]
[6] [7]
[8]
[9] [10]
[11]
[12]
[13]
Acknowledgements [14]
The authors thank Professor Deng for the kind supply of IL reagents. The authors also acknowledge financial support from the National Natural Science Foundation of China (51575506 and 51505462).
[15]
[16]
Appendix. The structure analysis data of synthesized ionic liquids
[17]
[EMIM][MBT]. H NMR (400 MHz, DMSO-d6) δ 9.26 (s, 1H), 7.81 (s, 1H), 7.72 (s, 1H), 7.42 (m, 1H), 7.26 (d, J=8.0 Hz, 1H), 7.10 (m, 1H), 6.94 (m, 1H), 4.20 (q, J=7.3 Hz, 2H), 3.86 (s, 3H), 1.40 (t, J=7.3 Hz, 3H). 13C NMR (101 MHz, DMSO-d6) δ185.86, 154.99, 136.80, 124.84, 124.02, 122.42, 121.05, 119.65, 116.84, 44.62, 36.19, 15.60. m/z: 111.0948 ([EMIM]+), 388.1601 ([EMIM]2[MBT]+), 665.2264 ([EMIM]3[MBT]2+), 942.2950 ([EMIM]4[MBT]3+). [BMIM][MBT]. 1H NMR (400 MHz, DMSO-d6) δ 9.21 (s, 1H), 7.78 (s, 1H), 7.71 (s, 1H), 7.42 (d, J=7.5 Hz, 1H), 7.25 (d, J=7.9 Hz, 1H), 7.10 (m, 1H), 6.94 (m, 1H), 4.16 (t, J=7.2 Hz, 2H), 3.86 (s, 3H), 1.74 (m, 2H), 1.23 (m, 2H), 0.88 (t, J=7.4 Hz, 3H). 13C NMR (101 MHz, DMSO-d6) δ 185.95, 154.76, 137.05, 136.69, 124.86, 124.07, 122.74, 121.07, 119.67, 116.75, 49.00, 36.24, 31.85, 19.25, 13.73. m/z: 139.1243 ([BMIM]+), 444.2222 ([BMIM]2[MBT]+). [CHO][MBT]. 1H NMR (400 MHz, DMSO-d6) δ 7.44 (d, J=7.7 Hz, 1H), 7.28 (d, J=7.9 Hz, 1H), 7.12 (m, 1H), 6.97 (m, 1H), 5.29 (s, 2H), 3.88 (m, 2H), 3.47 (m, 2H), 3.17 (s, 9H). 13C NMR (101 MHz, DMSO-d6) δ 186.04, 154.56, 136.58, 125.03, 121.31, 119.82, 116.79, 67.52, 55.66, 53.73, 53.69, 53.66. m/z: 104.1108 ([CHO]+), 374.1923 ([CHO]2[MBT]+). 1
[18]
[19] [20]
[21] [22] [23]
[24]
[25]
[26]
[27] [28]
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The corrosion and lubrication properties of 2-Mercaptobenzothiazole functionalized ionic liquids for bronze ⁎
Yi Lia,b, Songwei Zhanga,c, , Qi Dinga,c, Dapeng Fenga, Baofeng Qina, Litian Hua, a b c
MARK
⁎
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China Qingdao Center of Resource Chemistry & New Material, Qingdao 266071, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Functionalized ionic liquids 2-Mercaptobenzothiazole Low corrosivity Lubrication performances
Aiming at a better understanding of the structure-function relationship of the molecular structure, corrosion and lubrication properties of ionic liquids (ILs), three 2-mercaptobenzothiazole functionalized ILs were synthesized and their corrosion and lubrication properties were investigated. Results of electrochemical and tribological experiments indicated that the functionalized ILs showed much lower corrosivity and better lubricity for bronze than conventional ILs [BMIM][BF4]. And the investigation revealed that ILs with lower corrosivity promised the better lubrication properties via reducing the severe corrosive wear. Results from the surface analysis by XPS and SEM-EDS suggested that the anti-wear and friction-reducing mechanisms of the functionalized ILs were ascribed to synergistic effect of adsorbed ILs film and tribochemical products formed on the worn surfaces.
1. Introduction Bronze materials were widely used as worm, gear, bearing, and other machinery parts, because of their outstanding anti-scuffing capacity, low melting point, and corrosion resistance, etc. While these devices face with the harsh conditions (high contact stress, high temperature, corrosive environment, etc.), the lubricating oil loses its efficacy, causing high friction and severe wear to Cu alloy frictional components [1,2]. Thus, the issue on lubrication of bronze materials under the rigorous service conditions was becoming a hot area of research in recent years. Ionic liquids (ILs) exhibit remarkable physicochemical characteristics, such as negligible vapor pressure, nonflammability, high thermal stability, broad liquid range, wide electrochemical window and excellent conductivity. Thus, these properties give ILs potential for a large range of applications like batteries, cellulose processing, solar cells, chemical reaction medium, lubricants [3–8]. ILs were first reported as versatile lubricants for various engineering materials in 2001 by Prof. Liu [9]. Since then, considerable researches have been implemented on different types of ILs, applying them as lubricants and lubricant additives. ILs exhibited good friction-reducing and anti-wear performances on different frictional pairs, which make it possible for ILs to be used under some extreme conditions like high vacuum and high temperature [6,10]. Unfortunately, machine devices would be corroded
⁎
or damaged by the corrosive acids generated from hydrolysis reaction of some conventional ILs [11], which restricts their application as lubricants or additives. In order to solve this problem, some low corrosive ILs were developed, e.g., benzotriazole ILs [12,13], dioctyl sulfosuccinate based ILs [14], phosphonate ILs [10,15], and so on. These ILs showed excellent tribological properties and low corrosive to steel based materials at room temperature [10,12–15]. Unfortunately, they cannot offer good feasibility to the application of IL as lubricants or additives for copper alloy devices under rigorous service conditions. It is a good choice to improve the corrosion resistance of frictional pairs effectively by adding corrosion inhibitor into the base stocks. Liu et al. [16] added benzotriazole (BTA) into ILs and found that BTA can greatly improve the tribological properties of ILs, because of the alleviation of corrosion. BTA would sublimate while used this kind of mixture at high temperature, which reduced the tribological performance [12]. 2-Mercaptobenzothiazole (MBT) was widely used as corrosion inhibitor for copper alloys, as a protective film could be formed through the strong binding of MBT molecules on metal surface, which produced the high corrosion inhibition efficiency to the copper alloys [17,18]. Inspired by MBT's corrosion inhibition properties, it is a possible route to design functionalized ILs with excellent lubrication performances and low corrosivity by introducing the MBT group into structure of the conventional ILs. In this study, three MBT functionalized ILs with promising low
Corresponding authors at: State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China. E-mail addresses: [email protected] (S. Zhang), [email protected] (L. Hu).
http://dx.doi.org/10.1016/j.triboint.2017.04.022 Received 3 January 2017; Received in revised form 8 April 2017; Accepted 15 April 2017 Available online 17 April 2017 0301-679X/ © 2017 Elsevier Ltd. All rights reserved.
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corrosivity were synthesized and evaluated by electrochemical measurements. Their lubrication performances as lubricants or lubricant additives in PEG base oil were assessed at elevated temperature. The structure-function relationship of the molecular structure, corrosion and lubrication properties of ILs discussed sufficiently. This work may make an obvious progress towards low corrosive ILs lubricants in the application of lubrication engineering for copper alloys. 2. Experimental section 2.1. Materials and specimens preparation The tested metal is bronze, the chemical composition of which was (wt%): Sn (5.44), Zn (4.45), Pb (3.94) and Cu for balance. The specimens used for corrosion tests and tribological tests were cut into Φ24.0×7.9 mm sizes. Before measurements, all the specimens were mechanically abraded and polished to gain a smooth surface with the roughness (Ra) about 0.025 µm. Then they were rinsed with distilled water and cleaned in acetone ultrasonically. After drying at room temperature, they were stored under N2 atmosphere. The IL reagents, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1-ethyl-3-methylimidazolium bromide and 1-butyl-3methylimidazolium bromide with the purity of ≥98% were obtained from Lanzhou Institute of Chemical Physics, CAS. The following chemicals were commercially available: 2-mercaptobenzothiazole (Aladdin), choline chloride (J & K), polyethylene glycol 200, short as PEG (Guangdong Xilong Chemical Regent Company), potassium hydroxide (Tianjin Kaitong Chemicals Regent Company), formulated polyether-based worm gear oil-220, short as POE 220 (Anshan Haihua Oil Chemical Company). These functionalized ILs were synthesized following previous methods in literatures [19,20]. The specific synthesis route is presented as follows: 16.725 g (0.1 mol) 2-mercaptobenzothiazole was mixed with 5.611 g (0.1 mol) potassium hydroxide in methanol firstly. Afterwards, a solution of methanol with 0.1 mol of 1-ethyl-3-methylimidazolium bromide (respectively, 1-butyl-3-methylimidazolium bromide, Choline chloride) was added into the mixtures. The reaction system was stirred magnetically at ambient temperature over 1 h. Then the insoluble solids were separated. The solution was evaporated under reduced pressure to remove methanol and the byproduct water. In the process of removing water, some insoluble solids precipitated. Thus, anhydrous acetone was used to extract the ILs. And then the acetone was evaporated to get the products. The obtained ILs were dried overnight at 80 °C under vacuum. The structures of synthesized functionalized ILs were characterized by 1H NMR, 13C NMR, HRMS and the data were listed in Appendix. The molecular structures of tested ILs were presented in Fig. 1. Their thermal properties were measured on STA449C Jupiter simultaneous TGA from 25 °C to 600 °C with a heating rate of 10 °C/min in N2 atmosphere.
Fig. 1. The molecular structures of tested ILs.
amplitude of 5 mV. Nyquist and Bode plots were obtained. The EIS parameters were collected by fitting the experimental results to an appropriate equivalent circuit using Gamry Echem. Analyst software. The potentiodynamic polarization results were automatically recorded by increasing the electrode potential from −350 mV to 350 mV with a scan rate of 0.5 mV/s, the data were also analyzed by Gamry Echem. Analyst software.
2.3. Tribological tests The lubrication properties of MBT functionalized ILs were assessed using an Optimol SRV-IV oscillating reciprocating friction and wear tester at an elevated temperature of 100 ℃. The lower stationary disc was bronze. And the upper ball was AISI 52100 steel with 10 mm diameter. The elastic properties of steel and bronze were listed in Table 1. The stress and deformation parameters of the ball-on-disc contact were also calculated according to the reference [21]. Before starting the tribological tests, 0.2 ml lubricants were dropped onto the steel/bronze contact area. Tribological tests were performed under normal conditions (load: 100 N, test duration: 30 min, amplitude: 1 mm, frequency: 25 Hz, relative humidity: 10–30%). The friction coefficient was monitored continuously by the SRV instrument. The average friction coefficient, which indicates the average value of friction coefficients during the whole friction process, was presented in the Results and Discussion section. Some oil samples exhibited fluctuant friction coefficient curves in the whole process without any steady period. Therefore, the average friction coefficient would be a better choice than the steady friction coefficient, while we made the comparison of friction-reducing properties of these oil samples. The wear volume of bronze disc was calculated using a noncontact surface mapping microscope profilometer MicroXAM-3D. The results for each test were presented as an average of three replicates.
2.2. Electrochemical tests Electrochemical experiments were carried out by using Gamry Reference 3000 advanced electrochemical workstation in a traditional three-electrode system at 298 K ± 2 K. A saturated calomel electrode (SCE) and a platinum electrode were utilized as reference and auxiliary electrode respectively. The electric potential values listed in this article were all referred to SCE. Before the electrochemical tests were made, the bronze electrode was dipped in tested electrolyte at open circult potential (OCP) for 1 h to get a steady state. The electrolytes are 2.0 wt % ILs in ethanol/water (40 wt% ethanol) solutions. The ethanol/water solutions are to imitate the similar environments in working conditions and to give ILs a better solubility. The exposed surface area of electrochemical specimen was 1.0 cm2. The electrochemical impedance spectroscopy (EIS) measurements were performed in frequency range of 104 Hz to 10−2 Hz using a sinusoidal AC perturbation with an
Table 1 Characteristics of steel ball and bronze disk.
122
Parameter
Steel
Bronze
Young's modulus (E)/GPa Poisson's ratio (ν) Contact radius (a)/mm Normal approach (δ)/mm The Maximum Hertzian Contact Pressure (P0)/GPa
219 0.300 0.21 4.41×10−3 1.08
110 0.300
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Fig. 2. TGA curves of tested ILs in N2 atmosphere.
2.4. Surface analysis A scanning electron microscope (SEM, JSM-5600LV) equipped with an energy-dispersive X-ray Spectrometer (EDS, KEVEX) attachment was utilized to analyze the morphology and chemical composition of worn surfaces. The X-ray photoelectron spectrometer (XPS) was carried out to test the chemical changes of elements on the worn surfaces. The exciting source of XPS was Al Kα radiation. The binding energy of contaminated carbon at 284.8 eV was used as the reference. Before scanning, the specimens were cleaned in ethanol ultrasonically. 3. Results and discussion 3.1. Properties of functionalized ILs The pyrolysis properties of ILs were examined systematically and the TGA curves are shown in Fig. 2. As shown in Table 2, the onset decomposition temperatures are 369 °C, 228 °C, 205 °C and 180 °C for [BMIM][BF4], [EMIM][MBT], [BMIM][MBT] and [CHO][MBT] respectively. Results illustrate that the conventional ILs [BMIM][BF4] exhibited much higher thermal stability than MBT functionalized ILs. Unfortunately, it always causes serious corrosion to many metallic materials, which is an obvious deficiency about [BMIM][BF4] [11,12]. For MBT functionalized ILs, Fig. 2 and Table 2 explicitly demonstrate that the ammonium ILs showed lower stability than imidazolium ILs. The high thermal stability of imidazolium ILs was ascribed to the higher intermolecular interactions, smaller free volume and compact structure of imidazolium ring [22]. In the alkylimidazolium ILs, the length of alkyl chain connecting to N, has a significant effect on the thermal stability. Increasing the substituted alkyl chain length from two to four decreased the thermal stability for imidazolium ILs. This can be ascribed to the large free volume and steric hindrance [23].
Fig. 3. EIS for bronze in ILs solution, a: Nyquist plots; b and c: Bode plots. (scatter plot: tested results; full line: fitted results).
temperature were obtained and shown in Fig. 3. To determine the impedance parameters, the equivalent circuit models given in Fig. 4 were employed to fit the obtained EIS data, and the fitted EIS parameters are presented in Table 3. In equivalent circuit models, constant phase element (CPE) is often used instead of double layer capacitance to give a satisfactory fit to the EIS data. The impedance of the CPE can be given as follows:
3.2. Corrosion properties 3.2.1. EIS measurements Nyquist and Bode plots of bronze in ILs solutions at room Table 2 Comparison of thermal stability of tested ILs. ILs
[BMIM][BF4] [EMIM][MBT] [BMIM][MBT] [CHO][MBT]
ZCPE = [Y0 ( jω)n ]−1
TGA temperature for % weight loss,°C Onset
20%
50%
80%
369 228 205 180
376 230 219 185
407 247 235 201
424 265 252 222
(1)
where Y0 is the CPE constant, n is the phase shift, j is the imaginary unit, and ω is the angular frequency. According to the values of n, CPE can describe an ideal capacitor (n=1), resistance (n=0), inductance (n=−1) and Warburg impedance (n=0.5). The equivalent circuit shown in Fig. 4a was used for fitting electrochemical responses of the [BMIM][BF4] solution on bronze, and model in Fig. 4b was suitable for the MBT functionalized IL 123
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Fig. 5. Potentiodynamic polarization curves for bronze in ILs solutions. Fig. 4. Equivalent circuit models used to fit the obtained impedance spectra. Table 4 Corrosion parameters obtained from potentiodynamic polarization curves of bronze in ILs solutions.
solutions. In the equivalent circuit models, CPE1 is representing the double layer capacitance (the value of n1 is close to unity), which is associated with the relaxation process at high frequency. The CPE2 and R2 at low frequency region are related to the adsorption/desorption process of the ILs on working electrode surface. Rs represents solution resistance, R1 is ascribed to the charge transfer resistance of the corrosion process at the solution/metal interface. The W in Fig. 4a is Warburg impedance representing the diffusion of metal ions through surface generated from corrosion process. The polarization resistance (Rp) is often used to assess the corrosivity of the selected solution and the inhibition efficiency of corrosion inhibitors in corrosion science [24,25]. The working electrode shows a better corrosion resistance when Rp has a higher value in ILs solution. In both models in Fig. 4, Rp values are the sum of all Rfitted parameters (Rp=R1+R2) except solution resistance Rs [26]. As can be seen from Table 3, [BMIM][BF4] exhibits more serious corrosivity than functionalized ILs, due to its smallest Rp value. The order of corrosivity for functionalized ILs is as follows: [EMIM][MBT] > [BMIM][MBT] > [CHO][MBT]. The low corrosivity of MBT functionalized ILs may be ascribed to the strong adsorbability of ILs on bronze surface, which formed an adsorbed layer to protect the working electrode from these corrosive factors.
ILs
βa/mV dec−1
-βc/mV dec−1
Ecorr/mVSCE
icorr/μA cm−2
[BMIM][BF4] [EMIM][MBT] [BMIM][MBT] [CHO][MBT]
251.3 299.9 223.1 299.3
94.08 478.3 228.2 304.3
−664.5 −593.0 −512.2 −385.2
8.364 2.732 1.325 0.4458
is oxygen reduction reaction, while anodic reaction is the dissolution reaction of active metal on the surface. The icorr values represent the material corrosion rates at Ecorr [27]. The low icorr values indicate the chemical benignity of ILs solutions towards metals at Ecorr. [BMIM][BF4] showed the most serious corrosivity, speculated from highest icorr values in Table 4. The values of icorr followed the order as listed: [EMIM][MBT] > [BMIM][MBT] > [CHO][MBT]. Moreover, [CHO][MBT] has the lowest corrosivity in all the tested ILs. Results of electrochemical tests indicate that MBT functionalized ILs showed extremely low corrosivity to bronze, which suggest that they may be suitable for using as lubricants or lubricant additives for the industrial applications in lubrication engineering. 3.3. Lubrication performances
3.2.2. Potentiodynamic polarization curves measurements Potentiodynamic polarization curves tests have been performed to gain further knowledge about the corrosivity of the functionalized ILs. Polarization curves of the tested solutions are presented in Fig. 5. The electrochemical parameters derived from the Tafel extrapolation of the data, such as cathodic Tafel slope (βc), anodic Tafel slope (βa), corrosion potential (Ecorr) and corrosion current density (icorr) are given in Table 4. As can be seen from Fig. 5, the polarization curves of MBT functionalized ILs are below [BMIM][BF4] ILs. Furthermore, the curves of MBT functionalized ILs are parallel with each other, which indicates that similar cathodic and anodic reactions occurred though they have different cations. In these neutral solutions, cathodic reaction
3.3.1. ILs as lubricants The MBT functionalized ILs as lubricants were researched in steel/ bronze contact at 100 ℃. Fig. 6a shows the evolution of friction coefficients with time. The friction coefficient of PEG base oil fluctuated sharply during the whole friction process, while four tested ILs showed relatively steady friction coefficient curves, which indicates PEG base oil had a poor lubrication effect for steel/bronze frictional pairs at elevated temperature. For [BMIM][BF4], it took a long period about 1000 s before the friction coefficient curve coming to a steady state. For MBT functionalized ILs, they all have short running-in time about
Table 3 Fitting results of EIS for bronze in ILs solutions. ILs
[BMIM][BF4] [EMIM] [MBT] [BMIM] [MBT] [CHO][MBT]
Yo1/10−6 S sn cm−2
n1
355.9 574.7
62.6 12.3
0.701 0.836
0.0286 1.01
32.2 43.1
0.0443 0.223
705.8
4.45
0.790
1.35
2.23
540.3
4.60
0.740
5.26
2.56
Rs/Ω cm2
R1/104 Ω cm2
Yo2/10−5 S sn cm−2
124
B3/10−2 s1/ cm−2
Rp/104 Ω cm2
2
2
1.57 7.36
9.43 –
3.54 –
0.185 1.75
0.555
40.4
–
–
5.39
0.749
44.0
–
–
9.67
n2
R2/103 Ω cm2
Yo3/10−4 S s1/ cm−2
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Fig. 6. (a) Evolution of friction coefficient with time at 100 N for PEG base oil and tested ILs at 100 °C; (b) Average friction coefficients and wear volumes of bronze discs lubricated by PEG base oil and tested ILs at 100 °C.
[BMIM][BF4] fluctuated sharply during the whole friction process, which resembles the friction coefficient curve of PEG base oil. It revealed that [BMIM][BF4] could not improve the friction-reducing properties effectively when PEG base oil suffered a lubrication failure in steel/bronze frictional pairs at 100 °C. In contrast, the MBT functionalized ILs exhibited lower friction coefficients and relatively stable curves, which indicated that they could remarkably reduce the friction at the elevated temperature while using as additives in PEG base oil. Fig. 7b shows the comparison of average friction coefficients and wear volumes of bronze discs lubricated with PEG base oil with and without ILs. The wear volume and average friction coefficient of PEG base oil were significantly reduced after addition of four ILs respectively. Furthermore, a 2.0 wt% dose of functionalized ILs in PEG base oil gave better anti-wear and friction-reducing properties than that of [BMIM][BF4]. Thus, the MBT functionalized ILs would obviously be a better choice than the conventional ILs [BMIM][BF4] using as lubricant additives for steel/bronze contact at elevated temperature. In a word, the MBT functionalized ILs exhibit lower friction coefficient and stable friction process when used as lubricants. Smaller wear volumes also presented after tribological tests than [BMIM][BF4] and PEG base oil. When used as lubricant additives in PEG base oil, MBT functionalized ILs could significantly improve the anti-wear and friction-reducing properties, and [CHO][MBT] performed the best. Owing to the severe tribocorrosion process, [BMIM] [BF4] exhibited higher friction coefficient and wear volumes than MBT
100 s, and the friction coefficient curves are more smooth and steady during the whole friction process, comparing with [BMIM][BF4]. Results from friction tests indicate that MBT functionalized ILs give more steady friction coefficients and show better lubrication properties than the conventional ILs [BMIM][BF4] and PEG base oil. Fig. 6b shows the wear volumes and the average friction coefficients of the bronze discs lubricated by ILs and PEG base oil respectively. All the tested ILs showed lower average friction coefficient than PEG base oil, which proved the better friction-reducing property of ILs than that of PEG base oil. Moreover, three MBT functionalized ILs give lower average friction coefficient than [BMIM][BF4], and the lowest friction coefficient is gained by [CHO][MBT]. It can also be seen in Fig. 6b that ILs exhibit lower wear volumes than PEG base oil, and the best antiwear property is obtained by [CHO][MBT]. The above results discussed confirmed the excellent lubrication properties of the MBT functionalized ILs as lubricants for steel/bronze contact at 100 °C. Furthermore, because of the low corrosivity and good lubrication, MBT functionalized ILs are more qualified lubricants than [BMIM][BF4]. 3.3.2. ILs as lubricant additives The lubrication performances of MBT functionalized ILs as additives of PEG base oil were explored for steel/bronze contact at 100 °C. Fig. 7a displays the evolution of friction coefficient as a function of time for PEG base oil with and without ILs. The friction coefficient of
Fig. 7. (a) Evolution of friction coefficient with time at 100 N for PEG base oil and PEG plus 2.0 wt% tested ILs at 100 °C; (b) Average friction coefficients and wear volumes of bronze discs lubricated by PEG base oil and PEG plus 2.0 wt% tested ILs at 100 °C.
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Fig. 8. SEM and 3D morphologies of worn surfaces lubricated by PEG base oil and different ILs: (a, b, c) PEG; (d, e, f) [BMIM][BF4]; (g, h, i) [EMIM][MBT]; (j, k, l) [BMIM][MBT]; (m, n, o) [CHO][MBT].
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Fig. 9. SEM and 3D morphologies of worn surfaces lubricated by PEG base oil and different ILs as additives in PEG base oil: (a, b, c) PEG base oil; (d, e, f) 2.0 wt% [BMIM][BF4]; (g, h, i) 2.0 wt% [EMIM][MBT]; (j, k, l) 2.0 wt% [BMIM][MBT]; (m, n, o) 2.0 wt% [CHO][MBT].
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Fig. 10. SEM micrographs and corresponding element mapping on the worn surface of bronze lubricated by PEG base oil and different ILs.
The worn surface lubricated by PEG base oil showed severe plastic deformation with adhesive wear on the bronze surface, implying the direct contact between the bronze disc and steel ball [23]. The wear scar of bronze disc lubricated by [BMIM][BF4] was also very rough with adhesive wear on the surface. However, the wear scars of bronze surface lubricated by MBT functionalized ILs were much smoother with shallow friction scratches. The [CHO][MBT] ILs produced the smoothest wear scars compared with PEG and other ILs. Fig. 9 shows the SEM images and 3D optical microscopic images of worn surfaces of bronze discs lubricated by PEG base oil with and without ILs. When adding 2.0 wt% [BMIM][BF4] into the PEG base oil, it showed no effect on reducing severe scuffing damages. However, scuffing was greatly alleviated, and relatively smooth surfaces with few some plastic deformation signs were obtained when lubricated by PEG with functionalized ILs. Results of surface analysis furtherly proved that the MBT functionalized ILs possess excellent anti-wear performances when used as lubricant additives in PEG base oil for steel/ bronze contact at elevated temperature. Take a comprehensive consideration of the corrosion behaviors, lubrication properties and worn surface features of ILs, it is reasonable to conclude that ILs with lower corrosivity might always promise the better lubrication properties via reducing the severe corrosive wear. EDS was utilized to explore the elements distribution and content on the worn surface of bronze discs lubricated by PEG base oil and four tested ILs respectively. In Fig. 10, we can see that the characteristic elements distribute uniformly on the worn surface. The elements
Table 5 The element content on worn surfaces lubricated by PEG and ILs. ILs
PEG [BMIM][BF4] [EMIM][MBT] [BMIM][MBT] [CHO][MBT]
Elements Content (wt%)* C
O
B
F
S
15.36 8.52 9.09 8.24 8.14
2.51 1.64 1.96 1.39 0.11
0 3.15 0 0 0
0 0.3 0 0 0
0 0 2.94 1.52 0.46
*Cu and alloying elements for balanced.
functionalized ILs using as lubricants or lubricant additives. It can be discovered from the corrosion and lubrication properties that the higher corrosive ILs [BMIM][BF4] showed poorer lubrication properties, while the MBT functionalized ILs with lower corrosivity exhibited better anti-wear and friction-reducing performances. Considering the predominant lubricating performances and extremely low corrosivity, MBT functionalized ILs as lubricants or lubricant additives can be excellent candidates for the industrial application of bronze materials at elevated temperature. 3.4. Surface analysis The SEM and 3D optical microscopic images of worn surfaces of bronze discs lubricated by PEG base oil and ILs are displayed in Fig. 8. 128
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Fig. 11. XPS spectra of Cu2p (a), O1s and S2p on the worn surfaces lubricated by different ILs: [EMIM][MBT] (b, e); [BMIM][MBT] (c, f); [CHO][MBT] (d, g).
elements was analyzed. Fig. 11 presents the XPS spectra of Cu2p, O1s and S2p. The binding energy of O1s peaks were at 531.7 eV and 530.6 eV, which are belong to Cu2O [18] and C–O bonds [1], respectively. The organic compounds containing C–O bonds may generate from the oxidation reaction of ILs fragments during friction process. The exocyclic S (exo-S) and endocyclic S (endo-S) have different binding energy in the MBT anion due to the distinct chemical environment [28]. The S2p binding energy differences for three functionalized ILs were shown in Table 6. The binding energy of exoS and endo-S are about 161.5 eV and 164.0 eV. We can see a tinge of difference on the binding energy of S atoms, which was caused by cations in the ILs. In the Fig. 11, exo-S and endo-S atoms could be found on the worn surfaces, which revealed that the functionalized ILs might adsorb on the worn surface by means of the combination between MBT anion and Cu element (elementary copper or oxide of copper) [18,28]. Furthermore, the S2p peak appears at approximately 162.4 eV may correspond to CuS [29].
Table 6 Comparison of binding energy of S2p in ILs and on the worn surfaces. S atoms
exo-S endo-S CuS
Ionic liquids (eV)
Worn surfaces (eV)
[EMIM] [MBT]
[BMIM] [MBT]
[CHO] [MBT]
[EMIM] [MBT]
[BMIM] [MBT]
[CHO] [MBT]
161.46 163.99 –
161.67 164.02 –
161.46 164.00 –
161.46 163.99 162.40
161.67 164.02 162.40
161.46 164.00 162.40
content on the worn surfaces of bronze discs lubricated by PEG base oil and ILs listed in Table 5. 3.15 wt% of elements B and 0.3 wt% of F were detected on the worn surface lubricated by [BMIM][BF4], which indicates that the protective films may be mainly generated from the degradation or corrosion of BF4− during the friction process. Sulphur is the characteristic element of MBT functionalized ILs and it was observed in the wear scars, which reveals the formation of tribochemical thin film composed of functionalized ILs or/and tribochemical products of ILs. S elements observed on all the worn surfaces lubricated by MBT functionalized ILs implies the different lubricating mechanism with [BMIM][BF4]. The different tribochemical reactions occurred with the bronze surface between MBT functionalized ILs and conventional ILs should be further investigated. To gain further insight into the role of functionalized ILs on bronze surface during the friction process, the chemical states of typical
3.5. Mechanism discussion During the electrochemical corrosion process, MBT functionalized ILs exhibited higher Rp and lower icorr values than those of [BMIM] [BF4]. Results indicated that for [BMIM][BF4] solution, a great number of Cu2+ ions were dissolved out from the exposed work electrode. As a result, serious corrosion was caused to bronze by [BMIM][BF4] 129
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Fig. 12. Schematic representation of MBT functionalized ILs adsorbed layer on bronze.
Fig. 13. (a) Evolution of friction coefficient with time at 100 N for PEG plus 2.0 wt% tested ILs, POE 220 oil and tested ILs at 100 °C; (b) Average friction coefficients and wear volumes of bronze discs lubricated by PEG plus 2.0 wt% tested ILs, POE 220 oil and tested ILs at 100 °C.
steel/bronze tribopairs. Therefore, we investigated the lubrication performance of the commercial formulated worm gear oil POE 220 and made a comparison with MBT functionalized ILs. As it can be seen in Fig. 13a, when used as additives, the friction curves of three oil samples were above that of the POE 220 oil, while the curves of pure ILs were comparable with that of POE 220 oil. The friction curve of [CHO][MBT] with the lowest corrosivity was at the bottom. As regards to wear volumes and average friction coefficients in Fig. 13b, POE 220 oil exhibited lower wear volume and average friction coefficient than PEG plus ILs oil samples. For pure ILs, [EMIM][MBT] had comparable wear volume value with POE 220 oil. [BMIM][MBT] and [CHO][MBT] showed lower wear volumes. What's more, [CHO][MBT] had the lowest average friction coefficient. Based on the comparisons above, MBT functionalized ILs used as base oil or as additives in PEG base oil both showed comparable properties with commercial formulated worm gear oil POE 220. Especially for [CHO][MBT], it showed better performances both on friction and wear comparisons. While when used in PEG base oil, the three ILs additives exhibited inferior behaviors, which could be ascribed to the additive package in formulated POE 220 oil. As a conclusion, the MBT functionalized ILs have the potential to be lubricants or additives in worm gear industries, but more investigation and improvement still need to be done in the future.
solution. In the MBT functionalized ILs solutions, dissolution rate of bronze was slowed down through decreasing the numbers of active sites on exposed bronze electrode. Furthermore, the occupation of active sites was attributed to the adsorption of MBT functionalized ILs, and the specific process was that MBT anion firstly adsorbed on the surface through a S-N bridge connection [17,18]; the counter cations of ILs may then adsorb on the surface by electrostatic attractions and generate the physicochemical adsorbed film. That's why the MBT functionalized ILs solutions exhibited lower corrosivity for bronze. Surface analysis after tribological tests reveals that not only the CuS and organic compounds containing C-O bonds formed on the worn surfaces during friction process, but also the formation of adsorption film composed of ILs existed on the worn surface. Combined with the corrosivity discussion, the lubricating mechanism was proposed in Fig. 12. During the friction process, CuS generated from ILs and bronze due to the tribochemical reaction. The organic compounds containing C–O bonds also generated. Cu2O, CuS, organic compounds containing C–O bonds and ILs adsorption film could protect bronze discs synergistically during friction process. That is why the MBT functionalized ILs with low corrosivity exhibit excellent anti-wear and frictionreducing abilities for steel/bronze contact at elevated temperature.
3.6. Potential applications as worm gear oils
4. Conclusion
The worm gear and worm were widely used as transmission system of machine, which were mostly consisted of steel worm and bronze worm gear [30]. This kind of contact was consistent with the tested
The results and discussion above can be concluded to the following 130
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points: (1) Three 2-Mercaptobenzothiazole functionalized ILs were synthesized and their structures were well characterized. (2) Results of EIS and potentiodynamic polarization curves measurements revealed that the MBT functionalized ILs showed extremely low corrosivity to bronze, which is much lower than conventional ILs [BMIM][BF4]. (3) No matter using as lubricants or lubricant additives in PEG base oil, MBT functionalized ILs showed excellent lubrication performances for steel/bronze contact at elevated temperature. Furtherly, MBT functionalized ILs exhibited better lubrication abilities than [BMIM] [BF4], and [CHO][MBT] performed the best. (4) Results of electrochemical and tribological tests revealed that ILs with lower corrosivity might always promise the better lubrication properties via reducing the severe corrosive wear. (5) The extremely low corrosivity and excellent lubrication performances were attributed to the formation of adsorbed ILs film and also the tribochemical products CuS, organic compounds containing C-O bonds and Cu2O generated during the friction process. (6) When used as lubricants or additives in PEG oil, MBT functionalized ILs exhibited comparable tribological performances with worm gear oil POE 220, having the potential application in worm gear industries.
[3]
[4] [5]
[6] [7]
[8]
[9] [10]
[11]
[12]
[13]
Acknowledgements [14]
The authors thank Professor Deng for the kind supply of IL reagents. The authors also acknowledge financial support from the National Natural Science Foundation of China (51575506 and 51505462).
[15]
[16]
Appendix. The structure analysis data of synthesized ionic liquids
[17]
[EMIM][MBT]. H NMR (400 MHz, DMSO-d6) δ 9.26 (s, 1H), 7.81 (s, 1H), 7.72 (s, 1H), 7.42 (m, 1H), 7.26 (d, J=8.0 Hz, 1H), 7.10 (m, 1H), 6.94 (m, 1H), 4.20 (q, J=7.3 Hz, 2H), 3.86 (s, 3H), 1.40 (t, J=7.3 Hz, 3H). 13C NMR (101 MHz, DMSO-d6) δ185.86, 154.99, 136.80, 124.84, 124.02, 122.42, 121.05, 119.65, 116.84, 44.62, 36.19, 15.60. m/z: 111.0948 ([EMIM]+), 388.1601 ([EMIM]2[MBT]+), 665.2264 ([EMIM]3[MBT]2+), 942.2950 ([EMIM]4[MBT]3+). [BMIM][MBT]. 1H NMR (400 MHz, DMSO-d6) δ 9.21 (s, 1H), 7.78 (s, 1H), 7.71 (s, 1H), 7.42 (d, J=7.5 Hz, 1H), 7.25 (d, J=7.9 Hz, 1H), 7.10 (m, 1H), 6.94 (m, 1H), 4.16 (t, J=7.2 Hz, 2H), 3.86 (s, 3H), 1.74 (m, 2H), 1.23 (m, 2H), 0.88 (t, J=7.4 Hz, 3H). 13C NMR (101 MHz, DMSO-d6) δ 185.95, 154.76, 137.05, 136.69, 124.86, 124.07, 122.74, 121.07, 119.67, 116.75, 49.00, 36.24, 31.85, 19.25, 13.73. m/z: 139.1243 ([BMIM]+), 444.2222 ([BMIM]2[MBT]+). [CHO][MBT]. 1H NMR (400 MHz, DMSO-d6) δ 7.44 (d, J=7.7 Hz, 1H), 7.28 (d, J=7.9 Hz, 1H), 7.12 (m, 1H), 6.97 (m, 1H), 5.29 (s, 2H), 3.88 (m, 2H), 3.47 (m, 2H), 3.17 (s, 9H). 13C NMR (101 MHz, DMSO-d6) δ 186.04, 154.56, 136.58, 125.03, 121.31, 119.82, 116.79, 67.52, 55.66, 53.73, 53.69, 53.66. m/z: 104.1108 ([CHO]+), 374.1923 ([CHO]2[MBT]+). 1
[18]
[19] [20]
[21] [22] [23]
[24]
[25]
[26]
[27] [28]
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