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Excellent lubrication performance and superior corrosion resistance of vinyl functionalized ionic liquid lubricants at elevated temperature
Tribology International 44 (2011) 1111–1117
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Excellent lubrication performance and superior corrosion resistance of vinyl functionalized ionic liquid lubricants at elevated temperature Dongmei Li, Meirong Cai, Dapeng Feng, Feng Zhou n, Weimin Liu n State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
a r t i c l e i n f o
abstract
Article history: Received 20 December 2010 Received in revised form 30 March 2011 Accepted 14 April 2011 Available online 28 April 2011
In order to improve the lubrication performance and inhibit the serious corrosivity of conventional ionic liquids (ILs) at elevated temperatures, a series of vinyl functionalized ILs were synthesized in this work. The corrosion behavior of the ILs was evaluated with copper sheet corrosion test and their tribological properties were investigated on an Optimol SRV-IV oscillating friction and wear tester at elevated temperatures. The results showed that ILs with vinyl group, such as 1-vinyl-3-butyl imidazolium tetrafluoroborate (VBImBF4), can reduce corrosion effectively and its extreme pressure reached up to 1500 N at 150 1C. Based on the XPS analysis, ILs with vinyl group could interact with the iron surface and a protecting layer would form on the surface of steel possibly. Thus, ILs lubricants with good lubricating performance and low corrosivity at elevated temperature were achieved. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Ionic liquids Alkene Low corrosivity Lubricity
1. Introduction Room temperature ionic liquids (ILs) have become increasingly popular in academia and industry since the end of last century [1–3]. Except their extensive applications in diverse fields, such as synthesis, catalysis, separation technology, electrochemistry, and nanotechnology, it is well established that ILs are potential versatile lubricants with excellent friction reduction and extreme pressure properties [4–15]. The high performance of ILs is normally attributed to the dipole character and the formation of boundary films and protective tribo-layers [16]. However, regarding the application of ILs under working conditions, the usage of traditional ILs lubricants usually suffers from the corrosivity and low efficiency at elevated temperature [17,18]. For the corrosivity, it might come from the generation of acid during rigorous sliding process and it was attempted to be alleviated by adding benzotriazole as additive, which is known to be an effective corrosion inhibitor [19]. However, the low sublimation point of benzotriazole, o100 1C, limits its application at elevated temperature or under reduced pressure. Apart from the addition of well known nitrogen and sulfur containing organic compounds as corrosion inhibitors [20,21], a possible route is to incorporate functional group into the ILs, which could interact with friction component strongly, thus protective tribo-layers with improved efficiency might form. Vinyl group should be an ideal candidate to improve the
n
Corresponding authors. E-mail addresses: [email protected] (F. Zhou), [email protected] (W. Liu).
0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.04.017
interaction between ILs and the surface friction component because it is well known that vinyl can interact with iron via coordination possibly [22–24]. Importantly, vinyl group itself is non-corrosive and the vinyl functionalized ILs are easy to be synthesized and purified. Here ILs with vinyl functional groups and alkyl chains of different length on the other side were synthesized and the corrosion behavior and tribological properties of these ILs at elevated temperatures were investigated. For comparison, typical ILs lubricants, i.e., HMImBF4 and HEImBF4, were tested under the same conditions, Scheme 1.
2. Materials and experimental procedures Three kinds of 1-alkyl-3-vinyl-imidazolium tetrafluoroborate ILs were synthesized according to literatures starting from N-vinyl-imidazole [25]. The molecule structures are shown in Scheme 1. The density, viscosity, and viscosity–temperature index of the ILs were measured by a SVM3000 Stabinger Viscometer. Thermogravimetric analysis was carried out on a STA 449C Jupiters-Simultaneous TG-DSC at a heating rate of 10 1C/min in air atmosphere over temperature range from ambient to about 800 1C. The friction and wear tests were carried out on an Optimal SRV-IV oscillating reciprocating friction and wear tester. The contact between the frictional pair was achieved by pressing the upper running ball (diameter 10 mm, AISI 52100 steel) against the lower stationary disk (24 mm 7.9 mm, AISI 52100 steel), which was driven to reciprocate at a given frequency and
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C 4H 9 N
C 6H 13 N
N BF4
VBImBF4
-
C 10 H21 N
N BF4
VHImBF4
-
C6 H 13 N
N BF4
-
VDImBF4
C6 H 13 N
N BF4-
N BF4-
HMImBF4
HEImBF4
Scheme 1. ILs synthesized and used in this work.
displacement. The experiments were conducted at the frequency of 25 Hz, at an amplitude of 1 mm, and relative humidity of 45–55%. Prior to the friction and wear test, 0.12 mL lubricant was dropped to the ball–disk contact area. The wear volume loss of the lower disk was measured by a MicroXAM 3D non-contact surface mapping profiler. The morphologies of the worn surfaces were analyzed by a JSM-5600 LV scanning electron microscope (SEM). The X-ray photoelectron spectrometer (XPS) analysis was carried out on a PHI-5702 multifunctional XPS, using AlKa radiation as the exciting source. The binding energies of the target elements were determined at a pass energy of 29.35 eV, with a resolution of about 70.3 eV, using the binding energy of contaminated carbon (C1s: 284.8 eV) as the reference. All the ILs were vacuum treated before using and the water contents in all the samples were o300 ppm measured by Kari Fischer coulometer (Metro 831 KF coulometer). The 1H and 13C NMR of the vinyl functionalized ILs synthesized in this manuscript were shown as follows. VBImBF4: 1H NMR (400.1 MHz, C3D6O): d ¼ 0.94 (t, 3H), 1.41 (m, 2H), 1.98 (m, 2H), 4.40 (t, 2H), 5.46 (q, 1H), 6.03 (t, 1H), 7.39 (t, 1H), 7.90 (s, 1H), 8.12 (s, 1H), 9.32 (s, 1H). 13C NMR (100.6 MHz, C3D6O): d ¼13.61, 19.92, 32.45, 50.55, 109.45, 120.25, 124.26, 129.71, 135.94. VHImBF4: 1H NMR (400.1 MHz, C3D6O): d ¼ 0.87 (t, 3H), 1.35 (m, 6H), 2.04 (m, 2H), 4.40 (t, 2H), 5.46 (q, 1H), 6.03 (t, 1H), 7.40 (t, 1H), 7.91 (s, 1H), 8.13 (s, 1H), 9.34 (s, 1H). 13C NMR (100.6 MHz, C3D6O): d ¼14.13, 22.98, 26.38, 30.46, 31.76, 50.81, 109.44, 120.33, 124.25, 129.72, 136.20. VDImBF4: 1H NMR (400.1 MHz, C3D6O): d ¼0.86 (t, 3H), 1.41 (m, 14H), 2.02 (m, 2H), 4.40 (t, 2H), 5.47 (q, 1H), 6.03 (t, 1H), 7.39 (t, 1H), 7.91 (s, 1H), 8.13 (s, 1H), 9.36 (s, 1H). 13C NMR (100.6 MHz, C3D6O): d ¼14.29, 23.23, 26.73, 30.53, 32.52, 50.81, 109.41, 120.32, 124.25, 129.72, 136.10. 3. Results 3.1. Physical properties of ILs The physical properties of the synthesized ILs, i.e., HMImBF4, HEImBF4, VBImBF4, and VDImBF4, are given in Table 1. All the ILs have moderate densities falling in the range 1064.0–1151.7 kg/m3. The viscosities are in the range of 49.778 and 533.008 cP at 40 1C and between 8.465 and 34.0744 cP at 100 1C. The viscosities of the ILs increase with increasing the length of side alkyl chains due to the increased van der Waals interactions between long alkyl chains. The decomposition temperatures of all the ILs are around 300 1C, indicating their outstanding thermal properties in air atmosphere and making them potential high temperature lubricants. It is worth noting that IL VBImBF4 has the similar kinematic viscosity, density, and decomposition temperature as the classical IL lubricant HMImBF4. Thus it could be used under similar working conditions as HMImBF4, but with much improved friction performance and extreme pressure at elevated temperatures. 3.2. Corrosion test For the corrosion tests, copper sheets were polished, cleaned and dried. The copper sheets were dipped into ILs and maintained at 100 or 200 1C for 10 h. For comparison, IL HMImBF4 and
Table 1 Physical properties of ILs HMImBF4, VBImBF4, and VDImBF4. ILs
Kviscosity (mm2/s) 40 1Ca
Kviscosity (mm2/s) 100 1C
Vindexb
d/kg/m3
Td/1Cc
HEImBF4 HMImBF4 VBImBF4 VDImBF4
49.778 76.157 94.520 533.008
8.465 10.661 10.915 34.074
146.4 126.3 99.5 97.17
1125.7 1151.7 1209.1 1064.0
359.5 354.5 314.0 297.9
a b c
Kinematic viscosity. Viscosity index. Decomposition temperature.
HEImBF4 were also checked under the same conditions. The photographs are shown in Fig. 1. Clearly, almost no corrosion occurred at 100 1C in ILs VBImBF4, VHImBF4, and HMImBF4 but slight corrosion was observed in IL VDImBF4, which indicates that the presence of long alkyl chain makes the IL more corrosive. If the experiment was performed at 200 1C, there is no obvious corrosion occurring in IL VBImBF4 but corrosion occurred in ILs VHImBF4, HEImBF4, and HMImBF4. For the corrosion in IL VDImBF4, it is already very serious. As to the reason for the difference in the corrosivity of ILs VBImBF4, VHImBF4, and VDImBF4, it should be explained that the long alkyl chain of VDImBF4 might prevent the interaction of vinyl group with steel surface due to its non-polar property. The corrosion induced attack to substrate was further characterized by checking the SEM morphologies of substrates after corrosion tests, Fig. 2. The SEM pictures of copper sheets after corrosion test at 100 1C were not shown here because they all have no corrosion or slight corrosion occurred. From the SEM pictures after corrosion test at 200 1C, it could be seen clearly that only slight corrosion occurred in IL VBImBF4 but a large number of etching pits appeared on copper sheets in VHImBF4, HMImBF4, and HEImBF4. For the corrosion test of IL VDImBF4, serious corrosion occurred. These results were in agreement with the surface appearance shown in Fig. 1. Therefore, IL VBImBF4 is less corrosive in comparison with the classical IL lubricants HMImBF4 and HEImBF4. 3.3. Tribological properties The tribological properties of ILs VBImBF4, VDImBF4, HMImBF4, and HEImBF4 were tested. Fig. 3 shows a load ramp test from 100 to 1600 N stepped by 100 N at room temperature, and the test duration for each load was 2 min. For wear volume and wear scar diameter measurement, the sliding was conducted at different normal loads for 30 min at room temperature. The corresponding results are given in Figs. 3–5. Clearly, the classical IL lubricants HMImBF4 and HEImBF4 exhibited the best performance and it could work stably till 1600 and 1200 N. For the vinyl functionalized ionic liquids, i.e., VBImBF4 and VDImBF4, they can only work below 500 N. This is probably due to the relative larger viscosities of vinylated ILs than HMImBF4, which makes viscous liquid unable to feed back to sliding track quickly. Meanwhile, these ILs exhibit similar friction coefficient, wear volume, and wear scar diameter. These results indicate that the introduction of vinyl group cannot improve the lubrication function of ILs lubricants at room temperature.
D. Li et al. / Tribology International 44 (2011) 1111–1117
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Fig. 1. Photographs of the corrosion test of ILs on copper sheets (A) 100 1C, 10 h. From left VBImBF4, VHImBF4, VDImBF4, HMImBF4, and HEImBF4 (B) 200 1C, 10 h. From left VBImBF4, VHImBF4, VDImBF4, HMImBF4, and HEImBF4.
Fig. 2. SEM morphologies on copper sheets after corrosion test in ILs at 200 1C for 10 h by (A) VBImBF4; (B) VHImBF4; (C) VDImBF4; (D) HMImBF4; (E) HEImBF4.
D. Li et al. / Tribology International 44 (2011) 1111–1117
0.25
VDImBF4 VBImBF4 HMImBF4 HEImBF4
1200 800
0.12
400
0.09
Friction coefficient
0.15
0 0.06 0
500
1000 Time/s
6.0
0.10
400
0.05
0 500
HMImBF4 HEImBF4
3.0
1500
1000 Time/s
Fig. 6. Evolution of friction coefficient with time during a load ramp test from 100 to 1600 N stepped by 100 N at 150 1C (stroke: 1 mm, frequency: 25 Hz, test duration for each load was 2 min).
Wear volume/10-4mm3
Wear volume/10-4mm3
800
VDImBF4 VBImBF4 HMImBF4 HEImBF4
200
VBImBF4
1200
0.15
0
VDImBF4
4.5
0.20
1500
Fig. 3. Evolution of friction coefficient with time during a load ramp test from 100 to 1600 N stepped by 100 N at r.t. (stroke: 1 mm, frequency: 25 Hz, test duration for each load was 2 min).
1600
VDImBF4 VBImBF4 HMImBF4 HEImBF4
1600
Load/N
Friction coefficient
0.18
Load/N
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160
120
80
40 1.5 0 100
200
300 Load/N
400
Fig. 4. Wear volumes of steel disks lubricated by various ILs VBImBF4, VDImBF4, HMImBF4, and HEImBF4 at room temperature (stroke: 1 mm, frequency: 25 Hz, duration: 30 min).
400
200
500
600 Load/N
800
1000
Fig. 7. Wear volumes of steel disks lubricated by various ILs VBImBF4, VDImBF4, HMImBF4, and HEImBF4 at 150 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min). The point at 900 N using VBImBF4: Start from 100 to 900 N stepped by 100 N, maintained 30 min, test duration for each load was 2 min.
1.4 VDImBF4
0.6
wear scar diameter/mm
VBImBF4 HMImBF4 wear scar diameter/mm
HEImBF4 0.5
0.4
1.2
VDImBF4 VBImBF4 HMImBF4 HEImBF4
1.0
0.8
0.6
0.4 0.3
200 100
200
300 Load/N
400
500
Fig. 5. Wear scar diameter of steel balls lubricated by various ILs VBImBF4, VDImBF4, HMImBF4, and HEImBF4 at room temperature (stroke: 1 mm, frequency: 25 Hz, duration: 30 min).
400
600
800
1000
Load/N Fig. 8. Wear scar diameter of steel balls lubricated by various ILs VBImBF4, VDImBF4, HMImBF4, and HEImBF4 at 150 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min). The point at 900 N using VBImBF4: Start from 100 to 900 N stepped by 100 N, maintained 30 min, test duration for each load was 2 min.
D. Li et al. / Tribology International 44 (2011) 1111–1117
The friction coefficients evolutions, wear volumes of the disks, and wear scar diameters of the steel balls lubricated by the ILs at 150 1C display a different scenario, Figs. 6–9. As shown in Fig. 6, IL VBImBF4 shows stable friction coefficient although its friction coefficient is a little bit higher than HMImBF4. The extreme pressure of IL VBImBF4 reached to 1500 N at 150 1C. This should be one of the highest extreme pressures IL lubricants can bear at high temperature, which was similar as HEImBF4, i.e., 1500 N, but less corrosive. At the same time, the friction coefficient using IL VBImBF4 as lubricant was lower than 0.15 at 1500 N. In contrast, using IL HMImBF4 lubricant will result in lubrication failure below
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Table 2 Binding energies of different elements after lubrication under different ILsa. ILs
B1s (eV)
F1s (eV)
Fe2p (eV)
N1s (eV)
HMImBF4 HEImBF4 VBImBF4 (900 N) VBImBF4 VDImBF4
d.d.b d.d. d.d. d.d. d.d.
685.09 685.09 685.09 684.68 684.56
710.56 711.50 710.89 711.10 710.31
d.d. 399.80 d.d. 400.10 d.d.
a b
The same lubrication conditions as in Fig. 10. Difficult to be determined.
Fig. 9. Morphological feature pictures of the steel surface lubricated for 30 min by (A) VBImBF4 (B) VBImBF4, (C) VDImBF4, (D) HEImBF4, (E) VBImBF4, and (F) HEImBF4. (A: room temperature, stroke: 1 mm, frequency: 25 Hz, 300 N, duration: 30 min; B, C, and D: 150 1C, stroke: 1 mm, frequency: 25 Hz, 300 N, duration: 30 min; E and F: 150 1, stroke: 1 mm, frequency: 25 Hz, duration: 30 min, start from 100 to 900 N stepped by 100 N, maintained 30 min, test duration for each load was 2 min).
(900N)VBImBF4 VDImBF4 HMImBF4 VBImBF4
F Intensity (a.u.)
Intensity (a.u.)
B
VDImBF4 HMImBF4 VBImBF4 HEImBF4 (900N)VBImBF4
HEImBF4 185
190 195 200 205 Binding energy (e.v.)
210
HMImBF4 VBImBF4 HEImBF4
(900N)VBImBF4 Intensity (a. u.)
VDImBF4
HEImBF4 VDImBF4 HMImBF4
(900N)VBImBF4 700
700
N
Fe Intensity (a. u.)
690 685 695 Binding energy (e.v.)
680
710 720 730 Binding energy (e.v.)
740
VBImBF4 395
400 405 410 Binding energy (e.v.)
415
Fig. 10. XPS spectra of B, F, Fe, and N of worn surface lubricated by VBImBF4, VDImBF4, HMImBF4, and HEImBF4 at 150 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min). (900 N) VBImBF4: Start from 100 to 900 N stepped by 100 N, maintained 30 min, 25 Hz, test duration for each load was 2 min.
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D. Li et al. / Tribology International 44 (2011) 1111–1117
Fig. 11. MS spectra of IL VBImBF4 before and after friction.
400 N at 150 1C. Thus, our vinyl functionalized IL VBImBF4 is a potential lubricant at high temperature with low corrosivity. Unfortunately, the wear volume using ILs VBImBF4 and HEImBF4 is larger than IL HMImBF4 at lower loadings. The difference is that sliding pairs lubricated with VBImBF4 and HEImBF4 have progressively decreased friction coefficient till 1500 N with no sign of failure, while sliding pairs lubricated with HMImBF4 show instable friction coefficient even below 500 N. It indicates that ILs with C2 group, i.e., ethyl or vinyl, can improve their stability at high temperature remarkably but the friction coefficient and wear volume of the steel are still not good enough. 3.4. Surface analysis Fig. 10 and Table 2 show the XPS analysis results of the worn surfaces of the steels lubricated with ILs HMImBF4, HEImBF4, VBImBF4, and VDImBF4. Due to the instability of IL lubricants at high temperature, samples after being lubricated at 200 N were analyzed by XPS. Further on, sample using IL VBImBF4 being lubricated at 900 N was also tested. From the spectra of nitrogen, it could be seen that more nitrogen elements were observed on the surface of sample lubricated with VBImBF4 at 900 N. In contrast, much less fluoride and iron elements were observed. These results clearly suggested that the steel surface was covered with a layer of vinyl functionalized imidazolium, in which possibly the vinyl group interacts with the steel surface well. More interestingly, if the sample VBImBF4 was directly used at 900 N, it cannot work stably. Comparing with the XPS spectra using VBImBF4 at 200 N, it could be hyphothesized that the interaction between vinyl functionalized imidazolium was formed during the lubrication progressively. Also, XPS measurement can give more information about the chemical state of the elements. For the B1s spectra, they are too weak and it is difficult to be discussed, but the F1s peaks of worn surfaces appear at 684.5–685.0 might be assigned to the BF4 . The Fe2p showed that iron elements on the worn surface are possibly FeO, Fe2O3, or FeF2. The N1s spectra might be attributed to that of imidazolium.
could be explained by the interaction between vinyl functionalized IL with the metal surface and a thin layer would form, which prevents the corrosion of metal surface at high temperature. The vinyl group could interact with the iron surface and a protecting layer would form on the surface of steel. However, for vinyl functionalized ILs with longer side chain, the hydrophobicity of the side chain would repulse the interaction and thus its performance would be worse than that with shorter side chain. Commonly, the vinyl group was considered to be instable and thus it is used very less in the lubricant designation. Therefore, we tried to analyze the IL VBImBF4 with MS to check its composition before and after lubrication measurement at 150 1C, Fig. 11. Clearly, only the cation peak 151 (m/z) of IL VBImBF4 could be observed. There is no clear evidence about the polymerization of the vinyl functionalized IL lubricant. It suggests that the vinyl functionalized IL lubricant was stable enough under the lubrication conditions. 5. Conclusions Vinyl functionalized imidazolium ILs were synthesized and used as lubricants at high temperature. Although it exhibits similar performance as IL HEImBF4 with up to 1500 N extreme pressure, this new lubricant is much less corrosive at high temperature/150 1C. This should be the first time to report that IL lubricant could work at high temperature with low corrosivity and high loading capacity. The good performance is supposed to be due to the interaction of vinyl group on the imidazolium ring with iron surface, which is helpful to compose ordered absorption film and the tribochemical reaction film.
Acknowledgment The authors would like to thank the financial support of this work by ‘‘973’’ Program (2007CB607601) and NSFC (50421502), ‘‘Hundred Talents Program’’ of CAS. The authors also thank Jiazheng Zhao and Bo Wang for their help with the SEM and XPS analyses. References
4. Discussion From the tribological data and the analysis of the morphological features of worn surfaces, we can see that vinyl functionalized IL with shorter alkyl chain has low corrosivity than the traditional ILs HMImBF4 and HEImBF4, and that with longer chain. We think this
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Excellent lubrication performance and superior corrosion resistance of vinyl functionalized ionic liquid lubricants at elevated temperature Dongmei Li, Meirong Cai, Dapeng Feng, Feng Zhou n, Weimin Liu n State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
a r t i c l e i n f o
abstract
Article history: Received 20 December 2010 Received in revised form 30 March 2011 Accepted 14 April 2011 Available online 28 April 2011
In order to improve the lubrication performance and inhibit the serious corrosivity of conventional ionic liquids (ILs) at elevated temperatures, a series of vinyl functionalized ILs were synthesized in this work. The corrosion behavior of the ILs was evaluated with copper sheet corrosion test and their tribological properties were investigated on an Optimol SRV-IV oscillating friction and wear tester at elevated temperatures. The results showed that ILs with vinyl group, such as 1-vinyl-3-butyl imidazolium tetrafluoroborate (VBImBF4), can reduce corrosion effectively and its extreme pressure reached up to 1500 N at 150 1C. Based on the XPS analysis, ILs with vinyl group could interact with the iron surface and a protecting layer would form on the surface of steel possibly. Thus, ILs lubricants with good lubricating performance and low corrosivity at elevated temperature were achieved. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Ionic liquids Alkene Low corrosivity Lubricity
1. Introduction Room temperature ionic liquids (ILs) have become increasingly popular in academia and industry since the end of last century [1–3]. Except their extensive applications in diverse fields, such as synthesis, catalysis, separation technology, electrochemistry, and nanotechnology, it is well established that ILs are potential versatile lubricants with excellent friction reduction and extreme pressure properties [4–15]. The high performance of ILs is normally attributed to the dipole character and the formation of boundary films and protective tribo-layers [16]. However, regarding the application of ILs under working conditions, the usage of traditional ILs lubricants usually suffers from the corrosivity and low efficiency at elevated temperature [17,18]. For the corrosivity, it might come from the generation of acid during rigorous sliding process and it was attempted to be alleviated by adding benzotriazole as additive, which is known to be an effective corrosion inhibitor [19]. However, the low sublimation point of benzotriazole, o100 1C, limits its application at elevated temperature or under reduced pressure. Apart from the addition of well known nitrogen and sulfur containing organic compounds as corrosion inhibitors [20,21], a possible route is to incorporate functional group into the ILs, which could interact with friction component strongly, thus protective tribo-layers with improved efficiency might form. Vinyl group should be an ideal candidate to improve the
n
Corresponding authors. E-mail addresses: [email protected] (F. Zhou), [email protected] (W. Liu).
0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.04.017
interaction between ILs and the surface friction component because it is well known that vinyl can interact with iron via coordination possibly [22–24]. Importantly, vinyl group itself is non-corrosive and the vinyl functionalized ILs are easy to be synthesized and purified. Here ILs with vinyl functional groups and alkyl chains of different length on the other side were synthesized and the corrosion behavior and tribological properties of these ILs at elevated temperatures were investigated. For comparison, typical ILs lubricants, i.e., HMImBF4 and HEImBF4, were tested under the same conditions, Scheme 1.
2. Materials and experimental procedures Three kinds of 1-alkyl-3-vinyl-imidazolium tetrafluoroborate ILs were synthesized according to literatures starting from N-vinyl-imidazole [25]. The molecule structures are shown in Scheme 1. The density, viscosity, and viscosity–temperature index of the ILs were measured by a SVM3000 Stabinger Viscometer. Thermogravimetric analysis was carried out on a STA 449C Jupiters-Simultaneous TG-DSC at a heating rate of 10 1C/min in air atmosphere over temperature range from ambient to about 800 1C. The friction and wear tests were carried out on an Optimal SRV-IV oscillating reciprocating friction and wear tester. The contact between the frictional pair was achieved by pressing the upper running ball (diameter 10 mm, AISI 52100 steel) against the lower stationary disk (24 mm 7.9 mm, AISI 52100 steel), which was driven to reciprocate at a given frequency and
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C 4H 9 N
C 6H 13 N
N BF4
VBImBF4
-
C 10 H21 N
N BF4
VHImBF4
-
C6 H 13 N
N BF4
-
VDImBF4
C6 H 13 N
N BF4-
N BF4-
HMImBF4
HEImBF4
Scheme 1. ILs synthesized and used in this work.
displacement. The experiments were conducted at the frequency of 25 Hz, at an amplitude of 1 mm, and relative humidity of 45–55%. Prior to the friction and wear test, 0.12 mL lubricant was dropped to the ball–disk contact area. The wear volume loss of the lower disk was measured by a MicroXAM 3D non-contact surface mapping profiler. The morphologies of the worn surfaces were analyzed by a JSM-5600 LV scanning electron microscope (SEM). The X-ray photoelectron spectrometer (XPS) analysis was carried out on a PHI-5702 multifunctional XPS, using AlKa radiation as the exciting source. The binding energies of the target elements were determined at a pass energy of 29.35 eV, with a resolution of about 70.3 eV, using the binding energy of contaminated carbon (C1s: 284.8 eV) as the reference. All the ILs were vacuum treated before using and the water contents in all the samples were o300 ppm measured by Kari Fischer coulometer (Metro 831 KF coulometer). The 1H and 13C NMR of the vinyl functionalized ILs synthesized in this manuscript were shown as follows. VBImBF4: 1H NMR (400.1 MHz, C3D6O): d ¼ 0.94 (t, 3H), 1.41 (m, 2H), 1.98 (m, 2H), 4.40 (t, 2H), 5.46 (q, 1H), 6.03 (t, 1H), 7.39 (t, 1H), 7.90 (s, 1H), 8.12 (s, 1H), 9.32 (s, 1H). 13C NMR (100.6 MHz, C3D6O): d ¼13.61, 19.92, 32.45, 50.55, 109.45, 120.25, 124.26, 129.71, 135.94. VHImBF4: 1H NMR (400.1 MHz, C3D6O): d ¼ 0.87 (t, 3H), 1.35 (m, 6H), 2.04 (m, 2H), 4.40 (t, 2H), 5.46 (q, 1H), 6.03 (t, 1H), 7.40 (t, 1H), 7.91 (s, 1H), 8.13 (s, 1H), 9.34 (s, 1H). 13C NMR (100.6 MHz, C3D6O): d ¼14.13, 22.98, 26.38, 30.46, 31.76, 50.81, 109.44, 120.33, 124.25, 129.72, 136.20. VDImBF4: 1H NMR (400.1 MHz, C3D6O): d ¼0.86 (t, 3H), 1.41 (m, 14H), 2.02 (m, 2H), 4.40 (t, 2H), 5.47 (q, 1H), 6.03 (t, 1H), 7.39 (t, 1H), 7.91 (s, 1H), 8.13 (s, 1H), 9.36 (s, 1H). 13C NMR (100.6 MHz, C3D6O): d ¼14.29, 23.23, 26.73, 30.53, 32.52, 50.81, 109.41, 120.32, 124.25, 129.72, 136.10. 3. Results 3.1. Physical properties of ILs The physical properties of the synthesized ILs, i.e., HMImBF4, HEImBF4, VBImBF4, and VDImBF4, are given in Table 1. All the ILs have moderate densities falling in the range 1064.0–1151.7 kg/m3. The viscosities are in the range of 49.778 and 533.008 cP at 40 1C and between 8.465 and 34.0744 cP at 100 1C. The viscosities of the ILs increase with increasing the length of side alkyl chains due to the increased van der Waals interactions between long alkyl chains. The decomposition temperatures of all the ILs are around 300 1C, indicating their outstanding thermal properties in air atmosphere and making them potential high temperature lubricants. It is worth noting that IL VBImBF4 has the similar kinematic viscosity, density, and decomposition temperature as the classical IL lubricant HMImBF4. Thus it could be used under similar working conditions as HMImBF4, but with much improved friction performance and extreme pressure at elevated temperatures. 3.2. Corrosion test For the corrosion tests, copper sheets were polished, cleaned and dried. The copper sheets were dipped into ILs and maintained at 100 or 200 1C for 10 h. For comparison, IL HMImBF4 and
Table 1 Physical properties of ILs HMImBF4, VBImBF4, and VDImBF4. ILs
Kviscosity (mm2/s) 40 1Ca
Kviscosity (mm2/s) 100 1C
Vindexb
d/kg/m3
Td/1Cc
HEImBF4 HMImBF4 VBImBF4 VDImBF4
49.778 76.157 94.520 533.008
8.465 10.661 10.915 34.074
146.4 126.3 99.5 97.17
1125.7 1151.7 1209.1 1064.0
359.5 354.5 314.0 297.9
a b c
Kinematic viscosity. Viscosity index. Decomposition temperature.
HEImBF4 were also checked under the same conditions. The photographs are shown in Fig. 1. Clearly, almost no corrosion occurred at 100 1C in ILs VBImBF4, VHImBF4, and HMImBF4 but slight corrosion was observed in IL VDImBF4, which indicates that the presence of long alkyl chain makes the IL more corrosive. If the experiment was performed at 200 1C, there is no obvious corrosion occurring in IL VBImBF4 but corrosion occurred in ILs VHImBF4, HEImBF4, and HMImBF4. For the corrosion in IL VDImBF4, it is already very serious. As to the reason for the difference in the corrosivity of ILs VBImBF4, VHImBF4, and VDImBF4, it should be explained that the long alkyl chain of VDImBF4 might prevent the interaction of vinyl group with steel surface due to its non-polar property. The corrosion induced attack to substrate was further characterized by checking the SEM morphologies of substrates after corrosion tests, Fig. 2. The SEM pictures of copper sheets after corrosion test at 100 1C were not shown here because they all have no corrosion or slight corrosion occurred. From the SEM pictures after corrosion test at 200 1C, it could be seen clearly that only slight corrosion occurred in IL VBImBF4 but a large number of etching pits appeared on copper sheets in VHImBF4, HMImBF4, and HEImBF4. For the corrosion test of IL VDImBF4, serious corrosion occurred. These results were in agreement with the surface appearance shown in Fig. 1. Therefore, IL VBImBF4 is less corrosive in comparison with the classical IL lubricants HMImBF4 and HEImBF4. 3.3. Tribological properties The tribological properties of ILs VBImBF4, VDImBF4, HMImBF4, and HEImBF4 were tested. Fig. 3 shows a load ramp test from 100 to 1600 N stepped by 100 N at room temperature, and the test duration for each load was 2 min. For wear volume and wear scar diameter measurement, the sliding was conducted at different normal loads for 30 min at room temperature. The corresponding results are given in Figs. 3–5. Clearly, the classical IL lubricants HMImBF4 and HEImBF4 exhibited the best performance and it could work stably till 1600 and 1200 N. For the vinyl functionalized ionic liquids, i.e., VBImBF4 and VDImBF4, they can only work below 500 N. This is probably due to the relative larger viscosities of vinylated ILs than HMImBF4, which makes viscous liquid unable to feed back to sliding track quickly. Meanwhile, these ILs exhibit similar friction coefficient, wear volume, and wear scar diameter. These results indicate that the introduction of vinyl group cannot improve the lubrication function of ILs lubricants at room temperature.
D. Li et al. / Tribology International 44 (2011) 1111–1117
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Fig. 1. Photographs of the corrosion test of ILs on copper sheets (A) 100 1C, 10 h. From left VBImBF4, VHImBF4, VDImBF4, HMImBF4, and HEImBF4 (B) 200 1C, 10 h. From left VBImBF4, VHImBF4, VDImBF4, HMImBF4, and HEImBF4.
Fig. 2. SEM morphologies on copper sheets after corrosion test in ILs at 200 1C for 10 h by (A) VBImBF4; (B) VHImBF4; (C) VDImBF4; (D) HMImBF4; (E) HEImBF4.
D. Li et al. / Tribology International 44 (2011) 1111–1117
0.25
VDImBF4 VBImBF4 HMImBF4 HEImBF4
1200 800
0.12
400
0.09
Friction coefficient
0.15
0 0.06 0
500
1000 Time/s
6.0
0.10
400
0.05
0 500
HMImBF4 HEImBF4
3.0
1500
1000 Time/s
Fig. 6. Evolution of friction coefficient with time during a load ramp test from 100 to 1600 N stepped by 100 N at 150 1C (stroke: 1 mm, frequency: 25 Hz, test duration for each load was 2 min).
Wear volume/10-4mm3
Wear volume/10-4mm3
800
VDImBF4 VBImBF4 HMImBF4 HEImBF4
200
VBImBF4
1200
0.15
0
VDImBF4
4.5
0.20
1500
Fig. 3. Evolution of friction coefficient with time during a load ramp test from 100 to 1600 N stepped by 100 N at r.t. (stroke: 1 mm, frequency: 25 Hz, test duration for each load was 2 min).
1600
VDImBF4 VBImBF4 HMImBF4 HEImBF4
1600
Load/N
Friction coefficient
0.18
Load/N
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160
120
80
40 1.5 0 100
200
300 Load/N
400
Fig. 4. Wear volumes of steel disks lubricated by various ILs VBImBF4, VDImBF4, HMImBF4, and HEImBF4 at room temperature (stroke: 1 mm, frequency: 25 Hz, duration: 30 min).
400
200
500
600 Load/N
800
1000
Fig. 7. Wear volumes of steel disks lubricated by various ILs VBImBF4, VDImBF4, HMImBF4, and HEImBF4 at 150 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min). The point at 900 N using VBImBF4: Start from 100 to 900 N stepped by 100 N, maintained 30 min, test duration for each load was 2 min.
1.4 VDImBF4
0.6
wear scar diameter/mm
VBImBF4 HMImBF4 wear scar diameter/mm
HEImBF4 0.5
0.4
1.2
VDImBF4 VBImBF4 HMImBF4 HEImBF4
1.0
0.8
0.6
0.4 0.3
200 100
200
300 Load/N
400
500
Fig. 5. Wear scar diameter of steel balls lubricated by various ILs VBImBF4, VDImBF4, HMImBF4, and HEImBF4 at room temperature (stroke: 1 mm, frequency: 25 Hz, duration: 30 min).
400
600
800
1000
Load/N Fig. 8. Wear scar diameter of steel balls lubricated by various ILs VBImBF4, VDImBF4, HMImBF4, and HEImBF4 at 150 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min). The point at 900 N using VBImBF4: Start from 100 to 900 N stepped by 100 N, maintained 30 min, test duration for each load was 2 min.
D. Li et al. / Tribology International 44 (2011) 1111–1117
The friction coefficients evolutions, wear volumes of the disks, and wear scar diameters of the steel balls lubricated by the ILs at 150 1C display a different scenario, Figs. 6–9. As shown in Fig. 6, IL VBImBF4 shows stable friction coefficient although its friction coefficient is a little bit higher than HMImBF4. The extreme pressure of IL VBImBF4 reached to 1500 N at 150 1C. This should be one of the highest extreme pressures IL lubricants can bear at high temperature, which was similar as HEImBF4, i.e., 1500 N, but less corrosive. At the same time, the friction coefficient using IL VBImBF4 as lubricant was lower than 0.15 at 1500 N. In contrast, using IL HMImBF4 lubricant will result in lubrication failure below
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Table 2 Binding energies of different elements after lubrication under different ILsa. ILs
B1s (eV)
F1s (eV)
Fe2p (eV)
N1s (eV)
HMImBF4 HEImBF4 VBImBF4 (900 N) VBImBF4 VDImBF4
d.d.b d.d. d.d. d.d. d.d.
685.09 685.09 685.09 684.68 684.56
710.56 711.50 710.89 711.10 710.31
d.d. 399.80 d.d. 400.10 d.d.
a b
The same lubrication conditions as in Fig. 10. Difficult to be determined.
Fig. 9. Morphological feature pictures of the steel surface lubricated for 30 min by (A) VBImBF4 (B) VBImBF4, (C) VDImBF4, (D) HEImBF4, (E) VBImBF4, and (F) HEImBF4. (A: room temperature, stroke: 1 mm, frequency: 25 Hz, 300 N, duration: 30 min; B, C, and D: 150 1C, stroke: 1 mm, frequency: 25 Hz, 300 N, duration: 30 min; E and F: 150 1, stroke: 1 mm, frequency: 25 Hz, duration: 30 min, start from 100 to 900 N stepped by 100 N, maintained 30 min, test duration for each load was 2 min).
(900N)VBImBF4 VDImBF4 HMImBF4 VBImBF4
F Intensity (a.u.)
Intensity (a.u.)
B
VDImBF4 HMImBF4 VBImBF4 HEImBF4 (900N)VBImBF4
HEImBF4 185
190 195 200 205 Binding energy (e.v.)
210
HMImBF4 VBImBF4 HEImBF4
(900N)VBImBF4 Intensity (a. u.)
VDImBF4
HEImBF4 VDImBF4 HMImBF4
(900N)VBImBF4 700
700
N
Fe Intensity (a. u.)
690 685 695 Binding energy (e.v.)
680
710 720 730 Binding energy (e.v.)
740
VBImBF4 395
400 405 410 Binding energy (e.v.)
415
Fig. 10. XPS spectra of B, F, Fe, and N of worn surface lubricated by VBImBF4, VDImBF4, HMImBF4, and HEImBF4 at 150 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min). (900 N) VBImBF4: Start from 100 to 900 N stepped by 100 N, maintained 30 min, 25 Hz, test duration for each load was 2 min.
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Fig. 11. MS spectra of IL VBImBF4 before and after friction.
400 N at 150 1C. Thus, our vinyl functionalized IL VBImBF4 is a potential lubricant at high temperature with low corrosivity. Unfortunately, the wear volume using ILs VBImBF4 and HEImBF4 is larger than IL HMImBF4 at lower loadings. The difference is that sliding pairs lubricated with VBImBF4 and HEImBF4 have progressively decreased friction coefficient till 1500 N with no sign of failure, while sliding pairs lubricated with HMImBF4 show instable friction coefficient even below 500 N. It indicates that ILs with C2 group, i.e., ethyl or vinyl, can improve their stability at high temperature remarkably but the friction coefficient and wear volume of the steel are still not good enough. 3.4. Surface analysis Fig. 10 and Table 2 show the XPS analysis results of the worn surfaces of the steels lubricated with ILs HMImBF4, HEImBF4, VBImBF4, and VDImBF4. Due to the instability of IL lubricants at high temperature, samples after being lubricated at 200 N were analyzed by XPS. Further on, sample using IL VBImBF4 being lubricated at 900 N was also tested. From the spectra of nitrogen, it could be seen that more nitrogen elements were observed on the surface of sample lubricated with VBImBF4 at 900 N. In contrast, much less fluoride and iron elements were observed. These results clearly suggested that the steel surface was covered with a layer of vinyl functionalized imidazolium, in which possibly the vinyl group interacts with the steel surface well. More interestingly, if the sample VBImBF4 was directly used at 900 N, it cannot work stably. Comparing with the XPS spectra using VBImBF4 at 200 N, it could be hyphothesized that the interaction between vinyl functionalized imidazolium was formed during the lubrication progressively. Also, XPS measurement can give more information about the chemical state of the elements. For the B1s spectra, they are too weak and it is difficult to be discussed, but the F1s peaks of worn surfaces appear at 684.5–685.0 might be assigned to the BF4 . The Fe2p showed that iron elements on the worn surface are possibly FeO, Fe2O3, or FeF2. The N1s spectra might be attributed to that of imidazolium.
could be explained by the interaction between vinyl functionalized IL with the metal surface and a thin layer would form, which prevents the corrosion of metal surface at high temperature. The vinyl group could interact with the iron surface and a protecting layer would form on the surface of steel. However, for vinyl functionalized ILs with longer side chain, the hydrophobicity of the side chain would repulse the interaction and thus its performance would be worse than that with shorter side chain. Commonly, the vinyl group was considered to be instable and thus it is used very less in the lubricant designation. Therefore, we tried to analyze the IL VBImBF4 with MS to check its composition before and after lubrication measurement at 150 1C, Fig. 11. Clearly, only the cation peak 151 (m/z) of IL VBImBF4 could be observed. There is no clear evidence about the polymerization of the vinyl functionalized IL lubricant. It suggests that the vinyl functionalized IL lubricant was stable enough under the lubrication conditions. 5. Conclusions Vinyl functionalized imidazolium ILs were synthesized and used as lubricants at high temperature. Although it exhibits similar performance as IL HEImBF4 with up to 1500 N extreme pressure, this new lubricant is much less corrosive at high temperature/150 1C. This should be the first time to report that IL lubricant could work at high temperature with low corrosivity and high loading capacity. The good performance is supposed to be due to the interaction of vinyl group on the imidazolium ring with iron surface, which is helpful to compose ordered absorption film and the tribochemical reaction film.
Acknowledgment The authors would like to thank the financial support of this work by ‘‘973’’ Program (2007CB607601) and NSFC (50421502), ‘‘Hundred Talents Program’’ of CAS. The authors also thank Jiazheng Zhao and Bo Wang for their help with the SEM and XPS analyses. References
4. Discussion From the tribological data and the analysis of the morphological features of worn surfaces, we can see that vinyl functionalized IL with shorter alkyl chain has low corrosivity than the traditional ILs HMImBF4 and HEImBF4, and that with longer chain. We think this
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