Tribological Performance Evaluation of Task-Specific Ionic Liquids

Article pubs.acs.org/IECR

Tribological Performance Evaluation of Task-Specific Ionic Liquids Derived from Amino Acids Praveen K. Khatri, Gananath D. Thakre,* and Suman L. Jain* Chemical Sciences Division, CSIR-Indian Institute of Petroleum, Mohkampur, Dehradun-248005, India ABSTRACT: Aspartic acid and glutamic acid derived ionic liquids were found to be efficient antiwear and friction-reducing additives for base oils. The tribological performance evaluation of lubricant blends, prepared as 0.15%, 0.3%, 1%, 1.5%, 2%, and 3% concentration by weight into the mineral base oil, was carried out on a four-ball tribotester. The concentration of ionic liquids along with the chain lengths of alkyl groups influenced the tribological performance of the formulated blends. The synthesized ionic liquids exhibited excellent antifriction and antiwear characteristics.

1. INTRODUCTION Ionic liquids are considered to be unique compounds owing to their properties and are achieving a rapidly growing research interest since the past decade.1−8 Due to their unique properties of nonflammability, nonvolatility, and outstanding thermooxidative stability, ionic liquids are being considered as promising lubricating fluids that can be used as lubricant base fluids, additives, or thin films.9−28 Several ionic liquids, derived from cations such as imidazolium, pyridinium, and ammonium and anions such as BF4 and bis(trifluoromethylsulfonyl)imide (TFSI), have been examined under boundary conditions.29−31 The lubrication properties of dialkylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids have been extensively studied,32 and it has been postulated that they can act as versatile lubricants for different sliding pairs and exhibit excellent frictionreducing and antiwear performance along with high loadcarrying capacity. However, dialkylimidazolium ionic liquids suffer from some limitations, such as lower oxidative stability at higher temperatures. Because of the presence of an aromatic ring in dialkylimidazolium, it is easily prone to oxidation and also liable to ring-open at relatively higher temperatures.33 The pyridine cation ionic liquids have also been studied and it has been reported that they cannot be used as good lubricants owing to their corrosive behavior.34 Similarly, phosphonium-based ionic liquids cannot be established as good lubricants from environmental viewpoints. Amino acids and their derivatives are the most abundant natural source and their applications in ionic liquids are gaining considerable interest due to their biodegradability, reduced toxicity, and high biocompatibility.35−40 Recently, amino acid derived functionalized dicarboxylic acids have been used to synthesize ionic liquids.41 The ionic structure of these functionalized dicarboxylic acids has been found to be beneficial when dissolved into the ionic liquids. In the present paper, we report amino acid derived ammonium cation based ionic liquids, in which the anions involved are derived from natural amino acids such as aspartic acid and glutamic acid. The synthesized ionic liquids were used as antiwear and friction-reducing agents for lubricating oils. The tribo-performance of ionic liquids has been investigated when blended as additives in a base oil. The quantity of ionic liquids required for study when used as a base is large as compared to the quantity required when used as an additive. When blended into © 2013 American Chemical Society

the base oils the representative characteristic performance over and above the performance of the base oil can be attributed to the additive and its concentration. Hence, with a small quantity the required tribo-performance can be easily studied. The chemical structures of synthesized ionic liquids are presented in Figure 1.

2. MATERIALS AND METHODS 2.1. Materials. All the reagents used in the experiment were of analytical grade. Tetrabutylammonium hydroxide was purchased from Aldrich and used as obtained. L-(+)-Aspartic acid and L-(+) glutamic acid were obtained from Across Organics and used without further purification. N-Benzylaspartic acid was synthesized as per the procedure available in the literature, starting from maleic anhydride.42 2.2. Synthesis and Characterization of Amino Acid Based Ionic Liquids. General Procedure for the Synthesis of Ionic Liquids. An aqueous solution of tetrabutylammonium hydroxide (40 wt % solution in water) was added dropwise to an amount of amino acid aqueous solution that was slightly in excess of equimolar. The reaction mixture was allowed to stir at room temperature for 12 h. At the end of the reaction, water was removed under reduced pressure by rotary evaporation at 55 °C. Further, to this concentrate were added 90 mL of CH3CN and 10 mL of CH3OH, and the mixture stirred vigorously. Excess of amino acid was filtered off and filtrate was concentrated to remove the solvents. The product was dried in vacuum for 2 days at 80 °C. The synthesized ionic liquids were characterized by means of elemental analysis (Table 1), TGA, and 1H NMR spectroscopy (Table 2). Table 1. Elemental Analysis Data of Synthesized Ionic Liquidsa

a

ionic liquid

C

H

N

IL1 IL2 IL3

60.28 (64.07) 64.03 (64.85) 68.47 (69.72)

11.10 (11.21) 11.17 (11.32) 10.22 (10.33)

7.48 (7.47) 7.24 (7.21) 7.06 (6.03)

Values in parentheses are calculated.

Received: Revised: Accepted: Published: 15829

July 6, 2013 September 11, 2013 October 17, 2013 October 17, 2013 dx.doi.org/10.1021/ie402141v | Ind. Eng. Chem. Res. 2013, 52, 15829−15837

Industrial & Engineering Chemistry Research

Article

Figure 1. Synthesized ionic liquids.

Figure 2. Four-ball tribotester.

Figure 4. TGA of lubricant blends. Figure 3. TGA curves of the synthesized ILs.

selected was mineral-based Neutral 150 oil, which was blended with the known quantities of the additive. The blend was homogenized by rigorous stirring on a magnetic hot plate at 30 °C for 20 min. The additives were easily dissolved into the base oil and resulted into a homogeneous and clear blend. The lubricant blends reported no separation before and after the test.

2.3. Lubricant Blend Preparation. Lubricant blends were prepared using ionic liquids as additives in mineral base oil. Ionic liquids were blended in the concentrations of 0.15%, 0.3%, 1%, 1.5%, 2%, and 3% by weight into the base oil. The base oil 15830

dx.doi.org/10.1021/ie402141v | Ind. Eng. Chem. Res. 2013, 52, 15829−15837


Article

Figure 5. Weight loss of IL1 with time at 75 °C.

Figure 7. Antifriction performance of ionic liquid blends at varying dosage.

Figure 6. Comparative assessment of antiwear behavior of ionic liquid blends at varying dosage.

The as prepared lubricant blends were tested for their tribological performance. 2.4. Experimental Setup. The tribological performance evaluation of the lubricant blends was carried out on the four-ball rolling contact fatigue tribotester (DUCOM India) shown in Figure 2. The tribotester utilizes four-ball geometry in a tetrahedral form. The top ball is fixed into the spindle and rotates at the predefined speed. The bottom three balls are fixed in a ball pot filled with lubricant. The four balls make three point contacts and the experimental conditions are so maintained that the elastohydrodynamic lubrication conditions are simulated. The AISI standard steel no. E-52100 ball test specimens with a diameter of 12.7 mm and hardness of Rockwell C 64−66 were used for the performance tests. The test conditions employed are as given in Table 3. The test temperature of 75 °C along with other operating conditions has been selected so as to keep them in tandem with the standard ASTM D4172 test procedure, which uses 75 °C as the standard test temperature to investigate the friction and wear behavior of lubricating oils. Friction encountered within the contact is continuously monitored and recorded using data acquision software. Wear in terms of wear scar diameters is measured using a 40× microscope at the end of the test. Each of the lubricant blends is tested twice and the average results of the two tests are reported.

Figure 8. Percentage reduction in friction for ionic liquid blends at varying dosage.

Figure 9. Percentage reduction in wear for ionic liquid blends at varying dosage.

dissolved into the base oil and resulted into a homogeneous, clear blend. The viscosity values of the lubricant blends for an optimum concentration (2% dose) are given in Table 5. Figures 3 and 4 show the TGA curves for the synthesized additives and blends (2% additive with base oil). The thermal stabilities of three ionic liquids and the blends were investigated under N2 atmosphere by the thermogravimetric analysis from 40 to 500 °C at a temperature rise of 10 °C/min. The TGA curves of synthesized ionic liquids IL1, IL2, and IL3 and their

3. RESULTS AND DISCUSSION 3.1. Viscosity and Thermal Stability of Additives and Blends. The values of viscosities of synthesized additives are given in Table 4. The additives (ionic liquids) were easily 15831



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Figure 10. Wear scars obtained on the ball test specimens lubricated with ionic liquid blends with varying dosages.

constant temperature of 75 °C with time (Figure 5). As shown in the Figure 5, no significant mass loss occurred with time, establishing the high thermal stability of the lubricant additives. 3.2. Tribological Performance. The synthesized ionic liquids have reported significant antifriction and antiwear performance when blended with a mineral-based base stock. Figure 6 depicts the comparative performance of antiwear characteristics of the ionic liquids when blended in varying concentrations on a percentage weight basis. At lower concentrations the antiwear performance is marginally different

blends (2% additive with base oil) reveal that all the three ionic liquids as well as blends possess good thermal stability and start to decompose around 180, 190, and 220 °C, respectively, evidence that the ionic liquids can be successfully used as an additive in the base oil without any separation/decomposition at temperatures below 180 °C. The operating in situ temperatures for industrial applications are well below this temperature. Hence, they are appropriate for use as additives for industrial applications. Furthermore, the thermal stability of the lubricant additive IL1 was studied by determining the mass loss at a 15832



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Figure 11. SEM micrographs of the worn out ball test specimens lubricated with ionic liquid blended lubricant.

behavior of IL1 at this concentration is due to high wear which was irreparable during test and hence the high friction was observed. The lubricant blends prepared using ionic liquid IL2 reported the lowest frictional resistance. At higher doses of ionic liquid concentration, i.e., 3%, IL2 has the lowest friction coefficient followed by IL1 and IL3. However, the friction behavior varied marginally for ionic liquid IL3, with the coefficient of friction varying between 0.08 and 0.12. Figure 8 shows the percentage reduction in friction using ionic liquids. The base oil is the reference mark and reported the coefficient of friction of 0.1469 at the end of the test. A maximum of 70% reduction in frictional resistance is reported for IL2, with a coefficient of friction of 0.0439, followed by IL1 (≈50% reduction) and IL3 (≈30% reduction), with coefficients of friction of 0.0619 and 0.0895, respectively. It is clear from the

with all the ionic liquid blends reporting similar wear scar diameters. However, with increased dosage the difference in the antiwear performance is substantial. At 3% concentration, the IL2 blended lubricant reported the lowest wear scar diameter. The difference in antiwear performance of IL2 is significant as compared to the lubricant blends prepared using IL3 and IL1 ionic liquids. From Figure 6 it is clear that the increased doses of ionic liquids enhanced the antiwear behavior of the lubricant blend. A similar trend is reported for the antifriction behavior. Figure 7 shows the antifriction performance of the ionic liquid blended lubricants. Friction encountered within the tribo contact is attributed to the friction due to interlayer shearing of lubricating films and the friction due to the actual metal to metal contact. At a concentration of 1.5%, IL1 has reported a higher wear with higher friction coefficient. The abnormal 15833



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Figure 12. EDX results for the worn out ball test specimens lubricated with 2% ionic liquid blended lubricant.

ionic liquids. Hence 2% may be considered as the optimum dose for better antiwear performance. On the basis of the results shown in Figures 8 and 9, it can be concluded that the ionic liquid IL2 has the best tribological performance characteristics as compared to IL3 and IL1. The reason for such a behavior can be attributed to the additional CH2 group present in IL2 in comparison to IL1. The additional CH2 group is capable of enhancing the antifriction behavior when compared with IL1. It has been reported in literature43,44 that friction coefficient decreases with an increase in chain length, and the same is observed in the present case. However, an additional CH2 group does not affect the linearity of the straight chain compound and therefore may not be able to influence the viscosity of the blend; it is due to this reason that there is no appreciable difference in the antiwear performance among the ionic liquids.

histogram that antifriction behavior is enhanced with an increase in concentration of ionic liquids in all the three cases. Figure 9 shows the percentage reduction in wear characteristics of lubricant blends. Wear of the steel balls is represented in terms of wear scar diameter as a standard practice followed by lubrication scientists and also used in ASTM D standard procedures. The volumetric loss of material in this case is very small, the wear ratio tending to be highly insignificant. Hence, the percentage change in the wear scar diameter of the steel balls is shown in Figure 9. The antiwear performances of all three ionic liquid blended lubricants are almost similar. The maximum reduction on the order of 30% in wear is observed at the concentration of 2%. The antiwear performance increased initially and then decreased with an increase in the dosage of 15834


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Table 3. Experimental Test Conditions for the Four-Ball Test parameter

value

load temperature speed test duration

392 N 75 °C 1200 rpm 1h

Table 4. Viscosity Values of Synthesized Additives kinematic viscosity (mm2/s) entry

ionic liquid

40 °C

100 °C

1 2 3

IL1 IL2 IL3

187.9 169.8 171.7

18.1 15.3 14.2

Table 5. Viscosity Values and Physicochemical Properties of Base Oil and Blends blend with 2% additive in base oil entry 1 2 3

characteristics viscosity at 40 °C, cSt viscosity at 100 °C, cSt density, ρ

IL1

IL2

IL3

33.30

base oil

33.684

33.933

33.789

5.49

5.5931

5.5386

5.5634

0.8812 (15 °C)

0.8680 (40 °C)

0.8696 (40 °C)

0.8700 (40 °C)

Compounds were dissolved in CDCl3.

N-Benzyl-protected ionic liquid (IL3) showed poor antifriction as well as antiwear properties, and one plausible explanation for this is that due to the introduction of an aromatic ring in IL3 its compatibility with base oil, which is primarily comprising of parrafins and cyclic paraffins, gets reduced by fewer interactions between the phenyl group and base oil molecules, which tends to retard its antifriction and antiwear behaviors. All three ionic liquids are equally capable of forming lubricating films to protect the contacting surfaces from damage, thereby lower wear scar diameters are reported. In order to have better insights on the surface failures, particularly wear, the worn surfaces of used ball test specimens were further investigated using SEM and EDX. Figure 10 shows the wear scars obtained on the ball test specimens after the test. The SEM analysis was undertaken to investigate the surface damage obtained after the wear of test specimens. Figure 11 shows the SEM micrographs of the worn out test specimens. The micrographs reveal smooth rubbing wear along with micropitting in some cases. IL2 blended in 2% resulted in smooth rubbing wear. The wear mechanism is adhesive wear, as the wear tracks obtained are smooth and the surfaces too are very smooth. IL1 at 2% concentrations reported slightly rigorous rubbing in the sliding direction. However, the severity of rubbing was higher in the case of IL3 with some micropitting. Micropitting is more prominent in samples lubricated with IL3. The micropitting on the surfaces of the used test specimens is confirmed by the presence of small pits on the surface, as seen in the SEM micrographs. However, no trace of corrosion was obtained on the used test specimens. The micrographs further reveals that due to the smooth surfaces formed by rubbing, substantial friction reduction took place in case of IL2 as compared to IL3 and IL1 blended lubricants. Figure 12 shows the results for the EDX analysis of the worn out test specimens. The EDX analysis shows that carbon, iron, chromium, and oxygen are prominent on the surface owing to the steel surface. Silicon and sulfur have been observed in traces

a

96.8 97 97.5 IL1 IL2 IL3

0.94 (t, CH3), 1.35 (sex, CH3CH2), 1.60 (quin, CH3CH2CH2), 2.38 (t, CH3CH2CH2CH2), 2.54 (d, CH2COO), 3.16 (t, CHCH2COO), 3.77 (m, CHNH2) 0.98 (t, CH3), 1.40 (m, CH3CH2), 1.65 (quin, CH3CH2CH2), 2.05 (q, CHCH2CH2COO), 2.40 (t, CH3CH2CH2CH2), 2.49 (t, CHCH2CH2COO), 3.25 (t, CHCH2CH2COO), 3.54 (m, CHNH2) 0.98 (t, CH3), 1.39 (m, CH3CH2), 1.60 (quin, CH3CH2CH2), 2.69 (t, CH3CH2CH2CH2), 3.22 (d, CH2COO), 3.45 (t, CHCH2COO), 3.96 (s, CH2C6H5), 7.28−7.40 (m, C6H5)

yield (%) ionic liquid

Table 2. 1H NMR Data of Synthesized Ionic Liquids

1

H NMR data (ppm)a


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(14) Jiménez, A. E.; Bermúdez, M. D.; Carrión, F. J.; Martínez-Nicolás, G. Room temperature ionic liquids as lubricant additives in steel− aluminium contacts: Influence of sliding velocity, normal load and temperature. Wear 2006, 261, 347. (15) Jimenez, A. E.; Bermudez, M. D. Imidazolium ionic liquids as additives of the synthetic ester propylene glycol dioleate in aluminium− steel lubrication. Wear 2008, 265, 787. (16) Yao, M.; Liang, Y. M.; Xia, Y.; Zhou, F. ACS Appl. Mater. Interfaces 2009, 1, 467. (17) Carrión, F. J.; Sanes, J.; Bermúdez, M. D. Effect of ionic liquid on the structure and tribological properties of polycarbonate−zinc oxide nanodispersion. Mater. Lett. 2007, 61, 4531. (18) Yu, B.; Zhou, F.; Mu, Z.; Liang, Y. M.; Liu, W. M. Tribological properties of ultra-thin ionic liquid films on single-crystal silicon wafers with functionalized surfaces. Tribol. Inter. 2006, 39, 879. (19) Yu, G.; Zhou, F.; Liu, W. M.; Liang, Y. M.; Yan, S. Preparation of functional ionic liquids and tribological investigation of their ultra-thin films. Wear 2006, 260, 1076. (20) Zhu, M.; Mo, J. Y.; Bai, M. Effect of the anion on the tribological properties of ionic liquid nano-films on surface-modified silicon wafers. Tribol. Lett. 2008, 29, 177. (21) Jiang, D.; Hu, L.; Feng, D. Tribological properties of crown-type phosphate ionic liquid as additive in poly(ethylene glycol) for steel/steel contacts. Ind. Lubri. Tribol. 2013, 65, 202. (22) Palacio, M.; Bhushan, B. Ultrathin Wear-resistant ionic liquid films for novel MEMS/NEMS applications. Adv. Mater. 2008, 20, 1194. (23) Song, Z.; Ming-Jin, F.; Liang, Y.; Zhou, F.; Liu, W. In-situ preparation of anti-corrosion ionic liquids as the lubricant additives in multiply-alkylated cyclopentanes. RSC Adv. 2013, 3, 21715. (24) de Souza, R. L.; de Faria, E. L.; Figueiredo, R. T.; Freitas, L. S.; Iglesias, M.; Mattedi, S.; Zanin, G. M.; dos Santos, O. A.; Coutinho, J. A.; Lima, Á . S.; Soares, C. M. Protic ionic liquid as additive on lipase immobilization using silica sol-gel. Enzyme Microb Technol. 2013, 52, 141. (25) Espinosa, T.; Sanes, J.; Jiménez, A.-E.; Bermúdez, M.-D. Surface interactions, corrosion processes and lubricating performance of protic and aprotic ionic liquids with OFHC copper. Appl. Surf. Sci. 2013, 273, 578. (26) Khare, V.; Pham, M.-Q.; Kumari, N.; Yoon, H.-S.; Kim, C.-S.; Park, J.-I.; Ahn, S.-H. Graphene−ionic liquid based hybrid nanomaterials as novel lubricant for low friction and wear. ACS Appl. Mater. Interfaces 2013, 5, 4063. (27) Kheireddin, B. A.; Lu, W.; Chen, I.-C.; Akbulut, M. Inorganic nanoparticle-based ionic liquid lubricants. Wear 2012, 303, 185. (28) Yu, B.; Bansal, D. G.; Qu, J.; Sun, X.; Luo, H.; Dai, S.; Blau, P. J.; Bunting, B. G.; Mordukhovich, G.; Smolenski, D. J. Oil-miscible and non-corrosive phosphonium-based ionic liquids as candidate lubricant additives. Wear 2012, 289, 58. (29) Kamimura, H.; Kubo, T.; Minami, I.; Mori, S. Effect and mechanism of additives for ionic liquids as new lubricants. Tribol. Inter. 2007, 40, 620. (30) Cai, M.; Liang, Y.; Zhou, F.; Liu, W. Anticorrosion imidazolium ionic liquids as the additive in poly(ethylene glycol) for steel/Cu−Sn alloy contacts. Faraday Discuss. 2012, 156, 147. (31) Cai, M.; Zhao, Z.; Liang, Y.; Zhou, F.; Liu, W. Alkyl imidazolium ionic liquids as friction reduction and anti-wear additive in polyurea grease for steel/steel contacts. Tribol. Lett. 2010, 40, 215. (32) Jimenez, A. E.; Bermudez, M. D.; Iglesias, P.; Carrion, F. J.; Martinez-Nicolas, G. 1-N-alkyl-3-methylimidazolium ionic liquids as neat lubricants and lubricant additives in steel−aluminium contacts. Wear 2006, 260, 766. (33) Liu, X. Q.; Zhou, F.; Liang, Y. M.; Liu, W. M. Tribological performance of phosphonium based ionic liquids for an aluminum-onsteel system and opinions on lubrication mechanism. Wear 2006, 261, 1174. (34) Bermudez, M. D.; Jimenez, A. E.; Nicolas, M. Study of surface interactions of ionic liquids with aluminium alloys in corrosion and erosion−corrosion processes. Appl. Surf. Sci. 2007, 253, 7295.

in some cases and may be considered as a contribution of dust and base oil, respectively. However, no contribution from the ionic liquids has been observed on the surface.

4. CONCLUSION In summary, the antiwear and friction-reducing properties of the ionic liquids derived from amino acids such as aspartic acid and glutamic acid have been demonstrated. The synthesized ionic liquids when added in concentrations of 0.15%, 0.3%, 1%, 1.5%, 2%, and 3% by weight into the mineral base oil showed significant enhancement of antifriction and antiwear performance. The antifriction properties enhanced with increasing dosage of ionic liquids, and significant reduction in friction is observed for IL2 blended lubricant. The reduction in wear is almost same for all the three ionic liquids, with a maximum reduction on the order of 30%. The SEM micrographs revealed rubbing wear of the surfaces for IL2 and IL1, while micropitting was observed for IL3. Hence, from the tribological performance studies IL2 is considered to possess the best antifriction and antiwear characteristics among the three ionic liquids

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AUTHOR INFORMATION

Corresponding Author

*Tel: +91-135-2525788. Fax: +91-135-2660202. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We kindly acknowledge Director IIP for his kind permission to publish these results. The Analytical Division of the Institute if kindly acknowledged for providing analysis of samples.

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REFERENCES

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Tribological Performance Evaluation of Task-Specific Ionic Liquids

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