steel contacts

Wear 266 (2009) 1224–1228

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Tribological behaviour of two imidazolium ionic liquids as lubricant additives for steel/steel contacts A. Hernández Battez a,∗ , R. González a , J.L. Viesca a , D. Blanco b , E. Asedegbega b , A. Osorio a a b

University of Oviedo, Department of Mechanical and Civil Engineering, Ctra. de Castiello s/n, 33204 Gijon, Spain University of Oviedo, Department of Chemical Engineering, C/Julian Claveria, 8. 33006 Oviedo, Spain

a r t i c l e

i n f o

Article history: Received 3 September 2008 Received in revised form 25 March 2009 Accepted 27 March 2009 Available online 5 April 2009 Keywords: Ionic liquids Lubricant additives Rheology Friction Sliding wear

a b s t r a c t In this paper two room-temperature ionic liquids (ILs), 1-hexyl-3-methylimidazolium tetrafluroborate [HMIM][BF4 ] and 1-hexyl-3-methylimidazolium hexafluorophosphate [HMIM][PF6 ], have been studied as 1%wt. additives of a mineral hydrocracking oil for steel–steel contacts. Rheological properties of the mixtures and base oil were determined over shear rates and temperatures ranging 1–1000 s−1 and 40–100 ◦ C, respectively. Friction and wear testing was made using a block-on-ring tribometer set for pure sliding contact and XPS was used to analyze wear surfaces. [HMIM][PF6 ] and [HMIM][BF4 ] increased the viscosity of the base oil and decreased friction and wear. Friction and wear reduction are related to reactivity of the anion of the ionic liquids with surfaces forming FeF3 , B2 O3 , and species such as P2 O5 or PO4 3− . © 2009 Elsevier B.V. All rights reserved.

1. Introduction Room-temperature ionic liquids (ILs) are salts with melting points lower than room temperature and they are generally formed by an organic cation and a weakly coordinating anion and are of current interest as solvents for clean chemical synthesis, in separation and extraction technologies and in the development of new materials. They are called green solvents because they have some unique characteristics including negligible volatility, nonflammability, high thermal and chemical stability, low melting point, broad liquid range, and controlled miscibility with organic compounds, which meets the demands of high performance lubricants. Components in ionic liquids strongly interact with each other through the Coulomb force; whereas normal liquids are bonded through Van der Waals force and/or hydrogen bonding force [1,2,3]. Only a few ILs of the more than 1000 known and more than 300 commercially available have been investigated as lubricants or additives. There is ample opportunity for the development of ILs as effective friction coefficient and EP improvers with very low reactivity to surfaces [4]. The past decade has seen explosive growth of studies on ionic liquids for their diverse applications as catalyst, liquid crystals, green solvent in organic synthesis, and in separations, electrochemistry, photochemistry, CO2 storage

∗ Corresponding author. Tel.: +34 985182669; fax: +34 985182433. E-mail address: [email protected] (A.H. Battez). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.03.043

devices, etc. Recently, it was found that alkylimidazolium ionic liquid could act as a versatile lubricant for different sliding pairs and exhibited excellent friction-reduction, anti-wear performance and high load-carrying capacity. In particular, alkylimidazolium tetrafluoroborates and hexafluorophosphates have shown promising lubricating properties as base oils for a variety of contacts [2,5]. Table 1 summarizes the results of the previous tribological studies carried out with these new lubricants. Now, the molecular design of wear-preventing and friction-reducing additives for ionic liquids has become in a new and promising research line [6]. Alkylimidazolium tetrafluoroborates could be used as a kind of novel versatile lubricant for the contact of steel–steel, steel–aluminium, steel–copper, steel–SiO2 , Si3 N4 –SiO2 , steel–Si (1 0 0), steel–sialon ceramics, and Si3 N4 –sialon ceramics because of they exhibited excellent friction-reduction, anti-wear performance and high load-carrying capacity [7]. Several research groups have recently focused on the application of ionic liquids as neat lubricants in aluminium lubrication. Jimenez and Bermudez reported on the lubricating ability of a series of ionic liquids, in particular imidazolium derivatives, in aluminium–steel contacts both as neat lubricants and as mineral oil additives [1]. Jimenez et al. [1,2,8] worked with seven ILs: [HMIM][PF6 ], [EMIM][BF4 ], [HMIM][BF4 ], [EMIM][CF3 SO3 ], [OMIM][BF4 ], [EMIM][4-CH3 C6 H4 SO3 ] and [BMIM][TFSI], as 1%wt. additives of the synthetic ester propylene glycol dioleate (PGDO) or a paraffinic–naftenic mineral base oil in pin-on-disk tests for AISI 52100 steel–ASTM 2011 aluminium contacts at 25 and 100 ◦ C under variable sliding speed. Friction coefficients for IL additives

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Table 1 Room-temperature ionic liquids used in previous works. Ionic liquids

Contact materials

Test

Function of IL

References

[HMIM][PF6 ], [EMIM][BF4 ], [HMIM][BF4 ], [EMIM][CF3 SO3 ], [OMIM][BF4 ], [EMIM][4-CH3 C6 H4 SO3 ], [BMIM][TFSI] B-TBP6, B-TBP8, B-TBP10, B-TBP12, [HEIM][PF6 ]

AISI 52100 steel–ASTM 2011 aluminium GCR15 Steel–Al 2024 alloy

Pin-on-disk

Additive

[1,2,8]

Neat lubricant

[5]

[BMIM][PF6 ], [HMIM][PF6 ], [HEIM][PF6 ], [OEIM][PF6 ], [OPIM][PF6 ] [HEIM][TFSI], [HEIM][BF4 ]

Steel–Steel

Optimol SRV oscillating friction and wear tester Optimol SRV oscillating friction and wear tester Optimol SRV oscillating friction and wear tester Ball-on-disk type reciprocating friction test and four-ball wear test Optimol SRV oscillating friction and wear tester Optimol-SRV oscillating friction and wear tester

Neat lubricant

[7]

Neat lubricant

[9]

Neat lubricant

[3]

Neat lubricant

[10]

Neat lubricant

[11]

[BMI][BF4 ] [DEME][BF4 ] [DEME][TFSI], [RP][TFSI] [OPIM][PF6 ] [DPPHIM][BF4 ], [DPPHIM][PF6 ], DPPOIM][BF4 ], [DPPOIM][PF6 ], [OPIM][PF6] [HEIM][BF4 ],

SAE52100 Steel–SAE52100 Steel SUJ2-SUJ2 (High Carbon Chromium Bearing Steel) 1Cr18Ni9Ti stainless Steel–SAE52100 Steel 52100 Steel–Al alloy

are similar or lower than for neat ILs, while wear rates for 1%wt. ILs can be several orders of magnitude lower than those for neat ILs. The exception is the long alkyl chain [OMIM][BF4 ], which always shows better lubricating ability as pure lubricant, probably due to its lower miscibility with the base oil. Liu et al. [5] evaluated tetraalkylphosphonium ionic liquids (BTBP6, B-TBP8, B-TBP10, B-TBP12) as a new kind of lubricant for the contacts of steel/Al using an Optimol SRV oscillating friction and wear tester at room temperature. The phosphonium ionic liquid showed excellent tribological performance and was superior to the conventional ionic liquid 1-ethyl-3-hexylimidazolium hexafluorophosphate [HEIM][PF6 ] in terms of anti-wear performance and load-carrying capacity. Wang et al. [7] prepared and evaluated ionic liquids from alkylimidazolium hexafluorophosphate as neat lubricants for the steel/steel contact. Their tribological properties were investigated with an Optimol SRV oscillating friction and wear tester in ambient condition. These synthetic ionic liquids show excellent tribological performance and are superior to the conventional lubricant of liquid paraffin, containing 2%wt. of zinc dialkyldithiophosphate (ZDDP), in terms of the friction-reduction ability and load-carrying capacity. Lu et al. [9] synthesized and evaluated the ionic liquid 1ethyl-3-hexylimidazolium-bis(trifluoromethylsulfonyl)-imide [HEIM][TFSI] as lubricant for steel/steel contact. This synthetic ionic liquid showed excellent tribological performance and was superior to the ionic liquid of alkylimidazolium tetrafluoroborate. Kamimura et al. [3] examined several ionic liquids, derived from cations such as imidazolium, pridinium, ammonium under boundary conditions. It was found that tribological properties of ionic liquids are better than those of conventional lubricants such as synthetic hydrocarbons, synthetic esters, and fluorinated ethers. Xia et al. [10] investigated the friction and wear properties of the modified and unmodified 1Cr18Ni9Ti stainless steel specimens sliding against SAE52100 steel under the lubrication of ionic liquid of 1-propyl-3-octylimidazolium hexafluorophosphate ([OPIM][PF6 ]) and polyalphaolefin (PAO) with an Optimol SRV oscillating friction and wear tester. The resultant surface protective films composed of various tribochemical products together with

the adsorbed boundary lubricating film contributed to reduce the friction and wear. Mu et al. [11] synthesized four imidazolium-based roomtemperature ionic liquids containing phosphonyl functional groups. The physical properties of the resulting synthetic products were evaluated, and their tribological behaviour as lubricants for an aluminum-on-steel sliding system was studied with an oscillating friction and wear tester. It was found that the synthesized ionic liquids had better friction-reducing and anti-wear ability for the aluminium-on-steel system than their non-functionalized counterparts (1-ethyl-3-hexylimidazolium tetrafluoroborate, coded as [HEIM][BF4 ], and 1-propyl-3-octylimidazolium hexafluorophosphate, coded as [OPIM][PF6 ]). In particular, they had much better load-carrying capacity. Ionic liquids were used as base oil with good results in most of the above-cited papers. However, the utilization of the ionic liquids as neat lubricants is not feasible from the economical point of view right now because of their high prices. Therefore, they would be more likely used as additives in the lubricant industry. Some papers [1,2,8] have reported the lubricating ability of a series of ionic liquids, in particular imidazolium derivatives, as mineral oil additives but for aluminium–steel contacts only. Now, in this paper the tribological behaviour of two imidazolium ionic liquids, [HMIM][PF6 ] and [HMIM][BF4 ], have been studied as 1%wt. additives of a mineral hydrocracking oil in a steel–steel contact.

2. Experimental The ionic liquids used in this work, Table 2, were commercially available from Solvent Innovations (Germany). Mineral Hydrocraking Oil (M2) was kindly provided by REPSOL-YPF, S.A. Viscosity measurements of the mixtures and base oil were carried out using a parallel plate HAAKE viscosimeter at 40, 60, 80 and 100 ◦ C with maximum rotor speed of 1000 s−1 . The main properties of the lubricant and specimens used in the experiments are listed in Table 3. All test-section components were ultrasonically cleaned with heptane for 5 min, rinsed in ethanol and dried with hot air

Table 2 Room-temperature ionic liquid nomenclature. Ionic liquids

Cation

R1

R2

Anion

Purity (%)

Water content (%)

[HMIM][BF4 ]

CH3

C6 H13

BF4 −

99

<1

[HMIM][PF6 ]

CH3

C6 H13

PF6 −

99

<1

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Table 3 Material properties. Materials

Properties

M2

Density (15 ◦ C): 0.838 g/cm3 (ASTM D 4052) Kinematic viscosity (40 ◦ C): 32.89 mm2 /s (ASTM D 445) Kinematic viscosity (100 ◦ C): 5.96 mm2 /s (ASTM D 445) Viscosity Index: 127 (ASTM D 2270)

Block – F1140 Steel

0.40–0.50%C, 0.15–0.40%Si, 0.50–0.80%Mn, <0.035%P, <0.035%S 0.16–0.21%C, 0.15–0.45%Si, 1.30–1.60%Mn, <0.045%P, <0.045%S

Ring – ST-52 – DIN 2391–BK

before and after tests. Friction and wear testing was undertaken using a TE53SLIM tribometer set for pure sliding contact, with a block-on-ring configuration at room temperature. Test blocks of 12.7 mm × 12.7 mm × 14 mm F1140 steel cubes with a hardness of about 266 HV500gf were run against a 60 mm diameter ST-52DIN2391-BK steel counterface ring, hardened in the range 280 HV500gf . All tests were run for a total distance of 1800 m at a sliding speed of 0.5 m/s and loads of 101, 165 and 214 N (corresponding to mean contact pressures of 0.08, 0.10 and 0.11 GPa, respectively and maximum contact pressures of 0.10, 0.13 and 0.14 GPa, respectively). Two tests were conducted for each sample and wear was quantified by weight loss (±0.1 mg). The friction coefficient was recorded throughout in each test by means of a load transducer positioned to measure the lateral force on the block specimen. The surface of the samples was analyzed by X-ray photoelectron spectroscopy (XPS) with a SPECS Phoibos 100 MCD5 system equipped with a hemispherical electron analyzer operating in a constant pass energy, using monochromatic Al K␣ radiation (h = 1486.7 eV). The background pressure in the analysis chamber was kept below 5 × 10−10 mbar during data acquisition. Survey scan spectra were made at a pass energy of 90 eV, while the individual high resolution spectra were taken at a pass energy of 30 eV. All spectra were calibrated using C 1s peak fixed at 284.6 eV. The binding energies (BE) of the F 1s, B 1s and P 2p core levels were used to reveal the chemical state of F, B and P species. 3. Results and discussion The results of viscosity measurements, Table 4, showed that the addition of 1%wt. of ionic liquids in the base oil increased the viscosity to similar values independently of the anion type. These increments are lower at increasing temperatures; however they can improve the film formation properties of the mixtures. These results clarify the fact that the tribological results are closely related to the anion type and its reactivity with surfaces. The increase in viscosity of a mineral oil with the addition of [HMIM][PF6 ] and [HMIM][BF4 ] was also reported in two previous works [2,8]. However, the addition of 1%wt. of these ILs in a synthetic ester propylene glycol dioleate (PGDO) lowered the viscosity of PGDO, both at room temperature and at 100 ◦ C [1]. Although this rheological behaviour is interesting, authors have not explained the mechanism or proposed a hypothesis for these results. Results in Fig. 1 show mean friction values after 1800 m of sliding distance. Real-time friction variations with sliding distance at

Fig. 1. Mean friction coefficient and wear.

Table 4 Viscosity values. Lubricant

M2 M2 + 1% [HMIM][BF4 ] M2 + 1% [HMIM][PF6 ]

Viscosity (mPa s) 40 ◦ C

60 ◦ C

80 ◦ C

100 ◦ C

26.9 28.5 28.2

12.8 14.0 13.9

7.4 8.0 7.9

4.9 5.1 5.1

Fig. 2. Friction vs. sliding distance under a load of 101 N.

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Fig. 5. Fe 2p XPS spectra of the wear surface after test with [HMIM][PF6 ] at 101 N.

Fig. 3. Friction vs. sliding distance under a load of 165 N.

Fig. 4. Friction vs. sliding distance under a load of 214 N.

room temperature are shown in Figs. 2–4. As can be observed both mixtures present lower friction coefficients than base oil at increasing loads, but [HMIM][PF6 ] clearly show a better behaviour. It can be also seen, in Fig. 1, that the wear results are similar to friction ones, so the influence of the anion type is evident. The friction coefficient and estimated oil-film thickness values (<0.2 ␮m) verify that tests were made under the mixed lubrication regimen [12], so we can expect to find the active elements of the ionic liquids at the surfaces. In order to gain further insights, XPS experiments were performed on all samples. The spectra reveal that the elements constituting the anionic component of the ionic liquid have binding energies different from that expected in the original anions Table 5 Results obtained from XPS measurements. Sample

Load [N]

[HMIM][BF4 ] [HMIM][PF6 ] [HMIM][BF4 ]

101 101 165

[HMIM][PF6 ] [HMIM][BF4 ] [HMIM][PF6 ] M2

165 214 214 214

Binding energy (eV)

(BF4 − , PF6 − ). Therefore, new compounds have been formed at the expense of the partial or total decomposition of the former existing materials. Binding energies of all samples are given in Table 5. As can be observed, the photoelectronic peak corresponding to F 1s varies within the range of 684.4–685.5 eV. These values indicate the presence of F− probably due to the formation of ionic compounds such as FeF2 or FeF3 [13]. In order to verify this, high resolution XPS of Fe 2p3/2 was studied as well. The results obtained, given in Fig. 5, show peaks at about 711.5 and 725 eV corresponding to 2p3/2 and 2p1/2 of Fe3+ or Fe2+ [7,13,14]. It is sometimes difficult to distinguish between these two species. The characteristic satellite peak at about 719 eV, which can be assigned to Fe3+ species [15], could clarify our doubts thereby demonstrating the existences of FeF3 . Hence, XPS gives us hints on the reaction between ionic liquid and iron during the sliding process with the formation of FeF3 . Apart from these iron fluorides, F could also be present in its original anionic form or as a fragment of this starting material. This was only observed in one of the samples studied. The presence of F 1s peaks at higher binding energies (689.4 eV in our case) is usually ascribed to covalently bonded F. Some authors have given values of 689.7 eV for BF3 compounds [16] while others talk about F–C bonds which appear at binding energies around 689.1 eV [17] or 689.6 eV [18]. In our case it was difficult to verify what covalent fluorine specie was present on one of our wear surfaces. The XPS results obtained from the study of B 1s and P 2p were also quite revealing. Very small amounts of boron were present on the wear surfaces of the samples tested with the [HMIM][BF4 ] ionic liquid. B 1s peaks which appeared around 192 eV indicated that the little existing B present at the surface of the sample could be assigned to oxidized boron (B2 O3 ) [1,19] and not as elemental B which would appear at a lower binding energy (187.2 eV) [20,21]. The same applies to P for the samples tested with [HMIM][PF6 ]; although, in this case, peaks are slightly more intense. The binding energies around 133.4 eV correspond to some oxidized species such as P2 O5 or PO4 3− as has been noted by other authors like Wang et al. [7] and not to P in its zero oxidation form which would display a peak at lower binding energy (130 eV) [22]. The above results coincide with previous studies on lubrication with ILs [7,9,11,19,23–26] which have reported a complex tribochemistry when tetrafluoroborate or hexafluorophosphate ionic liquids are used as base oils. These studies made for steel/steel contacts verified the formation of FeF2 , FePO4 and B2 O3 or BN tribofilms, when the ILs were used as base oil.

F 1s

B 1s

P 2p

685.5 684.5 684.9 (60.95) 689.4 (39.05) 684.9 684.4 684.7 –

191.4 – 191.4

– 133.4 –

4. Conclusions

– 191.6 – –

133.2 – 133.4 –

The addition of 1%wt. of the imidazolium ionic liquids [HMIM][BF4 ] and [HMIM][PF6 ] increase the viscosity of the mineral oil used as base oil, and so this viscosity increment could improve the film forming properties of the mixtures. Although this hypoth-

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esis was not studied in this paper, it would be an interesting future research line. The friction and wear reduction obtained with the ILs used as additives with respect to that of the neat lubricant can be taken into account by lubricant industries in order to replace current anti-wear additives for steel–steel contacts. If lubricants for high temperature applications (>200 ◦ C) are required, neat ILs can be the solution as has been shown in previously cited works. In the case of applications at lower temperatures, ILs would be wonderful candidates as anti-friction and anti-wear additives. [HMIM][BF4 ] and [HMIM][PF6 ] improve the friction and antiwear properties of the base oil through the reactivity of the anion with the steel surfaces. In the case of [HMIM][BF4 ], FeF3 and B2 O3 were found on the wear surfaces, although the amount of boron present was small. As for [HMIM][PF6 ], FeF3 was also identified as well as phosphorous species corresponding to P2 O5 or PO4 3− . In both cases although the quantity of B and P was very little, the above-mentioned species were found and they decreased friction and wear. Because of environmental issues, these results are important due to phosphorous reduction in lubricant formulation. So, the biodegradability of ILs as neat lubricants and mixtures when the ILs are used as additives have to be also explored in the future. Acknowledgement The authors wish to express their thanks to the University of Oviedo, Spain, for supporting this work within the framework of the Research Project UNOV-08-MB-11. References [1] A.E. Jimenez, M.D. Bermudez, Imidazolium ionic liquids as additives of the synthetic ester propylene glycol dioleate in aluminium–steel lubrication, Wear 265 (5–6) (2008) 787–798. [2] A.E. Jimenez, M.D. Bermudez, et al., 1-N-alkyl-3-methylimidazolium ionic liquids as neat lubricants and lubricant additives in steel–aluminium contacts, Wear 260 (2006) 766–782. [3] H. Kamimura, T. Kubo, et al., Effect and mechanism of additives for ionic liquids as new lubricants, Tribol. Int. 40 (2007) 620–625. [4] M. Fox, M. Priest, Tribological properties of ionic liquids as lubricants and additives. Part 1: synergistic tribofilm formation between ionic liquids and tricresylphosphate, Proc. IMechE Part J: J. Eng. Tribol. 222 (2008) 291– 303. [5] X. Liu, F. Zhou, et al., Tribological performance of phosphonium based ionic liquids for an aluminium-on-steel system and opinions on lubrication mechanism, Wear 261 (2006) 1174–1179.

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