1-N-alkyl -3-methylimidazolium ionic liquids as neat lubricants and lubricant additives in steel–aluminium contacts

Abstract

The influence of the alkyl chain length and of the anion on the lubricating ability has been studied for the room-temperature ionic liquids (IL) 1-n-alkyl-3-methylimidazolium X⁻ [X = PF₆; n = 6 (L-P106). X = BF₄; n = 2 (L102), 6 (L106), 8 (L108). X = CF₃SO₃; n = 2 (L-T102). X = (4-CH3C6H4SO3); n = 2 (L-To102)]. Neat IL have been used for AISI 52100 steel-ASTM 2011 aluminium contacts in pin-on-disk tests under variable sliding speed. While all IL give initial friction values lower than 0.15, real-time sharp friction increments related to tribochemical processes have been observed for L102 and L-P106, at room-temperature and at 100 °C. Electronic microscopy (SEM), energy dispersive (EDS) and X-ray photoelectron (XPS) spectroscopies show that wear scar surfaces are oxidized to Al₂O₃ and wear debris contain aluminium and iron (for L102) fluorides. For L-P106, the steel surface is covered with a P-containing tribolayer. A change of anion (L-T102; L-To102) reduces friction and wear, but the lowest values are obtained by increasing the alkyl chain length (L106; L108). When the more reactive L102 and L-P106 are used as 1 wt.% base oil additives at 25 °C, tribocorrosion processes are not observed and a friction reduction (69–75% for 1 wt.% L102) and a change from severe (10⁻³ mm³ m⁻¹) to mild wear (10⁻⁴ to 10⁻⁶ mm³ m⁻¹) is obtained with respect to the neat IL. 1 wt.% IL additives also show good lubricating performance at 100 °C.

Introduction

Room temperature ionic liquids are generally formed by an organic cation and a weakly coordinating anion [1] and are of current interest as solvents for clean chemical synthesis, in separation and extraction technologies and in the development of new materials [2], [3].

Among those more thoroughly studied are 1-n-alkyl-3-methylimidazolium salts [4]. A number of recent studies [5], [6], [7], [8], [9], [10], [11], [12] have raised interest on ionic liquids as novel lubricants. Table 1 reviews the results of the previous tribological studies carried out with these new lubricants. In particular, alkylimidazolium tetrafluoroborates and hexafluorophosphates (Fig. 1) have shown promising lubricating properties as base oils for a variety of contacts.

At the beginning of the present work [13], the only precedent in ionic liquid lubrication of steel/aluminium [5], was the use of neat 1-hexyl-3-methylimidazolium tetrafluoroborate (L106; Table 1; Fig. 1) as base oil. A friction coefficient of 0.04 was reported, but no wear data were given.

Very recently, Liu et al. [11], [12], have described a new series of hexafluorophosphate ionic liquids which give low friction and wear values when used as neat lubricants of steel-Al2024 contacts, under increasing normal load.

In spite of the huge amount of knowledge currently being generated about IL, studies on metallic corrosion by ionic liquids are still very scarce [14], [15] and, as far as we are aware, do not include tetrafluoroborate derivatives, but previous studies on lubrication with IL [5], [6], [7], [8], [9], [10], [11], [12] have reported a complex tribochemistry when tetrafluoroborate or hexafluorophosphate ionic liquids are used as base oils, with the formation of FeF₂, FePO₄ and B₂O₃ or BN tribofilms for steel/steel contacts.

In their recent work [11], [12], Liu et al. propose a mechanism for the formation of adsorbed layers and protective tribofilms to explain the lubricating ability of IL for the Al2024-AISI 52100 steel contact, but they also state the importance of preventing corrosive damage by IL or their tribochemical decomposition products. In the present study, we provide experimental evidence of the influence of anion nature and alkyl chain length on the tribological performance of commercial 1-n-alkyl-3-methylimidazolium IL (Table 2), when they are used as neat lubricants under increasing sliding speed and temperature, analyzing the tribochemical processes and their influence on wear and on the friction changes which take place during the tests.

We have examined the influence of the alkyl chain length in the series of 1-n-alkyl-3-methylimidazolium tetrafluoroborates, where n = 2 (L102), 6 (L106) or 8 (L108). The effect of the anion nature has been examined for a series of 1-ethyl-3-methyl imidazolium salts containing BF₄⁻ (L102), (CF₃SO₃⁻) (L-T102) or (4-CH₃C₆H₄SO₃⁻) (L-To102) and for 1-hexyl-3-methylimdazolium with BF₄⁻ (L-106) or PF₆⁻ (L-P106) anions (Table 2).

The lubricating ability of the room-temperature ionic liquids has been attributed to the formation of boundary films and protective tribolayers. At least for one of the ionic liquids used in the present study, 1-octyl-3-methylimidazolium tetrafluoroborate (L108), evidence has been found [16] of surface ordering with orientation of the alkyl chains and ionic headgroups. In fact, L108 has already shown a good lubricating performance for steel–ceramic contacts [6].

In the case of the 1-ethyl-3-methylimidazolium bifluoride [17] and nitrate [18] salts recent studies on the structural ordering in the liquid state have shown that the local and intermediate-range order resembles that of the solid state, due to electrostatic interactions which lead to charge-ordering effects. These results suggest that the short-chain ionic liquids could also be useful in lubrication.

We have previously studied the use of other ordered fluids such as neutral and ionic thermotropic liquid crystals as base oil additives in lubrication of steel–steel [19] and steel–aluminium [20] contacts.

The ability of IL to form ordered surface layers could be exploited to develop new lubricating additives which could expand their range of applications by replacing less effective conventional non-environmentally friendly additives. As far as we are aware, the only attempt to use ionic liquids as additives has been in ceramic lubrication as water additives [21], [22], [23].

In this paper we also describe the use of L-P 106 and L108 as 1 wt.% lubricant additives, comparing their performance with that of neat IL.