Tribological performance and lubrication mechanism of new gemini quaternary phosphonium ionic liquid lubricants

Ionic liquids are expected to be used as new high-performance lubricants because of their low volatility, high thermal stability, and high oxidation stability. It is well known that some ionic liquids exhibit excellent lubricity for metals. On the other hand, there is concern about the corrosiveness of ionic liquids caused by tribo-chemical reactions. In this study, the lubricating properties of seven kinds of commercially available ionic liquids were evaluated under steel-on-steel sliding contacts using an SRV oscillating sliding tester. The worn surfaces were analyzed by optical microscopy, confocal laser scanning microscopy, scanning electron microscopy with energy dispersive X-ray spectrometry, and X-ray photoelectron spectroscopy. The halogen-containing ionic liquids exhibited excellent lubricity for steel compared with the halogen-free ionic liquids. The X-ray photoelectron spectroscopy analytical results showed the formation of iron fluoride on sliding surfaces lubricated with the fluorine-containing ionic liquids. Formation of this iron-fluoride boundary layer reduced friction; however, corrosion occurred on the worn surfaces after the sliding. On the other hand, less corrosive damage was observed on the worn surfaces that slid under the dry-nitrogen atmospheric condition. These results suggested that the hydrofluoric acid, which was produced by the reaction between iron fluoride and contaminated water from the atmosphere, caused the corrosion on the worn surface. The halogen-free ionic liquids showed inferior lubricity compared with the halogen-containing ionic liquids, though they did not cause any remarkable corrosion. The corrosion-inhibition effect of the phosphorus element was shown in the halogen-containing ionic liquids composed of a phosphate anion or phosphonium cation. The friction-reduction effect was, however, inferior to that of the phosphorus-free ionic liquids. The tribo-chemical reaction of the phosphorus contained in the ionic liquids yielded a phosphate boundary layer that prevented the formation of iron halide on the sliding surface and enhanced the wear resistance.

Ionic liquids are considered as an ideal alternative to volatile organic solvents and chemical industries in the future, because they are non-volatile. Ionic liquids are also considered as new novel chemical agents and widely regarded as a greener alternative to many commonly used solvents. Ionic liquids have been studied for a wide range of synthetic applications and have attracted considerable interest for use as electrolytes in the areas of organic synthesis, catalysis, solar cell, fuel cells, electrodeposition and supercapacitors. However, some ionic liquids suffer from more or less some drawbacks such as toxicity, preparation and high cost in the process for use. Most recently, three types of ionic liquids are attracted much attentions specifically traditional ionic liquid, protic ionic liquid and deep eutectic solvent, where their preparation, mechanism and limitation were differentiated. However, those liquids are having their own advantages and limitations based on applications. Traditional ionic liquid and protic ionic liquid are highly cost and toxic for applied engineering research, but they consist of micro-biphasic systems composed of ionic compounds which have more varieties in the applications. The deep eutectic solvent is very economic for large-scale possessing but there are only limited ionic mixtures to certain application such as electrochemistry. Keywords: deep eutectic solvent, ionic liquids, protic ionic liquids, volatile organic solvents

Friction modifiers (FMs) are surface-active additives added to base fluids to reduce friction between rubbing surfaces. Their effectiveness depends on their interactions with rubbing surfaces and may be mitigated by the choice of the base fluid. In this work, the performance of an imidazolium ionic liquid (ImIL) additive in polyethylene-glycol (PEG) and 1,4-butanediol for lubricating steel/steel and diamond-like-carbon/diamond-like carbon (DLC—DLC) contacts were investigated. ImIL-containing PEG reduces friction more effectively in steel—steel than DLC—DLC contacts. In contrast, adding ImIL in 1,4-butanediol results in an increase in friction in steel—steel contacts. Results from the Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and focused ion beam-transmission electron microscopy (FIB-TEM) reveal that a surface film is formed on steel during rubbing in ImIL-containing PEG. This film consists of two layers. The top layer is composed of amorphous carbon and are easily removed during rubbing. The bottom layer, which contains iron oxide and nitride compound, adheres strongly on the steel surface. This film maintains its effectiveness in a steel—steel contact even after ImIL additives are depleted. Such film is not observed in 1,4-butanediol where the adsorption of ImIL is hindered, as suggested by the quartz crystal microbalance (QCM) measurements. No benefit is observed when the base fluid on its own is sufficiently lubricious, as in the case of DLC surfaces.This work provides fundamental insights on how compatibilities among base fluid, FM, and rubbing surface affect the performance of IL as surface active additives. It reveals the structure of an ionic liquid (IL) surface film, which is effective and durable. The knowledge is useful for guiding future IL additive development.

Tribological performance of fatty acid ammonium ionic liquids as anti-wear additives in water-based fluids

Ionic liquids as lubricants: An overview of recent developments

Minimizing the emissions of SO2 is highly important, because it is a significant source of atmospheric pollution that threatens environment and human health. SO2 can be absorbed by ionic liquids (ILs) due to their unique properties, such as ultra low vapor pressure, wide liquid temperature range, non-flammability, chemical stability, and tunable structure and properties. As a new sorption technology, ILs with useful functional groups are attracting the attention of a growing number of scientists and engineers in recent years. In this article, the recent advances on the studies of ILs in the capture of SO2 were reviewed, where traditional ILs and functionalized ILs were included. Traditional ILs with the anions such as BF4 - , PF6 - , Tf2N - , F - , Cl - , Br - and I - could uptake SO2 at ambient temperature and pressure through physical interaction between IL and SO2, but they could not be used under industrial conditions because of the weak interactions. Industrial condition means high temperature and low SO2 partial pressure, which needs strong interaction between IL and SO2. Functional ILs, including organic acid salt ILs, phenolate ILs, azolate ILs, ether-based ILs, amine-based ILs and multi-functional ILs, showed highly efficient SO2 absorption under ambient conditions. 1,1,3,3-Tetramethylguanidine lactate ([TMG][L]) was the first functional IL used to achieve about equal mole SO2 capacity under 40°C and 8% SO2 through chemical interaction. [SCN]-based ILs were proved to be highly efficient and reversible ILs for SO2 capture under low partial pressure by the single-site interaction mechanism. Although ether-based ILs could achieve much higher SO2 capacity due to their multiple ether groups, they could not be used to capture SO2 under industrial conditions because of the weak physical interaction. Among the functional ILs investigated, novel azolate ILs were found to have the two mole of chemical absorption capacity through two-site cooperative interactions under low partial pressure. Very recently, other kinds of multiple-site based ILs were developed, including sites on the independent cation and anion as well as those on the same anion. [Et2NEmim][Tetz] was found to be a kind of multi-functional IL that could absorb two mole of SO2 under low partial pressure through independent anion···SO2 and amine···SO2 interactions. Among the above mentioned functional ILs, the ILs based on [Tetz] and [SCN] anions were proved to be more efficient and highly reversible for the capture of SO2 under low SO2 partial pressure. The perspective for further researches on the design of functionalized ILs was also provided. Finally, some challenges in the absorption of SO2 from flue gas were indicated, which includes the high cost of ILs, the deep mechanism study of SO2 absorption, the investigations under high temperature and low SO2 partial pressure, as well as the effect of other species (such as CO2, NO x , O2, dust, and moisture) in flue gas. The development of alternative ILs that are able to achieve rapid and reversible SO2 capture in high capacity is always highly desired.

Removal of dimethyl disulfide via extraction using imidazolium-based phosphoric ionic liquids

A new class of more effective lubricants could lead to huge energy savings. Limited recent literature has suggested potential for using room-temperature ionic liquids as lubricants, however, only a few out of millions (or more) of species possible have been evaluated. In this study, a series of new protic alkylammonium ionic liquids were synthesized by neutralization and metathesis reactions, and have demonstrated promising lubricating properties as neat lubricants or lubricant additives, particularly for use with difficult-to-lubricate metals like aluminum. More than a 30% friction reduction was observed with ammonium-based ionic liquids compared to conventional hydrocarbon oils in reciprocating sliding tests of 52100 bearing steel on aluminum alloy 6061-T6511. The inherent polarity of ionic liquids is believed to provide strong adsorption to contact surfaces and can form a boundary lubricating film leading to friction and wear reductions. Other advantages of ionic liquids include (1) negligible volatility, (2) high thermal stability, (3) non-flammability, and (4) better intrinsic properties that may eliminate the need for more complex and expensive additive packages. With very flexible molecular structures, this new class of lubricants, particularly ammonium-based ionic liquids, can be tailored to fit a variety of applications.

Functional alkylimidazolium ionic liquids as lubricants for steel/aluminum contact: Influence of the functional groups on tribological performance

To enhance the lubricating and extreme pressure (EP) performance of base oils, two types of oil-soluble ionic liquids (ILs) with similar anion albeit dissimilar cations were synthesized. The physical properties of the prepared ILs were measured. The anticorrosion properties of ILs were assessed by conducting corrosion tests on steel discs and copper strips, which revealed the remarkable anticorrosion properties of the ILs in comparison with those of the commercial additive zinc dialkyldithiophosphate (ZDDP). The tribological properties of the two ILs as additives for poly-α-olefin-10 (PAO10) with various mass concentrations were investigated. The tribological test results indicate that these ILs as additives are capable of reducing friction and wear of sliding contacts remarkably as well as enhance the EP performance of blank PAO10. Under similar test conditions, these IL additives exhibit higher lubricating and anti-wear (AW) performances than those of ZDDP based additive package in PAO10. Subsequently, X-ray photoelectron spectroscopy (XPS) and energy dispersive spectrometer (EDS) were conducted to study the lubricating mechanism of the two ILs. The results indicate that the formation of tribochemical film plays the most crucial role in enhancing the lubricating and AW behavior of the mixture lubricants.

Effect of two halogen-free ionic liquids with different anions on the tribological properties of TC4

The performance of a series of novel room temperature ionic liquids (ILs) based on the trihexyl(tetradecyl)phosphoniumcation (P66614 +) and a number of novel anions have been studied in pin-on-disk tests using a 100Cr6 steel ball on AA2024 aluminium disks. The anions coupled to the (P66614 +) cation include diphenyl phosphate (DPP-), dibutyl phosphate (DBP-), bis (2,4,4-trimethyl pentyl) phosphinate (M3PPh-) and bis(2-ethyl hexyl) phosphate (BEH-). More traditional anions such as bis(trifluoromethanesulfonyl) amide (NTf2 -) and bromide (Br-) were also investigated. Experiments were conducted at various loads to assess the IL film forming abilities. The results suggest that the structure of the anion is important in forming a surface film that reduces the friction and wear of the aluminium disk. At 30N five of the six ILs tested showed a 30-90% reduction in wear, as determined from wear scar depth measurements, compared to fully formulated diesel oil. The IL lubricant with a diphenyl phosphate anion achieved the lowest wear coefficient, showing a better performance than a typical fluorine-containing IL anion, NTf2. To further investigate wear mechanisms and surface interactions the wear scars were analysed using a scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS).

Ionic liquids have been widely discussed as potential lubricants, however, their properties make them also very good potential candidates as lubricant additives (e.g., friction modifiers and anti-wear). In this work, the tribological study of two ionic liquids (tributylmethylphosphonium dimethylphosphate (PP), and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP)) as lubricant additives has been performed on stainless steel (AISI 316L) exposed to polar (water-glycol) and non-polar (polyalphaolefin) based lubricants under boundary lubricating conditions. The performance of these ionic liquids as lubricant additives has been compared to a classical organic friction modifier (dodecanoic acid (C12)). The water-glycol lubricant formulated with the two ionic liquids showed friction values higher than the same base lubricant formulated with dodecanoic acid, however, opposite results were observed for polyalphaolefin (PAO). A detailed surface chemical analysis using X-ray photoelectron spectroscopy (XPS) revealed differences in the passive/tribofilm thickness and chemical composition of the stainless steel surface tested in all lubricants. In the case of the polar lubricant additivated with ionic liquids, the tribochemical reaction accompanied by a tribocorrosion process led to the formation of an unstable passive/tribofilm resulting in high friction and wear. However, in the absence of tribocorrosion process (polyalphaolefin base lubricant), the tribochemical reaction led to the formation of a stable passive/tribofilm resulting in low friction and wear. A detailed surface and subsurface investigation of the microstructure using scanning electron microscopy equipped with a focused ion beam (SEM-FIB) showed that high wear rates resulted in thicker recrystallization region under the wear track surface. Among all lubricant additives tested in this work, BMP in non-polar lubricant media showed the best tribological performance.

XPS and ToF-SIMS analysis of the tribochemical absorbed films on steel surfaces lubricated with diketone

Five room temperature ionic liquids (ILs), 1-butyl-3-methylimidazolium hexafluorophosphate (L-P104), 1-hexyl-3-methylimidazolium hexafluorophosphate (L-P106), 1-octyl-3-methylimidazolium hexafluorophosphate (L-P108), 1-decyl-3-methylimidazolium hexafluorophosphate (L-P110), and 1-hexyl-3-methylimidazolium tetrafluoroborate (LB106) were studied as 1 wt% additives of polyurea grease for steel/steel contacts. Their tribological behaviors as additives of polyurea grease for steel/steel contacts were evaluated on an Optimol SRV-IV oscillating reciprocating friction and wear tester and an MRS-1J (G) four-ball tester at room and high temperatures. The friction test results showed that the ILs, as 1 wt% additives in polyurea grease for steel/steel contacts, had better friction reduction and anti-wear properties at high temperature than at room temperature, and ILs can significantly improve the friction reduction and anti-wear properties of polyurea grease compared with base grease containing 1 wt% of zinc dialkyldithiophosphate (T204). The excellent tribological properties are attributed to the formation of a surface protective film composed of FeF2, nitrides, and compound containing the P–O bonding on the lubricated metal surface by a tribochemical reaction. The ordered adsorbed films and good miscibility of ILs with the base grease also contributed to the excellent tribological properties. Wear mechanisms and worn steel surfaces were studied by a PHI-5702 multifunctional X-ray photoelectron spectrometer and a JSM-5600LV scanning electron microscope.