Tribological Properties Of Ionic Liquids Research Articles

Effect of Anionic Alkyl Chain Length on Tribological Properties of Ionic Liquids: Molecular Dynamics Simulations

Effect of Anionic Alkyl Chain Length on Tribological Properties of Ionic Liquids: Molecular Dynamics Simulations

Effect of temperature on the lubrication ability of two ammonium ionic liquids

The tribological properties of ionic liquids (ILs) have been widely investigated, and their excellent performance has been acknowledged by many researchers. However, their lubricity is closely linked to operating conditions. For wider applications, the complexity of their behavior must be understood. In this study, we compared the tribological properties of ILs at temperatures of 30 °C and 80 °C. Two ammonium ILs with different cation side-chain lengths, methyl-trioctylammonium [N1888] and tributylmethylammonium [N4441] and the same anion bis(trifluoromethylsulfonyl)imide [NTf2] were selected. Their tribological properties were investigated in the lubrication of three different friction pairs: bearing steel–bearing steel E 52100, bearing steel–stainless steel 304, and bearing steel–aluminum alloy 6082 T6. Tribotests were performed using a ball-on-plate reciprocating tribometer for 30 min at 4 N, 15 Hz, and a 1 mm stroke length. The worn surfaces were analyzed using optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results indicated diminished lubricity at higher test temperatures. The performance difference between the investigated temperatures depended on the friction pair, being almost negligible in bearing steel–bearing steel friction pairs and significantly different for bearing steel–aluminum friction pairs. The wear reduction ability was more sensitive to the test temperature than friction did. The investigated ILs were superior to the 5W–40 motor oil at both temperatures.

Polyaniline as an additive towards improving tribological properties and anti-corrosion performance of ionic liquids-based greases

PurposeThis paper aims to explore polyaniline (PANI) as a lubricant additive to improve the anti-corrosion and tribological properties of ionic liquids (ILs) for actual applications.Design/methodology/approachILs were synthesized by dissolving lithium salts in synthetic oil and were used as a base oil to prepare ILs-based greases. PANI was used as an additive. The tribological properties were investigated in detail and the anti-corrosion ability was also assessed via salt spray test. After friction test, the worn surfaces were characterized by scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy to analyze the lubrication mechanisms.FindingsPANI not only reduces the corrosion but also improves the friction reduction and anti-wear abilities of the ILs-based greases. The analysis indicates that the protective films generated on the worn surfaces were responsible for the preferable anti-corrosion and tribological properties.Originality/valueThis paper provides an effective approach to improve the anti-corrosion and tribological properties of ILs for actual applications.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-11-2019-0469/

Physicochemical and tribological properties of gemini-type halogen-free dicationic ionic liquids

A series of new halogen-free dicationic ionic liquids (ILs) with different alkyl chain lengths were prepared, and the relationship between the alkyl chain length, physicochemical and tribological properties of ILs, and their role as neat lubricant for steel-steel friction pairs, was investigated. Evaluation of stability during hydrolysis and copper strip corrosion test results show that synthetic ILs are stable and not corrosive to metal contacts, due to the halogen-free anions. The friction and wear test results indicate that ILs with long alkyl chains have excellent friction-reducing and anti-wear properties, especially at high temperatures. Based on the surface three-dimensional (3D) profiles, electrical contact resistance, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and the X-ray photoelectron spectrometry (XPS) analysis of the worn surfaces of steel discs, we can conclude that the efficiency of ILs is due to the formation of high quality tribofilms that consist of both tribochemical reaction and ordered absorption films.

Tribological properties of ionic liquids for steel/aluminum, steel/copper and steel/Si3N4 ceramic contacts under boundary lubrication

PurposeThe purpose of this paper is to study the influence of alkyl chain length and kind of anions of ionic liquids on the tribological properties with different materials as friction pairs (steel-aluminum, steel-copper and steel-Si3N4 ceramic).Design/methodology/approachTribological properties were evaluated by an optimol-SRV-IV reciprocation friction tester with a ball-on-block configuration at room temperature and high temperature, respectively. Friction-reducing and anti-wear properties of the ionic liquids for steel/aluminum, steel/copper and steel/ceramic contacts were evaluated on the ball-on-block reciprocating UMT-2MT tribometer. The morphologies of the worn surfaces were observed by a scanning electron microscope. The chemical states of several typical elements on the worn surfaces were examined by X-ray photoelectron spectroscopy.FindingsBoth the alkyl chain length and kind of anion influence the tribological properties of ionic liquids, especially for the length of alkyl chains. With the increase of alkyl chain length, the load carrying capacity of ionic liquids is improved at both room temperature and high temperature, and the friction reducing and antiwear behaviors are also significantly enhanced.Research limitations/implicationsThe paper presents potentially useful and highly efficient lubricants.Practical implicationsOwing to their good friction-reducing and wear resistance properties, these ionic liquids are promising candidates for versatile applications.Originality/valueThis work might provide a promising research direction for design and application of ionic liquids as lubricants.

Wetting and Tribological Properties of Ionic Liquids

A phenomenological study of the surface-wetting and tribological properties of various ionic liquids was conducted using molecular dynamics simulations. The surface-wetting capabilities of the liquids were studied by simulating the morphological transformation of an isolated liquid drop in vacuum to its equilibrium state on solid surface. The tribological properties of the liquids were probed examining their flow behaviors and viscosities in computational lubrication experiments. All liquids exhibited good surface-wetting properties, as demonstrated by the hemispherical shape of the droplets at equilibrium and the surface contact angles. Contact angles for all liquids were much lower than 90°. Lubrication experiments demonstrated a flow behavior for the liquids that depended on the magnitude of the applied shear rate. Three distinctive flow regimes were observed: Newtonian, thixotropic (non-Newtonian), and oversheared. The liquids' viscosities were determined in the Newtonian regime and agree well with experimental results and with previously reported values calculated using equilibrium simulations. The phenomenological approach implemented in this study allowed for the calculation of the transport properties of the liquids and produced values within the appropriate order of magnitude without the use of calculational artifacts. These results corroborate previous reports indicating that nonequilibrium techniques represent a more robust approach for the calculation of transport properties than do equilibrium methods based on time-correlation functions.

Inorganic nanoparticle-based ionic liquid lubricants

This work deals with the tribological properties of recently described mixtures of nanoparticles (NPs) and ionic liquids (ILs), specifically mixtures of SiO2 (silica) nanoparticles and 1-butyl-3-methylimidazolium (trifluoromethysulfony)imide. Friction force profiles, kinetic friction coefficients, friction traces, rheological properties, and wear behavior of these mixtures were compared with that of the pristine ionic liquids at various concentrations of nanoparticles for a tribo-pair of stainless steel ball and a steel surface. It was shown that NP concentration significantly influences the tribological properties of the NP-IL mixtures: the friction coefficient for the optimum NP concentration (0.05wt%) was ~35% less than that for high NP concentrations (>3wt%) and 25% less than that for low NP concentrations (<0.01wt%). At the optimum NP concentration, while the friction force was slightly lower for NP-IL mixture at low loads, the friction force was about 28% lower for NP-IL mixture at high loads, compared to the pristine ionic liquid. In addition, the wear volume was found to decrease by 24% upon the addition of the optimum amount of nanoparticles into ionic liquid. Overall, this study concludes that promising tribological properties of ionic liquids can be further enhanced by incorporating nanoparticles into ionic liquids.

Influence of structural variations on imidazolium‐based ionic liquids

ABSTRACTIn this work, a fundamental study of ionic liquids (ILs) is shown. The main goal was to establish relationships between rheological and tribological properties of ILs and to understand how these properties are linked to the molecular structure of ILs. Of particular interest were the effects occurring to a frictional system due to variations of the anions and the alkyl chain length of the imidazolium‐based cations. Tribological experiments were performed using a reciprocating ball‐on‐disc test machine. To understand the effects resulting from a variation of the structural characteristics of the used ILs on the rheological properties, studies were performed using a rheometer. The results obtained are very promising in terms of improving friction and wear behaviour by using ILs as lubricants. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Comparative study of the tribological properties of ionic liquids as additives of the attapulgite and bentone greases

ABSTRACTThree kinds of ionic liquids (1‐butyl‐3‐methylimidazolium hexafluorophosphate (L‐P104), 1‐hexyl‐3‐methyl imidazolium hexafluorophosphate (L‐P106) and 1‐octyl‐3‐methylimidazolium tetrafluoroborate (LB108)) were added to the attapulgite base grease and the bentone base grease to investigate and compare the tribological behaviours of the ionic liquids with the two base greases at room temperature and 150°C. Tribological tests were performed using a ball‐on‐plate reciprocating tribometer. The attapulgite base grease showed better wear resistance properties than that of bentone base grease by adding ionic liquids as additives. At same time, the attapulgite base grease showed excellent friction‐reducing and wear resistance properties at high temperature (150°C). Also, we discussed the tribological mechanism of the attapulgite base grease at both room temperature and 150°C from the aspect of the structure of the grease thicker. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Evaluation of Vapor Pressure and Ultra-High Vacuum Tribological Properties of Ionic Liquids

Ionic liquids are a class of salts that incorporate polyatomic anions and cations. These materials are typically viscous fluids at room temperature. The fluids are generally characterized as possessing negligible vapor pressures under ambient conditions. These beneficial properties have led us to study the effectiveness of ionic liquids containing both organic cations and anions for use as lubricants with space applications. This article presents vapor pressure measurements for four ionic liquids as well as friction coefficient data from a spiral orbit tribometer in the boundary lubrication regime under simulated space conditions using stainless steel tribocouples. In addition, we present the first tribological data on mixed ionic liquids and an ionic liquid additive. Postmortem infrared and Raman analyses of the balls and races indicates that the major degradation pathway for these organic ionic liquids is similar to those of other carbon-based liquid lubricants; that is, deterioration of the organic structure into amorphous graphitic carbon. The coefficients of friction and lifetimes of these lubricants are comparable to or better than those of several commonly used space oils.

The Tribological Properties of Ionic Liquids Composed of Trifluorotris(pentafluoroethyl) Phosphate as a Hydrophobic Anion

The tribological properties of trifluorotris(pentafluoroethyl) phosphate [(C2F5)3PF 3 − , FAP]-derived ionic liquids were evaluated under boundary conditions. The anion is hydrophobic in comparison with bis(trifluoromethylsulfonyl)imide [(CF3SO2)2N−, TFSI]. 1,3-Dialkylimidazolium salts of FAP provided much lower friction than 1,3-dialkylimidazolium salts of TFSI. In addition, the FAP salts exhibit better anti-wear properties than the TFSI salts. Another advantage of the FAP anion is availability of several cations to prepare ionic liquids. For example, tetraalkylphosphonium, N,N-dialkylpyrrolidium, and tetramethylisouronium salts of FAP provided friction coefficient of approximately 0.1. Straight-chain carboxylic acids as model friction-reducing additives improved the tribological properties of the FAP salts. Surface analyses were conducted to study the boundary film formed by rubbing. It was found that the boundary film is composed of adsorbed anion on uppermost surfaces and reacted anion on sub-surfaces. The model friction-reducing additives were found on the rubbed surfaces.

Tribological properties of ionic liquids as lubricants and additives. Part 1: Synergistic tribofilm formation between ionic liquids and tricresyl phosphate

The use of imidazolium tetrafluoroborate, IMM+BF4−, and hexafluorophosphate, IMM+PF6−, ionic liquids as lubricants was investigated at 25 and 100 °C to show lower friction coefficients but higher wear rates than a reference hydrocarbon lubricant. The ionic liquids readily form tribofilms at the lower temperature but have difficulty in forming partial films at the higher temperature. Wear tracks for a Plint TE77 reciprocating ball-on-plate test using ionic liquids show smoother surfaces, with small pits developing, compared to the reference hydrocarbon lubricant test result. Similar ionic liquids not containing fluorine were found to be less effective as lubricants. Addition of ionic liquids to a base grease and a formulated high temperature grease gave surprisingly large increases in the weld load for the Four Ball extreme pressure test. Little difference between base grease and base grease+5 per cent ionic liquid was observed for the TE77 test. There was no clear indication of the effect of alkyl substituent chain length on the imidazolium cation for the Four Ball test wear scar diameter. Addition of 1 per cent tricresyl phosphate (TCP) to ionic liquids rapidly establishes a tribofilm and reduces the wear volume by 64 per cent compared to the same test for the neat ionic liquid or neat TCP. Addition of 1 per cent TCP and 1 per cent ionic liquid to a Group III base oil also establishes a substantial tribofilm and reduces wear volumes compared to the base oil with 1 per cent TCP alone or the base oil with 1 per cent ionic liquid alone. Ionic liquids show promise as neat liquid lubricants by establishing a tribolayer chemically adsorbed to the steel surfaces. They are not as effective as a reference hydrocarbon lubricant in reducing wear of those surfaces by tribocorrosion. The fluorine-free ionic liquids investigated were not as effective as those containing fluorine. The addition of ionic liquids to grease, base or fully formulated, gave a substantial improvement in performance, which indicates a synergistic interaction with the additives present in the formulated grease. There is also clear evidence of a strong synergistic effect between ionic liquids and TCP, both for the neat ionic liquids and for 1 per cent dilution of each respectively in a Group III base oil to give a thick tribofilm and substantially reduced wear in the TE77 ball-on-plate test. The nature of the synergy between ionic liquids and TCP requires further investigation.

Aspartic Acid-derived Wear-preventing and Friction-reducing Agents for Ionic Liquids

Abstract The first additives that improve the tribological properties of ionic liquids were designed. They were evaluated by a ball-on-flat type tribo-test under reciprocating motion. Tetraalkylammonium and tetraalkylphosphonium salts of N-protected aspartic acid was dissolved in 1-alkyl-3-methylimidzolium bis(trifluoromethylsulfonyl)imide. They prevented wear remarkably and reduced friction considerably. The salt of N-protected glutamic acid prevented wear but did not reduce friction under the conditions we have evaluated. A new model of boundary film composed of liquid clathrate structure is proposed herein.

Tribology of Ionic Liquids

The tribological properties of ionic liquids were outlined from the viewpoint of tribo-chemistry. The tribological properties of two hydrophobic anions, trifluorotris (pentafluoroethyl) phosphate (FAP) and bis (trifluoromethylsulfonyl) imide (TFSI), were compared by a pendulum type friction test. FAP salts of imidazolium possess much better lubricity than TFSI salts of imidazolium. Tribo-chemical reactions of the ionic liquids were studied by time-of-flight secondary ion mass spectroscopy. It was found that content of phosphorus on the inside wear track was higher than that on the outside wear track. The results indicate that reaction of FAP took place under the tribo-chemical conditions. Straight-chain carboxylic acids were found to improve tribological properties of the ionic liquids. Direct interaction of the acid with the rubbed surfaces was confirmed by using stable isotopic tracers.

Effect and mechanism of additives for ionic liquids as new lubricants

Ionic liquids are unique compounds, which exhibit low viscosity, non-flammability, low vapor pressure, and extremely high thermal stability. Therefore, they are expected as candidates for advanced lubricants. Several ionic liquids, derived from cations such as imidazolium, pridinium, ammonium, and anions such as BF 4 - , bis(trifluoromethylsulfonyl)imide ( TFSI - ) were examined 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. Careful analysis of worn surface revealed that tribochemical reactions of ionic liquids take place under these conditions. For example, organic fluoride, iron fluoride, iron sulfate derived from anionic moiety of the ionic liquids were detected by the instrumental surface-analysis with TOF-SIMS and XPS. Tricresylphosphate (TCP) and dibenzyldisulfide (DBDS) were found to improve anti-wear properties of ionic liquids to some extent. Interestingly, both additive-derived compounds and anionic moiety derived ones were detected on the worn surface. Moreover, additive response of ionic liquids was found to be superior to those for conventional lubricants. It was speculated that the unique tribochemical reactions will lead us to design tailor-made lubricants. Lubrication mechanism of ionic liquids is discussed from the viewpoint of tribochemistry.