Special Issue "Ionic Liquids: Friction and Lubrication Mechanisms"

A special issue of Lubricants (ISSN 2075-4442).

Deadline for manuscript submissions: closed (30 June 2018).

Special Issue Editor

Guest Editor
Prof. Dr. Rosa M. Espinosa-Marzal

Department of Civil and Environmental Engineering, University of Illinois at Urbana−Champaign, Urbana 61801, Illinois, USA
Website | E-Mail
Interests: friction; adhesion; lubrication; surface forces; nanotribology; tribochemistry; soft matter nanoconfinement; ionic liquids; hydrogels; surface forces apparatus

Special Issue Information

Dear Colleagues,

The significant influence of friction, lubrication, and wear on the efficiency and lifetime of machinery has been recognized since the mid-1960s, and the development of novel and better base oils and additives is one of the approaches chosen to improve efficiency and lifetime. The properties of surface-adsorbed and confined lubricant molecules play a key role in determining the performance of such boundary lubricants. Ionic liquids display novel self-assembling behavior at the solid–liquid interface, where they remain firmly surface-adsorbed under high normal and shear stresses. Such appealing properties have inspired increasing research efforts in the field of tribology of ionic liquids over the past fifteen years. However, it has emerged that ILs actually exhibit a rather large variability in their tribological behaviors. Although many works have demonstrated the lubricious properties of several ionic liquids, generalizations are not presently possible, as the molecular structure and chemical composition of the ionic liquid, as well as the substrate’s surface chemistry and topography, affect the properties of adsorbed films, and, in turn, the lubrication mechanisms and tribochemical reactivity.

This Special Issue will show current advances and future trends in ionic liquids in the field of tribology emphasizing the underlying friction and lubrication mechanisms. Contributions are welcome dealing with mechanisms under mild tribological conditions, as well as under high pressures and temperatures with the additional effects of tribochemical reactions.

Prof. Dr. Rosa Maria Espinosa-Marzal
Guest Editor

Manuscript Submission Information

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Keywords

  • ionic liquids
  • nanotribology
  • friction
  • lubrication
  • tribochemistry

Published Papers (7 papers)

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Research

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Open AccessArticle
Influence of Water on Tribolayer Growth When Lubricating Steel with a Fluorinated Phosphonium Dicyanamide Ionic Liquid
Received: 4 March 2019 / Revised: 18 March 2019 / Accepted: 19 March 2019 / Published: 22 March 2019
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Abstract
This work aims to elucidate the role of environmental humidity on the tribological behavior of steel surfaces lubricated with an ionic liquid comprised of a fluorinated phosphonium cation—tributyl-3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl-phosphonium—and a dicyanamide anion (i.e. N(CN)2). Ball-on-disk tribotests were carried out at room [...] Read more.
This work aims to elucidate the role of environmental humidity on the tribological behavior of steel surfaces lubricated with an ionic liquid comprised of a fluorinated phosphonium cation—tributyl-3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl-phosphonium—and a dicyanamide anion (i.e. N(CN)2). Ball-on-disk tribotests were carried out at room temperature and at various levels of relative humidity (RH). Water was found to be required to promote the formation of a tribofilm over the contact area. The reaction layer exhibited a patchy morphology, which resembles that observed formed with conventional antiwear additives such as ZnDTP. A surface-chemical analysis of the tribofilm indicated that the tribofilm is composed of fluorides, oxides, and phosphates, pointing to a stress-induced degradation of the ions and corrosion of the sliding counterparts, which is enabled by the presence of water at the sliding interface. Full article
(This article belongs to the Special Issue Ionic Liquids: Friction and Lubrication Mechanisms)
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Open AccessCommunication
Tribological Properties of Double-Network Gels Substituted by Ionic Liquids
Received: 9 September 2018 / Revised: 1 October 2018 / Accepted: 6 October 2018 / Published: 8 October 2018
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Abstract
Since human body joints have a gel-like structure with low friction that persists for several decades, hydrogels have attracted much interest for developing low-friction materials. However, such advantages can hardly be realized in industrial usage because water in the gel evaporates easily and [...] Read more.
Since human body joints have a gel-like structure with low friction that persists for several decades, hydrogels have attracted much interest for developing low-friction materials. However, such advantages can hardly be realized in industrial usage because water in the gel evaporates easily and the gel deswells. The substitution of water with an ionic liquid (IL) is one of the effective ways to overcome this problem. In this study, we substituted water in a double network (DN) hydrogel with 3-ethyl-1-methyl-imidazolium ethylsulfate (EMI-EtSulf), a hydrophilic IL, via a simple solvent exchange method to obtain a DN ion gel. A compressive test and thermogravimetric analysis showed that the DN ion gel has a high compression fracture stress and improved thermal properties, with the difference in 10% loss of temperature being ΔT10 = 234 °C. A friction test conducted using a reciprocating tribometer showed that the friction of a glass ball/DN ion gel was relatively higher than that of a glass ball/DN hydrogel. Because the minimum coefficient of friction (COF) value increased after substitution, the increase in polymer adhesion caused by the electrostatic shielding of the surface moieties of glass and poly 2-acrylamidomethylpropanesulfonic acid (PAMPS) was considered the main contributor to the high friction. As the COF value decreased with increasing temperature, the DN ion gel can achieve low friction via the restriction of polymer adhesion at high temperatures, which is difficult in the DN hydrogel owing to drying. Full article
(This article belongs to the Special Issue Ionic Liquids: Friction and Lubrication Mechanisms)
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Open AccessArticle
Meta-Analysis Comparing Wettability Parameters and the Effect of Wettability on Friction Coefficient in Lubrication
Received: 26 June 2018 / Revised: 30 July 2018 / Accepted: 8 August 2018 / Published: 15 August 2018
Cited by 2 | PDF Full-text (4928 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
This work presents a meta-analysis that compares the suitability of various parameters used to characterize wettability in tribological systems. It also examines the relationship between wettability and the friction factor for multiple lubricant-surface pairings. The characterization of wetting behavior was similar when using [...] Read more.
This work presents a meta-analysis that compares the suitability of various parameters used to characterize wettability in tribological systems. It also examines the relationship between wettability and the friction factor for multiple lubricant-surface pairings. The characterization of wetting behavior was similar when using the contact angle between a lubricant and surface and various dimensional and dimensionless formulations of a spreading parameter. It was possible to identify hydrodynamic, boundary, and mixed lubrication regimes by combining a dimensionless wettability parameter with the specific film thickness for a variety of neat ionic liquids and magnetorheological fluids in contact with metallic, thermoplastic, and elastic surfaces. This characterization was possible using multiple dimensionless wettability parameters, but those that can be fully determined using only the contact angle may be preferred by experimentalists. The use of dimensional and dimensionless wettability parameters that included polar and disperse components of surface tension and surface energy did not appear to provide additional insight into the wettability or frictional performance for the tribological system examined here. Full article
(This article belongs to the Special Issue Ionic Liquids: Friction and Lubrication Mechanisms)
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Open AccessArticle
Evaluation of Friction Behavior and Surface Interactions of Cyano-Based Ionic Liquids under Different Sliding Contacts and High Vacuum Condition
Received: 29 June 2018 / Revised: 2 August 2018 / Accepted: 8 August 2018 / Published: 9 August 2018
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Abstract
The friction coefficients of ionic liquids were evaluated by many investigations. Most investigations used fluorine-based ionic liquids as lubricants. However, these ionic liquids produce the corrosion wear. This investigation focuses on the use of cyano-based ionic liquids as lubricants. Compared to fluorine-based ionic [...] Read more.
The friction coefficients of ionic liquids were evaluated by many investigations. Most investigations used fluorine-based ionic liquids as lubricants. However, these ionic liquids produce the corrosion wear. This investigation focuses on the use of cyano-based ionic liquids as lubricants. Compared to fluorine-based ionic liquids, cyano-based ionic liquids exhibit high friction coefficients against steel material. This work examines how the friction coefficients of cyano-based ionic liquids are influenced by the type of sliding material used (AISI 52100, TiO2, and tetrahedral amorphous carbon). TiO2 lubricated with 1-ethyl-3-methylimidazolium tricyanomethanide, and ta-C lubricated with 1-butyl-1methylpyrrolidinium tetracyanoborate exhibited very low friction coefficients, smaller than fluorine-based ionic liquids. Time-of-Flight Secondary Ion Mass Spectrometry analysis showed that anions adsorb onto the worn surface, suggesting that anion adsorption is a critical parameter influencing friction coefficients. Quadrupole Mass Spectrometry measurements revealed that cations decompose on the nascent surface, preventing adsorption on the worn surface. These results suggest that low friction coefficients require the decomposition of cations and adsorption of anions. The reactivity of nascent surface changes with the sliding material used due to varying catalytic activity of the nascent surfaces. Full article
(This article belongs to the Special Issue Ionic Liquids: Friction and Lubrication Mechanisms)
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Open AccessArticle
Study of the Lubricating Ability of Protic Ionic Liquid on an Aluminum–Steel Contact
Received: 6 July 2018 / Revised: 22 July 2018 / Accepted: 26 July 2018 / Published: 28 July 2018
Cited by 3 | PDF Full-text (8534 KB) | HTML Full-text | XML Full-text
Abstract
Contact friction between moving components leads to severe wear and failure of engineering parts, resulting in large economic losses. The lubricating ability of the protic ionic liquid, tri-[bis(2-hydroxyethylammonium)] citrate (DCi), was studied as a neat lubricant and as an additive in a mineral [...] Read more.
Contact friction between moving components leads to severe wear and failure of engineering parts, resulting in large economic losses. The lubricating ability of the protic ionic liquid, tri-[bis(2-hydroxyethylammonium)] citrate (DCi), was studied as a neat lubricant and as an additive in a mineral oil (MO) at various sliding velocities and constant load on an aluminum–steel contact using a pin-on-disk tribometer. Tribological tests were also performed at different concentrations of DCi. When DCi was used as an additive in MO, friction coefficient and wear volume were reduced for each sliding velocity, with a maximum friction and wear reduction of 16% and 40%, respectively, when 2 wt % DCi was added to MO at a sliding velocity of 0.15 m/s. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were also applied to analyze the wear mechanism of the interface lubricated by MO and DCi as additive. Full article
(This article belongs to the Special Issue Ionic Liquids: Friction and Lubrication Mechanisms)
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Open AccessArticle
Molecular Mechanisms Underlying Lubrication by Ionic Liquids: Activated Slip and Flow
Received: 19 June 2018 / Revised: 13 July 2018 / Accepted: 17 July 2018 / Published: 20 July 2018
Cited by 2 | PDF Full-text (3364 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The present study provides molecular insight into the mechanisms underlying energy dissipation and lubrication of a smooth contact lubricated by an ionic liquid. We have performed normal and lateral force measurements with a surface forces apparatus and by colloidal probe atomic force microscopy [...] Read more.
The present study provides molecular insight into the mechanisms underlying energy dissipation and lubrication of a smooth contact lubricated by an ionic liquid. We have performed normal and lateral force measurements with a surface forces apparatus and by colloidal probe atomic force microscopy on the following model systems: 1-ethyl-3-methyl imidazolium bis-(trifluoro-methylsulfonyl) imide, in dry state and in equilibrium with ambient (humid) air; the surface was either bare mica or functionalized with a polymer brush. The velocity-dependence of the friction force reveals two different regimes of lubrication, boundary-film lubrication, with distinct characteristics for each model system, and fluid-film lubrication above a transition velocity V. The underlying mechanisms of energy dissipation are evaluated with molecular models for stress-activated slip and flow, respectively. The stress-activated slip assumes that two boundary layers (composed of ions/water strongly adsorbed to the surface) slide past each other; the dynamics of interionic interactions at the slip plane and the strength of the interaction dictate the change in friction -decreasing, increasing or remaining constant- with velocity in the boundary-film lubrication regime. Above a transition velocity V, friction monotonically increases with velocity in the three model systems. Here, multiple layers of ions slide past each other (“flow”) under a shear stress and friction depends on a shear-activation volume that is significantly affected by confinement. The proposed friction model provides a molecular perspective of the lubrication of smooth contacts by ionic liquids and allows identifying the physical parameters that control friction. Full article
(This article belongs to the Special Issue Ionic Liquids: Friction and Lubrication Mechanisms)
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Review

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Open AccessReview
Are Ionic Liquids Good Boundary Lubricants? A Molecular Perspective
Received: 18 December 2017 / Revised: 4 January 2018 / Accepted: 10 January 2018 / Published: 16 January 2018
Cited by 4 | PDF Full-text (3192 KB) | HTML Full-text | XML Full-text
Abstract
The application of ionic liquids as lubricants has attracted substantial interest over the past decade and this has produced a rich literature. The aim of this review is to summarize the main findings about frictional behavior of ionic liquids in the boundary lubrication [...] Read more.
The application of ionic liquids as lubricants has attracted substantial interest over the past decade and this has produced a rich literature. The aim of this review is to summarize the main findings about frictional behavior of ionic liquids in the boundary lubrication regime. We first recall why the unusual properties of ionic liquids make them very promising lubricants, and the molecular mechanisms at the origin of their lubricating behavior. We then point out the main challenges to be overcome in order to optimise ionic liquid lubricant performance for common applications. We finally discuss their use in the context of electroactive lubrication. Full article
(This article belongs to the Special Issue Ionic Liquids: Friction and Lubrication Mechanisms)
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