This tutorial review outlines current state of the art research on ionic liquid lubricants. Ionic liquids (ILs) were first reported as very promising high-performance lubricants in 2001 and have attracted considerable attention in the field of tribology since then because of their remarkable lubrication and anti-wear capabilities as compared with lubrication oils in general use; in recent times we have seen dramatically increased interest in the topic. The review starts with a brief introduction to ILs and fluid lubrication, and then discusses in more detail the tribological properties of IL lubricants, either as lubrication oils, additives or thin films. As well as lubrication mechanisms, some current problems and potential solutions are tentatively discussed.

Ionic liquid lubricants: designed chemistry for engineering applications

Ionic liquids (ILs) including ambient-temperature molten salts, which exist in the liquid state even at room temperature, have a long research history. However, their applications were once limited because ILs were considered as highly moisture-sensitive solvents that should be handled in a glove box. After the first synthesis of moisture-stable ILs in 1992, their unique physicochemical properties became known in all scientific fields. ILs are composed solely of ions and exhibit several specific liquid-like properties, e.g., some ILs enable dissolution of insoluble bio-related materials and the use as tailor-made lubricants in industrial applications under extreme physicochemical conditions. Hybridization of ILs and other materials provides quasi-solid materials, which can be used to fabricate highly functional devices. ILs are also used as reaction media for electrochemical and chemical synthesis of nanomaterials. In addition, the negligible vapor pressure of ILs allows the fabrication of electrochemical devices that are operated under ambient conditions, and many liquid-vacuum technologies, such as X-ray photoelectron spectroscopy (XPS) analysis of liquids, electron microscopy of liquids, and sputtering and physical vapor deposition onto liquids. In this article, we review recent studies on ILs that are employed as functional advanced materials, advanced mediums for materials production, and components for preparing highly functional materials.

New frontiers in materials science opened by ionic liquids.

Current research on room-temperature ionic liquids as lubricants is described. Ionic liquids possess excellent properties such as non-volatility, non-flammability, and thermo-oxidative stability. The potential use of ionic liquids as lubricants was first proposed in 2001 and approximately 70 articles pertaining to fundamental research on ionic liquids have been published through May 2009. A large majority of the cations examined in this area are derived from 1,3-dialkylimidazolium, with a higher alkyl group on the imidazolium cation being beneficial for good lubrication, while it reduces the thermo-oxidative stability. Hydrophobic anions provide both good lubricity and significant thermo-oxidative stability. The anions decompose through a tribochemical reaction to generate metal fluoride on the rubbed surface. Additive technology to improve lubricity is also explained. An introduction to tribology as an interdisciplinary field of lubrication is also provided.

Ionic liquids in tribology.

Due to ever increasing demands on lubricants, such as increased service intervals, reduced volumes and reduced emissions, there is a need to develop new lubricants and improved wear additives. Ionic liquids (ILs) are room temperature molten salts that have recently been shown to offer many advantages in this area. The application of ILs as lubricants in a diverse range of systems has found that these materials can show remarkable protection against wear and significantly reduce friction in the neat state. Recently, some researchers have shown that a small family of ILs can also be incorporated into non-polar base oils, replacing traditional anti-wear additives, with excellent performance of the neat IL being maintained. ILs consist of large asymmetrical ions that may readily adsorb onto a metal surface and produce a thin, protective film under boundary lubrication conditions. Under extreme pressure conditions, certain IL compounds can also react to form a protective tribofilm, in particular when fluorine, phosphorus or boron atoms are present in the constituent ions.

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A Review of Ionic Liquid Lubricants

Roles of nanoparticles in oil lubrication

Effect of Ag nanoparticles additive on the tribological behavior of multialkylated cyclopentanes (MACs)

A nickel aluminide (Ni3Al) coating on a carbon steel surface was fabricated by the self-propagating high-temperature synthesis (SHS) casting route. Coating phases were determined by using X-ray diffraction (XRD). The microstructure of the coating and bonding interface between the substrate and the coating, and the distribution of elements adjacent to the interface, were analyzed by using electron probe microanalysis (EPMA). The microhardness of the coating was measured. The tribological properties of the coating at different elevated temperatures were investigated and the worn surfaces at different temperatures were analyzed by EPMA. The elemental chemical state of the worn surface was determined by using X-ray photoelectron spectroscopy (XPS). The results show that the coating was composed of Ni3Al phase, and that its microstructure was dense and pure. Elements had diffused mutually at both sides of the interface and a metallurgical bonding interface was formed. The heat of the reaction coarsened the grain size of the substrate near the interface. The hardness of the coating is higher than that of the substrate. Wear loss and friction coefficient of the coating vary with the testing temperature. The coating had a good resistance to oxidation at elevated temperature. Oxidation of the coating occurred as a result of friction at the temperature of 873 K. The mechanisms of combustion synthesis of Ni3Al coatings on carbon steel substrates are discussed in detail. It is pointed out that the combustion temperature and the wettability between the product of the reaction and the substrate are significant to obtain the coating.

A study of Ni3Al coating on carbon steel surface via the SHS casting route

Ionic liquids (ILs) are considered as a new kind of lubricant for micro/nanoelectromechanical system (M/NEMS) due to their excellent thermal and electrical conductivity. However, so far, only few reports have investigated the tribological behavior of molecular thin films of various ILs. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers-thick film on a substrate is a critical step for their application in MEMS/NEMS devices. To this end, four kinds of ionic liquid carrying methyl, hydroxyl, nitrile, and carboxyl group were synthesized and these molecular thin films were prepared on single crystal silicon wafer by dip-coating method. Film thickness was determined by ellipsometric method. The chemical composition and morphology were characterized by the means of multi-technique X-ray photoelectron spectrometric analysis, and atomic force microscopic (AFM) analysis, respectively. The nano- and microtribological properties of the ionic liquid films were investigated. The morphologies of wear tracks of IL films were examined using a 3D non-contact interferometric microscope. The influence of temperature on friction and adhesion behavior at nanoscale, and the effect of sliding frequency and load on friction coefficient, load bearing capacity, and anti-wear durability at microscale were studied. Corresponding tribological mechanisms of IL films were investigated by AFM and ball-on-plane microtribotester. Friction reduction, adhesion resistance, and durability of IL films were dependent on their cation chemical structures, wettability, and ambient environment.

Nano/Microtribological Properties of Ultrathin Functionalized Imidazolium Wear-Resistant Ionic Liquid Films on Single Crystal Silicon

Ionic liquids (ILs) are considered as a new kind of lubricant for micro/nanoelectromechanical system (M/ NEMS) due to their excellent thermal and electrical con- ductivity. However, so far, only few reports have investigated the tribological behavior of molecular thin films of various ILs. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers- thick film on a substrate is a critical step for their appli- cation in MEMS/NEMS devices. To this end, four kinds of ionic liquid carrying methyl, hydroxyl, nitrile, and carboxyl group were synthesized and these molecular thin films were prepared on single crystal silicon wafer by dip- coating method. Film thickness was determined by elli- psometric method. The chemical composition and morphology were characterized by the means of multi- technique X-ray photoelectron spectrometric analysis, and atomic force microscopic (AFM) analysis, respectively. The nano- and microtribological properties of the ionic liquid films were investigated. The morphologies of wear tracks of IL films were examined using a 3D non-contact interferometric microscope. The influence of temperature on friction and adhesion behavior at nanoscale, and the effect of sliding frequency and load on friction coefficient, load bearing capacity, and anti-wear durability at micro- scale were studied. Corresponding tribological mechanisms of IL films were investigated by AFM and ball-on-plane microtribotester. Friction reduction, adhesion resistance, and durability of IL films were dependent on their cation chemical structures, wettability, and ambient environment.

Nano/Microtribological Properties of Ultrathin Functionalized Imidazolium Wear-Resistant Ionic Liquid Films on Single

The films of three kinds of 3-butyl-1-methylimidazolium base ionic liquids with thickness of 2 nm were prepared on hydroxyl-terminated and amino-terminated Si substrates by dip-coating method. As anions, tetrafluoroborate, hexafluorophosphate, and adipate, respectively, were chosen. The tribological performances of these thin films were examined by the determination of the film durability and friction coefficient by means of a UMT-2MT tribometer using a steel ball as counterpart. The morphologies of worn surfaces were investigated by a non-contact interferometric microscope. The findings showed that 3-butyl-1-methyl-imidazolium hexafluorophosphate having the poorest hydrophilicity of the ionic liquids exhibited the best tribological properties on aminated Si surface at 0.4 N and 4 Hz.

Mingwu Bai

Papers

Effect of Ag nanoparticles additive on the tribological behavior of multialkylated cyclopentanes (MACs)

A study of Ni3Al coating on carbon steel surface via the SHS casting route

Nano/Microtribological Properties of Ultrathin Functionalized Imidazolium Wear-Resistant Ionic Liquid Films on Single Crystal Silicon

Nano/Microtribological Properties of Ultrathin Functionalized Imidazolium Wear-Resistant Ionic Liquid Films on Single

Effect of the Anion on the Tribological Properties of Ionic Liquid Nano-Films on Surface-Modified Silicon Wafers