Ionic Liquid Lubricants Research Articles

S1107-1-1 固化潤滑油およびグリースの超高圧レオロジー特性新評価法([S1107-1]トライボロジーにおける実験・計測・解析手法の新展開)

High-pressure rheological properties of solidified lubricant oils and greases were evaluated by an originally proposed new method, that is, the large plastic deformation analysis of metal microspheres (about 0.07 mm) in these lubricants, occurring due to non-hydrostatic pressurization of a diamond-anvil pressure cell. The solidification pressures obtained by observing aluminum sphere deformation were 0.2-0.7 GPa for traction grease and oils, 1.3-1.8 GPa for synthetic base oils and their urea or lithium greases including ionic liquid and recent aerospace lubricants. The solidification pressures were almost the same values among base oils and their greases. By substituting the solidification pressures and pressure-viscosity coefficients in the Barus equation, viscosities at the solidification pressure were obtained. The values were different for each oil and were within the order of 10^4〜10^7 Pas. Based on several assumptions, rough equivalent traction coefficients of the solidified oils could be estimated up to 4 GPa.

ChemInform Abstract: Ionic Liquid Lubricants: Designed Chemistry for Engineering Applications

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Lubrication of Steel/Steel Contacts by Choline Chloride Ionic Liquids

There is a growing interest in the use of ionic liquids to provide lubrication for challenging contacts. This study is an initial assessment of the application of two ionic liquids based on choline chloride cations to be used as ionic liquid lubricants for engineering contacts, in this case steel on steel. These ionic liquids, termed ethaline and reline, have anions of ethylene glycol and urea, respectively, and are available at relatively low costs and in high quantities. In order to assess the lubrication performance of the ionic liquids, lubricated reciprocating sliding wear tests were conducted between M2 tool steel samples and a steel stylus. Initial tests conducted at a sliding speed of 0.005 m s−1 and 30 N showed that ionic liquids could provide low friction lubrication, comparable to that of SAE 5W30 friction modifier free engine oil under the same test conditions; however, lubrication was lost after short sliding distances. Further testing with higher sliding speed/lower load and varying sample surface textures showed that ionic liquid lubrication could be better maintained in high-speed/low-load testing and by increasing the roughness and therefore surface area of the sample. It was also observed that the choline chloride/urea ionic liquid formed a residual film when tested on iron silicate peened samples, and that this film may promote lubrication.

Tribological Characterization of Several Silicon-Based Materials Under Ionic-Liquids Lubrication

The tribological behaviors of three silicon-based materials (low temperature silicon oxide (LTO), polysilicon (Poly Si) and silicon nitride (Si3N4) films) under ion liquids (ILs) lubrication have been investigated by varying the applied load and the sliding velocity. An atomic force microscope and a nanoindentor were used to characterize the deposited films, and the worn surfaces after frictional tests were analyzed by an optical microscope, a scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS) and an X-ray photoelectron spectrometer (XPS). The results show the best lubricating properties of the IL lubricants are obtained at the intermediate load of 150 g for the three tribo-pairs. For the Si3N4/Si3N4 tribo-pair, the coefficients of friction (COFs) and the wear rates under ILs lubrication are larger than those under dry friction. The COF values decrease with sliding velocity, and the COFs of the Si3N4/Si3N4 tribo-pair are greater than the ones of the other two tribo-pairs (LTO/Si3N4 and Poly Si/Si3N4) at low velocities. The COF of 1-ethyl-3-methylimidazolium tetrafluoroborate is nearly 3 times larger than the other two IL lubricants (1-butyl-2,3-dimethylimidazolium tetrafluoroborate and N-butylpyridinium tetrafluoroborate) for the Si3N4/Si3N4 tribo-pair. The differences in COFs between the latter two lubricants for the three tribo-pairs are negligible. Different mechanisms of these results have been analyzed in the paper.

Frictional Dynamics of Alkylsilane Monolayers on SiO2: Effect of 1-n-Butyl-3-methylimidazolium Nitrate as a Lubricant

The effect of 1-n-butyl-3-methylimidazolium nitrate ionic liquid (IL) on the shear dynamics and tribological properties of contacting ordered alkylsilane self-assembled monolayers (SAM) on SiO(2) surfaces was studied using molecular dynamics simulation. Use of the IL as a lubricant was found to reduce the friction between the contacting monolayers in the studied range of applied loads. At low applied loads, complex behavior in the dependency of the frictional force on the normal load in both lubricated and dry monolayers is observed and is shown to be due to the rearrangement of the SAM chains under shear. In contrast, this dependency was found to be linear at high normal loads. The simulations also indicate that the ordering of the SAM chains may be disrupted by penetrating lubricant molecules, suggesting that the IL could incorporate into a damaged area of the SAM coating and restore tribological properties. To our knowledge, this is the first simulation study of the effect of an IL lubricant on the frictional dynamics of SAMs.

Ionic liquid lubricants: designed chemistry for engineering applications

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 Liquids as Lubricants of Titanium–Steel Contact

The friction and wear behavior of grade 3 titanium have been studied against AISI 52100 steel at room temperature and at 100 °C, in the presence of six ionic liquid (IL) lubricants, four imidazolium ILs, 1-ethyl-3-methylimidazolium tetrafluoroborate (L102), 1-octyl,-3-methylimidazolium tetrafluoroborate (L108), 1-hexyl, 3-methylimidazolium hexafluorophosphate (L-P106) and 1-benzyl,3-methylimidazolium chloride (ClB), and two quaternary ammonium salts, the chloride derivative AMMOENG™ 101 (AM-101) and the dihydrogenphosphate AMMOENG™ 112 (AM-112), and compared with that of a mineral base oil. At room temperature, all ILs, except L102, give similar mean friction values, below 0.20, with a 60% reduction with respect to the mineral oil. All ILs, except L102, also reduce titanium wear rates. The poor performance of the short alkyl chain tetrafluoroborate L102 is due to tribocorrosion. The best antiwear performance at room temperature is found for the imidazolium chloride (ClB), although corrosion of the AISI 52100 steel ball is observed. At 100 °C, L-P106 maintains the room temperature friction values and shows a 80% wear rate reduction with respect to room temperature. L-108 fails at 100 °C after a sliding distance of 200 m due to decomposition and tribocorrosion. The friction and wear mechanisms and surface interactions are discussed from friction–sliding distance curves, SEM, EDS and XPS analysis, and XRD data.

Lubrication of Inconel 600 with ionic liquids at high temperature

The friction and wear behavior of Inconel 600 against AISI 52100 steel have been studied in the presence of three ionic liquid (IL) lubricants, two imidazolium derivatives, 1-methyl-3-octylimidazolium tetrafluoroborate (L108) and 1-methyl-3-hexylimidazolium hexafluorophosphate (L-P106), and the quaternary ammonium chloride AMMOENG ™101 (AM-101), and compared with a mineral base oil at room temperature. The IL lubricants have been studied at high temperature. While the ammonium derivative has been used at 100 and 200 °C, the higher thermal stability of the imidazolium derivatives allows them to be used up to 300 °C. The results show that the imidazolium derivatives are the best lubricants at all temperatures. A temperature increase from 100 to 200 °C reduces wear rates for all ILs due to a transition form abrasive to adhesive wear and formation of a tribolayer on the Inconel wear track. At 300 °C, the hexafluorophosphate derivative produces tribocorrosion attack on the steel ball and severe wear of Inconel 600 due to decomposition of the IL. The wear mechanisms and surface interactions are discussed in terms of IL-metal surface interactions from SEM, EDX and XPS data.

High-Temperature Tribological Properties of 2-Substituted Imidazolium Ionic Liquids for Si3N4-Steel Contacts

Novel ionic liquid lubricants based on monocationic and dicationic 2-substituted imidazolium ionic liquids were synthesized and evaluated as lubricants at 250 °C. Results showed that both monocationic and dicationic 2-substituted imidazolium ionic liquids with bis(trifluoromethylsulfonyl)imide anion have excellent friction-reducing and anti-wear properties at high temperature, which is ascribed to the high chemical and thermal inertness.

The tribological performance of BCN films under ionic liquids lubrication

Boron carbon nitride films were deposited onto silicon substrates by medium frequency magnetron sputtering from graphite and boron targets with Ar and N 2 as feedstock. The three elements of B, C and N were bonded to each other and an atomic-level hybridized B–C–N had been formed in the films. The tribological performances of the boron carbon nitride film with 1-butyl, 3-methylimidazolium tetrafluoroborate ionic liquid as lubricant and the electrochemical corrosive behaviors of the BCN film were investigated. The boron carbon nitride film demonstrated excellent tribological properties and corrosion resistance as compared with diamond like carbon film. An extensive discussion of the effect of film intrinsically structure on both lubrication and corrosion under ionic liquid condition is given. In addition, the interrelation between the tribological properties and corrosion resistance is illustrated.

A Comparative Study on the Tribological Behavior of Nanocrystalline Nickel and Coarse-grained Nickel Coatings under Ionic Liquid Lubrication

Tribological properties of nanocrystalline (NC) nickel coating and coarse-grained nickel coating were evaluated using a ball-on-flat sliding tester. Results indicated that the NC nickel coating had excellent tribological properties. This was partly attributed to the high hardness of the NC nickel coating and the tribochemical interactions between rubbing pairs and ionic liquid.

Ionic-Liquid Lubrication of Sliding MEMS Contacts: Comparison of AFM Liquid Cell and Device-Level Tests

Lubrication of microelectromechanical systems (MEMS) became very critical as the devices became complex and its reliability began to deteriorate. In this paper, ionic liquids (ILs) with low volatility and high environmental stability were investigated as lubricants for sliding MEMS devices. A method that is based on atomic force microscopy (AFM) with a liquid cell was developed to study friction and wear properties of surfaces lubricated with ILs, having a systematic variation in molecular geometry and chemistry. Six-member pyridinium and five-member imidazolium rings are compared as cations in ethyl methyl pyridinium and ethyl methyl imidazolium ethyl sulfate; influence of short and long alkyl chain lengths on lubrication is studied with butyl methyl pyrrolidinium and hexyl methyl pyrrolidinium bis(trifluro methyl sulfonyl) imide. Formation of a surface-screening cation layer was discovered and linked to low friction and wear of IL-lubricated hydrogenated-silicon (H-Si) substrates. Several promising IL lubricants were identified from the AFM study and were tested in real MEMS motor devices. The friction and wear data obtained for these tests showed good correlation with the failure life span of lubricated MEMS motors. This supports a conclusion that the AFM-liquid-cell technique can be used in screening IL lubricants for MEMS devices.

Influence of temperature on PA 6–steel contacts in the presence of an ionic liquid lubricant

In this paper we study the tribological properties of polyamide (PA 6) containing the ionic liquid 1-hexyl, 3-methyl imidazolium hexafluorophosphate (IL) as 3 wt.% additive dispersed in the polymer matrix. Pin-on-disc tests for neat PA 6 and for PA 6 + 3 wt.%IL blend discs have been carried out against AISI 52100 and AISI 316L steel pins at variable temperatures (−35, 25 and 67 °C). At these temperatures the relationship between the loss modulus and the storage modulus as determined by dynamic mechanical analysis (DMA) are the same for both PA 6 and PA 6 + 3%IL. Neat PA 6 shows a friction decrease both at high and low temperature with respect to room temperature which could be explained by the effect of thermal softening and low temperature hardening, respectively. In contrast, PA 6 + 3%IL maintains a low friction coefficient in the whole range of temperatures with values similar or even lower to that obtained when IL is used as external lubricant. Wear rates for both materials are negligible under the conditions used for variable temperature tests. Friction coefficients and wear rates under increasing normal loads and sliding distances are always lower for PA 6 + 3%IL. Surface interactions and wear mechanisms are discussed from SEM micrographs and EDS analysis.

Ionic liquid lubrication of electrodeposited nickel–Si 3N 4 composite coatings

Nano-Si 3N 4 particles were electrodeposited with nickel on copper substrate from a Ni bath. The friction and wear properties of the Ni/Si 3N 4 composite coating were evaluated while being lubricated with several various oils using a ball-on-disk sliding tester. The morphologies of the worn surfaces were observed using a scanning electron microscope. The chemical states of several typical elements on the worn surfaces were examined by means of X-ray photoelectron spectroscopy (XPS). Results indicated that the electrodeposited Ni/Si 3N 4 composition coating had excellent tribological properties while being lubricated with the ionic liquid. This was partly attributed to the high hardness of the electrodeposited nickel composite coating containing nano-sized Si 3N 4 and the tribochemical reaction between the lubricant and the sliding surface.

Tribological Behaviors of Amorphous Cr Coatings Electrodeposited from Cr(III) Baths under Ionic Liquid Lubrication

The amorphous chromium (a-Cr) coatings with thickness of 50 μm were electrodeposited from Cr(III) instead of Cr(VI) bath. The electrochemical and tribological behaviors of the a-Cr coatings in the ionic liquid were investigated. Possessing high hardness, steady passive film, and few grain boundary structures, the a-Cr coatings exhibit much better corrosion and wear resistance compared with Fe, Cu, and nanocrystalline Cr coatings under the ionic liquid lubrication. © 2007 The Electrochemical Society. [DOI: 10.1149/1.2746129] All rights reserved.

Surface interactions and tribochemical processes in Ionic Liquid lubrication of aluminium-steel contacts

Room-temperature Ionic Liquids (ILs) are being studied as new lubricants in a variety of sliding contacts. One of their more interesting tribological applications is that of steel-aluminium lubrication. In this work we present the surface interaction mechanisms and tribocorrosion processes which have been found in the use of ILs formed by a 1-alkyl-3-methylimidazolium cation and a fluorine-containing anion as neat lubricants of 52100 steel-2011 aluminium pairs. The results obtained by electron microscopy (SEM), Energy Disperssive Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS) techniques are discussed as functions of Ionic Liquid (IL) composition and properties.

Benzotriazole as the additive for ionic liquid lubricant: one pathway towards actual application of ionic liquids

In this paper, we report on the first tribological evaluation of the room temperature ionic liquids (RTILs) compatible lubricant additive. Benzotriazole (BTA) was chosen for study in that it shows good miscibility with imidazole ionic liquids because of similar molecular structure. BTA can greatly improve the tribological behaviors of ionic liquids carrying hexafluorophosphate anions for Steel/Cu–Sn alloy sliding pair mainly because of the alleviation of corrosion. The worn surface of the bronze was investigated by X-ray photoelectron spectroscopy (XPS), which revealed complex tribochemical reactions during the sliding process. A protective film comprised of [Cu(–C6H5N3)] and Cu2O is formed. Strong interaction between benzotriazole and the surface of Cu alloy was proposed to account for the excellent anti-wear and anti-corrosion improvement capability.

Tribological properties of plasma nitrided stainless steel against SAE52100 steel under ionic liquid lubrication condition

1Cr18Ni9Ti stainless steel was modified by plasma nitriding. The phase composition of the plasma nitrided layer was examined by means of X-ray diffraction. The friction and wear properties of the modified and unmodified 1Cr18Ni9Ti stainless steel specimens sliding against SAE52100 steel under the lubrication of ionic liquid of 1-ethyl-3-hexylimidazolium hexafluorophosphate (L-P308) and poly α-olefin (PAO) were investigated on an Optimol SRV oscillating friction and wear tester, with the interactions among the modified surface layer and the ionic liquids and PAO to be focused on. The morphologies of the worn surfaces were observed using a scanning electron microscope. The chemical states of several typical elements on the worn surfaces of the modified steel surfaces were examined by means of X-ray photoelectron spectroscopy. Results showed that the modified sample had better anti-wear abilities than the unmodified one, but the modified sample had a slightly higher friction coefficient than the untreated one. This was partly attributed to the change in the hardness and phase composition of the stainless steel surfaces after plasma nitriding and tribochemical reactions between the steel and the lubricant. The resultant surface protective films composed of various tribochemical products together with the adsorbed boundary lubricating film contributed to reduce the friction and wear.

Ionic Liquid Lubrication Effects on Ceramics in a Water Environment

Ionic liquids were studied to determine their effectiveness as boundary lubricant additives for water. The chemical and tribochemical reactions that govern their behavior were probed to understand lubrication mechanisms. Under water lubricated conditions, silicon nitride ceramics are characterized by a running-in period of high friction, during which time the surface is modified causing a dramatic decrease in friction and wear. Two mechanisms have been proposed to explain the friction and wear behavior. Si3N4 sliding against itself may result in tribochemical reactions that form a hydrated silicon oxide layer on the surface of the sliding contact. This film has been suggested to mediate friction and wear. Others have suggested that tribo-dissolution of SiO2 results in an ultra smooth surface and after a running-in period of high wear, the lubrication mode becomes hydrodynamic. The goal of this study was to examine the effects that ionic liquids have on the friction and wear properties of Si3N4, in particular their effects on the running-in period. Tribological properties were evaluated using pin-on-disk and reciprocating tribometers. The tribological conditions of the tests were selected to produce mixed/hydrodynamic lubrication. The relative lubrication mode between mixed and hydrodynamic was controlled by the initial surface roughness. Solutions containing 2 wt% ionic liquids were produced for testing purposes. Chemical analysis of the sliding surfaces was accomplished with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The test specimens were 1 in diameter Si3N4 disks sliding against 1/4 in Si3N4 balls. The addition of ionic liquids to water resulted in dramatically reduced running-in periods for silicon nitride from thousands to the hundreds of cycles. Proposed mechanisms include the formation of BFx and PFx films on the surface and creation of an electric double layer of ionic liquid.