Tribological properties of alkyl and hydroxyl groups on the imidazolium tetrafluoroborate ionic liquids

We present an investigation of effect of alkyl chain length and hydroxyl group functionalisation on the tribological properties of imidazolium tetrafluoroborate ionic liquids. Four kinds of hydroxyl‐functionalised ionic liquids with the alkyl chain of C₁–C₁₀ were synthesised, and the relationships between their structure and physicochemical properties such as viscosity, thermal stability and corrosion, before and after incorporation of a hydroxyl group, were measured. In particular, the studies on their lubrication properties as ionic liquid (IL) lubricants for steel–steel contacts, including the friction coefficient, the wear volume and so on, were particularly emphasised. Moreover, an in‐depth exploration about the function mechanism and failure mode of the ILs during the friction process were studied using the scanning electron microscope and X‐ray photoelectron spectroscope technologies, and a proposed interaction model between ILs and steel substrate was presented. Copyright © 2012 John Wiley & Sons, Ltd.     

Lubrication Mechanism of Phosphonium Phosphate Ionic Liquid Additive in Alkylborane–Imidazole Complexes

The assessment of ionic liquids (ILs) as lubricants in several tribological systems has shown their ability to provide remarkable reduced friction and protection against wear, whether they are used as additives or in the neat form. However, their corrosion and limited solubility in non-polar hydrocarbon oils represent the bottleneck-limiting factors for the use of ILs as lubricants. Therefore, in order to tackle these problems, mixtures of alkylborane–imidazole complexes with one halogen-free IL as additive were used in this study. The knowledge of the additive–surface interactions and hence the understanding of tribological properties are an important issue for lubricant formulations and were also investigated in this work. Thus, combination effects between two ionic liquid additives, a halogenated and a halogen-free one, were evaluated by a ball-on-disc-type tribometer under boundary lubrication conditions. Effective friction reduction and anti-wear properties have been demonstrated in tribological investigations when adding between 0.7 and 3.4 wt% of the halogen-free IL into base fluid composed of alkylborane–imidazole complexes. X-ray photoelectron spectroscopy analyses of the steel specimens were conducted to study the correlation between tribological properties and chemical surface composition of the boundary films formed on the rubbing surface. This work suggests potential applications for using halogen-free ILs as additives for synthetic ionic liquid lubricants.     

Molecular interaction originating from polar functional group in lubricants and its relationship with their traction property under elastohydrodynamic lubrication

The correlation between molecular interaction and traction properties was investigated using a traction tester and in situ observation of elastohydrodynamic lubrication film with a micro‐Fourier transform infrared spectrometer. The sample oils used were polypropylene glycols (PPGs) with the end‐group of alcohol or ether and a synthetic hydrocarbon oil, poly‐α‐olefin. From the traction tests, it was found that the traction coefficient of PPG was sensitive to the end‐group. PPG with alcohol as the end‐group showed a higher traction coefficient than that with the ether group. In situ observation with a micro‐Fourier transform infrared was performed in order to investigate the molecular interaction of the lubricant oil. It was found that the hydrogen bonding of hydroxyl groups in PPG was strengthened by high pressure in the Hertzian contact region. These results suggest that the rheological properties in the elastohydrodynamic lubrication contact region were affected by the strengthened hydrogen bonding. Copyright © 2014 John Wiley & Sons, Ltd.     

Conductive and Tribological Properties of Lithium-Based Ionic Liquids as Grease Base Oil

This article reports several conductive greases prepared by ionic liquids (ILs) that are synthesized by mixing lithium tetrafluoroborate (LiBF₄) or lithium bis(trifluoromethane-sulfonyl) imide (LiNTf₂) in diglyme (G2) and tetraglyme (G4) with appropriate weight ratios at room temperature (RT). The ILs have good solution in poly(ethylene glycol-ran-propylene glycol) monobutyl ether (PAG) and thus can be used as a base oil for preparing grease for steel–steel contacts. The electrical conductive properties of the grease prepared with the mixed oil of PAG plus ILs were evaluated using the DDSJ-308A conductivity tester, GEST-121 volume surface resistance tester, and HLY-200A circuit resistance tester. Combining the free volume with viscosity, the conductivity is inversely proportional to viscosity. The tribological properties were investigated using an MFT-R4000 reciprocating friction and wear tester. The results demonstrated that the prepared greases possess better conductive and tribological properties than the commercial grease with Cu powder as an additive.     

Engine oil compositions based on PIB—succinimide combinations

Additive packages for engine oils were prepared using PIB—succinimide combinations with different properties. PIB—succinimide‐type dispersants of various average molecular weights and dominant side effects were selected for preparing multifunctional additive combinations. A high‐molecular‐weight PIB—polysuccinimide type as viscosity‐index improver, a PIB—polysuccinimide with strong basic character as a highly effective sludge dispersant, molybdenum‐containing poly‐alkylene—polysuccinic anhydride derivatives as dispersants with complementary antifriction efficiency, and some conventional detergent, antioxidant, antiwear, and anticorrosion engine oil additives were used in engine oil compositions of various viscosity grades. The technical and economic advantages of appropriately selected and balanced additive combinations were shown by bench test results of the rheological, detergent—dispersant, and antifriction properties of the test oils.     

Excellent lubrication performance and superior corrosion resistance of vinyl functionalized ionic liquid lubricants at elevated temperature

In order to improve the lubrication performance and inhibit the serious corrosivity of conventional ionic liquids (ILs) at elevated temperatures, a series of vinyl functionalized ILs were synthesized in this work. The corrosion behavior of the ILs was evaluated with copper sheet corrosion test and their tribological properties were investigated on an Optimol SRV-IV oscillating friction and wear tester at elevated temperatures. The results showed that ILs with vinyl group, such as 1-vinyl-3-butyl imidazolium tetrafluoroborate (VBImBF₄), can reduce corrosion effectively and its extreme pressure reached up to 1500 N at 150 °C. Based on the XPS analysis, ILs with vinyl group could interact with the iron surface and a protecting layer would form on the surface of steel possibly. Thus, ILs lubricants with good lubricating performance and low corrosivity at elevated temperature were achieved.     

Glasslike Transitions in Thin Polymer-Melt Films Due to Thickness Constraint

The shear properties of thin films of star and linear polyisoprene (PIP) melts under high pressure were investigated as a function of sliding velocity (shear rate) using the surface forces apparatus. The results were contrasted with their bulk rheological properties; effects of thickness constraint on the shear behavior were discussed. The melts of PIP in bulk exhibit Newtonian-like constant viscosity at least at low shear rates (frequencies), which suggests that individual molecules flow with lateral sliding motion. However, thin films of PIP melts show tribological features involving apparent shear-thinning behavior, indicative of the correlated motions in confined geometries. The shear-property change from bulk rheological behavior to thin-film tribological behavior along with the thickness decrease reflects the physical states and their transitions in the systems; the thickness constraint induces glasslike transitions. Effects of molecular weights and molecular architecture (star-branched or linear) on the shear properties are also discussed.     

Ionic Liquids as Novel Lubricants and Additives for Diesel Engine Applications

The lubricating properties of two ionic liquids (ILs) with the same anion but different cations, one ammonium IL [C₈H₁₇]₃NH.Tf₂N and one imidazolium IL C₁₀mim.Tf₂N, were evaluated both in neat form and as oil additives. Experiments were conducted using a standardized reciprocating sliding test with a segment of a Cr-plated diesel engine piston ring against a gray cast iron flat specimen. The cast iron surface was prepared with simulated honing marks as on a typical internal combustion engine cylinder liner. The selected ILs were benchmarked against conventional hydrocarbon oils. Substantial friction and wear reductions, up to 55% and 34%, respectively, were achieved for the neat ILs compared to a fully formulated 15W40 engine oil. Adding 5 vol% ILs into mineral oil has demonstrated significant improvement in the lubricity. One blend even outperformed the 15W40 engine oil with 9% lower friction and 34% less wear. Lubrication regime modeling, worn surface morphology examination, and surface chemical analysis were conducted to help understand the lubricating mechanisms for ILs. Results suggest great potential for using ionic liquids as base lubricants or lubricant additives for diesel engine applications.     

Assessment of green cleaning effectiveness on polychrome surfaces by MALDI‐TOF mass spectrometry and microscopic imaging

This article proposes an innovative methodology which employs nondestructive techniques to assess the effectiveness of new formulations based on ionic liquids, as alternative solvents for enzymes (proteases), for the removal of proteinaceous materials from painted surfaces during restoration treatments. Ionic liquids (ILs), also known as “designer” solvents, because of their peculiar properties which can be adjusted by selecting different cation‐anion combinations, are potentially green solvents due totheir low vapour pressure. In this study, two ionic liquids were selected: IL1 (1‐butyl‐3‐methylimidazolium tetrafluoroborate ([BMIM][BF₄])) and IL2 (1‐ethyl‐3‐methylimidazolium ethylsulphate ([EMIM][EtSO₄])). New formulations were prepared with these ILs and two different proteases (E): one acid (E1—pepsin) and one alkaline (E2—obtained from Aspergillus sojae). These formulations were tested on tempera and oil mock‐up samples, prepared in accordance with historically documented recipes, and covered with two different types of protein‐based varnishes (egg white and isinglass—fish glue). A noninvasive multiscale imaging methodology was applied before and after the treatment to evaluate the cleaning's effectiveness. Different microscopic techniques—optical microscopy (OM) with visible and fluorescent light, scanning electron microscopy (SEM) and atomic force microscopy (AFM)—together with Matrix‐Assisted Laser Desorption/Ionization—Time of Flight Mass Spectrometry (MALDI‐TOF MS) were applied on areas cleaned with the new formulations (IL + E) and reference areas cleaned only with the commercial enzyme formulations (gels). MALDI‐TOF proved particularly very useful for comparing the diversity and abundance of peptides released by using different enzymatic systems. Microsc. Res. Tech. 77:574–585, 2014. © 2014 Wiley Periodicals, Inc.     

Ionic Liquid Lubricants: Basics and Applications

Interest in the tribological performance of ionic liquids (ILs) has increased significantly since they were first introduced as lubricants in 2001. The primary advantages of ILs over conventional lubricants lie in their better ability to form tribofilms, higher thermal stability, environmental friendliness, and adaptability to various applications. A remarkable reduction in friction and wear has been observed after the addition of ILs in oil- or water-based media and in grease, suggesting that ILs are promising candidate materials as neat lubricants as well as lubricant additives. Despite the relatively common utilization of ILs as lubricating media, their wider use is limited by their high cost and corrosive properties. This article provides a brief introduction to relevant IL structures and properties, focusing on recent applications of the materials in engineering tribology.     

New correlations for better monitoring the all-oil mud rheology by employing artificial neural networks

The rheological properties of the drilling fluid are crucial to the success of the drilling project. The traditional mud experiments normally performed by the mud engineers provide rheological data with a small resolution. Monitoring higher-resolution rheological properties is particularly important for all-oil mud because it is widely used with problematic drilled formations. The design and monitoring of the drilling fluid rheology is a critical issue for drilling, and therefore, this paper is a contribution to the effort to completely automate the process of highly accurate and real-time recording of the rheological mud properties. This paper aims to develop intelligent predictive models for the mud rheological properties using artificial neural networks [ANN] by linking the high-frequency mud parameters such as fluid density or mud weight [MWT] and Marsh funnel viscosity [MFV] with the rheological measurements of low frequency for drilling mud such as plastic viscosity [PV], yield point [YP], behavior indicator [n] and viscosity appearance [AV]. New empirical correlations have additionally been established to assess the rheological properties of water. In order to construct ANN models, data was obtained from 56 different wells during drilling operations of different drilling sections with various sizes. The data was fairly enough for building and testing the models as 369 data points were obtained. The models were optimized by trainlm which was the best training function and tansig was the best transfer function. 42 neurons in the hidden layer optimized AV and PV models where 34 neurons optimized all other rheological models [YP, n, R300, and R600]. ANN models presented good results as correlation coefficient [R] was 0.9 and an average absolute [AAPE] error of less than 8% for training and testing data sets. The new models were used to derive the empirical correlations for the estimation of rheological parameters. The empirical correlations were extracted to easily monitor the rheological properties of an all-oil mud system in real-time, which enables better control of the drilling activity by maintaining rheological properties at optimal values as well as early detection of other problems that might require immediate interactions, including well control and stuck pipe.     

Alkyl Imidazolium Ionic Liquids as Friction Reduction and Anti-Wear Additive in Polyurea Grease for Steel/Steel Contacts

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.     

Lubrication of DLC Coatings with Two Tris(pentafluoroethyl)trifluorophosphate Anion-Based Ionic Liquids

The lubrication of a Cr-DLC coating with ethyl-dimethyl-2-methoxyethylammonium tris(pentafluoroethyl)trifluoropho-sphate [(NEMM)MOE][FAP] and 1-butyl-1-methylpyrro-lidinium tris(pentafluoroethyl)trifluorophosphate [BMP] [FAP] ionic liquids (ILs) as 1 wt% additives to a polyalphaolefin (PAO 6) was studied. Zinc dialkyldithiophosphate (ZDDP) was also used as reference in order to evaluate the effectiveness of the ILs. Reciprocating ball-on-plate tribological tests at loads of 20 and 40 N were performed. The results showed that both ILs exhibited a friction reduction, especially at the lowest load tested. Antiwear properties were also improved; the PAO 6 + 1% [BMP][FAP] mixture was slightly better, close to the values for PAO 6 + 1% ZDDP. Scanning electron microscopy (SEM) images and X-ray photoelectron spectroscopy (XPS) analysis indicated that the additive–surface interaction was responsible for the tribological improvement.     

Tribological Testing and Thermal Analysis of an Alkyl Sulfate Series of Ionic Liquids for Use as Aerospace Lubricants

Due to their low vapor pressures, low melting points, high boiling points, high radiation resistance, and high thermal stability, room-temperature ionic liquids (ILs) appear to be suitable candidates as new aerospace lubricants for the upcoming return to the Moon and eventual Mars missions and for air and rotorcraft applications. In this study, three ILs with the same cation, 1-butyl-3-methylimidazolium, but different sulfate anions were tested using an ultra-high vacuum spiral orbit tribometer (SOT) and their thermal properties were determined by thermogravimetric analysis (TGA). Specifically, 1-butyl-3-methylimidazolium methyl sulfate, 1-butyl-3-methylimidazolium ethyl sulfate, and 1-butyl-3-methylimidazolium octyl sulfate were tested. The SOT experiments revealed that the lifetimes of the three ILs decreased with increasing alkyl substituent length on the sulfate anion. Infrared and Raman spectra were taken to detect unused ILs and graphitic degradation products, respectively, on worn parts. Post-run spectroscopic analysis indicated residual degraded, but still usable, ILs in all runs, coupled with varying amounts of amorphous graphitic material produced as the final degradation product of all ILs. SOT testing indicated that these ILs have lower friction coefficients and lifetimes greater than those of two commonly used perfluoropolyalkylether (PFPE) space lubricants. TGA showed that the methyl sulfate IL had the highest thermal stability in air and nitrogen. The vapor pressure of the methyl sulfate IL is as at least as low as Fomblin 815Z at 20°C.     

Friction and Wear Behavior of CF/PTFE Composites Lubricated by Choline Chloride Ionic Liquids

The use of ionic liquids (ILs) as lubricants has received increasing attention in recent years. The use of ILs, however, is limited by the corrosion problem and their potential toxic property. Here we present the results of our initial study on the tribological properties of carbon fiber (CF)-filled polytetrafluoroethylene (PTFE) composites, which have an excellent chemical resistance property, lubricated by choline chloride ILs. The difference between choline chloride ILs and water and hydraulic oil as lubricants was studied at the same time, as was the effect of the anion on the lubricating property of choline chloride ILs. The worn surface and transfer film of CF/PTFE composites were studied by scanning electron microscopy. Our results indicate that the lubricating property of choline chloride ILs is much better than that of water and hydraulic oil. The friction coefficient and wear rate of CF/PTFE composites lubricated with ILs were approximately 60 and 50 % lower than those under the dry friction condition. Among the three kinds of ILs tested, the best tribological properties of the CF/PTFE composites were found for those sliding in the mixture of 1,2-propanediol and choline chloride. The worn surface and transfer film of CF/PTFE composites were also much smoother than those under the dry friction, water lubrication, and hydraulic oil lubrication conditions.     

Computer modeling of rheological properties of base oil compositions with selected additives

An analysis of the rheological properties of various oils and their formulations with selected rheological additives (Visconyl-200 and ECA-6911) was performed. Base oils (OS and OU) obtained on a large laboratory scale were the primary subject of the investigation. They were produced according to classical and modified base oil technologies. The modification introduced an additional process of mild oxidation of atmospheric residue before vacuum distillation. Other oils, such as aromatic and saturated hydrocarbons separated from the OS and OU oils, as well as oil fractions (1S–5S and 1U-5U) obtained from the base oils by vacuum distillation, have also been investigated. Using the rheological properties determined for the distillate oil fractions formulated with the Visconyl and ECA additives, the empirical formulae describing the dependence of kinematic viscosity v_{50, c} and v_{100, c} and of the pour point of the compositions, on the basic physico-chemical properties of the base oils, and on the type and concentration of additive, have been found. The theoretical viscosity index values of the oil compositions were also characterised indirectly by using the empirical formulae describing kinematic viscosities of the compositions at 50 and 100°C. The formulae obtained were successfully tested with several selected base oils.     

Moisture and gamma‐ray irradiation effects on the mechanical properties of carbon fibre–reinforced plastics

The effects of γ‐irradiation and moisture absorption on the mechanical properties of carbon fibres–epoxy resin composites were studied. The properties dominated by the matrix and fibre–matrix interface (interlaminar and in‐plane shear strength) were measured at room temperature using standard tests. These tests were carried out before and after exposures to gamma irradiation and before and after immersion in water at 80°C during 21 days. The dosage of gamma irradiation was up to 11.7 MGy. The micrographs of surfaces fractured in performed tests were observed on a scanning electron microscope. They were analyzed with consulting the stated effects on mechanical properties and the measured values of the glass transition temperature of tested coupons before and after irradiation and immersion in water. The obtained results show that moisture and irradiation, if they act one after the other, have a significant influence on the degradation of matrix‐dominated mechanical properties of the tested carbon–epoxy composite.     

Study of the Conductivity and Tribological Performance of Ionic Liquid and Lithium Greases

Ionic liquid (IL) lubricating greases were prepared using 1-hexyl-3-methylimidazolium tetrafluoroborate and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide as base oil and polytetrafluoroethylene (PTFE) as thickener, respectively. Three kinds of lithium greases were also prepared using lithium ILs ([Li(PAG)]X) as base oil and PTFE as thickener. 1-Ethyl-3-methyl imidazolium hexafluorophosphate as an additive was added to the PAG grease, which was prepared using polyalkylene glycol monobutyl ether (PAG) as base oil and PTFE as thickener. The conductivities and tribological properties of the prepared lubricating greases were investigated in detail. Scanning electron microscopy and X-ray photoelectron spectroscopy were employed to explore the friction and wear mechanism. The results showed that the IL and lithium lubricating greases have conductivities and excellent tribological properties. Especially, IL greases have the highest conductivity. The excellent tribological properties are attributed to the formation of boundary films consisting of both tribo-chemical reaction films and physical absorption films, while high conductivities are attributed to the intrinsic electric fields of the ILs.     

The Influences of Methyl Group at C2 Position in Imidazolium Ring on Tribological Properties

Tribological properties of two kinds of imidazolium ionic liquids (ILs) of 1,2-dimethyl-3-hexylimidazolium bis(trifluoromethylsulfonyl)imide (L-F116) and 1-dimethyl-3-hexylimidazolium bis(trifluoromethylsulfonyl)imide (L-F106) were evaluated as lubricant additives in poly(ethylene glycol) (PEG) for the steel–steel sliding pair by using an Optimol-SRV oscillating friction and wear test at the room temperature. At the same time, their electrochemical corrosion behaviors were measured by Tafel polarization. The morphologies of the worn surfaces were observed using a scanning electron microscope (SEM). The chemical states of several typical elements on the worn surfaces were examined by means of X-ray photoelectron spectroscopy (XPS). The results show that corrosion phenomena of PEG containing ILs on pure copper are negligible compared to PEG at room temperature. The 2-substituted imidazolium IL L-F116 shows excellent tribological performance and is superior to L-F106 in terms of anti-wear performance and load-carrying capacity. The worn surfaces were characterized to have slight abrasion and the XPS results indicated the formation of tribochemical adsorption and chemical reaction films on the worn surfaces.