Effects of Ionic Liquid Additive Concentration on Scuffing and Wear in Oil-Starved EHL Contacts

Abstract

We show how various concentrations ranging from 0 to 5.0?wt% of the ionic liquid (IL) trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate added to a fully formulated aviation DOD-PRF-85734 gearbox lubricant affects the properties of the lubricant, including scuffing resistance under starved lubrication conditions, coefficient of friction (COF), wear rate, and lubricant chemical stability. Specimen surfaces were then characterized using scanning electron microscopy, energy-dispersive spectroscopy, and auger electron spectroscopy. Scuffing and wear properties were found to generally improve with IL concentration, which we hypothesize is due to the presence of a phosphorous-rich film observed on specimens exposed to IL.     

Tribological Performance of Halogen-Free Ionic Liquids in Steel–Steel and DLC–DLC Contacts

The tribological performance of halogen-free ionic liquids at steel–steel and diamond-like carbon (DLC)–DLC contacts was investigated. Hydrogenated amorphous carbon (a-C:H) and tetrahedral amorphous carbon (ta-C) were used as test specimens. Friction tests were carried out on steel–steel, a-C:H–a-C:H, and ta-C–ta-C contacts by using a reciprocating cylinder-on-disk tribotester lubricated with two different types of halogen-free ionic liquids: 1-ethyl-3-methylimidazolium dicyanamide ([BMIM][DCN]) and 1-butyl-3-methylimidazolium tricyanomethanide ([BMIM][TCC]). From the results of friction tests, the ta-C–ta-C tribopair lubricated with [BMIM][DCN] or [BMIM][TCC] exhibited an ultralow friction coefficient of 0.018–0.03. On the other hand, ultralow friction was not observed at the steel–steel and a-C:H–a-C:H contacts. Measurements obtained with a laser scanning microscope and an atomic force microscope (AFM) showed that a chemical reaction film, derived from the ionic liquid lubricant used, was formed on the steel surfaces. However, this chemical reaction film was not observed on either of the DLC surfaces. The AFM results showed that there were high-viscosity products on the ta-C surfaces, that the wear tracks on the ta-C surfaces exhibited low frictional properties, and that the ta-C surfaces were extremely smooth after the friction tests. Based on these results, it was concluded that an ionic liquid–derived adsorbed film formed on the ta-C surface and resulted in the ultralow friction when lubricated with a halogen-free ionic liquid.     

Friction property study of the surface of ZDDP and MoDTC antiwear additive films using AFM/LFM and force curve methods

The friction properties and material differences of the surface of ZDDP and MoDTC antiwear additive films, which give clear evidence of different friction coefficients in a pin-on-disc test, have been studied using atomic force microscopy (AFM)/lateral force microscopy (LFM) and force curve methods. The AFM/LFM observations show that the friction force on the surface of MoDTC additive films over the sliding area of a steel disc is lower and the friction force of ZDDP additive films is higher than that of afilmless area. Lateral force scope-trace evaluations reveal that the ratio of the friction forces on the surface of the ZDDP film, the filmless area, and the MoDTC film under the same normal force is approximately 1.5:1.0:0.7. Force curve measurements indicate that the surface materials of the ZDDP film, thefilmless area, and the MoDTC film differ according to their attractive forces, that is 29 nN for the ZDDP film, 22 nN for the filmless area, and 12 nN for the MoDTC film. These results correspond to the friction behaviour in the pin-on-disc test and also agree with the idea of the formation of solid MoS₂ lubricant from MoDTC additives on the surface of the antiwear film.     

Improvement of boundary lubrication properties of diamond-like carbon (DLC) films due to metal addition

The effects of added materials such as metals like titanium (Ti), molybdenum (Mo) and iron (Fe) diamond-like carbon (DLC) films on boundary lubrication and microtribological properties were investigated. The nanoindentation hardness and microwear resistance can be improved by adding the proper metal to DLC films, as evaluated by atomic force microscopy (AFM). Boundary lubrication properties of DLC films with metals are improved as comparing with DLC films without metal under lubricant with both MoDTC and ZDDP additives. Moreover, lower friction coefficient of μ=0.03 than carburized steel is exhibited with the appropriate quantity of Ti added. The tribochemical reactant was formed on the sliding surface of the Ti-containing DLC film like as carburized steel. Higher mechanical damping materials containing elements, such as Mo, Zn, P and S, formed tribochemical reactors as observed by X-ray photoemission spectroscopy (XPS) and AFM force modulation methods.     

Comparison of friction measurements using the atomic force microscope and the surface forces apparatus: the issue of scale

Results are presented of lateral force measurements using the atomic force microscope (AFM) and the surface forces apparatus (SFA). Two different probes are used in the AFM measurements; a sharp silicon nitride tip (radius R20 nm) and a glass ball (R15 m). The lateral force is measured between the (silicon nitride or glass) probe and a mica surface which has been coated by a thin lubricant film. In the SFA, a thin lubricant film separates two molecularly smooth mica surfaces (R1 cm) which are slid relative to each other. Perfluoropolyether (PFPE) and polydimethylsiloxane (PDMS) were used as the lubricant films. In the SFA where the contact diameter is largest, the PFPE film shows much lower friction than PDMS. As the size of the probe decreases, the difference in the measured friction decreases. For sharp AFM tips, no clear distinction between the tribological properties of the films can be made. Hence, the measured coefficient of friction varies according to the length scale probed, at least for small dimensions.     

Erratum to: Response Characteristics of the Potential-Controlled Friction of ZrO<Subscript>2</Subscript>/Stainless Steel Tribopairs in Sodium Dodecyl Sulfate Aqueous Solutions

The tribological properties of room temperature ionic liquids containing tetraalkylphosphonium cations were evaluated on the basis of the chemical structure of their salts. The tribochemistry of these ionic liquids was discussed on the basis of the results of tribo-tests and surface analyses. The tribological properties of the tetraalkylphosphonium salts examined in this work were observed to be better than those of 1,3-alkylimidazolium salts. The structure of the alkyl group in the phosphonium cation also has a slight effect on the tribological properties of the salts. During a friction test carried out under low-load conditions, the phosphonium cation was oxidized to phosphate to form a boundary film. This film inhibited the reaction of the bis(trifluoromethanesulfonyl)amide anion that yielded metal fluoride on the rubbed surfaces. The combination of the phosphonium cation with a phosphate anion or thiophosphate anion resulted in a better lubricant than 1,3-alkylimidazolium bis(trifluoromethanesulfonyl)amide. The reactions of the phosphate anion and thiophosphate anion yielded a phosphate boundary film that exhibited better tribological properties than those of the fluoride boundary film.     

Macro and microtribological studies of CrO2 video tapes

Atomic force microscopy (AFM) and its modification-friction force microscopy (FFM) are becoming increasingly important in the understanding of friction, wear, lubrication and nanomechanical property measurements. We describe modified AFM and FFM techniques and present data on microtribological studies of two CrO₂ video tapes. Macro-scale friction measurements were also made on the two tapes. We have observed that macro-scale friction is higher than that on micro-scale. Lower values of micro-scale friction as compared to macro-scale friction may be because of less ploughing contribution in micro-scale friction measurements. The directionality effects observed in micro-scale friction may come from the asymmetrical transfer of wipe material or from the calendering. Differences in the macro-scale friction in the two tapes appear to correlate with the changes in scratch resistance, wear resistance and the hardness of the tape surface on a micro-scale to nano-scales. It is demonstrated that AFM/FFM techniques are valuable in the fundamental understanding of tribology of magnetic tapes.     

Atomic Force Microscopy and Raman Spectroscopy Investigation of Additive Interactions Responsible for Anti-Wear Film Formation in a Lubricated Contact

Rational formulation of lubricants requires an understanding of additive interactions that impact antiwear film qualities such as thickness, topography, and friction. In an effort to understand the complex additive interactions responsible for formation of anti-wear and friction-reducing films, atomic force microscopy (AFM) in conjunction with Raman microscopy has been used to conduct a nanoscale investigation of the wear tracks formed by a high-frequency reciprocating rig (HFRR) in the presence of various commercial lubricant additives combinations. Of the additives examined, zinc dithiophosphate (ZnDTP)-based additives are found to be solely responsible for the formation of a thick (hundreds of nm) film that exhibits a pitted topography. Addition of a molybdenum-based friction modifier to the lubricant blend reduces the film thickness considerably and reacts to produce MoS ₂ on the surface, suggesting an interaction with the zinc dithiophosphate–based additive that prevents antiwear film formation. Formation of MoS ₂ , found only in the wear track, is consistent with a dramatic reduction of friction coefficient measured in the HFRR. Subsequent addition of borated dispersants to the lubricant reveals a further reduction in friction coefficient and a modest return of anti-wear film. However, addition of detergents to the formulation increases the friction coefficient and also promotes the formation of an anti-wear film. Nanoindentation measurements on the bulk properties of the anti-wear films determined that all of the anti-wear films had similar modulus and hardness measurements which were lower than that of the parent steel material, but did not correlate with the friction measurements obtained from the HFRR. This indicates that nanoscale measurements on material properties of the film are necessary to elucidate friction properties of the interface, and that these properties cannot be determined from macroscale measurements on the bulk film.

     

硅基微机械表面粘附及摩擦性能的AFM试验研究

在Si（100）基片上制备了十八烷基三氯硅烷（OTS）分子润滑膜，并用原子力显微镜（AFM）对比研究了施加OTS膜前后的硅表面的粘附、摩擦磨损性能。试验考虑了相对湿度和扫描速度对粘附、摩擦性能的影响。结果表明，相对于硅构件来讲，OTS膜表面粘附力较小，具有较小的摩擦因数，呈现较好的润滑性能；硅构件受湿度变化的影响比OTS膜明显。微构件的摩擦性能由于水合化学作用生成Si（OH）。润滑膜，使得其受相互间运动速度影响很大。OTS膜不仅是一种耐磨性较好的润滑膜，而且有良好的稳定性。     

FTIR spectroscopy and nanotribological comparative studies: influence of the adsorbed water layers on the tribological behaviour

Under ambient conditions, a water film is always present on a silica substrate and generates additional capillary forces between the nanotip and the studied surface. In the present paper, we report AFM measurements of pull-off and friction forces as a function of the temperature and a comparative FTIR spectroscopy study. The AFM results show a net decrease of the forces as the temperature increases, while the IR spectroscopy indicates that the liquid film is removed at high temperature. Consequently, we deduce that a liquid neck is created between the tip and the surface and that the forces measured are mostly capillary forces. The present work shows that temperature studies with AFM can be a useful way to probe the influence of the capillary force in turn to characterize surface properties.     

Surface chemistry of fluorine containing ionic liquids on steel substrates at elevated temperature using Mössbauer spectroscopy

Ionic liquids have properties that make them attractive as solvents for many chemical synthesis and catalysis reactions. Consequently, research has focused on their application as advanced solvents. Recently, ionic liquids were shown to have promise as a lubricant due to many of the same properties that make them useful as solvents. The focus of this paper is to study the surface chemistry of ionic liquid lubricated steel in sliding contact to temperatures from room to 300 °C. Tribological properties were evaluated using a pin on disk tribometer with high temperature capability (up to 800 °C). Chemistry was studied using Mössbauer spectroscopy and X-ray photoelectron spectroscopy. Samples used for tribological evaluation were 1 inch diameter polished M50 disks. Samples used for studying the surface chemistry were enriched ⁵⁷Fe grown via thermal evaporation. Some ⁵⁷Fe samples were oxidized to Fe₂O₃ and Fe₃O₄ prior to treatment with ionic liquids. The metallic and oxidized ⁵⁷Fe samples were then reacted with ionic liquids at elevated temperatures. Three ionic liquids were used in this study; 1-n-ethyl-3-methylimidazolium tetrafluoroborate (BF₄), 1,2-di-methyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide (TFMS), and 1,2-di-methyl-3-butylimidazolium hexafluorophosphate (PF₆). This study was focused on understanding the high temperature stability of the liquids in contact with metal and under tribological stress. Therefore, the friction data was collected in the boundary (or mixed boundary/EHL) lubrication region to enhance surface contact. BF₄ provided a friction coefficient of 0.04 for both the room and 100 °C tests and varied between 0.07 and 0.2 for the 300 °C test. The results from TFMS lubrication showed a friction coefficient of 0.025 at room temperature and 0.1 at 100 °C. The 300 °C test friction coefficient ranged between 0.1 and 0.3. Chemical analysis of the surface revealed corrosion of the surface due to reaction between the ionic liquids and steel/iron substrates.     

Tribological behaviors of composite molecular deposition films

Polyallylamine hydrochloride (PAH)/graphite oxides (GO) ultrathin film and the multilayer film of PAH incorporated with TiO₂ enwrapped by polyacrylic acid (PAA), namely PAH/PAA(TiO₂) composite film, were prepared by molecular deposition (MD) method in laboratory. Both of them were heated to change the film forming dynamic force from electrostatic force to covalent bond so as to increase the bonding strength of the films. The structure and nanotribological properties of the films were analyzed by atomic force microscope (AFM) and ultraviolet-visible (UV) spectroscopy. It was found that these films had a much smaller friction force than their substrates and the friction force was dependent on the morphology and/or hardness of the films.     

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 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.     

Studies on Degradation Mechanisms of Tape Lubricants in a High Vacuum Environment

Degradation and tribological performance of several tape lubricants were studied during sliding in a high vacuum environment. Gaseous products generated from the head–tape interface and friction force were detected and monitored as function of sliding distance by using a quadrupole mass spectrometer and strain gauges. Chemical analyses of the commercial tape lubricants were done to better understand the degradation processes and degradation products in the sliding tests. Experimental results showed that the commercial metal evaporated (ME) tape lubricants include fatty acid and perfluoropolyether (PFPE) compounds. The degradation products from the commercial tape include small fluorine containing fragments from the PFPE lubricant and small hydrocarbon fragment from fatty acid. In addition, three other species, H₂, C₂H₅ and CO₂ were detected in the case of ME tapes and they were generated from the degradation of the diamond-like carbon (DLC) coating on the tape surface. The commercial metal-particle (MP) tape used fatty acid and fatty acid ester lubricants. Only hydrocarbon fragments were detected in the MP tapes' sliding tests, no H₂, C₂H₅ and CO₂ were found. The coefficient of friction of these two tapes increased with sliding and depletion of lubricant on the tape surface. The trend of the coefficient of friction was different for ME and MP tapes because of the differences in the lubricant and lubricant distribution on these two tape surfaces. Z-Dol and stearic acid were used to lubricate ME tapes, their test results were analyzed and compared with the commercial tapes' test results.     

Effect of tetraalkylphosphonium based ionic liquids as lubricants on the tribological performance of a steel-on-steel system

A series of asymmetrical tetraalkylphosphonium ionic liquids were synthesized and evaluated as a new kind of lubricant for the contact of steel/steel using an Optimol SRV oscillating friction and wear tester in ambient condition. The phosphonium ionic liquid shows excellent tribological performance when being used as the lubricating oil, and is superior to the conventional high temperature lubricants X-1P and perfluoropolyether (PFPE) in terms of anti-wear performance and load-carrying capacity. The chemical compositions of the boundary film generated on different contact surfaces were analyzed on a scanning electron microscope with a Kevex energy dispersive X-ray analyzer attachment (SEM/EDS) and X-ray photoelectron spectrometer (XPS). The friction–reduction and anti-wear mechanism of tetraalkylphosphonium as the lubricant were proposed to originate from the active elements P in the tetraalkylphosphonium ionic liquids reacting with the fresh surface to form a reaction film onto specimen surface, an extreme-pressure film with lower shearing strength, which leads to lower friction coefficient, and good wear resistance.     

C60自组装单分子膜的制备及其磨擦特性

利用胺基与C60分子的加成反应,在3-胺基丙基-三乙氧基硅烷(APS)的自组装单分子膜(SAMs)表面上成功的制备了与基底化学键结合的C60-SAMs.其表面水接触角约为76°,膜厚约为1.15 nm,AFM形貌像显示其表面光滑、均匀,基本不含缺陷.摩擦学结果表明,APS自组装单分子膜由于其分子链短,膜的有序性差,表面颗粒聚集物及"针孔"等缺陷多,而不具有润滑作用.当在其上形成C60单分子层膜后,表现出优异的摩擦学性能,摩擦系数约为0.09～0.13,在给定实验条件下抗磨损寿命大于10 000次,有望作为微型机械的边界润滑材料使用.     

Influence of temperature on tribological behaviour of ionic liquids as lubricants and lubricant additives

Ionic liquids are low‐melting‐point salts composed entirely of ions, and many of them are liquids at room temperature. In recent years, studies have indicated that they might be good candidates for lubricants, either in neat or additive form. In this work, a sulfate‐based ionic liquid with a pyrollidinium cation was studied as neat lubricant and as additive for glycerol in lubrication of steel–steel contacts. Glycerol was chosen as the base oil because of its high polarity, which allows full miscibility with polar ionic liquids. Tests were performed on an oscillating friction and wear tribometer. The coefficients of friction and wear were measured. The tests were run at room temperature, 50 °C, 100 °C and 150 °C. By using profilometry, optical microscopy, scanning electron microscope and atomic force microscopy (AFM) analyses, it was shown that the ionic liquid plays an important role in the friction and wear reduction, as well as in the smoothening of the worn surface. Copyright © 2013 John Wiley & Sons, Ltd.     

Friction properties of perfluorinated polyethers for hot‐dipped tin low‐level separable electrical contacts

Tin coatings are used as a final protection for cuprous substrates involved in low‐level electrical contacts. High friction forces and fretting phenomena remain a cause of electrical failure of connector terminals, and so should be minimised. A lubricant layer can improve the life and reliability of connector terminals but such a layer must ensure low and stable resistance values even under severe temperature conditions. Work has been undertaken to investigate the lubricant layer most suitable for the protection of tinned electrical contacts. Perfluorinated polyethers (PFPEs) were chosen for their inertness. The aim was to correlate various physico‐chemical properties of branched or linear PFPEs, with their frictional and electrical properties. The long‐term electrical properties of a contact are shown to be linked to the initial wear mode in the contact.     

Tribological Analysis of a Hydrodynamic Journal Bearing under the Influence of Synthetic and Biolubricants

This study investigated the tribological characteristics of journal bearings exclusively for automotive applications under the influence of a synthetic lubricant (SAE20W40) and chemically modified rapeseed oil (CMRO) as a biolubricant, dispersed with TiO₂, WS₂, and CuO nanoparticles used as antiwear additive. The effects of synthetic and nanobased biolubricants on the tribological behavior of the hydrodynamic journal bearing were examined using a journal bearing test rig by measuring the coefficient of friction, oil film thickness, and wear under a load of 10 kN and a speed of 3,000 rpm. The test results show that CuO nanoadditives that are added to the biolubricant exhibit outstanding wear and friction reduction behavior, better than that with synthetic lubricants as well as other nanobased biolubricants. The inclusion of CuO nanoparticles in the biolubricant decreased the coefficient of friction by 27% and wear by about 47% compared to a synthetic lubricant. Additionally, investigations were performed using atomic force microscopy (AFM) and scanning electron microscopy (SEM) to study the surface morphology and surface roughness behavior of the tested bearing surfaces.     

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.