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.     

Effect of Imidazolium Ionic Liquid Additives on Lubrication Performance of Propylene Carbonate under Different Electrical Potentials

Three different ionic liquids (ILs), 1-octyl-3-methylimidazolium tetrafluoroborate ([OMIm]BF₄), 1-octyl-3-methylimidazolium hexafluorophosphate ([OMIm]PF₆) and 1-decyl-3-methylimidazolium hexafluorophosphate ([DMIm]PF₆), were used as additives in the base ester propylene carbonate (PC) for the lubrication of AISI 4340 steel surfaces. Ball-on-disk friction tests were done under different electrical potentials to investigate the synergetic effect of IL concentration and electrical potential on lubrication performance, and electrochemical and ellipsometric tests were conducted to explore the adsorption of IL additives at different potentials. The friction reduction and anti-wear performance of the tested three IL/PC solutions illustrated similar dependence on electrical potential. In the potential range from ?0.6 to +0.6 V, friction coefficient increases rapidly. When the potential is more negative than ?0.6 V, friction coefficient is at the lower level of about 0.13. When the potential is greater than +0.6 V, friction coefficient is at the higher level of about 0.2. The electrochemical test results show that [DMIm]PF₆/PC solution is the lowest in corrosion against AISI 4340 steel among the three tested lubricants. The wear of steel surface in 0.5 mM [DMIm]PF₆/PC solution is reduced when electrical potential is shifted to ?1.0 V comparing with that at open-circuit potential. The potential-dependent friction and wear behaviors are explained in terms of the variation of the adsorbed ion species and the surface concentration of the adsorbed ions under different additive concentration and electrical potential conditions.     

Tribological Interaction of Plasma-Functionalized Polytetrafluoroethylene Nanoparticles with ZDDP and Ionic Liquids

Polytetrafluoroethylene (PTFE) nanoparticles were coated with consecutive plasma deposited siliceous and methacrylate coatings. Secondary zinc dialkyldithiophosphate (ZDDP), phosphonium cation and phosphate anion ionic liquid (IL), and IL with phosphonium cation and dithiophosphate anion were mixed with the functionalized nanoparticles. Tribological studies were carried out for seven separate formulations including base oil, oils with only additives, and oils with additives and functionalized PTFE particles. Results indicate strong synergistic interactions of ZDDP and ILs with functionalized nanoparticles providing enhanced friction and wear performance. Chemical analysis of the tribofilms using X-ray photoelectron spectroscopy and X-ray absorption near edge structure spectroscopy indicates functionalized PTFE nanoparticles interact synergistically with ZDDP and ILs to form silicon- and fluorine-doped tribofilms resulting in superior tribological performance.     

Ionic Liquids as Lubricants of Titanium–Steel Contact. Part 3. Ti6Al4V Lubricated with Imidazolium Ionic Liquids with Different Alkyl Chain Lengths

The tribological behaviour and surface interactions of Ti6Al4V sliding against AISI 52100 steel have been studied in the presence of three commercial methylimidazolium (mim) room-temperature ionic liquids (ILs) containing the same anion, bis(trifluoromethylsulfonyl)amide, [(CF₃SO₂)₂N] (Tf₂N), and cations with increasing alkyl chain length, 1-ethyl-3-methylimidazolium [C₂mim], 1-butyl-3-methylimidazolium [C₄mim] and 1-octyl-3-methylimidazolium [C₈mim]. Increasing alkyl chain length increases viscosity whilst reducing the onset temperature for thermal degradation in air, the surface tension and the molecular polarity of the ILs. At room temperature, the tribological performance of the three ILs has been compared with that of a mineral oil (MO). The results show the reduction of the running-in period for the ILs with respect to the MO. In contrast with previously described results for IL lubrication, wear rates for Ti6Al4V at room temperature increase as the alkyl chain length of the ILs increases. The maximum wear reduction, of a 39%, with respect to MO is obtained for the [C₂mim] cation, with only two carbon atoms on the lateral chain. This was the IL selected for the tests at 100 °C. At this temperature, the reduction of the mean friction coefficient with respect to the MO is higher than 50%, whilst the wear rate of Ti6Al4V is reduced by 78%. The friction-sliding distance records for the IL at 100 °C show sharp transitions, related to formation of wear debris and surface interactions between the Tf₂N anion and the aluminium present in the Ti6Al4V alloy. Surface tribolayers and wear debris have been studied by SEM–EDX observations and XPS analysis.     

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.     

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.     

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.     

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.     

Lubrication of TiN,CrN and DLC PVD Coatings with 1-Butyl-1-Methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate

This article studies 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ionic liquid ([BMP][FAP]) as neat lubricant and 1 wt% additive to a polyalphaolefin (PAO 6) in the lubrication of TiN, CrN and diamond-like carbon (DLC) PVD coatings. Friction and wear tests were made using a ball-on-plate reciprocating tribometer under high-pressure conditions. Experiment results showed a slight friction reduction with the use of the [BMP][FAP] ionic liquid as oil additive compared with test made with neat PAO 6. [BMP][FAP] as neat lubricant provoked the lowest friction coefficients and an important wear reduction. Interactions of the ionic liquid with the surface and tribofilms formation contributed to the increase in the load-carrying capacity.     

Tribological Performance and Mechanism of Phosphate Ionic Liquids as Additives in Three Base Oils for Steel-on-aluminum Contact

Butylammonium dibutylphosphate and tetrabutylammonium dibutylphosphate ionic liquids (ILs) were evaluated as antiwear additives for steel-on-aluminum contact in three different base oils, a polyalphaolefin, an ester oil and an IL 1-methy-3-hexylimidazolium hexafluorophosphate, respectively, with similar viscosity and different polarities. The friction experiments were carried out on an Optimal SRV-IV oscillating reciprocating friction and wear tester at room temperature. Results indicate phosphate ILs can effectively improve the tribological properties of the base oil, especially the antiwear property, as additives for steel/aluminum contacts. For the base oils PAO10 and PAO40 with different viscosities, the higher viscosity of PAO40 can be beneficial to reducing the friction coefficient. The worn surface morphologies and chemical compositions of wear scars were analyzed by a JSM-5600LV scanning electron microscope and PHI-5702 multifunctional X-ray photoelectron spectrometer (XPS). The XPS analysis results illustrate that the phosphate IL additives in the base oils with different polarities exhibit the same tribological mechanism. A synergy exists between the adsorbed layers and boundary-lubricating films generated from the tribochemical reaction of IL and the substrate surface, which may reduce the friction coefficient and wear volume of the friction pairs.     

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.     

FAP− Anion Ionic Liquids Used in the Lubrication of a Steel–Steel Contact

This study compares the tribological behavior of two ionic liquids ([BMP][FAP] and [(NEMM)MOE][FAP]) used as oil additive for the lubrication of a steel–steel contact. Friction and wear experiments were performed using a HFRR test machine. Friction coefficient and electrical contact resistance were measured during the tests, and the wear surface was analyzed by confocal microscopy and XPS. The tribological results showed that both ionic liquids used as additive decrease friction and wear but the [BMP][FAP] had a better performance than the [(NEMM)MOE][FAP] due to its higher reactivity with the steel.     

不同润滑条件下HIP-Si3N4/GCr15摩擦副摩擦特性的研究

在干摩擦、水润滑、锂基脂润滑和锂基脂＋MoS2润滑不同条件下,利用MPX-2000型盘销式摩擦磨损试验机,对HIP－Si3N4/GCr15摩擦副进行了摩擦磨损性能的对比试验研究,通过扫描电子显微镜对试件的表面形貌进行观察分析。结果表明:磨损速率的表现,Si3N4为水润滑＞干摩擦＞锂基脂润滑＞锂基脂＋MoS2润滑;GCr15为干摩擦＞水润滑＞锂基脂润滑＞锂基脂＋MoS2润滑。在锂基脂＋MoS2润滑条件下,HIP－Si3N4/GCr15摩擦副摩擦特性最佳。     

Tribological characteristics of bisphenol S bis(diphenyl phosphate) as a high‐performance antiwear additive in lubricating greases at elevated temperature

Bisphenol S bis(diphenyl phosphate) (BSDP) was synthesised and characterised, and its tribological behaviours as additives in polyurea grease and lithium complex grease were evaluated for steel/steel contact at 200 °C. The results indicated that BSDP could dramatically reduce the friction and wear of sliding pairs in the base grease of polyurea, and the tribological performances of BSDP in polyurea grease were significantly superior to the normally used molybdenum disulfide‐based additive package. Furthermore, BSDP in polyurea grease has better tribological behaviour than that in lithium complex grease at a constant load of 100 N. X‐ray photoelectron spectroscopy analysis indicated that boundary lubrication films composed of Fe(OH)O, Fe₂O₃, Fe₃O₄ and FePO₄ compounds containing the P–O bonds and nitride compounds were formed on the worn surface, which resulted in excellent friction reduction and antiwear performance. Copyright © 2016 John Wiley & Sons, Ltd.     

Investigation of the Stability and Tribological Performance of Ionic Nanoliquids

In previous studies, nanoparticles and ionic liquids (ILs) have been used separately as oil additives and have been shown to reduce friction and wear. In this study, the stability and tribological performance of ionic nanoliquids (INLs), which combine nanoparticles and miscible ionic liquids together as oil additives, were investigated. Zinc oxide nanoparticles were dispersed in a mixture of polyalphaolefin (PAO) oil and ionic liquid (tetradecyltrihexylphosphonium bis (2,4,4-trimethylpentyl phosphinate)) using an ultrasonic homogenizer. The coefficient of friction was studied using a tribometer with a ball-on-disk configuration. The wear track was measured using a Bruker Contour GT-K Optical Profiler and the wear volume was calculated. The effects of concentration and load on the coefficient of friction and wear were investigated. The effects of ultrasonic mixing time and nanoparticle concentration on the stability of the solutions were also studied.     

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.     

纳微米硼酸盐添加剂与含磷添加剂的摩擦化学机理

利用四球试验机考察了纳微米硼酸盐添加剂与含磷抗磨添加剂复配体系的摩擦学性能，以及摩擦改进剂对其摩擦学性能的影响，分析研究了摩擦表面膜的组成和化学状态，结果表明：纳微米硼酸盐添加剂和磷型添加剂复配体系产生对抗效应；而摩擦改进剂可以使其复配体系的抗磨减摩性能产生协同效应，其主要原因是摩擦改进剂可以明显地减缓元素P和Fe的反应速度，从而大大降低腐蚀（氧化）磨损，四球磨斑表面变得明显光滑，元素P、N的含量增高。     

Tribological properties of nano‐calcium borate as lithium grease additive

Nano‐calcium borate (NCB) with an average particle size of about 70 nm was synthesised via ethanol supercritical fluid drying technique, and the morphology and microstructures of as‐prepared particles were characterised by means of scanning electron microscope (SEM, JEOL LTD., Tokyo, Japan) and X‐ray powder diffraction. The friction and wear behaviour of the NCB as additive in lithium grease were evaluated with an Optimol‐SRV IV (Optimol Instruments Prüftechnik GmbH, Munich, Germany) oscillating friction and wear tester (SRV tester). The morphology and surface composition of the worn surfaces of lower discs after SRV test were analysed by SEM and X‐ray photoelectron spectroscopy (XPS, Physical Electronics, Inc., USA). The result demonstrated that the anti‐wear and load‐carrying capacities of the lithium grease were significantly improved, and the friction coefficient of the lithium grease decreased with the addition of NCB additive. The analytical results of XPS indicate that the good tribological performance of NCB is attributable to the formation of a boundary lubrication film composed of deposited NCB and the tribochemical reaction products such as B₂O₃, CaO and iron oxides on the rubbing surface. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

羟基硅酸铝润滑油添加剂对钢/铸铁摩擦副磨损性能的影响

采用球盘试验和销盘试验相结合的方法,研究了钢/铸铁摩擦副在不同油润滑条件下磨损性能的变化,考察了不同含量羟基硅酸铝添加剂的作用效果,采用扫描电镜、X射线衍射仪以及纳米压痕仪对试验样品进行了分析测试。结果表明,该金属陶瓷添加剂表现出显著的抗磨性能,提高了钢/铸铁摩擦副承载能力和使用寿命;随着添加剂含量的增加,抗磨作用更加明显,负磨损现象开始出现;该添加剂可以抑制凹坑和裂纹的扩展,从而使摩擦表面形貌得到改善,并通过改变摩擦表面层的结构使摩擦副的磨损性能得到显著提高。