Tribological behaviour of protic ionic liquids with ammonium salts modified LABSA as lubricants and additives

Two typical protic ionic liquids with ammonium salts modified linear alkylbenzene sulfonic acid (LABSA) were synthesised, and their tribological behaviours used as both lubricants compared with PAO10 and additives in PAO10 were evaluated with an Optimol SRV‐I oscillating reciprocating friction and wear tester. The results show that the two novel protic ionic liquids with ammonium salts modified LABSA exhibited excellent friction‐reducing and anti‐wear properties as lubricants and can also significantly improve the tribological performance of the base fluids as friction modifier additives. The worn surfaces were characterised by scanning electron microscope and X‐ray photoelectron spectroscopy, indicating that the excellent anti‐wear and friction‐reducing performance could attributed to the boundary lubrication films that could contain both the tribochemical film composed of organic amine decomposed from the protic ionic liquids with ammonium salts and the stable chemical absorbed film through the interactions between the sulfonate anions and surface metallic atoms during the sliding process. Copyright © 2012 John Wiley & Sons, Ltd.     

Room temperature ionic liquid 1-ethyl-3-hexylimidazolium-bis(trifluoromethylsulfonyl)-imide as lubricant for steel–steel contact

The ionic liquid 1-ethyl-3-hexylimidazolium-bis(trifluoromethylsulfonyl)-imide was synthesized and evaluated as lubricant for the contact of steel/steel. The tribological properties of the ionic liquid as lubricant were investigated on an Optimol SRV oscillating friction and wear tester in ambient condition. The synthetic ionic liquid shows excellent tribological performance and is superior to the ionic liquid of alkylimidazolium tetrafluoroborate and the conventional high temperature lubricants X-1P and PFPE in terms of antiwear performance and load-carrying capacity. The elemental composition and chemical nature of the boundary film generated on the steel surface were analyzed on a scanning electron microscope with a Kevex energy dispersive X-ray analyzer attachment (SEM/EDS) and X-ray photoelectron spectrometer (XPS), and the XPS results indicate the occurrence of a complicated tribochemical reaction of ionic liquid with iron on the rubbing surface during the sliding process, with the formation of FeS, organic fluoride, inorganic fluoride, etc.     

Tribological performances of two ionic liquids (1-ethyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium tetrafluoroborate) as nanolubricants on polyether-ether-ketone surface

This paper seeks to address the potential of using ionic liquids as nanolubricants on Polyether-ether-ketone (PEEK) surface. We characterize the tribological properties of two ionic liquids, namely1-ethyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium tetrafluoroborate. The tribological data are compared with those of perfluoropolyether (PFPE) and Multiply Alkylated Cyclopentanes (MAC) data obtained from previous study. Ionic liquids at lower concentration of 0.4wt% (in solution prior to deposition) showed lower wear lives as compared to those for PFPE and MAC. However, at higher concentration of 4wt%, these ionic liquids have the same wear lives as those of PFPE and MAC, but show considerably lower coefficients of friction. Mechanisms of nanolubrication for ionic lubricants vis-a-vis those of PFPE and MAC are explained.     

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.     

Tribological Behavior of PTFE Composites Filled with PEEK and Nano-Al2O3

In this study, the tribological properties of polytetrafluoroethylene (PTFE) composites filled with polyetheretherketone (PEEK) and nano-Al₂O₃ particles were studied using a block-on-ring wear tester. The tribological performance of the composites was affected by the experimental parameters (sliding speed, normal load, and environmental temperature) and the composites achieved a high-speed sliding friction state. The results showed that the PEEK and nano-Al₂O₃ particles significantly improved the wear resistance of the PTFE composites. In addition, the nano-Al₂O₃ particles increased the hardness of the composites and enhanced the mechanical properties to enable applications in a wider range of industrial fields. The effects of the sliding speed and normal load on the tribological properties were more significant than that of the environmental temperature. In addition, the entire wear process was divided into three stages (the initial wear stage, severe wear transition stage, and ultralow stable wear stage), according to the evolution of the tribological characteristics (wear rate, morphology of the worn surface and transfer film, and wear debris morphology).     

Tribological characteristics of low-pressure plasma-sprayed A12O3 coating from room temperature to 800 °C

The tribological characteristics of low-pressure plasma-sprayed (LPPS) Al₂O₃ coating sliding against alumina ball have been investigated from room temperature to 800 °C. These friction and wear data have been compared quantitatively with those of bulk sintered alumina to obtain a better understanding of wear mechanisms at elevated temperatures. The friction and wear of Al₂O₃ coating show a strong dependence on temperature, changing from a mild to a severe wear regime with the increase of temperature. The coefficient of friction at room temperature is approximately 0.17 to 0.42, depending on applied load. The tribochemical reaction between the coating surface and water vapor in the environment and the presence of the hydroxide film on the Al₂O₃ coating reduce the friction and wear at room temperature as contrasted to those of bulk sintered alumina. At intermediate temperatures, from 400 to 600 °C, the friction and wear behavior of Al₂O₃ coating depends on the inter-granular fracture and pull-out of Al₂O₃ grains. At above 700 °C, formation and deformation of fine grain layer, and abrasive wear in the form of removal of fine alumina grains further facilitate the friction and wear process of Al₂O₃ coating.     

Tribological Characteristics of Alkylimidazolium Diethyl Phosphates Ionic Liquids as Lubricants for Steel–Steel Contact

Tribological properties of three novel phosphates-based ionic liquids (ILs), i.e., 1,3-diethyl imidazolium diethylphosphate, 1-ethyl-3-butyl imidazolium diethylphosphate, and 1-ethyl-3-octyl imidazolium diethylphosphate, were evaluated as lubricants for the steel–steel sliding pair by using an Optimol-SRV oscillating friction and wear tester. The chemical compositions of the boundary films generated on steel contact surfaces were analyzed with the use of a scanning electron microscope (SEM) with a Kevex energy dispersive X-ray analyzer attachment (EDS), and an X-ray photoelectron spectrometer (XPS). The results showed that these phosphate functional group-containing ionic liquids exhibit excellent tribological performance especially at a moderate temperature. The imidazole phosphate ionic liquid has no corrosion to steel. The mechanism for the friction-reduction and anti-wear effect of ionic liquids has also been proposed and discussed from the tribochemistry point of view.     

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.     

Improvement in the tribological properties of imidazolium‐derived ionic liquids by additive technology

Molecular design of wear‐preventing and friction‐reducing additives for ionic liquids is described. The tribological properties of carboxylic acid‐derived additives were evaluated by a ball‐on‐flat type tribotest under reciprocating motion. Tetraalkylammonium and tetraalkylphosphonium salts of N‐benzyl‐protected aspartic acid were dissolved in 1‐alkyl‐3‐methylimidazolium‐derived ionic liquids. They prevented wear remarkably and reduced friction fairly. Influences of alkyl group in imidazolium molecule on additive response were observed. In tetraalkylammonium‐derived ionic liquids, the additive reduced wear but did not reduce friction under these conditions. The salt of N‐acetyl‐protected glutamic acid prevented wear, but did not reduce friction. Copyright © 2008 John Wiley & Sons, Ltd.     

Tribological evaluation of α, -diimidazoliumalkylene hexafluorophosphate ionic liquid and benzotriazole as additive

Ionic liquids of α, -diimidazoliumalkylene hexafluorophosphate were synthesized. The tribological properties of the synthetic ionic liquid and the ionic liquid contained additive for contacts of steel/steel were investigated by Optimol SRV oscillating friction and wear tester under ambient conditions. The synthetic ionic liquid presented low friction coefficients and small wear volumes, especially under higher temperatures. The ionic liquid doped with benzotriazole (BTA) showed excellent anti-wear ability. The worn surfaces and chemical nature of the boundary films generated on the metal surfaces were analyzed by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). SEM results showed slight abrasion on the worn surfaces and XPS results indicated the formation of FeF₂, FeF₃, Fe₃O₄, and FePO₄ by the tribochemical reactions of ionic liquid with iron during the sliding process.     

Tribological properties of spark-plasma-sintered ZrO2(Y2O3)-CaF2-Ag composites at elevated temperatures

Spark-plasma sintering is employed to synthesize self-lubricating ZrO₂(Y₂O₃) matrix composites with different additives of CaF₂ and Ag as solid lubricants by tailoring the composition and by adjusting the sintering temperature. The friction and wear behavior of ZrO₂(Y₂O₃) matrix composites have been investigated in dry sliding against an alumina ball from room temperature to 800 °C. The effective self-lubrication at different temperatures depends mainly on the content of various solid lubricants in the composites. The addition of 35 wt.% Ag and 30 wt.% CaF₂ in the ZrO₂(Y₂O₃) matrix can promote the formation of a well-covered lubricating film, and effectively reduce the friction and wear over the entire temperature range studied. The friction coefficients at low temperatures were at a minimum value for the composite containing 35 wt.% of silver. At this silver concentration, low and intermediate temperature lubricating properties are greatly improved without affecting high-temperature lubrication by the calcium fluoride in ZrO₂(Y₂O₃) matrix composites. The worn surfaces and transfer films formed during wear process have been characterized to identify the synergistic lubrication behavior of CaF₂ and Ag lubricants at different temperatures.     

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 behaviour and wear mechanism of MoS2–Cr coatings sliding against various counterbody

MoS₂–Cr coatings with different Cr contents have been deposited on high speed steel substrates by closed field unbalanced magnetron (CFUBM) sputtering. The tribological properties of the coatings have been tested against different counterbodies under dry conditions using an oscillating friction and wear tester. The coating microstructures, mechanical properties and wear resistance vary according to the Cr metal-content. MoS₂ tribological properties are improved with a Cr metal dopant in the MoS₂ matrix. The optimum Cr content varies with different counterbodies. Showing especially good tribological properties were MoS₂–Cr8% coating sliding against either AISI 1045 steel or AA 6061 aluminum alloy, and MoS₂–Cr5% coating sliding against bronze. Enhanced tribological behavior included low wear depth on coating, low wear width on counterbody, low friction coefficients and long durability.     

Tribological behavior of sialon ceramics sliding against steel lubricated by fluorine-containing oils

The friction and wear behavior of sialon ceramics sliding against steel and lubricated by perfluoropolyethers (PFPE), tetrakis (3-trifluoromethylphenoxy)-bis(4-fluoro-phenoxy)-cyclotriphosphazene (X-1P) and ionic liquid (1-methyl-3-octylimidazolium tetrafluoroborate, coded as L108) were investigated. It was found that the three fluorine-containing lubricants reduced friction coefficient and wear volume effectively. The effectiveness of the three lubricants in reducing wear volume could be ranked as L108>X-1P>PFPE. The antiwear films mainly consisting of organic oxyfluoride or carbonfluoride species and silicon fluoride are all observed for the three lubricants, while the degradation of PFPE during friction might account for the higher wear volume therewith. The lowest friction coefficient 0.065 was recorded for L108 under load of 0.5–400 N. This is dependent on the physically adsorbed ionic liquid on the rubbing surface and the formation of BN under the harsh conditions.     

Evaluation of Vapor Pressure and Ultra-High Vacuum Tribological Properties of Ionic Liquids

Ionic liquids are a class of salts that incorporate polyatomic anions and cations. These materials are typically viscous fluids at room temperature. The fluids are generally characterized as possessing negligible vapor pressures under ambient conditions. These beneficial properties have led us to study the effectiveness of ionic liquids containing both organic cations and anions for use as lubricants with space applications. This article presents vapor pressure measurements for four ionic liquids as well as friction coefficient data from a spiral orbit tribometer in the boundary lubrication regime under simulated space conditions using stainless steel tribocouples. In addition, we present the first tribological data on mixed ionic liquids and an ionic liquid additive. Postmortem infrared and Raman analyses of the balls and races indicates that the major degradation pathway for these organic ionic liquids is similar to those of other carbon-based liquid lubricants; that is, deterioration of the organic structure into amorphous graphitic carbon. The coefficients of friction and lifetimes of these lubricants are comparable to or better than those of several commonly used space oils.     

Tribological Properties of Phosphor Bronze and Nanocrystalline Nickel Coatings Under PAO + MoDTC and Ionic Liquid Lubricated Condition

The friction and wear properties of phosphor bronze and nanocrystalline nickel coatings were evaluated using a reciprocating ball-on-plates UMT-2MT sliding tester lubricated with ionic liquid and poly-alpha-olefin containing molybdenum dialkyl dithiocarbamate, respectively. The morphologies of the worn surfaces for the phosphor bronze and nanocrystalline nickel coatings 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. Results show that the phosphor bronze and nanocrystalline nickel coatings exhibited quite different tribological behaviors under different lubricants. Phosphor bronze plate shows higher friction coefficient (0.14) and wear rate (3.2 × 10⁻⁵ mm³/Nm) than nanocrystalline nickel coatings (average friction coefficient is 0.097, wear rate is 1.75 × 10⁻⁶ mm³/Nm) under poly-alpha-olefin containing molybdenum dialkyl dithiocarbamate lubricated conditions. The excellent tribological performance of nanocrystalline nickel coatings under above lubricant can be attributed to the formation of MoS₂ and MoO₃ on the sliding surface. a quite a number of C, O and F products on worn surface of phosphor bronze than NC nickel coatings can improve anti-wear properties while using ionic liquid as lubricant.     

Lubricating property of cyano-based ionic liquids against hard materials

Ionic liquids are expected to be used as a new lubricants and lubricant additives because of their unique properties. However, cyano-based ionic liquids have exhibited poor lubricating property with steel/steel contacts. We evaluated the lubricating properties of cyano-based ionic liquids with steel/hard materials contacts. TiO₂, Al₂O₃, and tetrahedral amorphous carbon (ta-C) DLC were used as hard materials. Six types of ionic liquids, as combination of two types of cations ([EMIM], [BMPL]) and three types of cyanide anions ([DCN], [TCC] and [TCB]), were selected. In sliding tests of steel/TiO₂ and steel/Al₂O₃ lubricated with [EMIM][DCN], [BMPL][DCN], [EMIM][TCC], [BMPL][TCC] exhibited low friction coefficients of less than 0.1. In addition, steel/Al₂O₃ and steel/ta-C DLC lubricated with [BMPL][TCB] exhibited very low friction coefficients less than 0.05. On the other hand, high friction coefficients were observed at steel/TiO₂ and steel/Al₂O₃ contacts lubricated with [EMIM][TCB] and steel/ta-C DLC contact lubricated with [EMIM] cation group. Peeling of the ta-C DLC was observed when [EMIM] cation group was used. ToF-SIMS analysis indicated that the anion was adsorbed on the worn surfaces in the case of low frictional conditions. However, both ions were hardly observed in the case of high frictional conditions. It is considered that the ionic liquids underwent tribo-decomposition on the worn surfaces at low friction coefficient. To evaluate the degree of tribo-decomposition, Thermogravimetric analysis (TGA) was used. TGA results indicated that [EMIM][TCB], which exhibited high friction coefficient, had the most highest stability among all ionic liquids. Low stability ionic liquids, however, showed a tendency for low friction coefficient. These results suggest that lubricating properties are related to the stability of ionic liquids.     

A study on friction and wear characteristics of nanometer Al2O3 /PEEK composites under the dry sliding condition

The friction and wear properties of the polyetheretherketone (PEEK) based composites filled with 5 mass% nanometer or micron Al₂O₃ with or without 10 mass% polytetrafluroethylene (PTFE) against the medium carbon steel (AISI 1045 steel) ring under the dry sliding condition at Amsler wear tester were examined. A constant sliding velocity of 0.42 m s⁻¹ and a load of 196 N were used in all experiments. The average diameter 250 μm PEEK powders, the 15 or 90 nm Al₂O₃ nano-particles or 500 nm Al₂O₃ particles and/or the PTFE fine powders of diameter 50 μm were mechanically mixed in alcohol, and then the block composite specimens were prepared by the heat compression moulding. The homogeneously dispersion of the Al₂O₃ nano-particles in PEEK matrix of the prepared composites was analyzed by the atomic force microscopy (AFM). The wear testing results showed that nanometer and micron Al₂O₃ reduced the wear coefficient of PEEK composites without PTFE effectively, but not reduced the friction coefficient. The filling of 10 mass% PTFE into pure PEEK resulted in a decrease of the friction coefficient and the wear coefficient of the filled composite simultaneously. However, when 10 mass% PTFE was filled into Al₂O₃/ PEEK composites, the friction coefficient was decreased and the wear coefficient increased. The worn scars on the tested composite specimen surfaces and steel ring surfaces were observed by scanning electron microscopy (SEM). A thin, uniform, and tenacious transferred film on the surface of the steel rings against the PEEK composites filled with 5 mass% 15 nm Al₂O₃ particles but without PTFE was formed. The components of the transferred films were detected by energy dispersive spectrometry (EDS). The results indicated that the nanometer Al₂O₃ as the filler, together with PEEK matrix, transferred to the counterpart ring surface during the sliding friction and wear. Therefore, the ability of Al₂O₃ to improve the wear resistant behaviors is closely related to the ability to improve the characteristics of the transfer film.     

Study on the Synthesis and Tribological Property of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Based Magnetic Fluids

In this paper, Fe₃O₄ based magnetic fluids with different particle concentrations were prepared by the co-precipitation technique. The size of the Fe₃O₄ nanoparticles is about 13 nm and their shape is spherical. The tribological performances of the fluids with different concentration Fe₃O₄ nanoparticles were evaluated in a MMW-1A four-ball machine. The results show that the tribological performance of magnetic fluids with proper Fe₃O₄ nanoparticles can be improved significantly. The maximum nonseized load (P _B) has been increased by 38.4% compared with carrier liquid. The wear scar diameter has been reduced from 0.68 mm to 0.53 mm and the relative percentage in friction coefficient has decreased to 31.3%. The optimal concentration of the Fe₃O₄ nanoparticles in the carrier liquid is about 4 wt.%.     

Tribological properties of crown‐type phosphate ionic liquids as lubricating additives in rapeseed oils

The friction and wear properties of rapeseed oils with different concentrations and crown‐type phosphate ionic liquids were studied by an Optimol SRV oscillating friction and wear tester. Crown‐type phosphate ionic liquids have better solubility than conventional ionic liquids in rapeseed oils. The tribological test results showed that the crown‐type phosphate ionic liquids as lubricating additives in rapeseed oil exhibited better tribological performance than the base oil for steel/steel friction pair under various loads. It is noted that the friction pair showed the least friction coefficient and wear volume when the concentration of ionic liquids was 1 wt.%. The better tribological properties of friction pair should be attributed to the effective boundary films formed in the worn surface, on various tribochemical products, organometallic products and iron oxides. Copyright © 2012 John Wiley & Sons, Ltd.