Comments on “Pressure–Viscosity Coefficient of Vegetable Oils” by Biresaw and Bantchev

The classical study of elastohydrodynamic lubrication has not employed a consistent definition of the pressure–viscosity coefficient (PVC) or alpha of the liquid. This has likely been the result of the inaccurate film thickness formulas that require a value of alpha and of inaccurate film thickness measurements. Practitioners have found it possible to reconcile formula with measurement, when viscosity has been measured in a viscometer, by choosing a definition of alpha from the extensive menu of definitions that have been used. The problem for tribology is that the term, PVC, has become largely meaningless because there is no generally accepted definition.     

Ionic Liquids as Lubricants of Titanium–Steel Contact

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

Pressure–Viscosity Coefficients for Polyalkylene Glycol Oils and Other Ester or Ionic Lubricants

We present in this article new viscosity and density data for polypropylene glycol monomethyl ether, which completes a series of articles where we have published dynamic viscosity data of poly(propylene glycol) dimethyl ethers, dipentaerythritol esters, and pentaerythritol esters. New dynamic viscosity measurements up to 60 MPa at five temperatures in the range of 303.15–373.15 K, and density values at temperatures ranging from 298.15 to 398.15 K up to 60 MPa are reported in addition to other physical properties that affect the behavior of the fluids in elastohydrodynamic lubrication regime, such as the viscosity index value, VI, the universal pressure–viscosity coefficient, α _film, and the temperature–viscosity coefficient, β. The experimental measurements were performed using a rotational automated viscometer Anton Paar Stabinger SVM3000, a rolling-ball viscometer Ruska 1602-830 for high pressures, and an automated Anton Paar DMA HPM vibrating-tube densimeter. Together with these data, we also present a comparison of the film-generating capability for the fluids above mentioned as well as for other five ionic liquids. We analyze the dependence of the molecular structure on the lubrication properties of these oils, which can help the lubricant engineers to develop products with enhanced performance.     

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.     

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.     

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.     

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 Liquids as Lubricants of Titanium–Steel Contact. Part 2: Friction, Wear and Surface Interactions at High Temperature

The tribological behaviour and surface interactions of titanium sliding against AISI 52100 steel have been studied at 200 and 300 °C in the presence of two commercial imidazolium room temperature ionic liquid (ILs): 1-octyl-3-methylimidazolium tetrafluoroborate (L108) and 1-hexyl-3-methylimidazolium hexafluorophosphate (LP106). L108 presents the higher thermal stability but gives higher friction coefficients and wear rates than LP106, with long running-in periods and high friction values, both at 200 and 300 °C. Friction and wear rates for LP106 are lower and decrease as the temperature increases from 25 to 200 °C. At 200 °C, LP106 shows a constant friction coefficient, without running-in, produces a mild wear on titanium and no surface damage on steel. LP106 fails at 300 °C, close to its degradation temperature, due to tribochemical decomposition through partial dissociation of the hexafluorophosphate anion, with formation of a phosphorus-rich layer on the steel ball, while the titanium wear track surface is heterogeneous, showing regions with the presence of fluoride and others with the presence of phosphate. When the steel ball is substituted for a ruby sphere under the same conditions at 300 °C, a low friction coefficient and mild wear is observed, due to the higher stability of the LP106 lubricant at the ruby–titanium interface. The friction coefficients, wear mechanisms and surface interactions have been studied by means of friction-distance records, SEM, EDX and XPS.     

Semi-Empirical Molecular Modeling of Ionic Liquid Tribology: Ionic Liquid–Hydroxylated Silicon Surface Interactions

The interactions between selected ionic liquids and modified silicon surfaces are modeled in this article using semi-empirical methods. The modeled ionic liquids include a series of ionic liquids consisting of imidazolium derivatives with Cl^? as the anion interacting with hydroxylated silicon wafers. A second series consists of symmetrical and asymmetrical dicationic imidazolium derivatives with PF₆ ^? or BF₄ ^? as the anion interacting with hydroxylated single crystal silicon wafers. The tribological properties of these ionic liquids and their interactions with silicon surfaces are modeled using a rolling hydroxylated silicon surface. The ionic liquids are allowed to form a complex with this surface, and the enthalpies of complex formation are seen to correlate with the tribological properties of the ionic liquids.     

The Prediction of Contact Pressure–Induced Film Thickness Decay in Starved Lubricated Rolling Bearings

Under starved conditions the thickness and distribution of the lubricant film in an elastohydrodynamically lubricated (EHL) contact is directly related to the distribution of lubricant on the track in the inlet to the contact. In starved lubricated rolling bearings this lubricant distribution is determined by many effects. The authors have developed a model to predict the oil lost from the track induced by EHL pressure with no replenishment. A complete bearing is modeled with multiple rolling element EHL contacts and with the applied load to the rolling elements varying along the circumference of the bearing. Results of the oil layer thickness on the track are presented for a ball bearing and a spherical roller bearing for different bearing loads and rotational speeds. The predicted layer thickness decay rate for a ball bearing is significantly larger than for a spherical roller bearing and the predicted effect of the bearing load on the decay rate is small compared to the effect of the rotational speed. The predicted decay periods due to the contact pressure effect are small compared to the observed (grease) life of bearings. The results show that a bearing cannot sustain an adequate layer of oil on the running track unless significant replenishment takes place.     

Semi-Empirical Molecular Modeling of Ionic Liquid Tribology: Ionic Liquid–Aluminum Oxide Surface Interactions

The interactions between the selected ionic liquids (ILs) and aluminum oxide surfaces are modeled in this report using theoretical methods. A wide range of ILs and their interactions with an aluminum oxide surface are modeled using the PM5 semi-empirical method. The ILs modeled in this study contain imidazolium (C_{3, 4, 6, 8 or 10}mim) or ammonium cations including (C₆H₁₃)₃NH⁺, (C₈H₁₇)₃NH⁺, C₈H₁₇NH₃ ⁺, (C₂H₅)₃NH⁺, and (C₈H₁₇)NH₃ ⁺. The anions include Cl⁻, Br⁻, PF₆ ⁻, (CF₃SO₂)₂N⁻, and (C₂F₅SO₂)₂N⁻. The interactions of these ILs with an Al–O surface are modeled in a stepwise manner. The lowest energy forms of the individual ILs are determined, and these ILs are allowed to form a complex with the Al–O surface. The resulting reaction enthalpies of ionic liquid-surface complex formation are seen to correlate with the tribological properties of the ILs. The strongest correlations occur within those ILs containing similar cations.     

Viscosity–temperature correlation for liquids

A new viscosity–temperature equation and corresponding chart have been developed to extend the range of the current ASTM viscosity–temperature charts. This new chart and equation extends the temperature and viscosity range for hydrocarbons and, for the first time, has the ability to extend to the low viscosity regime of halocarbons and low temperature fluids. The new equation and chart can linearize liquid viscosity data from 0.04 cSt and covers the temperature range from −210 to 500 °C for halocarbons and hydrocarbons. With a modification to the temperature scaling, the new equation also has the ability to fit liquid metal viscosity data. The new chart and equation cannot accurately linearize the viscosity with respect to temperature of fluids exhibiting strong molecular bonding (water, ammonia), fluids whose molecular structure consists of long coils (some long chained silicones), or fluid mixtures in which one fluid precipitates out of solution (wax precipitation).

Frictional Coefficient in Upsetting of a Thin Rigid–Plastic Plate of Square Cross Section

Russian Engineering Research - Abstract—A method is developed for calculating the frictional coefficient in the upsetting of a plate. Experiments confirm the calculations.     

Correlating Molecular Structure to the Behavior of Linear Styrene–Butadiene Viscosity Modifiers

The effect of linear styrene–butadiene polymer structure on the temperature–viscosity behavior of model polymer-base oil solutions is investigated using molecular dynamics simulations. Simulations of alternating, random, and block styrene–butadiene polymers in a dodecane solvent are used to calculate viscosity at 40 and 100 °C, reference temperatures for characterizing their function as viscosity modifiers. Mechanisms underlying this function are explored by quantifying the radius of gyration and intramolecular interactions of the polymers at the same reference temperatures. The block styrene–butadiene configuration exhibits the least change in viscosity with temperature, characteristic of a good viscosity modifier or viscosity index improver, and the behavior is correlated to the ability of this structure to form smaller coils with more intramolecular interactions at lower temperatures and then expand as temperature is increased. The results indicate that there is a correlation between styrene–butadiene polymer structure, additive function, and the mechanisms underlying that function.     

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.     

Effects of Surface Roughness and Wood Grain on the Friction Coefficient of Wooden Materials for Wood–Wood Frictional Pair

In order to investigate the effects of the surface roughness and wood grain on the friction coefficient of wooden materials, the friction coefficients of solid wood, medium-density fiberboard (MDF), and particle board (PB) with varying surface roughness were tested by a friction coefficient tester. The friction coefficients of solid wood for a wood–wood frictional pair were measured under varying wood grain (the orientation of fibrils). The results showed that the friction coefficients of the solid wood increased linearly with the arithmetic mean deviation of the surface profile (R_a). The friction coefficients of MDF and PB increased sharply at first and then stabilized with increasing R_a. The friction coefficient of solid wood was respectively maximized and minimized when the grain directions of two wood specimens were both perpendicular to the sliding direction and perpendicular to each other.     

The Lubricating Performance of an “in Situ” Process MoS2 Film in Air and in Liquids

The performance of a synthetic MoS₂ film, produced by electrodeposition of molybdic oxide followed by a temperature-pressure H₂S conversion to a molybdenum sulfide compound, is examined under extreme pressure conditions immersed in various fluids. Friction wear and EP characteristics, measured on various test machines, are compared to those of the fluids alone and also to conventional bonded films.

The fluids examined include: mineral oil, jet fuel, hydraulic fluid, silicone fluid.

The dry films include: burnished MoS₂ powder, MIL-L-8937 resin bonded film, MIL-L-8129 silicate bonded film and the synthetic “in situ” MoS₂ film.

The performance of the synthetic MoS₂ film on titanium and stainless steel is also examined.