Tribological characterization of environmentally adapted ester based fluids

Fundamental properties of six synthetic ester base fluids, suitable for the formulation of environmentally adapted lubricants, have been investigated. High pressure viscosity data for the test fluids were obtained through experimental measurements with a high pressure Couette rheometer. The temperature, pressure and viscosity data η(p, T) were parameterized against the Roelands pressure–viscosity equation. Thermal conductivity and specific heat capacity data were obtained using a transient hotwire method, and the EHD friction coefficient, γ, was obtained experimentally as well. The results from these measurements are reported, and the correlation between thermal properties, molecular structure, and the fluid rheology parameters, of the test fluids are discussed.     

Additive properties of saturated very long chain fatty acids in castor and jojoba oils

Tribological and oxidation stability properties of saturated very-long-chain-fatty acids present in castor and jojoba oils have been investigated by the four ball tribosystem, pressure differential scanning calorimetry, optical micrographs and electron probe micro-analysis (EPMA). Test results of the oils additized with eicosanoic and octadecanoic acids in the oils were compared with those of zinc dialkyldithiophosphate (ZDTP). Results showed that eicosanoic and octadecanoic acids possessed better antiwear and lubricity properties compared to the popular ZDTP, though with reduced oxidation stability. EPMA and optical micrographs of worn surface morphology showed that lubricating films formed by these acids were metallic salts whose breakdown was not associated with tribochemical wear surface modification of the mating pairs.     

Comparison of Antiwear Additive Response Among Several Base Oils of Different Polarities

The antiwear performance of tricresyl phosphate dissolved in 11 kinds of base oils of different polarities has been studied. A mineral oil (MO) and an alkylnaphthalene (AN) were used as non-or low-polarity base oils. Five polyol esters (POEs), i.e., three trimethylolpropane esters and two pentaerythritol esters, and four polyalkylene glycols (PAGs) were used as the high polarity base oils. A four-ball friction apparatus was used to determine the anti-wear performance, using commercially available bearing steel balls as test specimens. All the TCP-formulated base oils showed optimum concentration characteristics for minimizing wear. It is shown that the order of the optimum concentration for POEs and PAGs can be reasonably explained by the interaction between additive and base oils, by using their solubility parameters and molecular sizes. These oils' dielectric constants showed little correlation with optimum concentrations. The dielectric constant showed large effects on such non- or low-polarity oils as MO and AN. A good correlation of the optimum concentration for all the base oils was obtained when it was arranged as a function of (SP_base/SP_TCP)^14.2 (MW_base/MW_TCP)^?2.78 (ε)^18.6.     

Friction and wear performance of MoDTC‐containing and ester‐containing lubricants over steel surfaces under reciprocating conditions

Lubricant oil can be regarded as a complex mixture of base oils and additives, each one with its specific functions and behaviour. In this paper, the interaction of a molybdenum dialkyldithiocarbamate (MoDTC)‐based additive and combinations of a polyalphaolefin and a synthetic ester is investigated. A reciprocating ball‐on‐disc configuration was used for tribological tests. The effect of MoDTC is seen as a sharp drop in the coefficient of friction. This friction reduction is affected by the base fluid: the effect is more intense and lasts longer when the ester content is decreased. The applied normal force also affects the MoDTC effect, which is not sustainable at higher loads. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Influence of different surface modification technologies on friction of conformal tribopair in mixed and boundary lubrication regimes

Moving machine assemblies are generally designed to operate in full film lubrication regime to ensure high efficiency and durability of components. However, it is not always possible to ensure this owing to changes in operating conditions such as load, speed, and temperature. The overall frictional losses in machines are dependent on the operating lubrication regimes (boundary, mixed or full-film). The present work is thus aimed at investigating the role of different surface modification technologies on friction of a sliding bearing/roller tribopair both in boundary and mixed lubrication regimes. A special test rig comprising of two bearings was built for the experimental studies. Tribological tests were conducted in a wide speed range to enable studies in boundary and mixed lubrication regimes. The influence of application of different surface modification technologies on both the sliding bearing and the roller surfaces on friction has been studied. The rollers used in these studies were provided with five different coatings (hard DLCs and a soft self-lubricating coating). Additionally, two uncoated rollers having different surface roughness were also studied. Uncoated bearings were used in all tribopairs except two. These two bearings were coated with DLC and phosphate coatings respectively and uncoated rollers were the mating counterparts. Friction measurements were made on the new as well as the previously run-in surfaces. It was found that the rollers with self-lubricating coating resulted in lowest boundary friction closely followed by the rollers with the hardest DLC coatings. The DLC coating applied on to the bearing showed lower boundary friction after running-in. Mixed friction has been found to be mainly dependent on the surface topography characteristics of both the original and the run-in surfaces of bearings and rollers. The harder DLC coatings and the phosphated bearing showed the lowest mixed friction due to an efficient running-in of the bearing surface.     

Additive interaction and lubrication performance in a polar additive binary system

Additive interactions were studied between polar compounds, one of which decomposes at the frictional surface, on the lubrication performance of a binary system for polar straight chain hydrocarbons and aromatic compounds.As a comparison, an additive single component system was also studied. The lubricant performance of the blended oils was evaluated using a Falex lubricant testing machine and the adsorption characteristics of the additives on an iron surface were analysed.In the single component system, good correlation was obtained between the adsorption isotherm and the anti-seizure characteristics of the additives for both 1-octadecanol and 2-naphthol. In the binary system of 2-naphthol and 1-octadecanol, the lubrication characteristics of 2-naphthol appeared, producing heavy decomposition products of 2-naphthol because 2-naphthol was irreversibly (or chemically) adsorbed while 1-octadecanol was reversibly (or physically) adsorbed.Conversely, in the binary system of β-naphthol and stearic acid, the lubrication performance was poorer than that of the individual additive, since neither the decomposition products of the aromatics nor the adsorption film of stearic acid was sufficient against seizure. This was owing to the comparable adsorbability of the two compounds.     

The effects of iron particles on the friction and wear mechanisms of ceramics in ethanol

For the combinations of an Si₃N₄ pin and five kinds of ceramic disk (SiC, Si₃N₄, Al₂O₃, ZrO₂, TiC), a friction and wear test was carried out in ethanol and in ethanol containing iron particles (1 wt.%, average diameter d = 200 nm, D = 12 μm under cohered condition) under a load in the range 5.88–11.50 N, at a sliding velocity of 0.138–0.196 m s⁻¹. A topographical analysis was also performed on the microasperities of the wear surfaces to estimate the behavior of the iron particles, and the degree of surface damage. As a result, the following facts were found. (1) The addition of iron particles in ethanol decreased both the wear rates of SiC and TiC disks and the mating pins, and also decreased the wear rate of the Al₂O₃ disk but increased that of the mating pin. The addition increased the wear rates of both ZrO₂ and Si₃N₄ disks and the mating pins. (2) The average coefficients of friction with the addition of iron particles were greater than those without iron particles. (3) The wear rates of pin and disk depended on the topographies of wear surfaces and the wear index Γ.     

二烷基二硫代磷酸钕作为锂基脂添加剂的极压抗磨作用机理

合成了二正十六烷基二硫代磷酸阴离子与稀土离子（Ｎｄ＾３＋）的中性络合物，考察了具有类似配体结构的稀土和过渡金属（Ｚｎ）络合物对锂基脂摩擦学性能的影响，通过Ｘ光光电子能谱对添加剂的作用机理进行了研究。     

The combined effects of ZDDP, surface texture and hardness on the running-in of ferrous metals

The effects of surface characteristics including roughness, lay direction and hardness of rubbing pairs on the antiwear performance of secondary short chain ZDDPs under a boundary lubrication condition are studied experimentally. The antiwear performance of the ZDDPs is evaluated by the duration of running-in periods recorded in wear tests of specially prepared specimens. A running-in period is defined as the time interval from the beginning of a wear test to the time at which the contact resistance between the rubbing pair approaches infinity. The shorter running-in period yields the better antiwear performance. The wear tests were conducted with a reciprocating sliding contact made by flat-on-flat specimens which were made from grey cast iron, quenched medium carbon steel and bearing steel. Two lay directions of the surface texture, namely, parallel and perpendicular to the sliding direction were ground, which made four possible combinations in a rubbing action. The CLA roughness of the specimens ranged from 0.35 μm to 0.04 μm. It is found that to obtain a shorter running-in period and to enhance the antiwear performance of ZDDPs, the following rules should be obeyed. If the hardness numbers of the two rubbing members are near the same, the combination of their lay directions should be both in parallel but perpendicular to the sliding direction and, the roughness values should be smaller than 0.09 μm. If the hardness number of one member is much greater than that of the other, the soft member should be the smaller one and the surface of the hard member should be as smooth as possible. Under any circumstance, the smaller members should be chamfered.     

The influence of thin boundary films on real surface roughness in thin film, mixed EHD contact

The behaviour of thin viscous boundary films in the rough surface rolling–sliding point contact operated under thin film lubrication conditions have been observed by thin film colorimetric interferometry. Changes in film thickness distribution within the lubricated contact between steel ball and glass disc were studies with both mineral base oil and mineral oil formulated with non-functionalized polyalkylmethacrylate (PAMA). Recent studies by other researchers showed that this polymer-containing viscosity index improver exhibits some friction-reducing capabilities even though it forms only very thin boundary films on rubbing surfaces. Results obtained in the current study proved that thin viscous boundary films formed on rubbing surfaces can reduce asperities interactions of rubbing surfaces under very thin film conditions. Even though these boundary films do not separate rubbing surfaces completely, they still can provide some protection of contacting bodies against excessive friction and wear.     

Effects of fullerene C60 on the friction and wear characteristics of ceramics in ethanol

For the combinations of a pin of Si₃N₄ and five kinds of disk, the friction and wear test was carried out in ethanol, and in ethanol containing C₆₀ particles (1 wt%). A topographical analysis was also performed on the micro-asperities of the wear surfaces to estimate the behavior of C₆₀ particles, and the degree of surface damage. As a results, the following facts were found. (1) The addition of C₆₀ particles in ethanol decreased the wear rates of such ceramic disks as Al₂O₃, SiC and TiC and of the mating Si₃N₄ pins. (2) The addition of C₆₀ particles decreased the mean coefficient of friction for SiC, Si₃N₄, and TiC disks. (3) The wear rates of pin and disk depended on the topographies of worn surfaces such as the mean depth of micro-grooves, the mean tip radius of micro-asperities and so on.     

Additive efficiency and micellar structure of lubricants

This paper reports on the influence of the micellar structure of additives, for example antioxidant additives, on their performance in lubricants. It is well known that hydrocarbon lubricants have a complex liquid structure depending on the composition of the base oils and the additive packages used. The authors have elsewhere suggested a micellar inhibition mechanism that functions in cases where lubricant compositions comprise both antioxidant and detergent additives. A similar situation is created in strongly degraded lubricants when the micellar‐like structure of the hydrocarbons is due to the interaction of oxidation and degradation products. The mechanism of reverse micelle formation and its influence on the oxidation mechanism and oxidation inhibition activity of different additive types are discussed. The influence of the hydrocarbon chain length of the additives on their activity is also discussed.     

Tribological behavior of PEEK-based materials under mixed and boundary lubrication conditions

In modern industries, more and more mechanical components are exposed to mixed and even boundary lubrication conditions, inducing fast wear and even scuffing of the motion systems. In order to enhance the lifetime and reliability of the motion systems, replacing metal–metal friction pairs by metal–polymer ones can be one of the most effective approaches. The present work focuses on tribological behavior of pure polyetheretherketone (PEEK) and a formulated PEEK composite lubricated with diesel and engine oil. It was demonstrated that in mixed and boundary lubrication regimes the structure of PEEK materials affect significantly the tribological performance. Formation of a tribofilm on the surface of metallic counterbody plays an important role on the tribological behavior of the PEEK-based materials.     

Load-carrying synergism of binary additive systems: dibenzyl disulphide and halogenated hydrocarbons

In this study, the tribological performance of binary additive blends with dibenzyl disulphide and some chlorinated and brominated compounds was investigated. The tribological tests were done on a four-ball and ball-on-disc machines. For the test conditions explored, chlorinated paraffin (40% Cl), benzyl chloride, hexachloroethane, benzyl bromide and two brominated paraffins which contained 29 and 49 mass% of bromine in the presence of dibenzyl disulphide produced strong synergism with regard to their load-carrying capacities. Load-carring synergism increased with the increase of both additive concentrations. Some chlorinated and brominated hydrocarbons produced almost no synergism. Chlorinated hydrocarbons with chlorine atoms on adjacent carbon atoms and when between carbon atoms bonded with chlorine there are one or two no chlorinated carbon atoms did not exhibit antiwear and extreme pressure synergism. Increased chlorine substitution in ethane increased synergism. Amounts of iron sulphide, benzyl thiol and elemental sulphur formed from dibenzyl disulphide as well as sulphur surface concentrations for brominated compounds were strongly reduced in the case of binary additive systems' strongly improved load-carrying properties. The bromine surface concentration after wear test is very low, and independent of additive concentration. A possible explanation for the load-carrying synergism of DBDS and halogenated hydrocarbons binary additive systems is proposed.     

Friction and wear bench tests of different engine liner surface finishes

Six different bore finishes of combustion engines were bench-tested against PVD-coated rings. Ring and liner specimens were removed from actual Heavy-Duty Diesel engine. The test was conducted in a CETR UMT-2 reciprocating tester. Current bore honings, e.g. slide honing, as well as recently developed structured laser were tested.For friction bench tests, the specimens were tested at different reciprocating speeds and loads. When lubricant condition approached the boundary regime (speed/load tending to zero), most of the finishes showed the usual boundary friction coefficient 0.11. When the conditions tended to the hydrodynamic regime, the specimen removed from the non-laser region of the laser-structured finish showed a remarkably low friction coefficient, followed by the laser-structured ones.For wear bench tests, the lubricant was doped with hard particles, and higher load and speed were used to accelerate the wear rate. Wear tests ran for 4 h and, as expected, friction coefficient reduced during the wear test due to break-in. In general, friction during the wear test followed the friction test ranking. The remarkable exception was the UV laser finish, which initially presented one of the highest coefficient and after 0.5 h of testing turned to be one of the lowest. Liner and ring wear was measured post-test by profilemeter. In general, the smoother surfaces presented lower ring and bore wear.Greenwood asperity parameters and Patir and Cheng flow factors were calculated from two measured surfaces. A computer model was applied to calculate the oil film thickness and friction for the different speed and load conditions. The calculated friction results showed good adherence to the measured data.     

A review of the effect of metals on the thermo‐oxidative stability of perfluoropolyalkylether lubricants

The structural attributes and degradation mechanisms are reviewed for Fomblin Z®, Demnum^TM, and Krytox® type perfluoropolyalkylether (PFPAE) fluids both in the presence and absence of metals/alloys. Static and tribological environment effects on fluid degradation mechanisms are discussed. Two solution paths for fluid degradation inhibition are reviewed: the addition of additives to the PFPAE fluid and reducing the reactivity of metals/alloys by the deposition of less reactive coatings to their surface. Continuing research for PFPAE fluids is discussed. Copyright © 2006 John Wiley & Sons, Ltd.     

Friction and wear behavior of diamond-like carbon coating on plasma nitrided mild steel under boundary lubrication

The friction and wear properties of synthetic ionic liquid functionalized borate esters as additives in poly-alpha-olefin (PAO) were measured for diamond-like carbon (DLC) coating on plasma nitrided AISI 1045 steel. Results show that the borate esters gave much better friction–reduction and antiwear properties for DLC coating/steel and DLC coating/DLC coating sliding pairs than zinc dialkyldithiophosphate (ZDDP). In addition the DLC coating had much better wear resistance than the nitrided mild steel substrate, indicating that duplicate surface modification was more effective in significantly increasing the wear resistance of mild steel.     

The effect of branching on slip and rheological properties of lubricants in molecular dynamics simulation of Couette shear flow

Molecularly thin liquid films of alkane in extreme conditions in a thin film lubrication regime have been investigated. To get an insight into the effects of molecular architecture in the behaviour of these thin lubricant films we have studied six different molecules, mainly isomers of C₃₀. In this work the effect of branching on rheological properties and behaviour of lubricant film is examined. Our study shows viscosity and normal stress effects depend on the degree of branching. Dynamics of the molecules and their orientation are also affected by the degree of branching. A weaker layering near the wall is observed for branched molecules. Slip between the wall and lubricant film also was larger for branched molecules. Branched molecules had less tendency to change their orientation under the flow. The results obtained here could be helpful in designing new lubricants at the molecular level.     

Large‐scale specimen testing on friction and wear of pure and internally lubricated cast polyamides

Due to the casting process for nylons, their composition can easily be modified to cover a wide range of mechanical properties and applications, especially as large wear surfaces in, for example, crane guidances. Presently, selection tests for working conditions up to 40MPa are presented on pure Na‐catalysed polyamides, oil‐filled polyamides with homogeneous oil dispersions and holes in the surface containing oil lubricant and two types of thermoplastic solid‐lubricated polyamides. Pure polyamides are, however, prone to high and unstable sliding at pressures as low as 10MPa with brittle fracture and lumpy transfer. Oil lubrication is not able to remove the sliding instabilities as oil supply to the sliding interface is controlled by migration effects that are restricted by deformation and thermal softening or melting of the polyamide matrix. Although friction and wear are lower and more stable for samples with oil supplied through lubricating holes, additional running‐in phenomena are attributed to a relatively thick transfer film that is brittle and easily peels off. A continuous thick molten film or island‐like deposition occurs on the polyamide surface. Solid lubricants are able to stabilize friction and lower wear down to the formation of a thin and coherent transfer film. However, increasing the amount of lubricants induces lower mechanical properties and higher deformation of the test samples. The differences in transfer behaviour are discussed with reference to optical microscopy and calculations of bulk and flash temperatures. Copyright © 2006 John Wiley & Sons, Ltd.