In mixed lubricated contact, the asperity contact regions may be separated by a thin layer of lubricant film. Simulations of mixed lubrication often use empirical formulae to describe the change of density with pressure in the lubricated areas. However, these models may not be applicable to molecularly thin films at asperity contact regions. We present a molecular simulation approach for density characterization of the fluid at asperity contact regions in mixed lubrication. The resultant density-pressure relationship is compared to the traditional Dowson-Higginson model. It is found that the two models predict comparable densities at low pressures but diverge at higher pressures.

Superabsorbent polymers and one or more solid and/or liquid lubricants, with and without lubricant additives, were formulated into a variety of solid and variable-viscosity lubricant compositions. A series of laboratory trials showed that superabsorbent polymer-based solid (e.g., agglomerated) lubricant compositions and variable-viscosity greases were more effective in reducing friction between moving surfaces than non-superabsorbent polymer-based lubricants.

This paper shows the effect of ultrathin lubricant films between sliding bodies on Acoustic Emission (AE) signals induced by the sliding friction. Experiments were conducted with a ball-on-disk friction tester to measure the friction coefficient, the raw AE signals and the root-mean-squarevalues of the AE signals (the AErms signals). The ball was a glass ball of 5mm diameter. The disk was a magnetic disk used for 2.5 inch HDD with a DLC protective layer on its surface, and was coated with PFPE Z-dol 4000 about 1.5nm thick. The AErms signals kept a low level for some time after the start of the test, and then increased. Its time variation was similar to that of friction coefficient. After the friction test, the circumferential distribution of the lubricant film thickness was measured with an ellipsometer. The distribution demonstrated the reduction of the lubricant film thickness at the circumferential position where the magnitude of AE signals became large. These facts showed that the AE signals correlated well with the lubricant film thickness.

Vapor phase lubrication (VPL) has been proposed as a method for lubricating high temperature engines. During VPL, lubricants are delivered through the vapor phase to high temperature engine parts and react on their surfaces to deposit a thin, solid, lubricating film. Although ceramics such as SiC are desirable materials for high temperature applications, their surfaces are unreactive for the decomposition of TCP and thus not amenable to vapor phase lubrication. As a means of activating the SiC surface for TCP decomposition we have used chemical vapor deposition of Fe from Fe(CO) 5 . Modification of the SiC surface with adsorbed Fe accelerates subsequent decomposition of TCP and deposition of P and C onto the surface. In the temperature range 500–800 K, m -TCP decomposes more readily on Fe-coated SiC surfaces than on SiC surfaces. The C and P deposition rates depend on the thickness of the Fe film and are further enhanced by oxidation of the Fe. This work provides a proof-of-concept demonstration of the feasibility of using vapor phase lubrication for ceramics.

The knowledge of wear kinetics of thin soft lubricant or hard coatings under alternated sliding contact is of great interest for many industrial applications. Because the coating endurance is related to the substrate reaching condition, it has been shown that classical wear volume descriptions are not appropriated. A local damage description, based on a local friction dissipated energy density variable is then introduced. It consists to compare the coating endurance (i.e. number of sliding cycles) versus the maximum local energy density dissipated through the interface per sliding cycle. A “life time vs maximum dissipated energy density master curve” is obtained and rationalized through a parabolic evolution. The coating endurance is modelized through a simple ratio between an energy capacity variable, representative of the durability of the studied coating. Applied to hard coatings (TiN) the stability of this approach has been confirmed for solid lubricant coatings.

Dimethyl Ether (DME) has been known as a ultra clean smokeless fuel for diesel engines for over a decade now. A significant challenge is to ensure full lifetime for the diesel engine injection equipment when handling the low boiling point DME as this fuel has very low viscosity and lubricity. Pressurised lubricity evaluators show that boundary lubrication additives increase DME lubricity significantly but recent work indicate that adequate performance in quasi-boundary lubrication is necessary for reducing the wear effectively in real pumps. In this regime where the pump surfaces are only separated by a few molecular layers fuel molecular structure and viscosity seem to be important properties. Earlier work showed that addition of low lubricity alkanes raised the high lubricity of a boundary lubricant to an even higher level. In the present work the long linear alkane n-hexadecane was added to DME in different proportions. The lubricities of the blends do not show any minimum but reflect the lubricity of pure n-hexadecane. It is not conclusive whether DME is a boundary or a quasi-boundary lubricant but clarifying tests are currently in progress.

A series of cooperative interlaboratory tests (RR, round robins) was conducted annuallly from 1997 to 2005 by the DIN 51834 Working Group on Tribological Tests using a Translatory Oscillation Apparatus ( SRV ™ ® ) in cooperation with ASTM D02. The statistical analysis of these test results shows the influence of cleaning solvent and seizure criteria on the tribological properties regarding O.K.-load of the lubricants tested. Coefficients of friction and wear results are ranked according to the effects of ten different cleaning solvents, where isopropanol gave the lowest values and isoparaffin solvents the highest. The wear volume is more descriptive than the wear scar diameter. The effect of machine model on coefficients of friction varied from about 0.2% to 0.9% of mean. Wear results were not affected. The load step interval of 1, 3 or 6 minutes influences the pass load of solid bonded films.

Spring constants and damping coefficients of a molecularly thin liquid bridge of PFPE lubricant intervening between a diamond probe tip and diamond like carbon (DLC) surface of a magnetic disk are identified through regression analysis of tip damping vibration. Perfluoropolyether (PFPE) lubricants having functional end groups were used to form the liquid bridge between the DLC surface and a probe tip with the notably small curvature radius of 0.1 μm. The tip was both retracted from and extended toward the disk surface at four different, progressive distances to attain varied elongation of the bridge, and was made to vibrate at each step to provide damping waveforms. By applying regression analysis to observed waveforms, the spring constant and the damping coefficient of the liquid bridge were identified. It is interesting to note that both the absolute value of the spring constant and the frequency-multiplied damping decrease with bridge elongation, and after reaching the minimum, those values begin to increase.

We experimentally and analytically investigated detailed characteristics of the bouncing vibrations of a flying head slider in a near-contact regime. In the experiment, we found that the hysteresis of touch-down and take-off pressure and the rate of instability become small as the pitch angle increases. Moreover, we measured the 3-dimensional slider motion by using two laser Doppler vibrometers simultaneously and found that the bouncing vibration is a coupled vibration between translation and pitch with a small phase shift. These experimental features can be explained analytically if we consider strong shear force due to lubricant and small amount of microwaviness for the previous two-degrees of freedom slider model with nonlinear air bearing stiffness.

In this study, the lubricant viscosity from a diesel engine operating on various percentages of coconut oil blended fuel is investigated to determine the suitable mixture appropriate for a diesel engine. The coconut oil was blended with conventional diesel fuel at an amount varying from ten to fifty percent. The lubricant used in this study is the conventional commercially available lubricant (SAE 40). After proper blending of diesel with coconut oil, the blended fuels are loaded into the fuel tank. The engine is operated at half throttle setting running at 1600 rpm for 100 hours. At every ten hour interval, one lubricant sample was collected and analyzed using a viscometer at 100 °C and 40 °C. The results show that the viscosity for all the blended fuel remain about constant throughout the 100 hours test. However, fifty percent coconut oil blended fuel at 40 °C shows slightly lower viscosity level in comparison to other percentages of blended fuel system. It is expected this is due to high fuel dilution in lube oil. As a result, ten to thirty percent coconut oil blended fuels seems suitable for diesel engine because it’s able to prolong lubricant life span by maintaining its viscosity.

Vapor phase lubrication (VPL) integrates media lubrication with the vacuum processing steps used throughout most of the hard disk media fabrication process. This avoids exposure of the unlubricated a-CH x overcoat surface to the ambient air and airborne contamination. In vapor lubrication the a-CH x surface can be oxidized under controlled conditions immediately prior to lubricant adsorption. The kinetics of a-CH x oxidation have been studied using x-ray photoemission spectroscopy in an apparatus that allows oxidation of freshly deposited a-CH x films. The dissociative sticking coefficient of oxygen is ∼10 −6 and the initial oxidation kinetics can be described by a Langmuir-Hinshelwood mechanism.

In order to grasp the possibility of evaluating shear properties for solidified lubricants under high pressure, plastic deformations of metal micro-spheres (about 0.07mm) in solidified lubricants were evaluated by employing a diamond-anvil pressure cell (DAC). Large deformations (2–5 times larger than the original sphere dimensions) were observed for CVT oil and ester oil up to 6 GPa at 23–25°C. Deformation starting pressure agreed with the solidified pressure. These deformations were caused by the non-hydrostatic pressure in the solidified lubricants. Shear stresses of the solidified lubricants were tentatively and roughly estimated from the plastic deformations of the spheres based on some assumptions. They almost agreed with the mean shear stress (traction force / hertzian contact area) from traction test.

Plane steady isothermal and non-isothermal EHL problems for non-Newtonian lubricants in a line contact are considered. Two cases of lubricant rheologies are studied. Namely, lubricants for which (a) the shear stress is any explicitly given nonlinear function of the shear strain and (b) the shear strain is any explicitly given nonlinear function of the shear stress are analyzed. The isothermal EHL problem is reduced to solution of a nonlinear equation for the sliding shear stress f, generalized Reynolds equation for pressure p, and the equation for gap h as well as the classic balance and boundary conditions. The non-isothermal EHL problem is reduced to solution of the above mentioned nonlinear equations and the nonlinear equations for temperatures of the lubricant T and of the contact surfaces T w1 and T w2 . The EHL problems are considered in the case of heavily loaded contact when the rolling shear stress in lubricant is much smaller than the lubricant sliding shear stress [1–7]. Therefore, the problems contain a small parameter represented by the ratio of the characteristic rolling and sliding shear stresses. That leads to the opportunity to use the perturbation methods for simplifying the EHL problem formulations.

As well recognized, a functional surface finish can determine how well the surface wears, transmits heat, and distributes a lubricant. Among the large number of surface parameters available to describe surface finish requirements, the arithmetic mean of surface roughness, Ra, is the most specified surface finish parameter across most industries and applications. However, Ra may not fully describe the functional requirement of the surface. This study is focused on the selection of surface parameters based on local contact pressure, an important indicator of friction and wear. Surface parameters are obtained from measured or generated three-dimensional topographies. Surface performance responses, evaluated by local contact pressures, are calculated from micro-contact analysis based on the rough surfaces. The analysis of correlation, indicated by Pearson correlation coefficient that is a measure of correlation strength of linear relationship, is performed among these surface parameters and between surface parameters and responses. A set of functional parameters is identified to properly specify a surface evaluated by local contact pressure.

To measure the properties of lubricant under high pressure, a diamond-anvil pressure cell was employed. With a falling sphere viscometry, viscosities were measured up to 2 GPa at up to 200 °C for traction oils. The results showed linearity between logarithmic viscosity and pressure at any temperatures. At 200 °C, lubricant viscosity-pressure coefficient fell to between 1/4 and 1/3rd of the value at 40°C. A simple expression for the dependence of high pressure viscosity on temperature has been suggested. Pressure temperature phase diagram of liquid-solid transition was plotted from the obtained viscosity. The change in volume due to solidification was estimated up to 6 GPa.

Room temperature ionic liquids (RTILs) are molten salts with melting points at or below room temperature. RTILs have recently been recognized as novel lubricants. Only a few have previously been evaluated.

The thermal properties of engine oil are important traits affecting the ability of the oil to transfer heat from the engine. The larger the thermal conductivity and specific heat, the more efficiently the oil will transfer heat. In this work, we measured the thermal conductivity and specific heat of a conventional mineral oil-based diesel engine lubricant and a Group V-based LHR diesel engine lubricant as a function of temperature. We also measured the specific heat of ethylene glycol. The measured values are compared with manufacturers’ data for typical heat transfer fluids. The Group V-based engine oil had a higher thermal conductivity and slightly lower specific heat than the mineral oil-based engine oil. Both engine oils had values comparable to high-temperature heat transfer fluids.

Self-assembled monolayers (SAMs) have received considerable recent attention as molecular-level lubricants in, for example, micro-electro-mechanical systems (MEMS).[1] Of particular interest as tribological films have been SAMs terminated by fluorocarbon groups, because of their inert nature and enhanced thermal stability. Surprisingly however, fluorocarbon films were shown to actually produce higher coefficients of friction (relative to CH 3 -terminated films) in atomic force microscopy (AFM) studies. Subsequent work has concluded that the increased van der Waals radius of the fluorine groups (∼45%) causes a steric disruption of the order of the molecular surface giving rise to an increased friction.[2] We present results from a direct comparison of the adhesive, mechanical and frictional properties of SAMs terminated by CF 3 and CH 3 groups using both Interfacial Force Microscopy[3] (IFM) and the AFM. IFM results are shown for a two micron tungsten tip interacting with C 16 alkylthiol molecules assembled on Au(111) single-crystal surfaces. AFM results involve a ∼20 nm tip interacting with the same two molecules assembled on Au films deposited on mica surfaces. A direct AFM comparison is accomplished by using a “nanografting technique”.[4]

An experimental study was performed to measure the static friction coefficient under different experimental conditions. These include different surface roughness conditions, the effect of dwell time, the effect of acceleration (sliding velocity) as well as the presence of traces of lubricant and wear debris at the interface. The static friction tester provides accurate measurement of friction, normal and lateral forces at the interface (using a high dynamic bandwidth piezoelectric force transducer) as well as precise motion control and measurement of the interface motion. The experimental measurements were subsequently compared with an improved elastic-plastic rough surface static friction model, and it was found that the model captures the experimental measurements well, especially in terms of surface roughness. However, the data also shows the limitations of the model as it fails to accurately capture the effects of experimental conditions such as the presence of wear debris and start up velocity.

The influence of lubricant characteristics and operating parameters on the elastohydrodynamic lubrication in hypoid gears is investigated. The full thermal elastohydrodynamic analysis of lubrication is applied, based on the simultaneous solution of the Reynolds, elasticity, energy, and Laplace’s equations. The oil viscosity variation with respect to pressure and temperature and the density variation with respect to pressure are included. Using a computer algorithm, the influence of oil viscosity, pressure-viscosity and temperature-viscosity exponents, supplied oil temperature, speed and minimum oil film thickness on maximum oil film pressure and temperature, EHD load carrying capacity, and power losses in the oil film is investigated. The obtained results are presented and discussed.