R. P. Glovnea

Study of Zinc Dialkydithiophosphate Antiwear Film Formation and Removal Processes, Part I: Experimental

Two recent trends in engine oil formulation are a progressive reduction in phosphorus concentration so as to reduce its impact on the de-NO x catalyst, and an increase in dispersant concentration to control the level of lubricant viscosity increase over extended drain intervals. Unfortunately, both of these trends make it more difficult to generate and retain effective antiwear films on lubricated surfaces and both thus strengthen the need to understand the processes by which antiwear films are formed, and removed, during rubbing. This article and its companion outline a study of the kinetics of antiwear film growth and removal. In Part I, a test method for monitoring antiwear film thickness during rolling/sliding is described and employed to explore how various factors, including operating temperature, antiwear additive type and concentration, and the presence of dispersant, influence both the formation and removal of the tribofilms formed by the antiwear additive zinc dialkyldithiophosphate (ZDDP). Part II then analyzes the obtained results to derive a kinetic model of ZDDP film formation and removal (Fujita, et al. [1]. Presented at the STLE Annual Meeting in Toronto, Ontario, Canada May 17-20, 2005 Review led by Elaine Yamaguchi

A Low Friction Bearing Based on Liquid Slip at the Wall

In recent years it has been shown experimentally by a number of workers that simple, Newtonian liquids can slip against solid surfaces when the latter are both very smooth and lyophobic. It has also been shown theoretically how, based on a half-wetted bearing principle, this phenomenon may be used to significantly reduce friction in lubricated sliding contacts and thus make possible the hydrodynamic lubrication of very low load contacts. This paper describes the experimental validation of this concept. A low load bearing is constructed and the influence of surface roughness and the wetting properties of the surfaces on friction are investigated over a wide range of sliding speeds. It is shown that liquid slip can be used to considerably reduce friction in full film, hydrodynamic conditions.

Measurement of Sub-Nanometer Lubricant Films Using Ultra-Thin Film Interferometry

The ultra-thin film interferometric method of measuring the thickness of very thin films in lubricated contacts has been refined so as to be able to measure films down to 0.3 nm with a standard deviation of 0.15 nm. The main remaining source of measurement variation for films below 3 nm thick is the surface roughness of the contacting solids. This modified technique has been applied to study the film-forming properties of three fluids, hexadecane, a dilute solution of surfactant in hexadecane, and cyclohexane. Purified hexadecane shows a very slightly enhanced oil-film thickness below 1 nm. The long-chain surfactant forms a boundary film 2 nm thick. Cyclohexane behaves as though it forms a surface layer about 1 nm thick with viscosity three times the bulk fluid viscosity.

The Effects of Three-Dimensional Model Surface Roughness Features on Lubricant Film Thickness in EHL Contacts

The Spacer Layer Imaging method has been used to investigate the influence of three-dimensional roughness features on the thickness and shape of elastohydrodynamic (EHL) films. An array of near-hemispherical bumps was employed to represent asperities. A micro-EHL film developed at the bumps whose orientation depended on that of the inlet boundary at the location at which the bump had entered the contact. Rolling-sliding conditions induced a micro-EHL film with a classical horseshoe shape at the bumps. The flow of lubricant around the bumps appeared to differ between thin and thick films.

Elastohydrodynamic film formation at the start-up of the motion

Abstract This paper describes an experimental study of elastohydrodynamic (EHD) lubricating film formation during the start-up of motion of a point contact from rest. EHD film thickness was measured using ultra-thin optical interferometry. It was found that film thickness behaviour depends strongly upon acceleration. When motion starts, most lubricants form a front that travels across the contact with its initial thickness unchanged. At some point, depending upon the acceleration, a second front often forms, so that the lubricant film has a characteristic stepped profile. This behaviour occurs in both pure sliding and pure rolling conditions. The development of film profile over time can be used to chart the speed of motion of the fluid film in the contact. In pure rolling conditions, the first film front travels through the conjunction at a speed lower than the entrainment speed in the first half of the contact and higher in the second half. In pure sliding it travels at a velocity higher than the entrainment speed across the whole width of the contact. The leading edge moves as a core of lubricant with a velocity constant through 60 per cent of its thickness. At very high accelerations, the central film thickness shows damped oscillations about the steady state final entrainment speed value. These oscillations are similar to those found theoretically by other workers for an accelerated slider bearing using a Navier-Stokes analysis. It is believed that some of the non-classical EHD behaviour observed on start-up may result from the finite rate of momentum transfer across the fluid film.

Compression of a Single Transverse Ridge in a Circular Elastohydrodynamic Contact

A detailed experimental study has been made of the behavior of a 100 nm high transversely oriented ridge in an elastohydrodynamic (EHD) contact. Ultra-thin film interferometry has been used to measure film profiles accurately over a very wide range of lubricant film thicknesses, from a few nanometers up to nearly one micron. This enables the recovery of the amplitude of the inlet perturbation geometry with increasing EHD film thickness to be quantified and compared with numerical predictions. In pure rolling under very thin film conditions, corresponding to a smooth surface EHD film thickness of 10 nm, the surfaces near the ridge were squashed down, leading to a constriction in the film of only about 9 percent of the height of the un-deformed ridge. As the EHD film thickness increased, this deformation recovered until the ridge constriction regained about 90 percent of its original height at film thicknesses of about 1 μm. However this relatively rapid recovery only occurred in pure rolling and is attributed to the local perturbation of film convergence which the ridge generates while in the inlet region. This propagates through the contact at the mean speed of the surfaces and-in pure rolling-acts to diminish the effect of local squeeze. When sliding was present, the ridge remained almost fully deformed even when the mean film thickness was as much as twice the height of the original ridge. In this case, the ridge travels through the contact at a different speed from the mean of the two surfaces. The consequent decoupling of the ridge and the convergence perturbation results in a large local pressure due to squeeze which acts to inhibit recovery of the ridge. The general trend of the behavior of the lubricated ridge is shown to be in good agreement with earlier theoretical results.

Elastohydrodynamic film collapse during rapid deceleration: Part I Experimental results

This paper describes a study of the behavior of elastohydrodynamic lubricated contacts subjected to rapid halting. Experiments have been carried out using ultrathin interferometry coupled to a high-speed camera to measure the change in lubricant film thickness and shape during fast, controlled deceleration, both in pure sliding and pure rolling conditions. Film collapse is seen to occur in two stages. The first persists throughout the deceleration period and, during this stage the film geometry remains almost constant across the contact. In this stage of film collapse, the film thickness lags behind the value predicted from steady-state theory, which means that when motion ceases, a thicker than expected film is present. The second stage of film collapse ensues when the entrainment speed falls below a critical value of approximately 0.002 m/s and is characterized by the formation of a central entrapment and classical, normal approach, squeeze behavior.

Elastohydrodynamic Film Collapse During Rapid Deceleration. Part II—Theoretical Analysis and Comparison of Theory and Experiment

This paper presents a theoretical model for the behavior of elastohydrodynamic films subjected to transient speed conditions, based on Grubins analytical solution for elastohydrodynamic lubrication. This model is applied to predict film thickness in high deceleration conditions. The models predictions are compared with the experimental results presented in an accompanying paper entitled Elastohydrodynamic Film Collapse During Rapid Deceleration.

The Influence of Lubricant Upon EHD Film Behavior During Sudden Halting of Motion

Although steady state elastohydrodynamic (EHD) lubrication is quite well understood both from the theoretical and from the experimental point of view, studies of transient effects in EHD are currently far less developed. This paper describes an experimental investigation into EHD film behavior during sudden halting of motion. A technique has been devised which enables both central lubricant film thickness and film thickness profiles to be measured every millisecond during halting of a ball on flat, sliding contact. This has enabled detailed information of influence of lubricant on film collapse during halting to be obtained. It is shown that film collapse occurs in two stages. The first is a very rapid reduction in film thickness with only very small changes in film geometry and thus pressure distribution. This is followed, as soon as entrainment ceases, by the formation of a lubricant entrapment, and subsequent slow leakage of fluid from the central film region. This paper focussed on the formation of this entrapment and the influence of the rheological properties of the lubricant, i.e. viscosity and pressure-viscosity coefficient, on its development and behavior. Presented at the 54th Annual Meeting Las Vegas, Nevada May 23–27, 1999

Lubrication of Rough Surfaces by a Boundary Film-Forming Viscosity Modifier Additive

In previous work it was shown that some functionalized polymers used as viscosity index improvers are able to form thick boundary lubricating films. This behavior results from adsorption of the polymer on metal surfaces to form a layer of enhanced viscosity adjacent to the surface. In the current work the behavior of one such polymer in rough surface contact conditions is studied, using both model and real rough surfaces. It is found that the polymer is able to form a thick boundary film in rough surface contact, just as it does with smooth surfaces. It is also shown that the effect of this boundary film is to significantly reduce friction in rolling-sliding, rough surface, lubricated contact.