Application of Spectroscopic Reflectometry to Elastohydrodynamic Lubrication Films Study

Optical measurement techniques have been successfully used for elastohydrodynamic (EHD) lubricant films studies for several decades and have significantly helped to understand the lubrication mechanisms within highly loaded machine contacts. Nevertheless, there are still many phenomena waiting for the explanation and new experimental approaches and measurements techniques are developed. Recent studies have provided promising results as to the application of spectroscopic reflectometry to the study of EHD films. Nevertheless, some simplifications were introduced. The main aim of this study was to develop a physically correct approach that could provide the additional information about the properties of lubricant film within highly loaded contacts. The principal part of this article was devoted to the effort to develop and verify the optical design suitable for such applications. This verification was carried out within lubricated contact formed between a steel barrel and sapphire disc without any semi-reflective layer. This simplified optical arrangement has enabled to obtain the correct lubricant film data and verify the applicability of the spectroscopic reflectometry for EHD lubrication films study. It represents the first step in this application of spectroscopic reflectometry and further research in the field of the behavior of thin solid films under high contact pressures is necessary to enable thin film measurements.     

Influence of surface roughness features on mixed‐film lubrication

An optical technique (three‐dimensional spacer layer imaging) has been developed to map accurately lubricant film thickness in thin‐film elastohydrodynamic (EHD) contacts. This experimental technique has been used to study the influence of surface roughness features, asperity height, and slope on EHD film thickness and pressure. Single ridges transverse to the entrainment direction were used to represent asperities. It was found that the ridges with lower slopes generate films of greater minimum thickness. Below a certain entrainment speed, the minimum film thickness declined at a rate dependent on the ridge slope. At low speeds, the ridges with higher slopes entrapped a larger volume of lubricant ahead of the ridge and along the entrainment direction. For all speeds, the highest ridges entrapped the most lubricant. Both ridge slope and ridge height had a negligible effect on mean film thickness in the contact. Asperity pressure increased with higher ridge slope, but was not influenced by entrainment speed. An increase in pressure was found where lubricant is entrapped upstream of a ridge.     

Elastohydrodynamic Film Thickness Mapping by Computer Differential Colorimetry

This paper describes a new experimental technique for the study of elastohydrodynamic (EHD) lubricant films. This technique, which is based on the computer processing of EHD chromatic interferograms, uses a combination of image analysis and differential colorimetry for film thickness evaluation. This approach overcomes some major limitations of conventional optical interferometry and allows the precise mapping of lubricant film thickness distribution in EHD contacts, including transient and quasistatic phenomena. The technique has been used for the evaluation of chromatic interference patterns obtained from a conventional optical test rig for rolling point contacts. Three-dimensional representations of lubricant film thickness and shape with high accuracy and spatial resolution have been obtained. The technique's accuracy has been checked and a comparison with conventional monochromatic interferometry has been done for validation. The technique's resolution has been confirmed through the observation of local film thickening just before the EHD exit constriction for both pure rolling and sliding conditions.     

The effect of surface texturing on thin EHD lubrication films

Surface texturing has been successfully used for conformal contacts in many tribological applications in an effort to diminish friction and wear. However, the use of such a surface modifications are still in nascent as far as highly loaded contacts between non-conformal surfaces are concerned. It is mainly caused by the fact that the presence of such micro-features within these contacts can significantly influence the pressure distribution within the contact. Nevertheless, it has been shown in recent studies that the surface texturing can also have beneficial tribological effects if the depth of micro-features is properly designed. This paper is devoted to the experimental study of the effect of the micro-dents of various depths on thin lubrication films to find an experimental evidence of the micro-feature depth threshold for surface texturing applications in highly loaded non-conformal surfaces. The behaviour of an array of micro-dents within thin EHD contacts has been studied by thin film colorimetric interferometry. The influence of surface texturing on lubricant film formation has been observed under sliding/rolling conditions. The significant effect of micro-dents depth on lubricant film thickness is observed for positive slide-to-roll ratio when the disc is moving faster than the micro-textured ball. The presence of deep micro-dents within lubricated contact results in film thickness reduction downstream. As the depth of micro-dents is reduced, this effect diminishes and beneficial effect of micro-dents on film thickness formation has been observed. No significant influence of micro-dents depth on lubricant film shape has been observed in case of negative slide-to-roll conditions when micro-dents do not cause film thickness reduction regardless of their depths.     

Boundary Film Formation by Lubricant Base Fluids

The measurement of lubricant film thicknesses in a rolling steel ball on glass flat contact, down to one nanometer is now possible by using ultrathin film interferometry. This technique has been used to study the film-forming properties of a range of synthetic base fluids in the mixed elastohydrodynamic and boundary lubrication regimes.

For a very highly purified sample of hexadecane, it was found that classical EHD theory was obeyed down to less than one nanometer, indicating that any boundary film formed by this fluid was less than I nm thick. Most other synthetic fluids tested gave thicker films than predicted from EHD theory under very thin film conditions. This deviation from theory occurred at between 1 and 10 nm for different fluids. The effect can be interpreted as resulting from the presence of boundary layers, one or two molecular layers thick of the fluid on each solid surface, which are more viscous than the bulk lubricant.     

Thin film lubrication of sliding point contacts of AISI 52100 steel

The mechanism of thin film lubrication of sliding point contacts of AISI 52100 steel has been studied as a function of load, sliding speed, composition and temperature of the lubricant.Below certain critical combinations of Hertzian pressure, speed and temperature the surfaces are kept apart by an elastohydrodynamic lubricant film. The load carrying capacity of this film depends primarily on the effective viscosity of the lubricant in the contact region which decreases with bulk oil temperature and with increasing sliding speed, because of friction induced thermal effects. After breakdown of the EHD film, boundary lubrication may still prevent severe adhesive wear. The transition from the boundary lubricated regime towards the regime of severe adhesive wear is a function of load (normal force), speed and bulk oil temperature and possibly depends on the conjunction temperature. Irrespective of the initial lubrication condition, oxidation of the steel surfaces leads to the (re)establishment of low friction, mild wear conditions.     

Modélisation du contact lubrifié—exemple de la mise en forme des métaux

Lubricated contact modelling — the example of metal forming processes. The theory of hydrodynamic (HD) lubrication was developed after the derivation of the Reynolds' equation in 1886. It allowed us to understand firstly low pressure lubricated contacts, then, when coupled with elastic deformation equations, high pressure lubricated contacts in elasto-hydrodynamic (EHD) lubrication. In the 60's, the same techniques were transposed to plasto-hydrodynamic lubrication (PHD), where one of the solids in contact is under plastic deformation, such as in metal forming processes. With the example of wire-drawing, the application of PHD models to metal forming is recalled. The importance of temperature effects on film formation and evolution was then an incentive to transposing thermo-EHD models into thermo-PHD. The roughness–lubricant flow coupling is then described, in EHD then in PHD. Finally, models of the mixed lubrication regime are addressed, whereby average contact pressure is borne partly by solid–solid, or micro-EHD, or boundary films, and partly by the hydrodynamic or hydrostatic pressure in the lubricant. Following a discussion of a few results on strip cold rolling, perspectives for evolution of these mixed regime PHD models are presented, in view of recent mixed-EHD models from the literature.     

Thin lubricating films behaviour at very high contact pressure

Thin film colorimetric interferometry has been used to examine the behaviour of thin elastohydrodynamic (EHD) lubricant films under very high contact pressures of the order of 0.5–3 GPa. It has been shown that at moderate pressures, the variation of film thickness with speed follows the Hamrock and Dowson prediction down to one nanometer. As the load is increased, however, thin films behave differently from the prediction of the conventional EHD theory. For a certain lubricant and operational conditions, there is a critical rolling speed below which a reduction of film thickness is observed. This behaviour is very similar to that previously predicted computationally by Zhu.     

Numerical modeling an elastohydrodynamic contact of shaped rollers with allowance for misalignment of their axes

A method of numerically solving an elastohydrodynamic (EHD) contact of shaped rollers with allowance for misalignment of their axes in a plane perpendicular to the rolling direction is advanced. The mode of EHD lubrication is typical of such friction assemblies as roller bearings and gearings, in which the contacting elastic bodies are separated by a lubricant film and deformed under the action of an external load. Results of numerical modeling demonstrate the significant effect of the misalignment angle on the distribution of pressure and thickness of the lubricant film in the EHD contact and can be used further to analyze friction in a contact area and the stress tensor in a subsurface layer. The mathematical model of the EHD contact is described through nonlinear integro-differential equations and inequalities. The computational algorithm is based on Newton’s method.     

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.     

Oscillations induced in EHD film thickness by a step in entrainment speed

Transient elastohydrodynamic (EHD) lubrication conditions occur in the contacts of many machine elements, such as gears, cams, and reciprocating devices, as a result of their working cycles. These conditions also occur in rolling‐element bearings at the onset or cessation of motion. The aspect of film thickness in elastohydrodynamically lubricated contacts subjected to a very rapid change in entrainment speed has not received much attention from researchers, probably because it is seen as less problematic than a sudden fall of the entrainment speed, which theoretically can lead to film failure. For a sudden stop, however, it has been shown previously that the lubricant forms an entrapment, which is able to protect the contact in many cases when the motion resumes. In this paper, EHD film behaviour under sudden acceleration is investigated; the study covers three cases ‐ starting from zero film, starting from an entrapped film, and starting from a continuous, steady film.     

The shear stress properties of ester lubricants in elastohydrodynamic contacts

This paper describes a systematic study into the influence of molecular structure on lubricant shear stress in elastohydrodynamic (EHD) contacts. An infrared emission technique has been employed to measure surface temperatures in an EHD contact and thence to determine the shear stress profile of lubricant films therein. The effect of structure on shear stress has been investigated by comparing the behaviour of a range of well-characterised, closely related, ester base fluids. Considerable variations in shear stress response to EHD conditions have been observed, depending upon the type and structure of the ester.     

Fractionation of liquid lubricants at solid surfaces

The film-forming properties of lubricant base fluid mixtures in elastohydrodynamic contacts have been studied using ultrathin film interferometry. It has been shown that in binary mixtures where one of the components is more polar than the other, the EHD film thickness formed in the very thin film (< 10 nm region) is controlled by the viscosity of the polar component rather than the viscosity of the blend. This means that a mixture of a highly viscous ester in less viscous hydrocarbon gives thicker than predicted lubricant films in the sub 20 nm region and vice versa. This phenomenon can be ascribed to the fractionation of the lubricant mixtures close to the surface caused by lubricant molecule/surface van der Waals forces.     

Thin-Film Lubrication of Dented Surfaces

Surface microfeatures introduced to conformal surfaces have been proved in the last decade to provide beneficial tribological performances. They were found to significantly improve load capacity, wear resistance, or friction coefficient in applications that involve mechanical seals, piston rings, thrust bearings, or ultra-high-density magnetic disc drives. Recent studies have suggested that such an approach could be used to improve the lubrication capabilities under thin-film lubrication of highly loaded non-conformal contacts. However, surface micro-features influence the film thickness and pressure distribution within concentrated contacts that could result in surface failures. In this paper, thin-film colorimetric interferometry has been used to study the effects of an artificially produced micro-dent on film thickness distribution within thin-film lubricated contacts. Obtained results have shown that the behavior of dented surfaces significantly depends on the slide-to-roll ratio. An increase in the lubricant film thickness has been observed just upstream of the trailing edge of the micro-dent when the disc is moving slower than the ball with the micro-dent. In the reverse conditions, for a positive slide-to-roll ratio, the presence of the micro-dent within the concentrated contact results in the film thickness reduction located downstream of the leading edge of the micro-dent. This reduction can cause the local film breakdown of very thin films. Nevertheless, it has been observed that highly viscous boundary films can avoid it and rubbing surfaces have been completely separated using the formulated oil even under very thin lubrication conditions.     

Substrate surface energy effects on a liquid lubricant film at nanometre scalesubstrate surface energy effects on a liquid lubricant film at nanometre scale

In this paper is discussed the effect of the physical characteristics of substrate surfaces on the lubrication properties of thin films at nanometre scale. Different coatings with different surface energies have been formed on the surface of a steel ball by means of plasma assisted sedimentation (PAS). The ball was put in a pure rolling system in point contact, where the lubricant film is measured by relative optical interference intensity (ROII). Experimental results show that the film thickness is closely related to the substrate surface energy when the film is in the nanometre scale, and that the combined surface roughness in the contact region is closely related to the liquid lubricant film thickness and the contact pressure. The thinner the film and the higher the contact pressure, the smaller will be the combined surface roughness. Lastly, the relationship between critical film thickness and its influencing factors is discussed.     

Effect of surface texturing on elastohydrodynamically lubricated contact under transient speed conditions

Effect of surface topography modifications on lubrication film thickness within non-conformal lubricated contact operated under transient speed conditions is observed. Optical test rig is used to observe the lubricant film behaviour between the flat surface of a chromium coated glass disc and a steel ball under simplified operational conditions modelling the cam and tappet contact. Numerical simulation was used to be able to choose the operating conditions suitable for experiments. An array of micro-dents was produced on the ball surface to be able to demonstrate the effect of surface topography on lubrication film formation. Experiments were carried out under elastohydrodynamic lubrication conditions. Obtained results have shown that surface texturing could represent the way how to increase lubrication efficiency of rolling/sliding non-conformal contacts under transient operational conditions through the lubricant emitted from micro-dents. It was found that the lubricant emitted from the micro-dents helps to separate rubbing surfaces especially under thin film lubrication conditions where the rubbing surfaces moves in the opposite direction.     

Lubricant Flow in an Elastohydrodynamic Contact Using Fluorescence

It is well-documented that parameters, such as film thickness and temperature in EHL contacts, can be measured experimentally using a range of techniques include optical interferometry, ultrasonics, capacitance and infrared emission. Considerably less is known, however, about the flow of lubricant through such contacts. Information about lubricant flow would greatly benefit the prediction of friction in machine components. This article describes initial steps to develop fluorescence as a means of observing lubricant flow. An EHL contact was produced between a steel ball and a glass disc and viewed using a fluorescence microscope. The entrained lubricant was dyed using a fluorescent species, so that when illuminated with laser light, a fluorescence intensity map could be viewed. When the contact was fully flooded with dyed lubricant, the fluorescence intensity within the contact correlated well with optical interferometric film thickness measurements under the same conditions. This suggests useful possibilities for mapping film thickness in contacts where conventional optical methods are impractical, such as between rough surfaces and within soft contacts. In order to observe how lubricant flows in an EHL contact, fluorescer-containing lubricant was placed on the out-of-contact track. The boundary between fluorescent and non-fluorescent lubricant was then entrained into the contact and the passage of the boundary through the contact was monitored.     

Basics of mixed lubrication

This paper reviews recent research in mixed lubrication, focusing on the current understanding of lubricant entrainment, and thus film thickness, and of friction in thin‐film, rough‐surface lubricated contacts. By combining research using optical interferometry on the experimental side and numerical modelling on the theoretical side, we now have a reasonable understanding of micro‐elastohydrodynamic lubrication, although design rules are still lacking. The regime of true mixed lubrication, where there are both elastohydrodynamic and boundary lubricated regions within a single contact, remains quite poorly understood. New experimental techniques as well as new information about very thin‐film rheology under high‐strain and high‐pressure conditions are probably needed before much further progress can be made in this area.     

Non-newtonian thermal analysis of an EHD contact lubricated with MIL-L-23699 oil

A thermal and non-Newtonian fluid model under elastohydrodynamic lubrication conditions is proposed, integrating some particularities, such as the separation between hydrodynamic and dissipative phenomena inside the contact. The concept of apparent viscosity is used to introduce the non-Newtonian behaviour of the lubricant and the thermal behaviour of the contact into the Reynolds equation, acting as a link element between the hydrodynamic and dissipative components of the EHD film, independently of the rheological and thermal models considered. The apparent viscosity enables the application of the rheological model better adapted to each lubricant, without appealing to special formulations of the EHD problem.The Newton–Raphson technique is used to obtain the lubricant film geometry and the pressure distribution inside the EHD contact. The shear stresses developed in the fluid film are evaluated assuming the non-linear Maxwell rheological model. The surfaces and lubricant temperature distributions are determined using the simplified Houpert's method, applied to the inlet contact zone, and the thermal method proposed by Tevaarwerk is applied in the high pressure contact zone.The non-Newtonian thermal EHD model is applied to the analysis of a contact lubricated with MIL-L-23699 oil. Significant results are obtained for the centre and minimum film thickness, for the inlet shear heating and film thickness reduction factor (φ_T), for the temperature rise of the lubricant and of the surfaces and for the friction coefficient inside the contact, considering wide ranges of the operating conditions (maximum Hertzian pressure, inlet oil temperature, rolling speed and slide-to-roll ratio).Finally, the numerical traction curves determined are compared with the corresponding experimental results, showing very good correlation.     

Measurement of the Rheology of Lubricant Films Within Elastohydrodynamic Contacts

There is growing need for a reliable model of the rheological response of lubricants in elastohydrodynamic (EHD) contacts, not only to predict behaviour in full-film EHD conditions, but also for use in modelling mixed-film lubrication. One barrier to developing such a model is that measurements of friction actually represent averaged values over the whole, lubricated contact under study. However the fluid film conditions of temperature, pressure and strain rate generally vary over such contacts, which makes it difficult to determine constitutive shear-stress equations from friction measurements. This paper examines the various different techniques used to study the origins of EHD friction and the underlying film rheology. It then describes and applies a technique for obtaining the temperature rise maps of both solid surfaces in a rolling-sliding EHD contacts and thus shear-stress and friction maps. The work shows that the shear stress of the traction fluid studied increases approximately linearly with pressure and decreases approximately linearly with temperature in the high-pressure central region of EHD contacts.