I. Křupka

An Automatic System for Real-Time Evaluation of EHD Film Thickness and Shape Based on the Colorimetric Interferometry

This paper presents an automatic system for the real-time evaluation of elastohydrodynamic (EHD) film thickness and shape that incorporates an advanced experimental apparatus controlled by a computer with an extensive interferogram processing software. This software coordinates the data acquisition and instrument control and also provides real-time data processing and displaying. Film thickness evaluation technique based on the colorimetric interferometry successfully overcomes limitations of conventional optical interferometry. Colorimetric interferometry combining conventional chromatic interferometry with image processing and differential colorimetry gives instantaneous detailed information on EHD film thickness and shape for various regimes. This approach makes it possible to determine film thickness in the range of 1 to 800 nm with a resolution of about 1 nm. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, Octob...

In situ measurements of thin films in bovine serum lubricated contacts using optical interferometry

The aim of this study is to consider the relevance of in situ measurements of bovine serum film thickness in the optical test device that could be related to the function of the artificial hip joint. It is mainly focussed on the effect of the hydrophobicity or hydrophilicity of the transparent surface and the effect of its geometry. Film thickness measurements were performed using ball-on-disc and lens-on-disc configurations of optical test device as a function of time. Chromatic interferograms were recorded with a high-speed complementary metal-oxide semiconductor digital camera and evaluated with thin film colorimetric interferometry. It was clarified that a chromium layer covering the glass disc has a hydrophobic behaviour which supports the adsorption of proteins contained in the bovine serum solution, thereby a thicker lubricating film is formed. On the contrary, the protein film formation was not observed when the disc was covered with a silica layer having a hydrophilic behaviour. In this case, a very thin lubricating film was formed only due to the hydrodynamic effect. Metal and ceramic balls have no substantial effect on lubricant film formation although their contact surfaces have relatively different wettability. It was confirmed that conformity of contacting surfaces and kinematic conditions has fundamental effect on bovine serum film formation. In the ball-on-disc configuration, the lubricant film is formed predominantly due to protein aggregations, which pass through the contact zone and increase the film thickness. In the more conformal ball-on-lens configuration, the lubricant film is formed predominantly due to hydrodynamic effect, thereby the film thickness is kept constant during measurement.

Experimental Study of Microtextured Surfaces Operating Under Thin-Film EHD Lubrication Conditions

The effect of microdents within thin elastohydrodynamics (EHD) contacts has been studied by two measurement techniques. Phase-shifting interferometry was used to obtain topography of microtextured surface and thin-film colorimetric interferometry provided detailed information about film thickness changes within a lubricated contact. The behavior of microdents has been observed for positive slide-to-roll ratios when the disk is moving faster than the microtextured ball. The depth of microdents has been found to play significant role as to the lubrication films efficiency. The presence of deep microdents within lubricated contact results in film thickness reduction downstream that can even cause lubrication film breakdown. As the depth of microdents is reduced, this effect diminishes and beneficial effect of microdents on film thickness formation has been observed. No such an effect of microdent depth on lubricant film shape has been observed in case of negative slide-to-roll conditions when microdents do not cause film thickness reduction regardless of their depths. Obtained results suggest that surface texturing using microdents of an appropriate depth could help to increase lubrication films capabilities.

Study of film formation in bovine serum lubricated contacts under rolling/sliding conditions

The aim of this study is to perform a detailed experimental analysis of lubricant film thickness of bovine serum within the contact between the artificial metal and ceramic heads (balls) and the glass disc to analyse the effect of proteins on film formation under various rolling/sliding conditions. Lubricant film observation of bovine serum solutions was carried out using an optical test rig. Chromatic interferograms were recorded with a high-speed CMOS digital camera and evaluated with thin film colorimetric interferometry. Film thickness was studied as a function of time. Under pure rolling conditions, film thickness increases with time as well as with rolling distance for all mean speeds and for both materials of the balls; however the metal ball always forms a thicker lubricating film in comparison to the ceramic ball. Under rolling/sliding conditions, when the disc is faster than the ball, the formation of lubricant film thickness is different compared to pure rolling conditions. At first, film thickness increases rapidly with a rolling/sliding distance for all mean speeds. When maximum film thickness is reached, then this effect is lost and film thickness starts to fall and finally, at the end of the measurement, film thickness drops down to a few nanometres. For the metal ball, maximum values of central film thicknesses are proportional to the mean speed; however this is not observed with the ceramic ball. An absolutely different formation of bovine serum film thickness is observed when the ball is faster than the disc. Under this condition, the protein layer is very thin for both materials of balls, and central film thickness reaches only about a few nanometres. Local protein spots are formed in a very small area of the contact zone and reach the thickness between 20 and 25 nm for the metal ball and 5 nm for the ceramic ball. From the performed experiments under rolling/sliding conditions, it is obvious that the formation of lubricant film thickness is markedly dependent on kinematic conditions acting in the contact, especially on the positive and negative slide-to-roll ratio and the mean speed. In addition, the material of the artificial head has a certain influence on the formation of bovine serum lubricating film.

Experimental Study of Lubricant Film Thickness Behavior in the Vicinity of Real Asperities Passing through Lubricated Contact

This article presents an experimental study of the influence of real surface micro-geometry on the film thickness in a circular elastohydrodynamic (EHD) contact formed between a real, random, rough surface of steel ball and smooth glass disk. Phase shifting interferometry was used to measure in situ initial undeformed rough surface profiles, whereas thin film colorimetric interferometry provided accurate information about micro-EHD film thickness behavior over a wide range of rolling speeds. Two real roughness features were studied in detail—a 56-nm-high ridge and a 90-nm-deep groove, both transversely oriented to the direction of surface motion. It was shown that the ridge is heavily deformed in a loaded contact and its height increases with increasing rolling speed. The asperity tip film thickness behavior is quite similar to the contact average film thickness when the film thickness is higher than the undeformed ridge height. However, below this limit the film is thicker than what the EHD theory predicts. For the groove, a local reduction in film thickness at the leading edge was observed. When the groove is passing through the EHD conjunction, it maintains its undeformed shape. The behavior of both roughness features studied shows good agreement with previous experimental observations conducted using an artificially produced ridge and groove.

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 techniques accuracy has been checked and a comparison with conventional monochromatic interferometry has been done for validation. The techniques resolution has been confirmed through the observation of lo...

Effect of Surface Texturing on Very Thin Film EHD Lubricated Contacts

Micro-dents of various depths and shapes have been produced on rubbing surfaces to find out whether there could be some operational conditions suitable for surface texturing applications in highly loaded non-conformal contacts. The significant effect of the size and shape of the micro-dents has been observed as a function of the slide-to-roll ratio. Shallow micro-features within concentrated contact were found to increase the film thickness by supplying more lubricant to the contact under rolling/sliding conditions. However, in many cases this effect was accompanied with local film thickness reduction that even caused lubrication film breakdown under thin film lubrication conditions. Longitudinally and transversely oriented short grooves were found not to be completely suitable for the surface texturing applications from this point of view. Conversely, small circular micro-dents seem to be more promising as to their use to help to separate rubbing surfaces when a micro-textured surface is moving faster than a non-textured one and the lubrication films become very thin. Nevertheless, it would require a well-balanced design, taking into account many factors as the lubrication regime, the slide-to-roll ratio conditions, the rolling contact fatigue, the technology of surface texturing patterns creation, etc.

Evidence of Plug Flow in Rolling–Sliding Elastohydrodynamic Contact

The existence of shear bands, boundary slippage or shear localization was observed several times by various researchers. At the same time, the concept of limiting shear stress is almost exclusively used for traction modeling without clear explanation of physical phenomena that it is associated with. Despite it, the classic linear distribution of speed through-film profile is often considered in current numerical analyses and in our thinking about experimental results. It seems there is a small effort to unify our approaches to meet general concept that can explain a wide range of phenomena. This paper presents experimental results that point out on irregular distribution of through-film speed profile. The observations show an existence of cohesive core of entrainment speed surrounded by two shear zones located closely to the surfaces. This result represents experimental evidence for plug flow mechanism in highly loaded rolling–sliding elastohydrodynamic contact. Possible mechanisms of shear localization are discussed.

Effect of Surface Velocity Directions on Elastohydrodynamic Film Shape

This article is focused on the effects of the angle between lubricant entrainment velocity and sliding velocity on elastohydrodynamic film thickness distribution. Thin-film colorimetric interferometry was used to evaluate the film thickness distribution in smooth glass–steel contacts to provide basic data on the effects of the slide–roll ratio and the direction of sliding with respect to entrainment velocity. It was observed that as the sliding perpendicular to the entrainment velocity increased, the overall film thickness was reduced and asymmetry of the film profile with respect to the direction of the entrainment velocity increased. The asymmetry of the film profile with respect to the direction of the entrainment velocity increased with the entrainment speed or the overall film thickness. When the speed of the glass disk was larger than that of the steel ball, a dimple was formed even if there was a difference in direction between the entrainment and sliding velocities. A part of the dimple was exhausted from the elastohydrodynamic lubrication (EHL) conjunction as the angle between the entrainment and sliding velocities approached 90°.

Prediction of Real Rough Surface Deformation in Pure Rolling EHL Contact: Comparison with Experiment

The ability to predict the in-contact deformation of surface topography is very important for the design of machine components with respect to minimizing the friction and wear of rubbing surfaces. In this study the amplitude attenuation principle is verified as a simple tool for this purpose. Measured lubricant film profiles are compared with prediction based on this principle. From the results obtained it appears that the amplitude attenuation principle provides reasonable estimation of the deformation of rubbing surfaces that can be used for the prediction of in-contact behavior of surface roughness. Good agreement was obtained under pure rolling conditions, which provides a good initial point for the other studies under rolling/sliding conditions where the wear of rubbing surfaces is of key importance.