Ivan Krupka

Thin Film Colorimetric Interferometry

Measurement technique for the study of very thin lubrication films down to one nanometer in a point contact between a steel ball and a transparent disc is used to explore the relationship between central and minimum film thickness and rolling speed at the interface between elastohydrodynamic and boundary lubrication for a series of lubricating fluids. This technique based on the colorimetric interferometry combines powerful film thickness mapping capabilities with high accuracy. It was confirmed that both hexadecane and mineral base oil obey the linear relationship between log central and minimum film thickness and log rolling speed predicted by elastohydrodynamic theory down to approximately one nanometer. Conversely, squalane and additive-treated mineral base oil showed film thickness enhancement at slow speeds caused by boundary layers formation within the lubricant film. Obtained experimental data was used for the determination of pressure-viscosity coefficients of test fluids. The measurement technique also enabled us to produce information about the influence of boundary layers on film thickness shape. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Seattle, Washington, October 1–4, 2000

Differential colorimetry: tool for evaluation of chromatic interference patterns

One of the oldest and simplest techniques for the determining the thickness of a thin transparent film is based on the evaluation of the interference colors produced by the film. Though its accuracy is limited and the technique itself has been superseded by many advanced techniques, it still plays an irreplaceable role in observing moving thin fluid lubricant films. A computer-aided system for the reconstructing the shape of a thin fluid film from chromatic interference fringes is presented conceptually, and parts of the system are demonstrated. Quasistatic fluid films were generated in an experimental apparatus operating as a twolayer Fizeau interferometer. Chromatic patterns produced by the Tolansky method were photographed and digitized. The CIELAB color difference equation was used for comparing interferograms with the digital color chart to determine the film thickness. Three-dimensional mesh surface plots of the film shape with high thickness resolution were generated using techniques of image processing and computer graphics. The limitations and accuracy of the proposed system are discussed, and its validity was checked by observing a lubricant’s ability to create a coherent film under various conditions.

Fabrication and characterization of DLC coated microdimples on hip prosthesis heads

Diamond like carbon (DLC) is applied as a thin film onto substrates to obtain desired surface properties such as increased hardness and corrosion resistance, and decreased friction and wear rate. Microdimple is an advanced surface modification technique enhancing the tribological performance. In this study, DLC coated microdimples were fabricated on hip prosthesis heads and their mechanical, material and surface properties were characterized. An Electro discharge machining (EDM) oriented microdrilling was utilized to fabricate a defined microdimple array (diameter of 300 µm, depth of 70 µm, and pitch of 900 µm) on stainless steel (SS) hip prosthesis heads. The dimpled surfaces were then coated by hydrogenated amorphous carbon (a-C:H) and tetrahedral amorphous carbon (Ta-C) layers by using a magnetron sputtering technology. A preliminary tribology test was conducted on these fabricated surfaces against a ceramic ball in simulated hip joint conditions. It was found that the fabricated dimples were perpendicular to the spherical surfaces and no cutting-tools wear debris was detected inside the individual dimples. The a-C:H and Ta-C coatings increased the hardness at both the dimple edges and the nondimpled region. The tribology test showed a significant reduction in friction coefficient for coated surfaces regardless of microdimple arrays: the lowest friction coefficient was found for the a-C:H samples (µ = 0.084), followed by Ta-C (µ = 0.119), as compared to the SS surface (µ = 0.248).

Comment on “History, Origins and Prediction of Elastohydrodynamic Friction” by Spikes and Jie

Progress in the classical field of EHL has for decades been paralyzed by the assumption that shear thinning should be indistinguishable from the shear dependence of the viscosity of a liquid heated by viscous dissipation and that the parameters of this simple shear dependence can be obtained from the shape of a friction curve. In the last few years, by abandoning this assumption and employing real viscosity measured with viscometers, there has been revolutionary progress in predicting film thickness and friction. Now, Spikes and Jie conclude that the previous assumption has as much merit as the use of viscosity measured in viscometers. This suggestion may be popular among those who wish to ignore viscometer measurements in favor of extracting properties from friction curves. However, within the subject article, there are numerous misstatements of fact and misrepresentations by omission, and the recent progress using real viscosity is not acknowledged. The debate has degenerated into a friction curve fitting competition which is not helpful. The great progress of the last few years would not have been possible using the concepts and methods espoused in this article.

Experimental Evaluation of EHD Film Shape and Its Comparison With Numerical Solution

Colorimetric interferomentry has been applied to the study of EHD lubrication of point contacts under pure rolling conditions to obtain lubricant film shapes with high accuracy and resolution. An RGB CCD camera together with an extensive image processing software has enabled real time evaluation of chromatic interferograms. The classical numerical isothermal solution of EHD lubrication of point contacts has been used for the comparison with three-dimensional representations of film thickness distributions obtained from experiments. A good agreement was found between experimental and numerical EHD film shapes by comparing lubricant film profiles and positions of minimum film thickness. Both experimental results and numerical solution confirm the ratio between central and minimum film thickness to change significantly with operating conditions.

Experimental Study of Central and Minimum Elastohydrodynamic Film Thickness by Colorimetric Interferometry Technique

Recently developed colorimetric interferometry technique was used for the study of both minimum and central film thicknesses for a wide range of operating parameters. Over 300 film thickness maps were obtained for the combination of four values of the materials parameter G, five values of the load parameter W and many values of the speed parameter U. The use of a spacer layer extended the range of film thickness measurement down to 5 nm. An excellent agreement was found between experimental values and data obtained from numerical solution presented by Venner and ten Napel, especially for thin lubrication films. An increase in a speed exponent with increasing material parameter G was observed for both central and minimum film thicknesses. The minimum film thickness and, thereby, the ratio between central and minimum film thickness was confirmed to be of a stronger dependence on material and load dimensionless parameters than Hamrock and Dowson equations predict. Presented at the 54th Annual Meeting Las Vegas, Nevada May 23–27, 1999

Effects of surface topography on lubrication film formation within elastohydrodynamic and mixed lubricated non-conformal contacts

Abstract This article focuses on the effects of surface topography modifications on lubrication film thickness within non-conformal lubricated contact. An optical test rig is used to study lubricant film behaviour between the flat surface of a chromium-coated glass disc and a steel ball. 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 and mixed lubrication using steady-state and transient operational conditions (start-up and reversal motion). The obtained results showed that surface texturing could represent a method of increasing the lubrication efficiency of rolling/sliding non-conformal contacts under operational conditions when the lubrication film is not able to separate rubbing surfaces. Lubricant emitted from the micro-dent helps in reducing asperities interaction. Moreover, the effect of surface micro-cavities has been observed within starved non-conformal contact under pure rolling conditions. An increase in film thickness within severe starved contact was localized upstream of the micro-dents, and was recently predicted numerically. This effect was further amplified as lubricant released from micro-dents remains on the ball surface and again enters into contact after the ball turns round.

Tribological investigation of ultra-high molecular weight polyethylene against advanced ceramic surfaces in total hip joint replacement

The aim of the study was to investigate whether a modified ceramic head surface could reduce the friction and wear rate of simulated ceramic-on-polyethylene hip joints. To address this aim, ultra-high molecular weight polyethylene (UHMWPE) was made to slide on aluminium oxide (Al2O3), dimpled Al2O3, diamond-like carbon (DLC) coated and DLC-coated dimpled substrates. The experiment condition was replicated to simulate artificial hip joints in terms of contact pressure, speed and temperature. UHMWPE on non-dimpled Al2O3 showed lower friction coefficient and wear rate compared to other advanced surfaces. Lower wettability, and higher hardness and surface adhesion of DLC resulted in increased friction and wear. The high difference in modulus of elasticity and hardness between UHMWPE and both, Al2O3 and DLC, reduced the effectiveness of textured surface techniques in friction and wear reduction. Therefore, no tribological benefit was found by fabricating either DLC coating or surface texturing on hard surface when rubbed against softer UHMWPE.

Fundamentals of thermal elastohydrodynamic lubrication in Si3N4 and steel circular contacts

The Si3N4 and steel are one of representative material combinations used in actual machine elements. The fundamental tribo-characteristics in Si3N4 and steel circular contacts are described through non-Newtonian thermal elastohydrodynamic lubrication analysis. The central film thickness and pressure spike are markedly influenced by the sign of the slide-roll ratio, i.e. by the positive and negative thermal viscosity wedge actions, while the minimum film thickness is very little affected by them. The thermal expansion of the fluid is also an important factor controlling the film thickness. Even if dimensionless parameters U, G, and W are fixed, the coefficient of traction and the film thickness are very much influenced by the equivalent radius of contact bodies.

Pressure Increase in Elliptical Impact Elastohydrodynamic Lubrication Contacts With Longitudinal Asperities

The phenomena that occur when an elliptical steel body impacts a stationary steel plate with surface asperities are discussed through isothermal Newtonian numerical analysis using sinusoidal roughness. The ridges of the surface asperities produce large local pressures, especially at a large ellipticity ratio, when the surfaces are approaching each other under the applied load. The values of the local pressures are larger when the ridges are along the major axis than when the ridges are along the minor axis. Furthermore, as the loading speed increases, the pressure increases. As a result, the microgrooves are produced in the ridges and the horseshoe-shaped constrictions are formed at the ridges located around the contact edge.