prof. Ing. Martin Vrbka, Ph.D.
E-mail:   Martin.Vrbka@vut.cz  WWW:   http://www.uk.fme.vutbr.cz Dept.:   Institute of Machine and Industrial Design Dept. of Tribology Position:   Professor Room:   A3/607

Supervised courses:

• Biomechanic Systems (ZBS)
• Diploma Design Seminar (YDS)

Publications:

• CHOUDHURY, D.; CHING, H.A.; MAMAT, A.B.; ČÍŽEK, J.; ABU OSMAN, N.A.; VRBKA, M.; HARTL, M.; KŘUPKA, I.:
  Fabrication and characterization of DLC coated microdimples on hip prosthesis heads, Wiley Periodicals, Inc.
  journal article in Web of Science
• VRBKA, M.; NEČAS, D.; HARTL, M.; KŘUPKA, I.; URBAN, F.; GALLO, J.:
  Visualization of lubricating films between artificial head and cup with respect to real geometry, Elsevier
  journal article in Scopus
• CHOUDHURY, D.; URBAN, F.; VRBKA, M.; HARTL, M.; KŘUPKA, I.:
  A novel tribological study on DLC-coated micro-dimpled Orthopedics implant interface, Elsevier Ltd.
  journal article in Web of Science
• VRBKA, M.; KŘUPKA, I.; HARTL, M.; NÁVRAT, T.; GALLO, J.; GALANDÁKOVÁ, A.:
  In situ measurements of thin films in bovine serum lubricated contacts using optical interferometry, SAGE Publications Ltd
  journal article in Web of Science
• VRBKA, M.; NÁVRAT, T.; KŘUPKA, I.; HARTL, M.; ŠPERKA, P.; GALLO, J.:
  Study of film formation in bovine serum lubricated contacts under rolling/sliding conditions, SAGE Publications Ltd
  journal article in Web of Science
• VRBKA, M.; KŘUPKA, I.; SVOBODA, P.; ŠPERKA, P.; NÁVRAT, T.; HARTL, M.; NOHAVA, J.:
  Effect of shot peening on rolling contact fatigue and lubricant film thickness within mixed lubricated non-conformal rolling/sliding contacts,
  Tribology International, Vol.44, (2011), No.12, pp.1726-1735, ISSN 0301-679X, Elsevier
  journal article - other
• VRBKA, M.; ŠAMÁNEK, O.; ŠPERKA, P.; NÁVRAT, T.; KŘUPKA, I.; HARTL, M.:
  Effect of surface texturing on rolling contact fatigue within mixed lubricated non-conformal rolling/sliding contacts, Elsevier
  journal article in Web of Science
• VRBKA, M.; KŘUPKA, I.; ŠAMÁNEK, O.; SVOBODA, P.; VAVERKA, M.; HARTL, M.:
  Effect of surface texturing on lubrication film formation and rolling contact fatigue within mixed lubricated non-conformal contacts,
  MECCANICA, Vol.2010, (2010), No.06, pp.1-8, ISSN 0025-6455, Springer
  journal article - other
• KŘUPKA, I.; HARTL, M.; VRBKA, M.; VAVERKA, M.; POLIŠČUK, R.:
  Effect of surface dents on contact pressure in elastohydrodynamic contacts,
  PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART J-JOURNAL OF ENGINEERING TRIBOLOGY, Vol.223, (2009), No.4, pp.683-693, ISSN 1350-6501, Professional Engineering Publishing
  journal article - other
• KŘUPKA, I.; HARTL, M.; VRBKA, M.:
  Effect of surface texturing on mixed lubricated non-conformal contacts,
  Tribology International, Vol.41, (2008), No.11, pp.1063-1073, ISSN 0301-679X, Elsevier
  journal article - other
• VAVERKA, M.; VRBKA, M.; POLIŠČUK, R.; KŘUPKA, I.; HARTL, M.:
  Numerical evaluation of pressure from experimentally measured film thickness in EHL point contact,
  Lubrication Science, Vol.20, (2008), No.1, pp.47-59, ISSN 0954-0075, John Wiley & Sons, Ltd.
  journal article - other

List of publications at Portal BUT

Abstracts of most important papers:

• VRBKA, M.; NEČAS, D.; HARTL, M.; KŘUPKA, I.; URBAN, F.; GALLO, J.:
  Visualization of lubricating films between artificial head and cup with respect to real geometry, Elsevier
  journal article in Scopus
  The aim of this study is to propose a novel experimental approach enabling in-situ observation of film formation within hip joint replacements with respect to real geometry (including radial clearance) of rubbing surfaces. A pendulum hip joint simulator in combination with thin film colorimetric interferometry was employed for film thickness evaluation between metal femoral head and glass acetabular cup lubricated by bovine serum solution. Glass acetabulum was developed according to dimensions of artificial cup so the real radial clearance between components was considered. The pendulum, deflected at an initial position, was released and allowed to oscillate freely in the flexion-extension plane; therefore the transient character of motion was considered. Maximum central film thickness of 232 nm was measured at the beginning of the experiment. After a short time it decreased and became quite stable (around 90 nm) until the end of the measurement. The preliminary results shown that novel experimental approach seems to be a very powerful tool for studying lubrication processes within artificial hip joints while considering different loading and kinematic conditions, influence of geometry, clearance and material combination of contact pairs.
• VRBKA, M.; KŘUPKA, I.; HARTL, M.; NÁVRAT, T.; GALLO, J.; GALANDÁKOVÁ, A.:
  In situ measurements of thin films in bovine serum lubricated contacts using optical interferometry, SAGE Publications Ltd
  journal article in Web of Science
  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.
• VRBKA, M.; NÁVRAT, T.; KŘUPKA, I.; HARTL, M.; ŠPERKA, P.; GALLO, J.:
  Study of film formation in bovine serum lubricated contacts under rolling/sliding conditions, SAGE Publications Ltd
  journal article in Web of Science
  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.
• VRBKA, M.; KŘUPKA, I.; SVOBODA, P.; ŠPERKA, P.; NÁVRAT, T.; HARTL, M.; NOHAVA, J.:
  Effect of shot peening on rolling contact fatigue and lubricant film thickness within mixed lubricated non-conformal rolling/sliding contacts,
  Tribology International, Vol.44, (2011), No.12, pp.1726-1735, ISSN 0301-679X, Elsevier
  journal article - other
  The effect of shot peening on rolling contact fatigue (RCF) and lubricant film thickness within nonconformal rolling/sliding contacts operated under mixed lubrication conditions was observed in this study. Rolling contact fatigue tests and film thickness measurements were carried out using specimens with modified surface topography by shot peening process using glass beads having diameter between 0.07 and 0.11 mm. It has been shown that the effect of shot peening on RCF has no positive effect even if shot peened surface of the roller exhibited somewhat higher hardness in contrast to the grounded surface. The reduction of RCF may be caused due to asperities interactions because after shot peening the surface roughness of the roller was increased. Film thickness measurements confirmed that the contact is realized actually only between asperity peaks of shot peened ball and smooth disc. Conversely, no negative effect on RCF was observed when the shot peened surface of the roller was polished. The polish of asperity peaks causes the creation of lands and micro-cavities, which may be employed as lubricant micro-reservoirs. From film thickness measurements it has been observed that lubricant emitted by shallow micro-cavities can provide the local increase in lubrication film thickness, which thereby reduces asperities interactions. Similar results were obtained for start-up conditions where the squeeze lubricant enlarges film thickness and reduces surface interactions. From the obtained results, it can be suggested that properly designed surface topography modification could help to increase the efficiency of lubrication films leading to the enhancement of contact fatigue life of non-conformal mixed lubricated rolling/sliding contacts.
• VRBKA, M.; ŠAMÁNEK, O.; ŠPERKA, P.; NÁVRAT, T.; KŘUPKA, I.; HARTL, M.:
  Effect of surface texturing on rolling contact fatigue within mixed lubricated non-conformal rolling/sliding contacts, Elsevier
  journal article in Web of Science
  The rolling contact fatigue (RCF) life of highly loaded machine components is significantly influenced by the surface roughness features so that there is a continuous effort to design the topography of rubbing surfaces to enhance lubrication efficiency and prolong the operation of machine components. It can be suggested from the recent experimental results that lubricant emitted from shallow micro-dents could effectively lift off the real roughness features and reduce the asperities interactions within rolling/sliding mixed lubricated contacts. Thereby the additional supply of lubricant from surface features could help to reduce the risk of surface damage through the reduction of the interaction of rubbing surfaces during start-up or starvation. However, the introduction of such roughness features into the rubbing surfaces of highly loaded non-conformal contacts should consider not only the effects on lubrication film thickness but also on RCF. That is why this study is focused on the effects of surface texturing on RCF within non-conformal rolling/sliding contacts operated under mixed lubrication conditions. The principal task has been whether possible beneficial effect on film thickness is not accompanied by the reduction in RCF life. Textures with various sizes of micro-dents and their arrangement within the contacts have been considered. It has been found that results obtained with textured surfaces have exhibited no obvious reduction in RCF. Conversely, some increase in RCF using textured surfaces was observed that could be attributed to the positive contribution of micro-dents working as lubricant micro-reservoirs that reduce asperities interactions. Nevertheless, further experiments are necessary to confirm this possible beneficial contribution of surface texturing on RCF.