Mapping of film thickness in bovine serum lubricated contacts

conference paper

English

At present bovine serum (BS) is commonly used as a model of synovial fluid lubricant for interpretation of wear and friction properties of artificial joints. Mavraki and Cann (2011) analysed lubricant film thickness behaviour of BS using ball-on-disc device for various operational conditions. They found that under high pressure rolling tests, BS initially formed thinner films over the speed range and in subsequent speed sweeps thicker films were formed at low speeds. This behaviour was imputed to formation of an adsorbed protein layer, which causes high-viscosity film. The aim of this study is to perform detail experimental mapping of lubricating film thickness of BS within a whole contact zone between artificial femoral head and glass disc and analysed effect of proteins on film formation. Mapping of lubricating film of 50% BS (Sigma-Aldrich, B9433, with a total protein content of 37.5 mg/ml) was observed using an optical test rig in which a circular contact is formed between a glass disc and a metal head of total hip joint replacement. The lower surface of the disc is coated with a thin semi-reflective chromium layer and the upper side has an antireflective coating. The artificial femoral head (AESCULAP NK430K) made from Cobalt-Chromium forged alloy (CoCr29Mo / ISO 5832-12) has 28 mm in diameter and was delivered from original package of manufacturer. The metal head was rotated by servomotor against the disc to provide pure rolling conditions. The film thickness was studied as a function of mean speed over a both increasing and decreasing speed range of 5-40 mm/s. Experiments were realised at room temperature of 24 C under steady state load of 5 N corresponding to mean Hertzian pressure of 180 MPa. The contact formed between the glass disc and the metal head was illuminated by halogen lamp. Obtained chromatic interferograms were recorded with a 3CCD digital camera and evaluated with thin film colorimetric interferometry. The film thickness against the mean rolling speed is plotted in Fig. 1. The results labelled UP1 and UP2 correspond to the speed increase; DOWN1 and DOWN2 label the speed decrease. At first, BS forms a fairly thin film and its thickness slightly increases with increasing rolling speed (UP1). However, after some time the film thickness starts to fluctuate (UP2) as proteins start to form highly viscous film on the rubbing surfaces. This film is not homogeneous so that two different film thickness regions can be found within the contact. Thicker protein film that increases over the time even during the speed decrease (DOWN2) and thinner film that during speed decrease (DOWN1) follows the initial film thickness tendency (UP1). During measurements the formation of proteins were observed within the higher pressure contact zone only. At the end of the test the fairly thick protein film was measured up to 95 nm. Due to challenging of initial study the more complex research work is carried out at the present time.

Film thickness, Bovine serum, Hip joint, Femoral head

VRBKA, M.; ZIMMERMAN, M.; NÁVRAT, T.; KŘUPKA, I.; HARTL, M.

18. 9. 2011

Imperial College

Londýn

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