Tribocorrosion under fluid lubrication: modeling wear of CoCrMo artificial hip joints

Wear is still the key problem causing the failure and limiting the lifetime of artificial hip joints, especially for the polymer acetabular cup in the metal-on-polymer and ceramic-on-polymer articulations. To reduce the wear, metal-on-metal articulation has been developed but the continuous release of nano-sized metal ions and particles into the body is of long-term concern of the patient's health. The implanted artificial hip joints are surrounded by synovial fluid, which on one hand acts as lubricant, alleviating the wear of implants but on the other hand introduces corrosion to the metal components. The interaction between wear and corrosion, called tribocorrosion, has been proposed as one of the crucial degradation mechanisms of metal implants. Tribocorrosion models have been developed in the past neglecting however lubrication effects. This constitutes the motivation for this work which aims at developing a composite wear model considering both tribocorrosion and lubrication effects to quantitatively describe and predict material degradation of passive metals (typically CoCrMo alloys) in hip joints. Based on the plastic deformation of the contacting asperities, an existing tribocorrosion model was expanded in order to include the lubrication effects by replacing the total normal load by the effective normal load which is the load carried only by the contacting asperities. The difference between these two loads is the load carried by the hydrodynamic fluid film flowing through the asperity junctions. The effective normal load was related to the total normal load based on Dowson's empirical running-in wear and minimum hydrodynamic film thickness correlation, which was derived from a large number of CoCrMo metal-on-metal hip joint simulator results. The composite model was then calibrated using well-controlled tribocorrosion experiments carried out in a dedicated tribometer from the literature. The calibrated model was found predicting precisely wear rates observed in tribometers and the running-in wear rates of metal-on-metal hip joints tested in simulators published in the literature. The model also allows identifying the dominating wear mechanisms (mechanical or chemical) and evaluating the influence of well-defined material, mechanical, electrochemical and physical parameters. The model could be successfully applied to other CoCr alloys of different carbide concentrations provided plastic deformation of the asperities remains the prevailing wear mechanism, as postulated in the model. In an attempt to further generalize the model, Dowson's empirical correlation was tentatively replaced by a mechanistic approach describing the real contact area and thus the effective normal force as a function of the topography of the contact surface and the hydrodynamic film thickness. The validity of the approach was assessed using tribocorrosion experiments carried out in H2SO4 - glycerol solutions exhibiting different viscosities. The topography was described using experimental Abbott - Firestone curves of the wear surface profile after cut-off of the waviness. The experimental mechanical and chemical wear rates were found to be consistent with this model approach. This shows that surface topography should be included in more general tribocorrosion models. However, tools to anticipate the evolution of surface topography during wear are not available at present and this limits the possibility of the model to predict tribocorrosion rate.

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