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

Tribocorrosion of passive metals in the mixed lubrication regime: theoretical model and application to metal-on-metal artificial hip joints

Wear is a key problem limiting the lifetime and performance of total hip arthroplasty. The synovial fluid, acting as lubricant, can alleviate the wear of implants but also introduces corrosion. The interaction of mechanical wear and electrochemical corrosion, nominally tribocorrosion, has been proposed as one of the crucial degradation mechanisms of implants and recently saw significant progress in mechanistic understanding and modeling. This study presents a predictive wear model based on a mechanistic approach of tribocorrosion and lubrication and taking into account clinical relevant parameters such as normal load, velocity and clearance. Predicted wear rate corresponds well to the running-in wear rate from simulator experimental results and allows to identify the dominating wear mechanism and to evaluate the influence of several parameters. (C) 2014 Elsevier B.V. All rights reserved

A lubricated tribocorrosion model incorporating surface roughness

This study presents an improvement of an existing tribocorrosion model developed for passive CoCrMo alloys. This model is based on an empirical formalism established by Duncan Dowson in his pioneering works on the relation between wear and elasto-hydrodynamic lubrication. The improvement consists in introducing surface topography features allowing for a mechanistic relation between lubrication and wear. The effective normal force accounting for the plastic deformation of passive metals during lubricated tribocorrosion was described through the real contact area, which in turn was related to the worn surface topography (roughness) and the elasto-hydrodynamic film thickness. The modified model was applied to results from dedicated tribocorrosion experiments obtained by varying the lubricant viscosity and the contacting surface roughness. Good correlations were found between the mechanical and chemical wear rates and corresponding variables, which validated the model. Further development of the model should include boundary film effects, third bodies build-up and time dependent evolution of the worn surface

Tribocorrosion of a CoCrMo alloy in sulfuric acid - Glycerol mixtures

This work reports on the tribocorrosion properties of a CoCrMo alloy sliding under imposed passive potential against alumina in sulfuric acid-glycerol mixtures exhibiting different viscosities and thus, in principle, lubrication properties. As expected, wear accelerated corrosion and friction decreased with increasing glycerol content of the mixture. These results could be successfully rationalized using a recently published tribocorrosion model which considers the interplay of hydrodynamic lubrication with mechanical and chemical deterioration phenomena. The comparison of results with a theoretical model showed that the mixing of glycerol into sulfuric acid aqueous solution promoted lubrication but also that glycerol mechanically weakens the CoCrMo alloy

Assessment of a recent tribocorrosion model for wear of metal-on-metal hip joints: Comparison between model predictions and simulator results

A composite running-in wear model for metal-on-metal artificial hip joints, which combines tribo-corrosion and lubrication aspects, was published recently. In order to check the quality of the model prediction, wear rates from nineteen well-controlled simulator wear studies were summarized and compared to the model predicted values. The results showed that the simulator wear results correlate well with the model predicted values. By estimating the maximum wear rate, the model can be used clinically to mitigate the failure risk of metal-on-metal hip joints. Furthermore, this study demonstrates the roles of the involved crucial parameters, giving tutorial suggestions of the input parameters and output values for the wear prediction of metal-on-metal artificial hip joints. (C) 2016 Elsevier B.V. All rights reserved

Modeling tribocorrosion of passive metals - A review

Tribocorrosion is a material degradation phenomenon resulting from interactive effects between wear and corrosion. It is commonly found in engineering applications (e.g. biomedical implants and marine equipment) which involve relative motion of contacting metals in a corrosive environment. In this study, models describing tribocorrosion of passive metals in sliding contacts were reviewed. Different categories of models (two-body or three-body contact models, lubricated tribocorrosion model, empirical models, multi-degradation models) were found in the literature. Through the identification of relevant chemo-mechanical degradation mechanisms, robust analytical expressions accurately predicting the overall material loss in tribocorrosion have been developed. Numerical methods have been used to describe time dependent transitions in tribocorrosion. Possibilities and limits of the proposed models in the literature as well as future trends are discussed in this review

Effect of surface films on tribologically induced metallurgical transformations of steel in oil lubricated contacts

This study was initiated with the aim to assess the influence of boundary films on the tribologically induced metallurgical transformations and deformations of metals in lubricated contacts and their effect on wear. For this, the tribological behavior of a carbon steel contact was studied in two commercially available oils expected to form boundary films of quite different chemistry and structure. Surface analysis by AES and XPS revealed that under the investigated conditions the first oil led to the formation of an iron-zinc oxide surface film while the second to a calcium-carbon-oxygen rich film. The results show that, without influencing the coefficient of friction, the nature of the formed films significantly affected the metallurgical transformations (characterized by electron microscopy of focused ion beam cross sections) occurring in the near surface region of the metal and the corresponding wear response. The effect of boundary films on wear was attributed to their capability to influence the plastic flow of the nano-grained structures generated in the studied tribological contacts. (C) 2016 Elsevier B.V. All rights reserved

Influence of Bovine Serum Albumin (BSA) on the Tribocorrosion Behaviour of a Low Carbon CoCrMo Alloy in Simulated Body Fluids

Tribocorrosion, as the interaction between mechanical wear and electrochemical corrosion, has been found to be the main problem causing the failure and limiting the lifetime of metal-on-metal artificial hip joints. Better understanding of the tribocorrosion mechanisms of CoCrMo alloys is needed in order to reduce the degradation of this alloy, especially in the presence of proteins as one of the organic components present in synovial fluid. In this study, tribocorrosion tests of a low carbon CoCrMo alloy in phosphate buffer solution (PBS) with and without bovine serum albumin (BSA) in two different concentrations at different applied potentials (passive and cathodic) were carried out. The results show that the effect of proteins on wear and friction was concentration and potential dependent. In the cathodic domain (absence of very thin passive film), wear was very low in all solutions and the friction was significantly reduced by the addition of BSA to PBS even at low BSA concentrations. However, in the passive domain, the friction and wear were found not to be affected when the BSA concentration was 0.5 g/L, while they were reduced when increasing the BSA concentration to 36 g/L. The tribocorrosion results were rationalized through an existing tribocorrosion model and the effect of BSA on wear and friction was explained by the consideration of physical factors such as changes in viscosity and double layer structure, because in the present results no tribofilm formation was observed

Tribological and tribocorrosion behavior of nickel sliding against oxide ceramics

Nickel is widely used as a coating material in electroplating because of its good corrosion and wear resistance even at high temperature. Tribocorrosion has been identified as one of the main degradation mechanisms of nickel. This study evaluated the tribocorrosion behavior of pure nickel in citrate buffer solution (pH 4.5) under controlled mechanical, chemical and electrochemical conditions as well as wear of pure nickel in dry condition sliding against alumina or zirconia balls, respectively. The results showed that the wear of nickel was higher in passive domain due to wear accelerated corrosion compared to that in the cathodic domain. Surprisingly, both alumina and zirconia were worn by nickel in tribocorrosion condition under passive potentials as well as in dry condition. However, in the cathodic domain, negligible wear was observed on the alumina and zirconia balls. The formation of mixed nickel and aluminum/zirconium oxides was proposed as the wear mechanism of alumina and zirconia