Metal-on-metal (MoM) hip joint replacements have been increasingly used for younger and moreactive patients in recent years due to their improved wear performance compared to conventionalmetal-on-polymer bearings. MoM bearings operate at body temperature within a corrosive jointenvironment and therefore are inevitably being subjected to wear and corrosion as well as thecombined action of tribo-corrosion. Issues such as metal sensitivity/metallosis associated with highlevels of metal ion release triggered by the wear and corrosion products remain critical concerns.During the past few decades, significant research has been conducted into understanding thewear/lubrication mechanisms within the MoM hip joints in order to improve their performance andthereby prolonging their life. However, not much attention has been given to the combined effect ofwear and corrosion of such devices in the hip joint environment, in addition, the role of third bodyparticles and the effects of proteins have not been well understood.In this work, a systemic approach is presented for the first time for the mapping of abrasion andtribo-corrosion performance of a cast CoCrMo (F75) in simulated hip joint environments. Theeffects of third body particles have been studied in the MoM context using 4 ?m SiC, 1 ?m and 300nm Al2O3, as well as sub-micron BaSO4. Modified tribo-testers (micro-abrasion,nanoindenter/scratching) incorporating a novel electrochemical cell have been used to monitor theabrasion-corrosion behaviour of the alloy in situ. The effects of solution chemistry, abrasives size /concentration and presence of proteins on the wear / corrosion level, wear-corrosion mechanisms,and the depassivation/repassivation kinetics of the CoCrMo have been explored. A variety of surfaceand sub-surface characterization techniques have been employed to identify the microstructual wearmechanism interactions. Results show that the change of protein concentration (0, 25% and 50%bovine serum) and pH (pH 7.4 and pH 4.0) of the test solutions can significantly influence theprotein adsorption behaviour, which subsequently influence the wear rates (synergy), wearmechanisms as well as the wear-induced corrosion currents of the CoCrMo. For abrasion-corrosiontests, reducing abrasive size from 4 ?m to 300 nm and/or abrasive volume concentration from 0.238vol% to 0.006 vol% results in different abrasion-corrosion wear mechanisms (rolling or groovingabrasion) and the average wear-induced corrosion currents show a linear correlation with wear ratesfor 4 ?m and 1 ?m abrasives. For low volume concentration ( 0.03 vol%) slurries containingbovine serum, organo-metallic conglomerates have been found within the wear scars. Theseconglomerates help separate the surfaces, impose less damage to the surface passive film and polishthe wear scars through a chemical mechanical polishing mechanism. In addition, tribo-corrosiontests at micro-/nano- scales reveal the effects of single abrasive particle on the surface/sub-surfacemicrostructual change. This investigation has revealed the nanoscale wear mechanisms that generatenanoscale wear debris, the mechanical mixing of the surface nanostructure with adsorbed denaturedprotein and also the slip/dislocation systems that are present near and on abraded surfaces that arelikely to disrupt the surface passive films. The findings give a better understanding of the evolutionof the sub-surface nanocrystalline structures and tribo-layers formation seen for the retrievedimplants. This near surface nanostructure layer and phase transformation might offer better wearresistance through these inherent self-protecting mechanisms (i.e. increased hardness); conversely, itmay become the precursors to debris ejection and enhanced ion-release into the CoCrMo joints.This work established an experimental technique that gives greater understanding of the tribocorrosionbehaviour of cast CoCrMo in simulated hip joint environments. In particular, the roles ofthird body abrasive particles and proteins have been addressed, which are relevant to clinicalapplications. The material multi-scale wear mechanisms as well as the evolution of the surface / subsurfacemicrostructures and tribo-layers have been elucidated, which provide new insights into the invivo wear mechanisms of CoCrMo. The findings of this study may provide some importantindications for improved MoM joint materials, design, manufacture and evaluation.
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