Abrasion-corrosion of cast CoCrMo in simulated hip joint environments


Sun, Dan (2009) Abrasion-corrosion of cast CoCrMo in simulated hip joint environments. University of Southampton, School of Engineering Sciences, Doctoral Thesis , 264pp.

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Description/Abstract

Metal-on-metal (MoM) hip joint replacements have been increasingly used for younger and more
active patients in recent years due to their improved wear performance compared to conventional
metal-on-polymer bearings. MoM bearings operate at body temperature within a corrosive joint
environment and therefore are inevitably being subjected to wear and corrosion as well as the
combined action of tribo-corrosion. Issues such as metal sensitivity/metallosis associated with high
levels 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 the
wear/lubrication mechanisms within the MoM hip joints in order to improve their performance and
thereby prolonging their life. However, not much attention has been given to the combined effect of
wear and corrosion of such devices in the hip joint environment, in addition, the role of third body
particles 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 and
tribo-corrosion performance of a cast CoCrMo (F75) in simulated hip joint environments. The
effects of third body particles have been studied in the MoM context using 4 μm SiC, 1 μm and 300
nm Al2O3, as well as sub-micron BaSO4. Modified tribo-testers (micro-abrasion,
nanoindenter/scratching) incorporating a novel electrochemical cell have been used to monitor the
abrasion-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 surface
and sub-surface characterization techniques have been employed to identify the microstructual wear
mechanism 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 the
protein adsorption behaviour, which subsequently influence the wear rates (synergy), wear
mechanisms as well as the wear-induced corrosion currents of the CoCrMo. For abrasion-corrosion
tests, reducing abrasive size from 4 μm to 300 nm and/or abrasive volume concentration from 0.238
vol% to 0.006 vol% results in different abrasion-corrosion wear mechanisms (rolling or grooving
abrasion) and the average wear-induced corrosion currents show a linear correlation with wear rates
for 4 μm and 1 μm abrasives. For low volume concentration (< 0.03 vol%) slurries containing
bovine serum, organo-metallic conglomerates have been found within the wear scars. These
conglomerates help separate the surfaces, impose less damage to the surface passive film and polish
the wear scars through a chemical mechanical polishing mechanism. In addition, tribo-corrosion
tests at micro-/nano- scales reveal the effects of single abrasive particle on the surface/sub-surface
microstructual change. This investigation has revealed the nanoscale wear mechanisms that generate
nanoscale wear debris, the mechanical mixing of the surface nanostructure with adsorbed denatured
protein and also the slip/dislocation systems that are present near and on abraded surfaces that are
likely to disrupt the surface passive films. The findings give a better understanding of the evolution
of the sub-surface nanocrystalline structures and tribo-layers formation seen for the retrieved
implants. This near surface nanostructure layer and phase transformation might offer better wear
resistance through these inherent self-protecting mechanisms (i.e. increased hardness); conversely, it
may 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 tribocorrosion
behaviour of cast CoCrMo in simulated hip joint environments. In particular, the roles of
third body abrasive particles and proteins have been addressed, which are relevant to clinical
applications. The material multi-scale wear mechanisms as well as the evolution of the surface / subsurface
microstructures and tribo-layers have been elucidated, which provide new insights into the in
vivo wear mechanisms of CoCrMo. The findings of this study may provide some important
indications for improved MoM joint materials, design, manufacture and evaluation.

Item Type: Thesis (Doctoral)
Subjects: R Medicine > RD Surgery
T Technology > TA Engineering (General). Civil engineering (General)
Q Science > QH Natural history > QH301 Biology
Divisions: University Structure - Pre August 2011 > School of Engineering Sciences > Bioengineering Sciences
University Structure - Pre August 2011 > School of Engineering Sciences > Engineering Materials & Surface Engineering
ePrint ID: 67337
Date Deposited: 04 Sep 2009
Last Modified: 27 Mar 2014 18:48
URI: http://eprints.soton.ac.uk/id/eprint/67337

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