Understanding the wear and tribocorrosion processes and mechanisms of titanium alloys in bovine serum solution

With economic and health care improvements, the number of elderly
people demanding failed tissue replacement growing rapidly due to aged
population increases in representative countries. To date, people at the age of 65
have a life expectance of 17.9 years and implants have on average 15 years of
durability. It has been given focus on the use of titanium in biomaterials owing to
its properties, such as low density, high corrosion resistance and biocompatibility.
The chemical stability as well as corrosion resistance and fast repassivation in a
wide range of environments are due to the formation of a protective passive film.
Furthermore, titanium alloys are free of toxic elements in their composition.
Nevertheless, titanium–based alloys show poor tribological properties and the
failures have been related to that characteristic. The failure rate of replacements
is a consequence of the poor knowledge of the degradation mechanism. While
wear and corrosion have long been identified as the problem limiting the long–
term endurance of orthopaedic implants there remains a lack of understanding
about the fundamental mechanisms and effects of tribocorrosion.
The aim of this work is to analyse four different titanium–based alloys Ti–
13Nb–13Zr (αβ alloy), Ti–12Mo–6Zr–2Fe (Near β alloy), Ti–35Nb–13Ta–4.6Zr
aged at 400oC (β alloy) and Ti–35Nb–13Ta–4.6Zr aged at 300oC (βω alloy) over
wear tests at cathodic potential (–1 V vs OCP), open circuit potential and anodic
potential (0.3 V vs OCP) at 0.5N, 1N and 2N normal load to understand the
mechanisms and phenomena that occur when composition and production
procedures change as a result of wear in a body simulated fluid approaching
factors not well investigated in the literature.
This work is divided in two parts. The first part characterizes the
tribocorrosion behaviour of those four titanium alloys at 0.5N and the second part
compares these results to the effect of increasing normal load to 1N and 2N as
well as the synergistic and mechanistic approach to analyse the material loss.
All alloys present a good corrosion resistance, but they become more
active with rubbing contact. At 0.5N, 1N and 2N the COF does vary with
electrochemical condition and material composition. All alloys show similar wear
behaviour that changes only with applied potential, namely, the material loss,
specific wear rate and wear rate increase with load and are lower at anodic
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potential than at OCP or cathodic conditions. This suggests the formation of a
tribofilm that acts as a lubricant reducing friction. The worn surface presented the
same ploughing characteristics with no debris, reflecting abrasive wear as the
main wear mechanism and a rougher surface at anodic potential. The organic
layer was identified by backscattered electron images and confirmed by Raman
spectroscopy in all electrochemical conditions and normal loads.
The mechanistic approach identified that mechanical wear was the
dominant material removal mechanism in all of these alloys, with the
electrochemical contribution irrelevant at all applied potentials. The
electrochemical and mechanical contributions increased with normal load. In
addition, the synergistic approach identified that wear enhanced corrosion rates
and corrosion has a positive effect of reducing wear rate on these alloys and for
this these alloys present an antagonistic effect. The synergistic approach
confirms that mechanical wear is the predominant factor on material loss.
The reduced elastic modulus and nanohardness of the unworn and worn
surfaces were measured by nanoindentation. Worn surfaces present higher
values of these mechanical properties due to the formation of a nanocrystalline
area at the subsurface. All alloys experienced minor α’’ and ω phase induced
transformation due to strain hardening, except the αβ alloy.