Page 338 - Vibrational Spectroscopic Imaging for Biomedical Applications

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behavior of the 180 and 260 cm Raman bands of monoclinic and tetrag-
onal zirconia polymorphs, respectively. Upon stress-induced tetragonal-
to-monoclinic transformation, significant volume expansion occurs,
which in turn induces a compressive stress field that shields the crack
mouth. In other words, partly stabilized zirconia materials are capable to
release the tensile crack-tip stress through a self-induced highly com-
pressive stress field arising from polymorphic transformation.
Besides the importance of such visualization from a basic materi-
als science point of view, the knowledge of the amount of polymorph
transformation and of the residual stress magnitude is fundamental
for improving the microstructural design of zirconia ceramics and for
correctly understanding the difference of their mechanical properties.
From this perspective, the Raman spectroscopic evaluation can be
considered as complementary to macroscopic fracture mechanics
characterizations of critical stress intensity factor at the onset of crack
initiation and of rising R-curve behavior.

10.4.2 Residual Stress Patterns on Ceramic-Bearing
Surfaces of Artificial Hip Joints
The finite element method has been for many years the main tool used
by materials technologists to solve engineering problems related to
stress and strain analysis of static or dynamic loaded contacts. In the
field of arthroplasty, the finite element method has been used to analyze
mechanical and thermal responses of acetabular cup and femoral head
components for total hip replacement. 39–41 Numerical evaluations have
been based on the concept that the wear volume is proportional to the
product of contact load and sliding distance, the proportionality con-
stant being referred to as the specific wear rate between the head and
the cup-bearing surfaces. According to such numerical studies, hip joint
simulators have been designed to reproduce as much reliably as pos-
sible the sliding kinetics taking place between the head and the cup.
However, although the results obtained from the model are useful in
understanding the causes of wear in hip prostheses and they may con-
tribute to predict the overall joint performance, they have shown insuf-
ficient to fully represent the complex kinetics of in vivo loaded
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hip-bearing surfaces and thus to predict their actual wear rate. In par-
ticular, it is very difficult to predict residual stresses by the finite ele-
ment method, although such type of stresses are very closely related to
wear phenomena. It should also be noted that the environment of ortho-
paedic implants sometimes induces additional effects as head/cup
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microseparation and systematic multiaxial microdisplacements 44,45 at
the contact of the modular prosthesis components. These additional
effects may contribute to the total lifetime of the implant in a nonnegli-
gible way, especially when artificial joints are made of brittle materials
such as ceramics. The necessary optimization of orthopaedic device life-
time thus requires a better knowledge of the damages induced by wear
contact. In this context, confocal Raman spectroscopy might provide