Page 384 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOCERAMICS 361

σ= Ad −n (15.1)
σ =σ e −BP (15.2)
P 0
For example, decreasing the grain size of Al O from 4 to 7 μm increases strength by approximately
2
3
20 percent (Dorre and Dawihl, 1980). With advances in ceramic processing, it is now possible to
fabricate alumina with grain sizes approximately 1 μm and small grain size distributions, material
characteristics that increase strength.
The amount of wear in alumina-alumina bearing couples can be as much as 10 times less than in
metal-polyethylene systems (Davidson, 1993; Kumar et al., 1991; Lusty et al., 2007). The coeffi-
cients of friction of alumina-alumina and alumina-polyethylene are less than that of metal-
polyethylene, because of alumina’s low surface roughness and wettability (Boutin et al., 1988;
Semlitsch et al., 1977).
The major limitations of alumina are its low tensile and bending strengths and fracture toughness.
As a consequence, alumina is sensitive to stress concentrations and overloading. Clinically retrieved
alumina total hip replacements exhibit damage caused by fatigue, impact, or overload (Walter and
Lang, 1986). Many ceramic failures can be attributed to materials processing or design deficiencies,
and can be minimized through better materials choice and quality control.

15.2.3 Zirconia
Yttrium oxide partially stabilized zirconia (YPSZ) is an alternative to alumina, and there are approx-
imately 150,000 zirconia components in clinical use (Christel et al., 1989; Cales and Stefani, 1995).
YPSZ has a higher toughness than alumina, since it can be transformation toughened, and is used in
bulk form or as a coating (Filiaggi et al., 1996).
At room temperature, pure zirconia has a monoclinic crystal symmetry. Upon heating, it trans-
forms to a tetragonal phase at approximately 1000 to 1100°C, and then to a cubic phase at approxi-
mately 2000°C (Fig. 15.2). A partially reversible volumetric shrinkage (density increase) of 3 to 10
percent occurs during the monoclinic to tetragonal transformation (Christel et al., 1989). The volu-
metric changes resulting from the phase transformations can lead to residual stresses and cracking.
Furthermore, because of the large volume reduction, pure zirconia cannot be sintered. However, sin-
tering and phase transformations can be controlled via the addition of stabilizing oxides. Yttrium
oxide (Y O ) serves as a stabilizer for the tetragonal phase such that upon cooling, the tetragonal
2
3
crystals are maintained in a metastable state and do not transform back to a monoclinic structure.
The tetragonal to monoclinic transformation and volume change are also prevented by neighboring
grains inducing compressive stresses on one another (Christel et al., 1989).
The modulus of partially stabilized zirconia is approximately half that of alumina, while the bending
strength and fracture toughness are 2 to 3 and 2 times greater, respectively (Table 15.1). The relatively
high strength and toughness are a result of transformation toughening, a mechanism that manifests itself
as follows (Fig. 15.3): crack nucleation and propagation lead to locally elevated stresses and energy in
the tetragonal crystals surrounding the crack tip. The elevated energy induces the metastable tetragonal
grains to transform into monoclinic grains in this part of the microstructure. Since the monoclinic grains
are larger than the tetragonal grains, there is a local volume increase, compressive stresses are induced,
more energy is needed to advance the crack, and crack blunting occurs.
The wear rate of YPSZ on UHMWPE can be 5 times less than the wear rate of alumina on
UHMWPE, depending on experimental conditions (Kumar et al., 1991; Davidson, 1993; Derbyshire
et al., 1994). Wear resistance is a function of grain size, surface roughness, and residual compressive
stresses induced by the phase transformation. The increased mechanical and tribological properties
of zirconia may allow for smaller diameter femoral heads to be used in comparison to alumina.
Partially stabilized zirconia is typically shaped by cold isostatic pressing and then densified by
sintering. Sintering may be performed with or without a subsequent hot isostatic pressing (HIP-ing)
cycle. The material is usually presintered until approximately 95 percent dense and then HIP-ed to
remove residual porosity (Christel et al., 1989). Sintering can be performed without inducing grain
growth, and final grain sizes can be less than 1 μm.