Biomedical Engineering Reference
In-Depth Information
Fig. 9.6
Microstructure of a
composite ceramic. By
courtesy of CeramTec AG,
Medical Products
2
μ
m
Fig. 9.7
Reinforcing mechanism in alumina-zirconia composite.
Grey
:alumina,
light grey
:
tetragonal zirconia;
dark grey
: monoclinic zirconia. By courtesy of CeramTec AG, Medical
Products
its 75wt% (80vol%) responsible for the order of magnitude of most of its properties.
Grain size is not forgotten either (see Table
9.2
and length bar in Fig.
9.6
). Platelets
are the third salient aspect of its anatomy.
To explain how the feedback system is operating, we should read Fig.
9.7
from
left to right. The high tensile stress at the crack tip triggers a phase transformation
of tetragonal zirconia (light grey) to its monoclinic counterpart (dark grey). The
accompanying volume expansion leads to compressive stresses which are blocking
crack propagation. For more crystallographic details, see Appendix
B
. The complete
story is beautifully described in a review paper by Hannink et al. [
282
].
Why is this happening? A quick look at Table
9.3
informs us that the monoclinic
form is about 2.3% less dense, or occupies about 4% more volume. Tensile stresses
(or reduced pressure) around the crack tip favor the transformation to the under
this condition thermodynamically most stable monoclinic phase. A true automatic
feedback system!
The presence, however, of tetragonal ZrO
2
is not obvious and needs to be sta-
bilized. At a concentration of Y
2
O
3
2mol% in ZrO
2
at room temperature, the
monoclinic phase is stable and the tetragonal phase is metastable. The strain energy
produced by the stiff alumina matrix on the embedded
small
zirconia particles,
however, retains the tetragonal phase. This mechanism is called
mechanical stabi-
lization
. 'Small particles' is emphasized because size matters for a successful phase
<
