Biomedical Engineering Reference
In-Depth Information
A composition gradient in alumina and zirconia was engineered to obtain
a pure alumina surface region and a homogeneous alumina/zirconia core with
intermediate continuously graded regions allowing to generate thermal residual
compressive stresses on the surface after sintering. It has been shown here that
functionally graded Al
2
O
3
/ZrO
2
ball-heads and acetabular cup inserts can be pro-
cessed by electrophoretic deposition (EPD). In order to experimentally verify the
sintering behavior of complex FGM components, fi ve millimeters thick Al
2
O
3
/
ZrO
2
FGM discs were processed by EPD and analyzed. For this purpose a mathe-
matical model to predict the composition profi le was established. Defect-free
Al
2
O
3
/ZrO
2
FGM discs with a diameter of more than 35 millimeters and a thick-
ness of more than fi ve millimeters were successfully produced.
A sequence of counter-electrodes was necessary to EPD complex shaped
FGM components. The design of the counter-electrodes was supported by electri-
cal fi eld calculations to generate a constant electric fi eld at the surface of the
deposition electrode. To obtain deposits with a shape different from the deposi-
tion electrode, the deposit was allowed to grow up to the counter-electrode.
After deposition, the wet green ball-heads and acetabular cup inserts had to
be removed from the electrode and dried. For the ball-shaped components, the
drying step was very critical due to the inhomogeneous thickness of the compo-
nent. For drying these components, a colloidal isopressing procedure was devel-
oped, whereby the solvent was pressed out of the green body under an isostatic
pressure. In contrast, no such special procedure was needed for the acetabular
cups, which have relatively uniform thickness.
After drying, the green EPD powder compacts were sintered by pressureless
sintering in air. The sintering kinetics were studied and an optimal sinter cycle
(one hour at 1550 °C) was found to ensure that the grain size of the Al
2
O
3
grains
remains below 2
m. Al
2
O
3
/ZrO
2
FGM discs with pure Al
2
O
3
at the surface, a
homogeneous Al
2
O
3
/ZrO
2
core and graded interlayers contain internal defects
when the zirconia content in the core exceeds 25 vol. %.
μ
10.9 FUTURE PERSPECTIVES
Whereas the research summarized above has shown the feasibility of incorporat-
ing a functional gradient in hip joint components by EPD, further development of
the process is necessary to implement it on an industrial scale, to further optimize
the different steps in the processing in order to minimize processing defects,
and to verify performance of the actual components in terms of load-bearing
capability and wear resistance.
The commercial introduction of advanced alumina composites in hip pros-
thesis is presently ongoing. A zirconia toughened and platelet reinforced alumina
is presently available from Ceramtec A.G. under the trade name BIOLOX delta
[Merkert 2003]. This material is also a candidate for other prostheses such as knee
prostheses where the material also sees a very high load. Its long term reliability
has been documented. The functionally graded material concept could also be
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