FUNCTIONALLY GRADED ALL CERAMIC HIP JOINT - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 336

Figure 10.14. Electrode set-up for EPD of cup inserts. [Anné, 2005a].
To prevent irregular deposition at the counter electrode, an insulating PTFE
cap is placed (Figure 10.14) in-between the deposit and the counter-electrode. In
this insulating cap, small excisions are made to conduct the current in order to
maintain the E-fi eld, as already explained in section 10.6.2. During EPD, this insu-
lating cap is rotated.
For the EPD of the cup inserts, a sequence of three counter-electrodes
is used (Figure 10.15). These counter-electrodes have the shape of a sphere
(diameter = 18 mm) with a different insulating PTFE cap. The procedure to EPD
FGM cup inserts is similar to that for the ball-heads. Homogeneous as well as
FGM cup inserts could be successfully produced with a good surface fi nish, as
shown in Figure 10.16.
For FGM acetabular cup inserts, a 135 ml starting suspension I, containing
222 g/l Al 2 O 3 , is pumped through the deposition cell (see Figure 10.4) by a peri-
staltic pump one at a rate of two ml/s. The counter-electrode is a ball-shaped
electrode with a large insulating cap, positioned in the middle of the cell (step one
of Figure 10.15 ). After 300 s of deposition at 120V, 90 ml of suspension II with
222 g/l of an Al 2 O 3 /ZrO 2 (45 vol. % Al 2 O 3 ) mixture is added to the circulating
suspension by pump two (Figure 10.4) at a constant speed during 340 s , creating a
gradient in the deposit composition (step two of Figure 10.15). Subsequently, the
suspension is circulated for 1800 s without any further additions (step three of
Figure 10.15). Afterwards, 100 ml of the circulating suspension is removed and the
insulating cap is replaced with a smaller insulating cap II. 65 ml of suspension III
with 230 g/l Al 2 O 3 was added at a constant speed for 350 s to the circulating sus-
pension (step four of Figure 10.15). In the fi nal step (step fi ve of Figure 10.15), the
counter-electrode is replaced with a smaller insulating cap III and the whole sus-
pension is replaced with 50 ml of suspension IV with a solids loading of 200 g/l
Al 2 O 3 and EPD is continued for 500 s to form the pure outside alumina layer.
Drying is performed in air without crack formation due to the more homoge-
neous thickness of the cup insert compared to the ball-heads. For the latter, drying