Biomedical Engineering Reference
In-Depth Information
in the total free energy of powder compacts. The fi rst step towards the goal of
reduced energy is pores to be eliminated from the system because the specifi c
surface energy of pores is larger than the grain boundary energy. Grain growth is
driven by surface energy and the process of grain boundary elimination often
begins before the process of pore elimination is completed. Discontinuous grain
growth, that is, a few grains growing at a very large rate at the expense of all other
grains, should be avoided since as a result pores are entrapped within the grains
and such pores can not be eliminated. An optimal sinter cycle has to be applied to
reach fully dense ceramics and to ensure that the grain size of the Al
2
O
3
remains
below two
m to obtain high strength [Willmann, 1996; ISO 6474].
Densifi cation of graded materials is a challenge because cracks and crack-
like defects are often observed in ceramic multi-layers as a result of mismatch
stresses [Cheng and Raj, 1989; Hillman et al., 1996; Cai et al., 1997a; Cai et al.,
1997b; Cai et al., 1998]. These mismatch stresses originate from the difference in
sintering rate (densifi cation kinetics) and thermal expansion coeffi cients [Hillman
et al., 1996].
In the FGM components, similar defects as reported in literature [Cheng
and Raj, 1989; Hillman et al., 1996; Cai et al., 1997a; Cai et al., 1997b; Cai et al.,
1998] were detected when a too-large zirconia concentration difference between
the surface layer and core was present. The critical composition in the core of
FGM Al
2
O
3
/ZrO
2
discs was investigated to prevent defects during the sintering
cycle.
μ
10.7.2 Sintering Kinetics of Al
2
O
3
, ZrO
2
and Their Composites
The addition of ZrO
2
to Al
2
O
3
has an effect on the densifi cation behavior and the
grain size development of the matrix. To study the differences on the densifi cation
behavior, the shrinkage of Al
2
O
3
, ZrO
2
and Al
2
O
3
/ZrO
2
(80 vol. % Al
2
O
3
) dry
pressed samples (cold isostatically pressed at 300 MPa) is recorded as a function
of temperature by means of a high temperature dilatometer (Netsch 402C) in air
at a heating rate of 3 °C/min.
The experimental densifi cation curves are plotted in Figure 10.18, as a func-
tion of the sintering temperature. The densifi cation of the pure ZrO
2
material
starts at lower temperatures than for the two-phase and the Al
2
O
3
material. Fur-
thermore, the curve of the pure ZrO
2
reaches a plateau at temperatures above
1500 °C and the sintered material is approaching the theoretical density.
During the fi rst stage, densifi cation in the ZrO
2
material is faster than in the
two - phase and Al
2
O
3
material. In the intermediate stage, the gradient of the den-
sifi cation curve is steeper for the two-phase material than for pure Al
2
O
3
. The
two - phase material densifi es faster during this stage. At the end of the fi nal stage,
the total shrinkage is larger for the two-phase material than for pure Al
2
O
3
. The
larger densifi cation shrinkage of the pure ZrO
2
and Al
2
O
3
/ZrO
2
can be explained
by the lower green density compared to the pure Al
2
O
3
.
The addition of ZrO
2
to the Al
2
O
3
matrix does not only infl uence the densifi -
cation behavior, but also the grain size of the Al
2
O
3
matrix (Figure 10.19). The
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