Biomedical Engineering Reference
In-Depth Information
zirconia polycrystal ceramic (Y-TZP) with a grain size of 0.4
m was
reported by Wakai et al. (1986). Since then, a large number of fine-grained
polycrystalline ceramics and ceramic composites with superplastic beha-
viour have been developed (Melendez-Martinez and Dominguez-Rodriguez
2004, Wei Chen and Xue 1990).
Several techniques have been developed to achieve ceramics and
composites with a fine microstructure, as follows.
μ
.
One technique to suppress grain growth is based upon the inclusion of
dispersed phases into the ceramics. In this regard, a multiphase ceramic
composite containing 40 vol% ZrO
2
, 30 vol% spinel and 30 vol% Al
2
O
3
has been sintered and superplastically deformed to 1050% at 1650
Cat
a strain rate of 0.4 s
1
(Kim et al. 2001). Other zirconia-based
composites have also been fabricated with inhibition of grain growth
(Wang et al. 2003, Yonn and Chen 1990). With this technique, it is
possible to deform the ceramics, with their microstructure unchanged, at
a temperature at which grain growth would be important in ceramics
without dispersed phases.
8
.
Another technique makes use of ultra-fine powders sintered under
stress-assisted conditions so that the sintering temperature is reduced
(Mayo 1996), for instance hot isostatic pressing, hot-pressing, sinter
forging, or techniques with a fast heating and cooling ramp, such as
spark plasma sintering (SPS) or microwave sintering, which avoid grain
growth by reducing the time or temperature of sintering. Very fast
densification has been reported in oxides and Si
3
N
4
-based ceramics by
SPS (Shen et al. 2003). Microwaves have also been used to sinter PSZ
(Wilson and Kunz 1988) and alumina (Mizuno et al. 2004).
.
The sintering of Y
2
O
3
nanocrystalline ceramics (d=60
m) has been
achieved through a two-step sintering method. The first step is pre-
sintering at high temperatures to obtain ceramics with intermediate
density values of 70-80%. Secondly, suppression of grain growth is
achieved by sintering at lower temperatures than those used in the first
step, exploiting the difference in kinetics between grain boundary
diffusion, which ultimately controls sintering, grain boundary migra-
tion, and grain growth (Chen and Wang 2000).
μ
There are a great number of papers dealing with the influence of grain
boundary segregation on superplasticity in Y-TZP. It has been shown that
the superplastic flow stress at 1400
C of 3Y-TZP doped with different
cations is correlated with the ionic radius of the dopant (Mimurada et al.
2001, Nakatani et al. 2003). Cations with a smaller ionic size decrease the
flow stress, whereas those with large ionic sizes increase the flow stress. The
authors suggested that the flow stress is determined by grain boundary
diffusivity, which is affected by segregation of the dopant. The same
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