Biomedical Engineering Reference
In-Depth Information
were composed of more uniform and homogeneous elongated grains. On the
other hand, above 25 vol% SiC, the growth of elongated grains decreased
with the increase of SiC content, and equiaxed and fine Si
3
N
4
grains were
developed.
Nano-nano composites were also studied for the Si
3
N
4
/SiC system,
including nano-size SiC particles of over approximately 40 vol% SiC
(Niihara 1991). It was reported that elongated Si
3
N
4
grains were formed by
the solution-diffusion-reprecipitation process during the liquid-phase
sintering. From this growth mechanism of elongated Si
3
N
4
grains and the
microstructural change observed in this system, it is reasonable to think that
the nano-size SiC particles dispersed within the Si
3
N
4
grains act as nuclei for
β
-Si
3
N
4
growth in the solution-diffusion-reprecipitation process. The
decrease of elongated grains observed at higher SiC content can thus be
explained by the excessive nuclei site of
-Si
3
N
4
in the above-mentioned
processes. In other words, intragranular SiC particles are thought to be
trapped in the Si
3
N
4
grains during the solution-diffusion-reprecipitation
process in the sintering of Si
3
N
4
. The mechanical properties of Si
3
N
4
/SiC
nanocomposites will be expected to improve by this morphology change of
Si
3
N
4
grains (Tani et al. 1986). For Si
3
N
4
/32 vol% SiC nanocomposites,
fracture toughness and strength were improved with the increase of nano-
size SiC content.
It has been reported that grain boundary sliding and/or cavitation are
responsible for the high-temperature strength degradation of the monolithic
ceramics. The fracture strength of monolithic Si
3
N
4
ceramics degrades
suddenly above 1200
β
C because of grain boundary sliding and/or cavitation
caused by softening of the grain boundary impurity phase. However, Si
3
N
4
/
SiC nanocomposites did not exhibit strong degradation of strength up to
1400
8
8
C. The fracture strength at 1400
8
C was over 1000MPa and about
8
900MPa even at 1500
C for the 32 vol% SiC nanocomposite in which the
nano-size SiC particles were dispersed not only within the Si
3
N
4
matrix but
also at the grain boundaries (Evans and Blumenthal 1983).
Nano-size SiC particles located at the grain boundaries of the Si
3
N
4
matrix make a direct bonding with the matrix in spite of grain boundary
impurity phases, and then improve the high-temperature mechanical
properties and also probably the oxidation resistance. Some Si
3
N
4
/SiC
nano/nano composites have exhibited superplastic deformation at approxi-
mately 1600
C. Superplasticity was provided by controlling the sintering
conditions. The important points are to retain the
8
-Si
3
N
4
phase and to
incorporate the SiC particles at approximately 40 vol% (Niihara 1991).
α
