Biomedical Engineering Reference
In-Depth Information
by Venkatachari and Raj (1987). This technique avoids cavities and voids
because sinter forging is produced by compression. High strength and high
fracture toughness of Si
3
N
4
have been achieved by superplastic sinter
forging due to the reduction of flaw size and grain alignment (Kondo et al.
1999).
1.7
Densification for the fabrication of thermal shock
resistant ceramic nanocomposites
Densification of graded powder compacts is a major challenge for achieving
ceramic nanocomposites with high mechanical strength. Properly densified
ceramic-based nanocomposites have received much attention in various
high-temperature structural applications. Some ceramics/ceramic nanocom-
posites in which the nanometre-size second phase was dispersed within the
matrix grains were successfully fabricated using a 1989b conventional
powder metallurgical technique by Niihara and colleagues (Niihara et al.
1988a, 1989b, 1990a, Yanai et al. 1990). They showed that Al
2
O
3
-, Si
3
N
4
-,
MgO- and B
4
C-based nanocomposites exhibited high fracture toughness
and strength at room temperature, good creep resistance and no
degradation of high-temperature strength. Among these, Al
2
O
3
/SiC
nanocomposites were reported to possess high fracture toughness and
strength over 1000MPa, high reliability and good high-temperature
mechanical properties (Niihara 1991, Niihara and Nakahira 1989, 1990b).
The densification and grain growth behaviours of Al
2
O
3
in a temperature
range of 1000-1800
CinN
2
atmosphere during hot-pressing Al
2
O
3
/5 vol%
SiC nanocomposites were investigated by Nakahira and Niihara (1992).
Fully dense (
8
C. On
the other hand, for Al
2
O
3
/5 vol% SiC nanocomposites, the relative densities
exceeded the value of approximately 90% at 1300
>
98%) monolithic Al
2
O
3
was prepared by them at 1500
8
8
8
C and 95% at 1400
C.
Al
2
O
3
/5 vol% SiC nanocomposites hot-pressed at and above 1400
8
C
contained no open pores. The fully dense (
>
99%) nanocomposites were
prepared at 1600
C. The main challenge associated with powder processing
is often densification of the graded powder compact. Sintering rates differ
with position and uneven shrinkage may lead to warping and cracking,
unless sophisticated sintering techniques are employed. Almost all ceramic/
ceramic bulk functionally graded materials (FGMs) are sintered by
conventional pressureless sintering (Cichocki and Trumble 1998, Marple
and Boulanger 1994, Wu et al. 1996) or hot-pressing (Kawai and
Wakamatsu 1995, Vanmeensel et al. 2004), depending on the sintering
properties of the two components. In metal/ceramic FGMs with a
continuous metal phase and a discontinuous ceramic phase, the sintering
8
