Biomedical Engineering Reference
In-Depth Information
not only mechanical properties such as hardness, fracture strength,
toughness and reliability at room temperature but also high-temperature
mechanical properties such as hardness, strength, and creep and fatigue
fracture resistances. The nano-nano composites were composed of
dispersoids and matrix grains of nanometre-size. The primary purpose of
this composite was to add new functions such as machinability and
superplasticity (as in metals) to ceramics.
1.3.1 Processing
The processing of thermally stable ceramic nanocomposites is based mainly
on results obtained on Al
2
O
3
/SiC and Si
3
N
4
/SiC systems. The Al
2
O
3
/SiC
nanocomposite system has been researched intensively because it has been
reported to have significantly improved mechanical properties over basic
Al
2
O
3
(Davidge et al. 1997, Jhao et al. 1993, Niihara, 1991). In Al
2
O
3
/SiC
nanocomposites, SiC 'nano' phases of order 100-200 nm in size were added
into the Al
2
O
3
matrix (typically 1-15 vol% SiC) and dispersed during
sintering or hot-pressing within the matrix grains or at the grain boundaries.
Although hot-pressed Al
2
O
3
/SiC materials have been reported to have the
most improved mechanical properties, pressureless sintered Al
2
O
3
/SiC
nanocomposites also have shown improvements in mechanical properties
(Anya and Roberts 1996, 1997, Jeong and Niihara K, 1997, Stearns et al.
1992). Processing of Al
2
O
3
with 1, 2.5 and 5 vol% SiC nanocomposites by
pressureless sintering has been reported to have significantly improved
mechanical and thermal shock resistance properties (Jhao et al. 1993).
Researchers have also succeeded to process other oxide and non-oxide based
ceramic nanocomposites, such as Al
2
O
3/
Si
3
N
4
,A
2
O
3
/TiC, mullite/SiC,
B
4
C/SiC, B
4
C/TiB
2
, SiC/amorphous SiC, Si
3
N
4
/SiC and so on by usual
powder metallurgical techniques such as pressureless sintering, hot-pressing
and hot isostatic pressing. The differences in fundamental mechanisms are
very clear between solid-state sintering and liquid-phase sintering. Solid-
state sintering processes were specifically applied for the processing of oxide
based nanocomposite materials and liquid-phase sintering processes have
been applied to the processing of non-oxide based nanocomposite materials
(Niihara 1991).
1.3.2 Effect of the use of dopants and other additives
Systematic experimental investigations are still necessary for the use of
dopants and other additives for the enhancement of mechanical properties
of thermally resistant ceramic nanocomposites. Several investigators have
studied the effect of the use of dopants for the enhancement of thermal
stability of ceramic materials by controlling the grain boundary mobility
