Biomedical Engineering Reference
In-Depth Information
and passed through a 250
m screen. The mixtures were then hot-pressed in
carbon dies of 40 nm diameter. The hot-pressing conditions were 1400-
1700
μ
C with an applied pressure of 30MPa for 1 h under a vacuum of less
than 10
4
Torr (1 Torr=133.322 Pa). The fracture strength of Al
2
O
3
/
7.5 vol% Mo nanocomposites hot-pressed at 1400
8
C exhibited a maximum
value of 884MPa, which was 1.5 times larger than that of the monolithic
Al
2
O
3
prepared under the same conditions.
When the hot-pressing temperature was increased to 1700
8
C, strength
decreased to around 450MPa, which corresponded to grain growth of the
Al
2
O
3
matrix. Thus, in Al
2
O
3
/Mo nanocomposites, strengthening was
assumed to be mainly attributed to the inhibition of grain growth of the
Al
2
O
3
matrix in the presence of nano-sized molybdenum particles. Many
voids were observed in the Al
2
O
3
/7.5 vol% Mo nanocomposites hot-pressed
at any temperature. Because of such voids, large molybdenum particles
seemed to be pulling out from the grain boundaries or triple junctions of the
Al
2
O
3
, which was observed in the microstructure. These results suggest that
the bonding strength of the interfaces between the Al
2
O
3
grains and
molybdenum particles is not strong. On the other hand, the fracture mode of
monolithic Al
2
O
3
was mainly intergranular, but in the case of a large grain
size, a transgranular fracture mode was observed.
When hot-pressed at a lower sintering temperature of 1400
8
C, the fracture
toughness for the Al
2
O
3
/Mo nanocomposites was around 4.4MPa.m
1/2
;
nevertheless, a significant improvement in fracture toughness was observed
with increasing molybdenum content and hot-pressing temperatutre.
Fracture toughness of the Al
2
O
3
/20 vol% Mo nanocomposites hot-pressed
at 1700
8
C exhibited a maximum value of 7.6MPa.m
1/2
, which was 1.8 times
larger than that of the monolithic Al
2
O
3
hot-pressed at 1600
8
C (Nawa et al.
1994b). The Al
2
O
3
/5 vol%Mo nanocomposites exhibited maximum value of
19.2GPa when hot-pressed at 1400
8
8
C. This hardening is supposed to be
accounted for by the contribution of the fine-grained microstructure of the
Al
2
O
3
matrix (Sargent and Page 1978) and/or the intergranular nano-
structure of Al
2
O
3
/Mo nanocomposites, in which nanometre-sized molyb-
denum particles are dispersed within the Al
2
O
3
grains and at the grain
boundaries. In fact, the monolithic Al
2
O
3
used in this nanocomposite
showed a negative dependence of grain size on hardness and the nanometre-
sized metal particle will itself cause hardening (Wierenga et al. 1984). The
investigators characterised samples of Al
2
O
3
/Mo nanocomposites to show
the variation of Vickers hardness with molybdenum content; it was found
that the hardness values of monolithic and molybdenum polycrystals were
18.5 and 2.1GPa, respectively. For nanocomposites containing 5-15 vol%
Mo hot-pressed at lower sintering temperatures, higher values of hardness
were observed than those predicted by the rule of mixtures.
