Biomedical Engineering Reference
In-Depth Information
1.4.6 Metal nanoparticle dispersed nanocomposites
To improve mechanical properties, particularly fracture strength and
toughness for high-temperature applications, many attempts have been
made to provide ceramic matrix composites incorporating second-phase
dispersions such as particulates, platelets, whiskers or fibres (Becher and
Wei 1984, Claussen et al. 1977, Prewo et al. 1986, Uchiyama et al. 1986). In
this case, ceramic/ceramic systems are the most active field of ceramic matrix
composite research. On the other hand, there are also some investigations
regarding ceramic/metal composites which have incorporated secondary
metal phase dispersions such as tungsten, molybdenum, titanium, chro-
mium, nickel, etc. (Breval et al. 1985, Cho et al. 1980, Hing 1980, McHugh
et al. 1966, Naerheim 1986, Rankin et al. 1971). However, for both types of
composites, fracture strength and toughness have not been improved
simultaneously. This is mainly due to the fact that the addition of second-
phase dispersions generally causes an enlargement of the flaw size in the
composites. Therefore, these composites have advanced from a micro-order
dispersion to a nano-order dispersion, especially in the ceramic/ceramic
systems in an attempt to alleviate this problem.
Ceramic/metal nanocomposites consisted of an oxide ceramic and either
refractory metal such as in the Al
2
O
3
/W, Al
2
O
3
/Mo and ZrO
2
/Mo systems
or a metal with a low melting point such as Al
2
O
3
/Ni, Al
2
O
3
/Cr, Al
2
O
3
/Co,
Al
2
O
3
/Fe and Al
2
O
3
/Cu systems. These composites were fabricated by hot-
pressing the matrix and oxide powders (Ji and Yeomans 2002, Nawa et al.
1994a, 1994b, Oh et al. 2001, Sekino and Niihara 1995, 1997, Sekino et al.
1997). In recent years, nanocomposites, in which nanometre-sized metal
particles are dispersed within the ceramic matrix grains and at the grain
boundaries, have shown significant improvements in mechanical properties,
such as fracture strength, hardness and creep resistance, even at high
temperatures (Niihara 1991, Niihara et al. 1986, 1988b). In the first
approach, Al
2
O
3
has been well examined as a structural ceramic and has
been studied in ceramic/ceramic nanocomposite systems such as Al
2
O
3
/SiC
(Niihara et al. 1986, 1988b).
1.4.7 Al
2
O
3
/Mo nanocomposites
Molybdenum particles are one of the refractory metals and have a lower
thermal expansion coefficient than the Al
2
O
3
matrix. A conventional
powder metallurgical technique was used to prepare Al
2
O
3
/Mo nanocom-
posites. The nanocomposites were made by using
α
-Al
2
O
3
powder as the
matrix and different vol% of molybdenum powder as particulate reinforce-
ment (5, 7.5, 10, 15 and 20 vol%). The powder mixtures were ball milled
using zirconia milling media in acetone for 24 h. These slurries were dried
