Biomedical Engineering Reference
In-Depth Information
5 vol% SiC nanocomposites after annealing at 1300
C, and then the fracture
strength was further improved from 1050MPa to 1550MPa (Yongli 2006).
8
1.4.4 Al
2
O
3
/SWCNT nanocomposites
Multifunctional composites of polycrystalline alumina ceramics and single-
wall carbon nanotubes (SWCNTs) have been shown to possess unique grain
boundary structures (Grossman 1972, Holand et al. 1983, Kawamura 1979,
Niihara 1986). These hierarchical structures comprise a three-dimensional
network of two-dimensional nets made up of random one-dimensional
SWCNTs bundles, embedded within the alumina grain boundaries (Uno
et al. 1991). Composites with such unprecedented structures exhibited
unusual mechanical properties, including enhanced high-temperature creep
resistance (Uno et al. 1992). SWCNTs having an elongation to failure of 20-
30% with a combination of stiffness (Young's modulus of 1.5 TPa) show a
tensile strength well above 100GPa. The flexibility of SWCNTs is
remarkable and the bending may be fully reversible up to a critical angle
as large as 110
8
. It has been observed that Al
2
O
3
/10 vol% SWCNTs
nanocomposites with uniformly distributed SWCNTs at the grain bound-
aries show creep resistance over two orders of magnitude larger relative to
pure Al
2
O
3
of the same grain size (Katagiri et al. 1987). This dramatic
increase of creep resistance with partial obstruction of grain boundary
sliding by the presence of SWCNTs at the grain boundaries was understood
by the investigators to be a new class of high-temperature resistant
nanocomposites. In this study, the effect of uniformity of distribution of the
SWCNTs at alumina grain boundaries on the creep behaviour of Al
2
O
3
/
SWCNTs nanocomposites was investigated. Several uniaxial compression
creep tests were performed to study the high-temperature mechanical
properties and thermal stability during deformation of the composites, using
a method and equipment described elsewhere (Nose and Fujii 1987) at
temperatures of 1300-1350
) range 26-256MPa. The
temperatures used were high enough for diffusion processes to occur, but
sufficiently low to avoid grain growth. An argon (Ar) gas atmosphere was
used to prevent oxidation of the SWCNTs.
Zhan et al. (2003a, 2003b) fabricated fully dense nanocomposites of
SWCNTs with a nanocrystalline alumina matrix at sintering temperatures
as low as 1150
8
C and in the stress (
σ
C by spark plasma sintering (SPS). The introduction of
SWCNTs led to a refinement of grain size. Most of the
8
-Al
2
O
3
grains were
in the nanocrystalline range, around 200 nm. The fracture toughness of the
Al
2
O
3
/5.7 vol% SWCNTs nanocomposites was more than twice that of pure
alumina and there was almost no decrease in hardness. A toughness of
9.7MPa.m
1/2
, nearly three times that of pure alumina, was achieved in
α
