Biomedical Engineering Reference
In-Depth Information
Al
2
O
3
/10 vol% SWCNT nanocomposites when sintered under identical
conditions.
1.4.5 Al
2
O
3
/ZrO
2
nanocomposites
The phase diagram shows that the miscibility of Al
2
O
3
and ZrO
2
phases is
very similar. There is also evidence that the grain boundary structure is
stable due to the presence of two phases. It was felt necessary to have these
two phases in one microstructure of nano-size. Cotton and Mayo (1996)
claimed that no increase in toughness occurs in ZrO
2
nanocomposites with
density values close to theoretical unless the materials are heat-treated such
that grains become susceptible to a phase transformation. Recent studies
have indicated that a different mechanism (rather than phase transforma-
tion) of toughening must be operating. Ferroelectric domain switching is
responsible for the great increase of toughness of Al
2
O
3
/ZrO
2
nanocompo-
sites, which is now well established as a mechanism for enhanced toughness
without undergoing transformation in ZrO
2
. Physically speaking, ferro-
elasticity is similar to ferromagnetism or ferroelectricity. Drawing the
analogy, ferroelasticity can be characterized by the existence of permanent
strain and an energy dissipating hysteresis loop between the stress and strain
axes. In such materials, new domains or twins can be nucleated depending
on the state of stress.
From the literature on superplastic deformation of metals, it can be
concluded that nano-nano composites in which the constituent phases have
similar grain sizes are preferred. Indeed, fine-grained (submicron but larger
than nanocrystalline) Al
2
O
3
/ZrO
2
nanocomposites have been superplasti-
cally deformed. The average value of toughness in HIP sintered Al
2
O
3
/ZrO
2
nanocomposites was calculated to be 8.38MPa.m
1/2
. A conventional
ceramic material cracks substantially under such a load. This means that
the nano-nano composites were actually deforming plastically under load
(Bhaduri and Bhaduri 1997). A combination of very rapid sintering at a
heating rate of 500
8
C/min and a sintering temperature as low as 1100
8
C for
3min by the SPS technique, and mechanical milling of the starting
-Al
2
O
3
nanopowder via a high-energy ball milling (HEBM) process, can also result
in a fully dense nanocrystalline alumina matrix ceramic nanocomposite. The
grain sizes for the matrix and the toughening phase were 96 and 265 nm,
respectively. With regard to toughening, a great improvement in fracture
toughness (8.9MPa.m
1
γ
=
2
) was observed in the fully dense nanocomposites,
nearly three times as tough as pure nanocrystalline alumina (152 nm,
3.03MPa. m
1/2
) (Zhan et al. 2003c).
