Biomedical Engineering Reference
In-Depth Information
with an alumina and zirconia precursor, respectively. The obtained
nanocomposites show a fine nanostructure where the matrix is formed by
Ce-TZP crystals. Nano-sized alumina crystals (
20 nm) are retained inside
Ce-TZP grains and other alumina grains (200-300 nm) are homogeneously
distributed in the Ce-TZP matrix. They claimed that these nanocomposites
show high flexural strength and fracture toughness values.
De Aza et al. (2002) obtained ZTA composites with submicrometre
alumina grains and mainly intergranular, nano-sized zirconia particles. They
stated that this leads to a high portion of tetragonal phase retained at room
temperature (after sintering) with the ability to transform under applied
stress. Therefore, the dominant toughening mechanism in these composites
is considered to be transformation toughening. They concluded that ZTA
composites present a higher reliability than monolithics, due to the
combination of the advantages of both alumina and zirconia.
The addition of alumina to zirconia can also drastically reduce the aging
kinetics. Chevalier et al. (2000) obtained composites with 1.7 vol%
(2.5 wt%) zirconia nanoparticles. It has been proved (Chevalier et al.,
2011) that alumina-zirconia nanocomposites exhibit a very limited surface
damage after 7million cycles in a hip simulator. Moreover, these
nanocomposites show crack resistance under static and cyclic fatigue well
beyond that of all existing biomedical-grade ceramics. Chevalier et al. stated
that, associated with expected full stability in vivo, the overall set of results
ensures potential future development for these kinds of new nanocomposites
in the field of orthopaedics.
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YTZP-CNT nanocomposites
As has been already mentioned, there is a critical zirconia grain size below
which no tetragonal to monoclinic phase transformation occurs. Therefore,
by decreasing the grain size, the sensitivity to aging of the material can be
avoided but, at the same time, the transformation toughness mechanism
that gives zirconia its exceptional mechanical properties will be lost.
Garmendia et al. (2009, 2010, 2011) studied the addition of carbon
nanotubes (CNTs) to a zirconia matrix as reinforcing agents to maintain, or
even increase, toughness in spite of the decrease in grain size. The CNTs
dispersed in the ceramic matrix act as fracture energy dissipating sites
through mechanisms such as crack deviation in the CNT and ceramic matrix
interface, crack bonding by the CNT, and CNT pull-out in fracture surfaces.
In any case, to really improve the mechanical properties of a ceramic matrix,
a good dispersion of the CNTs in the matrix and chemical bonding between
the CNTs and the zirconia are needed. In order to reach these requirements,
zirconia-coated CNTs have been introduced. In this way, nanozirconia
nanocomposites reinforced with CNTs with improved mechanical proper-
