Biomedical Engineering Reference
In-Depth Information
to monoclinic transformation. When the grain size is below 0.5
μ
m, a slow
transformation occurs. With grain sizes under 0.2
m the martensitic
transformation is not promoted, therefore reducing the possibility of
cracking (Evans and Heuer, 1980; Gutknecht et al., 2007). Therefore, by
reducing the zirconia grain size, aging resistance is increased. But, on the
other hand, the transformation toughening mechanism that gives zirconia
its exceptional mechanical properties will be lost. With the development of
zirconia-alumina nanocomposites, the combination of both aging resistance
and enhanced mechanical properties is promising. During recent years,
several zirconia-alumina composites and nanocomposites have been
developed and have shown significant improvement in toughness, strength
and aging resistance (Menezes and Kiminami, 2008; Nevarez-Rascon et al.,
2009).
Two kinds of composites can be prepared in the zirconia-alumina system
(De Aza, 2002): an alumina matrix reinforced with zirconia particles
(zirconia-toughened alumina, ZTA) or a phase-stabilized zirconia matrix
reinforced with alumina particles, known as alumina-toughened zirconia
(ATZ). Composites with high fracture toughness are suitable in the ATZ
system while composites with high hardness and relatively low fracture
toughness belong to the ZTA system (Nevarez-Rascon et al., 2009).
Zirconia-alumina composites are available in the market, such as the ZTA
Biolox
®
Delta (CeramTec GmbH, Germany) and the ATZ BIO HIP
®
(Metoxit AG, Switzerland).
For biomedical applications, ZTA is the most popular system. The
dispersion of zirconia grains as a discrete second phase in the alumina
matrix is a widely studied way of improving the mechanical properties of
alumina (Schehl et al., 2002). Significant improvements in toughness,
strength and aging resistance have been shown, which are due to the
dispersion of metastable tetragonal zirconia particles in the alumina matrix,
which transform into the stable monoclinic phase under loading (Menezes
and Kiminami, 2008). Other reinforcement mechanisms have been
identified, such as microcrack toughening, compressive surface stresses
and crack deflection (Laurent et al., 1996).
Nawa et al. (1998) developed 10mol% Ce-TZP/Al
2
O
3
nanocomposites
(with both phases being of nanometre scale) that exhibit high resistance to
aging, complete biocompatibility and a high wear resistance (Uchida et al.,
2002; Tanaka et al., 2002, 2003). Moreover, Benzaid et al. (2008) showed
that the cyclic fatigue threshold of these nanocomposites stands above that
measured in conventional biomedical-grade alumina and zirconia.
Therefore, they state that Ce-TZP/Al
2
O
3
nanocomposites may be con-
sidered as an option for future biomedical applications.
Benzaid et al. (2008) also proposed a nano-nano Ce-TZP/Al
2
O
3
composite. The starting nanopowders (Ce-TZP and alumina) were coated
μ
