Biomedical Engineering Reference
In-Depth Information
.
layered/laminated composites: two, three or multiple layers based on
oxides, nitrides, etc.
Most of these materials are treated using classical powder processing,
sintering, hot-pressing or hot isostatic pressing method, with or without
sintering additives in monolithic form or with different volume fractions in
the reinforcing phase. During the last decade, these methods have
significantly improved control of the microstructure (grain size and shape,
defects, flaws and intergranular phases in terms of amount, distribution and
microchemistry). This has been achieved through the production of fine and
pure powders, the design of the starting composition and the development
of controlled powder processing and densification procedures.
4.6 Wear of ceramic nanocomposites
Despite improvements in fracture toughness and strength of modern
ceramic materials, they are still predominantly used in applications that
make use of their exceptional hardness and chemical and thermal stability.
Typical examples are cutting tools, ceramic seals, ball bearings, etc. (i.e.
applications without high loads but with intense mechanical contact). Such
applications require high wear resistance and good tribological behaviour.
There are two advantages in using nanocomposites in this area: firstly, the
improvement in fracture toughness and hardness can improve the wear
resistance and, secondly, the addition of appropriate secondary phases can
lower the friction coefficient due to the development of a self-lubricating
effect.
4.6.1 Nanocomposites with carbon nanofilaments
Recently, efforts to develop ceramics (based on alumina, silicon nitride and
zirconia) with carbon filamentous phases (carbon nanofibres (CNFs) and
nanotubes (CNTs)) have produced interesting results in the area of
tribological properties (An et al., 2003; Lim et al., 2005; Bala´ zsi et al.,
2006; Yamamoto et al., 2008; Hvizdosˇ et al., 2010, 2011). It has been shown
that in self-mated tribological pairs, the coefficient of friction could be
significantly reduced, from values between 0.5 and 0.8 typical for the dry
sliding of ceramics, down to 0.2-0.3 (Fig. 4.20) (Hvizdosˇ et al., 2011). This
reduction, however, was generally accompanied by a lowering of the wear
resistance (Fig. 4.21) due to a less than optimal microstructure, which results
from difficulties with the incorporation of CNTs/CNFs. However, Hvizdosˇ
et al. (2011) also found an optimum for fractions of around 5% CNT in
Si
3
N
4
/CNT nanocomposite, where the wear resistance improved, probably
