Biomedical Engineering Reference
In-Depth Information
.
incorporation of fibers or whiskers that bridge the crack faces just
behind the crack tip
.
designing microstructures with elongated grains that act as bridges
between crack faces just behind the crack tip
.
incorporating second phase particles, which deflect the crack, making it
travel a more tortuous path
.
incorporating secondary phases that undergo stress-induced volume
expansion that forces the crack faces together.
However, one of the most recent developments has been the distribution of
multiple phases in a ceramic composite at the nanoscopic length scale.
Owing to the prevalence of nanoscopic features, such composites are
referred to as ceramic nanocomposites.
The definition of nanocomposite material has broadened significantly to
encompass a large variety of systems such as one-dimensional (1D), two-
dimensional (2D), three-dimensional (3D) and amorphous materials, made
of distinctly dissimilar components and mixed at the nanometer scale. The
general class of nanocomposite organic/inorganic materials is a fast-growing
area of research. Reducing the sizes of structural features in materials leads
to a significant increase in the portion of surface/interface atoms. The
surface/interface energy essentially controls the properties of a solid of such
type. Interfaces provide a means to introduce non-homogeneity in the
material. This non-homogeneity acts as a significant modification of both
thermal and mechanical properties of the composites. Selective mixing of
materials in a highly tailored morphology with a high percentage of
interface area leads to materials with enhanced properties. The properties of
nanocomposite materials depend not only on the properties of their
individual parents but also on their morphology and interfacial character-
istics.
Nanocomposites find their use in various applications because of the
improvements in the properties over the simpler structures. As an example,
for components used in a gas turbine engine, a lifetime up to 10 000 h and a
retained strength of
C have been
postulated, together with negligible creep rate. Furthermore, at elevated
temperatures, the material must exhibit high resistance to thermal shock,
oxidation, and subcritical crack growth. Ceramic nanocomposites have been
shown to be extremely important for such future applications. Advanced
bulk ceramic composite materials that can withstand high temperatures
(
300MPa at a temperature of 1400
8
~
C) without degradation or oxidation can also be used for
applications such as structural parts of motor engines, catalytic heat
exchangers, nuclear power plants, and combustion systems, besides their use
in fossil energy conversion power plants. These hard, high-temperature-
>
1500
8
