Biomedical Engineering Reference
In-Depth Information
attractive for making dense ceramic products with a negligible residual
glassy grain boundary phase and hence better high-temperature properties.
However, HP and HIP techniques are very costly. They become economic
only when a large number of samples are involved (Niihara and Hirai 1986).
Composite formation with nanostructured coatings based on ceramic
powders is very attractive for cutting tools and wear-resistant tools.
Pulsed electric current sintering (PECS) technique can be used for the
fabrication of such nanocomposites.
1.2.2 Thermal shock and flame retardant behaviour of
ceramic nanocomposites
Ceramic materials have a greater thermal shock sensitivity than metals and
can suffer catastrophic failure due to thermal shock because of their
unfavourable ratio of stiffness and thermal expansion to strength and
thermal diffusivity, and their limited plastic deformation. The stress field
that develops in a thermally shocked component can be explained by
calculating the thermal shock induced stresses along the x-andy-axes of a
plate. Considering a plate with Young's modulus E, Poisson's ratio
n
and
coefficient of thermal expansion (CTE)
, initially held at temperature T
i
:if
the top and bottom surfaces of the plate come into sudden contact with a
medium of lower temperature T
α
they will cool and try to contract.
However, the inner part of the plate initially remains at a higher
temperature, which hinders contraction of the outer surfaces, giving rise
to tensile surface stresses balanced by a distribution of compressive stresses
at the interior. By contrast, if the surfaces come into contact with a medium
of higher temperature T
∞
they will try to expand. As the interior will be at a
lower temperature, it will constrain expansion of the surfaces, thus giving
rise to compressive surface stresses balanced by a distribution of tensile
stresses at the interior (Kastritseas et al. 2006).
If perfect heat transfer between the surfaces and the medium is assumed
(i.e. if B
i
→∞
∞
) the surface immediately adopts the new temperature while the
interior of the plate remains at T
i
. Following Munz and Fett (1999), this
case corresponds to having a plate that can expand freely in the z-direction
with suppressed expansion in the x- and y-directions. In the absence of
displacement restrictions, the plate would expand along the x- and y-
directions by thermal strains of:
e
x
¼ a
ð
T
?
T
i
Þ
½
1
:
2
e
y
¼ a
ð
T
?
T
i
Þ
½
1
:
3
Since thermal expansion in both directions is completely suppressed, elastic
