Biomedical Engineering Reference
In-Depth Information
where l is a characteristic material dimension (e.g. the half-thickness of a
plate), h is the HTC between the body and the medium and k is the thermal
conductivity of the body. The larger the value of Bi, the larger is the rate of
heat transfer between a medium of different temperature and the body. The
sudden temperature change (
T) that generates non-linear temperature
gradients in a body and, as a consequence, thermal stresses is termed
'thermal shock'. If
Δ
T is positive (i.e. the temperature reduces) the material
is subjected to a cold shock, whereas if
Δ
T is negative the material is
subjected to a hot shock. The term refers to a single thermal cycle (N=1)in
contrast to terms such as thermal cycling, cyclic thermal shock and thermal
fatigue, which apply to multiple thermal cycles (N
Δ
>
1) (Kastritseas et al.
2006).
Although ceramic materials based on oxides, non-oxides, nitrides,
carbides of silicon, aluminium, titanium and zirconium, alumina, etc.
possess some very desirable characteristics (e.g. high strength and hardness,
excellent high-temperature structural applications, chemical inertness, wear
resistance and low density), they are not very robust under tensile and
impact loading and, unlike metals, they do not show any plasticity and are
prone to catastrophic failure under mechanical or thermal loading (thermal
shock) (Banerjee and Bose 2006). Many researchers have focused attention
on both oxide and non-oxide ceramic materials for the improvement of
microstructure. However, many problems are not yet solved. For instance,
non-oxide ceramic materials such as Si
3
N
4
and SiC suffer from degradation
of mechanical properties at high temperature due to slow crack growth
caused by the softening of grain boundary impurity phases associated with
sintering additives. Oxide ceramic materials such as Al
2
O
3
, MgO and ZrO
2
suffer from relatively low fracture toughness and strength, significant
strength degradation at high temperatures and poor creep, fatigue and
thermal shock resistance.
Attempts have been made by many researchers to solve these problems, as
well as brittleness problems, by incorporating secondary phases such as
particulates, platelets, whiskers and fibres in the ceramic materials (Becher
and Wei 1984, Buljun et al. 1987, Claussen 1985, Greskovich and Palm
1980, Homeney et al. 1987, Izaki et al. 1980, Lange 1973, 1974, 1982,
Nakahira et al. 1987, Niihara et al. 1986, Nishida et al. 1987, Shalek et al.
1986, Tiegs and Bechere 1987, Wahi and Ilschner 1980). For non-oxide
ceramic materials such as Si
3
N
4
, SiC has been incorporated for a second-
phase dispersion to take advantage of the characteristics of both phases.
(Becher and Wei 1984, Homeney et al. 1987, Nakahira et al. 1987, Tiegs and
Bechere 1987). Recently, the Si
3
N
4
/SiC whisker composite system has
demonstrated significant improvements in fracture toughness and high-
temperature fracture behaviour, but with a decrease in fracture strength
(Becher and Wei 1984, Nakahira et al. 1987) For the SiC particulate
