Biomedical Engineering Reference
In-Depth Information
nontoxic and soluble
: the material dissolves and is replaced by tissue (e.g., domain C
in Fig.
9.14
).
Hardness of ceramics is dictated by the partial covalency of the chemical bonds
in Al
2
O
3
. Covalent bonds are stronger than metallic bonds where the atoms are held
together by a 'sea' of electrons. Difference in electronegativity determines how the
binding electrons are shared by the partners aluminum or silicon and oxygen.
4
So,
strength is another
pro
. And by the way, none of the bonding electrons are in the
conduction band, which makes most ceramics extremely good electrical insulators.
Moreover, the bonds are directional and that makes slipping of one crystal plane over
another unfavorable. Not so for metals for which malleability is intrinsically linked
to the easy traveling of dislocations. Thus, ceramics are
brittle
which is mostly a
con
. It entails that shaping by techniques used for metals like forging are excluded.
A real unpleasant consequence of brittleness is inadvertent fracturing at subcritical
stress levels, which limits their use in bending. Crack propagation in alloys is mostly
progressive but fast to catastrophical in ceramics.
A few words more about fracture.
Fast fracture will occur
when in a material
subjected to a tensile stress
, a crack reaches some critical length
a
. Tabulated
values of fracture toughness are calculated by equation:
p
aY D
p
EG
C
D K
C
;
(9.4)
where
E
is the elasticity modulus,
G
C
the energy absorbed in m
ak
ing a unit area
is a critical flaw factor, equal to
p
of crack, called
toughne
ss
.
Y
for internal or
volume defects and to
p
for surface defects. Both are material properties. High
toughness means that it takes more energy to make a crack propagate. The left term
is abbreviated by the symbol
=2
K
IC
when in
the test mode the stress is perpendicular to the crack). The units are MPa m
1=2
or
MN m
3=2
.
A design engineer aims to be able to calculate the survival probability of a
ceramic object. For this purpose, he/she will not use Gaussian but
Weibull statis-
tics
. In particular for Properties Tables of ceramics, the Weibull modulus is given,
usually represented by the symbol
K
C
and called
fracture toughness
(or
. Passing over the theory of this function, it suf-
fices to mention that for engineering ceramics
m
100. An
example of its practical use in reliability testing is demonstrated in Fig.
9.1
. We learn
from this figure that the results are dependent on the mechanical solicitation mode:
m ' 24:8
m>
10 and for metals
m
, 25.4 and 22.3 and the characteristic strength 353, 386 and 339 MPa,
respectively, for the test modes four-point-, three-point-flexure or uniform-pressure-
on-disk method. From the constant
Y
in (
9.4
), it is clear that the shape of the test
4
Electronegativity is a term introduced by Linus Pauling in his very famous topic (first edition in
1939)
The Nature of the Chemical Bond
, third edn., Cornell University Press, Ithaca, NY, 1960
or recently revised editions. Ionicity of a bond is in most cases only partial. Alumina is ionic for
56% and zirconia for 67%.
LiF
is the prototype example of an ionic solid, diamond is the other
covalent extreme.
