Mechanical properties - Orthopaedic Biomaterials in Research and Practice - page 58

case, a stream of moving electrons). The resultant positive ions
pack together into high-density planes and structures, surrounded
by a cloud of delocalized electrons. Metallic bonds are very strong
but not particularly oriented in space. Planes of metallic atoms
can slide* over each other fairly easily, producing ductile deforma-
tion but generally associated with considerable toughness, owing
to the strength of the metallic bond and the ease of re-establishing
it after adjacent planes of atoms move one atomic diameter.
2. Ionic. Some pairs of atoms can easily exchange one or more elec-
trons, producing filled outer shells in each and a resultant pair of
positive and negatively charged ions that are strongly attracted.
Metal atoms in particular can donate electrons to atoms of gaseous
elements such as nitrogen, oxygen, and chlorine. The result is a
dense three-dimensional structure with oriented bonds but with a
high degree of order imposed by alternation of ionic type. These
materials are extremely strong but, at body temperature, tend to
fail catastrophically along a plane of symmetry when the ultimate
strain is exceeded. Thus, the ionic bond contributes brittle behav-
ior to solids.
If these compounds are stable in the presence of water, they
are called ceramics , such as aluminum oxide (alumina), zirconium
oxide, and silicon nitride. If they are appreciably soluble in water,
they are called salts , such as sodium chloride.
3. Covalent. Some atoms can fill their outer electronic shells on a
virtual basis by sharing electrons with an adjacent uncharged
atom. This is represented in Figure 3.3 by a slight overlap of the
circles representing the atoms. The resulting bond is strong but
very directional, owing to the details of electronic structure in the
atom. The result can be long chains of covalently bonded atoms,
with carbon being the best and most common example. The resul-
tant chains possess relatively fixed angles between bonds on a
particular atom but permit easy rotation around single-exchange
bonds and are quite strong. Therefore, these solids possess very
large molecules (polymers) that can deform easily but break only
with difficulty, exhibiting significant plastic behavior.
4. van der Waals. Hydrogen in particular can participate in a very
weak form of nonexchange bonding with oxygen and a number
of other atoms. This is weaker and less directional than the cova-
lent bond, but the very great quantity of hydrogen in hydrocarbons
(polymers containing carbon, hydrogen, and oxygen, including
proteins and other structural molecules in tissues as well as most
man-made polymers) produces large contributions to the cohesion
of materials. Polymers (long-chain molecules made up of identi-
cal or repetitive molecular groups) have strong covalently bonded
* The process is called “slip” and the motion planes are called “slip planes.” Slip requires
the application of shear stresses; reference to Chapter 2 will reassure the reader that in
all but uniform (hydrostatic) compression, shear stresses result from any external force.