10 Appendix
10.1 Bonding Types and Strength
Metallic bonding is explained
through the existence of free, mobile electrons between the
atoms. This model accounts for their properties such as electrical
conductivity and ductility. Due to the non-directional nature
of the metallic bond, metals typically develop dense packing
and high co-ordination figures. The ductility of metallic
materials can be modelled by slipping between the most densely
packed planes without any change in the state of the bond.
As a rule, the metallic bond is weaker than ionic or covalent
bonds.
In ceramic materials the
bonding is generally a hybrid of ionic and covalent
bonds. Whereas the ionic bond is distinguished by
the transfer of an electron between a metal and non-metallic
atom, the electron pair of a purely covalent bond is shared
between non-metallic atoms. The strong bonding forces of ceramic
materials result in properties such as high elastic modulus
and hardness, high melting points, low thermal expansion,
and good chemical resistance. The covalent bond is a highly
directional; the orientation and distance of neighbouring
atoms are precisely determined. The stability of ionic and
covalent bonded solids is highly dependent on the maintenance
of the atomic geometries and explains the lack of plastic
deformation under load (low fracture toughness). Ceramic materials
are hard and therefore brittle. Their brittle fracture behaviour
must be kept in mind by the designer when calculating stresses.
Organic polymers consist
of long chains of molecules which are either tangled or ordered
at room temperature. Intermolecular forces known as van
der Waals forces are found almost exclusively between
non-crosslinked polymers. These are up to two orders of magnitude
smaller than covalent forces, and can be easily overcome by
thermal motion. This underlies their low heat resistance,
low elastic modulus, and high thermal expansion coefficient,
and is also responsible for the outstanding elastic and plastic
strain values of these materials. Like ceramics, polymers
have good chemical resistance, electrical and thermal isolation
properties. They display a noticeable brittleness at low temperatures.
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