Breviary Technical Ceramics






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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