As discussed in the introduction, ceramics and related
materials cover a wide range of objects. Ceramics are a little
more complex than metallic structures, which is why metals were
covered first. A ceramic has traditionally been defined as “an
inorganic, nonmetallic solid that is prepared from powdered materials
and is fabricated into products through the application of heat.
Most ceramics are made up of two or more elements. This is called
a compound. For example, alumina (Al2O3) is a compound made up
of aluminum atoms and oxygen atoms.
The two most common chemical bonds for ceramic materials
are covalent and ionic. The bonding of atoms together is much
stronger in covalent and ionic bonding than in metallic. This
is why ceramics generally have the following properties: high
hardness, high compressive strength, and chemical inertness. This
strong bonding also accounts for the less attractive properties
of ceramics, such as low ductility and low tensile strength. The
absence of free electrons is responsible for making most ceramics
poor conductors of electricity and heat.
However, it should be noted that the crystal structures
of ceramics are many and varied and this results in a very wide
range of properties. For example, while ceramics are perceived
as electrical and thermal insulators, ceramic oxide (initially
based on Y-Ba-Cu-O) is the basis for high temperature superconductivity.
Diamond and silicon carbide have a higher thermal conductivity
than aluminum or copper. Control of the microstructure can overcome
inherent stiffness to allow the production of ceramic springs,
and ceramic composites which have been produced with a fracture
toughness about half that of steel. Also, the atomic structures
are often of low symmetry that gives some ceramics interesting
electromechanical properties like piezoelectricity, which is used
in sensors and transducers.
The structure of most ceramics varies from relatively
simple to very complex. The microstructure can be entirely glassy
(glasses only); entirely crystalline; or a combination of crystalline
and glassy. In the latter case, the glassy phase usually surrounds
small crystals, bonding them together. The main compositional
classes of engineering ceramics are the oxides, nitrides and carbides.