Technical ceramics are often subdivided into
groups in accordance with the definitions mentioned above.
However since this does not permit unambiguous classification,
they are alternatively grouped according to their mineralogical
or chemical composition.
The following groups belong to the materials
defined as technical ceramics:
- silicate ceramics
- oxide ceramics
- non-oxide ceramics
Silicate ceramics, as the
oldest group amongst all the ceramics, represent the largest
proportion of fine ceramic products. The major components
of these polyphase materials are clay and kaolin, feldspar
and soapstone as silicate sources. Additionally such components
as alumina and zircon are used to achieve special properties
such as higher strength. During sintering a large proportion
(> 20%) of glass phase material, with silicon dioxide (SiO2)
as the major component, is formed in addition to the crystalline
Included in the silicate ceramic materials
- cordierite and
Due to the relatively low sintering temperatures,
the good understanding of how to control the process, and
the ready availability of the natural raw materials, silicate
ceramics are much cheaper than the oxide or non-oxide ceramics.
The latter require expensive synthetic powders and high sintering
Silicate ceramics are found, for example,
in heat engineering applications, measurement and control
engineering, process and environmental technologies, high
and low voltage applications with typical uses such as insulators,
fuse cartridges, catalysts, enclosures and in a wide range
of applications in the electrical equipment industry. Silicate
ceramics also continue to be used as refractory materials.
Oxide ceramics are defined
as all materials that are principally composed of a single
phase and a single component (>90 %) metal oxide. These
materials have little or no glass phase. The raw materials
are synthetic products with a high purity. At very high sintering
temperatures a uniform microstructure is created which is
responsible for the improved properties.
Some examples of oxide ceramics include
- as a single-material system
- aluminium oxide,
- magnesium oxide
- zirconium oxide,
- titanium dioxide (as a capacitor material)
- and as a multi-material system
o mixed oxide ceramics
- aluminium titanate
- lead zirconium titanate (piezo-ceramics)
o and dispersion ceramics
- aluminium oxide reinforced with zirconium
(ZTA - Al2O3/ZrO2).
Oxide ceramics are found in the electrical
and electronics industries, and often as structural ceramics,
i. e.i.e. for non-electrical applications. They offer the
typical properties suited to these applications, such as high
fracture toughness, wear resistance, high-temperature resistance
and corrosion resistance.
Non-oxide ceramics include
ceramic materials based on compounds of boron, carbon, nitrogen
and silicon. (Products made of amorphous graphite do not
belong to this category!)
Non-oxide ceramics usually contain a high
proportion of covalent compounds. This allows their use at
very high temperatures, results in a very high elastic modulus,
and provides high strength and hardness combined with excellent
resistance to corrosion and wear.
The most important non-oxide ceramics are:
- silicon carbide,
- silicon nitride,
- aluminium nitride,
- boron carbide and
- boron nitride.