Ceramics

The term ‘ceramics’ refers to a large number of inorganic compounds, mostly oxides or carbides of metal or semi-metals (semiconductors). Their properties are completely different from the initial metallic properties. Above all, ceramics are not meltable (but in most cases withstand very high temperatures) and excellent insulators. Historically, the term ceramics comprises malleable clay materials which are baked in a kiln to harden. These historic ceramics are primarily

• Clay: 
  Clay is based on aluminium silicates. To this day, a large number of articles of daily use are made from clay: vessels, dishes, building materials (roof tiles), furnaces (for the high temperature resistance).
• Porcelain:
  Clay which contains kaolin can be used in porcelain manufacture. Kaolin is a weathering product of feldspar. Depending on the deposit, it contains minerals in different combinations.

Technical ceramics: Due to their high hardness, ceramics are of great importance in technology for use in abrasives and polishing agents and in cutting tools for hard materials. This goes in particular for the minerals:

• Aluminium oxide Al₂O₃ (corundum) for abrasives
• Zirconium dioxide ZnO₂ : for high-performance abrasives
• Silicon carbide (SiC) for high-performance abrasives

The second major application for technical ceramics are fabricated part with the following characteristic properties:

• temperature resistance
• small heat expansion
• good insulators
• high hardness
• high resistance to chemicals
• low friction and abrasion
• In addition to these outstanding properties, it is also characteristic of all ceramics that they are brittle and break easily under bending and stretching loads.

Fabricated parts are produced by sintering, as of late also by injection moulding and subsequent sintering. A process of growing importance is ceramic coating in a CVD and PACVD process in low-pressure plasma. Among the technical ceramics, the following are of particular importance:

• Beryllium oxide (BeO) for fabricated parts with highest temperature resistance (crucibles, kilns)
• Silicon carbides (SiC) as bearing material. For highest requirements, carbon fibre reinforcement silicon carbide is available.
• Boron carbide and boron nitride extreme hardness (diamond-like)
• Zirconium dioxide ZnO₂ : For medical implants

Silicon carbide and boron carbide can be generated by CVD and PACVD.