14.2.1 History
The word ceramic can be traced back to the Greek term keramos, meaning “a potter” or “pottery.” Keramos, in turn, is related to an older Sanskrit root meaning “to burn.” Thus, the early Greeks used the term to mean, “burned stuff” or “burned earth” when referring to products obtained through the action of fire upon earthy materials (The American Ceramic Society) [Marinescu, Toenshoff, and Inasaki 2001].
14.2.2 Structure
The structure of ceramic crystals is among the most complex of all materials, containing various elements of different sizes. The bonding between these atoms is generally covalent (electron sharing, hence, strong bonds) and ionic (primary bonding between oppositely charged ions, thus, strong bonds). These bonds are much stronger than metallic bonds. Consequently, hardness and thermal and electrical resistance of ceramics may be significantly higher than those of metals [Inasaki 1998].
The properties of ceramics products are dependent on the chemical composition and atomic and microscale structure. Compositions of ceramic products vary widely, and both oxide and nonoxide materials are commonly used. In recent times, the composition, grain structure, and also the distribution and structure of porosity have been more carefully controlled to achieve greater product performance and reliability [Inasaki 1998].
14.2.3 Ceramic Groups
Ceramics are classified in the following subgroups [Marinescu et al. 2001]:
• Oxides (alumina, zirconia, and partially stabilized zirconia)
• Carbides (tungsten and titanium used for cutting tools, silicon carbide used as abrasives in grinding wheels)
• Nitrides (cubic boron nitride [CBN], titanium nitride, and silicon nitride)
• Sialon (silicon nitride with various additions of aluminum oxide, yttrium oxide, and titanium carbide)
• Cermets (ceramics bonded with a metallic phase)
• Silicates are products of the reaction of silica with oxides of aluminum, magnesium, calcium, potassium, sodium, and iron such as clay, asbestos, mica, and silicate glasses.