atomic and molecular nature of ceramic (industrial ceramics) materials (materials processing) and their resulting characteristics and performance in industrial applications.

Industrial ceramics are commonly understood to be all industrially used materials that are inorganic, nonmetallic solids. Usually they are metal oxides (that is, compounds of metallic elements and oxygen), but many ceramics (especially advanced ceramics) are compounds of metallic elements and carbon, nitrogen, or sulfur. In atomic structure they are most often crystalline, although they also may contain a combination of glassy and crystalline phases. These structures and chemical ingredients, though various, result in universally recognized ceramic-like properties of enduring utility, including the following: mechanical strength in spite of brittleness; chemical durability against the deteriorating effects of oxygen, water, acids, bases, salts, and organic solvents; hardness, contributing to resistance against wear; thermal and electrical conductivity considerably lower than that of metals; and an ability to take a decorative finish.

In this article the relation between the properties of ceramics and their chemical and structural nature is described. Before such a description is attempted, though, it must be pointed out that there are exceptions to several of the defining characteristics outlined above. In chemical composition, for instance, diamond and graphite, which are two different forms of carbon, are considered to be ceramics even though they are not composed of inorganic compounds. There also are exceptions to the stereotypical properties ascribed to ceramics. To return to the example of diamond, this material, though considered to be a ceramic, has a thermal conductivity higher than that of copper—a property the jeweler uses to differentiate between true diamond and simulants such as cubic zirconia (a single-crystal form of zirconium dioxide). Indeed, many ceramics are quite conductive electrically. For instance, a polycrystalline (many-grained) version of zirconia is used as an oxygen sensor in automobile engines owing to its ionic conductivity. Also, copper oxide-based ceramics have been shown to have superconducting properties. Even the well-known brittleness of ceramics has its exceptions. For example, certain composite ceramics that contain whiskers, fibres, or particulates that interfere with crack propagation display flaw tolerance and toughness rivaling that of metals.

Nevertheless, despite such exceptions, ceramics generally display the properties of hardness, refractoriness (high melting point), low conductivity, and brittleness. These properties are intimately related to certain types of chemical bonding and crystal structures found in the material. chemical bonding and crystal structure are addressed in turn below.

The second limitation on the types of crystal structure that can be adopted by ionically bonded atoms is based on a law of physics—that the crystal must remain electrically neutral. This law of electroneutrality results in the formation of very specific stoichiometries—that is, specific ratios of cations to anions that maintain a net balance between positive and negative charge. In fact, anions are known to pack around cations, and cations around anions, in order to eliminate local charge imbalance. This phenomenon is referred to as coordination.

Most of the primary chemical bonds found in ceramic materials are actually a mixture of ionic and covalent types. The larger the electronegativity difference between anion and cation (that is, the greater the difference in potential to accept or donate electrons), the more nearly ionic is the bonding (that is, the more likely are electrons to be transferred, forming positively charged cations and negatively charged anions). Conversely, small differences in electronegativity lead to a sharing of electrons, as found in covalent bonds.

Secondary bonds also are important in certain ceramics. For example, in diamond, a single-crystal form of carbon, all bonds are primary, but in graphite, a polycrystalline form of carbon, there are primary bonds within sheets of crystal grains and secondary bonds between the sheets. The relatively weak secondary bonds allow the sheets to slide past one another, giving graphite the lubricity for which it is well known. It is the primary bonds in ceramics that make them among the strongest, hardest, and most refractory materials known.

Ordinarily, ceramics are poor conductors of electricity and therefore make excellent insulators. Nonconductivity arises from the lack of “free” electrons such as those found in metals. In ionically bonded ceramics, bonding electrons are accepted by the electronegative elements, such as oxygen, and donated by the electropositive elements, usually a metal. The result is that all electrons are tightly bound to the ions in the structure, leaving no free electrons to conduct electricity. In covalent bonding, bonding electrons are similarly localized in the directional orbitals between the atoms, and there are no free electrons to conduct electricity.

There are two ways that ceramics can be made electrically conductive. At sufficiently high temperatures point defects such as oxygen vacancies can arise, leading to ionic conductivity. (This is pointed out in the case of zirconia, above.) In addition, the introduction of certain transition-metal elements (such as iron, copper, manganese, or cobalt), lanthanide elements (such as cerium), or actinide elements (such as uranium) can produce special electronic states in which mobile electrons or electron holes arise. The copper-based superconductors are a good example of conductive transition-metal oxide ceramics—in this case, conductivity arising at extremely low temperatures.

Unlike most metals (metal), nearly all ceramics are brittle at room temperature; i.e., when subjected to tension, they fail suddenly, with little or no plastic deformation prior to fracture. Metals, on the other hand, are ductile (that is, they deform and bend when subjected to stress), and they possess this extremely useful property owing to imperfections called dislocations within their crystal lattices. There are many kinds of dislocations. In one kind, known as an edge dislocation, an extra plane of atoms can be generated in a crystal structure, straining to the breaking point the bonds that hold the atoms together. If stress were applied to this structure, it might shear along a plane where the bonds were weakest, and the dislocation might slip to the next atomic position, where the bonds would be re-established. This slipping to a new position is at the heart of plastic deformation. Metals are usually ductile because dislocations are common and are normally easy to move.

In order for polycrystalline materials to be ductile, they must possess more than a minimum number of independent slip systems—that is, planes or directions along which slip can occur. The presence of slip systems allows the transfer of crystal deformations from one grain to the next. Metals typically have the required number of slip systems, even at room temperature. Ceramics, however, do not, and as a result they are notoriously brittle.

Glasses (glass), which lack a long-range periodic crystal structure altogether, are even more susceptible to brittle fracture than ceramics. Because of their similar physical attributes (including brittleness) and similar chemical constituents (e.g., oxides), inorganic glasses are considered to be ceramics in many countries of the world. Indeed, partial melting during the processing of many ceramics results in a significant glassy portion in the final makeup of many ceramic bodies (for instance, porcelains), and this portion is responsible for many desirable properties (e.g., liquid impermeability). Nevertheless, because of their unique processing and application, glasses are treated separately in the article industrial glass.

Unlike metals and glasses, which can be cast from the melt and subsequently rolled, drawn, or pressed into shape, ceramics must be made from powders. As pointed out above, ceramics are seldom deformable, especially at room temperature, and the microstructural modifications achieved by cold-working and recrystallizing metals are impossible with most ceramics. Instead, ceramics are usually made from powders, which are consolidated and densified by sintering. Sintering is a process whereby particles bond and coalesce under the influence of heat, leading to shrinkage and reduction in porosity. A similar process in metal manufacturing is referred to as powder metallurgy.

Powder processing is used to make products that are normally identified as traditional ceramics—namely, whitewares such as porcelain and china, structural clay products such as brick and tile, refractories for insulating and lining metallurgical furnaces and glass tanks, abrasives, and cements. It also is used in the production of advanced ceramics, including ceramics for electronic, magnetic, optical, nuclear, and biological applications. Traditional ceramics involve large volumes of product and relatively low value-added manufacturing. Advanced ceramics, on the other hand, tend to involve smaller volumes of product and higher value-added manufacturing.