The term ceramic refers to an inorganic mineral that becomes hard and brittle after being subjected to high temperatures. The process of ceramics (which is Greek for “potter’s clay,” or burned material, keramos ) is historically defined as the art and science of making and decorating pottery; another name for any ceramic (earthy) material that is pliable when worked but becomes rigid when subjected to heat. Ceramic materials are usually understood to be compounds of metallic and nonmetallic elements, though some are actually ionic salts, and others are insulators. These materials can be very complicated, as are, for example, clays, spinels, and common window glass. Many ceramic compounds have very high melting points.

Ceramics have a wide range of applications. They have been used as refractories, abrasives, ferroelectrics, piezoelectric transducers, magnets, building materials, and surface finishes. Humans still use ceramics. They find their way into the kitchen cupboard as bowls, plates, and cups, and on shelves as flower vases. Indeed, a ceramic cup is still the most environmentally friendly method for holding one’s morning coffee. Bricks have been used for years by the construction industry, and for lining of ore smelting furnaces. Ceramic blades are being tested in jet engine turbines and automobile engines. Ceramics stovetops are common. Perhaps the most famous use of ceramics is the ceramic tiles that coat the space shuttle’s outside frame, which are designed to absorb the intense heat inflicted upon the spacecraft during re-entry into the Earth’s atmosphere.

Unlike metals, there are really no heat treatments that can be used to modify the properties of ceramics, but their properties can be altered by changes in chemical composition. By carefully considering the choice of chemical composition; purity; particle size; uniformity, arrangement, and packing of atoms, high quality ceramics can be synthesized in a wide variety.

Traditional ceramics

Arguably, ceramics is one of the pillars upon which modern civilization was built, both literally and figuratively. Ceramics have been used by humans since antiquity. The earliest ceramic articles were made from naturally occurring materials such as clay minerals. It was discovered in prehistoric times that clay materials become malleable when water is added to them, and that a molded object can then be dried in the sun and hardened in a high-temperature

Table 1. Traditional Ceramics . (Thomson Gale.)
Traditional ceramics
Category	Examples
whitewares	dishes, plumbing materials, enamels, tiles
heavy clay products	brick, pottery, materials for the treatment and transport of sewage, water purification components.
refractories	brick, cements, crucibles, molds
construction	brick, plaster, blocks, concrete, tile glass, fiberglass

Table 2. Modern Ceramics .(Thomson Gale.)
Modern ceramics
Category	Examples
electronic	heating elements, dielectric materials, semiconductors, insulators, transducers, lasers, hermetic seals, igniters
aerospace and automotive	turbine components, heat exchangers, emission control
medical	prosthetics, controls
high-temperature structural	kiln furniture, braze fixtures, advanced refractories
nuclear	fuels, controls

fire. Many of the same raw materials that were used by the ancients are still used today in the production of traditional ceramics. Traditional ceramic applications include whitewares, heavy clay products, refractories, construction materials, abrasive products, and glass.

Clay minerals are hydrated compounds of aluminum oxide and silica. These materials have layered structures. Examples include kaolinite, halloysite, pyrophillite, and montmorillonite. They are all formed by the weathering of igneous rocks under the influence of water, dissolved CO₂, and organic acids. The largest sources of these clays were formed when feldspar was eroded from granite and deposited in lake beds, where it became altered to a clay.

Silica is a major ingredient in glass, glazes, enamels, refractories, abrasives, and whiteware. Its major source is quartz, which is made up primarily of sand, sandstone, and quartzite.

Feldspar is also used in the manufacture of glass, pottery, enamel, and other ceramic products. Other naturally occurring minerals used directly in ceramic production include talc, asbestos, wollastonite, and sillimanite.

Hydraulic cement

Hydraulic cements are set by interaction with water. Portland cement, the most common hydraulic cement, is primarily a water-free calcium silicate. It is slightly soluble in water and sets by a combination of solution precipitation and chemical reaction with water to form a hydrated composition. The ratio of water to cement in the initial mix greatly influences the strength of the final concrete: the lower the water-to-cement ratio, the higher the strength.

Glass

Glass is a ceramic material consisting of uniformly dispersed mixtures of silica, soda ash, and lime, that is often combined with metallic oxides of calcium, lead, lithium, cerium, etc. Glass is distinguished from solid ceramics by its lack of crystallinity. It is, in fact, a supercooled liquid. The atoms in glass remain disordered in the solid state, much as they are in the liquid state. Glasses are, thus, rigid structures whose atomic arrangements and properties depend on both composition and thermal history.

Group 16 elements (oxygen, sulphur, selenium, tellurium) are especially good candidates for glass formation. Oxygen is able to form stable bonds with silicon, boron, phosphorous, and arsenic and thereby form stable structures having oxygen atoms at the corners and one of the other atoms at the center. Pure oxide glasses are very stable because each oxygen atom is linked by electron bonds to two other atoms.

Various two-phase structures may exist as glasses. One such structure is a mixture of glass and crystal. These materials are converted into strong and durable ceramics that are part glass and part crystal by prolonged heat treatment.

Modern ceramics

In the twentieth century, scientists and engineers acquired a much better understanding of ceramics and their properties. They succeeded in producing ceramics with tailor-made properties. Modern ceramics include oxide ceramics, magnetic ceramics, ferroelectric ceramics, nuclear fuels, nitrides, carbides, and borides.

Aluminum oxide

Aluminum oxide (Al₂ O₃) occurs naturally in the mineral corundum, which in gem-quality form is known as the precious stones ruby and sapphire. Ruby and sapphire are known for their chemical inertness and hardness. Al₂ O₃ is produced in large quantities from the mineral bauxite. In the Bayer process, bauxite (primarily aluminum hydroxide mixed with iron hydroxide and other impurities) is selectively leached with caustic soda. Purified aluminum hydroxide is formed as a precipitate. This material is converted to aluminum oxide powder, which is used in the manufacture of aluminum-oxide-based ceramics. Aluminum oxide powder is used in the manufacture of porcelain, alumina laboratory ware, crucibles and metal casting molds, high temperature cements, wear-resistant parts (sleeves, tiles, seals), sandblast nozzles, and other products.

Magnesium oxide

Magnesium oxide (MgO) occurs naturally in the mineral periclase, but not in sufficient quantity to meet commercial demand. Most MgO powder is produced from MgCO₃ or from seawater. MgO is extracted from seawater as a hydroxide and, then, converted to the oxide. MgO powder finds extensive use in high temperature electrical insulation and in refractory brick.

Silicon carbide

Silicon carbide (SiC) has been found to occur naturally only as small green hexagonal plates in metallic iron. The same form of silicon carbide has been manufactured synthetically, however. In this process, SiO₂ sand is mixed with coke in a large elongated mound in which large carbon electrodes have been placed at either end. As electric current is passed between the electrodes, the coke is heated to about 3, 992°F (2, 200°C). The coke reacts with the SiO₂ to produce SiC plus CO gas. Heating continues until the reaction has completed in the mound. After cooling, the mound is broken up, and the green hexagonal SiC crystals, which are low in impurities and suitable for electronic applications, are removed. The lower purity material is used for abrasives. The outer layer of the mound is reused in the next batch. SiC can be formed from almost any source of silicon and carbon. It has been produced in the laboratory from silicon metal powder and sugar, and from rice hulls. SiC is used for high-temperature kiln furniture, electrical resistance heating elements, grinding wheels and abrasives, wear-resistance applications, and incinerator linings.

Silicon nitride

Silicon nitride does not occur naturally. Most of the powder commercially available has been produced by reacting silicon metal powder with nitrogen at temperatures between 2, 282°F (1, 250°C) and 2, 552°F (1, 400°C). The powder that is removed from the furnace is not ready to use. It is loosely bonded, and must be crushed and sized. The resulting powder contains impurities of Fe, Ca, and Al. Higher purity silicon nitride powder has been produced by reducing SiO₂ with carbon in a nitrogen environment, and by reaction of SiCl₄ with ammonia; these reactions produce a very fine powder. High purity silicon nitride powder has also been made by laser reactions in which a mixture of silane (SiH₄ ) and ammonia is exposed to laser light from a CO₂ laser. This produces spherical particles of silicon nitride of very fine size.

Processing

The raw materials for ceramics are chosen on the basis of desired purity, particle size distribution, reactivity, and form. Purity influences such high temperature properties as strength, stress rupture life, and oxidation resistance. Impurities may severely influence electrical, magnetic, and optical properties. Particle size distribution affects strength.

Binders may be added to the ceramic powder to add strength prior to densification. Lubricants reduce particle-tool friction during compaction. Other agents are added to promote flowability during shaping.

Forming processes

The ceramic powder along with suitable additives are placed in a die, to which pressure may be applied for compaction. Uniaxial pressing is often used for small shapes such as ceramics for electrical devices. Hydrostatic pressing (equivalent pressing from all sides) is often used for large objects.

Alternatively, the ceramic powder may be cast. Although molten ceramics may be cast into cooled metal plates and quenched to produce materials made up of very fine crystals with high material toughness, casting of ceramics is usually done at room temperature. The ceramic particles are first suspended in a liquid and then cast into a porous mold that removes the liquid, leaving a particulater compact in the mold.

Yet another method of shaping a ceramic involves plastic forming. In this process, a mixture of ceramic powder and additives is deformed under pressure. In

KEY TERMS

Ceramic —A hard, brittle substance produced by strongly heating a nonmetallic mineral or clay.

Glass —A ceramic material consisting of a uniformly dispersed mixture of silica, soda ash, and lime; and often combined with metallic oxides.

Refractory —Any substance with a very high melting point that is able to withstand very high temperatures.

Sintering —The bonding of adjacent surfaces of particles in a mass of metal powders by heating.

the case of pure oxides, carbides, and nitrides, an organic material is added in place of or in addition to water to make the ceramic mixture plastic. While forming the ceramic object, heat and pressure are usually applied simultaneously.

Sintering

Densification of the particulate ceramic compact is referred to as sintering. Sintering is essentially the removal of pores between particles, combined with particulate growth and strong bonding between adjacent particles. In order for sintering to occur, the particles must be able to flow, and there must be a source of energy to activate and sustain this material transport. Sintering can take place in the vapor, liquid, or solid phase, or in a reactive liquid.

Machining

The sintered material must frequently be machined to allow it to meet dimensional tolerances, to give it an improved surface finish, or to remove surface flaws. Machining must be done carefully to avoid brittle fracture. The machining tool must have a higher hardness than the ceramic. The ceramic material can be processed by mechanical, thermal, or chemical action.

Design considerations

When evaluating the suitability of a ceramic material for a particular application, it is first necessary to understand the requirements of the application. These requirements might typically be defined by the load that the material will experience, the stress distribution in the material, interface, frictional requirements, chemical environment and range of temperatures that the material will experience, and restriction on the final cost of the materials. Usually, one or two material properties will dictate the choice of a material for a particular application.

Historically most ceramic designs have been developed by empirical, or trial-and-error investigation. Only since the advent of the digital computer has it been possible to predict the properties of a particular ceramic material prior to actually producing it.