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Ceramics are one of the three most important types of engineering materials that are primarily synthetic. The other two are metals and plastics. Ceramics include such everyday materials as brick, cement, glass, and porcelain. They also include unusual materials used in electronics and spacecraft. Most ceramics are hard and can withstand heat and chemicals. These properties give them a variety of uses in industry.
Ceramic molecules are characterized by ionic or covalent bonding, or both. The metallic atoms release or share their outermost electrons to the nonmetallic atoms, and a strong attractive force exists within the molecules. The general properties that result from these bonding mechanisms include: high hardness and stiffness (even at elevated temperatures), brittleness (no ductility), electrically insulating (nonconducting), refractory (thermally resistant), and chemically inert.
Ceramics possess either a crystalline or noncrystalline structure. Most ceramics have a crystal structure, while glasses based on silica (SiO2) are amorphous. In certain cases, either structure can exist in the same ceramic material. For example, silica occurs in nature as crystalline quartz. When this mineral is melted and then cooled, it solidifies to form fused silica, which has a noncrystalline structure.
Ceramics have high moduli like metals, but unlike metals, they are brittle. Their "strength" in tension means the brittle fracture strength; in compression it is the brittle crushing strength, which is about 15 times larger. Because ceramics have no ductility, they have a low tolerance for stress concentrations (like holes or cracks) or for high-contact stresses (at clamping points, for instance). Ductile materials accommodate stress concentrations by deforming in a way that redistributes the load more evenly, and because of this, they can be used under static loads within a small margin of their yield strength. Ceramics cannot. Brittle materials always have a wide scatter in strength and the strength itself depends on the volume of material under load and the time for which it is applied. So ceramics are not as easy to design with as metals. Despite this, they have attractive features. They are stiff, hard, and abrasion-resistant (hence their use for bearings and cutting tools); they retain their strength to high temperatures; and they resist corrosion well.
Manufacturers make common ceramics from such minerals as clay, feldspar, silica, and talc. These minerals, called silicates, form most of the earth's crust. Clay is an important silicate. But it is not used in all ceramic materials. Glass, for example, is made from sand. Chemists make materials called advanced ceramics in the laboratory from compounds other than silicates. These compounds include alumina, silicon carbide, and barium titanate.
Most ceramic products, like their mineral ingredients, can withstand acids, gases, salts, water, and high temperatures. But not all ceramic products have the same properties. Common ceramics are good insulators--that is, they conduct electricity poorly. However, certain ceramics lose their electrical resistance and become superconductors when they are cooled. Some ceramic materials are magnetic. Engineers control the properties of ceramics by controlling the proportion and type of materials used.
Ceramic engineers continually develop new uses for ceramics. For example, porcelain is used to make false teeth and artificial bone joints. Uranium oxide ceramics serve as fuel elements for nuclear reactors. Cutting tools are made from silicon nitride. Refractories made from carbides are used to make parts for aircraft engines. Alumina is used in making certain types of lasers (instruments that produce intense light beams).
Making Ceramics. The clays and other minerals used in ceramics are dug from the earth and refined to improve their purity. Machines crush and grind the materials into fine particles. The particles are mixed in the proper proportion, and water or other liquid is added to produce a mixture that can be shaped. A gluelike substance is sometimes added to mixtures that do not contain clay. Glass and some refractory products are made by melting the particles and shaping them when they are molten.
The most common methods for shaping clay ceramics are slip casting, jiggering, extrusion, and pressing. In slip casting, the liquid mixture is poured into a mold that absorbs water. As the water is absorbed, a layer of ceramic particles is deposited onto the mold, forming such hollow items as teapots and vases. The excess liquid is then poured out of the mold. In jiggering, a machine presses the clay onto a rotating mold. Jiggering is used to make dinnerware. Extrusion shapes items into rods or tubes by forcing ceramic paste through a shaping tool called a die. In pressing, ceramic powder is pressed in a steel die or a rubber mold.
After the product has dried, it is strengthened by firing, a process that takes place in special furnaces called kilns. Ceramics are fired at temperatures ranging from about 1200 to 3000 degrees F (649 to 1649 degrees C). Firing hardens the product permanently and gives it strength, durability, and other desired qualities.
Manufacturers cover many ceramic products with a glassy coating called glaze. Glaze prevents the item from absorbing liquids and makes it smoother and easier to clean. Glazes are also used for decoration.
Common Applications
The properties of ceramics make them especially suitable for certain products. Products made of ceramic materials include abrasives (materials used for grinding), construction materials, dinnerware, electrical equipment, glass products, and refractories (heat-resistant materials).
Abrasives. Manufacturers use some extremely hard ceramic materials for cutting metals and for grinding, polishing, and sanding various surfaces. These ceramic materials include alumina and silicon carbide.
Construction Materials. Clay and shale are used in making strong, durable bricks and drainpipes for homes and other buildings. Tiles are made of clay and talc. Cement consists chiefly of calcium silicates and is used primarily in making concrete. Gypsum is used to produce plaster for the surfaces of walls and ceilings. Bathtubs, sinks, and toilets are made of porcelain, which consists chiefly of clay, feldspar, and quartz.
Dinnerware. Ceramics make excellent containers for food and drinks. They do not absorb liquids, and they resist acids, salts, detergents, and changes in temperature. Most ceramic dinnerware is made from a mixture of clays, feldspar, and quartz.
Electrical Equipment. Ceramics that do not conduct electricity are used as insulators in automobile spark plugs, on electric power lines, and in television sets. Such ceramics include alumina and porcelain. Another ceramic material, barium titanite, is used in making capacitors, which store electric charges in electronic equipment. Magnetic ceramics are used in electronic circuits and in electric motors. Complex electronic circuits are bonded on thin layers of alumina.
Glass Products. Glass is one of the most important materials, chiefly because of its transparency. Products made of glass include food containers, light bulbs, windows, and lenses for eyeglasses and telescopes. Fiberglass insulates the walls of many homes. Cables made of glass fibers transmit telephone calls and other information. The main ingredient in glass is silica.
A glasslike coating called porcelain enamel serves as a protective surface on many metal products. These products include such appliances as refrigerators, stoves, and washing machines. Porcelain enamel also makes outdoor signs weather-resistant.
Refractories. The property of heat-resistance makes refractories suitable for the manufacture of industrial boilers and furnaces, such as the furnaces used to make steel. Refractories shaped into tiles cover the surface of space shuttles, which must withstand the intense heat created by high speeds. Ceramics used in making refractories include alumina, magnesium oxide, silica, silicon carbide, and zirconium oxide.