Encyclopedia of 20th Century Technology

(Note: Sample material is taken from uncorrected proofs. Changes may be made prior to publication.)

Ceramic Materials

Ceramics are synthetic, inorganic, non-metallic solids that generally require high temperatures in their processing, and are used in many industrial applications. Many of them are metal oxides (compounds of metallic elements and oxygen) but others are compounds of metallic elements and carbon, nitrogen, or sulphur. Typically, ceramic materials have favorable engineering properties like high mechanical strength, chemical durability, hardness, low thermal and electrical conductivity, as well as relatively low density which make them comparatively light and suitable for applications like in automotive engines. (Magnetic ceramics, used in magnetic memory, are described in the entry on Alloys, Magnetic.) But there are mechanical drawbacks which scientists and engineers have, however, successfully reduced during the last few decades: brittleness and lack of ductility, poor resistance to mechanical and thermal shock, difficult to machine because of their hardness, and sensitivity to catastrophic failure because of the potential presence of microvoids.

One of the origins of manufacture of engineering ceramics is in techniques for firing clay for porcelain, tableware, and construction materials. But 20th century technology and the chemical, electrical, electronic, automotive, aerospace, medical, and nuclear industries have made new demands on high performance materials. In the electrical industry, for example, ceramic insulators were used in electric power lines from the 1900s and since the 1920s for spark plug insulators but microwave electronics for radio relays in telephone networks in the 1940s required insulators for higher frequencies. Naturally occurring steatites, a class of magnesium silicate minerals also known as soapstone, were known and used in the late 19th century as for burners in stoves and gas lights but were developed and processed as improved electrical insulators that had low loss at high frequencies and high temperatures. During and after World War II the applications of ceramics in electronics extended to compact capacitors, piezoelectric transducers for use in telecommunications, resistor and semi-conductor compositors as well as magnetic materials and other energy converters.

Apart from electric and electronic applications abrasives have been a field in which ceramic materials, in this case silicon carbide (SiC) and aluminum oxide (Al2O3), excelled. During the 19th century it became clear that abrasive products like natural sandstone used in grinding wheels no longer satisfied industrial demands. In 1891 Edward G. Acheson, an electrical engineer from Monongahela City in Pennsylvania, combined a mixture of clay and powdered coke in an electrical furnace. This resulted in shiny crystals (silicon carbide, SiC) which proved immensely suitable for polishing precious stone. Until the invention of boron carbides in 1928 silicon carbide had been the hardest synthetic material available. Silicon carbide's high thermal conductivity, high-temperature strength, low thermal expansion and resistance to chemical reaction made it the material of choice for manufacturing high temperature bricks and other refractories, for example for industrial boilers and furnaces, and tiles covering the space shuttles.

The manufacture of aluminum oxide abrasives followed the development of silicon carbide closely. In 1897 scientists at the Ampere Electro-Chemical company at Ampere, New Jersey, made the first successful attempts with rock bauxite, of which aluminum oxide is the main ingredient. Today aluminum oxide, sometimes with the addition of zirconium oxide, is indispensable for producing highly precise and ultrasmooth surfaces in the automotive and aerospace industries.

During the 20th century high performance engineering materials were increasingly required for structural applications. For several purposes in particularly erosive, corrosive or high temperature environments, materials such as metals, polymers or composites could not fulfill the demands made on them. In 1893 Emil Capitaine, a German engineer who played a role in the development of the internal combustion engine, suggested the use of porcelain and fire clay in engines, though it is not clear what for; a decade later engineers at the Deutz Motor Company in Cologne experimented with ceramic materials for use in stationary gas turbine blades. During World War II interests in new high performance materials grew rapidly, in Germany not so much for surpassing steel alloys but for replacing metals like chromium, nickel, and molybdenum which were in short supply in the armament industry. Experiments with aluminum oxide seemed promising for use in gas turbine blades but the material's susceptibility to thermal shock created insurmountable difficulties. In order to reduce these problems and to impart greater ductility and thermal shock resistance to aluminum oxide, engineers during and after World War II tried to mix ceramics with different metals to make composite materials. Although this did not yield the expected results at the time the idea of reinforcing a ceramic matrix with metal fibers proved useful. Metal components have also been made stronger by the incorporation of ceramic fibers, whiskers, or platelets; special ceramic fibers and whiskers are also incorporated in a ceramic matrix to increase toughness and reduce the risk of catastrophic failure due to incipient cracks and microvoids. Automobiles and aircraft engines of the future may have ceramic matrix composite components, such as brake discs and turbine parts for high-temperature jet engines.

After the war and to an extent based on research in Germany, it soon became clear that for many applications a ceramic material like aluminum oxide had too many drawbacks. Attention therefore shifted from oxide ceramics to ceramic materials such as silicon nitride or silicon carbide. During the 1970s the oil price shock accelerated interest in thermal efficiency of power plants, enhanced by environmental legislation in countries like the United States or Germany. If the combustion temperature is raised for greater efficiency, the turbine parts must be oxidation, impact, and thermal-shock resistant. From that time onwards government sponsored R&D programmes, especially in the United States, Japan, Britain, France and Germany, have advanced research on ceramic materials like silicon nitride and silicon carbide which, among other assets, are more oxidation resistant than super alloys. To date however, none of these ceramic materials has been successfully adapted to the proposed high-temperature gas turbines. Advanced structural ceramics have also been employed in nuclear power as heat-resistant control rods. In medicine ceramics are, not least because of their good biocompatibility, employed in medical and dental applications such as false teeth, implants, and joint replacements; in the automotive industry they are useful as catalysts, catalyst supports or sensors.

Apart from silicon nitride and silicon carbide zirconium oxide is an excellent engineering ceramics. Zirconium oxide offers chemical and corrosion resistance to temperatures well above those that aluminium oxide can cope with. Stabilized zirconium oxide produced by addition of calcium, magnesium or yttrium oxides exhibits particularly high strength and toughness. This and its low thermal conductivity led to applications in oxygen sensors and high temperature fuel cells. Over recent decades it has become possible to better cope with the intricacies of engineering ceramics materials but many questions are still unsolved. But the ceramics' first rate potentials in various demanding applications make these efforts worthwhile.