Ceramic Materials. Description

Ceramics are synthetic, inorganic, nonmetallic 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 sulfur.

Typically, ceramic materials have favorable engineering properties such as 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 application in, for example, automotive engines. (Magnetic ceramics, used in magnetic memory, are described in the entry on Alloys, Magnetic.)

There are mechanical drawbacks that scientists and engineers have successfully reduced during the last few decades: they are brittle and lack ductility, have poor resistance to mechanical and thermal shock, are difficult to machine because of their hardness, and are sensitive 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. However, twentieth 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 have been used in electric power lines since the 1900s and since the 1920s in spark plug insulators.

Naturally occurring steatites, a class of magnesium silicate minerals also known as soapstone, were known and used in the late nineteenth century 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.

They were therefore used as insulators in microwave electronics for radio relays in telephone networks in the 1940s. During and after World War II the applications of ceramics in electronics extended to compact capacitors, piezoelectric transducers for use in telecommunications, resistor and semiconductor 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 nineteenth 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, strength at high temperatures, low thermal expansion and resistance to chemical reaction made it the material of choice for manufacturing bricks and other refractories at high temperatures, for example for industrial boilers and furnaces, and tiles covering the space shuttles.

The manufacture of aluminum oxide abrasives closely followed the development of silicon carbide. In 1897 scientists at the Ampere ElectroChemical Company, New Jersey, made the first successful attempts at aluminum oxide manufacture 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 twentieth century, high-performance engineering materials were increasingly required for structural applications. 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 interest in new highperformance materials grew rapidly, in Germany not so much for surpassing steel alloys but for replacing metals such as 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 disks, and turbine parts for high-temperature jet engines.

After the war, it soon became clear that for many applications a ceramic material like aluminum oxide had too many drawbacks, a conclusion based largely on research in Germany. 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 such as the U.S. 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 onward, government-sponsored research and development programs, especially in the U.S., Japan, Britain, France and Germany, have advanced research on ceramic materials such as 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.

Because of their good biocompatibility, ceramics are 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 ceramic. Zirconium oxide offers chemical and corrosion resistance at far higher temperatures than aluminum oxide. 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 ceramic materials but many questions are still unsolved. The ceramics’ first-rate potential in various demanding applications make these efforts worthwhile.