The material. Alumina (Al2O3) is to technical ceramics what mild steel is to metals—cheap, easy to process, the workhorse of the industry. It is the material of spark plugs, electrical insulators, and ceramic substrates for microcircuits. In single crystal form, it is sapphire, used for watch faces and cockpit windows of high-speed aircraft. More usually it is made by pressing and sintering powder, giving grades ranging from 80-99.9% alumina; the rest is porosity, glassy impurities, or deliberately added components. Pure aluminas are white; impurities make them pink or green. The maximum operating temperature increases with increasing alumina content. Alumina has a low cost and a useful and broad set of properties: electrical insulation, high mechanical strength, good abrasion, and temperature resistance up to 1650°C, excellent chemical stability, and moderately high thermal conductivity, but it has limited thermal shock and impact resistance. Chromium oxide is added to improve abrasion resistance; sodium silicate, to improve processability but with some loss of electrical resistance. Competing materials are magnesia, silica, and borosilicate glass.
Composition
Al2O3, often with some porosity and some glassy phase.
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General properties
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On the left: alumina components for wear resistance and for high temperature use (Kyocera Industrial Ceramics Corp.). On the right: an alumina spark plug insulator.
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Electrical properties
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Typical uses. Electrical insulators and connector bodies; substrates; high temperature components; water faucet valves; mechanical seals; vacuum chambers and vessels; centrifuge linings; spur gears; fuse bodies; heating elements; plain bearings and other wear resistant components; cutting tools; substrates for microcircuits; spark plug insulators; tubes for sodium vapor lamps, thermal barrier coatings.
