Silicon Carbide

Abstract

AbstractSilicon carbide, SiC, is a crystalline material having a color that varies from nearly clear through pale yellow or green to black, depending on the amount of impurities. It occurs naturally only as the mineral moissanite in the meteorite iron of Canon Diablo, Arizona. The commercial product, which is made in an electric furnace, is usually obtained as an aggregate of iridescent crystals. The iridescence is caused by a thin layer of silica produced by superficial oxidation of the carbide. The loose black or green grain of commerce is prepared from the manufactured product by crushing and grading for size. Traditionally, the metallurgical, abrasive, and refractory industries are the largest users of silicon carbide. SiC is also used for heating elements in electric furnaces, in electronic devices, and in applications where its resistance to nuclear radiation damage is advantageous. The development of advanced pressureless sintering and complex shape‐forming technologies has led to silicon carbide becoming one of the most important structural ceramics. Silicon carbide has found wide acceptance in wear‐, erosion‐, and corrosion‐resistant applications; it has demonstrated excellent performance as a heat‐exchanger material; it is also being evaluated for prototype high temperature gas turbine engine component applications. The measurements made on commercial, polycrystalline products should not be interpreted as being representative of single‐crystal silicon carbide. The pressureless‐sintered silicon carbides, being essentially single‐phase, fine‐grained, and polycrystalline, have properties distinct from both single crystals and direct‐bonded silicon carbide refractories. Silicon carbide may crystallize in the cubic, hexagonal, or rhombohedral structure. Silicon carbide is a leading candidate material for rotating and static components in many gas turbine engine applications. As is the case for other ceramics, silicon carbide is brittle in nature. The strength of a silicon carbide component is determined by preexisting flaws introduced into the material during processing. Sintered silicon carbide retains its strength at elevated temperatures. Silicon carbide is a semiconductor: it has a conductivity between that of metals and insulators or dielectrics. Silicon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. There are several routes to preparing SiC powders. Methods that have been examined include growth by sublimation from the vapor phase, carbothermic reduction, and crystallization from a melt. The high modulus, high intrinsic strength, and temperature stability make SiC, in the form of whiskers, platelets, and fibers, a promising candidate reinforcement material for metal, polymer, and ceramic matrix composites. Silicon carbide has been described as a mild inhalation irritant. The threshold limit value for silicon carbide in the atmosphere is 5 mg/m3.