Carbides
Carbides of titanium, zirconium, boron and silicon are significant refractory materials. Though, the carbides have extremely high melting points so they lack oxidation resistance.
Carborundum, a well identified abrasive material is silicon carbide or SiC. Such material has extremely high hardness, high thermal conductivity, extremely high refractoriness, and has semi-conducting nature. Silicon Carbide's electrical conductivity is low at low temperature but raises by order of magnitude at high temperatures. When 60 percent sand and 40 percent coke along with some saw salt and dust and heated in an electric arc furnace at a temperature of about 1425oC, the resulting product is Silicon Carbide. The SiC removed from the furnace is a mass of interlocking crystals. Several shapes may be acquired when SiC is ground, mixed along with binding agent, fixed and shaped. The most widely and oldest utilized is the clay bonded material. Within this material refractory clay is added as the bonding agent. The bond in such combination softens at 1200-1500oC. Silicon and silicon nitride, are other binding materials. Silicon bonded Silicon Carbide also has its employ limited to a temperature below 1500oC due to loss in bond strength. Silicon nitride bonded SiC, though, retains its strength at great higher temperature and shows also much better shock resistance. Though a monoxide, a layer of SiO2 is formed on SiC. Such oxide layer protects SiC from more oxidation. Adding of 0.5 to 1% B throughout sintering helps uniform structure and imparts stability. SiC fibres have been utilized as reinforcement in metal and ceramics. Reaction bonded SiC is made via two distinct processes. In one procedure a compact of SiC, an organic and carbon binder is infiltrated via liquid Si that bonds the particles by formation of additional SiC. The free Si might be leached out along with acid to leave an open porous refractory. Thin walled coating or components of SiC is produced via decomposition of gas mixture containing both Silicon and C on a heated substrate. The fuel elements of high temperature gas cooled elements are produced via this process. Recently slip casting has also been utilized for silicon carbides. It has resulted in products of superior refractoriness, density, abrasion resistance and high strength, and good chemical resistance also. Silicon carbide refractories are characterized via low thermal expansion and high thermal conductivity. They tend to oxidize gradually in air at a temperature range of 900-1300oC.