Carbides
Carbides of silicon, boron, zirconium and titanium are important refractory materials. Although, the carbides have extremely high melting points, they lack oxidation resistance.
Carborundum, a well known abrasive material is silicon carbide (SiC). This material has very high hardness, high thermal conductivity, very high refractoriness, and has semi-conducting nature. At low temperature its electrical conductivity is low but increases by order of magnitude at high temperatures. When 60% sand and 40% coke with some saw dust and salt and heated in an electric arc furnace at a temperature of about 1425oC, the resulting product is SiC. The silicon carbide removed from the furnace is a mass of interlocking crystals. Various shapes might be obtained when SiC is ground, mixed with binding agent, shaped and fixed. The oldest and most widely utilized is the clay bonded material. In this material refractory clay is added as the bonding agent. The bond in this combination softens at 1200-1500oC. Silicon & silicon nitride, are other binding materials. Silicon bonded SiC also has its use limited to a temperature below 1500oC because of loss in bond strength. Silicon nitride bonded SiC, though, retains its strength at much higher temperature and also shows much better shock resistance. Although a monoxide, a layer of SiO2 is formed on SiC. These oxide layers defend SiC from further oxidation. Addition of 0.5 to 1% B during sintering helps uniform structure and imparts stability. SiC fibres have been used as reinforcement in metal and ceramics. Reaction bonded SiC is made by two distinct processes. In one procedure a compact of SiC, carbon and an organic binder is infiltrated by liquid Si, that bonds the particles by formation of additional SiC. The free Si might be leached out with acid to leave an open porous refractory. Thin walled components or coating of SiC is produced by decomposition of gas mixture containing both Si & carbon on a heated substrate. The fuel elements of high temperature gas cooled elements are generated by this method. Recently slip casting has also been utilized for silicon carbides. It has resulted in products of superior refractoriness, abrasion resistance, strength and high density, as well as good chemical resistance. Silicon carbide refractories are characterised by low thermal expansion & high thermal conductivity. They tend to oxidise gradually in air at a temperature range of 900-1300oC.
Boron Carbide (B4C) is refractory carbide in common use. This is highly useful in nuclear power reactors for its capacity to absorb thermal neutrons; this is also known for its great hardness and excellent wear resistance. In hot pressed condition it may achieve a tensile strength of 400 MPa. Due to its wear resistance and high strength it discovers use as shot blast nozzle and ballistic armour. The sintered product is porous & has coarse grain structure, shall lower tensile strength. Boron carbide containing 75% to 80% boron is graded as high purity whereas technical grades contain 67.5-75% boron. High purity boron carbide is converted into finished articles by hot pressing the powder. Alternatively, high purity boron carbide powder might be bonded with boric oxide, sodium silicate or with other silicate and then fired. The boron carbide readily reacts with oxygen at temperature higher than 980oC and as such its application is restricted to this temperature although its melting point is at 2450oC. Its high cost is a disadvantage.
For temperatures in close vicinity of 2000oC carbides of, W, Mo, Zr ,Ta, Nb and Ce are used both in reducing or neutral atmospheres. Carbides of Va ,Ti and Nb may be used up to temperature of 2500oC.