Nitrides, Silicides and Sulfides
Nitrides have very high melting point but begin to oxidise at low temperature. They also have poor resistance to chemicals. Boron nitride contain a graphite structure & hence exhibit lubricating properties. A form of boron carbide called as Borazon produced by compressing under high pressure of 68000 atmospheres at a temperature above 1700oC has hardness equivalent to that of diamond. Borazon also has a good resistance to oxidation and can withstand a temperature of 1950oC.
In its most commonly available form boron nitride differs from ceramic in that it has soft laminar structure which can be machined. Anisotropic boron nitride crystals might be deposited on a substrate to obtain coefficient of thermal expansion which may be 100 times greater parallel to substrate than perpendicular to it. Such deposited crystals also cause the composite to behave as thermal conductor better than normal ceramic. It is further noted that many molten metals do not wet boron nitride. For these reasons boron nitride is used for making crucible for melting metals. The cubic form of boron nitride is very hard and is used as abrasive.
Silicon nitride and its derivatives have high strength and hardness, good refractoriness, low coefficient to thermal expansion, high resistance and high thermal shock resistance to oxidation due to formation of silica film. Its fabrication, though, is difficult and hinders its use. Its not simply sintered as it begins to dissociate above 1850oC. Alternative fabrication methods are costly. Silicon powder that compacted in nitrogen atmosphere is sintered to give porous reaction bonded silicon nitride. A costlier scheme is hot pressing of silicon nitride powder mixed with 1 to 15% MgO which provides liquid phase at sintering temperatures. Thus Discover structure strong product is obtained.
Silicates are particularly notable because of their resistance to oxidation. This resistance is obtained because of formation of a glassy silica coating on the surface. Though, they have low melting point.Sulfides of cerium and thorium are used to crucibles for molten metals. They are prone to oxidation and hence must be used in vacuum or neutral atmosphere.