NITRIDE

binary compound of nitrogen with a more electropositive element, such as boron, silicon, and most metals. Certain metal nitrides are unstable, and most react with water to form ammonia and the oxide or hydroxide of the metal; but the nitrides of boron, vanadium, silicon, titanium, and tantalum are very refractory, resistant to chemical attack, and hardand thus are useful as abrasives and in making crucibles. Nitrides are formed during a process for hardening the surface of steel objects by heating them with ammonia at temperatures usually between 950 and 1,050 F (500 and 550 C) for 5 to 100 hours, depending on the depth of hardened case desired. See also surface hardening.