Borazon

[bawr-uh-zon, bohr-]

Borazon, a boron nitride allotrope, is the fourth hardest substance, after aggregated diamond nanorods, ultrahard fullerite, and diamond, and the third hardest artificial material. Borazon is a crystal created by heating equal quantities of boron and nitrogen at temperatures greater than 1800 °C (3300 °F) at 7 GPa (1 million lbf/in²). Borazon is the only substance other than those listed above that can scratch a diamond

(although lasers can cut diamond). A diamond will also scratch Borazon.

Borazon was first produced in 1957 by Robert H. Wentorf, Jr., a physical chemist working for the General Electric Company. In 1969, General Electric adopted the name Borazon as its trademark for the crystal.

Uses and production

Prior to the production of Borazon, also referred to as cubic boron nitride or CBN, diamond was the preferred abrasive used for grinding very hard superalloys but it could not be used effectively on steels because of its carbon solubility potential. Aluminium oxide was the conventional abrasive used on hardened steel tools.

Borazon replaced aluminium oxide for grinding hardened steels due to its far superior abrasive properties, comparable to that of diamond. Borazon is used in industrial applications to shape tools, as it can withstand temperatures greater than 2000 °C (3500 °F), much higher than that of a pure diamond at 871 °C (1600 °F). Other uses include jewellery designing, glass cutting and laceration of diamonds.

CBN-coated grinding wheels, referred to as Borozon wheels, are routinely used in the machining of hard ferrous metals, cast irons, and nickel-base and cobalt-base superalloys. They can grind more material, to a higher degree of accuracy, than any other abrasive. The limiting factor in the life of such tools is typically determined not by wear on the cutting surface but by its break-down and separation from the metal core resulting from failure of the bonding layer.