Cubic Boron Nitride (CBN)

In 1969 General Electric launched cubic boron nitride – called CBN for short – on to the market under the brand name "BORAZON" as a revolutionary development in the field of superabrasives. Produced in the retort, it achieves a Knoop intrinsic hardness of about 4,700 N/mm², i.e. it is more than half as hard again as diamond (7,000 N/mm²) and not quite twice as hard as silicon carbide (SiC) at 2,500 – 2,700 N/mm². The originally monocrystalline CBN, which chips rather block-like, was joined a few years later by the microcrystalline version with completely different, considerably finer chipping characteristics.

The spectrum of use of CBN starts at relatively soft but tough materials (from about 50 HRc) and extends up to very hard carbide-containing alloys (tool steels, special steels, HSS, etc.). For amorphous and ceramic materials CBN is of less suitability than for working carbides. In a vitrified bond, the wheels can be given different profiles and/or conditioned considerably more easily and economically.

Because of the long service life, conventional grinding wheels are now increasingly being replaced by those of CBN, especially where production series are long or recurring. G ratios of 1,200 to 15,000 can be achieved with CBN wheels, which is 500 to about 5,000 times more than what could be achieved with conventional grinding materials under favourable conditions of use. CBN is used in a 1-layer electroplated form or in a resinoid, brittle bronze or vitrified bond form. In the precision grinding sector, however, the trend is quite clearly in the direction of vitrified bonding.

CBN grinding wheels – with the exception of wheels with an electroplated bond – are produced in various concentrations, depending on the nature of their use. The term concentration means the carat weight per cm³ of coat volume.

Thermal properties of CBN

CBN has a higher thermic stability than diamond. Oxidation takes place only above about 1,400ēC. The usual relatively high grinding temperatures do not attack CBN chemically during working of either iron, nickel or cobalt. The excellent machining properties of CBN result in "cool" grinding. The good thermal conductivity of CBN – which is about five times the heat conductivity of copper – additionally assists this effect. When used correctly, the majority of the grinding heat generated is therefore absorbed by the CBN grinding wheel and released to the coolant emulsion and/or the surroundings. As a result, the workpiece heats considerably less compared to working with corundum, since the latter can absorb far less heat.

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