The synthesis of cubic boron nitride (cBN) was first carried out by R. H. Wentorf in 1957 [WENT75]. cBN was first produced industrially in 1968 and, in 1969, it was introduced to the market under the brand name Borazon by the US General Electric Company.
In contrast to grinding wheels with conventional abrasive materials, cBN grinding wheels are mainly built from an abrasive lining applied to a base body.
Among the most important advantages of cBN in comparison with diamond are its higher thermal stability and its suitability for grinding ferrous tools.
Manufacture
Cubic boron nitride is manufactured by the pyrolysis of boron chloride ammonia using a catalyst at pressures of 50 to 90 kbar and temperatures between 1800 and 2700 °C. The corresponding chemical reaction is shown in equation 3.2.
BCl3 • NH3 ^ BN + 3HCl (3.2)
After the pressing process, the pressed pieces are further treated chemically, whereby the residues of the synthesis process are dissolved in various acid baths. Subsequently, the purified cutting material is introduced into the grit preparation. There the grits are prepared by means of surface treatment or coatings on the respective application area [N. N.5]. Classification of the grits then proceeds, as for the conventional abrasives corundum and SiC, by sifting or, for microgrits, by sedimentation.
Cubic boron nitride is harder than conventional abrasives by a factor of 2 and has a much higher thermal conductivity (table 3-3) [DRUM84, N. N.5, N. N.83, VRIE72].
At higher temperatures, chemical reactions are possible when cBN is exposed to oxygen or water. Investigations have established that cBN grits become covered with boron oxide when in 1200 °C warm, dry air (equation 3.3) [CELY54]. This layer has a wear-inhibiting effect in the machining process.
4BN + 3O2 —— 2B2O3 + 2N2. (3.3)
Boron nitride grits heated under steam formed no boron oxide layer in laboratory experiments. They became cracked and scarred and lost mass [CELY54]. Above a temperature of about 1000 °C, a hydrolysis initiated, proceeding in accordance with the relation
BN + 3H2O — H3BO3 + NH3 (3.4)
This reaction has, however, not been observed in the grinding process. It is presumed, on the contrary, that this reaction does not occur in grinding due to minimal contact times.
Uses
Compared with synthetic and natural diamonds, boron nitride is technological and economical advantageous in the machining of ferrous tools. Contrasted with conventional abrasives, it has proven advantageous above all in the grinding of diffi — cult-to-machine steels with high amounts of alloy and a hardness of over 55 HRC.
The low wear of cBN grinding wheels also makes it possible to reach high form and dimensional tolerances. At the same time, especially in the case of steels which are difficult to machine, there is less influence on the workpiece surface layer in comparison to conventional grinding wheels. This can be ascribed to the higher thermal conductivity of cubic boron nitride, resulting, especially in machining high speed steel, in a longer service life compared to grinding these tools with conventional abrasives.