Extreme Hardness Achievements in Binderless Cubic Boron Nitride Tools

Binderless cubic boron nitride tools are available in two forms: single phase cBN and dual phase wBN/cBN (w is wurtzite phase). In this work, a novel heat treatment process involving surface heating using a continuous wave CO2 laser followed by tandem waterjet quenching of the laser beam path was applied to increase the hardness of both forms of boron nitride. Stress-induced phase transitions and nanometric grain sizes accompanying the rapid quench heat treatment enabled a hardness increase of 20% in single phase cBN (nominal 60 GPa) and 100% in dual phase wBN/cBN (nominal 75 GPa) that attest the ability of cBN to reach the hardness of polycrystalline diamond (65-80 GPa). The effects of laser heat treatment are identified by an examination of the changes in phase and microstructure by Raman spectroscopy, high resolution scanning electron microscopy and X-ray diffraction