Origin of superhard pure PcBN: A relation to the fragment and its plastic mechanism under the high pressure

Cubic boron nitride (cBN) as a superhard material has significant potential to provide solutions for the thermostability problems of the high-speed cutting tools to adapt the harsh mechanical working environments. Granularity-induced improvements in hardness and the thermal resistance of pure polycrystalline cBN (PcBN) is highly conductive to fragment and plastic behaviors of the pristine cBN particles in the first step of high-pressure induced synthesis process. Rapid detection of them with respect to different sizes of pristine cBN powders is realized by the analysis of the internal stress through the methods of X-ray diffraction (XRD) profile and Raman spectrum. It is deduced that the plastic deformation with significant fragment of the relatively coarse-grained PcBN was dominated by the effect of strain strengthening and then recrystallization, resulting in the optimized performance with a combination of the enhanced mechanical properties (Vickers hardness of Hv3958 and compressive strength of 2.38 GPa) and the high thermal resistance (1202 °C). The proposed fundamental plastic deformation theory of the strengthening polycrystalline cBN is of great guiding significance for the field of high-precision cutting tools.