Constructing coherent/semi-coherent phase boundaries to enhance toughness of superhard cBN-B composite

Abstract

Harsh synthesized pressure is required in constructing of excellent grain boundary (GB) connection for nano-polycrystalline cubic boron nitride (nPcBN) to yield high hardness and toughness. Therefore, it is urgent to fabricate robust nPcBN composite under relatively mild pressure by developing the hard nonmetal binders to form coherent/semi-coherent phase boundary (cPB, scPB) with cBN. Here, the slight content of nano‑boron was composited with nPcBN to achieve superhard property of 45.2 GPa and high fracture toughness 8.4 MPa‧m0.5 under a relatively mild condition of 5 GPa and 1800 °C. The hardness is comparable with single crystal cBN and the toughness is about three times higher than single crystal cBN. The nano boron connects the nano cBN grains by scPB, and nano boron filled the triangular space of cBN crystals, which enhance the strength and eliminate the defects to prohibit the crack trigger sources. This work provides a reference and a new strategy on composite engineering in superhard materials with high toughness by relatively mild synthesized pressure.