Synthesis of black cBN single crystal in hBN–Li3N–B system

Generally, cBN crystals exhibit many colors from white, yellow to black due to different impurities and defects present in them. In this paper, black cBN crystals with relatively integrated octahedral morphology were obtained in hBN–Li3N–B system under high temperature and high pressure. The color, morphology and property of the synthesized cBN crystals were extensively studied. X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the synthesized cBN single crystals. We found that boron entered into the synthesized black cBN crystals through the way of substituting N vacancy and resulted in a change in color and morphology. We brought out a different coloration mechanism compared with a previous study according to the results in this system. The residual stress in the synthesized cBN crystals, which was calculated by the results of Raman spectroscopy, decreased markedly with the increase in the content of additive boron.