Formation of flower-like MgO crystal and its effect on the photoluminescence of Mg-cBN ceramics

Cubic boron nitride (cBN) ceramics were prepared by sintering cBN micro-powder with magnesium at high temperature and high-pressure conditions (HTHP). Following the sintering, the ceramics were annealed in nitrogen atmosphere. Surface microstructure, phase compositions and photoluminescence of the ceramics were examined comprehensively using scanning electronic microscopy, energy-dispersive spectrometer, X-ray diffraction, and fluorescence spectrophotometer. Experimental results indicated that flower-like MgO crystals clustered in the annealed surface of the ceramics after annealing at 1100 °C for 2 h. Oxygen vacancies in MgO crystals increased the emission peak near 400 nm. The emission peak near 317 nm was attributed to oxygen vacancies in cBN crystals due to scavenging of O from cBN by Mg. The optical vibrational eigenmodes of hBN resulted in phonon replicas in photoluminescence spectra due to the interactions between LO phonon of the hBN and LO phonon of the MgO crystal lattice, or with the nitrogen vacancies in cBN grains.