Oxygen segregation at coherent grain boundaries of cubic boron nitride

Segregation of even a trace amount of impurities to grain boundaries (GBs) can often modify properties of polycrystalline materials. Here, we demonstrate, by a combined study of advanced transmission electron microscopy with atomistic first-principles calculations to two coherent Σ9 and Σ3 GBs of cubic boron nitride (BN), that the two GBs are inclined to trap oxygen, which induces notable electronic states at Fermi level in the forbidden band gap of bulk BN and lowers the GB adhesion energies significantly. Such GB weakening by oxygen segregation is attributed to the lessened charge transfer between grains and more ionic bonding nature at GB.