Hexagonal BN-encapsulated ZrB2 particle by nitride boronizing

A facile method based on an in situ reaction, called nitride boronizing, was first developed to directly coat ZrB2 particles with a nanoscale hexagonal boron nitride (h-BN) film. The crystallinity of the BN coating and the diameters of the ZrB2 particles could be tailored by changing the reaction conditions. A mechanism was proposed from thermodynamic calculations to clarify the formation of a core-shell nanostructure, i.e. the shock heat generated by the exothermal reaction. The core-shell ZrB2 powders showed excellent sinterability and the advantage in producing dense h-BN texture-free composites. Even when sintered at 2000 °C for 20 min and under a uniaxial pressure of 60 MPa, no texture created by h-BN was observed. The isotropic bending strength (∼300 MPa) of ZrB2-37 vol.% BN ceramics from core-shell precursor powders were investigated and compared with ceramics of the same nominal composition, but densified from commercial powders containing highly aligned h-BN grains. The nitride boronizing approach could be explored to synthesize other h-BN-coated IVB group transition metal borides.