High‐temperature carbothermal synthesis and characterization of graphite/h‐BN bimaterials

Abstract

AbstractHexagonal boron nitride (h‐BN) and graphite have similar crystal structures, comparable lattice parameters, and coefficients of thermal expansion, but vastly different electrical and thermal transport. Despite their key differences, it is possible to couple h‐BN and graphite in a bimaterial system allowing the unique properties of both materials to be utilized in a single component. Through a carbothermal reduction of B2O3 in nitrogen, the surface of graphite can be converted to h‐BN. This results in a layered system that is electrically insulating on the surface due to h‐BN, and more compliant as well as conductive within the substrate due to the graphite structural body. We discuss the high‐temperature synthesis and characterization of this layered material, focusing on the processing–microstructure relationship as well as the interface of graphite/h‐BN to assess the chemical and mechanical adhesion of the layers, and to establish how such properties are contingent on the reacting phase of B2O3. This is achieved by investigating the origin of h‐BN formation and the unwanted side reaction of boron carbide formation, through the evaluation of the thermochemistry and kinetics governing the carbothermic reactions. We establish that a reaction temperature and holding time of 1700°C for 18 h produced the thickest h‐BN layers which exhibited the highest fracture toughness over all lower temperature synthesis conditions.