Aluminium oxynitride–hexagonal boron nitride composites with anisotropic properties

Structure of hexagonal boron nitride (h-BN) consist of hexagonal layers with strong covalent bonds and weak van der Waals bonds between them. Such specific structure and usually plate-like grains cause anisotropic properties of h-BN based materials. The aim of the present work was preparation of composites in the aluminium oxynitride–hexagonal boron nitride system with anisotropic properties. SHS technique was used to obtain complex powders with both phases synthesized in situ. Mixtures of aluminium, aluminium oxide and different amount of boron were combusted in nitrogen and the powders were hot-pressed. The h-BN grains in the composites show plate-like shapes and crystallographic orientation. The specific microstructure and texture result in anisotropy of thermal properties; thermal conductivity was few times higher in the direction perpendicular to hot-pressing force and parallel to the longer diameter of h-BN grains. The texture effect is stronger than the effect of the h-BN content.