Isotropic hexagonal boron nitride ceramics with high thermal conductivity and strength

The conventional fabrication of bulk van der Waals materials, such as hexagonal boron nitride (hBN), faces significant challenges in sintering powder into structurally isotropic bulk forms. hBN has attracted considerable attention for applications under extreme conditions because of its excellent thermal stability, oxidation resistance, and electrical insulation. However, its strongly anisotropic layered structure typically results in poor mechanical performance and limited isotropy in bulk forms. In this study, we report the fabrication of isotropic hBN ceramics through the controlled transformation of cubic BN (cBN) powders under spark plasma sintering conditions. The coexistence of randomly oriented hBN sheets and residual turbostratic BN (tBN) phases produces an isotropic microstructure that yields balanced mechanical and thermal properties. The ceramics exhibit a compressive strength of ~130 MPa and a thermal conductivity of ~77 W·m−1·K−1, nearly independent of the measurement direction. This thermally driven cBN → hBN transformation provides a new strategy for tailoring the microstructure and overcoming the intrinsic anisotropy of layered ceramics, paving the way for advanced insulating components in harsh environments.