Cubic and hexagonal boron nitride phases and phase boundaries

Phase stability of boron nitride (BN) polymorphs at elevated temperature is perplexing due to their complex nucleation and growth kinetics, nevertheless, holds great significance in fundamental science and technology. Therefore, the phase-transformation of three-dimensional cubic BN (3D c-BN) to a two-dimensional hexagonal BN (2D h-BN) or vice versa, remains an exciting domain to explore. Here, we used temperature-dependent spark plasma sintering on 3D c-BN, enabling phase transformations to a mixed phase of 3D/2D c-BN/h-BN material and ultimately to 2D h-BN. The phase transformed 2D h-BN ceramic features an extremely high density reaching similar to 90% of the theoretical limit, and exhibits excellent room temperature thermal conductivity and mechanical properties. Our findings provide valuable fundamental insights into the complex phase diagram, the relative stability regimes and boundaries of 3D c-BN and 2D h-BN phase, with the exhibition of functional properties, pivotal for extreme environments sustainable material-based technology. We used temperature-dependent spark plasma sintering to induce phase transformations of metastable 3D c-BN to mixed-phase 3D/2D c-BN/h-BN and ultimately to the stable 2D h-BN phase at high temperature, useful for extreme-temperature technology.