Carbon source graphitization-driven morphology control of HfB2 powders synthesized by boro/carbothermal reduction

Comprehensive performance of ultra-high temperature ceramic composites is predominantly governed by purity, morphology and crystallinity of HfB 2 powders. Nevertheless, achieving controllable synthesis of high-quality HfB 2 ceramic powders remains challenging due to insufficient mechanistic insights into the evolution and growth of powder morphology. In this work, we systematically investigate the effects of diverse carbon sources on the morphology and crystallinity of HfB 2 powders during carbothermal reduction. The results demonstrate that the graphitization degree of carbon sources directly governs whether HfB 2 powder exists in particle or rod-like morphology. Microstructural evolution analysis indicates that the morphological differentiation in HfB 2 powders is predominantly determined by the graphitization degree of the carbon source, which directly controls the oriented attachment of HfO 2 particles in precursors and modulates the reaction temperature during subsequent carbothermal reduction processes. The elevated graphitization degree promotes a more orderly arrangement of HfO 2 nanograins. The HfB 2 powders maintain the anisotropic rod-like morphology inherent in their HfO 2 precursors throughout the growth process, resulting in high-crystallinity, rod-shaped ceramic powders. These crystalline, rod-shaped powders display remarkable antioxidant properties; their mass fraction decreases by only 2.25 % following non-isothermal oxidation at 1000 °C. This research holds significant implications for developing controllable preparation methods for ceramic powders of different morphologies and provides a theoretical foundation for enhancing the performance of ultra-high temperature ceramic materials.