Effects of Processing Parameters and Reaction Mechanism in Combustion Synthesis of HfC <i> _x </i> N <i> _y </i> Powders

ABSTRACT HfC x N y solid solution powders were synthesized by combustion synthesis using Hf and carbon black as reactants, with HfC or HfN as diluents under a nitrogen atmosphere. The effects of processing parameters on phase compositions and microstructures were systematically examined, and the reaction mechanism in the combustion synthesis of HfC x N y was elucidated. The results demonstrated that both phase compositions and microstructures were strongly dependent on the processing parameters. For undiluted samples with high Hf/C molar ratio and diluted samples with low HfC content, the microstructure featured HfC x N y crystals embedded in the unreacted molten Hf. At moderate HfC contents, HfO 2 formed and replaced molten Hf as the bonding phase. At high HfC contents, the microstructure evolved into agglomerates composed of HfC x N y crystals. In contrast, the introduction of HfN as a diluent led to more complex phase compositions. Based on thermodynamic calculations and experimental characterization, four reaction pathways for the formation of the HfC x N y solid solution have been proposed. In path 1, HfC x N y crystals are directly precipitated from Hf‐C‐N melt. In path 2, HfC crystals precipitate from Hf‐C melt, followed by the formation of HfC x N y phase through the reaction between HfC and N 2 . In path 3, HfC x N y phase forms through a solid‐state reaction between HfC and HfN. In path 4, HfC x N y phase forms via a solid‐state reaction between HfN and C. This study provides mechanistic insight into HfC x N y formation in combustion synthesis and offers guidance for tailoring high‐quality powder.