Effects of milling time and reductant content on the formation of HfC-HfB₂ composite powders synthesized via a solid-state reaction route

In this study, a solid-state reaction route that is a combination of high-energy ball milling (HEBM) and annealing was adopted to synthesize HfC-HfB₂ composite powders. The effects of HEBM duration (0 h, 1 h, 2 h, and 4 h) and excess reductant (Mg or C) amount on the reaction mechanism of Hf–B₂O₃–C–Mg quaternary powder system were meticulously investigated. Following the HEBM of powder blends, annealing was performed at 1600 °C for 16 h under Ar atmosphere. A purification step was conducted the annealed powders by dissolving them in an acidic solution. X-ray diffractometry (XRD) technique revealed that a small amount of Hf and C powders reacted to form HfC phase after 2 h of HEBM. After annealing and purification, in addition to the HfC phase, the HfB₂ and HfO₂ phases were observed in the powders. Besides, the intensities of XRD peaks belonging to the HfO₂ phase gradually decreased, and those of HfC increased in the annealed and purified powders with increasing milling duration and reductant amounts. Both milled and milled-annealed-purified powders exhibited a decrease in the particle size with an increase in the HEBM duration. Transmission electron microscopy (TEM) micrograph belonging to 4 h milled-annealed-purified powders containing 50 wt% excess Mg and 50 wt% excess C revealed the formation of HfC-HfB₂ composite powders whose particles ranged between 50 nm and 100 nm.