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 degrees 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.