Synthesis of Hf<sub>0.75</sub>Ta<sub>0.25</sub>B<sub>2</sub> for self-coating TPS

Ultra-high temperature ceramic materials (UHTCs) are important for designing high-performance aerospace vehicles that can withstand repeated exposures to high temperatures. UHTCs mixed with silicides often have well-controlled oxidation due to the formation of protective silicates, which create a regenerative outer protective layer. Borides are known to have high melting points, high hardness and reasonably good oxidation resistance. In this study, Hf0.75Ta0.25B2 (HTB) is proposed as a potential alternative to YSZ protective coatings and ZrB2- SiC composites via the formation of Hf6Ta2O17 (HTO) passivation layer. The objective of this paper is to explore the parameter space of HTB synthesis via borocarbothermal (BCTR). Effects on particle size, phase purity, and residual oxygen content were analyzed with parameters of atmosphere composition, reactant grain sizes, and differing reaction pathways. The BCTR of HfO2 and Ta2O5 were analyzed to predict HTB behavior. It was shown that both one-step and two-step reaction routes can yield HTB, but two-step yields a purer product. Nano B4C produced finer HTB and facilitated reaction completion. Using a reducing atmosphere also enhanced reaction completion