Synthesis of Hf6Ta2O17 superstructure via spark plasma sintering for improved oxidation resistance of multi-component ultra-high temperature ceramics

Ultra-high temperature ceramics (UHTCs) have shown aspiration to overcome challenges in the thermal protection system (TPS) by designing new materials referred to as multi-component UHTCs (MC-UHTCs) in the compositional space. MC-UHTCs have shown remarkable improvement in oxidation resistance due to the formation of the Hf6Ta2O17 superstructure during plasma exposure. Herein, the Hf6Ta2O17 superstructure is synthesized via a solid-state reaction between HfO2 and Ta2O5 powder mixtures during spark plasma sintering (SPS). The compositions chosen are 50 vol% of HfO2 -50 vol% of Ta2O5 (50HO-50TO) and 70 vol% of HfO2 -30 vol% of Ta2O5 (70HO-30TO). The phase quantification via Rietveld analysis showed Hf6Ta2O17 as a principal phase with some residual Ta2O5 phase in both the samples. The high-temperature thermal stability of the samples was evaluated using high-velocity plasma jet exposure for up to 3 min. 50HO-50TO was able to withstand the intense plasma condition, which is attributed to the higher content of the Hf6Ta2O17 phase (∼84%) and lower strain in the Ta2O5 phase. The augmentation in the Hf6Ta2O17 phase to 94.7% (in 50HO-50TO) post plasma exposure has been attributed to the invariant transformation from a liquid state to Hf6Ta2O17 at temperatures >2500 °C during testing. The mechanical integrity is elucidated from the insignificant change in the hardness ∼13.3 GPa before and 11.2 GPa after plasma exposure of the 50HO-50TO sample. As a result, the Hf6Ta2O17 superstructure's thermo-mechanical stability suggests developing novel oxidation-resistant MC-UHTCs in compositional space for reusable space vehicle applications.