Insights into (Hf-Zr-Ta-Nb)C thermal protective system: Ablation behavior, morphology evolution and atomic bonding

Utilizing first-principles calculations about A-B-O binary systems (A = Zr, Hf; B = Ta, Nb) and oxyacetylene ablation tests (2.4 MW/m², 30 s), the synergistic effects of Hf, Zr, Ta and Nb elements in (Hf-Zr-Ta-Nb)C ceramic were studied. From the aspect of experimental results, the (Hf-Zr-Ta-Nb)C ceramic exhibited the lowest average linear variation rate (−1.2 μm/s) and thinnest oxide layer (∼170 μm), only half the thickness of (Hf–Ta)C ceramic (∼330 μm). From the aspect of protective characteristic of HfO₂-based oxide layer, Ta enrichment will be easier to bring about structural instability of HfO₂ than Zr and Nb elements, attributed to the easily formed Hf₆Ta₂O₁₇ phase with relatively low melting point. As for ZrO₂-based solid solutions, the continuous enrichment of Ta and Nb will both cause structural instability, based on the increasing trend of calculated binding energies of Ta or Nb modified ZrO₂, accompanied by the spontaneous formation trend of Zr₆Ta₂O₁₇ and Zr₆Nb₂O₁₇ low-melting-point phases.