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 mu m/s) and thinnest oxide layer (-170 mu m), only half the thickness of (Hf-Ta)C ceramic (-330 mu m). 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 lowmelting -point phases.