In-situ phase evolution of multi-component boride to high-entropy ceramic upon ultra-high temperature ablation

Multi-component boride (Hf_0.5Zr_0.5)B₂-SmB₆-ErB₄-YB₆ (HZRB) utilized as a coating on SiC-coated carbon/carbon (C/C) composite was prepared by supersonic atmosphere plasma spraying. In-situ phase evolution of HZRB into (Hf_0.2Zr_0.2Sm_0.2Er_0.2Y_0.2)O_2-δ high-entropy oxide (HEO) was investigated after oxyacetylene ablation for 120 s. The first-principles calculations were applied to analyze the in-situ formation mechanism of HEO. The mixing Gibbs free energy change (∆G_mix) of HEO was calculated to be negative at 2573 K, indicating that the HEO can be generated upon the ablation temperature. Due to the lower Gibbs free energy change of reaction (∆_RG_m), (Hf_0.5Zr_0.5)B₂ will be oxidized to generate HfO₂ firstly, and other elements dissolved into the HfO₂ lattice to form HEO. The solution energies of Zr, Sm, Er and Y atoms are − 0.01, 6.28, 8.55 and 4.46 eV/atom, and corresponding solution reactions possess negative ∆_RG_m, indicating the possible solution sequence of these elements is Zr > Y > Sm > Er.