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

Multi-component boride (Hf0.5Zr0.5)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 (Hf0.2Zr0.2Sm0.2Er0.2Y0.2)O2-delta 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 (Delta Gmix) 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 (Delta RGm), (Hf0.5Zr0.5)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 corre-sponding solution reactions possess negative Delta RGm, indicating the possible solution sequence of these elements is Zr > Y > Sm > Er.