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-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 (∆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 (∆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 corresponding solution reactions possess negative ∆RGm, indicating the possible solution sequence of these elements is Zr > Y > Sm > Er.