Oxygen adsorption on ideal ZrB₂ and ZrC surfaces

To understand the initial oxidation of ZrB₂ and ZrC, the adsorption behaviors of an O₂ molecule on ideal ZrC (1 1 1) and ZrB₂ (0 0 0 1) surfaces were investigated by first-principle calculation, and thermal gravimetric and differential scanning calorimetry test was executed. The results reveal that the initial oxidation temperature of ZrC was 210 °C, while that of ZrB₂ was up to 500 °C since the adsorption energies of O₂ molecule on Zr–ZrC surface was higher than that on Zr–ZrB₂ surface. Meanwhile, the adsorbed O₂ molecule affected the C atom bonding with the oxidized Zr atom in O–Zr–ZrC, while the B atom bonding with oxidized Zr atom in O–Zr–ZrB₂ underwent little influence. Besides, the higher adsorption energy was obtained in O–C–ZrC than in O–B–ZrB₂, implying that the C atom in C–ZrC might be oxidized more easily than the B atom in B–ZrB₂. In addition, big holes (C vacancies) were formed when O₂ molecule adsorbed on the C–ZrC surface because of the generation of C–C dimer and C–O bond, which led to that the next O₂ molecule might penetrate the Zr atomic layer and react with the Zr atoms. While O₂ molecule doped into the B atomic layer in B–ZrB₂, and more O₂ molecules might be adsorbed on the B–ZrB₂ surface to generate a B₂O₃ structure to protect the inner material.