Oxidation behavior of (NbTaZrW)C high-entropy carbide at 800–1000 °C

To enrich the database of HECC oxidation and to assess the potential of the hard-and-tough (NbTaZrW)C for high temperature application in oxygen-containing environments, in the current study, the oxidation resistance and behavior of this W-incorporated high-entropy carbide ceramic (HECC) was evaluated and investigated. To achieve this end, isothermal oxidation tests were conducted in a temperature range of 800–1000 °C. Natures of the oxidation products were systematically characterized. The results show that, the oxidation of (NbTaZrW)C follows the parabolic rule, suggesting the abeyance of a diffusion-controlled oxidation process at 800–1000 °C. Mechanistically, the oxidation of (NbTaZrW)C is dominated by the inward diffusion of O₂ with no sign of outward diffusion of cation-site metallic elements. Varying with temperature, a few compositionally complex oxides, including (Nb_0.5Ta_0.5)₂Zr₆O₁₇, ((N_b0.5Ta_0.5)₂O₅)₁₅(WO₃)₂, (Nb_0.5Ta_0.5)₁₆W₁₈O₉₄, as well as (NbTaZrW)O_x with unknown structure. During oxidation, laminated structures with inter-laminate cracks tend to form due to the combined effects from residual stress and gaseous products. Our final comparison shows that, (NbTaZrW)C exhibits significantly lower oxidation resistance than a few other HECCs, such as (HfTaZrNb)C and (HfZrTaNbTi)C. The low oxidation resistance, as quantified by the extremely high k_p values, of (NbTaZrW)C suggest its unsuitability for application in high temperature oxygen-containing environment.