Oxidation mechanisms of multicomponent carbide ceramics

In this study, typical multicomponent carbide (MCC) ceramics, namely, (Hf_0.2Ti_0.2Zr_0.2Ta_0.2Nb_0.2)C (HTZTNC) and (Hf_0.25Zr_0.25Ta_0.25Nb_0.25)C (HZTNC) ceramics were used to investigate their oxidation rate-controlling step and the factors that determine the oxide morphologies. The results indicate that the outward diffusion of metal-containing substances is the rate-controlling step for the HTZTNC ceramics, with TiO being generated during prolonged oxidation. Conversely, the inward diffusion of the oxidant is the rate-controlling step for the HZTNC ceramics. The elements in the resultant oxide show a gradient distribution, which is consistent with the order in which they are oxidized. The vapor pressure of the corresponding oxides can also have an important effect on the structure of the oxide layer. Herein, it is concluded that the oxide morphologies are determined by both the oxidation chemical potentials and the vapor pressure. These findings can provide guidelines for improving the oxidation resistance of MCC ceramics.