Effect of La2O3 addition on long-term oxidation kinetics of ZrB2–SiC and HfB2–SiC ultra-high temperature ceramics

Long-term oxidation kinetics of SiC-reinforced UHTCs and La2O3-doped UHTCs over an intermediate temperature range (1400–1600 °C) reveal partially protective behavior for the former characterized by an oxidation kinetic exponent 1 < n < 2. In addition, unstable oxidation behavior was observed in HfB2-based UHTCs due to the presence of SiC agglomerates. On the other hand, La2O3-doped UHTCs were found to be protective over the whole temperature range studied (n = 2), in particular at 1600 °C, where oxidation kinetic exponents as high as 8 were observed as a consequence of formation of new oxidation protective particles, MeOxCy, where Me is Zr, Hf or Si. Adsorption of oxygen-containing species formed protective MeOxCy phases, which enhanced the thermal stability of the oxide scale as well as providing protection against oxidation for long exposure times at 1600 °C.