Abstract
Oxidation of hot-pressed ZrC was investigated in air in the 1073-1373 K range. The kinetics were linear at 1073 K, whereas at higher temperature samples initially followed linear kinetics before undergoing rapid oxidation leading to a Maltese cross shape of the oxide. The linear kinetics at 1073 K was governed by inward oxygen diffusion through an intermediate layer of constant thickness between ZrC and ZrO 2 which was comprised of amorphous carbon and ZrO 2 nanocrystals. Diffusion of oxygen through the intermediate layer was measured to be 9 9 10 À10 cm 2 s À1 using 18 O as a tracer in a double oxidation experiment in 16 O/ 18 O. Oxidation at 1073 and 1173 K produced samples made of m-ZrO 2 and either t-or c-ZrO 2 with an adherent intermediate layer made of amorphous carbon and ZrO 2 , whereas oxidation at 1273 and 1373 K produced samples with a voluminous oxide made of m-ZrO 2 showing a gap between ZrC and the oxide. A sub-stoichiometric zirconia layer was found at the gap at 1273 K and no carbon uptake was detected in this layer when compared with the top oxide layer. The loss of the intermediate layer and the slowdown of the linear rate constant (g m À2 s À1) at 1273 K compared to 1173 K was correlated with the preferential oxidation of carbon at the intermediate layer which would leave as CO and/or CO 2 leaving a gap between ZrC and substoichiometric zirconia.