Effect of moisture on the oxidation behavior of ZrB 2

Oxidation studies of ZrB 2 were performed under wet air and dry air conditions at 1200°C, 1400°C, and 1500°C for 1, 4, and 10 h. Compared to dry air, the presence of water vapor was found to enhance the oxidation kinetics by a factor of 7 to 30, depending on the temperature. Thermodynamic calculations suggested that water vapor promoted the formation of additional volatile species such as boric acid (HBO 2 ), in addition to boria (B 2 O 3 ) produced in dry air, which increased the evaporation rate of B 2 O 3 . Compared to dry air, the presence of water vapor leads to more rapid evaporation of boria and the transition from parabolic oxidation kinetic behavior (ie, rate controlled by diffusion through boria) to linear (ie, underlying ZrB 2 is directly exposed to the oxidizing environment) at shorter times and lower temperatures.