Effects of Phase Change and Oxygen Permeability in Oxide Scales on Oxidation Kinetics of ZrB 2 and HfB 2

A wide range of experimental data on the oxidation of ZrB 2 and HfB 2 as a function of temperature (800°–2500°C) is interpreted using a mechanistic model that relaxes two significant assumptions made in prior work. First, inclusion of the effect of volume change associated with monoclinic to tetragonal phase change of the MeO 2 phases is found to rationalize the observations by several investigators of abrupt changes in weight gain, recession, and oxygen consumed, as the temperature is raised through the transformation temperatures for ZrO 2 and HfO 2 . Second, the inclusion of oxygen permeability in ZrO 2 is found to rationalize the enhancement in oxidation behavior at very high temperatures (>1800°C) of ZrB 2 , while the effect of oxygen permeability in HfO 2 is negligible. Based on these considerations, the significant advantage of HfB 2 over ZrB 2 is credited to the higher transformation temperature and lower oxygen permeability of HfO 2 compared with ZrO 2 .