Microstructural characterization of ZrB2–SiC based UHTC tested in the MESOX plasma facility

Microstructures were investigated for ZrB2–SiC and ZrB2–HfB2–SiC ultra high temperature ceramics that were subjected to a high temperature plasma environment. Both materials were tested in the MESOX facility to determine the recombination coefficient for atomic oxygen up to 1750 °C in subsonic air plasma flow. Surfaces were analyzed before and after testing to gain a deeper insight of the surface catalytic properties of these materials. Microstructural analyses highlighted oxidation induced surface modification. Oxide layers were composed of silica with trace amounts of boron oxide and zirconia if the maximum temperature was lower than about 1550 °C and zirconia for higher temperatures. The differences in the oxide layer composition may account for the different catalytic behavior. In particular, the presence of a borosilicate glass layer on the surface of ZrB2–SiC materials guarantees atomic oxygen recombination coefficients that are relatively lower than the coefficients measured when only zirconia is present. The oxidation processes of ZrB2–HfB2–SiC materials, associated with catalytic tests carried out up to 1550 °C, lead to the formation of hafnia as well as silica, and zirconia. The higher recombination coefficients measured in the case of ZrB2–HfB2–SiC materials can be correlated with the presence of hafnia which is probably characterized by higher catalytic activity compared to zirconia. In any case, the investigated materials demonstrate a low catalytic activity over the inspected temperature range with maximum values of recombination coefficients close to 0.1.