Oxidation of ZrB ₂-SiC: Influence of SiC content on solid and liquid oxide phase formation

The effect of SiC concentration on the liquid and solid oxide phases formed during oxidation of ZrB ₂-SiC composites is investigated. Oxide-scale features called convection cells are formed from liquid and solid oxide reaction products upon oxidation of the ZrB ₂-SiC composites. These convection cells form in the outermost borosilicate oxide film of the oxide scale formed on the ZrB ₂-SiC during oxidation at high temperatures (≥1500°C). In this study, three ZrB ₂-SiC composites with different amounts of SiC were tested at 1550°C for various durations of time to study the effect of the SiC concentration particularly on the formation of the convection cell features. A calculated ternary phase diagram of a ZrO ₂-SiO ₂-B ₂O ₃ (BSZ) system was used for interpretation of the results. The convection cells formed during oxidation were fewer and less uniformly distributed for composites with a higher SiC concentration. This is because the convection cells are formed from ZrO ₂ precipitates from a BSZ oxide liquid that forms upon oxidation of the composite at 1550°C. Higher SiC-containing composites will have less dissolved ZrO ₂ because they have less B ₂O ₃, which results in a smaller amount of precipitated ZrO ₂ and consequently fewer convection cells.