Thermogravimetric analysis of the oxidation resistance of ZrB2-SiC and ZrB2-Sic-TaB2-based compositions in the 1500-1900 °C range

Theoretically dense ZrB2-SiC two-phase microstructures were isothermally oxidized for similar to 90 min in flowing air in the range 1500-1900 degrees C. Specimens with 30 mol% SiC formed distinctive reaction product layers that were highly protective; 28 mol% SiC-6 mol% TaB2 performed similarly. At and above 1700 degrees C, the composition with only 15 mol% SiC oxidized extensively because of deficient silicate liquid formation. Specimens with 60 mol% SiC were resistant to oxidation up to 1800 degrees C; at 1900 degrees C, this composition displayed periodic ruptures of the passivating layer by emerging gas bubbles. Oxide coating thicknesses calculated from weight loss data were consistent with those measured from scanning electron microscopy micrographs. A layer of ZrB2 devoid of SiC was argued to be from preferential removal of SiC by reaction of a silica oxidation product with adjacent unreacted SiC to form escaping gases.