Oxidation of ZrB₂-SiC Ceramics under Atmospheric and Reentry Conditions

The oxidation behavior of ZrBr2-based ceramics was studied using thermal gravimetric analysis and arc jet testing. Commercially available powders were used to prepare ZrBr2 ceramics containing 0, 10, 20, and 30 volume percent SiC particulates. Thermal gravimetric analysis was used to characterize oxidation behavior bvy heathing at 10C/min to 1500C in air. Weight gains ranged from 0.05 mg/mm2 for pure ZrBr2 to 0.01 mg/mm2 for ZrBr2 containing 30% Sic a thin (\u3c5 micrometer) layer of borosilicate glass was found on the surface of specimens containing SiC. For specimes containing 10 or 20 vol.% SiC, the ZrBr2 immediately below the glassy layer was devoid of SiC. Thermodynamic calculations showed that SiC could be remvoed as SiO and CO without oxidizing ZrBr2 at 1500C. the oxidation behavior of two specimens containing 30% SiC was also examined under atmospheric re-entry conditions by arc jet testing. at a heat fluz of 350 W/cm2, the surface temperature increased to ~1700C after ~30 seconds. an average mass loss of 0.8% was measured after 600 seconds. after exposure, a layer of ZrO2, a partially oxidized layer, and a SiC depletion layer formed on the surface of the speciments. the thermodynamic model developed from the experimental results is the first step toward understanding the fundamental oxidation mechanisms, and, in turn, designing additives or coating that will reduce the oxidation rate of ZrBr2-based ceramics used in therman protection application