Oxidation of ultrahigh temperature ceramics in water vapor

Ultrahigh temperature ceramics (UHTCs) including HfB2+20 vol % SiC (HS), ZrB2+20 vol % SiC (ZS), and ZrB2+30 vol % C+14 vol % SiC (ZCS) have been investigated for use as potential aeropropulsion engine materials. These materials were oxidized in water vapor (90%) using a cyclic vertical furnace at 1 atm. The total exposure time was 10 h at temperatures of 1200, 1300, and 1400degreesC. Chemically vapor deposited SiC was also evaluated as a baseline for comparison. Weight change, X-ray diffraction analyses, surface and cross-sectional scanning electron microscopy, and energy dispersive spectroscopy were performed. These results are compared with tests conducted in a stagnant air furnace at temperatures of 1327degreesC for 100 min, and with high pressure burner rig (HPBR) results at 1100 and 1300degreesC at 6 atm for 50 h. Low velocity water vapor does not contribute significantly to the oxidation rates of UHTCs when compared to stagnant air. The parabolic rate constants at 1300degreesC, range from 0.29-16.0 mg(2)/cm(4) h for HS and ZCS, respectively, with ZS results between these two values. Comparison of results for UHTCs tested in the furnace in 90% water vapor with HPBR results was difficult due to significant sample loss caused by spallation in the increased velocity of the HPBR. Total recession measurements are also reported for the two test environments. (C) 2004 The Electrochemical Society.