͑ZCS͒ 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 1400°C. 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 1327°C for 100 min, and with high pressure burner rig ͑HPBR͒ results at 1100 and 1300°C 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 1300°C, 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.

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Gaseous titanium hydroxide and oxyhydroxide species were studied with quantum chemical methods. The results are used in conjunction with an experimental transpiration study of titanium dioxide (TiO) in water vapor-containing environments at elevated temperatures to provide a thermodynamic description of the Ti(OH)(g) and TiO(OH)(g) species. The geometry and harmonic vibrational frequencies of these species were computed using the coupled-cluster singles and doubles method with a perturbative correction for connected triple substitutions [CCSD(T)]. For the OH bending and rotation, the B3LYP density functional theory was used to compute corrections to the harmonic approximations. These results were combined to determine the enthalpy of formation. Experimentally, the transpiration method was used with water contents from 0 to 76 mol % in oxygen or argon carrier gases for 20-250 h exposure times at 1473-1673 K. Results indicate that oxygen is not a key contributor to volatilization, and the primary reaction for volatilization in this temperature range is TiO(s) + HO(g) = TiO(OH)(g). Data were analyzed with both the second and third law methods using the thermal functions derived from the theoretical calculations. The third law enthalpy of formation at 298.15 K for TiO(OH)(g) at 298 K was -838.9 ± 6.5 kJ/mol, which compares favorably to the theoretical calculation of -838.7 ± 25 kJ/mol. We recommend the experimentally derived third law enthalpy of formation at 298.15 K for TiO(OH), the computed entropy of 320.67 J/mol·K, and the computed heat capacity [149.192 + (-0.02539)T + (8.28697 × 10)T + (-15614.05)/T + (-5.2182 × 10)/T] J/mol-K, where T is the temperature in K.