Abstract
We report herein on the oxidation kinetics and morphology of the oxide phases that form after long term (up to 1500 h) oxidation in ambient air of fine and coarse-grained samples of with 30 vol % TiC and with 30 vol % SiC in the 875-1200°C temperature range. In all cases, the oxidation resulted in a duplex scale, an outer rutile and an inner rutile/silica layer. At 875°C, and up to at least 100 h, the oxidation kinetics of the TiC samples are parabolic; at 925°C, the oxidation kinetics of the SiC are subparabolic, up to at least 500 h. The oxidation kinetics of all other runs are initially parabolic, but at times >30 h they become linear. The reason for the transition is not entirely clear, but could be due to the buildup of stresses in the external oxide scales. Comparison with previously published results indicate that the rate limiting step, when the oxidation kinetics are parabolic, is most probably the inward diffusion of oxygen and the simultaneous outward diffusion of titanium. The results also strongly suggest that the activation energies for diffusion of oxygen and titanium in rutile are almost identical over at least the 1000-1200°C temperature range. © 2003 The Electrochemical Society. All rights reserved.
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