The Oxidation Behavior of a Si-Ti-C-O Fiber with a Low Oxygen Content

Abstract

The Si-Ti-C-O fiber (STC (6)) with a low oxygen content of 6mass% was oxidized at temperatures in the range of 1273 to 1773K in order to measure its oxidation rate and thereby to elucidate its oxidation behavior. These results were compared to those of the commercial fibers having 18 and 13mass% O (STC (18) and STC (13)). The kinetic data of STC (6) were expressed by the contracting-cylinder formula. The activation energies were calculated to be 72 and 430kJ/mol at the low-temperature (T 1473K) regions, respectively. On the other hand, STC (18) and STC (13) had activation energies of oxidation of E=72kJ/mol and E=70kJ/mol within the whole temperature range, respectively. Oxidation of STC (6) with a low oxygen content is considered to be similar in kinetic behavior to that of pure SiC.The oxide film consisted of SiO2 and TiO2; and the former was amorphous during the low-temperature oxidation process tending to be crystallized into cristobalite at higher temperatures. The average size of crystallized β-SiC particles of the interface between the oxide film and unoxidized core fiber was larger than that of core of the fiber. This is because the progress of the crystallization in O2 stream is considered to have been suppressed when the fiber was completely covered with the oxide film. When the fiber was oxidized at 1773K, the interface between the oxide film and the unoxidized core fiber contained many microcracks; moreover, the core fiber lost 75% of its original strength at room temperature. It is quite possible that these defects were produced by a contraction due to transformation of cristobalite (α-β) during cooling from the oxidation temperatures.