Influence of oxide layer on oxidation of free carbon in Si-Ti-C-O fiber-bonded ceramics

Influence of a silica layer on oxidation of free carbon in Si-Ti-C-O fiber-bonded ceramics has been investigated. The rate of oxidation of the Si-Ti-C-O fibers has been measured by thermogravimetric analysis at temperatures ranging between 773 and 1573 K. The relative mass of the fibers increased parabolically and the kinetics of the oxidation followed the parabolic rate characteristic of diffusion controlled processes. The thickness of the silica layer, calculated according to a simple equation, well agreed well with the experimentally observed one. Pores created by the oxidation of free carbon were observed inside the fibers after etching the surface of a heat-treated Si-Ti-C-O fiber-bonded ceramics at 773 K. Using the parabolic rate constant and the volume expansion coefficient of the fiber, the time required to close the pores by the formation of silica layer was calculated. The thickness of the free carbon layer and the oxidation rate of the fibers were the key parameters for the analysis of the oxidation behavior of the materials.