Effect of Thermal History on Pyrolysis of Si-C-O Amorphous Fiber Coated with Oxide Film

The effect of thermal history on pyrolysis of Si-C-O fibers coated with oxide films was investigated. The fibers were oxidized at 1373, 1573 and 1773 K to form an oxide film about 4.7×10−1 μm in thickness, and were quenched to room temperature. These oxidized fibers (named fiber-a, -b and -c respectively) were heated rapidly up to 1573, 1673 and 1773 K in Ar. During these heat treatments, mass changes of the fibers were measured with a thermobalance method. The heat-treated fibers were examined by X-ray diffraction, SEM observation and the tensile test.At 1773 K, fibers-a and -b pyrolyzed completely, while the growth and coasening of β-SiC was not observed in fibers-c. Pyrolysis of cores of the fibers took place, due to both the preferential oxidation of free carbon and the crystallization of the oxide film into cristobalite. The tensile strengths of the as-oxidized fibers were about 80% of that of the original fibers. Fibers-c heated in Ar at 1773 K retained 56% of that of the original fibers. This gradual reductions in strength were caused by the formation of surface defects, such as cracks and pits. The oxide film formed at 1773 K was found to be resistant to the thermal stress and to be effective for suppressing the pyrolysis of Si-C-O fibers.