Improving the mechanical properties of amorphous silicon carbide fibers by forming a protective silicon dioxide layer

Abstract

Silicon carbide fiber is a representative structural material that has been studied as a major reinforcement material for ceramic matrix composites (CMCs). SiC fibers are classified by their operating temperature, manufacture process, composition, and crystallinity. Among the various SiC fibers, amorphous ones are composed of a Si–O–C amorphous matrix and nanoscale SiC crystals, and amorphous fibers exhibit amorphous fracture behavior. An amorphous fiber is almost always fractured by surface flaws generated during the manufacturing process. In this study, we aimed to strengthen the surface by etching the surface cracks and forming a SiO2 oxidation layer, and then evaluated the effect of the oxide layer thickness on the fracture behavior and control of surface defects. The passive oxidation layer was formed by exposing the fiber bundles to air atmosphere at a temperature higher than 1000 °C. The mechanical properties of the strengthened fibers were evaluated through single-filament tensile strength tests, and the fracture distribution was analyzed using Weibull distribution. Further, fractographic analyses were conducted on the cross-sectional microstructures observed by field-emission scanning electron microscopy. The surface-oxidized fibers exhibited enhanced flexibility owing to the crack-blunting effect of the SiO2 layer.