Improving the oxidation resistance of carbon fibers using silicon oxycarbide coatings

Silicon oxycarbide (SiOC) ceramic was coated on carbon fibers using a vinyl group-modified silicon alkoxide as the sol precursor. The coatings were characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and thermogravimetry. The mechanical properties of the carbon fibers were investigated by single fiber tensile tests. Results showed that the amorphous SiOC coating was composed of SiCxO4-x and a free carbon phase. The oxidation resistance of the carbon fibers was improved by the SiOC coatings. However, debonding of the coatings and surface cracks led to a reduction of tensile strength and an increase of Weibull modulus. The coating with a thickness of 200 nm increased the onset oxidation temperature by about 150 °C, but decreased the tensile strength from 3.18 to 2.32 GPa.