Degradation mechanism of amorphous silicon carbide fiber due to air-exposure at high temperatures

The degradation mechanism of the amorphous silicon carbide fiber, Tyranno-ZMI®, exposed in air at 1173-1873 K for 20ks were studied. The average strength of the bare fiber, which was prepared by etching away the oxidation layer on the fiber surface, decreased with increasing exposure temperature, especially when exposed at the temperature higher than 1673 K. The measurement of the crystallite size of β-SiC in the fiber with Sherrer method revealed that coarsening of the crystalline occurred in the fiber exposed at the temperatures higher than 1773K. The scanning electron microscope observation of the fiber surface showed that the many defects formed on the fiber surface. By introducing an artificial notch directly into the fiber specimens using a focused-ion(Ga+)-beam, the fracture toughness values of the as-supplied fiber and of the fiber exposed at 1673 and 1773 K were determined to be 1.8 ± 0.3, 1.9 ± 0.4 and 1.3 ± 0.4 MPa√m, respectively. Based on these results, the reason for the degradation of the fiber was attributed to the extension of the surface defect which was enhanced by the reduction in fracture toughness due to coarsening or the β-SiC crystalline.