Irradiation-induced shrinkage of highly crystalline SiC fibers

Composites consisting of a SiC matrix prepared by chemical vapor infiltration that was reinforced by Tyranno™ SA3rd (TSA3) or Hi-Nicalon™ Type S (HNLS) SiC fibers were irradiated with 5.1 MeV Si ions to 100 dpa at 300 °C. It was confirmed that the irradiated microstructure of TSA3 was stable up to 100 dpa. In contrast, the results of surface profilometry and cross-sectional transmission electron microscopy (TEM) showed that HNLS fibers underwent 0.8% shrinkage along the axis and 0.7% radial shrinkage. High-resolution TEM images of the highly damaged regions in the HNLS material revealed the complete loss of carbon ribbons initially distributed at the SiC grain boundaries. Ion-beam-induced diffusion of carbon into the SiC grains was indicated from the observations of the interdiffusion layer formed at the fiber/pyrocarbon interface. The π∗ peak of the carbon K-edge spectrum was found in the electron-energy-loss spectrum of the HNLS SiC grains that were irradiated above 60 dpa; this peak was not observed in unirradiated SiC fibers, further demonstrating that carbon sp2 bonding was only detected in HNLS SiC fibers. According to the changes in the bonding and the volume relaxation related to the carbon defects in SiC, an increase in the population of antisite CSi was likely the key underlying mechanism of the irradiation-induced shrinkage of the HNLS fibers.