Evaluation of neutron irradiated near-stoichiometric silicon carbide fiber composites

Composites have been fabricated by chemical vapor infiltration of silicon carbide (SiC) into SiC-based fiber preforms. Fibers were Ceramic Grade Nicalon^TM, Hi-Nicalon^TM and Hi-Nicalon^TM Type-S. Results are presented for two parallel studies on the effects of neutron irradiation on these materials. In the first study, neutron irradiation induced changes in mechanical properties, as measured by bend testing, for Hi-Nicalon^TM fiber materials of varied interphase structures is measured. Results indicate that both the Ceramic Grade Nicalon^TM and Hi-Nicalon^TM materials degrade substantially under irradiation, though the higher oxygen content Ceramic Grade fiber degrades more rapidly and more substantially. Of the three interfaces studied in the Hi-Nicalon^TM system, the multilayer SiC is the most radiation resistant. At a dose of approximately 1 dpa the mechanical property degradation of the Hi-Nicalon^TM composite is consistent with a fiber densification-induced debonding. At a dose of 10 dpa the properties continue to degrade raising the question of degradation in the CVD SiC matrix as well. Low-dose results on the Hi-Nicalon^TM Type-S fabricated material are encouraging, as they appear to not lose, and perhaps slightly increase, in ultimate bend strength. This result is consistent with the supposition that as the oxygen content in SiC-based fibers is reduced, the irradiation stability and hence composite performance under irradiation will improve.