RADIATION RESPONSE OF SIC-BASED FIBERS

Loss of strength in irradiated fiber-reinforced SiC/SiC composite gene rally is related to degradation in the reinforcing fiber. To assess fi ber degradation, the density and length changes were determined for fo ur types of SiC-based fibers (Tyranno, Nicalon CG, Hi Nicalon and Dow X) after high temperature (up to 1000 degrees C) and high dose (up to 80 dpa-SiC) irradiations. For the fibers with nonstoichiometric compos itions (the first three types in the list), the fiber densities increa sed from 6% to 12%. In contrast, a slight decrease in density (<1%) wa s observed for the Dow X fiber with a quasi-stoichiometric composition . Fiber length changes (0-5.6% shrinkage) suggested small mass losses (1-6%) had occurred for irradiated uncoated fibers. In contrast, exces sive linear shrinkage of the pyrocarbon-coated Nicalon CG and Tyranno fibers (7-9% and 16-32%, respectively) indicated that much larger mass losses (11-84%) had occurred for these coated fibers. Crystallization and crystal growth were observed to have taken place at fiber surface s by SEM and in the bulk by XRD, moreso for irradiated Nicalon CG than for Hi Nicalon fiber. The radiation response of the quasi-stoichiomet ric Dow X fiber was the most promising. Further testing of this type f iber is recommended. (C) 1998 Elsevier Science B.V. All rights reserve d.