Creep limitations of current polycrystalline ceramic fibers

The objective of this paper is to present an overview of the issues, property status, and potential for use of creep-prone polycrystalline ceramic fibers in thermostructural ceramic composites. Issues arise because the fine-grained microstructures of high-strength fibers can result in creep-related property changes, often at temperatures as low as 800°C. The underlying mechanism is grain boundary sliding controlled by grain size and grain boundary character, and thus by the fiber processing method. With the assumption of upper and lower limit creep requirements, the creep properties of a variety of current SiC and Al₂O₃ polycrystalline fibers are reviewed and discussed. Property evaluation is based on the results of a simple bend stress relaxation test which allows predictive creep equations to be developed for each fiber type describing the effects of time, temperature, and applied stress. It is shown that sintered SiC fibers with grain sizes below 1000 nm appear to offer the best performance in terms of strength and creep resistance. However, even these fibers may not be capable of long-term service above 1400°C.