Heat treatment effects on creep behavior of polycrystalline SiC fibers

Polycrystalline SiC fibers are being considered as potential reinforcement for ceramic matrix composites (CMCs). For these fibers with fine grain size, basic issues arise concerning thermo-mechanical properties and microstructural instability during fabrication and application of CMCs. To examine these issues, three commercially available SiC fibers were heat treated at elevated temperatures for 1 h in an Ar atmosphere. The creep resistance of SiC fibers was evaluated by the bend stress relaxation (BSR) method, and it was found that the creep resistance could be improved by heat treatment. Combining the results of the BSR tests with the results of X-ray diffraction examinations indicated that the creep resistance of SiC fibers is mainly related to the β-SiC grain size and the composition at or adjacent to the grain boundaries. Also, the apparent activation energy of creep for both Hi-Nicalon™ and Hi-Nicalon™ type S fiber increased with increasing heat treatment temperature. In the case of Tyranno™-SA fiber, the apparent activation energy of creep did not show an obvious dependence on the heat treatment temperature.