Stress Rupture of an Enhanced Nicalon/Silicon Carbide Composite at Intermediate Temperatures

Abstract

The stress rupture characteristics of an enhanced Nicalon/SiC composite at 900°C have been examined. This temperature has been identified as being in the regime wherein oxidation embrittlement is operative. The enhancement of the composite involves the use of a coating around the fiber tows, comprising a C‐rich matrix and B‐containing particulates. The efficacy of this oxidation protection scheme has been evaluated by comparing the stress rupture characteristics with those of both Nicalon/SiC composites without the enhancement and the fibers alone. Such comparisons indicate that a substantial portion of the strength loss is attributable to a degradation of the fibers, and that the performance of the enhanced material is marginally better than that of the reference (nonenhanced) composite. Moreover, at stress levels greater than the matrix cracking limit, oxidation embrittlement occurs rapidly and the rupture times (several hours) are short in relation to the targeted service lives of most ceramic composite components. The mechanisms associated with the embrittlement have been identified using scanning electron microscopy and Auger spectroscopy.