STRESS RUPTURE OF AN ENHANCED NICALON SILICON CARBIDE COMPOSITE AT INTERMEDIATE TEMPERATURES/

The stress rupture characteristics of an enhanced Nicalon/SiC composit e at 900 degrees C have been examined. This temperature has been ident ified as being in the regime wherein oxidation embrittlement is operat ive. The enhancement of the composite involves the use of a coating ar ound the fiber tows, comprising a C-rich matrix and B-containing parti culates, The efficacy of this oxidation protection Scheme has been eva luated by comparing the stress rupture characteristics with those of b oth Nicalon/SiC composites without the enhancement and the fibers alon e. Such comparisons indicate that a substantial portion of the strengt h loss is attributable to a degradation of the fibers, and that the pe rformance of the enhanced material is marginally better than that of t he reference (nonenhanced) composite. Moreover, at stress levels great er than the matrix cracking limit, oxidation embrittlement occurs rapi dly and the rupture times (several hours) are short in relation to the targeted service lives of most ceramic composite components. The mech anisms associated with the embrittlement have been identified using sc anning electron microscopy and Auger spectroscopy.