STRENGTH OF NICALON SILICON-CARBIDE FIBERS EXPOSED TO HIGH-TEMPERATURE GASEOUS ENVIRONMENTS

The effects of exposures to high-temperature gaseous atmospheres on the strength of Nicalon SiC fibers were investigated. The exposure conditions were as follows: (1) H2 with various P(H2O) for 10 h at 1000-degrees and 1200-degrees-C, and (2) air for 2 to 100 h at 800-degrees to 1400-degrees-C. Individual fibers were tested in tension following each exposure. The strengths of the fibers were strongly influenced by the exposure atmosphere and temperature, but less affected by time at temperature. When exposed in air, a SiO2 layer was formed on the surface, minimizing the degradation of strength. However, this beneficial effect was negated under conditions in which the SiO2 layer became too thick. The most severe degradation resulted from exposure to a reducing atmosphere, presumably due to the reduction of SiO2 inherent in the fibers.