THERMOMECHANICAL STABILITY OF NICALON FIBERS IN A CARBON-MONOXIDE ENVIRONMENT

Microstructural changes and the related strength degradation were inve stigated for the Nicalon ceramic fibre hear-treated in argon and in a CO/Ar mixture (with a CO partial pressure of 40 kPa) at 1200-1400 degr ees C and for different periods of time (0.5-30 h). Detailed structura l and compositional analyses were carried out using X-ray diffraction, TEM, SEM, and Auger electron spectroscopy. Strength degradation of th e Nicalon fibre after heat-treatments in argon resulted from the fibre decomposition and CO gas escape which produced surface defects, grain growth and intergranular porosity. On the other hand, the strength de gradation was effectively reduced for fibers heat-treated in an approp riate atmosphere of argon and CO, as long as the heat-treatment temper ature was kept below 1400 degrees C. Such conditions of heat-treatment in CO/Ar showed little effect on the grain size.