THERMOMECHANICAL STABILITY OF NICALON FIBERS IN A CARBON-MONOXIDE ENVIRONMENT

Microstructural changes and the related strength degradation were investigated 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 degrees C and for different periods of time (0.5-30 h). Detailed structural and compositional analyses were carried out using X-ray diffraction, TEM, SEM, and Auger electron spectroscopy. Strength degradation of the 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 degradation was effectively reduced for fibers heat-treated in an appropriate atmosphere of argon and CO, as long as the heat-treatment temperature was kept below 1400 degrees C. Such conditions of heat-treatment in CO/Ar showed little effect on the grain size.