Effect of ageing treatments at high temperatures on the microstructure and mechanical behaviour of 2D Nicalon/C/SiC composites .2. Ageing under CO and influence of a SiC seal-coating

2D-Nicalon/C/SiC composites fabricated by CVI either uncoated or protected with a SiC CVD seal-coating, have been aged at 1100-1300 degrees C under vacuum or carbon monoxide. They experienced a weight loss due to the decomposition of the SiO2xC1-x phase present in the fibers and to secondary reactions between carbon (from the fibers and the interphase) and the fiber decomposition products (CO and SiO). The change occurring in the chemical composition and microstructure of the composites has been studied by EPMA, AES and TEM/PEELS and in their mechanical behavior by tensile and push-out rests at ambient. The degradation process is slowed down by the CO pressure either applied externally to or generated within the composites as the result of the confinement of SiO/CO by the SiC coating. When aged between 1100 and 1200 degrees C, the composites fail at relatively high strain and stress with a plateau-like feature for the tensile curve whereas those aged at 1300 degrees C are brittle when treated under vacuum even with a SiC coating, or non-brittle with a low failure stress when treated under CO. (C) 1997 Published by Elsevier Science Limited All rights reserved.