EFFECT OF AGING TREATMENTS AT HIGH-TEMPERATURES ON THE MICROSTRUCTUREAND MECHANICAL-BEHAVIOR OF 2D NICALON C/SIC COMPOSITES .2. AGING UNDER CO AND INFLUENCE OF A SIC SEAL-COATING/

2D-Nicalon/C/SiC composites fabricated by CVI either uncoated or prote cted 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 interph ase) and the fiber decomposition products (CO and SiO). The change occ urring in the chemical composition and microstructure of the composite s has been studied by EPMA, AES and TEM/PEELS and in their mechanical behavior by tensile and push-out rests at ambient. The degradation pro cess is slowed down by the CO pressure either applied externally to or generated within the composites as the result of the confinement of S iO/CO by the SiC coating. When aged between 1100 and 1200 degrees C, t he 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 n on-brittle with a low failure stress when treated under CO. (C) 1997 P ublished by Elsevier Science Limited All rights reserved.