Chemical and mechanical degradation of Hi-Nicalon and Hi-Nicalon-S fibres under CVD/CVI BN processing conditions

摘要

The chemical and mechanical properties of SiC-based fibers are changed during the CVD-processing of Hi-Nicalon (or Hi-Nicalon-S)/BN/SiC composites as a result of chemical reactions between the gas phase and the fiber surface. The effects of heattreatments under argon, BF{sub}3 or NH{sub}3 (BN-precursors) on the fibers are studied under temperature-pressure conditions typically used in BN-CVI (900-1000°C; 1-10 kPa) on the basis of AES, XPS analyses and mechanical tensile tests on singlefalements. BF{sub}3 is the most aggressive species, lowering the tensile failure stress of the fiber by 60% for Hi-Nicalon and 40% for Hi-Nicalon-S (which is protected by a thin layer of carbon present at the fiber surface in the as-received material).The effect of NH{sub}3 is more limited (34% and 20% strength decrease for Hi-Nicalon and Hi-Nicalon-S, respectively). Surprisingly, a heat treatment in argon also significantly lowers the fiber strength. This effect might result from an incompletestabilization of the fibers during their processing.