The chemical and mechanical properties of SiC-based fibers arechanged during the CVIprocessing of Hi-Nicalon (or Hi- Nicalon-S)fBN/SiC composites as a result of chemical reactionsbetween the gas phase and the fiber surface. The effects of heattreatments under argon, BF3 or NH3 (BN-precursors) on the fibers arestudied under temperature-pressure conditions typically used inBN-CVl (900-1000 degC, l-10 kpa) on the basis of AES, XPS analysesand mechanical tensile tests on single falements. BF3 is the mostaggressive species, Iowering the tensile failure stress of the fiberby 60 for Hi-Nicalon and 40 for Hi-Nicalon-S (which is protected by athin layer of carbon present at the fiber surface in the as-receivedmaterial). The effect of NH3 is more limited (34 and 20 strengthdecrease for HiNicalon and Hi-Nicalon-S, respectively). Surprisingly,a heat treatment in argon also significantly lowers the fiberstrength. This effect might result from an incomplete stabilizationof the fibers during their processing.
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