Chemical and mechanical alterations of SiC Nicalon fiber properties during the CVD/CVI process for boron nitride

Abstract

To improve the interfacial properties in SiC/SiC composites, BN is an appropriate interphase material to control the fiber/matrix bond. Unfortunately, the gaseous phase (NH3,BF3,HF,Ar) used to deposit BN acts aggressively upon Si-C-O (ex-PCS) Nicalon fiber surfaces, and weakens that bond through the formation of a complex interfacial sequence (SiO2/C), which actually controls the localization of debonds. The reactions between each gas involved and the fiber surface have been studied. Further, if the fiber surface consists of SiC or any silicon-containing compound, the BF3 gas reacts through a substitution of the silicon by boron in the initial fiber composition. Then, the surface evolves from a (C,O,Si) composition to a (B,C,O,Si) glassy layer. Such a reaction occurs mainly before the BN nucleation, and it alters the reinforcing potential of fibers in composites. This boron-containing glass is shown to be very unstable in the presence of HF gas (the main reaction product).