Chemical vapor infiltration of the hexagonal boron nitride interphase for SiC fiber-Reinforced HP-Si₃N₄ matrix composite

This work describes the processing of SiC fiber-reinforced Si₃N₄ matrix composites with boron nitride (BN) interphase. The BN interphase was processed by chemical vapor infiltration (CVI) with BF₃/NH₃ gaseous precursors. The BN interphase modification involved the continuous treatment of Hi-Nicalon SiC fibers. The relations between (i) the processing parameters, (ii) the mechanisms controlling the kinetics of the CVI of the BN, and (iii) the structure of the deposited BN are presented. A single-or multi-layer BN interphase can be produced depending on the CVI conditions imposed on the fibers during the continuous process. A surface reaction mechanism controlling the CVI promotes a smooth, isotropic BN coating. An anisotropic BN coating can be produced when the CVI kinetics are controlled by a mass transport mechanism. With a controlled temperature gradient, the BN interphase is then made by stacking successive isotropic and anisotropic layers.