Design and processing of high temperature CMCs preforms, fabricated by weaving processes. Such n D-fiber architectures are well suited to rapid processing techniques and complex loading schemes [24, 25 2.2. Interphase materials In most CMCs, an interphase is used to control the FM-bonding during processing It is a key constituent with several functions, including FM load transfer, matrix microcrack deflection (the so-called fuse function) and diffusion barrier (in reactive FM systems). The design of the FM-interfacial zone in a given CMC for a given application is somewhat difficult, and it is rarely performed in an optimized manner. The best interphase materials might be those with a layered structure or microstructure, the layers being parallel to the fiber surface and weakly bonded to one another. and the whole interphase strongly adherent to the fiber [, 26]. The most commonly employed interphase consists of a thin layer(less than I um)of anisotropic pyrocarbon deposited on the fiber surface by CVD/CVI from a suitable hydrocarbon(Fig. la Pyrocarbon is an excellent interphase material from a mechanical standpoint However, it is oxidation prone. The oxidation of carbon starts at about 450C. It occurs with formation of gaseous species exclusively (CO or/and CO2)and hence with a weight loss, the material being progressively consumed (active oxidation ). It is also anisotropic and strongly depending on the occurrence of specific impuritie (such as alkali or alkaline earth cations) often present in carbonaceous materials. When used in a SiC/SiC composite exposed to an oxidizing atmosphere under cyclic loading, a pyrocarbon interphase can be consumed with a dramatic change in the FM interfacial bonding and hence in the overall mechanical behavior of the com- Figure 1. Interphases in SiC/SiC composites: (a) anisotropic pyrocarbon; (b) hex-BN: and (c)(PyC-SiC)1o multilayers according to [73].131I. and [33 respectively