It is worth addressing the processing method first because this information is useful for a better under­standing of the structure of SiC/SiC. The manufac­ture of long fiber-reinforced composites requires three main steps14’15’28’29:

1. preparation of fibrous preform,

2. fiber coating’ which provides an interface material (interphase), and

3. infiltration of the matrix.

2.12.3.1 Fibrous Preform

The preforms of SiC/SiC composites are made of refractory SiC-based continuous fibers. The latest near-stoichiometric SiC fibers (such as Hi-Nicalon type S and Tyranno-SA3 fibers) are the most appro­priate for those CVI SiC/SiC foreseen for nuclear applications. These fibers exhibit high strength, high stiffness, low density, and high thermal and chemical stability to withstand long exposures at high tempera — tures.30 Finally, the fiber diameter must be small (<20 pm) so that the fibers can be woven easily.

The fiber preforms may consist of

1. A simple stack of unidirectional fiber layers or fabrics (1D or 2D preforms).

2. A multidirectional fiber architecture (3D preforms). Weaving in four or five directions can also be used.

The 2D layers are stacked and kept together using a tool or using fibers in the orthogonal direction (3D preforms).