Ceramic matrix composite materials with long fibers reinforcement

Ceramic materials are hard but brittle. A great deal of work has been devoted to improving the toughness of ceramics, notably by dispersing particles or short fibers in the ceramic to deflect cracks or split them. It is also possible to produce tough ceramic materials by manufacturing composite materials with a ceramic matrix (CMC) and long fibers. The reinforcement provided by these fibers ensures the material's mechanical properties, while the ceramic matrix protects them from the environment. The various CMCs are designated by two headings separated by a slash (C/C, C/SiC, SiC/SiC...), the first designating the fiber material/the second that of the matrix. When the matrix includes additional phases, the main component of the matrix is mentioned.

These composites, which are mainly used in applications involving extreme conditions, are known as "thermostructural", meaning that they can be used as structural materials at high temperatures, and must therefore retain their mechanical properties under operating conditions. They are currently used mainly in the aerospace and nuclear industries. Depending on the application, they are subjected to operating temperatures ranging from 400 to over 2,000°C in oxidizing atmospheres (air, reactor combustion gases, etc.). They can be subjected to stresses ranging from simple vibrations to multiaxial stresses. Finally, in the nuclear field, they can be subjected to irradiation. The fibers, which are essentially made of carbon, silicon carbide or oxides (alumina, basalt, mullite), bear most of the applied load. They must therefore have the highest possible breaking strength and elastic modulus, combined with good fatigue resistance and low density (for aerospace applications). Depending on the type of application, they must also have good resistance to oxidation, creep and critical cracking (propagation of a crack, usually by corrosion, for applied stress intensity factors lower than the macroscopic stresses that trigger damage), high thermal conductivity and be compatible with the interphase or matrix, during processing or use. They can be woven, braided or assembled in the form of semi-finished products (felts, unidirectional webs, etc.), which constitute a preform for the final part. This preform is densified by a ceramic matrix, making the part dense and protecting the fibers from environmental aggression. Unlike conventional composites (organic or metal matrix CMO or CMM), in CMC the modulus of elasticity of the matrix is greater than or equal to that of the fiber, and the strain at break of the matrix is less than the strain at break of the fiber.

Ceramic materials are brittle, however, and a third component is introduced into these composites to "defragilize" them. This is a thin, easily-cleavable interphase between the...