Design and processing of high temperature CMCs shrinkage of the matrix occurring during the pyrolysis [54-57]. This important drawback can be overcome by loading the precursor with an inert filler(such as a fine powder of the matrix) or/and by repeating several times(n=6 to 10)the oregnation/curing/ pyrolysis sequence(hence increasing the processing time and our cost). Conversely, this process relies on simple technology 3.1.3. Slurry impregnation and hot pressing (SIHP). The SIHP-process can be used to fabricate a variety of CMCs. In a first step, a fiber tow(or tape)is impregnated with a slurry, i.e. a stable suspension of the matrix in a liquid containing a fugitive binder and various additives, and taken up on a drum, yielding after drying a prepreg-type material. After cutting and stacking, the prepreg layer assembly is hot-pressed in order to yield a dense matrix composite [40. Two important requirements should be fulfilled. First, the fibers should be stable under the hot pressing conditions, limiting the choice to the HM-carbon fibers and Hi-Nicalon type fibers. Second, the matrix should soften and fow during hot-pressing, which is typically the case for glass-ceramic matrices, as already mentioned, the hot pressing temperature being 1200-1400C. Conversely, sintering additives have to be added to the slurry, for refractory non-oxide matrices, such as SiC or Si3N4, hot pressing being performed at much higher temperatures (up to 1700-1800C, for SiC). with some risk of fiber degradation [58-611 3. 2. Gas phase routes In the gas phase routes, referred to as chemical vapor infiltration(CVI), the matrix is formed within the preform pore network, from a gaseous precursor, e.g.a hydrocarbon for carbon. CVI-processes involve mass transfer of reactants and products in the porous medium and homogeneous/heterogeneous reactions(Fig. 4) An important requirement to achieve a high degree of densification, is that the pore network remains open. CVD/CVI processes yield ceramic deposits of high quality from a structural and mechanical standpoint 3. 2./. Isothermal/isobaric CVI(I-CVI). In I-CVI. the reactant/product mass transfers in the preform are mainly by diffusion. In order to favor in-depth deposition, the process has to be conducted at low temperature(800-1000C)and low pressure(a few kPa or 10 kPa) to lower the reaction kinetics and enhance the mean free paths of the molecules in the porous preform, which hence poses advantages and drawbacks [62]. A variety of fibers can be used, even those of relatively low thermal stability, such as Nicalon-type or transition alumina fibers The fibers are not damaged as long as they are chemically compatible with the gas phase. The process is highly flexible but relatively slow, i.e. a large number of preforms(eventually of different sizes and shapes) can be treated simultaneously The composites usually display some residual porosity and a density gradient I-CVI can be used to deposit a variety of interphase and matrix materials(non