6 R. Masai posite. Under such conditions, interphases with an improved oxidation resistance should be employed, A first approach is to select a material displaying intrinsi cally a high oxidation resistance, ideally a layered refractory oxide compatible with both the fiber and the matrix. Indeed, the number of such oxide interphases is ex- tremely limited. Known examples are magnetoplumbite-type oxides, e.g. hibonite CaAl12O19), or mica-type oxides such as fluorophlogopite KMg3 (SiAlOo F2 or related phyllosiloxides(KMg2 Al(Si4 )012 [26-29]. Moreover, the deposition of a layered oxide thin film with a controlled microtexture(the layers being parallel to the fiber surface) by sol-gel or CVD/CVI techniques, is not straightforward. The thermal stability of layered oxides at high temperatures is often limited, which is typically the case for micas and related materials. Another potential interphase material is hexagonal BN which has a layered crystal structure similar to that of raphite but whose oxidation starts at about 850.C and is passive(formation of condensed B2O3)[30]. Hex-BN can be deposited by CVD/CVI from a variety of gaseous precursors, the most commonly used being BX3-NH3 mixtures(with X=F, CD. It is often turbostratic(as its pyrocarbon counterpart)and even amor phous, depending on the nature of the precursor and the T-P conditions. Highly ordered hex-BN has been deposited at low temperatures from BF3-NH3 but under conditions which are aggressive for SiC-based fibers(Fig 1b)131. However, hex BN is sensitive to moisture when poorly crystallized [32]. A second approach is to design self-healing multilayered(X-Y)n interphases, combining at a nm-scale a compliant X material such as pyrocarbon with a stiff glass-former Y, such as SiC (Fig. Ic), the formation of the glass at a high enough temperature, protecting the mechanical fuse against oxidation [26, 33]. The concept can even be extended to X-Y sequences in which the mechanical fuse is a glass-former itself, an example being the recently proposed (BN-SiC), multilayered interphases [34]. Finally, in a few CMCs, such as the C/C composites, an interphase is not used, the mechanical fuse being the FM interface itself which has been weakened enough by, for example, thermal treatment at very high temperature 2.3. Matrix and seal-coating Ceramic matrices also fall into two categories, namely the non-oxide and the oxide matrices. Among the first family, the carbon matrix (associated with carbon fibers) is the most commonly used material on the basis of cost, processing and properties considerations, C/C are by far the most developed CMCs and the only materials used in volume production. The carbon matrix can be formed either from iquid or gaseous precursors, with a variety of microtextures and hence a variety of properties [35, 36]. Isotropic carbon displays low mechanical characteristics Highly anisotropic carbons, such as those deposited by CVD/CvI with the so-cal rough laminar(RL) microtexture, exhibit after a graphitization treatment beyond 2000C. a high thermal conductivity The advantage of SiC and Si3N4 matrices lies in their oxidation resistance(they are protected by a silica scale up to 1500-1600 C). However, most CMCs ar