J. Bouix et al. /Composites Science and Technology 61(2001)355-362 as B,C/SiC and B4C/TiB, double layers [7]. in order to fabrication of performant aluminium matrix composites increase the fibre resistance against oxidation and to with a fibre-volume fraction of 0.50 by a squeeze-casting ensure good wetting of the fibres by liquid aluminium. technique. The tensile strength is multiplied by a factor The process involves two successive RCVD steps of 3. In these materials aluminium carbide is no The presence of these thin carbide coatings is able to detected at the fibre /metal interface. w down considerably the gasification of carbon fibres The technique is not limited to modify only the carbon during an oxidation exposure and their reactivity with fibre surface, it has been applied to surface treatment of liquid aluminium. For instance, the curves of thermo- Hi-Nicalon fibres: a thin layer of Si3 N4 has been gravimetric analysis (TGA)shown in Fig 3 confirm the obtained by reaction between silicon carbide and low oxidation resistance of the pristine T300 fibre ammonia gas at a temperature higher than 1000C heated under oxygen atmosphere at 600oC. They also prove that BC single layer and B4C/SiC double layer have better protective behaviour than SiC single layer 3. Coatings with a double function gainst oxidation. Furthermore, the use of fibres coated by boron carbide or by silicon carbide has permitted The protective coatings used as interphases in MMCs re generally carbides, nitrides or oxides, i.e. brittle materials which crack at a low level of strength. when the coating thickness is up to a critical value 'ecrit it has been shown that the tensile strength of fibres decreases when the coating thickness increases. The ecrit value depends on fibre type and the adhesion strength between the fibre and the coating; however, it does not depend on the coating composition. Typically, ecrit about 16 nm for a T300 fibre. These very thin coatings are protective only when the fabrication technique of MMCs requires a very short contact time between the fibre and the heated metal When a thicker interphase is ed, the same pro- blen tered in MMCs and cmcs. i.e. the crack formation in the brittle component and the pro pagation of the cracks in the fibres. It appears that the presence of a deflector, or a 'mechanical fuse' in brittle properties of the two kinds of composites. It is possible to deflect cracks either at the nanometric scale. for instance between the graphitic planes of pyrolytic carbon Fig. 2. SEM micrograph of the residue obtained from Sic-coated or turbostratic boron nitride, or at the macroscopic T300 fibre after complete consumption of carbon by oxidation scale, for instance at different interfaces in weakly bonded multi-layered coatings 3. 1. Double layered coating: pyrocarbon/carbide We have developed a new generation of a fibre coat ing. It consists of two stacked layers: a preliminary deposition of pyrocarbon(pyC) by low-pressure CVD technique on the fibre followed by a partial conversion of this carbon layer into carbide by RCvd treatment [8]. The very thin carbon layer between the fibre and the external carbide layer acts like a mechanical fuse. Fibres with a such double-layered coating are chemically inert and are more mechanically resistant than the pristine DO(pristine) T300(SiC) fibres. Data for the tensile tests performed on three 12 fibres and four coatings can be found in Table Time(h) M40 fibres have been coated with a pyrocarbon/silicon Fig 3. Weight losses of T300 fibres coated with carbides as a function carbide(pyC/SiC) dual layer. They have been used as a of time([=600C, pO2=I bar) reinforcing agent in an aluminium matrix ID-compositeas B4C/SiC and B4C/TiB2 double layers [7], in order to increase the ®bre resistance against oxidation and to ensure good wetting of the ®bres by liquid aluminium. The process involves two successive RCVD steps. The presence of these thin carbide coatings is able to slow down considerably the gasi®cation of carbon ®bres during an oxidation exposure and their reactivity with liquid aluminium. For instance, the curves of thermo￾gravimetric analysis (TGA) shown in Fig. 3 con®rm the low oxidation resistance of the pristine T300 ®bre heated under oxygen atmosphere at 600C. They also prove that B4C single layer and B4C/SiC double layer have better protective behaviour than SiC single layer against oxidation. Furthermore, the use of ®bres coated by boron carbide or by silicon carbide has permitted fabrication of performant aluminium matrix composites with a ®bre-volume fraction of 0.50 by a squeeze-casting technique. The tensile strength is multiplied by a factor of 3. In these materials, aluminium carbide is not detected at the ®bre/metal interface. The technique is not limited to modify only the carbon ®bre surface, it has been applied to surface treatment of Hi-Nicalon ®bres: a thin layer of Si3N4 has been obtained by reaction between silicon carbide and ammonia gas at a temperature higher than 1000C. 3. Coatings with a double function The protective coatings used as interphases in MMCs are generally carbides, nitrides or oxides, i.e. brittle materials which crack at a low level of strength. When the coating thickness is up to a critical value `ecrit' it has been shown that the tensile strength of ®bres decreases when the coating thickness increases. The ecrit value depends on ®bre type and the adhesion strength between the ®bre and the coating; however, it does not depend on the coating composition. Typically, ecrit is about 16 nm for a T300 ®bre. These very thin coatings are protective only when the fabrication technique of MMCs requires a very short contact time between the ®bre and the heated metal. When a thicker interphase is required, the same pro￾blems are encountered in MMCs and CMCs, i.e. the crack formation in the brittle component and the pro￾pagation of the cracks in the ®bres. It appears that the presence of a de¯ector, or a `mechanical fuse' in brittle interphase is essential for increasing the mechanical properties of the two kinds of composites. It is possible to de¯ect cracks either at the nanometric scale, for instance between the graphitic planes of pyrolytic carbon or turbostratic boron nitride, or at the macroscopic scale, for instance at di€erent interfaces in weakly bonded multi-layered coatings. 3.1. Double layered coating: pyrocarbon/carbide We have developed a new generation of a ®bre coat￾ing. It consists of two stacked layers: a preliminary deposition of pyrocarbon (pyC) by low-pressure CVD technique on the ®bre followed by a partial conversion of this carbon layer into carbide by RCVD treatment [8]. The very thin carbon layer between the ®bre and the external carbide layer acts like a mechanical fuse. Fibres with a such double-layered coating are chemically inert and are more mechanically resistant than the pristine ®bres. Data for the tensile tests performed on three ®bres and four coatings can be found in Table 1. M40 ®bres have been coated with a pyrocarbon/silicon carbide (pyC/SiC) dual layer. They have been used as a reinforcing agent in an aluminium matrix 1D-composite Fig. 2. SEM micrograph of the residue obtained from SiC-coated T300 ®bre after complete consumption of carbon by oxidation. Fig. 3. Weight losses of T300 ®bres coated with carbides as a function of time (t=600C, pO2=1 bar). J. Bouix et al. / Composites Science and Technology 61 (2001) 355±362 357