J. Bouix et al. Composites Science and Tech 61(2001)355-3062 the fibre through thin layers deposited by CVD and working as diffusion barriers, and on the matrix com- position or the processing conditions. In CMCs, we have used single or multilayers for controlling the strength of the interfacial bonding and having a good resistance to oxidation 2. Deposit of thin refractory layers on carbon fibres by RCVD The difficulty consists in achieving a thin coating on each individual filament of a bundle constituted of several thousands of single filaments of some micrometer dia 2 um meter. The efficiency of a coating as a diffusion barrier depends on its continuity and its regular thickness along the whole length of each filament. Specific preferential deposit on the external filaments to the detriment of those situated in the tow centre must be avoide The reactive CVD (or RCVD) is seen as a promising way of achieving the surface treatment and to obtain a carbide coating(M,C: SiC, TiC, B C)on carbon fibres The coatings were prepared by heating the fibres in a gas stream carrying hydrogen and the m element of the M,C (SiCl4, TiCla or BCl3, for instance), carbon being taken on the fibre itself. For a similar kind of carbon fibre, the thickness of the coating depends much more upon the nature of the carbide and on temperature than on reaction time. Typically, the RCvd time is about 1 min. The carbide coating grows by carbon diffusion from the fibre through the layer formed already, therefore continuity Fig 1. Chemical behat a filament in two diffe and its regularity are reached even when normal pressu metallic melts: (a) strong intera ALC, formation after is used for the deposition immersion for 15 min at 680C in nium;(b)no reaction and The optimal conditions can be foreseen by thermo no wetting after 5 h immersion at 730.C in pure magnesium. dynamic calculations. The method is based on the total ibb's energy minimization of the MCl- H2/C(graphite) resulting from the coupling of two constituents which systems for a given set of conditions(temperature, gas separately exhibit brittle failure. In an axial tensile test phase composition, mole number of carbon in contact for instance, the interfacial zone must deviate in mode II with I mol of the gas mixture. )and the theoretical results the cracks induced in the matrix, thus deferring the are corroborated with experiments on bulk graphite sub- failure of the fibres and that of the composite itself. This strates and on carbon fibres with different micro- 'mechanical fuse effect can be obtained only if the structures(ex-Pan and ex-Pitch). A detailed description interfacial bonding is not too strong, which allows the of the fibre coating equipment has been given in previous activation of energy-consuming phenomena like fibre/ publications [4-6 matrix decohesion, interfacial sliding or broken fibre The uniformity and the continuity of the coating are extraction On the other hand, if the interfacial bonding confirmed by sEM observation of the oxide shells becomes too weak. a loss of contact and load transfer obtained after oxidation of as-coated carbon fibres in occurs between fibre and matrix. Optimization of the air at a temperature higher than 600oC. The residue interface requires a compromise in which the residual shown in Fig. 2 corresponds to a T300 fibre coated with thermal stress has an important part a Sic layer of 50 nm, after complete consumption of In the Laboratoire des Multimateriaux et Interfaces, carbon. The photograph indicates a thin and con- ve work to optimize the interfaces in fibre-reinforced tinuous shell that replicates the crenulated morphology metallic- and ceramic-matrix composites In MMCs, the of the fibre. This observation is proof of a continuous problem consists mainly in controlling the interfacial che- initial carbide layer. The RCV technique has been mical reactivity by acting both on the surface properties of used to fabricate more complex protective coatings suchresulting from the coupling of two constituents which separately exhibit brittle failure. In an axial tensile test for instance, the interfacial zone must deviate in mode II the cracks induced in the matrix, thus deferring the failure of the ®bres and that of the composite itself. This `mechanical fuse' e€ect can be obtained only if the interfacial bonding is not too strong, which allows the activation of energy-consuming phenomena like ®bre/ matrix decohesion, interfacial sliding or broken ®bre extraction. On the other hand, if the interfacial bonding becomes too weak, a loss of contact and load transfer occurs between ®bre and matrix. Optimization of the interface requires a compromise in which the residual thermal stress has an important part. In the Laboratoire des MultimateÂriaux et Interfaces, we work to optimize the interfaces in ®bre-reinforced metallic- and ceramic-matrix composites. In MMCs, the problem consists mainly in controlling the interfacial che￾mical reactivity by acting both on the surface properties of the ®bre through thin layers deposited by CVD and working as di€usion barriers, and on the matrix com￾position or the processing conditions. In CMCs, we have used single or multilayers for controlling the strength of the interfacial bonding and having a good resistance to oxidation. 2. Deposit of thin refractory layers on carbon ®bres by RCVD The diculty consists in achieving a thin coating on each individual ®lament of a bundle constituted of several thousands of single ®laments of some micrometer dia￾meter. The eciency of a coating as a di€usion barrier depends on its continuity and its regular thickness along the whole length of each ®lament. Speci®c preferential deposit on the external ®laments to the detriment of those situated in the tow centre must be avoided. The reactive CVD (or RCVD) is seen as a promising way of achieving the surface treatment and to obtain a carbide coating (MnC: SiC, TiC, B4C) on carbon ®bres. The coatings were prepared by heating the ®bres in a gas stream carrying hydrogen and the M element of the MnC (SiCl4, TiCl4 or BCl3, for instance), carbon being taken on the ®bre itself. For a similar kind of carbon ®bre, the thickness of the coating depends much more upon the nature of the carbide and on temperature than on reaction time. Typically, the RCVD time is about 1 min. The carbide coating grows by carbon di€usion from the ®bre through the layer formed already, therefore the coating formation is self-regulated and its continuity and its regularity are reached even when normal pressure is used for the deposition. The optimal conditions can be foreseen by thermo￾dynamic calculations. The method is based on the total Gibb's energy minimization of the MClx/H2/C(graphite) systems for a given set of conditions (temperature, gas phase composition, mole number of carbon in contact with 1 mol of the gas mixture...) and the theoretical results are corroborated with experiments on bulk graphite sub￾strates and on carbon ®bres with di€erent micro￾structures (ex-Pan and ex-Pitch). A detailed description of the ®bre coating equipment has been given in previous publications [4±6]. The uniformity and the continuity of the coating are con®rmed by SEM observation of the oxide shells obtained after oxidation of as-coated carbon ®bres in air at a temperature higher than 600C. The residue shown in Fig. 2 corresponds to a T300 ®bre coated with a SiC layer of 50 nm, after complete consumption of carbon. The photograph indicates a thin and con￾tinuous shell that replicates the crenulated morphology of the ®bre. This observation is proof of a continuous initial carbide layer. The RCVD technique has been used to fabricate more complex protective coatings such Fig. 1. Chemical behaviour of a P55 carbon ®lament in two di€erent metallic melts: (a) strong interaction with Al4C3 formation after immersion for 15 min at 680C in pure aluminium; (b) no reaction and no wetting after 5 h immersion at 730C in pure magnesium. 356 J. Bouix et al. / Composites Science and Technology 61 (2001) 355±362