J. Bouix et al. /Composites Science and Technology 61(2001)355-362 of the very thin pyc layers or the stacking of when SiC is heated with pure aluminium, an invar- weakly-bonded layers of TiC. The SEM photograph in iant transformation involving four phases occurs at Fig. 6 demonstrates the presence of microcracks in the 650+3 C, i.e. at 10oC below the melting point of the carbide layers and their deflection at C/TiC interfaces metal. This transformation, which is of the quasi-peri tectic type, can be written a 3.3. Multilayers with different microstructures SiC+ Alsz2Al4C3+ L Another concept is proposed to produce fibre coat- ings and interphases in CMCs. It rests on a stacking of where Als designates metallic aluminium in the solid layers of the same chemical composition, providing that state and Lo an aluminium-rich Al/Si/ c liquid contain- successive layers have a different microstructure, for ing 1.5 at. of silicon and about I at ppm of carbon instance, a lamellar structure and an isotropic one BN is a component which fits that interphase type [16] According to the temperature during the deposition by low-pressure CVD process(LPCVD), boron nitride exhibits a more or less pronounced microtexture and hence, mechanical property anisotropic. We have shown that the use of a furnace with temperature gradients allows the deposition of a stacking of isotropic and anisotropic layers by a continuous process The SEM photographs shown in Figs. 7 and & corre spond to Hi-Nicalon fibre coated with a Bn trilayer deposited under the following conditions: BF3/NH mixture, fibre speed of 0.5 m/h. The fractured section observation of the same fibre demonstrates a decohesion between the coating and the fibre, between isotropic and anisotropic layers and in the isotropic layer The weak bonding between the fibre and coating causes the conservation of the fibre mechanical propertie 4. Interface reactivity control in carbon/aluminium and carbon/magnesium composites Fig. 6. Polished section of fibres coated by a(pyC Tic) multi-layer 4.1. Carbon/aluminium composites As pointed out in the Introduction, the main problem 1.2 in these composites is to avoid an excessive degradation of the reinforcing fibres by chemical reaction with the metal matrix during fabrication by melt-infiltration. To solve this problem, thin layers of the refractory carbides SiC, TiC and B, C have been deposited at the surface of the fibres by the rcvd process previously described. Pres- sure-infiltration of these coated fibres by liquid aluminium has resulted in composites with improved mechanical properties, showing thereby that the carbide coatings could effectively protect the underlying fibre from alu minium attack. To acquire a thorough understanding of nis protecting effect and render possible a better control of the chemical reactivity at the matrix/coating interface a detailed investigation of the chemical interactions in the al c/si Ti and Al/ c/B ternary systems has been carried out a thermodynamic approach of the chemical interac- tions in the al/c/si system under atmospheric pressure has revealed two important features [17]: Fig. 7. BN tri-layered coatingfuse of the very thin pyC layers or the stacking of weakly-bonded layers of TiC. The SEM photograph in Fig. 6 demonstrates the presence of microcracks in the carbide layers and their de¯ection at C/TiC interfaces. 3.3. Multilayers with di€erent microstructures. Another concept is proposed to produce ®bre coat￾ings and interphases in CMCs. It rests on a stacking of layers of the same chemical composition, providing that successive layers have a di€erent microstructure, for instance, a lamellar structure and an isotropic one. BN is a component which ®ts that interphase type [16]. According to the temperature during the deposition by low-pressure CVD process (LPCVD), boron nitride exhibits a more or less pronounced microtexture and, hence, mechanical property anisotropic. We have shown that the use of a furnace with temperature gradients allows the deposition of a stacking of isotropic and anisotropic layers by a continuous process. The SEM photographs shown in Figs. 7 and 8 corre￾spond to Hi-Nicalon ®bre coated with a BN trilayer deposited under the following conditions: BF3/NH3 mixture, ®bre speed of 0.5 m/h. The fractured section observation of the same ®bre demonstrates a decohesion between the coating and the ®bre, between isotropic and anisotropic layers and in the isotropic layer. The weak bonding between the ®bre and coating causes the conservation of the ®bre mechanical properties. 4. Interface reactivity control in carbon/aluminium and carbon/magnesium composites 4.1. Carbon/aluminium composites As pointed out in the Introduction, the main problem in these composites is to avoid an excessive degradation of the reinforcing ®bres by chemical reaction with the metal matrix during fabrication by melt-in®ltration. To solve this problem, thin layers of the refractory carbides SiC, TiC and B4C have been deposited at the surface of the ®bres by the RCVD process previously described. Pres￾sure-in®ltration of these coated ®bres by liquid aluminium has resulted in composites with improved mechanical properties, showing thereby that the carbide coatings could e€ectively protect the underlying ®bre from alu￾minium attack. To acquire a thorough understanding of this protecting e€ect and render possible a better control of the chemical reactivity at the matrix/coating interface, a detailed investigation of the chemical interactions in the Al/C/Si, Al/C/Ti and Al/C/B ternary systems has been carried out. A thermodynamic approach of the chemical interac￾tions in the Al/C/Si system under atmospheric pressure has revealed two important features [17]: . when SiC is heated with pure aluminium, an invar￾iant transformation involving four phases occurs at 6503C, i.e. at 10C below the melting point of the metal. This transformation, which is of the quasi-peri￾tectic type, can be written as: SiC ‡ Als ÿ!ÿAl4C3 ‡ L0 …1† where Als designates metallic aluminium in the solid state and L0 an aluminium-rich Al/Si/C liquid contain￾ing 1.5 at.% of silicon and about 1 at.ppm of carbon; Fig. 6. Polished section of ®bres coated by a (pyC /TiC) multi-layer. Fig. 7. BN tri-layered coating. J. Bouix et al. / Composites Science and Technology 61 (2001) 355±362 359