J. Bouix et al. Composites Science and Technology 61(2001)355-362 attacked at a slower rate than the disordered graphite constituting low. graphitized pan base fibres. BC coatings can then protect such fibres against aluminium attack during melt-infiltration. Another advantage to the use of B, C coatings would be that the ternary carbide Al3 BC eventually produced at the B. C/matrix interface is far less sensitive to hydrolysis than Al4C3 4.2. Carbon /magnesium composites Carbon fibres have been shown to be chemically inert in the presence of pure solid or liquid magnesium up to at least 730C [20]. Moreover, pure liquid magnesium Fig 9. The interface between M40B carbon filaments and a Mg/zr does not wet carbon(Fig. 1b). Then, contrarily to the alloy(0. 1 -).2 at y zr)after immersion for I5 min at 680.C: reactive preceding case, the main problem in the fabrication of wetting and formation of a ZrCr layer about 0.25 um in thickness. carbon/magnesium composites by melt-infiltration is to promote a controlled reaction at the metal/ fibre inter face in order to improve wetting and bonding Aluminium is the main addition element of the most commonly used magnesium foundry alloys AZ61 or AZ91(6 or 9 wt. of aluminium and I wt. of zinc). It was then logical to examine first the chemical behaviour of carbon fibres in the presence of liquid Mg/Al alloy Experiments realized at 730@C have shown that a che- mical reaction proceeds at the metal/fibre interface as soon as little amounts (less than lat. %)of aluminium are added to magnesium. The reaction product is not the expected aluminium carbide Al,C3, but a ternary carbide of magnesium and aluminium with the chemica Fig. 10. Crack propagation along the C/ZrCx interface in a M40B formula Al2MgC2 [21]. From an applied point of view, Mg/Zr composite promoting voluntarily such an interface reaction is not however, very interesting. Effectively, as Al4C3, the with the formation of a thin and continuous layer of ternary carbide Al, MgC? is formed via a dissolutio zirconium carbide ZrCx at the fibre surface(Fig. 9) precipitation mechanism in which deep dissolution pits Accordingly, this particular behaviour can be regarded are dug at the fibre surface, without any noticeable as a result of a reactive wetting process. The growth of improvement of wetting. Moreover, this ternary carbide the ZrCx layer proceeds by unidirectional solid-state is hydrolysed in humid air more rapidly than Al4C diffusion of carbon through it. As a consequence, Kir- Manganese is another carbide forming element often kendall voids are formed at the interface between the present in magnesium alloys at a level of about I at. % outer part of the carbon fibre and the surrounding ZrCx When carbon fibres are immersed in such liquid Mg-Mn layer. A weakened zone is then created in which cracks alloys, a continuous layer of manganese carbides can be deviated when a heavy stress is applied to the (Mn23 C6, Mns C2 and Mn, C3) is formed at the fibre material(Fig. 10). The rate at which the ZrCr layer surface via a crossed solid-state diffusion process. This grows is much slower than for Mg/Mn alloys. This renders growth process is more favourable to the preservation possible a precise control of the interface reactivity and of the mechanical properties of the fibres than a dis- consequently, of the strength of the bond between the solution/precipitation mechanism. However, carbides carbon fibre and the Mg/Zr alloy matrix formation proceeds at a rate too fast to be easily con trolled The most interesting results in terms of fibre/matrix 5. Conclusion interface tailoring have been obtained when zirconium was added to magnesium [22]. It has effectively been The optimization of the interface between fibre and observed that a liquid Mg-Zr alloy saturated in zirco- matrix is the essential condition for obtaining high per- nium(0.18 at. Zr at 730 C)can spread rapidly at the formance inorganic composite materials. With this carbon fibre surface and penetrate easily within the fila- objective, one can act on the surface state of the rein- ments of a fibre yarn, without severe damage to these forcement, on the matrix composition or on the processing filaments. Spreading and penetration are associated conditionsattacked at a slower rate than the disordered graphite constituting low-graphitized PAN base ®bres. B4C coatings can then protect such ®bres against aluminium attack during melt-in®ltration. Another advantage to the use of B4C coatings would be that the ternary carbide Al3BC eventually produced at the B4C/matrix interface is far less sensitive to hydrolysis than Al4C3. 4.2. Carbon/magnesium composites Carbon ®bres have been shown to be chemically inert in the presence of pure solid or liquid magnesium up to at least 730C [20]. Moreover, pure liquid magnesium does not wet carbon (Fig. 1b). Then, contrarily to the preceding case, the main problem in the fabrication of carbon/magnesium composites by melt-in®ltration is to promote a controlled reaction at the metal/®bre inter￾face in order to improve wetting and bonding. Aluminium is the main addition element of the most commonly used magnesium foundry alloys AZ61 or AZ91 (6 or 9 wt.% of aluminium and 1 wt.% of zinc). It was then logical to examine ®rst the chemical behaviour of carbon ®bres in the presence of liquid Mg/Al alloys. Experiments realized at 730C have shown that a che￾mical reaction proceeds at the metal/®bre interface as soon as little amounts (less than 1at.%) of aluminium are added to magnesium. The reaction product is not the expected aluminium carbide Al4C3, but a ternary carbide of magnesium and aluminium with the chemical formula Al2MgC2 [21]. From an applied point of view, promoting voluntarily such an interface reaction is not, however, very interesting. E€ectively, as Al4C3, the ternary carbide Al2MgC2 is formed via a dissolution/ precipitation mechanism in which deep dissolution pits are dug at the ®bre surface, without any noticeable improvement of wetting. Moreover, this ternary carbide is hydrolysed in humid air more rapidly than Al4C3. Manganese is another carbide forming element often present in magnesium alloys at a level of about 1 at.%. When carbon ®bres are immersed in such liquid Mg-Mn alloys, a continuous layer of manganese carbides (Mn23C6, Mn5C2 and Mn7C3) is formed at the ®bre surface via a crossed solid-state di€usion process. This growth process is more favourable to the preservation of the mechanical properties of the ®bres than a dis￾solution/precipitation mechanism. However, carbides formation proceeds at a rate too fast to be easily con￾trolled. The most interesting results in terms of ®bre/matrix interface tailoring have been obtained when zirconium was added to magnesium [22]. It has e€ectively been observed that a liquid Mg-Zr alloy saturated in zirco￾nium (0.18 at.% Zr at 730C) can spread rapidly at the carbon ®bre surface and penetrate easily within the ®la￾ments of a ®bre yarn, without severe damage to these ®laments. Spreading and penetration are associated with the formation of a thin and continuous layer of zirconium carbide ZrCx at the ®bre surface (Fig. 9). Accordingly, this particular behaviour can be regarded as a result of a reactive wetting process. The growth of the ZrCx layer proceeds by unidirectional solid-state di€usion of carbon through it. As a consequence, Kir￾kendall voids are formed at the interface between the outer part of the carbon ®bre and the surrounding ZrCx layer. A weakened zone is then created in which cracks can be deviated when a heavy stress is applied to the material (Fig. 10). The rate at which the ZrCx layer grows is much slower than for Mg/Mn alloys. This renders possible a precise control of the interface reactivity and, consequently, of the strength of the bond between the carbon ®bre and the Mg/Zr alloy matrix. 5. Conclusion The optimization of the interface between ®bre and matrix is the essential condition for obtaining high per￾formance inorganic composite materials. With this objective, one can act on the surface state of the rein￾forcement, on the matrix composition or on the processing conditions. Fig. 9. The interface between M40B carbon ®laments and a Mg/Zr alloy (0.15±0.2 at.% Zr) after immersion for 15 min at 680C: reactive wetting and formation of a ZrCx layer about 0.25 mm in thickness. Fig. 10. Crack propagation along the C/ZrCx interface in a M40B/ Mg/Zr composite. J. Bouix et al. / Composites Science and Technology 61 (2001) 355±362 361