S Jacques et al. Journal of the European Ceramic Society 20(2000)1929-1938 force(93 N) below the apparent proportional limit (111 deflect the matrix cracks. The key role of the tow tra- N). In these conditions, the composite failure results velling rate has been confirmed. Indeed, the variation of from a rapid oxidation of the Pyc interphase by air the substrate temperature and the infiltrated gaseous oxygen into CO(g)and/or CO2(g).21 phase depends upon this rate. As evidenced by tEM Concerning the other batches, the protection of the the thickness, the textures, the homogeneity and the N interphase and consequently of the minicomposite morphology of the different sublayers that constitute fibres toward oxidation is due to the formation of a the whole final interphase hence depend on it, and, as a B2O3 layer. This glass acts as a physical barrier against mater of fact, the global composite properties. Further oxygen. But, this protection depends on the moisture more, a too slow passing through raises the fibre to a content. Indeed, water reacts with B2O, to give volatile too high temperature, the fibre surface undergoes a lo boric acids HBO2(g), H3BO3 (g), and H3,O6(g), which crystallisation which results in an intense debonding decreases the protecting effect of the oxide layer Finally, thermal ageing tests in dry or moist air under For batch 3, the matrix cracks are deflected close to a static loading have confirmed that the BN interphases at about 250 nm(Fig 14, top). Although this behaviour compared with a classical pyrocarbon ntepnomposites ne interphase/matrix interface i.e. far"from the fibre significantly improve the lifetimes of the minico is not favourable to the mechanical properties; it allows in return, the fibre to be protected from air. For batch 2, the interphase/matrix bonding involves important tear- References ng. Finally, cracks allows air to come nearer to the fibre (middle of Fig. 14)than in the case of batch 3, which 1. Evans, A. G and Zok, F w, The physics and mechanics of decreases the lifetime. as for batch 1. the weak link is fibre-reinforced brittle matrix composites. J. Mater. Sci., 1994, ocated in the fibre surface(Fig 14, bottom); the fibre is 29,3857-3896 of course quickly exposed; which results in an even 2. Rebillat, F, Guette, A, Spit shorter lifetime Naslain, R, Oxidation resistance of SiC/SiC minicomposites with a highly crystallised BN interphase. J. Eur. Ceram Soc., 1998, 18, At this point, it is not possible to determine the best Ninterphase. Indeed, even if the oxidation resistance is 3. Rebillat, F,Guette, A.and Brosse, C.R,Chemical and considerably improved compared with the reference, the echanical alterations of Sic Nicalon fiber properties during the best minicomposites at room temperature(batch 2)de CVD/CVI process for boron nitride. Acta Mater. 1999. 47(5) not result in the best lifetimes at high temperature( those of batch 3). Hot dynamic cycle fatigue tests would allow Bouix. J. Continuous elaboration of boron nitride on SiC (Hi- to settle because the mechanical behaviour would play an Nicalon) fiber. Textures of deposited BN and properties of coated even more important part than in static test fibers. In Proceedings of the IIth Journees Nationales sur les opposites, J. Lamon, and D. Baptiste, D. 18-20 Novemb Arcachon, France, 1998, pp 393-404 5. Lopez-Marure, A, La Realisation et le Comportement dinter. 5. Conclusion phases BN a gradient de Proprietes, Application a des Composites Ceramiques. PhD thesis, University of Claude Bernard Lyon 1 ith a structural gradient were France. 1999 pared by using a one-step continuous dynamic CVI 6. Olivier, C, Elaboration et Etude du comportement mecanique de process within 1-D SiC/SiC minicomposites. The fibre tow travelled through a reactor that contains different Institut National des Sciences Appliquees of Lyon, France, 1998 7. Bertrand, S, Forio, P, Pailler, R and Lamon, J, Hi-Nicalon / SiC hot areas(TG-Cvi process). The infiltration occurred minicomposites with(Pyrocarbon/SiC)n Nanoscale multilayered while keeping the same conditions of gaseous phase nterphases. J. Am. Ceram Soc., 1999. 82(9), 2465-2476 (pressure, flow rates, composition) during the whole 8. Lamon, J, Rebillat, F. and Evans, A G, Microcomposite test experiment. A judicious choice of travelling rate com- procedure for evaluating the interface properties of ceramic bined with a well adapted temperature profile in the matrix composites. J. Am. Ceram. Soc., 1995, 78(2), 401-405. 9. Marshall. D. B. and Evans. A. G. Failure Mechanisms in Cera. sceptor allowed to obtain minicomposites with good Fiber/Ceramic-Matrix Composites. J. Am. Cera. Soc properties 1985.68(5),225-231 Indeed, tensile tests at room temperature have shown 10. Aveston, J, Cooper, G.A. and Kelly, A. Single and multiple that the BN interphases act as mechanical fuses and that fracture. In Proceedings of the Conference of the National Physical Laboratory, the Properties of Fiber Composites, IPC Science and the fibres are not chemically degraded. These good results are obtained thanks to a fibre coating starting in a 11. Lebrun, G.A. and Lamon, J, Influence de la distribution spatiale lowtemperature area. This non-aggressive treatment des fibres au sein d un composite sur le comportement mecanique conditions protect the fibre. Then, while going through a en traction. In Annales des Composites, 1995/4. AMAC, 1995, pp hottest area, the structural anisotropy of the bn infil 121-130. 12. Bertrand, S, Lamon, J, Pailler, R. and Goujard, S, Effect of trated in the tow can increase without fibre chemical eat-treatments on the microstructure and the properties of Hi- attack. This organised bn allows the interphase to Nicalon fibres. J. Eur. Ceram. Soc., submitted for publicationforce (93 N) below the apparent proportional limit (111 N). In these conditions, the composite failure results from a rapid oxidation of the PyC interphase by air oxygen into CO(g) and/or CO2(g).21 Concerning the other batches, the protection of the BN interphase and consequently of the minicomposite ®bres toward oxidation is due to the formation of a B2O3 layer. This glass acts as a physical barrier against oxygen. But, this protection depends on the moisture content. Indeed, water reacts with B2O3 to give volatile boric acids HBO2(g), H3BO3(g), and H3B3O6(g), which decreases the protecting e€ect of the oxide layer.22 For batch 3, the matrix cracks are de¯ected close to the interphase/matrix interface i.e. ``far'' from the ®bre at about 250 nm (Fig. 14, top). Although this behaviour is not favourable to the mechanical properties; it allows, in return, the ®bre to be protected from air. For batch 2, the interphase/matrix bonding involves important tear￾ing. Finally, cracks allows air to come nearer to the ®bre (middle of Fig. 14) than in the case of batch 3, which decreases the lifetime. As for batch 1, the weak link is located in the ®bre surface (Fig. 14, bottom); the ®bre is of course quickly exposed; which results in an even shorter lifetime. At this point, it is not possible to determine the best BN interphase. Indeed, even if the oxidation resistance is considerably improved compared with the reference, the best minicomposites at room temperature (batch 2) do not result in the best lifetimes at high temperature (those of batch 3). Hot dynamic cycle fatigue tests would allow to settle because the mechanical behaviour would play an even more important part than in static tests. 5. Conclusion BN interphases with a structural gradient were pre￾pared by using a one-step continuous dynamic CVI process within 1-D SiC/SiC minicomposites. The ®bre tow travelled through a reactor that contains di€erent hot areas (TG-CVI process). The in®ltration occurred while keeping the same conditions of gaseous phase (pressure, ¯ow rates, composition) during the whole experiment. A judicious choice of travelling rate com￾bined with a well adapted temperature pro®le in the susceptor allowed to obtain minicomposites with good properties. Indeed, tensile tests at room temperature have shown that the BN interphases act as mechanical fuses and that the ®bres are not chemically degraded. These good results are obtained thanks to a ®bre coating starting in a ``low'' temperature area. This non-aggressive treatment conditions protect the ®bre. Then, while going through a hottest area, the structural anisotropy of the BN in®l￾trated in the tow can increase without ®bre chemical attack. This organised BN allows the interphase to de¯ect the matrix cracks. The key role of the tow tra￾velling rate has been con®rmed. Indeed, the variation of the substrate temperature and the in®ltrated gaseous phase depends upon this rate. As evidenced by TEM, the thickness, the textures, the homogeneity and the morphology of the di€erent sublayers that constitute the whole ®nal interphase hence depend on it, and, as a mater of fact, the global composite properties. Further￾more, a too slow passing through raises the ®bre to a too high temperature, the ®bre surface undergoes a local crystallisation which results in an intense debonding. Finally, thermal ageing tests in dry or moist air under a static loading have con®rmed that the BN interphases signi®cantly improve the lifetimes of the minicomposites compared with a classical pyrocarbon interphase. References 1. Evans, A. G. and Zok, F. W., The physics and mechanics of ®bre-reinforced brittle matrix composites. J. Mater. Sci., 1994, 29, 3857±3896. 2. Rebillat, F., Guette, A., Espitalier, L., Debieuvre, C. and Naslain, R., Oxidation resistance of SiC/SiC minicomposites with a highly crystallised BN interphase. J. Eur. Ceram Soc., 1998, 18, 1809±1819. 3. Rebillat, F., Guette, A. and Brosse, C. R., Chemical and mechanical alterations of SiC Nicalon ®ber properties during the CVD/CVI process for boron nitride. Acta Mater., 1999, 47(5), 1685±1696. 4. Vincent, H., Lopez-Marure, A., Lamouroux, F., Vincent, C., and Bouix, J., Continuous elaboration of boron nitride on SiC (Hi￾Nicalon) ®ber. Textures of deposited BN and properties of coated ®bers. In Proceedings of the 11th JourneÂes Nationales sur les composites, J. Lamon, and D. Baptiste, D. 18±20 November, Arcachon, France, 1998, pp. 393±404. 5. Lopez-Marure, A., La ReÂalisation et le Comportement d'Inter￾phases BN aÁ Gradient de ProprieÂteÂs, Application aÁ des Composites CeÂramiques. PhD thesis, University of Claude Bernard Lyon 1, France, 1999. 6. Olivier, C., Elaboration et Etude du Comportement MeÂcanique de Composites Unidirectionnels C/Si3N4 et SiC/Si3N4. PhD thesis, Institut National des Sciences AppliqueÂes of Lyon, France, 1998. 7. Bertrand, S., Forio, P., Pailler, R. and Lamon, J., Hi-Nicalon/SiC minicomposites with (Pyrocarbon/SiC)n Nanoscale multilayered interphases. J. Am. Ceram. Soc., 1999, 82(9), 2465±2476. 8. Lamon, J., Rebillat, F. and Evans, A. G., Microcomposite test procedure for evaluating the interface properties of ceramic matrix composites. J. Am. Ceram. Soc., 1995, 78(2), 401±405. 9. Marshall, D. B. and Evans, A. G., Failure Mechanisms in Cera￾mic-Fiber/Ceramic-Matrix Composites. J. Am. Ceram. Soc., 1985, 68(5), 225±231. 10. Aveston, J., Cooper, G.A. and Kelly, A., Single and multiple fracture. In Proceedings of the Conference of the National Physical Laboratory, the Properties of Fiber Composites, IPC Science and Technology Press Ltd, Surrey, UK, 1971, pp. 15±26. 11. Lebrun, G.A. and Lamon, J., In¯uence de la distribution spatiale des ®bres au sein d'un composite sur le comportement meÂcanique en traction. In Annales des Composites, 1995/4. AMAC, 1995, pp. 121±130. 12. Bertrand, S., Lamon, J., Pailler, R. and Goujard, S., E€ect of heat-treatments on the microstructure and the properties of Hi￾Nicalon ®bres. J. Eur. Ceram. Soc., submitted for publication. S. Jacques et al. / Journal of the European Ceramic Society 20 (2000) 1929±1938 1937