Carbon fiber-reinforced YMaS glass- ceramic-matrix composites-IV fracture is less brittle and the decohesion between the references fibers and the matrix is wider(a100 nm). This is not a consequence of the influence of the residual thermal I. Bianchi, v. Sinkler. w. Goursat P. Monthioux. m. and stresses, because the outlines of the interface are not at Menessier, E, Carbon fiber-reinforced (YMAS)glass- all parallel, but very likely because of physico-chemical trength. J. Eur. Ceram. Soc., 1997, 17, 1485-1500 reactions between the fiber and the matrix, a release of V, Sinkler, w, Goursat, P, Monthioux, M. and nitrogen, occurring when the matrix is still viscous dur Menessier, E, Carbon fiber-reinforced (YMAS) glass- ing the pressing, prevents good adhesion between the ceramic matrix composites. Ill. Fiber-matrix interfaces fibers and the matrix, just as the possible oxidation of J. Eur. Ceram. Soc., accepted the fibers by the matrix at high temperature could lead 3. Gault, C, Ultrasonic non-destructive evaluation of widening of the decohesion. Except in delamination microstructural changes and degradation of ceramics at perature. Ir planes where some bundles of fibers are extracted, aterials Properties, ed. J. Holbrook and J. Buissier which could explain the more controlled rupture, the MRS Symposium Proceedings, Vol. 142, 1989, Pp 263-274 racture surfaces do not demonstrate any characteristic 4. Rouxel,T. Ceramiques de type nitrure de silicium et increase in the pull-out lengths. In fact, some contact erres azotes Ph. D. thesis, Limoges University, france zones are still maintained at this interface which hin- 5. Lataillade, J. L and Pouyet, J, Lois de comportement ders sliding during push-in tests and mechanical tests notions elementaires sur le calcul previ Additionally, the corrugated texture of T400H fibers'is materiaux composites. In Introduction aux materiaux likely to prevent the extraction of fibers composites, Tome 2: Matrices metalliques et ceramique, o ed R. Naslain. CNrS Frammniques ultrasonores du com- portement de composites ceramique- ceramique soumis a 5 CONCLUSION des solicitations thermiques. Ph. D. thesis, Limoges Uni- versity, France, 1987. The mechanical behavior of carbon-fiber-reinforced 7. Hasselman, D. P, and Singh, J. P, Analysis of thermal YMAS-matrix composites depends on their fabrication stress resistance of microcracked brittle ceramics, Ceram Bul.1979.58.856860 conditions. Indccd, the reactivity of carbon fibers with 8. Cutard, T, Caracterisation ultra- sonore a haute tem- the matrix and the thermal residual stresses influence perature et sous contrainte de traction de composites cer- the nature and strength of the fiber/matrix interface amique-ceramique. Ph. D. thesis, Limoges university, The calculation of residual thermal stresses requires the france, 1993 knowledge of the transverse and longitudinal coeffi- 9. Schapery, R. A, Thermal expansion coefficients of com- cients of thermal expansion of the pitch-based P25 fiber posite materials based on cncrgy principles. J, Compos Mater,1968,2,380404 and the PaN-based T400H fiber. which were deter 10. Rojstaczer, S, Cohn, D. and Marom, G, Thermal mined from dilatometric measurements. According to expansion of Kevlar fibres and composites. J. Mater. Sci Let,1985,4,1233-1236 the sintering thermal cycle, each parameter can play a 11. Hashin, Z and Rosen, B. W. The elastic moduli more or less predominant role. The observation of the interface by TEM, the microindentation of fibers, the 12. Rosen, B. w, Thermomechanical properties of fi 3132u analysis of changes in Youngs modulus and the calcu- composites. Proc. R. Soc. London, 1970, A319, 79-83 lation of thermal stresses have permitted a clarification 13. Hsueh, C. H. and Becher, P. F, Thermal expansion of the contribution of each parameter. It is shown that, coefficients of unidirectional fiber orced at low temperature(when the matrix is still vitreous), Am Ceram. Soc.1988. 71. C438-C44 14. Mikata, Y and Toya, M, Stress field in a coated con- the effects of thermal stresses are dominant. For higher tinuous fiber composite subjected to thermomechanical temperatures, the reactivity between the carbon fibers loadings, J. Compos. Mater, 1985, 19, 554-572 and the matrix is associated with the effects of residual 15. Timoshenko, S. and Goodier, J N, Theorie de r'elasticite stresses, inducing greater debondings Librairie Polytechnique Ch. Beranger, Paris, Liege, The main conclusions of this study are exploited to 16. Sinkler w. Monthioux. M. Bianchi. v. Goursat Menessier, E, Carbon fiber-reinforced YMAs glass- explain the behavior of carbon-fiber-reinforced YMAS- ceramic matrix composites. II: Structural changes in the matrix composites in dry friction. 4+ In particular, the matrix with temperature. J. Eur. Ceram. Soc., accepted friction coefficient changes and the formation of a third Tell. w D. G. and Apstein body at the interface between the rubbing parts are C.S., Exponential temperature dependence of Youngs rclated to the presence of microcracks and the cohesion modulus for several oxides. Phys. Rev., 1961, 122, 1754- of the fibers to the matrix 18. Bowles, D. E and Tompkins, S. S, Prediction of coeffi- ACKNOWLEDGEMENT 19. Wolff, E. G, Min, B. K. and Kural, M. H, Thermal ycling of a unidirectional graphite magnesium compo- The assistance of H. Lemercier(Laboratoire de materi- site.J. Mater.Sci,1985,20,1141-1149 20. Guild, F.J. Davy, P.J. and Hogg. P J, A model for aux Ceramiques-ENSCD) for Youngs modulus mea surements is gratefully acknowledged compression. Compos. Sci. TechnoL, 1989, 36, 7-26Carbon-fiber-reinforced YMAS glass-ceramic-matrix composites-IV 417 fracture is less brittle and the decohesion between the fibers and the matrix is wider (x 100 nm). This is not a consequence of the influence of the residual thermal stresses, because the outlines of the interface are not at all parallel, but very likely because of physico-chemical reactions between the fiber and the matrix. A release of nitrogen, occurring when the matrix is still viscous dur￾ing the pressing, prevents good adhesion between the fibers and the matrix, just as the possible oxidation of the fibers by the matrix at high temperature could lead to widening of the decohesion. Except in delamination planes where some bundles of fibers are extracted, which could explain the more controlled rupture, the fracture surfaces do not demonstrate any characteristic increase in the pull-out lengths. In fact, some contact zones are still maintained at this interface, which hin￾ders sliding during push-in tests and mechanical tests. Additionally, the corrugated texture of T400H fibers’ is likely to prevent the extraction of fibers. 5 CONCLUSION The mechanical behavior of carbon-fiber-reinforced YMAS-matrix composites depends on their fabrication conditions. Indeed, the reactivity of carbon fibers with the matrix and the thermal residual stresses influence the nature and strength of the fiber/matrix interface. The calculation of residual thermal stresses requires the knowledge of the transverse and longitudinal coeffi￾cients of thermal expansion of the pitch-based P25 fiber and the PAN-based T400H fiber, which were ‘deter￾mined from dilatometric measurements. According to the sintering thermal cycle, each parameter can play a more or less predominant role. The observation of the interface by TEM, the microindentation of fibers,* the analysis of changes in Young’s modulus and the calcu￾lation of thermal stresses have permitted a clarification of the contribution of each parameter. It is shown that, at low temperature (when the matrix is still vitreous), the effects of thermal stresses are dominant. For higher temperatures, the reactivity between the carbon fibers and the matrix is associated with the effects of residual stresses, inducing greater debondings. The main conclusions of this study are exploited to explain the behavior of carbon-fiber-reinforced YMAS￾matrix composites in dry friction.24 In particular, the friction coefficient changes and the formation of a third body at the interface between the rubbing parts are related to the presence of microcracks and the cohesion of the fibers to the matrix. ACKNOWLEDGEMENT The assistance of H. Lemercier (Laboratoire de Mattri￾aux Ctramiques-ENSCI) for Young’s modulus mea￾surements is gratefully acknowledged. REFERENCES 1. 2. 3. 4. 5. 6. 1 0. 1. 2. 3. 4. 15. 16. 17. 18. 19. 20. Bianchi, V., Sinkler, W., Goursat, P., Monthioux, M. and Menessier, E., Carbon fiber-reinforced (YMAS) glass￾ceramic matrix composites. I. Preparation, structure and fracture strength. J. Eur. Cerum. Sot., 1997,17, 1485-1500. Bianchi, V., Sinkler, W., Goursat, P., Monthioux, M. and Menessier, E., Carbon fiber-reinforced (YMAS) glass￾ceramic matrix composites. III. Fiber-matrix interfaces, J. Eur. Ceram. Sot., accepted. Gault, C., Ultrasonic non-destructive evaluation of microstructural changes and degradation of ceramics at high temperature. In Non-destructive Monitoring of Materials Properties, ed. J. Holbrook and J. Buissier. MRS Symposium Proceedings, Vol. 142, 1989, pp. 263-274. Rouxel, T., Ceramiques de type nitrure de silicium et verres azotes. Ph.D. thesis, Limoges University, France, 1990. Lataillade, J. L. and Pouyet, J., Lois de comportement: notions Clementaires sur le calcul previsionnel dans les materiaux composites. In Introduction aux materiaux composites, Tome 2: Matrices metalliques et ceramiques, ed. R. Naslain. CNRS, France, 1979, pp. 142-152. Lamidieu, P., Etude par techniques ultrasonores du com￾portement de composites ceramiqueceramique soumis a des sollicitations thermiques. Ph.D. thesis, Limoges Uni￾versity, France, 1987. Hasselman, D. P. and Singh, J. P., Analysis of thermal stress resistance of microcracked brittle ceramics. Ceram. BUN., 1979, 58, 856860. Cutard, T., Caracterisation ultra-sonore a haute tem￾perature et sous contrainte de traction de composites c&r￾amiqueciramique. Ph.D. thesis, Limoges University, France, 1993. Schapery, R. A., Thermal expansion coefficients of com￾posite materials based on energy principles. J. Compos. Mater., 1968, 2, 38&404. Rojstaczer, S., Cohn, D. and Marom, G., Thermal expansion of Kevlar fibres and composites. J. Mater. Sci. Lett., 1985, 4, 1233-1236. Hashin, Z. and Rosen, B. W., The elastic moduli of fiber￾reinforced materials. J. Appl. Mech., 1964, 31, 223-232. Rosen, B. W., Thermomechanical properties of fibrous composites. Proc. R. Sot. London, 1970, A319, 79-83. Hsueh, C. H. and Becher, P. F., Thermal expansion coefficients of unidirectional fiber-reinforced ceramics. J. Am. Ceram. Sot., 1988,71, C4388C441. Mikata, Y. and Toya, M., Stress field in a coated con￾tinuous fiber composite subjected to thermomechanical loadings. J. Compos. Mater., 1985, 19, 554572. Timoshenko, S. and Goodier, J. N., Theorie de l’elasticite. Librairie Polytechnique Ch. B&anger, Paris, Liege, 1961. Sinkler, W., Monthioux, M., Bianchi, V., Goursat, P. and Menessier, E., Carbon fiber-reinforced YMAS glass￾ceramic matrix composites. II: Structural changes in the matrix with temperature. J. Eur. Cerum. Sot., accepted. Watchman, J. B., Tefft, W. E., Lam, D. G. and Apstein, C. S., Exponential temperature dependence of Young’s modulus for several oxides. Phys. Rev., 1961, 122, 1754 1759. Bowles, D. E. and Tompkins, S. S., Prediction of coeffi￾cients of thermal expansion for unidirectional composites. J. Compos. Mater., 1989, 23, 37&388. Wolff, E. G., Min, B. K. and Kural, M. H., Thermal cycling of a unidirectional graphite magnesium compo￾site. J. Mater. Sci., 1985, 20, 1141-l 149. Guild, F. J., Davy, P. J. and Hogg, P. J., A model for unidirectional composites in longitudinal tension and compression. Compos. Sci. Technol., 1989, 36, 7-26