wwceramics. org/ACT and a very low wear. They are now proposed as an op- microstructure scale, a toughness of the order of 10 kj/ tion for cars in Europe and could be extended to other m, and a good fatigue resistance. They creep at high fields(trains, lifts, etc. temperature but with a creep rate that is lower than that Finally, Sic-matrix composites might become key of metals. Their thermal conductivity is relatively high structural materials in high temperature nuclear reactors when their constituents are well crystallized and their of the future(e.g, the tokamak fusion reactors in which residual porosity is low. Their oxidation resistance is heat generated by the fusion reaction is extracted better at 1000-1200oC than at lower temperatures due through the wall of the toroidal plasma chamber with to the formation of protective oxide scales(SiO2, a cooling agent such as gaseous helium, at a temperature B2O3), in the so-called passive regime. It can be im of 800-1000C), on the basis of their refractoriness, proved with multilayered self-healing matrices and coat HT-mechanical properties, high thermal conductivity, ings, based on the formation of Auid healing oxides. The and more importantly low activation under radia- oxidation resistance is degraded in wet atmospheres tion.36-39 This domain is becoming an active field of where specific EBCs needed to be used research, the main concerns being: the combined effect SiC-matrix composites are matured enough to be of temperature and radiation on the structure(with ei- utilized in a variety of applications including the hot ther a shrinkage or swelling) and mechanical properties structures of spacecraft, aerojet engines, and gas turbines of both SiC fiber and matrix, the nature of the inter- of cogeneration, some parts being already in volume (thick layers of carbon or BN being inappropriate production, with a weight gain of a 50% versus su under radiation), the effect of activable impurities in the peralloys and a durability that can be several thousands composites, the thermal conductivity, and the corrosion of hours. They are also promising materials for braking by residual or in situ formed gaseous species(oxygen or systems and HT nuclear reactors of the future helium in fusion reactors). It appears from preliminary data that the composites should be better fabricated with fibers and matrix consisting of crystalline p-SiC Acknowledgments with a low impurity content (additives introduced as The author acknowledges the collaboration during sintering aids in fibers or/and matrix being a subject of many years of the senior researchers from LCTS and concern), with either porous SiC,(PyC-SiC)m, or(BN- engineers from Snecma and CEA when he was in charge SiC)n multilayered interphases of low C or BN content of the research program on SiC-matrix composites at and displaying a low residual porosity(for hermeticity LCTS and the support of CNRS, Snecma, CEA, and thermal conductivity considerations). Further and Bordeaux University, and the Aquitaine Regional mentioned previously, large size structures could be Authority. CVI/RMI combined ick ing technologies(PIP/RMI or bricated by alread chniques and joinin References 1. F. Christin, R. Naslain, an SiC-matrix composites, i.e., C/SiC and SiC/SiC, d H. Lydrin. The Electrochemical can be fabricated from different carbon or sic fibers playing a variety of properties, usually by single(such 2. R Naslain, J. Y. Rossignol, P. Hagenmuller, F. Christin, L. Heraud, and J. ). lry. "Synthesis and Properties of New Composite Materials for H as Cvi)or combined processes(e.g, PIP/RMI or CVI RMD). They are damage-tolerant when the fibers and 3. I.J. Brennan, "Interfacial Characterization of Glass and Glass-ceras matrix are bonded together with an interphase that can atrix/Nicalon SiC Fiber Composites, Tailoring Multiphase and Comp be a single relatively thick layer of pyrocarbon (or BN) Ceramie. eds. R. E. Tressler, G. L Messing, C.G. Pantano, and R.E. wham. Plenum Press, New York, 549-570. 1986 a porous single layer of SiC, or an engineered multi- 4. R Naslain, The Design of the Fibre- Marix Interfacial Zone in Ceramic layered (X-Y)n interphase (with X= PyC or BN and y= SiC)offering more design Flexibility. The compos- 6.R Nalain, "Design, Preparation and Properies of Non-Oxide CMCs for latrix Composites, Actd Metal, 37[10]2567-2583(1989) ites display a nonlinear stress-strain behavior under Application in Engines and Nuclear Reactors: An Overview, Composite Sci ensile loading related to damaging phenomena at the Technol.,6155-170(2004)and a very low wear. They are now proposed as an op￾tion for cars in Europe and could be extended to other fields (trains, lifts, etc.). Finally, SiC-matrix composites might become key structural materials in high temperature nuclear reactors of the future (e.g., the tokamak fusion reactors in which heat generated by the fusion reaction is extracted through the wall of the toroidal plasma chamber with a cooling agent such as gaseous helium, at a temperature of 800–10001C), on the basis of their refractoriness, HT-mechanical properties, high thermal conductivity, and more importantly low activation under radia￾tion.36–39 This domain is becoming an active field of research, the main concerns being: the combined effect of temperature and radiation on the structure (with ei￾ther a shrinkage or swelling) and mechanical properties of both SiC fiber and matrix, the nature of the inter￾phase (thick layers of carbon or BN being inappropriate under radiation), the effect of activable impurities in the composites, the thermal conductivity, and the corrosion by residual or in situ formed gaseous species (oxygen or helium in fusion reactors). It appears from preliminary data that the composites should be better fabricated with fibers and matrix consisting of crystalline b-SiC with a low impurity content (additives introduced as sintering aids in fibers or/and matrix being a subject of concern), with either porous SiC, (PyC–SiC)n, or (BN– SiC)n multilayered interphases of low C or BN content, and displaying a low residual porosity (for hermeticity and thermal conductivity considerations). Further and as mentioned previously, large size structures could be fabricated by already existing technologies (PIP/RMI or CVI/RMI combined techniques and joining). Conclusion SiC-matrix composites, i.e., C/SiC and SiC/SiC, can be fabricated from different carbon or SiC fibers displaying a variety of properties, usually by single (such as CVI) or combined processes (e.g., PIP/RMI or CVI/ RMI). They are damage-tolerant when the fibers and matrix are bonded together with an interphase that can be a single relatively thick layer of pyrocarbon (or BN), a porous single layer of SiC, or an engineered multi￾layered (X–Y )n interphase (with X 5 PyC or BN and Y 5 SiC) offering more design flexibility. The compos￾ites display a nonlinear stress–strain behavior under tensile loading related to damaging phenomena at the microstructure scale, a toughness of the order of 10 kJ/ m2 , and a good fatigue resistance. They creep at high temperature but with a creep rate that is lower than that of metals. Their thermal conductivity is relatively high when their constituents are well crystallized and their residual porosity is low. Their oxidation resistance is better at 1000–12001C than at lower temperatures due to the formation of protective oxide scales (SiO2, B2O3), in the so-called passive regime. It can be im￾proved with multilayered self-healing matrices and coat￾ings, based on the formation of fluid healing oxides. The oxidation resistance is degraded in wet atmospheres where specific EBCs needed to be used. SiC-matrix composites are matured enough to be utilized in a variety of applications including the hot structures of spacecraft, aerojet engines, and gas turbines of cogeneration, some parts being already in volume production, with a weight gain of
50% versus su￾peralloys and a durability that can be several thousands of hours. They are also promising materials for braking systems and HT nuclear reactors of the future. Acknowledgments The author acknowledges the collaboration during many years of the senior researchers from LCTS and engineers from Snecma and CEA when he was in charge of the research program on SiC-matrix composites at LCTS and the support of CNRS, Snecma, CEA, Bordeaux University, and the Aquitaine Regional Authority. References 1. F. Christin, R. Naslain, and C. Bernard, ‘‘A Thermodynamic and Experi￾mental Approach of Silicon Carbide CVD. Application to the CVD-Infil￾tration of Porous Carbon Composites,’’ Proceedings of the 7th International Conference on CVD. eds. T. O. Sedwick and H. Lydtin. The Electrochemical Society, Princeton, 499–514, 1979. 2. R. Naslain, J. Y. Rossignol, P. Hagenmuller, F. Christin, L. Heraud, and J. J. Choury, ‘‘Synthesis and Properties of New Composite Materials for High Temperature Applications Based on Carbon Fibers and C–SiC or C–TiC Hybrid Matrices,’’ Rev. Chim. Mine´rale, 18 544–564 (1981). 3. J. J. Brennan, ‘‘Interfacial Characterization of Glass and Glass–Ceramic Matrix/Nicalon SiC Fiber Composites,’’ Tailoring Multiphase and Composite Ceramics. eds. R. E. Tressler, G. L. Messing, C. G. Pantano, and R. E. Newnham. Plenum Press, New York, 549–570, 1986. 4. R. Naslain, ‘‘The Design of the Fibre–Matrix Interfacial Zone in Ceramic Matrix Composites,’’ Composites Part A, 29A 1145–1155 (1998). 5. A. G. Evans and D. B. Marshall, ‘‘The Mechanical Behavior of Ceramic Matrix Composites,’’ Acta Metall., 37 [10] 2567–2583 (1989). 6. R. Naslain, ‘‘Design, Preparation and Properties of Non-Oxide CMCs for Application in Engines and Nuclear Reactors: An Overview,’’ Composite Sci. Technol., 64 155–170 (2004). www.ceramics.org/ACT SiC-Matrix Composites: Application 83