Availableonlineatwww.sciencedirect.com DIRECTO COMPOSITES SCIENCE SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 64(2004)155-170 www.elsevier.com/locate/compscitech Review article Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview* R. Naslain* aboratoire des Composites Thermostructuraux, University Bordeaux I, Domaine Universitaire, 3 Allee de La boetie, 33600 Pessac, france Received 30 April 2003; accepted 3 June 200 Dedicated to Professor Anthony G. Evans Sic-based ceramic matrix composites, consisting of carbon or SiC fibers embedded in a Sic-matrix, are tough ceramics when the fiber/matrix bonding is properly optimized through the use of a thin interphase. They are fabricated according to different proces- sing routes(chemical vapor infiltration, polymer impregnation/pyrolysis, liquid silicon infiltration or slurry impregnation /hot pressing) each of them displaying advantages and drawbacks which are briefly discussed. Sic-matrix composites are highly tailor- ble materials in terms of fiber-type(carbon fibers of Sic-based fibers such as Si-C-o. SiC +C or quasi-stoichiometric SiC rein- forcements), interphase (pyrocarbon or hexagonal BN, as well as(Pyc-SiC)n or (BN-SiC)n multilayered interphases), matrix (simple Sic or matrices with improved oxidation resistance, such as self-healing matrices)and coatings( SiC or engineered multi layered coatings). The potential of Sic-matrix composites for application in advanced aerojet engines(after-burner hot section), gas turbine of electrical power/steam cogeneration(combustion chamber) and inner wall of the plasma chamber of nuclear fusion reaction, all of them corresponding to very severe conditions is discussed. C 2003 Elsevier Ltd. All rights reserved Keywords: A. Ceramic-matrix composites(CMC); SiC-matrix composites: B Interphases Contents 1. Introduction 2. Processing 156 2.1. The gas phase route 156 2.2. The liquid phase routes 2.3. The ceramic route 2.4. Hybrid processes 3. Material design 3. 1. Fibers .160 3.2. Interphases 3.3. Matrices 3. 4. Coat w Presented at Multifunctional Materials and Structures: Present Status and Future Perspectives a Symposium in Honor of A.G. Evans on the occasion of his 60th birthday, Max-Planck Institute fur Metallforschung, Stuttgart, 16-20 March 2003 *Tel:+33-5-5684-4706;fax:+33-5-5684-1225 E-Imail address: admin(a Icts. u-bordeaux fr(R. Naslain). .3538/S- see front matter C 2003 Elsevier Ltd. All rights reserved 10.1016/S0266-3538(03)00230=6
Review Article Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview§ R. Naslain* Laboratoire des Composites Thermostructuraux, University Bordeaux 1, Domaine Universitaire, 3 Alle´e de La Boe´tie, 33600 Pessac, France Received 30 April 2003; accepted 3 June 2003 Dedicated to Professor Anthony G. Evans Abstract SiC-based ceramic matrix composites, consisting of carbon or SiC fibers embedded in a SiC-matrix, are tough ceramics when the fiber/matrix bonding is properly optimized through the use of a thin interphase. They are fabricated according to different processing routes (chemical vapor infiltration, polymer impregnation/pyrolysis, liquid silicon infiltration or slurry impregnation/hot pressing) each of them displaying advantages and drawbacks which are briefly discussed. SiC-matrix composites are highly tailorable materials in terms of fiber-type (carbon fibers of SiC-based fibers such as Si–C–O, SiC+C or quasi-stoichiometric SiC reinforcements), interphase (pyrocarbon or hexagonal BN, as well as (PyC–SiC)n or (BN–SiC)n multilayered interphases), matrix (simple SiC or matrices with improved oxidation resistance, such as self-healing matrices) and coatings (SiC or engineered multilayered coatings). The potential of SiC-matrix composites for application in advanced aerojet engines (after-burner hot section), gas turbine of electrical power/steam cogeneration (combustion chamber) and inner wall of the plasma chamber of nuclear fusion reaction, all of them corresponding to very severe conditions is discussed. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Ceramic-matrix composites (CMC); SiC-matrix composites; B. Interphases 0266-3538/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0266-3538(03)00230-6 Composites Science and Technology 64 (2004) 155–170 www.elsevier.com/locate/compscitech Contents 1. Introduction ............................................................................................................................................................................... 156 2. Processing................................................................................................................................................................................... 156 2.1. The gas phase route........................................................................................................................................................... 156 2.2. The liquid phase routes ..................................................................................................................................................... 158 2.3. The ceramic route.............................................................................................................................................................. 158 2.4. Hybrid processes ............................................................................................................................................................... 159 3. Material design........................................................................................................................................................................... 160 3.1. Fibers................................................................................................................................................................................. 160 3.2. Interphases ........................................................................................................................................................................ 161 3.3. Matrices............................................................................................................................................................................. 162 3.4. Coatings............................................................................................................................................................................. 163 § Presented at Multifunctional Materials and Structures: Present Status and Future Perspectives a Symposium in Honor of A.G. Evans on the occasion of his 60th birthday, Max-Planck Institute fu¨r Metallforschung, Stuttgart, 16–20 March 2003. * Tel.: +33-5-5684-4706; fax: +33-5-5684-1225. E-mail address: admin@lcts.u-bordeaux.fr (R. Naslain)
R. Naslain/ Composites Science and Technology 64(2004)155-170 4. Examples of potential application 4.1. Aerojet engines and stationary gas turbines ++,,+++ 4.2. Nuclear fusion reactors 5. Summary. l67 References 1.Introduction ing atmospheres(combustion gas, for example) which is required of the applications previously Non-oxide CMCs, i.e. mainly those consisting of a tioned, being of several hundreds or thousands hours Sic-based matrix reinforced with either carbon or and even more. This problem has been addressed via the Sic-fibers and which will be referred to as C/SiC and design of innovative self-healing interphases and matri- SiC/SiC composites, have been extensively studied dur- s ough the use of specific coatings, with the ing the last two decades since their discovery in the mid result that the durability of these CMCs in sever seventies [1-3]. These tough ceramics have the potential environments is now good enough for applications in for being used up to about 1500oC, as structural mate- aeronautic engines [7, 8 rials, in different fields including advanced engines, gas The use of SiC/SiC composites in high temperature turbines for power/steam co-generation, heat exchan- nuclear reactors of the future, in place of monolithic gers, heat treatment and materials growth furnaces, as SiC, such as the first wall, blanket and divertor of well as nuclear reactors of the future nuclear fusion reactor, is another very challenging The main advantage of CMCs with respect to their potential application requiring among others a high monolithic counterparts lies in the fact that they are thermal conductivity, an excellent hermeticity with tough although their constituents are intrinsically brit- respect to gases(cooling gas fluids or gaseous species tle. This key property is achieved through a proper formed by nuclear reactions) and low residual radio- design of the fiber/matrix(FM) interface arresting and activity. Preliminary research in this field appears to be deflecting cracks formed under load in the brittle matrix very encouraging [9, 10 and preventing the early failure of the fibrous reinfor The aim of the present contribution is to give an cement [4. In its classical treatment, crack deflection is overview of the state of the art in the material design, controlled via the deposition of a thin layer of a com- processing and properties control of Sic-matrix during pliant material with a low shear strength, on the fiber the last few years and underlying the weak points which surface, referred to as the interphase and acting as a still require a significant effort of research mechanical fuse (to protect the fiber). It has been pos- tulated that the best interphase materials might be those with a layered crystal structure, such as pyrocarbon 2. Processing (PyC) or hexagonal boron nitride (hex-BN), or a layered microstructure, such as (PyC-SiC)n or Sic-matrix composites are processed according to (BN-SiC)n, the layers being deposited parallel to the (1)a gas phase route, also referred to as chemical vap fiber surface and the interphase strongly bonded to the infiltration(Cvi),(2)a liquid phase route including the fiber [5, 6]. From a mechanical standpoint, these CMCs polymer impregnation/pyrolysis(PIP)and liquid silicon are damageable elastic materials, i.e. when loaded at a infiltration (LSi)also called (reactive)melt infiltration high enough level, microcracking and FM-debonding (RMI or MD) processes, as well as (3)a ceramic route. occur, which are responsible for a stiffness lowering and i.e. a technique combining the impregnation of the non-linear stress-strain behavior. On the one hand, reinforcement with a slurry and a sintering step at high these damaging phenomena are beneficial since they are temperature and high pressure. Each of these routes has at the origin of the non-brittle character of these advantages and draw backs that will be briefly discussed ceramics On the other hand, they are detrimental since they favor the in-depth diffusion of oxygen towards the 2. 1. The gas phase route oxidation-prone interphase and fibers which in turn may embrittle the composites In the gas phase route, the different constituents of Hence, an important challenge has been to improve the composite, i.e. the interphase, the matrix and the the oxidation resistance of these non-oxide CMCs, the external coating, are successively deposited from gas lifetime under load at high temperatures and in oxidiz- eous precursors at moderate temperatures (900-1 100C)
1. Introduction Non-oxide CMCs, i.e. mainly those consisting of a SiC-based matrix reinforced with either carbon or SiC-fibers and which will be referred to as C/SiC and SiC/SiC composites, have been extensively studied during the last two decades since their discovery in the mid seventies [1–3]. These tough ceramics have the potential for being used up to about 1500 C, as structural materials, in different fields including advanced engines, gas turbines for power/steam co-generation, heat exchangers, heat treatment and materials growth furnaces, as well as nuclear reactors of the future. The main advantage of CMCs with respect to their monolithic counterparts lies in the fact that they are tough although their constituents are intrinsically brittle. This key property is achieved through a proper design of the fiber/matrix (FM) interface arresting and deflecting cracks formed under load in the brittle matrix and preventing the early failure of the fibrous reinforcement [4]. In its classical treatment, crack deflection is controlled via the deposition of a thin layer of a compliant material with a low shear strength, on the fiber surface, referred to as the interphase and acting as a mechanical fuse (to protect the fiber). It has been postulated that the best interphase materials might be those with a layered crystal structure, such as pyrocarbon (PyC) or hexagonal boron nitride (hex-BN), or a layered microstructure, such as (PyC–SiC)n or (BN–SiC)n, the layers being deposited parallel to the fiber surface and the interphase strongly bonded to the fiber [5,6]. From a mechanical standpoint, these CMCs are damageable elastic materials, i.e. when loaded at a high enough level, microcracking and FM-debonding occur, which are responsible for a stiffness lowering and non-linear stress–strain behavior. On the one hand, these damaging phenomena are beneficial since they are at the origin of the non-brittle character of these ceramics. On the other hand, they are detrimental since they favor the in-depth diffusion of oxygen towards the oxidation-prone interphase and fibers which in turn may embrittle the composites. Hence, an important challenge has been to improve the oxidation resistance of these non-oxide CMCs, the lifetime under load at high temperatures and in oxidizing atmospheres (combustion gas, for example) which is required in some of the applications previously mentioned, being of several hundreds or thousands hours and even more. This problem has been addressed via the design of innovative self-healing interphases and matrices and through the use of specific coatings, with the result that the durability of these CMCs in severe environments is now good enough for applications in aeronautic engines [7,8]. The use of SiC/SiC composites in high temperature nuclear reactors of the future, in place of monolithic SiC, such as the first wall, blanket and divertor of nuclear fusion reactor, is another very challenging potential application requiring among others a high thermal conductivity, an excellent hermeticity with respect to gases (cooling gas fluids or gaseous species formed by nuclear reactions) and low residual radioactivity. Preliminary research in this field appears to be very encouraging [9,10]. The aim of the present contribution is to give an overview of the state of the art in the material design, processing and properties control of SiC-matrix during the last few years and underlying the weak points which still require a significant effort of research. 2. Processing SiC-matrix composites are processed according to: (1) a gas phase route, also referred to as chemical vapor infiltration (CVI), (2) a liquid phase route including the polymer impregnation/pyrolysis (PIP) and liquid silicon infiltration (LSI) also called (reactive) melt infiltration (RMI or MI) processes, as well as (3) a ceramic route, i.e. a technique combining the impregnation of the reinforcement with a slurry and a sintering step at high temperature and high pressure. Each of these routes has advantages and drawbacks that will be briefly discussed. 2.1. The gas phase route In the gas phase route, the different constituents of the composite, i.e. the interphase, the matrix and the external coating, are successively deposited from gaseous precursors at moderate temperatures (900–1100 C) 4. Examples of potential application.............................................................................................................................................. 163 4.1. Aerojet engines and stationary gas turbines......................................................................................................................163 4.2. Nuclear fusion reactors ..................................................................................................................................................... 165 5. Summary .................................................................................................................................................................................... 167 References ....................................................................................................................................................................................... 168 156 R. Naslain / Composites Science and Technology 64 (2004) 155–170
in/ Composites Science and Technology 64(2004)155-170 and under reduced pressures (or sometimes at the yielding near-net-shape parts(Fig. 1). On the other atmospheric pressure). The starting material is a porous hand, I-CvI is a relatively slow technique since it has to nD-fiber preform(with usually n=2 or 3), self standing be performed at low deposition rate in order to avoid a or maintained with a tooling (at least at the beginning of too rapid sealing of the pore entrance by the deposit the densification process). During the densification steps The densification rate is improved by applying a pres- (CVI-steps), the interphase and then the SiC-matrix, are sure gradient to the preform, i.e. by replacing the deposited on the fiber surface, within the pore network slow diffusion mass transfer by the much faster con of the preform, according to the following overall vection mass transfer within the pore network (PG- equations(written for the main constituents of a Sic- CVI) or by introducing an inverse temperature gra matrix composite dient (TG-CVI) as mentioned above or by combining both of them(F-CVI with F standing for forced) CH3SiCl3g (1) [13-15]. Another efficient way to increase the densifi- cation rate is to immerse the heated fiber preform in a boiling liquid precursor at reflux (calefaction pro- 2CxHy(g) Lx C(s)+ yH (2) cess)[16]. In the so-called pressure-pulsed CVI-pro- cess(P-CVD), the fiber preform is filled and evacuated BXa+ NH BN(s)+3HX(g) periodically, with a residence time of the gaseous pre in the preform of the order of a few seconds Although P-CVI has been first presented as a way to with X=F CI shorten the overall densification duration of the pre- form [17] its main interest may rather be in its ability to The key point is to maintain the preform porosity yield highly engineered interphases or matrices througl open until the end of the densification process the sequencial use of several precursors [Eqs. (1)and(2) [1, 2, 11, 12]. This is achieved by keeping the pore entran- or Eqs. (1)and (3), for example], as further discussed in ces at a low enough temperature, i.e. by applying an the materials design section [18]. However, increasing inverse temperature gradient to the preform, or by per- the deposition rate is often at the expense of the flex- nodical surface machining(to re-open the pore entrances ibility of the process. Finally, the CvI-process whatever as they become sealed by the deposit). The CVI-process its version results in composites which display sig- displays a number of advantages but also a few draw- nificant residual porosity(typically, backs. It yields deposits with a high purity and well- open) and hence a relatively low thermal conductivity controlled composition and microstructure, as further (although SiC is intrinsically an excellent heat con- discussed in the materials design section. In its most ductor) and a poor hermeticity with respect to gas and common version, the I-CVI technique(I standing for liquid fluids. Despite its drawbacks, CVI is a matured isothermal /isobaric), it is a highly flexible process inas- enough processing route for SiC-matrix composites, much as a large number of preforms, eventually of dif- which has been already transferred to the plant level ferent shapes and sizes can be treated simultaneously [ 19] MTS r preform with DSiC/c/SiC vacuum Z Fig. 1. Schematic of the I-CVI process for the fabrication of C/Sic or SiC/SiC composites from a nD-fiber preform and gaseous precursors
and under reduced pressures (or sometimes at the atmospheric pressure). The starting material is a porous nD-fiber preform (with usually n=2or 3), self standing or maintained with a tooling (at least at the beginning of the densification process). During the densification steps (CVI-steps), the interphase and then the SiC-matrix, are deposited on the fiber surface, within the pore network of the preform, according to the following overall equations (written for the main constituents of a SiCmatrix composite): CH3SiCl3ð Þ g ! H2 SiCð Þs þ 3HClð Þ g ð1Þ 2CxHy gð Þ ! 2xCð Þs þ yH2ð Þ g ð2Þ BX3ð Þ g þ NH3ð Þ g ! BNð Þs þ 3HXð Þ g with X ¼ F; Cl ð3Þ The key point is to maintain the preform porosity open until the end of the densification process [1,2,11,12]. This is achieved by keeping the pore entrances at a low enough temperature, i.e. by applying an inverse temperature gradient to the preform, or by periodical surface machining (to re-open the pore entrances as they become sealed by the deposit). The CVI-process displays a number of advantages but also a few drawbacks. It yields deposits with a high purity and wellcontrolled composition and microstructure, as further discussed in the materials design section. In its most common version, the I-CVI technique (I standing for isothermal/isobaric), it is a highly flexible process inasmuch as a large number of preforms, eventually of different shapes and sizes can be treated simultaneously yielding near-net-shape parts (Fig. 1). On the other hand, I-CVI is a relatively slow technique since it has to be performed at low deposition rate in order to avoid a too rapid sealing of the pore entrance by the deposit. The densification rate is improved by applying a pressure gradient to the preform, i.e. by replacing the slow diffusion mass transfer by the much faster convection mass transfer within the pore network (PGCVI) or by introducing an inverse temperature gradient (TG-CVI) as mentioned above or by combining both of them (F-CVI with F standing for forced) [13–15]. Another efficient way to increase the densifi- cation rate is to immerse the heated fiber preform in a boiling liquid precursor at reflux (calefaction process) [16]. In the so-called pressure-pulsed CVI-process (P-CVI), the fiber preform is filled and evacuated periodically, with a residence time of the gaseous precursor in the preform of the order of a few seconds. Although P-CVI has been first presented as a way to shorten the overall densification duration of the preform [17], its main interest may rather be in its ability to yield highly engineered interphases or matrices through the sequencial use of several precursors [Eqs. (1) and (2) or Eqs. (1) and (3), for example], as further discussed in the materials design section [18]. However, increasing the deposition rate is often at the expense of the flexibility of the process. Finally, the CVI-process whatever its version results in composites which display significant residual porosity (typically, 10–15%, mainly open) and hence a relatively low thermal conductivity (although SiC is intrinsically an excellent heat conductor) and a poor hermeticity with respect to gas and liquid fluids. Despite its drawbacks, CVI is a matured enough processing route for SiC-matrix composites, which has been already transferred to the plant level [19]. Fig. 1. Schematic of the I-CVI process for the fabrication of C/SiC or SiC/SiC composites from a nD-fiber preform and gaseous precursors. R. Naslain / Composites Science and Technology 64 (2004) 155–170 157
R. Naslain/ Composites Science and Technology 64(2004)155-170 e route 2D-fabrics or even lD-fiber tows. i.e. to the ceramic route that will be discussed later on [24-27 There are two different liquid phase routes for the In the LSI-process, also referred to as the rMi or MI fabrication of Sic-matrix composites depending on process(depending on whether the infiltration is reactive hether the precursor is a Si-C based polymer, such as a or not), a porous nD-fiber preform(with n=1, 2 or 3)is polycarbosila ne or PCS(PIP-process)or liq uld silicon first consolidated with a carbon deposit by Cvi [Eq(2) pure or alloyed (Lsi or rMi process) or by PIP utilizing in this latter case a liquid carbon pre- In the PIP-process, a fiber preform, which can be a cursor such as a phenolic resin or a pitch(Fig. 2). In a 3D-preform similar to those used in CVI or more sim- second step, the residual open porosity is filled with liquid ply a stack of 2D-fabrics or lD-plies, is impregnated silicon(mp=1410 C)or with a liquid silicon-based alloy, with a Si-C precursor (in the molten state or in solu- which climbs by capillary forces in the pore network [28- tion in an organic solvent), e.g. under vacuum by resin 31]. Liquid silicon and its related alloys spontaneously wet transfer moulding(RTM), a technique commonly used carbon, with which they react according to the following for polymer matrix composites [20-23]. Different pre- equation, written for pure silicon cursors are employed such as polycarbosilanes(PCS)or C)+Sio, SiCs display a low enough viscosity to flow in the pore net- with an evolution of heat and a volume expansion. work of the fiber preform and a high ceramic yield After Despite its apparent simplicity and short processing uring to render the precursor infusible, which is achieved time, the RMi-process raises some difficulties. Firstly hermally or under radiation (y-rays or E-beam), the the infiltration temperature is relatively high(typically, green body is pyrolyzed at a temperature ranging from 1400-1600oC)which means that only the fibers with a 1000 to 1200C(this relatively low temperature being high thermal stability, namely HM carbon fibers or compatible with the use of fibers of limited thermal quasi-stoichiometric SiC fibers prepared at high tem- stability). Assuming that the precursor is a Yajima's peratures [such as Tyranno SA(from Ube Industries, type PCs, the pyrolysis results in a matrix which is a Japan) or Sylramic fibers(from Dow Corning, USA) SiC+C mixture or pure SiC depending on the nature of can be employed. Secondly, liquid silicon is a corrosive he atmosphere, according to the following overall medium with respect to PyC or hex-BN interphases as well as to the fibers themselves. Hence, specific inter [(CH3)SiH-CH21, nSiC+nC+3nH, phases acting both as mechanical fuse(crack deflection) (4) and diffusion barrier, such as dual hex-BN/SiC inter- [(CH3)SiH-CH21- nSiC +nCH +H, phases should be used. Further and as in CVI, the pores entrances should remain open until the end of the den sification process(which is here very fast compared to the ceramic yield being 89.6% and 68.9%, respectively. CVI)requiring specific care in the management of the Actually, the ceramic yield is in between, part of the liquid silicon flow to and in the fiber preform. Finally, carbon being lost as gaseous species even when the s ce, wnIch limits its refractoriness and creep resistance. To the matrix formed by RMi often contains free silicon olysis is conducted under an inert atmosphere. Hend there is a significant shrinkage during pyrolysis. The minimize the free silicon content, different treatments pyrolytic residue is porous and the porosity largely op have been suggested (silicon vaporization at high tem- [the gaseous species formed according to Eqs. (4)or(5) perature under vacuum, leaching treatment or use of have to escape from the composite, creating porosity]. silicon-metal alloys instead of pure silicon, the alloying In order to achieve a high enough densifiction level, element entrapping the silicon in excess in a refractory several impregnation/pyrolysis sequences(typically 6 to silicide, such as MoSi2)[29, 32]. On the other hand, the 10, and even more) have therefore to be performed RMI-process displays some important advantages: it is which is time consuming and costly. One way to reduce a fast densification technique and it yields composites the number of PI-P cycles is to load the liquid polymer with almost no residual open porosity(and hence with precursor with a filler, that is a powder with a fine an excellent hermeticity with respect to gas and liquid granulometry, which can be pure silicon carbide or a fluids)as well as a high thermal conductivity. It is used, mixture of SiC with additives, e.g. a boron-bearing spe- in a complementary manner, with either CVI or PIP, to cies such as boron carbide to entrap oxygen at medium fill the residual porosity inherent to those techniques temperatures in service conditions, as discussed in the materials design section. However, loading the liquid 2.3. The ceramic route precursor with a powder considerably increases its visc- osity and may render impossible the complete impreg. In the ceramic route, the matrix precursor is a slurry, nation of a complex nD-fiber preform. In such a case, i.e. a stable suspension of a p-sic powder in a liquid one has to move to more simple fiber arrangements: which also contains sintering additives and a fugitive
2.2. The liquid phase routes There are two different liquid phase routes for the fabrication of SiC-matrix composites depending on whether the precursor is a Si-C based polymer, such as a polycarbosilane or PCS (PIP-process) or liquid silicon, pure or alloyed (LSI or RMI process). In the PIP-process, a fiber preform, which can be a 3D-preform similar to those used in CVI or more simply a stack of 2D-fabrics or 1D-plies, is impregnated with a Si–C precursor (in the molten state or in solution in an organic solvent), e.g. under vacuum by resin transfer moulding (RTM), a technique commonly used for polymer matrix composites [20–23]. Different precursors are employed such as polycarbosilanes (PCS) or poly (vinylsilanes). The precursor should wet the fibers, display a low enough viscosity to flow in the pore network of the fiber preform and a high ceramic yield. After curing to render the precursor infusible, which is achieved thermally or under radiation (g-rays or E-beam), the green body is pyrolyzed at a temperature ranging from 1000 to 1200 C (this relatively low temperature being compatible with the use of fibers of limited thermal stability). Assuming that the precursor is a Yajima’s type PCS, the pyrolysis results in a matrix which is a SiC+C mixture or pure SiC depending on the nature of the atmosphere, according to the following overall equations: ½ ð Þ CH3 SiH CH2 n ! Ar nSiC þ nC þ 3nH% 2 ð4Þ ½ ! ð Þ CH3 SiH CH2 H2 nSiC þ nCH% 4 þ H% 2 ð5Þ the ceramic yield being 89.6% and 68.9%, respectively. Actually, the ceramic yield is in between, part of the carbon being lost as gaseous species even when the pyrolysis is conducted under an inert atmosphere. Hence, there is a significant shrinkage during pyrolysis. The pyrolytic residue is porous and the porosity largely open [the gaseous species formed according to Eqs. (4) or (5) have to escape from the composite, creating porosity]. In order to achieve a high enough densifiction level, several impregnation/pyrolysis sequences (typically 6 to 10, and even more) have therefore to be performed which is time consuming and costly. One way to reduce the number of PI-P cycles is to load the liquid polymer precursor with a filler, that is a powder with a fine granulometry, which can be pure silicon carbide or a mixture of SiC with additives, e.g. a boron-bearing species such as boron carbide to entrap oxygen at medium temperatures in service conditions, as discussed in the materials design section. However, loading the liquid precursor with a powder considerably increases its viscosity and may render impossible the complete impregnation of a complex nD-fiber preform. In such a case, one has to move to more simple fiber arrangements: 2D-fabrics or even 1D-fiber tows, i.e. to the ceramic route that will be discussed later on [24–27]. In the LSI-process, also referred to as the RMI or MI process (depending on whether the infiltration is reactive or not), a porous nD-fiber preform (with n=1, 2or 3) is first consolidated with a carbon deposit by CVI [Eq. (2)] or by PIP utilizing in this latter case a liquid carbon precursor such as a phenolic resin or a pitch (Fig. 2). In a second step, the residual open porosity is filled with liquid silicon (mp=1410 C) or with a liquid silicon-based alloy, which climbs by capillary forces in the pore network [28– 31]. Liquid silicon and its related alloys spontaneously wet carbon, with which they react according to the following equation, written for pure silicon: Cð Þs þ Sið Þl ! SiCð Þs ð6Þ with an evolution of heat and a volume expansion. Despite its apparent simplicity and short processing time, the RMI-process raises some difficulties. Firstly, the infiltration temperature is relatively high (typically, 1400–1600 C) which means that only the fibers with a high thermal stability, namely HM carbon fibers or quasi-stoichiometric SiC fibers prepared at high temperatures [such as Tyranno SA (from Ube Industries, Japan) or Sylramic fibers (from Dow Corning, USA)] can be employed. Secondly, liquid silicon is a corrosive medium with respect to PyC or hex-BN interphases as well as to the fibers themselves. Hence, specific interphases acting both as mechanical fuse (crack deflection) and diffusion barrier, such as dual hex-BN/SiC interphases should be used. Further and as in CVI, the pores entrances should remain open until the end of the densification process (which is here very fast compared to CVI) requiring specific care in the management of the liquid silicon flow to and in the fiber preform. Finally, the matrix formed by RMI often contains free silicon which limits its refractoriness and creep resistance. To minimize the free silicon content, different treatments have been suggested (silicon vaporization at high temperature under vacuum, leaching treatment or use of silicon–metal alloys instead of pure silicon, the alloying element entrapping the silicon in excess in a refractory silicide, such as MoSi2) [29,32]. On the other hand, the RMI-process displays some important advantages: it is a fast densification technique and it yields composites with almost no residual open porosity (and hence with an excellent hermeticity with respect to gas and liquid fluids) as well as a high thermal conductivity. It is used, in a complementary manner, with either CVI or PIP, to fill the residual porosity inherent to those techniques. 2.3. The ceramic route In the ceramic route, the matrix precursor is a slurry, i.e. a stable suspension of a b-SiC powder in a liquid which also contains sintering additives and a fugitive 158 R. Naslain / Composites Science and Technology 64 (2004) 155–170
R. Naslain/ Composites Science and Technology 64(2004)155-170 888 =皇 → filtrate 4001450cto 882 si+C→Sc) g a Sic-matrix composite by the liquid silicon infiltration process:(a) preparation of a 2D-fiber preform by a prepreg route b)consolidation of the material by pyrolysis and(c)infiltration of liquid silicon, according to Corman et al. [30] binder. The reinforcement, e. g. a continuous fiber tow cesses are sometimes used to opitimize the densification (uncoated or coated with an appropriate interphase)is of fiber preforms or/and the microstructure of the com- impregnated with the slurry and wound on a drum, posites. Firstly, CvI (in its I-CVi or P-CVI versions)is yielding a 1D-prepreg-type intermediate product. After the method of choice for the deposition of simple or drying, the layers are stacked in the die of a unidirec- highly engineered interphases whatever the technique(s) tional press and the composite sintered at high tem- further employed for the infiltration of the Sic-matrix perature under pressure [33] since it yields deposits of relatively uniform composi- It is well known that the sintering of SiC powder is tion, structure and thickness, even with preforms of difficult and requires very high temperatures, even in the complex fiber architecture. Secondly and as previously presence of sintering aids. Furthermore, since it is per- mentioned, LSI (in its MI or RMi versions)is utilized to formed here under pressure (to achieve low residual fill the residual open porosity of composites prepared porosity), the combined effect of high temperature and either by CVI or PIP, in order to increase both the high pressure was considered for a long time as a source thermal conductivity and hermeticity of the composites of too severe fiber degradation and this route more or Finally, more complex flow charts have been suggested less disregarded. However, it has been shown recently to densify nD-preforms combining, for example, an that the use of nanosized p-Sic powder(with a particu- impregnation with a Sic powder slurry (to introduce late size of N30 nm) and oxide sintering additives rapidly a significant amount of Sic in the preform), a few (Al,O3 and Y2O3)forming transient eutectics with Sio at relatively low temperature, considerably helps the sintering of the SiC-matrix which was effective at Coated fiber fabric M1780oC and 15-20 MPa. Further, the HM carbon fibers utilized in the first experiments [33] could be eplaced by the quasi-stoichiometric SiC fibers whose high thermal stability is compatible with these some- impregnation what mild sintering conditions. SiC/PyC/SiC compo- sites fabricated by this So-called NITE process(for nano filtration and transient eutectics) display a very low residual porosity, high mechanical and thermal proper ties as well as an excellent hermeticity with respect to gaseous cooling fluids such as helium(Fig 3)[34-36]. It Pressure then appears that this process could be attractive if the fiber volume fraction (presently: 20%) could be increased, the relatively thick PyC interphase(800 nm) replaced by a more appropriate material and the 1D-fiber architecture by a multidirectional fiber arrangement Products 2. 4. Hybrid processes Fig. 3. Flow chart of the NITE-process for the fabrication of sic Each of the processes discussed previously displaying matrix composites from nanosize Sic particle slurry with oxide sin advantages and drawbacks, hybrid (or combined) pro- tering additives and pressure sintering, according to Katoh et al. [36]
binder. The reinforcement, e.g. a continuous fiber tow (uncoated or coated with an appropriate interphase) is impregnated with the slurry and wound on a drum, yielding a 1D-prepreg-type intermediate product. After drying, the layers are stacked in the die of a unidirectional press and the composite sintered at high temperature under pressure [33]. It is well known that the sintering of SiC powder is difficult and requires very high temperatures, even in the presence of sintering aids. Furthermore, since it is performed here under pressure (to achieve low residual porosity), the combined effect of high temperature and high pressure was considered for a long time as a source of too severe fiber degradation and this route more or less disregarded. However, it has been shown recently that the use of nanosized b-SiC powder (with a particulate size of 30 nm) and oxide sintering additives (Al2O3 and Y2O3) forming transient eutectics with SiO2 at relatively low temperature, considerably helps the sintering of the SiC-matrix which was effective at 1780 C and 15–20 MPa. Further, the HM carbon fibers utilized in the first experiments [33] could be replaced by the quasi-stoichiometric SiC fibers whose high thermal stability is compatible with these somewhat mild sintering conditions. SiC/PyC/SiC composites fabricated by this so-called NITE process (for nano infiltration and transient eutectics) display a very low residual porosity, high mechanical and thermal properties as well as an excellent hermeticity with respect to gaseous cooling fluids such as helium (Fig. 3) [34–36]. It then appears that this process could be attractive if the fiber volume fraction (presently: 20%) could be increased, the relatively thick PyC interphase (800 nm) replaced by a more appropriate material and the 1D-fiber architecture by a multidirectional fiber arrangement. 2.4. Hybrid processes Each of the processes discussed previously displaying advantages and drawbacks, hybrid (or combined) processes are sometimes used to opitimize the densification of fiber preforms or/and the microstructure of the composites. Firstly, CVI (in its I-CVI or P-CVI versions) is the method of choice for the deposition of simple or highly engineered interphases whatever the technique(s) further employed for the infiltration of the SiC-matrix, since it yields deposits of relatively uniform composition, structure and thickness, even with preforms of complex fiber architecture. Secondly and as previously mentioned, LSI (in its MI or RMI versions) is utilized to fill the residual open porosity of composites prepared either by CVI or PIP, in order to increase both the thermal conductivity and hermeticity of the composites. Finally, more complex flow charts have been suggested to densify nD-preforms combining, for example, an impregnation with a SiC powder slurry (to introduce rapidly a significant amount of SiC in the preform), a few Fig. 3. Flow chart of the NITE-process for the fabrication of SiCmatrix composites from nanosize SiC particle slurry with oxide sintering additives and pressure sintering, according to Katoh et al. [36]. Fig. 2. Fabrication of a SiC-matrix composite by the liquid silicon infiltration process: (a) preparation of a 2D-fiber preform by a prepreg route from a matrix slurry, (b) consolidation of the material by pyrolysis and (c) infiltration of liquid silicon, according to Corman et al. [30]. R. Naslain / Composites Science and Technology 64 (2004) 155–170 159