COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 59(1999)1073-1085 Oxidation-resistant carbon-fiber-reinforced ceramic-matrix composites F. Lamouroux a,*S. Bertrand R. Pailler a.R. Naslain a.M. Cataldi Laboratoire des Composites Thermostructuraux, UMR 47(CNRS-SEP-UBl), Universite Bordeaux 1, 3 allee La boetie, 33600 Pessac, france sOciete Europeenne de Propulsion Les Cing Chemins, BP 37, 33165 Saint-Medard-en-Jalles, Fre Received 18 December 1997; accepted 10 September 1998 Abstract A multilayer Si-B-C ceramic matrix has been developed to improve the oxidation resistance and the lifetime in an oxygen environment of carbon-fiber-reinforced ceramic-matrix composites. This concept has been applied to a multidirectional fibrous carbon preform. The paper deals with the processing, the mechanical behaviour and the oxidation resistance of a carbon- fiber- reinforced multilayer ceramic-matrix composite. The efficiency of this matrix is compared to that of the classical anti-oxidation ystems based on an external coating usually employed to reduce the oxygen permeation. The efficiency of idation protection of the multilayer ceramic matrix is still evident in spite of the complex architecture of the fibrous preform and the damage of the matrix. The experimental results show that a strong improvement in the lifetime of such composites under thermomechanical loading and in an oxygen environment is obtained when a multilayer matrix is used to protect the carbon reinforcement against oxidation. C 1999 Elsevier Science Ltd. All rights reserved Keywords: A Ceramics; Composites; Oxidation; Carbon; Multilayer 1. ntroduction materials are both tough and heat resistant. The increase of related to the use of The development of advanced technologies in the phase layer between the fiber and the matrix [1]. This fields of energy, aeronautics and space requires new interphase permits the adjustment of the difference in materials with increasingly high performance. The thermomechanical properties between the fibers and intermediate and high-temperature applications in the matrix. Without an interphase, CMCs behave as brittle field of propulsion have promoted the development of materials in the same manner as monolithic ceramics thermostructural materials able to withstand thermo- The most commonly used interphase material in CMCs mechanical loading in extreme environments over short is pyrolytic carbon. Indeed, cracks propagating in the periods. But some technological progresses come up brittle matrix towards the fiber are deviated in the against the problem of the too-short lifetimes of thes interphase because of its strong mechanical anisotropy materials when exposed to aggressive environments. When graphitic planes of pyrolytic carbon are oriented This is the case for jet engines or heat exchangers at parallel to the fiber surface, these cracks are deviated high temperatures(>1200 C). The improvement of along the fiber, this phenomenon delaying the final materials for long-term applications is thus one of the rupture of the reinforcement and that of the composite main problems to solve in order to progress in the field The load is thus supported by the fiber reinforcement in of energy production. Potential materials for high-tem- spite of the presence of cracks inside the ceramic matrix perature applications are the ceramic-matrix composites However. CMCs exhibit two main drawbacks. The (CMCS). They combine the thermal and chemical thermal stability of most polycrystalline ceramic fibers resistance of monolithic ceramics with the mechanical is too low for use at temperatures higher than 1200C strength of carbon or ceramic reinforcements. Such [2-7. As an example, Sic or oxide-based fibers, which are the most commonly used ceramic fibers in CMCs, start to creep beyond 1200oC because of changes in the polycrystalline microstructure with temperature. The 0266-3538/99/S- see front matter C 1999 Elsevier Science Ltd. All rights reserved. PlI:S0266-3538(98)00146-8
Oxidation-resistant carbon-®ber-reinforced ceramic-matrix composites F. Lamouroux a,*, S. Bertranda , R. Pailler a , R. Naslaina , M. Cataldi b a Laboratoire des Composites Thermostructuraux, UMR 47 (CNRS-SEP-UB1), Universite Bordeaux 1, 3 alleÂe La BoeÈtie, 33600 Pessac, France bSocieÂte EuropeÂenne de Propulsion Les Cinq Chemins, BP 37, 33165 Saint-MeÂdard-en-Jalles, France Received 18 December 1997; accepted 10 September 1998 Abstract A multilayer Si±B±C ceramic matrix has been developed to improve the oxidation resistance and the lifetime in an oxygen environment of carbon-®ber-reinforced ceramic-matrix composites. This concept has been applied to a multidirectional ®brous carbon preform. The paper deals with the processing, the mechanical behaviour and the oxidation resistance of a carbon-®berreinforced multilayer ceramic-matrix composite. The eciency of this matrix is compared to that of the classical anti-oxidation systems based on an external coating usually employed to reduce the oxygen permeation. The eciency of the oxidation protection of the multilayer ceramic matrix is still evident in spite of the complex architecture of the ®brous preform and the damage of the matrix. The experimental results show that a strong improvement in the lifetime of such composites under thermomechanical loading and in an oxygen environment is obtained when a multilayer matrix is used to protect the carbon reinforcement against oxidation. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramics; Composites; Oxidation; Carbon; Multilayer 1. Introduction The development of advanced technologies in the ®elds of energy, aeronautics and space requires new materials with increasingly high performance. The intermediate and high-temperature applications in the ®eld of propulsion have promoted the development of thermostructural materials able to withstand thermomechanical loading in extreme environments over short periods. But some technological progresses come up against the problem of the too-short lifetimes of these materials when exposed to aggressive environments. This is the case for jet engines or heat exchangers at high temperatures (>1200C). The improvement of materials for long-term applications is thus one of the main problems to solve in order to progress in the ®eld of energy production. Potential materials for high-temperature applications are the ceramic-matrix composites (CMCs). They combine the thermal and chemical resistance of monolithic ceramics with the mechanical strength of carbon or ceramic reinforcements. Such materials are both tough and heat resistant. The increase of toughness in CMCs is related to the use of an interphase layer between the ®ber and the matrix [1]. This interphase permits the adjustment of the dierence in thermomechanical properties between the ®bers and matrix. Without an interphase, CMCs behave as brittle materials in the same manner as monolithic ceramics. The most commonly used interphase material in CMCs is pyrolytic carbon. Indeed, cracks propagating in the brittle matrix towards the ®ber are deviated in the interphase because of its strong mechanical anisotropy. When graphitic planes of pyrolytic carbon are oriented parallel to the ®ber surface, these cracks are deviated along the ®ber, this phenomenon delaying the ®nal rupture of the reinforcement and that of the composite. The load is thus supported by the ®ber reinforcement in spite of the presence of cracks inside the ceramic matrix. However, CMCs exhibit two main drawbacks. The thermal stability of most polycrystalline ceramic ®bers is too low for use at temperatures higher than 1200C [2±7]. As an example, SiC or oxide-based ®bers, which are the most commonly used ceramic ®bers in CMCs, start to creep beyond 1200C because of changes in the polycrystalline microstructure with temperature. The Composites Science and Technology 59 (1999) 1073±1085 0266-3538/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(98)00146-8 * Corresponding author
F Lamouroux et al. Composites Science and Technology 59(1999)1073-1085 mechanical properties of the composites fall when the oxidation resistance, it has been observed in previous temperature and the duration of exposure increases work that this matrix protects the carbon reinforcement [8-14]. The only reinforcement actually conceivable for even during the damage phase. This kind of matrix has long-term(>1000 h)and high-temperature applications already led to significant improvements in the lifetimes (>1200 C)are carbon fibers. However, another draw- of CMCs, but this progress has been achieved only for back is the low oxidation resistance of CMCs made model composite comprising a carbon- fiber tow and a with carbon fibers and/ or carbon interphase [15-19]. functional ceramic matrix. In the present work, the Oxidation of carbon starts at temperatures as low as concept of the functional ceramic matrix is applied to a 00oC, which dramatically limits the lifetime beyond more complex fibrous structure made of interlocked 500C. When CMCs are made partly of carbon, the woven fabrics called 2. 5D structure [36]. In this paper, lifetime of the material depends on the efficiency of the the concept of the functional matrix is first presented anti-oxidation systems used to reduce the oxygen per The main results of a previous study concerning the meability. These systems consist of an oxide coating or model composite are then summarized and compared to an oxide-forming layer allowing a crack-sealing process those obtained with the interlocked woven structure [20-32]. In such systems, the glassy phases flow in the matrix cracks and lead to a strong decrease in oxygen permeation. Unfortunately, the durability of these sys- 2. Concept of a functional matrix tems is very limited because of changes in composition of the external protective layers induced by gasification Damage is an intrinsic feature of the mechanical at high temperature and or low pressure. The lifetime behaviour of tough CMCs. In the present work, it is of carbon-fiber-reinforced ceramic-matrix composites is proposed to use this damage to improve the oxidation thus strongly related to the efficiency of the anti-oxidation resistance of the composite. This approach is based on system. ()better control of the crack propagation inside the The present work deals with the improvement of the ceramic matrix and (ii) the use of several sealants to oxidation resistance of carbon-fiber-reinforced ceramic- improve the efficiency of the sealing process. A multi matrix composites. It is aimed at designing CMCs layer matrix concept has been chosen to take simulta- which are able to withstand thermomechanical loading neous account of these two parameters. Such a matrix in an oxygen environment over a long period. A new consists of ceramic layers alternating with thin layers of kind of ceramic matrix which is more functional than a material which can act as a 'mechanical fuse' to trigger the conventional ones is proposed. Indeed, the classical crack deviation. Such a multilayer ceramic-matrix com ceramic matrices protect the carbon reinforcement only posite(MCMC) is expected to promote the multiplication in case of the undamaged state. The microcracking of of crack deviations in order (i) to increase the length of the matrix induces a decrease in the oxidation resistance the diffusion path of oxygen and (ii) to improve the seal of the composite because oxygen diffuses through the ing rate because of the narrowness of the debonding crack towards the carbon fibers and/ or interphase. zone[ Fig. 1(a)]. Consequently, oxygen cannot have direct Moreover in such conventional CMCs crack deviation access to the carbon fiber surfaces in contrast to what i inside the interphase is often accompanied by relaxation commonly observed in monolayered matrices [Fig. 1(b) of the matrix which renders the fiber reinforcement sen- Moreover, this multilayered matrix is less sensitive than sitive to overloading in an oxidizing environment [15-18]. a monolayered matrix to the cracks opening during Matrix damage occurs during thermomechanical mechanical loading in oxidizing atmosphere because the loading. If the composite components possess similar sealing zone acts in a debonding area which does not coefficients of thermal expansion(CTE), thermal loading affect the sealing efficiency [Fig. 2(a)and(b) does not affect the state of stress of the material Another advantage of a multilayered matrix results Unfortunately, such is not the case for carbon-rein- from the spreading of the mechanical fuse layers all forced CMCs for which the CTE mismatch induces through the matrix. This feature allows a better adjust matrix microcracking during the cooling-down stage of ment of the CTEs of the fibers and the matrix Optimi- processing. The functional matrix developed in this sation of the load transfer between each ceramic layers work offers several advantages [33-35]. From a ther- induces a better stress distribution and consequently momechanical point of view, it partly accommodates progressive crack propagation through the matrix. In the cte difference existing between the fibers and the this way, the damage of a ceramic layer becomes inde- matrix. It allows a progressive release of stresses during pendent of that of the other layers. If the load transfer is crack propagation in the matrix, in contrast with the well adjusted, the external ceramic layers car classical matrix for which crack propagation is cata damaged before the internal ones. Such a configur strophic. Moreover, with such a functional matrix, the allows a better sealing process because it occurs far from work of friction in the damage stage is more intensive the surface of the carbon fibers which, moreover, are and the energy dissipated is thus increased Concerning protected by the undamaged inner layers(Fig 3)
mechanical properties of the composites fall when the temperature and the duration of exposure increases [8±14]. The only reinforcement actually conceivable for long-term (>1000 h) and high-temperature applications (>1200C) are carbon ®bers. However, another drawback is the low oxidation resistance of CMCs made with carbon ®bers and/or carbon interphase [15±19]. Oxidation of carbon starts at temperatures as low as 500C, which dramatically limits the lifetime beyond 500C. When CMCs are made partly of carbon, the lifetime of the material depends on the eciency of the anti-oxidation systems used to reduce the oxygen permeability. These systems consist of an oxide coating or an oxide-forming layer allowing a crack-sealing process [20±32]. In such systems, the glassy phases ¯ow in the matrix cracks and lead to a strong decrease in oxygen permeation. Unfortunately, the durability of these systems is very limited because of changes in composition of the external protective layers induced by gasi®cation at high temperature and/or low pressure. The lifetime of carbon-®ber-reinforced ceramic-matrix composites is thus strongly related to the eciency of the anti-oxidation system. The present work deals with the improvement of the oxidation resistance of carbon-®ber-reinforced ceramicmatrix composites. It is aimed at designing CMCs which are able to withstand thermomechanical loading in an oxygen environment over a long period. A new kind of ceramic matrix which is more functional than the conventional ones is proposed. Indeed, the classical ceramic matrices protect the carbon reinforcement only in case of the undamaged state. The microcracking of the matrix induces a decrease in the oxidation resistance of the composite because oxygen diuses through the crack towards the carbon ®bers and/or interphase. Moreover, in such conventional CMCs, crack deviation inside the interphase is often accompanied by relaxation of the matrix which renders the ®ber reinforcement sensitive to overloading in an oxidizing environment [15±18]. Matrix damage occurs during thermomechanical loading. If the composite components possess similar coecients of thermal expansion (CTE), thermal loading does not aect the state of stress of the material. Unfortunately, such is not the case for carbon-reinforced CMCs for which the CTE mismatch induces matrix microcracking during the cooling-down stage of processing. The functional matrix developed in this work oers several advantages [33±35]. From a thermomechanical point of view, it partly accommodates the CTE dierence existing between the ®bers and the matrix. It allows a progressive release of stresses during crack propagation in the matrix, in contrast with the classical matrix for which crack propagation is catastrophic. Moreover, with such a functional matrix, the work of friction in the damage stage is more intensive and the energy dissipated is thus increased. Concerning oxidation resistance, it has been observed in previous work that this matrix protects the carbon reinforcement even during the damage phase. This kind of matrix has already led to signi®cant improvements in the lifetimes of CMCs, but this progress has been achieved only for a model composite comprising a carbon-®ber tow and a functional ceramic matrix. In the present work, the concept of the functional ceramic matrix is applied to a more complex ®brous structure made of interlocked woven fabrics called 2.5D structure [36]. In this paper, the concept of the functional matrix is ®rst presented. The main results of a previous study concerning the model composite are then summarized and compared to those obtained with the interlocked woven structure. 2. Concept of a functional matrix Damage is an intrinsic feature of the mechanical behaviour of tough CMCs. In the present work, it is proposed to use this damage to improve the oxidation resistance of the composite. This approach is based on (i) better control of the crack propagation inside the ceramic matrix and (ii) the use of several sealants to improve the eciency of the sealing process. A multilayer matrix concept has been chosen to take simultaneous account of these two parameters. Such a matrix consists of ceramic layers alternating with thin layers of a material which can act as a `mechanical fuse' to trigger crack deviation. Such a multilayer ceramic-matrix composite (MCMC) is expected to promote the multiplication of crack deviations in order (i) to increase the length of the diusion path of oxygen and (ii) to improve the sealing rate because of the narrowness of the debonding zone [Fig. 1(a)]. Consequently, oxygen cannot have direct access to the carbon ®ber surfaces in contrast to what is commonly observed in monolayered matrices [Fig. 1(b)]. Moreover, this multilayered matrix is less sensitive than a monolayered matrix to the cracks opening during mechanical loading in oxidizing atmosphere because the sealing zone acts in a debonding area which does not aect the sealing eciency [Fig. 2(a) and (b)]. Another advantage of a multilayered matrix results from the spreading of the mechanical fuse layers all through the matrix. This feature allows a better adjustment of the CTEs of the ®bers and the matrix. Optimisation of the load transfer between each ceramic layers induces a better stress distribution and consequently progressive crack propagation through the matrix. In this way, the damage of a ceramic layer becomes independent of that of the other layers. If the load transfer is well adjusted, the external ceramic layers can be damaged before the internal ones. Such a con®guration allows a better sealing process because it occurs far from the surface of the carbon ®bers which, moreover, are protected by the undamaged inner layers (Fig. 3). 1074 F. Lamouroux et al. / Composites Science and Technology 59 (1999) 1073±1085
F Lamouroux et al. Composites Science and Technology 59(1999)1073-1085 crack deflection matrix cracking (debonding ceramIc fuse material S;1 carbon fiber Fig. 1. Schematic representation of the composition, the damage propagation and the oxygen path diffusion(a)in multilayered CMC and(b)in UNLOADED LOADED rack slidin k opening (a) UNLOADED LOADED sealing zone crack opening: no sealing process Fig. 2. Schematic of the crack opening under mechanical loading and the sealing process in the multilayered(a)and monolayered (b) ceramic matrix With such a concept, it is possible to introduce several applied to different corrosive environments for which ceramic sealants in the same matrix. The temperature the choice of the ceramic layers has to be optimised. The range corresponding to an efficient protection against multilayered matrix studied in the present work is oxidation is then expanded. This approach can also be bifunctional. It is based on three materials: two ceramics
With such a concept, it is possible to introduce several ceramic sealants in the same matrix. The temperature range corresponding to an ecient protection against oxidation is then expanded. This approach can also be applied to dierent corrosive environments for which the choice of the ceramic layers has to be optimised. The multilayered matrix studied in the present work is bifunctional. It is based on three materials: two ceramics Fig. 2. Schematic of the crack opening under mechanical loading and the sealing process in the multilayered (a) and monolayered (b) ceramic matrix composites. Fig. 1. Schematic representation of the composition, the damage propagation and the oxygen path diusion (a) in multilayered CMC and (b) in monolayered CMC. F. Lamouroux et al. / Composites Science and Technology 59 (1999) 1073±1085 1075
F Lamouroux et al. Composites Science and Technology 59(1999)1073-1085 Fig 3. Schematic of the progressive propagation of damage through the multilayered matrix with a level of stress increasing from(a)to(d). for the sealing process separated by a mechanical fuse 3. Experimental procedure material. Boron and silicon carbides are used as cera- mic sealants because of the efficiency of their oxides The multilayered matrix is infiltrated into the inter- against the oxidation of carbon in distinct temperature locked woven fibrous preform by pulsed chemical ranges. The stoichiometry of the boron carbide is close vapour infiltration(P-CVI)[39-42]. This process con- to B13C2, but the generic term B4C will be used in sists in the repetition as a function of time of a sequence order to consider all the composition range of the boron of three basic steps: (i) the injection of a gaseous pre- carbide. Boron oxide produced during the oxidation cursor inside the deposition chamber of a CVI reactor, of the boron carbide efficiently protects the carbon (ii)a reaction step in a static gaseous environment and when the temperature is lower than 800C whereas (iii) an evacuation of the gaseous products. Such a protection by silica is achieved when the temperature is process avoids the accumulation of gas products inside higher than 1200oC. Between 800 and 1200 C, oxida- the porosity which usually inhibits the reaction in iso- tion protection is assumed to result from the formation baric processes. Moreover, P-CVI allows a regular and of a borosilicate phase. The fuse material could be uniform deposition of successive layers which is one of chosen among pyrolytic carbon, boron nitride or the main factors to control in the multilayered matrix boron-doped pyrolytic carbon BCl-x(with x&o, The P-CVI conditions have been reported elsewhere because of the strong anisotropy of their mechanical [33]. One of the functions of the multilayered matrix properties [1, 37, 38]. Considering the nature of the cera- being to protect the carbon fibers from oxidation of mic layers, boron-doped pyrolytic carbon has been corrosion, the objective at the beginning of the P-CVI chosen. The multilayered matrix is then made of process is to cover each carbon fiber with the first layer the repetition of a unit sequence S, comprising four sequence. As processing progresses, the fiber preform layers: B Cl-x(fuse)/B4 C(sealant)/BCI-x(fuse)/Sic porosity decreases and the other layer sequences coat groups of fibers, then fiber tows until the last layer
for the sealing process separated by a mechanical fuse material. Boron and silicon carbides are used as ceramic sealants because of the eciency of their oxides against the oxidation of carbon in distinct temperature ranges. The stoichiometry of the boron carbide is close to B13C2, but the generic term B4C will be used in order to consider all the composition range of the boron carbide. Boron oxide produced during the oxidation of the boron carbide eciently protects the carbon when the temperature is lower than 800C whereas protection by silica is achieved when the temperature is higher than 1200C. Between 800 and 1200C, oxidation protection is assumed to result from the formation of a borosilicate phase. The fuse material could be chosen among pyrolytic carbon, boron nitride or boron-doped pyrolytic carbon BxC1ÿx (with x&0,1) because of the strong anisotropy of their mechanical properties [1,37,38]. Considering the nature of the ceramic layers, boron-doped pyrolytic carbon has been chosen. The multilayered matrix is then made of the repetition of a unit sequence S, comprising four layers: BxC1ÿx (fuse)/B4C (sealant)/BxC1ÿx (fuse)/SiC (sealant). 3. Experimental procedure The multilayered matrix is in®ltrated into the interlocked woven ®brous preform by pulsed chemical vapour in®ltration (P-CVI) [39±42]. This process consists in the repetition as a function of time of a sequence of three basic steps: (i) the injection of a gaseous precursor inside the deposition chamber of a CVI reactor, (ii) a reaction step in a static gaseous environment and (iii) an evacuation of the gaseous products. Such a process avoids the accumulation of gas products inside the porosity which usually inhibits the reaction in isobaric processes. Moreover, P-CVI allows a regular and uniform deposition of successive layers which is one of the main factors to control in the multilayered matrix. The P-CVI conditions have been reported elsewhere [33]. One of the functions of the multilayered matrix being to protect the carbon ®bers from oxidation or corrosion, the objective at the beginning of the P-CVI process is to cover each carbon ®ber with the ®rst layer sequence. As processing progresses, the ®ber preform porosity decreases and the other layer sequences coat groups of ®bers, then ®ber tows until the last layer Fig. 3. Schematic of the progressive propagation of damage through the multilayered matrix with a level of stress increasing from (a) to (d). 1076 F. Lamouroux et al. / Composites Science and Technology 59 (1999) 1073±1085
F Lamouroux et al/ Composites Science and Technology 59(1999)1073-1085 equence constitutes the external protective layer of the S=(BCy/B13C2/Bxc/SiC final composite with xoO. l and yo09 Morphological investigations have been performed during and after processing by means of optical and scanning electron microscopies (SEM). The oxidation esistance of the 2. 5D composite has been characterised by thermogravimetric analyses in air(100 kPa) in the temperature range 600-1000C. Tensile tests have been performed at room temperature. They were based on unloading-reloading cycles at several levels of strain order to follow (i) changes in the damage with change in the longitudinal elastic modulus and (ii) the work of friction occurring at the interfaces which is related to the width of the hysteresis loops. The effi ciency of the multilayered matrix in the 2.5D structure for the protection against oxidation has been evaluated by means of lifetime tests in four-point bending loading in air and at 600oC. This temperature has been selec- ted because it corresponds to a critical temperature in conventional cmcs reinforced with carbon fibers Indeed, at low temperatures(T<800C), the matrix is characterised by the presence of large cracks which are not closed by the thermal expansion of the matrix and/ or the sealing process. Consequently, the oxidation rate increases and a uniform degradation of the whole carbon reinforcement induces a rapid decrease in the mechanical strength of the composite. At higher temperatures, the oxidation rate is controlled by diffusion mechanisms The degradation of the fibrous preform is thus non by overloading of the inner fibrous plies or by a notch carbon fiber mage of the multilayered matrix deposited on a single uniform and the final rupture of the composite occurs Fig 4. SEM effect induced by the degradation of the first ply. It has been shown that uniform degradation occurring at low 5% for S, to 20% for S4, in order to better control the temperatures was the most critical factor affecting the damage propagation and then to optimise the oxidation mechanical strength. Comparative tests have been carried protection of the fiber. Fig. 5 shows a transverse section out on conventional ceramic-matrix composites, i.e. of a fiber tow densified with a multilayered matrix. 2.5D C/SiC protected against oxidation by an external Each carbon fiber is coated by the two first sequences OxIde (single arrow). The third sequence concerns a group of fibers(double arrow) and finally the last sequence con- sists in the external coating of the whole tow(triple arrow). Such a configuration of the matrix is more effi cient because oxidation protection is ensured at several 4.1. Unidirectional model composites levels. In the case of a monolayered SiC-matrix composite, a thick carbon interphase is necessary to achieved The main results already obtained with a unidirec- mechanical fuse function (&l um). The morphology of tional model minicomposite(infiltrated fiber tow) are the tows is thus close to that of C/c tows coated by referred to for the purpose of the discussion [33, 34 Sic layer. For such a composite, the protection against Carbon fiber tows were densified with four S-sequences oxidation is only performed at the scale of the fiber tow of increasing thickness. The morphology of the as-pro- If oxygen has access to a fiber, then the whole fiber tow cessed multilayered matrix is shown in Fig 4 at the scale is exposed to oxidation because the fibers are connected of a single carbon fiber. The micrograph shows the by the carbon interphase. In a multilayered matrix, a concentric feature of the smooth successive layers. The distinct interphase between the fiber and the matrix is two first sequences S and S2 have a thickness of the not necessary. The matrix can be directly deposited on order of l um whereas the two others S, and S4 have a the carbon fiber which allows an oxidation protection at thickness of 4 and 6 um, respectively. The volume fraction the scale of the fiber. In this case, the oxidation of one of fuse material(boron-doped carbon) increases from fiber does not induce the degradation of the whole fiber
sequence constitutes the external protective layer of the ®nal composite. Morphological investigations have been performed during and after processing by means of optical and scanning electron microscopies (SEM). The oxidation resistance of the 2.5D composite has been characterised by thermogravimetric analyses in air (100 kPa) in the temperature range 600±1000C. Tensile tests have been performed at room temperature. They were based on unloading-reloading cycles at several levels of strain in order to follow (i) changes in the damage with change in the longitudinal elastic modulus and (ii) the intensity of the work of friction occurring at the interfaces which is related to the width of the hysteresis loops. The e- ciency of the multilayered matrix in the 2.5D structure for the protection against oxidation has been evaluated by means of lifetime tests in four-point bending loading in air and at 600C. This temperature has been selected because it corresponds to a critical temperature in conventional CMCs reinforced with carbon ®bers. Indeed, at low temperatures (T<800C), the matrix is characterised by the presence of large cracks which are not closed by the thermal expansion of the matrix and/ or the sealing process. Consequently, the oxidation rate increases and a uniform degradation of the whole carbon reinforcement induces a rapid decrease in the mechanical strength of the composite. At higher temperatures, the oxidation rate is controlled by diusion mechanisms. The degradation of the ®brous preform is thus nonuniform and the ®nal rupture of the composite occurs by overloading of the inner ®brous plies or by a notch eect induced by the degradation of the ®rst ply. It has been shown that uniform degradation occurring at low temperatures was the most critical factor aecting the mechanical strength. Comparative tests have been carried out on conventional ceramic-matrix composites, i.e. 2.5D C/SiC protected against oxidation by an external oxide sealant layers. 4. Results 4.1. Unidirectional model composites The main results already obtained with a unidirectional model minicomposite (in®ltrated ®ber tow) are referred to for the purpose of the discussion [33,34]. Carbon ®ber tows were densi®ed with four S-sequences of increasing thickness. The morphology of the as-processed multilayered matrix is shown in Fig. 4 at the scale of a single carbon ®ber. The micrograph shows the concentric feature of the smooth successive layers. The two ®rst sequences S1 and S2 have a thickness of the order of 1 mm whereas the two others S3 and S4 have a thickness of 4 and 6 mm, respectively. The volume fraction of fuse material (boron-doped carbon) increases from 5% for S1 to 20% for S4, in order to better control the damage propagation and then to optimise the oxidation protection of the ®ber. Fig. 5 shows a transverse section of a ®ber tow densi®ed with a multilayered matrix. Each carbon ®ber is coated by the two ®rst sequences (single arrow). The third sequence concerns a group of ®bers (double arrow) and ®nally the last sequence consists in the external coating of the whole tow (triple arrow). Such a con®guration of the matrix is more e- cient because oxidation protection is ensured at several levels. In the case of a monolayered SiC-matrix composite, a thick carbon interphase is necessary to achieved the mechanical fuse function (&1 mm). The morphology of the tows is thus close to that of C/C tows coated by a SiC layer. For such a composite, the protection against oxidation is only performed at the scale of the ®ber tow. If oxygen has access to a ®ber, then the whole ®ber tow is exposed to oxidation because the ®bers are connected by the carbon interphase. In a multilayered matrix, a distinct interphase between the ®ber and the matrix is not necessary. The matrix can be directly deposited on the carbon ®ber which allows an oxidation protection at the scale of the ®ber. In this case, the oxidation of one ®ber does not induce the degradation of the whole ®ber Fig. 4. SEM image of the multilayered matrix deposited on a single carbon ®ber. F. Lamouroux et al. / Composites Science and Technology 59 (1999) 1073±1085 1077