COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 61(2001)355-362 www.elsevier.com/locate/compscitech Physico-chemistry of interfaces in inorganic-matrix composites J. Bouix *, M.P. Berthet, F. Bosselet, R. Favre, M. Peronnet, O. Rapaud, J.C. Viala C. Vincent.H.Ⅴ Incent Laboratoire des Multimateriaux et Interfaces, UMR CNRS 5615, Universite Claude Bernard Lyon 1, 43 bvd 11 Novembre 1918, F69622 villeurbanne Cedex. france Received 26 May 1999: received in revised form 20 October 1999; accepted 2 May 2000 Abstract The performances of metal matrix composites(MMCs)or ceramic matrix composites(CMCs)are mited by the char acteristics of the fibre/matrix interface or more generally those of the interfacial zone. Concerning M optimization of this zone involves control of the chemical reactivity between the reinforcement and the matrix, which te usually an out-o quilibrium system. In the case of CMCs, it is possible to obtain a non-brittle material by associating two brittle components and to exhibit a good resistance to oxidation. The physical chemist is able to offer a significant contribution for solving these problems by ting on the reinforcement surface, the matrix composition or the manufacturing conditions of the composite.@ 2001Elsevier Science Ltd. All rights reserved 如mm小oMmu 1. Introduction a dissolution-growth process and does not ave an protecting effect on the fibre which is attacked in deptl All of the recent studies show that the mechanical and Consequently, the mechanical properties fall away and thermomechanical behaviour of ceramic and metallic- the composite is highly sensitive to corrosion by humid matrix composites depends widely on the nature of the air with emission of methane. It is therefore necessary to interfacial bonding which forms between the reinforce- limit this reactivity as far as possible. Conversely ment, consisting for instance of fibres, and the matrix between the same fibres and molten magnesium, neither chemical reactivity nor wettability are noticed(Fig. 1b) Generally speaking, this bonding must be strong In this last case, it will be necessary to create a strictly nough to provide good load transfer from the matrix controlled reactivity at the fibre/matrix interface [4] to the fibres, but weak enough to deflect cracks along In ceramic-matrix composites (CMCs), chemical the interface and to avoid their propagation through the reactivity between the fibre and the matrix can also be fibre with a brittle failure of the composite an important topic. For example, when carbon or silicon In ceramic-fibre-reinforced metallic-matrix compo- carbide fibres(Nicalon)are associated with an oxide sites(MMCs), the strength of the interfacial bond base matrix, interfacial oxidation-reduction reactions depends generally on the chemical interactions occurring may proceed at high temperature. In C/C, Sic/Sic or composite. For instance, low-graphitized carbon fibres no longer take place. However, the tion reactions can between fibre and matrix during the fabrication of the C/SiC CMCs, such oxidation-reduction reactions can x-PAN T 300 or ex-Pitch P 55) have a high tensile area remains a preferential path for oxygen diffusion and strength but they are highly reactive with oxygen and therefore constitutes a weak point concerning the resis- with metals like aluminium giving the carbide Al4C3 As tance to oxidation In the case of composites working in shown in Fig. la, this carbide forms as large crystals by air and at high temperature, it is therefore important to make use of oxidation-resistant or self-repairable inter phases. Nevertheless, the main function of the interface consists in conferring a non-brittle behaviour on materials 0266-3538/01/S. see front matter C 2001 Elsevier Science Ltd. All rights reserved PII:S0266-3538(00)00107-X
Physico-chemistry of interfaces in inorganic-matrix composites J. Bouix *, M.P. Berthet, F. Bosselet, R. Favre, M. Peronnet, O. Rapaud, J.C. Viala, C. Vincent, H. Vincent Laboratoire des MultimateÂriaux et Interfaces, UMR CNRS 5615, Universite Claude Bernard Lyon 1, 43 bvd 11 Novembre 1918, F69622 Villeurbanne Cedex, France Received 26 May 1999; received in revised form 20 October 1999; accepted 2 May 2000 Abstract The performances of metal matrix composites (MMCs) or ceramic matrix composites (CMCs) are usually limited by the characteristics of the ®bre/matrix interface or more generally those of the interfacial zone. Concerning MMCs, the optimization of this zone involves control of the chemical reactivity between the reinforcement and the matrix, which constitute usually an out-ofequilibrium system. In the case of CMCs, it is possible to obtain a non-brittle material by associating two brittle components and to exhibit a good resistance to oxidation. The physical chemist is able to oer a signi®cant contribution for solving these problems by acting on the reinforcement surface, the matrix composition or the manufacturing conditions of the composite. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon ®bres; A. Ceramic-matrix composites (CMCs); A. Metal-matrix composites (MMCs); B. Interfaces; E. Chemical vapour deposition (CVD) 1. Introduction All of the recent studies show that the mechanical and thermomechanical behaviour of ceramic and metallicmatrix composites depends widely on the nature of the interfacial bonding which forms between the reinforcement, consisting for instance of ®bres, and the matrix [1±3]. Generally speaking, this bonding must be strong enough to provide good load transfer from the matrix to the ®bres, but weak enough to de¯ect cracks along the interface and to avoid their propagation through the ®bre with a brittle failure of the composite. In ceramic-®bre-reinforced metallic-matrix composites (MMCs), the strength of the interfacial bond depends generally on the chemical interactions occurring between ®bre and matrix during the fabrication of the composite. For instance, low-graphitized carbon ®bres (ex-PAN T 300 or ex-Pitch P 55) have a high tensile strength but they are highly reactive with oxygen and with metals like aluminium giving the carbide Al4C3. As shown in Fig. 1a, this carbide forms as large crystals by a dissolution-growth process and does not have any protecting eect on the ®bre which is attacked in depth. Consequently, the mechanical properties fall away and the composite is highly sensitive to corrosion by humid air with emission of methane. It is therefore necessary to limit this reactivity as far as possible. Conversely, between the same ®bres and molten magnesium, neither chemical reactivity nor wettability are noticed (Fig. 1b). In this last case, it will be necessary to create a strictly controlled reactivity at the ®bre/matrix interface [4]. In ceramic-matrix composites (CMCs), chemical reactivity between the ®bre and the matrix can also be an important topic. For example, when carbon or silicon carbide ®bres (Nicalon) are associated with an oxide base matrix, interfacial oxidation-reduction reactions may proceed at high temperature. In C/C, SiC/SiC or C/SiC CMCs, such oxidation-reduction reactions can no longer take place. However, the ®bre/matrix interfacial area remains a preferential path for oxygen diusion and therefore constitutes a weak point concerning the resistance to oxidation. In the case of composites working in air and at high temperature, it is therefore important to make use of oxidation-resistant or self-repairable interphases. Nevertheless, the main function of the interface consists in conferring a non-brittle behaviour on materials 0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(00)00107-X Composites Science and Technology 61 (2001) 355±362 www.elsevier.com/locate/compscitech * Corresponding author
J. Bouix et al. Composites Science and Tech 61(2001)355-3062 the fibre through thin layers deposited by CVD and working as diffusion barriers, and on the matrix com- position or the processing conditions. In CMCs, we have used single or multilayers for controlling the strength of the interfacial bonding and having a good resistance to oxidation 2. Deposit of thin refractory layers on carbon fibres by RCVD The difficulty consists in achieving a thin coating on each individual filament of a bundle constituted of several thousands of single filaments of some micrometer dia 2 um meter. The efficiency of a coating as a diffusion barrier depends on its continuity and its regular thickness along the whole length of each filament. Specific preferential deposit on the external filaments to the detriment of those situated in the tow centre must be avoide The reactive CVD (or RCVD) is seen as a promising way of achieving the surface treatment and to obtain a carbide coating(M,C: SiC, TiC, B C)on carbon fibres The coatings were prepared by heating the fibres in a gas stream carrying hydrogen and the m element of the M,C (SiCl4, TiCla or BCl3, for instance), carbon being taken on the fibre itself. For a similar kind of carbon fibre, the thickness of the coating depends much more upon the nature of the carbide and on temperature than on reaction time. Typically, the RCvd time is about 1 min. The carbide coating grows by carbon diffusion from the fibre through the layer formed already, therefore continuity Fig 1. Chemical behat a filament in two diffe and its regularity are reached even when normal pressu metallic melts: (a) strong intera ALC, formation after is used for the deposition immersion for 15 min at 680C in nium;(b)no reaction and The optimal conditions can be foreseen by thermo no wetting after 5 h immersion at 730.C in pure magnesium. dynamic calculations. The method is based on the total ibb's energy minimization of the MCl- H2/C(graphite) resulting from the coupling of two constituents which systems for a given set of conditions(temperature, gas separately exhibit brittle failure. In an axial tensile test phase composition, mole number of carbon in contact for instance, the interfacial zone must deviate in mode II with I mol of the gas mixture. )and the theoretical results the cracks induced in the matrix, thus deferring the are corroborated with experiments on bulk graphite sub- failure of the fibres and that of the composite itself. This strates and on carbon fibres with different micro- 'mechanical fuse effect can be obtained only if the structures(ex-Pan and ex-Pitch). A detailed description interfacial bonding is not too strong, which allows the of the fibre coating equipment has been given in previous activation of energy-consuming phenomena like fibre/ publications [4-6 matrix decohesion, interfacial sliding or broken fibre The uniformity and the continuity of the coating are extraction On the other hand, if the interfacial bonding confirmed by sEM observation of the oxide shells becomes too weak. a loss of contact and load transfer obtained after oxidation of as-coated carbon fibres in occurs between fibre and matrix. Optimization of the air at a temperature higher than 600oC. The residue interface requires a compromise in which the residual shown in Fig. 2 corresponds to a T300 fibre coated with thermal stress has an important part a Sic layer of 50 nm, after complete consumption of In the Laboratoire des Multimateriaux et Interfaces, carbon. The photograph indicates a thin and con- ve work to optimize the interfaces in fibre-reinforced tinuous shell that replicates the crenulated morphology metallic- and ceramic-matrix composites In MMCs, the of the fibre. This observation is proof of a continuous problem consists mainly in controlling the interfacial che- initial carbide layer. The RCV technique has been mical reactivity by acting both on the surface properties of used to fabricate more complex protective coatings such
resulting from the coupling of two constituents which separately exhibit brittle failure. In an axial tensile test for instance, the interfacial zone must deviate in mode II the cracks induced in the matrix, thus deferring the failure of the ®bres and that of the composite itself. This `mechanical fuse' eect can be obtained only if the interfacial bonding is not too strong, which allows the activation of energy-consuming phenomena like ®bre/ matrix decohesion, interfacial sliding or broken ®bre extraction. On the other hand, if the interfacial bonding becomes too weak, a loss of contact and load transfer occurs between ®bre and matrix. Optimization of the interface requires a compromise in which the residual thermal stress has an important part. In the Laboratoire des MultimateÂriaux et Interfaces, we work to optimize the interfaces in ®bre-reinforced metallic- and ceramic-matrix composites. In MMCs, the problem consists mainly in controlling the interfacial chemical reactivity by acting both on the surface properties of the ®bre through thin layers deposited by CVD and working as diusion barriers, and on the matrix composition or the processing conditions. In CMCs, we have used single or multilayers for controlling the strength of the interfacial bonding and having a good resistance to oxidation. 2. Deposit of thin refractory layers on carbon ®bres by RCVD The diculty consists in achieving a thin coating on each individual ®lament of a bundle constituted of several thousands of single ®laments of some micrometer diameter. The eciency of a coating as a diusion barrier depends on its continuity and its regular thickness along the whole length of each ®lament. Speci®c preferential deposit on the external ®laments to the detriment of those situated in the tow centre must be avoided. The reactive CVD (or RCVD) is seen as a promising way of achieving the surface treatment and to obtain a carbide coating (MnC: SiC, TiC, B4C) on carbon ®bres. The coatings were prepared by heating the ®bres in a gas stream carrying hydrogen and the M element of the MnC (SiCl4, TiCl4 or BCl3, for instance), carbon being taken on the ®bre itself. For a similar kind of carbon ®bre, the thickness of the coating depends much more upon the nature of the carbide and on temperature than on reaction time. Typically, the RCVD time is about 1 min. The carbide coating grows by carbon diusion from the ®bre through the layer formed already, therefore the coating formation is self-regulated and its continuity and its regularity are reached even when normal pressure is used for the deposition. The optimal conditions can be foreseen by thermodynamic calculations. The method is based on the total Gibb's energy minimization of the MClx/H2/C(graphite) systems for a given set of conditions (temperature, gas phase composition, mole number of carbon in contact with 1 mol of the gas mixture...) and the theoretical results are corroborated with experiments on bulk graphite substrates and on carbon ®bres with dierent microstructures (ex-Pan and ex-Pitch). A detailed description of the ®bre coating equipment has been given in previous publications [4±6]. The uniformity and the continuity of the coating are con®rmed by SEM observation of the oxide shells obtained after oxidation of as-coated carbon ®bres in air at a temperature higher than 600C. The residue shown in Fig. 2 corresponds to a T300 ®bre coated with a SiC layer of 50 nm, after complete consumption of carbon. The photograph indicates a thin and continuous shell that replicates the crenulated morphology of the ®bre. This observation is proof of a continuous initial carbide layer. The RCVD technique has been used to fabricate more complex protective coatings such Fig. 1. Chemical behaviour of a P55 carbon ®lament in two dierent metallic melts: (a) strong interaction with Al4C3 formation after immersion for 15 min at 680C in pure aluminium; (b) no reaction and no wetting after 5 h immersion at 730C in pure magnesium. 356 J. Bouix et al. / Composites Science and Technology 61 (2001) 355±362
J. Bouix et al. /Composites Science and Technology 61(2001)355-362 as B,C/SiC and B4C/TiB, double layers [7]. in order to fabrication of performant aluminium matrix composites increase the fibre resistance against oxidation and to with a fibre-volume fraction of 0.50 by a squeeze-casting ensure good wetting of the fibres by liquid aluminium. technique. The tensile strength is multiplied by a factor The process involves two successive RCVD steps of 3. In these materials aluminium carbide is no The presence of these thin carbide coatings is able to detected at the fibre /metal interface. w down considerably the gasification of carbon fibres The technique is not limited to modify only the carbon during an oxidation exposure and their reactivity with fibre surface, it has been applied to surface treatment of liquid aluminium. For instance, the curves of thermo- Hi-Nicalon fibres: a thin layer of Si3 N4 has been gravimetric analysis (TGA)shown in Fig 3 confirm the obtained by reaction between silicon carbide and low oxidation resistance of the pristine T300 fibre ammonia gas at a temperature higher than 1000C heated under oxygen atmosphere at 600oC. They also prove that BC single layer and B4C/SiC double layer have better protective behaviour than SiC single layer 3. Coatings with a double function gainst oxidation. Furthermore, the use of fibres coated by boron carbide or by silicon carbide has permitted The protective coatings used as interphases in MMCs re generally carbides, nitrides or oxides, i.e. brittle materials which crack at a low level of strength. when the coating thickness is up to a critical value 'ecrit it has been shown that the tensile strength of fibres decreases when the coating thickness increases. The ecrit value depends on fibre type and the adhesion strength between the fibre and the coating; however, it does not depend on the coating composition. Typically, ecrit about 16 nm for a T300 fibre. These very thin coatings are protective only when the fabrication technique of MMCs requires a very short contact time between the fibre and the heated metal When a thicker interphase is ed, the same pro- blen tered in MMCs and cmcs. i.e. the crack formation in the brittle component and the pro pagation of the cracks in the fibres. It appears that the presence of a deflector, or a 'mechanical fuse' in brittle properties of the two kinds of composites. It is possible to deflect cracks either at the nanometric scale. for instance between the graphitic planes of pyrolytic carbon Fig. 2. SEM micrograph of the residue obtained from Sic-coated or turbostratic boron nitride, or at the macroscopic T300 fibre after complete consumption of carbon by oxidation scale, for instance at different interfaces in weakly bonded multi-layered coatings 3. 1. Double layered coating: pyrocarbon/carbide We have developed a new generation of a fibre coat ing. It consists of two stacked layers: a preliminary deposition of pyrocarbon(pyC) by low-pressure CVD technique on the fibre followed by a partial conversion of this carbon layer into carbide by RCvd treatment [8]. The very thin carbon layer between the fibre and the external carbide layer acts like a mechanical fuse. Fibres with a such double-layered coating are chemically inert and are more mechanically resistant than the pristine DO(pristine) T300(SiC) fibres. Data for the tensile tests performed on three 12 fibres and four coatings can be found in Table Time(h) M40 fibres have been coated with a pyrocarbon/silicon Fig 3. Weight losses of T300 fibres coated with carbides as a function carbide(pyC/SiC) dual layer. They have been used as a of time([=600C, pO2=I bar) reinforcing agent in an aluminium matrix ID-composite
as B4C/SiC and B4C/TiB2 double layers [7], in order to increase the ®bre resistance against oxidation and to ensure good wetting of the ®bres by liquid aluminium. The process involves two successive RCVD steps. The presence of these thin carbide coatings is able to slow down considerably the gasi®cation of carbon ®bres during an oxidation exposure and their reactivity with liquid aluminium. For instance, the curves of thermogravimetric analysis (TGA) shown in Fig. 3 con®rm the low oxidation resistance of the pristine T300 ®bre heated under oxygen atmosphere at 600C. They also prove that B4C single layer and B4C/SiC double layer have better protective behaviour than SiC single layer against oxidation. Furthermore, the use of ®bres coated by boron carbide or by silicon carbide has permitted fabrication of performant aluminium matrix composites with a ®bre-volume fraction of 0.50 by a squeeze-casting technique. The tensile strength is multiplied by a factor of 3. In these materials, aluminium carbide is not detected at the ®bre/metal interface. The technique is not limited to modify only the carbon ®bre surface, it has been applied to surface treatment of Hi-Nicalon ®bres: a thin layer of Si3N4 has been obtained by reaction between silicon carbide and ammonia gas at a temperature higher than 1000C. 3. Coatings with a double function The protective coatings used as interphases in MMCs are generally carbides, nitrides or oxides, i.e. brittle materials which crack at a low level of strength. When the coating thickness is up to a critical value `ecrit' it has been shown that the tensile strength of ®bres decreases when the coating thickness increases. The ecrit value depends on ®bre type and the adhesion strength between the ®bre and the coating; however, it does not depend on the coating composition. Typically, ecrit is about 16 nm for a T300 ®bre. These very thin coatings are protective only when the fabrication technique of MMCs requires a very short contact time between the ®bre and the heated metal. When a thicker interphase is required, the same problems are encountered in MMCs and CMCs, i.e. the crack formation in the brittle component and the propagation of the cracks in the ®bres. It appears that the presence of a de¯ector, or a `mechanical fuse' in brittle interphase is essential for increasing the mechanical properties of the two kinds of composites. It is possible to de¯ect cracks either at the nanometric scale, for instance between the graphitic planes of pyrolytic carbon or turbostratic boron nitride, or at the macroscopic scale, for instance at dierent interfaces in weakly bonded multi-layered coatings. 3.1. Double layered coating: pyrocarbon/carbide We have developed a new generation of a ®bre coating. It consists of two stacked layers: a preliminary deposition of pyrocarbon (pyC) by low-pressure CVD technique on the ®bre followed by a partial conversion of this carbon layer into carbide by RCVD treatment [8]. The very thin carbon layer between the ®bre and the external carbide layer acts like a mechanical fuse. Fibres with a such double-layered coating are chemically inert and are more mechanically resistant than the pristine ®bres. Data for the tensile tests performed on three ®bres and four coatings can be found in Table 1. M40 ®bres have been coated with a pyrocarbon/silicon carbide (pyC/SiC) dual layer. They have been used as a reinforcing agent in an aluminium matrix 1D-composite Fig. 2. SEM micrograph of the residue obtained from SiC-coated T300 ®bre after complete consumption of carbon by oxidation. Fig. 3. Weight losses of T300 ®bres coated with carbides as a function of time (t=600C, pO2=1 bar). J. Bouix et al. / Composites Science and Technology 61 (2001) 355±362 357
358 J. Bouix et al. /Composites Science and Technology 61(2001)355-362 fabricated by medium-pressure foundry method. The valves opening and closing. It is possible to vary the obtained composite has a tensile strength of 1300 MPa, relative thickness of each layer by modifying the number close to the value predicted by the mixture law of pulses and the residence time of the precursors in the reactor. The seM photographs in Figs. 4 and 5 corre- 3. 2. Multi-layered coatings with different components spond, one to a coating consisted in seven(pyc/TiC) sequences and the other to a An improvement of the interphase quality consists in For these two examples of coatings, the first and the last promoting the multi-fissuration in the interphase and sub-layers are pyC. These photographs also show the deflecting the cracks parallel to the fibre; therefore the weak bonding between the different sub-layers after the idea of increasing the rupture surface number by a fracture of the fibre stacking of sequences of two layers of different comp The tensile strength value of the as-coated FT500 nents, one constituted of pyC (or hexagonal BN), the fibres(3300 MPa) is close to that of the pristine fibre other constituted of carbide. This idea has been pro-(3560 MPa)and confirms the property of mechanica posed by Diefendorf et al. [9 who have studied the effect of a coating consisted in a(py C/SiC)sequence on fibres. There exists few examples of multi-layer coatings available in literature. Further, the idea of multi-layers has been extended for the fabrication of matrices [10] Table 2 collects some data [10-15 A laminated coating constituted of distinct thin sub- layers of carbides, insulated or not by thin carbon layers is the new concept that we develop now. We can give the example of a(C/TiC)n coating on FT500 fibre. The process relies on the Pressure pulsed-CVD and the RCVD techniques. It consists in performing a series of cycles within a short period of time: each cycle includes etting the reactor under vacuum, gas injection and deposition reaction. The reactor is filled periodically with a C3H/Ar mixture or with a TiCl4/H2 one and evacuated after a chosen reaction time. The titanium carbide coating is performed by RCVD process, i. e by consumption of a part or the whole of the pyC layer which is used as the reactive substrate Gas introduction into the reactor and its evacuation are performed by pneumatic valves, and a controller is used to monitor Table I Tensile strengths (MPa) of pyC (LPCVD)/ carbide (RCVD)-coated Pristine pyc pyC/SiC pyC/TiC pyC/B4C fibres 3150 42503200 2700 34003200 560 43003900 3480 Table 2 Examples of multilayers available in literature References Interphases pyC/SiC [,12] BN/SIC [3 [14 Matrix pyC/SiC 5
fabricated by medium-pressure foundry method. The obtained composite has a tensile strength of 1300 MPa, close to the value predicted by the mixture law. 3.2. Multi-layered coatings with dierent components An improvement of the interphase quality consists in promoting the multi-®ssuration in the interphase and de¯ecting the cracks parallel to the ®bre; therefore the idea of increasing the rupture surface number by a stacking of sequences of two layers of dierent components, one constituted of pyC (or hexagonal BN), the other constituted of carbide. This idea has been proposed by Diefendorf et al. [9] who have studied the eect of a coating consisted in a (pyC/SiC) sequence on ®bres. There exists few examples of multi-layer coatings available in literature. Further, the idea of multi-layers has been extended for the fabrication of matrices [10]. Table 2 collects some data [10±15]. A laminated coating constituted of distinct thin sublayers of carbides, insulated or not by thin carbon layers, is the new concept that we develop now. We can give the example of a (C/TiC)n coating on FT500 ®bre. The process relies on the Pressure pulsed-CVD and the RCVD techniques. It consists in performing a series of cycles within a short period of time: each cycle includes setting the reactor under vacuum, gas injection and deposition reaction. The reactor is ®lled periodically with a C3H8/Ar mixture or with a TiCl4/H2 one and evacuated after a chosen reaction time. The titanium carbide coating is performed by RCVD process, i.e. by consumption of a part or the whole of the pyC layer which is used as the reactive substrate. Gas introduction into the reactor and its evacuation are performed by pneumatic valves, and a controller is used to monitor valves opening and closing. It is possible to vary the relative thickness of each layer by modifying the number of pulses and the residence time of the precursors in the reactor. The SEM photographs in Figs. 4 and 5 correspond, one to a coating consisted in seven (pyC/TiC) sequences and the other to a (TiC/TiC)n multi-layer. For these two examples of coatings, the ®rst and the last sub-layers are pyC. These photographs also show the weak bonding between the dierent sub-layers after the fracture of the ®bre. The tensile strength value of the as-coated FT500 ®bres (3300 MPa) is close to that of the pristine ®bre (3560 MPa) and con®rms the property of mechanical Table 1 Tensile strengths (MPa) of pyC (LPCVD)/carbide (RCVD)-coated ®bres Pristine ®bres pyC pyC/SiC pyC/TiC pyC/B4C T300-99 3150 4250 3200 2700 3800 M40 B 2740 3400 3200 3750 ± FT500 3560 4300 3900 ± 3480 Table 2 Examples of multilayers available in literature References Interphases pyC/SiC [11,12] BN/SiC [13] BN/Si3N4 [14] Matrix pyC/SiC [10,15] Fig. 5. Fractured surface of (TiC/TiC)-coated ®bre. Fig. 4. Fractured surface of (pyC/TiC)-coated ®bre. 358 J. Bouix et al. / Composites Science and Technology 61 (2001) 355±362
J. Bouix et al. /Composites Science and Technology 61(2001)355-362 of the very thin pyc layers or the stacking of when SiC is heated with pure aluminium, an invar- weakly-bonded layers of TiC. The SEM photograph in iant transformation involving four phases occurs at Fig. 6 demonstrates the presence of microcracks in the 650+3 C, i.e. at 10oC below the melting point of the carbide layers and their deflection at C/TiC interfaces metal. This transformation, which is of the quasi-peri tectic type, can be written a 3.3. Multilayers with different microstructures SiC+ Alsz2Al4C3+ L Another concept is proposed to produce fibre coat- ings and interphases in CMCs. It rests on a stacking of where Als designates metallic aluminium in the solid layers of the same chemical composition, providing that state and Lo an aluminium-rich Al/Si/ c liquid contain- successive layers have a different microstructure, for ing 1.5 at. of silicon and about I at ppm of carbon instance, a lamellar structure and an isotropic one BN is a component which fits that interphase type [16] According to the temperature during the deposition by low-pressure CVD process(LPCVD), boron nitride exhibits a more or less pronounced microtexture and hence, mechanical property anisotropic. We have shown that the use of a furnace with temperature gradients allows the deposition of a stacking of isotropic and anisotropic layers by a continuous process The SEM photographs shown in Figs. 7 and & corre spond to Hi-Nicalon fibre coated with a Bn trilayer deposited under the following conditions: BF3/NH mixture, fibre speed of 0.5 m/h. The fractured section observation of the same fibre demonstrates a decohesion between the coating and the fibre, between isotropic and anisotropic layers and in the isotropic layer The weak bonding between the fibre and coating causes the conservation of the fibre mechanical propertie 4. Interface reactivity control in carbon/aluminium and carbon/magnesium composites Fig. 6. Polished section of fibres coated by a(pyC Tic) multi-layer 4.1. Carbon/aluminium composites As pointed out in the Introduction, the main problem 1.2 in these composites is to avoid an excessive degradation of the reinforcing fibres by chemical reaction with the metal matrix during fabrication by melt-infiltration. To solve this problem, thin layers of the refractory carbides SiC, TiC and B, C have been deposited at the surface of the fibres by the rcvd process previously described. Pres- sure-infiltration of these coated fibres by liquid aluminium has resulted in composites with improved mechanical properties, showing thereby that the carbide coatings could effectively protect the underlying fibre from alu minium attack. To acquire a thorough understanding of nis protecting effect and render possible a better control of the chemical reactivity at the matrix/coating interface a detailed investigation of the chemical interactions in the al c/si Ti and Al/ c/B ternary systems has been carried out a thermodynamic approach of the chemical interac- tions in the al/c/si system under atmospheric pressure has revealed two important features [17]: Fig. 7. BN tri-layered coating
fuse of the very thin pyC layers or the stacking of weakly-bonded layers of TiC. The SEM photograph in Fig. 6 demonstrates the presence of microcracks in the carbide layers and their de¯ection at C/TiC interfaces. 3.3. Multilayers with dierent microstructures. Another concept is proposed to produce ®bre coatings and interphases in CMCs. It rests on a stacking of layers of the same chemical composition, providing that successive layers have a dierent microstructure, for instance, a lamellar structure and an isotropic one. BN is a component which ®ts that interphase type [16]. According to the temperature during the deposition by low-pressure CVD process (LPCVD), boron nitride exhibits a more or less pronounced microtexture and, hence, mechanical property anisotropic. We have shown that the use of a furnace with temperature gradients allows the deposition of a stacking of isotropic and anisotropic layers by a continuous process. The SEM photographs shown in Figs. 7 and 8 correspond to Hi-Nicalon ®bre coated with a BN trilayer deposited under the following conditions: BF3/NH3 mixture, ®bre speed of 0.5 m/h. The fractured section observation of the same ®bre demonstrates a decohesion between the coating and the ®bre, between isotropic and anisotropic layers and in the isotropic layer. The weak bonding between the ®bre and coating causes the conservation of the ®bre mechanical properties. 4. Interface reactivity control in carbon/aluminium and carbon/magnesium composites 4.1. Carbon/aluminium composites As pointed out in the Introduction, the main problem in these composites is to avoid an excessive degradation of the reinforcing ®bres by chemical reaction with the metal matrix during fabrication by melt-in®ltration. To solve this problem, thin layers of the refractory carbides SiC, TiC and B4C have been deposited at the surface of the ®bres by the RCVD process previously described. Pressure-in®ltration of these coated ®bres by liquid aluminium has resulted in composites with improved mechanical properties, showing thereby that the carbide coatings could eectively protect the underlying ®bre from aluminium attack. To acquire a thorough understanding of this protecting eect and render possible a better control of the chemical reactivity at the matrix/coating interface, a detailed investigation of the chemical interactions in the Al/C/Si, Al/C/Ti and Al/C/B ternary systems has been carried out. A thermodynamic approach of the chemical interactions in the Al/C/Si system under atmospheric pressure has revealed two important features [17]: . when SiC is heated with pure aluminium, an invariant transformation involving four phases occurs at 6503C, i.e. at 10C below the melting point of the metal. This transformation, which is of the quasi-peritectic type, can be written as: SiC Als ÿ!ÿAl4C3 L0 1 where Als designates metallic aluminium in the solid state and L0 an aluminium-rich Al/Si/C liquid containing 1.5 at.% of silicon and about 1 at.ppm of carbon; Fig. 6. Polished section of ®bres coated by a (pyC /TiC) multi-layer. Fig. 7. BN tri-layered coating. J. Bouix et al. / Composites Science and Technology 61 (2001) 355±362 359