Availableonlineatwww.sciencedirect.com Aerospace BCIENCE RECTO Science Technolo ELSEVIER Aerospace Science and Technology 7(2003)135-146 Microstructures of ceramic composites ith glass-ceramic matrices reinforced by sic-based fibres Jean Vicens *, Gaelle Farizy, Jean-Louis Chermant LERMAT CNRS FRE 2149. 6 Boulevard Marechal Juin 14050 Caen Cedex france Received 14 February 2002; accepted 1 July 2002 Abstract A lot of studies have been carried out on the fibre/matrix interfaces in glass-ceramic matrix composites reinforced by SiC based fibres Chemical and structural analyses at the nanometer scale have shown that the fibre/matrix interface has a very complex structure consisting of several sublayers. The most important point is the existence of a thin carbon layer which is often found textured close to the fibre carbon layer acts as a mechanical fuse with a low or extremely low interfacial debonding energy. The mechanism responsible of the carbon formation and of the complex interfacial microstructure is still mater of controversy. This review will be focused on the microstructure of the interfacial region and on the different techniques which have been used to obtain chemical and microstructural parameters of the fibre/matrix interfaces in a large variety of glass-ceramic composites. The approaches concerning the sic/Bn dual-coated Nicalon SiC fibre-reinforced BMAS matrix composites will be described as well as the thermomechanical properties of this class of glass-ceramic composites and future esearch e 2002 Editions scientifiques et medicales Elsevier SAs. All rights reserved Keywords: Glass-ceramic matrix composite; Carbon interphase, SiC Nicalon fibre/HRTEM 1. Introduction where low density materials with high temperature capa- ility are needed: in st The new class of materials-ceramic matrix composites els with stifiners, high dimensional stability structures for (CMCs)-is concerned with a ceramic matrix reinforced by mirror or antenna) or in turbines(rear frame liners, mixer ceramic fibres, whiskers or particles. The matrix is made flow, petals, exhaust cones, . )[11, 19, 29, 32, 33]. Unfortu of either a monolithic ceramic(SiC, Al2O3, Si3N4,. ) or nately many CMCs with a glass-ceramic matrix have shown a glass-ceramic. The first ones are prepared from ceramic during high temperature and long term tests that some mor- routes(melting or chemical vapor infiltration-CVI-, poly- phological and chemical changes arise in the matrix mi- mer infiltration-PIP-processes)and the second ones result crostructure, leading also to the development of some vis- from the glass route which is easier to produce and needs a cous phases( from a mechanical point of view)at tempera lower temperature. Among the large variety of CMCs, com- tures higher than 1273 K. That is the reason why today only posites with long ceramic fibres have been extensively in- CMCs with a monolithic matrix are considered for applica- vestigated over the last decade because of their interesting tions at high temperatures and high stresses [10).Neverthe- behavior at high temperatures. Many research activities in less, mainly due to their low density, high corrosion resis- the field of the aeronautical and space domains concern the tance and cheaper process cost, CMCs with a glass-ceramic development of new equipment able to be used in severe matrix have a potential field of applications in a domain of conditions, such as high temperature, high stress level, ag- low low stresses and low temperatures(873-1073 K)[2].The gressive environment: this was the challenge of the ceramic present paper will focus on CMCs with a glass-ceramic ma- matrix composites reinforced by ceramic fibres CMCs have trix reinforced by Sic Nicalon fibres. It is well known that potential applications in relation to the aerospace sector CMCs are tough when the fibre-matrix bonding is controlled during processing, via the use of an interphase [31] Corresponding author. In CMCs fabricated by CVl, the design of the fibre/matrix interfacial zone is based on precoated fibres where a weak 1270-9638/02/S-see front matter o 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved i:10.1016/1270-9638(02)01178-1
Aerospace Science and Technology 7 (2003) 135–146 www.elsevier.com/locate/aescte Microstructures of ceramic composites with glass–ceramic matrices reinforced by SiC-based fibres Jean Vicens ∗, Gaëlle Farizy, Jean-Louis Chermant LERMAT, CNRS FRE 2149, 6 Boulevard Maréchal Juin, 14050 Caen Cedex, France Received 14 February 2002; accepted 1 July 2002 Abstract A lot of studies have been carried out on the fibre/matrix interfaces in glass–ceramic matrix composites reinforced by SiC based fibres. Chemical and structural analyses at the nanometer scale have shown that the fibre/matrix interface has a very complex structure consisting of several sublayers. The most important point is the existence of a thin carbon layer which is often found textured close to the fibre. This carbon layer acts as a mechanical fuse with a low or extremely low interfacial debonding energy. The mechanism responsible of the carbon formation and of the complex interfacial microstructure is still mater of controversy. This review will be focused on the microstructure of the interfacial region and on the different techniques which have been used to obtain chemical and microstructural parameters of the fibre/matrix interfaces in a large variety of glass–ceramic composites. The approaches concerning the SiC/BN dual-coated Nicalon SiC fibre-reinforced BMAS matrix composites will be described as well as the thermomechanical properties of this class of glass–ceramic composites and future research. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Glass–ceramic matrix composite; Carbon interphase; SiC Nicalon fibre/HRTEM 1. Introduction The new class of materials – ceramic matrix composites (CMCs) – is concerned with a ceramic matrix reinforced by ceramic fibres, whiskers or particles. The matrix is made of either a monolithic ceramic (SiC, Al2O3, Si3N4,...) or a glass–ceramic. The first ones are prepared from ceramic routes (melting or chemical vapor infiltration -CVI-, polymer infiltration -PIP- processes) and the second ones result from the glass route which is easier to produce and needs a lower temperature. Among the large variety of CMCs, composites with long ceramic fibres have been extensively investigated over the last decade because of their interesting behavior at high temperatures. Many research activities in the field of the aeronautical and space domains concern the development of new equipment able to be used in severe conditions, such as high temperature, high stress level, aggressive environment: this was the challenge of the ceramic matrix composites reinforced by ceramic fibres. CMCs have potential applications in relation to the aerospace sector * Corresponding author. E-mail address: jean.vicens@ismra.fr (J. Vicens). where low density materials with high temperature capability are needed: in structures (air intakes, structural panels with stiffners, high dimensional stability structures for mirror or antenna) or in turbines (rear frame liners, mixer flow, petals, exhaust cones,...) [11,19,29,32,33]. Unfortunately many CMCs with a glass–ceramic matrix have shown during high temperature and long term tests that some morphological and chemical changes arise in the matrix microstructure, leading also to the development of some viscous phases (from a mechanical point of view) at temperatures higher than 1273 K. That is the reason why today only CMCs with a monolithic matrix are considered for applications at high temperatures and high stresses [10]. Nevertheless, mainly due to their low density, high corrosion resistance and cheaper process cost, CMCs with a glass–ceramic matrix have a potential field of applications in a domain of low stresses and low temperatures (873–1073 K) [2]. The present paper will focus on CMCs with a glass–ceramic matrix reinforced by SiC Nicalon fibres. It is well known that CMCs are tough when the fibre-matrix bonding is controlled during processing, via the use of an interphase [31]. In CMCs fabricated by CVI, the design of the fibre/matrix interfacial zone is based on precoated fibres where a weak 1270-9638/02/$ – see front matter 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S1270-9638(02)01178-1
J Vicens et al. Aerospace Science and Technology 7(2003)135-146 interface(a pyrocarbon interphase for example)is deposited matrix composition the following phases are observed: eu- on the fibre prior to the matrix. To improve the oxidation cryptite (Li2O2-Al2O3-2SiO2), spodumene(LiO2-Al2O3- resistance, the use of multilayer interphases for example 4SiO2), petalite (LiO2-Al2O3-8SiO2), cordierite(2Mgo (PyC-SiC)n has been developed [16]. A self healing process 2Al2O3-5SiO2), mullite (3Al2O3-2SiO2), yttrium disilicate of the ceramic matrix can also be achieved by addition of (Y2Si2O7), barium osumilite(BaMg2 Al6SigO30).. The boron to pyrocarbon because oxidation of boron gives rise complexity of the matrix microstructure has been reviewed to a low melting glass healing the microcracks [20] for several ternary and quaternary systems [ 14] In CMCs fabricated by the glass route, the weak interface between the fibre and the matrix results in a chemical reaction during the high temperature step of the composite 3. Interfacial zone in SiC Nicalon/glass-ceramic processing. Many studies have been performed on the composites. Experimental results complex interfacial zone to determine the microstructure and the chemical composition. This paper presents a review or SiC Nicalon/glass-ceramic composites display a com- Nicalon fibres/glass matrices and on the new generation of plex multilayer fibre/matrix interfacial zone. In-situ reac- such composites where a BN/SiC dual-coated interphases tions occur at the fibre/glass ceramic interfaces during the have been deposited on the fibre to increase strength and hot pressing step, typically 1473-1673K. The interfacial ermal stability between 1273-1473K[9 zone is the result of an oxidation of the fibre surface by oxygen of the matrix. Many studies have been performed in order to characterize very carefully the different multi- 2. Materials and techniques layers observed at the contact zone in a large variety of composites. Transmission electron microscopy (TEM) and The reinforcement in glass and glass-ceramic matrix high resolution transmission electron microscopy (HRTEM composites is mainly the Nicalon SiC-O fibres(NLM are the main techniques used to investigate the interfacial 202, from Nippon Carbon, Tokyo)[37]. More recently Hi- layer microstructure on cross sections of composites. The Nicalon fibres were also used [45]. As described below, cross-sections were prepared by mechanical grinding and the key point is the formation of a pyrocarbon layer at the thinning by ion-milling. Chemical compositions of inter- fibre/matrix interface because SiC Nicalon/glass-ceramic facial layers were then determined by Energy Dispersive composites are reactive systems during the processing of X-ray Spectroscopy(EDX) and Electron Energy Low Spec the composite. Others types of fibres were occasionally used troscopy(EELS)on thin foils. The composition and the as reinforcement, for example HPZ (Si-C-0-N)fibres [23] chemistry of the interfacial layer were fully obtained using and Tyranno fibres containing 2%Ti [7]. Reaction products, complementary techniques. Investigations were performed Si2N2O or TiC with HPZ fibres or Tyranno respectively, can by Secondary-lon Mass Spectrometry (SIMS)and by Auger be formed between the matrix and the fibre depending on the Electron Spectroscopy(AES) on fibres extracted from the composites by dissolution of the matrix in a hydrofluoric Glass and glass-ceramic matrices are silicates which acid bath. X-rays Photoelectron Spectroscopy(XPS)analy- exhibit thermal expansion coefficients close to those of the ses on fibres extracted from the composite too were carried SiC fibres(3-5 10-6.K-). Examples of matrices include out to confirm EELS and AES results. These complemen- pure"silica, low-expansion borosilicate glasses(Pyrex)or tary techniques were developed particularly, for example, by Duran glasses(B2O3-Na20-SiO2)and alumino-silicates Lancin et al. [22], Ponthieu et al. 35] and Le strat[25, 26 LAS (Li2O-Al2O3-SiO2), CAS(Cao-Al2O3-Sio2), on Nicalon/LAS and Nicalon/Pyrex composites. As shown BAS (Bao-Al2O3-SiO2), MAS (MgO-Al2O3-SiO2), below, these data were the bases of the different models pro- YMAS (Y2O3-MgO-Al2O3-SiO2), and a combination of posed for explaining the complex interface microstructur these matrices, for example MLAS and BMAs [31, 34] or formation currently found in glass-ceramic composites other type such as BsAs or celsian matrix (BaO-SrO- a key point from a mechanical point of view is the pres- Al2O3-SiO2)[1]. Different kinds of additives, fluxing or ence of a thin layer of carbon, often found textured. The refining agents(ZnO, AS2O3, Sb2 O5)or nucleating agents carbon-rich layer is relatively weak and consequently in- TiO2, Nb2O5)to favor the glass-ceramic conversion may creases the fracture toughness of the composite. It allows act also as a source of oxygen as pointed out by Naslain 31]. crack deflection along the fibre/matrix interface and load Their presence induces a change in the fibre/matrix inter- transfer to occur from matrix to fibre. The nature of the in- face microstructure(see for example Nicalon/LAS compos- terfacial zone, its thickness and its kinetics of growth de- ite with or without Nb2O5)[6,9] pend on many parameters such as the glass composition The composite in the glass process are prepared accord- the hot-pressing condition.. Two examples of hRTEM ing to a pre-preg route comprising a slurry impregnation and studies associated with EDX analyses concerning the mi- hot-pressing steps. Special route such as tape casting has crostructure of the different layers up to the nanoscale and also been used in some cases [28]. After processing, the their composition are described afterwards. They have been glass-ceramic matrices crystallized and depending on the taken in the SiC Nicalon/YMAS composites fabricated
136 J. Vicens et al. / Aerospace Science and Technology 7 (2003) 135–146 interface (a pyrocarbon interphase for example) is deposited on the fibre prior to the matrix. To improve the oxidation resistance, the use of multilayer interphases for example (PyC–SiC)n has been developed [16]. A self healing process of the ceramic matrix can also be achieved by addition of boron to pyrocarbon because oxidation of boron gives rise to a low melting glass healing the microcracks [20]. In CMCs fabricated by the glass route, the weak interface between the fibre and the matrix results in a chemical reaction during the high temperature step of the composite processing. Many studies have been performed on the complex interfacial zone to determine the microstructure and the chemical composition. This paper presents a review on Nicalon fibres/glass matrices and on the new generation of such composites where a BN/SiC dual-coated interphases have been deposited on the fibre to increase strength and thermal stability between 1273–1473 K [9]. 2. Materials and techniques The reinforcement in glass and glass–ceramic matrix composites is mainly the Nicalon Si–C–O fibres (NLM 202, from Nippon Carbon, Tokyo) [37]. More recently HiNicalon fibres were also used [45]. As described below, the key point is the formation of a pyrocarbon layer at the fibre/matrix interface because SiC Nicalon/glass–ceramic composites are reactive systems during the processing of the composite. Others types of fibres were occasionally used as reinforcement, for example HPZ (Si–C–O–N) fibres [23] and Tyranno fibres containing 2% Ti [7]. Reaction products, Si2N2O or TiC with HPZ fibres or Tyranno respectively, can be formed between the matrix and the fibre depending on the glass composition. Glass and glass–ceramic matrices are silicates which exhibit thermal expansion coefficients close to those of the SiC fibres (3–5 10−6·K−1). Examples of matrices include “pure” silica, low-expansion borosilicate glasses (Pyrex) or Duran glasses (B2O3–Na2O–SiO2) and alumino-silicates: LAS (Li2O–Al2O3–SiO2), CAS (CaO–Al2O3–SiO2), BAS (BaO–Al2O3–SiO2), MAS (MgO–Al2O3–SiO2), YMAS (Y2O3–MgO–Al2O3–SiO2), and a combination of these matrices, for example MLAS and BMAS [31,34] or other type such as BSAS or celsian matrix (BaO–SrO– Al2O3–SiO2) [1]. Different kinds of additives, fluxing or refining agents (ZnO, As2O3, Sb2O5) or nucleating agents (TiO2, Nb2O5) to favor the glass–ceramic conversion may act also as a source of oxygen as pointed out by Naslain [31]. Their presence induces a change in the fibre/matrix interface microstructure (see for example Nicalon/LAS composite with or without Nb2O5) [6,9]. The composite in the glass process are prepared according to a pre-preg route comprising a slurry impregnation and hot-pressing steps. Special route such as tape casting has also been used in some cases [28]. After processing, the glass–ceramic matrices crystallized and depending on the matrix composition the following phases are observed: eucryptite (Li2O2–Al2O3–2SiO2), spodumene (LiO2–Al2O3– 4SiO2), petalite (LiO2–Al2O3–8SiO2), cordierite (2MgO– 2Al2O3–5SiO2),mullite (3Al2O3–2SiO2),yttrium disilicate (Y2Si2O7), barium osumilite (BaMg2Al6Si9O30).... The complexity of the matrix microstructure has been reviewed for several ternary and quaternary systems [14]. 3. Interfacial zone in SiC Nicalon/glass–ceramic composites. Experimental results SiC Nicalon/glass–ceramic composites display a complex multilayer fibre/matrix interfacial zone. In-situ reactions occur at the fibre/glass ceramic interfaces during the hot pressing step, typically 1473–1673 K. The interfacial zone is the result of an oxidation of the fibre surface by oxygen of the matrix. Many studies have been performed in order to characterize very carefully the different multilayers observed at the contact zone in a large variety of composites. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) are the main techniques used to investigate the interfacial layer microstructure on cross sections of composites. The cross-sections were prepared by mechanical grinding and thinning by ion-milling. Chemical compositions of interfacial layers were then determined by Energy Dispersive X-ray Spectroscopy (EDX) and Electron Energy Low Spectroscopy (EELS) on thin foils. The composition and the chemistry of the interfacial layer were fully obtained using complementary techniques. Investigations were performed by Secondary-Ion Mass Spectrometry (SIMS) and by Auger Electron Spectroscopy (AES) on fibres extracted from the composites by dissolution of the matrix in a hydrofluoric acid bath. X-rays Photoelectron Spectroscopy (XPS) analyses on fibres extracted from the composite too were carried out to confirm EELS and AES results. These complementary techniques were developed particularly, for example, by Lancin et al. [22], Ponthieu et al. [35] and Le Strat [25,26] on Nicalon/LAS and Nicalon/Pyrex composites. As shown below, these data were the bases of the different models proposed for explaining the complex interface microstructure formation currently found in glass–ceramic composites. A key point from a mechanical point of view is the presence of a thin layer of carbon, often found textured. The carbon-rich layer is relatively weak and consequently increases the fracture toughness of the composite. It allows crack deflection along the fibre/matrix interface and load transfer to occur from matrix to fibre. The nature of the interfacial zone, its thickness and its kinetics of growth depend on many parameters such as the glass composition, the hot-pressing condition.... Two examples of HRTEM studies associated with EDX analyses concerning the microstructure of the different layers up to the nanoscale and their composition are described afterwards. They have been taken in the SiC Nicalon/YMAS composites fabricated by
J, Vicens et al./Aerospace Science and Technology 7(2003)135-146 TL F 50 nm Fig. 1. Transmission electron micrograph of fibre/matrix interface in SiC Nicalon/ YMAS composite. Two interphase sublayers with bright(CL) and dark (TL) contrasts have been formed during processing. Ceramiques Composites(Bazet) and ONERA(Estab- species of the matrix reacting with the SiC nanocrystals of lishment of Palaiseau) in France [15, 46-48 and in Sic the fibre. Oxidation of Sic is known to result either in Sio Nicalon/MLAS composites fabricated by tape-casting [28] and volatile Co or in volatile Sio and CO, depending on TEM and HRTEM observations of the fibre/matrix in- the temperature and oxygen pressure. In the particular case terface reveal that different interphases are formed dur- of Sic Nicalon fibre two mechanisms have been proposed ing processing. An example of a fiber-matrix interphase is This will be explained in the following, but in both cases shown in Fig. 1: the matrix is located in the upper part of the formation of SiO2 and C has been observed. Recent works micrograph. In this case, the matrix is formed by cordierite have shown that a silicon oxicarbide phase can also be phase with small ZrO2 crystals. Two distinct nano-scale sub- formed by oxidation of the SiC crystals of the SiC Nicalon layers are clearly imaged at the interface, both of them are fibre [22, 25, 26, 35, 36 continuous. The sublayer on the matrix side(mean average The microstructure of the interface in the dark layer thickness of 80 nm) has a bright contrast, while the one on denoted transition layer(Tl) has been studied by HRTEM the fibre side(mean average thickness of 100 nm)is dark. An example is shown in Fig 3a and 3b, which illustrates They have been denoted carbon layer(CL) and transition the microstructure modification in the transition layer(TL) layer(TL) respectively. At low magnification, very small compared to the microstructure of the carbon interface cracks can be viewed at the contact zone between the matrix layer(CL). Indeed, the contact zone between these two and the bright sublayer. This may be due to differences be- interphases(CL and TL) is precisely observed in the tween the elastic constants and the slight difference of ther- area shown in Fig. 3a and an enlarged part of Tl is mal expansion coefficients of the fibre and the matrix shown in Fig. 3b. The turbostratic carbon is still visible A HRTEM micrograph of the bright sublayer belonging in Fig. 3b whereas a large amount of SiC crystals(two to another interface is presented in Fig. 2. Lattice fringes crystals have been arrowed in Fig. 3b)is imaged inside are visible in the whole interface layer with a lattice spacing an amorphous matrix. The Sic crystals in the tL are close to 0.35 nm. As confirmed by EDX analyses(see nanometer-sized and slightly larger than those observed below ) these lattice fringes correspond to(0002)carbon the Nicalon NLM 202 fibre, but their density is lower planes and the microstructure of this layer is typical of a The formation of a transition layer between the carbon turbostratic carbon with a microporous morphology. At the interphase and the Nicalon fibre core has already been the carbon planes have a tendency to be oriented parallel te Nicalon/Duran(B203-Na2O-SiO2)composite [17an. o contact zone between the matrix and the carbon interphase, observed in SiC Nicalon/LAS composite [22, 35], in Si the interface over a distance of 6 nm in this example SiC Nicalon/Pyrex composite[25, 2( The formation of this carbon layer results from reaction The chemical composition of both sublayers has been between matrix and fibre during processing, the oxygen etermined by local EDX analyses across the fibre/matrix
J. Vicens et al. / Aerospace Science and Technology 7 (2003) 135–146 137 Fig. 1. Transmission electron micrograph of fibre/matrix interface in SiC Nicalon/ YMAS composite. Two interphase sublayers with bright (CL) and dark (TL) contrasts have been formed during processing. Céramiques & Composites (Bazet) and ONERA (Establishment of Palaiseau) in France [15,46–48] and in SiC Nicalon/MLAS composites fabricated by tape-casting [28]. TEM and HRTEM observations of the fibre/matrix interface reveal that different interphases are formed during processing. An example of a fiber-matrix interphase is shown in Fig. 1: the matrix is located in the upper part of the micrograph. In this case, the matrix is formed by cordierite phase with small ZrO2 crystals. Two distinct nano-scale sublayers are clearly imaged at the interface, both of them are continuous. The sublayer on the matrix side (mean average thickness of 80 nm) has a bright contrast, while the one on the fibre side (mean average thickness of 100 nm) is dark. They have been denoted carbon layer (CL) and transition layer (TL) respectively. At low magnification, very small cracks can be viewed at the contact zone between the matrix and the bright sublayer. This may be due to differences between the elastic constants and the slight difference of thermal expansion coefficients of the fibre and the matrix. A HRTEM micrograph of the bright sublayer belonging to another interface is presented in Fig. 2. Lattice fringes are visible in the whole interface layer with a lattice spacing close to 0.35 nm. As confirmed by EDX analyses (see below), these lattice fringes correspond to (0002) carbon planes and the microstructure of this layer is typical of a turbostratic carbon with a microporous morphology. At the contact zone between the matrix and the carbon interphase, the carbon planes have a tendency to be oriented parallel to the interface over a distance of ∼6 nm in this example. The formation of this carbon layer results from reaction between matrix and fibre during processing, the oxygen species of the matrix reacting with the SiC nanocrystals of the fibre. Oxidation of SiC is known to result either in SiO2 and volatile CO or in volatile SiO and CO, depending on the temperature and oxygen pressure. In the particular case of SiC Nicalon fibre two mechanisms have been proposed. This will be explained in the following, but in both cases formation of SiO2 and C has been observed. Recent works have shown that a silicon oxicarbide phase can also be formed by oxidation of the SiC crystals of the SiC Nicalon fibre [22,25,26,35,36]. The microstructure of the interface in the dark layer denoted transition layer (TL) has been studied by HRTEM. An example is shown in Fig. 3a and 3b, which illustrates the microstructure modification in the transition layer (TL) compared to the microstructure of the carbon interface layer (CL). Indeed, the contact zone between these two interphases (CL and TL) is precisely observed in the area shown in Fig. 3a and an enlarged part of TL is shown in Fig. 3b. The turbostratic carbon is still visible in Fig. 3b whereas a large amount of SiC crystals (two crystals have been arrowed in Fig. 3b) is imaged inside an amorphous matrix. The SiC crystals in the TL are nanometer-sized and slightly larger than those observed in the Nicalon NLM 202 fibre, but their density is lower. The formation of a transition layer between the carbon interphase and the Nicalon fibre core has already been observed in SiC Nicalon/LAS composite [22,35], in SiC Nicalon/Duran (B2O3–Na2O–SiO2) composite [17] and in SiC Nicalon/Pyrex composite [25,26]. The chemical composition of both sublayers has been determined by local EDX analyses across the fibre/matrix
J Vicens et al. Aerospace Science and Technology 7(2003)135-146 CL M nm Fig. 2. HRTEM micrograph of the carbon layer where turbostratic carbon can be seen at the contact zone with the fibre. ole of a co fibre interface in the SiC Nicalon/YMAS. Yttrium di- centration profile taken at the FM interface in a SiC silicate(Y2Si2O7), and cordierite(Mg2 Al4 Sis O18)are the Nicalon/MLAS composite fabricated by tape-casting [28]is main phases crystallized in the matrix. Spectra taken at given in Fig. 5. This concentration profile was taken pre- the fibre surface (at 100 nm from the CL/TL interface) cisely through a SiC Nicalon/spodumene interface where (Fig. 4a), in the dark sublayer(Fig. 4b), in the carbon both sublayers(TL and Cl)were clearly imaged as in Fig. I interphase(CL)(Fig. 4c), at the carbon/matrix interfaces for the Sic Nicalon/Y MAS composite. The first layer(TL) CL/Mg2 Al4 SisO18, )and in Mg2 Al4SisO18Fig 4d and 4e, 400 nm) is enriched in oxygen and displays some amount
138 J. Vicens et al. / Aerospace Science and Technology 7 (2003) 135–146 Fig. 2. HRTEM micrograph of the carbon layer where turbostratic carbon can be seen at the contact zone with the fibre. interface in the following configurations: Mg2Al4Si5O18 fibre interface in the SiC Nicalon/YMAS. Yttrium disilicate (Y2Si2O7), and cordierite (Mg2Al4Si5O18) are the main phases crystallized in the matrix. Spectra taken at the fibre surface (at 100 nm from the CL/TL interface) (Fig. 4a), in the dark sublayer (Fig. 4b), in the carbon interphase (CL) (Fig. 4c), at the carbon/matrix interfaces (CL/Mg2Al4Si5O18, ) and in Mg2Al4Si5O18 (Fig. 4d and 4e, respectively) are displayed. Another example of a concentration profile taken at the FM interface in a SiC Nicalon/MLAS composite fabricated by tape-casting [28] is given in Fig. 5. This concentration profile was taken precisely through a SiC Nicalon/spodumene interface where both sublayers (TL and CL) were clearly imaged as in Fig. 1 for the SiC Nicalon/YMAS composite. The first layer (TL) (∼400 nm) is enriched in oxygen and displays some amount
J, Vicens et al./Aerospace Science and Technology 7(2003)135-146 液b整 Fig 3.(a) HRTEM image of the contact zone between CL(upper part) and tl (lower part). (b) Enlargement of the precedent zone showing the turbostratic carbon and SiC nanocrystals in TL. of Al and Mg. The carbon layer(100 nm) also contains fibres can vary slightly from one batch to another [36]. The oxygen. A very thin silica layer(20 nm) was found at the dark sublayer(TL)is characterised by an enrichment in CL/matrix interface oxygen and significant quantities of Mg and Al compared The following conclusions can be derived from all these to the fibre surface. The chemical nature of the carbon results obtained in Nicalon/YMAS or MLAS composites. units phase CL is confirmed by EDX The carbon layer The fibre surface is oxygen enriched compared with the contains also oxygen, silicon, magnesium and aluminium fibre core. This is in agreement with previous results which elements, however the silicon content remains low. The have clearly shown an enrichment in oxygen at the surface matrix close to the carbon layer exhibits an enrichment in of the fibres, although the chemical content of Sic Nicalon silicon. The transition layer appears as a diffusion zone for
J. Vicens et al. / Aerospace Science and Technology 7 (2003) 135–146 139 Fig. 3. (a) HRTEM image of the contact zone between CL (upper part) and TL (lower part). (b) Enlargement of the precedent zone showing the turbostratic carbon and SiC nanocrystals in TL. of Al and Mg. The carbon layer (∼100 nm) also contains oxygen. A very thin silica layer (∼20 nm) was found at the CL/matrix interface. The following conclusions can be derived from all these results obtained in Nicalon/YMAS or MLAS composites. The fibre surface is oxygen enriched compared with the fibre core. This is in agreement with previous results which have clearly shown an enrichment in oxygen at the surface of the fibres, although the chemical content of SiC Nicalon fibres can vary slightly from one batch to another [36]. The dark sublayer (TL) is characterised by an enrichment in oxygen and significant quantities of Mg and Al compared to the fibre surface. The chemical nature of the carbon units phase CL is confirmed by EDX. The carbon layer contains also oxygen, silicon, magnesium and aluminium elements, however the silicon content remains low. The matrix close to the carbon layer exhibits an enrichment in silicon. The transition layer appears as a diffusion zone for