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
J Vicens et al. Aerospace Science and Technology 7(2003)135-146 Si Matrix d|兽 1入 Fig 4. EDX spectra taken: (a) at the fibre surface(at 100 nm from the CL/TL interface);(b)in the dark sublayer, (c)in the carbon interface; (d) at the carbon natrix interface(CL-Mg2Al4Sis O18);(e)in Mg2Al4Sis O18 300 2900 10 00 00-300-200-100 0 300400500600700 fibre interphase spodumene SiOz Fig. 5. Concentration profile taken through a SiC Nicalon/spodumene interface in a SiC Nicalon/MLAS composite fabricated by tape-casting matrix elements, except for yttrium in SiC Nicalon/YMAs using a combination of several analytical techniques(AES which does not diffuse into the fibre. Recently a silicon EELS, XPS)[22, 25, 26, 35]. The evolutions of the sublayers oxycarbide phase resulting from SiC oxidation has been with the temperature and duration of processing have also claimed to be formed in the transition layers observed in Sic been studied in SiC Nicalon/Pyrex composites Nicalon/LAS [22, 351, SiC Nicalon Duran [17, 18] and SiC In conclusion, in glass or glass ceramic composites Nicalon Pyrex composites [25, 26]. This has been shown by thin layer of carbon often textured acts as a mechanical
140 J. Vicens et al. / Aerospace Science and Technology 7 (2003) 135–146 Fig. 4. EDX spectra taken: (a) at the fibre surface (at 100 nm from the CL/TL interface); (b) in the dark sublayer; (c) in the carbon interface; (d) at the carbon matrix interface (CL–Mg2Al4Si5O18); (e) in Mg2Al4Si5O18. Fig. 5. Concentration profile taken through a SiC Nicalon/spodumene interface in a SiC Nicalon/MLAS composite fabricated by tape-casting. matrix elements, except for yttrium in SiC Nicalon/YMAS which does not diffuse into the fibre. Recently a silicon oxycarbide phase resulting from SiC oxidation has been claimed to be formed in the transition layers observed in SiC Nicalon/LAS [22,35], SiC Nicalon Duran [17,18] and SiC Nicalon Pyrex composites [25,26]. This has been shown by using a combination of several analytical techniques (AES, EELS, XPS) [22,25,26,35]. The evolutions of the sublayers with the temperature and duration of processing have also been studied in SiC Nicalon/Pyrex composites. In conclusion, in glass or glass ceramic composites the thin layer of carbon often textured acts as a mechanical
J, Vicens et al./Aerospace Science and Technology 7(2003)135-146 14 500pm Fig 6 SEM micrograph of a fractured creep sample at 1223 K under 150 MPa. CL N Fig. 7. TEM micrograph of a totally debonded FM interface of the fractured crept sample shown in Fig. 6 fuse with very low interfacial debonding energy values(r) in air(Fig. 6). At the surface of the specimens exposed (up to and low medium interfacial shear resistance(ti) depending 400 um to the surface) the oxidation of the carbon induces on the state of residual stresses [4, 7, 13, 27]. The weak fi- silica formation at the fibre/matrix interface and a brittle be- bre/interphase interface(carbon-fibre)promotes a large pull- havior of the components(Fig. 6). A TEM micrograph of a out of fibres(up to 1 mm) as observed in Nicalon/YMAs totally debonded fibre/matrix interface of a fractured crept composites fractured during creep tests in air at 1223 K un- sample is shown in Fig. 7. The interface debonding took der 150 MPa[15]. An example is shown on the fractured su place at the contact zone between the carbon interphase and face of a specimen of Sic Nicalon/Y MAS composite cre the matrix which was enriched in silica. The formation of
J. Vicens et al. / Aerospace Science and Technology 7 (2003) 135–146 141 Fig. 6. SEM micrograph of a fractured creep sample at 1223 K under 150 MPa. Fig. 7. TEM micrograph of a totally debonded FM interface of the fractured crept sample shown in Fig. 6. fuse with very low interfacial debonding energy values (Γ ) and low medium interfacial shear resistance (τi) depending on the state of residual stresses [4,7,13,27]. The weak fi- bre/interphase interface (carbon-fibre) promotes a large pullout of fibres (up to 1 mm) as observed in Nicalon/YMAS composites fractured during creep tests in air at 1223 K under 150 MPa [15]. An example is shown on the fractured surface of a specimen of SiC Nicalon/YMAS composite crept in air (Fig. 6). At the surface of the specimens exposed (up to 400 µm to the surface) the oxidation of the carbon induces silica formation at the fibre/matrix interface and a brittle behavior of the components (Fig. 6). A TEM micrograph of a totally debonded fibre/matrix interface of a fractured crept sample is shown in Fig. 7. The interface debonding took place at the contact zone between the carbon interphase and the matrix which was enriched in silica. The formation of
42 J Vicens et al. Aerospace Science and Technology 7(2003)135-146 M Sio F 0.1m Fig. 8. Silica layer formed at the fibre/matrix interface in a sample annealed at 1223 K for 50 h in silica(0.3 um) at the fibre/matrix interface can be also ob- According to these authors, the silica activity in the glass- served by TEM after the creep test in the oxidized part of the ceramic matrix is suggested to play a primary role to sample(Fig. 8) control the chemical reaction which creates the carbon laver The formation of the carbon layer by reaction(1)is rate- limited by the diffusion of silica from the fibre to the 4. Models for layers formation on glass-ceramic matrix through the silica activity gradient. This gradient is composites determined by the basicity of the matrix glass. As pointed out by Le Strat and Le Strat et al. [25, 26], the Cooper- A first detailed investigation of layer formation at the Chyung model is based upon the hypotheses that Sic fibre/matrix interface has been performed by Cooper Chyung [12] in SiC/LAS and SiC/CAS composites. TEM SIC, C and Sioz, implying that the activities of these phases studies have revealed a carbon-rich reaction layer in all are equal to unity. Several studies have subsequently shown that Sic Nicalon fibres contain a significant amount of the generation of silica and carbon was thermodynamically silica oxycarbide and a very small amount of sioz [211 favored by the following chemical reaction is not thermodynamically stable [24]. The microstructural SiC(s)+ O2(g)- SiOz(s)+ C(s) observation of a silica-rich region between the matrix and
142 J. Vicens et al. / Aerospace Science and Technology 7 (2003) 135–146 Fig. 8. Silica layer formed at the fibre/matrix interface in a sample annealed at 1223 K for 50 h in air. silica (∼0.3 µm) at the fibre/matrix interface can be also observed by TEM after the creep test in the oxidized part of the sample (Fig. 8). 4. Models for layers formation on glass–ceramic composites A first detailed investigation of layer formation at the fibre/matrix interface has been performed by Cooper et Chyung [12] in SiC/LAS and SiC/CAS composites. TEM studies have revealed a carbon-rich reaction layer in all composites. According to thermochemical considerations, the generation of silica and carbon was thermodynamically favored by the following chemical reaction: SiC(s) + O2(g) → SiO2(S) + C(s) (1) According to these authors, the silica activity in the glass– ceramic matrix is suggested to play a primary role to control the chemical reaction which creates the carbon layer. The formation of the carbon layer by reaction (1) is ratelimited by the diffusion of silica from the fibre to the matrix through the silica activity gradient. This gradient is determined by the basicity of the matrix glass. As pointed out by Le Strat and Le Strat et al. [25,26], the Cooper– Chyung model is based upon the hypotheses that SiC Nicalon fibres are thermodynamically stable and contain SiC, C and SiO2, implying that the activities of these phases are equal to unity. Several studies have subsequently shown that SiC Nicalon fibres contain a significant amount of silica oxycarbide and a very small amount of SiO2 [21]. Other studies have demonstrated that the SiC Nicalon fibre is not thermodynamically stable [24]. The microstructural observation of a silica-rich region between the matrix and
J, Vicens et al./Aerospace Science and Technology 7(2003)135-146 nearby pure crystalline carbon layer as well as the planar some carbon units embedded in an amorphous oxycarbide face between these reaction phases gave rise to new intergranular phase [211 statements. These observations were performed in SIC/CAS As proposed by Le Strat et al. and Le Strat [25, 26 the by Bonney and Cooper [5]. They proposed a solid state reaction begins by oxygen diffusion from the glass to the displacement reaction being rate-limited by the diffusion fibre and inside the fibre through the silicon oxycarbide of oxygen through the silica layer. Other models including phase where it exchanges with silicon or carbon. At the a passive oxidation reaction of Sic by Co has also been same time, silicon and carbon elements diffuse through the proposed [3, 30]. Benson et al. 3] supposed two steps in concentration gradient from the fibre to the glass. Oxygen carbon layer formation, depending on the partial pressure of which migrates in the oxycarbide phase comes in contact CO. The first stage is the formation of an initial carbon layer with SiC, forming a new oxycarbide phase. The carbon and by the reaction of Sic and Oz corresponding to reaction(1). silicon resulting from the oxidation of SiC and SiO.Cy and In the second stage, the carbon layer formation is controlled the pre-existing carbon in the fibres migrate towards the by the two following chemical reactions fibre surface. Silicon is incorporated in the glass forming 2Cs+O2(g)→2COg) SiOz. The carbon crystallizes at the glass interface and the carbon layer grows at the fibre surface. The oxygen SiCs+2 CO(g)- 3Cs SiO(s) ( amount available for diffusion decreases progressively in at the matrix/fibre interface and at the carba e spectively the glass. This slows down the reaction rate. A phase reorganization then occurs leading to the formation of the faces.In this model the carbon layer thickness depends on two sublayers. In the transition layer, the oxycarbide phase the oxygen diffusion rate inside the matrix and on that of tends to an equilibrium composition by rejecting some CO(g) in the carbon layer elements in excess. This induces the formation of an oxygen A phenomenological model describing the kinetics of rich silicon oxycarbide phase(45 at %), the growing of the carbon interfaces formation at SiC-glass interfaces of SiC nanocrystals and the carbon unit formation SIC/LAS and SiC/CAS composites has been presented by Qi et al. [38]. The model shows how variables(process pa- rameters, addition of oxides such as As203, melting history 5. ic/Bn dual-coated Nicalon-fibre-reinforced which can modify the oxygen activity in the glass)influence glass-ceramic matrix composites the carbon layer thickness and its stability Recently a model has been published by Hahnel et al. [18]. 5.1. The concept of the dual Sic/BN interphase They proposed a multiple-step reaction to describe their ob- ervations of the complex phases arrangement(carbon, car- This part describes recent works performed by the team bon and silica, silica) between the fibre and the matrix(Sic of J.J. Brennan at the United Technologies Research Cen- Nicalon/Duran composite). The thermodynamic approach ter, East Harford, Connecticut, USA, on a new Sic/bn dual ields to a four stage model of layer formation including the coated Sic Nicalon fibre reinforced BMAS matrix com- active oxidation of sic posites. Mechanical behavior and associated microstructure An interesting work performed in the SiC/Pyrex compos- changes were very carefully found out [6,, 40-43, 491 [25, 26] described a mechanism to explain the two transi- The embrittlement of the glass-ceramic matrix which tions and carbon layers formation very often observed at the appears in oxidizing environment can be either the con- fibre/glass interface in many composites [17, 22, 35]. This has sequence of oxygen diffusion to the fibre/matrix interface not been clearly explained in previous models. In this model via matrix microcracks reacting with the carbon layer or a three points were emphasized. A first point concerns with the"pipeline diffusion"from cut ends of fibres exposed to air at formation of the TL oxygen-rich interphase located between the composite surface. To maintain an oxidative stability at the CL and the fibre as illustrated before. Recent results sup- high temperatures, one approach consists of using fibre coat- ported the existence of Sio, Cy phases instead of SiO2 in ings applied to the fibres prior to composite processing Such TL. These oxycarbides are the same phases as those in the coatings should have two key functions: a mechanical fuse SiC Nicalon fibres but with higher contents compared with to allow crack deflection and load transfer from matrix to fi- SiC Nicalon fibre. A second point of this description is the bre and an improvement of the oxidation resistance. Studies resence of a certain amount of dissolved oxygen atoms in were carried out to replace the carbon-rich zone with a sim the glass with a fast diffusion rate(10-8 cm-s-)and ilar weakly bonded coating which also improves oxidative of non-bridging oxygen atoms with diffusion coefficients stability. CVD coatings such as BN and Sic or BN were de- between 10-14-10-12 cm s-I. It was also suggested that posited on SiC Nicalon fibres, on SiN-C HPZ fibre(Dow the diffusing species are atomic oxygen instead of O2(g) or Corning Corp )[44] and extended to composite with Hi of Co(g). From these considerations, diffusion lengths( Dt) Nicalon fibre [1]. These coated fibres were incorporated into were calculated using the hot pressing duration of the com- LAS, BMAS [8, 40] and BsAs [l]. The BN coatings(100- posites. A third point concerns the particular microstruc- 300 nm thick) were either amorphous or partly turbostratic ture of the Nicalon fibre consisting of SiC nanocrystals with as it is usually the case for low temperature BN-CVD. The
J. Vicens et al. / Aerospace Science and Technology 7 (2003) 135–146 143 the nearby pure crystalline carbon layer as well as the planar interface between these reaction phases gave rise to new statements. These observations were performed in SiC/CAS by Bonney and Cooper [5]. They proposed a solid state displacement reaction being rate-limited by the diffusion of oxygen through the silica layer. Other models including a passive oxidation reaction of SiC by CO has also been proposed [3,30]. Benson et al. [3] supposed two steps in the carbon layer formation, depending on the partial pressure of CO. The first stage is the formation of an initial carbon layer by the reaction of SiC and O2 corresponding to reaction (1). In the second stage, the carbon layer formation is controlled by the two following chemical reactions: 2Cs + O2(g) → 2 CO(g) (2) SiCs + 2 CO(g) → 3Cs + SiO2(s) (3) Reactions (2) and (3) are supposed to occur respectively at the matrix/fibre interface and at the carbon/fibre interfaces. In this model the carbon layer thickness depends on the oxygen diffusion rate inside the matrix and on that of CO(g) in the carbon layer. A phenomenological model describing the kinetics of carbon interfaces formation at SiC–glass interfaces of SiC/LAS and SiC/CAS composites has been presented by Qi et al. [38]. The model shows how variables (process parameters, addition of oxides such as As2O3, melting history which can modify the oxygen activity in the glass) influence the carbon layer thickness and its stability. Recently a model has been published by Hähnel et al. [18]. They proposed a multiple-step reaction to describe their observations of the complex phases arrangement (carbon, carbon and silica, silica) between the fibre and the matrix (SiC Nicalon/Duran composite). The thermodynamic approach yields to a four stage model of layer formation including the active oxidation of SiC. An interesting work performed in the SiC/Pyrex composite [25,26] described a mechanism to explain the two transitions and carbon layers formation very often observed at the fibre/glass interface in many composites [17,22,35]. This has not been clearly explained in previous models. In this model three points were emphasized. A first point concerns with the formation of the TL oxygen-rich interphase located between the CL and the fibre as illustrated before. Recent results supported the existence of SiOxCy phases instead of SiO2 in TL. These oxycarbides are the same phases as those in the SiC Nicalon fibres but with higher contents compared with SiC Nicalon fibre. A second point of this description is the presence of a certain amount of dissolved oxygen atoms in the glass with a fast diffusion rate (∼10−8 cm−1·s−1) and of non-bridging oxygen atoms with diffusion coefficients between 10−14–10−12 cm·s−1. It was also suggested that the diffusing species are atomic oxygen instead of O2(g) or of CO(g). From these considerations, diffusion lengths (Dt) were calculated using the hot pressing duration of the composites. A third point concerns the particular microstructure of the Nicalon fibre consisting of SiC nanocrystals with some carbon units embedded in an amorphous oxycarbide intergranular phase [21]. As proposed by Le Strat et al. and Le Strat [25,26], the reaction begins by oxygen diffusion from the glass to the fibre and inside the fibre through the silicon oxycarbide phase where it exchanges with silicon or carbon. At the same time, silicon and carbon elements diffuse through the concentration gradient from the fibre to the glass. Oxygen which migrates in the oxycarbide phase comes in contact with SiC, forming a new oxycarbide phase. The carbon and silicon resulting from the oxidation of SiC and SiOxCy and the pre-existing carbon in the fibres migrate towards the fibre surface. Silicon is incorporated in the glass forming SiO2. The carbon crystallizes at the glass interface and the carbon layer grows at the fibre surface. The oxygen amount available for diffusion decreases progressively in the glass. This slows down the reaction rate. A phase reorganization then occurs leading to the formation of the two sublayers. In the transition layer, the oxycarbide phase tends to an equilibrium composition by rejecting some elements in excess. This induces the formation of an oxygenrich silicon oxycarbide phase (45 at %), the growing of the SiC nanocrystals and the carbon unit formation. 5. SiC/BN dual-coated Nicalon-fibre-reinforced glass–ceramic matrix composites 5.1. The concept of the dual SiC/BN interphase This part describes recent works performed by the team of J.J. Brennan at the United Technologies Research Center, East Harford, Connecticut, USA, on a new SiC/BN dual coated SiC Nicalon fibre reinforced BMAS matrix composites. Mechanical behavior and associated microstructure changes were very carefully found out [6,8,40–43,49]. The embrittlement of the glass–ceramic matrix which appears in oxidizing environment can be either the consequence of oxygen diffusion to the fibre/matrix interface via matrix microcracks reacting with the carbon layer or a “pipeline diffusion” from cut ends of fibres exposed to air at the composite surface. To maintain an oxidative stability at high temperatures, one approach consists of using fibre coatings applied to the fibres prior to composite processing. Such coatings should have two key functions: a mechanical fuse to allow crack deflection and load transfer from matrix to fi- bre and an improvement of the oxidation resistance. Studies were carried out to replace the carbon-rich zone with a similar weakly bonded coating which also improves oxidative stability. CVD coatings such as BN and SiC or BN were deposited on SiC Nicalon fibres, on SiN–C HPZ fibre (Dow Corning Corp.) [44] and extended to composite with Hi Nicalon fibre [1]. These coated fibres were incorporated into LAS, BMAS [8,40] and BSAS [1]. The BN coatings (100– 300 nm thick) were either amorphous or partly turbostratic as it is usually the case for low temperature BN-CVD. The
J Vicens et al. Aerospace Science and Technology 7(2003)135-146 BN coating was close to stoichiometric BN with an excess of studied by Sun et al. [42]. Below 1408K, the constant carbon In BN(C)/LAS composites, Si and Al diffuse from creep rates were extremely low(10-9.s-)and at 1473K the matrix into the coating introducing the crystallization constant creep rates were an order magnitude higher. The of BN, and bore diffuses into the matrix in opposite di- 0/90 fibre-reinforced composites exhibited long creep- rection impeding the glass-ceramic conversion. The resid- strain recovery. From the microstructure investigations, it ual boron-bearing glassy regions degrade the high tempera- was concluded that the dual sic/bn coating provides an ture strength. In order to prevent interdiffusion phenomena, effective barrier to reaction and diffusion. Moreover the a Sic diffusion barrier(100-250 nm thick) was deposited n coating allows debonding to occur with an extensive CVD on the BN(C)coating. These studies performed on Si ibre pull-out of the fibres. Tensile fatigue experiments were Nicalon/LAS and Sic Nicalon/BMAS composites fabricated also conducted. The composites survived 10 cycles without with such BN/SiC dual interphase showed that such com- fracture up to 1473 K under a maximum stress slightly posites have significant potential properties for applications higher than the proportional limit stress of the matrix to-1473 K. They could withstand stress levels higher than More recently Widjaja et al. [49] found a creep-induced the proportional limit at high temperatures for long period residual stress strengthening mechanism in SiC Nicalon- This short review demonstrates the interest of the dual fibre/BMAS composites After creep experiment at 1373K, interphase concept: one layer(BN) coating as an oxidation the BMAs matrix can be put in compression by the elastic resistant and mechanical fuse and the other(SiC) as an recovery of the fibres if the load is removed at room oxidation resistant diffusion barrier. One problem which temperature. This increases the stress at which matrix ne Bn layer which is attribute g of the SiC layer over cracking begins. The state of residual stresses was supported ed to thermally induced by X-ray diffraction results. It was shown also that the stresses [40] creep-load transfer process did not embrittle the fibre/matrix interface because fibre pull-out behavior was maintained 5.2. Mechanical behaviors and microstructure of sic/BM One problem which demands further studies is the debond dual-coated Nicalon/BMAS composites ing of the SiC overlayer from the BN layer during matrix infiltration [40]. Different coatings were also investigated Sic/Bn dual-coated Nicalon-fibre-reinforced BMAS ma- thinner SiC/BN coatings, Si3N4/BN coatings and BN coat trix composites(0/90 cross plies) were studied by Sun et ings alone for example[43]- Si3N4 is not suitable because it al.[41]. The mechanical properties of the composites were dissolves in the BMAs matrix during composite fabrication evaluated by three point bending and tensile testing at both It was concluded that further improvements in the perfor- room temperature and high temperatures(up to 1573 K)in mance of Sic/Bn coating may be expected through an in- r The composite strength was excellent up to 1473 K, the crease of the SiC coating with a bn thickness of 350 nm ultimate strength at 1473 K and the elastic modulus were The introduction of roughness with Bn underlayers may found respectively to be 565 MPa and 69 GPa. Mechanical also increase adhesion of the Sic overlayer properties decrease significantly at 1573 K because of ma- trix softening. A degradation of the mechanical properties occurs after annealing in air for 500 h at 1473 K 6. Conclusion ush-out experiments showed a strongly fibre/matrix interface with a debonding energy (J/m)equal to 8. J/m Considerable interest has been placed upon and a frictional sliding stress t 139 MPa in the ceramized opment of glass and glass ceramic matrix compe state. This is much higher than values observed in classical intermediate-temperature applications(1473 K). This ex glass ceramic composites [31] plains the numerous varieties of glass-ceramic composites Interfaces were studied by TEM and observations were reinforced by SiC fibres which have been fabricated. De- correlated with mechanical properties. It was shown that a spite these high number of different types of composites, nanoscale silica/carbon sublayer was formed at the BN/Sic this class of composites displays very typical fibre-matrix Nicalon fibre interface during long-term exposure to oxy- interphase characteristics. a quite good description of the gen at high temperature. This sublayer appearing between complex nature of the fibre-matrix interfacial zone is now 1373 K and 1473 K was supposed to be responsible for the available. This has been possible using a combination of decrease in the fibre/matrix bonding strength at high tem- a high number of very specific analytical techniques up perature. But good interfacial properties are maintained at the nanometer scale on thin foils of composites or on ex 1373 K for long-term exposures tracted fibres from the matrix. Using these complementary techniques and by studying composites with different ma- 5.3. Bending creep behaviourof sic/BN coated trices, a mechanism for explaining the developed interfaces fibre/BMAS composites and the kinetic of formation has been proposed Among the different glass-ceramic matrix composites The flexural creep and fatigue behavior of the the sic/bn dual coated Sic Nicalon fibre reinforced BMas composites and the associated microstructure changes matrix composites emerges as very good candidates for low-
144 J. Vicens et al. / Aerospace Science and Technology 7 (2003) 135–146 BN coating was close to stoichiometric BN with an excess of carbon. In BN(C)/LAS composites, Si and Al diffuse from the matrix into the coating introducing the crystallization of BN, and bore diffuses into the matrix in opposite direction impeding the glass–ceramic conversion. The residual boron-bearing glassy regions degrade the high temperature strength. In order to prevent interdiffusion phenomena, a SiC diffusion barrier (100–250 nm thick) was deposited by CVD on the BN(C) coating. These studies performed on SiC Nicalon/LAS and SiC Nicalon/BMAS composites fabricated with such BN/SiC dual interphase showed that such composites have significant potential properties for applications to ∼1473 K. They could withstand stress levels higher than the proportional limit at high temperatures for long period. This short review demonstrates the interest of the dual interphase concept: one layer (BN) coating as an oxidation resistant and mechanical fuse and the other (SiC) as an oxidation resistant diffusion barrier. One problem which needs more study is the debonding of the SiC layer over the BN layer which is attributed to thermally induced stresses [40]. 5.2. Mechanical behaviors and microstructure of SiC/BN dual-coated Nicalon/BMAS composites SiC/BN dual-coated Nicalon-fibre-reinforced BMAS matrix composites (0/90◦ cross plies) were studied by Sun et al. [41]. The mechanical properties of the composites were evaluated by three point bending and tensile testing at both room temperature and high temperatures (up to 1573 K) in air. The composite strength was excellent up to 1473 K, the ultimate strength at 1473 K and the elastic modulus were found respectively to be 565 MPa and 69 GPa. Mechanical properties decrease significantly at 1573 K because of matrix softening. A degradation of the mechanical properties occurs after annealing in air for 500 h at 1473 K. Push-out experiments showed a strongly fibre/matrix interface with a debonding energy (J/m2) equal to 8.65 J/m2 and a frictional sliding stress τ ∼ 139 MPa in the ceramized state. This is much higher than values observed in classical glass ceramic composites [31]. Interfaces were studied by TEM and observations were correlated with mechanical properties. It was shown that a nanoscale silica/carbon sublayer was formed at the BN/SiC Nicalon fibre interface during long-term exposure to oxygen at high temperature. This sublayer appearing between 1373 K and 1473 K was supposed to be responsible for the decrease in the fibre/matrix bonding strength at high temperature. But good interfacial properties are maintained at 1373 K for long-term exposures. 5.3. Bending creep behaviour of SiC/BN coated fibre/BMAS composites The flexural creep and fatigue behavior of the same composites and the associated microstructure changes were studied by Sun et al. [42]. Below 1408 K, the constant creep rates were extremely low (∼10−9·s−1) and at 1473 K constant creep rates were an order magnitude higher. The 0/90◦ fibre-reinforced composites exhibited long creepstrain recovery. From the microstructure investigations, it was concluded that the dual SiC/BN coating provides an effective barrier to reaction and diffusion. Moreover the BN coating allows debonding to occur with an extensive fibre pull-out of the fibres. Tensile fatigue experiments were also conducted. The composites survived 105 cycles without fracture up to 1473 K under a maximum stress slightly higher than the proportional limit stress of the matrix. More recently Widjaja et al. [49] found a creep-induced residual stress strengthening mechanism in SiC Nicalon- fibre/BMAS composites. After creep experiment at 1373 K, the BMAS matrix can be put in compression by the elastic recovery of the fibres if the load is removed at room temperature. This increases the stress at which matrix cracking begins. The state of residual stresses was supported by X-ray diffraction results. It was shown also that the creep-load transfer process did not embrittle the fibre/matrix interface because fibre pull-out behavior was maintained. One problem which demands further studies is the debonding of the SiC overlayer from the BN layer during matrix infiltration [40]. Different coatings were also investigated: thinner SiC/BN coatings, Si3N4/BN coatings and BN coatings alone for example [43]. Si3N4 is not suitable because it dissolves in the BMAS matrix during composite fabrication. It was concluded that further improvements in the performance of SiC/BN coating may be expected through an increase of the SiC coating with a BN thickness of 350 nm. The introduction of roughness with BN underlayers may also increase adhesion of the SiC overlayer. 6. Conclusion Considerable interest has been placed upon the development of glass and glass ceramic matrix composites for intermediate-temperature applications (∼1473 K). This explains the numerous varieties of glass–ceramic composites reinforced by SiC fibres which have been fabricated. Despite these high number of different types of composites, this class of composites displays very typical fibre-matrix interphase characteristics. A quite good description of the complex nature of the fibre-matrix interfacial zone is now available. This has been possible using a combination of a high number of very specific analytical techniques up to the nanometer scale on thin foils of composites or on extracted fibres from the matrix. Using these complementary techniques and by studying composites with different matrices, a mechanism for explaining the developed interfaces and the kinetic of formation has been proposed. Among the different glass–ceramic matrix composites, the SiC/BN dual coated SiC Nicalon fibre reinforced BMAS matrix composites emerges as very good candidates for low-