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 fabricated136 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 Hi￾Nicalon 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 inter￾face microstructure (see for example Nicalon/LAS compos￾ite with or without Nb2O5) [6,9]. The composite in the glass process are prepared accord￾ing 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: eu￾cryptite (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 com￾plex multilayer fibre/matrix interfacial zone. In-situ reac￾tions 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 multi￾layers 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 inter￾facial layers were then determined by Energy Dispersive X-ray Spectroscopy (EDX) and Electron Energy Low Spec￾troscopy (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) analy￾ses on fibres extracted from the composite too were carried out to confirm EELS and AES results. These complemen￾tary 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 pro￾posed for explaining the complex interface microstructure formation currently found in glass–ceramic composites. A key point from a mechanical point of view is the pres￾ence of a thin layer of carbon, often found textured. The carbon-rich layer is relatively weak and consequently in￾creases 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 in￾terfacial zone, its thickness and its kinetics of growth de￾pend on many parameters such as the glass composition, the hot-pressing condition.... Two examples of HRTEM studies associated with EDX analyses concerning the mi￾crostructure 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