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/matrixJ. 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 (Estab￾lishment 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 in￾terface reveal that different interphases are formed dur￾ing 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 sub￾layers 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 be￾tween the elastic constants and the slight difference of ther￾mal 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