PHILOSOPHICAL MAGAZINE A. 1998. VOL. 78. No. 1. 189-202 Sic Nicalon fibre-glass matrix composites: interphases and their mechanism of formation By E. LE STRAT Laborator d Etudes des materiaux, 29 avenue de la Division Leclerc, Office National d Etudes et de recherches Aerospatiale, Chatillon, France M. LANCIN.N. FOURCHES-COULON and C. marhic ntre de Micro caracteris2m比Hom4 channe, s [Received 16 June 1997 and accepted 17 November 1997] ABSTRACT The fibre-matrix reaction that occurs during the processing of a Nicalon SiC fibre-Pyrex glass matrix composite is analysed. Interphases are characterized by means of various complementary techniques: electron diffraction, HRTEM, EDX, EELS, SIMS and XPS. Neither the results of the present study nor those previously obtained for Nicalon SiC fibre-LAS(Li2O-Al2O-SiO,) glass or available model of reaction. An alternative reaction mechanism is suggested and oxidize SiC and Sio Cy in the fibre. The reaction yields carbon and silicon oxycarbide in the fibre and SiO which dissolves into the matrix. When the oxygen in excess in the matrix is consumed, the reaction stops and the phases on in the reaction layer which generates two interphases one carbon rich and the other silicon oxycarbide rich. These interphases are observed at the fibre periphery in all glass or glass-ceramic matrix composit §1. INTRODUCTION Glass or glass-ceramic matrix composites are developed for thermostructural applications such as in aircraft parts submitted to severe thermomechanical stresses. Glass-ceramic is reinforced by means of long fibres that are essentially stronger and less brittle than the matrix. This reinforcement yields composites with high strength and a'pseudoplastic' strain sufficient to alleviate the catastrophic rupture mode of a brittle material. Amongst commercial fibres, the Sic nicalon 202 fibre has been successfully used in the last years. The mechanical properties of Nicalon-glass matrix composites depend on the nature of the interphases that develop during material processing(Brennan 1988, Lancin 1991). A good characterization of the reaction between the fibre and the matrix together with the understanding of this reaction are essential in order to elaborate composites with required specifications Studies of the interphases produced in the course of the fibre-matrix reaction have been conducted in a number of composites based on various matrices(Brennan 1988, Chen et al. 1989, Bonney and Cooper 1990 ). The most currently used are Li2O-Al2O-SiO,(LAS)or Cao-Al2O3-SiO2( CAS)containing some additives such as niobium, arsenic or yttrium. In order to explain the formation of interphases, one usually refers to the model proposed by Cooper and Chyung(1987) which 0141-8610/98 $1200 C 1998 Taylor Francis Ltd
PHILOSOPHICAL MAGAZINE A, 1998, VOL. 78, NO. 1, 189±202 SiC Nicalon ®bre±glass matrix composites: interphases and their mechanism of formation By E. Le Strat Laboratore d’Etudes des Materiaux, 29 avenue de la Division Leclerc, O ce National d’Etudes et de Recherches Ae rospatiales, Chatillon, France M. Lancin, N. Fourches-Coulon and C. Marhic Centre de Micro Caracte risations, Laboratoire de Physique Cristalline, Institut des Materiaux de Nantes, 2 rue de la HoussinieÁre, 44 322 Nantes, France [Received 16 June 1997 and accepted 17 November 1997] Abstract The ®bre±matrix reaction that occurs during the processing of a Nicalon SiC ®bre±Pyrex glass matrix composite is analysed. Interphases are characterized by means of various complementary techniques: electron di raction, HRTEM, EDX, EELS, SIMS and XPS. Neither the results of the present study nor those previously obtained for Nicalon SiC ®bre±LAS (Li2O±Al2O3±SiO2) glass or LAS + MAS (MgO±Al2O3±SiO2) glass±ceramic matrix composites support the available model of reaction. An alternative reaction mechanism is suggested whereby the dissolved and non-bridging oxygen atoms of the matrix di use and oxidize SiC and SiOxCy in the ®bre. The reaction yields carbon and silicon oxycarbide in the ®bre and SiO2 which dissolves into the matrix. When the oxygen in excess in the matrix is consumed, the reaction stops and the phases undergo a reorganization in the reaction layer which generates two interphases, one carbon rich and the other silicon oxycarbide rich. These interphases are observed at the ®bre periphery in all glass or glass±ceramic matrix composites. § 1. Introduction Glass or glass±ceramic matrix composites are developed for thermostructural applications such as in aircraft parts submitted to severe thermomechanical stresses. Glass±ceramic is reinforced by means of long ®bres that are essentially stronger and less brittle than the matrix. This reinforcement yields composites with high strength and a `pseudoplastic’ strain su cient to alleviate the catastrophic rupture mode of a brittle material. Amongst commercial ®bres, the SiC Nicalon 202 ®bre has been successfully used in the last years. The mechanical properties of Nicalon±glass matrix composites depend on the nature of the interphases that develop during material processing (Brennan 1988, Lancin 1991). A good characterization of the reaction between the ®bre and the matrix together with the understanding of this reaction are essential in order to elaborate composites with required speci®cations. Studies of the interphases produced in the course of the ®bre±matrix reaction have been conducted in a number of composites based on various matrices (Brennan 1988, Chen et al. 1989, Bonney and Cooper 1990). The most currently used are Li2O-Al2O3-SiO2 (LAS) or CaO±Al2O3-SiO2 (CAS) containing some additives such as niobium, arsenic or yttrium. In order to explain the formation of interphases, one usually refers to the model proposed by Cooper and Chyung (1987) which 0141±8610/98 $12.00 Ñ 1998 Taylor & Francis Ltd
E Le strat et al involves the oxidation of Sic grains of the fibre by 'excess oxygen of the matrix. This oxidation would occur according to the following reaction: SiC+O→C+SiO The Cooper-Chyung model is based upon the two fundamental hypotheses: firstly that the Nicalon fibre contains SiC, carbon and Sio and secondly that the fibres are hermodynamically stable, which in turn implies that the activities of the different phases are equal to unity. The diff usion of silicon, carbon and oxygen from the fibre to the matrix is controlled by gradients of activity between these. Several studies have subsequently shown that the fibre is actually made up of Sic and carbon but that it also contains a significant amount of silicon oxycarbide and very little Sioz (Lipowitz et al. 1987, Laffon et al. 1989, Porte and Sartre 1989, Bleay et al. 1992) Other studies have proved that the Nicalon fibre is not thermodynamically stable (Mah et al. 1984, Clark et al. 1985, Johnson et al. 1988, Le Coustumer et al. 1993) Every published study shows that, given the elaboration conditions, a carbon interphase develops whose thickness and structure depend on characteristics of the matrix( Brennan 1988, Cooper and Chyung 1987, Ponthieu et al. 1990). These stu- dies generally agree as to the formation of a NbC interphase between the carbon layer and the Nb2 Os-containing matrix. Other results are more controversial, how- ever. The formation of silica is difficult to demonstrate. The presence of Sio between the carbon phase and the matrix has been identified only rarely by the electron diffraction pattern(EDP) or energy dispersive X-ray spectroscopy DXS)( Cooper and Chyung 1987, Bonney and Cooper 1990, Doreau 1995, Hahnel et al. 1995) but silica could be incorporated in the majority of matrices and then be undetectable. Kumar and Knowles(1996) claim to have localized silica between the carbon phase and the fibre but the edP, on which this analysis relies, cannot discriminate carbon from SiO nor can electron-energy-loss spectroscopy (EELS)differentiate SiO Cy from Sio2+ SiC. By means of high-resolution trans- mission electron microscopy(HRTEM), EDXS, secondary-ion mass spect (SIMS), EELS, Auger electron spectroscopy(AES)and X-ray photoelectro roscopy(XPS), Ponthieu et al. (1990, 1994)and Lancin et al. (1994 have ide an interphase between carbon and the fibre in Nicalon-LAS composites, whose structure and composition differ from those of the fibre. This zone is enriched with oxygen compared with the fibre, it does not contain Sio but essentially SiO Cy as demonstrated by XPS analysis. Since then, Pippel et al.(1995)b EELS, Doreau(1995) by HRTEM and EDXS and Lancin et al.(1993)by HRTEM, EDXS, SIMS and eels have identified this interphase in other compo- sites. It is worth emphasizing that this oxygen-rich interphase can be spotted in every published AES or EDXs analysis even though its existence had not actually been noted(Brennan 1988, Chen et al. 1989, Bonney and Cooper 1990, Homeny et al. 1990). It is, however, not yet proved that the composition of this oxygen-enriched interphase, as revealed by XPS in Nicalon-LAS composites, remains the same gardless of the matrix. The nature and the localization of the phases resulting from the fibre-matrix reaction are thus a matter of debate and this holds true also for their kinetics of formation. According to Cooper and Chyung(1987), Homeny et al.(1990)and Hahnel et al.(1995), the thickness of the carbon interphase increases with increasing hot-pressing temperature or time. By contrast, Brennan(1988), Ponthieu(1990)and
involves the oxidation of SiC grains of the ®bre by `excess’ oxygen of the matrix. This oxidation would occur according to the following reaction: SiC + O2 ® C + SiO2. The Cooper±Chyung model is based upon the two fundamental hypotheses: ®rstly that the Nicalon ®bre contains SiC, carbon and SiO2 and secondly that the ®bres are thermodynamically stable, which in turn implies that the activities of the di erent phases are equal to unity. The di usion of silicon, carbon and oxygen from the ®bre to the matrix is controlled by gradients of activity between these. Several studies have subsequently shown that the ®bre is actually made up of SiC and carbon but that it also contains a signi®cant amount of silicon oxycarbide and very little SiO2 (Lipowitz et al. 1987, La on et al. 1989, Porte and Sartre 1989, Bleay et al. 1992). Other studies have proved that the Nicalon ®bre is not thermodynamically stable (Mah et al. 1984, Clark et al. 1985, Johnson et al. 1988, Le Coustumer et al. 1993). Every published study shows that, given the elaboration conditions, a carbon interphase develops whose thickness and structure depend on characteristics of the matrix (Brennan 1988, Cooper and Chyung 1987, Ponthieu et al. 1990). These studies generally agree as to the formation of a NbC interphase between the carbon layer and the Nb2O5-containing matrix. Other results are more controversial, however. The formation of silica is di cult to demonstrate. The presence of SiO2 between the carbon phase and the matrix has been identi®ed only rarely by the electron di raction pattern (EDP) or energy dispersive X-ray spectroscopy (EDXS) (Cooper and Chyung 1987, Bonney and Cooper 1990, Doreau 1995, HaÈhnel et al. 1995) but silica could be incorporated in the majority of matrices and then be undetectable. Kumar and Knowles (1996) claim to have localized silica between the carbon phase and the ®bre but the EDP, on which this analysis relies, cannot discriminate carbon from SiO2 nor can electron-energy-loss spectroscopy (EELS) di erentiate SiOxCy from SiO2 + SiC. By means of high-resolution transmission electron microscopy (HRTEM), EDXS, secondary-ion mass spectrometry (SIMS), EELS, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), Ponthieu et al. (1990, 1994) and Lancin et al. (1994) have identi®ed an interphase between carbon and the ®bre in Nicalon±LAS composites, whose structure and composition di er from those of the ®bre. This zone is enriched with oxygen compared with the ®bre; it does not contain SiO2 but essentially SiOxCy as demonstrated by XPS analysis. Since then, Pippel et al. (1995) by EELS, Doreau (1995) by HRTEM and EDXS and Lancin et al. (1993) by HRTEM, EDXS, SIMS and EELS have identi®ed this interphase in other composites. It is worth emphasizing that this oxygen-rich interphase can be spotted in every published AES or EDXS analysis even though its existence had not actually been noted (Brennan 1988, Chen et al. 1989, Bonney and Cooper 1990, Homeny et al. 1990). It is, however, not yet proved that the composition of this oxygen-enriched interphase, as revealed by XPS in Nicalon±LAS composites, remains the same regardless of the matrix. The nature and the localization of the phases resulting from the ®bre±matrix reaction are thus a matter of debate and this holds true also for their kinetics of formation. According to Cooper and Chyung (1987), Homeny et al. (1990) and HaÈhnel et al. (1995), the thickness of the carbon interphase increases with increasing hot-pressing temperature or time. By contrast, Brennan (1988), Ponthieu (1990) and 190 E. Le Strat et al
SiC Nicalon fibre-glass matrix composites Lancin et al.(1993) did not report any significant modification of the carbon inter phase with hot-pressing time in Nicalon-LAS The present study was initiated in order to shed further light on the properties of the interphases and on their formation kinetics in Nicalon-Pyrex composites. A Pyrex matrix was selected because of the simplicity of its composition. The absence of additions such as Nb O enables one to avoid, between carbon and the matrix. any secondary reaction which could complicate the interpretation of the experimen al data. §2. MATERIALS Pyrex consists essentially of Sio,(80 wt%)and B2O(13. 1 wt%) which are arranged as a network of tetrahedra connected by bridging oxygen atoms. Pyrex also contains modifying oxides, namely Na2O(3.5 wt%), Fe2O+ AlO(2.5 wt%) and Ko(1.15 wt%), which involve polar bondings between cations and non-brid- ging oxygen atoms, that is oxygen atoms linked to only one silicon or boron atom. Oxygen is also dissolved in the glass in the form of interstitial molecular oxygen. In Pyrex, the non-bridging oxygen concentration, as determined by the corresponding concentrations of sodium, iron, aluminium and potassium, is low(several atomic per cent). The concentration of dissolved oxygen is more difficult to estimate because it depends both on the glass and on its preparation conditions. Typically, this concen- tration should be higher than that of the non-bridging oxygen (Qi et al. 1993) Two composites were prepared at the Office National d Etudes et de recherches Aerospatiale de Chatillon by hot pressing, at 1398 K, two-dimensional Nicalon woven fibres impregnated with a suspension containing the Pyrex powder. In order to obtain a uniform temperature in the materials, the preforms were main- tained for 25 nm at 1398 K before hot pressing. Two times for hot pressing were chosen which were sufficiently different(5 and 235 min) for the expected thickness ariation in the reaction zone to be larger than the scatter of the measurements, by at least one order of magnitude 83. EXPERIMENTAL TECHNIQU ts, Electron microscopy studies were conducted on thin foils prepared by mechan- al grinding and subsequent ion thinning. The bright-field and high-resolution ere obtained with a Philips CM20 and a Hitachi field emission gun HF 2000. EDXS and EELS analyses were realized on the FEG equipped with a Si-Li Super Quantek diode(Kevex)and a parallel EELS detector( Gatan). The probe siz was 6nm and 2 nm for EDXS and EELS respectively SIMS was performed on fibres mechanically extracted from the composite hot pressed for 5 min. Analyses were realized with a Cameca IMS 4F spectrometer. The primary beam consisted of Cs ions extracted under a voltage of 10kv. The proce- dure adopted during the analyses has been described by Lancin et al. (1997). XPS analysis, alternating with ion erosion, was conducted on extracted fibres in order to reveal the possible depth-dependent phase modifications. The apparatus was a Leybold Heraeus LHS 12 spectrometer equipped with a Mg Ka source (1253.6eV). The fibres were extracted by matrix dissolution in hydrofluoric acid. By comparing mechanically and chemically extracted fibres we found that the che- mical treatment did not modify the surface composition of the fibres. Analysis was realized on fibre tows because the analysed area was approximately 4 mm x 10 mm. SiC and Sio Cy standards were used in order to obtain the energy of the Si 2p
Lancin et al. (1993) did not report any signi®cant modi®cation of the carbon interphase with hot-pressing time in Nicalon±LAS. The present study was initiated in order to shed further light on the properties of the interphases and on their formation kinetics in Nicalon±Pyrex composites. A Pyrex matrix was selected because of the simplicity of its composition. The absence of additions such as Nb2O5 enables one to avoid, between carbon and the matrix, any secondary reaction which could complicate the interpretation of the experimental data. § 2. Materials Pyrex consists essentially of SiO2 (80 wt%) and B2O3 (13.1 wt%) which are arranged as a network of tetrahedra connected by `bridging’ oxygen atoms. Pyrex also contains modifying oxides, namely Na2O (3.5 wt%), Fe2O3 + Al2O3 (2.5 wt%) and K2O (1.15 wt%), which involve polar bondings between cations and non-bridging oxygen atoms, that is oxygen atoms linked to only one silicon or boron atom. Oxygen is also dissolved in the glass in the form of interstitial molecular oxygen. In Pyrex, the non-bridging oxygen concentration, as determined by the corresponding concentrations of sodium, iron, aluminium and potassium, is low (several atomic per cent). The concentration of dissolved oxygen is more di cult to estimate because it depends both on the glass and on its preparation conditions. Typically, this concentration should be higher than that of the non-bridging oxygen (Qi et al. 1993). Two composites were prepared at the O ce National d’Etudes et de Recherches Ae rospatiales de Chatillon by hot pressing, at 1398 K, two-dimensional Nicalon woven ®bres impregnated with a suspension containing the Pyrex powder. In order to obtain a uniform temperature in the materials, the preforms were maintained for 25 nm at 1398 K before hot pressing. Two times for hot pressing were chosen which were su ciently di erent (5 and 235 min) for the expected thickness variation in the reaction zone to be larger than the scatter of the measurements, by at least one order of magnitude. § 3. Experimental techniques Electron microscopy studies were conducted on thin foils prepared by mechanical grinding and subsequent ion thinning. The bright-®eld and high-resolution images were obtained with a Philips CM20 and a Hitachi ®eld emission gun HF 2000. EDXS and EELS analyses were realized on the FEG equipped with a Si±Li Super Quantek diode (Kevex) and a parallel EELS detector (Gatan). The probe size was 6 nm and 2 nm for EDXS and EELS respectively. SIMS was performed on ®bres mechanically extracted from the composite hot pressed for 5 min. Analyses were realized with a Cameca IMS 4F spectrometer. The primary beam consisted of Cs+ ions extracted under a voltage of 10 kV. The procedure adopted during the analyses has been described by Lancin et al. (1997). XPS analysis, alternating with ion erosion, was conducted on extracted ®bres in order to reveal the possible depth-dependent phase modi®cations. The apparatus was a Leybold Heraeus LHS 12 spectrometer equipped with a Mg Ka source (1253.6 eV). The ®bres were extracted by matrix dissolution in hydro¯ uoric acid. By comparing mechanically and chemically extracted ®bres we found that the chemical treatment did not modify the surface composition of the ®bres. Analysis was realized on ®bre tows because the analysed area was approximately 4 mm ´ 10 mm. SiC and SiOxCy standards were used in order to obtain the energy of the Si 2p SiC Nicalon ®bre±glass matrix composites 191
E. Le strat et al transition and its full width at half-maximum. Nicalon fibres were also analysed against depth. No change in the Si 2p and C Is transitions was detected, showing that the sputtering did not induce any phase modification of the fibres 4. RESULTS The two composites have similar microstructures with slight differences, how ever Between the fibre and the matrix, we always observe a reaction layer consisting of two distinct interphases(figure 1). The interphase next to the matrix, namely the carbon layer(CL), is essentially composed of carbon. The following interphase exhibits varying microstructure and composition from the Cl to the unmodified fibre core(this intermediate interphase is called the transition layer(TL)). The thick ness of the Cl interphase varies from one fibre to the next, just as that of the Tl does but to a lesser extent than the CL. Typically, the Cl thickness is 30-40 nm and the TL thickness is 60-70nm. It is worth noting that we detect no thickness dependence of the reaction layer(RL)(which equals the Cl plus the Tl)upon the time t of hot The CL microstructure exhibits the characteristic features of microporous tur bostratic carbon (figure 2). The TL microstructure is similar to that of the fibre although these can be easily differentiated. Just as the fibre, the Tl contains Sic grains(figure 3). As distinct from what is found in the fibre, the density of Sic grains decreases in the Tl from the fibre inside towards the CL. sic grains are larger in the TL than in the fibre as shown in the high-resolution images and EDPs(figure 4) Moreover, the images and patterns reveal the existence of carbon structural units (SUs)in the Tl which are not detected in the fibre. The density and size of the carbon SUs in the CL, which features in its organization, slightly increases with Figure 1. Typical interface. The bright-field image shows two interphases with different contrast. The CL is the carbon interphase and the tl a transition layer where the microstructure and the composition vary from the unmodified fibre core to the Cl
transition and its full width at half-maximum. Nicalon ®bres were also analysed against depth. No change in the Si 2p and C 1s transitions was detected, showing that the sputtering did not induce any phase modi®cation of the ®bres. § 4. Results The two composites have similar microstructures with slight di erences, however. Between the ®bre and the matrix, we always observe a reaction layer consisting of two distinct interphases (®gure 1). The interphase next to the matrix, namely the carbon layer (CL), is essentially composed of carbon. The following interphase exhibits varying microstructure and composition from the CL to the unmodi®ed ®bre core (this intermediate interphase is called the transition layer (TL)). The thickness of the CL interphase varies from one ®bre to the next, just as that of the TL does but to a lesser extent than the CL. Typically, the CL thickness is 30±40 nm and the TL thickness is 60±70 nm. It is worth noting that we detect no thickness dependence of the reaction layer (RL) (which equals the CL plus the TL) upon the time t of hot pressing. The CL microstructure exhibits the characteristic features of microporous turbostratic carbon (®gure 2). The TL microstructure is similar to that of the ®bre although these can be easily di erentiated. Just as the ®bre, the TL contains SiC grains (®gure 3). As distinct from what is found in the ®bre, the density of SiC grains decreases in the TL from the ®bre inside towards the CL. SiC grains are larger in the TL than in the ®bre as shown in the high-resolution images and EDPs (®gure 4). Moreover, the images and patterns reveal the existence of carbon structural units (SUs) in the TL which are not detected in the ®bre. The density and size of the carbon SUs in the CL, which features in its organization, slightly increases with 192 E. Le Strat et al. Figure 1. Typical interface. The bright-®eld image shows two interphases with di erent contrast. The CL is the carbon interphase and the TL a transition layer where the microstructure and the composition vary from the unmodi®ed ®bre core to the CL
n Figure 2. Microstructure of the carbon interphase as shown by the HRTEM image of microporous turbostratic carbon increasing hot-pressing time. By contrast, the TL microstructure does not undergo significant changes The composition variations in the Rl is revealed by EDXS, SIMS and EELS EDXS analyses show that the CL contains about 2 at. %Si and 10 at. %O. The CL also contains traces of sodium(1 at. or less ) whereas traces of iron, aluminium a Common to all our analyses, the main characteristic of the Tl is its high oxygen content, which is actually markedly larger than in the fibre. In spite of the local concentration fluctuations, the eDXS analyses always show a more prominent oxygen concentration in the Tl than in the fibre(figure 5). The carbon, silicon and oxygen contents in the Tl amount to 45 at. % 23 at and 30 at. respectively. In the fibre, they amount to 50at %, 32 at and 18 at. % respectively. In electron- energy-loss spectra, the Si L2,3 edge structure at 116ev, characteristic of the Si-O- Si bond, is clearly more pronounced in the tl than in the fibre(figure 6). On SIMS profiles, the intensities of the C, Si and O signals are constant in the fibre whereas they vary significantly at the fibre periphery. The CL is revealed by the maximum in the C signal. The Tl is characterized by a peak in the O signal,a minimum in the C signal compared with the fibre and a shoulder in the Si signal (arrowed in figure 7). The TL can be characterized by a higher oxygen concentration and lower carbon and silicon contents than to those in the fibre The asymmetry of the C and o profiles shows that the variation in composi tion is more progressive between the CL and the tl than between the Cl and the matrix, which is well supported by HRTEM observations. The width of the tl car be derived from SIMS profiles. The uncertainty of this estimation mainly result from the location of the tl- fibre interface because the o-to-c and si-to-c ratios reach constant values at different depths(figure 7). We choose to take int account the variation in the Si -to-C ratio. Even so, the width(90 nm) is larger than that derived from hrtEM observations. SIMS profiles show in addition that the tl contains a low percentage of boron while this element is not detectable in the CL. The other matrix elements could not be identified by SIMs because inherent
increasing hot-pressing time. By contrast, the TL microstructure does not undergo signi®cant changes. The composition variations in the RL is revealed by EDXS, SIMS and EELS. EDXS analyses show that the CL contains about 2 at.%Si and 10 at.%O. The CL also contains traces of sodium (1 at.% or less), whereas traces of iron, aluminium and potassium are within the experimental uncertainty. Common to all our analyses, the main characteristic of the TL is its high oxygen content, which is actually markedly larger than in the ®bre. In spite of the local concentration ¯ uctuations, the EDXS analyses always show a more prominent oxygen concentration in the TL than in the ®bre (®gure 5). The carbon, silicon and oxygen contents in the TL amount to 45 at.%, 23 at.% and 30 at.% respectively. In the ®bre, they amount to 50 at.%, 32 at.% and 18 at.% respectively. In electronenergy-loss spectra, the Si L2,3 edge structure at 116 eV, characteristic of the Si±O± Si bond, is clearly more pronounced in the TL than in the ®bre (®gure 6). On SIMS pro®les, the intensities of the C - , Si- and O - signals are constant in the ®bre whereas they vary signi®cantly at the ®bre periphery. The CL is revealed by the maximum in the C - signal. The TL is characterized by a peak in the O - signal, a minimum in the C - signal compared with the ®bre and a shoulder in the Si- signal (arrowed in ®gure 7). The TL can be characterized by a higher oxygen concentration and lower carbon and silicon contents than to those in the ®bre. The asymmetry of the C - and O - pro®les shows that the variation in composition is more progressive between the CL and the TL than between the CL and the matrix, which is well supported by HRTEM observations. The width of the TL can be derived from SIMS pro®les. The uncertainty of this estimation mainly results from the location of the TL±®bre interface because the O - -to-C- and Si- -to-Cratios reach constant values at di erent depths (®gure 7). We choose to take into account the variation in the Si- -to-C- ratio. Even so, the width (90 nm) is larger than that derived from HRTEM observations. SIMS pro®les show in addition that the TL contains a low percentage of boron while this element is not detectable in the CL. The other matrix elements could not be identi®ed by SIMS because inherent SiC Nicalon ®bre±glass matrix composites 193 Figure 2. Microstructure of the carbon interphase as shown by the HRTEM image of microporous turbostratic carbon
E. Le strat et a s nm Figure 3. Microstructure of the fibre and of the TL HRTEM images reveal the similarity and difference between(a)the fibre and(b)the TL microstructure. The Sic grains(white arrowhead) are larger in the tl than in the fibre. Carbon SUs(black arrowheads )are detected in the tl but not in the fibre confusions between the studied masses By EDXS, the signals of the matrix elements are not detectable The influence of annealing time on the interphase composition which was studied by EDXS is not experimentally detectable The phase composition of the CL and of the tl was studied by XPS. As the contamination layer was removed by sputtering the amount of silicon remains
confusions between the studied masses. By EDXS, the signals of the matrix elements are not detectable. The in¯ uence of annealing time on the interphase composition which was studied by EDXS is not experimentally detectable. The phase composition of the CL and of the TL was studied by XPS. As the contamination layer was removed by sputtering, the amount of silicon remains 194 E. Le Strat et al. (a) (b) Figure 3. Microstructure of the ®bre and of the TL HRTEM images reveal the similarity and di erence between (a) the ®bre and (b) the TL microstructure. The SiC grains (white arrowhead) are larger in the TL than in the ®bre. Carbon SUs (black arrowheads) are detected in the TL but not in the ®bre
SiC Nicalon fibre-glass matrix composite 2 nm-1(b) 2 nm-l Figure 4. Diffraction patterns of the fibre and of the TL: (a) diffraction pattern of the fibre; (b)diff raction pattern of the TL with SiC (white arrowhead)and (0002)carbon ring (black arrowhead The SiC grains, which are larger in the TL than in the fibre, result in punctuated diff raction rings Fibre TL 0.8 0.6 0.4 c 0.2 Energy (kev) Figure 5. EDXS analysis of the fibre and of the TL. The energy-dispersive X-ray spectra show that the oxygen concentration is more important in the tl than in the fibre roughly constant and equal to 2 at. for 30 min and then continuously increases to about 30 at. %. As shown by EDXS, a silicon concentration of about 2 at. is characteristic of the CL and thus the silicon photoelectron signal is emitted from the CL. The Si 2p transition occurs at about 101 5ev (figure 8(c). It is surprising hat silicon is implied in Si-o-C bonds in view of the low silicon and oxygen con- tents in this interphase. When the amount of silicon increases, the silicon photoelec- trons are emitted from both the cl and the tl. the influence of the tl becomes more and more prominent when the sputtering time increases. Whatever the sputter
roughly constant and equal to 2 at.% for 30 min and then continuously increases to about 30 at.%. As shown by EDXS, a silicon concentration of about 2 at.% is characteristic of the CL and thus the silicon photoelectron signal is emitted from the CL. The Si 2p transition occurs at about 101.5 eV (®gure 8 (c)). It is surprising that silicon is implied in Si±O±C bonds in view of the low silicon and oxygen contents in this interphase. When the amount of silicon increases, the silicon photoelectrons are emitted from both the CL and the TL. The in¯ uence of the TL becomes more and more prominent when the sputtering time increases. Whatever the sputterSiC Nicalon ®bre±glass matrix composites 195 (a) (b) Figure 4. Di raction patterns of the ®bre and of the TL: (a) di raction pattern of the ®bre; (b) di raction pattern of the TL with SiC (white arrowhead) and (0002) carbon ring (black arrowhead): The SiC grains, which are larger in the TL than in the ®bre, result in punctuated di raction rings. Figure 5. EDXS analysis of the ®bre and of the TL. The energy-dispersive X-ray spectra show that the oxygen concentration is more important in the TL than in the ®bre
E Le str 8000 Si 116 6000 4000 TL 160 180 Energy Loss△E(ev) Figure 6. EELS analysis of the fibre and of the TL. The edge at 116ev is characteristic of SHO-Si bonding. It is more pronounced in the tl than in the fibre ing time, it is noteworthy that we can never detect the Si 2p transition characteristic of Sio. On the contrary, when the silicon concentration is high enough to allow spectrum decomposition(figure 8(b), we observe the 101. 3 eV Si 2p transition char- acteristic of the silicon oxycarbide identified in the fibre( figure 8(a). The 102 5eV ansition, also observed in the fibre, could be attributed to a suboxide Sio(x <2), as is usually the case. We ascribe it to the presence of a silicon oxycarbide containing more Si-O bonds than the preceeding silicon oxycarbide. We observed indeed the same 102 5eV transition in a silicon oxycarbide standard of the following composi tion: 19 at. Si, 44 at. %O and 37 at. %C. Despite uncertainties due to spectrum decomposition, XPS analysis proves that the TL contains some silicon carbide, some carbon and at least two silicon oxycarbides with distinct compositions. It also shows that, whereas the amounts of silicon oxycarbide increase, that of SiC decreases from the fibre to the cl in agreement with the high-resolution observa tions §5. DISCUSSION The characterization of the Nicalon-Pyrex composites confin ompletes the results obtained previously for Nicalon-LAS or LAS+ -Alok-Sio (MAS)(Ponthieu et al. 1990, 1994, Lancin et al. 1993). Two features should be emphasized. One is concerned with the formation of the TL oxygen-rich interphase located between the CL and the fibre. This oxygen-rich interphase is actually a characteristic of the reaction irrespective of the matrix(Brennan 1988, Chen et al. 1989, Bonney and Cooper 1990, Homeny et a. 1990, Doreau 1995). Our experiments show that the fibre-matrix reaction results in the formation of Sio Cy, thus support
ing time, it is noteworthy that we can never detect the Si 2p transition characteristic of SiO2. On the contrary, when the silicon concentration is high enough to allow spectrum decomposition (®gure 8 (b)), we observe the 101.3 eV Si 2p transition characteristic of the silicon oxycarbide identi®ed in the ®bre (®gure 8 (a)). The 102.5 eV transition, also observed in the ®bre, could be attributed to a suboxide SiOx (x <2), as is usually the case. We ascribe it to the presence of a silicon oxycarbide containing more Si±O bonds than the preceeding silicon oxycarbide. We observed indeed the same 102.5 eV transition in a silicon oxycarbide standard of the following composition: 19 at.%Si, 44 at.%O and 37 at.%C. Despite uncertainties due to spectrum decomposition, XPS analysis proves that the TL contains some silicon carbide, some carbon and at least two silicon oxycarbides with distinct compositions. It also shows that, whereas the amounts of silicon oxycarbide increase, that of SiC decreases from the ®bre to the CL, in agreement with the high-resolution observations. § 5. Discussion The characterization of the Nicalon±Pyrex composites con®rms and completes the results obtained previously for Nicalon±LAS or LAS + MgO±Al2O3-SiO2 (MAS) (Ponthieu et al. 1990, 1994, Lancin et al. 1993). Two features should be emphasized. One is concerned with the formation of the TL oxygen-rich interphase located between the CL and the ®bre. This oxygen-rich interphase is actually a characteristic of the reaction irrespective of the matrix (Brennan 1988, Chen et al. 1989, Bonney and Cooper 1990, Homeny et al. 1990, Doreau 1995). Our experiments show that the ®bre±matrix reaction results in the formation of SiOxCy, thus support- 196 E. Le Strat et al. Figure 6. EELS analysis of the ®bre and of the TL. The edge at 116 eV is characteristic of Si±O±Si bonding. It is more pronounced in the TL than in the ®bre
SiC Nicalon fibre-glass matrix composites Fiber B(50) SI/C= constant o/C O-/C-= constant 83 nm Figure 7. SIMS of extracted fibre. The sputtering rate was equal to 25 nm min taking into account the ion mixing(twice the projec ange of Cs in is about 20 nm). The CL-TL interface is located at the inflection points which are observed in the b, Si and o signals simultaneously. The L-fibre interface was located 20nm before the depth where the Si -to-c ratio reaches a constant value characteristic of the fibre ing recent results that suggest the existence of Sio. Cy instead of Sio, in the TL (Pippel et al. 1995). Moreover, our work shows that the tl contains the same silicon oxycarbides as the fibre but with higher contents than the fibre. The second note- worthy feature is related to the reaction kinetics. In agreement with Brennan(1988 ), our studies show that the reaction is very fast and that it stops soon. In order to propose its reactional scheme, we consider all the experimental data collected so far for the Nicalon-glass matrix composites. First of all, it is necessary to identify the origin of the oxygen responsible for the fibre oxidation. Some workers invoke the oxygen of the fibre itself(Benson et al. 1988 ) or that confined in the preform( Chaim and Heuer 1991). These hypotheses are questionable because of the role of the matrix in the Cl thickness. Such a role has been established by a number of workers(Brennan 1988, Chen et al. 1989, Bonney and Cooper 1990). In Nicalon- LAS composites for instance, Ponthieu et al.(1990) showed that by the addition of
ing recent results that suggest the existence of SiOxCy instead of SiO2 in the TL (Pippel et al. 1995). Moreover, our work shows that the TL contains the same silicon oxycarbides as the ®bre but with higher contents than the ®bre. The second noteworthy feature is related to the reaction kinetics. In agreement with Brennan (1988), our studies show that the reaction is very fast and that it stops soon. In order to propose its reactional scheme, we consider all the experimental data collected so far for the Nicalon±glass matrix composites. First of all, it is necessary to identify the origin of the oxygen responsible for the ®bre oxidation. Some workers invoke the oxygen of the ®bre itself (Benson et al. 1988) or that con®ned in the preform (Chaim and Heuer 1991). These hypotheses are questionable because of the role of the matrix in the CL thickness. Such a role has been established by a number of workers (Brennan 1988, Chen et al. 1989, Bonney and Cooper 1990). In Nicalon± LAS composites for instance, Ponthieu et al. (1990) showed that by the addition of SiC Nicalon ®bre±glass matrix composites 197 Figure 7. SIMS analysis of extracted ®bre. The sputtering rate was equal to 25 nm min- 1 . Interfaces were located by taking into account the ion mixing (twice the projected range of Cs+ in the target is about 20 nm). The CL±TL interface is located at the in¯ ection points which are observed in the B - , Si- and O- signals simultaneously. The TL±®bre interface was located 20 nm before the depth where the Si- -to-C- ratio reaches a constant value characteristic of the ®bre
E Le strat et al 1200 Fibr 1000 Sio C Sic SIO C 200 0斯需 Sio Sio C 104 102 Energy(ev) Figure 8. XPS analysis of the fibre, of the TL and of the Cl interphases. The peak at 1025eV often attributed to a suboxide SiOx (x <2) is also characteristic of oxygen-rich silicon oxycarbide(SiOx Cpe)(37 at. %C, 44 at. %O and 19 at. %Si)
198 E. Le Strat et al. Figure 8. XPS analysis of the ®bre, of the TL and of the CL interphases. The peak at 102.5 eV often attributed to a suboxide SiOx (x <2) is also characteristic of an oxygen-rich silicon oxycarbide (SiOx  Cy Â) (37 at.%C, 44 at.%O and 19 at.%Si)