CATERTALIA Pergamon Acta mater.48(200046094618 www.elsevier.com/locate/actamat THE CONCEPT OF A STRONG INTERFACE APPLIED TO SiC/SIC COMPOSITES WITH A BN INTERPHASE F. REBILLAT, J. LAMONT and A. GUETTE Laboratoire des Composites Thermostructuraux, UMR 5801, CNRS-SNECMA-CEA-UB1, 3, allee de la Boetie 33600 Pessac. france natrix composites(CMC). The concept of a strong interface has been established in SiC/SiC composit with pyrocarbon(Py C)or multilayered(PyC/SiC) fiber coatings(also referred to as interphases). The present reports an attempt directe applying the concept of a strong interface to SiC/SiC composites with BN coating(referred to as SiC/BN/SiC). Fiber bonding and frictional sliding were investigated by means of push-out tests performed on 2D-composites as well as on microcomposite samples, and tensile tests perfor- d on microcomposites. The stress-strain behavior of the SiC/BN/SiC composites and microcomposites is discussed with respect to interface characteristics and location of debonding either in the coating or in the fiber/coating interface. 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. AlI rights reserved. 1 INTRODUCTION expansion mismatch. Fiber/matrix interfaces in the most advanced ceramic matrix composites consist of of composites because load transfers from the matrix a thin coating layer(less than l um thick )of one or to the fiber and vice versa must occur through the several materials deposited on the fiber (interphase) interface. Therefore, it exerts a profound infuence Recently, SiC/SiC composites with strong interfaces upon the mechanical behavior and the lifetime. Thus, have been developed. The coating/fiber bond was sig- as a function of end use applications through optimiz. treated 3-5). Features of the mechanical behavior of SiC/SiC composites with strong fiber/coating inter- 6-1 In fiber-reinforced ceramic composites, most Experiments as well as models have demonstrated increase fracture toughness. The major contribution that a strong interface is beneficial to the strength, the to toughness is attributed to crack bridging and fiber toughness, the lifetime and the creep resistance 14, 6 composite strength. A high strength requires efficient mental trast, weak interfaces are shown to be det- he once ept of strong interfaces has been estab- load transfers which are obtained with strong inter- lished on Sic/C/SiC composites with PyC and multi- faces. This implies short debond cracks and/or sig- layered(Py C/SiC)fiber coatings. In the present paper, nificant sliding friction. These latter requirements, to be met for strong composites, are therefore incompat- it is applied to SiC/BN/SiC composites with boron ible with the former ones for tough composites, if nitride fiber coatings. BN is foreseen to be an alterna- toughening is based solely upon the above mentioned tive fiber coating to improve the oxidation resistance weak interface-based mechanisms of ceramic matrix composites at high temperature Fiber/matrix bonding results from diffusion or chemical reactions(chemical bonding)or from fiber 2. FEATURES OF STRONG INTERFACES VS WEAK clamping by residual stresses induced by thermal INTERFACES n prc uce of a hom all correspondence should be addressed. Tel: strong interface, we recall first the basic features of 844-703;fax:+33-556-841-225 interface phenomena in CMCs subject to an essen- ddress: admin@lcts. u-bordeaux fr(. Lamon) tially tensile load. These phenomena influence the 1359-6454100/520.00@ 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved PI:S1359-6454(00)00247-0
Acta mater. 48 (2000) 4609–4618 www.elsevier.com/locate/actamat THE CONCEPT OF A STRONG INTERFACE APPLIED TO SiC/SiC COMPOSITES WITH A BN INTERPHASE F. REBILLAT, J. LAMON† and A. GUETTE Laboratoire des Composites Thermostructuraux, UMR 5801, CNRS-SNECMA-CEA-UB1, 3, alle´e de la Boe´tie, 33600 Pessac, France Abstract—Strong interfaces have been shown to allow improvement of the mechanical properties of ceramic matrix composites (CMC). The concept of a strong interface has been established in SiC/SiC composites with pyrocarbon (PyC) or multilayered (PyC/SiC) fiber coatings (also referred to as interphases). The present paper reports an attempt directed at applying the concept of a strong interface to SiC/SiC composites with a BN coating (referred to as SiC/BN/SiC). Fiber bonding and frictional sliding were investigated by means of push-out tests performed on 2D-composites as well as on microcomposite samples, and tensile tests performed on microcomposites. The stress–strain behavior of the SiC/BN/SiC composites and microcomposites is discussed with respect to interface characteristics and location of debonding either in the coating or in the fiber/coating interface. 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Interphase; Interface; Composites 1. INTRODUCTION The fiber–matrix interfacial domain is a critical part of composites because load transfers from the matrix to the fiber and vice versa must occur through the interface. Therefore, it exerts a profound influence upon the mechanical behavior and the lifetime. Thus, it may be expected that composites could be tailored as a function of end use applications through optimization of interfaces. In fiber-reinforced ceramic composites, most authors promote the concept of weak interfaces to increase fracture toughness. The major contribution to toughness is attributed to crack bridging and fiber pull-out [1, 2]. Weak interfaces are detrimental to composite strength. A high strength requires efficient load transfers which are obtained with strong interfaces. This implies short debond cracks and/or significant sliding friction. These latter requirements, to be met for strong composites, are therefore incompatible with the former ones for tough composites, if toughening is based solely upon the above mentioned weak interface-based mechanisms. Fiber/matrix bonding results from diffusion or chemical reactions (chemical bonding) or from fiber clamping by residual stresses induced by thermal † To whom all correspondence should be addressed. Tel.: 133-556-844-703; fax: 133-556-841-225. E-mail address: admin@lcts.u-bordeaux.fr (J. Lamon) 1359-6454/00/$20.00 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S13 59-6454(00)00247-0 expansion mismatch. Fiber/matrix interfaces in the most advanced ceramic matrix composites consist of a thin coating layer (less than 1 µm thick) of one or several materials deposited on the fiber (interphase). Recently, SiC/SiC composites with strong interfaces have been developed. The coating/fiber bond was significantly stronger when fibers had been previously treated [3–5]. Features of the mechanical behavior of SiC/SiC composites with strong fiber/coating interfaces has been examined in several papers [4, 6–10]. Experiments as well as models have demonstrated that a strong interface is beneficial to the strength, the toughness, the lifetime and the creep resistance [4, 6– 11]. By contrast, weak interfaces are shown to be detrimental. The concept of strong interfaces has been established on SiC/C/SiC composites with PyC and multilayered (PyC/SiC) fiber coatings. In the present paper, it is applied to SiC/BN/SiC composites with boron nitride fiber coatings. BN is foreseen to be an alternative fiber coating to improve the oxidation resistance of ceramic matrix composites at high temperature. 2. FEATURES OF STRONG INTERFACES VS WEAK INTERFACES In order to properly introduce the concept of a strong interface, we recall first the basic features of interface phenomena in CMCs subject to an essentially tensile load. These phenomena influence the
REBILLAT et al: SiC/SIC COMPOSITES mechanical response of composites described by the tress -strain curve Fiber debonding results from the deflection of the cracks that initiate in the matrix(Fig. 1). Then sliding 5 of the fiber debonded in the interface determines the load transfers from the fiber to the matrix and vice 5 200 versa. The fiber sliding is influenced by the misfit E strain, the associated radial component of the ther mally induced residual stress-field, surface roughness Weak interfaces debond easily. A single long debond crack is located at the surface of the fibers in o LONGITUDINAL TE small interface shear stresses, load transfers through Fig. 2: Typical tensile stress-strain behaviors measured on 2D the debonded interfaces are poor. The matrix becomes ricated from untreated or treated Nicalon(ceramic grade)fit subjected to lower stresses and the volume of matrix ers: (a)strong fiber/coating interfaces and (b) that may experience further cracking is reduced the presence of long debonds. The cracks are gener- ally widely opened, whereas the crack spacing dis- tance at saturation as well as the pull out length tend (cohesive failure type, Fig. 1), into short and branched to be rather long (100 um). Toughening results multiple cracks [4, 12]. Short debonds as well as essentially from sliding friction along the debonds. Improved load transfers allow further cracking of the matrix via a scale effect 6, 7] leading to a hig However, due to poor load transfers and long density of matrix cracks(which are slightly opened debonds,the fibers carry most of the load, which Sliding friction within the coating as well as multiple reduces the composite strength. The corresponding cracking of the matrix increase energy absorption, tensile stress-strain curve exhibits a short curved domain limited by a stress at matrix cracking satu- leading to toughening. Limited debonding and fibers, leading to strengthening. The associated tensile In the presence of stronger fiber/coating bonds, the stress strain curve exhibits a wide curved domain and the stress at matrix cracking saturation is close to ulti mate failure(Fig. 2). Table 1 gives various values of the interfacial shear stresses measured using various debond crack methods on SiC/SiC composites with PyC-based fiber coating. It can be noticed that the interfacial shear stresses range between 10 and 20 MPa for the weak interfaces whereas they are larger than 100-300 MPa for the strong interfaces. Additional data can be found in[4,7,8,24,29] 3. SiC/BN/SiC COMPOSITES: TESTING METHODOLOGY AND MICROSTRUCTURAL ANALYSES Fiber 3. 1. Specimen preparation debond crack SIC/BN/SIC microcomposites and woven com posites were manufactured via chemical vapor infil- tration [13]. They were reinforced with either as received or treated(proprietary treatment, SNECMA/SEP, Bordeaux) SiC Nicalon fibers (NL 202 grade). The SiC/BN/SIC mIcrocosmos tes consist of a single fiber (15 um diameter ), coated with a boron nitride layer(0.3-0.9 um thick) and a Sic matrix deposited by CVD. They are tive of Fiber their counterparts in the 2D woven con they are produced using identical deposition conditions [13] Fig 1 Schematic diagram showing crack deflection when the A single or a bilayered BN fiber coating was fiber coating/interface is (a) strong or(b) weak. deposited from a BF3, NH3, Ar gas mixture (Table
4610 REBILLAT et al.: SiC/SiC COMPOSITES mechanical response of composites described by the stress–strain curve. Fiber debonding results from the deflection of the cracks that initiate in the matrix (Fig. 1). Then sliding of the fiber debonded in the interface determines the load transfers from the fiber to the matrix and vice versa. The fiber sliding is influenced by the misfit strain, the associated radial component of the thermally induced residual stress-field, surface roughness and debond length. Weak interfaces debond easily. A single long debond crack is located at the surface of the fibers in those composites exhibiting weak interfaces (adhesive failure type, Fig. 1). As a consequence of small interface shear stresses, load transfers through the debonded interfaces are poor. The matrix becomes subjected to lower stresses and the volume of matrix that may experience further cracking is reduced by the presence of long debonds. The cracks are generally widely opened, whereas the crack spacing distance at saturation as well as the pull out length tend to be rather long (>100 µm). Toughening results essentially from sliding friction along the debonds. However, due to poor load transfers and long debonds, the fibers carry most of the load, which reduces the composite strength. The corresponding tensile stress–strain curve exhibits a short curved domain limited by a stress at matrix cracking saturation which is significantly smaller than ultimate strength (Fig. 2). In the presence of stronger fiber/coating bonds, the matrix cracks are deflected within the coating Fig. 1. Schematic diagram showing crack deflection when the fiber coating/interface is (a) strong or (b) weak. Fig. 2. Typical tensile stress–strain behaviors measured on 2D SiC/SiC composites possessing PyC based interphases and fabricated from untreated or treated Nicalon (ceramic grade) fibers: (a) strong fiber/coating interfaces and (b) weak fiber/coating interfaces. (cohesive failure type, Fig. 1), into short and branched multiple cracks [4, 12]. Short debonds as well as improved load transfers allow further cracking of the matrix via a scale effect [6, 7] leading to a higher density of matrix cracks (which are slightly opened). Sliding friction within the coating as well as multiple cracking of the matrix increase energy absorption, leading to toughening. Limited debonding and improved load transfers reduce the load carried by the fibers, leading to strengthening. The associated tensile stress strain curve exhibits a wide curved domain and the stress at matrix cracking saturation is close to ultimate failure (Fig. 2). Table 1 gives various values of the interfacial shear stresses measured using various methods on SiC/SiC composites with PyC-based fiber coating. It can be noticed that the interfacial shear stresses range between 10 and 20 MPa for the weak interfaces whereas they are larger than 100–300 MPa for the strong interfaces. Additional data can be found in [4, 7, 8, 24, 29]. 3. SiC/BN/SiC COMPOSITES: TESTING METHODOLOGY AND MICROSTRUCTURAL ANALYSES 3.1. Specimen preparation SiC/BN/SiC microcomposites and woven composites were manufactured via chemical vapor infiltration [13]. They were reinforced with either asreceived or treated (proprietary treatment, SNECMA/SEP, Bordeaux) SiC Nicalon fibers (NL 202 grade). The SiC/BN/SiC microcomposites consist of a single fiber (15 µm diameter), coated with a boron nitride layer (0.3–0.9 µm thick) and a SiC matrix deposited by CVD. They are representative of their counterparts in the 2D woven composites, since they are produced using identical chemical vapor deposition conditions [13]. A single or a bilayered BN fiber coating was deposited from a BF3, NH3, Ar gas mixture (Table
REBILLAT et al: SiC/SIC COMPOSITES 4611 Table 1. Interfacial shear stress (MPa) measured using various methods on 2D-SiC/SiC composites with Py C based fiber coatings and reinforced with either as-received or treated fibers SiC/C/SiC )n/SiC loops)[24] domain Untreated fibers 21-115 4080 12-10 Treated fibers PyC(o 1) 165-273 PyC/SiC)4 2). The selected processing conditions have been shown to cause minimum damage to the fibers and to improve adhesion of the BN coating onto the fib- ers, and the microstructure [13]. In the bi-layered coating(referred to as BN4, Table 2 and Fig. 3), the first sublayer on the fiber is BN2 type(poorly crystallized), whereas the second one is BNI type (highly crystallized). Processing of BN2 involved the less aggressive gaseous phase, which led to a better contact between the fiber and the coating. The pro- essing conditions of BNI were found to be aggress- ive against the fibers [13]- (a) 3.2. Push-out tests Fiber bonding and frictional sliding in the 2D SiC/BN/SiC composites were investigated by means of single fiber push-out tests [14-17].500 um thick wedges were prepared using standard metallographic ONERA, France)was used. The load was applied to the top of the fiber using a flat bottom diamond cone (at a constant displacement rate of 0.1 um/s). The nterface characteristics were extracted from the experimental stress-fiber end displacement curves by fitting the push-out model of Hsueh [ 15], as discussed n a previous paper [16] Only a few push-out experiments could be carried out on the microcomposites owing to the difficulties involved in microcomposite handling, preparation and testing. Parallel-faced strips were cut out of the Fig 3. Images of BN coatings:(a)TEM-image and DEAS. microcomposites which had been previously embed- picture of the BNI coating showing the three-dimensional ded in glass [18](microcomposites 2)or in a ceramic xagonal structure; (b) SEM-image of bi-layered BN ement [19](microcomposites 2 and 4). A first series BN4)in a SiC/BN/SiC microcomposite showing dered BN2 layer and the BN1 layer with a three dimensional ordered hexagonal structure Table 2. Main characteristics ber coatings [13] Batch BN coating Number Coating of push out tests on microcomposites 2 used a Vickers zation adhesion diamond probe [18).A correction for indentor dis- placement was done. Most of the tests were push in rather than push-out tests, Push-out could not be BN1+BN2 Strong achieved on the specimens with a thickness exceeding 200 um. A number of push-in curves
REBILLAT et al.: SiC/SiC COMPOSITES 4611 Table 1. Interfacial shear stress (MPa) measured using various methods on 2D-SiC/SiC composites with PyC based fiber coatings and reinforced with either as-received or treated fibers SiC/C/SiC Interphase Crack spacing Crack spacing Tensile tests Tensile tests Push-out tests Push-out tests SiC/(C/SiC)n/SiC [30] [31] (hysteresis (curved (curved (plateau) loops) [24] domain) domain) [8, 29] [7, 24] [8, 29] Untreated fibers 2D woven PyC (0.1) 12 8 0.7 Microcomposites PyC(0.1) 3 4–20 Minicomposites PyC(0.1) 21–115 40–80 2D woven PyC(0.5) 4 14–16 12–10 (PyC/SiC)2 2 31 19.3 (PyC/SiC)4 9 28 12.5 Treated fibers 2D woven PyC(0.1) 203 140 190 165–273 PyC(0.5) 370 100–105 (PyC/SiC)2 150 133 (PyC/SiC)4 90 90 2). The selected processing conditions have been shown to cause minimum damage to the fibers and to improve adhesion of the BN coating onto the fibers, and the microstructure [13]. In the bi-layered coating (referred to as BN4, Table 2 and Fig. 3), the first sublayer on the fiber is BN2 type (poorly crystallized), whereas the second one is BN1 type (highly crystallized). Processing of BN2 involved the less aggressive gaseous phase, which led to a better contact between the fiber and the coating. The processing conditions of BN1 were found to be aggressive against the fibers [13]. 3.2. Push-out tests Fiber bonding and frictional sliding in the 2DSiC/BN/SiC composites were investigated by means of single fiber push-out tests [14–17]. 500 µm thick wedges were prepared using standard metallographic techniques. An interfacial test system (designed by ONERA, France) was used. The load was applied to the top of the fiber using a flat bottom diamond cone (at a constant displacement rate of 0.1 µm/s). The interface characteristics were extracted from the experimental stress–fiber end displacement curves by fitting the push-out model of Hsueh [15], as discussed in a previous paper [16]. Only a few push-out experiments could be carried out on the microcomposites owing to the difficulties involved in microcomposite handling, preparation and testing. Parallel-faced strips were cut out of the microcomposites which had been previously embedded in glass [18] (microcomposites 2) or in a ceramic cement [19] (microcomposites 2 and 4). A first series Table 2. Main characteristics of the BN fiber coatings [13] Batch BN coating Number of Degree of Coating BN layers crystallization adhesion 1 BN1 1 High Strong 2 BN2 1 Low Weak 4 BN4 2 BN11BN2 Strong Fig. 3. Images of BN coatings: (a) TEM-image and DEASpicture of the BN1 coating showing the three-dimensional ordered hexagonal structure; (b) SEM-image of bi-layered BN interphase (BN4) in a SiC/BN/SiC microcomposite showing poorly ordered BN2 layer and the BN1 layer with a threedimensional ordered hexagonal structure. of push out tests on microcomposites 2 used a Vickers diamond probe [18]. A correction for indentor displacement was done. Most of the tests were pushin rather than push-out tests. Push-out could not be achieved on the specimens with a thickness exceeding 200 µm. A number of push-in curves was unusable
REBILLAT et al: SiC/SIC COMPOSITES for analysis in that they exhibited features that were (a) inconsistent with the model. A second series of push- in tests on microcomposites 2 was then performed on thicker samples (290 um) using a flat-bottomed cone microcomposites 4(thickness 190 um). The inter faces characteristics were extracted by fitting the a Hsueh's model [15] to the push-in curves or to the curved domain and to the plateau of the push-out 3.3. Tensile tests Five microcomposites per batch were tested in ten sion by using a specific table-model testing machine designed and developed for fiber testing [20]). The sin- gle fiber tensile test procedure based on window frames with appropriate gauge lengths(generally 10 mm)was employed [21, 22]. The microcomposites were loaded up to failure, either monotonically or with unloading-reloading cycles at a low strain rate (0.1%mn-) The interface characteristics including the shear stress(T), the debond energy (Gis)and the debond length (la)were extracted from the stress-strain curves [22-24] and from hysteresis loops on Stain (o unloading-reloading [23, 24). Independent models Fig. 4. Tensile stress-strain curves measured on the were used in order to assess the results. These models sic/BN icrocomposites reinforced with: (a) are referred to as LRLC. ClR and lre according to fibers and(b) treated fiber authors s[22-24]. They derive from modelling the tensile, load-displacement behavior (LRLC microcomposites I and 4 reinforced with as-received ers, suggesting the presence of rather weak models)of microcomposites: the CLR model deter- fiber/matrix interactions, short debonds and small mines the energy dissipated in the friction phenomena densities of matrix cracks at saturation when com whereas the LRE one determines the crack opening ing with the microcomposites 2 which exhibit a displacement during unloading-reloading cycles After ultimate failure, the microcomposites were widely curved stress-strain behavior up to ultimate examined using scanning electron microscopy (SEM failure and larger stresses. saturation of matrix crack The composition of the surface of fibers was determ- (0.6%) ing generally occurred at rather large deformations ined from Auger electron spectroscopy(AES) depth- profile analyses of the pulled-out fibers 4.1.2. SEM fractography of microcomposites. The o The tensile tests on the SiC/BN/SiC woven com- numbers of matrix cracks identified on the microcom- posites(three test specimens per batch) were perfor- posites after ultimate failure were generally compara- med at a constant strain rate of 0.05% min-I Defor- ble with the numbers of load drops or of slope mations were measured using an extensometer decreases on the force-displacement curves(Table 3) length 25 mm). The dir ensions of the test speci The higher density of matrix cracks was observed in were as follows: thickness 3 mm width 8 mm microcomposites 2(Table 3) 100mn In the microcomposites reinforced with untreated fibers debonding was observed mainly at the fiber/BN nterface. The free surface of untreated fibers RESULTS which the BN interphase was deposited, is at least 4.1. Tensile tests on the SiC/BN/SiC microcomposites partly made of silica 3). The resulting fiber/BN inter 4.1.. Stress-strain curves. Most of the tensile bond 3, 13, 25, 26]. Similar features have been tressstrain curves( Fig. 4) exhibited a curved observed on SiC/C/SiC composites with a fiber coat- domain over a wide range of deformations(0.2- ing of anisotropic PyC [51 0.9%), and rather large strains-to-failure up to 1. 2% In the microcomposites reinforced with treated (Table 3). However, most of the microcomposites fibers, debonding was detected in the BN coating ith treated fibers essentially experienced premature only in microcomposites 4 with a bi-layered BN coat ing, which indicates that the weakest link is now a plateau-like behavior was observed for located in the interface between the BN sublayers. As
4612 REBILLAT et al.: SiC/SiC COMPOSITES for analysis in that they exhibited features that were inconsistent with the model. A second series of pushin tests on microcomposites 2 was then performed on thicker samples (290 µm) using a flat-bottomed cone. Push-out tests were successfull on samples of microcomposites 4 (thickness <190 µm). The interfaces characteristics were extracted by fitting the Hsueh’s model [15] to the push-in curves or to the curved domain and to the plateau of the push-out curves. 3.3. Tensile tests Five microcomposites per batch were tested in tension by using a specific table-model testing machine designed and developed for fiber testing [20]. The single fiber tensile test procedure based on window frames with appropriate gauge lengths (generally 10 mm) was employed [21, 22]. The microcomposites were loaded up to failure, either monotonically or with unloading–reloading cycles at a low strain rate (0.1% mn21 ). The interface characteristics including the shear stress (t), the debond energy (Gic) and the debond length (ld) were extracted from the stress–strain curves [22–24] and from hysteresis loops on unloading–reloading [23, 24]. Independent models were used in order to assess the results. These models are referred to as LRLC, CLR and LRE according to authors’ names [22–24]. They derive from modelling the tensile load–displacement behavior (LRLC model) or the hysteretic behavior (CLR and LRE models) of microcomposites: the CLR model determines the energy dissipated in the friction phenomena whereas the LRE one determines the crack opening displacement during unloading–reloading cycles. After ultimate failure, the microcomposites were examined using scanning electron microscopy (SEM). The composition of the surface of fibers was determined from Auger electron spectroscopy (AES) depthprofile analyses of the pulled-out fibers. The tensile tests on the SiC/BN/SiC woven composites (three test specimens per batch) were performed at a constant strain rate of 0.05% min21 . Deformations were measured using an extensometer (gauge length 25 mm). The dimensions of the test specimens were as follows: thickness 3 mm, width 8 mm, length 100 mm. 4. RESULTS 4.1. Tensile tests on the SiC/BN/SiC microcomposites 4.1.1. Stress–strain curves. Most of the tensile stress–strain curves (Fig. 4) exhibited a curved domain over a wide range of deformations (0.2– 0.9%), and rather large strains-to-failure up to 1.2% (Table 3). However, most of the microcomposites with treated fibers essentially experienced premature failures. A plateau-like behavior was observed for Fig. 4. Tensile stress–strain curves measured on the SiC/BN/SiC microcomposites reinforced with: (a) as-received fibers and (b) treated fibers. microcomposites 1 and 4 reinforced with as-received fibers, suggesting the presence of rather weak fiber/matrix interactions, short debonds and small densities of matrix cracks at saturation when comparing with the microcomposites 2 which exhibit a widely curved stress–strain behavior up to ultimate failure and larger stresses. Saturation of matrix cracking generally occurred at rather large deformations (>0.6%). 4.1.2. SEM fractography of microcomposites. The numbers of matrix cracks identified on the microcomposites after ultimate failure were generally comparable with the numbers of load drops or of slope decreases on the force–displacement curves (Table 3). The higher density of matrix cracks was observed in microcomposites 2 (Table 3). In the microcomposites reinforced with untreated fibers debonding was observed mainly at the fiber/BN interface. The free surface of untreated fibers, on which the BN interphase was deposited, is at least partly made of silica [3]. The resulting fiber/BN interface has been reported to correspond to a very weak bond [3, 13, 25, 26]. Similar features have been observed on SiC/C/SiC composites with a fiber coating of anisotropic PyC [5]. In the microcomposites reinforced with treated fibers, debonding was detected in the BN coating only in microcomposites 4 with a bi-layered BN coating, which indicates that the weakest link is now located in the interface between the BN sublayers. As
REBILLAT et al: SiC/SIC COMPOSITES Table 3. Main features of the stress-strain curves for the SiC/BN/SiC microcomposites and 2D woven composites Number of cracks at Interphase thickne Pr Failure stress(MPa) Failure strain (%) Microcomposites 0.850.18 BN4 S=as-received fibers, T=treated fibers. SEM previously reported for Pyrocarbon fiber coatings [4 1, treated fibers seem to give stronger fiber/BN bonds. However, the microcomposites with a single layer BN coating appear to be an exception to this 2 rule. since the interface crack was detected at the fiber/BN interface. This was attributed to the presence of a weakly bonded sublayer of carbon that formed on the fibers [27] 4.1.3. Auger electron spectroscopy analyses. AES depth-profile analyses of the pulled out fibers in nicrocomposites reinforced with untreated fibers, showed that the fiber surface is rich in free carbon A layer enriched in carbon and oxygen(probably con- sisting of silica) is present under this carbon layer Such a complex interfacial sequence has been already observed in 2D SiC/BN/SiC composites [26, 28]. The8 曰 CLR (Ioo 原cLR( envelop very thin carbon layer results from the attack of the o LRE (loops) fiber surface during BN processing [271 器LRE( envelop) 4.1.4. Extraction of interfacial properties from the stress-strain curve. The models provide compara ble estimates of interfacial shear stresses for the icrocomposites reinforced with untreated fibers (Fig. 5). A certain discrepancy may be observed for microcomposites 2. The interfacial shear stresses can be grouped into two distinct families(Fig. 5) [5 MPa for microcomposites 4, Fig. 5. Interfacial characteristics estimated using various mod- T210 MPa for microcomposites I and reinforced with untreated fibers The debond energy estimates range between I and 4.2. Tensile tests on the 2D SiC/BN/SiC composites The interfacial characteristics determined for the The stress-strain curves of the 2D sic/bn/sic microcomposites reinforced with treated fibers are composites also display a curved domain( Fig. 7). The Fig. 6. The interfacial shear stresses strains-to-failure are smaller than those measured on obtained for microcomposites I and 2 are larger than the microcomposites(Table 3). They are close those measured for the microcomposites reinforced 0.6% for composites I(reinforced with untreated with as-received fibers. A certain discrepancy is fibers) and 4(reinforced with as-received or treated observed on the data extracted using the LRE model fibers ), whereas the other composites failed at defor- [24]: T=400 MPa and Gie =70 J/m2 seem to be mations <0.2% overestimations although microcomposites I experi- enced a premature failure. The characteristics pro. 4.3. Push-out tests on the 2D SiC/BN/SiC composites vided by the other models seem to be more realistic: 43. 1. Composites reinforced with as-received 10<<50MPa,0<G<7Jm2(Fg6) fibers. The stresses to initiate and propagate the
REBILLAT et al.: SiC/SiC COMPOSITES 4613 Table 3. Main features of the stress–strain curves for the SiC/BN/SiC microcomposites and 2D woven composites Interphase Number of cracks at Failure stress (MPa) Failure strain (%) Interphase thickness Vf saturationb (µm) Sa Ta STST Microcomposites BN1 0.28 0.42 792 680 0.55 0.18 9 2 BN2 0.27 0.76 1368 1813 0.85 1.27 55 46 BN4 0.29 0.47 970 670 0.99 0.2 14 1 Composites BN1 0.5 0.40 220 32 0.58 0.06 BN2 0.3 0.40 210 110 0.38 0.11 BN4 0.5 0.40 200 210 0.5 0.064 a S5as-received fibers, T5treated fibers. b Determined by SEM. previously reported for Pyrocarbon fiber coatings [4, 5], treated fibers seem to give stronger fiber/BN bonds. However, the microcomposites with a single layer BN coating appear to be an exception to this rule, since the interface crack was detected at the fiber/BN interface. This was attributed to the presence of a weakly bonded sublayer of carbon that formed on the fibers [27]. 4.1.3. Auger electron spectroscopy analyses. AES depth-profile analyses of the pulled out fibers in microcomposites reinforced with untreated fibers, showed that the fiber surface is rich in free carbon. A layer enriched in carbon and oxygen (probably consisting of silica) is present under this carbon layer. Such a complex interfacial sequence has been already observed in 2D SiC/BN/SiC composites [26, 28]. The very thin carbon layer results from the attack of the fiber surface during BN processing [27]. 4.1.4. Extraction of interfacial properties from the stress–strain curve. The models provide comparable estimates of interfacial shear stresses for the microcomposites reinforced with untreated fibers (Fig. 5). A certain discrepancy may be observed for microcomposites 2. The interfacial shear stresses can be grouped into two distinct families (Fig. 5): t<5 MPa for microcomposites 4, t$10 MPa for microcomposites 1 and 2, The debond energy estimates range between 1 and 8 J/m2 (Fig. 5). The interfacial characteristics determined for the microcomposites reinforced with treated fibers are shown on Fig. 6. The interfacial shear stresses obtained for microcomposites 1 and 2 are larger than those measured for the microcomposites reinforced with as-received fibers. A certain discrepancy is observed on the data extracted using the LRE model [24]: t 5 400 MPa and Gic 5 70 J/m2 seem to be overestimations although microcomposites 1 experienced a premature failure. The characteristics provided by the other models seem to be more realistic: 10,t,50 MPa, 0,Gic,7 J/m2 (Fig. 6). Fig. 5. Interfacial characteristics estimated using various models for various BN interphases in SiC/BN/SiC microcomposites reinforced with untreated fibers. 4.2. Tensile tests on the 2D SiC/BN/SiC composites The stress–strain curves of the 2D SiC/BN/SiC composites also display a curved domain (Fig. 7). The strains-to-failure are smaller than those measured on the microcomposites (Table 3). They are close to 0.6% for composites 1 (reinforced with untreated fibers) and 4 (reinforced with as-received or treated fibers), whereas the other composites failed at deformations ,0.2%. 4.3. Push-out tests on the 2D SiC/BN/SiC composites 4.3.1. Composites reinforced with as-received fibers. The stresses to initiate and propagate the