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. As4612 REBILLAT et al.: SiC/SiC COMPOSITES for analysis in that they exhibited features that were inconsistent with the model. A second series of push￾in 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 inter￾faces 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 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% 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 deter￾mines 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 determ￾ined from Auger electron spectroscopy (AES) depth￾profile analyses of the pulled-out fibers. The tensile tests on the SiC/BN/SiC woven com￾posites (three test specimens per batch) were perfor￾med at a constant strain rate of 0.05% min21 . Defor￾mations 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 compar￾ing with the microcomposites 2 which exhibit a widely curved stress–strain behavior up to ultimate failure and larger stresses. Saturation of matrix crack￾ing generally occurred at rather large deformations (>0.6%). 4.1.2. SEM fractography of microcomposites. The numbers of matrix cracks identified on the microcom￾posites after ultimate failure were generally compara￾ble 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 inter￾face 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 coat￾ing 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 coat￾ing, which indicates that the weakest link is now located in the interface between the BN sublayers. As