REBILLAT et al: SiC/SIC COMPOSITES The debond energy estimates fall within the range 2 composite: t-75 m a 10 um thk obtained from the tensile tests(Fig. 5) The debond stress in microcomposites 2 is q igl 1500 MPa), as well as the interfacial shear stress (t= 90 MPa), which indicates a high resist- plateau ance to debonding. This may be related to the fact that the fiber could not be pushed out the matrix, even For microcomposites 4, a very good agreement can extracted from respectively the curved domain and the plateau of the push out curves. As previously, lower interfacial shear stress is obtained for this 4 o microcomposite with a double layer coating. Further more, push out of the fiber did not require a load as nd displacement single fiber push-out high as that(omac)applied during fiber pushing in the IC/BN/SIC and a 2D microcomposite SIC/BN (interphase BN4)reinforced with as- SEM showed that debonding essentially received fibers at the fiber/coating interface. Ger v smooth slid surfaces were observed. A was detected 5. DISCUSSION The difficulty to perform push-out tests on thin samples becomes still worse with microcomposites Large load transfers require very thin embedded lengths in order to permit push-out. The parallel-faced strip is not the most convenient test specimen because it is too fragile, but it remains the only usable sample geometry. During a push-in, unknown maximum debond length, damage of the fiber due to high com pressive stresses, bending of the sample and diamond meeting with the matrix are factors that affect the stress-strain curves, and the validity of the extracted characteristics. Then, getting well polished thin strips with a thickness <200 um is rare. This explains why the push-out tests could be carried out only on microcomposites 4 which possessed the weakest Interfaces The less aggressive gaseous phase was used for processing the BN2 coating. This coating has been 5um/ shown to adhere well to the fiber[13]. The resistance to propagation of the interface crack during the push- fiber pu 二 out tests appeared to be high, which suggests efficient load transfers under a tensile load. During the tensile as-received fibers and(b)com- tests, the composites 2 experienced a premature fail- ure whereas the microcomposites 2 exhibited a high strain at saturation. Both features are compatible with efficient load transfers. Ultimate failure of 2D 4.4. Push-out and push-in tests on microcomposites posites involves additionnal phenomena and it The few push-out and push-in curves that were affected by variability in fiber strength degradation suitable for analysis exhibited the features usually during processing. The debond stresses and th observed with 2D composites(Fig. 8). The interface interfacial shear stresses are larger than those parameters extracted from these curves are summar- obtained for composites and microcomposites 4 with ized in Table 4 a bi-layered coating, and they are comparable to those The interfacial shear larger than those estimated for comp xtracted from the tensile ain curves(Fig. cessed in more aggressive conditions. As previously 5). They are in excellent agreement with those esti- mentioned, the propagation of interface cracks during mated on the 2D-SiC/BN/SiC composites(Table 4). the push-out tests was easier for composites I than4616 REBILLAT et al.: SiC/SiC COMPOSITES Fig. 8. Stress–fiber end displacement single fiber push-out curves measured on a SiC/BN/SiC microcomposite and a 2D SiC/BN/SiC composite (interphase BN4) reinforced with as￾received fibers. Fig. 9. SEM micrographs showing protruding fibers after single fiber push-out tests performed on 2D-SiC/BN/SiC composites with an interphase made of two layers of boron nitride (BN4): (a) composite reinforced with as-received fibers and (b) com￾posite reinforced with treated fibers. 4.4. Push-out and push-in tests on microcomposites The few push-out and push-in curves that were suitable for analysis exhibited the features usually observed with 2D composites (Fig. 8). The interface parameters extracted from these curves are summar￾ized in Table 4. The interfacial shear stresses are larger than those extracted from the tensile stress–strain curves (Fig. 5). They are in excellent agreement with those esti￾mated on the 2D-SiC/BN/SiC composites (Table 4). The debond energy estimates fall within the range obtained from the tensile tests (Fig. 5). The debond stress in microcomposites 2 is quite high (sd 5 1500 MPa), as well as the interfacial shear stress (t 5 90 MPa), which indicates a high resist￾ance to debonding. This may be related to the fact that the fiber could not be pushed out the matrix, even with thin samples. For microcomposites 4, a very good agreement can be noticed between the interfacial shear stresses extracted from respectively the curved domain and the plateau of the push out curves. As previously, the lower interfacial shear stress is obtained for this microcomposite with a double layer coating. Further￾more, push out of the fiber did not require a load as high as that (smax) applied during fiber pushing in the microcomposites 2. SEM showed that debonding occurred essentially at the fiber/coating interface. Generally, really smooth slid surfaces were observed. A certain roughness was detected. 5. DISCUSSION The difficulty to perform push-out tests on thin samples becomes still worse with microcomposites. Large load transfers require very thin embedded lengths in order to permit push-out. The parallel-faced strip is not the most convenient test specimen because it is too fragile, but it remains the only usable sample geometry. During a push-in, unknown maximum debond length, damage of the fiber due to high com￾pressive stresses, bending of the sample and diamond meeting with the matrix are factors that affect the stress–strain curves, and the validity of the extracted characteristics. Then, getting well polished thin strips with a thickness ,200 µm is rare. This explains why the push-out tests could be carried out only on microcomposites 4 which possessed the weakest interfaces. The less aggressive gaseous phase was used for processing the BN2 coating. This coating has been shown to adhere well to the fiber [13]. The resistance to propagation of the interface crack during the push￾out tests appeared to be high, which suggests efficient load transfers under a tensile load. During the tensile tests, the composites 2 experienced a premature fail￾ure whereas the microcomposites 2 exhibited a high strain at saturation. Both features are compatible with efficient load transfers. Ultimate failure of 2D com￾posites involves additionnal phenomena and it is affected by variability in fiber strength degradation during processing. The debond stresses and the interfacial shear stresses are larger than those obtained for composites and microcomposites 4 with a bi-layered coating, and they are comparable to those estimated for composites 1 with an interphase pro￾cessed in more aggressive conditions. As previously mentioned, the propagation of interface cracks during the push-out tests was easier for composites 1 than