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 theREBILLAT 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 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]. 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 compara￾ble 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 experi￾enced a premature failure. The characteristics pro￾vided 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 mod￾els 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 defor￾mations ,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