4614 REBILLAT et al: SiC/SIC COMPOSITES 400十 bN 4 80 S CLR (loops) BN 4 Fig. 7. Tensile stress-strain curves measured on the Fig. 6. Interfacial characteristics estimated using various mod- SiC/BN/SiC 2D composites reinforced with:(a)as-received els for various BN interphases in SiC/BN/SiC microcomposites fibers and(b) treated fibers. reinforced with treated fibers those estimated for the composites reinforced with debond crack as well as the interfacial shear stresses untreated fibers. Unfortunately, interfacial properties are much higher than those measured on 2D woven could not be extracted from the curved domain of the h-out curve to bending of the sample as a Sic/C/SiC composites with a PyC-based fiber coating result of the high load required to cause debonding and as received fibers [8, 291(Table 4 and Fig. 8). The interfacial shear stress that was extracted from previously with the microcomposites, the largest the plateau is large([=140 MPa). Smaller interfacial stresses and the interfacial shear stresses thus appear shear and debond stresses were obtained for com- to be insensitive to the conditions of BN processing. posites 4(Table 4). These interfacial shear stresses are close to those measured on the composites However, the effect of friction seems to be the most reinforced with untreated fibers efficient in the composites 2, as indicated by the com- The magnitudes of debond stresses may be related parison of the respective values of the following spe- to the respective stress-strain curves displayed by cific parameters(Table 4): the applied maximum composites 2 and 4. The premature ultimate failure stress, the fiber-end displacement and the roughness of composites 2 may correspond to the large oavalue amplitude. The smallest interfacial shear stress and The stress-strain behavior of composites 4, which roughness amplitude() were estimated for the bi- close to that observed for the composites reinforced layered bn coat ble 4) ith untreated fibers, may reflect the similarity in the SEM revealed the following interestis ather rough SEM revealed that debonding took place at the g features:(i) in composites 1, the fiber surface was respective interface characteristics t and oa an ace shov composites 2, the slid surface was very smooth and between the coating and the fiber was not continuous fiber/coating interface(as is usually observed in such probably, as a result of the attack of the fiber by the opposites [18, 26), (ii)in composites 4, the debond gaseous phase crack was located in the interface between the Bn In composites 4, debonding occurred in the inter face between the BN sublayers. Figure 9 shows that a bn sublayer remains bonded to the fiber. SEM 4.3.2. Composites reinforced with treated fibers. examination of the fracture surface of 2D specimens A large debond stress was estimated for composites tested in tension also showed a BN sublayer stuck on 2(oa=2000 MPa, Table 4). It is much larger than the fibers4614 REBILLAT et al.: SiC/SiC COMPOSITES Fig. 6. Interfacial characteristics estimated using various mod￾els for various BN interphases in SiC/BN/SiC microcomposites reinforced with treated fibers. debond crack as well as the interfacial shear stresses are much higher than those measured on 2D woven SiC/C/SiC composites with a PyC-based fiber coating and as received fibers [8, 29] (Table 4 and Fig. 8). As previously with the microcomposites, the largest t were obtained for composites 1 and 2. The debond stresses and the interfacial shear stresses thus appear to be insensitive to the conditions of BN processing. However, the effect of friction seems to be the most efficient in the composites 2, as indicated by the com￾parison of the respective values of the following spe￾cific parameters (Table 4): the applied maximum stress, the fiber-end displacement and the roughness amplitude. The smallest interfacial shear stress and roughness amplitude (A) were estimated for the bi￾layered BN coating (Table 4). SEM revealed the following interesting features: (i) in composites 1, the fiber surface was rather rough and the matrix surface showed a lot of pores, (ii) in composites 2, the slid surface was very smooth and debonding seemed to have occurred at the fiber/coating interface (as is usually observed in such composites [18, 26]), (iii) in composites 4, the debond crack was located in the interface between the BN sublayers (Fig. 9). 4.3.2. Composites reinforced with treated fibers. A large debond stress was estimated for composites 2 (sd 5 2000 MPa, Table 4). It is much larger than Fig. 7. Tensile stress–strain curves measured on the SiC/BN/SiC 2D composites reinforced with: (a) as-received fibers and (b) treated fibers. those estimated for the composites reinforced with untreated fibers. Unfortunately, interfacial properties could not be extracted from the curved domain of the push-out curve, owing to bending of the sample as a result of the high load required to cause debonding. The interfacial shear stress that was extracted from the plateau is large (t<140 MPa). Smaller interfacial shear and debond stresses were obtained for com￾posites 4 (Table 4). These interfacial shear stresses are close to those measured on the composites reinforced with untreated fibers. The magnitudes of debond stresses may be related to the respective stress–strain curves displayed by composites 2 and 4. The premature ultimate failure of composites 2 may correspond to the large sd value. The stress–strain behavior of composites 4, which is close to that observed for the composites reinforced with untreated fibers, may reflect the similarity in the respective interface characteristics t and sd. SEM revealed that debonding took place at the fiber/BN interface in composites 2. The contact between the coating and the fiber was not continuous, probably, as a result of the attack of the fiber by the gaseous phase. In composites 4, debonding occurred in the inter￾face between the BN sublayers. Figure 9 shows that a BN sublayer remains bonded to the fiber. SEM examination of the fracture surface of 2D specimens tested in tension also showed a BN sublayer stuck on the fibers