REBILLAT et al: SiC/SIC COMPOSITES for composites 2. This result is also indicated by the behavior. The microcomposites 2 possessed the thinn- tress-strain tensile curves of microcomposites I est matrix layer (Table 3) which display a plateau-like feature when comparing Comparable interfacial characteristics and an ident th those of microcomposites 2. This suggests that ical tensile stress-strain behavior were obtained for the fiber BNI bond is weaker. The plateau-like fea- earlier SiC/BN/SiC composites with as-received ture was not observed with the 2D composites. Fur- Nicalon fibers but a different boron nitride [26] thermore, microcomposites I failed at a smaller A difference in ultimate failure can be noticed deformation, which may be attributed to the degra- when comparing the microcomposites and the com- dation of the fiber during Bn processing posites. This difference must be attributed to the The interface between the BN sublayers in com- mechanisms involved. As previously mentioned, th posites 4(with as-received or treated fibers), and in ultimate failure of 2D composites involves fiber inter microcomposites 4 with treated fibers, was found to actions and the individual break of the weakest fibers be comparatively weaker. The fact that the associated prior to instability. The ultimate failure of microcom interfacial shear stresses are the smallest is not posites is thus dictated by the filament strength reflected by the stress-strain curves of the 2D com-(which exhibits a wide statistical distribution), posites nor of the microcomposites (premature whereas that of 2D composites is determined by fiber failure). The interfacial shear stresses are not distinct tow strength(which exhibits a limited scatter) enough to influence the stress-strain behavior. This conclusion agrees with predictions [6, 71 With treated fibers, the fiber/coating bond was 6. CONCLUSION strengthened, as shown by the estimated interfacial In the SiC/BN/SiC microcomposites reinforced shear stresses and the debond stresses. In the com- with untreated fibers, debonding occurred between posites 2 reinforced with treated fibers, oa(which the fiber free surface and the coating. The fiber/BN ay be considered to be commensurate with the inter- interface is the weakest bond in the interfacial face bond strength) and t were significantly sequence. The presence of a very thin layer of carbon increased. In the composites 4 with bi-layered fiber between a SiO2-C mixture layer and the BN coating atings, debonding occurred in the interface between was detected by aeS depth profile analyses. This sub- the BN sublayers. Furthermore, the corresponding layer seems to affect the interface bond shear and debond stresses are small when comparing When the Bn coating was deposited on treated with those pertinent to the other SiC/BN/SiC com- fibers, the fiber/coating interface was stronger, but posites for which debonding took place in the crack deflection did not occur within the BN coating fiber/BN coating interface. It is particularly interest- Only bi-layered BN coatings with a weak interface ing to compare composites 2 and 4 for which a BN2 between the sublayers experienced crack deviation layer is on the fiber. It may be considered that the within the coating. The interface between the bn sub- strength of the fiber/BN2 bond was the same in both layers becomes the weakest link in the interfacial composites. Therefore it may be concluded that the sequence between the fiber and the matrix interface between the bn sublayers was weaker than The micromechanics based models used for the the fiber/BN2 bond analysis of the stress-strain behavior of th The tensile behavior of the 2D SiC/BN/SiC com- microcomposites provided comparable interfacial posites may be considered to agree with the interface shear stresses, despite a certain scatter in some cases haracteristics measured using push-out tests, since A certain discrepancy was observed for the interface close interfacial shear stresses were obtained for the fracture energy estimates, which fell within a range omposites that exhibited a comparable stress-strain of small values(1-10 J/m). Although there might be behavior, whereas the largest interfacial shear stresses some uncertainly in the t data estimated from tensile were observed for the composites 2 with treated fibers tests on microcomposites, they indicated stronger that experienced a premature failure. This conclusion fiber/matrix bonds in the microcomposites fabricated can be also drawn for the microcomposites reinforced with treated fibers with as-received fibers The data determined using the Hsueh's pushout The interfacial shear stresses are in a good corre- model were about one order of magnitude larger thar lation with the stress-strain behavior, except for those extracted from the tensile stress-strain behavior stresses were determined for microcomposites 2 has not been elucidated in the papes discrepancy which possessed the highest density of matrix cracks The tensile stress/strain curves determined with at saturation(Table 3)and gave the most pronounced microcomposites were similar to those obtained for curvature in the stress-strain curve. However, the the 2D composites. Tensile tests on microcomposite thermally-induced residual stresses, resulting from the may be regarded as an interesting approach to inter thermal expansion mismatch between the fibers and face design. Despite the difficulties to perform the the matrix, exert a certain influence. They are push-out tests on microcomposites, this sample increased by a low volume fraction of matrix, which geometry seems to be appropriate to estimate realistic xplain discrepancies in the stress-strain data on the fiber/matrix interfaces in compositesREBILLAT et al.: SiC/SiC COMPOSITES 4617 for composites 2. This result is also indicated by the stress–strain tensile curves of microcomposites 1 which display a plateau-like feature when comparing with those of microcomposites 2. This suggests that the fiber BN1 bond is weaker. The plateau-like fea￾ture was not observed with the 2D composites. Fur￾thermore, microcomposites 1 failed at a smaller deformation, which may be attributed to the degra￾dation of the fiber during BN processing. The interface between the BN sublayers in com￾posites 4 (with as-received or treated fibers), and in microcomposites 4 with treated fibers, was found to be comparatively weaker. The fact that the associated interfacial shear stresses are the smallest is not reflected by the stress–strain curves of the 2D com￾posites nor of the microcomposites (premature failure). The interfacial shear stresses are not distinct enough to influence the stress–strain behavior. This conclusion agrees with predictions [6, 7]. With treated fibers, the fiber/coating bond was strengthened, as shown by the estimated interfacial shear stresses and the debond stresses. In the com￾posites 2 reinforced with treated fibers, sd (which may be considered to be commensurate with the inter￾face bond strength) and t were significantly increased. In the composites 4 with bi-layered fiber coatings, debonding occurred in the interface between the BN sublayers. Furthermore, the corresponding shear and debond stresses are small when comparing with those pertinent to the other SiC/BN/SiC com￾posites for which debonding took place in the fiber/BN coating interface. It is particularly interest￾ing to compare composites 2 and 4 for which a BN2 layer is on the fiber. It may be considered that the strength of the fiber/BN2 bond was the same in both composites. Therefore it may be concluded that the interface between the BN sublayers was weaker than the fiber/BN2 bond. The tensile behavior of the 2D SiC/BN/SiC com￾posites may be considered to agree with the interface characteristics measured using push-out tests, since close interfacial shear stresses were obtained for the composites that exhibited a comparable stress–strain behavior, whereas the largest interfacial shear stresses were observed for the composites 2 with treated fibers that experienced a premature failure. This conclusion can be also drawn for the microcomposites reinforced with as-received fibers. The interfacial shear stresses are in a good corre￾lation with the stress–strain behavior, except for microcomposites 1. Thus, the largest interfacial shear stresses were determined for microcomposites 2 which possessed the highest density of matrix cracks at saturation (Table 3) and gave the most pronounced curvature in the stress–strain curve. However, the thermally-induced residual stresses, resulting from the thermal expansion mismatch between the fibers and the matrix, exert a certain influence. They are increased by a low volume fraction of matrix, which may explain discrepancies in the stress–strain behavior. The microcomposites 2 possessed the thinn￾est matrix layer (Table 3). Comparable interfacial characteristics and an ident￾ical tensile stress–strain behavior were obtained for earlier SiC/BN/SiC composites with as-received Nicalon fibers but a different boron nitride [26]. A difference in ultimate failure can be noticed when comparing the microcomposites and the com￾posites. This difference must be attributed to the mechanisms involved. As previously mentioned, the ultimate failure of 2D composites involves fiber inter￾actions and the individual break of the weakest fibers prior to instability. The ultimate failure of microcom￾posites is thus dictated by the filament strength (which exhibits a wide statistical distribution), whereas that of 2D composites is determined by fiber tow strength (which exhibits a limited scatter). 6. CONCLUSION In the SiC/BN/SiC microcomposites reinforced with untreated fibers, debonding occurred between the fiber free surface and the coating. The fiber/BN interface is the weakest bond in the interfacial sequence. The presence of a very thin layer of carbon between a SiO2–C mixture layer and the BN coating was detected by AES depth profile analyses. This sub￾layer seems to affect the interface bond. When the BN coating was deposited on treated fibers, the fiber/coating interface was stronger, but crack deflection did not occur within the BN coating. Only bi-layered BN coatings with a weak interface between the sublayers experienced crack deviation within the coating. The interface between the BN sub￾layers becomes the weakest link in the interfacial sequence between the fiber and the matrix. The micromechanics based models used for the analysis of the stress–strain behavior of the microcomposites provided comparable interfacial shear stresses, despite a certain scatter in some cases. A certain discrepancy was observed for the interface fracture energy estimates, which fell within a range of small values (1–10 J/m2 ). Although there might be some uncertainly in the t data estimated from tensile tests on microcomposites, they indicated stronger fiber/matrix bonds in the microcomposites fabricated with treated fibers. The data determined using the Hsueh’s pushout model were about one order of magnitude larger than those extracted from the tensile stress–strain behavior of microcomposites. The origin of this discrepancy has not been elucidated in the paper. The tensile stress/strain curves determined with microcomposites were similar to those obtained for the 2D composites. Tensile tests on microcomposites may be regarded as an interesting approach to inter￾face design. Despite the difficulties to perform the push-out tests on microcomposites, this sample geometry seems to be appropriate to estimate realistic data on the fiber/matrix interfaces in composites