September 1998 Multilayered Interphases in SiC/SiC CHI Composites with"Weak"and" Strong" Interfaces 3000 °531n 20 UCCUEKEf aeg( rcan 0 Fig. 7. Typical curve for a first push-back(composite B). The average frictional shear stress(T)may be estimated from the displacement of the top of the fiber is below the range of the platear accuracy of the equipment. The subsequent load decrease(re- lon(bHc))corresponds to debonding and sliding along a distance shorter than the embedded fiber length. At point( b),a ( critical stress level is reached at the crack tip and the resulting nonlinear portion(region(cHd) displays an upward curvature where t is the embedded length of the fiber and r is the fiber that is attributed to increasing compression along the debond radius. As previously mentioned, the curvature of the nonlinear that does not extend. From point(d)to point(e), the downward domain(region(bH(c) is dictated by the propagation of the curvature is indicative of controlled crack propagation. finally crack and sliding of the debonded fiber. Thus, fitting the in the load decrease from point(e) to the plateau(f) can be at- terfacial model of Hsueh b allows extraction of the interfacial tributed to the mechanisms that were previously described for shear stress. Fitting involves adjustment of three parameters composites reinforced with untreated fibers. Again, Hsueh's the equations have been detailed by Rebillat et al. ) the model b does not apply to push-out curves with an upward oefficient of friction(u), the residual clamping stress(o ), and curvature and a decreasing compliance, because this behavior the residual axial stress in the fiber(o,). 6 The interfacial sl is different from that described in the model. Therefore. the stress is then given by the following equation: frictional shear stress was estimated from the plateau using (2) Observation of protruding fibers using SEM revealed a where o.(the contribution of the Poisson expansion of the uther rough surface (Fig. 6). The debris from the surface have fiber)is derived from the applied load using the equation given by Hsueh. 16 (4) Push-Back Curves and Extraction of slidin The SEM image of a protruding fiber after a push-out test Properties for Composites Reinforced with Treated Fibers (Fig. 4(a)) indicates that the debonding occurs at the fiber/ The stress-displacement curves from push-back tests(Fig coating interface. The fiber surface has rather smooth features, 7)are similar to those from the standard push-out tests(Fig 3). s is usually observed for composites with weak interfaces. involving an initial linear region, a nonlinear region with a However, at higher magnification, a level of roughness is de- downward slope, a load decrease, and a plateau. A previous tected at the fiber surface(Fig. 4(b)) study on analogous two-dimensional(2D) SiC/SiC composites with a single carbon interphase 4 noted a similar push-back 3 Push-Out Curves and Extraction of interphase data behavior. The initial linear domain indicates the elastic defor- for Composites Reinforced with Treated Fibers In contrast to the"well-behaved"push-out curves shown in attributed to the progressive sliding of the fiber via the increas- Fig 3 for composites with untreated fibers, the push-out curves ing sliding distance as the stress overcomes the static frictional obtained for the composites with treated fibers are quite dif- resistance to sliding. The load decrease reflects movement of ferent and follow trends that differ from those assumed in the the fiber end prior to sliding over the length of the test push-out models. Figure 5 shows a typical plot of stress men, as indicated by the pseudo-plateau versus fiber-end displacement obtained for a composite rein- The push-back curve can be analyzed using Hsueh's push forced with treated fibers(composite B). This behavior has out model. 6 The reseating load decrease(point (d))in the been investigated on SiC/SiC composites that possess a single plateau shows that the fiber recovers its initial position before carbon interphase. The results can be applied to the compos- protruding from the other side. The wavelength of the reseating ites with multilayered interphases load decrease is directly related to the wavelength of roughness ss In the apparently linear portion orrig. 5(region (a(b), along the sliding surface. 21-25 rt cracks are created in the interphase. The presence of such Observations of the protruding fiber surface after a push short cracks in SiC/SiC composites that possess a single carbon back test using SEM show that the interphase is partially de- terphase was indicated by nanoindentation curves. The stroyed(Fig. 8). Fragments are present at the fiber end, and short cracks cannot be detected by microindentation because hese pieces seem to be made of carbon because crystal fractuThe average frictional shear stress (t) may be estimated from the plateau: tplateau = splateaur 2t (1) where t is the embedded length of the fiber and r is the fiber radius. As previously mentioned, the curvature of the nonlinear domain (region (b)–(c)) is dictated by the propagation of the crack and sliding of the debonded fiber. Thus, fitting the in￾terfacial model of Hsueh16 allows extraction of the interfacial shear stress. Fitting involves adjustment of three parameters (the equations have been detailed by Rebillat et al.14): the coefficient of friction (m), the residual clamping stress (sc), and the residual axial stress in the fiber (sz).16 The interfacial shear stress is then given by the following equation: t 4 −m(sc + sp) (2) where sp (the contribution of the Poisson expansion of the fiber) is derived from the applied load using the equation given by Hsueh.16 The SEM image of a protruding fiber after a push-out test (Fig. 4(a)) indicates that the debonding occurs at the fiber/ coating interface. The fiber surface has rather smooth features, as is usually observed for composites with weak interfaces. However, at higher magnification, a level of roughness is de￾tected at the fiber surface (Fig. 4(b)). (3) Push-Out Curves and Extraction of Interphase Data for Composites Reinforced with Treated Fibers In contrast to the ‘‘well-behaved’’ push-out curves shown in Fig. 3 for composites with untreated fibers, the push-out curves obtained for the composites with treated fibers are quite dif￾ferent and follow trends that differ from those assumed in the push-out models.17–20 Figure 5 shows a typical plot of stress versus fiber-end displacement obtained for a composite rein￾forced with treated fibers (composite B). This behavior has been investigated on SiC/SiC composites that possess a single carbon interphase.14 The results can be applied to the compos￾ites with multilayered interphases. In the apparently linear portion of Fig. 5 (region (a)–(b)), short cracks are created in the interphase. The presence of such short cracks in SiC/SiC composites that possess a single carbon interphase was indicated by nanoindentation curves.14 The short cracks cannot be detected by microindentation because the displacement of the top of the fiber is below the range of accuracy of the equipment. The subsequent load decrease (re￾gion (b)–(c)) corresponds to debonding and sliding along a distance shorter than the embedded fiber length. At point (b), a critical stress level is reached at the crack tip and the resulting nonlinear portion (region (c)–(d)) displays an upward curvature that is attributed to increasing compression along the debond that does not extend. From point (d) to point (e), the downward curvature is indicative of controlled crack propagation. Finally, the load decrease from point (e) to the plateau (f) can be at￾tributed to the mechanisms that were previously described for composites reinforced with untreated fibers. Again, Hsueh’s model16 does not apply to push-out curves with an upward curvature and a decreasing compliance, because this behavior is different from that described in the model. Therefore, the frictional shear stress was estimated from the plateau using Eq. (1). Observation of protruding fibers using SEM revealed a rather rough surface (Fig. 6). The debris from the surface have the appearance of carbon flakes.5–7 (4) Push-Back Curves and Extraction of Sliding Properties for Composites Reinforced with Treated Fibers The stress–displacement curves from push-back tests (Fig. 7) are similar to those from the standard push-out tests (Fig. 3), involving an initial linear region, a nonlinear region with a downward slope, a load decrease, and a plateau. A previous study on analogous two-dimensional (2D) SiC/SiC composites with a single carbon interphase14 noted a similar push-back behavior. The initial linear domain indicates the elastic defor￾mation of the fiber. The nonlinear domain (region (b)–(c)) is attributed to the progressive sliding of the fiber via the increas￾ing sliding distance as the stress overcomes the static frictional resistance to sliding. The load decrease reflects movement of the fiber end prior to sliding over the length of the test speci￾men, as indicated by the pseudo-plateau. The push-back curve can be analyzed using Hsueh’s push￾out model.16 The reseating load decrease (point (d)) in the plateau shows that the fiber recovers its initial position before protruding from the other side. The wavelength of the reseating load decrease is directly related to the wavelength of roughness along the sliding surface.21–25 Observations of the protruding fiber surface after a push￾back test using SEM show that the interphase is partially de￾stroyed (Fig. 8). Fragments are present at the fiber end, and these pieces seem to be made of carbon because crystal fracture Fig. 7. Typical curve for a first push-back (composite B). September 1998 Multilayered Interphases in SiC/SiC CVI Composites with ‘‘Weak’’ and ‘‘Strong’’ Interfaces 2319