April 1998 Interfacial Bond Strength in SiC/C/SiC Composite Materials (a) tal 10 um (b) (b) I um Fig. 6. SEM micrographs of (a) protruding fibers and(b)a matrix Fig. 7. SEM micrographs of protruding fibers after push-out tests sliding surface after a push-out test on a SiC/C/SiC composite rein- performed on a SiC/C/SiC composite reinforced with treated fibers forced with as-received fibers(material 1). phase has been torn during fiber sliding(Fig. 7); this is in ontrast to the featureless surfac untreated fibe (6) Nanoindentation Tests Performed on 6(a). The sliding surface on th side. as well SiC/SiC Composites Reinforced with Treated Fibers protruded fiber surface, shows crack initiated Figure 8(a) shows a comparison of experimental nanoinden- the carbon layer(top inset in Fig. 1) tation data on a Nicalon TM fiber and a second-order polynomial xpression of that relationship. The good fit indicates that the 5) Results of Alternate Approaches to polynomial can be used to subtract the effect of indentor pen- Deriving Interfacial Characteristics relatively indepen. etration into the fiber. Figure 8(b) shows an example of nanoindentation curve derived by subtracting the indentor pen- dent of the expression that was used. The interfacial shear etration distance from the raw experimental data. The end of strengths(Ts)increase modestly as the complexity of the rela- the linear portion marks the initiation of debonding. Tables Il tionship increases. The largest Ts values were obtained for the and IV show that the debonding stress that is determined by composites reinforced with treated fibers; Ts remained near nanoindentation is much smaller than that measured by micro- 150-185 MPa for the composite reinforced with as-received indentation(1000 MPa versus -3800 MPa). The fiber dis- fibers, and Ts 1000 or 1300 MPa for those reinforced with placement obtained after subtraction of the penetration distance treated fibers(composites P and J, respectively ) Furthermore, is small (150 nm). Even at maximum load (50 g), the applied s was not significantly affected by the sample thickness, ex- stress is less than the debonding stress during microindentation cept for composite P specimens that had a thickness that was Such a small displacement cannot be measured by microinden- not significantly larger than the fiber diameter anymore, as tation, mainly because of significant background pertubations assumed in the models However, the interfacial characteristics measured during un- The clamping stress and the interfacial shear stress, deter- loading(Table IV) are in agreement with those determined by mined using Eq(6)(Table III), are in excellent agreement with microindentation(Table Il). The discrepancy between loading those obtained using the plateau stress(Table II). They confirm and unloading may result from the very large residual stresses the increase in interfacial characteristics due to the use of calculated from the fitting method Furthermore. when unload- treated fibers is performed, influence of the diamond on fiber deforma-phase has been torn during fiber sliding (Fig. 7); this is in contrast to the featureless surface of the untreated fibers (Fig. 6(a)). The sliding surface on the matrix side, as well as the protruded fiber surface, shows that the crack initiated within the carbon layer (top inset in Fig. 1). (5) Results of Alternate Approaches to Deriving Interfacial Characteristics The results given in Table III seem to be relatively indepen￾dent of the expression that was used. The interfacial shear strengths (ts) increase modestly as the complexity of the rela￾tionship increases. The largest ts values were obtained for the composites reinforced with treated fibers; ts remained near 150–185 MPa for the composite reinforced with as-received fibers, and ts ≈ 1000 or 1300 MPa for those reinforced with treated fibers (composites P and J, respectively). Furthermore, ts was not significantly affected by the sample thickness, ex￾cept for composite P specimens that had a thickness that was not significantly larger than the fiber diameter anymore, as assumed in the models. The clamping stress and the interfacial shear stress, deter￾mined using Eq. (6) (Table III), are in excellent agreement with those obtained using the plateau stress (Table II). They confirm the increase in interfacial characteristics due to the use of treated fibers. (6) Nanoindentation Tests Performed on SiC/SiC Composites Reinforced with Treated Fibers Figure 8(a) shows a comparison of experimental nanoinden￾tation data on a Nicalon™ fiber and a second-order polynomial expression of that relationship. The good fit indicates that the polynomial can be used to subtract the effect of indentor pen￾etration into the fiber. Figure 8(b) shows an example of a nanoindentation curve derived by subtracting the indentor pen￾etration distance from the raw experimental data. The end of the linear portion marks the initiation of debonding. Tables II and IV show that the debonding stress that is determined by nanoindentation is much smaller than that measured by micro￾indentation (∼1000 MPa versus ∼3800 MPa). The fiber dis￾placement obtained after subtraction of the penetration distance is small (∼150 nm). Even at maximum load (∼50 g), the applied stress is less than the debonding stress during microindentation. Such a small displacement cannot be measured by microinden￾tation, mainly because of significant background pertubations. However, the interfacial characteristics measured during un￾loading (Table IV) are in agreement with those determined by microindentation (Table II). The discrepancy between loading and unloading may result from the very large residual stresses calculated from the fitting method. Furthermore, when unload￾ing is performed, influence of the diamond on fiber deforma￾Fig. 6. SEM micrographs of (a) protruding fibers and (b) a matrix sliding surface after a push-out test on a SiC/C/SiC composite rein￾forced with as-received fibers (material I). Fig. 7. SEM micrographs of protruding fibers after push-out tests performed on a SiC/C/SiC composite reinforced with treated fibers (material J). April 1998 Interfacial Bond Strength in SiC/C/SiC Composite Materials 971