April 1998 Interfacial Bond Strength in SiC/C/SiC Composite Materials 0 6000 4000 3000 thickness 186 um 2000 ,f,,,,f,,, 3 Displacement (um) ig. 3. Single-fiber push-out curve measured for a SiC/C/SiC composite(sample J)reinforced with treated Nicalon TM fibers ample thickness on the measured interfacial properties pro- with weak interfaces. However, evidence of a very small am- ided data on the test zone to be selected for the push-oI plitude roughness is detected via SEM at higher magnification tests. The thickest part of the samples seemed to be the most(Fig. 6(b)) appropriate to minimize the scatter in data, because of the to the debonding stress. This obser yamax"Plateau)was similar Finally, the load drop(Aodrop =omax presence of fibers that had been debonded during sam expected in such a preparation. The embedded length of the tested fibers(320-400 um)did not seem to influence the measured properties, which emain essentially constant as the sample thickness varies Interfacial Parameters for However, the standard deviation for almost all the parameters ated Fiber-Reinforced C is quite large(<30%), and, the maximum stress at the load Because of the lack of a pertinent model, only the following drop( Fig 4)did not increase with the embedded length arameters could be extracted from the push-out curves: the Comparable estimates of the interfacial shear stress were debonding stress, the maximum load, the displacement at the derived from the nonlinear part of the curves and from the maximum load, and the frictional shear stress from the plateau lateau (table ll and Fig. 5). These values are close to the uppe stress(Table II). The large force needed to push the fibers out ound in the range of interfacial shear stresses commonl of the matrix indicated a high resistance to the propagation of measured by this technique on Nicalon M-fiber/SiC-matrix nterfacial cracks. Hence, it was possible to satisfactorily push composites with a interphase of 0.5 Hm. 8, I9 out the fibers(without fiber damage or the indentor contacting The magnitude derived axial and radial clamping the matrix)only when the sample thickness was <180 um comparison to those generally com- Table II shows that, for material J, the debonding matrix combination(less than or equal to increases only slightly with the sample thickness, where 200 MPa). 10, I 1, 28 A logical reason for the high magnitude of maximum load increases substantial 4)over the g is that the Hsueh model does not consider the effect of ange of sample thicknesses examined. The frictional surface roughness during fiber sliding. The effect of surface stresses are not affected significantly(Fig. 5 roughness may be characterized through a radial compressive When the thickness of the carbon interphase is 0. 1 um(ma- stress at the interface that is superimposed on the radial residual terial P), data indicate that the composite is more resistant to stress component. The clamping stress o becomes the sum debonding and sliding (Table II). The first feature to be noticed of the thermally induced residual clamping stress and the is that very thin samples (i.e, 45-81 um thick) are required to roughness(A)was extracted using the following expression: 28 and the interfacial shear stress are significantly higher than EX1+vm)+Em(l-ve those measured for composite J(Figs. 4 and 5). Such high acial shea △a△T (15) ness. 17 Finally, other values, such as the debond and maximum stresses and interfacial shear stress, show a significant increase where parameter q= I for an infinite matrix. A q value of 1 with sample thickness, which also suggests that the debond was used, which may lead to an underestimation of A: in the stress for material P is larger than that for material presence of a finite matrix, a value of q that is commensurate The forces necessary for debonding(2500 MPa) and fo ith the matrix fraction may be preferred. An amplitude of 3 pushing out the fibers(4000 MPa) are uncommonly high for m(+15 nm)was thus obtained for the roughness A (Table Il). test specimens as thin as 150 um. The associated displace- This value is in agreement with the fiber roughness measured ments are also very small (1. I um). Under similar displace- Nicalon TM fibers by atomic force microscopy(AFM)(30 ments, the untreated fibers exhibit maximum stresses that are a nm(see More") factor of-3 smaller Examination of the fiber surface also seems to confirm these The sliding shear stresses for the treated fibers are >100 observations. The scanning electron microscopy(SEM)image MPa, which is an order of magnitude higher than those deter- of a pushed-out fiber( Fig. 6(a)) indicates that debonding oc- mined for the untreated fiber composites. Such high T values curred at the fiber surface (lower inset in Fig. 1). The micro- Iggest rgy dissipation by friction during fiber sliding ph shows that the fiber surface has rather smooth features, SEM images show that the surface of the pushed-out treated as is usually observed for Nicalon TM-reinforced composites fibers appears to be rather rough, which shows that the inter-sample thickness on the measured interfacial properties pro￾vided data on the test zone to be selected for the push-out tests.31 The thickest part of the samples seemed to be the most appropriate to minimize the scatter in data, because of the presence of fibers that had been debonded during sample preparation. The embedded length of the tested fibers (320–400 mm) did not seem to influence the measured properties, which remain essentially constant as the sample thickness varies. However, the standard deviation for almost all the parameters is quite large (#30%), and, the maximum stress at the load drop (Fig. 4) did not increase with the embedded length. Comparable estimates of the interfacial shear stress were derived from the nonlinear part of the curves and from the plateau (Table II and Fig. 5). These values are close to the upper bound in the range of interfacial shear stresses commonly measured by this technique on Nicalon™-fiber/SiC-matrix composites with a carbon interphase of 0.5 mm.18,19 The magnitude of the derived axial and radial clamping stresses are too high, in comparison to those generally com￾puted for this fiber–matrix combination (less than or equal to −200 MPa).10,11,28 A logical reason for the high magnitude of sc is that the Hsueh model does not consider the effect of surface roughness during fiber sliding. The effect of surface roughness may be characterized through a radial compressive stress at the interface that is superimposed on the radial residual stress component. The clamping stress sc becomes the sum of the thermally induced residual clamping stress and the compressive stress due to roughness. A magnitude for the roughness (A) was extracted using the following expression:28 A = H− scF Ef~1 + nm! + Em~1 − nf! qEmEf G − DaDTJr (15) where parameter q 4 1 for an infinite matrix. A q value of 1 was used, which may lead to an underestimation of A; in the presence of a finite matrix, a value of q that is commensurate with the matrix fraction may be preferred. An amplitude of 35 nm (±15 nm) was thus obtained for the roughness A (Table II). This value is in agreement with the fiber roughness measured on Nicalon™ fibers by atomic force microscopy (AFM) (30 nm (see More32)). Examination of the fiber surface also seems to confirm these observations. The scanning electron microscopy (SEM) image of a pushed-out fiber (Fig. 6(a)) indicates that debonding oc￾curred at the fiber surface (lower inset in Fig. 1). The micro￾graph shows that the fiber surface has rather smooth features, as is usually observed for Nicalon™-reinforced composites with weak interfaces. However, evidence of a very small am￾plitude roughness is detected via SEM at higher magnification (Fig. 6(b)). Finally, the load drop (Dsdrop 4 smax − splateau) was similar to the debonding stress. This observation is expected in such a system (Table II). (4) Interfacial Parameters for Treated Fiber-Reinforced Composites Because of the lack of a pertinent model, only the following parameters could be extracted from the push-out curves: the debonding stress, the maximum load, the displacement at the maximum load, and the frictional shear stress from the plateau stress (Table II). The large force needed to push the fibers out of the matrix indicated a high resistance to the propagation of interfacial cracks. Hence, it was possible to satisfactorily push out the fibers (without fiber damage or the indentor contacting the matrix) only when the sample thickness was <180 mm. Table II shows that, for material J, the debonding stress increases only slightly with the sample thickness, whereas the maximum load increases substantially (Fig. 4) over the short range of sample thicknesses examined. The frictional shear stresses are not affected significantly (Fig. 5). When the thickness of the carbon interphase is 0.1 mm (ma￾terial P), data indicate that the composite is more resistant to debonding and sliding (Table II). The first feature to be noticed is that very thin samples (i.e., 45–81 mm thick) are required to be able to push out the fibers. Furthermore, the maximum stress and the interfacial shear stress are significantly higher than those measured for composite J (Figs. 4 and 5). Such high interfacial shear stresses suggest an increased effect of rough￾ness.17 Finally, other values, such as the debond and maximum stresses and interfacial shear stress, show a significant increase with sample thickness, which also suggests that the debond stress for material P is larger than that for material J. The forces necessary for debonding (>2500 MPa) and for pushing out the fibers (>4000 MPa) are uncommonly high for test specimens as thin as 150 mm.31 The associated displace￾ments are also very small (∼1.1 mm). Under similar displace￾ments, the untreated fibers exhibit maximum stresses that are a factor of ∼3 smaller. The sliding shear stresses for the treated fibers are >100 MPa, which is an order of magnitude higher than those deter￾mined for the untreated fiber composites. Such high t values suggest large energy dissipation by friction during fiber sliding. SEM images show that the surface of the pushed-out treated fibers appears to be rather rough, which shows that the inter￾Fig. 3. Single-fiber push-out curve measured for a SiC/C/SiC composite (sample J) reinforced with treated Nicalon™ fibers. April 1998 Interfacial Bond Strength in SiC/C/SiC Composite Materials 969