ournal Am Ceram So, 81 14 965-78(1998 Interfacial Bond Strength in SiC/C/SiC Composite Materials, As Studied by Single-Fiber Push-Out Tests Francis Rebillat, Jacques Lamon, and Roger Naslain Laboratoire des Composites Thermostructuraux, UMR 47, CNRS-SEP-UB1, LCTS, 33600 Pessac, France Edgar Lara-Curzio, Mattison K. Ferber, and Theodore M. Besmann Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6064 The interfacial characteristics of SiC/C/SiC composites characterized by a transverse anisotropic microstructure(typi- with different fiber-coating bond strengths have been ir cally pyrolytic carbon(pyC)or boron nitride(BN))and a low vestigated using single-fiber push-out tests. Previous stud shear modulus(40 GPa). Carbon seems to be the most effec- ies have shown that weak or strong bonds can be obtained tive. However, in Nicalon M(Nippon Carbon, Tokyo, Japan) fiber/SiC matrix combinations. the weakest link is not neces the stress-strain behavior is improved with the treated fi- sarily the interphase. Depending on the fiber-coating bond, 9 bers. This effect results from multiple branching of the the fiber/coating interface, instead of the interphase, may act as cracks within the interphase. The model used to extract a mechanical fuse. In addition, the thermally induced radial interfacial characteristics from nanoindentation and micro- stresses are partially relieved by the interphase. 0, I The inter- indentation tests does not consider the presence of an in- facial bond ultimately influences various features of the com- terphase. However, the results highlight the significant ef- posite mechanical behavior, including ultimate strength, non fect of the interphase on the interfacial parameters, as well linear deformations, Youngs modulus(E), interlaminar shear as the effect of roughness along the sliding surfaces. For the strength, fracture toughness, and compressive strength. Thus, composite with treated fibers, the uncommon upward cur- the properties of the interfacial bond should be tailored to vature of the push-out curves is related to different modes maximize composite performance of crack propagation in the interphase. Different tech- It has been shown, on the basis of fracture statistics, that the niques are required to analyze the interfacial properties, number of matrix cracks increases as the interfacial shear stress such as nanoindentation and microindentation with push increases. As a consequence, saturation of matrix cracking out and push-back tests. has a tendency to occur close to ultimate failure. Furthermore, the highest stresses have been obtained when the interfacial L. Introduction shear stress is relatively high. In contrast, for low interfacial shear stresses, the tensile strain-strain behavior exhibits a pla B ASED on many experimental and theorical studies, it has Following the idea that strong interfacial bonding enhances been established that the mechanical behavior of ceramic composite properties, a new family of SiC/C/SiC materials was matrix composites is dependent not only on the intrinsic prop designed and processed with strengthened fiber-interphase interface. 1-3 Indeed, the nonlinear stress-strain behavior of a treated surface, 14, Is and the composites exhibit higher strength ceramic-fiber-ceramic-matrix composite is related to the de- and toughness, in comparison to their counterparts that have models-6 note the importance of frictional sliding along the been fabricated from as-received fibers 13, 16, 1 In the present paper, the interface and interphase character- debonded interface. Thus, a tough composite is described as istics of strongly bonded two-dimensional(2D)SiC/SiC com- ng fiber frictional sliding over long debond dis- tances, which results in significant fiber pullout and bridging of technique is the most appropriate method to directly measure the cracks propagating through the matrix. Generally, to en- the interfacial properties of these 2D woven composites. How- ourage the deviation of matrix cracks. the fiber-matrix bond is equired to be weak. Improved control of the fiber/matrix in- small-diameter fibers. These difficulties are compounded by terface is achieved via deposition of a layer of a compliant the sample-preparation requirements. The main advantages of material on the fibers. Appropriate interphase materials are ush-out tests are the relative simplicity of the test method and he fact that specific fibers in the composite can be probed Furthermore, the mathematics involved in the pull-out and R. Kerans--contributing editor ush-out models are equivalent, despite a difference in the sign of the Poisson's effect (i.e, during a push-out test, the fiber expands under compression, thus increasing the sliding stress, whereas it contracts under tension, thus reducing the sliding No. 191641. Received July The intent of the present paper is(i)to estimate ssistant Secretary les(as part of the either with as-received or treated fibers using single-f out tests, (ii) to establish correlations between pr lo. DE-AC05-960R22464 with and the stress-strain behavior determined previously, (iii) and Member. American mic Society to assess the concept of strong fiber-matrix bondingInterfacial Bond Strength in SiC/C/SiC Composite Materials, As Studied by Single-Fiber Push-Out Tests Francis Rebillat, Jacques Lamon,* and Roger Naslain* Laboratoire des Composites Thermostructuraux, UMR 47, CNRS-SEP-UB1, LCTS, 33600 Pessac, France Edgar Lara-Curzio, Mattison K. Ferber, and Theodore M. Besmann* Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831–6064 The interfacial characteristics of SiC/C/SiC composites with different fiber-coating bond strengths have been in￾vestigated using single-fiber push-out tests. Previous stud￾ies have shown that weak or strong bonds can be obtained by using as-received or treated fibers, respectively, and that the stress–strain behavior is improved with the treated fi￾bers. This effect results from multiple branching of the cracks within the interphase. The model used to extract interfacial characteristics from nanoindentation and micro￾indentation tests does not consider the presence of an in￾terphase. However, the results highlight the significant ef￾fect of the interphase on the interfacial parameters, as well as the effect of roughness along the sliding surfaces. For the composite with treated fibers, the uncommon upward cur￾vature of the push-out curves is related to different modes of crack propagation in the interphase. Different tech￾niques are required to analyze the interfacial properties, such as nanoindentation and microindentation with push￾out and push-back tests. I. Introduction BASED on many experimental and theorical studies, it has been established that the mechanical behavior of ceramic￾matrix composites is dependent not only on the intrinsic prop￾erties of the constituents but also largely on the fiber/matrix interface.1–3 Indeed, the nonlinear stress–strain behavior of a ceramic-fiber–ceramic-matrix composite is related to the de￾flection of matrix cracks into the fiber/matrix interface. Many models4–6 note the importance of frictional sliding along the debonded interface. Thus, a tough composite is described as experiencing fiber frictional sliding over long debond dis￾tances, which results in significant fiber pullout and bridging of the cracks propagating through the matrix.7 Generally, to en￾courage the deviation of matrix cracks, the fiber–matrix bond is required to be weak. Improved control of the fiber/matrix in￾terface is achieved via deposition of a layer of a compliant material on the fibers. Appropriate interphase materials are characterized by a transverse anisotropic microstructure (typi￾cally pyrolytic carbon (pyC) or boron nitride (BN)) and a low shear modulus (∼40 GPa). Carbon seems to be the most effec￾tive. However, in NicalonTM (Nippon Carbon, Tokyo, Japan) fiber/SiC matrix combinations, the weakest link is not neces￾sarily the interphase. Depending on the fiber–coating bond,8,9 the fiber/coating interface, instead of the interphase, may act as a mechanical fuse. In addition, the thermally induced radial stresses are partially relieved by the interphase.10,11 The inter￾facial bond ultimately influences various features of the com￾posite mechanical behavior, including ultimate strength, non￾linear deformations, Young’s modulus (E), interlaminar shear strength, fracture toughness, and compressive strength. Thus, the properties of the interfacial bond should be tailored to maximize composite performance. It has been shown, on the basis of fracture statistics, that the number of matrix cracks increases as the interfacial shear stress increases.12 As a consequence, saturation of matrix cracking has a tendency to occur close to ultimate failure. Furthermore, the highest stresses have been obtained when the interfacial shear stress is relatively high. In contrast, for low interfacial shear stresses, the tensile strain–strain behavior exhibits a pla￾teau, and saturation of matrix cracking occurs at lower stresses. Following the idea that strong interfacial bonding enhances composite properties, a new family of SiC/C/SiC materials was designed and processed with strengthened fiber–interphase bonds.13 These were obtained using Nicalon™ fibers with a treated surface,14,15 and the composites exhibit higher strength and toughness, in comparison to their counterparts that have been fabricated from as-received fibers.13,16,17 In the present paper, the interface and interphase character￾istics of strongly bonded two-dimensional (2D) SiC/SiC com￾posites13 are investigated using push-out tests. This testing technique is the most appropriate method to directly measure the interfacial properties of these 2D woven composites. How￾ever, the single-fiber push-out test is difficult to perform on small-diameter fibers. These difficulties are compounded by the sample-preparation requirements. The main advantages of push-out tests are the relative simplicity of the test method and the fact that specific fibers in the composite can be probed. Furthermore, the mathematics involved in the pull-out and push-out models are equivalent, despite a difference in the sign of the Poisson’s effect (i.e., during a push-out test, the fiber expands under compression, thus increasing the sliding stress, whereas it contracts under tension, thus reducing the sliding stress). The intent of the present paper is (i) to estimate the interfa￾cial characteristics for 2D SiC/C/SiC composites reinforced either with as-received or treated fibers using single-fiber push￾out tests, (ii) to establish correlations between push-out data and the stress–strain behavior determined previously, (iii) and to assess the concept of strong fiber–matrix bonding. R. J. Kerans—contributing editor Manuscript No. 191641. Received July 30, 1996; approved June 24, 1997. Supported by the LCTS (Pessac, France) and SEP through a grant given to author FR, as well as, at Oak Ridge National Laboratory, by the U.S. Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Trans￾portation Technologies (as part of the HTML User Program), and the U.S. Depart￾ment of Energy, Office of Fossil Energy, Advanced Research and Technology De￾velopment Materials Program (under Contract No. DE-AC05-96OR22464 with Lockheed Martin Energy Research). *Member, American Ceramic Society. J. Am. Ceram. Soc., 81 [4] 965–78 (1998) Journal 965