ournal 1. Am Ceram Soc, s1 [9) 2105-12(1998 Assessment of the Interfacial Properties of Ceramic-Matrix Composites Using Strain Partition under Load Jean-Marie Morvan and Stephane Baste Laboratoire de Mecanique Physique, Universite Bordeaux 1, CNRS UPRES A 5469, 33405 Talence, France The interfacial sliding stress is a key parameter in the glob- curate study of the fracture behavior requires identification of I behavior of ceramic-matrix composites. An ultrasonic the damage threshold(that is to say, the stress where the crack characterization, through the complete determination of begins ), the debonding stress, the sliding stress, and the he stiffness tensor along a tensile test, detects all the dam interfacial shear stress. Three basic parameters can be exhibited ge mechanisms: transverse matrix microcracking and the from the stress-strain curves the stress at the onset of matri existence of longitudinal cracks at the fiber/matrix inter- cracking, the stress at saturation, and the ultimate strength. face. It also allows a strain partition under load. A micro Identification of the interfacial shear stress, T, has given rise mechanical model is established and gives access to the to numerous works. It has even led to new measurement tech- value of the interfacial sliding stress during the entire ten niques such as microindentation. ,9 This technique allows sile test, whether the damage occurs at the mesostructural study of the interfacial properties by analyzing the stress- or at the microstructural level of the composite. displacement curves obtained from push-out or push- hrough tests of a single fiber or of a bundle of fibers. 1 Some works have also focused on the measurement of the pull-out length, 12 the use of the intercrack distance at saturation, or the T IS now commonly recognized that the macroscopic nonlin- information held in the hysteresis coming from cyclic loading Lear behavior of ceramic-matrix composites( CMCs)unde tensile loading is the result of the combination of three main analysis 4, I5 is also included in these methods. However, they amage mechanisms: I matrix microcracking normal to the ten- are not exempt from criticism. For example, the push-out test sile axis. deflection of these cracks at the fiber/matrix interface creates a Poisson effect opposite to the one created during and fiber fracture. Matrix microcracking induces a loss of stiff. monotonic loading, tests on microcomposites lack the structure ness. The fiber-matrix debonding leads to fiber-matrix slid Nevertheless, some basic trends have a tendency to emerge prevents the composite from failing prematurely When a fiber The interaction between the fiber and the matrix involves the breaks, the load sharing that results is done through the sliding esistance along the fiber. 2 Thus, the sliding resistance(T)is a he fibers and the matrix, as well as the interfacial properties key parameter in the global behavior The importance of the sliding phenomenon is related to the misfit strain, the fiber surface roughness, and the debond trix cracking, followed by interaction of these cracks with the length. o However, surface roughness can be considered as a fibers and interfaces. 3 Such interactions involve the debond radial compressive stress at the interface. 6 The main informa fracture energy and the sliding resistance within the debond. tion is that t varies with the sliding length. Actually, the pa- Therefore. the role of t rameters that govern friction are not all constant. Abrasion and Beyond the protection o the fibers during processing roughness effects are functions of the sliding distance. The he fiber coating can improve both the crack deflection and the fiber pullout decreases the friction force because the length over which the fiber slides also decreases vsteresis is de oughness. The interphase controls the fracture resistance, be endent upon both the friction force and the length over which cause it promotes the crack arrest by matrix crack deflection. It he fibers slide. 7 Thus, the volume of fibers is an important also limits the extension of the cracks by energy absorption parameter, because the area available for energy dissipation These mechanisms occur either at the matrix/interphase or in- ncreases with it. 7 The hysteresis is larger with lower values of terphase/fiber interfaces or in the interphase itself when it is thick enough. Although the response of the interface is com- creases through more-effective bridging. 7 Debris at the inter plex, it is usually idealized as two mechanisms: the initial faces can influence the friction coefficient during a test. 6 esistance to relative motion(usually, the debonding) and the Some work has also been conducted on the decrease of inter frictional sliding resistance. facial shear during fatigue tests. The observation is made that The stress-strain curves of CMCs clearly show that the glob. assumption of a constant shear stress and a constant number of cracks created in the longitudinal bundles due to interfacial of the constituent o he first cycles might be wrong. Further- al strain is the sum of an elastic and an inelastic strain. The broken fibers during hat results from the thermal mismatch Thus, the identification of the micromechanical parameters conceal the abrasion effect. In the same manner, the interface that govern these phenomena is of prime importance. An roughness has a tendency to counterbalance the Poisson effect From a quantitative standpoint, carbon or boron nitride coat ngs have a tendency to diminish the interfacial shear stren A G. Evans--contributing editor however. strength is still a function of the residual stresses Nevertheless. the results found in the literature exhibit large scatter. 2,3, 20 In a two-dimensional chemical-vapor-infiltrated Sic-SiC(2D CVI SiC-SiC) composite, for example, the in- Manuscript No 191124 Received March 19, 1997: approved December 23, 1997 terfacial sliding stress can vary within a range of 2-370 ME 2405Assessment of the Interfacial Properties of Ceramic-Matrix Composites Using Strain Partition under Load Jean-Marie Morvan and Stéphane Baste Laboratoire de Mécanique Physique, Université Bordeaux 1, CNRS UPRES A 5469, 33405 Talence, France The interfacial sliding stress is a key parameter in the glob￾al behavior of ceramic-matrix composites. An ultrasonic characterization, through the complete determination of the stiffness tensor along a tensile test, detects all the dam￾age mechanisms: transverse matrix microcracking and the existence of longitudinal cracks at the fiber/matrix inter￾face. It also allows a strain partition under load. A micro￾mechanical model is established and gives access to the value of the interfacial sliding stress during the entire ten￾sile test, whether the damage occurs at the mesostructural or at the microstructural level of the composite. I. Introduction I T IS now commonly recognized that the macroscopic nonlin￾ear behavior of ceramic-matrix composites (CMCs) under tensile loading is the result of the combination of three main damage mechanisms:1 matrix microcracking normal to the ten￾sile axis, deflection of these cracks at the fiber/matrix interface, and fiber fracture. Matrix microcracking induces a loss of stiff￾ness. The fiber–matrix debonding leads to fiber–matrix sliding, with friction depending on the nature of the interface, and prevents the composite from failing prematurely. When a fiber breaks, the load sharing that results is done through the sliding resistance along the fiber.2 Thus, the sliding resistance (t) is a key parameter in the global behavior. Thus, the fracture properties of CMCs are governed by ma￾trix cracking, followed by interaction of these cracks with the fibers and interfaces.3 Such interactions involve the debond fracture energy and the sliding resistance within the debond. Therefore, the role of the interphase is of first importance. Beyond the protection provided to the fibers during processing, the fiber coating can improve both the crack deflection and the stress transfer by modifying, among other things, the surface roughness.4 The interphase controls the fracture resistance, be￾cause it promotes the crack arrest by matrix crack deflection. It also limits the extension of the cracks by energy absorption. These mechanisms occur either at the matrix/interphase or in￾terphase/fiber interfaces or in the interphase itself when it is thick enough. Although the response of the interface is com￾plex, it is usually idealized as two mechanisms: the initial resistance to relative motion (usually, the debonding) and the frictional sliding resistance.5 The stress–strain curves of CMCs clearly show that the glob￾al strain is the sum of an elastic and an inelastic strain. The inelastic strain is controlled by the opening of the transverse cracks created in the longitudinal bundles due to interfacial debonding and fiber–matrix sliding.6 Thus, the identification of the micromechanical parameters that govern these phenomena is of prime importance. An ac￾curate study of the fracture behavior requires identification of the damage threshold (that is to say, the stress where the crack￾ing begins), the debonding stress, the sliding stress, and the interfacial shear stress. Three basic parameters can be exhibited from the stress–strain curves: the stress at the onset of matrix cracking, the stress at saturation, and the ultimate strength.7 Identification of the interfacial shear stress, t, has given rise to numerous works.2 It has even led to new measurement tech￾niques such as microindentation.8,9 This technique allows study of the interfacial properties by analyzing the stress– displacement curves10 obtained from push-out4 or push￾through tests of a single fiber or of a bundle of fibers.11 Some works have also focused on the measurement of the pull-out length,12 the use of the intercrack distance at saturation,3 or the information held in the hysteresis coming from cyclic loading of either microcomposites or real composites.13 Finite-element analysis14,15 is also included in these methods. However, they are not exempt from criticism. For example, the push-out test creates a Poisson effect opposite to the one created during monotonic loading; tests on microcomposites lack the structure effect.11 Nevertheless, some basic trends have a tendency to emerge. The interaction between the fiber and the matrix involves the misfit strain induced by the difference of properties between the fibers and the matrix, as well as the interfacial properties.7 The importance of the sliding phenomenon is related to the misfit strain, the fiber surface roughness, and the debond length.10 However, surface roughness can be considered as a radial compressive stress at the interface.16 The main informa￾tion is that t varies with the sliding length. Actually, the pa￾rameters that govern friction are not all constant. Abrasion and roughness effects are functions of the sliding distance. The fiber pullout decreases the friction force because the length over which the fiber slides also decreases.17 Hysteresis is de￾pendent upon both the friction force and the length over which the fibers slide.17 Thus, the volume of fibers is an important parameter, because the area available for energy dissipation increases with it.17 The hysteresis is larger with lower values of frictional strength; however, the opening of the cracks de￾creases through more-effective bridging.17 Debris at the inter￾faces can influence the friction coefficient during a test.16 Some work has also been conducted on the decrease of inter￾facial shear during fatigue tests.18 The observation is made that assumption of a constant shear stress and a constant number of broken fibers during the first cycles might be wrong. Further￾more, the compression that results from the thermal mismatch of the constituents, especially if the thermal expansion coeffi￾cient of the matrix is greater than that of the fiber, might conceal the abrasion effect. In the same manner, the interface roughness has a tendency to counterbalance the Poisson effect during pullout.19 From a quantitative standpoint, carbon or boron nitride coat￾ings have a tendency to diminish the interfacial shear strength; however, strength is still a function of the residual stresses.3 Nevertheless, the results found in the literature exhibit large scatter.2,3,20 In a two-dimensional chemical-vapor-infiltrated SiC–SiC (2D CVI SiC–SiC) composite, for example, the in￾terfacial sliding stress can vary within a range of 2–370 MPa.7 A. G. Evans—contributing editor Manuscript No. 191124. Received March 19, 1997; approved December 23, 1997. J. Am. Ceram. Soc., 81 [9] 2405–12 (1998) Journal 2405