J.Am. Cera.Soc,84141787-94(2001) journal Influence of Strong Fiber/Coating Interfaces on the mechanical Behavior and Lifetime of Hi-Nicalon/(PyC/SiC)/SiC Minicomposites Sebastien Bertrand, Rene Pailler, * and Jacques Lamon poratoire des Composites Thermostructuraux, UMR 5801(CNRS, SEP/SNECMA, UBL, CEA), 33600 Pessac. France Hi-Nicalon fiber-reinforced silicon carbide matrix minicom- PyC-based fiber coatings are the most common. Pyc displays a posites (Hi-Nicalon/SiC) with nanoscale multilayered (PyC layered microstructure, and it leads to high-strength-high- SiC)n fiber coatings(also referred to as interphases) have been toughness SiC/SiC composites. PyC, however, is not stable in an manufactured via pressure pulse chemical vapor infiltration oxidizing environment. Consequently, with a view to protect (P-CVI). Fiber/coating interfaces were strengthened by using treated fibers. The microstructures of the interphases as well ered Py C/SiC interphases has emerged as the propagation and deflection of cracks in the interfacial Multilayered PyC/SiC interphases in SiC/SiC composites have region were investigated by SEM and TEM. Interfacial shear been investigated by several authors. ,6., 7, 10-12 Composites with stress was estimated using various methods based on either the multilayered interphases display mechanical properties and life- width of hysteresis loops on unloading-reloading, crack spac- times at high temperatures that compare favorably with those of ing, or fitting of the force-deformation curve using their counterpart with a single-layer fiber coating. micromechanics-based model. Tensile behavior at room tem- The present paper inve tes the tensile mechanical behavior perature and lifetime in static fatigue in air at 700C were and lifetime of SiC/SiC minicomposites containing Hi-Nicalon related to the interphase/interface characteristics. fibers coated with nanoscale(PyC/SiC), multilayered interphases Strong fiber/coating interphases were obtained using treated fi- bers. Hi-Nicalon/SiC minicomposites reinforced with as-received L. Introduction fibers have been examined in a previous paper IL. Experimental ProceduR omposites with strong interfaces have been developed. The strong (1) Processing Conditions and Materials oating/fiber bond was obtained when the fibers had been prev Minicomposites are unidirectional composites reinforced with a ously treated. Features of the mechanical behavior of sic/sic single tow of fibers. The Hi-Nicalon tows consist of 500 filaments opposites with strong fiber/coating interfaces at room and el each having a diameter -13.5 um(+1.5 um). The minicompos vated temperatures have been examined in several papers ites were manufactured using either as-received-or as-treated Experiments as well as models have demonstrated that a strong Hi-Nicalon tows interface is beneficial to strength, toughness, lifetime, and creep he minicomposites have been produced via pressure pulse resistance. -By contrast, weak interfaces are shown to be chemical vapor infiltration(P-CVI). The P-CVI apparatus and the processing conditions were detailed. -The tows were mounted on SiC/SiC composites with strong fiber/coating bonds, SiC frames for deposition of the interphase and SiC matrix.The cracks are deflected within the coating cohesive failure tows were slightly twisted with a constant angle(I turn/5 cm)to short and branched multiple cracks. The associated ecrease the porosity and to increase the fiber volume fraction in short debonds and load transfers allow further cracking of the the minicomposites matrix via a scale effect related to the stressed volume of Two different interphases were deposited on the fibers(table D) uncracked matrix, leading to a higher density of matrix cracks (1)A(PyC/SiC)lo nanoscale multilayered coating consisting Sliding friction within the coating and multiple cracking of the of 10 Py C/SiC sequences. The thickness of each sublayer was matrix increase energy absorption, leading to toughening. Short e( PyC)=20 nm and e(SiC)=50 nm. PyC was deposited first on debonds and improved load transfers limit fiber overloading the fibers during matrix cracking, leading to strengthening of the composi (2)For comparison purposes, a single Pyc layer 100 nn The associated tensile stress-strain curve exhibits a wide curved domain, and the stress at matrix-cracking saturation is close The fiber volume fraction in the minicomposites was 40% ultimate failure. Values of the interfacial shear stress(T (+5%). The main properties of the minicomposite constituents are using various methods on SiC/SiC composites with sted in Table il. Additional data can be found elsewhere 2, based(PyC-based) fiber coating, range between 10 and weak interfaces, whereas they are 100-300 MPa interfaces. 3,4, 6 (2) Microstructural Characterization Surface analysis of the fibers was performed using electron spectroscopy(AES). The fiber/matrix interfacial F. Zok--contributing editor and longitudinal sections using SEM and HRTEM. Preparation of the thin foils was detailed elsewhere Manuscript No. 188914 Received November 29, 1999; approved November 2, Supported by SEP and CNRS through a grant given to S. Bertrand. Proprietary treatment(LCTS-SNECMA)Influence of Strong Fiber/Coating Interfaces on the Mechanical Behavior and Lifetime of Hi-Nicalon/(PyC/SiC)n/SiC Minicomposites Sebastien Bertrand, Rene Pailler,* and Jacques Lamon* Laboratoire des Composites Thermostructuraux, UMR 5801 (CNRS, SEP/SNECMA, UB1, CEA), 33600 Pessac, France Hi-Nicalon fiber-reinforced silicon carbide matrix minicom￾posites (Hi-Nicalon/SiC) with nanoscale multilayered (PyC/ SiC)n fiber coatings (also referred to as interphases) have been manufactured via pressure pulse chemical vapor infiltration (P-CVI). Fiber/coating interfaces were strengthened by using treated fibers. The microstructures of the interphases as well as the propagation and deflection of cracks in the interfacial region were investigated by SEM and TEM. Interfacial shear stress was estimated using various methods based on either the width of hysteresis loops on unloading–reloading, crack spac￾ing, or fitting of the force–deformation curve using a micromechanics-based model. Tensile behavior at room tem￾perature and lifetime in static fatigue in air at 700°C were related to the interphase/interface characteristics. I. Introduction FIBER/MATRIX interfaces in the most advanced ceramic matrix composites consist of a thin coating layer (micrometer-scale) of one or several materials deposited on the fiber. Recently, SiC/SiC composites with strong interfaces have been developed. The strong coating/fiber bond was obtained when the fibers had been previ￾ously treated.1–3 Features of the mechanical behavior of SiC/SiC composites with strong fiber/coating interfaces at room and ele￾vated temperatures have been examined in several papers.3–8 Experiments as well as models have demonstrated that a strong interface is beneficial to strength, toughness, lifetime, and creep resistance.3–9 By contrast, weak interfaces are shown to be detrimental. In those SiC/SiC composites with strong fiber/coating bonds, the matrix cracks are deflected within the coating (cohesive failure mode) into short and branched multiple cracks.3,10 The associated short debonds and load transfers allow further cracking of the matrix via a scale effect related to the stressed volume of uncracked matrix,4,5 leading to a higher density of matrix cracks. Sliding friction within the coating and multiple cracking of the matrix increase energy absorption, leading to toughening. Short debonds and improved load transfers limit fiber overloading during matrix cracking, leading to strengthening of the composite. The associated tensile stress–strain curve exhibits a wide curved domain, and the stress at matrix-cracking saturation is close to ultimate failure. Values of the interfacial shear stress (t), measured using various methods on SiC/SiC composites with pyrocarbon￾based (PyC-based) fiber coating, range between 10 and 20 MPa for weak interfaces, whereas they are 100–300 MPa for strong interfaces.3,4,6 PyC-based fiber coatings are the most common. PyC displays a layered microstructure, and it leads to high-strength–high￾toughness SiC/SiC composites.3 PyC, however, is not stable in an oxidizing environment. Consequently, with a view to protect PyC-based interphases against oxidation, the concept of multilay￾ered PyC/SiC interphases has emerged. Multilayered PyC/SiC interphases in SiC/SiC composites have been investigated by several authors.2,6,7,10–12 Composites with multilayered interphases display mechanical properties and life￾times at high temperatures that compare favorably with those of their counterpart with a single-layer fiber coating. The present paper investigates the tensile mechanical behavior and lifetime of SiC/SiC minicomposites containing Hi-Nicalon fibers coated with nanoscale (PyC/SiC)n multilayered interphases. Strong fiber/coating interphases were obtained using treated fi￾bers.† Hi-Nicalon/SiC minicomposites reinforced with as-received fibers have been examined in a previous paper.12 II. Experimental Procedures (1) Processing Conditions and Materials Minicomposites are unidirectional composites reinforced with a single tow of fibers. The Hi-Nicalon tows consist of 500 filaments, each having a diameter ;13.5 mm (61.5 mm). The minicompos￾ites were manufactured using either as-received12 or as-treated Hi-Nicalon tows.† The minicomposites have been produced via pressure pulsed chemical vapor infiltration (P-CVI). The P-CVI apparatus and the processing conditions were detailed.12 The tows were mounted on SiC frames for deposition of the interphase and SiC matrix. The tows were slightly twisted with a constant angle (1 turn/5 cm) to decrease the porosity and to increase the fiber volume fraction in the minicomposites. Two different interphases were deposited on the fibers (Table I): (1) A (PyC/SiC)10 nanoscale multilayered coating consisting of 10 PyC/SiC sequences. The thickness of each sublayer was e(PyC) 5 20 nm and e(SiC) 5 50 nm. PyC was deposited first on the fibers. (2) For comparison purposes, a single PyC layer 100 nm thick. The fiber volume fraction in the minicomposites was ;40% (65%). The main properties of the minicomposite constituents are listed in Table II. Additional data can be found elsewhere.12,13 (2) Microstructural Characterization Surface analysis of the fibers was performed using Auger electron spectroscopy (AES). The fiber/matrix interfacial region was examined on failed minicomposites using SEM and on cross and longitudinal sections using SEM and HRTEM. Preparation of the thin foils was detailed elsewhere.14 F. Zok—contributing editor Manuscript No. 188914. Received November 29, 1999; approved November 2, 2000. Supported by SEP and CNRS through a grant given to S. Bertrand. *Member, American Ceramic Society. † Proprietary treatment (LCTS-SNECMA). J. Am. Ceram. Soc., 84 [4] 787–94 (2001) 787 journal