ournal 1. Am Ceram Soc, s1 [9) 2315-26 (1998) Properties of Multilayered Interphases in SiC/SiC Chemical-Vapor-Infiltrated Composites with Weak"and"Strong"In Francis Rebillat, Jacques Lamon, and Roger Naslain Laboratoire des Composites Thermostructuraux(LCTS), UMR 5801, CNRS-SEP-UB1, 33600 Pessac, France Edgar Lara-Curzio, Mattison K Ferber, and Theodore M. Besmann Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6064 The interfacial properties of SiC/SiC composites with in- requires thinner carbon interphases, to reduce the amount of terphases that consist of (C-SiC) sequences deposited or oxidizing phase and to enhance the self-healing capability the fibers have been determined by single-fiber push-out the material via formation of SiO2. However, interphase thick- tests, The matrix has been reinforced with either as- ness must exceed a lower limit to allow deviation of the matrix received or treated Nicalon fibers. The measured interfa- cracks. An alternative concept of multilayered interphases was cial properties are correlated with the fiber-coating bond developed at LCTS, by intercalating SiC sublayers within strength and the number of interlayers. For the composites the carbon coating. Thus, a new family of SiC/SiC composites reinforced with as-received(weakly bonded) fibers, inter- with multilayered interphases has been produced. The thick facial characteristics are extracted from the nonlinear por ness of the carbon sublayers in the interphase is now as low as tion of the stress-displacement curve by fitting Hsueh's 0.05 um. It is expected that oxidation of the matrix and of the push-out model. The interfacial characteristics are con- SiC sublayers will ensure production of SiO, that will be suf- trolled by the carbon layer adjacent to the fiber. The re- ficient to seal the matrix cracks and the associated narrow gaps sistance to interface crack growth and fiber sliding in- between the successive SiC sublayers, thus preventing the creases as the number of(c-SiC) sequences increa complete oxidation of the carbon sublayers. Furthermore, much the osites reinforced with treated(strongly self-protection of the interphases should allow efficient load fibers, the push-out curves exhibit an uncommon transfer at high temperatures in aggressive environments curvature, which reflects different modes of interphase Good mechanical properties require that a balance in fiber/ cracking and a contribution of fiber roughness matrix interactions is found to maximize load transfer while retaining the ability of the fiber to debond and slide. -y Fiber/ . Introduction matrix interactions may be tailored by selecting an appropriate T IS now well acknowledged that the properties of fiber/ to matrix interfaces determine the mechanical behavior of forced with Nicalonas' (Nippon Carbon Co., Tokyo, Japan) brittle-matrix composites 1, 2 Damage tolerance results from the fibers that have been treated prior to deposition of the Pyc deviation of matrix cracks into the fiber/matrix interface. this henomenon can be controlled via deposition of a coating on bonded, whereas the fiber-carbon coating bond is weak the fibers. The most-efficient interphase materials exhibit an those composites reinforced with as-received fibers. Trans anisotropic microstructure and a low shear modulus (-30 GP mission electron microscopy(TEM) examination of the inter- Two materials are commonly used as interphases in SiC/SiC cial region in various families of SiC/SiC composites ha composites: pyrolytic carbon or pyrocarbon(PyC) and boron shown that the sublayers of SiOz and anisotropic carbon are no nitride(BN). Carbon has remained the most-efficient inter- present anymore at the surface of the fibers that have been hase.However,it is very sensitive to oxidation at tempera- treated before interphase deposition. o Furthermore, the surface oxygen, the carbon coating is consumed, which degrades the The primary objective of the reported research is to extract load-transfer capability. At temperatures >8000-1000oC, pas- interface and interphase properties pertinent to these families of ive oxidation of the SiC matrix occurs. The silica(SiO2) that forms can seal the matrix cracks, thus inhibiting further deg effects of multilayering the g single-fiber push-out tests. The SiC/SiC composites by usil interphase and strengthening the adation of the interphase fiber bonding on interphase cracking and fiber sliding have Improving the oxidation resistance of SiC/C/SiC composites been investigated at room temperature Il. Materials and Their Mechanical Properties R. Kerans--contributing editor The SiC/SiC composites were produced via chemical vapor infiltration(CVI) of preforms of either as-received or treated Nicalon fibers. The number of( C-SiC)sequences(n)depos- No. 191642. Received June 2, 1997; appro ited on the fibers was I(single carbon-layered interphase), 2, and 4 (Table 1). at Oak Ridge National Laboratory w For the first family of composites reinforced with as- ergy, Ofto En partment of Energy, Office of Fossil Energy, Advanced Technolo Program)under Contract No. DE-AC05-96OR22464 Lockheed Martin Energy Research, Inc. Member. American Ceramic Society Proprietary treatment by SEP 2315Properties of Multilayered Interphases in SiC/SiC Chemical-Vapor-Infiltrated Composites with ‘‘Weak’’ and ‘‘Strong’’ Interfaces Francis Rebillat,* Jacques Lamon,* and Roger Naslain* Laboratoire des Composites Thermostructuraux (LCTS), UMR 5801, CNRS-SEP-UB1, 33600 Pessac, France Edgar Lara-Curzio,* Mattison K. Ferber,* and Theodore M. Besmann* Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831–6064 The interfacial properties of SiC/SiC composites with in￾terphases that consist of (C–SiC) sequences deposited on the fibers have been determined by single-fiber push-out tests. The matrix has been reinforced with either as￾received or treated Nicalon fibers. The measured interfa￾cial properties are correlated with the fiber–coating bond strength and the number of interlayers. For the composites reinforced with as-received (weakly bonded) fibers, inter￾facial characteristics are extracted from the nonlinear por￾tion of the stress–displacement curve by fitting Hsueh’s push-out model. The interfacial characteristics are con￾trolled by the carbon layer adjacent to the fiber. The re￾sistance to interface crack growth and fiber sliding in￾creases as the number of (C–SiC) sequences increases. For the composites reinforced with treated (strongly bonded) fibers, the push-out curves exhibit an uncommon upward curvature, which reflects different modes of interphase cracking and a contribution of fiber roughness. I. Introduction I T IS now well acknowledged that the properties of fiber/ matrix interfaces determine the mechanical behavior of brittle-matrix composites.1,2 Damage tolerance results from the deviation of matrix cracks into the fiber/matrix interface. This phenomenon can be controlled via deposition of a coating on the fibers. The most-efficient interphase materials exhibit an anisotropic microstructure and a low shear modulus (∼30 GPa). Two materials are commonly used as interphases in SiC/SiC composites: pyrolytic carbon or pyrocarbon (PyC) and boron nitride (BN). Carbon has remained the most-efficient inter￾phase. However, it is very sensitive to oxidation at tempera￾tures >500°C. Therefore, when the interphase is exposed to oxygen, the carbon coating is consumed, which degrades the load-transfer capability. At temperatures >800°–1000°C, pas￾sive oxidation of the SiC matrix occurs. The silica (SiO2) that forms can seal the matrix cracks, thus inhibiting further deg￾radation of the interphase.3 Improving the oxidation resistance of SiC/C/SiC composites requires thinner carbon interphases, to reduce the amount of oxidizing phase and to enhance the self-healing capability of the material via formation of SiO2. However, interphase thick￾ness must exceed a lower limit to allow deviation of the matrix cracks. An alternative concept of multilayered interphases was developed at LCTS,4–9 by intercalating SiC sublayers within the carbon coating. Thus, a new family of SiC/SiC composites with multilayered interphases has been produced.5 The thick￾ness of the carbon sublayers in the interphase is now as low as 0.05 mm. It is expected that oxidation of the matrix and of the SiC sublayers will ensure production of SiO2 that will be suf￾ficient to seal the matrix cracks and the associated narrow gaps between the successive SiC sublayers, thus preventing the complete oxidation of the carbon sublayers. Furthermore, much self-protection of the interphases should allow efficient load transfer at high temperatures in aggressive environments. Good mechanical properties require that a balance in fiber/ matrix interactions is found to maximize load transfer while retaining the ability of the fiber to debond and slide.5–9 Fiber/ matrix interactions may be tailored by selecting an appropriate combination of constituents and by modifying the fiber surface topography. For this purpose, the SiC/SiC composites are re￾inforced with Nicalony (Nippon Carbon Co., Tokyo, Japan) fibers that have been treated prior to deposition of the PyC interphase. The fiber and the carbon coating are strongly bonded, whereas the fiber–carbon coating bond is weak in those composites reinforced with as-received fibers.5–8 Trans￾mission electron microscopy (TEM) examination of the inter￾facial region in various families of SiC/SiC composites has shown that the sublayers of SiO2 and anisotropic carbon are not present anymore at the surface of the fibers that have been treated before interphase deposition.10 Furthermore, the surface of these fibers seems to be rather smooth.8 The primary objective of the reported research is to extract interface and interphase properties pertinent to these families of SiC/SiC composites by using single-fiber push-out tests. The effects of multilayering the interphase and strengthening the fiber bonding on interphase cracking and fiber sliding have been investigated at room temperature. II. Materials and Their Mechanical Properties The SiC/SiC composites were produced via chemical vapor infiltration (CVI) of preforms of either as-received or treated† Nicalon fibers.5 The number of (C–SiC) sequences (n) depos￾ited on the fibers was 1 (single carbon-layered interphase), 2, and 4 (Table I). For the first family of composites reinforced with as￾R. J. Kerans—contributing editor Manuscript No. 191642. Received June 2, 1997; approved December 4, 1997. Supported by LCTS (Pessac, France) and SEP through a grant given to author FR. Work performed at Oak Ridge National Laboratory was supported by the U.S. De￾partment of Energy, Assistant Secretary for Energy Efficiency and Renewable En￾ergy, Office of Transportation Technologies as part of the HTML User Program (U.S. Department of Energy, Office of Fossil Energy, Advanced Research and Technology Development Materials Program) under Contract No. DE-AC05-96OR22464 with Lockheed Martin Energy Research, Inc. *Member, American Ceramic Society. † Proprietary treatment by SEP. J. Am. Ceram. Soc., 81 [9] 2315–26 (1998) Journal 2315