Part A: applied science g ELSEVIER Composites: Part A 33(2002)1467-1470 www.elsevier.com/locate/composite Microstructural investigation of interfaces in CMCs G. Boitier.s. Darzens. J.-L. ChermantJ. Vicens LERMAT. URA CNRS 1317. ISMRA. 6 Bd Marechal Juin. /4050 Caen Cedex. france Abstract This paper is focused on the importance of pyrocarbon interfaces in two types of ceramic matrix composites( Cr-SiC and SiCr-SiBC) during creep tests under argon. The development of micromechanisms which consume energy and then allow a damage tolerance, depends on norphology of the fiber/matrix interphase, which has been investigated by TEM and HRTEM 02 Elsevier Science Ltd. All rights reserved Keywords: A Ceramic-matrix composites(CMCs); B Interface/interphase; TEM; B Cree 1. Introduction multilayered matrix, based on Si-C, B-C and Si-B-C phases. All the fiber architectures have received a thin Ceramic matrix composites(CMCs) are a class of pyrolytic carbon deposit before the infiltration of the matrix. materials developed for aeronautics and space applications, Specimens were creep tested in tension, under a in a domain where superalloys cannot be used anymore. pressure of argon(500 mbar), in a temperature domain up to They have potential applications in structures(air intakes, 1673 K and a stress domain between 110 and 250 MPa [10, structural panels with stiffness, high dimensional stability 11]. Due to the mismatch of the coefficients of thermal structures for mirror or antenna, etc. ) or in turbines (re expansion between fibers and matrix, Cr-SiC in the as- frame liners, mixer flow, petals, exhaust cones, etc. ) or for received state has some matrix microcracks, which are not brakes [1-4]. But their mechanical behavior depends mainly on the fiber/matrix interfaces (or interphases! ) They can be characterized at the macroscopic level by debonding energy, I, and frictional interface shear stress, T, but understanding the different behaviors and obtained 3. Results values requires investigations at the micro-and nanoscopic scale using TEM and HRTEM. The micro- and nano- During creep these composites exhibit a typical damage structures can be correlated to the characteristics of the r creep[10-12], with matrix microcracking, fiber/matrix and and values, and then to the life time parameters [5-9). The yarn/matrix debonding, fiber and yarn bridging, fiber and scope of this paper is to present the complexity, the role, the yarn pull-out and rupture. So the deformation of these evolution and the importance of interfaces and interphase in two CMCs reinforced with continuous carbon or silicon proposed by Kachanov [13]. The microcrack network will carbide fibers: Cr-Sic and SiC-SibC, before and after depend on the interaction with the pyrolitic carbon(PyC) deposited on the fibers, for Cr-SiC and SiCr-SiBC, and also with the matrix multilayers for SiCr-SiBC: the role of these interphases appears as predominant 2. Materials and techniques The observation of the as-received composites reveals in both cases turbostratic carbon regions, globally parallel to Cr-SiC and SiCr-SiBC were fabricated by Snecma the fibers and the matrix(Fig. 1), as already observed [14, hemical vapor infiltration(CVD) processes in woven 2.5D oriented' in his classification of possible PyC interplases l Propulsion Solide (St Medard en Jalles, France)from 15] and classified by Despres [15] as ' isotropic globall ber architectures(ex-pan carbon fibers or Nicalon nlm Cr-SiC composites. In the case of Cr-SiC the observations 202 silicon carbide fibers). SiBC matrix is a self-healing and analysis of the PyC interphases reveal two types of interfacial sliding mechanisms[8-10). At 1473 K there is Corresponding author. the interphase rupture by debonding between two carbon 1359-835X/02/S-see front matter e 2002 Elsevier Science Ltd. All rights reserved. PI:S1359-835X(02)00147-1Microstructural investigation of interfaces in CMCs G. Boitier*, S. Darzens, J.-L. Chermant, J. Vicens LERMAT, URA CNRS 1317, ISMRA, 6 Bd Mare´chal Juin, 14050 Caen Cedex, France Abstract This paper is focused on the importance of pyrocarbon interfaces in two types of ceramic matrix composites (Cf–SiC and SiCf–SiBC), during creep tests under argon. The development of micromechanisms which consume energy and then allow a damage tolerance, depends on the morphology of the fiber/matrix interphase, which has been investigated by TEM and HRTEM. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramic-matrix composites (CMCs); B. Interface/interphase; TEM; B. Creep 1. Introduction Ceramic matrix composites (CMCs) are a class of materials developed for aeronautics and space applications, in a domain where superalloys cannot be used anymore. They have potential applications in structures (air intakes, structural panels with stiffners, high dimensional stability structures for mirror or antenna, etc.) or in turbines (rear frame liners, mixer flow, petals, exhaust cones, etc.), or for brakes [1–4]. But their mechanical behavior depends mainly on the fiber/matrix interfaces (or interphases!). They can be characterized at the macroscopic level by debonding energy, G; and frictional interface shear stress, t; but understanding the different behaviors and obtained values requires investigations at the micro- and nanoscopic scale using TEM and HRTEM. The micro- and nano￾structures can be correlated to the characteristics of the G and t values, and then to the life time parameters [5–9]. The scope of this paper is to present the complexity, the role, the evolution and the importance of interfaces and interphases in two CMCs reinforced with continuous carbon or silicon carbide fibers: Cf–SiC and SiCf–SiBC, before and after creep tests. 2. Materials and techniques Cf–SiC and SiCf–SiBC were fabricated by Snecma Propulsion Solide (St Me´dard en Jalles, France) from chemical vapor infiltration (CVI) processes in woven 2.5 D fiber architectures (ex-PAN carbon fibers or Nicalon NLM 202 silicon carbide fibers). SiBC matrix is a self-healing multilayered matrix, based on Si–C, B–C and Si–B–C phases. All the fiber architectures have received a thin pyrolytic carbon deposit before the infiltration of the matrix. Specimens were creep tested in tension, under a partial pressure of argon (500 mbar), in a temperature domain up to 1673 K and a stress domain between 110 and 250 MPa [10, 11]. Due to the mismatch of the coefficients of thermal expansion between fibers and matrix, Cf–SiC in the as￾received state has some matrix microcracks, which are not present in SiCf–SiBC composites. 3. Results During creep these composites exhibit a typical damage￾creep [10–12], with matrix microcracking, fiber/matrix and yarn/matrix debonding, fiber and yarn bridging, fiber and yarn pull-out and rupture. So the deformation of these CMCs can be analyzed in terms of damage mechanics, as proposed by Kachanov [13]. The microcrack network will depend on the interaction with the pyrolitic carbon (PyC) deposited on the fibers, for Cf–SiC and SiCf–SiBC, and also with the matrix multilayers for SiCf–SiBC: the role of these interphases appears as predominant. The observation of the as-received composites reveals in both cases turbostratic carbon regions, globally parallel to the fibers and the matrix (Fig. 1), as already observed [14, 15] and classified by Despre`s [15] as ‘isotropic globally oriented’ in his classification of possible PyC interphases in Cf–SiC composites. In the case of Cf–SiC the observations and analysis of the PyC interphases reveal two types of interfacial sliding mechanisms [8–10]. At 1473 K there is the interphase rupture by debonding between two carbon 1359-835X/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S1 35 9 -8 35 X( 02 )0 0 14 7 -1 Composites: Part A 33 (2002) 1467–1470 www.elsevier.com/locate/compositesa * Corresponding author