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)00147-1
Microstructural 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 nanostructures 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 asreceived state has some matrix microcracks, which are not present in SiCf–SiBC composites. 3. Results During creep these composites exhibit a typical damagecreep [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
G Boitier et al. / Composites: Part A 33 (2002)1467-1470 leaves then the interfacial sliding can be assimilated to a dry friction between two rough solids. Close to the fibers and the matrix the structure of the PyC is turbostratic type, with atomic carbon planes parallel to the fiber direction That has also clearly been evidenced by SEM observations of the surface of the fibers: that surface is very rough(see the small insert in Fig. 2a). Although at 1473 K the fiber/matrix interface is radially in compression on. under stress some lenticular pores appear(Fig. 2a). These lenticular pores due to local decohesion between carbon planes are, in fact, the nuclei for the microcrack development. When the debond- ing is not total, carbon ribbons bridge the two parts of the microcracks. At 1673K, the PyC interphase is a little degraded over about 100 nm from the matrix; there are a disappearing of the previous anisotropic texture, and a certain amorphization; then the interfacial sliding can be assimilated to a viscous flow (Fig. 2b). That was also confirmed by SEM observations: the fiber surfaces are smooth [8-10)(see the small insert in Fig. 2b) If observations are performed at a higher scale one notes that the nanotexture of the carbon fibers increases with the test temperature. Consequently it leads to an increase of the local molecular orientation of oriented volumes of carbon planes parallel to the fibers. Although this phenomenon has been called carbon fiber nanocreep'[16], this contribution of the fibers and matrix to the macroscopic deformation is negligible, but it can be considered as the nuclei of the dama In the case of Sicr-SiBC same type of featu observed, but a little more complex due to the existence of the different matrix layers based on the Si-B-C system. In addition to the damage observed in Cr-SiC composites, Fig. 1. HRTEM micrographs of the matrix/pyrocarbon interface in a there are also matrix microcrack deviations via some matrix as-received Cr-SiC (a) and of the matrix/pyrocarbon/fiber interfaces in multilayers where carbon exists [11]: this carbon is located -received SiCr-SiBC (b). at the interfaces of specific matrix layers and presents a thin 5 nm Fig. 2. HRTEM and SEM micrographs of the pyrocarbon interphase in Cr-SiC creep tested specimen under 220 MPa and in Ar:(a) with the presence of some lenticular pores at 1473 K:(b)and the presence of carbon rollings at 1673 K Inserts correspond to SEM images of the surface features of the SiCr fibers at these
leaves: then the interfacial sliding can be assimilated to a dry friction between two rough solids. Close to the fibers and the matrix the structure of the PyC is turbostratic type, with atomic carbon planes parallel to the fiber direction. That has also clearly been evidenced by SEM observations of the surface of the fibers: that surface is very rough (see the small insert in Fig. 2a). Although at 1473 K the fiber/matrix interface is radially in compression, under stress some lenticular pores appear (Fig. 2a). These lenticular pores due to local decohesions between carbon planes are, in fact, the nuclei for the microcrack development. When the debonding is not total, carbon ribbons bridge the two parts of the microcracks. At 1673 K, the PyC interphase is a little degraded over about 100 nm from the matrix; there are a disappearing of the previous anisotropic texture, and a certain amorphization; then the interfacial sliding can be assimilated to a viscous flow (Fig. 2b). That was also confirmed by SEM observations: the fiber surfaces are smooth [8–10] (see the small insert in Fig. 2b). If observations are performed at a higher scale one notes that the nanotexture of the carbon fibers increases with the test temperature. Consequently it leads to an increase of the local molecular orientation of oriented volumes of carbon planes parallel to the fibers. Although this phenomenon has been called ‘carbon fiber nanocreep’ [16], this contribution of the fibers and matrix to the macroscopic deformation is negligible, but it can be considered as the nuclei of the damage process. In the case of SiCf–SiBC same type of features are observed, but a little more complex due to the existence of the different matrix layers based on the Si–B–C system. In addition to the damage observed in Cf–SiC composites, there are also matrix microcrack deviations via some matrix multilayers where carbon exists [11]: this carbon is located at the interfaces of specific matrix layers and presents a thin Fig. 2. HRTEM and SEM micrographs of the pyrocarbon interphase in Cf–SiC creep tested specimen under 220 MPa and in Ar: (a) with the presence of some lenticular pores at 1473 K; (b) and the presence of carbon rollings at 1673 K. Inserts correspond to SEM images of the surface features of the SiCf fibers at these two temperatures. Fig. 1. HRTEM micrographs of the matrix/pyrocarbon interface in a as-received Cf–SiC (a) and of the matrix/pyrocarbon/fiber interfaces in a as-received SiCf–SiBC (b). 1468 G. Boitier et al. / Composites: Part A 33 (2002) 1467–1470
G Boitier et al./ Composites: Part A 33(2002)1467-1470 M M 200nm Fig. 3. TEM micrographs of the pyrocarbon interphase showing a microcrack deviation mode 1- mode II in a longitudinal section(parallel to the stress direction)(a), and a bridging of a microcrack by carbon ribbons(b), for SiCr-SiBC specimens creep tested at 1473 K, under 200 MPa and in A layer of PyC, from 10 to 100 nm thick, and oriented globally microcracks and, consequently, permits a correct damage parallel to these interfaces. The larger the interphase is, the tolerance. To propose a creep mechanism one must more pronounced the debonding between the matrix layers accurately analyze the carbon plane orientations, their evolution, and nano- and micro-mechanisms occurring at In the case of the pyrocarbon layer(between the silicon the different microstructural scales. Materials based on self- carbide fibers and the matrix), close to the matrix the carbon sealing matrix exhibit a significant improvement in structure is turbostratic type over a thickness of about 12 comparison with previous Cr-Sic or SiCr-SiC: they can 15 nm, i.e. 30-35 carbon atomic planes. Further away, the be considered as a class of material for gas turbine jet carbon structure is globally isotropic oriented, according to engines with, for example, lifetime higher than 100 h under the Despres nomenclature [15]. In this zone the carbon is 170 MPa at 1473 K [17] constituted of rollings made of about 10 carbon planes with some porosities and some amorphous carbon, which correspond to the texture class I [15. This interphaseAcknowledgements tructure is, at room temperature, in favor of phenomenon of debonding and sliding close to the interphase/matrix This work has been supported by Snecma Propulsion interfaces, due to the nature of the bounds of carbon planes Solide(Saint Medard en Jalles, France), by CNRS and of van der Waals type; such phenomenon is also operating Region de Basse-Normandie(SD). We wish to warmly during creep at high temperature. It is this geometry of thank Drs M.Bourgeon, E. Pestourie, J.M. Rouges, for microstructure, as in the case of Cr-SiC, which permits both fruitful discussions and for providing the specimens a mode I- mode Il crack deviation(Fig 3a)and a bridging some cracks by nano-ribbons of carbon planes(Fig. 3b), which are not an artifact as their diffraction patterns correspond to carbon: both features are nano-mechanisms References which consume energy and, then, permit a certain damage tolerance Cmm如xwNB几 Schneider H, editors. Hi油 Germany: Wiley-VCH; 2001. P. 731-43 [2 Luthra KL, Corman GS. In: Krenkel w, Naslain R, Schneider H 4. Discussion and conclusion editors. High temperature ceramic matrix composites, HT-CMC4 Weinheim, Germany: Wiley-VCH: 2001.P. 744-53 In this short paper one has shown that microscopic [3] Renz R, Krenkel w. Composites: from fundamentals to exploitatio ECCM 9, Brighton, UK. ECCM 9 CD ROM C 2000. IOM observations and analysis of thermo-mechanical or crept Communication Ltd: 4-7 June 2000 CMCs are the only way to access the different micro- [4] Renz R, Heidenreich B, Krenkel w, Schoppach A, Richter F. In: mechanisms occurring, for example when creep tests are Krenkel W, Naslain R, Schneider H, editors. High temperature performed. Creep tests give only a value of some ceramic matrix composites, HT-CMC4. Weinheim, Germany: mechanical parameters and concern only a macroscopic wley-VCH;2001.p.839-45 [5] Kerans RJ, Hay RS, Pagano NJ, Parthasarathy TA. Am Ceram Soc approach. For these materials, the pyrocarbon interphase Bull198968:429-42. play a key role for the development of the matrix [6 Kuntz M, Grathwohl G. Advd Engng Mater 2001; 3: 371-9
layer of PyC, from 10 to 100 nm thick, and oriented globally parallel to these interfaces. The larger the interphase is, the more pronounced the debonding between the matrix layers appears. In the case of the pyrocarbon layer (between the silicon carbide fibers and the matrix), close to the matrix the carbon structure is turbostratic type over a thickness of about 12– 15 nm, i.e. 30–35 carbon atomic planes. Further away, the carbon structure is globally isotropic oriented, according to the Despre`s nomenclature [15]. In this zone the carbon is constituted of rollings made of about 10 carbon planes with some porosities and some amorphous carbon, which correspond to the texture class I [15]. This interphase structure is, at room temperature, in favor of phenomenon of debonding and sliding close to the interphase/matrix interfaces, due to the nature of the bounds of carbon planes of van der Waals type; such phenomenon is also operating during creep at high temperature. It is this geometry of microstructure, as in the case of Cf–SiC, which permits both a mode I ! mode II crack deviation (Fig. 3a) and a bridging of some cracks by nano-ribbons of carbon planes (Fig. 3b), which are not an artifact as their diffraction patterns correspond to carbon: both features are nano-mechanisms which consume energy and, then, permit a certain damage tolerance. 4. Discussion and conclusion In this short paper one has shown that microscopic observations and analysis of thermo-mechanical or crept CMCs are the only way to access the different micromechanisms occurring, for example when creep tests are performed. Creep tests give only a value of some mechanical parameters and concern only a macroscopic approach. For these materials, the pyrocarbon interphases play a key role for the development of the matrix microcracks and, consequently, permits a correct damage tolerance. To propose a creep mechanism one must accurately analyze the carbon plane orientations, their evolution, and nano- and micro-mechanisms occurring at the different microstructural scales. Materials based on selfsealing matrix exhibit a significant improvement in comparison with previous Cf–SiC or SiCf–SiC: they can be considered as a class of material for gas turbine jet engines with, for example, lifetime higher than 100 h under 170 MPa at 1473 K [17]. Acknowledgements This work has been supported by Snecma Propulsion Solide (Saint Me´dard en Jalles, France), by CNRS and Re´gion de Basse-Normandie (SD). We wish to warmly thank Drs M. Bourgeon, E. Pestourie, J.M. Rouge`s, for fruitful discussions and for providing the specimens. References [1] Christin F. In: Krenkel W, Naslain R, Schneider H, editors. High temperature ceramic matrix composites, HT-CMC4. Weinheim, Germany: Wiley–VCH; 2001. p. 731–43. [2] Luthra KL, Corman GS. In: Krenkel W, Naslain R, Schneider H, editors. High temperature ceramic matrix composites, HT-CMC4. Weinheim, Germany: Wiley–VCH; 2001. p. 744–53. [3] Renz R, Krenkel W. Composites: from fundamentals to exploitation. ECCM 9, Brighton, UK. ECCM 9 CD ROM C 2000. IOM Communication Ltd; 4–7 June 2000. [4] Renz R, Heidenreich B, Krenkel W, Scho¨ppach A, Richter F. In: Krenkel W, Naslain R, Schneider H, editors. High temperature ceramic matrix composites, HT-CMC4. Weinheim, Germany: Wiley–VCH; 2001. p. 839–45. [5] Kerans RJ, Hay RS, Pagano NJ, Parthasarathy TA. Am Ceram Soc Bull 1989;68:429–42. [6] Kuntz M, Grathwohl G. Advd Engng Mater 2001;3:371–9. Fig. 3. TEM micrographs of the pyrocarbon interphase showing a microcrack deviation mode I ! mode II in a longitudinal section (parallel to the stress direction) (a), and a bridging of a microcrack by carbon ribbons (b), for SiCf–SiBC specimens creep tested at 1473 K, under 200 MPa and in Ar. G. Boitier et al. / Composites: Part A 33 (2002) 1467–1470 1469
G Boitier et al. / Composites: Part A 33 (2002)1467-1470 [7 Boitier G, Vicens J, Chermant JL. Mater Sci Engng 2000 A279: [13] Kachanov L Izv Akad Nauk SSR 1985: 8: 26-31 [14] Dupel P, Bourrat X, Pailler R Carbon 1995: 9: 1193-204. [8 Boitier G, Vicens J, Chermant JL. Mater Sci Engng [15] Despres JF. These de Doctorat of the University of Pau and of Pays de Adour: 1993 [9 Boitier G, Vicens J, Chermant JL. Mater Sci Engng [16] Boitier G, Vicens J, Chermant JL. Script Mater 1998: 38: 937-43 [17] Lamouroux F, Bouillon E, Cavalier JC, Spriet P, Habarou G. In: 10] Boitier G. These de Doctorat of the University of Caen: 1997. Krenkel W, Naslain R, Schneider H, editors. High temperature [11] Darzens S. These de Doctorat of the University of Caen: 2000. eramic matrix composites, HT-CMC4. Weinheim, Germany [12] Chermant JL. Sil Ind 1995: 60: 261-73 wley-VCH:2001.p.783-8
[7] Boitier G, Vicens J, Chermant JL. Mater Sci Engng 2000;A279: 73–80. [8] Boitier G, Vicens J, Chermant JL. Mater Sci Engng 2000;A289: 265–75. [9] Boitier G, Vicens J, Chermant JL. Mater Sci Engng 2001;A313: 53–63. [10] Boitier G. The`se de Doctorat of the University of Caen; 1997. [11] Darzens S. The`se de Doctorat of the University of Caen; 2000. [12] Chermant JL. Sil Ind 1995;60:261–73. [13] Kachanov L. Izv Akad Nauk SSR 1985;8:26–31. [14] Dupel P, Bourrat X, Pailler R. Carbon 1995;9:1193–204. [15] Despre`s JF. The`se de Doctorat of the University of Pau and of Pays de l’Adour; 1993. [16] Boitier G, Vicens J, Chermant JL. Script Mater 1998;38:937–43. [17] Lamouroux F, Bouillon E, Cavalier JC, Spriet P, Habarou G. In: Krenkel W, Naslain R, Schneider H, editors. High temperature ceramic matrix composites, HT-CMC4. Weinheim, Germany: Wiley–VCH; 2001. p. 783–8. 1470 G. Boitier et al. / Composites: Part A 33 (2002) 1467–1470