Microstructure characterization of CVI-densified carbon/carbon composites with various fiber distributions S. Dietrich a , J.-M. Gebert a , G. Stasiuk b , A. Wanner a , K.A. Weidenmann a , O. Deutschmann c , I. Tsukrov d , R. Piat b,c,⇑ a Institut für Angewandte Materialien-Werkstoffkunde (IAM-WK), Karlsruher Institut für Technologie (KIT), Kaiserstr. 12, 76131 Karlsruhe, Germany b Institut für Technische Mechanik, Karlsruher Institut für Technologie, Kaiserstr. 12, 76131 Karlsruhe, Germany c Institut für Technische Chemie und Polymerchemie, Karlsruher Institut für Technologie, Engesserstr. 18, 76131 Karlsruhe, Germany d University of New Hampshire, Durham, NH 03824, USA article info Article history: Received 5 March 2012 Received in revised form 23 July 2012 Accepted 16 August 2012 Available online 28 August 2012 Keywords: A. Carbon fibers B. Porosity C. Statistics E. Chemical vapor infiltration (CVI) Microcomputed tomography abstract Mechanical behavior of multi-phase composites is crucially influenced by volume fractions, orientation distributions and geometries of microconstituents. In the case of carbon–carbon composites manufac- tured by chemical vapor infiltration, the microconstituents are carbon fibers, pyrolytic carbon matrix, and pores. The local variable thickness of the pyrolytic carbon coating, distribution of the fibers and porosity are the main factors influencing the properties of these materials. Two types of fiber arrange- ments are considered in this paper: 2D laminated preform and random felt. The materials are character- ized by determining their densities and their fiber distribution functions, by establishing types of pyrolytic carbon matrix present in the composites, and by studying the porosity. A technique utilizing X-ray computed tomography for estimation of the orientation distribution of the fibers and pores with arbitrary shapes is developed. A methodology based on the processing of microstructure images with subsequent numerical simulation of the coating growth around the fibers is proposed for estimation of the local thickness of the coating. The obtained information is appropriate for micromechanical modeling and prediction of the overall thermo-mechanical properties of the studied composites. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Carbon/carbon composites (C/C) are materials which combine exceptional strength and stiffness (also by high temperatures about 2000 °C) with light weight, excellent refractory and corrosion properties, making them the material of choice for severe-environment applications, such as atmospheric reentry, solid rocket motor exhaust, and disk brakes in high performance military and commercial aircraft, high speed trains and racing cars. According to surveys of the C/C business prospectus, ‘‘there are a whole host of applications ideally suited to the properties of C/C, provided the price is lowered as a result of more efficient fabrica- tion’’ [1,2]. The mechanical behavior of these materials with complicated hierarchical structures is strongly influenced by the distribution and shape of their microconstituents: fibers, pores and pyrolytic carbon (PyC) coating. This fact is especially exciting for the reason that these parameters can be controlled by the manufacturing process and production of the materials with orientation dependent stiffness or strength is possible. One of the often used methods for production of the C/Cs is chemical vapor infiltration (CVI) of carbon fiber preforms. It pre- sents an example when a complex hierarchical microstructure is adjustable by manufacturing conditions [3,4]. In C/C, carbon fibers are embedded in a matrix of PyC which has a cylindrically layered structure [5] dependent on the deposition parameters. By changing the CVI parameters, e.g. pressure, temperature, precursor or resi- dence time, it is possible to obtain different textures of the PyC coating [3,4], and thus change the mechanical properties of the coating [6–8]. The main microstructural components of C/Cs are fibers, pores and PyC coating which deposits around carbon fibers during CVI process. The elastic and thermal properties of carbon fibers are transversally isotropic with sharp contrast between the longitudi- nal and transverse directions (e.g. for P-100-H carbon fibers, E long = 770 GPa and E trans = 7.1 GPa [9]). For this reason, deviation of fibers from their desired orientation might cause significant changes in the composite material response. Experimental studies of the material properties of C/Cs on a macro scale for different fiber architectures, different porosities were reported in [10], and the influence of void fraction on the flexural strength and modulus was studied. An analytical modeling 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.08.009 ⇑ Corresponding author. E-mail address: romana.piat@kit.edu (R. Piat). Composites Science and Technology 72 (2012) 1892–1900 Contents lists available at SciVerse ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech