Microcracks in Carbon/Carbon Composites: A Microtomography Investigation using Synchrotron Radiation
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Microcracks in Carbon/Carbon Composites: A Microtomography Investigation using Synchrotron Radiation Oskar Paris1, Herwig Peterlik2, Dieter Loidl2, Christoph Rau3 and Timm Weitkamp3 1 Metal Physics Institute, University of Leoben, and Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstrasse 12, A-8700 Leoben, Austria. 2 Materials Physics Institute, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria. 3 European Synchrotron Radiation Facility (ESRF), 6 rue Jules Horowitz, B. P. 220, F-38043 Grenoble Cédex, France. ABSTRACT The mechanical properties of brittle matrix composites such as carbon/carbon (C/C) are closely related to the generation and propagation of microcracks. A better understanding of the role of microcracking requires a quantification of the three-dimensional morphology of the crack network. In this study we demonstrate that phase contrast microtomography using synchrotron radiation is a unique tool to get 3D information about cracks in C/C. This is shown for three different C/C specimens subjected to different final heat treatment temperatures (HTT). The results are discussed qualitatively with respect to the influence of HTT on the distribution of microcracks and their relevance for the mechanical properties of C/C. INTRODUCTION The mechanical properties of many materials, such as biological and advanced technological materials, depend on structural features on different length scales. Synchrotron radiation from third generation sources offers unique possibilities to investigate the structure of such complex materials at different levels of hierarchical organization. Microbeam X-ray diffraction (µ-XRD) [1-3] and microbeam small-angle X-ray scattering (µ-SAXS) [4-6] for example allow to get structural information on the micrometer and the nanometer level, simultaneously. A second class of very recent characterization techniques are phase contrast imaging and -tomography using highly coherent, hard X-rays from undulator sources [7,8]. Utilizing phase contrast has the advantage to allow imaging of objects with very low absorption contrast, such as biological systems or composites consisting of light elements (e.g. carbon). Resolution is presently restricted to about 1 µm, however, the feasibility of a hard X-ray microscope with a resolution down to about 0.1 µm has recently been demonstrated [9]. Microtomography in particular, is a important new tool in materials science, since it allows to get three-dimensional (3D) bulk information with a resolution close to that of a light microscope. Recent applications of microtomography to materials science have been summarized in [10]. Carbon fiber reinforced carbon composites (C/C) are used as light-weight structural components mainly in aerospace industry for high temperature applications. Already the reinforcing fibers themselves, often designated as “nano-composites”, are known to exhibit a complex internal structure [3,11,12]. For the extraordinary mechanical properties of the composite material, fibers and matrix, as
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