40207_15 - CARBON-CARBON COMPOSITES John D Buckley 15 15.1 INTRODUCTION n Carbon-carbon(CC materials are a generic class of composites similar to

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CARBON-CARBON COMPOSITES 15 John D. Buckley 15.1 INTRODUCTION n Carbon-carbon (CC) materials are a generic class of composites similar to the graphite/epoxy family of polymer matrix composites. These materials can be made in a wide variety of forms, from one-dimensional 1 -D 2-D to n-dimensional, using unidirectional tows, tapes, or woven cloth (Fig. 15.1). Because of their multiformity, their mechanical properties can be readily tailored (Table 15.1). Carbon materials have high strength and stiffness potential as well as high thermal and chemical stability in inert environments. These materi- 3-D n-D als must, however, be protected with coatings and/or surface when used in an OXi- dizing environment. Fig. 15.1 Multiformity and general properties of carbon-fiber and carbon-matrix composites. Table 15.1 General properties of carbon-carbon composites Ultimate tensile >276 MPa >40 000 psi strength Modulus of >69 GPa >lo7 psi elasticity Melting point >41OO0C 7412°F Thermal ~11.5 W m-' K-' 6.64 conductivity h ft "F Linear thermal ~1.1 x 10"OC 6.1 x W7"F expansion Density <2990 kg m-3 186.6 lb/ft3 Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7 The development of CC materials began in 1958 and was nurtured under the US Air Force space plane program, Dyna-Soar and NASA's Apollo projects. It was not until the Space Shuttle Program that CC material systems were intensively researched. The criteria that led to the selection of CC composites as a ther- mal protection system were based on the following requirements: 1. maintenance of reproducible strength levels at 1650°C (3002°F); 2. sufficient stiffness to resist flight loads and large thermal gradients; 3. low coefficient of thermal expansion to min- imize induced thermal stresses; 4. oxidation resistance sufficient to limit strength reduction;
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334 Carbon-carbon composites 5. tolerance to impact damage; 6. manufacturing processes within the state of Carbon-carbon composites consist of a fibrous carbon substrate in a carbonaceous matrix. Although both constituents are the same ele- ment, this fact does not simplify composite behavior because the state of each constituent may range from carbon to graphite. Crystallographic carbon, namely graphite, consists of tightly bonded, hexagonal arranged carbon layers that are held together by weak van der Waals forces. The single crys- tal graphite structure is illustrated in Fig. 15.2 (Bokros, 1969). The atoms within the layer the art. plane or basal plane (a-b direction) have a covalent bond strength of =524 kJ/mol (Kanter, 1957), while the bonding energy between basal planes (c direction) is =7 kJ/mol (Dienes, 1952). The result is a crystal that is remarkable in its anisotropy, being almost isotropic within the basal plane but with c direction properties that differ by orders of magnitude. On a larger scale, carbon, in addi- tion to its two well-defined allotropic forms (diamond and graphite), can take any number of quasicrystalline forms ranging continu- ously from turbostratic (amorphous, glassy carbon) to a highly crystalline graphite (Fig.
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This note was uploaded on 03/16/2010 for the course MECHANICAL ME765401 taught by Professor Prof.sulis during the Spring '10 term at Institut Teknologi Bandung.

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40207_15 - CARBON-CARBON COMPOSITES John D Buckley 15 15.1 INTRODUCTION n Carbon-carbon(CC materials are a generic class of composites similar to

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