CARBON-CARBON MATERIALS AND COMPOSITES Edited by John D. Buckley National Aeronautics and Space Administration Langley Research Center Hampton, Virginia Dan D. Edie Clemson University Clemson, South Carolina np NOYES PUBLICATIONS Park Ridge, New Jersey, U.S.A. Copyright © 1993 by Noyés Publications No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any informa- tion storage and retrieval system, without permission in writing from the Publisher. Library of Congress Catalog Card Number: 92-35012 ISBN: 0-8155-1324-0 Printed in the United States Published in the United States of America by Noyes Publications Mill Road, Park Ridge, New Jersey 07656 10 987654321 Library of Congress Cataloging-in-Publication Data Carbon-carbon materials and composites / edited by John D. Buckley and Dan D. Edie. p. cm. Includes bibliographical references and index. ISBN 0-8155-1324-0 1. Carbon composites. I. Buckley, John D. II. Edie, Danny D. (Danny Dale), 1943- . TA418.9.C6C27 1993 620.1'93—dc20 92-35012 CIP Preface Carbon-carbon composites, which have been used extensively for missile applications, were a part of NASA's Apollo spacecraft heat shield system. The development of carbon-carbon materials began in 1958 and was nurtured under the U.S. Air Force space plane program, Dyna-Soar, and by numerous thermal protection systems developed by NASA for aerospace research, The purpose of this book is to present data and technology relating to the materials and structures developed for the production of carbon-carbon materials and composites. The text is composed of papers written by noted authors in their areas of expertise relating to the processes and production of these material systems and structures. The subject matter is arranged to lead the reader step by step through the materials processing, fabrication, structural analysis, and applications of typical carbon-carbon products. The information presented in the text is limited to data that can or has been published in the open literature including: fiber technology, matrix material, design of composite structures, manufacturing techniques, engineering mechanics, protective coatings, and structural applications using carbon-carbon materials and structures. The editors thank the authors for their contributions of time and effort in the development of this book. The use of trademarks or names of manufacturers in this book is for accurate reporting and does not constitute an official endorsement, either expressed or implied, of such products or manufacturers by the National Aeronautics and Space Administration. John D. Buckley NASA Langley Research Center Hampton, Virginia Dan D. Edie Clemson University Clemson, South Carolina v Contributors John D. Buckley Frank K, Ko NASA Langley Research Center Drexel University Hampton, Virginia Philadelphia, Pennsylvania Robert L* Burns N. Murdie Fiber Materials, Incorporated Southern Illinois University at Biddeford, Maine Carbondale Carbondale, Illinois Russell J. Diefendorf Clemson University Louis Rubin Clemson, South Carolina The Aerospace Corporation El Segundo, California J, Don Southern Illinois University at James E, Sheehan Carbondale General Atomics Carbondale, Illinois San Diego, California Dan D- Edie E.G. Stoner Clemson University Clemson University Clemson, South Carolina Clemson, South Carolina C.P, Ju MJL Wright Southern Illinois University at Southern Illinois University at Carbondale Carbondale Carbondale, Illinois Carbondale, Illinois John J* Kibler Materials Science Corporation Blue Bell, Pennsylvania vn Notice To the best of the Publisher's knowledge the information contained in this book is accurate; however, the Publisher assumes no responsibility nor liability for errors or any consequences arising from the use of the information contained herein. Final determination of the suitability of any information, procedure, or product for use contemplated by any user, and the manner of that use, is the sole responsibility of the user. The book is intended for informational purposes only. Carbon-carbon raw materials and processes could be potentially hazardous and due caution should always be exercised in the handling of materials and equipment. Expert advice should be obtained at all times when implementation is being considered. Mention of manufacturers' trademarks or trade names does not constitute endorsement by the authors, the U.S. government, or the Publisher. Chapter 1 Carbon-Carbon Overview* John D. Buckleyt NASA Langley Research Center Hampton, Virginia Introduction 1 Carbon Fibers 4 Carbon Fibers in Carbon Matrix 5 Discontinuous Fiber Composites 5 Continuous Fiber Composites 6 Chemical Vapor Deposition 7 Carbonized Organic Composites 9 Mechanical Properties 11 Applications 12 Conclusions 14 References 14 Bibliography 17 Introduction 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 to n-dimensional, using unidirectional tows, tapes, or woven cloth (fig. 1). Because of their multiformity, their mechanical properties can be readily tailored. Carbon materials have high strength and stiffness potential as well as high thermal and chemical stability in ^Similar version published in Ceramic Bulletin, vol. 67, no. 2, 1988, (©ACerSh I Member, the American Ceramic Society, 1 CC Materials and Composites 1-D 2-D 3-D n-D General properties of carbon-carbon composites Ultimate tensile strength >MPa Thermal conductivity ~11.5 W/(m-K) (>40 000 psi) Linear thermal expansion Modulus of elasticity >69 GPa «1.1 x 10"6°/C (>107 psi) Density <2990 kg/m3 Melting point >4100°C Figure L Multiformity and general properties of carbon-fiber and carbon- matrix composites. inert environments. These materials must, however, be protected with coatings and/or surface sealants when used in an oxidizing environment. The development of CC materials began in 1958 and was nurtured under the U.S. 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 thermal protection system were based on the following requirements: (1) maintenance of reproducible strength levels at 1650°C, (2) sufficient stiffness to resist flight loads and large thermal gradients, (3) low coefficient of thermal expansion to minimize induced thermal stresses, (4) oxidation resistance sufficient to limit strength reduction, (5) tolerance to impact damage, and (6) manufacturing processes within the state of the art. Carbon-carbon composites consist of a fibrous carbon substrate in a carbona- ceous matrix. Although both constituents are the same element, 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, hexagonally arranged carbon layers that are held together by weak van der Waals forces. The single crystal graphite structure is illustrated in figure 2 (ref. 1). The atoms within the layer plane or basal plane (a-b direction) have a covalent bond strength of ^524 kJ/mol (ref. 2), while the bonding energy between basal planes (c direction) is ~7 kJ/mol (ref. 3). 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, 2 Carbon-Carbon Overview in addition to its two well-defined allotropie forms (diamond and graphite), can take any number of quasicrystalline forms ranging continuously from turbostratic (amorphous, glassy carbon) to a highly crystalline graphite (fig. 3). Basal plane C 002 V-—►a Reference directions Figure 2. Tightly bonded, hexagonally arranged carbon layers (ref. 1) held together by weak van der Waals forces. d002 - 3-440 À d = 3.354 Â 002 (a) L <50Â (b) L >300Â C C Figure 3. Comparison of (a) carbon turbostratic structure with (b) 3-D graphite lattice (ref 1). The anisotropy of the graphite single crystal encompasses many structural forms of carbon. It ranges in the degree of preferred orientation of the crystallites and influences the porosity, among other variables. A broad range of properties is the result of this anisotropy, which is available in carbon material. In CC composites, this range of properties can extend to both constituents. Coupled with a variety of 3 CC Materials and Composites processing techniques that can be used in the fabrication of CC composites, great flexibility exists in the design of and the resultant properties to be obtained from CC composites. The wide range of properties of carbon materials can be shown when comparing the tensile moduli of commercially manufactured carbon fibers that range from 27.6 GPa (4 x 106 psi) to 690 GPa (100 x 106 psi). In fabrication, the fibers can be used in either continuous or discontinuous form. The directionality of the filaments can be varied ranging from unidirectional lay-ups to multidirectional weaves. The fiber volume used constitutes another variable. The higher the volume fraction of a specific high-strength fiber in a matrix, the greater the strength of the composite. The matrix can be formed via two basic approaches: (1) through the carbonization of an organic solid or liquid, such as a resin or pitch, and (2) through the chemical vapor deposition (CVD) of carbon from a hydrocarbon. A range of carbon structures can be obtained by either approach. Finally, heat treatment of the composite material at graphitization temperatures offers additional variability to the properties that can be obtained. Typically, there is an optimum graphitization temperature at which the highest strength can be obtained for a given composite composition of fiber and matrix (refs. 4 and 5). Carbon Fibers The properties of carbon fibers can vary over a wide range depending on the organic precursor and processing conditions used. At present, graphite fibers are produced from three precursor materials: rayon, polyacrylonitrile (PAN), and petroleum pitch. Fibers having a low modulus (27.6 GPa (4 x 106 psi)) are formed using a rayon precursor material that may be chemically pretreated by a sequence of heating steps. First, the fiber is heated to ^ 400°C to allow cellulose to pyrolyzeJ Carbonization^ is completed more rapidly at >1000°C. Upon completion of carbonization, the fiber is graphitized" by heating to >2000°C; the fiber is now, for all practical purposes, 100 percent carbon. High-modulus carbon fibers from rayon precursors are obtained by the additional process of stretching the carbon fibers at the final heat treatment temperature. High-modulus (344 GPa (50 x 106 psi)), high-strength (2.07 GPa (300 x 103 psi)) carbon fibers are typically made from PAN or, in some cases, mesophase pitch precursors. These fibers are processed similarly in a three-stage operation (fig. 4, ref. 6). The PAN fibers are initially stretched from 500 percent to 1300 percent and then stabilized (cross- linked) in an oxygen atmosphere at 200°C to 280°C (under tension). Carbonization t + Decomposition or chemical change by thermal conversion of organic materials to carbon and graphite. ^Continued heating of organic material to >1000 C to initiate ordering of the carbon structures produced by pyrolysis. "Continued heating of carbonized organic materials to the 2000 C to 3000 C range to produce a 100-percent graphite-ordered crystal structure. 4 Carbon-Carbon Overview of the fibers is conducted between 1000°C and 1600°C. Finally, graphitization is accomplished at >2500°C. Mesophase pitch fibers undergo the same processing procedure as PAN fibers but do not require an expensive stretching process during heat treatment to maintain preferred alignment of crystallites (fig. 4, ref. 6). Control of fiber shape has resulted in improved fiber strength (4.1 GPa (600000 psi)), see ref. 7, when produced from melt-spun, mesophase petroleum pitch (fig. 5, ref. 7). Round fibers using the same method had a strength of 2.1 GPa (300 x 103 psi), as shown in reference 4. Of the shapes studied, the c-shape and hollow fibers were found to be superior in strength to round solid and trilobal cross sections (refs. 4 and 7). PAN process vjtmiiiirzznK p-q Stretch Ezzzzzzzzzzzz^ [ PAN Thermoset Carbonize Graphltize Pitch process Petroleum pitch Ue){ Thermoset Carbonize Graphitize K>? ^5 Spool Epoxy sizing Surface treatment Figure 4, Carbon fiber production using PAN and pitch processes (ref. 6). Carbon Fibers in Carbon Matrix Addition of a matrix to carbon fiber, either through the carbonization of an organic precursor or by the deposition of pyrolytic carbon, is conducted at 800° C to 1500°C Subsequent heat treatment of the composite material may involve temperatures to 3000° Q Discontinuous Fiber Composites Fabrication of discontinuous fiber composites uses short carbon fibers combined with either a pyrolytic carbon or pyrolyzed organic matrix. This approach to CC composites generally does not have true fiber reinforcement as an objective. Rather, discontinuous fiber substrates have been used to: (1) increase fabrication capability of large-scale structures, (2) achieve a more nearly isotropic material, (3) increase the composite interlaminar tensile strength, and (4) along with continuous filament substrates, obtain a stronger composite by providing additional nucleation sites that serve to reduce composite porosity. 5