Table Of ContentCARBON-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
