ENGINEERED INTERFACES IN FIBER REINFORCED COMPOSITES JANG-KYO KIM docx

ENGINEERED
INTERFACES IN
FIBER
REINFORCED
COMPOSITES
JANG-KYO
KIM
&
YIU-WING
MA1
c
f
t


ENGINEERED
INTERFACES
IN
FIBER REINFORCED
COMPOSITES

ENGINEERED
INTERFACES
IN
FIBER REINFORCED
COMPOSITES
Jang-Kyo Kim
Department
of
Mechanical Engineering
Hong Kong University

of
Science and Technology
Clear Water Bay, Hong Kong
Yiu-Wing Mai
Centre for Advanced Materials Technology and
Department
of
Mechanical
&
Mechatronic Engineering
University
of
Sydney, NSW
2006,
Australia
1998
ELSEVIER
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Library
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Cataloging-in-Publica~on
Data
Kim, Jang-Kyo.
Engineered interfaces in fiber reinforced composites
/
Jang-Kyo
Kim and Yiu-Wing, Mai.

1st ed.
p. cm.
Includes index.
ISBN
0-08-042695-6 (hardcover)
1.
Fibrous composites.
I.
Mai,
Y.
W.,
1946-
.
11.

Title.
TA418.9.C6K55 1998
620,1'18 DC21 97-
5
2002
CIP
First edition 1998
ISBN 0-08-042695-6
0
1998 Elsevier Science Ltd
All rights reserved. No part of
this
publication may be reproduced, stored in a retrieval system or
transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical
photocopying, recording or otherwise, without permission in writing from the publishers.
Q
The paper used in this publication meets the requirements
of
ANSUNIS0
239.48-1992
(Permanence of Paper).
Printed in The Netherlands
It is
a
pleasure to write the foreword to this book. This work emphasizes for the first
time in one volume how interfaces in fibrous composites can be defined, measured,
improved and optimized. Many practitioners of composites technology will find in
this book the information they have been seeking
to
match fiber and matrix at the

interface, thereby obtaining the best mix of properties in the final application.
Composites engineering is a relatively young field in which the test methods and
measurement techniques are not yet fully developed. Even more important, the ideas
linking the properties of composites to the interface structure are still emerging. This
book not only reviews the historic and pragmatic methods for studying composites;
but it also presents the most recent theories and fundamental tests of interface
properties. This allows the reader to find the true framework of theory to fit his/her
observations.
The fact that two brittle materials can be brought together to give a tough product
is
the proof that interfaces are critical to composite properties. However, the
complexities of this process depend on the raw materials, on the surface chemistry of
the components, on the fabrication procedures, on the chemistry of hardening, and
on the damage and corrosion sustained in use. A wide view of material science,
chemistry, mechanics, process engineering and applications experience is necessary
to focus successfully on the role of the interface. The authors have demonstrated
such a global view in this volume.
I
have known Professor Mai for over
20
years. He is a foremost authority on
fracture mechanics of composite materials, having studied polymer composites,
cement, ceramic and natural composite systems, in the
US,
Britain, Australia and
Hong Kong. In particular, he has made memorable contributions to the
understanding
of
cracks and to the crack-inhibiting effects seen in fibrous
composites. He has previously coauthored two books on fracture. Professor Kim

originally worked in the composites industry and has returned during the past
10
years to study interface mechanisms more closely. He
is
currently working in the
Hong Kong University
of
Science
&
Technology.
In
summary, the topic of engineered interfaces in composites is an important one,
critical to the advance of the composites industry. Many practitioners from a range
of disciplines are seeking the information which can be found in this book. The
authors display the wide experience and theoretical knowledge necessary to provide
a critical view of the subject.
I
strongly recommend this volume to the composite
expert and student alike.
Kevin Kendall
Keele University, UK
May
1997
V

PREFACE
The study and application of composite materials are a truly interdisciplinary
endeavor that has been enriched by contributions from chemistry, physics, materials
scicncc, mcchanics and manufacturing cnginecring. The undcrstanding of thc
interface (or interphase) in composites is the central point of this interdisciplinary

effort. From the early development of composite materials of various nature, the
optimization of the interface has been of major importance. While there are many
reference books available on composite materials, few of them deal specifically with
the science and mechanics of the interface of fiber reinforced composites. Further,
many recent advances devoted solely to research in composite interfaces are
scattered in different published literature and have yet to be assembled in a readily
accessible form.
To
this end this book is an attempt to bring together recent
developments in the field, both from the materials science and mechanics
perspective, in a single convenient volume.
The central theme of this book is tailoring the interface properties to optimize the
mechanical performance and structural integrity of composites with enhanced
strength/stiffness and fracture toughness (or specific fracture resistance). It deals
mainly with interfaces in advanced composites made from high performance fibers,
such as glass, carbon, aramid, ultrahigh modulus polyethylene and some inorganic
(e.g.
B/W,
A1203, Sic) fibers, and matrix materials encompassing polymers, metals/
alloys and ceramics. The book is intended to provide a comprehensive treatment
of
composite interfaces in such a way that it should be of interest to materials scientists,
technologists and practising engineers, as well as graduate students and their
supervisors in advanced composites. We hope that this book will also serve as
a
valuable source of reference to all those involved in the design and research of
composite interfaces.
The book contains eight chapters of discussions on microstructure-property
relationships with underlying fundamental mechanics principles. In Chapter
1,

an
introduction is given to the nature and definition of interfaces in fiber reinforced
composites. Chapter
2
is devoted to the mechanisms of adhesion which are specific
to each fiber-matrix system, and the physico-chemical characterization of the
interface with regard to the origin of adhesion. The experimental techniques that
have been developed to assess the fiber-matrix interface bond quality on a
microscopic scale are presented in Chapter
3,
along with the techniques of
measuring
interlaminar/intralaminar
strengths and fracture toughness using bulk
composite laminates. The applicability and limitations associated with loading
geometry and interpretation of test data are compared. Chapter
4
presents
comprehensive theoretical analyses based on shear-lag models
of'
the single fiber
composite tests, with particular emphasis being placed on the interface debond
vii

VI11
Preface
process and the nature
of
the fiber-matrix interface bonding. Chapter
5

is devoted to
reviewing current techniques of fiber surface treatments which have been devised to
improve the bond strength and the fiber-matrix compatibility/stability during the
manufacturing processes of composites. The microfailure mechanisms and their
associated theories of fracture toughness of composites are discussed in Chapter
6.
The role of the interface and its effects on the mechanical performance of fiber
composites are addressed from several viewpoints. Recent research efforts to
augment the transverse and interlaminar fracture toughness by means of controlled
interfaces are presented in Chapters
7
and
8.
Three concepts of engineered interfaces
are put forward to explain the results obtained from fiber coatings. Among those
with special interest from the composite designer’s perspective are the effects
of
residual stresses arising from differential shrinkage between the composite
constituents, tough matrix materials, interleaves as delamination arresters and
three-dimensional fiber preforms.
We are grateful for assistance from many sources in the preparation of this book.
We acknowledge the invaluable contributions of many individuals with whom we
had the privilege and delight to work together: in particular the past and present
colleagues at the University of Sydney and the Hong Kong University of Science
&
Technology, including
C.A.
Baillie,
F.
Castino,

B.
Cotterell, K.A. Dransfield, S.L.
Gao,
Y.C. Gao, M.I. Hakeem, B.J. Kennedy, M.G. Lau, L.M. Leung,
H.Y.
Liu, R.
Lord,
I.M.
Low,
S.V.
Lu, D.B. Mackay, L. Ye and L.M. Zhou.
The
generous
financial support provided by many organizations, most notably the Australian
Research Council and the Hong Kong Research Grant Council, for performing the
research recorded in this book is greatly appreciated. Thanks are also due to all
those who have allowed
us
to reproduce photographs and diagrams from their
published work and to their publishers for the permission to use them.
Special thanks are also due to our technical writer Dr. Virginia Unkefer of the
Hong
Kong
University
of
Science
&
Technology for her help without which this
book would never have eventuated. Finally, we can never thank sufficiently our
family members, Hyang and Jong-Rin Kim, and Louisa Mai, for their patience and

understanding
of
our pressure to undertake and complete such a time-consuming
task.
Jang-Kyo Kim
Clear Water Bay, Hong Kong
May 1997
Yiu- Wing Mai
Sydney, Australia
May 1997
CONTENTS
Foreword
v
Preface
vii
Chapter
1.
Chapter
2.
2.1.
2.2.
2.2.1.
2.2.2.
2.2.3.
2.2.4.
2.2.5.
2.2.6.
2.3.1.
2.3.2.
2.3.3.

2.3.4.
2.3.5.
2.3.6.
2.3.7.
2.3.8.
2.3.9.
2.3.10.
2.3.1
1.
2.3.
Chapter
3.
3.1.
3.2.
Introduction
1
References 4
Characterization of Interface Properties
5
Introduction
5
Theories of Adhesion and Types of Bonding
5
Adsorption and Wetting 7
Interdiffusion 12
Electrostatic Attraction 13
Chemical Bonding 14
Reaction Bonding 14
Mechanical Bonding
16

Physico-chemical Characterization
of
Interfaces
Introduction 17
Infrared (IR) and Fourier Transform Infrared (FTIR)
Spectroscopy 18
Laser Raman Spectroscopy 21
X-Ray Photoelectron Spectroscopy (XPS) 24
Auger Electron Spectroscopy (AES) 26
Secondary Ion Mass Spectroscopy
(SIMS)
29
Ion Scattering Spectroscopy
(ISS)
30
Solid State Nuclear Magnetic Resonance (NMR) Spectroscopy
Wide-Angle X-Ray Scattering (WAXS) 32
Small-Angle Light Scattering (SALS) and Small-Angle X-ray Scattering
(SAXS) 33
Measurement of Contact Angle
34
References 38
1
7
3
1
Measurements of Interface/Interlaminar Properties
43
Introduction 43
The Mechanical Properties of Fiber-Matrix Interfaces 44

ix
X
3.2.1.
3.2.2.
3.2.3.
3.2.4.
3.2.5.
3.2.6.
3.2.7.
3.3.1.
3.3.2.
3.3.3.
3.3.4.
3.3.5.
3.3.6.
3.3.7.
3.3.8.
3.4.1.
3.4.2.
3.4.3.
3.4.4.
3.3.
3.4.
Contents
Introduction
44
Single Fiber Compression Test
44
Fiber Fragmentation Test
45

Fiber Pull-out Test
5
1
Microindentation (or Fiber Push-out) Test
Slice Compression Test
58
Comparison of Microcomposite Tests and Experimental Data
Interlaminar/Intralaminar
Properties
61
Introduction
61
Short Beam Shear Test
62
Iosipescu Shear Test
66
[
f
45"Is
Tensile Test
69
[
lo"]
Off-axis Tensile Test
70
Rail Shear Test
71
In-plane Lap-shear Test
72
Transverse Tensile Test

72
Interlaminar Fracture Toughness
74
Delamination
74
Mode
1
Interlaminar Fracture Tests (IFT)
76
Mode I1 Interlaminar Fracture Tests
Mode
I
Edge Delamination Tests
References
85
56
59
81
83
Chapter
4.
Micromechanics
of
Stress Transfer Across the Interface
93
4.1.
4.2.
4.2.1.
4.2.2.
4.2.3.

4.2.4.
4.3.1.
4.3.2.
4.3.3.
4.3.4.
4.3.5.
4.3.6.
4.3 7.
4.4.1.
4.4.2.
4.4.3.
4.5.1.
4.3.
4.4.
4.5.
Introduction
93
Fiber Fragmentation Test
94
Introduction
94
Early Shear-Lag Models
97
An Improved Model based on a Fracture Mechanics Approach
An Improved Model based on a Shear Strength Criterion
110
Fiber Pull-Out Test
125
Introduction 125
Solutions for Stress Distributions

128
Interface Debond Criterion and Partial Debond Stress
Instability
of
Debond Process
135
Characterization
of
Interface Properties
138
Multiple Fiber Composite Model
139
Two-way Debonding Phenomenon
147
Fiber
Push-out
150
Solutions for Stress Distributions
150
Debond Criterion and Debond Stresses
Comparisons between Fiber Pull-out and Fiber Push-out
Cyclic Loading in Fiber Pull-out and Fiber Push-out
Introduction
156
101
131
152
154
156
xi

Contents
4.5.2. Relative Displacements and Degradation Function 157
4.5.3. Degradation
of
Interface Frictional Properties 161
References 164
Chapter
5.
Surface Treatments of Fibers and Effects on Composite Properties
171
5.1.
5.2.
5.2.1.
5.2.2.
5.3.1.
5.3.2.
5.4.1.
5.4.2.
5.5.1.
5.5.2.
5.5.3.
5.5.4.
5.5.5.
5.5.6.
5.3.
5.4.
5.5.
Introduction
17
1

Glass Fibers and Silane Coupling Agents
Structure and Properties
of
Glass Fibers
Silane Treatments of Glass Fibers 174
Carbon Fibers 183
Structure and Properties of Carbon Fibers
Surface Treatments of Carbon Fibers
186
Polymeric Fibers 196
Aramid Fibers
196
Ultrahigh Modulus Polyethylene (UHMPE) Fibers 201
Inorganic Fibers 205
Introduction 205
Selection
of
Coating Materials and Coating Techniques 206
Carbon Fibers
210
Boron Fibers 214
Silicon Carbide (Sic) Fibers 216
Alumina
(A1,OJ
Fibers 223
References 228
172
172
183
Chapter

6.
Interface Mechanics and Fracture Toughness Theories
239
6.1.
6.1.1.
6.1.2.
6.1.3.
6.1.4.
6.1.5.
6.1.6.
6.1.7.
6.2.
6.2.1.
6.2.2.
6.2.3.
6.2.4.
6.3.1.
6.3.2.
6.3.
6.4.
Interface-related Fracture Toughness Theories 239
Introduction 239
Fiber-Matrix Interface Debonding in Mode
I1
Shear 242
Post-debond Friction 243
Stress Redistribution 243
Fiber Pull-out 243
Total Fracture Toughness Theories 245
Fracture of Ductile Fibers and Ductile Matrices 247

Toughness Theories for Short and Randomly Oriented Fiber
Composites 247
Introduction 247
Fiber Pull-out Dominant Fracture Mechanisms 248
Matrix Dominant Fracture Mechanisms 250
Total Fracture Toughness Theory
252
Fracture Toughness Maps 254
Continuous Fiber Composites 255
Short Fiber Composites 255
Crack-Interface Interactions 257
xii
Contents
6.4.1. Tensile Debonding Phenomenon 257
6.4.2. Transverse Cracking versus Longitudinal Splitting 260
6.4.3. Crack Growth Resistance (R-curve) Behavior
in Transverse Fracture 268
References 273
Chapter
7.
Improvement of Transverse Fracture Toughness
with Interface Control 279
7.1.
7.2.
7.2.1.
7.2.2.
7.2.3.
7.3.
7.3.1.
7.3.2.

7.4.
7.5.
7.5.1.
7.5.2.
Introduction 279
Fiber Coating and Intermittent Bonding Concept
-
Experimental
Studies 281
Intermittent Bonding Concept 282
Fiber Coating for Improved Energy Absorption Capability
Fiber Coating Techniques 293
Theoretical Studies of Interphase and Three Engineered
Interphase Concepts 295
Theoretical Studies
of
Interphase 296
Engineered Interface Concepts with Fiber Coating 300
Control of Laminar Interfaces-Delamination Promoters 306
Residual Stresses 308
Origin of Residual Stresses 308
Control of Residual Stresses 3 15
References 320
285
Chapter
8.
Improvement
of
Interlaminar Fracture Toughness
with Interface Control

329
8.1.
8.2.
8.2.1.
8.2.2.
8.2.3.
8.3.
8.3.1.
8.3.2.
8.4.1.
8.4.2.
8.4.3.
8.4.
Introduction 329
Effects of Matrix Materials on Interlaminar Fracture Resistance 330
Introduction 330
Correlations between Matrix Properties and Composite Interlaminar
Properties 332
Impact Resistance and Tolerance
of
Fiber Composites with Tough
Matrices 339
Delamination Resisters 342
Mechanics of Free-edge Delamination 342
Interleaving Techniques 345
Three-dimensional Textile Composites Concept 35
1
Introduction
351
Improvement

of
Interlaminar Fracture Toughness
Impact Response
of
Stitched Composites 357
References 360
354
Concents
Appendices 367
List
of
Symbols and Abbreviations 371
Author Index 377
Subject Index 391
xiii

Chapter
1
INTRODUCTION
Fiber composite technology is based on taking advantage of the high strength and
high stiffness of fibers, which are combined with matrix materials of similar/
dissimilar natures in various ways, creating inevitable interfaces. In fiber composites,
both the fiber and the matrix retain their original physical and chemical identities,
yet together they produce a combination of mechanical properties that cannot be
achieved with either of the constituents acting alone, due to the presence of an
interface between these two constituents. The growing number of uses for fiber
reinforced composites in many engineering applications has made the issue of
interfuce
(or more properly termed,
interphase

(Drzal et al., 1983)) a major focus of
interest in the design and manufacture of composite components.
A classic definition of the
interjiuce
in fiber composites is a surface formed by a
common boundary of reinforcing fiber and matrix that is in contact with and
maintains the bond in between for the transfer of loads. It has physical and
mechanical properties that are unique from those of the fiber or the matrix. In
contrast, the
interphase
is the geometrical surface of the classic fiber-matrix contact
as well as the region of finite volume extending therefrom, wherein the chemical,
physical and mechanical properties vary either continuously or in a stepwise manner
between those of the bulk fiber and matrix material. In other words, the interphase
exists from some point in the fiber through the actual interface into the matrix,
embracing all the volume altered during the consolidation or fabrication process
from the original fiber and matrix materials. Therefore, the earlier definition of
Metcalfe (1974) for interface can be used for interphase as well: “An interface is the
region of significantly changed chemical composition that constitutes the bond
between the matrix and reinforcement”. Fig.
1.1
schematically illustrates the
concept
of the interphase according to Drzal et al.
(1983).
Also shown in Fig.
1.1
are
the various processing conditions that are imposed on the interphase to allow
chemical reactions to take place and volumetric changes and residual stresses to be

generated. It is the latter definition
of
interface that is in general use in this book.
However, for analytical purposes in micromechanics the interface is still conve-
niently considered to be infinitely thin and the properties of the mating fiber and
matrix are isotropic and homogeneous.
1
2
Engineered interfaces in fiber reinforced composites
Thermal,
chemical,
mechanical
Surface layer
Fig.
I
.1. Schematic illustration of the components of the three-dimensional interphase between fiber and
matrix. After Drzal et al.
(1983).
The issue of understanding the composition and properties
of
interfaces in fiber
composite materials is still evolving despite the fact that there have been a great
number of publications devoted to research in this field. Part of the reason for this
evolution is the interdisciplinary nature of the subject. In addition to a number of
multi-disciplinary conferences held in the past
30
years on adhesion science in
general, several international conferences dealing solely with the fiber-matrix
interfaces, such as the
Internationai Conference

on
Composite Interfuces
(ICCI)
and
Interfacial Phenomenon
in
Composite Materials
(IPCM),
have been held since 1986.
These conferences have provided
a
centralized forum not only to discuss and
identify the important problems of the subject, but also to disseminate important
research results from various sources. They are thus leading the scattered research
and development efforts in a sensible direction, as well as helping to make significant
contributions toward the improvement
of
our fundamental understanding of
interfaces in polymer, metal and ceramic matrices composites.
Nevertheless, recent advances in research in this multi-disciplinary field have not
yet been collected together. While there are plenty of reference books available on
composite materials in general, few of them are devoted specifically to composite
interface science and mechanics. It is hoped that this book adds to the research effort
by bringing recent developments in the field together in one convenient single
volume. It is intended
to
create a comprehensive reference work from both the
materials science and mechanics perspectives.
It is well known that the properties of an interface are governed largely by the
chemical/morphological nature and physical/thermodynamic compatibility between

the two constituents and most often limit the overall performance
of
the bulk
Chapter
I.
Inlroduction
3
composite. There is now a considerable amount of evidential data rcgarding the
influences of interfaces on fracture toughness in both transverse and interlaminar
fractures, and strength and stiffness of fiber composites in various failure modes and
loading configurations (Kim and Mai, 1991; Drzal and Madhukar, 1993). although
the relationship between documented material properties and the actual perfor-
mances of composites is still in question. It follows therefore that a thorough
knowledge of the microstructure-property relationship at the interface region is an
essential key to the successful design and proper use of composite materials.
Further, the interface properties are becoming gradually accepted as design and
process variables to be tailored for particular end applications (Kim and Mai, 1993).
Although there is no simple quantitative relation known for interface optimization
of a given combination of fiber and matrix, various chemical-physical and
thermodynamic-mechanical principles along with previous experience are invalu-
able sources
of
information to design the interface qualitatively.
A
number of
potential solutions have been suggested to improve specific properties of the
composites, particularly the interface bond quality for efficient stress transfer and
the fracture resistance/damage tolerance
of
inherently brittle composites without

sacrificing other important mechanical properties.
This book is concerned mainly with interfaces in advanced composites made from
high performance fibers, such
as
glass, carbon, aramid and some other organic (e.g.
ultrahigh molecular weight
(UHMW)
polyethylene) and inorganic (e.g.
B/W,
A1203, Sic) fibers and useful matrix materials encompassing polymer, metals/
alloys and ceramics.
To
control the interface properly and thereby to provide the
composite with improved mechanical performance and structural integrity, it is
essential to understand the mechanisms of adhesion which are specific to each fiber-
matrix system, and the physico-chemical characterization of the interface with
regard to the origin of adhesion. This is the focus of Chapter
2.
A
number
of
theoretical and experimental methods developed to assess the quality of the interface
bond are summarized. Several common experimental techniques that have been
developed to assess the fiber-matrix interface bond quality on a microscopic scale of
the so-called ‘single fiber microcomposite test’, are presented in Chapter 3 along
with the
interlaminar/intralaminar
strengths and fracture toughness of various
failure modes using composite laminates. Their applicability and limitations are
critically discussed with regard to the loading geometry and interpretation of the test

data based on the underlying mechanics.
A
proper load transfer across the interface
region is also of particular importance in composites technology. Chapter
4
considers from the load transfer and fracture mechanics angles, extensive and in-
depth theoretical analyses based on
a
shcar-lag model for the single fiber composite
test with different loading geometry. Of special interest are the stress states in the
composite constituents and debond process along the interface depending on the
nature
of
the interface bond. This is followed in Chapter
5
by comparisons of the
theories with experimental results
of
several different composite systems. Particular
emphasis is placed on the various techniques of surface treatments on a range
of
technologically important fibers to improve bond strength as well as to enhance
fiber-matrix compatibility and stability during processing or fabrication
of
the
4
Engineered interfaces
in
jiber reinforced composites
composites. A review of the microfailure mechanisms and their associated theories

of fracture toughness of fiber composites in Chapter
6
identifies that a high bond
strength does not necessarily lead to a high fracture toughness. Instead a
compromise always has to be made in the bond strength to optimize the strength
and toughness. The role of the interface and its effects on the overall performance of
composites is addressed from several viewpoints. Novel methods to improve the
transverse fracture toughness of composites by means of controlled interfaces are
presented in Chapter
7.
The effects of residual stresses arising from the thermal
mismatch between the fiber and matrix and the shrinkage of the matrix material
upon cooling from the processing temperature are specifically discussed. Recent
advances in efforts
to
improve the interlaminar fracture toughness are also critically
reviewed
in
Chapter
8.
References
Drzal, L.T., Rich, M.J. and Lloyd,
P.F.
(1983).
Adhesion of graphite
fibers
to epoxy matrices. part
I.
The
role of fiber surface treatment.

J.
Adhesion
16,
1-30.
Drzal, L.T. and Madhukar, M. (1993). Fiber-matrix adhesion and its relationship
to
compositc
mechanical properties.
J.
Muter.
Sci.
28,
569-610.
Kim, J.K. and Mai, Y.W.
(1991).
High strength, high fracture toughness fiber composites with interface
control-a review.
Composites
Sci.
Technol.
41,
333-378.
Kim, J.K.
and Mai,
Y.W.
(1993).
Interfaces in composites. in
Structure
and
Properties

of
Fiber
Composites,
Materials Science and Technology, Series Vol.
13,
(T.W.
Chou ed.), VCH Publishers,
Weinheim, Germany, pp.
239-289.
Metcalfe,
A.G.
(1974).
Physical-chemical aspects of the interface. In
Interfaces
in
Metal
Matrix
Composites,
Composite Materials.
Vol.
1,
(A.G.
Metcalfe ed.),
New
York,
Academic Press,
pp.
65-
123.
Chapter

2
CHARACTERIZATION OF INTERFACES
2.1.
Introduction
The physico-chemical aspect of composite interfaces is a difficult subject and our
understanding of this feature is still far from complete. Two important topics will be
reviewed in this chapter. They are the theory
of
bonding at the fiber-matrix interface
and the analytical techniques to characterize the interface. The nature or origin of
the bonding between the fiber and matrix is discussed in terms of the theories of
adhesion with associated mechanisms of bonding. Examples of specific fiber-matrix
systems are provided along with their corresponding mechanisms of adhesion.
Various physico-chemical analytical techniques, which have been devised to
characterize the surface properties of fibers and composite interfaces, are also
extensively reviewed with corresponding analytical models for evaluation of the
experimental data. Advantages and limitations
of
each method are also presented.
Proper characterization of composite interfaces, whether it is for chemical,
physical
or
mechanical properties, is extremely difficult because most interfaces with
which we are concerned are buried inside the material. Furthermore, the
microscopic and often nanoscopic nature of interfaces in most useful advanced
fiber composites requires the characterization and measurement techniques to be of
ultrahigh magnification and resolution for sensible and accurate solutions. In
addition, cxperiments have to be carried out in
a
well-controlled environment using

sophisticated testing conditions (e.g. in a high vacuum chamber). There are many
difficulties often encountered in the physico-chemical analyses of surfaces.
2.2.
Theories
of
adhesion and types
of
bonding
The nature
of
bonding is not only dependent on the atomic arrangement,
molecular conformation and chemical constitution of the fiber and matrix, but also
on the morphological properties of the fiber and the diffusivity of elements in each
constituent.
It
follows therefore that the interface
is
specific
to
each fiber-matrix
system (Kim and Mai,
1991).
Adhesion in general can
be
attributed to mechanisms
including, but not restricted to, adsorption and wetting, electrostatic attraction,
5
6
Engineered interfaces in
fiber

reinforced composites
chemical bonding, reaction bonding, and exchange reaction bonding (Kim and Mai,
1993), which are schematically shown in Fig. 2.1 and discussed in the following
sections. In addition to the major mechanisms, hydrogen bonding, van der Waals
forces and other
low
energy forces may also be involved. All these mechanisms take
place at the interface region either in isolation, or, most likely, in combination
to
produce the final bond. Reviews on these major mechanisms can be found in many
references including Scolar (1974), Wake (1978), Kinloch (1980, 1982), Hull (1981),
Adamson (1982) and Kinloch et al. (1992) for polymer matrix composites; Metcalfe
(1974) for metal matrix composites
(MMCs);
and Naslain (1993) for ceramic matrix
composites
(CMCs).
More recently, mechanisms and mechanics modeling
of
interfaces in cementitious composites have received a lot
of
attention (see for
example, Maso, 1993; Cotterell and Mai, 1996).
Fig.
2.1.
Interface bonds formed (a) by molecular entanglement;
(b)
by electrostatic attraction; (c) by
interdiffusion of elements;
(d)

by chemical reaction between groups
A
on
one
surface and groups
B
on
the
other surface;
(e)
by chemical reaction following forming of
a
new compound(s), particularly
in
MMCs;
(f)
by mechanical interlocking. After
Hull
(1981)
and Naslain
(1993).
Chapter
2.
Characterization
of
interfaces
1
2.2.1.
Adsorption
and

wetting
Good wetting of fibers by matrix material during the impregnation stages of
fabrication is a prerequisite to proper consolidation of composites, particularly for
composites based
on
polymer resins and molten metals.
It
is well understood that
physical adsorption of gas molecules to solid surfaces is ascribed to the attraction
arising from the quantum mechanical effect due to the valence electrons present in
the constituents as a free gas. The physical attraction between electrically neutral
bodies is best described by the wetting of solid surfaces by liquids. Bonding due to
wetting involves very short-range interactions of electrons on an atomic scale which
develop only when the atoms of the constituents approach within a few atomic
diameters or are in contact with each other.
Wetting can be quantitatively expressed in terms of the thermodynamic work of
adhesion,
WA,
of
a liquid to a solid using the Dupre equation
WA
=
YI
+
?2
-
712
.
(2.1)
W,

represents a physical bond resulting from highly localized intermolecular
dispersion forces. It is equal
to
the sum of the surface free energies of the liquid,
yl
,
and the solid,
y2,
less the interfacial free energy,
y12.
It follows that Eq.
(2.1)
can be
related to a model of a liquid drop
on
a solid shown in Fig.
2.2.
Resolution of forces
in the horizontal direction at the point
A
where the three phases are in contact yields
Young’s equation
Ysv
=
YSL
+
YLV
cos
3
(2.2)

where
ysv,
ysL
and
yLv
are the surface free energies
of
the solid-vapor, solid-liquid
and liquid-vapor interfaces, respectively, and
8
is
the contact angle. Liquids that
form contact angles greater and less than
90”
are respectively called ‘non-wetting’
and ‘wetting’. If the liquid does not form a droplet, i.e.
8
=
O”,
it is termed
‘spreading’ and the relationship given by
Fiq.
(2.2)
becomes invalid. In this case, the
equilibrium is expressed by an inequality
Ysv
-
Yst
>
YLV

.
(2.3)
Vapor
‘A
Fig.
2.2.
Contact
angle,
I),
and
surface
energies,
yLv,
ysL
and
ysv.
for a
liquid drop on a
solid
surface.
8
Engineered interfaces
in jber reinforced composites
The surface energy of a solid (i.e. reinforcement in composites), ysv, must be greater
than that of a liquid (Le. matrix resin),
yLv,
for proper wetting to take place.
Table 2.1 gives values of surface energies for some fibers and polymer matrix
materials. Thus, glass and carbon fibers can be readily wetted by thermoset resins
like epoxy and polyester resins at room temperature unless the viscosity of the resin

is
too
high
(Hull,
1981), and by some thermoplastic resins (e.g. Nylon
6.6,
PET,
PMMA
and
PS).
In contrast, it is difficult
to
wet polyethylene fibers (of surface
energy approximately
31
mJ/m2) with any of these resins unless the fibers are surface
treated. For the same reason, carbon fibers are often coated with Ti-B (Amateau,
1976)
using a chemical vapor deposition process to allow wetting by an aluminum
matrix.
Combining Eqs. (2.1) and
(2.2)
yields the familiar Young-Dupre equation
The values of
WA
reflect directly the significance of energetics between the liquid and
solid phases, i.e. the higher the work of adhesion the stronger the interactions.
WA
can be determined in experiments by measuring the surface energy of the liquid,
yLv,

and the contact angle, 8. Details of the measurement techniques
of
the contact angle
are discussed in Section 2.3.11.
It should be noted that, in the above equations, the effects of adsorption of vapor
or gas on the solid surfaces are completely neglected. The amount of adsorption can
be quite large, and may approach or exceed the point of monolayer formation at
saturation. The spreading pressure,
ns,
which is the amount of the reduction in
surface energy
on
the solid surface due to the adsorption of vapor in equilibrium, is
given by (Adamson, 1982)
ns
=
Ys
-
Ysv
.
(2.5)
The subscript
s
indicates the hypothetical case of a solid in contact with a vacuum.
The importance of impure surfaces is well recognized in areas like brazing where the
difficulty of brazing aluminum is associated with the presence of an oxide film on the
surface. Therefore, Eq. (2.5) can be substituted in Eqs.
(2.1)
and
(2.2)

by introducing
the spreading pressure. The Young-Dupre equation is then modified to
Although the discussion of wettability presented above has focused
on
the
thermodynamics between the fiber surface and the liquid resin, real composite
systems consist of an extremely large number of small diameter fibers embedded in a
matrix. Adding to the issue of proper wetting of fiber surfaces by the resin, a key to
creating good adhesion at the fiber-matrix interface is infiltration of the resin into
the fiber tow during the fabrication process. The minute gaps present between the
fibers can create very large capillary forces, which are often characterized by a
pressure drop due to the surface energy acting in the small capillaries.
If
the liquid