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The role of the fiber-matrix interphase on composite properties
Vacuum/volume 4 1 / n u m b e r s Printed
7 - 9 / p a g e s 1615 to 1 6 1 8 / 1 9 9 0
0 0 4 2 - 2 0 7 X / 9 0 S 3 . 0 0 + .00
in Great Britain
© 1990 Pergamon Press plc
The role of the fiber-matrix interphase on composite properties L T Drzal, Michigan State University, Department of Chemical Engineering, Composite Materials and Structures Center, East Lansing, Michigan 48824-1326, USA
A review of recent work on the subject of the effect of fiber-matrix adhesion on composite properties has shown that previous research (which has postulated a relationship between a single parameter model of adhesion e.g. chemical bonding), has not been successful. Because of the complexity of the area where the fiber and matrix come in contact a new formalism based on a fiber-matrix interphase is postulated. Results of studies are presented which show the effect of and the interrelationships between various interfacial parameters on composite properties.
Introduction
Fiber matrix adhesion and its relation to composite properties has been the subject of continuing study ever since the birth of composite materials in the early 1960's. At first, 'acceptable' adhesion between fiber and matrix was a 'necessary' criterion for producing a composite with 'acceptable' mechanical properties. The choice of commercial composite materials was limited and the level of fiber-matrix adhesion was considered to be a secondary factor. Indeed many of the early micromechanical attempts at describing composite behavior relied on composite materials with an implicitly high level of fiber-matrix adhesion for experimental verification of these models. As the potential use of composite materials expanded however, the advent of new polymeric matrices and new higher performance reinforcing fibers coupled with the desire for the creation of composites made from metal and ceramic matrices led to the need for fundamental investigation of the mechanisms of adhesion between fiber and matrix and its effect on composite properties. Although the literature is full of proposed models of adhesion, none are effective in predicting the type of fiber surface treatment required for a given fiber-matrix combination or coupling the degree of adhesion to observed composite properties. The major reason for this lack of theoretical development lies in the oversimplification of the composition and nature of the fiber-matrix interface. Attention has been focused on single mechanisms which were reputed to be solely responsible for fiber-matrix adhesion. Much attention has been given to the surface chemical aspect of adhesion to the neglect of other related changes that occur in the regions adjacent to the actual fiber-matrix interface. Attempts at explaining adhesion by chemical forces, electrostatic interactions, surface energetic considerations, etc. were largely unsuccessful. A review of the models proposed compared against the growing experimental evidence that a region different in structure and composition near the fiber-matrix interface has lead to a re-examination of the role of surface and interfacial aspects of adhesion. Indeed it will be shown that the interrelationships between fiber, interface
and matrix create a complex region (the interphase) not easily amenable to predictive analysis. The concept of a two-dimensional interface between fiber and matrix has given way to the evolution of a three-dimensional region more properly termed an interphaseL Figure l is a schematic representation of an interphase illustrating some of the possible components. This interphase includes the twodimensional region of contact between fiber and matrix (the interface) but also must incorporate a region of some finite thickness extending on both sides of the interface. The boundaries are defined as extending from the point in the matrix where the local properties start to change from the bulk properties in the direction of the interface. This region includes matrix that may have chemical and morphological features different from the bulk matrix. It can include impurities, unreacted polymer components, non-polymerized matrix additives, etc. At the interface, not only can there be chemical and physical interactions between fiber and matrix, but also voids,
INTERPHASE
~ T h m e ra l chemical
i
B u L k
adhesive
/ ..... ~ / "
"
\ " '
~
PoLymer of
mechanicaL . . environme~~dffferent properties IL I~ ~ A d s o r b e d material x~.~ "SurfaceLayer ~BuLk od herend Figure 1. A schematic diagram showing a magnified view of a crosssection of the hypothetical fiber-matrix interface and all of its compo-
nents. The entire region is taken as the 'interphase'. The arrows represent the directions in which the thermal, mechanical or chemical environments can reach the 'interphase'. 1615
L T Drzal: Fiber-matrix interphase and composite properties
adsorbed gases and surface chemical groups can be concentrated. On the reinforcement side, morphological and chemical features can be different from the bulk. Imposed upon the interphase are the processing conditions which allow chemical reactions, volumetric changes and stresses to be generated. The resultant 'interphase' can be a very complex material which does not easily lend itself to analysis by a single parameter models. The research reported in this paper is part of an on-going effort to investigate and characterize the composition and structure of the reinforcing fiber-polymer matrix interphase in order to provide the underpinnings for a predictive description of fiber-matrix adhesion and its relation to composite performance. In this paper research results will be discussed which examine each of the component mechanisms which may exist in the interphase and which can affect the resulting fiber-matrix adhesion. In order to highlight and simplify these interrelationships results will be reported for a typical carbon fiber-epoxy matrix composite. The mechanisms discussed and the role of each on composite properties extrapolates to polymeric composite systems in general. Fiber surface treatment A starting point for any investigation into adhesion is an examination of the chemical and physical condition of the surface of the adherend (in this case the reinforcement) and any changes made during the surface preparation procedures. The 'A' type carbon fibers used in this study are produced by a high temperature inert gas graphitization of polyacrylonitrile fiber. The resulting carbon fiber is composed of turbostratic graphitic layers which are formed into lameilar ribbons oriented nearly parallel to the fiber axis as well as varying in orientation across the fiber diameter. The fiber has a tensile modulus of 245 GPa and a tensile strength of 3.7 GPa. The surface of these fibers has seen the catastrophic molecular rearrangement associated with graphitization. Adhesion of these fibers in their untreated state to a typical amine cured epoxy results in very low interfacial shear strength with failure taking place in the outer fiber surface layer2. Many surface treatments for improving the adhesion to carbon fibers have been proposed and various commercial ones are described in the patent literature 3. All involve an etching away of the native fiber surface and the addition of surface oxygen groups which increase the concentration to about twice the level initially present on the fiber surface before surface treatment. Adhesion to these surface treatment fibers results in a doubling of the adhesion level as measured by an interfacial shear strength test as shown in Figure 2. 2 This surface oxygen can be removed by hydrogen reduction at moderate temperatures. When the surface treated fibers have all of their oxygen groups removed by such a treatment, adhesion to this fiber is decreased but remains higher than one can get with the untreated fiber. Ultramicrotome sectioning and TEM examination of the failed fiber-matrix interface shows that the main function of the fiber surface treatment is to remove the initial defect laden surface and leave behind a structurally sound surface. This is, i.e. one which is capable of sustaining high mechanical loads without failure. This increase in structural integrity is responsible for the majority of the improvement in adhesion. The addition of the surface chemical oxygen groups is responsible for only 10% of the increase in adhesion. 1616
80
70
chemistry
50
.~.~/ ~"~'~
Surface morphoLogy
40 "C N c
30
~
AU
20 0
I 5
/
I 10
I
15 Surfoce oxygen (%)
I 20
Figure 2. A plot of the single fiber interfacial shear strength against the number of oxygen groups present on a carbon fiber surface as a function of various surface treatments. The AU represents the untreated fiber; AS--the fiber after surface treatment.
Interfacial chemistry Modern surface analytical techniques allow the chemical nature of the carbon fiber surface to be determined. X-ray photoelectron spectroscopy in particular provides not only atomic information but molecular information about the surface sites. Of far more importance than the surface composition of the fiber is the determination of the type and extent of chemical bonding between the matrix and the reinforcing fiber. Solid state analysis of the actual fiber-matrix interface is impossible. However, selection of appropriate model compounds may be useful in defining the upper bounds on the extent of chemical interaction. In a series of experiments, monofunctional epoxy compounds, amines and epoxy-amine adducts were dissolved in an inert aromatic solvent and placed in contact with carbon fibers under the same temperature conditions experienced in the solid state processing of the composite4. Afterwards, the fibers were extracted in the pure solvent, dried and then their surface composition was determined with XPS analysis. Comparison of the carbon fiber spectra before and after this exposure to the matrix constituents confirmed that chemical adsorption had taken place (Figure 3). Both the epoxy group and the amine group can chemically react with the surface oxygen groups. Chemical bonding would be expected to create a stronger interaction than physical bonding. On an absolute basis only about 4% of the surface sites of the carbon fiber are involved in chemical bonding. The magnitude of the bond strength for chemical bonds is very high but the quantity of bonds is low resulting in a small difference. This is consistent with the observation made earlier that removal of the surface oxygen groups resulted in a decrease of 10% in the level of adhesion. Surface finishes In addition to the use of surface treatments which are primarily chemical in nature, surface finishes or coatings are also used to affect fiber-matrix adhesion. The mechanism by which surface finishes operate is fundamentally different than the surface treatment mechanism. The finishes or coatings are hundreds of nanometers thick. The properties of this finish layer itself are imparted to the interphase and can control adhesion.
L T Drzal: Fiber-matrix interphase and composite properties 10-9--
AS 4 Fiber thermal epoxy treatment / /
;
• Low values of interfaciaL shear strength
~
• Shear faiLure near the f i b e r surface
Frictional sliding
7 5 4-3
• InterfacioL shear strength
2
near the matrix yield strength
1 0 I 09 ~ Oxygen region- AS4 Fiber as received 8-
[nterfaciaL crack propagation
/
\
6
C=i
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• Interphase of Less p l a s t i c i t y
,
0 539
Matrix crack propagation
'
, ~.,,,~-rJ 538
537
I--_....-.~ 536
535
i~,--......j~ 534
533
532
531
530
529
Binding energy leVI
Figure 3. The X-ray photoelectron spectra of the oxygen ls region of an AS fiber in the 'as-received' state compared to the same fiber after exposure to a monofunctional epoxy molecule documenting the chemical reaction that has taken place between the fiber surface chemical groups and the epoxy.
As an example, surface treated carbon fibers were coated with 100 nm of an epoxy resin without any curing agent 5. These finished fibers were processed as usual and the level of adhesion was measured with a single fiber critical length test. The level of interfacial shear stress that the interface could withstand increased by about 25%. Model compound measurements of strength, stiffness and fracture toughness of variable epoxyamine stoichiometries combined with optical micrographs of the fiber-matrix interface under stress shows that the increase in the adhesion level is due to better stress transfer between fiber and matrix because of the stiffer interface. However, as shown in Figure 4, the increased brittleness reduces the fracture toughness of the interphase resulting in matrix cracking6. Matrix
• StructuraLLy 'sound' surface
Figure 4. Schematic diagram illustrating the three failure modes de-
tected with increasing fiber-matrix adhesion. The drawings represent a single fiber in the epoxy matrix and subjected to shear causing the fiber to fracture. At low levelsof adhesion a frictional debonding and sliding of the matrix in relation to the fiber is observed; at intermediate levels interracial crack growth is detected; and at the highest levels matrix cracking perpendicular to the fiber axis is observed.
introduction of hydroxyl groups present in oligomers of epoxy. The resulting materials provided a range of moduli which decreased with increasing diamine molecular weight yet at the same time kept the interfacial chemistry the same throughout all of the fiber-matrix combinations. Measurement of the single fiber interfacial shear strength has shown a monotonic decrease in interfacial shear strength with decreasing matrix modulus. Figure 5 is a plot of this data. This type of dependence has been proposed through micromechanicai models of the interface which show that the interfacial stresses between fiber and matrix are dependent on the product of the shear modulus of the matrix times the strain-to-failure of the matrix 7. Such a plot has been constructed (Figure 6) and two regions are detected. A linear portion exists from the highest modulus matrix down to a 135
properties
Adhesion studies are often focussed on molecular events at the fiber-matrix interface to the exclusion of consideration of the structure and properties of the surrounding matrix on the observed phenomena. Since adhesion is dependent to a large degree on shear stress transfer from the polymer side of the interphase to the fiber, the properties of the matrix itself should play an important role in establishing the level of adhesion attainable in any fiber-matrix system. In a series of experiments, the same carbon fiber was used with a series of epoxy matrices, all of which were based on diepoxy-diamine chemistry. The diglycidyl ether of bisphenolA was chosen as the epoxy resin but various diamines of increasing length were selected as curing agents. The aromatic meta-phenylene diamine and diamino-diphenyl sulphone were used at stoichiometry as well as a series of polyether diamines (Jeffamines). The polyether amines were selected to avoid the
120 ~105 N
90
~
75
135
o Difunctiona( resin zs TetrafunctionoL resin o OifunctiormL resin • TetrafunctionaL resin -- ISS asa f (T)
120 105
o
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~
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i
~, 45 g
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~n E
30 ~
30
o 15 O 200
I 400
~ 600
800
1000
I
i
1200
1400
"o
1600
Shear modulus of matrix (MPo)
Figure 5. A plot of the single fiber interfacial shear strength and the
radial component of the interfacial shear stress against the shear moduli of various epoxy-amine matrices. 1617
L T Drzal: Fiber-matrix interphase and composite properties 15.5 o 13.0
Conclusions
TetrafunctionoL resin
z~ Difunctional resin
[ 10.5
~
8.0
5. 5
g 3.0 o L
"~ 5.0 0
I I I J I 1 2 3 4 5 Strain to b r e a k , SQRT(shear m o d u l u s } ( M P o ) ° 5
I 6
Figure 6. A plot of the single fiber interfacial shear strength against the product of the square root of the shear modulus times the strainto-break of the matrix.
plateau. At this point the dependency vanishes and further decreases in modulus do not reduce the level of fiber-matrix adhesion as measured by the interfacial shear strength. In this plateau region, the matrix has the properties of an elasticplastic material. Residual stresses In addition to chemical and structural considerations, the state of stresses which result from the processing of the material itself can influence the degree of fiber-matrix adhesion. In the case of carbon fibers, the coefficient of thermal expansion is quite small and can actually be negative. The fiber itself is anisotropic and the radial and longitudinal thermal expansions can be quite different. The matrix is isotropic but has a factor of thirty larger coefficient of thermal expansion than the fiber. This mismatch becomes increasingly significant as higher processing temperatures are reached. The absolute difference between the glass transition temperature and use temperature determines the magnitude of these stresses. Epoxy matrices also reduce their volume as they crosslink. This volumetric shrinkage contributes to the state of stress at the fiber-matrix interphase. For fibers surrounded by matrix, the resulting cure shrinkage produces a compressive interfacial force while for matrix confined between an array of fibers, a net tensile interfacial state of stress may exist. The resulting state of stress can reduce the level of adhesion attainable between fiber and matrix. Figure 5 also contains the calculation of the interfacial stress for the fiber-matrix combinations used in this study 8. It can be seen that the radial component of the stress changes in the same manner as the measured interfacial shear stress.
1618
As our analytical capability has allowed us to obtain significant new information about the microscopic region at the fibermatrix interface, it has become necessary to alter our understanding of this region. A two-dimensional, single parameter model of the interface is not valid. A three-dimensional region exists more aptly called an interphase. The simultaneous interaction of physical and chemical forces at the interface with the structural features of both the fiber and matrix in the interphase region combine to produce the level of adhesion. All or some of these constituents may be responsible for the degree of adhesion as well as the mode of interfacial failure. Surface treatments used with carbon fibers improve adhesion through a two-part mechanism. The most important aspect is the etching of the carbon fiber surface followed by the addition of surface chemical groups which have the potential for chemically interacting with the matrix. Surface finishes create an interphase which can have significantly different properties than the matrix and function to transfer stresses between fiber and matrix. The degree of chemical bonding that actually exists at the fiber-matrix interface is small. Normal processing conditions encountered with carbon fibers and epoxy matrices do not provide the reaction conditions for extensive chemical bonding. The mechanical properties of the interphase matrix provide an intrinsic limit on the maximum degree of adhesion attainable for a given fiber-matrix combination. In general this dependency is a function of the shear modulus of the matrix. Processing of a composite requires the fiber-matrix interface to undergo changes with temperature. The volumetric shrinkage of the matrix as well as the difference in coefficients of thermal expansion combine to produce an interfacial state of stress which may be detrimental to optimizing fiber-matrix adhesion.
References 1L T Drzal, Advances in Polymer Science I1 (Edited by K Dusek), Vol 75. Springer, Berlin (1985). 2 L T Drzal, M Rich and P Lloyd, J Adhesion 16, I (1983). 3 L B Donnet and R C Bansal, Carbon Fibers. Dekker, New York (1984). 4 K Hook, R Agrawal and L T Drzal, J Adhesion. Submitted. 5 L T Drzal, M Rich, M Koenig and P Lloyd, J Adhesion 16, 133 (1983). 6 L T Drzal and M J Rich, Research Advances in Composites in the United States and Japan, A S T M STP 864, p 16. Am Soc Testing and Materials, Philadelphia (1985). 7 B W Rosen, Fibre Composite Materials. Am Soc Metals, Metals Park, OH (1965). 8 j M Whitney and L T Drzal, Toughened Composites, A S T M STP 937, p 179. Am Soc Testing and Materials, Philadelphia (1987).
7 - 9 / p a g e s 1615 to 1 6 1 8 / 1 9 9 0
0 0 4 2 - 2 0 7 X / 9 0 S 3 . 0 0 + .00
in Great Britain
© 1990 Pergamon Press plc
The role of the fiber-matrix interphase on composite properties L T Drzal, Michigan State University, Department of Chemical Engineering, Composite Materials and Structures Center, East Lansing, Michigan 48824-1326, USA
A review of recent work on the subject of the effect of fiber-matrix adhesion on composite properties has shown that previous research (which has postulated a relationship between a single parameter model of adhesion e.g. chemical bonding), has not been successful. Because of the complexity of the area where the fiber and matrix come in contact a new formalism based on a fiber-matrix interphase is postulated. Results of studies are presented which show the effect of and the interrelationships between various interfacial parameters on composite properties.
Introduction
Fiber matrix adhesion and its relation to composite properties has been the subject of continuing study ever since the birth of composite materials in the early 1960's. At first, 'acceptable' adhesion between fiber and matrix was a 'necessary' criterion for producing a composite with 'acceptable' mechanical properties. The choice of commercial composite materials was limited and the level of fiber-matrix adhesion was considered to be a secondary factor. Indeed many of the early micromechanical attempts at describing composite behavior relied on composite materials with an implicitly high level of fiber-matrix adhesion for experimental verification of these models. As the potential use of composite materials expanded however, the advent of new polymeric matrices and new higher performance reinforcing fibers coupled with the desire for the creation of composites made from metal and ceramic matrices led to the need for fundamental investigation of the mechanisms of adhesion between fiber and matrix and its effect on composite properties. Although the literature is full of proposed models of adhesion, none are effective in predicting the type of fiber surface treatment required for a given fiber-matrix combination or coupling the degree of adhesion to observed composite properties. The major reason for this lack of theoretical development lies in the oversimplification of the composition and nature of the fiber-matrix interface. Attention has been focused on single mechanisms which were reputed to be solely responsible for fiber-matrix adhesion. Much attention has been given to the surface chemical aspect of adhesion to the neglect of other related changes that occur in the regions adjacent to the actual fiber-matrix interface. Attempts at explaining adhesion by chemical forces, electrostatic interactions, surface energetic considerations, etc. were largely unsuccessful. A review of the models proposed compared against the growing experimental evidence that a region different in structure and composition near the fiber-matrix interface has lead to a re-examination of the role of surface and interfacial aspects of adhesion. Indeed it will be shown that the interrelationships between fiber, interface
and matrix create a complex region (the interphase) not easily amenable to predictive analysis. The concept of a two-dimensional interface between fiber and matrix has given way to the evolution of a three-dimensional region more properly termed an interphaseL Figure l is a schematic representation of an interphase illustrating some of the possible components. This interphase includes the twodimensional region of contact between fiber and matrix (the interface) but also must incorporate a region of some finite thickness extending on both sides of the interface. The boundaries are defined as extending from the point in the matrix where the local properties start to change from the bulk properties in the direction of the interface. This region includes matrix that may have chemical and morphological features different from the bulk matrix. It can include impurities, unreacted polymer components, non-polymerized matrix additives, etc. At the interface, not only can there be chemical and physical interactions between fiber and matrix, but also voids,
INTERPHASE
~ T h m e ra l chemical
i
B u L k
adhesive
/ ..... ~ / "
"
\ " '
~
PoLymer of
mechanicaL . . environme~~dffferent properties IL I~ ~ A d s o r b e d material x~.~ "SurfaceLayer ~BuLk od herend Figure 1. A schematic diagram showing a magnified view of a crosssection of the hypothetical fiber-matrix interface and all of its compo-
nents. The entire region is taken as the 'interphase'. The arrows represent the directions in which the thermal, mechanical or chemical environments can reach the 'interphase'. 1615
L T Drzal: Fiber-matrix interphase and composite properties
adsorbed gases and surface chemical groups can be concentrated. On the reinforcement side, morphological and chemical features can be different from the bulk. Imposed upon the interphase are the processing conditions which allow chemical reactions, volumetric changes and stresses to be generated. The resultant 'interphase' can be a very complex material which does not easily lend itself to analysis by a single parameter models. The research reported in this paper is part of an on-going effort to investigate and characterize the composition and structure of the reinforcing fiber-polymer matrix interphase in order to provide the underpinnings for a predictive description of fiber-matrix adhesion and its relation to composite performance. In this paper research results will be discussed which examine each of the component mechanisms which may exist in the interphase and which can affect the resulting fiber-matrix adhesion. In order to highlight and simplify these interrelationships results will be reported for a typical carbon fiber-epoxy matrix composite. The mechanisms discussed and the role of each on composite properties extrapolates to polymeric composite systems in general. Fiber surface treatment A starting point for any investigation into adhesion is an examination of the chemical and physical condition of the surface of the adherend (in this case the reinforcement) and any changes made during the surface preparation procedures. The 'A' type carbon fibers used in this study are produced by a high temperature inert gas graphitization of polyacrylonitrile fiber. The resulting carbon fiber is composed of turbostratic graphitic layers which are formed into lameilar ribbons oriented nearly parallel to the fiber axis as well as varying in orientation across the fiber diameter. The fiber has a tensile modulus of 245 GPa and a tensile strength of 3.7 GPa. The surface of these fibers has seen the catastrophic molecular rearrangement associated with graphitization. Adhesion of these fibers in their untreated state to a typical amine cured epoxy results in very low interfacial shear strength with failure taking place in the outer fiber surface layer2. Many surface treatments for improving the adhesion to carbon fibers have been proposed and various commercial ones are described in the patent literature 3. All involve an etching away of the native fiber surface and the addition of surface oxygen groups which increase the concentration to about twice the level initially present on the fiber surface before surface treatment. Adhesion to these surface treatment fibers results in a doubling of the adhesion level as measured by an interfacial shear strength test as shown in Figure 2. 2 This surface oxygen can be removed by hydrogen reduction at moderate temperatures. When the surface treated fibers have all of their oxygen groups removed by such a treatment, adhesion to this fiber is decreased but remains higher than one can get with the untreated fiber. Ultramicrotome sectioning and TEM examination of the failed fiber-matrix interface shows that the main function of the fiber surface treatment is to remove the initial defect laden surface and leave behind a structurally sound surface. This is, i.e. one which is capable of sustaining high mechanical loads without failure. This increase in structural integrity is responsible for the majority of the improvement in adhesion. The addition of the surface chemical oxygen groups is responsible for only 10% of the increase in adhesion. 1616
80
70
chemistry
50
.~.~/ ~"~'~
Surface morphoLogy
40 "C N c
30
~
AU
20 0
I 5
/
I 10
I
15 Surfoce oxygen (%)
I 20
Figure 2. A plot of the single fiber interfacial shear strength against the number of oxygen groups present on a carbon fiber surface as a function of various surface treatments. The AU represents the untreated fiber; AS--the fiber after surface treatment.
Interfacial chemistry Modern surface analytical techniques allow the chemical nature of the carbon fiber surface to be determined. X-ray photoelectron spectroscopy in particular provides not only atomic information but molecular information about the surface sites. Of far more importance than the surface composition of the fiber is the determination of the type and extent of chemical bonding between the matrix and the reinforcing fiber. Solid state analysis of the actual fiber-matrix interface is impossible. However, selection of appropriate model compounds may be useful in defining the upper bounds on the extent of chemical interaction. In a series of experiments, monofunctional epoxy compounds, amines and epoxy-amine adducts were dissolved in an inert aromatic solvent and placed in contact with carbon fibers under the same temperature conditions experienced in the solid state processing of the composite4. Afterwards, the fibers were extracted in the pure solvent, dried and then their surface composition was determined with XPS analysis. Comparison of the carbon fiber spectra before and after this exposure to the matrix constituents confirmed that chemical adsorption had taken place (Figure 3). Both the epoxy group and the amine group can chemically react with the surface oxygen groups. Chemical bonding would be expected to create a stronger interaction than physical bonding. On an absolute basis only about 4% of the surface sites of the carbon fiber are involved in chemical bonding. The magnitude of the bond strength for chemical bonds is very high but the quantity of bonds is low resulting in a small difference. This is consistent with the observation made earlier that removal of the surface oxygen groups resulted in a decrease of 10% in the level of adhesion. Surface finishes In addition to the use of surface treatments which are primarily chemical in nature, surface finishes or coatings are also used to affect fiber-matrix adhesion. The mechanism by which surface finishes operate is fundamentally different than the surface treatment mechanism. The finishes or coatings are hundreds of nanometers thick. The properties of this finish layer itself are imparted to the interphase and can control adhesion.
L T Drzal: Fiber-matrix interphase and composite properties 10-9--
AS 4 Fiber thermal epoxy treatment / /
;
• Low values of interfaciaL shear strength
~
• Shear faiLure near the f i b e r surface
Frictional sliding
7 5 4-3
• InterfacioL shear strength
2
near the matrix yield strength
1 0 I 09 ~ Oxygen region- AS4 Fiber as received 8-
[nterfaciaL crack propagation
/
\
6
C=i
5 4 3 2
• Interphase of Less p l a s t i c i t y
,
0 539
Matrix crack propagation
'
, ~.,,,~-rJ 538
537
I--_....-.~ 536
535
i~,--......j~ 534
533
532
531
530
529
Binding energy leVI
Figure 3. The X-ray photoelectron spectra of the oxygen ls region of an AS fiber in the 'as-received' state compared to the same fiber after exposure to a monofunctional epoxy molecule documenting the chemical reaction that has taken place between the fiber surface chemical groups and the epoxy.
As an example, surface treated carbon fibers were coated with 100 nm of an epoxy resin without any curing agent 5. These finished fibers were processed as usual and the level of adhesion was measured with a single fiber critical length test. The level of interfacial shear stress that the interface could withstand increased by about 25%. Model compound measurements of strength, stiffness and fracture toughness of variable epoxyamine stoichiometries combined with optical micrographs of the fiber-matrix interface under stress shows that the increase in the adhesion level is due to better stress transfer between fiber and matrix because of the stiffer interface. However, as shown in Figure 4, the increased brittleness reduces the fracture toughness of the interphase resulting in matrix cracking6. Matrix
• StructuraLLy 'sound' surface
Figure 4. Schematic diagram illustrating the three failure modes de-
tected with increasing fiber-matrix adhesion. The drawings represent a single fiber in the epoxy matrix and subjected to shear causing the fiber to fracture. At low levelsof adhesion a frictional debonding and sliding of the matrix in relation to the fiber is observed; at intermediate levels interracial crack growth is detected; and at the highest levels matrix cracking perpendicular to the fiber axis is observed.
introduction of hydroxyl groups present in oligomers of epoxy. The resulting materials provided a range of moduli which decreased with increasing diamine molecular weight yet at the same time kept the interfacial chemistry the same throughout all of the fiber-matrix combinations. Measurement of the single fiber interfacial shear strength has shown a monotonic decrease in interfacial shear strength with decreasing matrix modulus. Figure 5 is a plot of this data. This type of dependence has been proposed through micromechanicai models of the interface which show that the interfacial stresses between fiber and matrix are dependent on the product of the shear modulus of the matrix times the strain-to-failure of the matrix 7. Such a plot has been constructed (Figure 6) and two regions are detected. A linear portion exists from the highest modulus matrix down to a 135
properties
Adhesion studies are often focussed on molecular events at the fiber-matrix interface to the exclusion of consideration of the structure and properties of the surrounding matrix on the observed phenomena. Since adhesion is dependent to a large degree on shear stress transfer from the polymer side of the interphase to the fiber, the properties of the matrix itself should play an important role in establishing the level of adhesion attainable in any fiber-matrix system. In a series of experiments, the same carbon fiber was used with a series of epoxy matrices, all of which were based on diepoxy-diamine chemistry. The diglycidyl ether of bisphenolA was chosen as the epoxy resin but various diamines of increasing length were selected as curing agents. The aromatic meta-phenylene diamine and diamino-diphenyl sulphone were used at stoichiometry as well as a series of polyether diamines (Jeffamines). The polyether amines were selected to avoid the
120 ~105 N
90
~
75
135
o Difunctiona( resin zs TetrafunctionoL resin o OifunctiormL resin • TetrafunctionaL resin -- ISS asa f (T)
120 105
o
75
~
-~60
>e
i
~, 45 g
90
-o
~u 6o
-~
45
~n E
30 ~
30
o 15 O 200
I 400
~ 600
800
1000
I
i
1200
1400
"o
1600
Shear modulus of matrix (MPo)
Figure 5. A plot of the single fiber interfacial shear strength and the
radial component of the interfacial shear stress against the shear moduli of various epoxy-amine matrices. 1617
L T Drzal: Fiber-matrix interphase and composite properties 15.5 o 13.0
Conclusions
TetrafunctionoL resin
z~ Difunctional resin
[ 10.5
~
8.0
5. 5
g 3.0 o L
"~ 5.0 0
I I I J I 1 2 3 4 5 Strain to b r e a k , SQRT(shear m o d u l u s } ( M P o ) ° 5
I 6
Figure 6. A plot of the single fiber interfacial shear strength against the product of the square root of the shear modulus times the strainto-break of the matrix.
plateau. At this point the dependency vanishes and further decreases in modulus do not reduce the level of fiber-matrix adhesion as measured by the interfacial shear strength. In this plateau region, the matrix has the properties of an elasticplastic material. Residual stresses In addition to chemical and structural considerations, the state of stresses which result from the processing of the material itself can influence the degree of fiber-matrix adhesion. In the case of carbon fibers, the coefficient of thermal expansion is quite small and can actually be negative. The fiber itself is anisotropic and the radial and longitudinal thermal expansions can be quite different. The matrix is isotropic but has a factor of thirty larger coefficient of thermal expansion than the fiber. This mismatch becomes increasingly significant as higher processing temperatures are reached. The absolute difference between the glass transition temperature and use temperature determines the magnitude of these stresses. Epoxy matrices also reduce their volume as they crosslink. This volumetric shrinkage contributes to the state of stress at the fiber-matrix interphase. For fibers surrounded by matrix, the resulting cure shrinkage produces a compressive interfacial force while for matrix confined between an array of fibers, a net tensile interfacial state of stress may exist. The resulting state of stress can reduce the level of adhesion attainable between fiber and matrix. Figure 5 also contains the calculation of the interfacial stress for the fiber-matrix combinations used in this study 8. It can be seen that the radial component of the stress changes in the same manner as the measured interfacial shear stress.
1618
As our analytical capability has allowed us to obtain significant new information about the microscopic region at the fibermatrix interface, it has become necessary to alter our understanding of this region. A two-dimensional, single parameter model of the interface is not valid. A three-dimensional region exists more aptly called an interphase. The simultaneous interaction of physical and chemical forces at the interface with the structural features of both the fiber and matrix in the interphase region combine to produce the level of adhesion. All or some of these constituents may be responsible for the degree of adhesion as well as the mode of interfacial failure. Surface treatments used with carbon fibers improve adhesion through a two-part mechanism. The most important aspect is the etching of the carbon fiber surface followed by the addition of surface chemical groups which have the potential for chemically interacting with the matrix. Surface finishes create an interphase which can have significantly different properties than the matrix and function to transfer stresses between fiber and matrix. The degree of chemical bonding that actually exists at the fiber-matrix interface is small. Normal processing conditions encountered with carbon fibers and epoxy matrices do not provide the reaction conditions for extensive chemical bonding. The mechanical properties of the interphase matrix provide an intrinsic limit on the maximum degree of adhesion attainable for a given fiber-matrix combination. In general this dependency is a function of the shear modulus of the matrix. Processing of a composite requires the fiber-matrix interface to undergo changes with temperature. The volumetric shrinkage of the matrix as well as the difference in coefficients of thermal expansion combine to produce an interfacial state of stress which may be detrimental to optimizing fiber-matrix adhesion.
References 1L T Drzal, Advances in Polymer Science I1 (Edited by K Dusek), Vol 75. Springer, Berlin (1985). 2 L T Drzal, M Rich and P Lloyd, J Adhesion 16, I (1983). 3 L B Donnet and R C Bansal, Carbon Fibers. Dekker, New York (1984). 4 K Hook, R Agrawal and L T Drzal, J Adhesion. Submitted. 5 L T Drzal, M Rich, M Koenig and P Lloyd, J Adhesion 16, 133 (1983). 6 L T Drzal and M J Rich, Research Advances in Composites in the United States and Japan, A S T M STP 864, p 16. Am Soc Testing and Materials, Philadelphia (1985). 7 B W Rosen, Fibre Composite Materials. Am Soc Metals, Metals Park, OH (1965). 8 j M Whitney and L T Drzal, Toughened Composites, A S T M STP 937, p 179. Am Soc Testing and Materials, Philadelphia (1987).