epoxy interfaces

Composites Science and Technology 46 (1993) 293-301

APPLICATION OF THE M I C R O B O N D TECHNIQUE: EFFECTS OF H Y G R O T H E R M A L E X P O S U R E ON CARBONF I B E R / E P O X Y INTERFACES* David A. Biro, Gerald Pleizier & Yves Deslandes Institute for Environmental Chemistry, National Research Council of Canada, Ottawa, Ontario, KIA OR6 Canada (Received 15 May 1991; revised version received 10 January 1992; accepted 11 March 1992)

Abstract The authors have used the microbond technique to study the effect of hygrothermal exposure on carbonfiber/epoxy microcomposites. Immersion of T300/ Epon 828 or AS4/Epon 828 composites in water at 80°C for several hours resulted in lower interracial shear strengths for both materials within the first hour of exposure. After 6h in hot water, the interracial shear strength of T3OO/Epon 828 samples was reduced by 41% while the AS4/Epon 828 samples suffered a loss of 20% in interracial strength. The frictional component of the sheared droplet sliding along the fiber after debonding was twice its original value for AS4/Epon 828 composites while the frictional component of T3OO/Epon 828 fibers tripled for the same exposure to hot/wet conditions. X-ray photoelectron spectroscopy indicated little change in the concentration of functional surface groups on the fibers following hygrothermal exposure. The diminution in shear strength is probably related to plasticization of the resin by water as well as reduction in mechanical interlocking pressures arising from thermal expansion mismatching of the fiber and matrix.

Keywords: microbond, interfaces, effects, XPS, shear strength

adsorption of composites as a result of exposure to hot/wet conditions has been reported by many workers. 1-6 Moisture in any form (liquid or vapor) is an unfriendly environment for adhesive joints in composite materials. Water adsorption is detrimental to the performance of most epoxy resins since a 1% increase in weight as a result of the presence of water can reduce the glass transition temperature by 10-20°C and affect the mechanical properties of a fiber/epoxy composite. Environmental effect studies on fiber reinforced composites has focused primarily on the macroscopic approach using prepreg material and laminates. Microscopic single-fiber specimens have been studied only recently.7-1° One advantage of using single-fiber specimens arises from the fact that only a small amount of material is required for the treatment and analysis. In this context, the small sample dimensions ensure uniform environmental exposure and diffusionlimited processes occur on a shorter timescale than in a bulk composite. ~° Therefore, the period of environmental conditioning may be substantially reduced in these cases and is more likely to be representative of accelerated aging processes. Singlefilament model systems probably do not reflect what is happening in an actual composite material but they can provide insight into the fundamental interactions at the interface. Any extrapolation to composites of larger dimensions remains purely speculative. One of the factors determining the mechanical performance of a CFRP composite is the effectiveness of the bonds and/or interactions between fiber and resin. This interphasial region of a composite transfers stresses from the matrix to the fibers under load bearing conditions. The interracial shear strength (IFSS) is a good measure of the effectiveness of the interactions at the interface. There are four micromechanical test methods used to measure the interfacial shear strength in polymer/fiber composites. The first is the single-fiber fragmentation test 5'H-13 in which a single fiber is axially embedded in a

environmental

INTRODUCTION Carbon fiber reinforced polymers (CFRP) are currently being used for many commercial structural and load bearing applications. An understanding of their response to environmental exposure would be an asset in predicting changes in properties particularly at the interface between the fiber and its surrounding matrix. Mechanical property degradation and water

* Issued as NRC# 32967. © 1993 Government of Canada. 293

294

David A. Biro, Gerald Pleizier, Yves Deslandes

dumbbell-shaped specimen of the polymer. The test coupon undergoes tensile loading with stresses transferred parallel to the fiber axis. The fiber tensile stress increases until the fracture strength is surpassed and the fiber breaks inside the matrix. ~2 A method which uses actual composite material is the microindentation or microdebonding test first described by Mandell et al. 14.15 Sectioned composite laminates are the specimens used in this test where a load applied to a probe pushes out an individual fiber segment from the supporting matrix. Single-fiber pull-out tests have been used extensively to investigate adhesion on model composite materials. 16-~9 In this technique, a single fiber is embedded a short distance into or through a disk or button. The adhesion strength is then calculated from the force required to pull-out the fiber divided by the embedded area of fiber in the matrix. In this simplest form, the force required to extract the fiber varies with the embedded length in the matrix. Successful pull-out experiments provide data on inteffacial shear strength, the work of fracture, and frictional components after interphasial adhesion failure. 16.17 A variation of the single-fiber pull-out test has been developed by Miller et al.2° This microbond technique involves depositing a small amount of resin on single fibers to form one or more discrete microdroplets on the surface. The cured microdroplets are then debonded in shear from the fiber. This method has been chosen since it is a rapid and simple technique for determining interracial shear strength and for easy hygrothermal treatment of samples. The authors previously reported the effect of thermoset cure and resin formulation containing a fortifier on the interracial shear strength of carbon-fiber/epoxy interfaces. 2~ In this paper, the relationship between fiber/matrix interactions and the interfacial shear strength of composites from an epoxy resin and carbon fibers is investigated with the microbond technique for hygrothermally treated samples. Surface characterisation was also used to examine the surface composition and morphology of the samples in this study. EXPERIMENTAL

Table 1. Cure schedules for the epoxy formulation

Cure schedule Cure I Cure II Cure III

Conditions 24 h at 60°C 2 h at 90°C, 1 h at 120°C 2 h at 60°C, 1 h at 120°C, 5 h at 180°C

deposition on to the carbon fibers. Small resin droplets were applied to the fiber using a fine-point applicator. As the applicator was withdrawn from the fiber, a small droplet formed around the fiber. A series of these microcomposites were then cured under nitrogen in a Linberg tube furnace according to the schedules in Table 1. The embedded length of the resin droplets on the fibers was determined by optical microscopy prior to and following the microbond experiment. These ranged from 40 to 100 #m. The fiber specimens were glued on brass tabs with a commercial room temperature curing epoxy (Lepage, QC, Canada). Unreinforced resins were degassed in suitable molds and cured under pressure in nitrogen in an oven. The mechanical properties of the resin formulation was measured according to ASTM D638 on dumbbellshaped specimens at 2 mm/min crosshead speed. The tensile strength of the cured resin was determined to be 95 MPa 21 when using cure schedule III. This value is greater than the 85-88 MPa value reported by the resin manufacturer, 22 but cure cycles differ. Hygrothermal treatment T300/Epon 828 and AS4/Epon 828 were subjected to hygrothermal aging at ambient (21°C) and elevated (80°C) temperature in distilled water. The specimens were exposed for ambient condition for 24 h. The elevated temperature exposure ranged from 1 to 24 h at 80°C. Following exposure, the microcomposites were removed from the water and placed in a vacuum desiccator overnight. There were 30-80 to microcomposite specimens for a given environment treatment. Control experiments were conducted concurrently. For each treatment, samples were examined following debonding by optical microscopy and SEM. After debonding no residual resin was present on the fiber at the original position of the microdroplet.

Materials

Commercial carbon fibers (Amoco, Thornel T300 (Lot no. 81697901) and Hercules AS4 (Lot no. 437-3N)) were marked 'unsized' and used as received in the microbond experiments. The fiber diameter was taken as the average of numerous fibers as determined by scanning electron microscopy (SEM). The commercial epoxy EPON~828 (Shell Chemical Co., Canada) and the aromatic amine curing agent TONOX ~ (Uniroyal Chemicals, Ontario) were mixed in 10:3 wt% ratio in an aluminum pan at 60°C before

Apparatus

The microbond technique has been previously described by other workers. TM Essentially, a microdroplet of polymeric resin was deposited on the surface on a fiber which was fastened to a brass tab hung on the crosshead of an Instron (model 1123). The upper surface of the droplet was gripped by two flush stainless steel blades secured to a microvise, as shown schematically in Fig. 1. The droplet was debonded in shear by an upward displacement of the

Hygrothermal exposure effect on CF /epoxy interfaces

295

resolution data were obtained with 29 eV pass energy, while survey spectra were acquired with 187 eV pass energy. The X-ray spot size was I m m × 3 mm. All spectra were referenced to the Cls peak for neutral carbon, which was assigned a value of 284.6 eV. SEMs were taken on a Jeol JSM-84 electron microscope.

T

TO LOAD CELL

SHEARING

l

BLAOES

RESULTS

I ~-

Pull-out analysis

MICRODROPLET

i

<

CARBON

FIBER

Fig. 1. Schematic diagram of the microbond test. The carbon fiber is suspended from the load cell mounted on a crosshead. The resin droplet is sheared away by two blades attached to a fixed microvise as the crosshead translates upwards. crosshead at a speed of 0.5 mm/min. The output of the Instron was interfaced to an Epson personal computer in order to display, store and analyze the data. The shearing force at the interface was transferred to the fiber/resin interface and was recorded by the load cell. The pull-out force was recorded as debonding proceeded, and the interfacial shear strength, 3, was calculated from the equation.

A typical trace for a microbond experiment is shown in Fig. 2. The sampling rate was 10Hz. Faster sampling rates (i.e. 100-200Hz) did not provide additional insight to the debonding process in the recorded traces. From the pull-out force, F, and the embedded length, l, r can be calculated according to eqn (1). The interfacial frictional component of the bead slipping along the fiber can be evaluated by substituting the values of f from Fig. 2 into eqn (1). For any given hygrothermai treatment, the values of r in this report are the average of 30-70 separate pull-out experiments or as the best fit linear regression slope from plots of debond force versus embedded length. In general, the interracial strength decreased with exposure time to hot/wet conditions.

Hygrothermai effects There was no statistically significant difference (a~= 0.05) between controls and samples soaked in room temperature water (100% relative humidity) for 24 h. The results of the hygrothermal exposures for the T300/Epon 828 and AS4/Epon 828 systems are presented in Table 2. Three different cure schedules were employed for this study. Since the shear strength of samples for cures II and III were virtually the same, only those for cure II are presented for clarity. The interfacial shear strengths of the microcomposites

F

:rdl

(1)

where F is the pull-out force, d is the diameter of the fiber, and l is the embedded length of the resin droplet. Drop size distributions were similar for all experiments. Resin cure and glass transition temperature were determined by differential scanning calorimetry (DSC) on a Dupont 910 coupled to a Dupont 1090 thermal analysis system.

0.10 0.08

Z v

o 0 LL

0.06

/ Fmax

/

/

0.04

/ /

0.02

!

t

f

Surface characterisation and morphology The surfaces of carbon fibers and epoxy resin were examined by X-ray photoelectron spectroscopy (XPS) which provides information on the surface composition of the fiber before and after hygrothermal treatment. Spectra were collected on a PHI-5500 spectrometer equipped with an X-ray monochromator using an aluminum anode (1486-6 eV). The pressure in the sample chamber was lower than 10 - 9 torr. High

0.00 0

I

I

!

|

I

3

6

9

12

15

18

Time (seconds) Fig. 2. Typical experimental trace of force versus time for a microbond experiment. The debond force is represented by F, while the frictional force following debonding is shown by f.

David A. Biro, Gerald Pleizier, Yves Deslandes

296

Table 2. Interracial shear strength of microcomposites following hygrothermal treatment Exposure time in 80°C water

(h)

Control 2 4 6 24

Interfacial shear strength of T300/Epon 828 microcomposites (MPa) Cure I

Cure II

Interracial shear strength of AS4/Epon 828 microcomposites (MPa) Cure III

54.0 + 1-0 (66) -42.2 + 1-0 (62) 33-2 + 1.5 (62) --

72-3 + 2.9 (20) 59.6 + 2.2 (24) 50.7 + 3.6 (18) 42-2 + 3.2 (30) 49.9 ± 2.0 (32)

77.7 + 2.6 (23) 62.6 ± 3.6 (33) 62.2 + 4.5 (21) 62.0 + 2.1 (47) 58.3 ± 2-7 (33)

Variations expressed as 95% confidence limits. Numbers in parentheses represent number of specimens for each data set.

were reduced with respect to the control samples when subjected to hot/wet environments. Curing the T300-based samples for 24 h at 60°C did not completely cure the epoxy resin as indicated by the exotherm on the first DSC scan. However, the droplets were solid and did not behave as a B-stage material. Immersing these samples in 80°C water for 4 and 6 h reduced the interracial shear strength by 22% and 39%, respectively. The shear strength of samples from cures II and III are the same, but r for cure I was lower than the other cures. The reduction in strength was also reflected in the lower Tg of the resin for cure I. Immediately following debonding, the microbond trace was dominated by a frictional component of the debonded droplet sliding down against the fiber. This frictional component increased from 15% to 30% of the peak debond force when cure I samples were immersed in 80°C water for 6 h. Curing schedule II resulted in materials with interracial properties which were less affected by the hygrothermal treatment. The interracial shear strength of T300/Epon 828 decreased to 82% of its original value following 2 h at 80°C, dropped to 70% after 4 h, then down to 59% of its original value after 6 h. The frictional component following debonding was 14% of the peak load for the control sample and increased to an average of 29% for samples treated for 6 h at 80°C in parallel with samples prepared by cure I. This increase in sliding friction was probably related to slightly rougher local topography of the fiber following hygrothermal treatment. There was no difference in the frictional component for T300 samples soaked in distilled water (21°C) for 24 h. Immersion in hot water for 2 4 h reduced ~ to 4 9 . 9 M P a as compared to 72-3 MPa for the dry samples. The IFSS of samples subjected to cure I became gradually weaker following the hygrothermal treatment while those samples prepared according to cure 1I were more resistant to the effects of accelerated aging. The interracial shear strength of A S 4 / E p o n 828 composites was determined to be 7 7 . 7 + 2 . 6 M P a

while that of the T300/Epon 828 composites was 72.3 + 2.9 MPa. The temporal response to hygrothermal treatment of both samples is shown in Fig. 3. Both composites displayed a reduction in T within the first hour of treatment. After 6 h in hot water, the interracial shear strength of T 3 0 0 / E p o n 828 samples was reduced by 41% while the A S 4 / E p o n 828 samples suffered a loss of 20% in interracial strength. The interracial shear strength values of the AS4 samples appeared to stabilize at 58-60 MPa while those of the T300 samples stabilized at a lower value of 50 MPa after 24h immersed in hot water. The frictional component following debonding, rf, increased with treatment under hot/wet conditions. The T 3 0 0 / E p o n 828 composites displayed an increase in rf from 4-7 to 12-4 MPa after 6 h in hot water while the A S 4 / E p o n 828 samples increased from 9.7 to 17.7 MPa for the same time period. Other sets of specimens were also exposed to methanol and acetone for a 24h period at room temperature to study the effect of organic solvents.

100

0..

80 .

t-

--5

tf

l

l

40

20

0

0

i

I

I

I

I

5

10

15

20

25

•

,

30

0

Treatment time in water @ 80 C (hrs)

Fig. 3. Plot of interracial shear strength versus treatment time in 80°C water for AS4/Epon 828 samples (&) and for T300/Epon 828 samples (11). Error bars represent 95% confidence interval.

Hygrothermal exposure effect on CF/epoxy interfaces Samples soaked in methanol for 2 4 h showed no significant change in interracial strength. The interfacial shear strength of samples soaked in acetone for 44 h was 18% weaker than control samples. Acetone swelled the cured resin, dissolves unreacted resin, reducing the resin mechanical properties and hence leading to poor interfacial properties. Some samples were exposed to sea water (Halifax, Nova Scotia) at 80°C for 6h. There was no statistically significant difference (o~ = 0-05) between samples treated in sea water and those treated in distilled water in this case.

01s

C1 s

c

c

A control

280

Surface charaeterisation The surface texture of a hygrothermally treated carbon-fiber/epoxy microdroplet appeared slightly rougher than the surface of a pristine carbon fiber, as shown in Fig. 4. The lack of residual resin and variation of fiber morphology on the debonded portion of the fiber by SEM pointed towards an

285

290

i 3OO

295

~

~i ~ j

I

I

I

I

525

530

535

540

Binding Energy (eV) Fig. 5. XPS spectra of the carbon C,s and oxygen O1, regions of T300 carbon fibers for the control and hygrothermally treated samples for 2 h (A), 4 (B), and 6 h (C) in 80°C water. adhesive failure at the interface rather than cohesive failure of the epoxy resin. XPS spectra of T300 carbon fibers (Cls and Ols) before and after hygrothermal aging are collected in Fig. 5. The surface of hygrothermally treated epoxy resin was examined by XPS, revealing a small increase

>k-

,

297

AS4

z w i-

280 282 284 286 288 290 292 294

I

>.

T300

(n

z w l-Z

280 282 284 286 288 290 292 294 BINDING ENERGY, eV Fig. 4. Scanning electron micrographs of epoxy bead on a T300 fiber after pull-out for a control sample (A) and for a 6 h hygrothermally treated sample (B).

Fig. 6. Deconvoluted XPS spectra of the carbon Cts region of T300 and AS4 carbon fibers (control samples) showing relative distribution of carbon-based functional groups on the surface of the fibers.

298

David A. Biro, Gerald Pleizier, Yves Deslandes

in the overall oxidation, as indicated by the O~s/CL~ ratio, from 0.1607 to 0.1760. A comparison of the C~s peak of T300 and AS4 carbon fibers is shown in Fig. 6. The AS4 fibers appear more oxidized than the T300 fibers, as indicated by the larger shoulder at higher binding energy from the 284.6eV graphite carbon peak. DISCUSSION The adhesive bonding in a fiber/epoxy composite can be weakened or destroyed by environmental influence. Glass-fiber/epoxy composites suffer substantial degradation as a result of the effect of water. 9'I° Water molecules can penetrate to the interface by diffusion through the resin via cracks and voids. They are attracted to the high concentration of hydroxyl groups on the surface of glass fibers. Like glass fibers, carbon fibers contain many functional groups on their surface; however, the effect of moisture on the interface is generally less than in the case for glass. The decrease in interfacial strength in carbonfiber/epoxy composites following hygrothermal treatment indicates that one of the primary roles of water is to diminish the integrity of the interactions at the interface. As the temperature of the environment increases, so does water absorption and permeability. The extent of property degradation is partially determined by the duration of exposure to hot/wet conditions. Prolonged exposure may result in irreversible damage via hydrolysis of chemical bonds or disruption of intermolecular hydrogen bonding. A 39-41% reduction in r was evaluated by the microbond method after the samples were immersed in 80°C water for 6 h (cures I and II). Under these conditions, absorbed moisture will begin to plasticize the small bead of resin, lowering its Tg. This was found to be the case since the Tg decreased from 117 to I06°C as indicated by concurrent DSC measurements on hygrothermally treated resin samples. The Tg of the epoxy resin decreased as a consequence of absorption of water which resulted in weight gains of 1.54-1.97% depending on the immersion period in hot water. The small sample dimensions lead to rapid Fickian diffusion of water to the interface, either through the resin bead or by capillary action near the fiber/resin meniscus area. Sorbed water probably swells the epoxy, reducing the radial pressure between the epoxy and the fiber, and thus weakening the interracial interactions. Moisture also tends to reduce the material modulus, especially at elevated temperature. Fracture mechanics Generally, a uniform stress distribution along the embedded fiber length is assumed during pull-out. However, the droplet probably deforms slightly as a

force is applied against the top of the bead causing a stress concentration in that area. In practice, this implies that a shear lag during the bond rupture process creates a non-uniform stress distribution along the embedded length of the fiber. A shear lag analysis first developed by Cox 23 considered the case of an elastic fiber in an elastic matrix. If the resin deforms elastically, then the interfacial shear stresses will be maximized near the droplet ends and then decay toward the center of the droplet. The fracture mechanics of such an adhesive interface failure for pull-out type experiments can be expressed by an energy model based on a hyperbolic function, w'24 2~ At short embedded lengths, the model predicts a debond force which is proportional to the embedded length. Longer embedded lengths of fiber follow a limiting plateau region until the applied force exceeds the tensile strength of the fiber. Fortunately only short embedded lengths of carbon fiber were used in the experiments described here. Miller et al. observed similar results for microbond experiments with carbon fibers. 2° More recently, Penn and Lee w and Piggott and Andison z6 have identified some of the factors affecting fiber pull-out experiments. They have successfully fitted pull-out data to slight variations of the hyperbolic function mentioned above. In addition, Piggottz5 concludes that friction at the fiber/polymer interphase plays an important role in the debonding process. Adhesion from mechanical interlocking can be attributed to thermal shrinkage differences during the curing process between the fiber and the resin, leading to a pressure perpendicular to the fiber/matrix interface, P±, as described by eqn (2): PL

=

(2)

(0~m -- ¢rf) A T E , .

Ett~

(l + Vm) + (1 -- Vy) Ej

where tr is the thermal expansion coefficient, E is Young's modulus, v is Poisson's ratio, and AT is the temperature difference between Tg and room temperature. The coefficient of thermal expansion and Poisson's ratio for the fiber were taken from the manufacturer's specifications27"z8 while those for the matrix were 50 × 10-6/°C and 0.34, respectively, from the literature valuesfl9 The elastic modulus of the matrix was determined by analysis of the stress/strain curves for the particular resin. In the microbond test, the shear slipping is prevented until r~ =/~P~, where/~ is the friction coefficient. The value of P (and hence the IFSS) should be lower for hygrothermally treated samples since the glass transition temperature of the resin is lower for treated samples and hence AT is smaller, leading to a lower value of r. Furthermore, the modulus of the resin tends to decrease with exposure to water, thus leading to a further decrease

Hygrothermal exposure effect on CF /epoxy interfaces in P. This radial pressure is only a part of the total interactions which encompass the IFSS of a composite. Following the debonding process, the frictional force can be related t o / ' 1 by ~f =/~P± as before. The calculated coefficient of friction for a microdroplet slipping along a T300 fiber was 0.4, while #AS4 = 0"8. The values for hygrothermally treated samples were slightly higher. This was attributed mainly to the AT of the resin, hence the 10% lower value of P± and a larger value of ~f. The values of r and frictional components indicate that the AS4/Epon 828 interface is mechanically stronger than that of the T300/Epon 828 system during and following pull-out experiments. The former system probably adheres by secondary interactions (van der Waals forces) between functional groups on the fiber surface and on the resin surface in addition to mechanical interlocking and chemical bonding. The roughness of the untreated T300 fiber probably plays a pivotal role in adhesion for these samples. The lack of resin on the fiber following debonding indicates adhesive failure of the interface. The only cohesive failure of the resin is shown by the small cone remaining at the top of the fiber following the pull-out experiment. The contribution of this small resin cone is taken into account by subtracting its length from the embedded length in the calculation of interracial shear strengths presented in this paper. The increase in frictional forces for the hygrothermally treated samples can be related to the morphology of the fiber and resin which were roughened by exposure to hot/wet conditions (Fig. 4). The rough fiber surface below the bead (leading edge) also impedes frictional sliding of the debonded microdroplet along the fiber, leading to higher values of observed ~f. Accurate accountability of the drop in • due to hygrothermal exposure must consider all parameters contributing to fiber/matrix adhesion. The large value of 95 MPa for the plain resin strength of Epon 828 is much larger than values reported in the literature for similar systems. This is the result of different cure cycles as compared to that specified by the manufacturer. 22 The use of the Tonox curing agent yields physical properties different from those given by the use of metaphenylenediamine or 4-4'-methylenedianiline-alone. It is also possible that the small dimension of the droplet on the fiber may cause it to have properties different from those of a larger plain resin specimen. The microbond technique provides a distribution of values of interracial shear strength for a given resin formulation on a fiber. A comparison of interracial shear strength values by the microbond and other techniques and workers is qualitative at best, considering the large amount of work carried out with carbon fibers and epoxy resins. This arises mainly because a wide variety of materials (both resins and

299

carbon fibers) have been tested with no two identical systems. Some of the highest IFSS values were obtained by McAlea and Basio using the microbond method with high modulus fibers) The remaining IFSS values for intermediate strength carbon fibers (AS1, AS4 and T300) ranged from 25 to 78 MPa. 12'13"16-19The values of ~"obtained in this study compare favourably with results from other workers. Favre and Jacques have determined the IFSS for T300 carbon fibers to be 150 MPa. Of course the resin formulation and cure, and method of evaluating T, differ from the present case. ~ The variability of arises principally from the different materials, cure schedules and techniques used to evaluate the interracial shear strength.

X-ray photoelectron spectroscopy (XPS) Most commercial carbon fibers are surface treated by a proprietary method to increase the fiber/resin interracial bond strength. The most common surface treatments are air oxidation and electrolysis which are particularly well suited to continuous production methods. Details of these industrial processes are of course proprietary. Exposure of carbon-fiber/epoxy samples to hot water may change the surface chemistry by introducing polar groups. Chemical bonding information can be obtained with XPS. It has been used to study functional groups on the surface of carbon fibers because of its surface sensitivity and capability of providing chemical information. Unfortunately, the interracial region is not directly accessible with XPS and only the surface prior to the formation of the interface can be studied. The C~s high resolution spectra for T300 fibers "as-received" and hygrothermally aged are presented in Fig. 5. The line shapes and peak positions were not significantly altered by hygrothermal treatment, indicating that most of the surface groups remained intact following the treatment. The Ols peak positions were shifted slightly yet line shapes remained unchanged, indicating that no observable changes occur at the carbon fiber surface during hygrothermal treatment for a few hours at 80°C. The XPS spectra of the epoxy resin shows slight increases in oxygen containing functional groups. Spectral deconvolution of the T300 and AS4 C~s photoelectron peaks yields information indicating that the latter is more highly oxidized than the T300 sample. The Hercules treatment of AS4 fibers seems to add more nitrogen to the fiber surface than the commercial treatment of Amoco with respect to the T300 fibers which may account for the variation in the Ols/C~ ratios. The deconvoluted C~s spectral information of both fibers is presented in Table 3. It is evident that the AS4 fiber has a larger percentage of oxygen-containing species than the T300 fibers. It is known that surface oxygen compounds such as

300

David A. Biro, Gerald Pleizier, Yves Deslandes Table 3. XPS spectral data for "13110and AS4 carbon fibers

Fiber

T300 AS4

Percentage of functional groups deconvoluted from C,~ signal C--H, C--C

C--OH, C--O

73.1 69.1

14.8 15.7

--~O 5.5 6.7

--COO

:r--~r*"

4.8 6-0

2-0 2.5

Aromatic 'shake-up' peak and CO3 peak together.

carboxylic acids and alcohols, for instance, react with either epoxies or amines forming esters, ethers, or amides. 29'3° The more oxidized surface of the AS4 fibers and the presence of nitrogen-containing groups on the fiber surface provides a stronger interface with the epoxy resin in these microcomposites. The morphologically smoother AS4 fibers should not provide the mechanical interlocking adhesion which appears more evident in T300 fibers. This latter rougher, more striated fiber surface probably adheres by mechanical interlocking between the matrix and the fiber as well as by other means described below. The mechanisms of adhesion in fiber reinforced composites are the result of complex physical, chemical, and mechanical phenomena. A polymer or material must adsorb and wet the fiber sufficiently in order for physical adhesion to take place. Specific atomic or molecular interactions at the interface will determine, in part, the interracial strength of composites. Covalent bonds between the fiber and polymer are believed to be the strongest since they are relatively high energy interactions (200-500 kJ/mol). Only a few bonds per unit area would yield a composite with good interracial shear strength. Unfortunately, these interactions are perhaps the most difficult to prove directly. If chemical bonds do not exist between the fiber and the resin, then adhesion can be attributed to van der Waals interactions and/or hydrogen bonding which are a function of the acid/base group distribution on the fiber surface and in the epoxy formulation. The lack of resin on the fiber following the debond process (Fig. 4), would suggest that van der Waals interactions are greater than chemical interactions between the materials in this case. Electrostatic interactions are also possible. Polymeric chain entanglements at the interface can contribute in part to mechanical adhesion. Other mechanical adhesions may result simply from the roughness of the carbon surface in intimate contact with resin as is probably the case with T300 fibers. Adhesion at the interface thus becomes a complex blend of the interactions described above. Hygrothermal treatment of carbon-fiber/epoxy composites reduces the interfacial shear strength in these materials primarily by disrupting these interactions. The microbond method has been used as a tool probe

the mechanical integrity of the interface. Although most commercial applications are based on large-scale components, this microbond technique, SEM, and XPS can be used to study the microscopic interface between fiber and resin. Microbond technique

The microbond technique as described in this paper was a method used to evaluate the interracial shear strength between carbon fibers and their epoxy microcomposites. It is a relatively new method and has been used by several workers 8'9"1°'~7'z°with various degrees of success. The small dimensions of the fiber and deposition of a droplet require precise handling by the experimenter. Careful examination of the microdroplets during the debonding by optical microscopy, and afterwards by electron microscopy, indicated that the shearing blades made contact with the upper portion of the droplet and it was assumed that most of the stress was transferred non-uniformly across the interface. Other workers have suggested the use of a circular area of contact as a variation of load application. 31 It was observed that the shearing blades should barely make contact with the carbon fiber as the crosshead translates upwards. This was accomplished visually with the aid of a microscope mounted on the Instron and mechanically by noting changes in the initial portion of the force versus time trace. Several variables will contribute to the scatter in the data aside from the local variations in surface topography along the carbon fiber. These include stress concentration during loading, droplet geometry, and non-uniform shear stress distribution across the interface as suggested by Wu and Claypool. 32 They used finite element analysis to characterise the microbond method. The present authors agree with their conclusions that the shearing blade distance remains constant. This distance should be as close as possible to the fiber diameter for a given fiber. Data should be rejected if the droplet and/or fiber are damaged after the test. It was observed that some droplets shattered if they were not fully cured resins. Although eqn (1) is simple, it is the most widely used format to obtain IFSS from the microbond test.S'9"10'17,20,31,32

Hygrothermal exposure effect on CF /epoxy interfaces CONCLUSION The application of the microbond technique for the mechanical characterisation of interfaces has been extended to environmental exposure of carbonfiber/epoxy microcomposites. The interracial shear strength of T 3 0 0 / E p o n 828 and A S 4 / E p o n 828 composites was reduced by 18-19% after 2 h in water at 80°C and by 3 9 - 4 1 % after 6h. The surface chemistry of the fiber and resin shows only small variations which are not solely responsible for the observed decreases in IFSS. The differences are attributed to mechanical interactions at the interface relating to friction, relaxation of induced stresses, plasticization of the resin by water absorption, and water diffusing to the interface causing stress concentration and damaging the material.

REFERENCES 1. Loos, A. C. & Springer, G. S., Moisture absorption of graphite-epoxy composition immersed in liquid. In Environmental effects on Composite Materials, Vol. L Ed. G. S. Springer. Technomic Publishing, Lancaster, PA, 1981, pp. 34-50. 2. Springer, G. S., Introduction. In Enoironmental Effects on Composite Materials, Vol. H, ed. G. S. Springer. Technomic Publishing, Lancaster, PA, p. 1. 3. Gibbins, M. N. & Hoffman, D. J., Environmental exposure effects on composite materials for commercial aircraft. NASA Contractor Report 3502, Jan. 1982. 4. Bascom, W. D., The surface chemistry of moistureinduced composite failure. In Composite Materials Vol. 6; Interfaces in Polymer Matrix Composites, ed. E. P. Plueddemann. Academic Press, New York, 1974, p. 79. 5. McMahon, P. E. & Ying, I., Effects of fiber/matrix interactions on the properties of graphite epoxy composites. NASA Contractor Report 3607, Sept. 1982. 6. Adams, D. F., Properties characterisation-mechanical/physical hygrothermal properties test methods. In Reference Book for Composites Technology Vol. 2, ed. S. M. Lee. Technomic Publishing, Lancaster, PA, 1989, p. 49. 7. Drzal, L. T., Rich, M. J. and Koenig, M. F., Adhesion of graphite fibers to epoxy matrices. III. The effect of hygrothermal exposure. J. Adhesion, 18 (1985) 49. 8. McAlea, K. P. & Besio, G. J., Adhesion between polybutylene terephalate and E-glass with a microdebond technique. Polymer Composites, 9 (1988) 265. 9. Koenig, J. L. & Emadipour, H., Mechanical characterisation of the interracial strength of glass-reinforced composites. Polymer Composites, 6 (1985) 142. 10. Gaur, U. & Miller, B., Effects of environmental exposure on fiber/epoxy interracial shear strength. Polymer Composites, 11 (1990) 217. 11. Favre, J. P. & Jacques, D., Stress transfer by shear in carbon fibre model composites. Part 1. Results of single

301

fibre fragmentation tests with thermosetting resins. J. Mater. Sci., 26 (1990) 1373. 12. Drzal, L. T., The effect of polymeric matrix mechanical properties on the fiber-matrix interfacial shear strength. Materials Sci. Eng. A, 126 (1990) 289. 13. Rich, M. J. & Drzal, L. T., Interfacial properties of some high-strain carbon fibres in an epoxy matrix. J. Reinf. Plast. Compos., 7 (1988) 145. 14. Mandell, J. F., Chen, J. H. & McGarry, F. J., Int. J. Adhesion Adhesioes, 1 (1980) 40. 15. Mandell, J. F., Grande, D. H., Tsiang, T. H. & McGarry, F. J., Modified microdebonding test for direct in situ fibre/matrix bond strength determination in fibre composites. Mater. Technik, 3 (1986) 125. 16. Favre, J. P. & Perrin, J., J. Mater. Sci., 7 (1972) 1113. 17. Penn, L. S. & Lee, S. M., Interpretation of experimental results in the single pull-out filament test. J. Compos. Technol. Res., 11 (1989) 23. 18. Piggott, M. R. & Andison, D., The carbon fibre-epoxy interface. J. Reinf. Plast. Compos., 6 (1987) 290. 19. Piggott, M. R., The interface in carbon fiber composites. Carbon, 27 (1989) 657. 20. Miller, B., Muff, P. & Rebenfeld, L., A microbond method for determination of the shear strength of a fiber/resin interface. Compos. Sci. Technol., 211 (1987) 17. 21. Biro, D. A., McLean, P. & Deslandes, Y., Polym. Eng. and Sci., 37 (1991) 1250. 22. Shell Chemical Company, Epon ~ Resin Structural Reference Manual, SC: 67-1tl. Shell Chemical Company, Houston, TX, pp. 79-80. 23. Cox, H. L., The elasticity and strength of paper and other fibrous materials. British J. Applied Physics, 3 (1952) 72. 24. Piggott, M. R. & Dai, S. R., Brittle failure at the fibre-matrix interface. In Interfaces in Polymer, Ceramic, and Metal Matrix Composites, ed. H. Ishida. Elsevier, Amsterdam, 1988, p. 481. 25. Piggott, M. R., Debonding and friction at fibrepolymer interfaces. I. Criteria for failure and sliding. Compos. Sci. Technol., 30 (1987) 295. 26. Piggott, M. R., Failure processes in the fibre-polymer interphase. Paper presented at 9th NRCC/IMI Symp. 'Composites '90', Nov. 1990, Boucherville, QC, Canada; Composite Sci. & Technol. (in press). 27. Amoco Performance Products, Product Information Sheet F-5409, Greenville, NC. 1988. 28. Hercules Incorporated, Product Data Sheet No. 847-5, Magagna, Utah. 1984. 29. Lee, H. & Neville, K., In Handbook of Epoxy Resins. Mcgraw-Hill, New York, 1967, appendix 5-1. 30. Yosomiya, R., Morimoto, K., Nakajima, A., Ikada, Y. & Suzuki, T., Interracial strength in composite materials. In Adhesion and Bonding in Composites. Marcel Dekker, New York, 1989 pp. 251. 31. Haaksma, R. A. & Cehelnik, M. J., Mater. Res. Soc. Symp. Proc., 170 (1990) 71. 32. Wu, H. F. & Claypool, C. M., J. Mater. Sci. Lett., 10 (1991) 269.