X ray photoelectron spectroscopy and contact angle measurements on graphite fiber composites pretreated in a number of different ways including mechanical, chemical, and light irradiation were analyzed. Data acquired on surface contamination as a result of fabrication techniques provides answers to the strength and durability of adhesively bonded composites. These techniques were shown to provide valuable information on surface analysis of pretreated composites prior to adhesive bonding and following lap shear fracture

Messick, D. L.

Wightman, J. P.

English

The role of the interfacial region in determining the bond strength and durability of composite bonds is discussed. The characterization of a variety of carbon fibers including Celion 6000 using both scanning electron microscopy and X-ray photoelectron spectroscopy is discussed. The emphasis is on composite bonding, that is, the adhesive bonding between composites in contrast to fiber-matrix interaction. The primary objective of the research is the characterization of composite surfaces before adhesive bonding and after fracture of bonded specimens. Work done on the analysis of composite samples pretreated in a number of ways prior to bonding is detailed

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SEMI-ANNUAL TECHNICAL REPORT
SURFACE ANALYSIS IN COMPOSITE BONDING
by
D. L. Messick and J. P. Wightman
Prepared for
National Aeronautics and Space Administration
October, 1982
v	 Grant NAG-1-248
NASA-Langley Research Center
Hampton, Virginia 23665
Materials Division
Donald J. Progar
Department of Chemistry
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061
^.	 2
INTRODUCTION
s
There is a continuing need to establish the role of the interfacial
region in determining the bond strength and durability of composite bonds. We
have reported (1,2) preliminary studies on the characterization of a variety
of carbon fibers including Celion 6000 using both scanning electron microscopy
s	 (SEM) and x-ray photoelectron spectroscopy (XPS or ESCA).
Indeed, Donnet has emphasized recently (3) the importance of the surface
characterization of fibers in attempting tc understand the properties of
composites. However, in the present research, the emphasis is on composite
bonding, that is, the adhesive bonding between composites in contrast to
fiber-matrix interaction. The primary objective of the research is the
characterization of composite surfaces before cdhesive bonding and after
fracture : F bonded specimens. This report details work done on the analysis
of composite samples pretreated in a number of ways prior to bonding.
EXPERIMENTAL
A. Samples. Twelve composites prepared from Celion 6000 carbon fibers and a
polyimide (LARC-160) were received from the NASA-Langi^y Research Center. The
composite set consisted of an untreated sample and eleven pretreated samples
as listed in Table I. A 0.5 cm diameter sample was punched from each
composite panel and photographed at 20X with a Bausch and Lomb optical
microscope prior to any analysis. The untreated sample designated il1A
delaminated on punching and hence a virgin internal surface was produced and
designated #16.
B. X-ray Photoelectron Spectroscopy (XPS). XPS studies of the com posites were
obtained with a Physical Electronics SAM 550 spectrometer using a Mg X-ray
3anode. The spectrometer is located at Poly-Scientific in Blacksburg. Punched
samples were mourned to the XPS stage with double-sided tape. A wide scan of
binding energies (0 to 1000 eV) was performed on Samples 1A and 1B initially.
Subsequent narrow scans were completed for the elements C, N, 0. S, F, Al,
Si, and K on all samples. The atomic fraction of each of these elements
present in the top 5 nm of the surface was calculated.
C. Scanning Electron Microscopy (SEM ). After obtaining XPS data, scanning
electron microscopy (SEM) was performed on either an Advanced Metals Research
Model 900 scanning electron microscope or a JEOI JSM-35-C scanning electron
microscope. Samples observed in the former microscope were coated with a
Au/Pd alloy in a Denton vacuum evaporator Model DV515. Remaining samples were
coated with Au prior to observation. All sample surfaces were photographed at
20X initially. Det W ed photomicrographs of characteristic and unique areas
r	 of the surface followed for each sample, in some cases to a magnification of
2000x.
D. Contact Angles. Five different liquids of varying surface tensions were
used for contact angle determinations. The liquids and respective surface
tensions (in mJlm2 ) are noted below: water (72.8). formamide (58.3);
methylene iodide (50.8); bromonaphthalene (44.6); n-hexadecane (27.6). A
droplet of each liquid approximately 5 mm (diameter) was placed on each
1.	 composite sheet. Contact angles were measured with a Gaertner- Scientific
goniometer• within 30 seconds after the introduction of the droplet. A second
replication was Limpleted for each liquid on each composite.
Data reduction of the measured contact angle(o)-surface tension (Y)
result% wa% done using the University IBM 1360 System. C ►• itical surface
tensions for each composite were obtained by extrapolation of cos ® vs Y
plot%.
14
RESULTS AND DISCUSSION
Research accomplished during this report period is summarized in Table 1.
Composite samples studied by particular techniques are indicated by X.
A. Scanning Electron Microscopy (SEM). SEM photomicrographs were used
to assess changes in surface topography of composite samples after different
pretreatments. Indeed, the mechanical pretreatments (Sample Nos. 2-6)
appeared to "break into" the fibers in contrast to the chemical pretreatments
(Nos. 7-9). Representative SEM photomicrographs will be included in the Final
Technical Report.
B. X-ray Photoelectron Spectroscopy (XPS or ESCA . An extensive XPS
study was done on the composite samples before and following different
pretreatments. Wide scan XPS spectra were obtained on Samples Nos. 1A, 18, 7,
8, and 9. The major, ohotopeaks were assigned to fluorine, oxygen, nitrogen
and carbon. The presence of large amounts of fluorine on the sur f ace of some
of the samples even after pretreatment is a striking result and emphasizes the
importance of surface analysis. In addition, trace amounts of calcium and
sodium were noted on Sample No. 7 and Nos. 1A and 7, resp. Aluminum, silicon
and gold were detected on some samples. It is presumed that these elements
are associated with the sample holder and further work is in progress to check
this point.
Narrow scan XPS spectra were obtained on all samples and in addition to
scanning for fluorine, oxygen, nitrogen and carbon, scans were also made for
potassium, sultur, aluminum and silicon. 	 These latter elements were suspected
surface impurities based on the known pretreatments. Potassium was not
deter Led on any sample; su 1 f ur appeared ds d tr• ac.e impur i ty but it may be
associated also with the sample holder-.
The quint i t at i ve rc-su I t s of thv XPS ana I ys i s are q i ni in Tab It s 11 and
I.
III. The binding energies (B.E.) in eV and the atomic fractions (A.F.) for
the F Is, 0 Is, N 1s and C is photopeaks are listed in Table II. Half of the
samples contained high concentrations of surface fluorine even following
pretreatment and in every case, a high binding energy photopeak around 292 eV
was observed in the C is spectrum. This is a characteristic of
carbon-fluorine bonding (4). Of particular interest is the fact that the
as-received or untreated composite sample(#r1A) has a large fluorine signal.
However, the fluorine photopeak is some 100 times smaller for a freshly
exposed surface (#r 1B) produced in delamination of the same sample.
The atomic fraction ratios a,e listed in Table III. There are large
differences in the F/C ratio for the various samples. The mechanically
pretreated composites generally have lower, F /C ratios than the chemically
pretreated composites. The longer Flashblast treated samples show a much
i
	 reduced fluorine signal. Further, the values of the 0/C ratio are fairly
constant except for the Flashblast pretreated Sample Nos. 11, 12 and 12W. A
parallel trend is noted in the N/C ratio. It appears as though the Flashblast
pretreatment carbonizes the surface region resulting in the removal of oxygen
and nitrogen contained in gaseous species possibly, for example, CO and HCN.
In summary, the surface fluorine is associated with the external
•=	 composite surfaces only which suggests the inclusion of fluorine during
molding. The order of removal of the surface fluorine species is longer
Flashblast > roechanical > chemical.
i•
	 Fracture studies at NASA-LARC (5) have been made on similar composites
pretreated in the same ways as above. The effect of surface contamination or.
bond strength is being evaluated currently.
`	 C. Critical Surface Tension. The critical surface tension of each
composite sample is listed in Table IV. The critical surface tension was
r
t
6determined using the Zisman approach (6). A direct correlation is suggested
between the surface fluor i ne concentration as measured by XPS and the value of
the critical surface tension. The results of these two independent techniques
are plotted in Figure 2. Indeed, the higher the surface fluorine
concentration, the lower , the critical surface tension. This result is
consistent with critical surface tensions reported for rluoropolymers (7).
REFERENCES
1. W. Chen ano J. P. Wightman, "A Fundamental Approach to Adhesion:
Synthesis, Surface Analysis, Thermodynamics and Mechanics", NASA
Report NSG-1124, NASA-LARC, Hampton, VA, January, 1979.
2. W. Chen and J. P. Wightman, P,-oc. 14th Biennial Conference on Carbon,
P. A. Thrower, Ed., p. 107, Slate College, Pa. (1979).
3. J. B. D-nnet, Abstracts, 4th Intl. Conference on Surface and Colloid
Science, Jerusalem, July, 1981.
4. K. Siegoahn et al., "ESCA-Atomic, Molecular and Solid State Structure
Studies by Means of Electron Spectroscopy", Almquist and Wiksells,
Upsalla (1967).
5. D. J. Progar, communication, August, 1982.
6. W. A. Zisman in "Contact Angle, Wettability and Adhesion", Adv. Chem.
Series #43, R. F. Gould, Ed., pp. 1-51, Am. Chem. Soc., Washington
(1964).
7. ibid, p. 16.
ALj
TABLE I
SUMMARY OF RESEARCH DONE ON COMPOSITES
Technique
Sample	 Sample
No.	 Pretreatment	 Punched	 OM	 SEM	 XPS	 CA
lA As received X X X X X
1B Delaminate X X X X X
2 120 Alf03
BlastedGrit
X X X X X
3 Boeing Grit X X X X X
Blasted
4 Glass Bead Blast X X X X X
5 600 SiC
Handsanded X X X X X
6 180 SiC
Handsanded X X X X X
1 Ethanolic KOH X X X X X
8	 NH2NH2•H20
	
X	 X	 X	 X	 X
9	 Conc. H2SO4 + 30% H2O2	 X	 X	 X	 X	 X
10	 Flashblast #1	 X X	 X X X
11	 Flashblast #2	 X X X X
12	 Flashblast #3	 X X X X
Fl ashblast
after washing(MeOH) X
Fl as ash
after washing X
ashhlast #3
after washing	 X^12W X X
OM - Optical Microscopy;	 SEM -	 Scanning
XPS -	 X-ray Photoelectron Spectroscopy;
Electron Microscopy;
CA -	 Contact Angle
TABLE II
j
f
XPS ANALYSIS OF COMPOSITES
Sample Photopeak(	 No. Is 0 is N Is c rs-
1A 689.0 531.8 399.8 (284.6)	 B.E.
0.19 0.11 0.030 0.66
	
A.F.
1B 688.8 532.4 400.2 (284.6)
0.002 0.11 0.020 0.86
2 689.0 531.4 399.8 (284.6)
0.13 O.li 0.020 0.73
3 689.0 532.0 400.0 (284.6)
0.060 0.15 0.023 0.75
4 689.2 4--1.8 400.0 (284.6)
0.12 0.12 0.024 0.73
5 689.4 532.2 400.2 (284.6)
0.025 0.13 0.020 0.80
6 689.0 531.8 400.0 (284.6)
0.027 0.12 0.032 0.81
7 689.2 531.8 399.8 (284.6)
0.26 C.10 0.012 0.63
8 689.2 531.8 399.6 (284.6)
0.20 0.10 0.041 0.64
9 689.2 532.0 400.0 1\284.6)
0.19 0.12 0.020 0.66
10 689.4 532.0 400.2 (284.6)
0.14 0.080 0.026 0.74
11 - 532.6 - (284.6)
NSP 0.053 NSP 0.93
12 689.2 532.4 400.0 (284.6)
0.006 0.078 0.010 0.89
12W - 532.4 400.0 (284.6)
NSP 0.071 0.021 0.89
1
TABLE III
XPS ATOMIC RATIOS FOR COMPOSITES
Sample Atomic Fraction Ratio
No. - O/C
1A 0.29 0.17 0.-'45
1B 0.0023 0.13 0.023
2 0.18 0.15 0.027
3 0.08 0.20 0.031
4 0.16 0.16 0.032
5 0.031 0.16 0.025
6 0.033 0.15 0.040
7 0.41 0.16 0.019
8 0.31 0.16 0.064
9 0.29 0.18 0.030
10 0.19 0.11 0.035
11 <0.001 0.057 <0.001
12 u.UL'67 0.088 0.011
2.2W <0.001 0.080 0.023
9!
TABLE IV
i
t
CRITICAL SURFACE TENSIONS OF COMPOSITES
Sample No. Critical Surface Tension (mJ/m2)
1A 23
2 31
3 37
4 33
5 35
6 40
7 23
8 28
9 31
10 37
11 40
12 40.5
f
ri
0.5
OF POOR QUALITY
0	 10	 20	 30	 40	 50
CR I 1 1 CAL SURF NC E I ENS I lIN (MJ/SO M)
FIG. I -CHANGES 1N THE RATIO OF FLUORINE TO CARBON
AS RELATED TO THE CRITICAL SURFACE TENSION
MEASURED IN MILLIJOULES PER SQUARE METER
r.
r
i
F 0.4
U
0
R
N
E 0. 3
0
c
A
R 0.2-,
B
0
N
R
A
T	 0. 1
I
0
0.0


Surface analysis in composite bonding

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