Graphite surfaces are not wet by pure copper. This lack of wetting is responsible for a debonding phenomenon that was found in continuous graphite fiber/copper matrix composites materials subjected to elevated temperatures. By suitably alloying copper, its capability to wet graphite surfaces can be enhanced. In situ measurements of graphite/copper alloy wetting angles were made using the sessile drop method. Interfacial energy values were calculated based upon these measurements

Devincent, Sandra M.

Ellis, David L.

Michal, Gary M.

English

NASA Technical Reports Server

PfQ07(0
NASA Contractor Report 187087
Graphite/Copper Alloy Interfacial
Energies Determined Using the
Sessile Drop Method
Sandra M. DeVincent, David L. Ellis, and Gary M. Michal
Case Western Reserve University
Cleveland, Ohio
March 1991
Prepared for
Lewis Research Center
Under Cooperative Agreement NCC3-94
NASA
National Aeronautics and
Space Administration
https://ntrs.nasa.gov/search.jsp?R=19910010971 2020-03-19T18:34:00+00:00Z
GRAPHITE/COPPER ALLOY INTERFACIAL ENERGIES DETERMINED USING
THE SESSILE DROP METHOD
Sandra M. DeVincent,' David L. Ellis,' and Gary M. Michal
Case Western Reserve University
Department of Materials Science and Engineering
Cleveland, Ohio 44115
ABSTRACT
Graphite surfaces are not wet by pure copper. This lack of wetting has been responsible for a
debonding phenomenon that has been found in continuous graphite fiber/copper matrix composite
materials subjected to elevated temperatures. By suitably alloying copper, its capability to wet
graphite surfaces can be enhanced. In situ measurements of graphite/copper alloy wetting angles
have been made using the sessile drop method. Interfacial energy values have been calculated
based upon these measurements.
INTRODUCTION
Continuous fiber reinforced graphite/copper (Gr/Cu) composites offer potential for improved
performance as high heat flux structures for elevated temperature applications requiring high
thermal conductivity. These composites are candidate materials for applications such as space
power radiator panels and as leading edge and engine heat exchanger materials for hypersonic
aircraft. Gr/Cu composites made from pitch-based fibers possess a high thermal conductivity
equivalent to copper. They also exhibit a high modulus of elasticity equivalent to beryllium, and
moderate density similar to titanium [ 1 ].
The fabrication and performance of composite materials designed for elevated temperature
applications is strongly influenced by the fiber matrix interface. A key aspect of metal/ceramic
interfaces is the metal/ceramic interfacial energy. However, often the ideal fiber/matrix
combinations, based upon their mechanical and physical properties, have high interfacial energies
[2]. The high interfacial energies relative to the surface energies of the fibers results in a lack of
wetting between the metals and fibers. This intrinsic lack of wetting causes difficulties in
production of the composites, as shown in Figure 1. More importantly, the lack of wetting can
lead to matrix fiber debonding and pore formation during the service life of the composites at
elevated temperatures [1, 31.
Wettability studies using the sessile drop method have been conducted for the Gr/Cu system.
Many investigators have noted that pure Cu does not wet Gr. Mortimer and Nicholas [4] have
reported that small additions of Cr reduce the contact angle formed by Cu on HX30 Gr and
vitreous carbon and that small additions of V reduce the contact angle on vitreous carbon in
studies conducted in vacuum. Nogi et al. 151 have conducted similar experiments in an Ar-51 ­12
atmosphere on single crystal Gr. They have concluded that Gr is wet by Cu-Cr alloys when the Cr
content is high enough to create a continuous Gr/Cu-Cr interface. Kishkoparov et al. [6]
conducted sessile drop tests in helium between 1250 and 1550°C using Gr powder. The alloying
'Graduate Student Intern at Lewis Research Center.
'NASA Resident Research Associate at Lewis Research Center.
additions investigated include Fe, Co, Al, Ni and Sri. They have reported that alloys with more
than 10% Fe or Co, and more than 3% Al wet Gr powder. The trends indicated by these studies
are similar, but there are variations in the data due to the differences in materials and experimental
techniques.
Figure 1. Copper 50 v/o P100 graphite fiber composite.
An experimental program has been conducted to assess the effect of alloying additions to Cu on
the wetting angle established between the Cu-alloys and the polished surfaces of monolithic Gr
utilizing a sessile drop technique. The alloying elements chosen are those which have limited
solubility in solid Cu and are potential carbide formers. Most of the alloying elements chosen
tend to strongly segregate to interfaces. Because the Gr/Cu composites are being considered for
thermal conductivity applications, the additions of alloying elements were minimized with the
intention of minimizing the thermal conductivity decrease. For this reason, an effective alloying
addition is defined as one that produces the greatest decrease in contact angle, from that of pure
Cu, for the lowest atomic percent addition.
EXPERIMENTAL PROCEDURE
Nominal additions of 0.5 at% and 1.0 at% of Cr, Fe, Hf, La, Mn, Nb, Si, Ta, Ti, V, Y and Zr were
alloyed with Cu to produce alloys for sessile drop evaluation. Tests were also run using pure Cu
as a standard.
Sessile drop tests were conducted in a 1 atm argon atmosphere. For each alloy, two to five
individual tests were run on separate occasions. Commercial grade H-490 Gr discs of 2.54 cm
diameter were polished to a 1 µm finish and placed directly on the pancake induction coil of the
furnace. A Cu-alloy test specimen of 1.27 cm diameter was placed on top of the Gr.
Temperatures were monitored using a two wavelength optical pyrometer measuring the
temperature of the Gr substrate. To minimize the thermal gradient the Gr was surrounded by
Al2O3 insulation. Separate tests with thermocouples indicated the thermal gradient between the
Gr and Cu was <_ 5°C.After achieving a vacuum < 4.6 x 10 -3 Pa, the furnace was back-filled with
argon. This procedure was repeated to allow additional flushing of the system. The Cu alloy and
the Gr were simultaneously heated to the melting point of the alloy. The test temperature was then
raised to a 50°C superheat and held for 3600 s. The test was recorded on videotape while the
temperature was recorded on a strip chart recorder. The contact angles were measured at 300 s
intervals during the test using a video monitor .
2
The angle measurements obtained were used in the Young-Dupre equation in order to calculate the
Gr/Cu interfacial energies. For Gr/Cu, the Young-Dupre equation is as follows:
yGr/Cu = YGr/v - 9(Cu/v cosh
where yGr/Cu is the Gr/Cu interfacial energy, yGr/v is the free surface energy of Gr, yCu/v is the
free surface energy of Cu and 0 is the wetting angle.
RESULTS
Chemical Analysis
The compositions of the alloys tested, their oxygen contents and the amount of addition not bed up
as an oxide are listed in Table 1. The chemical analyses of these alloys indicate considerable
variation from the intended targets of 0.5 at% and 1.0 at% due to difficulties encountered in
melting.
Table 1. Chemical composition of copper alloys
Available
Alloy no. Addition wt% at% O, ppm addition, at%
1 Si 0.3 0.68 49 0.671
2 Si 0.4 0.90 12 0.898
3 Ti 0.041 0.054 27 0.049
4 Ti 0.041 0.054 658 -0.076
5 V 0.3 0.37 34 0.357
6 V 0.8 1.00 57 0.977
7 Cr 0.4 0.49 10 0.487
8 Cr 0.5 0.61 66 0.593
9 Cr 0.8 0.98 13 0.977
10 Cr 0.9 1.10 16 1.106
11 Cr 1.0 1.22 11 1.217
12 Mn 0.4 0.46 26 0.45
13 M n 0.9 1.04 10 1.036
14 Fc 0.1 0.11 16 0.104
15 Fe 0.4 0.45 10 0.446
16 Y 0.2 0.14 37 0.13
17 Y 0.2 0.14 404 0.033
18 Zr 0.1 0.07 10 0.068
19 Zr 0.4 0.28 295 0.222
20 Nb 0.2 0.14 34 0.127
21 La 0.038 0.017 20 -0.002
22 H 1.0 0.36 46 0.351
23 H f 2.0 0.72 254 0.669
24 Ta 0.3 0.11 20 0.107
25 Ta 0.4 0.14 76 0.128
26 Ta 0.8 0.28 18 0.277
Contact/Wetting Angle Measurements
Photographs of the test specimens were taken from each of the videotapes. An example is shown
for pure Cu in Figure 2. Contact angle data as a function of alloying element are given in Table 2.
The values reported are for the equilibrium angle. Most alloys reached equilibrium within 100 s
after melting. Based on all the measurements, a pooled standard deviation for the contact angle
measurements was calculated to be 16 0. The scatter due to the optical measurement system is
much less than this value. The major source for error is believed to be the presence of surface
oxide on the Cu-alloy test specimen. The kinetics of oxide film formation on the Cu-alloy free
surface varies for each alloying element, and therefore the stability of the surface oxide also varies.
Additions of Cr, Hf, V, Y and Zr significantly decrease the wetting angle from that of pure Cu. As
indicated in Table 2, additions of less than 0.1 at% La or Ti result in a slightly decreased contact
angle, whereas much greater additions of many of the other alloying elements are needed to
produce the same effect. The contact angle measurements as a function of Cr addition are given in
Figure 3. By increasing the amount of Cr addition, the contact angle of the Cu alloy on Gr can be
reduced. The results for the Cu-Cr alloys correspond to those found by others [4, 5, 7].
Figure 2. Sessile drop test specimen. Pure copper: average contact angle 157°.
180
150
a^
ti
n 120
a^
m 90
a
v 60
m
U	 30
0
8 °
O
O
O
O O
0.0
	
0.2	 0.4	 0.6	 0.8	 1.0
	
1.2
Alloying addition, atomic % Cr
Figure 3. Contact angle as a function of Cr Addition.
4
Table 2. Graphite/Cu-alloy interfacial energy
Cu-alloy
addition at%
contact
angle
YGr/Cu
mJ/m2
Cu-alloy
addition at%
contact
angle
YGr/Cu
mJ/m2
Cu 157 1975 Fe 0.11 146 1858
0.45 135 1702
Si 0.68 157 1975
0.90 150 1906 Y 0.14 111 1256
0.14 144 1833
Ti 0.054 128 1585
0.054 119 1418 Zr 0.07 113 1298
0.28 111 1256
V 0.37 128 1585
1.00 45 -106 Nb 0.14 140 1778
Cr 0.49 140 1778 La 0.017 138 1748
0.61 114 1318
0.98 60 159 Hf 0.36 90 798
1.10 45 -106 0.72 80 576
1.22 41 -167
Ta 0.11 130 1620
Mn 0.46 142 1858 0.14 132 1654
1.04 135 1702 0.28 142 1806
Gr/Cu Interfacial Energy Calculations
Using the Young-Dupre equation and assuming values for the free surface energy of Cu and the
free surface energy of Gr, estimates of the Gr/Cu interfacial energy can be made. Values of 1279
mJ/m 2 for the Cu [8, 91 and 798 mJ/m 2 for the Gr [41 were used in the calculations. The value for
Cu was assumed to be constant with alloying level. Interfacial energy values are listed in Table 2
for the Cu-alloys.
Some degradation of Gr surfaces, indicated by discoloration found in the areas not covered by the
Cu-alloy test specimens, was observed. This may indicate some reaction was occurring at the Gr
surface which may change the value of YGr/,.
Scanning Electron Microscopy
Initial studies of the Gr/Cu-Cr interface have been conducted using the scanning electron
microscope. In the case of Cu-1.22 at%Cr, which wets and adheres to the Gr, a 3 µm thick
reaction layer forms at the interface as shown in Figure 4a. Energy dispersive spectroscopy (EDS)
analysis indicates that this is a Cr-rich layer as shown in Figure 4b.
DISCUSSION
Wetting Angle Measurements
5
The wetting angle measurements given in Table 2 indicate that La is most effective in reducing the
wetting angle per at% added, followed by Ti and Zr. Other elements that reduced the wetting
angle significantly include Cr, Hf, V and Y. All of the alloys melted within 50°C of the melting
temperature of pure Cu. However, the V alloys required a temperature of 1530°C to achieve a
completely liquid state. While microscopy of the interfaces has not been completed, it is believed
that these alloying elements segregated to the Gr/Cu interface and formed a reaction layer, most
likely a carbide layer. The degree of wetting is also believed to be affected by whether the carbide
layer is continuous or discontinuous as discussed by Nogi et al. [5]. Analysis of the Cu-Cr alloy
interfaces, such as the one shown in Figure 4a, confirm that the best wetting is obtained when a
continuous Cr carbide layer forms. Confirmation of the reaction layers and their compositions by
optical microscopy and SEM examination is currently ongoing.
(a. )	 (b.)
Figure 4. (a.) SEM micrograph. Cu-1.22 at% Cr sessile drop test specimen: reaction layer
approximately 3 µm. (b.) EDS spectrum from reaction layer.
Gr/Cu Interfacial Energy Calculations
The Gr/Cu interfacial energy was observed to be affected by alloying addition. The alloying
elements most effective in lowering the Gr/Cu interfacial energy are La, Ti and Zr. The value
calculated for pure Cu of 1975 mJ/m2 is decreased to 1418 mJ/m2 for an addition of only 0.054
at% Ti and to 1298 mJ/m 2 for 0.07 at% Zr. The Gr/Cu interfacial energy is also affected by the
amount of alloying element present. As seen in Table 2, increasing at% of all the additions
studied, with the exception of Ta, results in a greater decrease in interfacial energy.
The results of the calculations for the Gr/Cu interfacial energies shown in Table 2 indicate that, in
some cases, such as with Cr and V, negative values can be obtained. These values are obviously
incorrect. Three possible explanations exist for this discrepancy.
The first two possibilities concern the assumptions about the surface energies of the Cu and Gr.
The surface energy of the Cu was assumed to be constant with respect to both alloying element
and the amount of alloying element added. This assumption is probably not valid given the high
mobility of the alloying elements in the liquid Cu and the tendency of many of the elements to
segregate to interfaces. The presence of these alloying addition atoms at the Cu/vapor interface
2 .could decrease the surface energy to values considerably below the assumed value of 1279 mJ/m
The assumption that the surface energy of Gr is 798 mJ/m 2 may also not be correct. The surface
energy of Gr has a wide range of values depending on grade, contamination level, degree of
orientation, and the plane of the Gr exposed at the surface [4].
The third assumption is the question of which interfacial surface energy is actually being
measured. No allowance was made for the presence of a reaction layer at the interface such as that
shown in Figure 4a. To account for the reaction layer the surface energy of the reaction layer
should be used instead of the surface energy of Gr [7, 101. This in turn requires knowledge of the
interfacial reaction products and their surface energies. While the reaction products can be
determined experimentally, in most cases their surface energies are not known.
SUMMARY
Sessile drop tests have been conducted on twelve Cu-based alloys to determine if the wetting of Gr
by liquid Cu can be improved through alloying additions. Of the alloys studied, Fe, La, Mn, Nb,
Si , Ta and Ti did not wet H-490 Gr at the alloying levels examined. The additions of Hf, Y and
Zr decreased the wetting angles, but did not reduce the angles to much below 1 1 1° at the alloying
levels investigated. The additions of Cr and V at approximately the level of 1 at% were able to
enhance the wetting behavior such that wetting angles of 45°, or less, were produced. However,
because of the difficulties of dissolving V in liquid Cu, a temperature of 1530°C was needed to
achieve that degree of wetting.
ACKNOWLEDGEMENTS
The authors would like to acknowledge Mr. Ricky Rivera and Mr. Henry Geringer for the arc
melting and induction melting of the alloy bars, and Mr. Gerald Loveland for assistance with the
test set-up and design. The authors would also like to recognize support from the Defense
Advanced Research Projects Agency and the NASA Lewis Research Center in Cleveland, Ohio,
under the NASA Graduate Student Researchers Program under Grant no. 50-632.
REFERENCES
I D. McDanels and J. Diaz, "Exploratory Feasibility Studies of Graphite Fiber Reinforced
Copper Matrix Composites for Space Power Radiator Panels", NASA TM-102328 (1989).
2F. Weitzer, K. Remschnig, J. C. Schuster and P.Rogl, "Phase Equilibria and Structural
Chemistry in the Ternary Systems M-Si-N and M-B-N," J. Mat. Research, 5 [10] 2152-59 (1990).
3 B. Ash and S. Fowser, Personal communications.
4 D. Mortimer and M. Nicholas, "The Wetting of Carbon by Copper and Copper Alloys", J.
Mat. Sci., 5 [2] 149-155 (1970).
5 K. Nogi, Y. Osugi and K. Ogino, "A New Method of Wettability Measurement Utilizing a
Small Sample and Its Application to Graphite or a-SiC and Liquid Cu-Cr Alloy Systems," ISIJ, 30
[ 1164-9 (1990).
6N. Kishkoparov, V. Chentsov, I. Frishberg and V. Pastukhov, "Wetting of Natural
Graphite by Copper Alloys," Sov. P. Met. and Met. Cer., [11] 869-70 (1984).
7M. Nicholas and D. Mortimer, "The Wetting of Carbon by Liquid Metals and Alloys"; pp.
1-3 in proceedings of International Conference on Carbon Fibers, their Composites and
Applications, London, February 2-4, 1971.
8L. Murr, "Interfacial Free Energy"; p. 102 in Interfacial Phenomena in Metals and Alloys.
Addison-Wesley Publ. Co., Reading, MA, 1974.
9W. Kingery, "Surface Energy"; p. 368 in Property Measurements at High Temperatures.
Wiley and Sons, Inc., New York, 1959.
10R. Loehman, "Interfacial Reactions in Ceramic-Metal Systems," Am. Ceram. Soc. Bull.,
68 [4] 891-92 (1989).
NASA	 Report Documentation PageNational Aeronautics and
Space Administration
1.	 Report No. 2. Government Accession No. 3.	 Recipient's Catalog No.
NASA CR — 187087
4.	 Title and Subtitle 5. Report Date
Graphite/Copper Alloy Interfacial Energies Determined Using the Sessile Drop March 1991
Method
6. Performing Organization Code
7. Author(s) 8. Performing Organization Report No.
Sandra M. DeVincent, David L. Ellis, and Gary M. Michal None	 (E - 6076)
10. Work Unit No.
505-63-40
9. Performing Organization Name and Address
11. Contract or Grant No.Case Western Reserve University
Department of Materials Science and Engineering NCC3-94
Cleveland, Ohio 44115
13. Type of Report and Period Covered
Contractor Report
Final
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
14. Sponsoring Agency CodeLewis Research Center
Cleveland, Ohio 44135 - 3191
15. Supplementary Notes
Project Manager, Hugh R. Gray, Materials Division, NASA Lewis Research Center. Sandra M. DeVincent, Case Western
University and Graduate Student Intern at Lewis Research Center. David L. Ellis, Case Western Reserve University and
NASA Resident Research Associate at Lewis Research Center. Gary M. Michal, Case Western Reserve University.
Prepared for the Second International Ceramic Science and Technology Congress—Symposium on Composites, sponsored
by the American Ceramic Society, Orlando, Florida, November 12 - 15, 1990.
16. Abstract
Graphite surfaces are not wet by pure copper. This lack of wetting has been responsible for a debonding phenomenon that
has been found in continuous graphite fiber/copper matrix composite materials subjected to elevated temperatures. By
suitably alloying copper, its capability to wet graphite surfaces can be enhanced. In situ measurements of graphite/copper
alloy wetting angles have been made using the sessile drop method. Interfacial energy values have been calculated based
upon these measurements.
17. Key Words (Suggested by Author(s)) 18.	 Distribution Statement
Copper; Graphite; High conductivity composites; Interfacial Unclassified - Unlimited
energy; Wetting Subject Category 26
19.	 Security Classif.	 (of the report) 20.	 Security Classif. (of this page) 21.	 No. of pages 22. Price'
Unclassified Unclassified 10 A02
NASA FORM 1626 OCT 86	
*For sale by the National Technical Information Service, Springfield, Virginia 22161


Graphite/copper alloy interfacial energies determined using the sessile drop method

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