THE EFFECT OF CAR,BONSTR,UCTUREON COPPER,OXIDATION
AT A SLIDING ELECTR,ICAL CONTACT
P. M. Scrrnnnn aNn W. J. Spnv
Research, Laboratory, N o,tiono,l, C arban C ompang.¡
Di,uision of Uni,on Carbide Corporati,on, Pctrmct, Ohio
-
(Ma,nuscript received September f 8, l96l)
The surface chemistry involvecl in the formation of a copper oxido film at the interface of a sliding,
currenl carrying, carbon-copper contact has been studied. It was found that the rate determinine
temperature for the growth of this oxide layer was essentially the bulk temperature of the copper. An
explanation of observed differences in the steady state ühickness of the oxide layer formed bv á series of
electrographitic brush grades has been proposed in terms of the carbon structure. These differences are
specifically related to the number ofload bearing zones and true conduction spots (a spots) at the interface.
The study has been repeated at other bulk temperatures ofthe copper slip-ring and extended by observations ofa variety ofdifferent brushes. The orginal interpretation
is ccnsistent wiüh these added experimental results. but peculiar structural factors have been observed in the oxide films.
I.
INTR,ODUCTION
The known chemical activity of fresh carbon
surfaces made it of interest to determine
whether cuprous oxide rvould be affected by
carbon in a sliding electrical contact formed
by a carbon brush on a copper slip-ring. The
acid and baseabsorbingpropertiesofcarbonl,2
make possible one mode of chemical interaction when an adsorbed water film exists at the
interface.3 Direct oxidation or reduction
effects between the solids also are possible
since high local temperatures are known to
exist at zones of true contact in such sliding
systems.4 In addition to these direct effects,
the flow of current through the contact also
can alter the copper oxide layer in two ways
if the effective temperature of the copper is
low, say belou' 100'C. The applied voltage
produces an electric field that changes the
r B. Sleenberg, Atlsorption and, Erchange of Ions
on Acti,uated,Clutrcoal,Almquist and Wiksells (1944).
2 V. Gorten, D. Weiss and J. lVillis, Aust. J. CILem.
10, 2e5 (1957).
3 E. I. Shobert, A.I.E.E. Tro,ns.73,788 (August,
r954).
a F. P. Bowden and D. Ta,bor, The Fr'iction a,nd,
Lubrication of Solid,s, Oxforcl University Press (1950).
oxidation rate. fn addition, electrochemical
oxidation and reduction c&n occur when
moisture is present. These latter effects are
controlled by the ph¡zsical structure of the
carbon brush. An initial attempt to study
such a system has been reported in a separate
publication5 and the present paper is concerned with continued work on the subject.
A sliding electrical contact permits the
interaction of solids in a manner that is
unique in several respects. First, the mechanism of wear is such that fresh surfaces are
being produced continually at the common
interface. There is a finite probability of
interaction between the solids before these
fresh surfaces are passivated by adsorbed
films from the surrounding atmosphere.
Second, the pressures and temperatures at
zones of true contact can be very high,
approaching the yield point and melting point
of the solids. Third, when current flo,w's
through the contact, electric fields and true
current densities are produced that are larger
than can be studied readily in stationarv
systems.
553
5 W . J . S p r y a n d P . M . S c h e r e r ,W e a r 4 , 1 3 7 ( 1 9 6 I
).
554
FIFTII
CABBON
CONFERENCD
The physical state of the interface possesses
the following characteristics that can be
described with particular emphasis on the
materials used in this experiment. The
electrical contacts were formed with National
Carbon electrographitic brushes of the SA
series operating on oxygen free, high conductivity copper. With a spring loading of 0.21 kg
on an SA-25 brush of 0.51 x 1.90 cm cross
section, true contact is obtained against a
copper-copper oxide surface at about 60 load
bearing zones. In a sliding system the load
bearing zonesvary in area in a random fashion,
but if a roughly circular shape is assumed per
zone, the average diameter is about 6.6 x
10-a cm and the total area in true contact is
about 1.9 x 10-o cmz. The SA seriesbecomes
progressively "softer" as the SA number
increasesand, with the same 0.21 kg force, an
SA-50 brush would contact the copper at
about 600 separate zones. During the process
of wear, fresh surfaces of carbon are produced
at these contact zones and the extent of the
carbon-copper oxide interaction can be
related to the number present.
A review of the current conduction mechanism between sliding solids is useful in
understanding the experimental results. It
has been shown by Holm6 that most current
flows through contact, zones at which carbon
and copper touch. These are designated as a
spots and at a few such spots (called major a
spots) the copper melts. When the effects of
mechanical vibration are minimized or absent,
and at reasonable current densities, the major
a spotsareremarkably stable.T In the present
study operating conditions were chosen such
that one or two stable major a spots would
exist. The primary flow of current was thereby
confined to a well-defined region ofthe copper
oxide. IJnder such conditions a relatively
stable electrical circuit was established.
Current through the major a spots stabilized
the contact drop while, over the rest of the
oxide surface, conduction occurred through
randomly located ordinary a spots or through
the copper oxide film.
It is also worthwhile to review the mechanism of copper oxidation in order to understand the relationship between the measured
thickness of copper oxide and the effect of
different carbon brushes on this oxide. The
rate limiting step is the diffusion of copper
ions through the oxide. At temperatures
below about 100'C, this diffusion rate is
controlled by the electric field produced by
surface oxygen ions adsorbed from the
surrounding atmosphere. At temperatures
somewhat above this value, thermal energy
alone permits a sufficient diffusion of the
copper ions.
As a consequence of this mechanism of
copper oxidation, two electrical effects can
modify the oxide in a sliding electrical contact
if the effective surface temperature is low.
These are controlled by the physical characteristics of the interface as well as by the conduction mechanism. tr'irst, if a surface layer
of water exists, or a hydrated surface of
cuprous oxide, an ionic bridge is formed
between the solids and electrochemical oxidation or reduction can occur depending on the
relative carbon-copper polarity. Curr¿nt flow
is limited by the contact voltage and the
resistivity of the oxide. Second, on those
oxide areas where the surface is effectively
"d.y," and also over all regions near true
contact zoneswhere physical separation ofthe
solids is less than about 300 A, an electric
field exists which may be as large as 106Vicm.
This field can either aid or inhibib the electric
field of the adsorbed oxygen ions and thereby
affects the rate of oxidation.e At high temperatures these polarity sensitive effects are
expected to disappear since thermal diffusion
ofcopper ions now controls the oxidation rate.
6 R. Holm, Electric Contacts Ha,ndbook, Springer,
Berlin (1958).
z E. I. Shobert, A.I.E.E. Trans.73,788 (1954).
8 N. Cabrera and N. Motü, Rep. Progr. Phys. 12,
163 ( 1948-1949).
COPPDR, OXIDATION
AT A SLIDING EIJDCTRICAL CONTACT
If all of these effects are considered, it
becomes possible tr relate the thickness of
copper oxide formed beneath a positive or a
negative brush to various factors of brush
structure and to the effective temperature at
which oxidation occurs. One likely condition
is that the bonding forces between carbon and
cuprous oxide are large and that considerable
onergy is dissipated in rupturing each bond
during sliding. If this is so, high local tempe:atures qill rcsult at carbon-copper oxide load
bearing zones, and thermal diffusion ofcopper
ions through the oxide will be the rate controlling factor in copper oxidation. Consequently, the oxide thickness would be.
independent of the polarity of the brush and.
the high temperature oxidation-reduction
effect of carbon would be the important
chcmical structure factor. Conversely, if
local temperatures at load bearing zones are
low, the two electrical effects mentioned above
become important and the measured oxide
thicknesses would have two characteristics.
First, a strong polarity effect rvould be
observed, and second, the number of load
bearing zones at the interface would alter the
oxide thickness in a predictable manner
dependent on the physical structure of the
carbon.
, The marked polarity effect in measured
oxide thickness found in earlier work for a bulk
copper temperature of 70 "C is shown in X'ig. I .
A separate measurement of the growth of the
oxide layer vs. time also has indicated that the
effective temperature controlling the surface
chemistry at the carbon-copper interface is
verynearly the bulktemperature ofthe copper.
The change in the effect of brush polarity v-ith
brush grade as indicated by Fig. I also supports this conclusion. To understand this
variation with brush grade, the effects of both
eletrochemical action and the electric field
acrossthe interface must betaken into account.
As the average number of true contact areas
increases,the effect of the electric field across
the interface becomes relatively more important than electrochemical oxidation or
555
reduction. As a result, the ratio of oxide
thickness under the positive brush to that
under the negative brush should decrease.
Under these conditions the physical structure
of the carbon appears to alter indirectly the
surface chemistry.
R BE8
rL -*B*R*U *S Hl J
Frc. l. Film
thickness vs. brush polarity
the SA series.
for
If this interpretation of experimental
results is valid for the system, then the
polarity effect on the copper oxide would. be
fairly independent of temperature below
100'C. The present experiment was an
attempt to verify this point.
II.
EXPEBIMENTAL
A. Operating Cond.itions
Standardized operating conditions lvere
usedthroughout. Each brush set was operated
continuously for 48 hr on a 7.6 cm diameter
slip-ring made from oxygen free, high conductivity copper. The ring rotated at b500rpm
in filtered air at 20"C and 50o/o relative
humidity. To investigate the effect of the
electric field, and of the djrection of current
florv on the system, the positive and negative
brushes were operated on separate tracks on
the slip-ring. These operating conditions
556
I.IN'TH CAR,]]ON CONrORENCE
were selected so that stable non-sparking
operation rvaspossiblewith a variety of carbon
grades. In addition, care was taken to
minimize mechanical vibrations which could
causecontact separation. Any contact arcing
damages the oxide layer and masks the effect
of the carbon structure on the surface chemistry.
and X'lom.l5 Figure 2 illustrates the method.
A 0.0076 cm diameter platinum probe (D) is
placed on the oxide layer being measuredand
a ground clamp (Z) is fastened to the bare
copper of the slip-ring. Point, (G) is connected
SLIP-RINGCONNECTIONS
PLATINUMWIRE
.OO3..DIAMETER
RESTINGON SURFACE
B. MeasurementTechni,ques
The.experimental techniques commonly
used in oxidation studies are gravimetric,
coulometric and optical interference methods.
Both the gravimetric and the interference
methods are impractical for use with a sliding
system. The coulometric method, originally
developed by Evans and Bannistere and by
Mileyto was used as a control for the present
series of experiments. With proper techniquelr,rz,rathis method is very sensitive and
makes it possible to determine the amount of
cuprous and cupric oxide on one film. The
structure of the films formed under the con.
ditions of this experiment consisted of a layer
of cuprous oxide covered by a deposit of
carbon. This is consistent with the earlier
work of Van Brunt and Savage.la
In addition to this electrolyt'ic method of
determining the quantity of oxide formed at
the interface, another independent measurement wás made in each case. In the field of
electrical contacts a common technique for
examining surface films makes use of the fact
that a dielectric such as cuprous oxide will be
punctured when an applied electric fielc
exceeds a certain value. This technique has
been used by Holm,6 Shobert,T and Savage
e V. R. Evans and L. C. Bannister, Proc. Rog. Soe.
A125, t529,370.
1 0H . M i l e y , J . A m . C h e m . S o c . 5 9 , 2 6 2 6 ( f 9 3 7 ) .
11 W. E. Campbell and U. B. Thomas, ?rarzs.
Electrochem.,Soc.105, 303 (1958).
12J. A. AIIen, Ira,ns. Iaradag Soc.48,273 (1952).
rsT. Mills and U. R. Evans, J. Chem. Soc. 2182
(1e56).
ra C. Van Brunt
and R,. H. Savage, General
Electric Ret:'ieu 47, f 6 (f 944).
coPPER t..
E
HEAVYCLAMP
ON SIDESOF
SLIP-RING
NOT TOUCHING
THE FILM
SLIP-RING
A
c+
CuzO+
Cu+
SEC.A-A ENLARGED
VOLTAGE
FILM PUNCTURING
Frc. 2. Circuiú diagram for electrically puncturing copper oxide films.
electrically to (D), and (f1) is connectedfo @).
The voltage bet'ween points (G) and (11) is
slowly increased until dielectric puncture
occurs. At puncture, the voltmeter reading
drops abruptly. The maximum voltage attained at puncture divided by the dielectric
strength of the cuprous oxide is then used to
compute the film thickness. A dielectric
breakdown field of l0o V/cm was used as a
reasonablevalue.6
The nature of the film makes any single
dielectric breakdown measurement meaningless and the problem must be treated statistically. Figure 3 shows a typical result when
one hundred different points on both a
positive and negative fi,lm are plotted vs. the
number of times each breakdown voltage
occurred. The result is a Poisson distribution
of correct form and a number related to the
most probable puncture voltage can be
determined from the peak of the distribution.
15 R. H. Savage and D. G. Flom, General Electri,c
Reaiew2,59 (1955).
COPPER, OXIDATION
AT A SLIDING ELECTRICAL
O-NEGATIVE BRUSH
O-POSITIVE BRUSH
F
É
so
G
L
ozs
É
U
6
820
z
CONTACT
correlating the apparent film thicknesses
measured by the two methods. In the present
seriesof experiments the oxide layers were all
uniform and firmly bonded, except in particu_
lar instances noted later. In general, the
experimental results are reported on the basis
of the dielectric puncture voltage method.
The coulometric method was used as an
experimental check and to detect any
structure differences. A recent review of
problems associated with such measurements
has been assembled by Ronnquist and Fischmeister.l6
III.
o2468to
(VOLTS}
PUNCTURE
VOLTAGE
Frc. 3. Film thickness distribution
of copper
oxide ys. brush polarity for SA-25 carbon
brushes.
DDI
RESULTS
IJsing the techniques described above, the
SA series was examined with bulk slip-ring
temperatures controlled at 8b * 2"C, 70 !
zoc,50 + loC, and 40 + l.C. Figure 4 shows
Q+oo
The dielectric puncture and coulometric
a
reduction methods measure different aspects
a
L¡J
z 300
of the oxide layer. The dielectric puncture
Y
(J
method, since it depends on a probe making
I
F
mechanical contact with the oxide layer, is
2 20O
sensitive to the hardness and mechanical
)
LL
strength of the oxide. The coulometric
J
I too
reduction technique measuresthe total quanE
tity of oxide covering a given area without
F
u
indicating the mechanical properties of that
v
trl
layer. To characterize a given oxide layer,
25
30
35
40
45
50
both techniques are useful.
SA NUMBER
As a calibration, a uniform, firmly bonded
Frc. 4. tr'ilm thickness vs. temperature
for
oxide layer was formed on a copper strip
copper oxide on positive brush tracks.
slowly oxidized in an oxygen atmosphere at
100"C. Inthis caseboth of the above methods the results obtained on the positive brush
indicated the same oxide layer thickness. tracks and on the slip-ring. As the slip-ring
temperature was lowered below 70"C, it
Oxide layers which were formed either rapidly
at elevated temperatures or by the action of became impossible to obtain realistic film
measurements beneath the track of the
chemical oxidizing agents were not uniform
or firmly bonded. Results obtained by measur- negative brush. The dependence of the film
ing the distribution of dielectric puncture thickness beneath the positive brush versus
values were not, valid in such cases. This is brush grade was found to be independent of
attributed to the mechanical damage done to
the structurally weak film by the probe used.
16 A. Ronnquist and H. Fischmeister, J. Insti,
A measure of film structure can be made bv M e t a l s 8 9 , 6 5 ( 1 9 6 0 6 l ) .
Trn'Trr caRBoN CONFDRDNCE
558
temperatrrre as expected. The change in the
ratio of the thickness of the positive track to
that of the ring seemsto be a direct chemical
effect ofthe carbon. Figure 5 is a plot ofdata
for both positive and negative brush tracks at
70 and 85'C. The changing behavior of the
negative track seemsto be related to changes
in the rate of carbon deposition as a function
of temperature.
ftfggs¡..*
25
30
35
40
45
SA NUMBER
50
Frc. 5. X'ilm thickness vs. temperaüuro on positive and neqative brush tracks.
€ 2o0
a
u't
l,¡,1
Z, t<a
lJv
\¿
(J
I
t-
=
ro0
J
tJ;-t
<^
IY.
c)
l¡l
t¡J
temperature of 70 _f 2"C. IJnder this condition the negative brush controls the system.
Such a result clearly implies that the interpretation just proposed is not complete. If it
were, the common track oxide would attain a
thickness equal to the average value obtained
on the positive and negative tracks as shown
in n'ig. 1. The observed difference appears to
be related to another aspect of the measured
film structure. I]nder the conditions of this
set of experiments, carbon eventually is
deposited on the slip-ring from the negative
brush. This irreversibly conditions the surface
areas touched by the negative brush and
consequently controls the system. There is a
time factor, a temperature dependence, and
a relationship to carbon structure involved in
the deposition of carbon. However, to date it
has been impossible to obtain reliable data
concerning the amount of carbon deposited.
Figure 6 also indicates the agreement obtained
between coulometric and dielectric measurements of the oxide films in separate experiments. One point for SA-50 brushes is of
particular interest. The low puncture voltage
result is due to sparking in that group oftests.
In general, the coulometric result will be high
and the dielectric puncture voltage low when
sparking occurs. This is emphasized to indicate some of the pitfalls of film studies in this
area.
v
25
30
35
40
45
SA NUMBER
50
Fre. 6. Film
thickness measuremenüs when
positive and negative brushes are operated on a
eommon traek.
n'igure 6 shows film measurements obtained
when positive and negative brushes were
operated on a common track wiúh a slip-ring
CONCLUSIONS
In conclusion, this work is added evidence
that the effective temperature controlling the
chemistry at the copper-carbon interface is the
bulk temperature of the copper. The polarity
effect can be explained in terms of chemical
knowledge about the mechanism of copper
oxidation. The results obtained when brushes
operate on a common track indicate an
additional effect due to the polarity sensitive
deposition of'carbon on the slip-ring.