Note: Descriptions are shown in the official language in which they were submitted.
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M$THOD OF FO~ QÇLkoeOSI~E_ELE~BICAL
CONT~ HA~ING CARBONA~EQUS SECONDARY
This invention is related to electrical contacts
and a method of forming electrical contacts, for example,
used in switches and circuit breakers, and more particularly
to composite electrical contacts and a method of forming
composite electrical contacts having a metal matrix, such as
silver or copper, and a carbonaceous secondary phase, such as0 graphite, tungsten carbide, or mixtures thereof.
Back~roun~ of ~hc-~nyeD~isn
Electrical contacts, make, carry, and break
electrical circuits passing through, for example, circuit
breakers and switches. The contacts are made of either
elemental metal, alloys, or composites, for example, silver,
silver alloys, or silver composites. Some representative
materials added as a secondary phase in the matrix of
composite contacts are graphite, refractory metals such as
tungsten or tungsten carbide, and metal oxides such as tin
and cadmium. Graphite serves as a lubricant and ensures a
smooth, low-friction contact surface to prevent welding or
sticking when arcs form between contacts. Silver-graphite
composite contacts are soft compared to other types of
contact materials, and electrical and mechanical erosion is
more rapid. Refractory metals such as tungsten carbide offer
2S good mechanical wear resistance and resistance to arcing.
Silver-tungsten carbide composite contacts can withstand
higher currents and more arcing, with greater resistance to
sticking and erosion.
Silver-graphlte composite contacts are produced by
a well known press-sinter-re-press process. ~lended powders
of silver and graphite in a desired composition are compacted
to the contact shape by pressing at about 275 MPa. The
pressed compacts are sintered between about 700 C and 900 C
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in an inert or reducing atmosphere. After sintering the
compact is further densified by a second pressin~, or re-
pressing, at about 600 to 900 MPa. Sometimes properties are
modified by additional sintering, annealing, or re-pressing
steps.
U.S. Patent 4,699,763 discloses a method of forming
s:ilver-graphite composite electrical contacts comprised of
pressed and sintered powder having from about 0.5 to about 10
weight percent of graphite fiber particles, from about 0.1 to
about 3 weight percent of a powder wetting agent selected
from the group consisting of nickel, iron, cobalt, copper,
gold, and mixtures thereof, and the remainder silver. The
composite is formed by mixing the silver powder, graphite
fiber particles, and powder wetting agent in a solution of a
volatile hydrocarbon solvent and a lubricant selected from
the group consisting of polyethylene, paraffin, and stearic
acid. The mixture is dried to eliminate volatile solvent,
screened, and pressed at about 7.5 to 10 tons per square inch
to form a solid briquet. The solid briquet is heated to bake
out the lubricant, and sintered at a temperature of about
1500-F to 1700-F in a reducing atmosphere. The sintered
briquet is re-pressed at about 50 tons per square inch, re-
sintered at about 1500-F to 1700-F in a reducing atmosphere,
and again re-pressed under a pressure of about 50 to 60 tons
per square inch.
Electrical contacts can be subject to severe
mechanical and electrical strains. The service life may vary
from a few operations, for example in a detection system, to
100 million cycles in automotive vibrators or 40 years in
telephone relays. The strength of the composite contact is
important to provide a long service life when the contact
experiences many opening and closing cycles during the life
of the contact. The additional re-pressing and sintering
steps performed in making composite electrical contacts by
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the known methods are required to improve the strength of the
composite contact.
It is an object of this invention to provide a
simplified method of forming composite electrical contacts
having a conductor metal matrix and a carbonaceous secondary
phase of graphite, tungsten carbide, or mixtures thereof.
It is another object of this invention to provide a
simplified method of powder forming composite electrical
contacts having a silver matrix and a secondary phase of
graphite, tungsten carbide, or mixtures thereof.
It is a further object of this invention to provide
composite electrical contacts having a conductor metal matrix
and a carbonaceous secondary phase of graphite, tungsten
carbide, or mixtures thereof, wherein the conductor metal
matrix has improved strength.
~ri~f De~cLi~ion Q~ the, Tnventiom
An improved composite electrical contact is
comprised of about 0.5 to 50 volume percent of a carbonaceous
secondary phase, and the remainder a conductor metal matrix
of sintered conductor metal grains, the metal matrix being
substantially free of carbon in the matrix grain boundaries.
Freedom from carbon contamination in the matrix grain
boundaries provides improved interparticle bonding in the
matrix and a stronger composite electrical contact.
A method for forming the improved composite
electrical contacts comprises providing a powder comprised of
a predetermined volume percent of the carbonaceous particles,
and the remainder the conductor metal. The carbonaceous
particles having a coating of a metal from the group
consisting of silver, nickel, chromium, and mixtures thereof,
sufficient to minimize flaking of carbon from the particles.
Preferably, the coating is deposited by electroless plating,
and the carbonaceous particles are particles from the group
consisting o~ graphite, tungsten carbide, and mixtures
thereof. As used herein, the term "graphite" includes all
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forms of elemental carbon, including amorphous,
polycrystalline, or single crystalline powders, whiskers, or
fibers. The powder is consolidated into a compact of the
electrical contact by cold pressing, and sintered in a
reducing atmosphere or vacuum to increase density without
causing substantial gas formation within the compact.
Optionally, the sintered compact can be re-pressed to
incxease density.
B~ief DeS~Li~ti~n ^~
FIG. 1 is a graph of dilatometer measurements on a
silver compact heated over a range of temperatures up to
about 940 C.
Detailed Descri~tion of the Invention
We have discovered improved composite electrical
contacts, and a simplified method of forming the improved
composite electrical contacts having a conductor metal matrix
of silver or copper and a secondary phase of carbonaceous
particles from the group consisting of graphite, tungsten
carbide, or mixtures thereof. In the known methods of
forming such electrical contacts, the conductor metal powder
of silver or copper was blended with a powder of the
carbonaceous particles, pressed into a compact, and sintered
to form the contact. We found that during blending of the
conductor metal powder and carbonaceous particles, free
carbon flakes from the carbonaceous particles and coats the
conductor metal particles. The carbon contamination coating
the conductor metal particles becomes a barrier to good
interparticle bonding between the conductor metal particles
durlng pressing and sintering, and the composite matrix
strength su~fers.
In the method of this invention, carbon
contamination between conductor metal particles is minimized
due to the coating of silver, nickel, chromium, or mixtures
thereof on the carbonaceous particles, and interparticle
bonding is improved during sintering. As a result, sintering
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can be performed at reduced temperatures while obtaining
improved strength in the composite matrix, and the additional
re-pressing and re-sintering steps of the known methods are
not required to break up carbon contaminated interparticle
boundaries and achieve good bonding between matrix powder
particles. Although carbon is mostly bonded to tungsten in
tungsten carbide powders, we have found that there is at
least a few percent of free carbon that flakes from the
tungsten carbide particles to coat conductor metal particles
during powder blending. As a result, the method of this
invention is advantageously practiced in forming contacts
with a secondary phase of graphite, tungsten carbide, or
mixtures thereof.
In the method of this invention, a powder of the
composite contact is provided having carbonaceous particles
that are coated with a metal from the group consisting of
silver, nickel, chromium, and mixtures thereof. The coating
can be deposited on the carbonaceous particles, for example,
in an electroless plating solution. An electroless silver
plating solution is described in "Electroless Deposition Of
Silver Using Dimethylamine Borane", F. Pearlstein, R.F.
Weightman, Plating, February, 1974, pp. 159-157, and an
electroless nickel plating solution is described in U.S.
Patent 4,780,342, both incorporated herein by reference. A
suitable method of depositing the coatings on the
carbonaceous particles is shown, for example, in the above
cited references together with the following description of
silver coating graphite powder particles.
Graphite powder is activated by suspending the
powder with ultrasonic agitation in a palladium activation
solution for about 9 minutes at room temperature. For
example, the graphite powder is activated in a palladium
colloid suspension comprised of, by volume percent, about
77.4 percent water, about 22 percent hydrochloric acid, and
~5 about 0.6 percent of a palladium activator, such as Macuplex
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activator, D-34 available from MacDermid, Connecticut. The
activated graphite powder particles are rinsed, filtered
dried, and suspended in a silver electroless plating bath
using ultrasonic agitation to disperse the graphite
pa~rticles. A suitable electroless silver plating solution is
comprised of about 0.01 mole NaAg~CN)2, 0.02 mole NaCN, 0.02
mole NaOH, and 0.03 mole dimethylamine borane. The bath is
heated to about 55 C for about 30 minutes while the bath is
agitated and the silver in solution plates onto the graphite
particles. Typically, about 95 percent of the silver in the
solution is plated onto the graphite powder. The plated
powder is rinsed with water, filtered, and dried. A final
alcohol rinse can be performed to improve powder drying.
The carbonaceous powder used in the method of this
invention can be any desired particle size, however, the
carbonaceous particles in the composite contacts of this
invention are preferably about 1 to 15 microns, and most
preferably about 2 to 5 microns.
It should be understood that the coating on the
carbonaceous particles does not have to be a continuous
coating that completely envelopes the particles, rather the
coating can be discontinuous but of a type ~hat minimizes
flaking of carbon from the carbonaceous particle. For
example, graphite particles are generally in the form of
flakes, and a coating that at least substantially covexs the
edges of the flakes will minimize carbon flaking from the
graphite particle when it is abraded, for example during
powder handling, blending, or pressing. Electroless plating
provides such coating deposition of edges on angular or
flake-like particles and is advantageously used in the method
of this invention.
The powder provided in the method of this invention
can be comprised of a blend of coated carbonaceous particles
and conductor metal particles, i.e., silver powder blended
with silver coated graphite powder, or the powder can be
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comprised solely of coated carbonaceous particles.
Preferably, the powder is comprised of about 0.5 to 50 volume
percent of the carbonaceous particles, and the remainder is
the conductor metal. Powder handling including blending,
pouring, or pressing can be performed with minimal carbon
contamination of the surface of the conductor metal particles
because the carbonaceous secondary phase particles have a
coating that minimizes the flaking of carbon from the
carbonaceous particles. The coating is the conductor metal
or a metal that contributes desirable properties to the
electrical contact, and is comprised of a metal selected from
the group consisting of silver, nickel, chromium, and
mixtures thereof. Preferably, powder handling is minimized
by using a powder that does not require blending. Powder
tha~ does not require blending is comprised of the
predetermined volume percent of carbonaceous particles, and
the remainder of conductor metal coated on the carbonaceous
particles. Such coated powders also provide a more
homogeneous distributio~ of the carbonaceous secondary phase
particles in the sintered composite contact as compared to
composite contacts formed from blended powders.
The powder, i.e., coated powder or blend of coated
powder and conductor metal powder, is compacted at a pressure
of about 135 to about 620 megapascals, MPa. The sintered
density of the pressed compact increases as the pressure of
powder compaction increases up to about 550 MPa., therefore,
powder compaction i5 preferably at about 415 to 550 MPa.
Compaction can be in a metal die so that consolidation
pressure is applied uni-axially or bi-axially, in an elastic
mold so that consolidatlon pressure is applied isostatically,
or a comblnation of die and mold pressing.
The compact is sintered in a reducing atmosphere or
vacuum to increase density without causing substantial gas
formation within the compact. Preferably, sintering is
performed in a hydrogen atmosphere above atmospheric
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pxessure. A suitable sintering temperature to increase
density without causing substantial gas formation within the
compact is shown by making reference to Figure 1. Figure 1
is a graph of dilatometer measurements on a silver powder
compact heated at 5 C per minute to 900 C, and l C per minute
to 940 C. At temperatures between about 400 C to 800 C
substantial densification occurs, and maximum densification
occurs at about 840 C to 910 C. Above about 910 C the silver
compact expands rapidly. It is believed the expansion is due
to entrapment of gas formins within the compact, which is
entrapped as bubbles that remain as matrix porosity upon
cooling. Therefore, sintering is performed between about
700 to 910 C to densify a silver matrix compact without
causing substantial gas phase formation within the compact.
A silver powder compact was used to show how to obtain the
sintering temperature for a silver matrix composite. Similar
dilatometer measurements can be made on other conductor metal
matrix materials such as copper to obtain the sintering
temperature for increasing density without causing
substantial gas formation within the compact.
The composite contacts formed by the method of this
invention have a uniform distribution of the secondary phase
carbonaceous particles in the conductor metal matrix. When
graphite is the secondary phase in a composite conductor, the
re-pressing and re-sintering steps of the prior known methods
cause extensive deformation and elongation of the graphite
particles so that the secondary phase is in the form of
stringers in the conductor metal matrix. Because the re-
pressing and re-sintering steps are not required in the
method of this invention, substantially anisotropic graphite
particles can remain in the conductor metal matrix composite
contact formed by the method of this invention. Therefore,
composite conductors having a spherical secondary phase of
graphite can be formed by the method of this invention.
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Various features and advantages of the composite
contacts and method of this invention are further shown in
the following Examples.
Exlmel Ql
A graphite powder having an average particle size
5 of about 2.5 microns was obtained from Lonza, Inc., N.J., and
blended with a silver powder having an average particle size
of about 1.7 microns, to form a blended powder comprised of
about 95 weight percent silver and S weight percent graphite,
i.e., about 21.6 volume percent graphite. Some of the
blended powder was poured in a 1.27 centimeter square metal
die and pressed at about 275.8 MPa into a compact about 0.165
centimeter in thickness. The compact was heated at about 4 C
per minute in vacuum to about 593 C, and a reducing gas
comprised of 96 percent nitrogen and 40 percent hydrogen was
introduced to a pressure of about 9 torr. The compact was
heated to a temperature range of about 7~9 C to 760 C and
held for 30 minutes, and furnace cooled. The sintered
compact was re-pressed in a metal die at about 551.6 MPa. to
form a composite contact.
EX~m~ ?
Some of the graphite powder from Example 1 was
electroles~ silver plated by Chemet Corp., Ma, to form a
coated powder comprised of about 95 weight percent silver,
and 5 weight percent graphite, i.e., about 21.6 volume
percent graphite. Some of the coated powder was pressed into
two compacts and sintered as described above in Example 1 to
form two composite contacts.
ExamDle 3
Some of the silver coated graphite powder from
Example 2 was pressed, slntered, and re-pressed as in Example
1 to form two composite contact.
Example 4
Some of the silver coated graphite powder from
Example 2 was poured in a 1.27 centimeter square metal die,
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and pressed at about 137.9 MPa. into a compact about 0.165
centimeters in thickness. The compact was heated to S00 C in
a hydrogen atmosphere at about 10-C per minute, held at 500 C
for 1 hour, and the atmosphere was pumped out to form a
vacuum. The temperature was increased to 850 C at about 10-C
per minute, held at 850 C for 30 minutes to sinter the
compact, and furnace cooled to room temperature forming two
composite contacts.
E~m~le 5
A second compact was formed as described in Example
~, and the sintered compact was cold isostatically pressed at
about 379 MPa forming a composite contact.
Exam~le 6
A coated powder comprised of 90 weight percent
silver, 7 weight percent nickel, and 3 weight percent
graphite was formed by electroless plating a nickel coating
on graphite powder, and blending the nickel coated powder
with a silver powder. Some of the graphite powder from
Example 1 was activated in a palladium colloid suspension
comprised of, by volume percent, about 77.4 percent water,
about 22 percent hydrochloric acid, and about 0.6 percent
Macuplex activator, D-34. The activated graphite powder
particles were rinsed, filtered, dried, and suspended in a
nickel electroless plating bath using ultrasonic agitation to
disperse the graphite particles. The electroless plating
bath was comprised of about 0.1 molar nickel acetate, about
0.45 molar KHCO3, about 0.50 molar X2C03, about 0.50 molar
K2HPO4, about 0.25 molar KOH, and about 0.65 molar N2H4-H2O.
The bath was agitated while heating to about 75 C for about
30 minutes, about 95 percent of the nickel in the solution
was plated onto the graphite powder to form a powder of about
70 weight percent nlckel and 30 weight percent carbon. The
plated powder was rinsed with water, filtered, and dried.
A silver powder having an average particle size of
about 1.7 microns was blended with the nickel coated graphite
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particles to form a powder of about 90 weight percent silver,
and 10 weight percent nickel coated graphite particles. The
blended powder was pressed in a one-half inch diameter die at
about 138 MPa. to form a compact having a 1.270 centimeter
diameter, a 0.269 centimeter length, and a calculated density
of 5.91 grams per cubic centimeter. The pressed compact was
heated to 850 C in a hydrogen atmosphere at a rate of about
lO C per minute, held at 850 C for 30 minutes to sinter the
compact, and furnace cooled to room temperature. The
sintered composite contact had a calculated density of about
7.38 grams per cubic centimeter. The sintered composite
contact was isostatically pressed at 344.8 MPa. to a
calculated density of about 8.44 grams per cubic centimeter.
~m~le 7
Some of the nickel plated graphite powder from
Example 6 was lightly ground in a quartz mortar pestle to
break up particle aggregates, and blended with a silver
powder having an average particle size of about 1.7 microns
to form a blended powder of about 16.67 weight percent nickel
coated graphite, and 83.3 weight percent silver. The blended
powder was pressed at about 68.9 MPa. to form a compact of
about 1.661 by 1.659 by 0.241 centimeters, having a
calculated density of about 6.14 grams per cubic centimeter.
The pressed compact was heated to 900 C in a hydrogen
atmosphere at a rate of about lO C per minute, held at 400 C
for 30 minutes, and the atmosphere was pumped out to form a
vacuum. The temperature was increased to 850 C at about 18-C
per minute, held for 30 minutes to sinter the compact, and
furnace cooled to room temperature. The sintered composite
contact had a calculated density of about 7.73 grams per
cubic centimeter. The sintered composite contact was
isostatically pressed at about 379.2 MPa. to calculated
density of about 8.40 grams per cubic centimeter.
The interparticle bonding or matrix strength, of
the sintered compacts formed in Examples 1 through 5 was
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measured in a cross break strength test. In the cross break
test, the 1.27 centimeters square by 0.165 centimeter thick
compacts were broken in a three point bend arrangement. The
composite contact was placed on parallel supports separated
about 0.953 centimeters, and a compression force from a knife
edge shape punch was applied at the middle of the contact
between the supports. The load required to break the sample
was divided by the width of the sample to form a unit of
cross break strength, pound per inch. The cross break
strength of the compacts from Examples 1-5 is shown below in
Table 1 along with the hardness of some of the contacts as
measured on the Rockwell 15 T scale.
Table l
Cross Break Strength of Composite Contacts
Example Sample Powder Cross Break Hardness
No. No. Process tlb./inch) R~15T)
1 1 Blend Ag and 50-68 50-55
Gr PSRD
2 1 Ag Coated Gr 130 17-20
PS 150 42
3 1 Ag Coated Gr 163
PSRD
2 Ag Coated Gr 150-193 65
PSRD
4 1 Ag Coated Gr 105
1 Ag Coated Gr 113-140 40-50
PSRI
PS - Cold press and sinter
PSRD - Cold press, sinter, and repress in a die
PSRI - Cold press, sinter, and repress isostatically
Silver-graphite compacts formed by the known method
in Example 1 have about half of the cross break strength of
sllver-graphite contacts formed by the method of this
invention in Examples 2-5. In addition, it is shown in
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Example 2 that the improved cross break strength is found
after sintering without repressing the sintered compact.
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