CA2059919A1

CA2059919A1 - Method of forming composite electrical contacts having carbonaceaous secondary phase

Info

Publication number: CA2059919A1
Application number: CA 2059919
Authority: CA
Inventors: Minyoung Lee; Gary Charles Buczkowski; Charles Dominic Iacovangelo
Original assignee: Individual
Current assignee: General Electric Co
Priority date: 1991-02-27
Filing date: 1992-01-23
Publication date: 1992-08-28

Abstract

RD-19,547 ABSTRACT OF THE DISCLOSURE

Improved composite electrical contacts are comprised of about 0.5 to 50 volume percent of a carbonaceous secondary phase, and the remainder a conductor metal matrix, the metal matrix being substantially free of carbon in the matrix grain boundaries. The composite contacts are formed by 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 carbonaceous particles. The powder is consolidated into a compact, and sintered in a reducing atmosphere or vacuum to increase density without causing substantial gas formation within the compact.

Description

~95g9~
RD-19,547 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 as 0 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

-` 20~919 RD-19,547 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 2V~9~9 RD-19,547 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 2 ~
RD-19,547 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 2 ~
RD-19,547 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 2 ~ 9 RD-l9, 547 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 RD-19,547 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 : ` `' .~ . .
"' RD-19,547 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.

2~5~
RD-19,547 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, _g_ .

, , .

2 ~
RD-19,547 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 2~5~ 3 RD-19,547 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 RD-19,547 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 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 ~9~1.9 RD-19,547 Example 2 that the improved cross break strength is found after sintering without repressing the sintered compact.

: -13-, , .' ', ' " , ',' '' , ' .

Claims

1. A method of forming a composite electrical contact having a conductor metal matrix and a secondary phase of carbonaceous particles, comprising:
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 carbonaceous particles;
consolidating the powder into a compact; and sintering the compact in a reducing atmosphere or vacuum to increase density without causing substantial gas formation within the compact.

2. The method of claim 1 wherein the conductor metal is silver.

3. The method of claim 2 wherein the carbonaceous particles are particles from the group consisting of graphite, tungsten carbide, and mixtures thereof.

4. The method of claim 3 wherein the predetermined volume percent is about 0.5 to 50 volume percent.

5. The method of claim 4 wherein the coating is deposited by electroless plating.

6. The method of claim 5 further comprising the step of pressing the sintered compact to increase density.

7. A method of forming a composite electrical contact comprising:
providing a powder comprised of about 0.5 to 50 volume percent of a secondary phase of particles from the group consisting of graphite, tungsten carbide, and mixtures thereof, and the remainder silver, the secondary phase particles having a coating of a material from the group consisting of silver, nickel, chromium, and mixtures thereof, sufficient to minimize flaking of carbon from the secondary phase particles;

RD-19,547 consolidating the powder into a compact of the electrical contact; and sintering the compact in a hydrogen atmosphere or vacuum at a temperature between about 820 to 910°C.

8. The method of claim 7 wherein the coating is deposited by electroless plating.

9. The method of claim 8 further comprising the step of pressing the sintered compact to increase density.

10. The method of claim 8 wherein the secondary phase particles have a first coating of nickel and a second coating of silver.

11. The method of claim 7 wherein the powder is a blend of silver powder and silver coated secondary phase particles.

12. The method of claim 7 wherein the powder is a blend of silver powder and nickel coated secondary phase particles.

13. A composite electrical contact comprising, about 0.5 to 50 volume percent of a secondary phase of carbonaceous particles, and the remainder a conductor metal matrix of sintered conductor metal grains, the matrix being substantially free of carbon in the matrix grain boundaries.

14. The composite electrical contact of claim 13 wherein the carbonaceous particles are from the group consisting of graphite, tungsten carbide, and mixtures thereof.

15. The composite electrical contact of claim 13 wherein the conductor metal is silver.

RD-19,547

16. The invention as defined in any of the preceding claims including any further features of novelty disclosed.