US3508320A

US3508320A - Electrical contact materials and method of making same

Info

Publication number: US3508320A
Application number: US735952*A
Authority: US
Inventors: Philip L Blue
Original assignee: PR Mallory and Co Inc
Current assignee: Duracell Inc USA
Priority date: 1968-04-17
Filing date: 1968-04-17
Publication date: 1970-04-28
Anticipated expiration: 1987-04-28

Description

April 28, 1970 P. 1.. BLUE 3,508,320

ELECTRICAL CONTACT MATERIALS AND METHOD OF MAKING SAME Original Filed April 5, 1967 STEP BLEND POWDERS l2 I0 STEP 2 PR ES 3 INTO BARS STEP3 SINTER STEP 4 COLD ROLL l2 |O STEP 5 I U ug Q'r-Q K.) o DWDOQ RE SlNTER 0% gm ifo iy 0 Q wa i w w c g o- O o% 7 1:

STEP 6 COLD ROLL STEP 7 ANN EAL STEPS CYCLES OF COLD ROLLING 8n ANNEALING INVENTOR PH/L/P L. BLUE ATTORNEY United States Patent Int. Cl. B22f 3/24 US. Cl. 29-4205 8 Claims ABSTRACT OF THE DISCLOSURE An electrical contact material consisting of a thin cold rolled material constructed of silver and a refractory material wherein the refractory particles are in intimate contact with a continuous silver matrix.

This is a division of application Ser. No. 627,962, filed Apr. 3, 1967, now abandoned.

Silver is an ideal electrical contact material because of its high electrical conductivity and excellent heat dissipating properties. However, when subjected to large short-circuit currents, on the order of 5000 amps, for example, it is subject to severe arc erosion. Consequently, it has been the practice to fabricate such heavy duty contacts from mixtures of silver and a high melting point, refractory material such as tungsten. With this combination the tungsten, while it carries a substantial amount of current, acts as a skeleton for holding the silver. The tungsten phase also provides greater resistance to are erosion.

Prior to this invention, silver-tungsten electrodes were for the most part deficient in many respects, the deficiencies arising mainly from the methods by which the contacts were produced. Such deficiencies include but are not limited to relatively poor electrical conductivity due to the oxide barrier between the tungsten particles and the silver matrix, discontinuity of the silver matrix, inability to make thin strips or sheets of the material, and non-uniformity in physical properties such as hardness and density.

Among the objects of the present invention is the provision of an electrical contact material composed of silver and a refractory material taken from the class consisting of tungsten, tungsten carbide, and molybdenum wherein the refractory particles are in intimate contact with the silver matrix.

Another object of the invention is to provide such a contact material which is fabricated as a thin body with suflicient ductility so that it can be readily formed into desired shapes.

Still another object of the invention is to provide such 'a contact material which has more uniform physical properties such as those of hardness, density and lack of porosity.

Another object of the invention is to provide a bar rolling technique for forming such contact material.

Still another object of the invention is to provide a bar rolling technique wherein powders of the refractory material are blended and pressed into bars, sintered, cold rolled, resintered and cold rolled with or without intermittent annealing treatments.

With the above and other objects in view, which will appear as the description proceeds, this invention resides in a novel article of manufacture and a process for making the same such as substantially described herein and more particularly defined by the appended claims, it being understood that such changes in the precise embodiment 3,508,320 Patented Apr. 28, 1970 ice of the invention here disclosed may be made as come within the scope of the claims.

In the drawings:

FIGURE 1 is a flow sheet showing the various steps m forming the novel electrical contact material;

FIGURE 2 is a cross section of a silver-tungsten electrical contact material showing the microstructure resulting from a commonly used sintering method; and

FIGURE 3 is a cross section of a silver-tungsten con tact material showing the microstructure formed by using the method of invention.

The invention in its broadest aspect contemplates providing as an article of manufacture, an electrical contact material having an active contact face fabricated of silver and a refractory material taken from the class consisting of tungsten, tungsten carbide, and molybdenum, the powder particle size of the refractory material being from 4-20 microns with the refractory particles being in intimate contact with a continuous silver matrix to yield a contact having a high electrical conductivity. Also, in its broadest aspect, the invention contemplates providing a process for forming such contact which comprises blending a mixture of powders of a refractory material taken from the class consisting of tungsten, tungsten carbide, and molybdenum and silver powders, said refractory powders having a particle size of from 4-20 microns as measured by F.A.P.S. analysis, pressing said mixtures into bars. sintering said bars in a non-oxidizing atmosphere, cold rolling said bars in a single pass to yield about a one-third reduction in the bars thickness, resintering and cold rolling said bars, annealing said bars to remove all rolled grain structure, and thereafter alternately cold rolling and annealing said bars in cycles to yield reductions in thickness of from 10-30 percent in each cycle until the desired physical properties are obtained.

Referring now to FIGURE 1, the first step in making the novel electrical contact material is that of blending the powders together. To this end, powders of silver which preferably have a particle size of from 8-10 microns by F.A.P.S. analysis are blended with the refractory powder having a particle size of from 4-20 microns by F.A.P.S. analysis. The particle size of the refractory is particularly critical. In general, if the particle size becomes excessively large, a good intimate electrical contact is diflicult to achieve between the refractory and the silver and the electrical load carrying capability of the contact is reduced; while on the other hand, if the tungsten particle size becomes too small, the material becomes brittle and cracks develop during the rolling operation. Preferably the particle size of the tungsten should be from about 6-20 microns, while for the tungsten carbide and molybdenum, it should be about 4-12 microns.

In general, the composition, that is, the weight percent of the refractory and silver, is dependent upon the electrical properties desired and, in the case of the present invention, the rolling ability of the mixture. With too little silver, the rolling operation becomes very difiicult due to the high refractory material content. On the other hand, with too much silver, the fine particles of silver reduce the materials current carrying capacity. In general, silver in an amount of from about 20 to percent by weight with the balance being the refractory material has been found to be suitable. Table I shows the ranges and the preferred percentages of silver for the named refractory materials of the invention.

The mixture is then, as shown in FIGURE 1, pressed into bars of a suitable shape by placing the mixture into a mold and applying pressure to it.

The bars are then, in step three, sintered in a nonoxidizing atmosphere in a furnace of either the muffie or open element type. The atmosphere may be a neutral atmosphere such as pure nitrogen, but a reducing atmosphere such as dissociated ammonia or pure hydrogen is preferred from the standpoint of reducing the tendency for the formation of oxide layers on the tungsten.

Sintering temperatures depend upon the particle size of the refractory material and composition of mix, the temperature, in general, being inversely proportional to both. The larger particle sizes, within the aforementioned range, tend to cause bleed-out of the silver or to cause the bars to deform. In such case, solid phase sintering at a temperature near the melting point of the silver (about 980 C.) is adequate. Bars which may be sintered above the melting point (liquid-phase) of silver are sintered in the range of from about 1000 C.1l30 C. Optimum temperature conditions are dependent upon the silver refractory composition of the mixture. And, in addition, enough refractory structure must be present to hold the silver matrix. For example, with a mixture of 50 percent silver, 50 percent tungsten liquid phase sintering at temperatures up to 1130 C. can be used. With a 90% Ag./% W. mixture solid phase sintering with temperatures of from 900 C. to 950 C. would be used regardless of particle size. With a 35%/65% mixture, liquid phase sintering with temperatures up to 1130 C. would be used. In any event the maximum temperature should be about 1130 C. to prevent excessive vaporization of the silver.

A sintering temperature for 50% silver, balance molybdenum would be 1100 C., as would the sintering temperature for a 65 silver, balance tungsten carbide composite. Greater silver contents within the ranges shown in Table I should be sintered at 940 C.

Again referring to the drawing, after the bars have been sintered, they are cold rolled through a suitable rolling mill, the roll gap being set to about two-thirds of the bars thickness to yield a one-third reduction in thickness in a single pass. With a one-third reduction, optimum economic rolling conditions are achieved without having a tendency for the bars to crack, especially along the edges.

As shown in step 5, following the cold rolling, the material is resintered. This step'completely relieves the stresses of rolling and promotes rapid grain growth in the silver matrix to exclude voids. This void exclusion is a result of grain growth in the solid phase sinter and refractory particle wetting in liquid phase sintering. More important, the resinter step at this point provides for a continuous silver matrix, uninterrupted by a refractory skeleton as exists in prior art infiltrated materials. This continuous silver matrix provides better electrical properties with no expense of hardness. Electrical conductivity tests consistently indicate superior conductivity for these rolled silver-refractory materials over materials of the same composition produced by so-called standard techniques.

In the cold rolling and resintering steps lie the heart of the invention. They are the steps that yield the novel electrical contact grain structure shown in FIGURE 3 wherein the refractory particles 10 are in intimate contact with the continuous silver matrix 12. More specifically, it has been found that the cold rolling substantially eliminates the oxide barrier normally formed on the refractory particles, thus leaving a direct or intimate contact between the refractory particles and the silver matrix. While not desiring to be so limited, it is felt that the cold rolling step sets up an abrasive action between the refractory particles so as to cause the substantial elimination of the oxide barrier. Although there would be an abrasive action with hot rolling, there would still be the tendency to create an oxide layer due to the heat involved. The substantial elimination of the oxide barrier yields a better electrical contact between the tungsten particles and the silver matrix thus yielding increased electrical conductivity. As previously noted, the resintering step promotes grain growth so as to eliminate voids in the material.

Thus these two steps have eliminated two of the major defects of prior art electrical contact materials which, as shown in FIGURE 2, includes voids 14, and substantially no intimate contact between the refractory particles 10 and silver matrix 12. 7

As shown in FIGURE 1, in the next two steps the body is rolled and annealed and then as shown in the last step the rolling and annealing is continued in cycles with a 1030 percent reduction in thickness in each cycle until the desired properties of hardness, density, thickness, etc., are obtained. The optimum amount of reduction in thickness is a balance of reducing the thickness as quickly as possible without causing cracks in the rolled body and is dependent upon the silver-refractory composition of the original mixture.

For a percent by weight of silver, 10 percent tungsten mixture a 30 percent reduction would be optimum; for a 50%/50% mixture, 15 percent would be optimum; for a 35% /65% mixture, 10 percent would be optimum; and for a 27% 73% mixture, a 10 percent reduction per cycle would be optimum. For a 50% by weight silver- 50% molybdenum composite a 15% reduction would be optimum. For a 65% by weight silver-35% tungsten carbide composite, a 10% reduction would be optimum.

Annealing for all compositions consists of heating the rolled body in a reducing atmosphere such as dissociated ammonia at a temperature of about 900 C. for about one half hour. This procedure completely removes the rolled grain structure, permitting further reductions in thickness without cracking or splitting the bar.

Using the method herein described, electrical contacts of a silver-refractory compositionhave been formed in very thin continuous strips. These strips, which can be blanked out to form electrical contacts of various sizes and shapes, not only have better electrical conductivity due to the intimate contact of the refractory. particles with the silver matrix, but also have a more uniform density and hardness than those of the prior art. This can be more clearly shown by the following examples and accompanying test data.

Example 1 A powder mixture of 50% silver-50% tungsten by weight was pressed and sintered to a bar size of .125 inch in thickness. The particle size of the tungsten was about 6 microns, and the silver about 10 microns. The powders were pressed with a pressure of from about 20-25 ton/m They were sintered in an atmosphere of dissociated ammonia for about 20 minutes at a temperature of about 1130 C. The pressed bars were then cold rolled in air at a 30% thickness reduction for one pass. The bars were then resintered in an atmosphere of dissociated ammonia for about 5-10 minutes at a temperature of about 1000 C. After resintering the bars were then alternately cold rolled and annealed until a strip having a thickness of about .031 inch was produced. The annealing was done at a temperature of about 900 C. for about one half hour in an atmosphere of dissociated ammonia. The cold rolling was done in passes with a 10-15% reduction in each pass.

Example 2 A powder mixture of 65 silver-35% tungsten carbide by weight was pressed and sintered to a bar size of .100 inch in thickness. The particle size of the tungsten carbide was about 4 microns, and the silver about 10 microns. The powders were pressed with a pressure of from about 20-25 ton/in. They were sintered in an atmosphere of dissociated ammonia for about 15 minutes at a temperature of about 1300 C. The pressed bars were then cold rolled in air at a 30% thickness reduction for one pass. The bars were then r'esintered in an atmosphere of dissociated ammonia for about -10 minutes at a temperature of about 1200 C. After resintering the bars were then alternately cold rolled and annealed until a strip having a thickness of about .031 inch was produced. The annealing was done at a temperature of 900 C. for about one half hour in an atmosphere of dissociated ammonia. The cold rolling was done in passes having a reduction in each pass.

Example 3 A powder mixture of 60% silver-40% molybdenum by weight was pressed and sintered to a bar size of .125 inch in thickness. The particle size of the molybdenum was about 4 microns, and the silver about 10-12 microns. The powders were pressed with a pressure of from about 20-25 ton/m They were sintered in an atmosphere of dissociated ammonia for about 20 minutes at a temperature of about 1200 C. The pressed bars were then cold rolled in air at about a 30% thickness reduction for one pass. The bars were then resintered in an atmosphere of dissociated ammonia for about 5 minutes at a temperature of about 1100 C. After resintering the bars were then alternately cold rolled and annealed until a strip having a thickness of about .031 inch was produced. The annealing was done at a temperature of about 900 C. for about one half hour in an atmosphere of dissociated ammonia. The cold rolling was done in passes having a reduction of about 10% in each pass.

The electrical contact materials noted in the examples were then formed into suitable electrical contacts for testing, the contacts being blanked out of the strips. The contacts were tested along with contacts formed by standard priorart infiltrating techniques to give a basis of comparison. The contacts, which were in the form of Weld buttons, were compared for weight loss and voltage drop.

The sample weld buttons were mounted and tested in an RBM appliance relay. Four pairs of contacts from each lot were tested. The test parameters were:

Contact closed force30 grams Contact opening force75 grams Contact gap.020" minimum Relay overtravel--.030" minimum Operations-150,000 at 45/minute Circuit voltagel20 volts AC Load currentAg-Mo, 5 amps; Ag-WC, 5 amps; Ag-W,

9 amps TABLE II Avg. wt. loss per Composition cycle, 10- Avg. voltage drop Ag-W (Std.). 3. 69 421 Ag-W (Exp. 1). 3.35 405 AgWC (Std.) 3.13 323 Ag-WC (Exp 2). 3. 97 260 Ag-Mo (Std 2.05 523 Ag-Mo (Exp. 3) 1. 36 417 Electrical conductivity comparisons were made of materials of the present invention and materials made by standard infiltrating techniques using eddy current methods. The materials were compared as a percentage of the International Annealed Copper Standard (I.A.C.S.). The results are shown in Table III.

It is readily seen from Table II that the materials of the present invention show appreciably less voltage drop compared to that of the standard infiltrated material. Such decrease in voltage drop indicates a lower resistance at the contact interface. Such lower resistance means that less heat is being generated, thus prolonging contact life. Except for the tungsten carbide-silver combination, the materials of the invention showed less weight loss than that of the standard materials.

It is clearly shown by Table III that the materials of the invention have a much higher conductive value than that of commonly used electrical contact materials.

In addition to the advantages shown by the above-noted tests, it should be understood that the material of the present invention is much easier to fabricate into useful electrical contacts because of the ease in blanking out the contacts from the strip or sheets into which the present material is formed.

The electrical contact material of the present invention, as hereinbefore described, is merely illustrative and not exhaustive in scope. It is intended that all matter contained in the above description and shown in the accompanying drawings shall be interposed as illustrative and not in a limiting sense.

What is claimed is:

1. A process for forming thin electrical contact material which comprises: blending a mixture of powder of a refractory material taken from the class consisting of tungsten, tungsten carbide, and molybdenum; and silver powders, said silver being present in an amount of 20 to by weight of said mixture; said refractory powders having a particle size of from about 4 to 20 microns as measured by F.A.P.S. analysis; pressing said mixture into bars; sintering said bars in a non-oxidizing atmosphere; cold-rolling said bars in a single path to yield at least about /a reduction in the bars thickness, resintering said bars; and thereafter alternately rolling and annealing said bars in cycles to yield reduction of thickness of about 10 to 30% in each cycle.

2. A process for forming a thin electrical contact material according to claim 1 in which said powder mixture contains 50% tungsten and 50% silver by weight.

3. A process for forming a thin electrical contact material according to claim 1 in which said powder mixture contains 90% silver by weight, the balance being tungsten.

4. A process for forming a thin electrical contact material according to claim 1 in which said powder mixture contains 35% silver by weight, the balance being tungsten.

5. A process for forming a thin electrical contact material according to claim 1 in which said powder mixture contains 27% silver by weight, the balance being tungsten.

6. A process for forming a thin electrical contact material according to claim 1 in which said powder mixture consists of from 5080% by weight of silver of said mixture, the balance being a refractory taken from the group consisting of tungsten carbide and molybdenum.

7. A process for forming a thin electrical contact material according to claim 6 wherein said powder mixture consists of 50% silver and 50% tungsten carbide by weight of said mixture.

8. A process for forming a thin electrical contact ma- 2,621,123 12/ 1952 Hoyer 75221 terial according to claim 6 wherein said powder mixture 3,335,002 8/1967 Clarke 75221 XR consists of 65% silver and 35% molybdenum.

CARL D. QUARFORTH, Primary Examiner References Cited 5 A. J. STEINER, Assistant Examiner UNITED STATES PATENTS 2,148,040 2/1939 Schwarzkopf 75-221 XR 2,313,070 3/1943 Hensel 7s 221 XR 29-630; 75-204, 208,214, 221