US3351440A - Electrical contact materials - Google Patents

Description

United States Patent 3,351,440 ELECTRICAL CONTACT MATERIALS Francis J. Clauss, Athe'rton, Henry W. Lavendel, Palo Alto, and Edward Bruce, San Leandro, Califi, assignors to Lockheed Aircraft Corporation, Burbank, Calif. No Drawing. Filed Jan. 24, 1966, Ser. No. 522,357 2 Claims. (Cl. 29-1825) ABSTRACT OF THE DISCLOSURE Electrical contact materials and elements fabricated therefrom for transmitting electrical power across moving elements of machinery. The materials and elements exhibit excellent electrical noise characteristics and wear qualities in high vacuum and are particularly useful for operation in such environments.

The art is replete with suitable sliding electrical contact materials for transmitting electrical power across moving elements of machinery. In addition to silver-graphite materials which enjoy widespread commercial use, a variety of other contact materials are disclosed in US. Patents 2,162,380, 2,418,811, 2,621,123 and 2,736,830. Unfortunately, such materials have not been evaluated for operation in the high vacuums encountered in space. Although investigations have shown sliding electrical contacts to have advantages over other techniques for transmitting electrical power in space, such investigations have also shown that the wear qualities and electrical noise characteristics of electrical contact materials are generally adversely aifected when such materials are operated at pressures of 10 torr an-d less.

In accordance with the invention a new sliding electrical contact material has been developed for operation in vacuum. More particularly, the material comprises a :sintered mixture containing by weight 1 to 15' percent copper, 7.5 to 25 percent molybdenum-disulfide and 60 to 91.5 percent silver.

The eflectiveness of this material was demonstrated by operating sliding electrical contacts formed of the material in ultra-high vacuum in an actual slip ring assembly and measuring their noise and wear characteristics. The slip ring assembly used during the evaluation had 24 rings .measuring 2.25 inches in diameter by 0.1875 inch wide formed of either electro-deposited fine silver or gold. Each ring was contacted by two identical electrical contact brushes so that power could be brought in and out on the same ring. The brushes were in the form of buttons that 'were soldered to a beryllium-copper armpThe contacting faces of the brushes were 0.090 '-0.005) inch wide by 0150 -0 005) inch long giving a projected contact area .of 0.0135 square inch and were contoured to mate with .both as a conductor and as a cantilever spring for pressing the contact button against the ring with the amount of brush contact force being set by the deflection of the spring.

To obtain the desired vacuum a vacuum chamber was utilized which was a stainless steel cylinder measuring 12 inches diameter by 22 inches long. The chamber was pumped by a 360 liter/second ion pump (Ultek Model 336) through a 6-inch diameter leg. The chamber was provided with a valve (Veeco R50PSS) for initially evacuating it to a pressure low enough for the ion pump to be energized. Initial roughing was by means of a mechanical pump provided with a liquid nitrogen finger for trapping oil vapors and preventing contamination of the vacuum system. After the ion pump had been put into operation, the roughing valve was closed and the roughing pump removed from the system. Power to the ion pump came from the 60 c.p.s., 1l5-v., A-C house supply through an ion pump power supply (Ultek Model 1 8-1000). The pump was mounted in a vertical position on a rigid frame. The slip ring assembly was mounted on a 12-inch diameter flange that contained hermetically sealed feed-throughs for making all electrical connections. The axis of the slipring assembly was horizontal. The slip ring was rotated by a hermetically-sealed magnetic drive which was attached to the 12-inch diameter flange by means of a smaller flange. All attachments and checkouts were made to the 12-inch diameter flange before it was mounted to the chamber. All flanges were sealed with copper metal crush gaskets to eliminate contamination by outgassing from elastomeric seals. The magnetic drive was operated by D-C motor through a pair of pulleys providing the proper speed reduction. Power through the D-C motor came from the 60 c.p.s., 115-v., A-C house supply through a rectifier and control circuit which allowed the speed of the motor to be adjusted to give the exact speed desired on the slip ring shaft. The motor was mounted with vibration-damping mounts. The chamber was provided with an ionization gauge (Veeco RGK) for making pressure measurements within the chamber and for serving as a check onpressure measurements made by means of the pump current reading. The ionization gauge was connected to an ionization gauge control circuit (Veeco RG31A). Power to the ionization gauge control circuit came from the 60 c.p.s. 1l5-v., A-C house supply.

Current was adjusted to the desiredlevel by means of the variable autotransformer, a separate transformer being used for each ring and associated pair of brushes. A resistive load having a resistance considerably higher than that of the slip ring and brushes was inserted in series to maintain an essentially constant current despite minor fluctuations in the resistance of the brush contacts. The amount of current passing through the slip ring was determined from the voltage drop across a calibrated manganin shunt wire placed in serieswith the slip ring. Brush noise was measured by monitoring the potential drop across the brush pairs. An audio frequency microvolter (Type 548C Audio Frequency Microvolter of General Radio Company) was used to measure the potential a voltameter (Hewlett-Packard Model 400D vacuum-tube voltameter) and into a spectrum analyzer (Panoramic Analyzer Model TMI-l of Panoramic Radio Products). In use the hum-bucking circuit of the audio frequency microvolter was adjusted to eliminate the 60-c.p.s. wave form from the oscilloscope pattern which in turn minimized the voltameter reading.

During actual operation a rotational speed of 60 r.p.m. was used giving a peripheral sliding velocity of 424 inches per minute. All tests were conducted at room temperature and at a pressure below 10" torr. A current of 4.05 amperes was used giving a current density of 300 amperes per square inch of projected brush contact area. Brush contact forces were either 2.16 ounces or 1.30 ounces giving contact pressures of 10 psi. and 6 p.s.i., respectively. The brush thickness was measured before and after testing and the difiference recorded as the brush wear.

The following table sets forth the results of Test Run 1 of the preceding-described appartus. Test Run 1 contained 15 pairs of brushes of silve-r-copper-molybdenum disulfide composites and for comparative purposes, 4 pairs of brushes of silver-graphite. All brushes were operated at 60 r.p.m. against 2.25 inch diameter slip rings of electrodeposited fine silver. Copper content of the silver-coppermolybdenum disulfide brushes was varied from to 15 percent by weight and the molybdenum-disulfide content from 2.5 to 15 percent by weight. The purpose of this test was to compare the general performance in vacuum of silver-copper-molybdenum disulfide brushes with commerical silver-graphite material and to determine the approximate amounts of molybdenum disulfide for best performance. The apparatus was run in air for 20 hours and then operated for 500 hours in a vacuum of from 2X10 to 2 10- torr. Final dimensions were measured on both brushes of a given composition in 7 cases. In 12 cases, denoted by an asterisk in the table, final dimensions were obtained for only one brush due to one of the two brushes breaking off the brush holder during the test run. Where final dimensions were obtained on both brushes the brush wear in inches in the table is the average wear of the two brushes.

TEST RUN 1 Composition (percent by wt.)

Brush Wear (in.)

A G On MOS: Graphite The results of Test Run 1 indicate that molybdenum disulfide contents of 2.5% and 5% generally resulted in high brush wear whereas molybdenum disulfide contents of 10% and 15% resulted in low brush Wear. The higher copper contents also tended to favor lower brush wear, although the effect was less than that of molybdenum disulfide. The results also show that silver-graphite brushes are inadequate for use in vacuum due to excessive brush wear.

The following table sets forth the results obtained in Test Run 2 of the aforementioned apparatus. This test utilized 24 pairs of brushes of silver-copper-molybdenum disulfide composites. Since Test Run 1 established the unsuitability of silver-graphite brushes, these brushes were not included in Test Run 2. For this test the apparatus was run for 20 hours in air and then operated for 630 hours in vacuum of 7 10 torr, dropping to 1 10- torr at the end of the test. All brushes completed the full time of testing without a single failure. All brushes operated at approximately 70 F. throughout the entire test. For one period of approximately 10 hours after an initial 10 hours in operation in vacuum, the slip rings were not rotated and the current was conducted through the circuits. No evidence of welding or burning from this condition of operation was observed. Compositional changes in the composites were made in small steps in order to define the optimum composition more exactly. Although most of the brushes operated against silver rings, 8 pairs of brushes operated against gold rings so that the effect of ring material could be evaluated. As indicated in the table, gold rings and silver rings wore about equally but gold rings usually caused greater brush wear than silver rings. All brushes were operated at 60 r.p.m. against 2.25-inch diameter slip rings. The average brush wear in inches for each pair of identical brushes is set forth in the table.

TEST RUN 2 Composition (percent by wt.) Ring Average Brush Material Wear (in.)

Ag Cu M05:

92. 5 0 7. 5 A G 0. 005 90. 0 0 10. 0 AG 0. 007 90. 0 0 l0. 0 AG 0.0095 90. 0 0 10. 0 Au 0. 005 90. 0 0 10. 0 Au 0. 0058 87. 5 0 12. 5 AG 0. 0065 85. 0 0 15. 0 AG 0. 005 85. 0 0 15.0 A G 0. 0162 85. 0 0 15. 0 Au 0. 014 85. 0 0 l5. 0 Au 0. 025 90. 0 2. 5 7. 5 AG 0. 005 87. 5 2. 5 10. 0 AG 0. 0075 85. 0 2. 5 l2. 5 AG 0. 0082 82. 5 2. 5 15. 0 A G 0. 007 87. 5 5 7. 5 AG 0. 0075 85. 0 5 10. 0 AG 0. 0055 85. 0 5 10. 0 AG 0. 0075 85. 0 5 10.0 All 0. 011 85. O 5 10. 0 Au 0. 0138 82. 5 5 12. 5 A G 0. 010 80. 0 5 15. 0 AG 0. 0035 80.0 5 l5. 0 AG 0. 012 80. 0 5 15. 0 A11 0. 0105 80. 0 5 15. 0 Au 0. 0232 As shown in the table, brushes containing 7.5 percent molybdenum-disulfide consistently suffered low brush wear. Since Test Run 1 established that brushes containing 2.5 percent and 5 percent molybdenum disulfide suffered high brush wear, a lower molybdenum disulfide limit of 7.5 percent by weight is dictated in the compositions of the invention. Although not shown in the tables, Test Runs 1 and 2 established that as the molybdenum disulfide concentration is increased above 7.5 percent, ring Wear is decreased. As will be subsequently discussed, increasing the molybdenum disulfide concentration in the compositions of the invention beneficially decreases electrical noise. Accordingly, a preferred minimum molybdenum disulfide concentration in the compositions of the invention is 10 percent.

As further shown by Test Run 2 for each copper level increasing molybdenum disulfide concentrations tend to increase brush wear. This effect is most pronounced for brushes containing no copper. A balance, therefore, between electrical noise and ring wear, on one hand, and brush wear, on the other hand, dictates a maximum molybdenum disulfide content in the compositions of the invention of 25 percent and preferably 15 percent.

To compensate from the increase in brush wear due to increasing the molybdenum disulfide content in the compositions of the invention, minimum copper inclusions in the compositions of the invention are required.

Additionally, although not shown, it was determined in Test Run 2 that copper inclusions in the compositions of the invention are requiredto insure reproducibility of contact elements having uniform properties. For example, the four indicated brushes in the table containing percent copper and 15 percent molybdenum disulfide exhibit a wide scattering in wear property: 0.007 inch, 0.0025 inch, 0.0215 inch and 0.011 inch, respectively. In contrast, the two indicated brushes in the table utilized to obtain brush wear for they 2.5 percent copper and 15 percent molybdenum disulfide compositions exhibited consistent wear characteristics of 0.007 inch and 0.0075 inch, respectively. Itwas further determined in conjunction with fabricating the brushes utilized in Test Run 2 that the copper-free brushes were the most difficult to machine properly and in general tended to crumble. For these reasons a minimum copper inclusion of 1% is dictated in the compositions of the invention with a lower copper limit of 2% by weight being preferred.

Electrical noise measurements made during Test Run 1 are shown in the following table. The measurements were made after 500 hours of operation in vacuum. For comparative purposes there is set forth in the table the noise measurement associated with the silver graphite brush obtained after operating for 20 hours in air (designated as (a) on the table) and the noise measurements obtained on a silver graphite brush after 500 hours of operation in vacuum (designated as (b) in the table).

TABLE A Composition (percent by wt.)

Noise tmv.) AG Cu MoSz Graphite As previously discussed, Table A shows that increasing the molybdenum disulfide content of the compositions of the invention decreases the electrical noise associated with the compositions. Table A also shows that the general electrical noise level in vacuum of the better silvercopper-molybdenum disulfide brush composites of the invention is as good as the noise level in air of commercially available silver graphite brush materials. The unsuitability of the silver-graphite brushes for use in vacuum where low noise levels are required is established in the table. Table A also shows that the noise level of the silver-copper-molybdenum disulfide brush composites increases with increasing copper content. Accordingly, a maximum copper content of 15% is dictated for the electrical contact material of the invention with a maximum copper content of 5% being preferred. A preference for the 5% copper level is also dictated by the determination in conjunction with Test .Run 1 that ring wear increases with increasing copper content. In this regard, however, it was found that the ring wear for a 15% copper level was less than the wear caused by the silvergraphite brushes utilized in Test Run 1.

Conventional powder metallurgical sintering techniques are utilized to form the materials of the invention. Briefly, such techniques involve dry mixing the component powders in the desired proportions, the powders preferably having a particle size less than 325 mesh to promote sintering and to insure homogeneity in the formed product. Initially, mixing only the metallic components and then blending the molybdenum-disulfide into this mixture maximizes uniform distribution of the components in the initialmix and in the formed product; The mixture is then cold pressed under pressures in the order of 20,000 p.s.i. to 100,000 p.s.i. with a pressure of 40,000 p.s.i. generally being used. Pressures lower than 20,000 p.s.i. result in a green compact having insufiicient mechanical strength to be handled. Pressures in excess of 100,000 p.s.i. generally result in laminations in the green compact. The green compact is then sintered in a vacuum below 1 10- millimeters of mercury at temperatures in the order of 600 C. to 950 C. for approximately 0.25 to 2 hours. A vacuum is utilized to remove gases dissolved in the silver and to prevent further absorption of gas by'the silver. Below 600 C. only slight sintering occurs between silver and copper to the detriment of densification of the formed product. Maximum temperatures are limited by the melting point of silver, 960 C. The upper temperature limit is also limited by the amount of copper utilized and accordingly by the eutectic tem perature at which a liquid phase is formed between silver and copper. Generally temperatures in the lower temperature range are utilized. A temperature of 650 C. is found to favor formation of a silver-copper solid solution which is the preferred wear resistant, conductive matrix for the molybdenum-disulfide lubricant. Sintering times in the order of 0.25 hours are suitable for the upper range of temperatures with times up to two hours being used for the lower range of temperatures. At this stage of processing the formed electrical contact is of sufiicient strength and homogeneity to be used. Further densification and strengthening can be achieved by an additional pressing step followed by an additional sintering step carried out in accordance with the preceding teachings although such further processing steps do not significantly improve performance of the materials. To facilitate forming solder contacts to the formed body, a 5 to 10 mm. layer of copper or silver powder can be put on top of the initial powder mixture prior to the initial cold pressing step. During the subsequent processing steps, limited diffusion of copper provides a diffusion bond between this layer and the contact material.

In one specific embodiment of this process grams of silver and 5 grams of copper were dry mixed with mortar and pestle. Ten grams of molybdenum disulfide were then added to this mixture with continued mixing. The resultant mixture was then pressed under 40,000 p.s.i. The green compact exhibited a density of 8.14 grams/ cc. The green compact was then loaded into a vacuum furnace which was evacuated to 1X10" mm. of mercury and heated to a temperature of 650 C. Heating continued for two hours. At the end of this time the product was furnace-cooled in vacuum to room temperature. The product was then removed from the furnace and repressed under 10,000 p.s.i. which gave a density of 8.95 grams per cc. for the material. The material was then resintered at 650 C. to further promote formation of a coppersilver solid solution. The resintering step did not appreciably change the density of the material. The resultant electrical contact material had a composition on a weight percent basis of 85% silver, 5% copper and 10% molybdenum disulfide and the characteristics set forth in the tables.

Although the invention has been described with reference to specific embodiments, these embodiments are to be construed as illustrative only and not as limiting in any manner the scope and spirit of the invention as defined by the appended claims.

What is claimed is:

1. An electrical contact material exhibiting low wear and excellent noise characteristics in vacuums below l 10 torr consisting essentially of a sintered mixture of 60 to 91.5 percent by weight silver, 1 to 15 percent by weight copper and 7.5 to 25 percent by weight molybdenum disulfide.

2. A composition in accordance with claim 1 wherein said mixture consists essentially of on a percent by weight 7 8 basis of 80 to 88 percent silver, 2 to 5 percent copper FOREIGN PATENTS and 10 to 15 percent molybdenum disulfide. 465,142 5/1950 Canada.

747,662 4/1956 Great Britain.

Referencescited 386,609 5/1963 Japan.

UNITED STATES PATENTS 5 1 190 199 4 1965 Germany 2,145,690 1/1939 Hensel 29-182.5 674,128 6/ 1952 Great Britain- 2,200,855 5/1940 R b 29-1825 2 2 005 11/1944 75 173 L. DEWAYNE RUTLEDGE, Przmary Exammer. 2,982,619 5/ 1961 Long 29182.5 X BENJAMIN R. PADGETT, CARL D. QUARFORTH, 3,061,756 10/1962 Henderson 29-182.5 X 10 Examiners, 3,191,278 6/1965 Kendall 29-1825 3,306,715 2/1967 Schwmer 29 182.5 A. I. STEINER, Assistant Examiner.

Claims (1)

1. AN ELECTRICAL CONTACT MATERIAL EXHIBITING LOW WEAR AND EXCELLENT NOISE CHARACTERISTICS IN VACUUM BELOW 1X10**-5 TORR CONSISTING ESSENTIALLY OF A SINTERED MIXTURE OF 60 TO 91.5 PERCENT BY WEIGHT SILVER, 1 TO 15 PERCENT BY WEIGHT COPPER AND 7.5 TO 25 PERCENT BY WEIGHT MOLYBDENUM DISULFIDE.