US3128540A - Electrical contact - Google Patents

Description

United States Patent Ofifice 3,128,540 Patented Apr. 14, 1964 3,128,540 ELEfiTRICAL CONTACT Childress B. Gwyn, J12, Export, and Norman S. Hoyer, Pittsburgh, Pa., assignors to Gibson Electric Company, Delmont, Pa., a corporation of Pennsylvania No Drawing. Filed Sept. 19, 1960, Ser. No. 56,690 7 Claims. (Cl. 29182) This invention relates to improved sintered metal compositions for electrical make and break contacts.

It is known in the art to produce electrical contact elements which comprise tungsten or molybdenum or their mixtures impregnated or bonded with silver or copper. The general characteristics of these prior products is well known in the art and the present invention relates to contact elements having improved characteristics.

Contact elements comprising tungsten and silver have the tendency to form cumulative and deleterious oxides on the working faces during their continued use. If these oxides are not continuously removed by high mechanical pressures, they can cause failures in the apparatus in which they are being used. While contact elements comprising molybdenum and silver show a rapid wear by mechanical action and thus exhibit a lower tendency to cumulative oxidation, they are rapidly and detrimentally affected by ambient high humidity atmospheres, and in general will not withstand as severe arcing and welding conditions as will contact elements made of tungsten and silver.

Mixtures of tungsten, molybdenum and silver have been proposed in the hope of producing a synergistic action which might combine the best features of tungstensilver and molybdenum-silver combinations, while eliminating some of their less desirable properties. The desired result was not obtained, nor was it obtained when carbides of molybdenum and/ or tungsten were substituted in whole or in part for tungsten and molybdenum.

According to this invention it has been found, that when a binary alloy of tungsten and molybdenum is used in the preparation of the electrical contact elements, instead of mixtures of tungsten and molybdenum, the above discussed uncontrollable and inconsistent detrimental features are sufficiently overcome. The contact elements of this invention show consistent and satisfactory service performance.

Some of the additional advantages of the contact elements of this invention can be listed as follows: (a) in continuous use they show very uniform temperature characteristics and low temperatures, while the prior products show an erratic pattern of values with occasional high peak temperatures; (b) they have lower density; they yield in thinner sections mechanical wear life equal to thicker sections of the prior products. The different thicknesses may range about 10 percent or more, while maintaining equal wear characteristics; (d) in view of their density and reduction in thickness, their raw material cost is lower; (2) they work well at considerably lower unit pressures than those required for the above prior contact elements; (f) the rate of rise of interfacial contact resistance between the mating surfaces, is initially lower with the products of this invention as compared with the prior art contacts. This advantage prevails substantially throughout their overall service life.

Other advantages will become apparent from thedescription of the invention which follows.

The electrical make and break contact elements of the present invention comprise a binary alloy of tungsten and molybdenum. The alloys are prepared prior to the manufacturing of the contact elements.

The composition of the binary alloy may range from about 2 percent to about 98 percent of tungsten, the balance to 100 percent being molybdenum. All percents in this specification are weight percents. A preferred range is from about 35 percent to about 95 percent of tungsten, the balance to 100 percent being molybdenum.

It should be understood that the above applies to tungsten and molybdenum carbide alloys as well as to the tungsten-molybdenum alloys.

It is advantageous to use the binary alloys in combination with a conducting metal component, which melts at comparatively low temperatures. Such metal components are silver or copper. Mixtures of silver and copper and alloys of silver and copper can also be used.

The low temperature melting conducting metal component, such as silver, copper, their mixtures and alloys, in the contact element may range from about 10 percent to about 90 percent, the balance to 100 percent being the binary alloy component. In the binary alloy, as stated above, a preferred proportion is from about 35 percent to about 95 percent of tungsten, the balance to 100 percent being molybdenum.

The proportion of binary alloy to low temperature melting conducting metal is balanced against the electro mechanical service requirements which they are required to meet. For example, for automotive type circuit breakers, handling normal currents up to 40 amperes at 12 volts D.C., our preferred composition by weight percents is silver of about 90 percent and the binary alloy of tungsten-molybdenum of about 10 percent. For household fuse replacement type circuit breal-zers, which carry normal currents up to 50 amperes 60 cycle 120 volts A.C., our preferred composition by weight percents is silver of about 50 percent and binary alloy of about 50 percent. For large air-break power circuit breakers normally handling current or" about 100 amperes or more in voltage ranges well over 120 AC, we prefer compositions by weight percent of about 25 percent silver and of about percent binary tungsten-molybdenum alloy. The composition of the binary alloy also varies to meet specific requirements. Examples are:

Molybdenum 10% and tungsten Molybdenum 40% and tungsten 60% Molybdenum 5% and tungsten Generally speaking, for lower range currents in the order of l to 15 amperes, a comparatively higher molybdenum content is preferred; whereas for services in the range of to 1000 amperes or more, a comparatively high tungsten content is preferred. Likewise with 10W unit pressures on the contacts as closed under normal operating conditions, a high moiybdenum alloy content is preferable, whereas with higher unit pressures increased tungsten percentages are preferred.

in certain contact applications further processing of the binary ailoys of tungsten and molybdenum to form the carbides of the alloyed metals, is of advantage. If to these carbides a low temperature melting conducting metal is added, such as silver or copper, contact elements are obtained which have superior operational and wear characteristics. Such an application is, for example, an aircraft relay, or certain types of electric elevator control service components.

The preparation of the contact elements of this invention follows the following procedure. First the binary alloy of tungsten and molybdenum is prepared. The alloy is powderized to the proper particle size. Next the powdered material is compacted. Forms and pressure are usually used for this purpose. Next the compacted forms are sintered at elevated temperatures. When low temperature melting conducting metals are co-used with the binary alloys of this invention, they are incorporated in powder form prior to the compacting step,

by intermixing the respective powders. An infiltrating step follows advantageously, either with or without the further addition of the infiltrating low temperature conducting metal. The pressing range in the compacting operation is from about to about 50 tons pressure per square inch. In most cases about 30 tons per square inch pressure is useful. The optimum pressure for a given combination will depend on the composition of the binary alloy and the final density of the contact element desired. The sintering temperature depends largely on the nature of the infiltrating or impregnating low temperature conducting metal, i.e. whether it is silver or copper, or their alloy or mixture. The sintering temperature ranges normally from about 1800 F. to about 2500 F. In most cases it ranges around 2200 F. Sintering times vary from about 2 minutes to about 30 minutes, or in some cases to even more.

The binary alloys of molybdenum and tungsten of this invention are presently available in commercial quantities from Sylvania Electric Corporation. Specific examples of suitable alloys would be as follows:

Molybdenum Tungsten Content, Content, percent percent These binary alloys are designated as 100 percent minus 325 mesh, which means that the entire quantity passes through a 325 mesh screen. In some cases the coarseness may range to a 150 mesh type powder and in other cases finer than 325 mesh quality has advantages.

As an illustration the following represents contact compositions where particularly satisfactory results were ob- Similar results were obtained by replacing the binary alloy with its carbide.

In order to illustrate the improvement in behavior of the compositions of this invention versus prior products, six contact elements were prepared as follows:

In Other Metal Other How Other Silver, Metal Metal Was percent orMetals, Added Moly. Tungs.

percent Content, Content,

percent percent 50 50 mixture, r 50 50 50 50 do 30 70 50 50 50 50 5O 70 50 50 100 50 50 none Sintering Electrical Temperature, Density Grams conductivity,

F. Per cc. percent I.A.C.S.

These six contact elements were tested in a standard Westinghouse 20 ampere snap-open, snap-close Quicklag circuit breaker. Table 2 below shows the temperature rise during operation of the test circuit breaker. The tests were carried. out at room ambient at 30 C. The degrees are in centrigrades. The number of operations is shown in thousands. The load was 20 amperes, 220

volts A.C.

Number of operations It will be apparent from the above test results that contact elements of designation C and D, which contain the binary alloys of this invention, remain substantially uniform and show a lower temperature rise. In contrast thereto the contact elements of designation A and B, which contain mixtures of molybdenum and tungsten and of designation E and P, which contain molybdenum only or tungsten only, show a higher temperature rise. On the other hand, (C) and (D), the products of the present invention show a consistently lower temperature rise. All six contact elements have the same silver content.

In the foregoing, the present invention has been described only in connection with preferred embodiments thereof. Many variations and modifications of the principles of this invention within the scope of the description herein are obvious. Accordingly, it is preferred to be bound not by the specific disclosure herein, but only by the appending claims.

We claim:

1. An electrical make and break contact surface element constituted of from about 10% to 90% by weight of a low melting conducting metal component selected from the group consisting of silver, copper, their mixtures and alloys, and a binary alloy component consisting essentially of from about 2% to about 98% of tungsten by weight, the balance of said binary alloy component consisting essentially of molybdenum.

2. The electrical contact element as set forth in claim 1, in which the binary alloy component is a carbide formed from a tungsten-molybdenum alloy.

3. The electrical contact element as set forth in claim 1, in which the conducting metal component is silver.

4. The electrical contact element as set forth in claim 1, in which the conducting metal component is copper.

5. The electrical contact element as set forth in claim 1, in which the binary alloy component consists of from about 35% to about 95% of tungsten by weight, the balance to 100 weight percent of the alloy being molybdenum.

6. An arcing tip which comprises the product of 2,391,456 claim 1. 2,648,747 7. A circuit breaker comprising the product of claim 1 2,851,381 as the working faces used for circuit closure and opening. 2,983,996

UNITED STATES PATENTS 1,236,384 Fahrenwald Aug. 7, 1917 6 Hensel Dec. 25, 1945 Graves Aug. 11, 1953 Hoyer Sept. 9, 1958 Neely et a1 May 16, 1961 OTHER REFERENCES Schwarzkopf: Powder Metallurgy, Its Physics and Production, The Macmillan Co., New York, 1947, pp.

Claims (1)

1. AN ELECTRICAL MAKE AND BREAK CONTACT SURFACE ELEMENT CONSTITUTED OF FROM ABOUT 10% TO 90% BY WEIGHT OF A LOW MELTING CONDUCTING METAL COMPONENT SELECTED FROM THE GROUP CONSISTING OF SILVER, COPPER, THEIR MIXTURES AND ALLOYS, AND A BINARY ALLOY COMPONENT CONSISTING ESSENTIALLY OF FROM ABOUT 2% TO ABOUT 98% TOF TUNGSTEN BY WEIGHT, THE BALANCE OF SAID BINARY ALLOY COMPONENT CONSISTING ESSENTIALLY OF MOLYBDENUM.