US2199458A - Electric contact - Google Patents

Description

Patented May 2, i545 UNHTED STATE hittgifi PATENT @FFEQE ELEC'ERHC CONTACT Mallory & (30., Inc., poration of Delaware Indianapolis, lind., a cor- No Drawing. Application August 15, 1939, Serial No. 290,222

4 Claims.

This invention relates to electric contacts.

An object of the invention is to improve the characteristics of silver base electric contacts.

Another object of the invention is to provide a silver base contact material which is highly resistant to tarnishing, and therefore will not form a layer of high contact resistance.

A still further object is to provide a new contact material which can be used under severe electrical conditions without welding.

It is a further object of the invention to provide a contact capable of operating on high frequency at heavy current values, without objectionable contact metal transfer.

Other objects of the invention will be apparent from the following description, taken in connection with the appended claims.

The present invention comprises the combination of elements, methods of manufacture and the product thereof, brought out and exemplified in the disclosure hereinafter set forth, the scope of the invention being indicated in the appended claims.

While a preferred embodiment of the invention is described herein, it is contemplated that considerable variation may be made in the method of procedure and the combination of elements, without departing from the spirit of the invention.

The present invention comprises an improvement in electrical contacts which contain silver as the preponderant metalby weight. According to the invention, the contacts are improved by the addition of metals of second group of the periodic system of elements in the fourth, sixth, and eighth series, namely calcium, barium, and strontium.

Accordingly, the contacts may be binary compositions containing calcium, barium, and strontium in approximately the. following proportions istics of silver alloys containing additional ingredients, by the addition of calcium, barium or strontium,thus silver alloys containing beryllium, magnesium, zinc, cadmium, copper, gold, nickel, manganese, thorium, platinum group elements,

silicon, titanium, zirconium, tin, indium, can be improved by the addition of calcium, barium, or strontium, in substantially the proportions indicated above.

The calcium, barium or strontium are added 5 in proportions greater than are necessary as deoxidizers alone, so that the indicated proportion of these elements is retained as part of the finished alloy. Hence, especially in cases where additional ingredients are added to the alloy it may 10 be necessary to initially add amounts of these elements somewhat higher than the amount desired and in these cases the added calcium, barium or strontium may be Within the range .5 to 5% or 1 to 5%.

The silver content, where other contact metals are present, should preferably amount to not less than about The alloys of the present invention may be cast into desirable shapes and forms for the manufacture of electrical contacts. They may be cold worked by swaging, drawing, or rolling in reductions not exceeeding 5% in area per pass or a total reduction of not over 15% in area, The allowable reduction increases as the proportion 5 of added element is decreased.

It is entirely within the scope of this invention to employ ternary and quaternary alloys of silver containing any two or all three of the elements calcium, barium, and strontium up to 30 a total added percentage of 5% since we have found that the binary alloys of these metals with silver up to 5% by weight form eutectic systems similar in nature. Intermetallic compounds are formed in the binary systems which seem to be ,35 Ag4Ba, Ag-iCa, and Ag4Sr.

The metals calcium, barium and strontium have melting and boiling points as follows:

Calcium Strontium Barium 40 0. C. C. Melting point 810 800 860 Boiling point 1, 170 1, 150 l,

51' alloys. This method prevents undue oxidaon of the calcium and facilitates high recovery calcium when the melting is carried out in a :ducing or neutral atmosphere. The alloy iould be cast in pre-heated molds, and care .ust be taken to keep the pouring temperature or a little below, 960 C. If the alloy is poured hot, porosity of the casting results.

There is an additional advantage in using the Jove-mentioned pressed-disc master alloys. If ie discs are present to high density, they will nk, or may be pushed, to the bottom of the melt 1d therefore form the desired alloy with a mini- .um of deterioration by oxidation. Calcium power may be used in the preparation of the above ascribed master alloy discs. in which case it is esirable to mix the calcirun and silver powders a nitrogen or other similar neutral atmosphere I prevent oxidation and absorption of moisture. )me hydrogen may be present in the mixing ;mosphere, if desired.

In making silver-calcium alloys, the molten lver may be deoxidized before the silver-calum master discs are added. If a deoxidant is i be used magnesium may be recommended for re purpose, particularly when the final alloy is be used for electrical contacts, as in the presit invention.

We found it unnecessary to use any deoxidizer our work since all melting was carried out in 1 atmosphere of hydrogen. At temperatures 210w 700 0., pure hydrogen should not be kept contact too long with high-calcium metal marials, for calcium hydride may be formed which fairly stable up to approximately 675 C. This recaution is particularly directed toward calum metal powder and pressed powder materials mtaining calcium metal which has not been illy alloyed. In a number of contact applications, silver is ixed with refractory materials, taken from the pup of tungsten and molybdenum, or their com- )unds particularly carbides, and with metals ,ken from the iron group, such as nickel, co- L115 and iron. These materials are prepared ther by mixing the powders together, pressing ,me, and hot and cold working same into suit- )le forms from which contacts can be prepared by first pressing the refractory or iron group etals, and then impregnating same with a. silar-calcium alloy. Since calcium is available in )wder form, it can be readily incorporated into ie powder mix, and during elevated temperalres, will alloy and diffuse into the silver re ilting in a homogeneous alloy. Certain care is to be taken in producing these products, in 'der to avoid oxidation of the calcium as deribed above in connection with melting tech- .que. Sintering in the presence of metallic calum we have found to be an excellent proctive method.

In the preceding paragraphs, various alloy- -eparation methods have been described using ilcium as an example. Since, as stated before, 1e addition metals of the present invention are milar in behavior, barium and strontium may substituted wherever calcium is mentioned in 1e above description.

Using the methods described above, test alloys are prepared for contact investigations. The loys were cast and cold-swaged into rod form. mtacts were machined from these rods for the sts described below.

These contact tests were made to compare the erformance of fine silver contacts with the behavior of contacts containing 2.5% of calcium, barium and strontium in binary alloys, under identical conditions of voltage, current, type of lead, contact pressure and frequency of operation wherein the current was gradually increased at intervals of 4 minutes until failure by welding took place.

Taking silver plus 2.5% calcium as a typical example, it was found that the material would withstand a 25% heavier current lead than fine silver. Specifically, where fine silver contacts would fail by welding at 12 amperes, our new contact alloy would carry 15 amperes before failure'took place. By increasing the proportion of calcium to 3% by weight, it was found that the limiting current was increased to 1'7 amperes, and the transfer was further decreased. Under the same conditions, binary silver alloys containing barium and strontium had limiting currents in the range of 13 to 16 amperes, depending upon the amount of addition metal present. Alloys of the present invention were investigated over the range set forth in the appended claims. These tests were made at 12 volts D. C. 150 grams contact pressure, and on resistive-inductive load consisting of a rheostat in series with a 0.1 millihenry inductance.

The alloys containing 0.5% of addition metal maintained low contact resistance. This was investigated by operating the contacts 72,000 times at a frequency of 20 operations per minute on resistive load at values shown below:

These values are accepted as low contact resistance in the art, especially at the contact pressure employed, which was 50 grams.

We have found that contact alloys of the present invention are useful in differential combinations with other contact materials on D. C. circuits. It is possible to employ our alloys to advantage for the negative (cathode) contact in conjunction with positive contacts made of other materials of different composition. For resistive loads, we may use positive contact alloys such as silver-copper nickel, silver-gold-nickel, silvertungsten, etc. For inductive loads, it is advisable to use positive contact materials such as silvercadmium, silver-cadmium-nickel, silvergoldnickel, etc.

The improvement obtained by such use of the alloys containing calcium, strontium, or barium is an increase in limiting current with a decrease in transfer of material between contacts. The increased limiting current, we believe, is due to anti-wetting properties conferred on the contacts by the oxides of calcium, barium, or strontium. The decreased transfer is obtained by using a positive contact alloy which has low transfer tendency under the type of electrical load imposed on the contacts.

The alloys of the present invention have a very satisfactory hardness. As a mater of fact, values such as 90 to 95 Rockwell F, can be readily obtained. With such hardness values, the contacts will show not only good electrical erosion resistance, but also mechanical wear resistance. In many cases, these contacts are used for relays where mechanical wear in the order of a very small fraction of an inch, will mean failure.

In cases where additional hardness is required to Withstand mechanical wear, we have incorporated small amounts of copper and/or nickel into the above-described binary compositions. We have also added small amounts of cobalt or manganese for the same purpose. The percentage of nickel, cobalt or manganese is kept below 1%. Copper may be added, however, to the extent of 10%. These materials act to restrain grain growth and to inhibit softening at high temperatures.

The alloys were also tested for tarnish resistance by placing same in a. chamber through which hydrogen sulphide was passed. For comparison, fine silver and some of the standard silver alloys were tested. It was found that while fine silver would show a very heavy sulphide coat, within a period of 24 hours, the alloys of the present invention maintained abright surface for the same length of time, and showed only a slight tarnish film at the end of 72 hours. When testing the contact resistance, it was found that the improved alloys of the present invention showed contact resistance values of the same order, after the sulphiding test, as were obtained before sulphiding.

The present alloy has been found particularly useful for such applications as sensitive relays, thermostatic control contacts, overload relay contacts, differential contacts in general, etc.

Because of their lower specific gravity and decreased proportion of silver, alloys of the present invention are lower in cost per given contact shape, than contacts fabricated from fine silver.

While the present invention as to its objects and advantages has been described herein as carried out in specified embodiments thereof, it is not desired to be limited thereby, but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. An electric contact element formed of an alloy comprising .02 to 5% of material selected from the group of elements consisting of calcium, barium and strontium as an essential alloying ingredient, and the balance predominantly silver.

2. An electric contact element formed of an alloy comprising .02 to 5% of material selected from the group of elements consisting of calcium, barium and strontium as an essential alloying ingredient, and the balance predominantly silver, the silver content of said alloy amounting to not less thanabout 75%.

3. An electric contact formed of a heterogeneous bonded metal composition of finely divided refractory material selected from the group consisting of tungsten and molybdenum and their refractory compounds interspersed with and bonded by an alloy of .02 to 5% of material selected from the group of elements consisting of calcium, barium and strontium as an essential alloying ingredient and the balance substantially all silver.

4. An electric contact material formed of a heterogeneous bonded metal composition of finely divided metal selected from the group con-