US 3314771 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
April 13, 1967 H. HOFFMANN ETAL 3,314,771
CONTACT OF COPPER WITH BRASS AND TIN LAYERS Filed March 11, 1964 United States Patent Germany Filed Mar. 11, 1964, Ser. No. 351,037 Claims priority, application Germany, Sept. 11, 1963, L 45,823, L 45,824
4 Claims. (Cl. 29-183.5)
provided with a coating whose pu pose it is to maintain the contact resistance of the contact element constant.
Contact elements of the type used in switching equipment, i.e., the elements whose surfaces are used to establish the actual galvanic contact, are conventionally made of a material which has a low specific resistance. The contact surfaces should remain metallically clean for as long as possible so that, irrespective of the contact pres sure, the contact resistance, which results in a voltage drop across the contact element and in a heat loss, is not unduly increased but remains within given tolerances. It
alloys, are preferred. Such contact elements, however, have the drawback that they are vulnerable to sulphur or sulphur-containing substances. This drawback becomes particularly significant in the case of contact elements associated with switching equipment which is not operated very frequently, so that the contacts are not subjected to the automatic wiping and self-cleaning action which is built into many types of switches. Contact elements made of a base metal, as, for example, copper, and provided with a coating of a noble metal, such as silver, are subjected to the same drawback.
Other attempts have been made to render contact elements more resistant to tarnishing. One way of doing this is to convert the surface of the contact elements into a chromate compound. But here, too, sulfide layers will, in the course of time, be formed, particularly if the switch operates in an atmosphere of agressive media, which sulfide layers increase the contact resistance.
Another way of reducing tarnish has been to use, instead of coatings of a noble metal, a coating made of a base metal, as, for example, tin. Coatings of this type element, and experience has contact element, i.e., the voltage drop across it, becomes progressively worse.
German Patent No. 830,269 shows slide bearings provided with tin running surfaces. In order to prevent the tin from diffusing into the basic metal, layers of iron,
ning in of the slide bearings.
In the case of tin-plated copper contact elements oper- 3,314,771 Patented Apr. 18, 1967 ating in a sulphurous atmosphere, no sulfide is formed and the voltage drop does not increase. Such a contact element will, however, operate satisfactorily only so long as it is subjected to relatively low temperatures, i.e., so long as the contact element is subjected to low loads. during operation, the normal operatlng temperature becomes higherdue, for example, to increased contact resistance resulting from low contact pressure the operating temperature with a coating made of tin, this coating being applied onto a barrier layer which is between the coating and the basic metal, which barrier layer prevents the tin from diffusing into the basic metal so that the electrical characteristics of the contact element remain unchanged. If the barrier layer were made of iron, nickel and cobalt and the alloys thereof (see above-mentioned German Patent No. 830,- 269), such barrier layers these layers would, of FeSn and Ni Sn which would increase the electrical resistance. Therefore, the dilfusion barrier layers used in contact elements according to the instant invention are as brass, tin-zinc, and tin-lead.
crease even at relatively high operating temperatures of, for example, 200 C.
The provision of the barrier layer also prevents the discoloration of the tin coating which is a normal conappa-rent upon consideration of the following description when taken in conjunction with the accompanying drawings in which:
FIGURE 1 is a sectional view of a contact element according to the present invention which is provided with multiple layers and coatings.
FIGURE 2 is a sectional view of a contact: element according to the instant invention which is provided with mgs.
Referring now to the drawings and to FIGURE 1 thereof in particular, the same shows a contract element whose basic slab or body portion 10 is made of a base metal such as copper. The basic portion 10 has applied to it a barrier layer 2 made of brass ii. of 63% copper, 37% zinc, a tin-zinc alloy f.i. of tin, 25% zinc, or a tin-lead alloy f.i. of 10% tin, lead. Applied over the barrier layer 11 is the tin layer 12, both of layers 11 and 12 being applied galvanically.
The contact element as described so far is rendered even more oxydation-resistant, even when subjected to higher temperatures for extended periods of time, by applying a plurality of sets of barrier layers and tin coatings, i.e., by applying to the basic metal a barrier layer, a tin coating, a further barrier layer overlying the first tin coating, a further tin coating overlying the second barrier layer, and so on. Even though a tin coating is not, or not always, absolutely porous-free, a medium tending to cause deterioration of the contact element will, despite the most unfavorable conditions, be prevented from adversely affecting the surface of the contact element inasmuch as there are a plurality of barrier layers which prevent the tin coating from diffusing into the basic metal.
FIGURE 1 thus shows a five-layer contact element, consisting not only of the copper base 10, and a first brass barrier layer 11, a first tin coating 12, but also of a second brass layer 13, and a second tin coating 14. The various layers and coatings are applied galvanically, one on top of the other so that all of the layers and coatings are bonded together.
The contact element of FIGURE 1 can be used as a pressure-type contact element, as an inserta-ble slide-type contact element, or as a contact blade. As stated above, the advantages realized by the present invention are of particular significance for use in switch gear which is operated but infrequently.
FIGURE 2 shows a multiple layer contact element in which at least one of the layers serves as a diffusion layer which does not diffuse with the basic metal (copper, copper alloy, or iron), but with the material of which the coating is made, namely, the tin. As a result, even the most unfavorable conditions (aggressive sulphurous atmosphere, high temperature and long exposure thereto, slightly porous tin coating) will not result in any free copper appearing at the contact surface of the contact element.
In the multiple-layer arrangement intermetallic phrases rather than pure metals or alloys are used as the barrier layers. Here, the first barrier layer 21 applied to the basic metal 20, e.g., copper or brass, is a metal which can not form any diffusion layer but a mixed crystal, as, for example, silver. Applied over layer 21 is a layer 22 of tin which can form a silver diffusion layer with the layer 21. The third layer 23 is brass, and the fourth layer 24 is the final tin coating. The contact element of FIGURE 2, comprising the basic copper or brass, thus has a multiple layer arrangement of silver-tin-brass-tin, the thickness of the silver being between 1 and microns and preferably 5 microns, the thickness of the first tin layer being between 5 and 15 microns and preferably microns, the thickness of the brass layer being between 1 and 5 microns and preferably 5 microns, and the thickness of the final tin layer being between 5 and 20 microns and preferably microns.
Before use, the contact element is heat-treated at 200 C. for a period of between 6 and 200 hours and preferably 48 hours, thereby to make certain that the difiusion layer is formed between the silver and the tin. A thus-formed contact element has been found to be suitable for use in sulphur-containing atmospheres because the intermetallic phases of silver and tin are stable.
The present invention is not strictly limited for use with contact elements but is applicable for other types of current conductors as well; moreover, the present invention is generally applicable wherever the corrosion of a tin coating is to be prevented.
It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.
What is claimed is:
1. A contact element comprising a body made of copper, a first brass layer on and bonded to said body, a first tin coating on and bonded to said first brass layer, a second brass layer on and bonded to said first tin coating, and a second tin coating on and bonded to said second brass layer.
2. A contact element comprising a body made of copper, a silver layer on and bonded to said body, a first tin layer on and bonded to said silver layer, a brass layer on and bonded to said first tin layer, and a second tin layer on and bonded to said brass layer.
3. A contact element as defined in claim 2 wherein said silver layer has a thickness of between 1 and 5 microns, wherein said first tin layer has a thickness of between 5 and 15 microns, wherein said brass layer has a thickness of between 1 and 5 microns, and wherein said second tin layer has a thickness of between 5 and 20 microns.
4. A contact element as defined in claim 2 wherein said silver layer has a thickness of 5 microns, wherein said first tin layer has a thickness of 10 microns, wherein said brass layer has a thickness of 5 microns, and wherein said second tin layer has a thickness of 15 microns.
References Cited by the Examiner UNITED STATES PATENTS HYLAND BIZOT, Primary Examiner.