Ag-SnO Alloy composite electrical contact

Composite electrical contact composed of a copper base portion clad with a contact portion of Ag-SnO alloy. The contact and base portions are fusion bonded by an interfacial alloy layer of Ag and Cu and an adjacent diffusion layer of Cu as the result of heat-treatment of the contact at the eutectic temperature of Ag and Cu.

This invention relates to an electrical contact composed of a base portion 
made from non-precious metals such as copper and clad with Ag-SnO alloy 
metals as a contact portion. 
More particularly, this invention relates to a novel composite electrical 
contact of the kind mentioned above, in which the contact and base 
portions are fusion bonded to each other by means of a Ag-Cu interfacial 
alloy layer between the said two portions, and also by means of a 
diffusion layer of Cu which extends deeply into the matrix of the contact 
portion, said two layers having been proposed by subjecting the composite 
electric contact for a comparatively short period of time to a temperature 
about or higher than the eutectic temperature of Ag and Cu, viz., 
779.degree. C. This heat treatment, which would be comparable with liquid 
phase sintering production of a contact, not only produces the 
aforementioned layers which firmly affiliate the contact and base 
portions, but also keeps an even dispersion of metallic oxides throughout 
the matrix of the contact portion of Ag-SnO. It shall be noted also that 
said heat treatment, which can be held after or simultaneously with 
binding of the base and contact portions by cold or hot press working, can 
release from the contact physical stresses produced in the contact by the 
press working. 
It has been widely known to produce a clad electrical contact by clothing a 
base portion, such as for example a copper rivet-shape stem, with a thin 
plate of contact material made from silver-metallic oxides alloys, such as 
for example Ag-CdO alloys, by means of cold press. This clothing operation 
is firstly for improving physical and electrical properties of a composite 
electrical contact, especially its anti-welding property, by means of a Ag 
contact portion alloyed with metallic oxides, and secondly for minimizing 
the amount of expensive Ag in a contact and for increasing proportionately 
the amount of Cu in the contact. 
The first point mentioned above as an advantage of the clothing operation 
is partly affirmative. However, the second point is questionable in its 
real merit. For example, in the case of a conventional contact of the type 
having a Ag-CdO contact portion and a Cu base portion mechanically bonded 
to each other by cold press, the contact portion often accidentally 
separates from the base portion when the former is only partially consumed 
(e.g. to only half its thickness). This occurs because the binding force 
between the contact and base portions relies primarily upon mechanical or 
physical stress to cause the metal to become plastic and to flow between 
the two portions, and hence the resulting bond, which has been 
extrinsically weakened by metallic oxides existing about the interface 
between the two portions and pulverized by the cold press binding process, 
inherently is not strong enough to stand up against the different contact 
conditions which occur with the consumption of the contact. Assumed that 
the thickness or volume of the Ag-CdO contact portion is equal to the base 
portion, the life of this type of composite contact is only one fourth the 
life of a comparative solid contact, which is made wholly from Ag-CdO 
alloys. Hence, it is doubtful whether the saving of a half amount of 
Ag-CdO or Ag can compensate for the shortening of its brief life 
expectancy, when considered in the light of the expensive and laborious 
steps and processes that are involved in manufacturing such a bimetallic 
composite contact. 
In order to eliminate the drawbacks mentioned above, it has been tried by 
the present inventor to fusion bond a most conventional Ag-CdO alloy 
contact portion to a Cu base portion. This trial failed as no prior 
literature has disclosed a composite electrical contact composed of a 
contact portion of Ag-internal oxidized metal-and of a Cu base portion 
which are fusion or diffusion bonded to each other. In case of a composite 
electrical contact comprising a Ag-CdO contact portion, for example, such 
fusion bonding was found to be impossible, because cadmium oxides 
dispersed in the Ag matrix of the contact portion decomposed or sublimed 
at a temperature lower than the melting point of said Ag matrix metal, and 
the cadmium oxides therefore produced a segregation or barrier layer 
facing the inner surface of the Cu base portion, which is a detriment to a 
contact of this kind. 
The applicant herein previously developed a specific Ag-SnO electrical 
contact material as disclosed in his U.S. Pat. No. 3,933,485. This 
specific electrical contact comprises an alloy having metal oxides 
precipitated therein as the result of internal oxidation, and being 
composed of about 5 to about 10% by weight of tin, and 1.0 to about 6% by 
weight of indium, the balance being silver. This alloy may contain a trace 
amount of less than 0.5% by weight of ferrous or alkali earth metals. 
Applicant has also invented another specific Ag-SnO electrical contact 
material comprising an alloy having metal oxides precipitated therein as 
the result of internal oxidation and being composed of 1.5 to 6% by weight 
of tin, 0.5 to 3% by weight of indium, the total amount by weight of said 
tin and indium being less than 2 to 6%, a trace amount of less than 0.5% 
by weight of iron family element, and the balance being silver. 
The inventor has now found that the Ag-SnO electrical contact materials, 
which he has invented as mentioned above, can advantageously be employed 
as a contact portion of a composite electrical contact composed of a 
contact portion of Ag-metal oxides, and a Cu basic portion, said two 
portions being fusion or diffusion bonded each to the other. This results 
from the findings that tin oxides of the aforementioned Ag-SnO alloys are 
stable in an oxygen atmosphere and at a temperature up to about 
2,000.degree. C., and sublime at the boiling point of Ag; that these metal 
oxides are hard enough to withstand a butt press joining without producing 
undesirable pulverization of said oxides but instead producing the wedging 
of the oxides into the Cu base portion, which results in preventing the 
abutting surfaces of the base and contact portions from sliding 
transversely to the longitudinal axes of said two portions during the butt 
press joining operation, and also results in mechanically strengthening 
the butt joint between the two portions; that a heat treatment of the 
portions at a temperature about the eutectic temperature of Ag and Cu, 
which produces the Cu diffusion into the Ag-SnO alloy matrix of the 
contact portion, does not adversely affect alloy structures of the base 
portion but works to release the two portions from stresses accompanied 
with the joining press work; and that the resultant bimetallic composite 
contact will have a useful life proportionate to the overall volume or 
thickness of the contact portion--i.e., will be useful until the contact 
portion has been substantially completely consumed. 
It has been found also that among the aforementioned Ag-SnO electrical 
contact materials, those containing ferrous or alkali earth metals such as 
nickel and cobalt are most preferable as materials for the contact portion 
of the present invention, because such elements urge metal oxides to 
produce about spherical nuclei, causing the resultant Ag-SnO alloy to have 
a good elongation percentage, which is a prerequisite for the cold butt 
joining of said alloy to a Cu base portion, utilizing plastic metallic 
flows therebetween. 
For example, the following alloys, which were made by wires of 2 mm in 
diameter and subjected to internal oxidation under oxidizing atmosphere at 
750.degree. C. for 24 hours, have respective elongation percentages as 
follows: 
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Sn 1.5% - In 0.5% - Ni 0.1% - balance Ag 
(elongation - 11%) 
Sn 3% - In 1.3% - Ni 0.1% - balance Ag 
(elongation - 8%) 
Sn 4.2% - In 1.8% - Ni 0.1% - Ag 
(elongation - 4%) 
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Hence, it is an object of this invention to provide a composite electrical 
contact composed of a contact portion made from an alloy having metal 
oxides precipitated therein as the result of internal oxidation, and being 
composed of 1.5 to 10% by weight of tin, 0.5 to 6% by weight of indium, 
and a trace amount of less than 0.5% by weight of iron family element 
metals, the balance being silver, and a base portion of copper or copper 
alloy clad with said contact portion, said two portions having an 
interfacial Ag-Cu alloy layer and said contact portion having a diffusion 
layer of Cu adjacent to said alloy layer as the result of heat-treatment 
of said two portions for a short period of time and at a temperature about 
and higher than the eutectic temperature of Ag and Cu.

EXAMPLE 1 
A composite electrical contact of the rivet type (Sample A) was prepared 
from a contact portion of 5 mm diameter and 0.5 mm. thickness made from a 
Ag-SnO alloy of Sn 5%-In 1.8%-Ni 0.3%-Ag, and a copper base portion having 
a base part of 5 mm diameter and 1.0 mm thickness and an integral shank 
part of 2.5 mm diameter and 2.5 mm length by cladding the base with the 
contact portion by cold press butt joining. This Sample A was heated at 
800.degree. C. for 10 seconds at atmospheric conditions. The Sample A thus 
heat-treated was cut and etched, and observed by an electron microscope at 
a magnification of .times.2800. There were observed an interfacial alloy 
layer of Ag-Cu of about 7/u thickness around the boundary of the base and 
contact portions (and immediately adjacent the copper base), and a thicker 
diffusion layer of Cu into the contact portion at a depth of about 51/u 
adjacent to the aforementioned Ag-Cu alloy layer, and at the side thereof 
remote from the Cu base or matrix. It was also observed that the alloy 
structure of the contact portion, especially its metal oxides and the even 
distribution thereof, were not adversely affected by said Cu diffusion. 
Sample B, having a construction similar to the Sample A but utilizing 
Ag-CdO 13%, was prepared without the above-noted heat-treatment. 
Both samples A and B were subjected to switching tests of 50,000 cycles (AC 
200 V, 75 A, for 0.6 second with a switching cycle of 3 seconds). Average 
loss of weight in Sample A was 20.66 mg., while in Sample B 29.52 mg. 
Noticeable part of the contact portion of Sample B was broken off. 
EXAMPLE 2 
Sample A of Example 1, Sample C of dimensional structures same to the 
Sample A, but made wholly from Ag-CdO 13%, and a sample D of dimensions 
same as Sample A but made wholly (i.e. both contact and base portions) 
from Ag-SnO alloy of Sn 5%-In 1.8%-Ni 0.3%-Ag, were tested for 2000 cycles 
under AC 200 V, 32 A, 60 cycles per minute, contact force of 100 g. and 
opening force of 80 g. Average hardness (HRF) of these three Samples when 
measured vertically from contact surfaces were 80 (Sample A), 107 (Sample 
C), and 110 (Sample D). 
Times of welding and average force (g) are as follows. 
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Sample A 19 times 117g 
Sample C 21 times 138g 
Sample D 4 times 228g 
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Contact resistances (m.OMEGA.) of the samples are as follows: 
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At 1000 cycles - 
Sample A 2.7 m.OMEGA. 
Sample C 3.0 m.OMEGA. 
Sample D 3.2 m.OMEGA. 
At 1500 cycles - 
Sample A 1.9 m.OMEGA. 
Sample C 3.8 m.OMEGA. 
Sample D 5.3 m.OMEGA. 
At 2000 cycles - 
Sample A 3.7 m.OMEGA. 
Sample C 4.3 m.OMEGA. 
Sample D 4.5 m.OMEGA. 
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Thus low contact resistance and good thermal dissipation were observed in 
Sample A. 
In the present invention instead of Cu and its alloy, Al and Ni and their 
alloys can be employed also as a base portion. The heat-treatment of the 
contact for producing a fusion binding therein can, of course, be made 
simultaneously with a cold forging operation of the contact, for example 
by providing heat to cold forging dies.