CA1296883C

CA1296883C - Internal oxidized ag-sno system alloy electrical contact materials, and manufacturing method thereof

Info

Publication number: CA1296883C
Application number: CA000515771A
Authority: CA
Inventors: Akira Shibata
Original assignee: Chugai Electric Industrial Co Ltd
Current assignee: Chugai Electric Industrial Co Ltd
Priority date: 1985-08-30
Filing date: 1986-08-12
Publication date: 1992-03-10
Anticipated expiration: 2009-03-10
Also published as: IN165226B; DK414686D0; ES556445A0; PT82516B; NO861703L; BR8602289A; DK414686A; ES8708252A1; CN1014329B; PT82516A; CN86103279A; EP0219924A1; AU6188886A; AU581338B2

Abstract

Abstract of the Disclosure Electrical contact materials made from an internally oxidized Ag alloy containing 0.5 to 12 weight % of Sn.
Internal oxidized structures which have been produced in the alloy at the forwardmost area along a progressive direction of internal oxidation and which are fine and free from the segregation of tin oxides, are employed as contact surfaces.
Another surface opposite to said contact surfaces may be brazeable by having it subjected to a reduction or decompo-sition treatment of metal oxides about said another surface.
As an embodiment, the alloy is internal oxidized by having it sandwiched between pure silver thin layers, and is cut horizontally right in two, simultaneously removing the dep-letion layer from the internally oxidized alloy.

Description

-~ 1296~

BACKGROU~3D OF THE INVENTION

Field of the invention The present invention relates to internally oxidized Ag-SnO system alloy electrical contacts and to a method of manufacturing the same.
Ag alloys which contain 0.5 to 12 weight ~ of Sn and which have been internal oxidized, are widely used as electrical contact materials in various electrical devices such as switches, contactors, relays and circuit breakers.
These Ag alloys which are molten, cast, and rolled or drawn, and are generally in the form of thin plates with or without backing thin pure Ag plates joined to a side of the Ag alloy thin plates, are internally oxidized by subjecting them to an oxygen atmosphere under a pressure.
They are different from those sintered Ag-metal oxides alloys which are made by mixing matric Ag powders with powders of the metal oxides and sintering them. One of their noticeable differences is that the former, viz.
internal oxidized Ag-Sn system alloys are far superior to the latter in respect to structural density, while the latter has the more uniform dispersion of metal oxides than the former. The latter may be very readily consumed in too rapid and frequent switching operations. oxygen which has penetrated into the Ag alloys as time passes, oxidizes metallic solute elements in the alloys and precipitates them as minute metallic oxides distributed in their Ag matrices.
Said metallic oxidized precipitates afford refractoriness and , . .

, 9~83 .

consequently anti-welding properties to the Ag alloys. The backing thin pure Ag plates when they are employed, work as mediums for brazing the oxidized Ag alloy contact materials to support or base metals of electrical contacts.
It has been observed, however, that when Ag alloys of the above-mentioned kind are internal oxidized, metallic solute elements in the Ag alloys do not precipitate and distribute evenly in their Ag matrices, but they tend to precipitate at a high concentration about outer areas which are suhjected directly to oxygen. Such precipitation of 10 metallic oxides at outer areas produces their segregations about the outer areas, particularly at top surfaces, and bring in turn depletion layers of an unnegligible thickness which lie between the top and bottom surfaces of the Ag alloys, when they are internally oxidized from both sides thereof. The segregations of metallic oxides at a high concentration about outer sur~aces of electrical contact materials make the outer surfaces physically too hard, and produces electrically a high contact resistance of the mate-rials especially at an initial stage of operations and 20 consequently an excessive temperature raise. In practice, such segregations about the outer areas are often shaved off by files and so on. This is not only laborious, but also it makes difficult to reuse filings of the outer areas, since they are contaminated by ~ilings of the files.
Compared to internal oxidized Ag-Sn system alloys, internal oxidized Ag-Cd system alloys which had competed with the Ag-Sn system alloys, have a more uniform dispersion , .. . .

of metal oxides. This is chiefly because that the diffusion velocity of Cd in a silver matrix is inherently well balan-ced with the diffusion velocity of oxygen in the internal oxidation, while they are not so in the case of internal oxidation of Ag-Sn system alloys. In other words, electri-cal contact materials made of internal oxidized Ag-Cd system alloy and methods for preparing them can hardly be referen-ces which are utilizable for the preparation of Ag-Sn system alloys and the internal oxidation thereof.
At all events, the segregation of tin oxides about 10 contact surfaces makes them too hard, and often brings about cracks of the surfaces. High electrical contact resistances especially of an initial stage of operations of electrical contacts made from internal oxidized Ag-Sn alloys result from the segregation or excessive concentration of tin oxides about top surfaces. Unduly high raise of temperature of contacts results also from the segregation.
In order to avoid the production of such segregations, there were invented by the present inventor certain methods such as disclosed in V.S. Patent No. 4,457,787 in which 20 vac~ant lattice voids are produced in Ag alloys by having them absorbed with hydrogen and the like, and in the course of internal oxidation, solute melts fill in the voids and precipitate as oxides at the innumerable oxide nuclei on an atomic s¢ale, without diffusing about much but only to such extent that they reach most adjacent voids, and consequently without any segregation and depletion thereof, and U.S.

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Patent No. 4,472,211 in which a high contact resistance which is caused by high concentration or supersaturation o~
metal oxides including tin oxides about a contact surface, is avoided by having solute metals sublimated, reduced or extracted about the contact surface before the internal oxidation thereof.
The aforementioned depletion layers in which metallic oxides lack completely or they are extremely thin, can hardly stand up to severe switching operations, since they have poor refractoriness. Therefore, when a contact material having a depletion layer between its upper contact surface and lower surface is used till its wear reaches the depletion layer, its life ends. This means that while the lower half of the contact material which lies below the depletion layer can join with the upper half above the depletion layer to disperse heat generated with switching operations and to give a desired height of the material, it can not be active as a contact surface. Often, the existence of such lower half of the contact material is meaningless.
The object of the present invention is, therefore, to provide internal oxidized An-SnO system alloy electrical contacts having contact surfaces of a moderate initial contact resistance and having no depletion layer, and a method of manufacturing such excellent contacts, not using such methods as disclosed in the above-mentioned U.S.
Patents which methods are difficult to ade~uately control.
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Brief Summary of the Invention It has been found by the present inventor that though internally oxidized structures of an An-Sn system alloy about its surface or surfaces with which oxygen comes to contact first and from which it penetrates into the alloy, are rough, the deeper they lie in the alloy, the finer they become. In other words, the internal oxidized structures which have been produced in the alloy at the forwardmost area along a progressive direction of internal oxidation, are fine and free from the segregation of tin oxides. They 10 are, therefore, most suited as contact surfaces.
It has been observed by the inventor that along with the progressive direction of internal oxidation, grain sizes of tin oxides precipitated in Ag matrices become gradually larger~ Hence, the contrast between the Ag matrices and the tin oxides becomes clearer or more remarkable in the progre-ssive direction of internal oxidation, which contrast can be expressed by the above words that the internal oxidized structure at the forwardmost area along the progressive direction of internal oxidation is most fine. The larger 20 the size of precipitates of tin oxides has, the larger area the Ag matrices can occupy so that lower electrical contact resistances are assured and unduly high raise of temperature of contacts can accordingly be avoided. Granting that a concentration of Sn throughout an alloy or from the rearmost area to the forwardmost area of internal oxidation of the alloy is constant, the forwardmost area which is consisted : ,,, ,, . ,. - .

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of Ag matrices and one grain ~for exampl~ only) o~ tin oxides of a certain weight % of the Ag matrices can afford to the Ag matrices larger contact surfaces, compared to the rearmost area which is consisted of the ten grains ~for example) of the same weight % in total and Ag matrices.
It shall be noted also that the larger precipitates of tin oxides are, the lesser the strain to be produced in the tin oxides with the internal oxidation becomes, so that and whereby precipitates come to have a moderate hardness which can scarcely bring about cracks of contact surfaces. 10 In view of the above, "the forwardmost area along a progressive direction of internal oxidation" used in the specification and claims can readily be comprehended micros-copically by those skilled in this art and can define this invention.
Such fine internal oxidized Ag-Sn alloy structures at the front or forwardmost area of internal oxidation appear, when the alloy is oxidized from both sides, centrally in the alloy with a depletion zone therebetween, and when the alloy is oxidized from a single side, at a bottom opposite to a 20 surface from which oxygen penetrates into the alloy. Since the depletion zone or a zone where tin oxides are poor lies usually next to the forwardmost area of internal oxidation, said area which is employed in this invention as a contact surface, should be free from the above zones. -Typical constituents of Ag-Sn alloys employable in this invention are those comprising of Ag matrices, 0.5 - 12 weight % of Sn, and 0.5 - 15 weight % of In, and those comprising of Ag ~atrices, 3 - 12 weight % of Sn, and 0.~1 - less than 1.5 weight ~ of Bi. Said constituents may contain one or more metallic elements selected from 0.1 - 5 weight ~ of Cd, 0.1 - 2 weight % of Zn, 0.1 - 2 weight ~ of Sb, and 0.01 - 2 weight ~ of Pb. In the case of the above-men-tioned latter constituents, 0.1 - less than 2 weight % of In may be contained. Further, they may contain less than 0.5 weight % of one or more elements of iron family.

More particularly, the invention first proposes a method of making an internally oxidized Ag-SnO system alloy electrical contact which comprises:
preparing an Ag alloy comprising a silver matrix and either 0~5 - 12 weight % of Sn and 0.5 - weight % of In or 3 to 12 weight % of Sn and 0.01 - less than 1.5 weight % of Bi:
completely internally oxidizing the alloy;
the alloy resulting from such an internal oxida-tion comprising a plurality of areas extending parallel to a surface of the alloy from which oxygen was diffused to achieve said internal oxidation, said areas extending adjacent to each other in the progressive direction of said internal oxidation and including in said progressive direction an external oxide seyregation area, an internally oxidized intermediate area, a forwardmost area of internal oxidation and an internal oxide depletion area;
and forming contact surfaces by cutting or shaving the alloy remotely from the surface from which oxygen was diffused into the alloy to achieve internal oxydation thereof so that the oxide depletion area is removed from the alloy and the forwardmost area is exposed to form a contact surfaceO
the invention also proposes a method of making an internally oxidized Ag-SnO system alloy electrical contact, which com-prises:

;~ ~ 7 ~Z~883 preparing an Ag alloy layer having a thickness at least twice as thick as a preselected thickness a~d compri-sing a silver matrix and either 0.5 - 12 weight % of Sn and 0.5 - 15 weight % of In or 3 to 12 weight % Sn and 0.01 -less than 1.5 weight % of Bi;
fixedly sandwiching the alloy layer between thin pure silver layers;
completely internally oxidizing said alloy layer to form an oxide depletion layer intermediate its cpposed surfaces;
slitting said alloy to a desired configuration after internal oxidation thereof;
cutting the alloy into two parts along said deple-tion layer; and removing the depletion layer left on one of said parts, wherein said one part after such a removal forming said contact.
Of course, the invention further proposes contacts obtained by any of the above mentioned methods.

As aforesaid, the Ag alloy may be prepared to a flat plate or disk having a height which is at least twice a desired final height and to while may be added the height of a depletion layer which is expected to be produced when the Ag-alloy is completely internal-oxidized. This Ag-alloy is backed at its both surfaces by thin pure Ag layers.

Then, the so prepared Ag-alloy is completely internal oxidized in an oxygen atmosphere under a pressure and at an elevated temperature.

During the internal oxidation of the Ag-alloy, 7a lZ96B~3 the backing thin pure Ag layers work as follows.

Since the partial pressure of oxygen, which has been dissolved into silver at the elevated temperature, is comparatively low, and since an amount of oxygen which diffu-ses through the silver is constant at a predetermined specific temperature, and under an oxygen atmosphere of a predeter-mined specific pressure, an amount of oxygen which shall be diffused into a metal alloy via the silver for oxidizing the former, can readily and freely be controlled. In additi~

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: 7b , to this advantage, since the oxygen in this instance is diffused into the metal alloy through the silver, and conse-quently at a selected direction of paths of oxygen, crystal-line metallic grains oxidized and precipitated in the metal alloy are not arranged at random but can be prismatically aligned in the paths of oxygen. Since these prismatically aligned metallic oxides are also in parallel with electric current paths passing through the internal oxidized Ag alloy contact mat~rial, electric resistance by the material is lowered. 10 The completely internal oxidlzed Ag alloy plate or disk havlng a depletion layer which lies centrally and transver-sely to the axis or height of plate or disk, is cut along said depletion layer by a super hard and high speed cutting device such as a mill with a width more than the width of the depletion layer. Unlikely to the conventional sanding off of segregation of metal oxides from outer surfaces of oxidized Ag alloys, said cutting operation does not give any contamination to cut surfaces and a cut-off portion of the Ag alloy which includes the depletion layer. 20 Two parts thus cut off from the plate or disk have respectively a completely internal oxidized Ag alloy body having a fresh contact surface of a moderate hardness and inltial resistance and a pure silver backing at its bottom surface, and having no depletion layer.

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~ ~ 2968~3 Preferred Embodiments Example 1.
(1) Ag-Sn 8~-In ~%

(2) Ag-Sn 8~-In 4~-Cd 0.5%

(3) Ag-Sn 7%-Bi 0.5~

(4) Ag-Sn 7%-Bi 0.5%-Zn 0.3%
Alloys of the above (1) to (4) were melted in a high frequency melting furnace at about 1f100 to 1,200C and poured into molds for obtaining ingots of about 5 Kg each.
Each ingot was stripped at its one surface. Then, each 10 ~ingot was butted opposite at its stripped surface to a nickel plate by means of a hydraulic press, and rolled to a plate of about 2.2 mm with the nickel back of about O.l mm.
Each plate was subjected to an oxygen atmosphere for 200 hours and at 650C so that the plate was completely internal-oxidized. Since the nickel back is unoxidizable, internal oxidation progressed from the stripped surface only. Segregation of tin oxides was observed around the stripped surface. The internal oxidized structures which had been produced in the plate at the forwardmost area along 20 the progressive direction of internal oxidation, viz., in this instance about 2 mm deep from the stripped surface, were;extremely fine and completely free from the segregation of metal oxides. A depletlon zone or a zone where tin oxides are poor next to said forwardmost area with a depth of about O.l mm.
Each of the internal oxidized plates were placed in a , . ... ..

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H2 gas atmosphere and heated at 750C for ten minutes, so that metal oxides about the stripped surface were reduced or decomposed whereby the stripped surface could be brazeable to a movable or stationary contact base.
~ he nickel plate can be replaced by other metals ~,7hich are not oxidizable, and the reduction or decomposition of metal oxides about the stripped surface may be made by heating in a flux or im~ersing it into an acid solution.
Then, the plates were horizontally cut at a plane of 0.2 mm from the bottom surface. And, plates were slitted to obtain 10 square electrical contacts of 5 mm sides and of a thickness of 1.9 mm, having the forwardmost areas of internal oxida-tion along its progressive direction as contact surfaces, and the reduced or decomposed stripped surfaces as backs.
Instead of slitting the plates after the internal oxi-dation, they may be cut or pressed out to desired configura-tion before the internal oxidation.
In order to compare the above electrical contacts made in accordance with this invention, the following contacts were made. 20 (5) Ag-Sn 8%-In 4%

(6) Ag-Sn 8%-ln 4%-Cd 0.5%

(7) Ag-Sn 7%-Bi 0.5%
~) Ag-Sn 7%-Bi 0.5%-Zn 0.3%
Similarly to the above example, the above alloys (5) to (8) were prepared to ingots. Then, each ingot was butted at its stripped surface to a pure silver plate by means of a hydraulic press, platen of which was heated at about 440C, <.~,r~

Z9~Ei 83 and rolled to a plate of about 2 mm thickness, while it was annealed at about 600C, at every stages of rolling rates of 30 ~ in reduction~
Each plate was internally oxidized in an oxygen atmos-phere for 200 hours and at 650C. Then, internally oxidized plates were pressed by a punch of 6 mm in diameter to obtain electrical contacts of 2 mm in thickness which were backed with a thin silver layer.
The above contact samples of alloys 11) to (4) of this invention and of alloys ~5) to (8) of prior known samples 10 were checked of their contact surface hardness, and of their initial contact resistance with the ~ollowing conditions.
Initial contact resistance:
Contact pressure - 400g Current - DC 6V, 1A

________________________________________________ Samples Hardness (HR "F") ________________________________________________ (1) 69 - 82 ~2) 67 - 74 20 (3) 64 - 76 (4) "of this invention" 67 - 76 (5) 95 - 105 (6~ 93 - 9~
(7) 90 - 100 (8) "of prior known samples" 90 - 100 ________________________ ______________________ ,~,,, .

lZ968~3 ________________________________________________ Samples Initial contact resistance (~ Q) ______________________ _________________________ (1) ~.6 - 2.1 (2) 0~6 - 2.1 ~3) 1.5 - 1.4 (4) "of this invention" 0.5 - 1.6 (5~ 1.2 - 2.2 (6) 1.2 - 2.2 (7) 0.7 - 2.1 (8) "of prior known samples" 1.7 - 2.2 ____________________ ___________________________ Thus, it is known from the above tables that the con-tact materials made in accordance with this invention have 10 moderate hardness and lower initial contact resistance, compared to corresponding prior-known contact materials.
Example 2 An alloy ingot of Ag-Sn 8%-In 4% was drawn to a wire of 5 mm in diameter, from which there were prepared a number of pieces each having a body portion of 5 mm in diameter and 3.3 mm length, which was integrally provided at its both sides with projections of 2.5 mm in diameter and 1 mm in height. Those pieces were completely internally oxidized, and then transversely to thelr axes cut right in two by a 20 mill with kerf of 0.3 mm so that~such rivet-shaped contact materials each having a contact head of 5 mm diameter and 1.5 mm height with a shank of 2.5 mm diameter and 1 mm ::
height, which were characterized by making the forwardmost areas of internal oxidation as contact surfaces, were pro-duced. The pieces may be subjected to H2 gas before or after they were cut to two so that the shank can be brazeab-.
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le to a contact support metal as described in Example 1.
The rivet-shaped contact materials thus obtained, had sxcellent physical and electrical characteristics, compared to their correspondent conventional contact materials. It has been observed that their hardness was about 30 % less than that of conventional ones, and their initial contact resistance was as much as 50 % less.
Example 3 ~ 9) Ag-Sn 8%-In 4%
l10~ Ag-Sn 8%-In 4%-Cd 0.5% 10 t11) Ag-Sn 7%-Bi 0.5%
(12) Ag-Sn 7%-Bi 0.5%-Zn 0.3%
Alloys of the above (9) to (12) were melted in a high frequency melting furnace at about 1,100 to 1,200C and poured into molds for obtaining ingots of about 5 Kg. Each ingot was stripped at its both surfaces. Then, each ingot was butted at its stripped both surfaces to pure silver plates by means of a hydraulic press, platens of which were heated at about 400C, and rolled to a plate of 3.1 mm thickness. while it was annealed at about 500C at every 20 stages of rolling rates of 30 % in reduction.
Each plate had one of the above alloys t9), (10), (11) and (12) of 2.5 mm thickness joined at its both surfaces by the pure silver layer of 0.3 mm thickness.
Each plate was completely internally oxidized in an oxygen atmosphere for 200 hours and at 650C. The plate had centrally a depletion layer of about 0.1 - 0~2 mm thickness.
Then, the plates were horizontally cut right in two by a . .

` 1%~6883 mill with a Xerf of 0.5 mm. And, the plates were slitted to obtain square electrical contacts of 5 mm sides and of a thickness of 1 mm, which were backed at one of the surfaces with a thin silver layer of 0.3 mm.
Instead of slitting the plates after the internal oxi-dation, they may be cut or pressed out to desired configura-tions before the internal oxidation.
The above contact samples of alloys (9) to (12~ of this invention and of alloys (5) to (8~ of prior known samples (Example 1) were checked of their contact surface hardness, 10 and of their initial contact resistance with the following conditions.
Initial contact resistance:
Contact pressure - 400g Cu~rent - DC 6V, 1A

Samples Hardness (HR "F") ________________________________________________ (9) 69 - 80 (10) 67 - 72 20 (11) 64 - 75 (12) "of this invention" 67 - 75 t5) 95 105 (5) 93 - 94 (7) 90 - 100 (8) "of prior known samples" 90 - 100 ~lL29~ 33 ________________________________________________ Samples Initial contact resistance (m a ) _______ ________________________________________ Ig) 0.6 - 2.0 (10) 0.6 - 2.0 (11) 1.5 - 1.3 (12) "of this invention" 0.5 - 1.4 l5) 1.2 - 2.2 (6) 1.2 - 2.2 0.7 - 2~1 ~8) "of prior known samples" 1.7 - 2.2 Thus, it is known from the above tables that the con- 10 tact materials made in accordance with this invention have moderate hardness and lower initial contact resistance, compared to corresponding prior-known contact materials.
Though in the above examples, Ag-Sn syste~ alloys were prepared by a melting method and then subjected to internal oxidation, they can be prepared powdermetallurgically prefe-rably with subsequent forging and then be subjected to internal oxidation. It is a matter of course that since internal oxidation mechanisms in the case of the latter aLloys work exactly same to the case of the former alloys, 2D
this invention as defined in the claims undoubtedly cover the latter alloys too. It shall be noted also that though the electrical contact materials (9) to (12) obtained in Example 3 in accordance with this invention in which they were contacted to oxygen not directly but indirectly through pure silver screens, had less rough oxidation structures at their surfaces which were immediately next to said silver ,~ , - ~.Z96~ 33 screens and hence came to contact first with the oxygen, compared to the internal oxidized structures around the stripped surfaces of the alloys (1) to (4) of Example 1;
their forwardmost areas along the progressive direction of internal oxidation had finer structures which were clearly distinctive by microscopic observations as aforementioned.

Claims

1. An internally oxidized Ag-SnO system alloy electrical contact, said contact being made by a) subjecting to a complete internal oxidation an Ag - alloy comprising a silver matrix and:
either 0.5 to 12 weight % of Sn and 0.5 to 15 weight % of In;
or 3 to 12 weight % of Sn and 0.01 to less than 1.5 weight % of Bi, the alloy resulting from such an internal oxida-tion comprising a plurality of areas extending parallel to a surface of the alloy from which oxygen was diffused to achieve said internal oxidation, said areas extending adjacent to each other in the progressive direction of said internal oxidation and including in said progressive direction, an ex-ternal oxide segregation area, an internally oxidized intermediate area, a forwardmost area of internal oxidation and an internal oxide deple-tion area; and (b) subsequently cutting or shaving the alloy remotely from the surface thereof from which oxygen was diffused to achieve internal oxidation, so that the oxide depletion area is removed from the alloy and the forwardmost area is exposed to form a contact surface.

2. An electrical contact as claimed in claim 1, in which the surface from which oxygen was diffused into the alloy to achieve internal oxidation thereof, is subjected to a chemical reaction so that metal oxides thereabout are reduced or decomposed to make said surface brazeable.

3. An electrical contact as claimed in claim 1 or 2, wherein said alloy further comprises one or more metallic elements selected from the group consisting of 0.1 - 5 weight % of Cd, 0.1 - 2 weight % of Zn, 0.1 - 2 weight % of Sb, 0.01 - 2 weight % of Pb, and 0.1 - less than 2 weight of In.

4. An Ag-SnO system alloy electrical contact having an internally oxidized Ag alloy layer of a preselected thickness, said contact being made from oxidized Ag alloy having a thickness between opposed surfaces thereof, at least twice that of said preselected thickness, said Ag alloy comprising a silver matrix containing:

either 0.5 - 12 weight % of Sn and 0.5 - 15 weight % of In;
or 3 to 12 weight % of Sn and 0.01 - less than 1.5 weight % of Bi, said Ag alloy being fixedly sandwiched between thin layers of pure silver and being completely internally oxidized to form an oxide depletion layer intermediate its opposed surfaces, said Ag alloy being slitted to a desired configura-tion after internal oxidation and then cut in two parts along said depletion, layer one of said parts forming said contact after full removal of the depletion layer left on it.

5. An electrical contact material as claimed in claim 4, wherein said Ag alloy further comprises one or more metallic elements selected from the group consisting of 0.1 - 5 weight % of Cd, 0.1 - 2 weight % of Zn, 0.1 -2 weight % of Sb, 0.01 - 2 weight % of Pb, and 0.1 - less than 2 weight % of In.

6. A method of making an internally oxidized Ag-SnO system alloy electrical contact which comprises:
preparing an Ag alloy comprising a silver matrix and either 0.5 - 12 weight % of Sn and 0.5 - weight % of In or 3 to 12 weight % of Sn and 0.01 - less than 1.5 weight % of Bi;
completely internally oxidizing the alloy;
the alloy resulting from such an internal oxida-tion comprising a plurality of areas extending parallel to a surface of the alloy from which oxygen was diffused to achieve said internal oxidation, said areas extending adjacent to each other in the progressive direction of said internal oxidation and including in said progressive direction, an external oxide segregation area, an internally oxidized intermediate area, a forwardmost area of internal oxidation and an internal oxide depletion area; and forming contact surfaces by cutting or shaving the alloy remotely from the surface from which oxygen was diffused into the alloy to achieve internal oxydation thereof so that the oxide depletion area is removed from the alloy and the forwardmost area is exposed to form a contact surface.

7. A method as claimed in claim 6, wherein the Ag alloy which is prepared further comprises one or more metallic elements selected from the group consisting of 0.1 - 5 weight % of Cd, 0.1 - 2 weight % of Zn, 0.1 - 2 weight % of Sb, 0.01 - 2 weight % of Pb, and 0.1 - less than 2 weight % of In.

8. A method of making an internally oxidized Ag-SnO system alloy electrical contact, which comprises:
preparing an Ag alloy layer having a thickness at least twice as thick as a preselected thickness and compri-sing a silver matrix and either 0.5 - 12 weight % of Sn and 0.5 - 15 weight % of In or 3 to 12 weight % Sn and 0.01 -less than 1.5 weight % of Bi;
fixedly sandwiching the alloy layer between thin pure silver layers:
completely internally oxidizing said alloy layer to form an oxide depletion layer intermediate its opposed surfaces;
slitting said alloy to a desired configuration after internal oxidation thereof;
cutting the alloy into two parts along said deple-tion layer; and removing the depletion layer left on one of said parts, wherein said one part after such a removal forms said contact.

9. A method as claimed in claim 8, wherein the Ag alloy which is prepared further comprises one or more metallic elements selected from a group consisting of 0.1 - 5 weight % of Cd, 0.1 - 2 weight % of Zn, 0.1 - 2 weight % of Sb, 0.01 - 2 weight % of Pb, and 0.1 - less than 2 weight % of In.