Lately, 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. Typical constituent of such Ag alloys are those comprising of Ag matrices, 0.5-12 weight % of Sn, and 0.5-15 weight % of In, and those comprising of Ag matrices, 3-12 weight % of Sn, and 0.01--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-mentioned latter constituents, 0.1--less than 2 weight % of In many be contained.
These Ag alloys which 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. Oxygen which has penetrated into the Ag alloys as time passes, oxidizes metallic solute elements in the alloys and precipitates them as minute metallic oxide distributed in their Ag matrices. Said metallic oxidized precipitates afford refractoriness and consequently anti-welding to the Ag alloys. The backing thin pure Ag plates 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 not covered by pure Ag layers but are subjected directly to oxygen. Such precipitation of 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. The segregations of metallic oxides at a high concentration about outer surfaces of electrical contact materials make the outer surfaces physically too hard, and produce electrically a high contact resistance of the materials especially at an initial stage of operations and consequently an excessive temperature raise. In practice, such segregations about the outer areas are 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 filings of the files.
In order to avoid the production of such segregations, there were invented by the present inventor certain methods such as disclosed in U.S. Pat. No. 4,457,787 in which vacant lattice voids are produced in Ag alloys by having them absorved with hydrogen and the like, and in the course of internal oxidation, solute metals fill in the voids and precipitate as oxides at the innumerable oxide nuclei on an atomic scale, 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. Pat. No. 4,472,211 in which a high contact resistance which is caused by high concentration or supersaturation of 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.
This invention is, therefore, to provide internal oxidized Ag-SnO system alloy electrical contact materials having contact surfaces of a moderate initial contact resistance and having no depletion layer, and a method of manufacturing such excellent contact materials, not using such methods as disclosed in the above-mentioned U.S. Patents which methods are difficult to adequately control.