Semifinished product for electric contacts made of a composite m

Details Semifinished product for electric contacts made of a composite m Semifinished product for electric contacts made of a composite m

1990-09-17

1994-11-01

Walsh, Donald P.

Stock material or miscellaneous articles

All metal or with adjacent metals

Having metal particles

75246, 75247, 419 19, 419 38, 419 41, 419 42, 419 43, 428547, 428548, 428567, 428568, B22F 316

Patent

active

053606736

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to a semifinished product for electric contacts made of a composite material based on silver-tin oxide and to a powder-metallurgical process of making said product.


STATE OF THE ART

This invention is based on a process having the features set forth in the prior art part of claim 1 and on a semifinished product having the features set forth in the prior art part of claim 14.
At the present time, contact materials based on silver-tin oxide have the best prospects of superseding the contact materials based on silver-cadmium oxide which have proved satisfactory but have fallen into disrepute owing to the toxicity of cadmium. The high importance gained by contact elements made of silver-cadmium oxide in low-voltage switchgear, particularly in motor contactors, is due to the fact that they optimally combine a long life, low tendency to weld, consistently low contact resistance (resulting in a low contact temperature rise), effective arc quenching and good workability. Presently known contact elements based on silver-tin oxide are closest to the contact elements made of silver-cadmium oxide as regards the combination of their properties but do not simultaneously attain such favorable properties in all respects stated hereinbefore.
It is known (DE-26 59 012 B2) that an extremely fine distribution of the metal oxides in the silver matrix will result in favorable properties of the contact. For this reason silver-cadmium oxide materials are often made by an internal oxidation of a silver-cadmium alloy. But it is generally not possible to make a semifinished product of silver-tin oxide by an internal oxidation of a corresponding workpiece made of a silver-tin alloy because a complete oxidation of the tin present in the interior of the workpiece will be obstructed by the formation of passive layers so that the oxidation is virtually restricted to a surface layer. The formation of a passivating layer can substantially be suppressed by an addition of other oxidizable metals, particularly indium or bismuth (DE-A 29 08 923). Contact elements made of such materials may be superior to elements made of silver-cadmium oxide as regards the life under AC3 and AC4 testing conditions (defined in IEC Standard 158-1). But the former exhibit in switchgear a higher contact temperature rise, by which the life of the switchgear may adversely be affected. Moreover, the internally oxidized contact elements cannot subsequently be deformed.
It is also known to make contact materials of silver-tin oxide by powder metallurgy in that a silver powder is mixed with a tin oxide powder, silver-tin oxide blanks are formed in that the powder mixture is compacted and sintered, and the blanks are shaped by extruding or extruding and rolling. In comparison with a silver-cadmium oxide contact material such a material which has been made by powder metallurgy and additionally contains tungsten oxide or molybdenum oxide in/small amounts may exhibit approximately the same good properties as regards contact temperature rise and superior properties in the AC4 life test but will be inferior in the AC3 life test. But it is difficult to deform the blanks by rolling or extruding because the tin oxide particles in the silver-tin oxide composite material will greatly obstruct its plastic deformation. A further difficulty is due to the fact that the working of the silver-tin oxide becomes increasingly difficult as the tin oxide is more finely dispersed in the material because in that case the tin oxide particles will more effectively obstruct the plastic deformation of the composite material as it is worked. In order to improve the workability it has been proposed in DE-A 29 52 128 to anneal the tin oxide powder at 900.degree. to 1600.degree. C. before it is mixed with the silver powder and thus to coarsen the tin oxide powder particles so that they will less strongly obstruct the subsequent working of the composite material. But the improved workability will involve a partial deterioration of the switching properties of th

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