US2621123A - Method of sintering silver contact material - Google Patents

Description

Patented Dec. 9, 1952 Ne-o STAT -METHOD OF SINTERING SILVER CONTACT MATERIAL No Drawing. Application April 23, 1949, Serial No. 89,371

1 Claim. 1

My invention relates to an electrical contact material, and more particularly to a novel powdered silver copper graphite mixture for contact materials.

Heretofore powdered silver graphite has been commonly employed as contact materials in circuit breakers. Silver is a good conductor and therefore keeps the RI drop at the contacts to a minimum. Graphite was added to impart to the silver the non-welding property necessary during arcing when the circuit breaker opens on short circuit conditions.

Silver, however, while a good conductor, has defects in that it materially tends to soften the contact material and increases the danger of welding despite the presence of the graphite.

I have discovered that by the addition of a mixture of fine powdered copper and fine powdered graphite to fine powdered silver, I can appreciably increase the hardness of the end product resulting in a substantial reduction in the rate of mechanical and electrical wear.

Accordingly, an object of my invention is to provide a novel mixture of powdered silver, copper and graphite.

In carrying out my invention, I have found from experimentation that a desirable composition is an end product of 47 /2% of silver, 47 of copper, and 5% of graphite. To obtain this end result I start with a mix of 47.125% copper, 47.125% silver, and 5.75% graphite. The additional of graphite I have found necessary due to loss of graphite during the processing. This occurs because some of the graphite tends to combine with oxygen, metallic oxides and minute quantities of oxygen carried by the sintering atmosphere, as will be described hereinafter, to form carbon-monoxide and carbon-dioxide.

In my experimentation, I discovered that in mixing the powdered silver, copper, graphite in the proportions described above, the very fine grain and light graphite powder tended when the mixture was agitated to rise to the top of the mixture and separate from the other powders during processing. I overcome this defect by using extremely fine grain powders of the several metals.

Particularly when I used a fine silver powder which passes 100% through a 325 mesh and a fine grade of copper powder which passes 100% through a minus 325 mesh and as fine a graphite powder as I could secure, substantially no separation occurred during agitation.

The silver and copper powders of the above mix are dry tumbled for two hours, after which time the graphite is added and the whole mix tumbled for an additional fifteen minutes.

When I subjected this mix to a test for separation by repeatedly dropping the mixture a distance of approximately six inches and thereafter testing for homogeneity of the mix, I found substantially no separation of the graphite from the silver and copper powders.

The mixture is first subjected to a pressure of five tons per square inch resulting in a percent of theoretical density of approximately 62.8%. The compacted material is then sintered in a hydrogen atmosphere at 1350 F. for one-half hour, although this temperature is not found to be critical and may vary over a range. It was found that during this first sintering operation the compacted material shrank somewhat, resulting in an increase of several percent of theoretical density.

After sintering, the compacted material is then repressed again at 50 tons per square inch. The re-press percent of theoretical density was found to be in the range of 92.2 to theoretical density and the product then subjected to a resintering at 1350 F. in a hydrogen atmosphere for one-half hour, although this again is not found to be critical and may vary over a range.

The end product was found to have a hardness in the range of 43 to 57 on the superficial 15 T scale and hardnesses in the range of 72 to 82 on the Rockwell I-I scale.

Although in the above I have described specific and preferred percentages, temperatures, pressures, and time of operation, it will be understood that these may be modified somewhat without departing from the spirit of the invention.

Thus, for example, although I have 1350 F. as a preferred sintering temperature, I have experimentally successfully sintered at 1400 F.

I claim:

The method of producing a powdered contact material of silver 47.5%, copper 47.5%, and graphite 5% which comprises mixing powdered silver 47.125% and copper 47.125%, dry tumbling the mix for two hours, thereafter adding graphite 5.75 tumbling for fifteen minutes, subjecting the mix to a pressure of the order of five tons per square inch, sintering the mix in a hydrogen atmosphere at a temperature of the order of 1350" F. for approximately one half hour, re-pressing the material at a pressure of the order of 52 tons per square inch, and re-slntering the material in a hydrogen atmosphere at a temperature of the order of 1350 F. for about one half hour, the graphite percentage of the composition being reduced during the processing to the order of 5% 3 by virtue of loss inherent in the process due to Number combining with oxygen and oxides present. 2,200,854 NORMAN STANLEY HOYER. 2,255,120 2,300,558 REFERENCES CITED 7 5 2 319 240 The following references are of record in the 2,396,101 file of this patent:

UNITED STATES PATENTS Number Number Name Date 7 10 74 842,730 Viertel et a1 Jan. 29, 1907 332,914: 849,643 Speirs Apr. 9, 1907 346,973 1,605,432 Fredriksen 190v. 2, 1926 361,041 1,661,245 Williams Mar. 6, 1928 Name Date Ruben May 14, 1940 Kiefer et a1 Sept. 9, 1941 Driggs Nov. 3, 1942 Larsen et a1 May 18, 1943 Hensel et a1. Mar. 5, 1946 FOREIGN PATENTS Country Date France Dec. 13, 1932 Germany May 16, 1920 Germany Jan. '7, 1922 Great Britain Nov. 19, 1931