US2300558A - Contact alloys - Google Patents

Description

Patented Nov. 3, 1942 CONTACT ALLOYS Frank H. Drlggs, Highland Park, 111., assignmto Fansteel Metallurgical Corporation, North Chicago, Ill., a corporation of New York No Drawing. Application July 22, 1940, Serial No. 346,799

7 Claims.

This invention relates to electrical make-andbreak contacts adapted for use in many electrical contact applications.

It is an object of the invention to provide electrical make-and-break contacts out of a composition that heretofore has not been thought useful for this purpose.

It is a further object of the invention to provide electrical contacts which are harder, more durable and more resistant to oxidation than most of the contacts currently employed.

Yet another object of the invention is to provide a contact which is simple and cheap to produce in the forms commonly required by the trade.

For many years electrical make-and-break contacts have been provided of tungsten. This material has proved to be especially suitable in automobile ignition service and many other applications where hard, evenly oxidation-resistant contacts are required. Tungsten contacts in this field have almost entirely replaced the platinumiridium type of contacts, not only because they are less expensive but because they have definitely superior qualities. However, ,tungsten contacts are somewhat subject to oxidation, and to this extent they may well be improved upon. For certain other applications, tungsten is not sufliciently highly conductive, and for these applications it has been proposed to make up a porous body of tungsten and impregnate the pores thereof with more highly conductive metal such as silver or copper. No substantial degree of alloying takes place in bodies of this kind, the tungsten skeleton merely serves as a wear-resisting structure, while the current is carried largely by the silver or copper. In some cases tungsten carbide is substituted for the tungsten in order to obtain even higher hardness although the crushing strength of the material has been considerably lowered.

It has also been proposed to employ tungsten carbide bodies cemented by a binder, such as cobalt, for contact purposes. Cobalt, as distinguished from silver and copper, does form an alloy with the tungsten carbide.

I have found that an alloy composition of tungsten and tungsten carbide combines the virtues of practically all of these prior art compositions, and in addition has many other advantageous properties. Tungsten has an extremely high melting point, and therefore requires extensive and expensive special processing in order to produce solid bodies suitable for contact purposes. Tungsten carbide has a much lower melting point, and I have found that an addition of 10 to 30 per cent by weight of tungsten carbide to tungsten enables me to sinter the mixture at a sufiiciently low temperature to make the production of shaped bodies directly from the mixed powders a feasible and attractive project. Furthermore, the tungsten carbide employed produces a considerably harder body than a pure tungsten contact, thereby increasing the wear-resisting properties of the contact.

Another significant advantage which I obtain is the provision of a contact material which contains no foreign material and is at the same time entirely homogeneous. This is a great advantage over the tungsten carbide-cobalt type of contact in that it eliminates the cobalt which is a great deal more susceptible to oxidation than tungsten or tungsten carbide. It is also superior to the tungsten or tungsten carbide-silver compositions, intthat it is uniform and homogeneous throughou Last of all, I find there is a definite advantage in the high oxidation resistance of my novel contacts. One of the difliculties heretofore met with in the operation of tungsten contacts is their tendency to oxidize. The oxidation of my novel contact appears to take place in two steps, the first stage apparently transforming tungsten carbide into pure tungsten and an oxide of carbon, and the second step oxidizing the tungsten to tungsten oxide. Inasmuch as the weight of carbon required to combine with oxygen is less than /15 that of tungsten, it will be readily seen that the wear on my novel contacts due to oxidation is reduced, to say nothing of the wear resistance due to inherent hardness.

I produce my novel contact by admixing tungsten powder of an average particle size of 3 to 5 microns with tungsten carbide powder ranging from 5 micron size downward. I may, for example, employ 10 per cent by weight of tungsten carbide. The resulting powders are mixed by a mixing and screening process, or I may assist intimate mixture by dispersing the tungsten carbide in alcohol, and then mixing it. The resulting powder mixture, when dry, is pressed at high pressures of the order of 100,000 pounds per square inch. It is convenient to press the powder into the shape desired for the final contact application, providing however bodies of somewhat larger size than those required in order to allow for shrinkage.

The resulting bodies are then put into a suitable container, employing separating agent which may desirably be a coarse tungsten powder of 50 to mesh in order to prevent the individual bodies from fusing to each other. The pressed bodies are then sintered in a vacuum or neutral atmosphere. I have found that a temperature of at least 1800 C. should be used, and

not less than a fifteen-minute sintering period...

I prefer for the example given to employ a temperature of 2200 C. for to one hour.

It is not necessary to employ tungsten carbide itself in the production of this material. I may use carbon in the form of finely divided graphite or lamp black; and in the example above given .51 per cent of carbon is the equivalent of fore not surprising that the substitution of silicon for carbon may be made.

Since I may use up to around 30 to 35 per cent tungsten carbide, this corresponds to approximately 4 tungsten atoms for every carbon atom present. In the substitution of silicon for carbon, the substitution should be made on an atomic basis rather than a weight basis, since the carbon or silicon is present on a chemical basis.

In order to demonstrate the effectiveness of my new material, I have tested it in comparison with commercially available materials. My first test was performed with an automobile horn. Four horns were employed with each of the tested materials using the rated voltage of 6% volts. The contacts were deemed to have failed when the horn refused to blow.

Re indicates the contact resistance at the end of the test and was measured by the voltmeterammeter method at 20 amperes, the rated operating condition of this type of horn.

Pitting grades are measured as follows:

(1) Flat, or nearly so. (2) Smooth, shallow pit. (3) Deep, sharp-edged pit.

A second test was run on a standard commercially available electric shaver operated on '110 volts A. C. This test was conducted for a period continuous test hours at an average speed of about 3100 R.P. M. Final contact resistance was measured at 2 amperes, the normal operating amperage of this type of device.

Having thus described my invention and the mode of operation which I have determined now to be best, what I wish to have protected by United States Letters Patent is set forth in the following claims.

I claim: a

1. An electrical contact element consisting, except for minor impurities, of tungsten and at least one element from the group consisting of carbon and silicon in the ratio of from about 4 tungsten "atoms to 1 of carbon, or silicon, down to about 100 tungsten atoms to 1 of carbon or silicon.

2. An electrical contact element consisting, except for minor impurities, of tungsten and carbon in the ratio of from about 4 tungsten atoms to 1 of carbon, down to about 100 tungsten atoms to 1 of carbon.

3. An electrical contact element consisting, except for minor impurities, of tungsten. and silicon in the ratio of from about 4 tungsten atoms to 1 of silicon, down to about 100 tungsten atoms to 1 of silicon.

4. An electrical contact consisting, except for minor impurities of 0.1 to 1.0 per cent carbon and the rest tungsten.

' 5. An electrical contact consisting, except for minor impurities of about 0.6 per cent carbon and the rest tungsten.

6. A sintered-to-shape electric contact body comprising not more than 2.5 per cent of an element from the group consisting of carbon andsilicon, and tungsten substantially the remainder. 7. An electrical contact consisting, except for minor impurities, of 0.15 to 3.5 per cent silicon and the rest tungsten.

FRANK H. DRIGGS.