US2983996A

US2983996A - Copper-tungsten-molybdenum contact materials

Info

Publication number: US2983996A
Application number: US751984A
Authority: US
Inventors: Lloyd F Neely; Ang Choh-Yi
Original assignee: PR Mallory and Co Inc
Current assignee: Duracell Inc USA
Priority date: 1958-07-30
Filing date: 1958-07-30
Publication date: 1961-05-16
Anticipated expiration: 1978-05-16
Also published as: GB857569A

Description

May 16, 1961 F. NEELY ETAL 2,983,996 COPPER-TUNGSTEN-MOLYBDENUM CONTACT MATERIALS Filed July 50, 1958 INVENTORS LLOYD F. NEELY CHOH-Yl ANG ATTORNEY COPPER-TUNGSTEN-MOLYBDENUNI CONTACT MATERIALS Filed July 30, 1958, Ser. No. 751,984

'8 Claims. (Cl. 29-182) This invention generally relates to contact materials, and, more particularly, to electric make-and-break contacts formed of a copper-tungsten-molybdenum composition.

Electric contacts comprising a refractory constituent, such as tungsten, and a high conductivity constituent, such as copper, have been used in the electrical arts for many years and on a very substantial scale. The principal advantage of contacts of this type is that they combine the substantial hardness and other desirable characteristics of the refractory metal with the high thermal and electrical conductivity of the high conductivity metal. However, these contacts would erode very badly when used for certain applications, for example in circuit breakers for interrupting currents of high intensity, causing operating difiiculties and requiring frequent replacement of the contacts at considerable expense.

It is an object of the present invention to improve contacts of the described general type.

It is another object of the present invention to provide novel and improved electric contact materials having excellent electrical and mechanical properties, especially greatly improved resistance to are erosion.

The invention also contemplates electric make-andbreak contacts having greatly improved characteristics, including superior resistance to are erosion, which can be readily manufactured and sold at a cost below that of conventional contacts.

Other and further objects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying dr-awing, in which:

Fig. 1 is a photomicrograph showing the microstructure of a preferred contact composition of the invention at a magnification of 100 diameters; and

Fig. 2 is a similar photomicrograph showing the microstructure of the said composition at a magnification of 500 diameters.

Broadly stated, in accordance with the principles of the present invention, there is provided a novel contact material which has excellent electrical and mechanical properties and is particularly characterized by a high resistance to are erosion. The novel contact material can be used as high current make-and-break contacts in circuit breakers, as pressure exerting welding electrode, or as medium to heavy duty contacts or electrodes in any application where the prime requisite is a low erosion rate. These new results and advantages are obtained by a metal composition containing copper, tungsten and molybdenum within a well-defined and narrow range. Thus, it has been found that materials essentially composed of 28% to 36% by weight of copper, 32% to 36% by weight of tungsten, and 32% to 36% by Weight of molybdenum consistently provide the advantages of the invention, the preferred composition range being about 30% to about 34% by weight of copper,'about 33% to about 35% by weight of tungsten, and about 33% to about 35 by weight of molybdenum.

2,983,996 Patented May 16, 1961 Various methods may be used for making the contact compositions of the invention, of which the press-sinterinfiltration method has been found to provide highly satisfactory results. In carrying this method into prac tice, the preferred raw materials are tungsten powder and molybdenum powder not over 5 microns in Fisher average particle size and copper powder minus 100 mesh. A mixture of these powders in suitable proportions, such as 41% by weight of tungsten powder, 41% by weight of molybdenum powder, and 18% by weight of copper powder, is blended thoroughly by ball milling for 6-12 hours, using a ceramic mill and balls. The powder mix is then screened through a 40 mesh sieve. Binders, such as Amberol (an oil-soluble phenol-formaldehyde and maleic glyceride resin), and/or lubricants may be added to the powders in amounts as high as 2% by weight to improve powder flow and apparent density and to increase the green strength of the pressed compacts.

Compacts of desired shape are pressed from the blended powder mixture by means of conventional powder metallurgical procedures. The pressed density of the compacts may be varied from 7 to 9 gm./cc. Keeping sintering conditions constant, the final copper content after infiltration is roughly inversely proportional to the pressed density. The higher the pressed density, the lower is the porosity to be replaced by copper during infiltration. The composition ranges given in the foregoing cover the practical range of pressed density here described.

Either batch or stoker type sintering operations may be employed in sintering the pressed compacts. The sintering temperature may be 1300 to 1350 C. and the time at heat 6 to 20 minutes. The compacts can be placed in either graphite or metal (such as molybdenum) boats, with or without Alundum sand. The furnace atmosphere must be either reducing or inert. Together with the variation in pressed density, sintering conditions may be varied slightly to obtain the desired linear shrinkages, which will affect tooling and also final composition analysis. The nominal range of linear shrinkages expected from the processing conditions specified in the foregoing is 1 to 6% The furnacing conditions of sintering also apply to the infiltration step, except that the temperature is lowered to 1200 to 1275 C. The calculated amount of copper" with some excess is placed on top of the compact, or, if desired, at the ends in the case of a bar, or applied in any other manner practiced by a powder metallurgical shop. The sintered compacts may have some of their sides, or faces, covered by an impervious refractory paste, such as kaolin, to control the direction or amount of bleeding of the copper. The infiltrated parts usually have some excess of copper, which is machined ofi.

Another satisfactory method of preparing the contact materials of the invention is the straight press and sinter procedure without subsequent infiltration. In this case, the correct amounts of tungsten, molybdenum and copper powders to make up the final composition, for example 30% by weight of copper, 35% by weight of tungsten, and 35 by weight of molybdenum are thoroughly blended and then fritted at about 1100 to 1200 C. in a reducing atmosphere into slightly sintered cakes. These cakes are broken up by pulverization and then screened. Binders, lubricants, or both, may be added to the powder. The compacts pressed fromsaid powder are then sintered to a density over of the theoretical density at some temperature between 1350 and 1425 C. for not less than 30 minutes.

Other methods of preparing the contact materials of theinvention are simultaneous sintering and infiltration of compacts pressed from the constituent powders, or hot pressing.

v A comparison of properties between the novel contact material A (composed of 31.85% by weight of copper,

Copper analyses, percent by weight 31. 85 32. 10 Hardness, Rockwell B 90-95 86-90 Conductivity, percent IACS 39-41 30-40 Average weight loss, mg./operatin 2.62 6. 89 Average volume, loss, cc.X /operat1on 2. 33 4. 90

The average Weight and volume losses per operation e have been optained from results of short circuit arc erosion tests carried out under oil. The short circuit test consisted of 3.0.interruptions of 1,400 -amperes,.2.40.volts, A.C. The current flow was limited to one-half cycle by an electronic timer which controlled the energizing of the circuit.

From the foregoing tabulation, the superiority of the contact material of the invention over the conventional material, Which is extensively used as heavy duty electrical contacts, is clearly apparent. Particularly striking is the improvement of the resistance to are erosion which is probably the most important requirement in contacts of the described type. Comparative tests indicate that the contacts ofthe invention are about 27% more erosion resistant, on weight basis, than the conventional contacts.

In preparing the contact material of the invention referred to in the foregoing table, the ball milling time was 7-8 hours. The compacts were pressed to a density of 8.75 g./cc. and sintered at 1325 C. for 12 minutes. 'Ihereupon the sintered compacts were infiltrated at 1250 C. for 8 minutes. Typical properties of the novel contact material prepared by the method just described will appear from the following:

Density 11.25 g./cc., nominal. Hardness 95 Rockwell B. Electrical conductivity 42i4% IACS.

Figs. 1 and 2 of the drawing show the microstructure of the contact material of the invention obtained at a magnification of 100 and 500 diameters, respectively, with an etchant of NaOH+K Fe(CN) The white areas are the infiltrant copper and the clearly visible grains are particles of tungsten and molybdenum. There is pnac tically no alloying between copper and tungsten or molyb denum. Excellent physical and mechanical properties are derived from the uniformity of microstructure, clearly apparent in Fig. 1, and from the homogeneity of bond: ing between grains of the refractory metals, with-a minimum amount of residual porosity.

It will be noted that the contact materials of the invention provide a number of important advantages. First of all, the electrical and mechanical properties of the novel contacts, particularly their resistance to are erosion, are

greatly superior to those of comparable conventional contact materials composed of copper and tungsten in the absence of molybdenum. Although both tungsten and.

molybdenum have about the same electrical conductivity at room temperature, molybdenum has a lower temperature coeflicient of electrical resistivity and a lower electron work function than tungsten. Furthermore, molyb-.

denum has higher thermal conductivity than tungsten. Together with tungsten, which has a high melting point, and copper, which is an excellent electrical and thermal conductor, the presence of molybdenum within a clearlysurface temperature during arcing because of its lower work function and its greater capacity to conduct away the heat. The loss of material during arcing is therefore reduced to a minimum.

It is also to be observed that, as the preferred procms of making the contacts of the invention is closely similar to that of making conventional tungsten-copper materials, there is no significant difference in the cost of manufacture of the two types of materials. A substantial amount of saving is realized, however, in the cost of raw materials. Molybdenum powder is about 11% cheaper per pound than tungsten powder and its density is only about half of that of tungsten. A simple calculation will show that, as the cost of making contacts is based on volume, the cost saving on raw material per given piece of electrical contact embodying the invention is about 24%. A cost saving of such magnitude is, of course, of great commercial value in a highly competitive art as that of electrical contacts.

Although the present invention has been disclosed in connection with preferred embodiments thereof, variations' and modifications may be resorted to by those 1 skilled in the art without departing from the principles of the invention. All of these variations and modifications are considered to be within the true spirit and scope of the present invention as disclosed in the foregoing description and defined by the appended claims.

What is claimed is: 1. A metal composition particularly suitable for electric contacts and pressure exerting welding electrodes which comprises 28% to 36% by weight of copper, 32% to 36% by weight of tungsten, and 32% to 36% by weight of molybdenum, said tungsten and said molybdenum being substantially unalloyed with each other.

2. A metal composition particularly suitable for electric contacts and pressure exerting welding electrodes essentially composed of 30% to 34% by weight of copper,

33%. to 35% by weight of tungsten, and 33% to 35% by weight of molybdenum, said tungsten and said molyb denum being substantially unalloyed with each other.

3. An electric maKeand-break contact made of a metal composition essentially composed of about 30% by weight of copper, about 35 by weight of tungsten, and about 35 by weight of molybdenum, said tungsten and said molybdenum being substantially unalloyed with each other.

4., An electric make-and-break contact having high resistanceto arc erosion essentially composed of 28% to v 36% by weight of copper, 32% to 36% by weight of tungsten, and 32% to 36% by weight of molybdenum, said tungsten and molybdenum being substantially unalloyed with each otherand being in the form of a porous matrix, the pores of which are substantially filled with saidcopper.

5. An electric make-and-break contact characterized by high. resistance, to are erosionformed .of a pressed and sintered porous compact of tungsten and molybdenum infiltrated with "copper, said tungsten and said molybdenum being substantially unalloyed with each other and each constituting about 32% to about 36% by weight of the contact, the balance of said contact being substantially all copper.

6. An electric make-and-break contact characterized by high resistance to are erosion formed of a pressed and sintered porous compact of tungsten, molybdenum and copper which is infiltrated with an additional amount of copper, said tungsten and said molybdenum being substantially unalloyed with each other and each constituting about 32% to about 36% by weight of the contact, the

defined range in the novel contact material makes it highly V resistant to are erosion. In comparison with the straight.

copper-tungsten materials of the prior art, the-contact material of the invention is characterized by a much lower.

" about 3 6%.. by weigh't'of tungsten, and about 32% to'.

about 36% by weight of molybdenum, the balance being- References Cited in the file of this patent UNITED STATES PATENTS Fahrenwald Aug. 7, 1917 Hoyer Sept. 9, 1958 OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, vol. I, 1949, 0 page 204; vol. 2, 1950, pages 197-201.

Claims

1. A METAL COMPOSITION PARTICULARLY SUITABLE FOR ELECTRIC CONTACTS AND PRESSURE EXERTING WELDING ELECTRODES WHICH COMPRISES 28% TO 36% BY WEIGHT OF COPPER, 32% TO 36% BY WEIGHT OF TUNGSTEN, AND 32% TO 36% BY WEIGHT OF MOLYBDENUM, SAID TUNGSTEN AND SAID MOLYDBENUM BEING SUBSTANTIALLY UNALLOYED WITH EACH OTHER.