US1958357A - Electrical make-and-break contacts - Google Patents

Description

May 8, 1934. J. A. `wElGER ELECTRICAL MAKE-AND-BREAK CONTACTSv Filed Deo. 8, 1931 :Snnentor I8u Gftornegs Patented May s, i934 UNITED STATES PATENT foi-*Fica 15 Claims. (CL 14s-11.5)

i This invention relates to electrical make-and break contacts, and more particularly to electrical make-and-break contacts of tungsten and a method of producing the same.

'I'he primary objects of the -invention are the provision of electrical make-and-break contacts having Agenerally improved characteristics, andv capable of carryingl and breaking larger currents than` has heretofore been possible with the use of contacts common to prior practice while still.

maintaining satisfactory service life, have a decreased arcing tendency, give superior perfomance at high and low operating speeds, and have improved contact resistance characteristics .during performance.

The invention Afurther contemplates, among other objects, the provision of electrical makeand-break contacts in which the deleterious effects usually incident to arcing are pronouncedly reduced, and pitting of the contact faces, fusion and transfer of material from cooperating contacts are eliminated to a'very substantial degree under the conditions to which such contacts are ordinarily subjected.

A further object of the invention is the provi- 5 -sion of tungsten contacts andmethod of treating them, wherein and whereby the usual body structure is so modified as to have improved working characteristics throughout and to present at the contact face a minimum amount of the substance .or substances which adversely affect the eflicient functioning of the contact.

A still further object of the invention is the provision of such contacts and such a method of treating them, wherein and whereby an' interf locking of the grain structure is insured and a reduction `of the impurities and material in the grain boundary condition to a minimum is obtainedby a treatment with a compound of silicon.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

'I'he invention accordingly comprises the several steps and theV relation of one or more of such 'steps with respect to each of the others, and the article possessing the features, propapplications, of which the use in automobile contacts.

erties, and the relation of elements, which are exempled in the following detailed disclosure, and the scope of theapplicationof which will be indicated in the claim. 6o

For a fuller understanding of the nature and objects of the invention reference should be had to the followingdetailed description taken in connection with the accompanying drawing, in which:

, Fig. 1 illustrates a portion of the contact surface of a typical contact such as hadrheretofore been used, magnified 1000 diameters;

Fig. 2 illustrates a portion of a contact surface of a tungsten contact material in which the grains are interlocked, magnified 1,000 diameters;

Fig. 3 illustrates a portion of the contact surface of a tungsten contact prepared from the material depicted in Fig. 2, heat treated to ob- 7.. tain a large grain4 structure, magnified 29 .dio ameters; and

Fig. 4 illustrates a portion of the contact surface of a tungsten contact which has been-treated in accordance with the present invention to obtain an interlocked grain structure, magnified 29 diameters.

'Heretofore electrical make-and-break contacts of tungsten have been extensively used in various ignition systems may be taken as representative. Such contacts up to the present time have been customarily made of wrought tungsten, either by sawing contact discs' from a wrought tungsten rod, or by punching them from a rolled tungsten sheet.

Such contacts develop a number of faults in service. For example, the repeated breaking of current which is required results in periodic arcing, attended with a transfer of metal from one cooperating contact to the other, with consequent pitting of one contact face and building up of projections on the other. Also, the continued4 transfer of material from one contact to another often brings about a localization of the contact area, with a resultant increase in current density at such restricted area of contact. This condition, in the case of prior contacts, renders more pronounced the deteriorating action of the arc. The transfer of material under these circumstances sometimes becomes so great that a sucient amount thereof is built up on one of themake-and-break contacts to prevent Y the breaking of the circuit by separation of the Such factors of deterioration bring 110 and the fact that an increase in the amount of current broken at their faces hastens their deterioration. Consequently the permissible current densities and amperages which have heretofore been usable in ignition and similar make-and-break systems have been restrictively limited to an amount commensurate with satisfactory service life of the contacts. That is, with contacts of the typel hitherto in general use, unless the contact surface was comparatively large, and the amperage comparatively low, the service life of the contacts was unduly shortened.

A proper understanding of these shortcomings in prior contacts, and of the nature of the present invention whereby such unsatisfactory characteristics are eliminated or greatly mitigated, calls for consideration of the microstructure of the contact metal. To study the microstructure,

' the contact surface is polished and then lightly etched with a suitable chemical reagent which serves to develop the grain structure and the boundaries between the grains.

The drawings forming a part of this application are representations of the various structures, shown under the microscope.

Microscopic examination of the surface of the best prior art tungsten contacts, as illustrated in Fig. l, shows the presence of very fine grains, in the neighborhood of 20,000 per square millimeter, each of which is surrounded and separated from adjacent grains by material characv terized by properties different from those of the main body of the metallic grains. Throughout this specication and in the accompanying claims, such material will 'be referred to as material in the grain boundary condition, Whether it is produced in the course of the grain structure formation and actually denes the surfaces of the several grains, or is produced in the working of the contact metal, as by swaging, rolling, grind- .boundary condition) exposed on theY working surface of such contacts is large in proportion to the surface area of the contacts, and in addition the strain-hardening of the ,material of the grains l0 may contribute to the amount of material in the grain boundary condition.

'Y It has been found that the material in the grain boundary condition is much more active physically and chemically than the material at the center of the grains. Whether this be due to greater concentration of impurities at the grain boundaries or to amorphous condition of the grain boundary material, or to some other cause,

is not as yet positively established. But whatever may be its precise nature, this material in the grain boundary condition has properties different from those of the grains themselves.

Many observations .of make-and-break contacts of tungsten of the types heretofore generally used, have indicated that the eiects of deterioration in actual service manifest themselves rst and more markedly 'at the grain boundaries separating the individual lgrains in the Working surface of the contact. It appears that the material in the grain boundary condition has lower resistance to the deteriorating inuences imposed under service conditions than has the material comprising the grain proper. It Volatilizes more readily than the material of the grains themselves, and these facts together with the other properties peculiar to it, appear to be largely responsible for the arc deterioration and other faulty characteristics of contacts heretofore used.

A structure in which the grains are equiaxed, a condition depicted in Fig. 3, has been found to increase the likelihood of poor operating characteristics. that, since the electric arc between two such contacts gradually etches away impurities and materialin the grain boundary condition, the grains become loosened and eventually transfer to the opposite contact, resulting in the pittingof one contact and a building up on the other contact.

In the application of Childress B. Gwyn, Serial No. 551,671, led July 18, 1931 and assigned to P. R. Mallory & Co., Incorporated, methods are disclosed which reduce the material in the grain boundary condition, thus considerably enhancing the operating characteristics of contacts so treated. In accordance with the present invention, it has been found that their operating characteristics can be further improved by insuring an interlocked grain structure and by treating This is believed to be due to the fact the contact material with an ingredient or in- Y gredients which stimulate and accelerate grain growth, and reduce to a minimum impurities and material in the grain boundary condition.

In accordance with the presentinvention, a greatly improved contact material may be obtained by increasing the size of the grains to a size of less than 2,000 per square millimeter'by heat treatment, care -being taken to regulate the time and temperature to grow the grains without equiaxing them to insure an interlocked grain structure. The equiaxing temperature for tungsten is about 2400o C., for a short duration.

The operating characteristics of such a structure can be further improved in accordance with the present invention by treating metal from which the contacts are' to be formed, in such a manner as to reduce the impurities and material in the ygrain boundary condition. Such a reduction is performed in the preferred form by treating the material from which the contacts are to be formed with an ingredient which reduces such impurities and which can then be volatilized to leave the metal in a substantially pure form. It hasbeen found that a compound of silicon, such as, for example, sodium silicate, which can be reduced to.silicon oxide by heat treatment, will not only perform these desired functions, but -will, in addition, stimulate and accelerate grain-growth. y

Such a method may bespracticed in the following manner, although it is to be understood that the present invention is not to be limited thereto, in regard to the number of steps taken, their order, and the specific values set forth in each step. in the treatment, since it is obvious that the invention may be practiced by other specific methods, or the desired features of the V'tungsten contacts may be obtainedv in varying degrees by the practice of methods of this same general character, differing from the preferred method chiefly in specic values of the treatments constituting the different steps of the method, or the .order in which they are taken. A The preferred method is practiced by addin to pure tungsten in the oxide form a small amount of silicon oxide, approximately .5 per cent in the form of sodium silicate. 'I'he mixture is thoroughly mixed to insure an even distribution of the silicon compound throughout the mass of the oxide of tungsten. This mixture is then fired, preferably in a silica crucible, at approximately 1100 C., for about one hour,- to agglomerate the finely divided particles into coarser particles and to convert the sodium silicate into silicon oxide. 'Ihe fired mixture is then reduced by heating in a reducing atmosphere, such as, for example, hydrogen, at a temperature increasing from about 400 C., to about 900 C., for a considerable period of time, such as, for about two hours. The mixture is then removed from the reduction furnace and passed through a sieve of about 200 mesh, after which it is pressed into slugs by considerable pressure, approximating about ten tons per square inch.

The pressed slug is then baked in a non-oxidizing atmosphere at a temperature of about 1250 C., for a short period of time, preferably about fifteen minutes, and is then heat treated -in a non-oxidizing atmosphere, preferably hydrogen, by passing an electric current through it, the current being increased gradually until the slug is heated to about between 3000 C., and 3200 C., the heat being maintained at the maximum temperature for a short period of time, such as, for about ten minutes, after which the ow of current is discontinued and the slug allowed to cool rapidly.

During this heat treatment, the silicon com- 1 pound reduces the impurities, including oxygen,

in the tungsten, decreases the amount of material in the grain boundary condition and in addition, this ingredient accelerates and stimulates the grain/ growth, thereby producing a material with a..l relatively large grain structure. After performing its desired functions this ingredient is vlargely volatilized by this heat treatment.

The treated slugs are then hot-swaged into rods, the temperature being accurately controlled to avoid equiaxing the grain structure of the metal and to insure the production of a strain worked metal with distorted and interlocked L grains, a condition depicted in Fig. 2. The swaged rods are then heated in a non-oxidizing atmosphere, preferably of hydrogen, by passing an electric current through them, the metal being quickly heated to a temperature of about 2400 C., the temperature being held at the maximumvalue for a short period of time, approximaterly two minutes, after which the metal is cooled rapidly. This latter heat treatment is performed to obtain an increase in the size of the grains, the temperature being increased to the maximum Value depicts sucha material having a grain count of about per square millimeter. Advantageous results are obtained through the use of electrical make-and-break contact material having a grain count much higher than that of the specic material depicted, produced by the practice of the above outlined method, the upper limit being about 2,000 per square millimeter. Since the last step in the method outlined above is chiefly the determining factor in this respect, any desired grain size within the limits set forth may be obtained by proper control of this heat treatment.

Comparative tests were conducted to determine the relative merits ofV various .types of contacts, including those made in accordance with the present invention. These contacts were operated at 8.75 volts across the contacts and approximately 6.25 break amperes, making and breaking the circuit 2,200 times per minute, and showed the following results after 61 hours of operation:

y 1. Standard fine grain tungsten contacts of 20,000 grains per square millimeter, showed 16 per cent failures;

2. Large grain equiaxed tungsten contacts with no reduction in the material in the grain boundary condition showed 33 per cent failurevs; and

3. Distorted interlocked grain tungsten contacts, retreated to reduce the material in the grain boundary condition, showed no failures.

Electrical make-and-break contacts, made in accordance with the present invention, efficiently attain the objects set forth above in a very admirable manner, improved service life and operation at much higher current densities being apparently attributable to the facts that since the material in the grain boundary condition has been materially reduced, very limited areas of this material' are subjected to the action of the arc, the result being that the arc will wander more uniformly over the entire surface of the contact, Vinstead of concentrating on one local spot; and even after the wearing away of the material in the grain boundary condition, the grains are securely held in their positions by the mechanical interlock. i

Electrical make-and-break contacts, made in accordance with the present invention, can be further improved by further treatment of the contact surfaces thereof to additionally reduce the material in the grain boundary condition. 125 Such a treatment has been disclosed in the application of Childress B. Gwyn, Serial No. 551,671, mentioned above.

Since certain changes may be made in the contacts and in the process of manufacture described 130 above without departing from the scope of theinvention, it iswintended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sensei yIt is also to be understood that the following claims are intended to cover all of the generic and specific featuresof the invention herein described and all statements of the scope of the invention which as a matter' of language might be said to fall therebetween.

Having described my invention,` what I claim as new and desire to secure by LettersPatent, is:

1. An electrical make-and-break contact of tungsten having an interlocked grain structure 145 of relatively large grains and the impurities and material in the grain boundary condition reduced 'to a minimum by the volatilization by heat of a previously incorporated Water soluble alkali metal silicate which stimulates graingrowth. w

2. An electrical make-and-break contact of tungsten having an interlocked grain structure of relatively large grains and the impurities and material in the grain boundary condition reduced to a minimum by the volatilization by heat of previously incorporated silicon oxide formed by heat conversion of a water soluble alkali metal silicate which stimulates grain growth.

3. An electrical make-and-break contact of tungsten having an interlocked grain structure and the impurities and material in the grain boundary condition reduced to a minimum.

4. An electrical make-and-break contact of tungsten having a structure of relatively large interlocked grains in which the impurities and material in the grain boundary condition have been reduced to a minimum.

5. An electrical make-and-break contact having a tungsten face of an interlocked grain structure substantially free from material in the grain boundary condition.

6. An electrical make-and-break contact having a tungsten face of an interlocked grain structure with a grain count of less than 2000 per square millimeter and being substantially free from material in the grain boundary condition.

'7. The method of producing an electrical makeand-break contact material comprising mixing, with an oxide of tungsten a water soluble alkali metal silicate, heat treating the mixture to convert the silicate to an oxide of silicon and to reduce the tungsten oxide to tungsten, heat-treating at a temperature of between about 88 per cent and 95 per cent of the melting point of tungsten to cause the silicon oxide to perform a grain growth accelerating action, remove impurities and material'in the grain boundary condition and to volatilize the silicon oxide, and mechanically 1 working and heat-treating the tungsten at a temperature above the equiaxing temperature of. tungsten to obtain a body in which the grains are of a desired size and interlocked, the temperatures during the latter heat treatment' being rapidly carried through the equiaxing temperature of tungsten.

8. The method of producing an electrical makeand-break contact material comprising mixing with an oxide of tungsten a water soluble'alkali metal silicate, firing the mixture to convert the silicate to an oxide of silicon, reducingthe mixture in a reducing atmosphere with heat to change the` tungsten oxide to tungsten, pressing the resultant mixture into a slug, heat-treating the slug at a temperature between aboutv 3000" C. and 3200 C. to cause the silicon oxide to perform a grain growth accelerating action and remove impurities in the material in the grain boundary condition, maintaining the heat at a maximum temperaturegto volatilize the silicon oxide and hot swaging the slug of tungsten into' a rod ofv strain workedcontact material vat a'temperatureE which insures an interlocked grain structure.

9. The method of producing an electrical makeand-break contact material comprising mixing with an oxide of tungsten a waterv soluble alkali metal silicate, converting the silicate -to` silicon oxide by heat reducing the mixture in a reducing atmosphere with heat to change the tungsten oxide tov tungsten, pressing the resultant mixture into a slug, heat treating the slug at a temperature between about 3000? C. and 3200 C. to cause the silicon oxide to perform a grain growth accelerating action and remove impurities and material in the grain boundary condition, maintaining the heat at a maximum to volatilize the silicon oxide, hot swaging the slug of tungsten into a rod of strain worked contact material at a temperature which insures an interlocked grain structure, and heat treating the rod-in a non-oxidizing atmosphere at a temperature which insures a desired grain growth after passing quickly through the equiaxing temperature to prevent destruction of the interlocked grain structure.

10. 'I'he method of producing an electrical make-and-break contact' material comprising mixing tungsten oxide with sodium silicate, treating with heat to convert the sodium silicate to silicon oxide,.reducing the. mixture in a reducing atmosphere with heat to reduce the tungsten oxide to tungsten, pressing the resultant mixture'into a slug,` heat treating the slug at a temperature between about 3000 C. and 3200 C. to cause the silicon oxide to perform a grain growth accelerating action and remove the impurities and material in the grain boundary condition, maintaining the heat at a maximum temperature to volatilize the silicon oxide and hot swaging the slug of tungsten into a rod of strain worked contact material at a temperature which insures an interlocked grain structure.

l1. In a method of producing make-and-break contact material, the step comprising heating at a temperature between aboutk300o C. and 3200 C. to accelerate and stimulate the grain growth and to remove impurities and vmaterial in the grain boundary condition a mixture of tungsten oxide and an oxide of silicon formed by heat conversion -of a water soluble alkali metal silicate.

12. In a method of producing an electrical make-and-break contact material, the steps comprising heating at a temperature between about 3000 C. and 3200 C. to accelerate and stimulate grain growth and to remove impurities and the materialin the grain boundary condition a mixture of tungsten oxide and an oxide of silicon formed by heat conversion of a water soluble4 -grains of relatively large size obtained by heating to a temperature above the equiaxing temperature, equiaxing being avoided by passing rapidly through the equiaxing temperature.

14. The method of producing an electrical make-and-break contact material comprising mixing tungsten oxide and a water soluble alkali metal silicate, ring the mixture to convert the silicate to an oxide of silicon, heating theresultant mixture of tungsten oxide and silicon oxide in a reducing atmosphere at a temperature between about 800 C. and 1000 C. to reduce the tungsten oxide to tungsten, ,forming a slug from the mixture, heating the slug at a temperature Vbetween about 88 per cent and 95 .per cent ofthe fusing temperature lof tungsten to volatilizezthe y silicon oxide and cause it to decrease the amount of material i'nthe grain boundary condition and accelerate and stimulate grain growth, hot swaging the slug to form a strain worked rod having f temperature to promote grain growth without equiaxing the grains.

15. 'I'he method of producing `an electrical make-and-break contact material comprising mixing tungsten oxide with an oxide of silicon in the form of a water soluble alkali metal silicate, iiring the mixture at a temperature of about 1100 -C. for a time suicient to convert the silicate to silicon oxide, ring the mixture in a reducing atmosphere at a temperature of about between 800 C. and 1000 C. for a time suicient to reduce the tungsten oxide to tungsten, pressing the mixture into a slug, heating the slug in a non-oxidizing atmosphere at a temperature between about 3000 C. and 3200" C. for a short period of time to volatilize the silicon oxide and 4cause it to decrease the amount of material in the grain boundary condition and accelerate and stimulate grain growth, hot swaging the slug into a rod of strain worked metal with distorted and interlocked grains, quickly heating the strain' worked rod to a temperature higher than the equiaxing temperature to increase the size of the grains and rapidly cooling the rod.

JOSEPH A. WEIGER.

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