EP0579670B1 - Material for electric contacts of silver with carbon - Google Patents

Abstract

Powder-metallurgically produced material or extruded semi-finished product for electric contacts of silver or a silver-based metal material with 0.5 to 10 wt. % carbon and 0 to 2 wt. % of an additional metal. The material contains powdered carbon in combination with carbon fibres in the mass ratio of 10:1 to 1:10, whereby the diameter of the powder particles is on average smaller than half the length of the fibres .

Description

The invention is based on a contact material with the features specified in the preamble of claim 1. Such a material is disclosed in A. Keil et al., Electrical contacts and their material ", Springer-Verlag (1984), p. 195.

Contact materials based on silver with carbon, in particular with graphite, have found widespread use in the field of circuit breakers in low-voltage power engineering because they offer high security against welding of the contacts. In the majority of cases, the contact material contains the carbon in powder form. Since silver and carbon are not soluble in one another in either the solid or the liquid state, such materials can only be produced by powder metallurgy. It is known to mix silver powder and graphite powder with one another, to press individual parts out of the mixture, to sinter and press them, or to cold-isostatically press blocks from the powder mixture, to sinter and to extrude them, whereby the graphite particles are oriented in the extrusion direction to form fiber-like agglomerates (cf. A-Keil et al, loc. cit., and the company publication "GRAPHOR Kontaktwerkstoffe aus Silber-Graphit", printed by the applicant, with the print reference 4/90), which are common in the literature are simply called graphite fibers. The formation of this fibrous structure is particularly pronounced in AgC materials which are produced by repeated extrusion of sheathed wires filled with graphite powder (cf. K. Müller and D. Stöckel, DE-Z. "Metall" 36 (1982), p. 743 ).

The very high welding resistance of the silver graphite materials is offset by an unsatisfactory erosion resistance as a disadvantage. With increasing graphite content, not only the resistance to welding increases, but also the erosion. High welding resistance and low erosion are therefore mutually exclusive requirements for silver-graphite contact materials.

The graphite powder causes a kind of dispersion hardening in the contact material, so that the material is not very ductile and subsequent shaping of the contact pieces is very complex.

Occasionally, attempts have been made to achieve higher erosion resistance of contact materials by incorporating fibers made of a high-melting material (US Pat. Nos. 3,254,189, 4,699,763, 2057 618, 0 205 897). EP-A-0 205 897 discloses arc electrodes in which carbon fibers are embedded in a carbon matrix. Such electrodes are intended for switching high voltage, but are not suitable for circuit breakers in low-voltage power engineering. DE-A-20 57 618 goes from continuous carbon or graphite threads or from a "wool" of carbon threads which are impregnated with molten silver or copper, optionally with an addition of 0.5 to 4% by weight of platelet-shaped graphite to improve the lubricating properties when used for sliding contacts. Since copper, silver and their alloys do not wet graphite, a carbide-forming additive such as titanium must be used. However, practice has shown that even when such a wetting agent is used, the production of corresponding materials by infiltration of a fiber bundle or a wool from carbon threads is extremely difficult. These difficulties can be avoided by the method described in US-A-4,699,763. Here silver powder, graphite fibers and various additives are mixed to form a slip and processed into contact plates in several powder-metallurgical work steps. The application technology test of such materials, which contain their carbon content in the form of real carbon fibers in accordance with DE-A-20 57 618 or graphite fibers in accordance with US-A-4,699,763, shows that the erosion resistance compared to a composite material which is produced with graphite powder is significantly increased, but the welding resistance is drastically deteriorated. For this reason, no noteworthy practical use has become known for the materials produced according to US-A-4,699,763.

The invention has for its object to provide a contact material based on silver with carbon or graphite, which is superior to the known contact materials based on silver and graphite powder in terms of erosion and processability, but not in terms of welding resistance has the serious disadvantages of a contact material based on silver and carbon fibers.

This object is achieved by a material with the features specified in claim 1. Advantageous developments of the invention are the subject of the dependent claims.

The contact material according to the invention is characterized in that the carbon is present in it in the form of fiber pieces in combination with a portion in the form of a powder. Surprisingly, it has been shown that in the material according to the invention, the values for the erosion and the welding resistance are considerably more favorable than would result from the selected ratios of carbon fibers to carbon powder using the mixture rule; The combined use of carbon fibers and carbon powder leads to an effect that was not foreseeable from the known effect of the individual components.

The carbon fiber content must not be too low, because otherwise the beneficial effect on the reduction of the burn-up and the increase in ductility is too low. In contrast, the proportion of carbon powder must not be too small, because otherwise the welding strength is insufficient. On the other hand, the content of carbon powder must not be too high, because otherwise the material is too poorly deformable. These aspects mean that, with a total carbon content of 0.5 to 10% by weight, the mass ratio of the carbon powder to the carbon fibers is limited to values between 10: 1 and 1:10, preferably to values between 1: 3 and 3: 1 is. At the same time, care should be taken to ensure that the carbon powder is clearly distinguishable not only from the particle shape, but also from the particle size, from the fibers or their fragments, since this greatly favors the achievement of the effect according to the invention. The fiber pieces should be at least twice as long in the contact material as the graphite powder particles are in diameter. The length of the fiber pieces is preferably 10 to 100 times the average diameter of the carbon powder particles. The diameter of the fibers should be at least twice as large as the powder particles on average. The fiber diameter is expediently in the range from 1 to 50 »m, preferably in the range from 4 to 25» m. Commercially available powders with an average particle diameter of 0.2 to 40 »m, preferably of 1 to 10» m, can be used as carbon or graphite powder. The carbon fibers or the graphite fibers can after known methods can be produced. The length in which they are used must be so small that the fibers can be mixed with the silver powder evenly. Fibers with a length of 30 to 6000 »m are suitable, preferably the fibers are used in lengths of not more than 500» m. The fibers are broken up into smaller pieces by the pressing process, in particular by the preferably downstream extrusion process, so that the average fiber length in the finished contact material is less than the average initial length of the fibers.

The coarse fiber content in the contact material ensures its ductility and erosion resistance; The desired welding strength in combination with the fiber portion is ensured by the powdery fine portion of the carbon, which for this purpose can be significantly lower than in a material that contains no carbon fibers, but only carbon or graphite powder.

The metal matrix of the material according to the invention expediently consists of silver; it can also consist of a silver-based alloy, ie an alloy consisting predominantly of silver, the other alloy partner of which is selected in terms of type and quantity in such a way that it does not reduce the electrical conductivity too much. Copper and nickel are particularly suitable as alloy metals of silver. Instead of alloying this metal, it can also be powder metallurgically combined with the silver.

The carbon content in the material should not exceed 10% by weight. It should be noted that the density of carbon is only about 2 g / cm³ less than that of silver, so that the volume fraction of carbon is significantly higher than its weight fraction . With a content of more than 10% by weight of carbon, the material becomes too brittle, with a content of less than 0.5% by weight of carbon, its effect on improving the welding safety is too small.

To reduce the burnup, the material according to the invention preferably contains no more than 2% by weight of one or more additional metals, namely bismuth, calcium, lead, antimony and / or tellurium. Metallic additives to a silver graphite material are already disclosed in US Pat. No. 4,699,763; However, there is nickel, iron, cobalt, copper and / or gold, with which the burn-up is not to be reduced, but rather the sintering together of the powder particles is to be facilitated (they serve as a wetting aid). The additive metal is preferably used in an amount of at least 0.05%. Smaller additions show no significant effect. More than 2% by weight of the additive metal should not be added because otherwise the electrical conductivity of the contact material will drop too much.

The optimal carbon content is between 2 and 7% by weight, the optimal mass ratio of carbon fibers to carbon powder is between 1: 1 and 3: 1.

The carbon can be used in different modifications, the powder e.g. in the form of soot. The material behaves most favorably if both the carbon powder and the carbon fibers consist of graphite.

The contact material according to the invention not only has the advantage of optimally combining welding resistance and low erosion, its ductility also makes it easier to process, in particular to deform subsequently, which makes the manufacture of contact pieces and their connection to contact carriers easier and cheaper.

Because the material according to the invention is so ductile, semifinished products can even be produced in a simple manner from the material according to the invention, which have a silver backing from the outset, which they need in order to be soldered or welded onto contact carriers. While conventional silver-graphite contact materials are bonded to a silver sintered layer using single-press technology, or extruded contact materials are provided with a solderable backside by burning out the graphite on one side (DE-B: "Electrical contacts and their materials", A. Keil et al., Springer-Verlag 1984, pp. 195 and 196), a semifinished product according to the invention with a silver back can be produced simply by composite extrusion molding, by sheathing a preferably cylindrical block made of the material according to the invention with silver and then inserting it into a reverse extrusion press, which produces a composite extrusion that is still in the die of the extrusion press or is divided lengthways thereafter. Alternatively, the block can also be coated with an AgNi material. In this embodiment, there are additional technological advantages when the contact plates are applied to contact carriers by resistance welding.

Examples:

• 1.
  96.2% by weight of commercially available silver powder, 2.3% by weight of graphitized carbon fibers with a diameter of 15 »m and 1.5% by weight of graphite powder with an average particle diameter of 2» m are dry mixed, cold isostatically into one Pressed bolt, the bolt sintered under a protective gas, surrounded with a sheath made of silver with 10% by weight of nickel and processed by backward extrusion into strips with a thickness of 2.5 mm and a width of 20 mm, which then have a final thickness of 0 , 8 mm are rolled. These strips can be divided lengthways according to the desired contact widths, the contact pieces chopped off and easily welded onto contact carriers.
  A cut of this material parallel to the extrusion direction is shown in FIGS. 3 and 4; in Figure 3 with 50x magnification, in Figure 4 with 500x magnification. The combination of the fibrous coarse fraction with the powdery fine fraction of graphite in the silver matrix can be clearly seen.
• 2nd
  95% by weight of commercially available silver powder, 3.5% by weight of pyrolytically produced carbon fibers, 1% by weight of graphite powder with an average particle size of approximately 1 μm and 0.5% by weight of bismuth powder are mixed with one another and then with processed the steps given in the first example to a band-shaped semi-finished contact.

Comparative examples:

For comparison, two band-shaped semi-finished contact products were produced, which had the same composition as in Example 1, but the total carbon content of 3.8% in one case consisted only of graphite powder and in the other case only of graphitized carbon fibers. These semi-finished products were compared with the semi-finished product according to Example 1 in terms of burn-off and resistance to welding. The results are shown in FIGS. 1 and 2. FIG. 1 shows that the welding forces in the semifinished product according to the invention are much closer to those of the comparative semifinished product which only contains carbon powder than in the comparative semifinished product which only contains carbon fibers. FIG. 2 shows that the semi-finished product according to the invention is almost as good when burned up as the comparatively produced semi-finished product which only contained carbon fibers.

Claims (11)

1. A powder-metallurgically made composite material for electric contacts, consisting of silver or a silver-containing alloy or a silver-containing composite metal as a metal component and of 0.5 to 10% by weight of carbon, characterized in that carbon powder in combination with carbon fibers in a mass ratio from 10 : 1 to 1: 10 are powder-metallurgically processed together with the pulverulent metal component to form a material in which the average length of the carbon fibers is more than twice the average diameter of the carbon powder particles.
2. A material according to claim 1,
   characterized in that the length of the fibers exceeds the average diameter of the powder particles by a factor of ten to one hundred.
3. A material according to claim 1 or 2,
   characterized in that the diameter of the fibers is at least twice the average diameter of the carbon powder particles.
4. A material according to any of the preceding claims,
   characterized in that the fiber diameter exceeds the average diameter of the powder particles by a factor from four to twenty.
5. A material according to any of the preceding claims,
   characterized in that the average fiber diameter is between 4 and 25 micrometers and the average powder particle diameter ist between 1 and 10 micrometers.
6. A material according to any of the preceding claims,
   characterized in that the mass ratio of carbon fibers to carbon powder is between 1 : 3 and 3: 1, preferably between 1 : 1 and 3 : 1.
7. A material according to any of the preceding claims,
   characterized in that the total carbon content is from 2 to 7% by weight.
8. A material according to any of the preceding claims,
   characterized in that the silver-base material contains copper and/or nickel.
9. A material according to any of the preceding claims,
   characterized in that it contains 0 to 2% by weight of a metallic additive and that the metallic additive is one or more of the metals Bi, Ca, Pb, Sb, and Te.
10. A material according to any of the preceding claims,
    characterized in that the metallic additive is present in an amount of at least 0.05% by weight.
11. A process of manufacturing a semi-finished product for electric contacts,
    characterized in that a material according to any of claims 1 to 10 is joined by coextrusion to silver or silver-nickel, which constitutes the backing of the semi-finished product.

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