EP0736217B1 - Sintered contact material, process for producing the same and contact pads made thereof - Google Patents

Abstract

A sintered contact material made of silver and nickel is characterised in that it contains from 5 to 50 % by mass nickel, and in that nickel is homogeneously distributed in the silver structure as particles of 1 νm to 10 νm average size (d</>). An appropriate process for producing this sintered contact material is characterised in that the nickel is introduced into the silver structure before sintering, as when an alloy is mechanically produced. This process takes place in an air atmosphere. Contact pads made of this material may be extruded as strips or sections, compression moulded as individual contact pieces or be designed as two-layered pieces.

Description

The invention relates to a two-component sintered contact material made of silver and nickel, to the method for its production and to contact pads made therefrom and their use.

In the past, contact materials made of silver (Ag) and nickel (Ni) have proven themselves for switching currents in switchgear in energy technology. The manufacture of such contact materials as well as the manufacture and testing of related contact pieces is described in Int. J. Powder Metallurgy and Powder Technology, Vol. 12 (1976), p. 219-228, described in detail.

In the prior art, silver and nickel powder are usually mixed wet in a mixer, dried, compression-molded and sintered under a reducing atmosphere to produce a contact material made of silver and nickel. The fineness of the structure essentially depends on the size of the starting powder used. Such relationships are described in detail in the monograph by H. Schreiner "Powder Metallurgy of Electrical Contacts", Springer-Verlag (1976), pages 105 to 140. In particular, an AgNi material with average grain sizes of 1 µm produced using precipitation powder is specified.

EP-A-0 462 617 discloses a silver-based contact material which forms a three-component material with the components silver (Ag), nickel (Ni) and nickel oxide (NiO), the nickel in the range from 0.5 to 39. 9% by weight, the nickel oxide in the range from 0.14 to 7% by weight and the silver as the balance. To produce such a material worked according to a combined melting / powder metallurgical process, in which a part is melted as an Ag / Ni powder mixture and subjected to water atomization and mixed with further carbonyl nickel and then subjected to processing including internal oxidation and at least one forming. This means that Ni particles and NiO particles are present in a microscopically inhomogeneous distribution in the finished material.

It has already been assumed that in the case of contact materials made of silver and nickel, the nickel particles must be as small and finely divided as possible in the silver so that the contact has good switching properties. In principle, the known method of mechanical alloying lends itself to this. JP-OS 66/33090 already discloses a process for producing materials for electrical contacts based on silver, in which a metal is selected as a further component which has little or no solubility in silver. The latter metal is in particular nickel, iron, tungsten, or another metal which does not form a mixed crystal with silver or in which there is a tendency to segregate according to the state diagram for thermodynamic reasons.

JP-OS 6633090 strives for a mixed crystal-like constitution of the material. For this purpose, electrolyte / silver powder and carbonyl-nickel powder are mixed in a ball mill with steel balls under so-called styrene gas over longer periods, for example up to 300 hours, in order to obtain a mechanically alloyed powder. The powder obtained in this way should have grain sizes below 0.01 μm. The disappearance of nickel reflections and thus the presence of an amorphous alloy was confirmed in an X-ray diffraction analysis. When producing contacts from an alloy powder produced in this way with alternating sintering and pressing steps, secondary precipitates should be able to occur, but the grain size of the nickel particles should be limited to 1 µm.

It was found that when using mechanically alloyed silver-nickel powders with the amorphous character described above, undesirable side effects can occur, which lead to comparatively poor contact properties.

The object of the invention is to provide a remedy here. The aim is to create a contact material made of silver and nickel, which has improved contact properties compared to conventional silver-nickel materials. At the same time, the associated manufacturing process and corresponding contact requirements should be specified.

< 10 µm, durch einen Mahlvorgang nach Art des mechanischen Legierens erzeugt, in weitgehend homogener Verteilung vorliegen.The object is achieved according to the invention in a two-component sintered contact material made of silver and nickel in that the mass fraction of nickel is between 5 and 50%, and in the silver structure nickel particles with average particle sizes of 1 μm < $\overline{\mathit{\text{d}}}$

<10 µm, produced by a grinding process in the manner of mechanical alloying, are present in a largely homogeneous distribution.

< 5 µm, insbesondere $\overline{\mathit{\text{d}}}$

< 3 µm. Bei den angegebenen Teilchengrößenverteilungen sollte der mittlere Abstand $\overline{\mathit{\text{D}}}$

der Nickelteilchen zwischen 5 und 10 µm liegen.Preferably the mean particle size is nickel $\overline{\mathit{\text{d}}}$

<5 µm, especially $\overline{\mathit{\text{d}}}$

<3 µm. With the specified particle size distributions, the mean distance should be $\overline{\mathit{\text{D}}}$

the nickel particles are between 5 and 10 µm.

The process for producing the specified two-component sintered contact material made of silver and nickel is characterized according to the invention in that, prior to sintering, the nickel is introduced into the silver structure in the manner of mechanical alloying, this process taking place in an air atmosphere. Either silver powder and nickel powder or granules of silver and nickel are used as starting materials. Particle size distributions are preferred 500 microns, preferably less than 100 microns, especially less than 50 microns, in question. Mixing in the manner of mechanical alloying takes place in a ball mill until a lamellar structure has formed with Ni lamella widths very much smaller than the particle diameter of the starting powder. With such a degree of refinement of the structure, one is already in the range of the detection limit of a light microscope.

In the invention, contact layers can be produced from the silver-nickel powder produced in the manner of mechanical alloying by compression molding, such as extrusion or molding technology, and sintering under a reducing atmosphere. The contact pads are preferably designed as strips or profiles or as contact pieces and are used in a switching device in power engineering.

In contrast to the prior art, mechanical alloying is not carried out under protective gas in the invention. Instead, normal atmospheric air is used. Mixing does not take place for as long as possible, in particular in JP-OS 6633090, in order to obtain an alloy powder which is as fine as possible. Rather, it is consciously used to carry out the mechanical alloying process in air. This creates oxide skins on the particles, which have the same effect as sweat-reducing additives. Furthermore, the oxides on the surface of the particles contribute to the embrittlement of the composite particles and thereby to faster structure refinement. The mechanical alloying process is considerably shortened compared to mechanical alloying under inert gas.

• Figur 1 das Schliffbild eines Werkstoffes AgNilO und
• Figur 2 das Schliffbild eines Werkstoffes AgNi40.

Further details and advantages of the invention result from the following description of exemplary embodiments, reference being made to micrographs with associated enlargement of the detail and a table with the results of an electrical test. They show in 400x magnification

• 1 shows the micrograph of a material AgNilO and
• Figure 2 shows the micrograph of a material AgNi40.

Silver powder with a particle size distribution <300 µm and nickel powder with a particle size distribution <150 µm are used as starting materials for the production of AgNilO and AgNi40. After appropriate weighing, the powders are placed in a ball mill (attritor) and mechanically alloyed there until the nickel that forms is <3 µm in size and is homogeneously present in the silver. The ball mill works in an air atmosphere and without waxes as further additives.

The structure refinement resulting from mechanical alloying is accompanied by a change in the powder particle shape and size. By processing in an air atmosphere, it is consciously accepted that oxide skins form on the particles.

After mixing in the manner of mechanical alloying, contact layers are produced in a known manner by compression molding and sintering in a reducing atmosphere. Alternatively, extrusion for the production of strips or profiles or the so-called molding technology for the production of individual contact pieces can be considered as a method of pressure forming. It is also advantageous to produce two-layer contact pads or contact pieces with a first layer made of silver-nickel and a second layer made of pure silver, in order to ensure secure connection technology with the contact piece carrier.

≈ 3 µm der mittlere Abstand $\overline{\mathit{\text{D}}}$

zweier Partikel etwa beim Doppelten, also bei $\overline{\mathit{\text{D}}}$

= 6 µm liegt. Auch dieser Wert $\overline{\mathit{\text{D}}}$

ist ein signifikanter Parameter zur Kennzeichnung des Werkstoffes.The micrographs according to FIG. 1 and FIG. 2 show the material AgNi10 on the one hand and AgNi40 on the other. The homogeneous distribution of the nickel particles, whose mean particle sizes are approximately 3 μm in FIG. 1 and consistently <10 μm in FIG. 2, becomes clear. It can be seen from the image detail in FIG. 1 that nickel particles with a particle size of the order of magnitude $\overline{\mathit{\text{d}}}$

≈ 3 µm the mean distance $\overline{\mathit{\text{D}}}$

of two particles, for example, in double, ie in $\overline{\mathit{\text{D}}}$

= 6 µm. This value too $\overline{\mathit{\text{D}}}$

is a significant parameter for the identification of the material.

The table shows measured values for welding force Fs, burn-up A and the contact resistances Rk when switching on and off. The switching properties of contacts 2 and 4 produced according to the invention are shown using the example of the material compositions AgNi10 and AgNi40, which are compared with the properties of conventionally produced contacts No. 1 and No. 3 of the same composition are.

The electrical test was carried out on crowned contacts (r = 80 mm) measuring 10 mm x 10 mm with 1000 switch-on and switch-off processes under AC 1000 A, 220 V, cosϕ = 0.4 and the contact force 60 N. The bounce time of the first three Jumps were 5 ms with a closing speed of 1.0 m / s and an opening speed of 0.8 m / s with a switch-on angle of 0 ° and a switch-off angle of 80 ° and a blowing field B = 0.5 T / A. The contact resistance test was carried out under 10 A. The burnup was determined by weighing both contact pieces and averaging. The volume burnup was derived from this, taking into account the theoretical density.

The table clearly shows that the contact materials No. 2 and No. 4 produced by the method according to the invention are distinguished by lower welding force values and by considerably lower burn-off rates.

Extensive studies have shown that when using mechanically alloyed silver-nickel material for switch contacts, a switching structure that is rich in nickel compared to conventionally manufactured materials of the same composition is formed, since the finely divided nickel can be dissolved to a greater extent in the melt in the short arc exposure time. This nickel is separated out again when the melt cools down.

The nickel-rich melt resulting from the silver-nickel material according to the invention compared to a previously known AgNi material of the same nickel concentration has a higher viscosity. This means that less material is sprayed during melting, which means that contact burn-off is less with mechanically alloyed materials. Furthermore, in the case of the higher-viscosity melt, the gas dissolved in the melt only becomes too released to a lesser extent, so that when the material solidifies, pores are formed in the switching structure which reduce the mechanical strength and thus the welding force.

Claims (18)

1. Two-component sintered contact material comprising silver and nickel, characterized in that the mass fraction of nickel is between 5 and 50 %, and in that the nickel particles are present in the silver microstructure with average particle sizes ( $\overline{\mathit{\text{d}}}$

   

   ) 1 µm < $\overline{\mathit{\text{d}}}$

   

   < 10 µm, produced by a mechanical alloying-type grinding process, in largely homogeneous dispersion.
2. Two-component sintered contact material according to Claim 1, characterized in that the average particle size ( $\overline{\mathit{\text{d}}}$

   

   ) of the nickel is $\overline{\mathit{\text{d}}}$

   

   < 5 µm.
3. Two-component sintered contact material according to Claim 1, characterized in that the average particle size ( $\overline{\mathit{\text{d}}}$

   

   ) of the nickel is $\overline{\mathit{\text{d}}}$

   

   < 3 µm.
4. Two-component sintered contact material according to Claim 1, characterized in that the average mutual distance ( $\overline{\mathit{\text{D}}}$

   

   ) of the nickel particles is between 5 and 10 µm.
5. Method for preparing a two-component sintered contact material comprising silver and nickel in accordance with Claim 1 or any one of Claims 2 to 4, which involves subjecting a mixture of silver powder and nickel powder to at least one sintering process as a strength-increasing heat treatment, characterized in that prior to the sintering process the nickel is introduced into the silver microstructure, by way of a mechanical alloying-type process under an air atmosphere.
6. Method according to Claim 5, characterized in that the mechanical alloying makes use either of silver powder and nickel powder or a granular material comprising silver and nickel.
7. Method according to Claim 6, characterized in that the nickel powder or the granular material used has a particle size distribution < 500 µm.
8. Method according to Claim 7, characterized in that the nickel powder or the granular material used has a particle size distribution < 100 µm.
9. Method according to Claim 8, characterized in that the nickel powder or the granular material used has a particle size distribution < 50 µm.
10. Method according to any one of Claims 5 to 9, characterized in that the mechanical alloying in a ball mill is continued until the lamellar microstructure formed comprises nickel lamella widths which are very much smaller than the particle diameter of the nickel starting powder, preferably < 1 µm.
11. Method according to Claim 5, characterized in that, to produce contact facings, the mechanically alloyed powder is pressure-moulded and is sintered under a reducing atmosphere.
12. Method according to Claim 11, characterized in that during the sintering process spheroidization of the nickel lamellae takes place to form globular particles having a particle size distribution ( $\overline{\mathit{\text{d}}}$

    

    ) 1 µm < $\overline{\mathit{\text{d}}}$

    

    < 10 µm and a particle distance ( $\overline{\mathit{\text{D}}}$

    

    ) between 5 and 10 µm.
13. Method according to Claim 11, characterized in that the pressure-moulding is effected by extrusion.
14. Method according to Claim 11, characterized in that the pressure-moulding is carried out as a shaped-part technique for contact pieces.
15. Contact facing made of a two-component sintered contact material according to Claim 1 or any one of Claims 2 to 4, produced according to a method in accordance with Claim 13, characterized by being formed as strips or sections.
16. Contact facing made of a two-component sintered contact material according to Claim 1 or any one of Claims 2 to 4, produced according to a method in accordance with Claim 14, characterized by being formed as contact pieces.
17. Contact facing according to Claim 15 or 16, characterized by being formed as a two-layer structure with a first layer of silver-nickel and a second layer of pure silver.
18. Use of a contact facing according to any one of Claims 15 to 17 in a power engineering switching device.

Images