GB2154064A

GB2154064A - Composite materials

Info

Publication number: GB2154064A
Application number: GB08501301A
Authority: GB
Inventors: Dieter Stockel; Hubert Claus; Peter Tautzenberger
Original assignee: Rau G GmbH and Co
Current assignee: Rau G GmbH and Co
Priority date: 1984-01-21
Filing date: 1985-01-18
Publication date: 1985-08-29
Also published as: CH665729A5; DE3402091A1; FR2558640B1; FR2558640A1; GB2154064B; DE3402091C2; GB8501301D0; ATA13485A; AT399062B

Abstract

A composite material for electrical switch contact elements for use in power-supply systems is improved by acquiring an optimum combination of resistance to fusion, resistance to burn-off and low generation of heat. For this purpose, particles 1 of a non- metallic component e.g. graphite are coated with a base metallic component 2 e.g. nickel or copper. and these coated particles are embedded in a noble-metal component e.g. silver to form a matrix 3. <IMAGE>

Description

SPECIFICATION Composite material for electrical switch contact elements The invention concerns a composite material for electrical switch contact elements for use in power-supply systems, which material com prises a first metallic component consisting of noble metal, in which are heterogeneously embedded a second base-metal component and a non-metallic component in particle form.

A thin-layer composite material which can be used for a variety of purposes, including the formation of contact coatings which resist spark erosion, is disclosed in DE-AS 24 48 738. Therein, particles of metallic powder, e.g. molybdenum, are provided with a metallic coating of silver, for example, and are formed into a thin-layer composite material and applied to a metallic carrier by sintering.

This specification does not mention the embedding of non-metallic components.

A sintered material for electrical contact elements in switch gear used in power-supply systems forms the subject matter of DE 31 16 442 Al. This material consists of two basic metals, copper and silver, which are separated from each other in the microstructure. Metallic oxides, e.g. CdO and Bi2O3, are embedded in the basic metals. The contact elements concerned are intended to provide high resistance to fusion and to burn-off and are designed to heat up only to a limited extent and to provide good conductivity. However, the results so achieved do not appear to be satisfactory for all applications. Such composite materials fail particularly when the contact elements concerned are required to be absolutely reliable as regards resistance to fusion.

The choice of the components of the composite materials is mainly determined by the principal requirements imposed upon the contact elements. Thus for example, known silver/graphite composite materials for electrical contact elements are used in direct-current and alternating-current switch gear in low-voltage power-supply systems when high resistance to fusing is of main importance. The graphite component of the fixed contact elements is then 3 to 5% wt. (K. Miller and D.

Stöckel: "Ein neues Verfahren zur Herstellung komplexer graphithaltiger Verbundwerkstoffe", DE-Z Metall 7/1982 pp 743-746).

The resistance to fusion of these materials is excellent both when switching on and in the closed condition when very high short-circuit currents are used. Silver/graphite composite materials containing 0.5 to 5% wt. of graphite exhibit a low contact resistance, since no solid oxides form on the surface of the contact. However, a high graphite content, that increases resistance to fusion, results in a considerable decrease in resistance to burnoff.

The resistance to burn-off of silver/graphite composite materials of this kind can be increased in the known manner by the addition of nickel, copper or copper oxide. Such materials are generally produced by powdermetallurgy methods involving sintering which results in a statistically uncontrolled distribution of the C and Ni particles in the silver matrix. Particularly when having high nickel contents, these composite materials, in comparison with binary silver/graphite composite materials, suffer from the serious disadvantage of heating up to a greater extent when continuous current is applied, with greater generation of heat being caused by oxidation and therefore increasing electrical resistance of the base-metal component.

The present invention seeks to provide an improved composite material for electrical contact elements of the initially stated kind, in which high resistance to fusion is combined with high resistance to burn-off and reduced heating-up.

According to the invention there is provided a composite material for electrical switch contact elements for use in power-supply systems, which material comprises a first metallic component consisting of noble metal, in which are heterogeneously embedded a second base-metal component and a non-metallic component in particle form, characterised in that the particles of the non-metallic component are coated with the base-metal component, and in that these coated particles are embedded in th noble-metal component which forms a matrix.

In this way a composite material is obtained which, when used for producing electrical contact elements, surprisingly exhibits an advantageous combination as regards resistance to fusion, burn-off and heating-up. These advantageous properties can be explained by the fact that, among other things, when use is made of coated non-metallic particles in the metallic matrix, oxidation of the base metal can be suppressed by reason of its local bonding with the non-metallic component and, possibly, because of its reducing effect.

For this reason, the proportion of base metal in such a composite material can be considerably increased and a marked saving in the proportion of noble metal can be achieved.

The particles of the non-metallic components are advantageously coated with the baser of the two metallic components, the nobler of these components expediently being used as the material for the matrix.

Depending upon how they are produced, the particles may be of different forms, e.g. as irregular fractions or of spherical shape. The thickness of the coating, which may be expediently applied by electroplating, is in the range of 10-80 jum when the mean particle size is 0.1-0.3 mm. Although a completely solid coating of the particles is generally aimed at, an advantageous effect can also be achieved in some circumstances with a partially perforate coating. When the component used for coating is present in a high proportion, it may also be expedient to add a further amount of this component in the form of a powder of suitable particle size.

The expression "metallic components" is intended to include pure metals as well as alloys based on a metallic component. The non-metallic component consists of graphite, though the new form of composite material can be extended to offer considerable advantages when use is made of other non-metallic components such as oxides (e.g. CdO) and carbides (e.g. WC).

The coated particles can be advantageously distributed in the matrix in a statistically uniform manner. In another possibly expedient arrangement, the coated particles can be em bedded in the form of strands for example, arranged in a preferential direction. Yet another advantageous arrangement consists in providing the strands with a jacket of a metal lic material which may be of the same material as that of the component used for the coating.

An advantageous combination can be obtained if the non-metallic component consists of graphite, and if the component used for jacketing consists of nickel, and the compo nent used as the matrix consists of silver. In this connection, a graphite content of 1 to 10%, a nickel content of 5 to 80% and a silver content of 10 to 90% of the weight of the finished composite material appear to offer advantages. A combination of 3% wt.

graphite, 35% wt. nickel and 62% wt. silver was tested.

An alternative advantageous composition of a composite material of this kind can be achieved if, in the above-mentioned components and compositions, the nickel content is in each case replaced by a copper content likewise of between 5 and 80% wt., and preferably 35% wt. The graphite and silver contents remain within the stated limits.

Forms of the composite material are illustrated diagrammatically and by way of example only in the accompanying drawings, in which: Figure 1 is an isometric representation of a contact element of composite material with embedded particles, and Figure 2 is an isometric representation of a contact element of composite material with parallel strands embedded therein.

Fig. 1 shows a contact element incorporating granular particles 1 of graphite which have a mean diameter of 0.1 5 mm and are provided with a coating 2 of nickel having a thickness of 40 jum. The particles are contained in a matrix 3 of pure silver. The proportion of graphite is 3%, the proportion of nickel 35% and the proportion of silver 62% of the weight of the final composite material.

In the form shown in Fig. 2, the size of graphite particles 1 and the thickness of the coating 2 are unaltered. The coated particles were, however, first compacted to form strands 4 and these were embedded in the silver matrix 3 in an aligned arrangement.

The production of such a composite material will now be described in greater detail by reference to an example.

Graphite powder having a particle size of < 0.15 mm was provided with a nickel coating, having a thickness of 40 ,um, by means of a known electrolysis method. The coated graphite powder was pressed into tubes of pure silver (diameter 9 mm, wall thickness 0.3 mm), and a bundle of these filled tubes was then subjected to plastic shaping by a four-stage indirect extrusion process to provide a composite wire having a diameter of 5 mm. The composite wire produced in the first extrusion stage was cut into a number of lengths, and the composite wires with the strands of coated particles embedded therein were bundled in the same way as were the original tubes, and were further shaped. After four extrusion stages the composite material, whose structure is illustrated in Fig. 2, contained 500,000 strands embedded in the silver matrix, each of which strands contained the graphite particles coated with nickel.

Diffusion annealing may also be used to achieve an advantageous metallic bond between the metallic matrix material and the coating material.

Claims

1. A composite material for electrical switch contact elements for use in powersupply systems, which material comprises a first metallic component consisting of noble metal, in which are heterogeneously embedded a second base-metal component and a non-metallic component in particle form, characterised in that the particles of the nonmetallic component are coated with the basemetal component, and in that these coated particles are embedded in the noble-metal component which forms a matrix.

2. A composite material according to claim 1 wherein the coated particles are distributed in the matrix in a statistically uniform manner.

3. A composite material according to claim 1 wherein the coated particles are embedded in the matrix in a preferential direction.

4. A composite material according to claim 3 wherein the coated particles are arranged in strands which are embedded in the matrix and are arranged directionally therein.

5. A composite material according to any one of claims 1 to 4 wherein the non-metallic component consists of graphite, the component used for the coating consists of nickel, and the component used as a matrix consists of silver.

6. A composite material according to claim 5 wherein: the graphite component comprises 1-10% the nickel component 5-80% and the silver component 10-90% of the weight of the finished composite material.

7. A composite material according to claim 6 wherein: the graphite component amounts to 3% wt.

the nickel component to 35% wt.

and the silver component to 62% wt.

8. A composite material according to any one of claims 1 to 4 wherein the non-metallic component consists of graphite, the component used for coating consists of copper, and the component used as a matrix consists of silver.

9. A composite material according to claim 8 wherein: the graphite component comprises 1-10% the copper component 5-80% and the silver component 10-90% of the weight of the finished composite material.

10. A composite material according to claim 9 wherein: the graphite component amounts to 3% wt.

the copper component to 35% wt.

and the silver component to 62% wt.

11. A composite material according to any one of the preceding claims wherein the mean particle size is < 0.15 mm, and the thickness of the coating is 40 ym.

1 2. A composite material according to claim 1 substantially as hereinbefore de scribe.