WO2013074038A1

WO2013074038A1 - Electrical contact with embedded solid lubricant particles

Info

Publication number: WO2013074038A1
Application number: PCT/SE2012/051275
Authority: WO
Inventors: Benny ANDRÉ
Original assignee: Andre Benny
Priority date: 2011-11-17
Filing date: 2012-11-19
Publication date: 2013-05-23

Abstract

An electrical contact (1) comprising a substrate (2) at least on one side provided with a noble metal contact layer (3). The contact layer (3) comprises a solid lubricant zone (4) close to an upper surface of the contact layer, with solid lubricant particles (5) embedded in the noble metal in the solid lubricant zone (4). A method of manufacturing such a contact comprising mechanically embedding solid lubricant particles into the contact layer (3), thereby forming a solid lubricant zone (4), or plating a solid lubricant zone (4).

Description

Electrical contact with embedded solid lubricant particles

TECHNICAL FIELD OF THE INVENTION AND BACKGROUND ART

The present invention relates to an electrical contact comprising a noble metal contact layer.

For electrical connectors the contact resistance decides the efficiency of the connection. The contact resistance in turn depends on the inherent resistivity of the materials in the two contacting bodies, any surface layers on top of the materials and the area of real contact. The most commonly used materials for high performance electrical connectors are copper coated with noble metals. Noble metal coatings are used largely because of their very low inherent resistivity and the fact that they do not form any thick insulating layers such as oxides on their surfaces. The contact area and pressure distribution depend on the load between the mating surfaces and the geometry of the surfaces. If the pressure is high enough to plastically yield one of the surfaces, the surface will deform, the contact area will increase and the pressure will be lowered until it is balanced by the hardness of the surface. Also in this case the noble metals are a very good choice because of their softness, allowing for large areas of intimate contact. The drawbacks of the noble metals are mainly that they are prone to cause high friction, the high cost of the raw material and, in some cases, the low wear resistance.

High friction and high wear rate are due to the fact that if a connector uses the same noble metal on both mating surfaces the surfaces tend to weld together. This means they are of limited use in sliding connectors in systems where additional lubrication is not possible or allowed.

The solution to this, without losing the ability to form large conducting area, is to introduce oil or grease as lubricant in the contact. This is, however, associated with both additional handling and environmental issues. Another way to reduce friction in sliding contacts is to change the material of only one of the mating surfaces. For the other surface a material that resists cold welding with the noble metal could be chosen, resulting in a lower friction coefficient during sliding and likely also lower wear rate. Thin ceramic PVD coatings have been proposed to achieve this. Albeit addressing the aforementioned material properties adequately, the problem with PVD coatings is that the coating processes are complicated in that coatings must be deposited in a vacuum chamber and that the equipment for this poses an expensive threshold.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electrical contact which has improved properties with respect to contact resistance, friction and wear rate, and which is suitable for use in sliding contacts. This object is achieved with an electrical contact according to claim 1. A second object of the present invention is to provide cost-efficient methods of manufacturing such a contact. This object is achieved by methods according to claims 6 and 8. Advantageous embodiments of the different aspects of the invention are disclosed in the dependent claims.

An electrical contact according to the invention comprises a substrate at least on one side provided with a noble metal contact layer. The contact layer comprises a solid lubricant zone close to an upper surface of the contact layer, with solid lubricant particles embedded in the noble metal in the solid lubricant zone.

By introducing solid lubricant particles, low friction coefficients can be achieved while still retaining a low contact resistance thanks to the noble metal. The wear resistance is increased, thereby increasing the lifetime of the contact.

According to a preferred embodiment, the solid lubricant particles comprise transition metal disulphides and/or diselenides such as tungsten disulphide, molybdenum disulphide, etc. These materials have a layered structure where the bonds between some layers are very weak. Coatings containing these materials have shown to provide very low friction coefficients through easy shear between the basal planes of the sulphides. According to another embodiment of the invention, the solid lubricant particles exhibit an inorganic fullerene-like or nano-tube structure.

In a preferred embodiment, the solid lubricant particles comprise tungsten disulphide in an inorganic fullerene-like WS₂ structure (IF-WS₂). Thanks to the semi-conducting properties of IF-WS₂, the solid lubricant particles in this embodiment only make a minor contribution to the overall contact resistance. A further benefit of the closed structure of IF- WS₂ is a high oxidation resistance. The invention also relates to a method of manufacturing an electrical contact, the electrical contact comprising a noble metal contact layer, the method comprising the steps of:

- providing a substrate at least on one side provided with a noble metal contact layer;

- providing solid lubricant particles on the surface of the contact layer;

- mechanically embedding the solid lubricant particles into the contact layer, thereby forming a solid lubricant zone in the contact layer.

The invention also relates to a method of manufacturing an electrical contact, the electrical contact comprising a noble metal contact layer, the method comprising a step of plating a solid lubricant zone, which solid lubricant zone comprises a noble metal and solid lubricant particles.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in detail with reference to the attached drawings, wherein

Figure 1 schematically shows a cross section of a contact according to the invention, Figure 2a schematically illustrates a method of manufacturing a contact according to an embodiment of the invention,

Figure 2b schematically illustrates a method of manufacturing a contact according to another embodiment of the invention,

Figure 3a shows a SEM image of a contact according to an embodiment of the invention, Figure 3b shows a SEM image of a contact according to another embodiment of the invention,

Figure 3c shows a SEM image of a contact according to another embodiment of the invention,

Figure 3d shows a SEM image of a contact according to another embodiment of the invention,

Figure 4a shows friction coefficient for a contact according to the invention,

Figure 4b shows contact resistance for a contact according to the invention, Figure 4c shows friction coefficient for a prior art contact,

Figure 4d shows contact resistance for a prior art contact,

Figure 5a shows a SEM image of wear marks on a prior art contact,

Figure 5b shows a SEM image of wear marks on a prior art contact,

Figure 5c shows a SEM image of wear marks on a contact according to the invention, and Figure 5d shows a SEM image of wear marks on a contact according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A section from an electrical contact according to the invention is schematically depicted in fig .1. A contact 1 according to the invention comprises a substrate 2 of conventional type, such as copper. The substrate 2 is at least on one side provided with a noble metal contact layer 3, preferably of silver or gold, or alloys thereof. The contact layer 3 has a thickness in the range 1 μιη-1 mm. The contact layer 3 comprises a solid lubricant zone 4 close to the upper surface of the contact layer 3. The thickness of the solid lubricant zone 4 is in the order of up to 1/5 of the contact layer 3. Embedded in the solid lubricant zone 4 are solid lubricant particles 5. The solid lubricant particles 5 should be in the size range of 10 nm-1 mm, and distributed both laterally and in depth of the solid lubricant zone 4. The solid lubricant particles may be in different forms and may for example comprise nanoparticles, flakes or agglomerates of nanoparticles. In a preferred embodiment the solid lubricant particles comprise transition metal disulphides and/or diselenides (tungsten disulphide, molybdenum disulphide, etc.) and more preferably transition metal disulphides exhibiting an inorganic fullerene-like or nano-tube structure. The invention is exemplified, but not limited to, tungsten disulphide in an IF-WS₂ structure (Inorganic Fullerene-like WS₂), which is tungsten disulphide with curved basal planes that form spheres in a closed structure. Powders of IF-WS₂ are commercially available, consisting of nanoparticles some 100 nm in diameter built up of agglomerated spheres.

The contact layer 3 could also include small amounts of other inclusions. One example of such material that could be beneficial, for example in humid air applications, is carbon in all its different structures. The contact according to the invention may also comprise other layers known in the art, for example a nickel layer in-between the substrate 2 and the contact layer 3.

To reach the situation with well distributed solid lubricant particles 5 in the solid lubricant zone 4, embedding during plating, such as electroplating, could be used. Another way of distributing the solid lubricant particles is to mechanically push the particles into an existing contact layer surface. One example of mechanical distribution of particles in the solid lubricant zone is shown in fig. 2a and b. In this example, silver with IF-WS₂ is produced by spreading IF-WS₂ nanoparticles over the surface 10 of a polished stainless steel plate 1 1. Then an electrical contact in the form of a cylinder 12 covered with a silver contact layer is placed between that surface 10 and a flat tool 13' with a flat working surface 14' or a structured tool 13" with a structured working surface 14". The particles, or rather agglomerates of particles, are then pushed into the silver contact layer by loading the tool 13', 13" while moving it back and forth, making the cylinder 12 roll between the tool 13, 13' and the particle covered steel surface 10. By using the structured tool 13", the particles reach deeper down in the silver contact layer.

The electrical contact according to the invention may of course be of any shape, and the method of mechanically embedding the particles into the contact layer could by the skilled person easily be adapted to suit the shape of the electrical contact. The solid lubricant particles could for example be embedded into a planar contact layer the by means of a tool in the form of a cylinder, with or without surface structure.

Thorough investigations clearly indicate that the concept of embedding solid lubricant particles with a layered structure such as transition-metal disulfides or diselenides (WS₂, MoS₂, etc.) into noble metals, such as silver or gold or alloys thereof, has a clear effect on wear resistance and friction. During tests carried out, four different kinds of samples in the form of silver cylinder surfaces with IF-WS₂ were produced. This was achieved by using the flat tool 13' shown in fig. 2a for some of the samples and the structured tool 13" shown in fig. 2b for some of the samples, and by removing some of the particles on some samples. In this way, two kinds of samples with varying surface coverage of IF-WS₂ were produced using the flat tool, and two kinds of samples with varying surface coverage were produced by the structured tool, resulting in particles being embedded deeper into the surface. Representative surfaces of the different kinds of samples can be seen in fig. 3a-3d. Fig 3a shows high coverage and shallow depth of IF-WS₂, fig. 3b shows medium coverage and shallow depth of IF-WS₂, fig. 3c shows high coverage and large depth of IF-WS₂, and fig. 3d shows low coverage and large depth of IF-WS₂. The agglomerates of nanoparticles can be seen as patches or in groves and have bright contrast in these figures.

Triboelectrical testing in reciprocating sliding was used for continuous combined friction and contact resistance measurement. The testing equipment was using a crossed cylinder setup where the lower cylinder was driven back and forth while the upper cylinder was stationary held using a strain gage that continuously measured the friction force. A load was applied by a spring mechanism that was connected to a strain gauged in one end. This strain gauge was used to measure the applied load. The equipment was also capable of continuously measuring the contact resistance. This was done by a four point measurement. The ends of the cylinders were connected to wires, one loop driving a current through the contact and one loop was used to measure the potential drop in the part of the loops that was common for both loops.

The tests of the new concept, to combine silver with embedding of IF- WS₂, were conducted in a test setup of this type with the following set up: A load of 20 N between the two identically prepared crossed cylinders was applied using a spring. During the test the bottom cylinder was sliding back and forth with a stroke of 4 mm and a frequency of 4 Hz.

The modified cylindrical silver contacts according to the invention were compared tribologically and electrically with reference silver in reciprocating sliding of crossed cylinders. The reference silver had a scattering coefficient of friction between 0.8-1.2 during the whole test, which for the reference pure silver was 300 or 600 strokes respectively. Already after 300 strokes the copper from the substrate could be seen through a heavily worn contact area. The contact resistance was of course low, around 50 μΩ before it was worn through.

All modified silver surfaces according to the invention could last much longer while keeping the contact resistance at a low level. The best performing specimens were those with structured silver and a low coverage of IF-WS₂. These cylinders lasted 8000 strokes, the steady state friction was 0.3 and the contact resistance was low from start to finish, never exceeding values of about 200 μΩ. A comparison of the best working samples and the reference self mating silver can be found in fig. 4a-4d. Fig. 4a and 4b respectively shows the coefficient of friction and the contact resistance versus the number of strokes for two of the best of the silver+IF-WS₂ samples according to the invention. Fig. 4c and 4d respectively shows the coefficient of friction and the contact resistance versus the number of strokes for two of the reference self mating silver samples. It should be noted that different scales are used in fig. 4a-4d.

Fig. 5a and 5b are SEM images showing wear marks on self mating silver sample, i.e. a silver sample without the solid lubricant zone. Fig. 5c and 5d are SEM images showing wear marks on the best silver+IF-WS₂ sample, i.e. a sample including a solid lubricant zone. What should be pointed out is that the silver-silver samples in fig. 5a have been subjected to 300 strokes, while the silver+IF-WS₂ samples in fig. 5b have been subjected to 8000 strokes. It can be clearly seen from the SEM images that the wear marks on the samples that include a solid lubricant zone are much less prominent than the wear marks on the samples without a solid lubricant zone. The solid lubricant zone can thus be concluded to remarkably increase the wear resistance of the contact layer.

The use of the structured tool 13", producing a structured silver surface, was expected to embed IF-WS₂ deeper down in the surface. The effect of this seemed to be that the low friction coefficient lasts longer compared to the samples prepared using the flat tool 13', producing an unstructured silver surface. This is likely because in the case of the structured silver surface, IF-WS₂ could remain in the wear mark thanks to pits created using the structured tool 13", the pits acting as reservoirs of IF-WS₂.

Applying only a low surface coverage of IF-WS₂ on structured silver gave the best results for both the friction coefficient and the contact resistance. The friction coefficient started at 0.2 and then quickly increased to 0.4, after which it began to decrease. This is probably because the low coverage of IF-WS₂ initially was insufficient to provide a low friction, but the reservoirs soon had the effect of providing IF-WS₂ to allow for growing a thin tribofilm that governed low friction. The contact resistance results show that the amount of IF-WS₂ was low enough to allow for low contact resistance right from the beginning to the end of the tests.

The invention is of course not in any way restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention such as defined in the appended claims.

Claims

1. An electrical contact (1 ) comprising a substrate (2) at least on one side provided with a noble metal contact layer (3), characterised in that the contact layer (3) comprises a solid lubricant zone (4) close to an upper surface of the contact layer (3), with solid lubricant particles (5) embedded in the noble metal in the solid lubricant zone.

2. The electrical contact according to claim 1 , characterised in that the solid lubricant particles (5) comprise transition metal disulphides and/or diselenides.

3. The electrical contact according to claim 2, characterised in that the solid lubricant particles (5) exhibit an inorganic fullerene-like or nano- tube structure.

4. The electrical contact according to claim 2, characterised in that the solid lubricant particles (5) comprise tungsten disulphide in an inorganic fullerene-like WS₂ structure.

5. The electrical contact according to any of the previous claims, characterised in that the noble metal contact layer consists of gold or silver or alloys thereof.

6. A method of manufacturing an electrical contact, the electrical contact (1 ) comprising a noble metal contact layer (3), characterised in that the method comprises the steps of:

- providing a substrate (2) at least on one side provided with a noble metal contact layer (3),

- providing solid lubricant particles (5) on the surface of the contact layer (3);

- mechanically embedding the solid lubricant particles (5) into the contact layer (3), thereby forming a solid lubricant zone (4) in the contact layer (3).

7. A method of manufacturing an electrical contact according to claim 6, characterised in that a tool (13") with a structured working surface (14") is used for mechanically embedding the solid lubricant particles (5) into the contact layer (3) by means of the structured working surface (14") of the tool (13"), such that the formed solid lubricant zone (3) obtains a structured surface.

8. A method of manufacturing an electrical contact, the electrical contact (1 ) comprising a noble metal contact layer (3), characterised in that the method comprises a step of plating a solid lubricant zone (4) which solid lubricant zone (4) comprises a noble metal and solid lubricant particles (5).

9. A method of manufacturing an electrical contact according to claim 8, characterised in that the solid lubricant zone (4) is plated onto a noble metal contact layer (3).