EP4657677A1

EP4657677A1 - Sliding contact member

Info

Publication number: EP4657677A1
Application number: EP24178826.4A
Authority: EP
Inventors: Felix Rosenburg; Christian Wiebel
Original assignee: SGL Carbon SE
Current assignee: SGL Carbon SE
Priority date: 2024-05-29
Filing date: 2024-05-29
Publication date: 2025-12-03
Also published as: WO2025247991A1

Abstract

The invention relates to a sliding contact member, a composition for producing the sliding contact member and a process for manufacturing the sliding contact member.

Description

FIELD OF THE INVENTION

BACKGROUND OF THE INVENTION

A sliding contact is an electrical contact, which conducts current between stationary and rotating parts. The contacts regularly consist of a brush (stationary part) and a slip-ring/commutator (rotating part). Typical applications include slip ring assemblies, which enable the transmission of data, power and signals, e.g. in wind turbines between the fixed part, the nacelle, and the rotating part composed of the hub. The slip ring rotates with the generator shaft and comprises several conductive rings, which are in contact with a set of stationary carbon/graphene brushes, one for each ring, picking up the current from the turning slip rings and sending it to the transformer. As the rotor turns with the blades of the wind turbine, the brushes maintain constant contact, allowing for continual transmission of power and signals, without hindering the mechanical movement of the wind turbine.
A stable contact voltage and a low wear rate are required for materials used in sliding electrical contacts. Copper and copper alloys are mainly used as materials for large scale applications. For niche applications, also precious metals are used.
Conventional sliding contact members frequently include lead or antimony additives to provide a good cleaning effect, to promote cooling of the contact spots of the sliding contact member during operation, and to provide good slidability against the mating contact member. However, these known additives are toxic, environmentally harmful and the addition results in a significant loss of hardness leading to an excessive wear rate drop. On the other hand, water- or oilbased lubricants without these additives, which might be less harmful to the environment, can regularly not be used as they hinder the electrical transport.
It is therefore an object of the present invention to provide an environmentally friendly sliding contact member, whose operating characteristics, such as hardness, resistivity, slidability and flexural strength are not adversely affected.

DESCRIPTION OF THE INVENTION

This aim is achieved in accordance with the invention by means of a sliding contact member as described in claim 1. The inventive sliding contact member has a lead content of ≤ 1 wt.-%, preferably less than ≤ 0.1 wt.-%, and comprises at least the following constituents:

A) Graphite,
B) copper and
C) a metal selected from the group consisting of zinc, tin, aluminum, silver and alloys thereof; or a metal component selected from the group consisting of MoS₂, WS₂, BN and metal stearates; as well as mixtures of the foregoing.

The weight proportions of the constituents A)-C) are as follows:

A) 2-28 wt.-%,
B) 72-97 wt.-% and
C) 0.1-10 wt.-%.

The term "graphite" refers to a carbon material that has undergone graphitization, i.e. the solid-state transformation of non-graphitic carbon into carbon material with stacked layers of graphite leading to a three-dimensional hexagonal crystalline long-range order.
The term "alloys thereof" relates to alloys comprising ≥ 50 wt.-% of one or more metals consisting of the group zinc, tin, aluminum and silver. Preferably, the content of the foregoing metals in such an alloy is ≥ 70 wt.-%, more preferably ≥ 80 wt.-% and most preferably ≥ 90 wt.-%.
The invention combines environmental friendliness with improved tribological properties, high hardness and flexural strength values, and lower wear rates of the sliding contact member. Thereby, service life of the sliding contact member can substantially be increased.
Without being bound by a theory, the inventors believe that during sintering of the composition for producing the inventive sliding contact member, the metal or metal component C) could be arranging in interstitial spaces, promoting the formation of sinter necks and creating tension between copper grains. The resulting distortion of the copper crystal lattice could lead to a high interaction of component C) with copper, enhancing the structural stability of the composition and improving the hardness, the flexural strength and the wear resistance of the sliding contact member. Further, resistivity is kept at a comparingly low level.
Different types of graphite can be used in the sliding contact member of the invention. Particularly preferred is the use of natural graphite, such as flake-, microcrystalline-, and/or vein-graphites, i.e. the graphite component A) comprises or consists of one or more of the foregoing graphite types.
In a preferred embodiment, the graphite comprises or consists of needle- and/or flake-shaped graphite.
In another embodiment, the graphite of the sliding contact member comprises or consists of a synthetic graphite, i.e. a material consisting of graphitic carbon which has been obtained by chemical synthesis, e.g. by chemical vapor deposition from hydrocarbons, by decomposition of thermally unstable carbides or by crystallizing from metal melts supersaturated with carbon.
In some embodiments, the graphite component A) comprises or consists of a mixture of natural and synthetic graphite. Preferably, the natural and/or synthetic graphite comprise(s) 70% or more, 80% or more, 90% or more, 95% or more, or 99% or more of flake- and/or needle-shaped graphite.
Graphite component A) is contained in the composition in an amount of at least 2 wt.-% to ensure a sufficient slidability. A 28 wt.-% of graphite component A) should not be exceeded as otherwise wear rate can be negatively affected. Particularly preferred is a graphite component A) content of 4-20 wt.-%, more preferred 5-15 wt.-% and most preferred 6-14 wt.-%.
The inventors have found that the use of microcrystalline graphite with a D₅₀ of from 20 to 200 µm, preferably 30 to 150 µm, most preferably 30 to 70 µm is particularly beneficial. Without being bound to a theory, it is thought that in this range, the particles are small enough to fill existing gaps and create a more uniform lubricating effect, but large enough that grain boundaries do not substantially impede the flow of electrons.
The D₅₀ value can be determined by laser diffraction in accordance with ISO 13320:2020.
In a preferred embodiment of the invention the graphite has an ash value determined by combustion in accordance with DIN 51903 of from 10,000 to 25,000 ppm, preferably in the range of 10,000 to 20,000 ppm. The inventors have found that impurities, such as minerals remaining from intermingling in the crude ore used in the graphite manufacturing process, interact with the copper and/or the component C) of the sliding contact member and have a positive impact on its performance. Without being bound by a theory the inventors believe that these minerals limit the build-up of a patina, which is - in general - beneficial to reduce abrasion and enhance service life of the sliding contact member. However, a too thick patina should be avoided as this may negatively impact the electrical and chemical properties of the sliding contact member. The above defined ranges lead to patina of appropriate thickness to ensure minimal abrasion while keeping the beneficial electrical and chemical properties of the sliding contact member.
Copper is the main constituent of the inventive sliding component and is indispensable due to its high electrical conductivity. To ensure a high electrical conductivity copper is contained with a proportion of ≥ 72 wt.-%. Preferably, the proportion of copper is in the range of 75-95 wt.-%, more preferably 80-95 wt.-% and most preferably 82-92 wt.-%.
Preferably, the copper of the sliding contact member has at least in part a dendritic structure. Such dendritic copper structures are regularly intertwined so that a very stable percolative network of copper is built in the sliding contact member.
As an additional component C) the composition comprises a metal selected from the group consisting of zinc, tin, aluminum, silver, and alloys thereof; or a metal component selected from the group consisting of MoS₂, WS₂, BN, metal stearates, such as calcium or sodium stearate. The composition may also comprise as component C) a mixture including two or more materials selected from the lists of metals, metal alloys and metal components as described above.
The inventors have found that the interaction of metal and/or metal components with copper results in a sliding contact member with enhanced wear resistance and better tribological properties in comparison to the compositions known from the prior art. These effects are particularly pronounced in the case of tin or tin alloys.
Preferably, the component C) comprises or consists of a metal selected from the group consisting of zinc, tin, aluminum, and their respective alloys; in particular the component (C) comprises or consists of zinc, tin, and/or aluminum. These metals have a low melting point, which favors their arrangement in interstitial spaces between copper particles already at a very early phase of the sintering process. This leads to an enhanced interaction of the components and a significant decrease of the wear rate. Particularly preferred is the use of tin and/or alloys thereof.
For significantly influencing the properties of the sliding contact member component C) has to be contained in the composition in an amount of at least 0.1 wt.-%. However, 10 wt.-% should not be exceeded, as a higher content negatively affects the wear rate of the sliding contact. Particularly preferred is a content of 1-8 wt.-%, more preferred 1.5-6 wt.-% and most preferred 1.5-4.5 wt.-%.
The sliding contact member comprises one or more sliding surfaces, which are to be contacted with another mating member of the sliding contact. In case the graphite comprises needle- and/or flake-shaped graphite crystals, the longitudinal axes of the crystals are at least partially oriented perpendicular to the one or more sliding surfaces. This leads to a significant release of the wear rate. The longitudinal axis is the axis of a body corresponding to the direction of its greatest extent.
The inventive chemical composition results in a high hardness and bending strength of the sliding contact member.
In a preferred embodiment of the invention the sliding contact member has a HR 10/60 hardness determined in accordance with DIN IEC 60413 / 303of ≥ 65, more preferably of ≥ 75, even more preferably of ≥ 80 and most preferably of ≥ 90, but regularly also not higher than 120.
In a preferred embodiment of the invention the sliding contact member has a bending strength determined in accordance with DIN IEC 60413 / 501of ≥ 70 N/mm², more preferably of ≥ 85 N/mm², even more preferably of ≥ 100 N/mm² and most preferably of ≥ 115 N/mm², but regularly also not higher than 150 N/mm².
In a preferred embodiment of the invention the sliding contact member has a density of ≥ 5.3 g/cm³, more preferably of ≥ 5.5 g/cm³, even more preferably of ≥ 5.7 g/cm³ and most preferably of ≥ 5.9 g/cm³, but regularly not higher than 7.0 g/ cm³.
In a preferred embodiment of the invention the lead and/or antimony weight content of the sliding contact member is ≤ 1000 ppm, preferably ≤ 100 ppm, more preferably ≤ 50 ppm and most preferably ≤ 10 ppm or even ≤ 1 ppm. Due to environmental aspects, the content of such metals should be kept as low as possible.
The inventive sliding contact can be used in a variety of applications. E.g. the sliding contact member can be part of a sliding contact for an electromechanical system from the group consisting of railroad traction motors, slip ring motors and generators; grounding return current device, in particular for railway earthing, lightning protectors, high current transporters for electrolytic coating, low-voltage motors, including actuators on machines and motor vehicles (e.g. fans, windshield wipers); motors in household appliances, audio and/or video recording and playback devices; current collectors for electric rail vehicles, potentiometers, sliders, angle sensors, acceleration sensors and incremental encoders.
Preferably, the sliding contact member is a slip ring, a contact brush, such as an earthing brush, a commutator for motors, a rotary switch, a contact carriage or an earthing contact for a railway earthing device.
In another preferred embodiment of the invention, the sliding contact member is a component of a crane current collector system such as a conductor bar crane electrification, e.g. a conductor bar or a collector.
A conductor bar is an essential component used to supply electrical power to the crane or gantry crane as it moves along its track. Conductor bars are typically made of metal, such as copper, and are mounted along the length of the crane's path. They provide an electrical connection to the crane's power collector.
As the crane moves, the power collector slides or rolls along the conductor bars, regularly with aid of a sliding shoe system, ensuring an uninterrupted supply of electricity to the crane's motors and control systems. This system allows the crane to operate flexibly along its entire range of movement without being restricted by power cables. Conductor bars electrification systems stand out due to their reliability, ease of maintenance, and suitability for a wide range of industrial environments.
In another preferred embodiment, the sliding contact member is a component of a current collector ("pickup") for an electrically powered transport system, such as or electric overhead crane systems. The primary function of the current collector is to transfer electrical power from the fixed installation (overhead line, third rail, etc.) to the moving vehicle in a safe, efficient, and reliable manner. A component can be the fixed installation or the moving part, i.e. the collector.
Preferably, the sliding contact member(s) of the invention are used for moving current consumers such as gantry cranes or carousels.
Therefore, the invention also relates to a rotatable structure for an electromechanical system including one or more sliding members of the present invention. Examples of such systems are cranes, in particular revolving cranes; wind turbines, rotating purifiers in sewage or water treatment facilities, carrousels, manipulators, wrapping machines, radar and antenna dishes, theatre stages, aerodrome beacons, rotating tanks, power shovels, radio telescopes, telemetry systems, heliostats, ferris wheels and cable reels.
In a preferred embodiment the sliding contact member comprises graphite, dendritic copper and tin in the following weight proportions:

Graphite 5-8 wt.-%,
dendritic copper 87-92 wt.-%, and
tin 3-5 wt.-%.

Such a sliding contact member is preferably a slip ring and/or preferably comprises a density of ≥ 6.0 g/cm³ and/or a resistivity of ≤ 0.16 µOhmm and/or a hardness (10/60) of ≥ 90 and/or a bending strength of ≥ 120 N/mm².
In another preferred embodiment, the sliding contact comprises graphite, dendritic copper and tin in the following weight proportions:

Graphite 10-15 wt.-%,
dendritic copper 82-88 wt.-%, and
tin 1-3 wt.-%.

Such a sliding contact member is preferably an earthing brush and/or preferably comprises a density of ≥ 5.0 g/cm³ and/or a resistivity of ≤ 0.16 µOhmm and/or a hardness (10/60) of ≥ 75 and/or a bending strength of ≥ 80 N/mm².
The invention also relates to a composition for producing an inventive sliding contact member. The composition comprises at least the following components:

A) Graphite,
B) copper and
C) a metal or metal component selected from the group consisting of zinc, tin, aluminum, silver, MoS₂, WS₂, BN, as well as alloys and mixtures of the foregoing.

Particularly preferred is the use of tin.
The weight proportions of the components are as follows:

A) 2-28 wt.-%, preferably 4-20 wt.-%, more preferred 5-15 wt.-% and most preferred 6-13 wt.-%,
B) 72-97 wt.-%, preferably 75-95 wt.-%, more preferably ≥ 80-95 wt.-% and most preferably ≥ 82-92 wt.-%, and
C) 0.1-10 wt.-%, preferably 1-8 wt.-%, more preferably 1.5-6 wt.-% and most preferably 1.5-4.5 wt.-%.

The invention also relates to a process for manufacturing an inventive sliding contact member comprising the following steps:

I. Providing a composition comprising or consisting of
1. A) Graphite,
2. B) copper and
3. C) a metal or metal component selected from the group consisting of zinc, tin, aluminum, silver, MoS₂, WS₂, BN, as well as alloys and mixtures of the foregoing
wherein the weight proportions of the components are as follows:
1. A) 2-28 wt.-%,
2. B) 72-97 wt.-% and
3. C) 0.1-10 wt.-%.
II. Mixing components A), B) and C), preferably in a ploughshare mixer, to obtain a mixture,
III. molding the mixture to obtain a shaped object, preferably under a pressing force of ≥ 10 kN/cm²,
IV. heat treating the shaped object at temperatures of ≥ 300°C, preferably at temperatures of 300°C to 700°C, to obtain a heat-treated shaped object.

Finally, the heat-treated shaped object can be machined, e.g. with aid of a CNC machine.
In step III., the pressing force is preferably ≥ 12 kN/cm², more preferably ≥ 15 kN/cm², even more preferably ≥ 18 kN/cm² and most preferably ≥ 20 kN/cm², but regularly also ≤ 40k N/cm².
The inventors have observed that the higher the pressing force, the higher the hardness, the higher the flexural strength, the higher the density and the lower the electrical resistivity of the sliding contact member. The foregoing finding supports the belief of the inventors that the metal or metal component C) arranges in interstitial spaces, resulting in distortion of the copper crystal lattice and high interaction of the constituents of the sliding contact member.
In order to avoid oxidation, which might have a detrimental effect on the properties of the sliding contact member, step IV. is preferably conducted in an inert or reductive atmosphere such as a H₂ atmosphere.
Preferably, heat treating of the shaped object in step IV. is performed at temperatures of ≤ 800°C, more preferably ≤ 600°C, to avoid copper melting, which may result in alloying of copper and component C).
If needle- and/or flake-shaped graphite(s), such as present in flake-, amorphous or vein graphite, are/is used in the manufacturing process, then preferably step II. is performed under a pressing force. Pressing gives the graphite a preferred direction parallel to the pressing die.
The graphite flakes must not lie parallel to the sliding surface, as this favors abrasion. Therefore, the final sliding contact member should be cut out of the heat-treated object in such a way that the graphite flakes are aligned perpendicular (= more than 30%, preferably more than 50% or even more than 70% of the flakes in terms of number exhibit an angle of 90°+/- 10°) to at least one of the one or more sliding surfaces. This reduces abrasion and increases the conductivity of the sliding contact member.

EXAMPLES

The invention will now be explained in more detail with the aid of manufacturing examples in accordance with the invention and with the aid of the accompanying figures.

Measurement methods

Resistivity

Resistivity of the samples was determined with a four-point measurement in accordance with DIN IEC 60413 / 402.

Hardness (10/60)

Rockwell-Hardness of the samples was determined in accordance with DIN IEC 60413 / 303.

Bending strength

Bending strength of the samples was determined in accordance with DIN IEC 60413 / 501.

Wear rate

Conductor rail test

The wear behavior was tested with a current collector system for a crane comprising a wheel with 10 conductor rails mounted thereon. Each conductor rail had two current collectors, which were contacted with an inventive or comparative sliding contact member, namely a carbon brush obtained via the below described inventive or comparative manufacturing example. The sliding speed was 2 m/s, the voltage 230 V (AC), the current 10 A and the test duration was 1007 h.

Pin-on-Disk (PoD)

A stationary pin is pressed against a rotating disk under the given load and with increasing temperature of the rotating disc. The coefficient of friction (CoF), wear and temperature of the pin are continuously monitored during the test.

Manufacturing examples

Inventive example I

A mixture consisting of 13 wt.-% flake-shaped natural graphite, 85 wt.-% dendritic copper powder and 2 wt.-% tin was introduced in a Lödige horizontal plowshare 50 L mixer, homogenized and cold pressed in a rectangular format a pressing force of ≥ 10 kN/cm². The pressed plates were thermally treated in a batch furnace at -500-600°C to avoid copper melting. The thermal treatment was performed under H₂ atmosphere. An earthing brush as shown in Fig. 1 was machined with aid of a CNC-machine.

Comparative example I

A mixture consisting of 12 wt.-% flake-shaped natural graphite, 82 wt.-% dendritic copper powder and 5 wt.-% lead and 1 wt.-% tin was introduced in a Lödige horizontal plowshare 50 L mixer, homogenized and cold pressed in a rectangular format a pressing force of ≥ 10 kN/cm². The pressed plates were thermally treated in a batch furnace at -500-600°C to avoid copper melting. The thermal treatment was performed under H₂ atmosphere. An earthing brush as shown in Fig. 1 was machined with aid of a CNC-machine.

	Inventive example	Comparative example
Density [g/cm³]	5.311	5.2
Resistivity [µOhmm]	0.15	<0.16
Hardness [10/60]	82.1	51-70
Bending strength [N/mm²]	87.3	46-68
Wear rate (PoD) [µm/100h]	1263	2176

Inventive example II

A mixture consisting of 6 wt.-% flake-shaped natural graphite, 90 wt.-% dendritic copper powder and 4 wt.-% tin was introduced in a Lödige horizontal plowshare 50 L mixer, homogenized and cold pressed in a rectangular format a pressing force of ≥ 10 kN/cm². The pressed plates were thermally treated in a batch furnace at -500-600°C to avoid copper melting. The thermal treatment was performed under H₂ atmosphere. A low voltage slip ring was machined out of the heat-treated object with aid of a CNC-machine.

Comparative example II

A mixture consisting of 3.5 wt.-% graphite, 84 wt.-% dendritic copper powder and 10 wt.-% lead and 2.5 wt.-% tin was introduced in a Lödige horizontal plowshare 50 L mixer, homogenized and cold pressed in a rectangular format a pressing force of ≥ 10 kN/cm². The pressed plates were thermally treated in a batch furnaceat ~500-600°C to avoid copper melting. The thermal treatment was performed under H₂ atmosphere. A low voltage slip ring was machined out of the heat-treated object with aid of a CNC-machine.

	Inventive example	Comparative example
Density [g/cm³]	6.132	6.271
Resistivity [µOhmm]	0.15	0.14
Hardness [10/60]	97	72
Bending strength [N/mm²]	130	108
Wear rate (PoD) [µm/100h]	2331	3724
Wear rate (Conductor rail test) [µm/100h]	175	293

Brief description of the Figures

Fig. 1 depicts a schematic representation of a sliding contact, namely a slip ring assembly, which is a combination of a slip ring and a carbon brush with respective holder.
Fig. 2 depicts a schematic representation of the inventive process for manufacturing a sliding contact member, in which a pressing force is applied in step III.
Fig. 3 depicts a schematic representation of the shaped body obtained by the pressing process shown in Fig. 2.

Detailed description of the Figures

Fig. 1 depicts a schematic representation of a sliding contact to be used in a wind energy plant, namely a slip ring assembly 1, which is a combination of a slip ring 2 with a carbon brush 3 and respective carbon brush holder 4 and lead wires 5. A slip ring assembly 1 is an electromechanical device transferring electricity from a rotating generator shaft to a transformer. The slip ring 2 rotates with movement of a generator shaft of the wind energy plant and comprises several conductive rings, which are in contact with a set of stationary carbon brushes 3, one for each ring, picking up the current from the turning slip rings and sending it via the lead wires 5 to the transformer. As the rotor turns with the blades of the wind turbine, the brushes 3 maintain constant contact, allowing for continual transmission of power and signals, without hindering the mechanical movement of the wind turbine. The slip ring assembly may comprise further elements such as a "brush rocker" to adjust the position of the brushes so they can make consistent electrical contact with the rotating slip ring as it moves or a housing (both not shown in Fig. 1).
In the slip ring assembly 1, i.e. the sliding contact of Fig. 1 , the carbon brush 3 is an inventive sliding contact member having a composition as defined in claim 1. However, also other components of the sliding contact, such as the slip ring can be made of the inventive composition.
Fig. 2 depicts a schematic representation of step III of an exemplary inventive process, in which the mixture 6 of graphite, copper and tin, is molded under a pressing force F, here applied via a press punch 7. Pressing gives the graphite a preferred direction parallel to the contact surface 8 of the press punch. Wherein a) shows the pressing process when the press 7 is moving downwards and the mixture 6 has not yet been contacted, b) shows the contacting state. The sliding surface 11 is shown in dashed style.
Fig. 3 depicts a schematic representation of the shaped body obtained by the pressing process shown in Fig. 2 . The graphite flakes 10 are aligned perpendicular to at least one of the one or more sliding surfaces 11. This reduces abrasion and increases the conductivity of the sliding contact member. Aligned perpendicular means in this respect that more than 30%, preferably more than 50% or even more than 70% of the flakes 10 in terms of number exhibit an angle of 90°+/- 10°. In the highlighted volume A, the perpendicular reference line, which is orthogonal to the sliding surface 11, is shown in dashed style.

REFERENCE SIGNS

1: Slip ring assembly
2: Slip ring
3: Carbon brush
4: Carbon brush holder
5: Lead wire
6: Mixture of graphite, copper and tin
7: Press punch
8: Contact surface of the press punch
9: Shaped object
10: Graphite flakes
11: Sliding surface

Claims

Sliding contact member with a lead content of ≤ 1 wt.-%, comprising at least the following constituents:
A) Graphite,

B) copper and

C) a metal selected from the group consisting of zinc, tin, aluminum, silver, and alloys thereof; and/or a metal component selected from the group consisting of MoS₂, WS₂, BN and metal stearates; preferably tin and alloys thereof,
characterized in that the weight proportions of the constituents are as follows:
A) 2-28 wt.-%,

B) 72-97 wt.-% and

C) 0.1-10 wt.-%.
Sliding contact member according to claim 1, wherein the graphite comprises or consists of needle- and/or flake-shaped graphite.
Sliding contact member according to claim 2, wherein the sliding contact member comprises one or more sliding surfaces and wherein the longitudinal axes of the needle- and/or flake-shaped graphite crystals are at least partially oriented perpendicular to the one or more sliding surfaces.
Sliding contact member according to any of the preceding claims, wherein the graphite has a D₅₀ value determined by laser diffraction in accordance with ISO 13320:2020 of from 50 to 200 µm.
Sliding contact member according to any of the preceding claims, wherein the graphite has an ash value determined by combustion in accordance with DIN 51903 of from 10,000 to 25,000 ppm.
Sliding contact member according to any of the preceding claims, wherein the copper has at least in part a dendritic structure.
Sliding contact member according to any of the preceding claims, wherein the sliding contact member has a HR 10/60 hardness determined in accordance with DIN 51917 of ≥ 65, preferably ≥ 80.
Sliding contact member according to any of the preceding claims, wherein the bending strength of the sliding contact member determined in accordance with DIN EN 843 is ≥ 70 N/mm², preferably ≥ 80 N/mm².
Sliding contact member according to any of the preceding claims, wherein the lead and/or antimony content of the sliding contact member is ≤ 100 ppm, preferably ≤ 10 ppm.
Sliding contact member according to any of the preceding claims, wherein the sliding contact member is a slip ring, a contact brush, a contact carriage or an earthing contact for a railway earthing device.
Rotatable structure for an electromechanical system including one or more sliding members as defined in any of the preceding claims.
Rotatable structure according to claim 11, wherein the electromechanical system is selected from the group consisting of cranes, in particular revolving cranes; wind turbines, rotating purifiers in sewage or water treatment facilities, carrousels, manipulators, wrapping machines, radar and antenna dishes, theatre stages, aerodrome beacons, rotating tanks, power shovels, radio telescopes, telemetry systems, heliostats, ferris wheels and cable reels.
Composition for producing a sliding contact member according to any of the preceding claims, comprising at least the following components:
A) Graphite,

B) copper and

C) a metal or metal component selected from the group consisting of zinc, tin, aluminum, silver, MoS₂, WS₂, BN, as well as alloys and mixtures of the foregoing, preferably tin, wherein the weight proportions of the components are as follows:
A) 2-28 wt.-%,

B) 72-97 wt.-% and

C) 0.1-10 wt.-%.
Process for manufacturing a sliding contact member according to any of claims 1-10, comprising the following steps:
I. Providing a composition according to claim 13,

II. mixing components A), B) and C), preferably in a ploughshare mixer, to obtain a mixture,

III. molding the mixture to obtain a shaped object, preferably under a pressing force of ≥ 10 kN/cm²,

IV. heat treating the shaped object at temperatures of ≥ 300°C, preferably at temperatures of 300°C to 700°C, to obtain a heat-treated shaped object;
and optionally

V. Machining the heat-treated shaped object.
Process according to claim 14, wherein step IV. is conducted in a H₂ atmosphere.