Conductive self-lubricating sliding element

Abstract

Sliding element with a metallic carrier material (10) and a sliding layer based on PTFE or thermoplastic applied thereon, wherein the sliding layer forms an interface (18) to the carrier material (10) and a sliding surface (20) for contact with a counter-rotator, wherein the sliding layer contains conductive particles (22) which each extend individually within the sliding layer at least from the interface (18) to the sliding surface (20), characterized in that the volume fraction of the conductive particles (22), based on the volume of the sliding layer, is 0.1 - 1.2%, preferably 0.2 - 0.9%.

Description

[0001] The invention relates to a self-lubricating sliding element with a metallic carrier material and a sliding layer based on PTFE or thermoplastic material applied thereon, in particular for lubricant-free but also for lubricated applications, wherein the sliding layer forms an interface to the carrier material and a free sliding surface for contact with a counter-rotator.

[0002] The typical structure of composite materials for self-lubricating sliding elements consists of a carrier material coated and / or impregnated with a plastic or plastic compound to create the sliding properties. The carrier material is metallic and consists of a support material such as a metal strip, e.g., steel, copper, aluminum, or their alloys, which is usually coated with a porous metal layer, e.g., bronze powder, or has a porous structure itself, such as fabric or expanded metal. Also worth mentioning are material combinations in which the fabric or expanded metal is bonded to the support material.

[0003] While these metallic components are electrically conductive, the plastic layers with tribologically effective additives typically applied to them as a sliding layer are not. PTFE, PPS, POM, PVDF, PFA, PEEK, for example, are particularly commonly used as plastic matrices; in addition, various polyamides, polyesters, PES, PAI, and others are also used. Their functionality can be roughly divided into composite materials, in which only a thin running-in layer is present over the metallic carrier material, so that after running-in, metallic components are exposed and make contact with the shaft, thus establishing conductivity. This is the case with many PTFE-based coatings, such as those described in DE 10 2013 227 187 B4.

[0004] Hinges often use very thin-walled materials with PTFE-based coatings, which are calibrated for zero play during assembly by using pins that are oversized relative to the bearing bush bore. These materials can have a structure consisting of a carrier material, porous bearing metal, and a plastic coating; however, materials produced by coating and impregnating metal mesh or expanded metal are also frequently used, as described in DE 101 47 292 B4. In the latter case, the PTFE-based coating should be so wear-resistant that the carrier material is not exposed over the entire service life. Otherwise, due to the equalizing friction values ​​on the sliding surface and bearing back, a tight fit can no longer be guaranteed and the bearing could, for example, migrate out of the housing.In most thermoplastic-based composites, as described for example in EP 3 087 142 B1, no carrier material is exposed over the lifetime.

[0005] However, there is increasing interest in self-lubricating bearing elements that prevent static charges on the components connected to the bearing elements or, in the case of electrostatic painting, ensure an electrical connection even after the components connected via these bearings have been installed. For such requirements, it is necessary that the sliding layer has sufficient conductivity from the outset. The conductivity of the bearings then eliminates the need for additional contacts on the components.

[0006] Common additives in PTFE include MoS2, hBN, ZnS, BaSO4, CaF2, Fe2O3, graphite, carbon black, and other plastics such as PFA, FEP, ETFE, PEEK, PPS, PAI, polyaramids, aromatic polyesters, hard materials such as glass, Si3N4, SiC, and carbon fibers. The use of fine lead powder was common, but is increasingly being phased out due to regulations regarding lead's toxicity. When thermoplastics form the matrix, PTFE is often added, along with portions of the other additives mentioned above.

[0007] Of these common additives, graphite, carbon black, and carbon fibers are conductive, but their proportion is usually insufficient to achieve conductivity. The use of carbon nanotubes has also been proposed in connection with plain bearings, for example, in EP 2 804 902 B1, but not to increase conductivity, but rather to improve tribological properties.

[0008] Increasing the proportion of conventional conductive additives such as carbon fibers, graphite, and carbon black, as described in EP 1 875 091 B1, or using fine metal powders until a percolation leading to conductivity is achieved, is possible, but this leads to a significant impairment of the load-bearing capacity and wear resistance of the materials, as well as to a deterioration of the processing properties in conventional PTFE dispersion-based, powder-based, or film-based coil coating processes. Although nanotubes enable conductivity even at lower concentrations, they are comparatively expensive and difficult to handle.

[0009] EP 2 069 65 1 B1 also proposed removing the top layer of the materials until the substrate material is exposed in order to increase thermal and, consequently, electrical conductivity. While this would create conductivity, with conventional PTFE-based materials on porous bronze it has the disadvantage that the initial formation of the transfer layer, which is important for the sliding process, is more difficult, while with thermoplastic-based materials the composition of the surface created by this measure is no longer sufficiently lubricating. With thin materials with a fabric or expanded metal framework without a carrier material, this would jeopardize the tight fit and thus the general function from the outset due to the equal friction on the sliding side and back.

[0010] Document WO 2012 / 154558 A2 discloses an assembly comprising a metal-polymer composite bushing having an outer metal layer with an inner surface, metal particles sintered to the inner surface, a polymer material between the metal particles, a finished inner surface jointly defined by the polymer material and exposed portions of the metal particles, a plurality of interstices formed between the metal particles and the polymer material, and a plurality of ribs defined by the polymer material and extending beyond the metal particles. The ribs occupy at least about 8% of the area of ​​a cylindrical reference surface that nominally corresponds to the finished inner surface. The assembly further includes a crankshaft having a journal at least partially received within the bushing and supported by the finished inner surface.The journal is polished to a surface finish of approximately 0.1 micrometers or less to reduce the wear rate of the bushing.

[0011] Document JP 2007-239838 A discloses a flanged cylindrical bearing bushing that can suppress fluctuations in dynamic characteristics as much as possible and does not cause backlash and the like, a manufacturing method therefor, and a hinge structure using the flanged cylindrical bearing bushing. Conductivity can be imparted to a multilayer bearing without machining such as cutting, and electrostatic plating can be performed on a door without wiring from a high-voltage source.

[0012] Document DE 34 14 095 A1 discloses a bearing that can be used in a scroll compressor of a freezer. The bearing comprises a porous bearing part provided between a shaft and a bearing bush and impregnated with a composite material consisting of a synthetic material, such as tetrafluoroethylene, and a lubricant. Before use, the surface of the bearing part is subjected to a machining process that removes the surface layer consisting exclusively of the composite material. As a result, both the bronze system alloy and the composite material are randomly distributed in the bearing surface in contact with the shaft. A lubricating device for supplying the bearing surface with lubricating oil is also provided.

[0013] Document EP 1 020 642 A1 discloses a porous bronze and a resin impregnated into the pores of the porous bronze formed on a support metal, forming a bearing. At a surface intended to be brought into contact with a crankshaft, the porous bronze and the resin are sparsely exposed. The proportion of the exposed area of ​​the porous bronze to the contact area is at least 5% and at most 60%. Thus, a bearing for a refrigeration compressor with high seizure resistance under boundary lubrication and low sliding wear, as well as a refrigeration compressor using this bearing, can be obtained.

[0014] JP 2001-221231 A discloses the provision of an environmentally friendly sliding material that is free of lead particles. This sliding material consists of polytetrafluoroethylene with a bismuth particle content of 3-40 vol.% and contains no lead particles. As a result, the sliding material exhibits a low coefficient of friction and excellent wear resistance.

[0015] The task is therefore to provide a self-lubricating sliding element that, on the one hand, has the tribological properties required especially for lubricant-free applications and, at the same time, has improved conductivity.

[0016] The problem is solved by a sliding element having the features of claim 1.

[0017] The sliding element according to the invention comprises a metallic carrier material and a sliding layer based on PTFE or thermoplastic applied thereto, wherein the sliding layer forms an interface with the carrier material and a free sliding surface for contact with a counter-rotator. It is characterized in that the sliding coating contains conductive particles, each of which extends individually within the sliding layer at least from the interface to the sliding surface in order to establish an electrical connection between the carrier material and the sliding surface, wherein the volume fraction of the conductive particles, based on the volume of the sliding layer, i.e. without the metallic carrier material, is 0.1-1.2%, preferably 0.2-0.9%.

[0018] The interface between the sliding layer and the substrate can be identified in a section (micrograph) through the sliding element using conventional optical aids (light or electron microscope) in a manner familiar to those skilled in the art. The distance between the interface and the sliding surface is referred to as the thickness of the sliding layer.

[0019] The conductive particles penetrate the entire sliding layer and each establishes electrical contact between the bearing housing via the metallic carrier material and the counter-rotor bearing on the sliding surface. This occurs right from the start and not only after part of the sliding layer has been removed. This requires that the conductive particles are at least as large as the thickness of the sliding layer. Unlike with conventional conductive fillers, the conductivity of the layer is therefore not primarily dependent on the amount of filler and the associated statistical distribution of the particles. The individual conductive particles create point-by-point electrical contacts along or in the sliding surface, which reliably establish the conductive connection between the connected components and at the same time do not impair the tribological properties and manufacturability of the material or the tight fit of the sliding elements.

[0020] Sliding elements are understood here to mean, in particular, radial bearing elements or flat sliding elements, such as plain bearing bushes, thrust washers, flanged bushes, bearing shells or sliding or guide elements of other geometries.

[0021] The conductive particles consist, for example, of metallic or conductive non-metallic materials. Suitable non-metallic materials include coarse graphite or short graphite fibers. Metallic materials such as copper or aluminum or alloys thereof are preferred, in particular bearing alloys made of copper-tin, copper-tin-zinc, copper-zinc, copper-aluminum, copper-nickel-silicon, copper-tin-bismuth, aluminum-tin, aluminum-tin-silicon, aluminum-tin-copper, aluminum-tin-nickel-manganese-copper, optionally with additional alloying additives and / or unavoidable impurities.

[0022] Conductive particles that can be used include bronze and aluminum chips, elongated, twisted particles produced as turning or milling chips from the corresponding materials, short bronze fibers, as well as spherical and spattered bronze particles, depending on the particle size required, readily available, or more cost-effective. Aluminum particles are particularly preferred for sliding layers created by powder coating, as they are less prone to segregation than bronze particles, for example, due to their density, which is more similar to that of plastic.

[0023] The size of the conductive particles is preferably selected such that the d50 value of the smallest dimension of the conductive particles is at least 90% of the layer thickness of the sliding layer and the d90 value of the smallest dimension of the particles is not greater than 300%, preferably not greater than 150%, of the layer thickness of the sliding layer.

[0024] The "smallest dimension" is defined as the shortest edge length of the smallest possible cuboid enclosing the particle. For ideally cylindrical fiber sections with a greater length than diameter, for example, the smallest dimension would therefore correspond to the fiber diameter. Particles can be isolated from a geometrically statistical particle distribution by sieving through longitudinal slits, for example, using a harp sieve with straight longitudinal wires.

[0025] The d50 value refers to the value of the statistical size distribution that 50% of the particles fall below or exceed. The d90 value is the value of the statistical size distribution that 90% of the particles fall below or 10% exceed.

[0026] The thickness of the sliding layer is the distance between the highest points of the metallic substrate (i.e., the interface) and the outer surface of the plastic-based sliding layer (i.e., the sliding surface). To determine the thickness of the sliding layer, the highest elevations of the interface and sliding surface over a visible length of > 1 mm can be used in a cross-section through a sliding element. In practice, the layer thickness can be determined as the difference between the thickness of the sliding element and the thickness of the metallic substrate, which can each be easily measured using a micrometer, for example. The layer thickness of the sliding layer is preferably 5 µm to 500 µm and particularly preferably 5 µm to 100 µm.

[0027] The d50 value of the smallest dimension of the conductive particles being at least 90% of the sliding layer thickness ensures that sufficient particles are present to penetrate the entire sliding layer and enable contact between the counter-pin and the substrate. The d90 value of the smallest dimension of the conductive particles being no greater than 300% (depending on the porosity of the metallic substrate at the interface, preferably no greater than 150%) of the sliding layer thickness ensures that the particles can be pressed into the substrate during coating, for example, by rolling, or plastically deformed in such a way that no disruptive surface roughness is created.

[0028] The inventive use of the conductive particles is possible in all embodiments of the generic self-lubricating sliding elements described in the prior art and is particularly advantageous in all applications in which the substrate material is not exposed during their service life. This applies in particular to medium-thick and thick sliding layers, which—as an alternative to the conductive particles used according to the invention—would otherwise be particularly impaired in their tribological stability by larger proportions of finely distributed conductive additives. Thick layers are understood to be layers of 120 µm or more, while thin layers are understood to be layers between 5 µm and 50 µm. The former are predominantly based on thermoplastics, the latter on PTFE. Layers with thicknesses of 50 to 120 µm are referred to as medium-thick layers and can be made from either PTFE or thermoplastics.In principle, the particles according to the invention are also useful for such thin layers when conductivity is required even before the substrate is exposed, for example, in hinges before painting in the assembled state or when static charges must be avoided regardless of the running-in state. For use in hinges for movable components, for example, sliding elements made of thin-walled materials without a support layer are of interest. The metallic carrier material consists exclusively of, for example, steel or bronze expanded metal or steel or bronze mesh, and a medium-thick PTFE-based sliding layer of over 50 µm is used.

[0029] The addition quantities of the particles ensure a preferred average density of 1 cm-2 to 10 cm-2 of the contacts made by the conductive particles along the sliding surface.

[0030] While thinner layers usually have a PTFE matrix and are produced from a dispersion by mixing with the additives and precipitation, thicker layers are often based on thermoplastic-based powder mixtures or compound films. Medium-thick layers can have either a PTFE matrix or a thermoplastic resin base. The conductive particles can be added using any manufacturing process, but the dispersion-based process is particularly suitable, as density-related segregation can be effectively counteracted by vigorous stirring during precipitation or late addition during solidification of the coating compound. In the further course of production, segregation is hindered by incorporation into the highly viscous coagulum.

[0031] Furthermore, the volume of the sliding layer preferably consists predominantly of PTFE, i.e. the proportion of PTFE relative to the entire sliding layer is more than 50 vol.%.

[0032] Further additives can be the known additives mentioned at the beginning, such as MoS2, hBN, ZnS, BaSO4, CaF2, Fe2O3, graphite, carbon black, as well as other plastics such as PFA, FEP, ETFE, PEEK, PPS, PAI, polyaramids, aromatic polyesters, hard materials such as glass, Si3N4, SiC, or even carbon fibers.

[0033] The metallic carrier material is preferably formed from a single support metal layer or a layered composite comprising a support metal layer and a bearing metal layer. The sliding element thus preferably represents either a three-layer system or a two-layer system.

[0034] The support metal layer or the bearing metal layer is preferably porous, at least near the interface. Regardless of the number of layers, the porous support metal layer or the bearing metal layer serves as an anchor for the sliding layer material, which is, in a sense, impregnated into the porous surface. The support metal layer or the bearing metal layer need not be porous throughout, but can be.

[0035] In a preferred variant of the three-layer system, the metallic carrier material consists of steel with sintered bronze.

[0036] In a preferred variant of the two-layer system, the metallic carrier material is formed exclusively from a single support metal layer made of steel, a copper alloy or an aluminum alloy.

[0037] The metallic carrier material particularly preferably consists exclusively of a metallic fabric or expanded metal made of steel, copper or aluminium or their alloys.

[0038] Sintered material as well as fabric or expanded metal are subsumed under the porous support metal or bearing metal layers.

[0039] Conductivity is particularly important in hinges, which often use thin-walled materials that can be calibrated to eliminate play during assembly by using pins that are oversized relative to the bearing bush bore. These materials can have a structure consisting of a carrier material, porous bearing metal, and a plastic coating. However, the carrier material often consists of metal mesh or expanded metal coated and / or impregnated with the sliding layer material.

[0040] According to a particularly preferred embodiment, a PTFE-based sliding layer is therefore applied to such a metallic fabric or expanded metal made of steel, copper or aluminum or their alloys.

[0041] The sliding elements according to the invention are therefore used in particular as hinges for lubricant-free joints, such as for trunk lids or the like. Since the sliding layer has sufficient conductivity for electrical contacting of the components connected to the sliding elements, unwanted electrostatic charging of the components is avoided from the outset and electrical contact is provided, for example, for the purpose of painting.

[0042] Embodiments of the invention are explained in more detail below with reference to the figures. They show: Fig.1 a schematic sectional view through a first embodiment of a 3-layer sliding element according to the invention; Fig. 2 a schematic sectional view through a second embodiment of a 3-layer sliding element according to the invention; Fig. 3 a schematic sectional view through a first embodiment of a 2-layer sliding element with metal mesh according to the invention; Fig. 4 a schematic sectional view through a second embodiment of a 2-layer sliding element with expanded metal according to the invention. Fig. 5 a diagram showing the course of the electrical resistance and the wear resistance as a function of the volume fraction of the conductive particles; and Fig. 6 a graphical comparison of the required volume fractions of different conductive fillers.

[0043] A first embodiment of a sliding element according to the invention in the form of a three-layer system is shown in Fig.1. It consists of a metallic carrier material 10, which in turn is constructed from a supporting metal layer 12, for example a steel supporting layer, and a bearing metal layer 14 arranged thereon. The bearing metal layer 14 consists, for example, of sintered bronze, illustrated as a coherent sintered body 15, and forms a porous partial layer of the carrier material 10. On the side of the bearing metal layer facing away from the supporting metal layer 12, a sliding layer 16, for example based on PTFE, is applied, forming an interface 18 to the carrier material 10. The free surface of the sliding layer forms the sliding surface 20. The sliding layer is a thin sliding layer with a thickness d of less than 50 µm. As already mentioned, the layer thickness can be determined by first measuring the layer thickness of the metallic carrier material (d T ) and after coating the thickness of the sliding element (d G) is measured, for example, using a micrometer screw, preferably several times at different points, and then the difference d=d is calculated from the two (average) values. G -d T is formed.

[0044] The sliding layer 16 contains isolated conductive particles 22 which extend within the sliding layer 16 at least from the interface 18 to the sliding surface 20 and establish an electrical connection between the carrier material 10 and the sliding surface 20.

[0045] It should be noted that the grain size of the sintered material and thus also the pore size of the metallic substrate 10 at the interface 18 are exaggerated for illustrative purposes. This clearly shows that the d90 value of the smallest dimension of the conductive particles can advantageously be up to 300% of the layer thickness d when the pore size of the metallic substrate is large relative to the layer thickness d.

[0046] A second embodiment of a sliding element according to the invention in the form of a three-layer system is shown in Fig. 2. Like the first embodiment, it consists of a metallic carrier material 10, which is constructed from a support metal layer 12 and a sintered bearing metal layer 14 arranged thereon. The bearing metal layer 14 also forms a porous partial layer of the carrier material 10. On the side of the bearing metal layer facing away from the support metal layer 12, a sliding layer 16, for example based on PTFE or based on a thermoplastic, is applied to form an interface 18. The free surface of the sliding layer forms the sliding surface 20. The sliding layer is a thick sliding layer with a thickness d of at least 50 µm.

[0047] The sliding layer 16 in turn contains isolated conductive particles 22 which extend within the sliding layer 16 at least from the interface 18 to the sliding surface 20 and establish an electrical connection between the carrier material 10 and the sliding surface 20.

[0048] This illustration shows that for the d90 value of the smallest dimension of the conductive particles, a d90 value of a maximum of 150% of the layer thickness d is sufficient for the interface with finer pores or for interfaces with low roughness in relation to the layer thickness d.

[0049] A third embodiment of a sliding element according to the invention in the form of a two-layer system is shown in Fig.3. It consists of a metallic carrier material 10, which consists exclusively of a metallic fabric 24, for example, made of steel, copper, or aluminum. The metallic fabric 24 thus forms the porous layer of the carrier material 10. A sliding layer 16, for example, based on PTFE or a thermoplastic, is applied to the metallic fabric 24, forming an interface 18. The free surface of the sliding layer forms the sliding surface 20. The sliding layer is a medium-thick or thick sliding layer with a thickness d of at least 50 µm.

[0050] The sliding layer 16 in turn contains isolated conductive particles 22 which extend within the sliding layer 16 at least from the interface 18 to the sliding surface 20 and establish an electrical connection between the carrier material 10 and the sliding surface 20.

[0051] A fourth embodiment of a sliding element according to the invention in the form of a two-layer system is shown in Fig. 4. It consists of a metallic carrier material 10, which consists exclusively of expanded metal 26, for example, made of steel, copper, or aluminum. The expanded metal 26 thus forms the porous layer of the carrier material 10. A sliding layer 16, for example, based on PTFE or a thermoplastic, is applied to the expanded metal 26, forming an interface 18. The free surface of the sliding layer forms the sliding surface 20. The sliding layer is a medium-thick or thick sliding layer with a thickness d of at least 50 µm.

[0052] The sliding layer 16 in turn contains isolated conductive particles 22 which extend within the sliding layer 16 at least from the interface 18 to the sliding surface 20 and establish an electrical connection between the carrier material 10 and the sliding surface 20.

[0053] As in the Fig. 3 and Fig. 4, the matrix material (PTFE or thermoplastic) of the sliding layer penetrates the open-pore structure of the metallic fabric 24 or the expanded metal 26 when it is applied or rolled on. Since the mesh size of the metallic fabric 24 as well as the pore size of the expanded metal 26 in the examples of Fig. 3 and Fig. 4 can also be larger than the smallest dimension of the conductive particles, these can also be found together with the matrix material within the open-pore structure, not shown here. Table 1 Example No. Substrat plastic base Additives [vol%] Layer thickness [µm] d50 value [µm] Resistance 1 Bronze fabric PTFE PPSO2 (25), hBN(5) 60-80 > 10 7 Oh 1a + Bz Chips (0.55) 60-80 100 < 10 4 Oh 2 Steel expanded metal PTFE PPSO2 (40) 60-80 > 10 7 Oh 2a + spattered Bz particles (0.33) 60-80 100 < 10 4 Oh 3 steel+porous bronze PTFE MoS2 (20) 10-30 > 10 7 Oh 3a + Bz fibers (0.11) 10-30 35 < 10 4 Oh 4 steel+porous bronze PPS PTFE (15), PPTA(5) 70-100 > 10 7 Oh 4a + spherical Bz particles (0.89) 70-100 100 < 10 4 Oh 5 steel+porous bronze PEEK PTFE (20) 250-300 > 10 7 Oh 5a + Al Chips (1.12) 250-300 300 < 10 4 Oh 6 Steel PVDF PTFE (20), PPTA(5) 200-250 > 10 7 Oh 6a + Al Chips (0.66) 200-250 300 < 10 4 Oh

[0054] Table 1 shows the layer structure of various examples of bearing elements, including the additives contained in the sliding layer, as well as the results of an electrical resistance measurement on these bearing elements. Examples 1 to 6 represent comparative examples without the conductive particles according to the invention. Examples 1a to 6a are exemplary embodiments of the invention with conductive particles.

[0055] Examples 1 and 1a contain a CuSn10 bronze mesh with a wire diameter of 0.25 mm as the substrate or carrier material. Examples 2 and 2a contain a steel expanded metal based on a 0.3 mm thick steel as the carrier material. Examples 3 to 6 and 3a to 6a use a bare, ground steel with a sintered bronze framework with a pore volume of 35% as the carrier material.

[0056] These substrates are combined with PTFE-based layers (Examples 1 to 3 and 1a-3a) and with thermoplastic-based layers (Examples 4 to 6 and 4a to 6a).

[0057] As shown in Table 1, bronze and aluminum chips, elongated, twisted particles produced as turning or milling chips of the respective materials, short bronze fibers, and spherical and spattered bronze particles were used as conductivity additives. The particle shape is selected based on availability and cost considerations, as well as the particle sizes required for the respective applications or sliding layer thicknesses. Therefore, the particles in the example combinations are generally interchangeable.

[0058] In the inventive examples shown in Table 1, the conductive particles are each selected such that the d50 value of the smallest dimension corresponds to at least 90% of the layer thickness. Due to the manufacturing process, the measurement results for the layer thicknesses vary when measured at multiple locations or on multiple pieces. Therefore, a range is given for this.

[0059] The electrical resistance across the entire thickness of the bearing element is measured using a device consisting of two blocks serving as contacts. This device both provides a contact area defined by the blocks and also applies a defined load to the material sample, as the material sample completely overlaps the edges of the contact blocks. This creates comparable contact conditions. The contact area is 28.3 cm. 2 and the sample is loaded with 220 N.

[0060] Fig.Figure 5 shows the electrical resistance and wear resistance curves for the sliding element according to Example 1 with increasing volume fraction of added conductive particles, in this case bronze chips. The amount corresponding to Example 1a is marked on the curve.

[0061] Wear resistance was measured on an oscillation test rig using bushings measuring 15 mm x 22 mm x 25 mm (width x inner diameter x outer diameter) at a load of 35 MPa and an average sliding speed of 0.07 m / s. The reduction in the thickness of the sliding layer in µm is normalized to the friction distance (bearing circumference x revolutions) in km.

[0062] The resistance measurement curve clearly shows that from approximately 0.05 vol.% bronze chips relative to the volume of the sliding layer, the electrical resistance quickly drops to a minimum (or the electrical conductivity quickly increases to a maximum). The wear measurement curve also clearly shows that with a volume fraction of bronze chips exceeding 1.2 vol.%, the wear resistance of the sliding layer drops sharply. From both observations, the preferred value range AA' for the volume fraction of conductive particles to be used relative to the volume of the sliding layer is 0.1 to 1.2 vol.%. The value range BB' from 0.2 to 0.9 vol.% is particularly preferred.

[0063] This quantity corresponds to approximately 1-10 contact points per cm 2on the sliding surface. This range of values ​​ensures sufficient conductivity regardless of the material type. Taking into account the size of the sliding elements or sliding surfaces, it is recommended for statistical reasons to select a higher density of contact points for very small plain bearings than for very large ones.

[0064] If one attempts to increase the conductivity of the sliding elements to the same level with other additives in the plastic sliding layer, for example, > 15 vol.% graphite or > 5 vol.% conductive carbon black are required. The diagram in Fig.Figure 6 illustrates this relationship between the volume fractions of different additives required for the same conductivity compared to the solution according to the invention. However, this significantly impairs the tribological properties of the sliding layers, making the sliding element unusable for a wide range of applications. This applies particularly to medium-thick and thick sliding layers, whose tribological stability is particularly impaired by larger proportions of finely distributed conductive additives. One area, as in Fig. 5, in which the tribological properties and the conductivity are simultaneously close to their achievable optimum, cannot be found by simple modification of known compositions.

[0065] In summary, it can be stated that only the addition of a few, larger conductive particles according to the invention makes a satisfactory solution possible with regard to conductivity and wear resistance.

[0066] The production of the exemplary sliding elements is described below: Examples 1-3, 1a-3a.

[0067] The production of the materials according to the invention and the reference materials based on PTFE is carried out in a known manner by • Pre-dispersion of the additives except PTFE and particles according to the invention in water and wetting agent using a suitable mixing tool such as a dissolver or similar; • Addition of an aqueous PTFE dispersion with a concentration between 20 and 40 wt.% and homogeneous mixing; • Coagulation of the PTFE by adding suitable salts, e.g. aluminum nitrate or acids, e.g. citric acid; • Stir to solidify the mass, if necessary removing water and adding a solvent (e.g. toluene) until the consistency required for rolling is reached; • The conductive particles are added after precipitation during the solidification of the mass. This prevents segregation due to higher density. • Rolling the mass onto the carrier material; • Heating the coated substrate in an oven to a temperature above the PTFE melting point (327°C), e.g. to 370°C for at least 90 s and • Rolling the hot coated substrate to the required final thickness.

[0068] Alternatively, other processes can be used, e.g. the production of a mixture of powdered PTFE with the additives in an organic solvent, which is then rolled onto the carrier material. Examples 4-6, 4a -6a

[0069] The production of the materials according to the invention and the reference materials based on thermoplastics can be carried out in a known manner, for example by the following steps: • Mix all components in powder form in a suitable powder mixer; • Spreading the powder mixture onto the carrier material using a doctor blade device; • Heating the sprinkled carrier material to a temperature above the melting temperature of the thermoplastic, for PEEK e.g. to 390°C; • Rolling the melt with the carrier material; • If necessary, further heating and holding at a thermoplastic-specific process temperature and • Rolling the hot coated substrate to the required final thickness.

[0070] Alternatively, other processes can also be used, e.g. the production of a plastic film by melt extrusion of the component mixture and rolling this film onto the carrier material.

Claims

[1] Sliding element with a metallic carrier material (10) and a sliding layer based on PTFE or thermoplastic material applied thereon, wherein the sliding layer forms an interface (18) to the carrier material (10) and a sliding surface (20) for contact with a counter-rotator, wherein the sliding layer contains conductive particles (22) which each extend individually within the sliding layer at least from the interface (18) to the sliding surface (20), characterized by that the volume fraction of the conductive particles (22), based on the volume of the sliding layer, is 0.1 - 1.2%, preferably 0.2 - 0.9%. [2] Sliding element according to claim 1, characterized by that the conductive particles (22) are metallic particles (22). [3] Sliding element according to claim 2, characterized by that the metallic particles (22) consist of copper, aluminum or alloys thereof. [4] Sliding element according to one of the preceding claims, characterized by that the d50 value of the smallest dimension of the particles (22) is at least 90% of the sliding layer thickness and the d90 value of the smallest dimension of the particles (22) is not greater than 300% of the sliding layer thickness. [5] Sliding element according to one of the preceding claims, characterized by that the average density of the contacts made by the conductive particles (22) along the sliding surface (20) is 1cm 2 -10cm 2 amounts. [6] Sliding element according to one of the preceding claims, characterized by that the volume of the sliding layer consists predominantly of PTFE. [7] Sliding element according to one of the preceding claims, characterized by that the sliding layer has a thickness of more than 50 µm. [8] Sliding element according to one of the preceding claims, characterized bythat the metallic carrier material (10) is formed from a single support metal layer (12) or a layer composite with a support metal layer (12) and a bearing metal layer (14). [9] Sliding element according to claim 8, characterized by that the support metal layer (12) or the bearing metal layer (14) is porous at least near the interface (18). [10] Sliding element according to one of claims 8 or 9, characterized by that the metallic carrier material (10) consists of steel with sintered bronze. [11] Sliding element according to one of claims 8 or 9, characterized by that the metallic carrier material (10) is formed exclusively from a single supporting metal layer (12) made of steel, a copper alloy or an aluminium alloy. [12] Sliding element according to one of claims 8 or 9, characterized bythat the metallic carrier material (10) consists exclusively of a metallic fabric (24) or expanded metal (26) made of steel, copper or aluminium or their alloys.

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