WO2008154978A2 - Method for producing a sliding element coated with silver in a structured manner, and sliding element obtained by said method - Google Patents

Abstract

The invention relates to a method for producing a sliding element coated in a structured manner. According to said method, in a first step, a silver-containing layer is electrolytically deposited on a sliding element as the main constituent in relation to the mass, and in a second step, a metal or metal alloy layer is then electrolytically deposited on the first layer. The current density in the first step is at least 1 A/dm2. The invention also relates to a sliding element coated in a structured manner.

Description

 Process for the preparation of a silver-structured coated sliding element and then available sliding element

The invention relates to a method for producing a structured coated sliding element, in particular a sliding bearing, in which metal or a metal alloy is deposited electrolytically in at least two steps on the sliding element, wherein in the first step a layer containing silver as a main component at an unusually high current density of at least 1 A / dm ^{2 is} electrodeposited to create a microstructure of the first layer.

Sliding elements which are subject to mechanical stresses in the form of friction, for example slide bearings, must have good sliding properties, high corrosion resistance and sufficient wear resistance.

To achieve these properties, sliding elements, in particular their running surfaces, can be provided with sliding layers in the form of electrolytically deposited metals or metal alloys. These coatings should on the one hand have sufficient ductility and show a low embrittlement tendency, especially under load and at high temperatures, and on the other hand have a high internal strength to withstand high loads. DE 197 54 221 A1 describes a layered composite material whose galvanically applied sliding layer, irrespective of the copper content, does not show any embrittlement even at relatively high temperatures, wherein the layered composite material has a sliding layer with 8-30% by weight copper, 60-97% by weight tin and 0.5-10 wt% cobalt.

To achieve good sliding properties of these electrodeposited coatings are usually provided with smooth as possible surfaces. The sliding properties of coated sliding elements can additionally be improved by a lubricant film. For better adhesion of this lubricant film to the surface and to achieve better emergency running properties structuring of the Running surfaces of such sliding elements are provided. In order to achieve a structuring of the surface of an electrodeposited metal or metal alloy layer, it is known to structure the surface of the sliding element to be coated before the electrolytic deposition mechanically or by means of a chemically generated etching.

However, these methods have the disadvantage that a time-consuming and costly patterning cut must be provided. Furthermore, in this way, as a rule, no microstructuring can be achieved. Since for the coating of sliding elements, such as plain bearings, no coatings of very hard metals such as chromium or titanium can be taken, moreover, in these methods of the prior art often the internal strength and thus the resilience of the electrodeposited metal coatings is insufficient.

It is therefore an object of the present invention to provide an improved process for producing a structured coated sliding element and a coated sliding element obtainable by this process which overcomes the above-described disadvantages of the prior art.

According to the invention, this object is achieved by a method for producing a structured coated sliding element, wherein on a sliding element in a first step, as a main component, based on the mass, silver-containing layer is deposited electrolytically and then on the first layer in a second step Metal or metal alloy layer is deposited electrolytically, wherein the current density in the first step is at least 1 A / dm ² .

With this method, surprisingly, a coated sliding member is obtainable which has a high load capacity and a good lubricant retention capability, avoiding the step of prior mechanical structuring of the surface of the sliding member to be coated.

In the production of electrochemical coatings, a coating current density suitable for the electrolyte system is generally used in order to achieve a trouble-free, uniform metal coating. These current densities are specific for the electrolytes and thus for the metals and metal alloys to be deposited electrolytically and are known to the person skilled in the art.

At these current densities, a metal or metal deposition layer is deposited which does not have its own microstructure. In the context of the invention it was found that above a current density of 1 A / dm ^2, a special microstructuring of an electrodeposited silver layer or an alloy layer containing silver as a main constituent (so-called silver-based layer) is produced in the form of a bud-shaped and / or dendritic structure, which is followed by subsequent electrolytic deposition of a second metal - or metal alloy layer leads to a highly resilient lubricious coating.

The second layer follows the microstructuring of the first layer, and the bud-shaped and / or dendritic structure of the first layer (base layer) thus results in a particular microstructuring of the surface of the second layer in this two-step process. to a microstructuring of the surface of the applied coating. Surprisingly, this coating leads to a high wear resistance of the coated sliding element, at the same time very good lubricant retention capacity of the surface.

The high wear resistance of the coating produced by the process according to the invention is, without being bound thereto according to the invention, due to the increase in the internal strength of the applied coating by a change in the growth direction of the crystallization of the electrodeposited silver or the electrodeposited silver alloy.

Furthermore, the hydrodynamics between a sliding member thus coated and a sliding partner is positively influenced by the improvement in the constancy of the lubricant. As a result, in addition, the emergency running properties and the resistance of the sliding element with respect to the adhesive wear are significantly increased. Another advantage of the coating process according to the invention is a shortening of the process, which results in a time saving and the production costs are reduced, since the previously required, prior mechanical or chemical etching generated structuring of the sliding member to be coated is no longer required.

For the purposes of the invention, a sliding element is understood to mean elements, in particular machine elements, which have a sliding surface for sliding engagement with a mating surface and are to be provided with a sliding and anti-wear layer. These may be metallic or non-metallic sliding elements. If a metal or metal alloy layer is to be deposited on a non-metallic sliding element, the sliding element is first rendered electrically conductive by applying a thin metal film. The coating according to the invention can be used for coating different sliding elements, for example sliding bearings, bushes cylinders, pistons, bolts, seals, Valves and Daickzylindern. Preferred sliding elements are sliding bearings, in particular sliding bearings for motor vehicles, for example crankshaft bearings, camshaft bearings or connecting rod bearings. The coating method according to the invention can be used for coating the entire surface of the sliding element or only for coating the sliding surfaces of the sliding element.

The sliding element may already be provided with additional metal or metal alloy layers before coating with the method according to the invention. For example, a plain bearing usually has the following layer structure: steel (material of the sliding bearing), bearing metal layer, optionally a dam layer, sliding layer, optionally an inlet layer. The bearing metal layer may be, for example, a copper alloy layer, in particular a sintered or cast copper alloy layer. The dam layer can serve as a diffusion barrier and the sliding layer in turn can be applied by the method according to the invention.

The first current density for depositing a layer containing as the main constituent, based on the mass, silver is preferably at least 1.5 A / dm ² , more preferably at least 2 A / dm ² , even more preferably at least 3 A / dm ^2, and most preferably at least 5 A / dm ² . At these current densities, particularly favorable microstructures of the silver layer or silver-based layer are obtained, which lead to highly loadable coated sliding surfaces.

The electrolytic deposition in the second step is preferably carried out at such a low current density that the resulting second layer of metal or a metal alloy does not have its own microstructuring.

In the context of the invention, a structure-free or microstructure-free coating or layer is understood to mean that the metal layer or metal alloy layer does not form its own microstructure, but deposits on the surface to be coated, following the surface structure of this coated surface. For example, a surface to be coated may be sandblasted and thus have a macrostructure, which then follows the coating in the electrolytic deposition without, however, additionally forming its own microstructuring.

The above-described current densities used in the second step for depositing a metal or metal alloy layer without its own microstructure depend on the metal to be deposited and the electrolyte used. According to the invention, the second current density for depositing a tin based on the mass as the main component or bismuth-containing layer no longer 4 A / dm ² (ampere per square decimeter), preferably not more than 3 A / dm ² , more preferably 0.5-3 A / dm ² and even more preferably 0.5-2.5 A. / dm ² . Tin as the main constituent, based on the mass, containing layers are tin layers which consist of 100 wt .-% or nearly 100 wt .-% of tin and contain only the usual impurities and alloys whose weight fraction of tin higher than the weight fraction of the remaining Alloy components is. For depositing a layer containing silver as the main constituent, based on the mass, the second current density is no longer 0.9 A / dm ² , preferably 0.3-0.75 A / dm ² , particularly preferably 0.5-0.75 A / dm ² . For depositing a layer containing as the main constituent, based on the mass, nickel, the second current density is no longer 7 A / dm ² , preferably not more than 6 A / dm ² and particularly preferably 1 -5 A / dm ² .

The surface structure of the coated sliding element, in particular the roughness of the surface and the density of the surface structures, can be controlled via the coating current density in the first step of the electrodeposition, which influences the expression of the buds and dendrites, and the layer thicknesses of the first layer and the second layer. The higher the current density in the first step, the greater the layer thickness of the first layer and the smaller the layer thickness of the second layer deposited there above, the rougher the preserving surface and the denser the surface structure obtained. In this way, the internal strength of the coating and the surface structure of the type of use of the sliding element and the lubricant to be used can be adapted. For example, the internal strength and the surface structure can be increased in the direction of a high load capacity, e.g. Crankshafts of high-revving internal combustion engines or in the direction of a high mileage, at e.g. Slide bearings are optimized by truck internal combustion engines.

Suitable coating materials for the second layer are all metals and metal alloys which have sufficient ductility and strength for the respective sliding element and its desired use. The first and second layers may comprise the same metal or metal alloy or different metals or metal alloys. If the first and second layers are to comprise different metals or metal alloys, the electrolyte is changed before performing the second electrodeposition step.

Suitable metals and metal alloys for the second layer are independently aluminum, antimony, bismuth, lead, iron, gold, copper, nickel, cobalt, silver, zinc and tin and their alloys. Preference is given to tin, silver and bismuth and their alloys, in particular those containing tin, silver or bismuth as main constituents. These Materials provide highly resilient and at the same time sufficiently ductile structured coatings according to the inventive method, which are particularly suitable for plain bearings.

Among the tin alloys, tin alloys containing the metals antimony, lead, copper, silver, bismuth, cobalt and / or nickel are preferred. Examples are tin-antimony alloys, tin-copper alloys, tin-bismuth alloys and tin-lead alloys, in particular those containing tin as the main constituent, based on the mass, and preferably additionally silver, cobalt, aluminum, iron and / or nickel. The tin content of the tin-based alloys is preferably 60-98% by weight.

Tin-copper alloys with 60-98% by weight of tin and 2-40% by weight of copper and tin-copper-cobalt alloys with 60-98% by weight tin, 1 are particularly preferred according to the invention for the second layer -30% by weight of copper and 1-10% by weight of cobalt, each of which may preferably additionally contain silver, bismuth and / or nickel as alloy constituents.

The electrolytic deposition on the surface of the slider takes place from an electrolyte. For the purposes of the invention, an electrolyte is understood to mean an aqueous solution whose electrical conductivity is brought about by electrolytic dissociation into ions. The electrolyte therefore contains the metal or metals for forming the metal alloy in the form of ions, and moreover the usual electrolyzers known to those skilled in the art, such as, for example, acids and salts and the remainder water.

The process of the invention is preferably carried out in an electrolyte containing 80-300 g / l (grams per liter) of methanesulfonic acid. Further, it is preferred that the electrolyte contains one or more mono- or polyhydroxybenzene (s) and / or beta-naphtholethoxylate. A particularly preferred electrolyte for carrying out the process of this invention comprises 1-150 g / l of depositable metal or ionic depositable metals, 80-300 g / l of methanesulfonic acid, preferably 80-250 g / l of methanesulfonic acid, 1-5 g / l of mono- or Polyhydroxybenzene, 30-45 g / l Cerolyt BMM-T (company Enthone) and the remainder water. Cerolyt BMM-T is an electrolyte additive containing ß-naphthol ethoxylate as its main constituent. As the polyhydroxybenzene, the di-, tri- and tetrahydroxybenzenes are suitable, preferred are dihydroxybenzenes, among which resorcinol is particularly preferred.

The process according to the invention can also be carried out in a cyanide electrolyte commonly used in the prior art. Cyanide electrolytes usually contain as main constituents of the dissolved components cyanide and hydroxide salts, in particular in the form of potassium or sodium cyanide and potassium or sodium hydroxide. In In a cyanide electrolyte, the above-mentioned current densities for producing silver layers or silver based on the composition of layers containing as the main component are preferably selected to be about twice as high. The first current density for producing a layer containing as the main constituent, based on the mass, silver is therefore preferably at least 2 A / dm ² , more preferably at least 4 A / dm ² , even more preferably at least 8 A / dm when using a cyanide electrolyte ^2, and most preferably at least 10 A / dm ² .

These electrolytes, in particular the methanesulfonic acid-containing electrolyte described above, have proven to be particularly suitable for the formation of the structuring and the internal strength of the coating according to the invention.

Over the electrolytic coating time, ie the period of deposition, the desired layer thickness can be adjusted. Usually, the coating time in the first step is 5 to 60 seconds and in the second step 5 to 25 minutes. The temperature of the electrolyte for the deposition of the metal or the metal alloy is usually 20- 90 ⁰ C, preferably 20-50 ⁰ C.

0.1 to 3 .mu.m for the first layer and 3 to 60 .mu.m for the second layer deposited thereon have proven to be suitable layer thicknesses. The layer thickness of the first layer is preferably in the range from 0.3 to 2 μm, particularly preferably in the range from 0.5 to 1.5 μm. The layer thickness of the second layer deposited thereon is preferably in the range from 4 to 25 μm and particularly preferably in the range from 5 to 12 μm.

In a particularly preferred embodiment, the first and / or the second layer additionally contain solid particles. By incorporating solid particles, surprisingly, the wear resistance and the emergency running properties of the coating according to the invention can be further improved.

The invention thus relates in a preferred embodiment to a method in which solid particles are incorporated into the first and / or the second layer. For the production of the solid particle-containing layer, the electrolytic deposition is carried out in the presence of solid particles. The solid particles are advantageously dispersed in the electrolyte. The solid particles deposit during deposition on unevenness of the surface and are encapsulated by the subsequently deposited metal and / or fixed to the surface. Hard particles of tungsten carbide, chromium carbide, aluminum oxide, silicon carbide, silicon nitride, boron carbide and / or diamond are preferably used as solid particles. The grain size of the solid particles is preferably in the range of 0.01 to 5 microns. Diamonds are particularly preferred as solid particles, and below this, in turn, those having a size in the range of 0.25-0.4 μm are preferred. Further, alumina particles having a particle size in the range of about 0.2-5 microns are preferred. Embedded diamond particles can be formed from mono- and / or polycrystalline diamond. With polycrystalline diamond, the better results are often achieved because a polycrystalline diamond has numerous slip planes due to the many different crystals. The solid particles or hard material particles can also be a mixture of solid particles of different types of substances in combination.

In order to improve the lubricity, solid lubricant particles may additionally be contained in the layer containing solid particles, whereby the layer can be adapted to the particular application. As solid lubricant particles, for example hexagonal boron nitride, graphite and / or polymer particles, in particular of polyethylene and / or polytetrafluoroethylene, can be incorporated into the layer.

The first and second electrolytic deposition steps may be repeated, if necessary a plurality of times, so that the first and second layers are mutually deposited. For example, in this way four, six, eight, ten or twelve layers can be deposited on each other, so that the first layer, which has a bud-shaped and / or dendritic structure, and the second layer are applied alternately on the sliding element. In this way, an optimal coating can be produced depending on the material and the desired use. In particular, by repeating the first and second steps, an even higher internal strength and thus a higher load-bearing capacity of the coating can be achieved.

Furthermore, in the first and / or the second layer, a gradient structure can be generated, for example, by coating at a current density gradient or a concentration gradient. Thus, for example, in the first layer a decreasing microstructure can be generated by a current density decreasing in the first step, and the microstructuring can be adapted to the material to be deposited and the desired use.

In the case of the coating method according to the invention, it may be favorable, under the first layer (base layer), which has a microstructuring, for better adhesion of this first layer and / or as a diffusion barrier on the sliding element, a lower layer of metal or a metal alloy, preferably by electrolytic deposition. It is therefore preferred in accordance with the invention to electrolytically deposit an underlayer of metal or a metal alloy prior to the first step of producing the first layer.

A diffusion barrier is particularly advantageous if the metal located under the first layer diffuses with increasing service life of the sliding element in the first layer and thereby deteriorates the properties of the sliding element. For example, copper often diffuses into over-deposited tin layers, causing brittle phases to form and the deposited tin layer can flake off.

For the underlayer, tin, nickel, iron, gold, cobalt, copper and their alloys are preferred. Particularly preferred are nickel and nickel alloys, since they act excellent as diffusion barriers. The backsheet is typically applied using conventional techniques, i. it does not have its own microstructuring. However, it is also possible for the lower layer to be applied electrolytically at a correspondingly high current density, so that the lower layer then has its own microstructuring.

In addition, an inlet layer can be applied over the second layer, which facilitates the running in of the sliding element. This may be a Wettere electrolytic deposited metal or metal alloy layer or applied by PVD or CVO process layer. As entry layers, electrolytically deposited molybdenum-based layers, PVD and CVD layers are preferred. It is therefore preferred according to the invention, following the second step for producing a second layer, to apply an inflow layer, preferably by electrolytic deposition or by a PVD or CVD method.

In the context of the invention, a PVD layer is understood to be a layer deposited on a sliding element by PVD (Physical Vapor Deposition). PVD processes are known per se to the person skilled in the art. The layer starting material is vaporized by laser, ion or electron beams or by arc discharge, usually under reduced pressure at about 1-1000 Pa, and the PVD layer formed by condensation of the material vapor on the substrate. If necessary, a suitable process gas can be supplied.

For the purposes of the invention, a CVD layer is understood to mean a layer deposited by CVD (Chemical Vapor Deposition) on a sliding element. CVD methods are known per se to the person skilled in the art. In a CVD process, a solid from the

Gas phase on the heated surface of a substrate by a chemical reaction deposited. As a rule, CVD processes are also carried out under reduced pressure at about 1 to 1000 Pa.

As PVD or CVD layers, all coatings obtainable by PVD or CVD processes are suitable according to the invention. Preferred PVD and CVD layers are AISn ₂₀ Cu and AISn _2O Fe.

The run-in layer is applied according to the invention, in a layer thickness, so that. After a certain period of use, in particular after the running-in phase, it has worn away so far that the underlying second layer, which has a structured surface, comes to light. In this way, after the removal of the upper run-in layer, at least a part of the material located in the depressions of the underlying structured layer remains, so that the surface then remains from the elevations of the structured second layer and the material of the applied run-in layer remaining in the depressions of this structured layer is formed.

The layer thickness of the inlet layer, for example the PVD or CVD layer, is preferably 4-12 μm, more preferably 4-8 μm, even more preferably 5-6 μm. Since the run-in layer is applied to a structured layer, in the context of the invention an under-run layer is also understood to mean a deposited material which completely or partially fills the valleys of the underlying structured layer and thereby completely covers or merely partially covers or merely covers the overlying structured layer completely or partially fills the valleys of the underlying structured layer without forming a continuous layer in the sense of a complete coating. The layer thickness in the latter case is the mean value of the height of the filling of the valleys.

The present invention further relates to a structured coated sliding element obtainable by the method according to the invention, in particular a plain bearing, e.g. Crankshaft, camshaft or connecting rod bearings.

The invention thus also relates to a structured coated sliding element, having a surface comprising a first layer applied to the surface, which contains silver as the main constituent, based on the mass, and a second layer of metal or a metal alloy applied thereover, wherein the first layer has a microstructuring.

The surface may be the entire surface of the slider or include only the sliding surface (s) of the slider. The second layer preferably has no own Microstructuring on. The second layer preferably contains tin, silver or bismuth, in particular as the main constituent, based on the composition. The microstructuring of the first layer is preferably bud-shaped and / or dendritic. The second layer follows the microstructure of the first layer and therefore has a surface microstructure.

In a preferred embodiment, the invention further relates to a structured coated sliding member comprising, in addition to the first and second layers, a lower layer of metal or a metal alloy disposed below the first layer. The underlayer can advantageously contribute to imparting better adhesion of the first layer to the sliding element and / or serve as a diffusion barrier.

Further, the invention relates to a coated sliding member comprising an enema layer over the second layer in addition to the first and second layers and optionally the subbing layer described above. The run-in layer is preferably an electrodeposited metal or metal alloy layer, a PVD or CVD layer.

The structured, coated sliding element according to the invention has the advantages described above in connection with the method. The suitable, preferred and particularly preferred embodiments described in the method according to the invention are likewise suitable, preferred and particularly preferred in the case of the coated sliding element according to the invention.

It is understood that the features mentioned above and those explained below can be used not only in the specified combinations, but also in other combinations or in isolation, without departing from the scope of the present invention.

The following examples illustrate the invention.

An electrolyte of the following composition is produced:

Ag ⁺ content (added as silver (I) methanesulfonate) 30 g / l

Methanesulfonic acid 250 g / l resorcinol 2.5 g / l

Cerolyt BMM-T (company Enthone) 30 g / l Subsequently, a sliding bearing is introduced into the electrolyte and the sliding bearing at 30 ° C for 30 seconds at a current density of 2 A / dm ² coated. It is then coated at a current density of 0.5 A / dm ² for a further 20 minutes.

Comparison Test:

An identical slide bearing is coated in the electrolyte having the same composition as described in the above example for 22 minutes at a current density of 0.5 A / dm ² .

Subsequently, the load capacity was tested with Underwood tests. Here a shaft with eccentric weights rotates in rigidly mounted connecting rods. The bearings in the connecting rods are formed by the test bearings. The test bearings have a wall thickness of 1, 4 mm and a diameter of 50 mm. The bearing width is used to set the specific load. The speed is 4000 revolutions / min. The maximum load capacity in megapascals (MPa) without sliding-layer fatigue after 250 h endurance test was measured.

The results are listed in the following table:

Die obigen Versuche zeigen, daß das nach dem erfindungsgemäßen Verfahren beschichtete Gleitlager eine deutlich verbesserte Belastbarkeit gegenüber dem nach herkömmlichen Verfahren beschichteten Gleitlager aufweist. Ferner zeigte das erfindungsgemäß beschichtete Gleitlager gegenüber dem im Vergleichsversuch beschichteten Gleitlager eine deutlich verbesserte Schmiermittelhaltefähigkeit, wie durch Inaugenscheinnahme überprüft wurde. Diese erhöhte Schmiermittelhaltefähigkeit verbessert die Gleiteigenschaften und die Notlaufeigenschaften des erfindungsgemäß beschichteten Gleitlagers.

The above experiments show that the sliding bearing coated according to the method according to the invention has a significantly improved load-bearing capacity compared with the slide bearing coated according to conventional methods. Furthermore, the plain bearing coated according to the invention exhibited a markedly improved lubricant retention capacity compared with the sliding bearing coated in the comparison test, as verified by visual inspection. This increased lubricant holding ability improves the sliding properties and the emergency running properties of the sliding bearing coated according to the invention.

Claims

claims 1. A process for producing a structured coated sliding element, wherein on a sliding element in a first step as a main constituent, based on the mass, silver-containing layer is electrodeposited and then electrolytically on the first layer in a second step, a metal or metal alloy layer is deposited, characterized in that the current density in the first step is at least 1 A / dm ² . 2. The method according to claim 1, characterized in that the current density in the first step is at least 3 A / dm ² . 3. The method according to claim 1 or 2, characterized in that is deposited in the first step of an electrolyte containing 80-300 g / l methanesulfonic acid. 4. The method according to any one of claims 1 to 3, characterized in that in the second step as a main component, based on the mass, tin or bismuth-containing layer at a current density of not more than 4 A / dm ² or as a main component, based is electrolytically deposited on the bulk, silver-containing layer at a current density of not more than 0.9 A / dm ² . 5. The method according to any one of claims 1 to 4, characterized in that the first layer has a layer thickness of 0.1 to 3 microns and the second layer has a layer thickness of 3 to 60 microns. 6. The method according to any one of claims 1 to 5, characterized in that on the sliding member before the first step, a lower layer of metal or a metal alloy is applied. 7. The method according to any one of claims 1 to 6, characterized in that after the second step, an inlet layer is applied. 8. The method according to claim 7, characterized in that the inlet layer is a PCD or CVD layer. 9. The method according to any one of claims 1 to 8, characterized in that after the second step, the first and second steps, if necessary, repeated several times. 10. The method according to any one of claims 1 to 9, characterized in that the first and / or second step is carried out in the presence of solid particles, which are incorporated in the first and / or second layer. 11. Coated sliding element, obtainable by a method according to one of claims 1 to 10. 12. A coated sliding member having a surface comprising a first layer applied to the surface containing, as a main component, by mass, silver and an overlying second layer of metal or a metal alloy, characterized in that the first layer is a microstructure having. 13. Coated sliding element according to claim 12, characterized in that the second layer does not have its own microstructuring. 14. Coated sliding element according to claim 12 or 13, characterized in that the second layer contains as the main constituent, based on the mass, tin, silver or bismuth.