Piston rings with hard chrome layer and improved running-in behavior

Abstract

A piston ring is provided, comprising a base body, wherein the base body has an inner circumferential surface, a first side surface, a second side surface, and an outer circumferential surface, wherein a first hard chrome layer having a crack network is deposited on the outer circumferential surface, the crack network of the first hard chrome layer having a crack density of 10-250 cracks / mm, and solid particles having a median particle size of 0.01-10 μm are embedded in the cracks of the first hard chrome layer, and a second hard chrome layer having a crack network is deposited on the first hard chrome layer, wherein the crack network of the second hard chrome layer has a crack density of 10-250 cracks / mm, and no solid particles are embedded in the cracks of the second hard chrome layer, cracks at the surface of the second hard chrome layer have an average width of 1-15 μm, the cracks at the surface of the second hard chrome layer are electrolytically extended, and the surface proportion of the cracks at the surface of the second hard chrome layer relative to the total surface of the second hard chrome layer is 3-25%.

Description

[0001] The invention relates to a piston ring having a hard chromium layer on the sliding surface and having improved running-in behavior, as well as a method for producing such a piston ring and its use in internal combustion engines.

[0002] Piston rings for internal combustion engines are exposed to high friction and high temperatures and must therefore have a surface that offers high wear and corrosion resistance, as well as a high scorch resistance, and in addition, good sliding properties. For this purpose, the outer circumferential surface (sliding surface) of the piston ring is usually provided with a wear protection layer, for example in the form of an electrolytically deposited hard chromium layer.

[0003] In order to improve the wear and corrosion resistance, solid particles can be embedded in the electroplated hard chromium layer. DE 3531410 A1 and EP 0217126 A1 describe electroplated hard chromium layers having a crack network with solid particles embedded in the cracks of the crack network.

[0004] However, the wear resistance of such a hard chromium solid particle layer is so high that the running-in period for piston rings takes a long time, typically exceeding 2,000 operating hours. During this time, the internal combustion engine is prone to malfunctions and exhibits high fuel consumption. This is particularly important for large piston rings, such as those installed in marine engines. Generally, piston rings with a diameter of approximately 120 to 1,000 mm are considered large.

[0005] The present invention is therefore based on the object of overcoming the above-mentioned disadvantages of the prior art and providing a piston ring whose running-in behavior is improved. Furthermore, the object of the present invention is to provide a method for producing such a piston ring.

[0006] According to the present invention, this object is achieved by a piston ring comprising a base body, wherein the base body has an inner circumferential surface, a first side surface, a second side surface, and an outer circumferential surface, wherein a first hard chrome layer having a crack network is deposited on the outer circumferential surface, the crack network of the first hard chrome layer having a crack density of 10-250 cracks / mm, and solid particles having a median particle size of 0.01-10 μm are embedded in the cracks of the first hard chrome layer, and a second hard chrome layer having a crack network is deposited on the first hard chrome layer, wherein the crack network of the second hard chrome layer has a crack density of 10-250 cracks / mm, and no solid particles are embedded in the cracks of the second hard chrome layer, cracks at the surface of the second hard chrome layer have an average width of 1-15 μm, the cracks at the surface of the second hard chrome layer are electrolytically extended, and the surface proportion of the cracks at the surface of the second hard chrome layer relative to the total surface of the second hard chrome layer is 3-25%.

[0007] The object is further achieved by a method for producing a piston ring, comprising the following steps:

[0008] (a) placing a piston ring having a substrate having an inner circumferential surface, a first side surface, a second side surface, and an outer circumferential surface in an electrolyte containing a chromium compound and solid particles having an average particle size of 0.01 to 10 μm,

[0009] (b) electrolytically depositing a first hard chromium layer on the outer circumferential surface, the first hard chromium layer having a crack network,

[0010] (c) reversing the direction of the current, wherein the cracks that have formed expand and the solid particles become embedded in the cracks,

[0011] (d) repeating steps (b) and (c) at least once to form a first hard chromium layer containing solid particles in the cracks,

[0012] (e) placing the piston ring in an electrolyte containing a chromium compound and no solid particles, and electrolytically depositing a second hard chromium layer having a crack network on the first hard chromium layer, and

[0013] (f) reversing the current direction, wherein cracks formed on the surface of the second hard chrome layer expand to an average width of 1-15 μm and the surface area of ​​the cracks is 3-25% relative to the total surface area of ​​the second hard chrome layer.

[0014] The above-described piston ring has the following advantages: it has cracks on its surface that are extended and therefore have a high surface area, enabling it to act as a lubricant reservoir. At the same time, the piston ring according to the invention has a particle-free hard chrome layer on its surface that is not as wear-resistant as the first hard chrome layer with solid particles (hard chrome-solid particle layer), whereby the overall running-in behavior is shortened and thus significantly improved. This reduces fuel consumption in particular during the first approximately 2,000 operating hours. In addition, the upper particle-free hard chrome layer is a hard chrome layer that has proven to be particularly suitable for piston rings. The upper particle-free hard chrome layer wears away during the running-in phase, so that after the piston ring has been run-in, the known advantageous hard chrome solid particle layer, i.e., the first hard chrome layer, can take over its task as a friction partner in the internal combustion engine. Steps (e) and (f) can be repeated if necessary, possibly multiple times. The thickness of the particle-free hard chrome layer can thus be adapted to the requirements.

[0015] The basic construction of a piston ring having inner and outer circumferential surfaces and side surfaces is described, for example, in DE 10 2011 084 052 A1.

[0016] For the purposes of the present invention, a hard chromium layer is understood to be an electrolytically (electroplated) deposited chromium layer. To form the hard chromium layer, the piston ring is placed in an electrolyte and connected to a cathode. The formation of hard chromium layers with or without solid particles is known and is described, for example, in EP 2 825 682 A1 and EP 2 260 127 A1.

[0017] Direct current or pulsating direct current is applied to the piston ring. In deposition step (b), a hard chromium layer having a crack network (microcrack network) is formed. Since solid particles should be embedded in the crack network, the electrolyte contains solid particles, and in polarity reversal step (c), the piston ring is connected to the anode and the cracks are extended so that the solid particles are embedded in the extended microcracks. In this case, the solid particles are preferably kept suspended in the electrolyte. When steps (b) and (c) are repeated, the cracks are closed in the subsequent deposition step (b), and another hard chromium layer with microcracks is deposited, which again has new cracks, and whose cracks can then be extended and filled with particles.

[0018] Each chromium layer (chromium layer) preferably has a thickness of about 6-20 μm, and by multiple reversals of the current direction (switching polarity) and deposition, the thickness of the first hard chromium layer can be adapted to the desired deployment purpose.

[0019] Following the deposition of the first hard chrome layer, a second hard chrome layer free of solid particles is deposited in step (e). At the end of the deposition, the polarity is reversed (current direction is reversed) in step (f), and the surface cracks are extended to a defined average width and a correspondingly high surface area.

[0020] The average width of cracks (crack width) at the surface of the second hard chrome layer is determined by measuring the widths of at least ten randomly selected cracks in the surface approximately perpendicular to the crack direction and calculating the arithmetic mean of these at least ten measured crack widths. For this measurement, a micrograph of the surface is used, in particular a micrograph of a sliding surface polishing disc, which can be prepared as described in the following examples.

[0021] The average width of the cracks at the surface of the second hard chromium layer is 1-15 μm, preferably 1.5-12 μm, particularly 2-10 μm and most preferably 3-9 μm.

[0022] The surface proportion of the cracks in or on the surface of the second hard chromium layer is also determined by means of a micrograph of the surface, in particular a micrograph of a grinding section of the surface. The cracks differ in color from the rest of the hard chromium layer, the cracks having a darker color, as can be seen in the Figure 1To determine the surface percentage of cracks, a surface area of ​​at least 40 μm x 40 μm is taken and the dark percentage, i.e., the percentage of surface cracks relative to the total surface area, is determined by measurement. This percentage is measured on at least three randomly selected squares of at least 40 μm x 40 μm, and the arithmetic average of these three measurements is determined. The value thus determined is the surface percentage of cracks in the surface of the second hard chrome layer.

[0023] The surface proportion of the cracks in the surface of the second hard chrome layer is 3-25%, preferably 5-20%, in particular 6-18%, each relative to the total surface of the second hard chrome layer.

[0024] In a preferred embodiment, the chromium compound in the electrolyte for depositing the hard chromium layer is a Cr(III) compound or a Cr(VI) compound, in particular a Cr(VI) compound. Preferably, the electrolyte contains a chromium compound, in particular a Cr(VI) compound, in an amount corresponding to 100-400 g / l, in particular 150-300 g / l, of chromic anhydride. Further preferably, the electrolyte contains 1-26 g / l, in particular 2-25 g / l, of one or more acids, such as sulfuric acid and / or aliphatic sulfonic acids. Preference is given to aliphatic sulfonic acids having 1-6 carbon atoms in the electrolyte, in particular in an amount of 1-18 g / l. Particular preference is given to aliphatic sulfonic acids having 1-4 carbon atoms, with methanesulfonic acid, ethanesulfonic acid, methanedisulfonic acid and ethanedisulfonic acid being particularly preferred. Most preferred is methanesulfonic acid. The electrolyte may also contain conventional electrolysis aids and catalysts that support chromium deposition. These may be present in the electrolyte in conventional amounts. The above-mentioned specified amounts of the individual components in the electrolyte are all relative to the total electrolyte. For chromium plating of cast iron, the electrolyte may additionally contain fluorides, such as potassium fluoride or potassium hexafluorosilicate.

[0025] In the deposition steps (b) and (e), the current density is preferably 10-200 A / dm 2 , particularly preferably 20-100A / dm 2 And the most preferred is 40-80A / dm 2 During the electrolytic deposition (electroplating deposition), the temperature may be 20-95° C., preferably 40-80° C. In the polarity switching steps (c) and (f), the current is also preferably 10-200 A / dm 2 , particularly preferably 20-100A / dm 2 And the most preferred is 40-80A / dm 2 .

[0026] The duration of the polarity switching step (c) is preferably 30-240 s, in particular 45-120 s. The duration of the crack propagation in step (f) is preferably 60-300 s, particularly preferably 120-240 s.

[0027] The duration of the deposition steps (b) and (e) is selected depending on the desired thickness of the respective hard chromium layer, wherein the thicker the layer, the higher the current density and current yield, and the higher the deposition time. In the meaning of the present invention, a hard chromium layer is understood to be an electrolytically deposited chromium layer.

[0028] To achieve a uniform distribution of the solid particles in the first hard chrome layer, steps (b) and (c) are repeated, with 1 to 50 repetitions, in particular 10 to 30 repetitions, being found to be advantageous. The first hard chrome layer preferably has a layer thickness of approximately 50 to 300 μm. A layer thickness of 60 to 200 μm, in particular 80 to 180 μm, is preferred.

[0029] Because the first hard chromium layer contains solid particles, it is also referred to as a hard chromium-solid particle layer within the context of the present invention. The piston ring base preferably consists of a metal or metal alloy, onto which the first hard chromium layer is deposited directly. Alternatively, an additional metal layer may be first deposited on the base as a base layer, followed by the deposition of the first hard chromium layer. In a preferred embodiment, the piston ring base consists of chromium steel containing more than 10 weight percent (wt%) chromium.

[0030] The particle-free hard chrome layer, or the second hard chrome layer, preferably has a thickness of 5 to 150 μm, particularly preferably 10 to 50 μm. The second hard chrome layer is preferably deposited directly on the first hard chrome layer. This means that, in this preferred embodiment, no further layers are present between the first and second hard chrome layers. If desired, a running-in layer, such as a PVD or CVD layer, may also be deposited on the outside of the second hard chrome layer.

[0031] To achieve high wear resistance for the first hard chrome layer, hard material particles are preferably used as solid particles. Within the meaning of the present invention, hard material particles are understood to be particles made of a material with a Mohs hardness of 9 or higher. Among these, hard material particles with a Mohs hardness of 9.2-10 are preferred, and those with a Mohs hardness of 9.4-10 are particularly preferred. The Mohs hardness is determined according to the Mohs hardness measurement known in the prior art. Particularly preferred hard material particles are those derived from diamond, tungsten carbide, chromium carbide, aluminum oxide, silicon carbide, silicon nitride, boron carbide, and / or cubic boron nitride.

[0032] In the method according to the invention, the amount of solid particles contained in the electrolyte can vary within a wide range. Here, it has been found to be advantageous to contain 0.1-200 g / l of solid particles in the electrolyte, in particular 1-100 g / l.

[0033] For the purposes of this invention, a crack network is a network of cracks known to form during the electrolytic deposition of chromium. Here, cracks form randomly at certain intervals in the chromium layer and are filled with chromium during the subsequent deposition. The cracks extend in all directions throughout the entire electrolytically deposited hard chromium layer.

[0034] The crack density of the first hard chrome layer and the second hard chrome layer is 10-250 / mm, particularly preferably 20-220 / mm, further preferably 30-200 / mm and most preferably 40-180 / mm. For determination, at least two line segments of at least 1 mm in length are placed in different directions on a micrograph of the sliding surface grinding disc, the crack density is determined by counting, and the arithmetic mean value is formed from these at least two counts. Figure 1 An example of a photograph of a suitable sliding surface grinding disc is shown in FIG.

[0035] The crack growth in step (f) results in a significantly higher surface area than in crack-free growth. These cracks can absorb lubricant, particularly oil, and contribute to the sliding properties of the piston ring according to the invention, as well as to improved emergency operating performance in the event of a potential lack of lubrication. The cracks on the surface of the second hard chrome layer are not filled with chromium during the electrolytic (electroplating) growth process; in other words, they remain open. A surface area of ​​5-20%, particularly 6-18%, has proven particularly advantageous.

[0036] The median particle size (particle size) of the solid particles is 0.01-10 μm, preferably 0.1-3 μm, particularly preferably 0.2-2 μm, in particular 0.2-1 μm. The average particle size (d 50 ). Median particle size (d 50 ) is the value at which 50% by volume have a smaller particle size and 50% by volume have a larger particle size relative to the specified value.

[0037] The solid particles preferably account for 0.1-20% by volume, particularly preferably 0.2-10% by volume, and in particular 0.3-5% by volume of the total volume of the first hard chromium layer. Preferably, the solid particles are selected from the group consisting of diamond, tungsten carbide, chromium carbide, aluminum oxide, silicon carbide, silica nitrid, boron carbide, and cubic boron nitride.

[0038] The present invention also relates to the use of the piston ring according to the invention in an internal combustion engine. To this end, the piston ring according to the invention is inserted into the piston of the internal combustion engine in a manner known to those skilled in the art. A preferred application is in marine diesel engines. In this case, piston rings with a diameter of 120 to 1000 mm (millimeters) are preferably used.

[0039] It goes without saying that the aforementioned known features and the features still to be explained below can be used not only in the combination specified but also in other combinations or on their own without departing from the scope of the present invention.

[0040] Figure 1 Micrograph of a sliding surface polishing disc of a hard chrome layer according to the invention showing a crack extending on the surface.

[0041] The present invention will be further described in the following examples, but the present invention is not limited to these examples. Example

[0042] A chromium electrolyte solution having the following composition was prepared:

[0043] 250g / l CrO3 (chromic acid)

[0044] 3.0g / l H2SO4 (sulfuric acid)

[0045] 4.2ml / l methanesulfonic acid

[0046] 50 g / l of single-crystal diamond particles with an average particle size of 0.2 to 0.4 μm are dispersed in this chromium electrolyte at 60°C by stirring and kept suspended during the chromium plating. A piston ring made of chrome steel is placed in the electrolyte. The piston ring to be chromium plated is first connected to the cathode in the first stage and is heated at 60 A / dm 3 In the second stage, the polarity is reversed and the piston ring is chromium plated at a current density of 60A / dm 3 When the anodic layer is connected at a current density of 100 μm for 1 minute, the crack network in the previously deposited chromium layer expands and is embedded by diamond particles. This cycle, consisting of 10 minutes of cathodic chromium plating and 1 minute of anodic etching, is repeated 15 times, resulting in a hard chromium-diamond particle layer with a layer thickness of approximately 120 μm. The crack density is approximately 125 cracks / mm.

[0047] Subsequently, the piston ring was placed in an electrolyte having the same composition as given above, but without diamond particles, and the piston ring was first connected to the cathode and at 60 A / dm 2 The chromium plating was carried out for 30 minutes at a current density of 60A / dm. 2When the electrode was connected to the anode at a current density of 100 nm, a crack network on the surface expanded. The crack density on the surface was 121 cracks / mm and the average crack width was 4 μm.

[0048] In order to produce micrographs of the surface, the sliding surface of the piston ring was ground. For this purpose, the piston ring was ground on the surface with SiC wet sandpaper in the following grit sizes:

[0049] Granularity 220

[0050] Granularity 320

[0051] Particle size 600

[0052] Granularity 1200

[0053] Granularity 4000

[0054] Polishing was achieved with a 1 μm diamond suspension until the sample was scratch-free and sharp-edged.

[0055] Subsequently, micrographs of the surface of the sliding surface abrasive disc were generated.

Claims

1. A piston ring comprising a base body, wherein: The substrate has an inner circumferential surface, a first side surface, a second side surface, and an outer circumferential surface, and a first hard chrome layer having a crack network is deposited on the outer circumferential surface, wherein the crack network of the first hard chrome layer has a crack density of 10-250 cracks / mm, and solid particles with a median particle size of 0.01-10 μm are embedded in the cracks of the first hard chrome layer. A second hard chrome layer having a crack network is deposited on the first hard chrome layer, wherein the crack network of the second hard chrome layer has a crack density of 10-250 cracks / mm, characterized in that no solid particles are embedded in the cracks of the second hard chrome layer, cracks at the surface of the second hard chrome layer have an average width of 1-15 μm, the cracks at the surface of the second hard chrome layer are electrolytically extended, and the surface proportion of the cracks at the surface of the second hard chrome layer relative to the total surface of the second hard chrome layer is 3-25%.

2. The piston ring according to claim 1, characterized in that An average width of cracks at the surface of the second hard chrome layer is 2-10 μm, and a crack density of the second hard chrome layer is 30-200 cracks / mm.

3. The piston ring according to claim 1 or 2, characterized in that: The surface proportion of cracks at the surface of the second hard chrome layer is 5-20%.

4. The piston ring according to claim 1 or 2, characterized in that: The thickness of the first hard chromium layer is 60-200 μm.

5. The piston ring according to claim 1 or 2, characterized in that: The thickness of the second hard chromium layer is 5-150 μm.

6. The piston ring according to claim 1 or 2, characterized in that: The diameter of the piston ring is 120-1000 mm.

7. The piston ring according to claim 1 or 2, characterized in that: The solid particles are composed of diamond, tungsten carbide, chromium carbide, aluminum oxide, silicon carbide, silicon nitride, boron carbide and / or cubic boron nitride.

8. A method for producing a piston ring according to any one of claims 1 to 7, comprising the following steps: (a) placing a piston ring having a substrate having an inner circumferential surface, a first side surface, a second side surface, and an outer circumferential surface in an electrolyte containing a chromium compound and solid particles having an average particle size of 0.01-10 μm, (b) electrolytically depositing a first hard chromium layer on the outer circumferential surface, the first hard chromium layer having a crack network, (c) reversing the direction of the current, wherein the cracks that have formed expand and the solid particles become embedded in the cracks, (d) repeating steps (b) and (c) at least once to form a first hard chromium layer containing solid particles in the cracks, (e) placing the piston ring in an electrolyte containing a chromium compound and no solid particles, and electrolytically depositing a second hard chromium layer having a crack network with a crack density of 10 to 250 cracks / mm on the first hard chromium layer, and (f) reversing the current direction, wherein cracks formed on the surface of the second hard chrome layer expand to an average width of 1-15 μm and the surface area of ​​the cracks is 3-25% relative to the total surface area of ​​the second hard chrome layer.

9. Piston ring obtainable by the method according to claim 8.

10. Use of the piston ring according to any one of claims 1 to 7 or 9 in an internal combustion engine.

11. The use according to claim 10, wherein the internal combustion engine is a marine diesel engine.

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