Piston ring

Abstract

To provide a piston ring that is improved in terms of wear behavior and / or friction and / or production efforts.SOLUTION: A piston ring (10) has flank surfaces (14) and a running surface (12) which are coated. The uppermost layer of the running surface (12) is a hydrogen-containing or hydrogen-free DLC layer, and the uppermost layer of at least one flank surface (14) is a chromium layer. A method of producing a piston ring (10) includes forming a DLC layer as the uppermost layer of a running surface (12), and forming a chromium layer as the uppermost layer of at least one flank surface (14).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to piston rings.

[0002] High demands are placed on piston rings of internal combustion engines in terms of the lowest possible wear and the smallest possible friction. [Background technology]

[0003] In this respect, for particularly good wear behavior, piston rings with a DLC layer on the sliding surfaces, in other words the lateral surfaces, are known from DE 10 2014 213 822. The flank surfaces are nitrided but can have a coating.

[0004] DE 10 2007 038 188 A1 relates to a piston ring having a galvanic chromium layer onto which a PVD layer is applied. Summary of the Invention [Problem to be solved by the invention]

[0005] Against this background, the present invention is based on the object of providing piston rings which are improved in terms of wear behavior and / or friction and / or manufacturing effort. [Means for solving the problem]

[0006] This object is solved by a piston ring as set forth in claim 1.

[0007] Thus, the piston ring has a coated running surface and a flank surface, the uppermost layer of the running surface being a DLC layer and the uppermost layer of at least one flank surface being a chrome layer. Preferably, at least the lower flank surface, i.e., the flank surface of the piston ring intended to face away from the combustion chamber, is coated. The second flank surface may be uncoated or coated, and is particularly provided with a chrome layer. The DLC layer on the running surface ensures low wear and relatively small friction, and is also resistant to grinding burns. Compared to a DLC layer, the chrome layer on at least one flank surface can be applied relatively easily and cost-effectively, and also ensures low wear in this area.

[0008] Furthermore, the topic of friction reduction in internal combustion engines in general is becoming increasingly important, and a chrome layer on at least one flank surface advantageously contributes to this goal. The movement of the piston ring during operation also causes axial and / or radial friction, which is expected to be relatively small with the chrome layer according to the invention. A further advantage is that, compared to nitrided flanks, wear is reduced both on the piston ring flank surface and on the piston ring groove, especially under high loads and / or unfavorable lubrication conditions. Tests using steel pistons with a chrome coating according to the invention have shown a reduction in wear of more than 50% and a significantly reduced friction compared to chrome nitrided steel.

[0009] Reduced wear is also relevant in terms of so-called blow-by, i.e., unwanted oil flow in front of the oil wiper ring. High wear on the lower flank of the piston ring in the first groove causes increased blow-by. If this blow-by exceeds the oil-separating effect of the engine ventilation, oil is transported from the crankcase via the blow-by flow, and engine oil consumption increases unacceptably. Furthermore, if the wear on the flank is too great, the piston ring in the groove can rotate downward together with the sliding surface under load, and the lower edge of the sliding surface wipes off the oil in a relatively undesirable manner, increasing oil consumption. Finally, if there is excessive wear on the flank, optionally combined with wear in the piston ring groove, the piston ring can break due to excessive axial clearance in the groove.

[0010] Preferred embodiments of the piston ring according to the invention can be obtained from the further claims.

[0011] Particularly favorable properties are expected from DLC layers when they are hydrogen-free, however, hydrogen-containing DLC ​​layers may also be advantageous in certain applications.

[0012] This offers the advantage of a technological process in which the chrome layer is formed first, so that the DLC layer can be formed in the area above the chrome layer. This applies to both the sliding surface and the flank surface.

[0013] In particular, it may be advantageous for production if the DLC layer partially overlaps the chrome layer at least in the region of the peripheral edge, which may taper towards the peripheral edge and / or which may be processed at a defined angle.

[0014] A PVD (Physical Vapor Deposition) process is preferred for forming the DLC layer.

[0015] Particular advantages are expected, especially in terms of small wear, if the chrome layer has a hardness of at least 800 HV 0.1.

[0016] In view of possible particle deposits, it is preferred that the crack density of the chrome layer has a crack rate of 700 to 1200 cracks / cm.

[0017] For a specific reduction of friction in the region of the flanks, it is preferred that the chromium layer has a coefficient of friction that is at least 20% less than that of chromium nitride steel in oiled frictional contact.

[0018] Based on positive experience, it is preferred that the chromium layer is produced by galvanic formation.

[0019] In particular, the chromium layer can be treated, preferably in combination with a galvanic coating, so that the roughness of the flank surface is subsequently less than Rz4.

[0020] To further improve its properties, the chromium layer may have particle deposits, which may be embedded in particular within the crack network of the chromium layer.

[0021] The above object is further solved by a method as defined in claim 7, which corresponds in terms of its method steps to a piston designed according to the invention. It is to be noted here that all method steps given above or below for a piston ring according to the invention are also preferred and vice versa.

[0022] It is particularly advantageous for the process sequence if the chromium layer is formed, particularly galvanically, before the DLC layer, which avoids the problem of low electrical conductivity of DLC, which would interfere with the galvanic process, the undesirable formation of hydrogen gas during chromium plating, and the chemical aggressiveness of the chromium electrolyte to the DLC.

[0023] Examples of embodiments of the invention that are illustrated in the drawings are described in more detail below. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 shows a cross-sectional view through a piston ring according to the present invention. [Figure 2] FIG. 2 shows the marked area of ​​FIG. 1 on an enlarged scale. DETAILED DESCRIPTION OF THE INVENTION

[0025] Detailed Description of Preferred Embodiments of the Invention As can be seen in FIG. 1 , piston ring 10 typically has a rectangular cross-section having a running surface 12 and a flank surface extending substantially perpendicular thereto, the lower flank surface being designated 14. In the preferred embodiment shown, the lower flank surface 14 is coated with a chrome layer and the running surface with a DLC layer. Preferred dimensions are shown here, whereby the DLC layer overlaps the chrome layer, particularly in the transition region between running surface 12 and flank surface 14. In the example shown, the transition is rounded, but it may also be chamfered.

[0026] As can be seen particularly in FIG. 2, the chromium layer extends, decreasing in thickness, to the sliding surface 12 (vertical in the figure), and the DLC layer also extends, decreasing in thickness, to approximately the beginning of the flank surface 14 (horizontal in the figure).

Claims

1. 1. A piston ring (10) having at least one coated running surface (12) and flank surface (14), characterized in that a top layer of the running surface (12) is a hydrogen-containing or hydrogen-free DLC layer, and a top layer of at least one flank surface (14), preferably the lower flank surface (14), is a chrome layer, the chrome layer having a crack density with a crack rate of 700 to 1200 cracks / cm.

2. A piston ring (10) having at least one coated sliding surface (12) and flank surface (14), wherein a top layer of the sliding surface (12) is a hydrogen-containing or hydrogen-free DLC layer, and a top layer of at least one flank surface (14), preferably the lower flank surface (14), is a chrome layer, and wherein the DLC layer overlaps with the chrome layer in a transition region between the sliding surface and the flank surface.

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