Piston ring having a DLC-coated face and a chromium-coated lower ring flank, and method for producing same

Abstract

The invention relates to a piston ring (10) having at least one face (12), an upper flank area (15) (definition of "upper" in the direction of the piston crown or combustion chamber or cylinder head) and a lower flank area (14), wherein the face (12) is coated with a DLC layer, and wherein the lower flank area (14) is coated with a chromium layer, characterized in that the DLC layer and the chromium layer overlap in a region of a lower end of the face in the direction of the lower flank area (14).

Description

[0001] Applicant: Federal-Mogul Burscheid GmbH

[0002] Our reference number: 13226 DE

[0003] Piston ring with a DLC-coated running surface and a chrome-coated lower ring flank, and method for its manufacture.

[0004] The present invention relates to a piston ring as used in internal combustion engines. In particular, the present invention relates to a piston ring with a DLC (diamond-like carbon) coated running surface and a chromium-coated lower ring flank, wherein the two coatings overlap in a region of a lower outer ring edge and wherein the chromium coating extends over the DLC coating.

[0005] German patent application DE 102017221606 Al already discloses a piston ring in which one running surface is coated with a DLC layer and one lower flank is coated with a chromium layer. In this document, the electroplated chromium layer is applied before and thus beneath the DLC layer. This is due to the diamond layer, which is non-conductive and therefore cannot be coated by an electroplating process.

[0006] Due to the dynamic loads, it is particularly desirable at the lower edge of a piston ring that the outer lower edge of the piston ring can withstand the occurring loads. The different loads on the running surface on the one hand and especially on the lower ring flank on the other require different coatings, with the running surface being particularly protected against friction and the lower flank being particularly protected against the impact of the piston ring on the lower piston ring groove flank. It is therefore of interest to improve the wear resistance of the lower outer edge of the piston ring, and furthermore, it is of interest to keep the loads acting on the lower ring flank as low as possible.

[0007] According to the invention, a piston ring with the features of claim 1 is provided, and a manufacturing method for such a piston ring is further provided by the features of claim 10. Preferred embodiments are given in the dependent claims.

[0008] The piston ring of the present invention has a running surface, an upper flank surface, and a lower flank surface. The running surface is provided with a DLC layer, i.e., a "diamond-like carbon" layer, which is an amorphous carbon that mainly exhibits covalent atomic bonds but does not form the classic diamond lattice. The lower flank surface is provided with a chromium layer. The piston ring has a lower chamfer in a region at the lower end of the running surface, extending towards the lower flank surface. The DLC layer and the chromium layer overlap each other at least in a region of the lower chamfer, with the chromium layer and the DLC layer overlapping.

[0009] According to one embodiment of the present invention, the DLC layer extends over and covers the chromium layer. A PVD (physical vapor deposition) or CVD (chemical vapor deposition) process can be used relatively easily here. Non-conductive surfaces can also be coated by gas or plasma phase deposition.

[0010] According to a further embodiment of the present invention, the chromium layer extends over and covers the DLC layer. This can lead to problems, particularly with electroplating processes, if the DLC layer has insufficient conductivity for electroplating.

[0011] This ensures that the entire lower flank of the piston ring is covered by the chromium layer. The softer chromium layer interacts better with the lower piston ring groove flank than the DLC layer. The softer chromium layer does not pose the risk of cracking, particularly in the case of a thin DLC layer, when the combustion gases impact the piston ring due to slight deformation of the underlying chromium layer. If the chromium layer covers the DLC layer in an area where it is very thin, cracking in the DLC layer will not occur, as the chromium layer primarily protects the DLC layer in areas of low thickness.

[0012] The DLC layer can be hydrogen-containing or hydrogen-free.

[0013] There may be further layers beneath the chrome layer, for example an adhesion promoter.

[0014] Here and in the following, "top" is defined as the direction towards a piston crown, combustion chamber, or cylinder head. Accordingly, "bottom" is defined as the direction towards a piston skirt or crankcase. The lower surface of the piston ring is subjected to greater stress, as it must transfer the pressure generated by the combustion gases to a lower piston ring groove flank.

[0015] In the selected design, the brittle DLC layer is covered by the chromium layer, thus preventing cracking in the DLC layer under piston ring operating conditions, as the softer chromium layer can better absorb the forces generated by the combustion gases.

[0016] According to a preferred embodiment of the piston ring, the DLC layer contains boron, phosphorus, and / or nitrogen, thereby imparting a certain conductivity to the DLC layer, which allows it to be coated by an electroplating process. Non-galvanic, so-called "electroless" processes can also be used to coat a non-conductive DLC layer.

[0017] In another exemplary embodiment of the piston ring, it has a lower chamfer in a region of one lower end of the running surface, extending towards the lower flank surface. Preferably, the DLC layer and the chromium layer overlap at least in the region of the lower chamfer. The chamfer abuts directly onto the running surface. This definition of the chamfer is necessary to uniquely identify it within the piston ring profile.

[0018] In another exemplary embodiment of the piston ring, it is designed as a rectangular ring. Rectangular rings are piston rings with a (substantially) rectangular cross-section, whereby the two ring flanks form planes that run parallel to each other. The substantially rectangular cross-section can also be provided with internal chamfers or internal angles to achieve a desired twisting behavior. Here, minute rings, nose rings, and nose-minute rings are also considered rectangular rings, although the definition of the chamfer in nose and nose-minute rings is difficult and refers to the part of the nose recess that is closer to the horizontal. Here, the rectangular ring is defined by the fact that the upper flank surface and the lower flank surface are aligned parallel to each other. "Rectangular ring" here means that it is not a so-called trapezoidal ring.

[0019] According to another exemplary preferred embodiment of the piston ring, the upper flank surface and / or the lower flank surface has the shape of a slotted truncated cone shell, wherein the truncated cone shell(s) taper axially in the direction of the piston ring. The truncated cone-shaped flank surface is slotted radially or axially due to the annular gap. This embodiment relates to a double-sided trapezoidal (piston) ring or a single-sided trapezoidal ring.

[0020] Trapezoidal rings have the advantage that coking and deposits, especially during radial movements of the piston ring in the piston ring groove, are ground down.

[0021] In another exemplary embodiment, the upper flank surface has an angle between 0° and 14° to the radial plane. In another embodiment, the lower flank surface has an angle between 0° and 15° to the radial plane. In a further additional exemplary embodiment, the upper frustoconical flank surface has an angle between 3° and 13° to the radial plane. In a further additional exemplary embodiment, the lower frustoconical flank surface has an angle between 3° and 13° to the radial plane. In yet another additional exemplary embodiment, the upper frustoconical

[0022] The flank surface of the 5-sided ring has an angle between 10° and 12.5° to the radial plane. In another additional exemplary embodiment, the upper frustoconical flank surface has an angle between 10° and 12.5° to the radial plane. Overall, the piston ring can be designed as a rectangular ring, a trapezoidal ring, or a one-sided trapezoidal ring.

[0023] 10

[0024] In another exemplary embodiment of the piston ring, the lower flank surface has the shape of a slotted truncated cone shell, tapering axially towards the piston ring or a radial center plane of the piston ring. The lower flank surface is not provided with a facet, but rather forms an edge with chamfer 15. In a preferred embodiment, this edge may have a radius between 0.05 and 0.25 mm. This embodiment has no facet, and the lower chamfer and the lower flank surface form an edge or merge directly into one another. Ideally, the edges of the lower chamfer form angles of less than 45° with the lower flank surface and the running surface. In the case of a convex running surface 20, angles of more than 45° may also occur.In a further preferred embodiment, the lower chamfer angles to the lower flank surface and to the running surface can each be of the same size. The angles are each considered in a plane of a profile section that corresponds to an axial axis of rotational symmetry.

[0025] 25

[0026] In an additional exemplary embodiment of the piston ring, the lower flank surface has the shape of a slotted truncated cone shell, tapering axially towards the piston ring or a radial center plane of the piston ring. The lower flank surface is provided with a facet that lies (essentially) in a 30° radial plane of the piston ring. The facet has a radial dimension corresponding to between 1 / 100 and 1 / 10 of a radial dimension of the piston ring profile, and the facet is preferably arranged between the lower chamfer and the lower flank surface. This embodiment features a facet that forms a small horizontal radial surface on the trapezoidal piston ring and breaks an edge between the running surface and the lower flank surface. The facet can also serve to reduce the acute angle between the running surface and that of the lower flank surface by means of two chamfers.The chamfer and the facet break. Here, the chamfer and the facet form a common edge. The chamfer and the facet complement each other.

[0027] In another exemplary embodiment of the piston ring, a running surface of the piston ring exhibits a radius variation in the axial direction before coating. The running surface can have a symmetrical or an asymmetrical crown.

[0028] In another exemplary embodiment of the piston ring, a DLC layer in the area of ​​the piston ring's running surface exhibits a thickness variation in the axial direction. This thickness variation could be achieved by an orifice or a directional CVD. The thickness variation of the DLC layer can impart a symmetrical or asymmetrical crown to the running surface.

[0029] In another exemplary embodiment of the piston ring, the DLC layer exhibits a thickness variation in the area of ​​the chamfer(s), with the thickness of the DLC layer preferably decreasing towards the lower flank surface. Here, the DLC layer tapers progressively in the chamfer area. The DLC layer is covered by the chromium layer, which prevents the thin DLC layer from cracking near the lower or inner edge of the chamfer under load. The chromium layer over the DLC layer distributes the forces occurring, thus preventing bending in the DLC layer that could lead to cracking.

[0030] In another design of the piston ring, the chromium layer exhibits a thickness variation in the chamfer area, with the thickness of the chromium layer decreasing towards the lower edge of the running surface. Here, the thickness of the chromium layer decreases from the inside out. The chromium layer is preferably designed such that the sum of the thicknesses of the DLC layer and the chromium layer results in a substantially constant value. This design allows the piston rings to be manufactured to the correct shape in the chamfer area before coating.

[0031] According to a further aspect of the present invention, a method for manufacturing a coated piston ring is provided, as described above. The method comprises a piston ring which is first coated with a DLC layer on one running surface, and subsequently coated with a chromium layer on the lower flank surface. The DLC layer extends to a lower outer edge of the piston ring. The lower flank surface of the piston ring is coated with the chromium layer in such a way that it at least partially overlaps or superimposes the DLC layer. A challenge here is applying a chromium layer to the DLC layer.

[0032] In one exemplary embodiment of the process, the DLC layer is applied using a PVD process.

[0033] In one possible embodiment of the process, the chromium layer is chromatically formed. "Chromatic" here refers to a currentless plating process for producing chromium layers, also known as "electroless plating".

[0034] In another exemplary embodiment of the process, the chromium layer is formed electroplated. Electroplating processes are known in the art, but require a conductive substrate. This can be achieved, for example, by vapor deposition with a metal, in order to be able to electroplat a non-conductive DLC layer as well.

[0035] In a further embodiment of the present invention, the DLC layer comprises boron, phosphorus, and / or nitrogen in an amount that provides the conductivity necessary for electroplating with chromium. This makes it possible to electroplate the DLC layer without the need for a prior intermediate step to provide conductivity to the surface to be electroplated. A further advantage of this solution is that no other precautions need to be taken to electroplate the DLC layer. Another advantage of this embodiment is that only the plasma plating process of the DLC layer needs to be adapted, without the need for an additional, costly step.

[0036] A fifth step must be added to the manufacturing process.

[0037] According to a further embodiment of the manufacturing process for a piston ring as described above, the DLC layer is vapor-deposited with a metal layer before electroplating, preferably partially vapor-deposited. The vapor deposition with a metal coats the surface of the DLC layer in the areas where the chromium coating is to be deposited.

[0038] The invention is described below with reference to illustrations of preferred embodiments, which are not to scale.

[0039] 15

[0040] Figures 1A and 1B show a basic embodiment of a piston ring according to the invention.

[0041] Figures 2A to 2C show sectional views of embodiments of the invention.

[0042] 20 trapezoidal rings.

[0043] AFC

[0044] Figures 3A to 3F show partial sectional views of possible coatings.

[0045] Figures 4A to 4C show sectional views of embodiments of the invention.

[0046] 25 trapezoidal rings with a convex running surface.

[0047] Figures 5A to 5C show sectional views of embodiments of trapezoidal rings according to the invention.

[0048] 30 In the following, the same or similar reference symbols are used in both the description and the figures to refer to the same or similar components or elements.

[0049] Figure 1A shows a sectional view in a radial-axial plane through a piston ring 10 according to the invention. The piston ring 10 is designed as a rectangular ring.

[0050] The piston ring 10 has a chrome layer on the lower piston ring flank 14.

[0051] A chrome coating 42 is applied, which serves as wear protection against the piston ring impacting a lower piston ring groove flank. The running surface 12 of the piston ring 10 is provided with a DLC layer 40, which, due to its high hardness, offers excellent protection against wear caused by the running surface rubbing against the inside of the piston. At the lower outer corner of the running surface 12, or of the piston ring 10, the chrome layer 42 overlaps the DLC layer of the running surface. This combination of layers provides particularly good protection for the tapered lower end region of the DLC layer 40.

[0052] The piston ring 10 can also be provided with an internal recess such as an internal chamfer 32, which is shown here by a dashed line on an inner lower edge of the piston ring 10. The piston ring 10 can also have an internal recess such as an internal angle 33, as shown here by a dashed line on an inner upper edge of the profile of the piston ring 10.

[0053] Figure 1B shows an enlarged sectional view of the corner area of ​​the piston ring 10 to better illustrate the overlap area 44 of the chromium layer 42 and the DLC layer 40.

[0054] Figure 2A shows a section through a piston ring 10, which is designed as a trapezoidal ring 44, wherein the upper flank 15 and the lower flank 14 of the trapezoidal ring 44 form a truncated conical surface, except for the annular gap. As shown in Figures 1A and 1B, the running surface 12 is provided with the DLC layer 40 and the lower ring flank 12 has a chromium coating 42. The trapezoidal ring has no bevel, with the ring flanks transitioning into the running surface via a chamfer. As in Figures 1A and 1B, the DLC layer 40 and the chromium coating 42 overlap, with the overlap occurring in the region of the chamfer.

[0055] In Figure 2B, the upper ring flank of the trapezoidal ring 24 is also coated with a chromium layer 42 to protect the upper ring flank against wear. As in Figure 2A, the chromium layer 42 extends over the DLC layer 40 in the area of ​​the chamfers.

[0056] Figure 2C is essentially the same as Figure 2B, except that here the DCL layer 40 and the chromium layer 42 have an overlap area in the chamfer area, with the DLC layer 40 covering the chromium layer.

[0057] Figures 3A to 3F show partial sectional views of possible coating applications, where the chromium layer 42 has been applied to the DLC layer 40. In Figure 3A, the lower chamfer 16 essentially forms the overlap area 44. The thin, tapered DLC layer is overlaid by a thicker chromium layer, which in particular protects the thin part of the DLC layer against cracking.

[0058] In figure 3B, the DLC layer does not extend over the entire length of the chamfer 16. The end or lower edge of the DLC coating 40 is still covered and protected by the chrome layer 42.

[0059] Figure 3C presents the opposite case to Figure 3B, where the chromium layer 42 does not cover the entire chamfer 16 or the entire DLC coating in the chamfer area. Here, the overlap area is reduced. The end or lower edge of the DLC coating 40 is still covered and protected by the chromium layer 42.

[0060] Figure 3D shows only a slight overlap 44 of the DLC coating 40 by the chromium layer 42, while still achieving the preferred effect of a protected edge for the DLC coating 40. Figure 3E represents the opposite case to Figure 3C, where the chromium layer 42 extends beyond the lower chamfer 16 into the running surface. This case is undesirable; however, in operation, it can be assumed that the chromium layer 5 on the running surface will wear away quickly.

[0061] Figure 3F shows a defective coating without overlap, whereby this embodiment no longer falls within the scope of the claim, since the edges of the DLC layer 40 and the chrome layer do not overlap.

[0062] > 10

[0063] Figure 4A shows a sectional view of a trapezoidal ring, which is provided with a facet 18 / 19 between the chamfer and the piston ring flank at the top and bottom. The surfaces of the facets lie in radial planes.

[0064] 15 The running surface is coated with DLC layer 40. Before coating, the running surface formed a cylindrical circular area (except for the gap). The DLC layer was applied with a variable thickness, so that the outer surface of the DLC layer essentially has a convex shape.

[0065] 20 In Figure 4A, only the lower side of the piston ring is coated with the chrome layer 42, il*

[0066] ■r

[0067] In Figure 4B, both the side of the piston ring and the upper side of the piston ring 10 are coated with the chrome layer 42.

[0068] 25 Figure 4C represents an embodiment which is essentially the same as that shown in Figure 4A, except that the embodiment shown in Figure 4C does not have any facets.

[0069] Figure 5A shows a section through a piston ring 10, which is designed as a trapezoidal ring 44. The design of the piston ring in Figure 5A essentially corresponds to that of Figure 30 in Figure 4B. In contrast to Figure 4B, the trapezoidal ring 44 in Figure 5A has an upper facet 19 only on its upper surface; a lower facet is absent. This allows the piston ring to sit more securely on a lower piston ring groove flank. Here, both ring flanks are provided with a chrome coating 42, which overlaps the DLC layer on the running surface 12 in the area of ​​the chamfers. The trapezoidal ring does not have a lower facet. The ring flanks transition into the running surface via a chamfer.

[0070] Figure 5B shows a sectional view of a single-sided trapezoidal ring 26. The single-sided trapezoidal ring 26 is provided with an upper facet 19 between the upper chamfer and the piston ring flank. The facet surface lies in a radial plane. Since the single-sided trapezoidal ring 26 already has a flat lower flank 14, the lower flank does not have a facet.

[0071] The running surface is coated with DLC layer 40. Before coating, the running surface had a convex shape (except for the gap). The DLC layer was applied with a variable thickness, so that the outer surface of the DLC layer also has a predominantly convex shape, with the most heavily loaded area of ​​the running surface having a thicker DLC layer.

[0072] Figure 5C shows a sectional view of a piston ring designed as a rectangular ring 22. The DCL layer 40 and the chromium layer 42 have overlapping areas at the top and bottom in the chamfer region, with the DLC layer 40 being covered by the chromium layer 42. The thickness of the DLC layer 40 can be varied to achieve a directional coating by changing coating conditions such as apertures.

[0073] The exemplary embodiments are also intended to include combinations of features from individual embodiments as disclosed. List of reference numerals

[0074] 10 Piston ring / Piston ring body 12 Running surface

[0075] 14 lower flank surface

[0076] 15 upper flank surface

[0077] 16th chamfer (bottom)

[0078] 17th chamfer (top)

[0079] 18 facets (below)

[0080] 19 facets (above)

[0081] 20 convex running surface

[0082] 22 rectangular ring

[0083] 24 Trapezoidal ring

[0084] 26 One-sided trapezoidal ring

[0085] 30 internal angles

[0086] 32 Inner chamfer

[0087] 40 DLC layers

[0088] 42 Chrome layer

[0089] 44 Overlap area

Claims

Claims 1. Piston ring (10) with at least one running surface (12), an upper flank surface (15) (definition “upper” in the direction of the piston crown or combustion chamber or cylinder head) and a lower flank surface (14), wherein the running surface (12) is coated with a DLC layer, and wherein the lower flank surface (14) is coated with a chromium layer, characterized in that the DLC layer and the chromium layer overlap in a region of a lower end of the running surface in the direction of the lower flank surface (14).

2. Piston ring (10) according to claim 1, wherein the piston ring has a lower chamfer (16) in a region of a lower end of the running surface in the direction of the lower flank surface (14), and wherein the DLC layer and the chromium layer overlap at least in the region of the lower chamfer, and wherein the piston ring preferably has an upper chamfer (17) in a region of an upper end of the running surface in the direction of the upper flank surface (15), and wherein the DLC layer and the chromium layer overlap at least in the region of the upper chamfer (17).

3. Piston ring (10) according to claim 1 or 2, wherein the DLC layer extends over and covers the chromium layer.

4. Piston ring (10) according to claim 1 or 2, wherein the chromium layer extends over the DLC layer.

5. Piston ring (10) according to claims 1 to 3, characterized in that the DLC layer contains boron, phosphorus and / or nitrogen.

6. Piston ring (10) according to claim 1, 2, 3, 4 or 5, characterized in that the upper flank surface (15) and the lower flank surface (14) are aligned parallel to each other.

7. Piston ring (10) according to any one of claims 1 to 6, characterized in that the upper flank surface (15) and / or the lower flank surface (14) has the shape of a slotted truncated cone shell, wherein the truncated cone shell(s) taper in the axial direction towards the piston ring.

8. Piston ring (10) according to one of claim 7, characterized in that no facet is arranged between the lower flank surface (14) and the lower chamfer (16), wherein preferably the lower flank surface (14) transitions into the lower chamfer (16) by means of a rounding.

9. Piston ring (10) according to one of claim 7, characterized in that the lower flank surface (14) has the form of a slotted truncated cone shell which tapers in the axial direction towards the piston ring (or a radial median plane), and that the lower flank surface (14) has a facet which lies in a radial plane of the piston ring, wherein the facet has a radial dimension which corresponds to between 1 / 100 and 1 / 10 of a radial dimension of the profile of the piston ring, and wherein the facet is preferably arranged between the lower chamfer and the lower flank surface (14).

10. Piston ring (10) according to one of the preceding claims, characterized in that the running surface has a radius variation in the axial direction, wherein preferably the running surface has a symmetrical or an asymmetrical crown.

11. Piston ring (10) according to one of the preceding claims, characterized in that the DLC layer has a thickness variation in the area of ​​the chamfer, wherein the thickness of the DLC layer decreases in the direction of the lower flank surface (14).

12. Piston ring (10) according to one of the preceding claims, characterized in that the chromium layer has a thickness variation in the region of the chamfer, wherein the thickness of the chromium layer decreases towards the lower running surface.

13. Method for manufacturing a piston ring (10) according to one of the preceding claims, wherein first the running surface (12) is coated with the DLC layer and subsequently the lower flank surface (14) is coated with the chromium layer. wherein the lower flank surface (14) is coated with the chromium layer in such a way that it at least partially overlaps the DLC layer.

14. Method for manufacturing a piston ring (10) according to claim 13, characterized in that the DLC layer is applied by a PVD process.

15. Method for manufacturing a piston ring (10) according to claim 13 or 14, characterized in that the chromium layer is chromatically formed.

16. Method for manufacturing a piston ring (10) according to claim 13, 14 or 15, characterized in that the chromium layer is formed electroplated.

17. Method for manufacturing a piston ring (10) according to claim 16, characterized in that the DLC layer comprises boron, phosphorus and / or nitrogen and thereby provides a conductivity necessary for electroplating with chromium.

18. Method for manufacturing a piston ring (10) according to claim 16 or 17, characterized in that the DLC layer is vapor-deposited with a metal layer prior to electroplating in order to provide conductivity necessary for chromium plating.

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