Piston ring with shot-peened running-in layer and method for the production thereof

Abstract

A piston ring includes an annular body, a wear-resistant layer that is applied to the annular body by thermal spraying, and a strain-hardened run-in layer, made of an AlCuFe alloy, that is applied to the wear-resistant layer by thermal spraying.

Claims

1. A piston ring, comprising: an annular body; a thermally sprayed wear-resistant layer on the annular body; and a thermally sprayed run-in layer, made of an AlCuFe alloy, on the wear-resistant layer, wherein the run-in layer has imprints from shot peening, at which the run-in layer has a greater hardness due to strain hardening, wherein the run-in layer has a hardness HV of 150-250, wherein a coverage of 50-100% is achieved in the shot peening, and wherein the run-in layer has a roughness Rz between 60 μm and 95 μm.

2. The piston ring according to claim 1, wherein the run-in layer is applied to the wear-resistant layer by electric arc wire spraying.

3. The piston ring according to claim 1, further including a thermally sprayed adhesion promoter layer on the running surface of the annular body, wherein the wear-resistant layer is applied to the adhesion promoter layer.

4. The piston ring according to claim 1, wherein the wear-resistant layer and the run-in layer are applied in oversprayed form.

5. The piston ring according to claim 1, wherein the annular body has a flank with beveled edges.

6. The piston ring according to claim 5, wherein the beveled edges have an angle of 30 to 70°.

7. The piston ring according to claim 5, wherein the beveled edges have a width of 0.5 mm to 2.0 mm.

8. The piston ring according to claim 1 wherein the annular body has gas discharge slots with beveled edges.

9. The piston ring according to claim 8, wherein the beveled edges of the gas discharge slots have an angle of 30° to 70°.

10. The piston ring according to claim 8, wherein the beveled edges of the gas discharge slots have a width of 0.5 to 2.0 mm.

11. The piston ring according to claim 1, wherein the piston ring is a large bore piston ring.

12. A method for manufacturing a piston ring, comprising: providing an annular body; thermal spraying of a wear-resistant layer on the annular body; and thermal spraying of a run-in layer, made of an AlCuFe alloy, on the wear-resistant layer, and strain hardening the run-in layer by shot peening, wherein a coverage of 50-100% is achieved in the shot peening, wherein the run-in layer has a hardness HV of 150-250, and wherein the run-in layer has a roughness Rz between 60 μm and 95 μm.

13. The method according to claim 12, wherein the run-n layer is applied to the wear-resistant layer by electric arc wire spraying.

14. The method according to claim 12, wherein the thermal spraying of the wear-resistant layer encompasses one of: an electric arc wire coating process; a wire flame coating process; atmospheric plasma spraying (APS); and high-velocity oxygen fuel (HVOF) flame spraying.

15. The method according to claim 12, further comprising: providing the flank of the annular body with a bevel at its edges, wherein the bevel preferably has an angle of 30° to 70° and/or a width of 0.5 to 2.0 mm, and/or providing the gas discharge slots with a bevel at their edges, wherein the bevel preferably has an angle of 30° to 70° and/or a width of 0.5 to 2.0 mm.

Description

THE DRAWINGS

(1) The present invention is explained below with reference to figures of schematic illustrations of exemplary embodiments.

(2) FIGS. 1A through 1F show one embodiment of the method based on a sectional view of a piston ring during various machining steps.

(3) FIG. 2 shows a top view of a strain-hardened run-in layer at the running surface of one embodiment of a piston ring according to the invention.

DETAILED DESCRIPTION

(4) Identical or similar reference symbols are used in the description and in the figures to refer to identical or similar elements and components.

(5) Coatings on the running surfaces of piston rings, in addition to having good, temperature-resistant adhesion to the substrate and good, temperature-resistant binding within the layer (cohesion), must also be wear-resistant in the surface region that is in contact with the cylinder wall. In addition, their intrinsic wear in the run-in phase should be sufficient for a satisfactory adaptation to the countersurface. Furthermore, these layers should have high fracture resistance, and exhibit little or no fatigue behavior, even after long run times.

(6) In the present case, a strain-hardened run-in layer made of an AlCuFe alloy is used which is produced by electric arc wire spraying onto a piston ring. The strain hardening here is achieved by shot peening, resulting in smoothing and hardening of the run-in layer. The shot peening, i.e., the strain hardening, results in compaction of the sprayed-on run-in layer. The strain hardening produces compressive stresses in the run-in layer that result in an improved, stronger run-in layer. The strain hardening may affect practically the entire thickness of the run-in layer. The shot peening is to be carried out using steel shot blast pellets having a diameter of 0.5 to 1 mm so that a sufficiently smooth, hardened surface of the run-in layer may be achieved. The blasting pressure may be between 2.5 bar and 6 bar, preferably between 3 bar and 5.5 bar, more preferably between 4 bar and 5 bar. Shot peening may be carried out here at a distance between 50 and 200 mm from the surface of the run-in layer. It is possible to use a mass throughput of 1 to 3 kg blast pellets per minute in the shot peening. Rolling, knurling, or hammering may also be used as strain hardening processes instead of shot peening.

(7) The object of the present invention, therefore, is to provide the running surfaces of piston rings with wear-resistant layers, applied by a flame spraying process, that are able to withstand extreme stresses but which exhibit good run-in behavior. The method for producing the layers is intended to be preferably simple and cost-effective, and in particular to allow production of the wear-resistant coatings and also the run-in layer with properties that are coordinated with the particular application.

(8) According to the invention, in a first embodiment this object is achieved by a coating made up of at least two different spray layers situated one on top of the other: a wear-resistant layer, optionally an adhesion promoter layer, and an outer run-in layer. The adhesion and cohesion of the layers may be further improved by appropriately beveling edges at the ring flank and the use of gas discharge slots prior to the coating.

(9) FIGS. 1A and 1B show a cross-sectional view of how the piston ring or the annular body 2 is beveled. According to the invention, prior to the coating, the edges of the piston ring 2 may be provided with a bevel 10 on the running surface side (at the left in the figure). According to the invention, the angle α of the bevel 10 may be between 30° and 70°; an angle of 45° is shown in FIG. 2 by way of example. In addition, according to various embodiments of the invention the bevel 10 may have a width d of 0.5 to 2 mm. The faceting of edges is an optional step in the manufacture of the piston ring; i.e., the bevel is an optional feature for the finished piston ring according to the invention. The faceting may be carried out using any suitable known method. It is likewise possible to round off the running surface of the piston ring in order to obtain a barrel-shaped running surface. The faceting or shaping of the running surface may be carried out using any suitable known method.

(10) FIG. 1C shows a cross-sectional view of the step of applying the first layer of the coating. An adhesion promoter layer 4 is applied to the annular body 2. According to the invention, this takes place by means of a thermal spraying process, encompassing high-velocity oxygen fuel (HVOF) flame spraying, atmospheric plasma spraying (APS), electric arc wire coating, or wire flame coating processes. As an example, this is illustrated by an HVOF device 12. The adhesion promoter layer 4 is designed here as a nickel alloy.

(11) In FIG. 1D, a wear-resistant layer 6 is then applied to or over the adhesion promoter layer 4. This may take place, the same as the application of the adhesion promoter layer, using one of the above-mentioned thermal spraying process, it also being possible to use different processes for different respective layers. In the example shown, this is likewise achieved using an HVOF device 14. The layer design is FF, i.e., oversprayed.

(12) According to the first embodiment of the invention, the wear-resistant layer may be a molybdenum alloy with chromium carbide CrC or tungsten carbide WC or molybdenum carbide MoC.

(13) In FIG. 1E, a run-in layer 8 is then applied to or over the one wear-resistant layer 6. This is carried out by electric arc wire spraying, using an electric arc wire spraying device 16. The layer design here is likewise FF, i.e., oversprayed. The run-in layer includes aluminum-copper-iron (an AlCuFe alloy). The piston ring is preferably a large bore piston ring for a 2-stroke internal combustion engine.

(14) In FIG. 1F, the last applied run-in layer is post-processed on its surface or running surface by shot peening. The run-in layer is strain-hardened at the running surface, as the result of which the run-in phase of the engine is extended, or the abrasion may be reduced by using a thinner run-in layer. The shot peening device 26 is used for the strain hardening.

(15) In FIG. 1F, the finished piston ring is illustrated in a cross-sectional view according to the first embodiment of the invention, comprising the annular body 2, the adhesion promoter layer 4, the wear-resistant layer 6, and the run-in layer 8.

(16) According to a second embodiment of the present invention, a piston ring comprises an annular body, a wear-resistant layer that is applied to the running surface of the annular body, and a strain-hardened AlCuFe alloy run-in layer that is applied to the wear-resistant layer. The AlCuFe alloy may be composed of these components and unavoidable impurities, or the AlCuFe alloy may include further alloying elements and solid lubricants such as carbon in small amounts.

(17) FIG. 2 illustrates an enlarged detail of the running surface of the piston ring according to the invention, the surface showing the imprints from the shot blast pellets 42 on the running surface or the run-in layer 8. The imprints 42 here show a large coverage, as the result of which the entire surface of the run-in layer 8 is strain-hardened.

(18) By use of the two different functional layers, the piston ring according to the invention provides a novel, advantageous combination of wear resistance (due to the wear-resistant layer) and favorable run-in properties (due to the AlCuFe run-in layer). The run-in layer 8 is strain-hardened after the spraying, and is removed by the abrasion that occurs in the run-in phase. The wear-resistant layer 6 prevents excessive wear under extreme operating conditions of the engine.

(19) The wear-resistant layer 6 preferably includes hard chromium, chromium with aluminum oxide ceramic (for example, CKS® from Federal-Mogul), or chromium with microdiamond (for example, GDC® from Federal-Mogul). The wear-resistant layer 6 may likewise be provided with a diamond-like carbon (DLC) layer or may include such a layer.

(20) The run-in layer 8 preferably has a layer thickness of 20 to 400 μm.

(21) The application of the wear-resistant layer preferably takes place using a thermal spraying process. The thermal spraying process is preferably atmospheric plasma spraying (APS, for example MKP) or high-velocity oxygen fuel (HVOF) flame spraying (for example, MKJet® from Federal-Mogul). The wear-resistant layer preferably includes hard chromium, chromium with aluminum oxide ceramic (for example, CKS® from Federal-Mogul), or chromium with microdiamond or a diamond-like carbon layer (for example, GDC® from Federal-Mogul).

(22) The wear-resistant layer is preferably activated by a blasting process or is activated thermally.

(23) The application of the run-in layer 8 takes place using a thermal coating process. The thermal coating process is preferably electric arc wire spraying.

(24) The run-in layer 8 preferably has a layer thickness of 20 to 400 μm.