PISTON RING WITH HARD CHROMIUM LAYER AND IMPROVED RUNNING-IN BEHAVIOUR

Abstract

A piston ring includes a base body having an inner circumferential surface, first and second flank surfaces and an outer circumferential surface, wherein a first hard chromium layer with a crack network is applied to the outer circumferential surface and has a crack density of 10-250 cracks per mm and solid particles having an average particle size of 0.01-10 μm embedded in cracks of the first hard chromium layer, a second hard chromium layer having a crack network applied to the first hard chromium layer and having a crack density of the crack network of 10-250 cracks per mm, no solid particles being embedded in the cracks thereof, where the cracks have an average width of 1-15 μm, the cracks are electrolytically expanded and the surface proportion of the cracks are 3-25% based on a total surface of the second hard chromium layer.

Claims

1.-10. (canceled)

11. A piston ring, comprising: a base body having an inner circumferential surface, a first flank surface, a second flank surface and an outer circumferential surface and a first hard chromium layer with a crack network is to the outer circumferential surface, wherein the crack network of the first hard chromium layer has a crack density of 10-250 cracks per mm and solid particles having an average particle size of 0.01-10 μm are embedded in the cracks of the first hard chromium layer, a second hard chromium layer having a crack network is applied on the first hard chromium layer, the crack density of the crack network of the second hard chromium layer being 10-250 cracks per mm, wherein no solid particles are embedded in the cracks of the second hard chromium layer, the cracks on the surface of the second hard chromium layer have an average width of 1-15 μm, the cracks on the surface of the second hard chromium layer are electrolytically expanded and the surface proportion of the cracks on the surface of the second hard chromium layer is 3-25%, based on a total surface of the second hard chromium layer.

12. The piston ring according to claim 11, wherein the average width of the cracks on the surface of the second hard chromium layer is 2-10 μm and the crack density of the second hard chromium layer is 30-200 cracks per mm.

13. The piston ring according to claim 12, wherein the surface proportion of the cracks on the surface of the second hard chromium layer is 5-20%.

14. The piston ring according to claim 13, wherein the thickness of the first hard chromium layer is 60-200 μm.

15. The piston ring according to claim 14, wherein the thickness of the second hard chromium layer is 5-150 μm.

16. The piston ring according to claim 15, wherein the diameter of the piston ring is 120-1000 mm.

17. The piston ring according to claim 16, wherein the solid particles comprise at least one of diamond, tungsten carbide, chromium carbide, alumina, silicon car-bide, silicon nitride, boron carbide and/or cubic boron nitride.

18. The piston ring according to claim 11, wherein the surface proportion of the cracks on the surface of the second hard chromium layer is 5-20%.

19. The piston ring according to claim 11, wherein the thickness of the first hard chromium layer is 60-200 μm.

20. The piston ring according to claim 11, wherein the thickness of the second hard chromium layer is 5-150 μm.

21. The piston ring according to claim 11, wherein the diameter of the piston ring is 120-1000 mm.

22. The piston ring according to claim 11, wherein the solid particles comprise at least one of diamond, tungsten carbide, chromium carbide, alumina, silicon car-bide, silicon nitride, boron carbide and/or cubic boron nitride.

23. A method of manufacturing a piston ring according to claim 11, comprising the steps of: (a) a piston ring having the base body comprising the inner peripheral surface, the first flank surface, the second flank surface and the outer circumferential surface placed in the electrolyte containing a chromium compound and solid particles having an average particle size of 0.01-10 μm; (b) providing the first hard chromium layer electrolytically deposited on the outer circumferential surface, where the first hard chromium layer has the crack network; (c) reversing a direction of electric current, wherein the solid particles are deposited within cracks of the crack network that have expanded; (d) repeating steps (b) and (c) at least once so that a first hard chromium layer is formed which contains solid particles in the cracks; (e) placing the piston ring in an electrolyte containing a chromium compound and no solid particles, and the second hard chromium layer is electrolytically deposited on the first hard chromium layer, the second hard chromium layer having the crack network with the crack density of 10-250 cracks per mm; and (f) reversing the direction of electric current, wherein cracks formed on the surface of the second hard chromium layer expand to the average width of 1-15 μm and the surface proportion of the cracks is 3-25%, based on the entire surface of the second hard chromium layer.

24. An internal combustion engine comprising a piston ring according to claim 11.

25. The internal combustion engine of claim 24, wherein the internal combustion engine comprises a diesel engine.

Description

[0039] FIG. 1 is a microscopic image of a tread section of a hard chromium layer according to the invention with expanded cracks on the surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] The invention will be explained in more detail in the following example, but without limiting the invention thereto.

Example

[0041] A chromium electrolyte with the following composition is prepared:

250 g/l CrO.sub.3 (chromic acid)
3.0 g/l H.sub.2SO.sub.4 (sulphuric acid)
4.2 ml/l methane sulfonic acid

[0042] In this chromium electrolyte at 60° C., 50 g/l of monocrystalline diamond particles with an average particle size of 0.2 to 0.4 μm are dispersed by stirring, kept in suspension during chromium plating and a chromium steel piston ring is introduced into the electrolyte. The piston ring to be chromium-plated is first cathodically operated in a first step and chromium-plated for 10 minutes at a current density of 60 A/dm.sup.3. In a second step, the polarity is reversed and, by anodically operating the piston ring for 1 minute at a current density of 60 A/dm.sup.3, the crack network of the previously deposited chromium layer is expanded and filled with diamond particles. This cycle, namely cathodic chromium plating for 10 min and anodic etching for 1 min, is repeated a total of 15 times, resulting in a hard chromium diamond particle layer with a thickness of about 120 μm. The crack density is about 125 cracks per mm.

[0043] The piston ring is then placed in an electrolyte with the same composition as above, but without diamond particles, and the piston ring is first cathodically operated and chromium-plated for 30 min at a current density of 60 A/dm.sup.2. Then, the polarity is reversed and the crack network at the surface is expanded by anodically operating the piston ring at a current density of 60 A/dm.sup.2. The crack density at the surface is 121 cracks per mm and the average width of the cracks is 4 μm.

[0044] To create a microscopic image of the surface, a polishing of the running surface of the piston ring is made. For this purpose, the piston ring is ground on the surface with SiC wet-grinding paper successively with the following grain sizes:

Grain size 220
Grain size 320
Grain size 600
Grain size 1200
Grain size 4000

[0045] Polishing is performed with a 1 μm diamond suspension until the specimen is scratch-free and contour sharp.

[0046] Subsequently, a microscopic image of the surface of the tread is taken.