Sliding element, in particular piston ring, having a coating and process for producing a sliding element

1. A sliding element which has on at least one running surface, from the inside outwards, a coating having a metal-containing adhesive layer and a tetrahedral hydrogen-free amorphous carbon layer with a thickness of at least 10 μm, wherein the tetrahedral hydrogen-free amorphous carbon has an sp3 content of hybridized carbon atoms of at least 40 at %, wherein the tetrahedral hydrogen-free amorphous carbon layer contains hydrogen in an amount of less than 0.5at %, and the sp3 content in the outer 1 μm to 3 μm of the tetrahedral hydrogen-free amorphous carbon layer is reduced.

2. The sliding element according to claim 1 , wherein the adhesive layer contains at least one of the following materials: chromium, titanium, chromium nitride and/or tungsten.

3. The sliding element according to claim 1 , wherein the adhesive layer has a thickness of from 0.1 μm to 1.0 μm.

4. The sliding element according to claim 1 , wherein the tetrahedral hydrogen-free amorphous carbon layer contains oxygen in an amount of less than 0.5 at %.

5. The sliding element according to claim 1 , wherein the hardness of the tetrahedral hydrogen-free amorphous carbon layer is not more than 55 GPa and the modulus of elasticity is not more than 550 GPa.

6. The sliding element according to claim 1 , wherein an outer 1 μm to 3 μm of the tetrahedral hydrogen-free amorphous carbon layer is doped.

7. The sliding element according to claim 1 , wherein an outer 1 μm to 3 μm of the tetrahedral hydrogen-free amorphous carbon layer is doped with at least one of the following elements: boron, oxygen and/or silicon.

8. The sliding element according to claim 1 , wherein the coating has a roughness depth Rz of <6 μm.

9. The sliding element according to claim 1 , wherein the coating has a reduced peak depth Rpk of <0.3 μm.

10. The sliding element according to claim 1 , wherein the base material is cast iron or steel.

11. The sliding element according to claim 1 , including a lower surface edge of the sliding element having a radius that is <0.1 mm.

12. A method for making a sliding element including applying a coating on a running surface of the sliding element with a metal-containing adhesive layer and a tetrahedral hydrogen-free amorphous carbon layer in a thickness of at least 10 μm wherein tetrahedral hydrogen-free amorphous carbon layer is formed to have sp3 content of hybridized carbon atoms of at least 40 at %, wherein the tetrahedral hydrogen-free amorphous carbon layer contains hydrogen in an amount of less than 0.5 at %, and reducing the sp3 content in the outer 1 μm to 3 μm of the tetrahedral hydrogen-free amorphous carbon layer.

13. The method according to claim 12 , wherein the adhesive layer is formed by at least one of a sputtering process, a thermal vapour deposition process or an electrical vapour deposition process.

14. The method according to claim 12 wherein the tetrahedral hydrogen-free amorphous carbon layer is formed by means of a vacuum laser arc process.

15. The method according to claim 12 , wherein the surface to be coated is cleaned by means of a metal ion sputtering process.

16. The method according to claim 12 , wherein the coating is smoothed by lapping, belt and/or brush polishing after it has been formed.

17. The sliding element according to claim 1 , comprising a piston ring.

18. The sliding element according to claim 1 , wherein the coating has a roughness depth R z of <2 μm.

19. The sliding element according to claim 1 , wherein the coating has a roughness depth R z of <1 μm.

20. The sliding element according to claim 1 , including a lower surface edge of the sliding element having a radius that is 0.1 mm or less.

21. The method according to claim 12 wherein the sliding element is selected as a piston ring.

22. The method according to claim 12 wherein the adhesive layer is formed by an arc process.