Abstract:
A piston ring with a DLC coating comprises an adhesive layer on a base material from inside to outside, an amorphous metallic hydrocarbon layer and an amorphous non-metallic hydrocarbon layer, wherein primarily sp 2  bonds predominate in the metallic hydrocarbon layer and the non-metallic hydrocarbon layer comprises sp 2  and sp 3  bonds, the sp 3  bond fraction being higher than in the metallic hydrocarbon layer so that an electrical resistance of the DLC coating arises, said resistance being in the range greater than 5000 ohms, in particular greater than 5000 kohms.

Description:
TECHNICAL FIELD 
     The invention relates to a piston ring having a novel coating. 
     Piston rings are used in internal combustion engines in order to provide as perfect a sealing as possible between the piston that moves up and down and the cylinder wall. Furthermore, piston rings serve to wipe off the oil located on the cylinder wall and to maintain the usability of the oil for lubrication. The fundamental requirements placed on piston rings are the lowest possible friction and the longest possible service life under the, in part, extreme conditions during the operation of an internal combustion engine. This implies a wear behavior in which the piston ring meets the requirements placed on it as long as possible. 
     RELATED ART 
     WO 2007/020139 A1 describes a substrate that may be a piston ring, having an adhesive layer, an intermediate layer of tetrahedral carbon, and an outer layer of amorphous carbon. 
     DE 10 2005 063 123 B3 of applicant pertains to a sliding element, for example a piston ring, which comprises a wear layer and a run-in layer. 
     WO 2006/125683 A1 relates to a piston ring having a coating which comprises, from inside to outside, a first intermediate layer having at least one element of group IVB, VB or VIB, a second intermediate layer having a diamond-like, nanocomposite composition, and a diamond-like carbon layer. 
     SUMMARY OF THE INVENTION 
     An object underlying the invention is to provide a piston ring which can be manufactured with less effort than the piston rings known in the prior art and which at the same time meets the requirements regarding the friction values and/or service life. 
     Accordingly, the piston ring comprises, from inside to outside, an adhesive layer applied to a base material, a metal-containing amorphous carbon layer and a metal-free amorphous carbon layer. Due to this layer structure, the structure common in the prior art with a run-in layer and a separately formed wear protection layer can be dispensed with. The known wear protection layers are mostly formed as nitrided layers and require the use of steels having high contents of chromium, which are comparably expensive. Compared to this, high contents of chromium are not required according to the invention, and therefore the novel piston ring can be produced cost-effectively. Thus, a fundamental concept of the invention can also be seen in that the novel carbon coating on the substrate is only provided with a thin adhesive layer to promote adhesion between the substrate and the carbon layer, without interposition of a separate wear protection layer. 
    
    
     DETAILED DESCRIPTION 
     Tests have shown that the friction coefficients for the piston ring according to the invention are permanently low, as will be described in detail below. In this respect, the novel piston ring accordingly offers a long service life. The same is true for wear, for which it has been found that it is within limits that guarantee a long service life. The metal-containing amorphous carbon layer can be designated in accordance with the conventional designations (cf. VDI Guideline 2840) by the abbreviation a-C:H:Me. The corresponding designation for the metal-free amorphous carbon layer is a-C:H. The metal-free carbon layer can be applied, for example, as part of a separate process step within a PACVD (plasma-assisted chemical vapor deposition) method. Both amorphous carbon layers are DLC (diamond-like carbon) layers. 
     In the metal-containing carbon layer there are predominately sp 2  bonds. Due to this property, it could be found that the piston ring according to the invention has the properties desired for the piston ring. In particular, the proportions range from 20% to 70% of the total percentage of the carbon bonds. 
     With regard to the metal-free carbon layer, the sp 3  bond fraction is higher than the sp 3  bond fraction of the metal-containing carbon layer. A preferred bond fraction of sp 3  fractions is provided when an increased electrical resistance (Ohm) develops. While conventional DLC coatings have an electrical resistance in a range of less than 100 Ohm, the preferred increased electric resistance of the layer according to the invention is greater than 5000 Ohm, in particular greater than 5000 kOhm and preferably less than 10 kOhm. 
     Steel or cast iron has been found to be particularly favorable as the base material of the piston ring according to the invention. 
     The adhesive layer preferably contains chromium and can in particular consist predominantly of chromium. It has been found that such an adhesive layer comprises the desired properties promoting adhesion between the base material and the metal-containing amorphous carbon layer. 
     Based on test results, a maximum thickness of 2 μm, preferably approximately 0.5 μm, is currently favored for the adhesive layer. 
     As regards the metal contained in the metal-containing carbon layer, tungsten is currently favored. Particularly good properties were found for this in tests. 
     This similarly applies to that preferred embodiment in which the metal content within the metal-containing carbon layer decreases towards the outside. 
     With regard to the thickness, it is preferred for the metal-containing carbon layer to have a thickness of from approximately 1.5 to approximately 17 μm. In particular, good behavior was found with a layer thickness of approximately 1.5 μm. 
     Particularly good properties were moreover found for that embodiment in which the metal-containing carbon layer contains nanocrystalline carbide phases. 
     Furthermore, if the metal-free carbon layer has a thickness of from approximately 1 μm to approximately 15 μm, in particular approximately 1 μm, the piston ring provided therewith was found to have good properties. 
     The metal-containing and/or the metal-free carbon layer preferably also contains hydrogen. The available hydrogen has an influence on the degree of cross-linking of the carbon, thereby resulting in particularly hard layers. 
     Regarding the total thickness of the layers applied to the base material, this is currently preferred to be &gt;approximately 2.9 μm and preferably &lt;approximately 30 μm. This advantageously constitutes a difference over the previously known DLC layers, which are mostly thinner. Owing to the thicknesses that can be realized according to the invention, the advantage can be realized that this does not require a separate wear protection layer since the DLC layers act both as run-in and wear protection layers. 
     The surface hardness of the piston ring designed according to the invention is preferably from 1800 to 2800 HV, preferably up to 2400 HV 0.002. Measurement is carried out in this case using a nanoindenter with HV pl 0.002. The abbreviation pl stands for plastic, and the corresponding method is known to the persons skilled in the art. In particular applications, measurement using a nanoindenter with HV pl 0.02 is also possible. 
     It is currently preferred for the adhesive layer, which preferably consists of chromium, to be applied by means of metal vapor deposition. 
     It is currently preferred that the metal-containing and/or metal-free carbon layer is applied by means of a PACVD (plasma-assisted chemical vapor deposition) method. 
     Moreover, reference is made with regard to all of the features of the piston ring according to the invention, the coating provided thereon and the sliding partners preferably cooperating therewith to the aforementioned DE 10 2005 063 123 B3 or WO 2007/079834 A1, or US2009/001669 which is based thereon, the disclosure of which is made the subject matter of the present application with regard to the cited features. 
     EXAMPLES 
     A piston ring according to the invention having a coating with a total thickness of approximately 2.9 μm was produced, for which a hardness of 1935 HV 0.002 was determined. The piston ring was subjected to a wear test together with a piston ring having a nitrided layer as a wear protection layer and a mass-produced DLC-coated piston ring. This test was carried out at 190° C., a load of 50 N and using Castrol VP1 as oil. Silitec S260 was used for the bore. The corresponding product is known to the persons skilled in the art. After six hours, a wear of approximately 1.0 μm was determined for the piston ring according to the invention, and a wear of approximately 5.3 μm was determined for the mass-produced DLC-coated piston ring. The wear of the sliding partner usually increases in the case of a coating that is more resistant to wear. The coating according to the invention surprisingly does not show increased wear of the sliding partner such as the cylinder sleeve of the cylinder. 
     The wear of the cylinder sleeve was 2.7 μm for the piston ring according to the invention, and 2.8 μm for the mass-produced, DLC-coated piston ring. Moreover, the coefficient of friction was found to be significantly reduced for the piston ring according to the invention. Compared to a conventional DLC-coated piston ring, the coefficient of friction was found to be reduced by up to 30%. 
     Thus, it has been shown that advantageous properties are achieved for the piston ring according to the invention. As mentioned above, this applies in particular to the coefficient of friction, especially in the longer term, the run-in behavior and scuff resistance, which as a whole results in a durable piston ring.