Sliding element having a coating

1. Technical Field

The invention relates to a sliding element, in particular a piston ring, having a coating on at least one running surface.

2. Related Art

Sliding elements such as, for example, piston rings, pistons or cylinder sleeves in internal combustion engines must work over a long service life, both with as little friction as possible and with only a low level of wear. It is the case that the friction, which is directly associated with the fuel consumption in the case of internal combustion engines, can be kept to a low level by coatings of DLC (diamond-like carbon). Further, layer thicknesses of up to 40 μm can be achieved in principle. Nevertheless, in the case of layer thicknesses of more than 5 μm, there is the problem that the layer properties alter, for example in respect of the structure and composition of the layer, such that the required service life is not achieved. This applies likewise to layer thickness of less than 5 μm.

Also known in this connection are PVD coatings based on a hard material, which comprise at least chromium nitride. Although such layers do have the necessary resistance to wear, they nevertheless do not have the required low coefficients of friction.

A sliding element having a DLC coating with a good running-in behaviour is known from DE 10 2005 063 123 B3. Overall, however, the durability of a low coefficient of friction could be improved further.

DE 10 2008 016 864 relates to a sliding element having a coating that comprises, from the inside outwards, an adhesive layer, a metal-containing DLC layer, and a metal-free DLC layer.

DE 197 35 962 A1 discloses a guide bush and a method for realising a hard carbon film on the inner surface of the guide bush, wherein a plasma CVD process is used to realise a hard carbon film of hydrogenated amorphous carbon on the inner surface.

Finally, WO 2006/125683 A1 discloses a piston ring comprising, from the inside outwards, a layer comprising an element of the groups IVB, VB or VIB, an intermediate layer comprising a diamond-like nanocomposite composition, and a DLC layer.

Against this background, the invention is based on the object of providing a sliding element that is improved further in respect of the combination of coefficient of friction and wear properties.

Accordingly, this sliding element has, on at least one running surface, a coating that, from the inside outwards, comprises an adhesive layer, a DLC layer containing metal, in particular tungsten, and a metal-free DLC layer, the ratio of the thickness between the metal-free DLC layer and that containing metal being between 0.7 and 1.5, and/or the ratio of the thicknesses between the metal-free DLC layer and the coating as a whole being between 0.4 and 0.6. The adhesive layer is preferably a chromium adhesive layer. The DLC layer containing metal comprises amorphous carbon, and can be designated by a-C:H:Me and, as the preferred DLC layer containing tungsten, can be designated as a-C:H:W. The outermost, or top, layer likewise comprises amorphous carbon, and can be denoted by a-C:H. Particularly good properties in respect of friction and wear were ascertained in the case of the values described. These tribological properties can be influenced, towards a longer service life, by a thicker top layer. If this top layer becomes too thick in comparison with the middle layer, however, the wear values become worse. It was possible to ascertain particularly good wear values if the middle and the top layer are of a virtually identical thickness, such that a thickness ratio of approximately 1.0, in particular 0.9 to 1.1, or a thickness ratio of the top layer to the total layer of approximately 0.5, in particular 0.45 to 0.55, is preferred here. In respect of the friction, for coatings within the said ranges it was possible to ascertain coefficients of friction that satisfactorily meet the demands arising in an internal combustion engine and that, in particular, are largely constant. Outside these ranges, by contrast, high friction coefficient peaks and a non-constant friction characteristic were ascertained even after a short period of time.

As an explanation for this behaviour, to which, however, the invention is not limited, it is at present considered that the metal-free DLC layer first introduces into the overall system, i.e. into the coating as a whole, very high internal stresses that, in the case of the metal-containing DLC layer being of a layer thickness similar to the thickness of the outermost layer, can be compensated in such a way that the coating is realised in an optimal manner with regard to the combination between strength and toughness. A sliding element coated therewith, in particular a piston ring, thus has a good resistance to wear. If the layer thickness ratio between the metal-free and the metal-containing DLC layer is <0.7, and/or if the ratio of the layer thickness of the top layer to that of the total layer is <0.4, the service life of the sliding element is too short, since the outermost, metal-free DLC layer, while having a high resistance to wear, is nevertheless of an insufficient layer thickness. In contrast to this, if the layer thickness ratio between the metal-free and the metal-containing DLC layer is >1.5, and/or if the ratio of the thickness of the top layer to that of the total layer is >0.6, the thickness of the metal-containing DLC layer is not sufficient to compensate the internal stresses. This results in premature wear of the DLC layer as a whole, despite the great thickness of the outermost layer, or in flaking-off of the DLC layer, as a result of excessively high loads during operation.

While the coating is realised, at least partially, on at least one running surface of the sliding element, it can extend over the entire running surface and, in particular, can also be realised, wholly or partially, on surfaces that adjoin the running surfaces, such as, for example, the flanks of a piston ring and/or at the transition from the running surface to the surfaces adjoining the latter.

At present, cast iron or steel is preferred as a base material of the sliding element, in particular of the piston ring. It was possible to ascertain particularly good properties for these materials.

In respect of the hardness of the layer, values of 1700 HV0.02 to 2900 HV0.02 are preferred for the metal-free (a-C:H-, top-) DLC layer, and/or of 800 to 1600 HV0.02 for the metal-containing (a-C:H:Me-) DLC layer, since the requirements were met satisfactorily with these values.

Both the metal-containing and the metal-free DLC layer can contain hydrogen, this having proved advantageous in tests.

Further, for the DLC layer containing tungsten, it is preferred that it contain nanocrystalline tungsten carbide precipitates, whereby the properties are further enhanced.

For the thickness of the adhesive layer, which, in particular, is a chromium adhesive layer, a value of maximally 1 μm is preferred.

A total coating thickness of 5 μm to 40 μm is preferred so that the described balance between the coefficient of friction and the wear properties can be achieved in a particularly satisfactory manner.

In respect of efficient realisation of the coating, it is at present preferred that the adhesive layer be effected by metal vapour deposition.

Favourable production of the coating according to the invention is further ensured, in respect of the metal-containing and/or metal-free DLC layer, if these layers are realised by means of a PA-CVD process.

Preferred exemplary embodiments of the invention are explained more fully in the following with reference to the drawings, wherein:

FIG. 1 shows, schematically, the structure of the coating according to the invention;

FIG. 2 shows a representation of the relative wear in the case of two examples according to the invention and one comparative example; and

FIG. 3 a representation of the relative coefficient of friction in the case of two examples according to the invention and one comparative example.

As represented schematically in FIG. 1 , the coating according to the invention comprises, from the inside outwards, an adhesive layer 10 , a middle layer 12 and a top layer 14 , the thicknesses and thickness ratios of which are within the range stated above.

The properties of the coating according to the invention were investigated on the basis of two examples and one comparative example. Tests were performed with the following coatings:

Examples 1 and 3 in this case are examples according to the invention, and Example 2 is a comparative example. The studies were performed for the system “piston ring/honed grey cast iron cylinder sleeve, lubricated”. FIG. 2 shows the relative wear values of the piston rings. As can be seen from FIG. 2 , the examples according to the invention have a significantly lesser wear than the comparative example 2, in particular approximately only 20%, or less, of the wear occurring in the case of the comparative example 2.

As shown by FIG. 3 , in the case of the examples according to the invention, moreover, the coefficient of friction is largely constant over time, whereas, in the case of the comparative example, on the one hand, the coefficient of friction is higher, and has very high peak values, over the entire course. On the other hand, the coefficient of friction fluctuates extensively. Further, the coating according to this comparative example can be said to have failed before completion of the test period.