Abstract:
The invention relates to a gliding element of an internal combustion engine, especially a piston ring, having a DLC coating of the ta-C type which has at least one residual stress gradient.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to a sliding element in an internal combustion engine, especially a piston ring. 
     When reducing the carbon dioxide emissions of internal combustion engines the fuel consumption plays an essential role. This is affected essentially by the friction losses in the engine, in particular in the area of the pistons, for example on the piston rings. There is therefore a requirement for sliding elements in internal combustion engines, especially piston rings, which have the most favourable possible friction characteristics over their whole life span. With regard to the whole life span, the running-in characteristics, any possible lubrication deficiency and any burn mark formation, which respectively lead to changed tribological properties, are to be taken into account. 
     2. Related Art Prior Art 
     In the aforementioned Technical field PVD coatings on a hard material base are known which have good resistance to wear, but are in need of improvement with regard to the friction coefficients. 
     A layer structure emerges from DE 10 2005 063 123 B3 which, from the outside to the inside, has a running-in layer, an adhesive layer and a wear protection layer. However, the characteristics of the friction properties over the life span have proven to still be capable of improvement. 
     SUMMARY OF THE INVENTION 
     A sliding element of an internal combustion engine, in particular a piston ring, has a coating which reliably has favourable friction properties over the longest period of time possible. 
     Therefore, the latter has a DLC coating of the ta-C type which has changing residual stresses over its thickness. In other words, at least one residual stress gradient is formed over the thickness of the coating. As is known, “DLC” stands for Diamond-Like Carbon. The ta-C type is characterised by a tetrahedral structure, is free from hydrogen and is defined, for example, in VDI guideline 2840. This type of layer provides on the one hand good friction characteristics, and moreover, for the following reasons has a particularly long life span. 
     On the one hand the variation in residual stresses over the thickness of the layer makes it possible to produce large layer thickness, for example greater than 10 μm, without problems occurring with regard to the adhesion or brittleness of the layer. Specifically, one has been able to establish that, for example, zones with low layer residual stresses relax or relieve the whole layer composite, i.e. the in particular multi-layered coating provided, locally. In this way, with high thrust load stresses, as occur when used in an internal combustion engine, the elastic limit of the DLC coating is not exceeded. Accordingly, the wear of the coating can be successfully limited. 
     For the cross-over from the coating to the base material of the sliding element a negative residual stress gradient in this, i.e. an inner region of the coating, as viewed from the outside to the inside, has proved to be advantageous. In other words, the residual stresses decrease towards the base material to a low value so as to achieve a favourable stress cross-over to the base material and good adhesion of the layer. 
     For a region lying to the outside a positive residual stress gradient, as viewed from the outside to the inside, has proved to be advantageous. In other words, the residual stress lies on the outer surface of the coating on a comparably low level, and this has proved to be advantageous for favourable running-in characteristics. From here, however, the residual stresses preferably increase strongly towards the inside so that the effects described above can be achieved. 
     For a centrally lying region of the coating, i.e. not right on the outside and not right on the inside, a negative residual stress gradient, which is preferably smaller than the negative residual stress gradient in the inner region of the coating, i.e. lying towards the base material, has proved to be of value. In this way the residual stress lies with the highest level a comparably long way to the outside on the coating, and this allows one to expect favourable characteristics. 
     In particular with sliding elements for which one can expect an extremely high surface pressure, a constant low residual stress profile in an innermost region, i.e. directly against the base material, is favourable. 
     Furthermore, by means of a likewise constant, comparably low residual stress profile that is, however, preferably at a higher level than in the innermost region, and which is provided on the outside of the coating, the running-in characteristics can be further improved. 
     In order to avoid the spread of cracks in the coating, extensively alternating residual stresses in the middle region have proved to be advantageous. In connection with this, the extent of the zones with low residual stresses can be smaller than, equal to or greater than the extent of the zones with high residual stress. 
     For the periodicity, i.e. the thickness, between the start of a region with low residual stress, over a region with high residual stress to the start of the next region with low residual stress, values of 0.1 to 1 μm are conceivable. 
     Overall, by means of the measures described, coatings with a thickness of 10 μm or greater can be produced, the effect of which on the one hand is favourable running-in characteristics, but at the same time constitutes a sufficient layer thickness so as to ensure a long life span with favourable friction properties of the coating after unavoidable wear. 
     For the generation of the different residual stresses a change, for example, in the ratio between sp2 and sp3 hybridised carbon atoms is advantageous. In particular, the pressure residual stresses can be increased by increasing the sp3 portion, and this enables overall the formation of a residual stress gradient. 
     This applies similarly with an increasing density so that for a change to the density of the layer over the thickness of the latter it is expected to be possible to change the residual stresses advantageously over the thickness of the layer. 
     Finally, it has been considered to change the hardness of the layer over the thickness of the latter because a greater degree of hardness results in greater pressure residual stresses, and so in this way the desired residual stress gradients can also be set. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following embodiments of the invention are described, as examples, in more detail with reference to the drawings. These show as follows: 
         FIG. 1  an illustration of the residual stress profile over the thickness of a DLC coating in a first embodiment; and 
         FIG. 2  an illustration of the residual stress profile over the thickness of a DLC coating in a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the figures the residual stresses of the DLC coating are respectively marked over the thickness of the coating, an outermost region of the coating lying on the left, and an innermost region of the coating on the right in the diagrams. In other words, the base material, for example steel or grey iron, adjoins the coating with the residual stresses shown on the right. 
     In the embodiment of  FIG. 1  (from the outside to the inside) negative residual stress gradient is formed starting from the inner region or the “base” of the coating (zone III). In other words, the residual forces at the cross-over to the base material are particularly low in order to achieve a good stress cross-over and good adhesion of the coating. In a middle region (zone II) the residual stress increases further, but with a lower gradient, to the highest level. On the outside (zone I) the residual stresses decrease greatly, in other words a high positive residual stress gradient is formed from the outside to the inside, and this leads to favourable running-in characteristics. 
     This applies in the same way to the embodiment of  FIG. 2  in which in an outermost zone I the residual stresses are initially constant at a low level, and then (zone II) increase greatly. This provides a stress-dependant cross-over between zone I and a zone III in which the residual stresses alternate extensively. In particular, with thrust stress loading a constantly high residual stress state has proven to be at risk of cracks. The alternating residual stresses in zone III prevent the spread of cracks. In this region the periodicity α can be, for example, between 0.1 and 1 μm. Similarly to the embodiment of  FIG. 1 , a zone IV with a strongly negative residual stress gradient adjoins the base material (on the right in the figure), and directly adjacent to the base material there is a zone V with a constantly low residual stress profile in order to achieve a good stress cross-over to the base material and good adhesion. The residual stress level in zone V can in particular be lower than that in the outermost zone I, and in zone III the residual stresses can vary between absolute maximum values and a level a little above the level of zone I.