Abstract:
The invention relates to a sliding element, in particular a piston ring, preferably made of cast iron or steel, comprising a coating having a CrN, an Me(C x N y ), and a DLC layer extending from the inside to the outside, wherein the DLC layer is either metal-free or consists of a metal-containing substructure and a metal-free DLC top layer. The invention further relates to a combination of such a sliding element with an iron-based mating miming element.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a sliding element, in particular a piston ring, comprising a coating and a combination of a sliding element with a mating running element. 
         [0002]    Numerous technical applications entail sliding elements in sliding contact with mating running elements. The combination of a piston ring with a cylinder liner can be considered to be a typical application. If the cylinders or cylinder liners are, for example, made of aluminum-silicon alloys, which is in particular the case with spark-ignition engines, DLC (diamond like carbon) coating systems have proven their worth with respect to wear and friction loss. However, known DLC layer systems need to be improved with respect to service life and suitability for use with diesel or highly supercharged spark-ignition engines which usually have iron-based cylinder liners. Due to the significantly higher cylinder pressures and in particular in combination with direct injection, there is a greater proportion of mixed friction conditions. The decisive factor for the inadequate suitability of DLC coating systems for situations of this kind is considered to be the low layer thickness which is usually less than 5 μm. 
       PRIOR ART 
       [0003]    DE 10 2005 063 123 B3 discloses a DLC coating with a run-in layer. With respect to higher layer thicknesses, PVD coatings are also known, in particular based on CrN with layer thicknesses of 10-30 μm. Although these do result in an improved service life, there is a deterioration in the friction loss and resistance to wear, in particular in the case of insufficient lubrication and scuff resistance. On the other hand, due to their amorphous structure, DLC layer systems have the advantage that they are substantially chemically inactive with metal surfaces and hence have an extremely low adhesion tendency with respect to the mating running element. 
         [0004]    DE 296 05 666 U1 relates to a component in the region of an internal combustion engine coated with a metal-free, amorphous carbon layer with graduated hardness over the layer thickness. 
         [0005]    DE 10 2008 016 864 B3 discloses a piston ring comprising an adhesive layer, a metal-containing amorphous carbon layer and a metal-free amorphous carbon layer extending from the inside to the outer side. 
         [0006]    WO 2009/106201 A1 relates to a component of an internal combustion engine, with which a DLC coupling agent layer, a DLC functional layer and a DLC run-in layer are present on a substrate. 
         [0007]    Finally, WO 2006/125683 A1 discloses a piston ring comprising a layer with an element from the groups IVB, VB or VIB, an intermediate layer with a diamond-like nanocomposite composition and a DLC layer extending from the inside to the outer side. 
       DESCRIPTION OF THE INVENTION 
       [0008]    The invention is based on the object of providing a sliding element, in particular a piston ring, which in particular when used in diesel or highly supercharged spark-ignition engines with, for example, iron-based mating running elements, fulfils the requirements with respect to service life and friction loss. It will also disclose a suitable combination of a sliding element with mating running element. 
         [0009]    This object is achieved by the sliding element described in claim  1 . 
         [0010]    This sliding element is characterised, on the one hand, by a combination of a CrN layer and a DLC layer on the outer side. The DLC layer is either metal-free or comprises a metal-containing substructure and a metal-free DLC top layer. In other words, the DLC layer is at least partially metal-free and, in particular on the outer surface, comprises a metal-free DLC layer. Moreover, extensive tests have shown that the coating described displays particularly good characteristics if a Me(C x N y ) layer is provided as an intermediate layer between the CrN layer and the DLC layer. Tests with this layer structure have shown that the wear characteristics on the sliding element, in particular a piston ring, are improved compared with the known structure comprising a Cr adhesive layer and a DLC top layer. This can significantly improve the service life. Tests have also shown that it is also possible to significantly reduce the relative coefficient of friction. Hence, overall, the described layer structure can provide an improved sliding element. It is stressed that the CrN layer is preferably applied to the basic piston material without an adhesive layer, for example by metal vapour deposition or sputtering. However, in special applications, the CrN layer can be applied to the basic material of the sliding element by means of an adhesive layer. 
         [0011]    For the sake of clarity, it should be noted that Me stands for metal and this can be, for example, tungsten, chromium, titanium or silicon. In addition, carbon and nitrogen can be present in the described layer in virtually any proportion, that is both x and y can be within the range of 0-99 atom %. In addition, in particular at least in regions, the coating can be provided on in particular at least one running surface. In addition, the coating can extend into the transition to surfaces and to surfaces adjacent to the running surfaces. This relates, for example, to the running edges of a piston ring. 
         [0012]    Preferred further developments are described in the other claims. 
         [0013]    Values of 1100-1900 HV 0.002 have been found to be suitable for the hardness of the CrN layer or the substructure. This embodiment can be combined with a hardness of the metal-free DLC layer of 1700-2900 HV 0.002 and/or a hardness of the metal-containing DLC layer of 800-1600 HV 0.002. 
         [0014]    It is also expected that particularly good properties of the DLC-layer, in particular the metal-containing and/or the metal-free DLC layer, will be obtained if it contains hydrogen. 
         [0015]    The metal-containing DLC layer can also in an advantageous way comprise nanocrystalline metal carbide precipitations, such as, for example, WC, CrC, SiC, GeC or TiC. 
         [0016]    Advantageous properties have also been identified for CrN layers with a thickness of 1-30 μm, Me(C x N y ) layers with a thickness of a maximum of 2 μm and an overall layer thickness of 5-40 μm. 
         [0017]    With respect to the roughness, to further improve the properties, it is preferable for the top layer forming the surface to have roughness parameters Rz&lt;5 μm and/or Rpk&lt;0.8 μm. 
         [0018]    In a proven way, the metal-containing and/or metal-free DLC layer can be produced by PA-CVD methods. 
         [0019]    The invention also relates to a combination of at least one sliding element as described above with a mating running element, in particular a cylinder or a cylinder liner of an internal combustion engine, in particular a diesel or highly supercharged spark-ignition engine, wherein the mating running element is iron-based. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The following describes a preferred exemplary embodiment with reference to the drawings, which show: 
           [0021]      FIG. 1  a layer structure according to the invention 
           [0022]      FIG. 2  the relative wear on a piston ring and a cylinder liner and 
           [0023]      FIG. 3  the relative coefficient of friction of different layer systems. 
       
    
    
     DETAILED DESCRIPTION OF A PREFERRED EXEMPLARY EMBODIMENT OF THE INVENTION 
       [0024]    As  FIG. 1  shows, a CrN layer  12  is applied to the basic material  10  of the sliding element. The adhesive layer  14  shown in  FIG. 1  does not necessarily have to be provided. In particular, this is preferably omitted. By means of a Me(C x N y ) intermediate layer  16 , a DLC layer  18  is provided on the outer side with a metal-free DLC layer on the outer side. 
         [0025]    Tests with comparative examples were performed with a structure of this kind, without the adhesive layer  14 . The table shows the layer systems. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Layer structure 
                   
               
             
          
           
               
                   
                   
                   
                   
                 Running 
                   
               
               
                   
                 Substrate 
                   
                 Inter- 
                 surface 
                 Total layer 
               
               
                   
                 Adhesive 
                   
                 mediate 
                 Top 
                 thickness 
               
               
                   
                 layer 
                 Substructure 
                 layer 
                 layer 
                 [μm] 
               
               
                   
                   
               
             
          
           
               
                 DLC 
                 Cr 
                   
                   
                 DLC 
                 3.5 
               
               
                 series 
               
               
                 CNDLC01 
                 — 
                 CrN - 01 
                 Me(CN) x   
                 DLC 
                 8.8 
               
               
                 CNDLC02 
                 Cr 
                 CrON - 02 
                 Cr 
                 DLC 
                 14.5 
               
               
                 CNDLC03 
                 Cr 
                 Cr—N - 03 
                 Cr 
                 DLC 
                 14.6 
               
               
                 CNDLC04 
                 Cr 
                 CrON - 04 
                 Cr 
                 DLC 
                 14.3 
               
               
                   
               
             
          
         
       
     
         [0026]    Here, the system “DLC series” is a standard DLC layer system without CrN. These layer systems were applied to a piston ring, and the tribological behaviour investigated in combination with a honed cast iron cylinder liner in lubricated condition.  FIG. 2  shows the relative wear values for the piston ring (at the top) and for the cylinder liner. The percentage values shown indicate the percentage of the original DLC layer remaining after the end of the test. If this value is more than 100 percent, in addition to the DLC layer, the CrN or CrON layer is also at least partially worn. The highest wear value is used as the reference value for the relative wear of the cylinder liner and hence defined as 100 percent. 
         [0027]    As  FIG. 2  shows, the coating according to the invention with a Me(C x N y ) intermediate layer has the lowest relative wear. In the case of wear to the cylinder liner, only the known DLC coating results in a lower value. However, the wear of the cylinder liner is also in an acceptable range for the example according to the invention. 
         [0028]    Investigations were also performed into the coefficients of friction and the results are shown in  FIG. 3 . Here the example according to the invention has the lowest relative coefficient of friction. Hence, this requirement is also fulfilled in addition to the requirement for comparatively low wear and hence a long service life.