Coating with enhanced sliding properties

Abstract

The present invention relates to coated sliding parts having coating systems which allow better sliding performance under dry and/or under lubricated conditions. The coating systems according to the present invention being characterized by having an outermost layer which—is a smooth oxide-containing layer in case of sliding applications under lubricated conditions, or—is a self-lubricated layer comprising molybdenum nitride, in case of sliding applications under dry or lubricated conditions, is a self lubricated layer with a structured surface comprising a multitude of essentially circular recesses with diameters of several micrometers or below, the recesses randomly distributed over the surface.

Claims

1. A sliding component having a sliding surface to be exposed to relative movement with respect to another component under dry or lubricated conditions, wherein the sliding surface of the sliding component is at least partially coated with a coating system comprising a nitride-containing running-in layer deposited as outermost layer, wherein, the outermost layer comprises molybdenum nitride and at least one element or a mixture of elements whose melting point is lower than the molybdenum melting point, or the outermost layer consists of molybdenum oxynitride having an element composition Mo.sub.dO.sub.eN.sub.f, with d+e+f=1, f >e, and d >e, where d, e and f are the concentrations in atomic percent of molybdenum, oxygen and nitrogen, respectively.

2. A sliding component having a sliding surface to be exposed to relative movement with respect to another component under dry or lubricated conditions, wherein the sliding surface of the sliding component is at least partially coated with a coating system comprising a nitride-containing running-in layer deposited as outermost layer, wherein the outermost layer exhibits a multilayer architecture consisting of a combination of molybdenum nitride and molybdenum oxide single layers, wherein the molybdenum oxide single layers have element composition according to the formula Mo.sub.vO.sub.w with v≥w, where v and w are the concentration in atomic percent of molybdenum and oxygen, respectively, and/or a combination of oxynitride single layers having different element compositions along the running-in layer thickness.

3. The sliding component according to claim 2, wherein the thickness of the single layers in the multilayer architecture is smaller than 300 nm.

4. The sliding component according to claim 1, wherein the coating system further comprises: at least one bonding strength layer for improving coating adhesion, deposited directly on the substrate, and/or at least one metal nitride containing layer or deposited directly on the substrate or if given on the bonding strength layer.

5. The sliding component according to claim 1, wherein the nitride-containing running-in layer is an arc-PVD-deposited layer comprising droplets embedded in the layer.

6. A sliding component having a sliding surface exposed to relative movement with respect to another component under dry or lubricated conditions, wherein the sliding surface of the sliding component is at least partially coated with a coating system comprising an outermost layer, wherein the outermost layer is a self-lubricated layer with a structured surface comprising a multitude of circular recesses with diameters of several micrometers or less, the recesses randomly distributed over the surface, wherein the self-lubricated layer consists of a molybdenum oxynitride layer or a layer having element composition M.sub.Oh-Z.sub.LMPiN.sub.j with j+h+i≈1 and j >h >i, where Z.sub.LMP is one element or a mixture of elements whose melting point is lower than the molybdenum melting point, and j, h and i are the element concentration in atomic percentage of nitrogen, molybdenum and Z.sub.LMP.

7. The sliding component according to claim 6, wherein the coating system further comprises: at least one metal oxide containing layer arranged directly under the outermost layer, or at least one structured metal oxide containing layer arranged directly under the outermost layer.

8. A method for producing the sliding component according to claim 6, comprising the step of depositing by arc PVD a layer forming an outermost layer comprising essentially Mo—Z.sub.LMPN where Z.sub.LMP is an element or mixture of elements having lower melting point than molybdenum, wherein the arc PVD deposited technique involves a reactive deposition process in which at least one target comprising Mo and Z.sub.LMP is arc-evaporated in nitrogen atmosphere thereby creating an ensemble of droplets which at least partially does not adhere to the surface leading to a surface with a multitude of essentially circular recesses with diameters of several micrometers or below, the recesses randomly distributed over the surface.

9. A tribological system comprising at least a first and a second sliding component, each sliding component having respectively at least one sliding surface to be exposed to tribological contact, characterized in that, the sliding surface of the first sliding component and/or the sliding surface of the second sliding component is at least partially coated according to claim 1.

10. The sliding component according to claim 3, wherein the thickness of the single layers in the multilayer architecture is smaller than 150 nm and at least the thickness of at least one single layer is smaller than 100 nm.

11. The sliding component according to claim 4, wherein the coating system further comprises at least one metal oxide containing layer deposited on the bonding strength layer or on the metal nitride containing layer, and at least one metal nitride containing layer deposited on the metal oxide containing layer.

12. The sliding component according to claim 11, wherein the at least one metal oxide containing layer deposited on the bonding strength layer or on the metal nitride containing layer comprises a zircon oxide, aluminum chromium carbon oxide, or aluminum chromium oxide layer.

13. The sliding component according to claim 6, wherein Z.sub.LMP is Cu.

14. The sliding component according to claim 7, wherein the coating system further comprises at least one bonding strength layer for improving coating adhesion, deposited directly on the substrate and at least one metal nitride containing layer deposited directly on the substrate or on the bonding strength layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: Fracture cross section of the unpolished Cr.sub.1.0N.sub.1.0 coating (sample A).

(2) FIG. 2: Fracture cross section of the unpolished ta-C coating (sample B).

(3) FIG. 3: Fracture cross section of the unpolished Al.sub.0.76Mo.sub.0.24N.sub.1.15 coating (sample C).

(4) FIG. 4: Fracture cross section of the unpolished Mo.sub.1.0N.sub.1.0 coating (example D).

(5) FIG. 5: Fracture cross section of the unpolished Mo.sub.1.0N.sub.1-xO.sub.x coating (sample E).

(6) FIG. 6a: Fracture cross section of an unpolished Mo.sub.0.85Cu.sub.0.15N.sub.1.0 coating (sample F).

(7) FIG. 6b: SEM surface micrograph of an unpolished Mo.sub.0.85Cu.sub.0.15N.sub.1.0 coating (sample F).

(8) FIG. 7: Coefficient of friction in function of time in the reciprocating wear test for unpolished samples and under dry conditions.

(9) FIG. 8a: Wear track (above) and counter-part wear (below) for unpolished samples and under dry conditions for Cr.sub.1.0N.sub.1.0, ta-C and Al.sub.0.76Mo.sub.0.24N.sub.1.15 (from left to right).

(10) FIG. 8b: Wear track (above) and counter-part wear (below) for unpolished samples and under dry conditions for Mo.sub.1.0N.sub.1.0, Mo.sub.1.0N.sub.1-xO.sub.x and Mo.sub.0.85Cu.sub.0.15N.sub.1.0 (from left to right).

(11) FIG. 9: Coefficient of friction in function of time in reciprocating wear test for polished samples and lubricated conditions

(12) FIG. 10a: Wear track (above) and counter-part wear (below) for polished samples and lubricated conditions for Cr.sub.1.0N.sub.1.0, ta-C and Al.sub.0.76Mo.sub.0.24N.sub.1.15 (from left to right).

(13) FIG. 10b: Wear track (above) and counter-part wear (below) for polished samples and lubricated conditions for Mo.sub.1.0N.sub.1.0, Mo.sub.1.0N.sub.1-xO.sub.x and Mo.sub.0.85Cu.sub.0.15N.sub.1.0 (from left to right).

(14) FIG. 11a: Coefficient of friction in function of time in the reciprocating wear test of Zr—O coating parts under dry-unpolished, dry-polished and polished-lubricated conditions.

(15) FIG. 11b: Coefficient of friction in function of time in the reciprocating wear test of Al—Cr—C—O coated parts under dry-unpolished, dry-polished and polished-lubricated conditions.

(16) FIG. 11c: Coefficient of friction in function of time in the reciprocating wear test of Al—Cr—O coated parts under dry-unpolished, dry-polished and polished-lubricated conditions.

(17) FIG. 12: Wear track (above) and counter-part wear (below) for polished samples and lubricated conditions for Zr—O, Al—Cr—C—O and Al—Cr—O (from left to right).

(18) FIG. 13a: Draft of a coating for sliding parts which must be used under lubricated conditions according to the first aspect of the present invention.

(19) FIG. 13b: Draft of a coating for sliding parts which can be used under dry as well as under lubricated conditions according to the second aspect of the present invention having a metal nitride containing layer 5′.

(20) FIG. 13c: Draft of a coating for sliding parts which can be used under dry as well as under lubricated conditions according to the second aspect of the present invention having a metal oxide containing layer 6′.

(21) FIG. 13d: Draft of a coating for sliding parts which can be used under dry (if 9″ is a self-lubricated layer) as well as under lubricated conditions (if 9″ is a metal oxide containing layer as well as if 9″ is a self-lubricated layer) according to the third aspect of the present invention having one structured layer 9″ as an outermost layer.

(22) FIG. 13e: Draft of a coating for sliding parts which can be used under dry as well as under lubricated conditions according to the third aspect of the present invention having two structured layers 9″. The outermost structured layer is a self-lubricated layer and the lower structured layer 9″ is for example a structured metal oxide containing layer.

EXAMPLES OF THE INVENTION

(23) Coating Deposition and Characterization Methods

(24) The deposition of the coatings was performed in an INNOVA production system of OC Oerlikon Balzers AG. The substrates (polished disks of hardened steel and polished tungsten carbide inserts) were wet-chemical cleaned before deposition. After evacuation of the process chamber below 10.sup.−5 mbar, standard heating and etching steps were performed to ensure a good layer adhesion to the substrate. Elemental or composite metallic targets were utilized in combination with the appropriate reactive gases which were fed to the chamber via gas flow controllers. The chromium targets were produced by GfE Metalle and Materialien GmbH, the graphite targets by Steinemann AG, and the Mo, Al—Mo and Mo—Cu targets by PLANSEE Composite Materials GmbH. The coatings were mostly deposited on CrN interfaces. This interface was utilized because it forms a good adhesion layer to steel. In addition, CrN was also utilized as standard for the comparison with the other synthesized coatings because for this material many investigations had been accomplished in the past. The depositions of the coatings were performed under conditions similar to them described elsewhere. The synthesized layers represent a wide spectrum of materials: very hard ta-C, soft CrN and Mo-based coatings.

(25) Surface roughness was characterized by measurements of the mean roughness depth R.sub.z, the roughness average R.sub.a, the reduced peak height R.sub.pk, the reduced valley depth R.sub.vk and the material portions Mr1 and Mr2 of the coated samples before and after polishing according to the EN ISO standards utilizing a stylus instrument (Mahr Perthometer M1. The tip radius of the used stylus is 5 μm, the evaluation length was set with ln=4 mm (lr=0.8). The averaged surface roughness was calculated from three single measurements per sample and for the bare hardened steel substrate.

(26) Optical microscopy (Olympus MX40) was utilized to investigate the wear track after SRV testing for identification of material transfer from the counter-part and removal of coating materials. The wear volume of the counter-part was calculated from the wear diameter of the counter-part.

(27) A Zeiss LEO 1530 Gemini scanning electron microscope (SEM) equipped with a detector for Energy Dispersive X-ray (EDX) Analysis (from EDAX) was employed to examine the surface morphology and the fracture cross-section of the layers and to perform the compositional analysis of the material.

(28) The layer composition was analyzed by Rutherford Backscattering Spectrometry (RBS) at the 6 MeV tandem accelerator of the Federal Institute of Technology in Zurich. The measurements were performed using a 2 MeV, .sup.4He beam and a silicon surface barrier detector under 165°. The collected data were evaluated using the RUMP program. Elastic Recoil Detection (ERD) was utilized to estimate the hydrogen content of the ta-C coating by forward scattering.

(29) The indentation hardness (H.sub.IT) and the indentation modulus (E.sub.IT) at room temperature were determined by Martens hardness measurements (Fisherscope H100c) following the ISO14577-1 guidelines.

(30) It is beyond the scope of this work to investigate the crystal structure of the synthesized layers. However, selected results are mentioned to allow a comparison with references. In this case, the measurements were performed on a Bruker-AXS D8 Advance diffractometer with Göbel-Mirror and a solid state point detector using Cu K.sub.α-radiation in the θ-2θ-mode. The ICDD-data base was used to identify the crystallographic phases being present in the coatings.

(31) The wear behaviour of the deposited coatings was investigated using a reciprocating wear tester (SRV®, Optimol Instruments). Detailed information about the test and the set-up can be found elsewhere. A spherical counter body is oscillating under load against the coated polished hardened steel sample. Hardened steel balls (1.3505, Grade 25, 60-68 HRC) with 10 mm diameter were utilized as counter body (Spheric-Trafalgar Ltd.). In the test, a load of 20 N, a frequency of 5 Hz and a stroke of 1 mm was employed. The friction force is continuously recorded by sensors at the test block so that the coefficient of friction can be calculated knowing the normal force. The tests were performed under dry and lubricated sliding conditions whereby 0W20 molybdenum dialkylthiocarbamate (0W20 Mo-DTC) oil was used as lubricant. All tests were accomplished at room temperature (23° C.±5° C.). The total duration of a test for unpolished substrates and dry conditions was 12.5 min, consisting of a running-in time of 2.5 min and the testing time of 10 min. During the running-in, the load was continuously increased from 2 N to 20 N. Due to the reduced wear for polished samples and under lubricated conditions, a duration of the test of 122.5 min was chosen including the same running-in procedure of 2.5 min. Polishing was performed manually. In a first step, the sample surfaces were treated with Scotch-Brite™ and afterwards polished with a polishing mob.

(32) After the test the wear of the coated sample and the counter body were evaluated by optical microscopy.

(33) Coating Properties

(34) Table 1 summarizes the most relevant parameters of the cathodic arc deposition process: the cathode material, the utilized process gases and the deposition (substrate) temperature. With exception of the ta-C synthesis, only reactive gases without the addition of inert gases were utilized. In most cases, the functional layer was deposited on a Cr.sub.xN interface. Only for ta-C, a thin Cr interface was utilized. The thickness of the interface and the functional layers are also given in Table 1. The compositions of the synthesized coatings were measured by RBS and ERD. While for the metallic composition an error of ±3% is estimated, the nitrogen content can only be estimated with an error of ±10%. EDX was utilized to double-check the metallic composition in the layers and confirms the RBS results. The RBS spectrum for Mo.sub.1.0N.sub.1-xO.sub.x coating could be well simulated for a multilayer structure consisting of bilayers of Mo.sub.1.0N.sub.1.0 (120 nm) and Mo.sub.1.00N.sub.0.94O.sub.0.06 (60 nm), respectively. The metallic ratio in the coatings produced from composite targets does not reflect for all samples the metallic ratio of the utilized targets. Sample C shows a pronounced “loss” of Al in the synthesized Al.sub.0.76Mo.sub.0.24N.sub.1.15 compared to the target composition of Al.sub.80Mo.sub.20. The table contains also the mechanical properties of the coatings obtained by microhardness measurements, the indentation modulus E.sub.IT and the indentation microhardness H.sub.IT.

(35) In Table 2, the parameters describing surface roughness (R.sub.z, R.sub.a, R.sub.pk, R.sub.vk, Mr1 and Mr2) of the samples before and after polishing are listed. The roughness of the coatings deposited by arc evaporation is increased with coating thickness caused by the consecutive incorporation of droplets. However, the surface roughness of the synthesized coatings depends not alone on coating thickness. It is also influenced e.g. by the melting point of the target material, the fabrication method of the target, the target composition and the magnetic field of the arc source. Fracture cross section SEM (X-SEM) micrographs of the coatings deposited on tungsten carbide substrates are displayed in FIGS. 1 through 6 to illustrate the typical morphology of coatings. The Cr.sub.xN adhesive layer can easily be distinguished from the functional layer by its darker colour in the SEM images (FIG. 3-6). FIG. 1 shows the X-SEM micrograph of sample A. The thick Cr.sub.1.0N.sub.1.0 coating with an R.sub.z of 3.28 μm is characterized by a large number of droplets and openings in the layer indicating a loose integration of the droplets into the coating. The analysis of the crystal structure by XRD (not shown here) reveals face-centred cubic chromium nitride (IDD 03-065-2899) with some minor addition of body-centred cubic chromium (IDD 01-089-4055) which stem probably from droplets. This is in accordance with the results obtained in previous investigations for arc evaporated coatings utilizing low bias voltages. A glassy microstructure is found for the ta-C layer (FIG. 2) with a thickness of only 1.7 μm and an R.sub.z value of 1.73 μm. The X-SEM of Al.sub.0.76Mo.sub.0.24N.sub.1.15 (FIG. 3) is characterized by a coarse microstructure and many defects in the grown layer. In comparison, the fracture cross-section of Mo.sub.1.0N.sub.1.0 (FIG. 4) and Mo.sub.1.0N.sub.1-xO.sub.x (FIG. 5) coatings are denser and indicate less droplet generation during growth. These three coatings have a thickness of about 15 μm and R.sub.z values between 3.46 μm and 4.31 μm. The surface roughness of Mo.sub.0.85Cu.sub.0.15N.sub.1.0 (sample F) also falls in this roughness range (R.sub.z=3.75 μm), although its layer thickness (FIG. 6a) is only 4.6 μm. This relatively high surface roughness can be explained by the presence of Cu-enriched droplets at the surface of the sample. These droplets were generated during the deposition and were loosely incorporated into the surface of the coating during growth (FIG. 6b). Therefore, the six samples fabricated by cathodic arc evaporation exhibit a wide spectrum of features with respect to morphology and surface roughness and may illustrate the usefulness of the reciprocating wear test for its ability of coating classification.

(36) The measured surface roughness after polishing is also given in Table 2. A tendency for a classification of the materials can already be deduced by comparing the R.sub.pk values before and after polishing. These values characterize the reduction of the surface roughness for large R.sub.z values in the Abott-Firestone Curve. A pronounced decrease in this value from 0.03 to 0.04 μm is found for samples A, D and F. The values of sample C (0.17 μm) and sample E (0.11 μm) are higher. No decrease is observed for sample B (ta-C) from the initial R.sub.pk value of 0.53 μm to to even somewhat higher 0.55 μm.

(37) The different coating materials were investigated by the reciprocating wear test under dry-unpolished and lubricated-polished conditions. The counter-part material for all measurements was 100Cr6 steel. The test provides information about the time dependency of the CoF, the wear behaviour of the coated substrate and of its tribological counter-part. The wear volume which is deduced from the wear cap diameter created at the counter-part can be utilized to quantify counter-part wear. Optical microscopy was used to study material transfer, either from the counter-part to the coating or vice versa. FIG. 7 shows the CoF as a function of time for the investigated materials. The CoF at the end of the test (CoF.sub.fin) is listed in Table 3 for dry-unpolished as well as lubricated-polished conditions. The table also lists the measured wear cap diameter and the counter-part wear volume. For the dry-unpolished experiment, the CoF curves in FIG. 7 suggest three categories of materials: The lowest friction coefficient with a tendency to further decrease was observed on the ta-C coated sample. However, the curve shows peaks which likely have been caused by the hard droplets of the coating. The highest friction is observed for Cr.sub.1.0N.sub.1.0 and Al.sub.0.76Mo.sub.0.24N.sub.1.15 for which CoF values of approximately 0.8 and above were obtained. The MoN-based coatings, although having similar surface roughness to samples A and C, have lower and share similar friction coefficients (between 0.60 and 0.65), resulting in a medium friction range. The investigation of wear after the test by optical microscopy is shown in FIGS. 8a and b. The figures compare the surface of the coating (above) and the wear cap of the counter-part (below) for all samples after the test. The Cr.sub.1.0N.sub.1.0 (sample A) shows considerably material transfer from the counter-part to the wear track. This is different for sample B (ta-C). The wear track was flattened with no pronounced material transfer after the test. In contrast, the counter-part received carbon from the coating. The micrograph of sample C suggests a combination of both and indicates an undefined tribological situation, an assumption which is also supported by its non-constant CoF behaviour (FIG. 7). For the Mo—N based coating materials, no remarkable material transfer from the counter-part to the coated samples is visible. There is, however, material transfer from the coating to the counter-part. The wear of the counter-part is different among these samples. The wear volume is highest for Mo.sub.1.0N.sub.1.0 (1.83*10.sup.7 μm.sup.3) and lowest for Mo.sub.0.85Cu.sub.0.15N.sub.1.0 (1.36*10.sup.6 μm.sup.3). The latter shows the lowest wear of the counter-part for unpolished surfaces and dry sliding conditions.

(38) The time dependence of the CoF for the polished samples under lubricated conditions is depicted in FIG. 9 for which the test duration was increased to 122.5 min. The CoF.sub.fin of ta-C is reduced to 0.10. Despite the Mo-DTC lubrication, the curve is characterized by spikes. This coincides with the already discussed insufficient polishing which did not remove the strongly adherent droplets from the amorphous ta-C matrix. During graphitization of the ta-C which is indicated by the coverage of the counter-part, the gradual release of the droplets may produce these spikes. Sample C (Al.sub.0.76Mo.sub.0.24N.sub.1.15) has the largest value with CoF.sub.fin of 0.17. In comparison to the amorphous appearance of sample B, the X-SEM micrograph of sample C shows a large number of grains and droplets in the layer and at the surface. This supports crack propagation and the generation of debris. Cr.sub.1.0N.sub.1.0 and Mo.sub.1.0N.sub.1.0 have the lowest friction coefficients with 0.07, while Mo.sub.1.0N.sub.1-xO.sub.x and Mo.sub.0.85Cu.sub.0.15N.sub.1.0 share a somewhat higher CoF.sub.fin of 0.09. The micrographs of the wear tracks and the counter-part are shown in FIGS. 10a and b. The wear track of sample A does not indicate any coating wear and also the counter-part gives the impression of no wear. The cap of the counter-part which is shown by the micrograph is rather attributed to a reversible deformation of this area during the test than to real material removal. Sample B shows reduced wear (7.29*10.sup.5 μm.sup.3) of the counter-part under lubricated conditions. The ta-C coating seems stable, although at about 1500 s a step-like decrease of the CoF is visible which is probably to a change in contact area. However, some scratch traces can be seen from the hard carbon droplets which are released during the test. Al.sub.0.76Mo.sub.0.24N.sub.1.15 also forms a stable coating for which no wear can be detected by optical microscopy. The counter-part wear is highest among the investigated samples. The debris generation already discussed above or sharp sized hard droplets may prevent a better polishing. Based on the wear cap diameter, samples D, E and F show a wear behaviour similar to Cr.sub.1.0N.sub.1.0. The slight brownish coloration of the counter-part for the Mo.sub.1.0N.sub.1.0 sample is attributed to a degradation or decomposition of the oil or the additive which does not affect the coating stability in the test. Also Mo.sub.0.85Cu.sub.0.15N.sub.1.0 (sample F) generates a wear cap similar to Cr.sub.1.0N.sub.1.0. Outstanding is the situation for the Mo.sub.1.0N.sub.1-xO.sub.x samples. A wear of the counter-part is not detectable by optical microscopy and the ring is attributed to the reversible deformation of the counter-part. Despite of the different indentation hardnesses and indentation moduli of the coatings, the similar size of the visible wear cap diameters of samples A, D, E and F suggest this assumption. It was verified for sample F by an evaluation of the contact area which stems from elastic deformation only. The given test parameters (diameter of the counter-part and the load), the thickness of the Cr.sub.xN interface and of the Mo.sub.1.0N.sub.1-xO.sub.x coating and the mechanical constants given in Table 1 were utilized for this contact evaluation. The E.sub.n of the hardened steel disk (substrate) and of the steel counter-part were measured to be 231 GPa and 222 GPa, respectively. Poisson's ratios of 0.25 and 0.3 were assumed for the coating and the hardened steel (disk and counter-part). The elastic contact region was evaluated applying a layered contact model which uses an extended Hertzian approach. As result, a contact diameter of 104.9 μm was obtained, which coincides rather perfectly with the measured radius (97 μm) of the circle in FIG. 10b (middle part). Similar good agreement has also been found for the wear diameters of the other three samples.

(39) Findings from the Experiments and from the Coating Analysis

(40) Reactive cathodic arc evaporation is a versatile approach to synthesize complex thin film materials. There is no need for a sophisticated control of the reactive gases to avoid target poisoning like it is needed in sputtering technology. Additionally, composite targets support a nearly unlimited coating design which can be obtained by this method. A comparison of the chemical composition of the metallic constituents in the synthesized material with the target material demonstrates both possibilities: the target composition could be maintained for sample F (Mo.sub.0.85Cu.sub.0.15N.sub.1.0), while for sample C (Al.sub.0.76Mo.sub.0.24N.sub.1.15) a remarkable “loss” of Al was measured. Due to the low substrate bias of only −40 V and the exclusive utilization of nitrogen and no argon gas, re-sputtering from the substrate site during deposition is unlikely.

(41) The explanation of the effect of aluminium loss in the coating needs more detailed investigations. Important is that the composition of the deposited layers is reproducibly controlled by the target composition as it is shown in our experiments. It is believed that future requirements dictate very specific material design which involves besides nitrides and oxi-nitrides also pure oxides for the optimization of tribological systems. Cathodic reactive arc evaporation combines the ease of reactive gas control with the freedom in target design to respond to these requirements. The generation of droplets associated with non-filtered cathodic arc deposition process evoke scepticism about the utilization of the coatings for tribological applications. This is indeed a problem for materials which are difficult to polish (ta-C) by standard methods or which exhibit sharp and hard contours or produce hard debris also under lubricated conditions (Al.sub.0.76Mo.sub.0.24N.sub.1.15). However, droplet formation may also be utilized to create holes in the coating by intention or to generate droplets which can be easily removed during polishing. FIG. 6b shows the surface of the Mo.sub.0.85Cu.sub.0.15N.sub.1.0 sample. The surface of the coating is characterized by a number of droplets and by holes. During polishing, the large droplets can be easily removed which is reflected in a decrease of Mr1 from 15.8 to 9.1, i.e. by more than 40%. This means that large Rz values are decreased drastically. The reduction in Mr2 from 91.6 to 89.0 (decrease of less than 3%) is, however, only marginal. This means, that the “valley” character of the coating is maintained while large peaks are removed. The “valleys” are also maintained for ta-C. But in this case it is due to the difficulties in polishing which results in a nearly unchanged surface (even small increase in Mr1). “Valleys” may serve as reservoirs for the lubricant and improve the tribological performance of the system. A target design promoting the creation of “soft” droplets which are only loosely integrated into the harder coating matrix is therefore an additional potential for the arc deposition technology.

(42) The reciprocating wear test was selected for the investigations because it has the potential to model wear behaviour in engines. The test can be conducted under different sliding conditions and can be varied with respect to counter-part material, lubricant, additives, temperature, and contact pressure. The test results indicate that all investigated coatings have too high surface roughness in the as deposited state resulting in material transfer or too high coefficients of friction. During running-in the material transfer or material transformations are different from those at the end of the test because of the higher contact pressure and the continuous changing contact conditions. We restricted ourselves on the status at the end of the test. For samples A and C, material transfer from the counter-part to the coating is visible which may represent an effect similar to “scuffing”. For ta-C, a material transfer to the counter-part is observed which indicates that the hard ta-C is graphitized, leading to a form of solid lubrication. The coverage of the counter-part by this graphitized carbon is responsible for the low friction coefficient under dry conditions. It demonstrates the potential of ta-C for tribological applications under dry conditions. Polishing of the ta-C surface is, however, difficult and may not allow low cost applications. The coatings with a CoF between 0.6 and 0.65 do not show a material transfer to the coating at the end of the test. This may indicate “no scuffing” effects under dry or deficient lubrication, which is an important requirement for engine applications. However, Mo.sub.1.0N.sub.1.0 and Mo.sub.1.0N.sub.1-xO.sub.x cause high wear of the counter-part due to their high surface roughness and hardness. If there exist a solid lubrication effect for these two materials it is only active for the coated sample but not for the counter-part. ta-C (3.40*10.sup.6 μm.sup.3) and Mo.sub.0.85Cu.sub.0.15N.sub.1.0 (1.36*10.sup.6 μm.sup.3) provoke the lowest wear of the counter-part among the unpolished samples although these coatings possess a high microhardness. Even the soft Cr.sub.1.0N.sub.1.0 induces slightly higher wear. The optical micrographs (FIGS. 8a and b) show for samples B and F a coverage of the counter-part by material which stems from the coated surface and reduce the counter-part wear. The results suggest that a post-treatment of the coatings deposited by arc evaporation is necessary or strongly recommended. However, surface finishing methods for e.g. piston rings (e.g. for Cr.sub.xN) have been already in production since years and may easily applied also to new coating materials.

(43) The results from the reciprocating wear test for the polished samples under lubricated conditions show an increase in performance for most of the materials. All coatings have low or no wear. Only in the ta-C layer, a few scratch patterns can be observed which again demonstrate wear by the very hard droplets. This and the small effect of the polishing (only 10% reduction in roughness) result in remarkable wear of the softer 100Cr6 steel counter-part. The counter-part wear for samples A, D, E and F is negligible. The wear cap diameter visualizes only the elastic deformation of the counter-part.

(44) In summary, the investigations show that cathodic arc evaporation can realize a large variety of coating materials which may be utilized to optimize tribological systems in reciprocating engine applications. The results from the reciprocating wear test may be used as a first step of optimization or a pre-selection of coatings for further and more expensive tests or application in real engines. Mo-based materials showed promising results under the test conditions, especially for Mo.sub.0.85Cu.sub.0.15N.sub.1.0 and for Mo.sub.1.0N.sub.1-xO.sub.x multilayer structures. For Mo.sub.0.85Cu.sub.0.15N.sub.1.0, it seems possible to turn the disadvantage of droplet generation in cathodic arc evaporation into an advantage by removing weakly adhering droplets by polishing and keeping the holes in the coating which may act as lubricant reservoirs.

(45) Further Important Aspects of the Present Invention

(46) FIG. 7 shows that a few coating materials prevent the material transfer from the counter-part to the coated part. This is an important aspect in the design of tribological systems because material transfer means wear and reduced life time of the components in tribological system. Besides ta-C, the favoured coating materials to prevent the material transfer are MoN-based. Responsible for this is the solid lubrication which occurs for these materials at the surface under sliding conditions: graphitization for ta-C and oxidation for MoN-based materials. Solid lubrication can also be achieved or improved from soft material if these materials can be incorporated in hard coatings. This is the case if MoN is doped by Cu with the effect that not only material transfer to the coated part is prevented, but also the wear of the counter-part under dry and unpolished conditions is reduced. In this case obviously two effects play a role in improving sliding conditions. The first is again the self-lubricating effect caused by copper under high contact pressure based on a form of liquefaction. Depending on counter-part material, a coverage or partial coverage of the counter-part with copper will happen. In addition, another effect can be observed in the Mo—Cu—N coating. This is the creation of “valleys” or holes in the coating stemming from droplets during the evaporation of the target material. It is in this case a desirable by-product of arc evaporation from a composite target. These droplets are only loosely incorporated in the coating (in contrast to ta-C) and form already before polishing a surface with a lot of holes. The copper content of the droplets is higher compared to the Cu content in the matrix coating and the mechanical integration of the soft droplets in the matrix is only insufficient. This is at least partial due to the difference in the hardnesses between matrix coating and droplet and is also reflected by the marginal impact of polishing on Mr2. It indicates that already before polishing, a matrix coating with reservoirs for lubricants of solid and/or liquid nature was generated. This is an important design tool for coated surfaces and rather obvious for liquid lubrication. However, it becomes an essential tool for designing surfaces utilized in high temperature application for which normal lubricants are not suitable due to their high vapour pressure or insufficient chemical stability. For those applications, the “valleys” might be formed in the solid lubricant coating (Mo—Cu—N).

(47) However, the utilization of solid lubricant coatings may not always deliver the best solution. This is indicated for the results displayed in FIG. 9. While the MoN-based coatings with “built-in” or designed self-lubrication properties (Mo—Cu—N and Mo—O—N) show CoF.sub.final of 0.09, the values for Mo—N and Cr—N are only 0.07. As discussed above, all four coatings show no or only negligible wear. However, sliding conditions are better for the pure metal-nitride coatings. This is believed to be the result of better wettability of the metal-nitride surfaces by the lubricant and its additives.

(48) [The issues discussed above suggest the following technological improvements for coated surfaces under sliding dry as well as lubricated and polished as well as unpolished conditions:

(49) The combination of a running-in layer of Mo—Cu—N or Mo—O—N with Cr—N or another metal-nitride coating.

(50) In additional experiments, oxide coatings produced by cathodic reactive arc evaporation were investigated with respect to their sliding properties by the reciprocal wear test. The CoF for Zr—O, Al—Cr—C—O, and Al—Cr—O coatings are shown in FIGS. 11a, b and c, respectively. The coatings have some behaviour in common: they show very high CoF for dry conditions with only minor difference for unpolished and polished surfaces. This is probably due to the formation of debris of the oxide coatings in non-lubricated conditions. However, in the lubricated experiments with polished surfaces similar or even reduced wear is visible compared to the MoN-based or CrN-based coatings. For the Al—Cr—O coating, the wear cap diameter is lowest and only about 180 μm.

(51) This suggests a second technological improvement:

(52) The combination of a self-lubricant coating (Mo—Cu—N or M-O—N) with an oxide containing coating to improve the running-in properties of the oxide coatings. The oxide coating can be polished before the deposition of the self-lubricant coating or the self-lubricated coating must be thick enough to reduce the intrinsic surface roughness of the oxide coating underneath.

(53) It is known from literature that many oxide coatings form temperature stable compounds with much better mechanical stability at high temperatures than normal metal-nitride coatings do. In experiments not shown here, pure Cr—N coatings are completely unstable under the utilized sliding conditions at 200° C., while e.g. Al—Cr—O coatings show no wear. In these test, an alumina counter-part was used.

(54) In experiments for which the temperature was increased to 800° C. (again with the alumina counter-part), it could be observed that the CoF of the Al—Cr—O coating could be reduced from 0.6 at RT to 0.4 (dry conditions). Although this is a reduction of 30%, it is of course far from the CoF which can be obtained under lubricated condition at RT. At 800° C., however, normal lubrication does not work. Although, the inventors are not able to explain the self-lubrication process at high temperatures, sliding tests showed that the creation of “valleys” either in the oxide coating or in the self-lubricating coating improves the sliding conditions. The fraction of “valleys” (Mr2) could also be increased by polishing of the oxide coatings or the combination of self-lubricated overcoat and oxide coating.

(55) Based on the investigations above, this suggests another technological improvement:

(56) The combination of a temperature stable solid-lubrication coating with holes in the matrix coating covering the temperature stable oxide coating which may also show holes in the surface.

(57) The coating systems comprising a self-lubricated outermost layer according to the present invention could be particularly beneficial for applications in tribological systems which must be operated under dry conditions. For example, for applications in tribological systems which are operated at elevated temperatures at which it is not possible to use any lubricant. In the case of a sliding tribological system, it comprises at least a first and a second sliding component and each sliding component has at least one sliding surface to be exposed to tribological contact (relative movement of one sliding surface in respect to the other). For avoiding or reducing wear of the involved sliding components, the sliding surface of the first sliding component and/or the sliding surface of the second sliding component should be coated (at least partially) with a coating system according to any of the embodiments of the present invention which includes a self-lubricated outermost layer.

(58) Tables

(59) TABLE-US-00001 TABLE 1 The most relevant deposition parameters, the chemical composition of the coatings and the mechanical properties of the coatings. Thickness Inter face/ Functional Compositional Analysis Cathode Process T.sub.DEP Layer of the Coating H.sub.IT E.sub.IT Sample Material Gases [C] [μm] RBS EDX [N/mm.sup.2] [GPa] A Cr N.sub.2 450   0/16.1 Cr.sub.1.0N.sub.1.0 Cr 13728 ± 1064 280 ± 12 B Graphite Ar 150 <0.1/1.7  C (H < 1 at. %) C 52394 ± 7195 409 ± 31 C Al.sub.80Mo.sub.20 N.sub.2 450 5.2/10.6 Al.sub.0.76Mo.sub.0.24N.sub.1.15 Al.sub.74Mo.sub.26 27674 ± 2148 257 ± 18 D Mo N.sub.2 450 4.6/9.8  Mo.sub.1.0N.sub.1.0 Mo 19243 ± 1918 409 ± 58 E Mo N.sub.2, O.sub.2 450 4.8/11.2 Mo.sub.1.0N.sub.1−xO.sub.x Mo 28814 ± 1527 387 ± 15 F Mo.sub.85Cu.sub.15 N.sub.2 400 1.7/2.9  Mo.sub.0.85Cu.sub.0.15N.sub.1.0 Mo.sub.85Cu.sub.15 28439 ± 2648 351 ± 34

(60) TABLE-US-00002 TABLE 2 Measurement of the surface roughness before and after polishing. Surface roughness Surface roughness before polishing after polishing R.sub.z R.sub.a R.sub.pk R.sub.vk R.sub.z R.sub.a R.sub.pk R.sub.vk Sample [μm] [μm] [μm] [μm] Mr1 % Mr2 % [μm] [μm] [μm] [μm] Mr1 % Mr2 % A 3.28 0.43 1.01 0.24 18.3 94.1 0.27 0.02 0.03 0.04 9.5 89.1 B 1.73 0.23 0.53 0.26 17.6 93.8 1.65 0.23 0.55 0.18 20.5 94.3 C 4.29 0.54 1.32 0.70 15.0 93.2 1.36 0.11 0.17 0.25 10.8 86.3 D 4.31 0.46 1.43 0.21 17.5 94.1 0.46 0.03 0.03 0.13 8.7 81.9 E 3.46 0.44 1.19 0.22 17.9 94.4 0.64 0.07 0.11 0.15 12.6 88.9 F 3.75 0.36 1.17 0.29 15.8 91.6 0.57 0.03 0.04 0.10 9.1 89.0 Steel 0.19 0.02 0.03 0.02 9.31 90.44 n/a n/a n/a n/a n/a n/a Substrate

(61) TABLE-US-00003 TABLE 3 Characterization of the tribology by the SRV test for unpolished samples under dry conditions and polished samples under lubricated conditions. The wear volume was calculated after [24, page 17(2)]. Unpolished/Dry Conditions Polished/Lubricated Conditions (after 12.5 min) (after 122.5 min) Ø Wear Ø Wear Cap Cap Material Transfer [μm] Material Transfer [μm] .fwdarw. Wear .fwdarw. Wear .fwdarw. counter Volume .fwdarw. counter Volume Sample CoF.sub.fin coating part [μm.sup.3] CoF.sub.fin coating part [μm.sup.3] A 0.79 Yes No 876 0.07 No No 219 5.78 * 10.sup.6 2.26 * 10.sup.4 B 0.18 No Yes 767 0.10 Scratches Yes 522 3.40 * 10.sup.6 7.29 * 10.sup.5 C 0.87 Yes Yes 1135  0.17 No Yes 1009  1.63 * 10.sup.7 1.02 * 10.sup.7 D 0.64 No Yes 1169  0.07 No (Yes) 225 1.83 * 10.sup.7 2.52 * 10.sup.4 E 0.64 No Yes 1076  0.09 No No 206 1.32 * 10.sup.7 1.77 * 10.sup.4 F 0.63 No Yes 610 0.09 No No 194 1.36 * 10.sup.6 1.39 * 10.sup.4