Friction components, mechanical systems incorporating such friction components, and methods of operation.

Abstract

DEPCT6330 / 08 / 2562 This invention involves a friction piece (10) suitable for operation. In a lubricating medium at temperatures above 200 degrees Celsius, the workpiece (10) is composed of metal surfaces (12) and Outer coating (14) which consists of tungsten carbide filled with nitrogen WC(N) Containing a nitrogen atomic ratio between 5 and 12%, the invention also involved a mechanical system. (1) Comprising the aforementioned workpieces (10), this invention concerns a method for Use the above workpiece(10) ----------------------------------------------------------- DEPCT63 This invention involves a friction piece (10) suitable for operation. In a lubricating medium at temperatures above 200 degrees Celsius, the workpiece (10) is composed of metal surfaces (12) and The outer coating (14) is made of tungsten carbide filled with nitrogen WC(N) containing With an atomic ratio of nitrogen between 5 and 12%, the invention also involved a mechanical system. (1) Comprising the aforementioned workpieces (10), this invention concerns a method for Give the work piece as mentioned above (10) -----------------------------------------------------------

Description

[0001] FRICTION PART, MECHANICAL SYSTEM COMPRISING SUCH A FRICTION PART, AND METHOD OF IMPLEMENTATION

[0002] The present invention relates to a friction part capable of being subjected to high temperatures in a lubricated environment. The invention also relates to a mechanical system comprising such a part. The invention further relates to a method for implementing such a part.

[0003] The field of the invention is that of metallic friction parts coated with a thin layer, allowing a reduction of friction in a lubricated environment.

[0004] As a non-limiting example, the friction part could be a piston ring in an automotive engine, and more specifically the top ring. In practice, these rings are subjected to operating temperatures exceeding 200°C, in a potentially lean lubricated environment.

[0005] According to other non-limiting examples, the thin-film coating can be applied to a piston pin, piston skirt, any other piston component, or any other part that might be subjected to high-temperature operation. In industrial applications, the presence of a lubricating film separating two friction surfaces results in low wear levels, thanks to the complete absence of contact between these two parts.

[0006] In some cases, a continuous film cannot be maintained over time, leading to direct interactions between the two parts. Discontinuity in the lubricant film is the main characteristic of lubrication under "mixed regime" and "limit regime" conditions.

[0007] In the automotive industry, DLC (Diamond-Like Carbon) thin-film coatings are widely used to reduce friction between parts. In practice, the reduction of friction between DLC-coated parts occurs primarily in mixed lubrication systems.

[0008] A key characteristic of DLC coatings is that surface roughness decreases over time, tending towards particularly low values. Thus, the lubrication transition from the limit regime to the mixed regime, or from the mixed regime to the hydrodynamic regime, shifts towards lower speed values. Unlike DLC-coated surfaces, initially polished steel surfaces experience an increase in roughness over time. By reacting with the surfaces, ZDTP-type anti-wear additives form islands that locally increase roughness.

[0009] Some lubricated contacts operate continuously at temperatures above 200°C, and transiently at even higher temperatures. These contacts are rather poorly lubricated, which explains the high temperatures observed at them.

[0010] Under such operating conditions, the wear of a DLC coating is relatively rapid. This wear exhibits stochastic behavior, likely due to the fact that lubrication is quite low and poorly controlled.

[0011] The DLC coating then exhibits wear through polishing, and the coating layer is consumed via a mechanism similar to oxidation. The carbon and hydrogen in the coating layer combine with ambient oxygen to form water and carbon dioxide. The depletion of lubricant and the increase in temperature at the contact interface between the friction parts lead to accelerated oxidation of the DLC coating, until its complete disappearance. The poorly controlled lubrication, combined with high temperatures, thus results in an increased wear rate.

[0012] Today, two approaches address the problems described above. The first approach involves using coatings made of chromium alloys or chromium nitrides, which offer high resistance under high-temperature, low-lubrication operating conditions, at the expense of friction reduction. This approach corresponds to older designs of mechanical systems, such as automotive engine combustion chamber rings, commonly coated with chromium-based layers. While this approach does not reduce friction losses compared to uncoated steel, it does protect surfaces against wear and seizing.

[0013] The second approach would involve improving the lubrication associated with a DLC coating, which would allow for better heat dissipation and protection of the coating against oxygen. This approach would require modifying the design of the mechanical systems for distributing the lubricant, as well as increasing the lubricant flow rates, and therefore the energy consumed to ensure its circulation. In practice, this second approach is not implemented due to the increased design costs and the fact that the energy efficiency provided by the coating would be offset by the effort required to achieve ample lubrication.

[0014] The aim of the present invention is to provide an improved friction part, remedying the above drawbacks.

[0015] For this purpose, the invention relates to a friction part, comprising a metallic surface and an external coating layer, characterized in that the external coating layer is composed of nitrogen-doped tungsten carbide WC(N) with between 5 and 12 atomic % of nitrogen.

[0016] Thus, the invention makes it possible to reduce the friction experienced by the coating layer in a lubricated environment, in particular in mixed lubrication regimes, while improving its resistance to high stress temperatures, especially above 200°C.

[0017] Compared to a DLC coating, the coating materials according to the invention are much more resistant to hot wear, due to their better thermal stability and resistance to oxidation, while providing a satisfactory reduction in friction.

[0018] Compared with a coating of the type chromium nitride CrN or tungsten carbide WC (with 6% Co by mass), the nitrogen-doped tungsten carbide WC(N) coating materials according to the invention make it possible to lower friction, while providing satisfactory oxidation resistance.

[0019] Some advantageous features of the invention, taken individually or in combination, are detailed below.

[0020] Preferably, the outer coating layer comprises at least one element selected from cobalt, nickel, and iron, for a total of between 5 and 11 atomic percent. The additive element may be pure and present between 5 and 11 atomic percent in the coating layer. Alternatively, several additive elements may form a mixture, for a total of between 5 and 11 atomic percent in the coating layer. In a particular embodiment, the outer coating layer comprises:

[0021] between 40 and 43 atomic percent of carbon,

[0022] between 40 and 43 atomic percent of tungsten,

[0023] - between 5 and 12 atomic percent of nitrogen,

[0024] at least one element chosen from cobalt, nickel and iron, for a total between 5 and 11 atomic percent,

[0025] other compounds for a total of between 0 and 10 atomic percent. According to a preferred embodiment, the outer coating layer is made of nitrogen-doped tungsten carbide WC(N), with:

[0026] between 40 and 43 atomic percent of carbon,

[0027] between 40 and 43 atomic percent of tungsten,

[0028] between 7 and 9 atomic percent of cobalt,

[0029] - between 5 and 12 atomic percent of nitrogen,

[0030] other compounds for a total between 0 and 8 atomic %.

[0031] In practice, the external coating layer is formed by vacuum spraying of a WC tungsten carbide target, with the introduction of a nitrogen flow.

[0032] Cobalt, nickel and / or iron constitute an element of the target.

[0033] Typically, the manufacturing of the friction part includes a step of pickling the metal surface to be coated, in particular ion etching, before the application of the outer coating layer. According to a particular embodiment, the metal surface is coated only with the outer coating layer, excluding the presence of an undercoat between the metal surface and the outer coating layer.

[0034] Alternatively, the metal surface is coated with at least one underlayer formed beneath the outer coating layer. This underlayer is, for example, composed of chromium or chromium nitride.

[0035] The invention also relates to a mechanical system comprising a first friction element as described above, a second friction element disposed in lubricated contact with the first friction element, and a lubricant disposed at a lubricated contact interface between the friction elements. In a particular embodiment, the second friction element is similar to the first friction element. That is, the second friction element has a metallic surface and an external coating layer deposited on the metallic surface, the external coating layer of the second friction element being composed of nitrogen-doped tungsten carbide WC(N) with between 5 and 12 atomic percent nitrogen.

[0036] Preferably, the outer coating layer of the first friction piece and the outer coating layer of the second friction piece have the same composition.

[0037] Alternatively, the facing coating layers can have different compositions, but with 5 to 12 atomic percent nitrogen.

[0038] The invention also relates to a method for implementing a friction part as mentioned above. The method is characterized in that it comprises steps consisting of:

[0039] arrange a second friction piece in lubricated contact with the friction piece;

[0040] apply a lubricant to a lubricated contact interface between the friction parts; and

[0041] actuate the friction parts, so that the lubricated contact interface between the friction parts reaches a temperature above 200 °C.

[0042] The invention will be better understood upon reading the following description, given solely by way of non-limiting example and made with reference to the accompanying drawings in which:

[0043] Figure 1 is a partial schematic representation of a mechanical system according to the invention, in cross-section;

[0044] Figure 2 is a representation analogous to Figure 1, showing a mechanical system conforming to a second embodiment of the invention;

[0045] Figure 3 is a representation analogous to Figure 1, showing a mechanical system conforming to a third embodiment of the invention.

[0046] Figure 4 is a schematic representation of a test bench, used to identify coating materials conforming to the invention;

[0047] Figures 5, 6, and 7 are graphs illustrating different stages of identifying coating materials according to the invention. Figure 1 shows, partially and schematically, a mechanical system 1 according to the invention.

[0048] The mechanical system 1 comprises two friction parts 10 and 20 separated by a lubricant 3 at their contact interface 2. In practice, this interface 2 is subjected to operating temperatures exceeding 200°C. Under these conditions, the lubricant 3 is likely to become depleted.

[0049] Parts 10 and 20 are subject to relative translational and / or rotational motion. For the sake of simplicity, parts 10 and 20 can have any shape and arrangement suitable for the intended application, in other words, suitable for the type of mechanical system 1. Parts 10 and 20 are made of metal, preferably steel.

[0050] The part 10 comprises a body 11, an external surface 12, and an external coating layer 14 deposited on the surface 12. The layer 14 has an internal surface 15 and an external surface 16. The surface 15 is bonded to the metal surface 12 during the deposition of the layer 14. The surface 16 faces the part 20 at the lubricated contact interface 2.

[0051] Part 20 comprises a body 21 and an external surface 22. Part 20 has no external coating layer. Surface 22 faces part 10 at the lubricated contact interface 2.

[0052] In boundary or mixed regime, the surface 16 of the coating layer 14 and the surface 22 of the part 20 are in contact in places, at the lubricated contact interface 2 between the parts 10 and 20. Thus, the lubricant 3 forms a discontinuous film between the surfaces 16 and 22, at the interface 2.

[0053] The lubricant 3 is chosen according to the intended application, in other words, according to the type of mechanical system 1. For example, the lubricant 3 could be a commercially available automotive lubricant, such as an SAE 5W30 oil containing conventional additives. According to the invention, the coating layer 14 is a nitrogen-doped tungsten carbide compound WC(N), with between 5 and 12 atomic percent nitrogen included.

[0054] In practice, such a coating layer 14 is well suited for operation in a lubricated environment at temperatures above 200°C. Thus, the part 10 incorporating the coating layer 14 is suitable for operation in a lubricated environment at temperatures above 200°C. Unexpectedly, doping tungsten carbide layers with nitrogen leads to a reduction in friction in a lubricated environment, compared to conventional materials, including undoped or lightly doped WC tungsten carbide (atomic percentage of nitrogen less than 5%).

[0055] Figures 2 and 3 show, partially and schematically, mechanical systems 1 conforming to other embodiments of the invention. The constituent elements of system 1, which are similar to the first embodiment, bear the same numerical references. Only the differences with the first embodiment are detailed below.

[0056] In Figure 2, part 10 has a sub-layer 18 formed between the body 11 and the coating layer 14. The surface 12 is coated first with the sub-layer 18, then with the coating layer 14. By way of non-limiting example, the sub-layer may be composed of chromium or chromium nitride. Chromium nitride may be deposited by magnetron reactive sputtering.

[0057] In Figure 3, part 20 has an external coating layer 24 deposited on surface 22. Layer 24 has an internal surface 25 and an external surface 26. Surface 25 is bonded to the metal surface 22 during the deposition of layer 24. Surface 26 faces part 10 at the lubricated contact interface 2. The lubricant 3 forms a discontinuous film between surfaces 16 and 26 of the coating layers 14 and 24, at the lubricated contact interface 2 between parts 10 and 20.

[0058] According to a particular embodiment, part 20 also conforms to the invention. In other words, the external coating layer 24 has a nitrogen-doped tungsten carbide composition WC(N) with between 5 and 12 atomic percent nitrogen. In this particular case, where both surfaces 12 and 22 of parts 10 and 20 are coated, layers 14 and 24 preferably have a similar composition.

[0059] Alternatively, part 20 may include an external coating layer 24 having a different composition than layer 14.

[0060] Furthermore, part 20 may include an underlayer formed between the body 21 and the coating layer 24, as described above for part 10 with reference to Figure 2. The method for identifying coating materials capable of withstanding lubrication and high-temperature conditions is detailed below. This method consists of depositing a coating layer onto steel test specimens, then characterizing the different coatings, firstly, by friction in a lubricant and, secondly, by resistance to a temperature of 350°C.

[0061] The coating materials tested are: chromium nitride CrN, tungsten carbide WC, nitrogen-doped tungsten carbides WC(N), DLC type a- C:H.

[0062] Prior to coating, all test specimens undergo the same preparation steps. The specimens are degreased and positioned on a rotating sample holder with a planetary motion, according to state-of-the-art practices for vacuum deposition of hard thin films. The vacuum chamber is then pumped. This pumping is carried out simultaneously with heating to 150°C, which activates desorption processes and improves vacuum quality. After 2.5 hours of heating, the pressure measured by a Penning gauge is verified to be less than 2 x 10⁻¹¹ bar. "5mbar. The steel specimens are then ionically cleaned in an argon plasma for one hour under a voltage of -150V applied to the specimens. After cleaning, the passivation oxide on the specimens has disappeared, and the surfaces are ready to receive a deposit. The chromium nitride is deposited by reactive magnetron sputtering. During the final stages of the ionic cleaning of the specimen, the magnetron cathode is activated at 5kW behind a pre-sputtering shield to clean its surface for 5 minutes. At the end of the cleaning, the ionic assist plasma is maintained, the bias voltage is reduced to -50V, and the shield is opened to begin depositing a thin layer of chromium. During the deposition of this layer, the intensity emitted by the chromium atoms in the plasma is measured at a wavelength of 520 nm.This light intensity is standardized to 100, and a flow of nitrogen is introduced until the light emitted by the chromium decreases to 50% of the intensity emitted by pure argon. After 3 hours of deposition, a CrN layer 2.7 μm thick and with a hardness of 1900 Hv under 10 mN is obtained.

[0063] The tungsten carbide deposition is achieved by spraying a tungsten carbide target containing 6% cobalt binder by mass. The target is cleaned under a protective cover for 5 minutes at the end of the ionic cleaning process. Once the ionic cleaning is complete, the ion-assisted plasma is switched off and the pre-spraying cover is opened to begin the WC deposition. After 3 hours of deposition, a layer 2.1 μm thick with a hardness of 1900 Hv under 10 mN is obtained.

[0064] Nitrogen-doped tungsten carbide deposits are made by introducing a flow of reactive gas into the sprayed stream.

[0065] Regardless of the nitrogen flow rate, starting from 60 sccm of nitrogen, the coating layer composition is stable, and EDX analyses indicate a composition of 40 at.% carbon, 12 at.% nitrogen, 8 at.% cobalt, and 40 at.% tungsten. Under these conditions, nitrogen-doped deposits exhibit a hardness of 2900 Hv, significantly higher than that of a WC deposit without reactive gas.

[0066] The characteristics of the deposits are summarized in Table 1 below. The deposits labeled WC0 to WC4 are carried out with increasing nitrogen flow rates until nitrogen saturation is reached (60 sccm in the specific case of the deposition conditions used here). Table 1 - Nitrogen-doped tungsten carbide deposits

[0067]

[0068] Finally, the aC:H type DLC coating is achieved by combining PVD deposition technology to deposit a WC-based underlayer, which is progressively enriched with carbon to obtain a composition suitable for promoting the adhesion of the PACVD-formed DLC layer, as described in document WO2012 / 156647. The resulting coating consists of a 0.8 μm thick W-based layer, on which a 2.2 μm thick DLC layer is deposited. The total coating thickness is 3 μm and the surface hardness is 3200 Hv.

[0069] Figures 4 to 7 illustrate an initial characterization test, consisting of a series of tribological tests on the specimens. Figure 4 is a schematic representation of a test bench 30, constituting a tribometer. The objective of the test series is to demonstrate the potential of specific coating materials to reduce friction in a lubricated environment, compared to steel-on-steel contact. To test only the effects of the coatings, it is ensured that the tests are carried out under identical conditions. In particular, all the specimens have the same initial roughness.

[0070] The test bench 30 is used to characterize friction within a mechanical system 40 consisting of two steel specimens 41 and 42, namely a cylinder 41 and a disc 42. Successive tests involve several systems 40, and therefore several specimens 41 and 42. In a reference system 40, the specimens 41 and 42 are uncoated, allowing for the characterization of steel-on-steel contact. In each of the other systems 40, the specimens 41 and 42 are coated with the same material. The test of two coated specimens 41 and 42 rubbing against each other corresponds to the configuration in Figure 3.

[0071] The 41 cylinders used for the tests are cylindrical steel rollers

[0072] 100Cr6, from a roller bearing. The cylinders 41 have a diameter of 10 mm. The surface of the cylinders 41 is polished on at least one generatrix until an arithmetic mean roughness Ra < 0.02 μηη is obtained, then this generatrix is ​​coated with the material to be tested.

[0073] The 42 discs used for the tests have a diameter of 25 mm and a thickness of 5 mm. A flat surface of each 42 disc is polished until an arithmetic mean roughness Ra < 0.02 μηη is obtained, then coated with the material to be tested.

[0074] To perform a test, the disc 42 is installed in a tank 32 filled with a commercial automotive lubricant, in this case an SAE 5W30 oil 43 containing standard additives. This oil 43 provides a coefficient of friction of approximately 0.12-0.13 for steel-to-steel contact under the lubrication limit. The disc 42 is fixed in the tank 32 by being immersed in the oil 43.

[0075] The cylinder 41 is mounted on a support 31 suspended by flexible steel blades, which allow the cylinder 41 to self-align with the polished surface of the disc 42 when the test specimens 41 and 42 are brought into contact. This self-alignment system is essential for the proper conduct of the tests, as it ensures that the geometry of the contact between the specimens 41 and 42 is perfectly controlled, so as not to introduce bias into the friction measurement. Aligning the specimens 41 and 42 in a cylinder-to-plane contact is the main challenge of these tests.

[0076] The support 31 of the cylinder 41 is connected to the rest of the tribometer by means of a piezoelectric sensor 33, placed opposite the lubricated contact between the coated generatrix of the cylinder 41 and the coated flat surface of the disk 42. The sensor 33 makes it possible to measure the tangential force Ft, from which the coefficient of friction is determined.

[0077] The lubricated contact interface between cylinder 41 and disc 42 measures 4 mm in length along the generatrix and 35 μm in contact width in the friction direction. Applying a normal load Fn of 21 N results in a contact pressure of 200 MPa. Besides the alignment of specimens 41 and 42, the contact pressure is a crucial factor in determining the coefficient of friction.

[0078] From a kinematic point of view, cylinder 41 wears faster than disc 42, regardless of the coating material. Therefore, it is crucial to ensure that cylinder 41 shows no significant wear after the test. Indeed, wear on cylinder 41 after the test would lead to a decrease in contact pressure due to an increased contact area. The observed reduction in friction would then no longer be solely attributable to the coating material, but also to the drop in contact pressure caused by wear. To avoid this problem, the coated disc 42 is lightly polished with 1200-grit SiC abrasive paper. This polishing removes surface peaks, resulting in a significant reduction in the wear rate of cylinder 41. The width of the friction trace is checked after the test. The test is rejected if the width of this trace exceeds 45 µm.In all the results presented, the width of the friction trace is between 35 and 45 μηη, indicating that no significant wear has occurred and therefore that the friction is indeed caused by the nature of the coating materials.

[0079] The disc 42 is free to move in translation following a reciprocating linear motion, thanks to a translation mechanism 34. In this case, the disc 42 is fixed in the tank 32, which is mounted on a ball rail 35 and connected by a rod 36 to a rotating eccentric element 37. The eccentric element 37 is driven in rotation by a motor, not shown for the sake of simplicity. This mechanism 34 imparts a reciprocating linear motion to the tank 32, and therefore to the disc 42, over a stroke of 10 mm, following a sinusoidal law.

[0080] The oil 43 is heated to 110°C in the tank 32, then the mechanism 34 drives the tank 32 in a reciprocating motion. A preliminary test of 1 hour is carried out under a normal load Fn of 21 N, with a rotational speed of the eccentric element 37 of 300 revolutions per minute. This preliminary test serves to break in the surfaces and stabilize the friction.

[0081] After obtaining stabilized friction, 3-minute tests are carried out at rotational speeds of the eccentric element 37 of 300, 450, 600, and 750 revolutions per minute. From these tests, the evolution of the friction coefficient as a function of the translational speed of the disk 42 is extracted.

[0082] Figure 5 is a graph illustrating a measurement carried out with the piezoelectric sensor 33 equipping the test bench 30, for a given coating material.

[0083] The x-axis represents time T in seconds. The left y-axis represents the tangential force Ft in newtons. The right y-axis represents the translational velocity V of disk 42 in mm / s.

[0084] The graph in Figure 5 shows a sinusoidal curve V42 representing the translational velocity V of the disk 42 and a curve Ft40 representing the tangential force Ft measured by the sensor 33, as a function of time T.

[0085] Figure 6 shows a curve Cfi representing the evolution of the friction coefficient Cf as a function of the instantaneous translational velocity V of the disk 42, based on measurements taken for a given coating material.

[0086] The x-axis represents the instantaneous velocity V in mm / s obtained from curve V42, while the y-axis represents the instantaneous friction coefficient Cf determined from curve Ft40. Figure 7 is a graph illustrating the evolution of the friction coefficient Cf for different coating materials. Each of the curves shown on this graph corresponds to the Cfi curve in Figure 6, for a given coating material.

[0087] The x-axis represents the instantaneous velocity V in mm / s, while the y-axis represents the instantaneous coefficient of friction Cf.

[0088] The graph in Figure 7 represents:

[0089] - a reference CfO curve corresponding to the result obtained with uncoated specimens 41 and 42 (steel / steel contact);

[0090] - a Cf1 curve obtained with chromium nitride coated specimens 41 and 42 (CrN / CrN contact);

[0091] - a Cf2 curve obtained with specimens 41 and 42 coated with undoped tungsten carbide reference WC0 (WC / WC contact);

[0092] - a Cf3 curve obtained with test specimens 41 and 42 coated with WC doped with nitrogen at 2 atomic % of reference WC1 (contact WC(N) / WC(N));

[0093] - a Cf4 curve obtained with specimens 41 and 42 coated with nitrogen-doped tungsten carbide at 5 atomic % reference WC2 (contact WC(N) / WC(N)); - a Cf5 curve obtained with specimens 41 and 42 coated with nitrogen-doped tungsten carbide at 9 atomic % reference WC3 (contact WC(N) / WC(N));

[0094] - a Cf6 curve obtained with test specimens 41 and 42 coated with 12% nitrogen-doped tungsten carbide reference WC4 (contact WC(N) / WC(N));

[0095] - a Cf7 curve obtained with test specimens 41 and 42 coated with DLC of type aC:H (DLC / DLC contact).

[0096] Two types of evolution of the coefficient of friction Cf can be distinguished in figure 7.

[0097] A first group of materials includes steel, CrN, undoped WC and the

[0098] WC(N) is lightly doped (nitrogen atomic percentage less than 5%). These materials are characterized by a coefficient of friction Cf greater than 0.12, which decreases only slightly as the sliding speed V increases. This level of friction is quite typical of the limit lubrication regime and the shearing of anti-wear reaction films (tribofilms) that grow on the surfaces, formed from the ZnDTP additives contained in oil 43. ZnDTP can decompose by reacting on a surface and form a tribofilm of Zn polyphosphates. Although the friction surfaces are initially polished, the friction in the limit lubrication regime is induced by the roughness of the tribofilms generated as islands. Observation of the planes and cylinders by optical microscopy reveals the presence of these islands, which appear blue and brown depending on their thickness.

[0099] A second group of materials comprises nitrogen-doped WC layers with a nitrogen content of 5 atomic percent or higher, corresponding to WC2, WC3, WC4, and aC:H type DLC. These materials are characterized by a coefficient of friction Cf that decreases as the sliding speed V increases. This behavior is typical of the mixed lubrication regime. The reduction in friction in this regime is made possible by the fact that the friction surfaces remain smooth after the test. The rate at which the coefficient of friction decreases reflects the roughness of the test specimens. Of all the materials tested here, DLC induces the greatest reduction in friction. Observation of the friction surfaces after testing does not clearly show the presence of tribofilms. These films are either absent or significantly thinner than conventional tribofilms.A smaller thickness leads to limiting the roughness on initially polished surfaces, since it is the island structure and the thickness that produce roughness.

[0100] For DLC, its non-metallic nature likely leads to its low affinity for ZnDTP. Doping tungsten carbide with oxygen or nitrogen appears to significantly reduce its affinity for ZnDTP. Unexpectedly, adding nitrogen to a WCCo coating above 5 atomic percent results in lower friction compared to undoped or lightly doped layers. A second characterization test of the coating materials is performed using coated flat specimens.

[0101] The test involves heating the test specimens to 350°C in air for 2 hours. The thickness of the coating layer is measured before heating, and then again after heating, to verify that the material has remained intact. Choosing a temperature of 350°C accelerates the oxidation process and allows the materials to be classified in a test lasting only a few hours, instead of several tens of hours.

[0102] Table 2 below shows the thickness measurements of each of the coating layers before and after heating. The deposit thicknesses are characterized using the heat sink.

[0103] Table 2 - Coating thicknesses before and after heating

[0104] Apart from the DLC, all coating layers exhibit the same thickness after heating (within measurement accuracy), indicating that their oxidation is negligible. Furthermore, this is confirmed by the fact that their visual appearance remains unchanged. The temperature and duration are too low for oxidation to be detectable. Conversely, the entire aC:H DLC coating has disappeared through oxidation. Only the WCC-type underlayer, which serves as the DLC's adhesion layer, remains. Visual examination of the specimen is sufficient to demonstrate the oxidation and the disappearance of the DLC, as the initially black surface has acquired a metallic sheen after oxidation. Table 3 below presents the characteristics of the coatings determined by the various tests described above. In particular, it shows the evolution of the coefficient of friction between sliding speeds of 50 mm / s and 300 mm / s, as well as the coatings' resistance to a moderate temperature of 350°C.

[0105] Table 3 - Coating characteristics

[0106]

[0107] CrN coatings, undoped WC (ref WC0), and lightly doped WC(N) (ref WC1) are hard coatings not conforming to the invention. Friction in a lubricated environment does not lead to a reduction in friction compared to polished, uncoated steel. These materials are capable of withstanding a moderate temperature of 350°C.

[0108] The DLC coating, widely used in lubricated environments, is also not in accordance with the invention. Indeed, the reduction in friction in a lubricated environment is dramatic, but exposure to a temperature of 350°C leads to the destruction of the coating by oxidation.

[0109] The WC(N) coatings of reference WC2, WC3, and WC4 conform to the invention. Unlike undoped or lightly doped WC, a significant reduction in friction is observed in a lubricated environment. Although this reduction is not as dramatic as with DLC, these materials have the advantage of not degrading upon exposure to a temperature of 350°C.

[0110] Those skilled in the art will understand that the technical characteristics of the various embodiments and variants mentioned in the description above can be combined, in whole or in part. Thus, the friction part 10 can be adapted in terms of cost, functionality, and performance.

Claims

DEPCT6330 / 08 / 25621. A friction instrument (10) with a metallic surface (12) and an outer coating (14), characterized by the outer coating (14) being composed of nitrogen-doped tungsten carbide WC(N) with a nitrogen atomic ratio between 5 and 12%.

2. A friction instrument (10) under Reputation 1, characterized by the outer coating (14) being composed of at least one element chosen from cobalt, nickel and iron, for an overall atomic ratio between 5 and 12%.

3. A friction instrument (10) under either Reputation 1 or 2, characterized by the outer coating (14) being composed of: - a carbon atomic ratio between 40 and 43%, - a tungsten atomic ratio between 40 and 43%, - a nitrogen atomic ratio between 5 and 12%, - at least one element chosen from cobalt, nickel and iron for an overall atomic ratio between 5 and 12%, - another compound for an overall atomic ratio between 0 and 10%.4.Friction pieces (10) under any of the claims 1 to 3, characterized by an outer coating (14) composed of: - an atomic ratio of carbon between 40 and 43%, - an atomic ratio of tungsten between 40 and 43%, - an atomic ratio of nitrogen between 5 and 12%, - an atomic ratio of cobalt between 7 and 9%, - an other compound for all atomic ratios between 0 and 8%.

5. Friction pieces (10) under any of the claims 1 to 4, characterized by a metal surface (12) coated with only the outer coating (14).

6. Friction pieces (10) under any of the claims 1 to 4, characterized by a metal surface (12) coated with at least one sublayer (18) made beneath the outer coating (14).

7. Friction pieces under claim 6, characterized by a sublayer (18) composed of chromium or chromium nitride. 8.Mechanical system (1), which consists of: - first friction element (10) according to one of the claims 1 to 7, - second friction element (20) which is arranged in lubricating contact with the first friction element (10); - lubricant (3) which is arranged at the lubricating contact interface (2) between the friction elements (10;20).

9. Mechanical system (1) according to claim 8, which is characterized by the second friction element (20) consisting of a metal surface (22) which is arranged in direct lubricating contact with the first friction element (10).

10. Mechanical system (1) According to claim 8, the characteristic is that the second friction element (20) is composed of a metal surface (22) and an outer coating (24) which is deposited on the metal surface (22), the outer coating (24) of the second friction element (20) is composed of nitrogen-doped tungsten carbide WC(N) with a nitrogen atomic ratio between 5 and 12%11. The mechanical system (1) according to claim 10, the characteristic is that the outer coating (14) of the first friction element (10) and the outer coating (24) of the second friction element (20) have the same composition12.The method for applying friction material (10) under any of the claims 1 to 7, which is characterized by the following steps: -arranging the second friction material (20) in lubricating contact with friction material (10); -arranging the lubricant (3) at the lubricating contact interface (2) between friction materials (10;20); and - driving the friction material (10;20), such that the lubricating contact interface (2) between friction materials (10;20) reaches a temperature above 200 degrees Celsius. --------------------------------------------------------DEPCT631. A friction instrument (10) with a metallic surface (12) and an outer coating (14), characterized by an outer coating (14) incorporating nitrogen-doped tungsten carbide WC(N) with a nitrogen atomic ratio between 5 and 12%.

2. A friction instrument (10) according to claim 1, characterized by an outer coating (14) incorporating at least one element chosen from cobalt, nickel and iron, for an overall atomic ratio between 5 and 12%.3.Friction pieces (10) under one of the claims 1 or 2, which are characterized by an outer coating (14) composed of: - an atomic ratio of carbon between 40 and 43%, - an atomic ratio of tungsten between 40 and 43%, - an atomic ratio of nitrogen between 5 and 12%, - at least one element chosen from cobalt, nickel and iron for an overall atomic ratio between 5 and 12%, - another compound for an overall atomic ratio between 0 and 10%.

4. Friction pieces (10) under one of the claims 1 or 2, which are characterized by an outer coating (14) composed of: - an atomic ratio of carbon between 40 and 43%, - an atomic ratio of tungsten between 40 and 43%, - an atomic ratio of nitrogen between 5 and 12%, - at least one element chosen from cobalt, nickel and iron for an overall atomic ratio between 5 and 12%, - another compound for an overall atomic ratio between 0 and 10%. In one of the claims 1 through 3, the characteristic is that the outer coating (14) is composed of: - an atomic ratio of carbon between 40 and 43%, - an atomic ratio of tungsten between 40 and 43%, - an atomic ratio of nitrogen between 5 and 12%, - an atomic ratio of cobalt between 7 and 9%, - other compounds for all atomic ratios between 0 and 8%.

5. Friction workpieces (10) under one of the claims 1 through 4, are characterized that the metal surface (12) is coated with only the outer coating (14). 6.

7. A friction element (10) under one of the claims 1 to 4, characterized by a metal surface (12) coated with at least one sublayer (18) made beneath the outer coating (14).

8. A friction element under claim 6, characterized by a sublayer (18) made of chromium or chromium nitride.

9. A mechanical system (1) made of: - a first friction element (10) under one of the claims 1 to 7, - a second friction element (20) arranged in lubricating contact with the first friction element (10); - a lubricant (3) arranged at the lubricating contact interface (2) between the friction elements (10;20).

10. A mechanical system (1) under claim 8, characterized by a second friction element (20) made of a metal surface (22) arranged in direct lubricating contact with the first friction element (10).The mechanical system (1) according to claim 8, characterized by the second friction element (20) being composed of a metal surface (22) and an outer coating (24) which is deposited on the metal surface (22), the outer coating (24) of the second friction element (20) being composed of nitrogen-doped tungsten carbide WC(N) with a nitrogen atomic ratio between 5 and 12%11. The mechanical system (1) according to claim 10, characterized by the outer coating (14) of the first friction element (10) and the outer coating (24) of the second friction element (20) having the same composition12.The method for applying friction material (10) under any of the claims 1 to 7, which is characterized by the following steps: -arranging the second friction material (20) in lubricating contact with friction material (10); -arranging the lubricant (3) at the lubricating contact interface (2) between friction materials (10;20); and - driving the friction material (10;20), such that the lubricating contact interface (2) between friction materials (10;20) reaches a temperature above 200 degrees Celsius.