PART COATED WITH A NON-HYDROGENATED AMORPHIC CARBON COATING ON AN UNDERCOAT CONTAINING CHROMIUM, CARBON AND SILICON

Description

[0001] The present invention relates to a coated part comprising a metallic substrate coated with an underlayer and a coating of non-hydrogenated amorphous carbon, which is deposited on the underlayer comprising chromium, carbon and silicon.

[0002] The parts, which have a coating, considered here are, for example, friction components for the automotive, aeronautical or space sectors.

[0003] In the automotive field, this includes, for example, timing components such as rocker arms, tappets, or cams to reduce friction between these parts. It can also include piston pins, to reduce wear and protect surfaces against seizing.

[0004] A coating such as the one described here can also be applied to components such as piston rings, piston skirts, and cylinder liners.

[0005] In the preceding non-limiting examples, coatings are often required to operate in a lubricated environment.

[0006] Of course, amorphous carbon coatings, whether hydrogenated or not, have numerous applications that are not limited to components for the automotive, aerospace, or space industries. Guiding or sliding elements, such as those found on molds for plastics processing, can also be coated with such a material to minimize wear and friction without lubrication.

[0007] Amorphous carbon coatings are often called "DLC" (for "Diamond Like Carbon"). They refer to carbon materials generally obtained in the form of a thin film and by vacuum deposition technologies.

[0008] These coatings can, for example, be classified into two families: those containing hydrogen (H) and those without hydrogen.

[0009] Among hydrogen-based coatings, DLC coatings of significant industrial interest include: The "aC:H" coatings (for "hydrogenated amorphous carbon"). These coatings are generally produced by plasma-assisted chemical vapor deposition of a gaseous carbon precursor (which is, for example, acetylene (C2H2)).

[0010] Among hydrogen-free coatings, DLC coatings of significant industrial interest include: "aC" coatings (for "amorphous carbon") are generally produced by magnetron sputtering of a graphite target. And, most notably, "ta-C" coatings (for "tetrahedral amorphous carbon") are generally produced by arc evaporation of a graphite target.

[0011] The three types of coating mentioned above are therefore each obtained through a different technology.

[0012] In addition, currently, for each type of DLC coating such as those mentioned above (made with different technologies, as illustrated above), it is often necessary to use a specific undercoat for the coating to adhere to a given substrate.

[0013] For example, in the case of ta-C coatings, adhesion is obtained by bombarding, at very high energy at the beginning of deposition (i.e. on the order of a few kilo-electronvolts), carbon (C) ions onto a substrate (this is described for example in the document: Tetrahedrally Bonded Amorphous Carbon Films I, Basics, Structure and Preparation, Bernd Schultrich, ©< Springer-Verlag GmbH Germany 2018 p.552).

[0014] The substrate is optionally pre-coated with a thin layer of metallic chromium (Cr), sometimes with a flash of chromium (Cr), which notably promotes good adhesion to a steel substrate. This chromium layer is, however, optional and can therefore be omitted.

[0015] According to the state of the art, optimal adhesion is obtained by bombarding relatively cold substrates with high-energy carbon (C) ions.

[0016] This condition is restrictive because the initial steps preceding this bombardment (degassing of the substrates and the machine by heating, ion cleaning, and deposition of a Cr flash) lead to heating of the parts, and this heating is detrimental to the adhesion of the ta-C layer. It is therefore desirable to cool the substrates before bombarding them with energetic carbon ions. However, cooling parts (in this case, substrates) under vacuum is often inefficient and takes a long time to reach the desired temperature for acceptable adhesion, typically several hours.

[0017] Furthermore, as mentioned previously, adhesion is achieved by bombarding substrates previously coated with a flash of chromium or not with high-energy carbon ions.

[0018] The high voltage required to accelerate the ions increases the risk of electrical arcing on the parts or the workpiece holders (also called substrate holders), which can lead to part destruction or loss of adhesion on the parts exposed to the arc. Furthermore, if the part polarization generator detects an arc and shuts down, carbon ion deposition occurs without acceleration, which impairs the adhesion of the carbon coating to the substrates then exposed to deposition.

[0019] Another drawback of this high-energy carbon ion bombardment phase is the heating of the parts being coated, resulting from the transfer of energy from the ions to the part. The combination of the high carbon ion density on the substrate and the high energy induces a high power density applied to the substrate and therefore a rapid increase in its temperature.

[0020] Besides the fact that this temperature rise can be detrimental to the characteristics of mechanical components whose tempering temperatures are low, i.e. typically between 150°C and 220°C, it is also critical for the mechanical properties of ta-C which collapse above a deposition temperature of about 200°C.

[0021] Usual procedures can therefore lead to risks of overheating of parts.

[0022] Thus, the present invention aims to resolve at least partially the aforementioned drawbacks.

[0023] In particular, an object of the invention is to provide a part, comprising a non-hydrogenated DLC coating, which has fairly high mechanical properties and whose DLC coating has satisfactory adhesion to the substrate.

[0024] The present invention also aims to provide a part that can be obtained without a cooling step before deposition in the case of ta-C coating for example, that is to say whose deposition process to obtain such a part makes it possible to do away with the cooling step, before deposition of the ta-C coating, and with the use of high-energy ion bombardment currently used to make the ta-C coating adhere.

[0025] To this end, a part is proposed according to a first aspect, comprising a metallic substrate, a non-hydrogenated amorphous carbon coating, of type ta-C or even aC, coating the substrate, and a sub-layer based on chromium (Cr), carbon (C) and silicon (Si) disposed between the metallic substrate and the amorphous carbon coating and on which the amorphous carbon coating is applied, characterized in that the sub-layer has the following atomic proportions at its interface with the amorphous carbon coating (i.e. on the surface of the sub-layer): A ratio between silicon content and chromium content (Si / Cr) of between 0.35 and 0.60, and a ratio between carbon content and silicon content (C / Si) of between 2.5 and 3.5.

[0026] Such a sublayer composition has contents which are measurable, for example, by EDX (Energy Dispersive X-Ray Spectrometry) analysis in a scanning electron microscope (SEM), or by GDOES (Glow Discharge Optical Emission Spectroscopy).

[0027] It has been shown that such an undercoat makes it possible to obtain a coating adhesion result noted as HF1, which is stable over time.

[0028] Furthermore, it has become apparent that such a sublayer makes it possible to dispense with a cooling step, at least in the case of ta-C deposition, because, contrary to the state of the art, it is possible to start the deposition of the carbon coating at very low bias voltages compared to the hundreds of volts according to the state of the art.

[0029] It also emerged that such a sub-layer allows a transition of mechanical properties between a DLC coating of the ta-C or aC type and a metallic substrate.

[0030] Moreover, such a sub-layer has also proven to be particularly advantageous for a DLC coating of hydrogenated amorphous carbon, that is to say in particular of the aC:H type.

[0031] Such a sublayer then appears as a layer with a composition gradient based mainly on chromium (Cr), silicon (Si) and carbon (C).

[0032] The underlayer is progressively enriched (from the substrate towards the DLC coating) in silicon (Si) and carbon (C), until a composition allows the coating to adhere, as described above.

[0033] In one particular example, the ratio between the silicon content and the chromium content (Si / Cr) of the sublayer in the vicinity of the interface with the DLC is between 0.38 and 0.60, or even between 0.40 and 0.60.

[0034] In one particular example, the ratio between the carbon content and the silicon content (C / Si) of the sublayer in the vicinity of the interface with the DLC is between 2.8 and 3.2, or even between 2.9 and 3.1.

[0035] The undercoat may optionally contain nitrogen (N). This is particularly advantageous if the part also has a chromium nitride layer, as described below.

[0036] Thus, in an interesting example of implementation, the sublayer also includes nitrogen (N) atoms, with a ratio between nitrogen content and chromium content (N / Cr) being less than 0.70 in the vicinity of the interface with the DLC, i.e. at the interface between the sublayer and the amorphous carbon coating.

[0037] According to advantageous examples, the ratio between nitrogen content and chromium content (N / Cr) is between 0.26 and 0.70, or even between 0.29 and 0.67, or even between 0.35 and 0.65, at the interface between the underlayer and the amorphous carbon coating.

[0038] According to advantageous examples, the ratio between silicon content and chromium content (Si / Cr) is between 0.40 and 0.55, or even between 0.45 and 0.55, at the interface between the underlayer and the amorphous carbon coating.

[0039] In a preferred example, the sublayer, with or without nitrogen, has a thickness of a few tenths of a micrometer; preferably a thickness equal to or less than about 1.1 µm, for example between about 0.2 µm and 1.1 µm, preferably between about 0.3 µm and 0.6 µm.

[0040] In practice, beyond 1.1 µm, columnar development occurs, which is detrimental to the adhesion of the underlayer, and below 0.2 µm, the underlayer does not produce its effect as an adaptation layer.

[0041] The amorphous carbon coating, for example, has a thickness equal to or greater than about 0.3 µm, or even about 0.5 µm, or even about 1 µm, or even 1.5 µm.

[0042] The amorphous carbon coating, for example, has a thickness equal to or less than about 10 µm, or even 8 µm, or even 3.5 µm.

[0043] The amorphous carbon coating, for example, has a thickness of between approximately 1.5 µm and approximately 3.5 µm, but can reach 8 µm when such a coating is applied to a segment, for example.

[0044] The metallic substrate is, for example, steel or other metallic alloys.

[0045] In interesting embodiment examples, the part also includes a chrome-based layer, deposited on the substrate and on which the underlayer is formed.

[0046] The chromium-based layer is for example a chromium (Cr) layer and / or a chromium nitride layer, for example CrN or Cr2N, or any intermediate compound.

[0047] Preferably, the part comprises a layer of chromium (Cr), or a layer of chromium (Cr) followed by a layer of chromium nitride (e.g. CrN or Cr2N, or any intermediate compound).

[0048] Preferably, the chromium-based layer has a thickness of a few tenths of a micrometer, preferably a thickness equal to or less than about 1 µm, or even 0.6 µm, for example between about 0.1 µm and 0.5 µm, or even between about 0.3 µm and 0.5 µm.

[0049] The table below presents different tests, numbered 1 to 15. The atomic proportions, measured by EDX, are those of the sublayer in the vicinity of the interface with the coating (keeping in mind that the sublayer has a composition gradient, the target composition is that towards which it tends at the interface with the DLC coating).

[0050] In all the tests, it was noted that the adhesion behavior of DLC on the undercoat is linked to the composition of the undercoat surface.

[0051] The presence of nitrogen on the surface is not a determining factor for DLC adhesion. Indeed, at similar nitrogen (N / Cr) ratios (examples 9, 12, and 13), adhesion can be considered good or poor. A relatively high nitrogen content can impair adhesion, as in example 11. The absence of nitrogen can lead to good adhesion (examples 4 to 6, 14, and 15) or poor adhesion (examples 1 to 3, 7, and 8).

[0052] In contrast, the proportion of chromium proved to be a more decisive factor. The proportion of chromium is defined by the Si / Cr ratio, and the N / Cr ratio if nitrogen is present. A relatively high proportion of chromium does not appear to be suitable for adhesion (e.g., examples 1 to 3). A relatively low proportion of chromium also does not appear to be suitable for DLC adhesion (e.g., examples 7 and 8).

[0053] Thus, while the ratio of Si / Cr compositions is between 0.35 and 0.6, all the DLC layers deposited on these sub-layers proved to be adherent (examples 4, 5, 6, 9, 10, 13, 14 and 15).

[0054] To obtain a coating as described above, vacuum deposition equipment as described below is used.

[0055] Vacuum deposition equipment mainly consists of a chamber, a pumping system, a heating system, configured to pump, heat the parts (substrate) and the inside of the chamber, in order to accelerate the desorption of gases and quickly obtain a vacuum, considered to be of quality, in the chamber.

[0056] The deposition equipment also includes a substrate carrier adapted, from the point of view of geometry, electrical polarization and kinematics, to the parts, or the portion of the parts to be coated.

[0057] The vacuum deposition equipment also includes an ionic stripping system configured to bombard the parts (substrates) to be coated with argon (Ar) ions, in order to remove a passivation layer generally present on the metallic substrates to be coated.

[0058] For ta-C coatings, many different ion etching technologies are suitable. The same applies to a-C coatings.

[0059] The vacuum deposition equipment also includes a magnetron cathode, equipped with a chromium target, to generate the chromium-based layers.

[0060] Preferably, the ionic cleaning system is configured to operate simultaneously with the magnetron cathode. This allows the end of the ionic cleaning process to be used to pre-spray the magnetron cathode, which is equipped with a chromium target.

[0061] Such equipment is therefore particularly interesting because it also allows for the application of suitable aC:H type coatings.

[0062] For example, the plasma source such as that described in document FR 2 995 493 can be implemented to carry out efficient ionic stripping of the parts to be coated and to coat them with a DLC coating of type ta-C, or aC, or even aC:H.

[0063] The sub-layer deposition step is, for example, configured to produce a sub-layer with a composition as described previously.

[0064] The sub-layer deposition step is, for example, configured to further produce a sub-layer with a thickness as described previously.

[0065] In one implementation example, the process may optionally include a metallic chromium deposition step, for example a chromium spraying step.

[0066] Optionally, this metallic chromium deposition step includes a nitrogen introduction step simultaneously with the chromium spraying step so as to obtain a chromium nitride layer, for example CrN or Cr2N or any intermediate compound.

[0067] Such a chromium-based layer, possibly with nitrogen, is deposited with a thickness of a few tenths of a micrometer, as described previously.

[0068] The deposition process continues with the introduction of an organosilicon gas, that is, a gas containing at least silicon, typically tetramethylsilane (also called TMS, with the formula (Si(CH3)4), which may contain traces of oxygen), which is the easiest to use, or a mixture of silane and a hydrocarbon. While not the only option, TMS is by far the preferred choice due to its relatively high chemical stability and high volatility, allowing for easy application using a mass flow meter.

[0069] If a chromium-based layer (Cr, and / or CrN or Cr₂N) has been previously deposited, the organosilicon gas is introduced at an increasing flow rate until a value is reached at which the silicon content of the sublayer is at least approximately 0.35 times its chromium content and at most approximately 0.60 times the chromium content in the vicinity of the interface. The ratio of carbon content to silicon content is also between 2.5 and 3.5 in the vicinity of the interface.

[0070] When a chromium-based layer with nitrogen is used, the amount of nitrogen injected can be gradually reduced as the amount of organosilicon gas increases. The amount of nitrogen does not necessarily have to be reduced to zero but must become significantly lower than that of the organosilicon gas. The nitrogen introduced to produce a CrN (or Cr₂N) layer can also be abruptly reduced to zero before introducing the organosilicon precursor. However, a gradual decrease in nitrogen is the preferred method because it allows for a gradual transition of nitrogen into the sublayer.

[0071] For example, considering a CrN layer, the N / Cr ratio is then, for example, 1, and therefore the amount of nitrogen is possibly considered excessive. Considering a Cr2N layer, the N / Cr ratio is then, for example, 0.5, and in such a case, this ratio can be maintained.

[0072] During the development of the various vacuum thin films described previously (chrome-based layer, underlayer or DLC coating), the bias voltage of the substrate carrier is generally between -50 V and -100 V (volt).

[0073] The partial pressure of argon during the deposition of these layers is preferably between 0.2 Pa and 0.4 Pa.

[0074] When the organosilicon gas flow rate reaches the required level, the magnetron cathode power supply is switched off, and the reactive gases (i.e., the organosilicon gas, or the organosilicon gas and nitrogen as appropriate) are stopped. The argon flow rate, if present, is reduced to a low value or even to zero to begin ta-C deposition using arc sources, or a-C deposition.

Claims

1. Part comprising a metal substrate, a non-hydrogenated amorphous carbon coating, of ta-C type or even of a-C type, coating the substrate, and a sub-layer based on chromium (Cr), carbon (C) and silicon (Si) disposed between the metal substrate and the amorphous carbon coating and on which the amorphous carbon coating is applied, characterized in that the sub-layer comprises the following atomic ratios at its interface with the amorphous carbon coating: - A ratio between the silicon content and the chromium content (Si / Cr) comprised between 0.35 and 0.60, and - A ratio between the carbon content and the silicon content (Si / Cr) comprised between 2.5 and 3.5.

2. Part according to claim 1, characterized in that the ratio between the silicon content (Si) and the chromium content (Cr) (Si / Cr) of the sub-layer is comprised between 0.38 and 0.6.

3. Part according to any one of claims 1 or 2, characterized in that the ratio between the carbon content (C) and the silicon content (Si) (Si / Cr) of the sub-layer is comprised between 2.8 and 3.2, or even between 2.9 and 3.1.

4. Part according to any one of claims 1 to 3, characterized in that the sub-layer further comprises atoms of nitrogen (N), a ratio between the nitrogen content and the chromium content (N / Cr) being less than 0.70 at the interface between the sub-layer and the amorphous carbon coating.

5. Part according to claim 4, characterized in that the ratio between the nitrogen content and the chromium content (N / Cr) is comprised between 0.26 and 0.70 and the ratio between the silicon content and the chromium content (Si / Cr) is comprised between 0.40 and 0.55 at the interface between the sub-layer and the amorphous carbon coating.

6. Part according to any one of claims 1 to 5, characterized in that the sub-layer has a thickness equal to or less than 1.1 µm, for example comprised between 0.2 µm and 1.1 µm, preferably comprised between 0.3 µm and 0.6 µm.

7. Part according to any one of claims 1 to 6, characterized in that the amorphous carbon coating has a thickness equal to or greater than 0.3 µm, or even than 0.5 µm, or possibly even than 1 µm.

8. Part according to any one of claims 1 to 7, characterized in that the amorphous carbon coating has a thickness comprised between 1.5 µm and 3.5 µm.

9. Part according to any one of claims 1 to 8, characterized in that it further comprises a chromium-based layer, deposited on the substrate and on which the sub-layer is formed, the chromium-based layer being a layer of chromium (Cr) and / or a layer of chromium nitride, for example CrN or Cr2N, or any intermediate compound.

10. Part according to claim 9, characterized in that the chromium-based layer has a thickness of a few tenths of a micrometer, preferably a thickness equal to or less than 1 µm, or even 0.6 µm, for example comprised between 0.1 µm and 0.5 µm, or even between 0.3 µm and 0.5 µm.