System comprising two parts in lubricated contact and use of a lubricant in such a system

Abstract

The invention relates to a system (1) comprising two parts (10, 20) in lubricated contact by means of a lubricant (30), wherein at least a first of the two parts (10) comprises a substrate (11) and an amorphous carbon-based coating (12) applied to the surface of the substrate (11), wherein the amorphous carbon-based coating (12) of the first of the two parts (10) comprises at least one DLC coating (13), and wherein the lubricant (30) comprises at least one base (31), the base (31) being composed of mono-glycol, the content of the base (31) in the lubricant (30) being at least 50 wt.% of the lubricant (30). The invention also relates to a valvetrain system comprising such a system (1) and to the use of a lubricant in a mechanical system comprising two parts in contact with one another.

Description

[0001] A system comprising two lubricated contacting parts and the use of a lubricant in such a system

[0002] The invention relates to a system comprising two parts in contact in which the contact is lubricated by a lubricant.

[0003] It also relates to the use of such a system, and in particular of a lubricant in such a system.

[0004] It falls under the category of mechanical systems lubricated by oils.

[0005] It applies, for example, to lubricated systems of internal combustion engine components, automotive transmission components, or even pumps; but other lubricated mechanical systems are conceivable.

[0006] One objective, for example, is to provide a lubricated system in which mechanical friction losses (MFL) are relatively low, or even reduced, compared to traditional systems in a given application.

[0007] To reduce mechanical friction losses in a mechanical system lubricated with oils, it is common practice to use surface coatings such as amorphous carbon coatings or to add friction-modifying additives when the system is likely to operate in boundary or mixed lubrication regimes.

[0008] To reduce the PMF of mechanical systems lubricated by oil and operating in hydrodynamic or elasto-hydrodynamic lubrication regime, it is common practice, for example, to reduce the viscosity of the oil.

[0009] Glycols, and in particular polyethylene glycols, can be used as additives in oil-based or water-based lubricants, especially to adjust viscosity.

[0010] An oil-based lubricant, or a water-based lubricant, here refers to a lubricant containing at least 50 wt.% of oil, or water, respectively.

[0011] In general, in this description, a percentage is a mass percentage, designated "wt.%" or "%wt." interchangeably, unless otherwise specified.

[0012] An additive typically has a fairly low concentration in a lubricant, i.e. on the order of a few mass percent at most.

[0013] Glycols in oil-based or water-based lubricants typically have a concentration of approximately 10 wt.% or less. Polyalkylene glycols, and particularly polyethylene glycols, can be used as a base for lubricants. These lubricants have glycol bases composed of molecules with a molecular weight of 200 g / mol or greater, and often between 400 and 1000 g / mol.

[0014] Document FR3134813A1, for example, concerns an aqueous lubricant composition comprising, among other things, water and a polyalkylene glycol.

[0015] However, reducing the viscosity of an oil to reduce TMF over a high-speed operating range may be accompanied by more frequent limit and mixed operating speeds over lower-speed operating ranges, resulting in more TMF and an increased risk of component damage (wear that can lead to seizing).

[0016] Furthermore, friction-modifying additives can be expensive.

[0017] Furthermore, these are often compounds that can generate sulfated, sulfurous or phosphorus ash which can damage the exhaust after-treatment systems of internal combustion engines.

[0018] Moreover, friction-modifying additives have a limited effect over time (they are consumed), which necessitates regular oil changes to replace the oil and add new additives.

[0019] On the other hand, most of the oils currently used in mechanical systems contain a large proportion of components that are petrochemical products and are harmful to the environment.

[0020] As such, by 2040, the various environmental regulations impacting lubricants will require manufacturers formulating these lubricants to replace at least about 30% of the approximately 8000 molecules traditionally used in lubricants.

[0021] At the level of contacting parts in the mechanical system, surface coatings such as amorphous carbon coatings can be used to reduce friction, although the primary function sought is generally resistance to wear and seizing. Surface coatings are typically chosen for their compatibility with the oil selected as the lubricant, but the oil is not formulated to fully utilize coated surfaces such as amorphous carbon coatings, resulting in relatively small friction reductions.

[0022] The present invention aims to improve, at least in part, the aforementioned drawbacks, potentially leading to other advantages. To this end, a mechanical system comprising two parts and a lubricant is proposed, according to a first aspect. The two parts are in contact with each other, and the contact between the two parts is lubricated by the lubricant.

[0023] At least one of the two parts includes a substrate and an amorphous carbon-based coating applied to the surface of the substrate, and the amorphous carbon-based coating of the first of the two parts includes at least one DLC coating (for "diamond like carbon").

[0024] The term DLC encompasses several types of coatings: amorphous carbon (aC), tetrahedral amorphous carbon (ta-C), hydrogenated forms of these two (aC:H and ta-C:H), as well as doped forms of these materials (dopants: metals, Si, N, B, O, F). A review of these materials and their properties can be found, for example, in J. Vetter, Surf. Coat. Technol. 257 (2014) pp. 213-240.

[0025] The lubricant here comprises at least one base, the base being made of mono-glycol, and a content of the base in the lubricant is at least 50 wt.% of the lubricant, for example between 50 wt.% and 100 wt.% of the lubricant.

[0026] Such a system includes, for example, at least one of the contact surfaces which is coated with an amorphous carbon coating, in particular DLC, for example of the aC:H type (i.e. hydrogenated amorphous carbon).

[0027] Using a glycol-based lubricant instead of mineral or synthetic oil-based lubricants helps to reduce PMF and ensure lubrication of the system.

[0028] Such a system, thanks to the formulation of the lubricant in combination with contacting parts surfaces of which at least one part has a DLC surface coating, makes it possible to achieve, under certain tribological conditions indicated later, a coefficient of friction considered to be extremely low in service, for example equal to or less than about 0.01.

[0029] Such a system offers, for example, at least some of the following advantages:

[0030] - Reduction of friction: very low coefficient of friction (less than 0.01) under tribological conditions where engine oils give coefficients of friction between 0.01 and 0.15;

[0031] - Thermal control: one of the roles of the lubricant is to dissipate the heat generated in the contact and to control the temperature of the mechanical components; compared to a traditional engine oil, lubricants such as ethylene glycol or propylene glycol are fluids with lower viscosities and higher thermal conductivities which make them better heat transfer fluids;

[0032] - Products such as ethylene glycol and propylene glycol are biodegradable in a few days (often less than a few tens of days); they are therefore more environmentally friendly than the mineral or synthetic base oils traditionally used.

[0033] A content indicated as a percentage here refers to a mass percentage, marked "wt.%" or "%wt.".

[0034] According to one embodiment, the content of the base in the lubricant is more particularly at least 60 wt.% of the lubricant, or even 70 wt.%, or even 80 wt.%, or even 90 wt.%.

[0035] In one particular embodiment, the lubricant consists of the base alone, i.e. the base content in the lubricant is approximately 100 wt.%, without considering any possible impurities.

[0036] According to an example embodiment, the base has a viscosity less than or equal to 50 cSt, when measured at 40°C.

[0037] According to an example embodiment, the base has a viscosity less than or equal to 10 cSt, when measured at 100°C.

[0038] A glycol here refers to a hydrocarbon, linear or branched, containing two hydroxyl functions.

[0039] The base thus comprises at least one mono-glycol, i.e. mono glycol of at least one type, or of several types, i.e. a mixture of mono-glycols.

[0040] In one particular embodiment, the base comprises 100 wt.% of a mono-glycol.

[0041] The base then consists of only one mono-glycol, i.e. a mono-glycol of only one type.

[0042] For example, at least one monoglycol of the base has a low molar mass.

[0043] According to an example embodiment, the base has an average molar mass less than 100 g / mol, for example between 40 g / mol and 100 g / mol.

[0044] For example, ethylene glycol has a molar mass of approximately 62 g / mol.

[0045] For example, propylene glycol has a molar mass of approximately 76 g / mol.

[0046] For example, methanediol has a molar mass of approximately 48 g / mol.

[0047] For example, butanediol has a molar mass of approximately 90 g / mol.

[0048] In contrast, pentanediol has a molar mass of approximately 104 g / mol. Pentanediol can, for example, be used in mixtures with a lighter monoglycol. Furthermore, pentanediol can also be advantageous for use in heated applications.

[0049] For example, pentane-1,5-diol has a melting point of about -18°C and a kinematic viscosity of about 129 cSt at 20°C and 78 cSt at 30°C.

[0050] Probably because of their very low viscosity, glycols with a molecular weight of less than 200 g / mol, in particular monoethylene glycols and monopropylene glycols, are rarely considered on their own as a basis for lubricants for mechanical systems.

[0051] However, when coupled with DLC-type surface coatings, low molecular weight glycols have proven to be very effective lubricants for reducing PMFs in such mechanical systems.

[0052] In one particular embodiment, at least one mono-glycol of the base comprises a monoethylene glycol (EG) and / or a monopropylene glycol (PG).

[0053] Monopropylene glycol (PG) can also be referred to as propane-1,2-diol.

[0054] In one example embodiment, the lubricant includes at least one additive.

[0055] The additive includes, for example, at least one of the following: water, glycol, glycerol, viscosity modifier, anti-wear (AW), "extreme pressure" (EP), anti-oxidant, anti-corrosion, anti-foam, detergent, anti-emulsifier, friction modifier, acid neutralizer.

[0056] An additive designated as "extreme pressure (EP)" is an additive that helps limit the risk of wear and seizing in a mechanical system under high contact pressures. This type of additive can be found in traditional gearbox lubricants where its purpose is to reduce the risk of wear and seizing of the contacts between gear teeth.

[0057] If the lubricant is 100 wt.% base, then there are no additives.

[0058] If the lubricant contains at least one additive, the content of the additive in the lubricant is, for example, at most 50 wt.% of the lubricant, for example between 0 wt.% and 50 wt.% of the lubricant.

[0059] In one particular embodiment, the water content (as an additive) in the lubricant is at most 25 wt.% of the lubricant, for example between 0 wt.% and 25 wt.% of the lubricant.

[0060] In one particular embodiment, the lubricant comprises a mixture of monoethylene glycol (PG) and glycerol, specifically a mixture comprising 70 wt.% monoethylene glycol (PG) and 30 wt.% glycerol. In one embodiment, the lubricant contains a maximum of 3 wt.% impurities.

[0061] An impurity here refers to any element whose presence represents a relatively small quantity, and which is neither intentional nor controlled.

[0062] This analysis considers an approximate level of impurities in the lubricant before use. Indeed, the nature and concentration of impurities are generally unknown, but there is a tolerance level for impurity content in the lubricant. Furthermore, the lubricant is considered before use because its composition typically changes over time due to oxidation of some initial components, the formation of debris, etc.

[0063] A free surface of the DLC coating of the amorphous carbon-based coating of the first of the two parts forms the contact surface of the first part with a second of the two parts of the system.

[0064] For example, the amorphous carbon-based coating has a thickness of between approximately 0.5 pm and 10 pm, more specifically, for example, between approximately 1 pm and 4 pm.

[0065] For example, the DLC coating has a thickness of between approximately 0.5 pm and 10 pm, more specifically for example between approximately 1 pm and 4 pm.

[0066] In one embodiment, a free surface, i.e. configured to be in contact with the lubricant, of the first of the two parts, in particular of the amorphous carbon-based coating, has a roughness Ra of less than 0.2 pm, for example between 0.01 pm and 0.2 pm, preferably less than 0.05 pm, for example between 0.01 pm and 0.05 pm.

[0067] Such roughness is measured for example by standard profilometry, for example according to the ISO13565-1 standard.

[0068] Here, the roughness is preferably as low as possible.

[0069] For example, at least the first part has a surface hardness of at least 1500 Hv, or even 2000 Hv. For example, the surface hardness of the first part is equal to or less than 10000 Hv, or even 8000 Hv.

[0070] The hardness "Hv" here refers to Vickers hardness.

[0071] The amorphous carbon-based coating consists at least of the DLC coating. However, according to one embodiment, the amorphous carbon-based coating of the first of the two parts may also include at least one sub-layer. Such a sub-layer is optional, and its inclusion depends on the stress conditions and the nature of the parts, particularly the substrate to which it can be applied.

[0072] Such an underlayer aims in particular to improve the adhesion of the deposit and to accommodate any differences in mechanical properties between the substrate and the DLC coating.

[0073] The underlayer is then placed between the substrate and the DLC coating.

[0074] For example, the underlayer is placed on the surface of the substrate.

[0075] The DLC coating is then, for example, placed on the underlayer.

[0076] At least one underlayer can be of any type.

[0077] According to a particular example, at least one sublayer comprises at least one of the following materials: tungsten carbide (WC), carbon-enriched tungsten carbide (WCC), chromium (Cr), chromium nitride (CrN), hydrogenated amorphous silicon carbide (SiCH) or hydrogenated chromium-silicon carbonitride (CrSiCHN) as described in WO2019 / 241720A1.

[0078] For example, the underlayer is ceramic, for example chromium nitride. In one embodiment, the underlayer has a thickness less than or equal to 2 µm, or even less than or equal to 1 µm. It is, for example, between 0.01 µm and 2 µm, in particular between 0.01 µm and 1 µm.

[0079] In a particular embodiment, depending on the application concerned, the substrate of the first of the two parts comprises, for example, one of the following:

[0080] - at least one ceramic, for example silicon nitride, silicon carbide (SiC), alumina (Al2O3), zirconia (ZrOz), or

[0081] - a metal, for example iron, aluminum and / or copper, or

[0082] - an alloy, for example steel, an aluminum alloy or a copper alloy, or

[0083] - a polymer, for example a poly-ether-ether-ketone (PEEK), a polyamide-imide (PAI) or a butadiene-acrylonitrile (NBR).

[0084] In one embodiment, the second of the two parts of the system includes at least one substrate.

[0085] In a particular embodiment, depending on the application concerned, the substrate of the second of the two parts comprises, for example, one of the following:

[0086] - at least one ceramic, for example silicon nitride, silicon carbide (SiC), alumina (Al2O3), zirconia (ZrO2), or

[0087] - a metal, for example iron, aluminum and / or copper, or

[0088] - an alloy, for example a steel, an aluminium alloy or a copper alloy, or - a polymer, for example a poly-ether-ether-ketone (PEEK), a polyamide-imide (PAI) or a butadiene-acrylonitrile (NBR).

[0089] In one embodiment example, the second of the two parts of the system has an amorphous carbon-based coating.

[0090] For example, the amorphous carbon-based coating is applied to the surface of the substrate of the second of the two parts.

[0091] For example, the amorphous carbon-based coating of the second of the two parts includes at least one DLC (for "diamond like carbon") coating.

[0092] A free surface of the DLC coating of the amorphous carbon-based coating of the second of the two parts then forms the contact surface of the second part with the first of the two parts of the system, i.e. incidentally with the lubricant.

[0093] For example, the amorphous carbon-based coating of the second part has a thickness of between approximately 0.5 pm and 10 pm, more specifically, for example, between approximately 1 pm and 4 pm.

[0094] For example, the DLC coating of the second part has a thickness of between approximately 0.5 pm and 10 pm, more specifically for example between approximately 1 pm and 4 pm.

[0095] In one example embodiment, a free surface, i.e. in contact with the lubricant, of the second of the two parts has a roughness Ra of less than 0.2 pm, for example between 0.01 pm and 0.2 pm, preferably less than 0.05 pm, between 0.01 pm and 0.05 pm.

[0096] Such roughness is also measured, for example, by standard profilometry, for example according to the ISO13565-1 standard.

[0097] Here, the roughness of the second of the two pieces is also preferably as low as possible.

[0098] For example, the second of the two parts in the system has a surface hardness of at least 1500 Hv, or even 2000 Hv. For example, the surface hardness of the second part is equal to or less than 10000 Hv, or even 8000 Hv.

[0099] The amorphous carbon-based coating of the second part is at least made up of the DLC coating.

[0100] However, according to one embodiment, the amorphous carbon-based coating of the second of the two parts may also include at least one sub-layer. Such a sub-layer is intended, in particular, to improve the adhesion of the coating and to accommodate any differences in mechanical properties between the substrate and the DLC coating. The sub-layer is then placed between the substrate and the DLC coating of the second of the two parts.

[0101] For example, the underlayer is placed on the surface of the substrate of the second piece.

[0102] The DLC coating of the second part is then, for example, placed on the underlayer of the second part.

[0103] The at least one underlayer of the second piece can be of any type.

[0104] According to a particular example, at least one sublayer of the second part comprises at least one of the following materials: tungsten carbide (WC), carbon-enriched tungsten carbide (WCC), chromium (Cr), chromium nitride (CrN), hydrogenated amorphous silicon carbide (SiCH) or hydrogenated chromium-silicon carbonitride (CrSiCHN) as described in WO2019 / 241720A1.

[0105] For example, the underlayer of the second piece is ceramic, for example chromium nitride.

[0106] In one embodiment example, the underlayer of the second part has a thickness less than or equal to 2 pm, or even less than or equal to 1 pm. For example, it is between 0.01 pm and 2 pm, in particular between 0.01 pm and 1 pm.

[0107] Also proposed, according to another aspect of the invention, is a valve distribution system comprising a cam and a valve pusher element, for example a pusher, a rocker arm, a rocker arm or other, characterized in that it comprises a system having all or part of the characteristics described above, in which the first of the two parts of the system forms the cam and a second of the two parts of the system forms the valve pusher element.

[0108] In one embodiment, at least one of the cam and pusher element of a valve may be made of steel.

[0109] In one embodiment example, the DLC coating of at least one of the cam and pusher element of a valve may have a roughness Ra between 0.01 pm and 0.05 pm, for example about 0.03 pm.

[0110] In one embodiment, the DLC coating of at least one of the cam and the pusher element of a valve can be lubricated with monoethylene glycol. In another embodiment, the cam has at least one tungsten carbide (WC) sublayer, and / or at least one carbon-enriched tungsten carbide (WCC) sublayer. In such a configuration, the PMFs of the valve train are significantly reduced compared to a case where the cam and the pusher element of a valve coated with the same aC:H type DLC coating are lubricated with a 0W30 grade engine oil.

[0111] Also proposed, according to yet another aspect of the invention, is the use of a lubricant in a mechanical system comprising two parts, the two parts being in contact with each other, and the contact being lubricated by the lubricant.

[0112] At least one of the two parts includes a substrate and an amorphous carbon-based coating applied to the surface of the substrate, and the amorphous carbon-based coating of the first of the two parts here includes at least a DLC (for "diamond like carbon") coating.

[0113] The lubricant here comprises at least one base, the base being made of mono-glycol, and a content of the base in the lubricant is at least 50 wt.% of the lubricant, for example between 50 wt.% and 100 wt.% of the lubricant.

[0114] Using such a glycol-based lubricant as a replacement for mineral or synthetic oil makes it possible to reduce PMF and ensure lubrication of the system.

[0115] In a specific use case, the system also operates under at least one of the following parameters:

[0116] - a sliding speed between the two parts of between 0 m / s and 50 m / s, preferably between 0.1 and 10 m / s; and / or

[0117] - a fluid entrainment velocity between 0.01 m / s and 50 m / s, preferably between 0.1 m / s and 5 m / s; and / or

[0118] - a contact pressure between the two parts of between 1 MPa and 3 GPa, preferably between 10 MPa and 1 GPa.

[0119] The sliding speed refers to a difference in speed between the surface of the first of the two parts of the system and the surface of the second of the two parts of the system, the speeds being considered in a reference frame of the contact area.

[0120] If the system is a bearing, the theoretical sliding speed is then approximately 0 m / s.

[0121] The drive speed refers to an average of the surface speeds of the two parts of the system, the speeds being considered in a reference frame of the contact zone. The invention, according to an exemplary embodiment, will be well understood and its advantages will become clearer upon reading the detailed description that follows, given by way of example and in no way limiting, with reference to the accompanying drawings in which: Figure 1 schematically represents a system comprising two parts and a lubricant according to a first exemplary embodiment of the invention;

[0122] Figure 2 schematically represents a system comprising two parts and a lubricant according to a second embodiment of the invention;

[0123] Figure 3 presents a comparison of the evolution of power dissipated by friction, in Watts (W), i.e. mechanical losses by friction, as a function of the rotational speed of a camshaft in a system as illustrated in Figure 2 lubricated by monoethylene glycol or a fully formulated 0W30 grade engine oil with a friction modifier additive (MoDTC, i.e. molybdenum dithiocarbamate), the cam and tappet being coated with DLC;

[0124] Figure 4 schematically represents a system comprising two parts 10, 20 and a lubricant 30 according to a third embodiment of the invention;

[0125] Figure 5 presents a comparison of the evolution of the coefficient of friction as a function of the sliding speed between the parts in a system as illustrated in Figure 4, lubricated by monoethylene glycol, monopropylene glycol, butane-1,4-diol, engine oil 1 (fully formulated 5W30 grade oil without friction modifier additive) and engine oil 2 (fully formulated 0W30 grade oil with a MoDTC type friction modifier additive), at 80°C, under an average contact pressure of 390 MPa, the cylinder being coated with a DLC coating of type aC:H with a roughness Ra of about 0.03 pm and the surface being coated with a DLC coating of type aC:H with a roughness Ra of about 0.01 pm;

[0126] Figure 6 presents a comparison of the evolution of the coefficient of friction as a function of the sliding speed between the parts in a system as illustrated in Figure 4 lubricated by a mixture of 52 wt. of monoethylene glycol and 48 wt. of monopropylene glycol, a mixture of 70 wt. of monoethylene glycol and 30 wt. of glycerol, engine oil 1 and engine oil 2, at 80°C, under an average contact pressure of 390 MPa, the cylinder being coated with a DLC coating of type aC:H with a roughness Ra equal to about 0.03 pm and the plane being coated with a DLC coating of type aC:H with a roughness Ra equal to about 0.01 pm;

[0127] Figure 7 presents a comparison of the evolution of the coefficient of friction as a function of the sliding speed between the parts in a system as illustrated in Figure 4 lubricated by monoethylene glycol, monopropylene glycol, engine oil 1 and engine oil 2, at 80°C, under an average contact pressure of 390 MPa, the cylinder being made of uncoated steel with a roughness equal to about 0.02 pm and the surface being coated with a DLC coating of type aC:H with a roughness Ra equal to about 0.01 pm;

[0128] Figure 8 presents a comparison of the evolution of the coefficient of friction as a function of the sliding speed between the parts in a system as illustrated in Figure 4 lubricated by monoethylene glycol, a mixture of 85 wt. of monoethylene glycol and 15 wt. of water, a mixture of 75 wt. of monoethylene glycol and 25 wt. of water, engine oil 1 and engine oil 2, at 80°C, under an average contact pressure of 390 MPa, the cylinder being made of uncoated steel with a roughness equal to about 0.02 pm and the surface being coated with a DLC coating of type aC:H with a roughness Ra equal to about 0.01 pm;

[0129] Figure 9 presents a comparison of the evolution of the coefficient of friction as a function of the sliding speed between the parts in a system as illustrated in Figure 4 lubricated by monoethylene glycol, and engine oil 1, at 80°C, under an average contact pressure of 390 MPa, the cylinder being made of uncoated steel with a roughness equal to about 0.02 pm and the plane being coated with a DLC coating of type aC (non-hydrogenated amorphous carbon) with a roughness Ra equal to about 0.02 pm;

[0130] Figure 10 presents a comparison of the evolution of the coefficient of friction as a function of the sliding speed between the parts in a system as illustrated in Figure 4, lubricated by monoethylene glycol and engine oil 1, at 80°C, under an average contact pressure of 390 MPa, the cylinder being coated with a ta-C type DLC coating (non-hydrogenated tetrahedral amorphous carbon) with a roughness Ra of approximately 0.01 pm and the surface being coated with a ta-C type DLC coating with a roughness Ra of approximately 0.03 pm; and

[0131] Figure 11 presents a comparison of the evolution of the coefficient of friction as a function of the sliding speed between the parts in a system as illustrated in Figure 4 lubricated by monoethylene glycol, and engine oil 1, at 80°C, under an average contact pressure of 100 MPa, the cylinder being made of aluminum alloy 2017A having a roughness Ra equal to about 0.08 pm and the surface being coated with a DLC coating of type aC:H with a roughness Ra equal to about 0.01 pm.

[0132] Identical elements shown in the aforementioned figures are identified by identical numerical references. Table 1 gives the dynamic viscosities at 80°C of the different lubricants mentioned in Figure 3 and in Figures 5 to 11.

[0133] [Table 1]

[0134] Figure 1 schematically illustrates a system 1 comprising two parts 10, 20 and a lubricant 30, according to an example of an embodiment of the present invention.

[0135] The two parts 10, 20 are considered to be in contact, and the contact between the two parts is lubricated by the lubricant 30.

[0136] The first of the two pieces, named first piece 10, includes a substrate 11.

[0137] The substrate 11 of the first part 10 is, for example, formed of at least one of: - a ceramic, for example a silicon nitride, a silicon carbide (SiC), an alumina (Al2O3), a zirconia (ZrO2), or

[0138] - a metal, for example iron, aluminum and / or copper, or

[0139] - an alloy, for example a steel, an aluminium alloy or a copper alloy, or - a polymer, for example a poly-ether-ether-ketone (PEEK), a polyamide-imide (PAI) or a butadiene-acrylonitrile (NBR).

[0140] The first part 10 further includes here an amorphous carbon-based coating 12 applied to the surface of the substrate 11. The amorphous carbon-based coating 12 is thus here in contact with the lubricant 30.

[0141] The first part 10 thus includes here a free surface 15 which is formed by a surface of the amorphous carbon-based coating 12.

[0142] The free surface 15, for example, has a roughness Ra of less than 0.2 pm.

[0143] The amorphous carbon-based coating 12 of the first part 10 here has a DLC coating 13 (“diamond-like carbon”). Here, more specifically, the DLC coating 13 is thus in contact with the lubricant 30.

[0144] The free surface 15 is therefore formed here by a surface of the DLC coating 13.

[0145] In the illustrated embodiment example, the amorphous carbon-based coating 12 of the first part 10 also includes here an underlayer 14.

[0146] The underlayer 14 is positioned between the substrate 11 and the DLC coating 13.

[0147] Similarly here, a second of the two pieces, named second piece 20, includes a substrate 21.

[0148] The substrate 21 of the second part 20 is, for example, formed of at least one of the following:

[0149] - a ceramic, for example silicon nitride, silicon carbide (SiC), alumina (Al2O3), zirconia (ZrO2), or

[0150] - a metal, for example iron, aluminum and / or copper, or

[0151] - an alloy, for example steel, an aluminum alloy or a copper alloy, or

[0152] - a polymer, for example a poly-ether-ether-ketone (PEEK), a polyamide-imide (PAI) or a butadiene-acrylonitrile (NBR).

[0153] The second part 20 here includes an amorphous carbon-based coating 22 applied to the surface of the substrate 21. The amorphous carbon-based coating 22 is thus in contact with the lubricant 30.

[0154] The second part 20 here includes a free surface 25 which is formed by a surface of the amorphous carbon-based coating 22.

[0155] For example, the free surface 25 has a roughness Ra of less than 0.2 pm.

[0156] The amorphous carbon-based coating 22 of the second part 20 here has a DLC coating 23 (“diamond-like carbon”). Here, more specifically, the DLC coating 23 is thus in contact with the lubricant 30.

[0157] The free surface 25 is therefore formed here by a surface of the DLC coating 23. In the illustrated embodiment example, the amorphous carbon-based coating 22 of the second part 20 also includes here a sub-layer 24.

[0158] The underlayer 24 is positioned between the substrate 21 and the DLC coating 23.

[0159] Lubricant 30 comprises at least one base 31, at a content of at least 50 wt.% of lubricant 30.

[0160] According to one embodiment of the invention, base 31 consists of mono-glycol.

[0161] It can be made up of 100 wt.% of a mono-glycol, or a mixture.

[0162] The mono-glycol of base 31 may for example include a monoethylene glycol (EG) and / or a monopropylene glycol (PG).

[0163] Lubricant 30 may also possibly contain at least one additive 32.

[0164] For example, additive 32 may contain at least one of the following: water, glycol, glycerol, viscosity modifier, anti-wear (AW), extreme pressure (EP), anti-oxidant, anti-corrosion, anti-foam, detergent, anti-emulsifier, friction modifier, acid neutralizer.

[0165] The content of additive 32 in lubricant 30 is then at most 50 wt.% of lubricant 30.

[0166] Figure 2 schematically illustrates part of a valve distribution system according to an example of an embodiment of the invention.

[0167] The valve distribution system here comprises a cam and a valve pusher element, the first part 10 of the system 1 described previously thus forming here the cam and the second part 20 forming here the valve pusher element.

[0168] In the example shown here, the push element of a valve is a pusher, but it could be a rocker arm, a rocker, or something else.

[0169] Figure 3 presents a comparison of the evolution of power dissipated by friction, in Watts (W), i.e. mechanical losses by friction, as a function of the rotational speed of a camshaft in a system as illustrated in Figure 2, in which lubricant 30 is either monoethylene glycol or a 0W30 grade engine oil with a friction modifier additive (of the MoDTC type), also referred to here as "engine oil 2".

[0170] In this particular embodiment, the first part 10 (the cam) and the second part 20 (the pusher) are coated with DLC 13, 23. The first part has a tungsten carbide and carbon-enriched tungsten carbide underlayer and a roughness Ra of 0.04 µm. The second part has a chromium and chromium nitride underlayer and a roughness Ra of 0.03 µm. As described previously, such underlayers are optional and could be omitted under other conditions.

[0171] According to this figure, with lubricant 30 consisting of 0W30 grade engine oil with a friction modifier additive (MoDTC), the higher the speed, from approximately 350 rpm to approximately 2200 rpm, the greater the power dissipated by friction; here it increases from approximately 15W to 68W.

[0172] With lubricant 30 consisting of monoethylene glycol, when the speed increases in the same range, i.e. from about 350 rpm to about 2200 rpm, the power dissipated by friction decreases over the speed range from about 350 rpm to 750 rpm, from about 15W to about 10W, and then increases only slightly between 750 rpm and 2200 rpm, from about 10W to about 24W.

[0173] It therefore appears that at 2200 rpm, using monoethylene glycol instead of a 0W30 grade engine oil with a friction modifier additive (MoDTC) reduces the power dissipated by friction from approximately 68W to 24W. The power dissipated is thus reduced by almost a factor of three.

[0174] Figure 4 schematically represents a system 1 comprising two parts 10, 20 and a lubricant 30 according to a second embodiment of the invention.

[0175] In this example, the first part is a cylinder and the second part is a flat surface, and the system is lubricated with a glycol-based lubricant. The cylinder rotates about its axis of revolution, and the flat surface has a low-amplitude, low-speed (approximately 1 mm / s) reciprocating linear motion. This low-amplitude reciprocating linear motion of the flat surface maintains a linear contact configuration even when the surface wears, thus preserving the desired contact pressure during operation.

[0176] For example, a sliding contact between a cylindrical part and a surface under an average contact pressure of 390 MPa, lubricated by a glycol-based lubricant instead of engine oil, reduces the maximum permissible force (MPF) and ensures system lubrication. Various examples are illustrated with reference to Figures 5 to 11.

[0177] Figure 5 shows a comparison of the evolution of the coefficient of friction as a function of the sliding speed between two parts in a system as illustrated in Figure 4, for different lubricants used, at 80°C, under an average contact pressure of 390 MPa. In the example in Figure 5, the first part 10 (the cylinder) and the second part 20 (the flat) are coated with a DLC coating of the aC:H type. The first part has a tungsten carbide (WC) and carbon-enriched tungsten carbide (WC / C) sublayer and has a roughness Ra of approximately 0.03 pm. The second part has a chromium, chromium nitride (CrN), WC, and WC / C sublayer with a roughness Ra of approximately 0.01 pm.

[0178] The different lubricants used here are: a monoethylene glycol, a monopropylene glycol, a butane-1,4-diol, engine oil 1 (5W30 grade engine oil without friction modifier additive), and engine oil 2 (commercial 0W30 grade engine oil containing a MoDTC type friction modifier additive).

[0179] When the first part and the second part are coated with a DLC coating of type aC:H having a roughness Ra less than 0.05 pm, the use of monoethylene glycol, monopropylene or butane-1,4-diol as lubricant makes it possible to achieve a coefficient of friction of the order of 0.005 over at least part of the sliding speed range [0.67; 2.7 m / s].

[0180] Over this same speed range, the use of engine oil to lubricate the cylinder and the surface coated with DLC aC:H leads at best to a coefficient of friction of 0.01.

[0181] Unexpectedly, monoethylene glycol, monopropylene glycol and butane-1,4-diol show lower coefficients than engine oils over at least part of the speed range investigated under tribological conditions favorable to a mixed lubrication regime (i.e. due to high contact pressure and relatively low fluid drive speed) although the three lubricants mentioned have a lower dynamic viscosity than the two engine oils.

[0182] Figure 6 shows a comparison of the evolution of the coefficient of friction as a function of the sliding speed between the two parts in a system as illustrated in Figure 4, in the same configuration as in Figure 5, i.e., at 8CTC, under an average contact pressure of 390 MPa. The first and second parts are coated with a DLC coating of type aC:H. The first part has a tungsten carbide (WC) and carbon-enriched tungsten carbide (WC / C) sub-layer and has a roughness Ra of approximately 0.03 pm. The second part has a chromium, chromium nitride (CrN), WC, and WC / C sub-layer and has a roughness Ra of approximately 0.01 pm. The different lubricants used are: a mixture of 52 wt. monoethyl glycol and 48 wt. monopropylene glycol, and a mixture of 70 wt. of monoethylene glycol and 30 wt. glycerol, engine oil 1, and engine oil 2.In this same configuration of contacting materials (cylinder and plate coated with DLC), the use of a mixture of 52 wt. of monoethylene glycol and 48 wt. of monopropylene glycol or a mixture of 70 wt. of monoethylene glycol and 30 wt. of glycerol also makes it possible to achieve a very low coefficient of friction (0.005 or less) compared to oils 1 and 2, as illustrated in Figure 6.

[0183] Figure 7 shows a comparison of the evolution of the coefficient of friction as a function of the sliding speed between two parts in a system as illustrated in Figure 4, at 80°C, under an average contact pressure of 390 MPa. The first part is made of uncoated steel with a roughness of approximately 0.02 pm, and the second part is coated with a DLC coating of type aC:H and has a roughness Ra of approximately 0.01 pm. The second part also has an undercoat of chromium, chromium nitride, WC, and WC / C.

[0184] The different lubricants used here are: monoethylene glycol, monopropylene glycol, engine oil 1 and engine oil 2.

[0185] When the first steel part is uncoated with a roughness Ra less than 0.05 pm and the second part is coated with a DLC coating of type aC:H, the use of monoethylene glycol or monopropylene glycol makes it possible to achieve a coefficient of friction less than 0.01 whereas engine oils 1 and 2 give at best a coefficient of friction of 0.02, as illustrated in Figure 7.

[0186] Here too, unexpectedly, monoethylene glycol and monopropylene glycol show lower coefficients than engine oils over at least part of the speed range investigated under tribological conditions favorable to a mixed lubrication regime, although the two lubricants mentioned have a lower dynamic viscosity than the two engine oils.

[0187] Figure 8 presents a comparison of the evolution of the coefficient of friction as a function of the sliding speed between the two parts in a system as illustrated in Figure 4, at 80 D C, under an average contact pressure of 390 MPa, the first steel part being uncoated with a roughness of approximately 0.02 µm and the second part being coated with a DLC coating of type aC:H and having a roughness Ra of less than 0.01 µm. The second part also has an undercoat of chromium, chromium nitride, WC, and WC / C. The different lubricants used here are: monoethylene glycol, a mixture of 85 wt. monoethylene glycol and 15 wt. water, a mixture of 75 wt. monoethylene glycol and 25 wt. water, engine oil 1, and engine oil 2.

[0188] In this same configuration of contacting materials (first part uncoated and second part DLC coated), the use of monoethylene glycol-water mixtures with up to 25 wt.% water also makes it possible to achieve very low coefficients of friction (0.005 or less) compared to oils 1 and 2, as illustrated in Figure 8. Unexpectedly, having a water content of up to 25 wt.% does not degrade the friction behavior of the system, even though the addition of water leads to a reduction in the lubricant's viscosity.

[0189] Figures 9 and 10 present a comparison of the evolution of the coefficient of friction as a function of the sliding speed between the two parts in a system as illustrated in Figure 4, at 80°C, under an average contact pressure of 390 MPa.

[0190] In Figure 9, the first part is made of uncoated steel with a roughness of approximately 0.02 pm, and the second part is coated with a DLC coating of type aC and has a roughness Ra of approximately 0.02 pm. The second part has a chromium and chromium nitride underlayer.

[0191] For Figure 10, the first and second parts are made of steel coated with a ta-C type DLC coating with a chromium and chromium nitride underlayer. The roughness Ra of the first part is 0.01 pm and the roughness Ra of the second part is 0.03 pm.

[0192] The different lubricants used here are: a monoethylene glycol, and engine oil 1. The use of other types of DLC coatings such as aC, or ta-C, with a lubricant according to the invention also makes it possible to achieve very low coefficients of friction (0.005 or less) compared to those obtained with engine oil 1, in particular as soon as the sliding speed becomes higher, i.e. here beyond about 1.5 m / s for the configuration of figure 9, and beyond about 1.65 m / s for the configuration of figure 10.

[0193] Figure 11 presents a comparison of the evolution of the coefficient of friction as a function of the sliding speed between the two parts in a system as illustrated in Figure 4, at 80°C, under an average contact pressure of 100 MPa, the first part being made of 2017A aluminum alloy having a roughness Ra of 0.08 pm and the second part being coated with a DLC coating of type aC:H comprising an underlayer of chromium, chromium nitride, WC and WC / C and having a roughness Ra equal to about 0.01 pm.

[0194] The different lubricants used here are: monoethylene glycol, and engine oil 1.

[0195] Over the illustrated sliding speed range [0.67 m / s ; 2.7 m / s], the coefficient of friction obtained with monoethylene glycol is between 0.04 and 0.05, while that obtained with engine oil 1 is 0.07, as shown in Figure 11.

[0196] Here too, unexpectedly, monoethylene glycol shows lower coefficients than engine oil over the speed range investigated under tribological conditions favorable to a mixed lubrication regime, although monoethylene glycol has a lower dynamic viscosity than engine oil.

Claims

DEMANDS 1. System (1) comprising two parts (10, 20) and a lubricant (30), the two parts being in contact, and the contact between the two parts being lubricated by the lubricant (30), at least one of the two parts (10) comprising a substrate (11) and an amorphous carbon-based coating (12) applied to the surface of the substrate (11), the amorphous carbon-based coating (12) of the first of the two parts (10) comprising at least one diamond-type carbon (DLC) coating (13), and the lubricant (30) comprising at least one base (31), the base (31) being mono-glycol, a content of the base (31) in the lubricant (30) being at least 50 wt.% of the lubricant (30).

2. System (1) according to claim 1, wherein the base (31) has an average molar mass of less than 100 g / mol.

3. System (1) according to any one of claims 1 or 2, wherein the base (31) has a viscosity less than or equal to 50 cSt at 40°C.

4. System (1) according to any one of claims 1 to 3, wherein the base (31) has a viscosity less than or equal to 10 cSt at 100°C.

5. System (1) according to any one of claims 1 to 4, wherein the mono-glycol of the base (31) comprises a monoethylene glycol (EG) and / or a monopropylene glycol (PG).

6. System (1) according to any one of claims 1 to 5, wherein the lubricant (30) comprises at least one additive (32), the additive (32) comprising at least one of: water, glycol, glycerol, viscosity modifier, anti-wear (AW), extreme pressure (EP), anti-oxidation, anti-corrosion, anti-foam, detergent, anti-emulsifier, friction modifier, acid neutralizer.

7. System (1) according to claim 6, wherein a content of the additive (32) in the lubricant (30) is at most 50 wt.% of the lubricant.

8. System (1) according to claim 7, wherein a water content in the lubricant (30) is at most 25 wt.% of the lubricant (30).

9. System (1) according to any one of claims 6 or 7, wherein the lubricant (30) comprises a mixture of monoethylene glycol (PG) and glycerol, in particular a mixture comprising 70 wt.% of monoethylene glycol (PG) and 30 wt.% of glycerol.

10. System (1) according to any one of claims 1 to 9, wherein the lubricant (30) comprises a maximum of 3 wt.% of impurities.

11. System (1) according to any one of claims 1 to 10, wherein the amorphous carbon-based coating (12) of the first of the two parts (10) comprises a sub-layer (14), the sub-layer (14) being disposed between the substrate (11) and the DLC coating (13).

12. System (1) according to any one of claims 1 to 11, wherein a free surface (15) of the first of the two parts (10) has a roughness Ra of less than 0.2 pm.

13. System (1) according to any one of claims 1 to 12, wherein a free surface (25) of a second of the two parts (20) has a roughness Ra of less than 0.2 pm.

14. System (1) according to any one of claims 1 to 13, wherein the substrate (11) of the first of the two parts (10) comprises one of: - at least one ceramic, for example silicon nitride, silicon carbide (SiC), alumina (Al2O3), zirconia (Zr2), or - a metal, for example iron, aluminum and / or copper, or - an alloy, for example steel, an aluminum alloy or a copper alloy, or - a polymer, for example a poly-ether-ether-ketone (PEEK), a polyamide-imide (PAI) or a butadiene-acrylonitrile (NBR).

15. System (1) according to any one of claims 1 to 14, wherein a second of the two parts (20) comprises at least one substrate (21), the substrate (21) of the second of the two parts comprising one of: - at least one ceramic, for example silicon nitride, silicon carbide (SiC), alumina (Al2O3), zirconia (ZrO2), or - a metal, for example iron, aluminum and / or copper, or - an alloy, for example steel, an aluminum alloy or a copper alloy, or a polymer, for example a poly-ether-ether-ketone (PEEK), a polyamide-imide (PAI) or a butadiene-acrylonitrile (NBR).

16. System (1) according to any one of claims 1 to 15, wherein a second of the two parts (20) comprises an amorphous carbon-based coating (22), for example comprising a DLC coating (23), the amorphous carbon-based coating (22) being applied to the surface of a substrate (21) of the second of the two parts, and an undercoat (24), the undercoat (24) then being disposed between a substrate (21) and the DLC coating (23).

17. Valve distribution system comprising a cam and a valve pusher element, characterized in that it comprises a system (1) according to any one of claims 1 to 16, the first of two parts (10) of the system (1) forming the cam and a second of two parts (20) forming the valve pusher element, for example a pusher, a rocker arm, a rocker arm, or other.

18. Use of a lubricant in a mechanical system comprising two parts, the two parts being in contact with each other, and the contact being lubricated by the lubricant, at least one of the two parts comprising a substrate and an amorphous carbon-based coating applied to the surface of the substrate, and the amorphous carbon-based coating of the first of the two parts comprising at least a DLC coating, the lubricant comprising at least one base, the base being mono-glycol, and a content of the base in the lubricant being at least 50 wt.% of the lubricant.

19. Use of a lubricant according to claim 18, wherein the system further operates under at least one of the following parameters: - a sliding speed between the two parts between 0 m / s and 50 m / s; and / or - a fluid entrainment velocity between 0.01 m / s and 50 m / s; and / or - a contact pressure between the two parts of between 1 MPa and 3 GPa.

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