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

Abstract

The invention relates to a system (1) comprising two contacting parts (10, 20) lubricated by a 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 DLC coating (13), and the lubricant (30) comprising at least one base (31), the base (31) being monoglycol, the content of the base (31) in the lubricant (30) being at least 50 wt.% of the lubricant (30). It also relates to a valve distribution system comprising such a system (1), and to the use of a lubricant in a mechanical system comprising two parts in contact with each other. Figure for the abstract: Fig. 1

Description

Title of the invention: System comprising two parts in lubricated contact and use of a lubricant in such a system

[0001] The invention relates to a system comprising two parts in contact in which the contact is lubricated by a lubricant.

[0002] It also relates to the use of such a system, and in particular of a lubricant in such a system.

[0003] It falls within the category of mechanical systems lubricated by oils.

[0004] 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.

[0005] One objective is, for example, to provide a lubricated system in which mechanical friction losses (MFRs) are relatively low, or even reduced, compared to traditional systems in a given application.

[0006] To reduce mechanical losses due to friction 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.

[0007] 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.

[0008] Glycols, and in particular polyethylene glycols, can be used as additives in oil-based lubricants or in water-based lubricants, in particular to adjust viscosity.

[0009] An oil-based lubricant, or a water-based lubricant, herein means a lubricant comprising at least 50 wt.% of oil, or water, respectively.

[0010] Generally, in this description, a percentage is a mass percentage, designated "wt.%" or "%wt." interchangeably, unless otherwise specified.

[0011] An additive generally has a fairly low concentration in a lubricant, i.e. on the order of a few mass percent at most.

[0012] Glycols in an oil-based or water-based lubricant therefore have, for example, a content less than or equal to about 10 wt.%.

[0013] On the other hand, polyalkylene glycols, and in particular polyethylene glycols, can be used as a base for a lubricant. This is then referred to as a lubricant in which the glycol base is composed of molecules with a molecular weight greater than or equal to 200 g / mol, and often between 400 and 1000 g / mol.

[0014] However, reducing the viscosity of an oil to reduce TMF over a high-speed operating range can be accompanied by more frequent limit and mixed operating conditions over lower-speed operating ranges, resulting in more TMF and an increased risk of damage to components (wear that can lead to seizing).

[0015] Moreover, friction-modifying additives can have a high cost.

[0016] Moreover, these are often compounds that can generate sulfated, sulfurous or phosphorus ash which can damage the exhaust gas aftertreatment systems of internal combustion engines.

[0017] Moreover, friction modifier additives have a limited effect over time (they are consumed) which necessitates regular oil changes to replace the oil and add new additives.

[0018] 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.

[0019] 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 conventionally used in lubricants.

[0020] 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. Generally, surface coatings are chosen to be compatible with the oil selected as a lubricant, but the oil is not formulated to fully utilize coated surfaces such as amorphous carbon coatings, resulting in relatively small friction reductions.

[0021] The present invention thus aims to improve at least in part the aforementioned disadvantages, which may lead to other advantages.

[0022] To this end, a mechanical system comprising two parts and a lubricant is proposed according to a first aspect, the two parts being in contact with each other, and the contact between the two parts being lubricated by the lubricant.

[0023] At least one of the two parts comprises 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 comprises at least a DLC coating (for “diamond like carbon”).

[0024] The term DLC encompasses several types of coating: 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, for example, be found 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] The use of a glycol-based lubricant as a replacement for an oil, whether mineral or synthetic, thus makes it possible to reduce PMF and ensure lubrication of the system.

[0028] Such a system, thanks to the formulation of the lubricant in combination with contacting part 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: - 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; - 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; - 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.

[0030] A content indicated as a percentage here refers to a mass percentage, marked "wt. %" or "%wt.".

[0031] 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.%.

[0032] In one particular embodiment, the lubricant consists of the base alone, that is to say that the content of the base in the lubricant is about 100 wt.%, without considering any possible impurities.

[0033] According to one embodiment, the base has a viscosity less than or equal to 50 cSt, when measured at 40°C.

[0034] According to one embodiment, the base has a viscosity less than or equal to 10 cSt, when measured at 100°C.

[0035] A glycol here refers to a hydrocarbon, linear or branched, comprising two hydroxyl functions.

[0036] 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.

[0037] In one particular embodiment, the base comprises 100 wt.% of a mono-glycol.

[0038] The base then consists of only one mono-glycol, i.e. a mono-glycol of only one type.

[0039] For example, at least one mono-glycol of the base has a low molar mass.

[0040] According to one embodiment, the base has an average molar mass less than 100 g / mol, for example between 40 g / mol and 100 g / mol.

[0041] For example, ethylene glycol has a molar mass of about 62 g / mol.

[0042] For example, propylene glycol has a molar mass of approximately 76 g / mol.

[0043] For example, a methanediol has a molar mass of about 48 g / mol.

[0044] For example, butanediol has a molar mass of approximately 90 g / mol.

[0045] In contrast, pentanediol has a molar mass of approximately 104 g / mol. Pentanediol can, for example, be used in a mixture with a lighter monoglycol.

[0046] Furthermore, pentanediol may also be of interest for use in a heated application.

[0047] 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.

[0048] 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.

[0049] However, coupled with DLC-type surface coatings, low molecular weight glycols have proven to be very effective lubricants for reducing PMFs in such mechanical systems.

[0050] In a particular embodiment, at least one mono-glycol of the base comprises a monoethylene glycol (EG) and / or a monopropylene glycol (PG).

[0051] Monopropylene glycol (PG) may also be referred to as propane-1,2-diol.

[0052] In one embodiment, the lubricant comprises at least one additive.

[0053] The additive comprises, for example, at least one of the following: water, glycol, glycerol, a viscosity modifier, an anti-wear (AW), an "extreme pressure" (EP), an anti-oxidation, an anti-corrosion, an anti-foam, a detergent, an anti-emulsifier, a friction modifier, an acid neutralizer.

[0054] An additive designated "extreme pressure (EP)" is an additive that helps to limit the risks of wear and seizing of a mechanical system under high contact pressures. This type of additive can be found in a traditional gearbox lubricant where its purpose is to limit the risks of wear and seizing of the contacts between gear teeth.

[0055] If the lubricant consists of 100 wt.% of the base, then there is no additive.

[0056] If the lubricant contains at least one additive, a 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.

[0057] In a 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.

[0058] In a particular embodiment, the lubricant 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.

[0059] In one embodiment, the lubricant contains a maximum of 3 wt.% of impurities.

[0060] An impurity here means any element whose presence represents a relatively small quantity, and which is neither intentional nor controlled.

[0061] Here, we consider an approximate level of impurities in the lubricant before its use. Indeed, the nature and content of the impurities are generally unknown, but there is a tolerance regarding the impurity content in the lubricant. Furthermore, we consider the lubricant before use because its composition generally changes over time due to the oxidation of certain initial components, the appearance of debris, etc.

[0062] 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.

[0063] For example, the amorphous carbon-based coating has a thickness of between approximately 0.5 pm and 10 pm, more particularly, for example, between approximately 1 pm and 4 pm.

[0064] For example, the DLC coating has a thickness of between approximately 0.5 pm and 10 pm, more particularly for example between approximately 1 pm and 4 pm.

[0065] 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.

[0066] Such roughness is measured for example by standard profilometry, for example according to ISO13565-1.

[0067] Here, the roughness is preferably as low as possible.

[0068] 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.

[0069] The hardness "Hv" here refers to Vickers hardness.

[0070] The amorphous carbon-based coating is at least made up of the DLC coating.

[0071] However, according to one embodiment, the amorphous carbon-based coating of the first of the two parts may also include at least one underlayer.

[0072] Such an underlayer is optional and the choice of its presence depends on the stress conditions and the nature of the parts, in particular the substrate on which it can be applied.

[0073] Such an underlayer is intended in particular to improve the adhesion of the deposit and to accommodate any differences in mechanical properties between the substrate and the DLC coating.

[0074] The underlayer is then placed between the substrate and the DLC coating.

[0075] For example, the underlayer is arranged on the surface of the substrate.

[0076] The DLC coating is then, for example, placed on the underlayer.

[0077] The at least one sub-layer can be of any nature.

[0078] 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 and silicon carbonitride (CrSiCHN) as described in WO2019 / 241720A1.

[0079] For example, the underlayer is ceramic, for example chromium nitride.

[0080] In one embodiment, the underlayer 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.

[0081] 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: - 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).

[0082] In one embodiment, the second of the two parts of the system comprises at least one substrate.

[0083] 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: - 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).

[0084] In one embodiment, the second of the two parts of the system includes an amorphous carbon-based coating.

[0085] For example, the amorphous carbon-based coating is applied to the surface of the substrate of the second of the two parts.

[0086] For example, the amorphous carbon-based coating of the second of the two parts includes at least one DLC (for “diamond like carbon”) coating.

[0087] 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.

[0088] For example, the amorphous carbon-based coating of the second part has a thickness of between approximately 0.5 pm and 10 pm, more particularly for example between approximately 1 pm and 4 pm.

[0089] For example, the DLC coating of the second part has a thickness of between approximately 0.5 pm and 10 pm, more particularly for example between approximately 1 pm and 4 pm.

[0090] In one 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.

[0091] Such roughness is also measured for example by standard profilometry, for example according to the ISO13565-1 standard.

[0092] Here, the roughness of the second of the two parts is also preferably as low as possible.

[0093] For example, the second of the two parts of 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.

[0094] The amorphous carbon-based coating of the second part is at least made up of the DLC coating.

[0095] However, according to one embodiment, the amorphous carbon-based coating of the second of the two parts may further comprise at least one underlayer.

[0096] Such an underlayer is intended in particular to improve the adhesion of the deposit and to accommodate any differences in mechanical properties between the substrate and the DLC coating.

[0097] The underlayer is then placed between the substrate and the DLC coating of the second of the two parts.

[0098] For example, the underlayer is arranged on the surface of the substrate of the second part.

[0099] The DLC coating of the second part is then, for example, disposed on the sub- layer of the second piece.

[0100] At least one underlayer of the second part can be of any kind.

[0101] 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 and silicon carbonitride (CrSiCHN) as described in WO2019 / 241720A1.

[0102] For example, the underlayer of the second part is ceramic, for example chromium nitride.

[0103] In one embodiment, 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.

[0104] Also proposed, according to another aspect of the invention, is a valve distribution system comprising a cam and a valve push 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 push element.

[0105] In one embodiment, at least one of the cam and the pusher element of a valve may be made of steel.

[0106] In one embodiment, the DLC coating of at least one of the cam and the pusher element of a valve may have a roughness Ra between 0.01 pm and 0.05 pm, for example about 0.03 pm.

[0107] In one embodiment, the DLC coating of at least one of the cam and pusher element of a valve can be lubricated by a monoethylene glycol.

[0108] In one embodiment, the cam comprises at least one tungsten carbide (WC) sublayer, and / or at least one carbon-enriched tungsten carbide (WCC) sublayer.

[0109] In such a configuration, the PMFs of the valve distribution system are greatly reduced compared to a case where the cam and the push element of a valve coated with the same DLC coating of type aC:H are lubricated by a 0W30 grade engine oil.

[0110] 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.

[0111] At least one of the two parts comprises 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 comprises at least a DLC (for “diamond like carbon”) coating.

[0112] 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.

[0113] The use of such a glycol-based lubricant as a replacement for an oil, whether mineral or synthetic, thus makes it possible to reduce PMF and ensure lubrication of the system.

[0114] In a particular use case, the system further operates under at least one of the following parameters: - 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 - 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 - a contact pressure between the two parts of between 1 MPa and 3 GPa, preferably between 10 MPa and 1 GPa.

[0115] 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.

[0116] If the system is a bearing, the theoretical sliding speed is then about 0 m / s.

[0117] The drive speed designates an average of the speeds of the surfaces of the two parts of the system, the speeds being considered in a reference frame of the contact zone.

[0118] The invention, according to an exemplary embodiment, will be better understood and its advantages will become more apparent upon reading the following detailed description, given by way of example and not limitation, with reference to the accompanying drawings in which:

[0119] Fig. 1 schematically represents a system comprising two parts and a lubricant according to a first embodiment of the invention;

[0120] Fig. 2 schematically represents a system comprising two parts and a lubricant according to a second embodiment of the invention;

[0121] Fig. 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 Fig. 2 lubricated by monoethylene glycol or a fully formulated 0W30 grade motor oil with a friction modifier additive (MoDTC, i.e. molybdenum dithiocarbamate), the cam and tappet being coated with DLC;

[0122] Fig. 4 schematically represents a system comprising two parts 10, 20 and a lubricant 30 according to a third embodiment of the invention;

[0123] 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;

[0124] 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.% monoethylene glycol and 48 wt.% monopropylene glycol, a mixture of 70 wt.% monoethylene glycol and 30 wt.% 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;

[0125] Figure 7 shows 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 [Fig.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 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;

[0126] 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.% monoethylene glycol and 15 wt.% water, a mixture of 75 wt.% monoethylene glycol and 25 wt.% 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;

[0127] Fig. 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 Fig. 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 surface being coated with a DLC coating of type aC (non-hydrogenated amorphous carbon) with a roughness Ra equal to about 0.02 pm;

[0128] 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

[0129] Fig. 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 Fig. 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 of about 0.08 pm and the surface being coated with a DLC coating of type aC:H with a roughness Ra of about 0.01 pm.

[0130] Identical elements represented in the aforementioned figures are identified by identical numerical references.

[0131] Table 1 gives the dynamic viscosities at 80°C of the different lubricants mentioned in [Fig.3] and in Figures 5 to 11.

[0132] [Tables 1] Lubricant Dynamic viscosity at 80°C (mPa.s) Kinematic viscosity at 80°C (cSt) Kinematic viscosity at 40°C (cSt) Kinematic viscosity at 100°C (cSt) Engine oil 1 (5W30 grade, fully formulated) 12.3 15.6 55.6 9.9 Engine oil 2 (0W30 grade, fully formulated) 12.1 15.1 50.1 9.8 Monoethylene glycol 3.2 3.0 8.5 2.2 Monopropylene glycol 4.2 4.3 18.6 2.5 Butane-1,4-diol 8.7 8.7 32.2 5.5 52%wt. monoethylene glycol-48%wt. monopropylene glycol 3.7 3.8 13.5 2.3 70%wt. Monoethylene glycol-30% wt. glycerol 7.5 6.6 35.6 3.8 85% wt. monoethylene glycol-15% wt. water 2.6 2.5 8.8 1.5 75% wt. monoethylene glycol-25% wt. water 2.0 1.9 6.2 1.2

[0133] Fig. 1 schematically illustrates a system 1 comprising two parts 10, 20 and a lubricant 30, according to an embodiment of the present invention.

[0134] The two parts 10, 20 are considered to be in contact, and the contact between the two parts is lubricated by the lubricant 30.

[0135] A first of the two parts, called first part 10, comprises a substrate 11.

[0136] 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 - 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).

[0137] The first part 10 further comprises 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.

[0138] The first part 10 thus includes here a free surface 15 which is formed by a surface of the amorphous carbon-based coating 12.

[0139] The free surface 15 has for example a roughness Ra of less than 0.2 pm.

[0140] The amorphous carbon-based coating 12 of the first part 10 comprises Here, a DLC 13 (“diamond like carbon”) coating is used. In this particular case, the DLC 13 coating is in contact with lubricant 30.

[0141] The free surface 15 is therefore here formed by a surface of the DLC coating 13.

[0142] In the illustrated embodiment, the amorphous carbon-based coating 12 of the first piece 10 also includes here an underlayer 14.

[0143] The underlayer 14 is disposed between the substrate 11 and the DLC coating 13.

[0144] Similarly here, a second of the two parts, named second part 20, comprises a substrate 21.

[0145] The substrate 21 of the second part 20 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 - 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).

[0146] 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.

[0147] The second part 20 here includes a free surface 25 which is formed by a surface of the amorphous carbon-based coating 22.

[0148] The free surface 25 has for example a roughness Ra of less than 0.2 pm.

[0149] The amorphous carbon-based coating 22 of the second part 20 here includes a DLC coating 23 (“diamond-like carbon”). Here, more specifically, the DLC coating 23 is thus in contact with the lubricant 30.

[0150] The free surface 25 is therefore here formed by a surface of the DLC coating 23.

[0151] In the illustrated embodiment, the amorphous carbon-based coating 22 of the second piece 20 also includes here an underlayer 24.

[0152] The underlayer 24 is disposed between the substrate 21 and the DLC coating 23.

[0153] The lubricant 30 comprises at least one base 31, at a content of at least 50 wt.% of the lubricant 30.

[0154] According to one embodiment of the invention, base 31 consists of mono-glycol.

[0155] It can consist of 100 wt.% of a mono-glycol, or a mixture.

[0156] The mono-glycol of base 31 may for example comprise a monoethylene glycol (EG) and / or a monopropylene glycol (PG).

[0157] The lubricant 30 may also optionally include at least one additive 32.

[0158] For example, additive 32 may comprise at least one of: water, a glycol, a glycerol, a viscosity modifier, an anti-wear (AW), an "extreme pressure" (EP), an anti-oxidant, an anti-corrosion, an anti-foam, a detergent, an anti-emulsifier, a friction modifier, an acid neutralizer.

[0159] The content of additive 32 in lubricant 30 is then at most 50 wt.% of lubricant 30.

[0160] Fig. 2 schematically illustrates part of a valve distribution system according to an example of an embodiment of the invention.

[0161] The valve distribution system here comprises a cam and a valve pusher element, the first part 10 of the system 1 described above thus forming here the cam and the second part 20 forming here the valve pusher element.

[0162] In the example illustrated here, the push element of a valve is a pusher, but it could be a rocker arm, a rocker, or something else.

[0163] 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 the lubricant 30 is either a monoethylene glycol or a 0W30 grade engine oil with a friction modifier additive (of the MoDTC type), also referred to herein as "engine oil 2".

[0164] 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 sub-layer of tungsten carbide and enriched tungsten carbide. The first part is made of carbon and has a roughness Ra of 0.04 pm. The second part has a chromium and chromium nitride underlayer and has a roughness Ra of 0.03 pm.

[0165] As described above, such sublayers are however optional and could be omitted under other conditions.

[0166] According to this figure, with lubricant 30 consisting of 0W30 grade engine oil with a friction modifier additive (MoDTC), the higher the speed, from about 350 rpm to about 2200 rpm, the greater the power dissipated by friction; here it increases from about 15W to 68W.

[0167] 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.

[0168] 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 3.

[0169] Fig. 4 schematically represents a system 1 comprising two parts 10, 20 and a lubricant 30 according to a second embodiment of the invention.

[0170] In this example, the first part is a cylinder and the second part is a plate, and the system is lubricated by a glycol-based lubricant. The cylinder rotates about its axis of revolution, and the plate has a low-amplitude, low-speed (approximately 1 mm / s) reciprocating linear motion. The low-amplitude reciprocating linear motion of the plate maintains a linear contact configuration even when the plate wears, thus preserving the desired contact pressure during operation.

[0171] By way of example, a sliding contact between a cylindrical part against a plane under an average contact pressure of 390 MPa, and lubricated by a glycol-based lubricant instead of engine oil, thus makes it possible to reduce the PMF and ensure lubrication of the system. Various examples are illustrated with reference to Figures 5 to 11.

[0172] Fig. 5 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 Fig. 4, for different lubricants used, at 80°C, under an average contact pressure of 390 MPa.

[0173] In the example of [Fig. 5], the first part 10 (the cylinder) and the second part 20 (the plane) are coated with a DLC coating of type aC:H. The first part has a tungsten carbide (WC) sublayer and tungsten carbide The first part is carbon-enriched (WC / C) and has a roughness Ra of approximately 0.03 gm. The second part has an underlayer of chromium, chromium nitride (CrN), WC, and WC / C with a roughness Ra of approximately 0.01 gm.

[0174] The different lubricants used here are: a monoethylene glycol, a monopropylene glycol, a butane-1,4-diol, a motor oil 1 (5W30 grade motor oil without friction modifier additive), and motor oil 2 (0W30 grade commercial motor oil containing a MoDTC type friction modifier additive).

[0175] 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 gm, 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].

[0176] Over this same speed range, the use of an 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.

[0177] 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.

[0178] 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 80°C, under an average contact pressure of 390 MPa, the first and second parts being 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 gm. The second part has a chromium, chromium nitride (CrN), WC, and WC / C sublayer and has a roughness Ra of approximately 0.01 gm.

[0179] The different lubricants used here are: 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.

[0180] In this same configuration of materials in contact (cylinder and plate coated with DLC), the use of a mixture of 52 wt.% monoethylene glycol and 48 wt. monopropylene glycol or a mixture of 70 wt. monoethylene glycol and 30 wt. Glycerol also allows for a very low coefficient of friction (0.005 or less) compared to oils 1 and 2, as illustrated in [Fig.6].

[0181] Figure 7 shows a comparison of the evolution of the coefficient of friction Depending on the sliding speed between the two parts in a system as illustrated [Fig. 4], at 80°C, under an average contact pressure of 390 MPa, the first part being uncoated steel with a roughness of approximately 0.02 pm and the second part being coated with a DLC coating of type aC:H and having a roughness Ra of approximately 0.01 pm. The second part also has an undercoat of chromium, chromium nitride, WC, and WC / C.

[0182] The different lubricants used here are: a monoethylene glycol, a monopropylene glycol, engine oil 1 and engine oil 2.

[0183] 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 motor oils 1 and 2 give at best a coefficient of friction of 0.02, as illustrated in [Fig.7].

[0184] 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.

[0185] Figure 8 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, at 80°C, under an average contact pressure of 390 MPa. The first steel part is uncoated 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 less than 0.01 pm. The second part also has an undercoat of chromium, chromium nitride, WC, and WC / C.

[0186] The different lubricants used here are: a 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.

[0187] In this same configuration of materials in contact (first uncoated part and second DLC coated part), the use of monoethylene glycol-water mixtures up to 25 wt.% water also makes it possible to achieve very low coefficients of friction (0.005 or less) compared with oils 1 and 2, as illustrated in [Fig.8].

[0188] Unexpectedly, having a water content of up to 25 wt.% does not degrade the friction behavior of the system although the addition of water leads to a reduction in the viscosity of the lubricant.

[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 [Fig.4], at 80°C, under an average contact pressure of 390 MPa.

[0190] For [Fig. 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 [Fig. 10] the first part and the second part 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.

[0193] 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 with 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 [Fig. 9], and beyond about 1.65 m / s for the configuration of [Fig. 10].

[0194] Fig. 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 Fig. 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.

[0195] The different lubricants used here are: a monoethylene glycol, and engine oil 1.

[0196] 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 [Fig.1 1].

[0197] 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 first 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-like 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 base (31) comprises 100 wt.% of a mono-glycol.

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

7. System (1) according to any one of claims 1 to 6, 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.

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

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

10. System (1) according to any one of claims 7 or 8, wherein the lubricant (30) comprises a mixture of monoethylene glycol (PG) and glycerol, in particular a mixture comprising 70 wt.% monoethylene glycol (PG) and 30 wt.% glycerol.

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

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

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

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

15. System (1) according to any one of claims 1 to 14, 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 (ZrO2), or - a metal, for example iron, aluminum and / or copper, or - an alloy, for example steel, aluminum alloy or copper alloy, or - a polymer, for example polyetheretherketone (PEEK), polyamide imide (PAI) or butadiene acrylonitrile (NBR).

16. System (1) according to any one of claims 1 to 15, 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).

17. System (1) according to any one of claims 1 to 16, 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).

18. 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 17, 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 tappet, a rocker arm, a rocker arm, or other.

19. 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 one 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.

20. Use of a lubricant according to claim 19, wherein the system further operates under at least one of the following parameters: - a sliding speed between the two parts of between 0 m / s and 50 m / s; and / or - a fluid entrainment speed of 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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