Friction-reduced sliding system

Abstract

Sliding system (10) with a sliding element (12) and a counterbody (14) in sliding contact with the sliding element (12), wherein the sliding element (12) has a surface (16) at least partially coated with amorphous carbon (DLC) which is in contact with the counterbody (14), characterized in that the surface (16) coated with the amorphous carbon is treated by running in with a first lubricant containing at least one oxygen-containing organic compound, wherein the first lubricant is replaced after running in by a friction-reducing lubricant with a hydrocarbon-based base oil, and wherein the first lubricant consists of the oxygen-containing compound, optionally in aqueous solution.

Description

[0001] The invention relates to a sliding system comprising a sliding element and a counter body in sliding contact with the sliding element, wherein the sliding element has a surface at least partially coated with amorphous carbon (DLC).

[0002] Amorphous carbon coatings, so-called "diamond-like carbon" or DLC coatings, are useful in many areas of mechanical engineering. In applications subject to high tribological stress, amorphous carbon coatings are applied to stressed components to reduce wear. The carbon coatings are applied using PVD and / or PA-CVD processes, for example.

[0003] In the following, the nomenclature according to the guideline of the Association of German Engineers on carbon coatings from November 2005 (VDI 2840) is used, which divides amorphous carbon coatings into two categories depending on the predominant hybridization of the carbon atoms in the carbon layer. 2 -hybridization, they are referred to as hydrogen-free amorphous carbon layers (aC). If such a layer contains metal atoms or hydrogen atoms in a proportion of more than 3 at.%, they are referred to as metal-containing or hydrogen-containing amorphous carbon layers (aC:Me or aC:H). If the carbon layer contains both metal and hydrogen atoms, they are referred to as metal-containing hydrogen-containing amorphous carbon layers (aC:H:Me). If there is a predominant sp 3If the carbon atoms in the carbon layer are hybridized, the layers are referred to as tetrahedral hydrogen-free amorphous carbon layers (ta-C). Here, too, the carbon layer can contain hydrogen atoms. In this case, they are referred to as tetrahedral hydrogen-containing amorphous carbon layers (ta-C:H). This list of different carbon layer types is, of course, not exhaustive. The various types of amorphous carbon layers exhibit different properties, and thus the choice of layer type for a specific application depends on the respective requirements of the carbon layer.

[0004] Sliding systems consist of several parts that move against each other and are manufactured individually. Due to manufacturing tolerances, the individual parts do not fit together precisely at the factory, so that increased wear occurs at the beginning of the service life of a sliding system until the individual components have adapted to one another. This adaptation period is referred to as running-in, and this running-in usually begins when the sliding system is first put into operation. In the following, however, the term running-in or running-in phase is used for a period in which the sliding system is operated with an initial lubricant that is not the lubricant that will be used long-term. This running-in preferably begins when the sliding system is first put into operation, but is not limited to this.

[0005] A generic sliding system is known from EP 1 510 594 A2. It states that surfaces treated with amorphous carbon are incompatible with conventional lubricants, which typically contain friction-reducing additives based on inorganic compounds such as molybdenum disulfide or organic molybdenum compounds. It is shown that the friction of such systems can be reduced by using lubricants containing organic oxygen-containing compounds. This effect is attributed to the formation of a tribological film containing functional groups such as ether bonds, oxides, and hydroxyl groups formed on the carbon surface.

[0006] DE 10 2011 003 254 A1 discloses a sliding element, in particular a piston ring, wherein the sliding element has, at least on one running surface from the inside out, a coating with a metal-containing adhesive layer and a DLC layer of the ta-C type with a thickness of at least 10 µm. In a method for producing the sliding element, in particular the piston ring, a coating with a metal-containing adhesive layer and a DLC layer of the ta-C type with a thickness of at least 10 µm is applied.

[0007] DE 23 04 583 A describes a running-in oil for piston engines based on naphthalene-based and / or paraffin-based and / or synthetic oils, wherein the running-in oil contains, in combination, as abrasion accelerator, a compound containing or releasing P-OH groups and, as an activator for this, an acid or a compound reacting as an acid under operating conditions.

[0008] Furthermore, EP 1 783 349 A1 provides a piston ring, a piston, a cylinder, and a piston pin that can realize a low friction coefficient without depending on the adsorption or reactions of additives contained in lubricating oils when used under wet conditions using lubricating oil. To this end, the piston ring, the piston, the cylinder, and the piston pin are provided with an amorphous hard carbon film whose Si content is 1 atomic % or more and 20 atomic % or less, and whose surface roughness R z 0.5 µm or less.

[0009] JP 2012-56165 A discloses a sliding member comprising a Si-containing diamond-like carbon layer formed on a base material and a polymer brush layer covalently bonded to the surface of the Si-containing diamond-like carbon layer on a sliding surface. Preferably, the polymer constituting the polymer brush layer is prepared by randomly copolymerizing a monomer containing a group with oleophilic properties and a monomer containing a group capable of forming a crosslinked structure.

[0010] Lubricants that contain a significant proportion of organic oxygen-containing compounds are not widely used. Therefore, sliding systems requiring these special lubricants are not yet suitable for mass-produced applications. Furthermore, even in a sliding system, such as an engine block, not all parts requiring lubrication are coated with amorphous carbon because this coating is expensive. The use of a lubricant specifically designed to reduce friction for parts coated with amorphous carbon can therefore adversely affect the tribological properties of uncoated parts.

[0011] The object of the invention is to provide a sliding system which has at least one sliding element with a surface at least partially coated with amorphous carbon and which can be used in combination with a conventional sliding system.

[0012] This problem is solved by a sliding system according to claim 1.

[0013] Advantageous embodiments of the invention emerge from the subclaims, which can optionally be combined with one another.

[0014] A first aspect of the invention relates to a sliding system comprising a sliding element and a counter-body in sliding contact with the sliding element. The sliding element has a surface at least partially coated with amorphous carbon, which is treated by running in with a first lubricant containing at least one oxygen-containing organic compound. After running in, the first lubricant is replaced by a conventional friction-reducing lubricant with a hydrocarbon-based base oil.

[0015] The invention is based on the finding that permanent lubrication of the sliding system with a lubricant containing at least one oxygen-containing organic compound is not necessary to permanently reduce the friction of a generic sliding system. Rather, it is sufficient to use a first lubricant containing an oxygen-containing organic compound during the running-in period, which is replaced after the running-in period with a friction-reducing lubricant containing conventional lubricant additives. Using the first lubricant during the running-in period improves the compatibility of the surfaces treated with amorphous carbon with conventional lubricants, so that these lubricants can be used permanently to reduce the friction of the sliding system.A chemical change in the surface of the carbon layer with the formation of functional groups such as ether bonds, oxides and hydroxyl groups on the surface is a possible explanation for the improved compatibility. After the formation of these functional groups on the carbon surface, no lubricant containing an oxygen-containing organic compound is required to maintain the friction reduction, so that after the running-in period is complete, conventional friction-reducing lubricants can also be used, optionally with common additives to reduce the friction of the sliding system.

[0016] By using conventional and thus widely available lubricants, the inventive sliding system is immediately suitable for mass-produced products, as it does not rely on the widespread availability of lubricants based on special oxygen-containing organic compounds. Furthermore, the use of conventional lubricants optimized for use with uncoated parts is advantageous for reducing friction in components that have not been coated with an amorphous carbon layer.

[0017] According to a preferred embodiment of the invention, the amorphous carbon is selected from the group consisting of aC, ta-C, aC:Me, aC:H, ta-C:H, and aC:H:Me. The carbon layer types in this group are well known and can be advantageously produced by PVD and / or PA-CVD processes.

[0018] Particularly preferably, the amorphous carbon consists of a ta-C carbon layer, since this is characterized by the predominant sp 3 -Hybridization of its carbon atoms gives it high strength and hardness.

[0019] The first lubricant preferably contains an oxygen-containing organic compound consisting of carbon, hydrogen and oxygen and is free of other heteroatoms.

[0020] The oxygen-containing compound preferably has at least one of the following functional groups: hydroxyl group, carboxyl group, ether bond, ester bond, ketone group, aldehyde group, and carbonyl group. Due to their oxygen content, these functional groups are particularly suitable for reacting with the amorphous carbon layer to form a stable tribological film.

[0021] The oxygen-containing compound is particularly preferably a monomeric organic compound. Compared to polymers, the reactivity of such monomeric compounds toward amorphous carbon layers is increased.

[0022] The oxygen-containing compound can, for example, be selected from the group consisting of alcohols, carboxylic acids, ethers, esters, ketones, aldehydes, and carbonates. These oxygen-containing compounds are well known and readily available as commodity chemicals.

[0023] According to a particularly preferred embodiment, the oxygen-containing compound comprises a C8 to C30 fatty acid ester of glycol or another polyhydric alcohol, preferably a C10 to C20 fatty acid ester, and most preferably an oleic acid ester of glycol, particularly preferably glycerol monooleate (GMO). Tests with lubricants containing this class of compounds have shown a significant reduction in the friction of the lubrication system during the running-in phase.

[0024] Furthermore, the oxygen-containing compounds mentioned in detail in EP 1 510 594 A2 can be used.

[0025] The first lubricant preferably contains the oxygen-containing compounds in a proportion of more than 25 wt.%, preferably at least 50 wt.% and particularly preferably at least 70 wt.%, in order to ensure sufficient reactivity towards the amorphous carbon layer to form the tribological film with oxygen-containing functional groups on the surface of the carbon layer.

[0026] According to the invention, the first lubricant consists of the oxygen-containing compound, optionally in aqueous solution, in order to ensure improved formation of the oxygen-containing functional groups on the surface of the carbon layer.

[0027] The friction-reducing lubricant is preferably a lubricating oil based on hydrocarbons and conventional additives in a proportion of 0 to 25 wt.%. The lubricating oil is preferably based on mineral oil. The lubricating oil is most preferably a conventional engine oil. This is particularly advantageous due to the widespread availability of such a lubricating oil and its proven tribological properties for metal-to-metal contacts and / or metal-to-plastic contacts.

[0028] According to a further aspect of the invention, a method for producing a friction-reduced sliding system is provided. This method comprises a running-in treatment of the sliding system with a first lubricant until the sliding system reaches a nearly constant, reduced coefficient of friction, and in which, after the running-in treatment, the first lubricant is replaced with a friction-reducing lubricant.

[0029] The same or similar advantages as those already described with regard to the sliding system according to the invention also apply to the method according to the invention and therefore require no further explanation.

[0030] The sliding system according to the invention is preferably used as a mechanical seal or as a piston ring-cylinder wall combination.

[0031] A further aspect of the invention relates to the use of the sliding system according to the invention in combination with conventional sliding systems that have metal-to-metal contacts and / or metal-to-plastic contacts. In these combinations, the same friction-reducing lubricant can advantageously be used for both the sliding system according to the invention and the conventional one.

[0032] Further advantages of the invention will become apparent from the following description of a preferred embodiment with reference to the drawings, which, however, should not be construed in a limiting sense. In the drawings: - Fig. 1 a schematic sketch of an exemplary sliding system; - Fig. 2 the friction coefficient curve of a sliding system in which only a lubricant containing an oxygen-containing organic compound was used; and - Fig. 3 the friction coefficient curve of a sliding system according to the invention in comparison with a sliding system not according to the invention.

[0033] In the Fig. 1 schematically illustrates a sliding system 10. A sliding element 12 is in sliding contact with a counter-body 14. The surface 16 of the sliding element 12 is at least partially coated with amorphous carbon, preferably ta-C. Not shown is the lubricant between the sliding element 12 and the counter-body 14, which reduces friction during movements of the sliding element 12 relative to the counter-body 14.

[0034] The Fig. 2 and Fig. 3 are measurement curves in which the coefficient of friction of various test setups is plotted against the running time. The test setups each consist of a sliding system 10 according to Fig. 1, in which the surface 16 of the sliding element 12 has been at least partially coated with amorphous carbon and which is initially operated with a first lubricant containing an oxygen-containing organic compound, preferably GMO, for running-in. The running-in extends from the start of the test setup until time T.

[0035] In the experimental setup according to Fig. 2, after the running-in period is completed at time T, the first lubricant is replaced with fresh lubricant of the same composition. The coefficient of friction drops rapidly from a relatively high value at the beginning to a nearly constant value, which it maintains even after the lubricant has been changed.

[0036] In Fig. Figure 3 shows the coefficient of friction curve of a test setup in which, after the end of the run-in period at time T, the first lubricant, preferably GMO, is replaced by a conventional hydrocarbon-based lubricant such as polyalphaolefin (PAO). For comparison, curve B of a test setup in which only the conventional lubricant was used is also shown.

[0037] As in the experimental setup according to Fig. 2, the coefficient of friction of the measurement curve A drops rapidly to a nearly constant value during the running-in period. A difference only occurs after the lubricant is changed at time T. After changing to a conventional lubricant, a brief increase in the coefficient of friction is observed, but the coefficient of friction quickly drops back to a nearly constant value.

[0038] In contrast, the measurement curve B of the comparison test shows only a slight decrease in the friction coefficient and then a continuous increase in the friction coefficient, so that the friction coefficient of the comparison test at the end of the test period is significantly higher than the friction coefficient of measurement curve A, which corresponds to a sliding system according to the invention.

[0039] A reduction in the friction of a sliding system with components at least partially coated with amorphous carbon is thus achieved by using a hydrocarbon-based lubricant after running in the sliding system using a lubricant containing oxygen-containing organic compounds.

[0040] To determine the Fig. 2 and Fig. Model tests were conducted using the measurement curves shown in Figure 3. The sliding system 10 consisted of a test disc 12 and a ball 14 made of 100Cr6 steel, with the surface 16 of the test disc 12 coated with ta-C carbon. The measurements were performed on a tribometer from Optimol Instruments.

[0041] A polyalphaolefin (PAO) was used as a friction-reducing lubricant, and glycerol monooleate (GMO) was chosen as an oxygen-containing lubricant.

[0042] During the tests, the ball 14 was moved oscillatory by the tribometer at a frequency of 50 Hz with a track length of 1 mm. Test disc 12 and ball 14 were pressed against each other with a normal force of 50 N. The temperature of the assembly was 120 °C. The time T at which the lubricant was exchanged varied between the tests and can be determined from the Fig. 2 and Fig.3 can be read.

Claims

[1] Sliding system (10) with a sliding element (12) and a counter-body (14) in sliding contact with the sliding element (12), wherein the sliding element (12) has a surface (16) coated at least partially with amorphous carbon (DLC) which is in contact with the counter-body (14), characterized by in that the surface (16) coated with the amorphous carbon is treated by running in with a first lubricant which contains at least one oxygen-containing organic compound, wherein the first lubricant is replaced after running in by a friction-reducing lubricant with a hydrocarbon-based base oil, and wherein the first lubricant consists of the oxygen-containing compound, optionally in aqueous solution. [2] Sliding system (10) according to claim 1, characterized by that the amorphous carbon is selected from the group consisting of aC, ta-C, aC:Me, aC:H, ta-C:H and aC:H:Me. [3] Sliding system (10) according to claim 1, characterized by that the amorphous carbon is a ta-C carbon layer. [4] Sliding system (10) according to one of the preceding claims, characterized by that the oxygen-containing organic compound consists of carbon, hydrogen and oxygen and is free of other heteroatoms. [5] Sliding system (10) according to one of the preceding claims, characterized by that the oxygen-containing compound has at least one of the following functional groups: hydroxyl group, carboxyl group, ether bond, ester bond, ketone group, aldehyde group or carbonyl group. [6] Sliding system (10) according to one of the preceding claims, characterized by that the oxygen-containing compound is selected from the group consisting of alcohols, carboxylic acids, ethers, esters, ketones, aldehydes and carbonates. [7] Sliding system (10) according to one of the preceding claims, characterized bythat the oxygen-containing compound is a C8 to C30 fatty acid ester of glycol. [8] Sliding system (10) according to one of the preceding claims, characterized by that the first lubricant contains the oxygen-containing compound in a proportion of more than 25% by weight, preferably at least 50% by weight. [9] Sliding system (10) according to one of the preceding claims, characterized by that the friction-reducing lubricant is a lubricating oil based on mineral oil and additives in a proportion of 0 to 25 wt.%. [10] A method for producing a friction-reduced sliding system (10) according to claim 1, wherein the sliding system (10) is subjected to a running-in treatment with the first lubricant until the sliding system (10) reaches a predetermined friction value, and wherein after the running-in treatment the first lubricant is replaced by the friction-reducing lubricant. [11] Use of a sliding system (10) according to one of the preceding claims as a mechanical seal or as a piston ring-cylinder wall combination. [12] Use of a sliding system (10) according to one of the preceding claims in combination with a metal-metal sliding system or a metal-plastic sliding system.

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