DE102017202620B4 - Cylinder for an internal combustion engine, tribological system and internal combustion engine with one - Google Patents

Abstract

Cylinder (30) for an internal combustion engine, comprising a cylinder space (40) and a cylinder wall (31), the cylinder wall (31) having a hard material coating (50) on the side facing the cylinder space (40) and the hard material coating (50) TiN, Ti (B, N) and TiB2 in the specified order starting from the cylinder wall (31).

Description

The invention relates to a tribological system of an internal combustion engine, as well as a cylinder for an internal combustion engine and an internal combustion engine with such a cylinder.

The function and manufacture of a component require materials that have to meet a wide range of requirements. The same extreme requirements are often not placed on the base material as on the surface or boundary layer. While the design of a component in the case of mechanical and mechanical thermal stress is based on strength aspects that relate primarily to the component volume, complex and irreversible processes in the boundary layer must be taken into account in the case of tribological stress, which acts on the contact surface through normal and tangential forces. These processes are influenced by numerous parameters, so that the tribological behavior can only be described as system-related behavior and, precisely because of the complex processes, is in many cases not accessible to a causal description.

A tribological system consists of three system elements: a base body, a counter body and an intermediate material (lubricant, abrasion). Enveloping substances (gas, dust) form the system-enveloping values. The physical quantities such as normal forces, relative speed and their progression, for example temperature and exposure time, are to be named as stress.

Piston engines are examples of tribological systems. Piston machines (also referred to as reciprocating piston machines) comprise a stationary component, namely a cylinder, and a piston that is movably arranged in the cylinder. The piston is essentially composed of a piston skirt, which is mounted so as to slide along the cylinder wall, and a piston head. The piston crown, together with the cylinder, encloses a closed combustion chamber, the volume of which changes due to the movement of the piston in accordance with the work cycle of the machine. The position of the piston in the housing determines the size of the combustion chamber. The combustion chamber is sealed off from the crankcase by means of sealing elements provided on the piston skirt. Gray cast iron and light metal alloys based on aluminum are widely used as materials for such pistons.

The piston machines that are most widespread today are internal combustion engines such as gasoline and diesel engines, which are used in particular in motor vehicles. In the case of motor vehicle engines, the piston must, among other things, transmit the gas forces of a fuel gas supplied to the combustion chamber to the connecting rod. In addition, it has the task of transferring the heat of combustion transferred to it to the coolant.

The development of modern internal combustion engines aims not only at a significant reduction in pollutants in order to meet future emissions regulations, but also at increasing the engine's performance. At the same time, the component load increases, especially in the internal combustion engine, due to a reduction in displacement with at least the same output (so-called downsizing) or due to non-optimal operating conditions due to the use of fuels with high sulfur contents or alcohol additives. Furthermore, intermittent operating conditions in vehicles with hybrid drives pose challenges to the component design, in particular to the structure of the tribological systems. In aluminum cylinder crankcases with coated raceways in particular, improved tribology is intended to ensure, in addition to lower friction due to optimized surface topographies, less wear and tear and greater seizure resistance. In addition, strongly fluctuating fuel qualities combined with intermittent operating conditions, such as those in hybrid and electric vehicles or in cooled exhaust gas recirculation systems for compliance with existing and future exhaust gas standards, under certain circumstances lead to corrosive processes and subsequently to damage to the cylinder liners.

As a rule, wear protection coatings applied by thermal spraying (APS, LDS, PTWA / RSW) are used in cylinder bores in order to increase the service life despite the problems mentioned above.

Thermal spray coatings require an adhesive topography for mechanical interlocking in the surface, which means that an additional, investment-intensive processing step is always required to produce roughened cylinder surfaces in order to apply spray coatings. Furthermore, in the LDS or PTWA / RSW process, no corrosion-resistant layers that are tribologically suitable for running the engine can be applied. With this process, only tribologically suitable steel layers can be applied. However, with a thickness of 200 - 500 µm, the layers are rather thick and must be reworked in several stages by machining and honing to a layer gauge of 100 - 150 µm. The coating process is therefore complex and has to be provided with many coating modules running in parallel, depending on the number of items.

The application of carbon layers in the form of so-called diamond-like is also possible coatings (DLC) known for reducing friction ( DE 11 2014 005 506 T5 ). The application of DLC layers during production in the HF-PACVD process leads to uneven layer thicknesses and can only be partially managed by extending the cylinder barrel and the associated additional mechanical post-processing. Alternatively, DLC layers are applied by means of DC pulse PACVD deposition. However, the adhesion of these thin DLC layers is relatively low, so that there is always the risk of spontaneous delamination. This risk is significantly increased when the surfaces are contaminated and by the so-called eggshell effect when the layer systems are exposed to specific loads. Another disadvantage is the fact that the cylinder liner itself has to serve as a reaction chamber or recipient. This makes the coating complex, expensive and, above all, time-consuming.

Out CN 10 592 59 39 A and DE 10 2014 218 757 A1 TiN systems are known as a coating for cylinder components, which are also deposited using CVD or PVD, for example.

The DE 101 28 055 A1 deals with the requirements of tribological systems of steam engines. Piston-cylinder systems are disclosed whose material compositions in the presence of water vapor, especially under high pressure and temperature conditions, are at the same time corrosion-resistant and durable and reduce the friction in the system. Systems of this type cannot be transferred to tribological systems in an internal combustion engine due to the different physical and, in particular, different chemical requirements on the system.

The invention is now based on the object of reducing or avoiding the disadvantages of the prior art. A tribological system is to be provided which is improved in terms of service life and corrosion resistance. In particular, the system should be adapted to the requirements in an internal combustion engine and should exhibit minimal friction.

The use of adapted, corrosion-inhibiting coating systems should offer the opportunity to provide robust cylinder liners for all operating conditions and regions of the world or even to open up new markets.

This object is achieved by a cylinder, a tribological system for an internal combustion engine and an internal combustion engine with such a system with the features of the independent claims.

Thus, a first aspect of the invention relates to a cylinder for an internal combustion engine, comprising a cylinder space and a cylinder wall, the cylinder wall having a hard material coating on the side facing the cylinder space and the hard material coating comprising TiN, TiB ₂ and / or Ti (N, B). The cylinder according to the invention has a very smooth surface with an optimal oil retention volume. The hard material coating is particularly durable on the cylinder walls used for internal combustion engines, since it shows good adhesive properties on their surface even without pretreatment. The particularly smooth surface of the coating in the range from 2000 to 4000 HV reduces the friction of a piston in the cylinder according to the invention to a minimum and thus increases the efficiency of the system and its service life. Furthermore, from a production point of view, the manufacture of the cylinder according to the invention is less challenging and less time-consuming than in known systems, since intermediate steps such as repeated removal of the layer, as well as high temperatures and pressure, are dispensed with. Rather, the hard material coating takes place from the gas phase directly on the surface in the defined, very small layer thickness. In particular, the cylinders according to the invention can be manufactured from conventional cylinders or cylinder liners by applying the hard material coating essential to the invention to the cylinder walls of the existing cylinder liners. For this purpose, no further pretreatment of the surfaces is advantageously required.

Titanium nitride (TiN) is a chemical compound of the two elements titanium and nitrogen. TiN - its formula - is a metallic hard material with a typical golden yellow color. The ceramic material is characterized by its very high hardness and corrosion resistance, which results in a number of technical applications. Titanium boride (TiB) is an inorganic chemical compound in the form of a technical ceramic of boron from the group of borides with titanium in the form of a technical ceramic. It occurs stoichiometrically in the empirical formula TiB ₂ and is characterized by high electrical conductivity with simultaneous non-wetting against light metal melts. When mixed with other ceramics, it increases the hardness of the mixed ceramics produced. Titanium boron (o) nitride (Ti (B, N) or Ti (N, B)) is a mixed system or a mixed ceramic made of TiB and TiN, whereby the ratio of boron to nitrogen can vary and, depending on the excess, one of the two elements in the lattice the physical properties are determined by the respective excess element.

The excellent properties in terms of hardness, smooth surface and corrosion resistance are further improved in mixed systems of the compounds mentioned, that is, hard material coatings which have at least two of the compounds TiN, TiB and Ti (N, B).

The cylinder wall of the cylinder according to the invention preferably comprises iron. The use of aluminum is less preferred because of the high deposition temperatures. It preferably comprises an iron alloy such as steel and gray cast iron. The coating is applied to this cylinder wall on the side facing the cylinder space.

The hard material coating is preferably applied to the surface by means of chemical vapor deposition (CVD). This can optionally be plasma-assisted (plasma assisted CVD; PACVD), with the cylinder liner being connected cathodically during the deposition. The coating produced optimally covers pores in the cylinder wall and shows a high degree of homogeneity even with very low layer thicknesses. In addition, it shows increased adhesion compared to layers applied with chemical bath deposition. Alternatively, the hard material coating is applied using PVD.

According to the invention it is further provided that the hard material coating has TiN, Ti (B, N) and TiB ₂ as sub-layers in the specified order. The list is to be understood beginning at the cylinder wall, so that TiB _{2 is arranged} on the side of the coating facing the cylinder space. Alternatively, an inverted arrangement of the lower layers is also formed, so that a layer stack comprising TiN-Ti (B, N) -TiB ₂ -Ti (B, N) or TiN-Ti (B, N) -TiB ₂ -Ti (B, N ) -TiN is formed.

TiN has an embedded structure and crystallizes in the salt lattice, with the titanium atoms forming a face-centered cubic lattice and the small nitrogen atoms being embedded in the octahedral gaps in the basic structure. The crystal structure that characterizes this metallic hard material exists only in the composite and not in the form of individual molecules, which is reflected in its insolubility in almost all, even aggressive solvents.

TiN adheres to the surface due to chemical deposition. The adhesion of TiN is significantly higher than that of TiB ₂ due to its lower hardness. An improvement in adhesion can be achieved by plasma nitriding carried out directly before coating. For example, depending on an N ₂ concentration during nitriding, either only a diffusion zone (free of connecting layers up to 0.5 mm deep, hardness increasing towards the edge) is created, which leads to very good adhesion of the layers applied later. Alternatively, a precipitation layer Fe ₄ N or Fe _2-3 N is produced in the edge area of the surface, which likewise leads to a significant improvement in the adhesion of the layers. This creates a slowly increasing hardness gradient, which preferably ends in a hardness gradient of a hard material layer system (TiN - Ti (B, N) - TiB ₂ ), so that an "eggshell effect" is avoided. TiB ₂ is significantly smoother, more chemically stable and harder than TiN, TiN has better adhesion to steel substrates.

It is also preferred if the hard material coating has a cover layer, in particular comprising TiB ₂ , on the side facing the cylinder space, since an advantageous, very smooth, chemically resistant and harder cover layer is then formed which offers particularly good wear resistance.

The hard material coating is particularly advantageously designed as a multilayer system or as a gradient system. In the present case, in contrast to the gradient system, a multilayer system is to be understood as meaning that the individual sublayers made of TiN, TiB ₂ and the mixed layer can be clearly delimited from one another. This means that within the individual sub-layers of the hard material coating there is largely a constant stoichiometric ratio of titanium, boron and nitrogen atoms to one another. It should be understood, however, that diffusion layers between them cannot be completely avoided. In contrast, the gradient system shows a course of the stoichiometric ratio over the entire layer thickness of the hard material coating. The formation of a gradient system is particularly preferred, since no boundary layers arise in the system that could negatively affect the adhesion.

In a preferred embodiment of the invention it is further provided that a connecting layer, in particular produced by a plasma nitriding process, is arranged between the cylinder wall and the hard material coating, which is preferably a nitride of a base material of the cylinder wall, for example an Fe ₄ N. The resulting duplex coating (combination of hard material coating essential to the invention and plasma nitriding layer or connecting layer) improves in particular the adhesion properties and thus the longevity of the layer and the tribological system. The risk of spontaneous delamination and the occurrence of the so-called eggshell effect is reduced or even avoided. Nitriding layers are particularly preferred. If the cylinder wall has an iron connection, it is through An iron nitride compound, in particular Fe ₄ N, is formed during the nitriding process.

Alternatively or additionally, the surface of the cylinder wall is physically or chemically pretreated. A pretreatment that roughen the surface is particularly preferred. For this purpose, a honing process is preferably used, which creates defined depressions in the surface, which are filled by the subsequently applied hard material coating. If the hard material coating becomes increasingly worn while the cylinder is in use, part of the coating remains in these depressions and also determines the tribological properties of the system, so that the effects of the layer are largely retained even if the layer wears.

The hard material coating particularly advantageously has a layer thickness of 1 to 30 μm, in particular 2 to 15 μm. These very small layer thicknesses can advantageously be applied in just one process step and are more durable compared to thicker layers and DLC layers with comparable layer thicknesses, since the so-called eggshell effect is avoided. They also take account of the need for reduced installation space.

Another aspect of the invention is a tribological system for an internal combustion engine, which has a base body and a counter body, the base body comprising a cylinder according to the invention. The tribological system according to the invention is optimized with regard to internal combustion engines and the physical and chemical conditions prevailing there. In particular, when the cylinder according to the invention interacts with a piston ring which has a carbon coating in the manner of a DLC (diamond-like carbon) on a surface facing the base body, the tribological properties are significantly increased compared to conventional internal combustion engines. The result is long-lasting and durable raceway systems that are corrosion-resistant and have very low friction. They are very robust and have a mileage of several million kilometers.

The invention further relates to an internal combustion engine for a motor vehicle, having a cylinder according to one of the preceding claims, and an internal combustion engine having the tribological system according to the invention. These show the improved properties described, especially when used in trucks.

Further preferred embodiments of the invention result from the other features mentioned in the subclaims.

The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless stated otherwise in the individual case.

• 1 eine schematische Schnittdarstellung eines tribologischen Systems in Form einer Kolbenmaschine in einer bevorzugten Ausgestaltung der Erfindung, und
• 2 eine schematische Darstellung einer beschichteten Zylinderwand eines Zylinders in einer bevorzugten Ausgestaltung der Erfindung.

The invention is explained below in exemplary embodiments with reference to the accompanying drawings. Show it:

• 1 a schematic sectional illustration of a tribological system in the form of a piston machine in a preferred embodiment of the invention, and
• 2 a schematic representation of a coated cylinder wall of a cylinder in a preferred embodiment of the invention.

A preferred embodiment of a tribological system using the example of a piston machine with a piston according to the invention is shown in the figure using a sectional illustration. 1 shows a piston machine indicated overall by 1 in the form of a reciprocating piston engine, preferably a diesel engine. The piston engine 1 includes a piston that is cylindrical here 10 with a diameter d _K , which is in a cylinder 30th is arranged axially movable.

In the embodiment shown, the piston 10 a piston skirt 12 on, which is designed in the form of a cylinder jacket, and a substantially circular planar piston head 11 . Piston crown 11 and piston skirt 12 are preferably formed in one piece and are preferably made of a light metal alloy. Aluminum alloys, in particular aluminum-silicon alloys, are particularly preferred. The piston 10 also has circumferential grooves, which sealing elements 13 so-called piston rings.

The piston 10 is connected to a crankshaft for driving a vehicle by a connecting rod (not shown). Also not shown are inlet and outlet channels and corresponding inlet and outlet valves and a fuel injector which are arranged in the area of the cylinder head.

The piston 10 , especially its piston crown 11 , and the cylinder 30th define a combustion chamber 40 whose volume is determined by the position of the piston 10 is determined. The combustion chamber 40 is part of the cylinder space 40 .

The piston crown 11 has a coating 20th with low thermal conductivity. The cylinder 30th points to one of the combustion chamber 40 facing side of a cylinder wall 31 an anti-friction hard material coating 50 on. The hard material coating 50 is at least in the area of the cylinder wall 31 arranged in operation with the piston 10 comes into contact and in which thus friction between pistons 10 and cylinder 30th is produced. The friction arises in particular between piston rings 13 and cylinder wall 31 . Whereby the cylinder 30th the base body and the piston rings 13 form the counter body in the sense of a tribological system. The hard material coating 50 reduces the friction on the side of the cylinder wall 31 . On the side of the piston rings 13 a special material selection can be made to further improve the tribological properties of the system or a coating can also be arranged. In the preferred embodiment shown, the piston rings are 13 coated with a DLC (diamond like carbon).

The hard material coating 50 is in a preferred embodiment in detail in 2 shown and also described there.

The hard material coating according to the invention 50 comprises titanium nitride (TiN), titanium boronitride (Ti (B, N)) and / or titanium boride (TiB ₂ ). A multilayer structure which has at least two of the materials mentioned is particularly preferred. The hard material coating 50 preferably forms layer thicknesses of 2 to 15 µm.

In the embodiment shown, this hard material coating is constructed as a single-layer system (TiN or Ti (B; N) or TiB ₂ ) or a multi-layer system. Alternatively, a gradient system can also be formed or a gradient system can be combined with a multilayer system, for example by using a gradient system composed of TiN-Ti (B, N) -TiB ₂ -Ti (B, N) and TiN-Ti (B, N) -TiB ₂ -Ti (B, N) -TiN is arranged in two sublayers on top of one another. In addition, a cover layer made of TiB _{2 can be} provided.

Between the cylinder wall 31 and the hard material coating 50 a diffusion zone with or without a connecting layer ( II , III ) be provided. This is, for example, N intercalation in the base material produced by means of the plasma nitriding process and, if necessary, an iron nitride compound of the form Fe ₄ N and / or Fe ₂ - ₃ N arranged above it, also produced by plasma nitriding. Between the layers and the cylinder wall 31 a diffusion layer forms II in which foreign atoms of the adjacent layer diffuse into the lattice of the adjacent layer. In particular, iron atoms of the material of the cylinder wall diffuse 31 into the iron nitride or titanium nitride lattice and nitrogen atoms occupied lattice sites in the material of the cylinder wall 31 or arrange themselves on interstitial spaces. This ensures a better adaptation of the lattice structures and increases the adhesion of the layers to one another.

The hard material coating 50 in the embodiment shown is one of TiN, Ti (B, N) and / or TiB ₂ ; TiN-Ti (B, N) -TiB ₂ , TiN-Ti (B, N) -TiB ₂ -Ti (B, N) or TiN-Ti (B, N) -TiB ₂ -Ti (B, N) - TiN existing structure, the limits of which form gradients in relation to the stoichiometric composition. Optionally, the hard material coating 50 on the the combustion chamber 40 facing side a cover layer V made of TiB ₂ .

The layer thickness of the hard material coating 50 results in the in 2 embodiment shown from the sum of the layer thicknesses from the hard material coating IV and the top layer V .

The cylinder according to the invention shown 30th can by applying the hard material coating, which is essential to the invention 50 on the honed cylinder wall 31 of a conventional cylinder (liner) with a good friction topology, in that the hard material coating is chemically, physically and, if necessary, plasma-assisted deposited from the gas phase in the specific composition and sequence.

1. 1. Fertigung eines Zylinderliners mit optimaler motorisch tauglicher Oberfläche mit anschließendem normalen Waschprozess nach Honung;
2. 2. Optional kann ein Sputterätzen als vorgeschaltete Reinigung in Beschichtungsanlage vorgesehen werden; und
3. 3. Je nach Ausführungsform der Erfindung findet ein Plasmanitrierprozess und direkt angeschlossener Schritt 4 in gleicher Anlage statt;
4. 4. Überführung/Chargierung des Liners in eine PACVD oder CVD-Anlage mit einem Vakuum von ungefähr 30 - 200 mbar, und Aufheizen der Kammer auf Beschichtungstemperatur. Diese ergibt sich aus der Zusammensetzung der Hartstoffbeschichtung;
5. 5. Abschließendes Abkühlen im Vakuum

For example, depending on the composition of the layer, the method comprises the following steps:

1. 1. Manufacture of a cylinder liner with an optimal motorized surface with a subsequent normal washing process after honing;
2. 2. Optionally, a sputter etching can be provided as an upstream cleaning in the coating system; and
3. 3. Depending on the embodiment of the invention, a plasma nitriding process and directly connected step 4 take place in the same system;
4. 4. Transfer / charging of the liner in a PACVD or CVD system with a vacuum of approximately 30 - 200 mbar, and the chamber is heated to the coating temperature. This results from the composition of the hard material coating;
5. 5. Final cooling in vacuum

The cylinder liners can then be installed directly.

The tribological system shown has been optimized with regard to the tribological properties. In particular, the arrangement of the hard material coating according to the invention leads to a long-lasting, low-friction system.

The layer systems produced have a very high level of adhesion, especially in combination with an upstream plasma nitriding process. Due to the very thin hard material layer, the previously generated surface, which is optimally tribologically produced for engine operation, is directly reproduced and permanently preserved. In the event that the hard material layer should wear off, it remains in the depressions in the surface and thus ensures an optimal layer-piston ring tribological system over the long term.

It is precisely the combination of the above-mentioned hard material layers on cylinder liners with DLC-coated piston rings that form long-lasting or durable raceway systems that are corrosion-resistant, have very low friction and are very robust.

Furthermore, the cylinder liners can be coated in conventional standard CVD or PACVD coating systems (liners connected cathodically) by contract coaters, i.e. batches with several 100 cylinder liners, which can be coated in large recipients like tool coating at low prices.

List of reference symbols

1

Piston machine tribological system

10

piston

11

Piston crown

12

Piston skirt

13

Sealing element

20th

Coating

30th

cylinder

31

Cylinder wall

40

Cylinder chamber, combustion chamber

50

Hard material coating

51

DLC coating

I.

Steel or cast iron

II

Diffusion zone

III

Link layer

IV

Hard material coating (TiN, Ti (B, N) and / or TiB ₂ ; TiN-Ti (B, N) -TiB ₂ , TiN-Ti (B, N) -TiB ₂ -Ti (B, N) or TiN-Ti (B, N) -TiB ₂ -Ti (B, N) -TiN)

V

TiB ₂ top layer

Claims (9)

Cylinder (30) for an internal combustion engine, comprising a cylinder space (40) and a cylinder wall (31), the cylinder wall (31) having a hard material coating (50) on the side facing the cylinder space (40) and the hard material coating (50) TiN, Ti (B, N) and TiB ₂ in the order given, starting from the cylinder wall (31). Cylinder (30) Claim 1 , characterized in that the hard material coating (50) is a coating applied to the surface by means of, optionally plasma-assisted chemical evaporation (CVD or PACVD). Cylinder (30) Claim 1 or 2 , characterized in that the hard material coating (50) is designed as a multilayer system or as a gradient system. Cylinder (30) according to one of the preceding claims, characterized in that between the cylinder wall (31) and the hard material coating (50) there is arranged a connecting layer (III), in particular produced by a plasma nitriding process, which is preferably a nitride of a base material of the cylinder wall ( 31). Cylinder (30) according to one of the preceding claims, characterized in that the hard material coating (50) has a layer thickness of 1 to 30 µm, in particular 2 to 15 µm. Tribological system (1) for an internal combustion engine having a base body and a counter body, the base body comprising a cylinder (30) according to one of the preceding claims. Tribological system (1) according to Claim 6 , characterized in that the counter-body comprises a piston ring (13) which has a DLC (diamond-like-carbon) on a surface facing the base body. Internal combustion engine for a motor vehicle, comprising a cylinder (30) according to one of the Claims 1 to 5 . Internal combustion engine after Claim 8 , having a tribological system (1) according to Claim 6 or 7th .

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