WO2015090267A1

WO2015090267A1 - Method for producing a protective layer on a thermally stressed component and component having such a protective layer

Info

Publication number: WO2015090267A1
Application number: PCT/DE2014/000637
Authority: WO
Inventors: Alexander Kopp; Christoph PTOCK
Original assignee: Meotec GmbH & Co. KG; Volkswagen Aktiengesellschaft
Priority date: 2013-12-17
Filing date: 2014-12-17
Publication date: 2015-06-25
Also published as: DE112014005973A5; EP3084048B1; EP3084048A1

Abstract

The invention relates to a method for producing a protective layer on a thermally stressed component, which consists at least partially of a valve metal, wherein the protective layer is produced by an electrochemical process. The method according to the invention is characterized in that the electrochemical process is a plasma-electrolytical oxidation using an electrolyte and connected to an electrical output. The invention furthermore relates to a component having a protective layer, wherein the protective layer is produced by a method according to the invention, wherein the component consists at least partially of a valve metal.

Description

PROCESS FOR PRODUCING A PROTECTION LAYER ON A THERMALLY LOADED COMPONENT AND COMPONENT WITH SUCH A PROTECTIVE LAYER

The present invention relates to a method for producing a protective layer on a thermally stressed component and to a component having such a protective layer. In particular, the present invention relates to an electrochemical method for producing an oxidation, wear or corrosion protective layer on a component of an internal combustion engine or a component of an exhaust system.

Such components are used in particular in motor vehicles. In the case of motor vehicles, there is an attempt to reduce the total weight of the vehicle and therefore its individual components in order to increase the efficiency. It therefore makes sense to resort to particularly lightweight materials, in particular so-called light metals such as aluminum, titanium, or their alloys. However, a problem or disadvantage of these materials is the relatively good thermal conductivity, so that the use of these materials, especially for components exposed to higher temperatures, for example, above 300 ° C are not readily possible. Due to the system, such high temperatures occur in motor vehicles in the internal combustion engine and in the exhaust system. As an example, an exhaust gas turbocharger may be mentioned, in which temperatures of over 900 ° C may occur. At such temperatures, so-called hot gas corrosion may occur due to the particularly hot gas (the hot gas).

In order to enable the use of such materials even with thermally stressed components, the surface must be provided with a protective layer, by which in particular the heat transfer coefficient is reduced. From the state of the art are here in particular spraying, for example, thermal spraying or

CONFIRMATION COPY Plasma spraying known. A disadvantage of this solution, however, is that in such sprayed coatings, the connection between the sprayed protective layer and the component by mechanical interlocking of the layer material (eg by flakes) on the substrate, ie the surface of the component, or by adhesion processes or diffusion processes comes about. In operation, it can therefore cause problems due to spalling or due to a lack of abrasion resistance. Also, the known spray methods are expensive and energy consuming. In particular, in the interior spraying, ie when applying a sprayed layer in a cavity, special injection molding tool is necessary, if a spray process is even possible. As an example, reference is made to a manifold of an exhaust system, which for this reason can not be made of any of the aforementioned materials, but is usually provided as a cast iron part or built stainless steel part.

For these reasons, the formation of an oxide layer has recently been proposed as a protective layer. For example, DE 10 2012 002 284 A1 shows a turbine wheel made of, on or in the surface of which a halide from the group of fluorine, chlorine or bromine is introduced or introduced and on the surface of which an oxidation layer is formed by the so-called halogen effect in the frame a heat treatment is formed. The halogens are applied in particular by ion implantation.

A disadvantage of such methods for producing a protective layer on a thermally stressed component based on the halogen effect is that the oxide layers formed are very thin. Consequently, there is only a limited improvement in the corrosion resistance so that the wear protection is not optimal. Furthermore, owing to the relatively thin oxide layer, no major influence on the electrical or thermal insulation of the component is to be expected. Alternatively, the formation of an oxide layer by electrochemical methods is proposed. From DE 10 2012 218 666 AI such a method is known. Herein, a turbine wheel of a turbocharger made of a titanium alloy is subjected to an electrochemical anodization, which builds up an oxide layer as a protective layer and thus protects the component against further oxidation. Furthermore, the component is thus protected against further environmental influences.

Although a technically usable layer is provided by the electrochemical anodization, which in contrast to previously mentioned method is not only inexpensive, but in addition to the oxidation also a wear protection and other positive properties, the usual limitations for this process remain. A disadvantage of the protective layer shown in DE 10 2012 218 666 Al is that the layer has a high pore content due to the process.

As a result, even with the use of optimal parameters, only a limited protection can be realized which, for example, does not approach the quality of a protective layer produced by ion implantation. Furthermore, as part of the anodization only limited wear and corrosion protection can be produced. A major drawback is the breakup and limited formation of the protective layer around edges and sharp transitions. Thus, in particular in the case of critical component areas, adequate protection due to a lack of formation of the protective layer is not given.

Against this background, it is the object of the present invention to provide an inexpensive method for producing a protective layer on a thermally stressed component, which allows the application of a protective layer even on hard to reach surfaces, has a good adhesion to the surface and thus an optimal oxidation , Wear and corrosion protection offers. It is another object of the invention to show a component with such a protective layer. The object is achieved by a method according to claim 1 and a component according to claim 20. Advantageous developments are described in the dependent claims.

According to the invention, a method is proposed for producing a protective layer on a thermally stressed component, which consists at least partially of a valve metal, wherein the protective layer is produced by an electrochemical process. The method according to the invention is characterized in that the electrochemical process is a plasma electrolytic oxidation (PEO) using an electrolyte and applying an electrical power. A metal metal is here understood to mean a metal in which the surface can be converted by an electrochemical process into an oxide ceramic layer or an oxide layer, such as titanium (Ti), aluminum (Al), magnesium (Mg) or zirconium (Zr ) or their alloys. In these metals, which are also known as "barrier-layer-forming metals", the surface reacts by sparking an electric power in a local plasma and forms an oxide ceramic or layer. or the electrolyte to an oxide ceramic or layer (for example, A1 ₂ 0 ₃ , spinels, mixed oxides, etc.).

A PEO process is an anodic oxidation process using a special modulated AC voltage, resulting in a temporary and localized spark discharge due to plasma discharges. The PEO process is therefore also referred to as anodic oxidation with spark discharge (ANOF). The resulting from the spark discharge local melting of the surface to be coated should lead to a particularly wear-resistant coating. An ANOF process or a PEO process according to the invention is a combined process from the fields of plasma technology and electrochemical mie, can be provided by the surfaces of components which are formed of so-called valve metals, with a protective layer of an oxide ceramic. In particular, native barrier layer formers such as aluminum, magnesium or titanium come into the selection as valve metals. The generation of the protective layer can be carried out in particular in aqueous electrolytes. The component to be oxidized is poled anodically and immersed in the electrolyte together with a counter electrode (cathode). The component initially forms a purely chemically induced passive layer. The growth of this passive layer can be achieved by applying a potential between the anodically poled component and the cathode. In this case, the oxide layer of the component to be coated will penetrate locally, wherein plasma-chemical solid-state reactions, the spark discharges, are triggered. This process does not take place nationwide but at those points where the thickness of the oxide layer and thus the local electrical resistance is lowest. Since the plasma reactions thus always take place at those points of the passive layer which locally have the lowest layer thickness, and there ensure a layer thickness growth, the surface is coated with a very uniform protective layer. In order to permanently break the increasing dielectric property of the growing oxide layer with a breakdown voltage, the applied electric potential is increased so long that the desired layer thickness of the protective layer is reached. The inventive method has the advantage that the layer formed according to its ceramic character has a defined thermal conductivity, which is well below the thermal conductivity of the substrate material, for example, aluminum num. As a result of the smaller coefficient of thermal conduction and the low thermal conductivity of the protective layer, higher wall temperatures are thus enabled, so that the surface provided with the protective layer is thermally insulated from the adjacent medium, for example hot gas. The protective layer produced by the method according to the invention is therefore constructed as follows: Adjacent to the substrate is a thin, dense and closed layer, the so-called barrier layer, followed by a compact and low-pore layer. This is followed by a porous and less compact layer which, depending on the layer thickness, becomes both more porous and more brittle. In particular, this layer is openly porous and characterized by small channels which are perpendicular to the surface and protrude from the surface to the adjacent barrier layer in the direction of the substrate. Additionally or alternatively, the layer has an interconnecting pore network and / or a non-interconnecting pore network, which is characterized by closed inclusions of air or electrolyte.

Advantageously, the electrolyte is an electrolyte wherein the electrolyte based on phosphoric acid (H3PO4), potassium hydroxide (KOH), water glass (Na ₂ SI03), deionized water, or a zirconium compound. An electrolyte base here is a substance from a variety of substances, the amount of g / L in addition to water and urotropin is most common in an electrolyte. Zirconium sulfate (ZrS0 ₄ ) or zirconium tungstate (ZrW0 ₄ ) is particularly suitable as a zirconium-containing compound. This has the advantage that with a deratigen electrolyte composition, a component of, for example, aluminum or titanium or of the corresponding alloys can be oxidized plasma-electrolytic. It is advantageous if the electrical power is voltage-controlled, the current is limited or current-controlled, the voltage is limited, or is power-controlled. Conveniently, the electrical power is applied at a frequency of 1 Hz to 10 kHz, in particular with a frequency of 1 Hz to 1000 Hz. It is advantageous if the voltage is applied in a range between 150 and 1500 volts, preferably in a range between 210 and 650 volts, and if the current with a current density in a range between 0.001 and 1000 A / dm ² , preferably in one Range between 0.5 to 15 A / dm ^{2 is} applied. It is conceivable that the applied current and / or the applied voltage is supermodulated by a higher-frequency current and / or a higher-frequency voltage. Furthermore, it is advantageous if the applied current and / or the applied voltage is regulated in the same way, or has the form of a symmetrical wave, an asymmetric wave, a rectangle or a trapezoid. Here, the characteristic shape is provided with a duty cycle and an offset in the range of 0 to 100% and can thus be designed both uni- and bipolar. In particular, the shape of a wave is advantageous.

It is also advantageous if a temperature in the range between 0 ° C and 80 ° C is selected as the process temperature for the PEO. More preferably, the temperature is between 18 ° C and 50 ° C.

The abovementioned process parameters make it possible for a particularly oxide-rich protective layer to grow closed on the component and thus to form a particularly dense and therefore safe protective layer. The component can thus be safely and long-term stable protected against external influences, for example against unwanted oxidation. Furthermore, with the inventive method component can be produced in mass production with corresponding quality requirements. Furthermore, a practicable production speed can be achieved in this way, which makes mass production possible at all. It is advantageous if the electrolyte is carried out as a dispersion, wherein one or more of the following particles are added to the electrolyte: Al ₂ O ₃ , T ₂ O ₂ , SIO ₂ , tungsten carbide (WC), ZrO ₂ , iron oxide, graphite and / or MoS ₂ . In this case, the electrolyte is applied to an above-mentioned electrolyte base by the addition of said particles. The particles may be either globular, ellipsoidal or sparse in the form of flakes or the like. Furthermore, the particles can be made of an oxide, a carbide or another material as long as the particles due to the fact that they are incorporated into the protective layer as a foreign body or react chemically, electrochemically or physically together with the substrate or the electrolyte to form a different compound.

In particular, particles of A1 ₂ 0 _3, Ti0 _2, Si0 _2, tungsten carbide (WC), Zr0 _2, iron oxide, have a significantly reduced thermal conductivity, so that the incorporation of these particles further improves the insulating effect of the protective layer in the protective layer. In particular, zirconium oxide (Zr0 ₂ ) has proved to be advantageous.

Furthermore, the friction value is reduced by the addition of lubricant particles such as graphite, MoS ₂ or by other corresponding particles, which are stored embedded in the protective layer.

In a particularly preferred embodiment of the method according to the invention can also be provided that in the protective layer particles are provided deviating from a Grundbeziehungsweise matrix material of the protective material, which in comparison to the base or matrix material of the protective layer has a relatively high or low thermal conductivity exhibit. It can be particularly preferably provided that both those particles are provided which have a relatively high thermal conductivity compared to the base or matrix material of the protective layer, as well as those which have a relatively low thermal conductivity. This aspect of the invention is based, on the one hand, on the recognition that the protective layer produced in the context of the method according to the invention represents an advantageous compromise with regard in particular to thermal insulation and durability, but alternative materials are present which are characterized by an even lower thermal conductivity and thus a lower thermal conductivity further improved thermal insulation. However, for various reasons, these can not be used for the complete formation of a protective layer. By introducing particles of one or more of these alternative materials in the protective layer produced according to the invention, the average thermal conductivity can be further lowered and thus the thermal insulating properties are further improved, without this to a relevant extent on the further advantageous properties of the inventive protective layer, ie in particular a good Durability and low surface roughness, negative impact. Therefore, it can be provided particularly advantageously that particles having a relatively low thermal conductivity are provided in the entire protective layer (based on the area and possibly also the layer thickness).

As the material for the particles with relatively low thermal conductivity come beispiels- as Y-stabilized zirconia (Zr (Y) ₂ 0), alumina (A1 ₂ 0 _3), spinel (Al ₂ 0 ₃ MgO), mullite (Al ₂ 0 ₃ / Si0 ₂ ), zirconium corundum (Al ₂ 0 ₃ / Zr0 ₂ ), titanium oxide (Ti0 ₂ ) or silicon oxide (Si0 ₂ ) and mixed ceramics with essential constituents of said oxides into consideration.

Even if the thermal conductivity of the introduced particles in their pure bulk state is not lower than that of the matrix, the thermal conductivity of the composite material of the protective layer formed from both can nevertheless be lower overall, since the introduced particles act as impurities for the propagation of the crystal oscillations (phonons). Act. In this respect, according to the invention, the specification "with relatively low thermal conductivity" is not limited exclusively to an actual material property of the particles, but should also include a heat conductivity reducing effect within the matrix.

On the other hand, the particles with a relatively high thermal conductivity can advantageously be used to avoid or reduce localized peaks of the wall temperature of the surface provided with the protective layer, as a result of these particles being able to achieve a relatively high local transition of heat energy from, for example, a combustion chamber or an exhaust gas guide as well as possible a larger area of the protective layer distributed becomes. As a result, the formation of locally high wall temperatures, which can have a negative effect on the ignition delay (ie the period between the injection of fuel into the combustion chamber and the ignition of the fuel), can be avoided. This may be sufficient if the particles with relatively high thermal conductivity in only one or more sections, but not in the entire protective layer (based on the area and preferably also the layer thickness) are provided. Such a localized provision of particles with relatively high thermal conductivity does not therefore have to be associated with a relevant deterioration in the mean thermal conductivity of the entire protective layer. A relatively large ignition delay achieved by avoiding locally high wall temperatures is of importance, in particular for self-igniting internal combustion engines, ie in particular diesel engines, so that the method according to the invention can be used particularly advantageously in the improvement of such a self-igniting internal combustion engine. However, there may also be advantages in using the method for improving spark-ignited internal combustion engines, in particular gasoline engines.

The material used for the particles with relatively high thermal conductivity is, for example, copper, iron, beryllium, aluminum, copper, silver, silicon, molybdenum, tungsten, carbon, beryllium oxide, beryllium nitrite, silicon nitrite and / or silicon carbide, as well as mixtures and / or alloys thereof consideration.

If both relatively low thermal conductivity particles and relatively high thermal conductivity particles are to be provided, their distribution in the protective layer should be such that the mean thermal conductivity of the protective layer, which is locally increased by the relatively high thermal conductivity particles, does not increase to a relevant one Heat transfer to the arranged below the protective layer region of the coated, the combustion chamber and / or the exhaust gas guide limiting- leads the component. This can be advantageously achieved in that the particles with relatively high thermal conductivity exclusively in a first, adjacent to the combustion chamber and / or the exhaust gas guide sub-layer of the protective layer and the particles with relatively low thermal conductivity in a second, from the combustion chamber and / or the exhaust system provided by the first sub-layer separated sub-layer. The particles with a relatively high thermal conductivity can then ensure the most uniform possible distribution of heat energy transferred into the protective layer within the first sub-layer, while the second sub-layer with the particles of relatively low thermal conductivity acts particularly well thermally insulated and consequently heat transfer from the first sub-layer reduces the area of the component below the protective covering.

Anodic oxidation under spark discharge makes it possible to arrange particles in the protective layer in a relatively simple manner. This is especially true in the case of an anodic oxidation with spark discharge by means of an alternating voltage, in which either the positive or negative voltage phases can be alternately used to attach the particles contained in the electrolyte to the growing protective layer, while the corresponding other voltage phases for the growing training the protective layer can be used.

The particle size of the particles can be in the range from 0.001 to 5000 μm, in particular in a range from 0.1 to 100 μm. Such particle sizes have proven to be practicable.

For uniform dispersion of the particles, an ultrasonic vibrator can be used. As a result, the dispersion of the particles in the electrolyte can be done inexpensively and quickly. Furthermore, the particles can be polarized by the use or addition of surfactants. The surfactants may be neutral, positive or in particular cationic see surfactants (eg Esterquads) be such that the polarized particles are drawn, for example, in the cathodic part of a half-wave to the surface and integrated in the anodic part of a half-wave - in the context of the spark discharge - in the surface.

The object is also achieved by a component with a protective layer, which was produced by the method according to the invention. According to the invention, the component consists at least partially of a valve metal or an alloy of a valve metal. Thus, it is advantageous if the component is made of aluminum, an aluminum alloy, magnesium, a magnesium alloy, titanium or a titanium alloy. In addition to components with the materials mentioned and components of other materials, especially metallic materials such as steel or cast iron, according to the invention can be coated.

Conveniently, the layer thickness of the protective layer is in a range between 1 μιη and 1500 μπι. Preferably, the layer thickness is in a range between see 25 μπι and 600 μηι.

According to the invention, the component may be a combustion chamber, an engine block, a crankcase, a crankcase interior, a cylinder liner, a cylinder head, an intake manifold, an exhaust manifold, a turbocharger compressor wheel, a turbocharger inner chamber, an exhaust gas recirculation or a cylinder piston. Thus, it is advantageous if an internal combustion engine and / or a motor vehicle is or are provided with a component according to the invention.

The partial or complete generation of a protective layer in the abovementioned components with a method according to the invention, at least on the surfaces adjacent to the medium, for example to the hot gas, thermally isolates the system boundary of the thermally loaded component. In other words, there is the advantage that the thermal conductivity at the system boundary is reduced. On the one hand, the component is thermally less stressed, on the other hand, the temperature of the medium (eg of the hot gas) can thus be maintained over a longer path and time period. Furthermore, this leads to the fact that certain aggregates of the automobile respond faster, and also the thermal energy stored in the gas is not purely thermally dissipated, but can be recovered by other aggregates or components.

In addition to thermal insulation, the protective films produced by the process of the invention have good resistance to wear, oxidation, erosion and corrosion, which is required in a number of components. Furthermore, as the life of the components is improved. This particularly affects the cylinder liner (Tribological wear due to solid state, transition, mixed and / or sliding friction), the compressor wheel of the turbocharger (erosion wear) or the manifold (corrosion resistance).

A further advantage of the method according to the invention lies in the applicability and ability to selectively and nevertheless homogeneous coating in cavities, channels or complex geometries with undercuts. In contrast to thermal spraying, in which the surface to be coated has to be directly reachable by the spray jet, in the process according to the invention a homogeneous protective layer is formed everywhere on the surface where the electrolyte wets the component surface. So undercuts or depressions or channels can be provided with a protective layer.

Due to the process, the surface is thus converted by reaction of the electrolyte with the substrate in contrast to galvanic methods. That is, there is no locally dependent on the prevailing field lines material deposition according to the local distribution of the current density instead - which, for example, in complex geometries and undercuts, the use of auxiliary electrodes are necessary would - but local spark gaps are generated wherever the process-related potential is present.

By using conductive electrolytes, this potential is distributed evenly across the electrolyte, and therefore even complicated geometries or internal surfaces can be homogeneously provided with a protective layer. Due to the process, care must be taken with a PEO for thorough mixing of the electrolyte. Furthermore, by means of a suitable system technology, protective layer can be produced selectively by means of PEO on the surface sections to be thermally insulated.

In addition, due to the ceramic character of the protective layers produced by PEO, they have an improved insulating effect due to a poorer thermal conductivity than the anodized surfaces known from the prior art. This is because the prior art surfaces do not have a classical ceramic structure and thus are rather hybrids. In contrast to anodised surfaces, protective layers produced with PEO do not have a regularly arranged pore pattern, but rather a chaotic pore network, which, in contrast to anodization, can also have interconnecting compounds.

The invention will be explained in more detail with reference to embodiments shown in the figures. Here are shown schematically:

1 shows a concept for the production of a protective layer

Component by flushing the internal channels;

2 shows a plant for producing a protective layer with a

method according to the invention for a cylinder piston head;

3 shows an internal combustion engine in a schematic representation; 4 shows a cross section through an internal combustion engine of the internal combustion engine. and

Fig. 5 shows an area of Fig. 4 in an enlarged view

FIG. 1 shows a concept 1 in the form of an electrolytic cell for producing a protective layer in a component 2. The component 2 may for example be a manifold. It is therefore the application of a method shown, in which not the entire component 2 is dipped into the electrolyte for the application of PEO, but the electrolyte is flushed through lying in the interior of the component 2 channels, so that on the inside of the channels of the component. 2 selectively a suitable protective layer is produced. For this purpose, the interior of the component 2 is sealed with two flanges 3, each having a seal. By means of a pump 4, the electrolyte is pumped through a line assembly 6 through the component 2. In this cycle, the electrolyte is cooled or tempered by the electrolytic cooling 5. Furthermore, the concept 1 has a power supply 7 as a power supply, which as shown can be a DC power supply or an AC power supply. About an electrical cable 8, the component 2 and a counter electrode 9 is connected. Via the flanges 3 and the counter electrode 9 is introduced into the space to be coated in the interior of the component 2. The counter electrode 9 is the cathode, the component 2 representing the anode 10. 2, a second embodiment is shown. Fig. 2 is in two parts, wherein in Fig. 2 - Part 1, the system 1 for producing a protective layer on a component 2, in this example a cylinder piston head, is shown, and in Fig. 2 Part 2 of the procedural part of the system with respect to Electrolyte.

As can be seen, the cylinder piston head 2 in the system 1 is charged with electrolyte, which is charged via a pump 4 through an inlet valve 11. The Circulation of the electrolyte is done via a discharge 12, for example via an extraction, wherein the suction tube shown is made of a stainless steel, for example made of V2A. Furthermore, the system 1 has a cooling system 13 for the cylinder piston head 2. The power supply 7 is designed such that the discharge 12 simultaneously represents the counter electrode 9, and the cylinder piston head 2, the anode 10th

The internal combustion engine shown in FIG. 3 comprises an internal combustion engine 110 which operates, for example, according to the diesel principle and is embodied, for example, as a four-cylinder reciprocating internal combustion engine. The internal combustion engine 110 is supplied with fresh gas (ambient air) via a fresh gas train 112. For this purpose, the fresh gas is compressed after being aspirated from the environment by means of a compressor 114. The compressed fresh gas is then passed through a charge air cooler 116, in which the fresh gas heated as a result of the compression is cooled until it reaches the desired temperature for entry into the internal combustion engine 110. Via a suction tube 118, the fresh gas enters into combustion chambers 120 of the internal combustion engine 110, in which this or the oxygen contained therein is burned in a known manner with fuel injected directly into the combustion chambers 120.

The exhaust gas produced during combustion of the fuel-fresh gas mixture is removed via an exhaust line 122 of the internal combustion engine. Exhaust line 122 includes an exhaust manifold 124 in which the exhaust gas flowing out of the individual combustion chambers 120 is brought together, and a turbine 126 arranged downstream thereof. Turbine 126 forms an exhaust gas turbocharger together with compressor 114 and is controlled by means of an adjustable bypass 128 (wastegate ) executed passable. The bypass 128 serves, in certain operating states of the internal combustion engine 110 leading to a large exhaust gas mass flow, to a part of the exhaust gas mass. Stream past the turbine 126 so as to limit the boost pressure in the fresh gas train 112.

In the exhaust line 122 downstream of the turbine 126, an exhaust aftertreatment device is further integrated. In this case, the exhaust aftertreatment devices may comprise, for example, an oxidation catalytic converter 130 and a particle filter 132.

4 shows a cross section through the internal combustion engine 110 in the region of a cylinder. The engine 110 includes a cylinder housing 134 that forms the individual cylinders. In each of the cylinders, a piston 136 is movably guided up and down. Above the cylinder housing 134, a cylinder head 138 connects. The cylinder housing 134, the cylinder head 138, and the pistons 136 are formed of aluminum alloys. Into the cylinder head 138, at least one intake passage 40 and at least one exhaust passage 142 are integrated for each cylinder. The intake ports 140 are part of the fresh gas train 112 of the internal combustion engine and connect the suction pipe 118 fluid-conductively with the respective cylinders. The exhaust passages 142 - are part of the exhaust line 122 and connect the respective cylinders to the exhaust manifold 124. Via gas exchange valves 144, introduction of the fresh gas into the cylinders and discharge of the exhaust gas from the cylinders are controlled in a known manner. In this case, the gas exchange valves 144 are actuated, for example, by means of one or more (not shown) camshafts. The combustion chambers 120 formed by the individual cylinders are each bounded by a portion of the inner wall of the associated cylinder, by the top of the associated piston 136, a portion of the underside of the cylinder head 138, and by the bottoms of the associated gas exchange valves 144.

In order to thermally insulate the combustion chambers 120, in particular on the surfaces formed by the upper sides (of main bodies) of the pistons 136, a protective Layer 146 applied by means of anodic oxidation with radio discharge. This protective layer 146 consists essentially of aluminum oxide (A1203), which forms in the context of anodic oxidation with radio discharge on the tops of the piston 136. The protective layer 146, which may have a layer thickness of, for example, about 200 μπι, already characterized basically by their formation of alumina by a high wear resistance and good thermal resistance, whereby their use for limiting the combustion chambers 120 of the engine 46 is possible. Furthermore, the protective layer 146 is also characterized by a relatively low thermal conductivity and a relatively low heat capacity in comparison to the aluminum alloy from which the pistons 136 are formed. This achieves the desired thermal insulation of the combustion chambers and consequently a relatively low heat transfer of gases in the combustion chambers 120 to the pistons 136. In order to further reduce a heat transfer from the combustion chambers to the base body of the pistons 136, it is provided to embed particles 148 of zirconium oxide, for example, in the protective layer 146 consisting of aluminum oxide as matrix material, which have an even lower thermal conductivity compared to the aluminum oxide. As is apparent from Fig. 5 is provided to provide the particles 148 of zirconia over the entire surface of the protective layer 146 in a (second) sub-layer, which extends between the surface of the body of the corresponding piston 136 and another, to the Combustion chamber 120 adjacent (first) sub-layer is arranged.

In the first sub-layer of the protective layer 146, no particles 148 of zirconium oxide are provided, but locally particles 150 of a material, for example copper, which in comparison with the aluminum oxide serving as matrix material are provided by a relative tively high thermal conductivity. It is envisaged to provide the particles 150 of copper in those regions of the first sub-layer of the protective layer 146 in which experience has shown that relatively high local wall temperatures may result during the operation of such an internal combustion engine. The particles 150 of copper serve to reduce such locally high wall temperatures by forwarding the increased introduction of heat energy at these points as well as possible to the entire second partial layer. FIG. 5 shows that the particles 150 made of copper can be arranged, for example, at the edge transitions of a piston recess 152 and in the region of a central elevation of the piston recess 152. FIG. 5 also shows that the density of the distribution of the particles 150 of copper, ie the number of particles per unit volume; in the formation of the protective layer 146 by means of anodic oxidation under spark discharge can be controlled (also possible for the particles 148 of zirconium oxide). It is thus provided that in those sections of the first part-layer in which particles 150 of copper are provided, a higher density of particles 150 in a central region and a decrease in the density of particles 150 towards the edge of the respective section are provided.

The subdivision of the protective layer 146 into the first sub-layer and the second sub-layer results merely from the different embedding of the different particles 148, 150 and the different functionalities for the protective layer 146 achieved thereby. A structural parting plane is not formed between the two sub-planes.

The particles 148, 150 may, for example, have a size of <5 μm.

In addition to the upper sides of the pistons 136, individual or all other surfaces bounding the combustion chambers 120 of the internal combustion engine 110 may also be provided with a corresponding protective layer 146 in order to further improve the thermal insulation of the combustion chambers 120. FIG. 4 shows by way of example that both the Inner walls of the cylinders (at least in those portions which define the combustion chambers 120), the corresponding portions of the underside of the cylinder head 138 and the bottoms of the gas exchange valves 144 may each be coated with a protective layer 146 formed by spark discharge anodization.

FIG. 4 also shows the possibility of providing the outlet ducts 142 of the internal combustion engine 110 serving as exhaust gas ducts with corresponding protective layers 146. Likewise, other surfaces of the exhaust tract 122 of the internal combustion engine serving for exhaust gas routing, for example walls of an exhaust manifold and / or a turbine of an exhaust gas turbocharger, can be provided with corresponding protective layers 146.

LIST OF REFERENCE NUMBERS

1 concept / electrolytic cell / plant

2 component

3 flange

4 pump

5 electrolytic cooling

6 line arrangement

7 energy supply

8 electrical cable line

9 counter electrode / cathode

10 anodes

11 inlet valve

12 exhaustion

13 cooling

110 internal combustion engine

112 fresh gas train

114 compressors

116 intercooler

118 intake manifold

120 combustion chamber

122 exhaust system

124 exhaust manifold

126 turbine

128 bypass

130 oxidation catalyst

132 particle filter cylinder housing

piston

inlet channel

exhaust port

Gas exchange valve

protective layer

Particles with relatively low thermal conductivity

Particles with relatively high thermal conductivity

piston bowl

Claims

Patent claims

1. Method for producing a protective layer on a thermally stressed component which consists at least partially of a valve metal, the protective layer being generated by an electrochemical process,

characterized in that

the electrochemical process is a plasma electrolytic oxidation using an electrolyte and applying electrical power.

Method according to claim 1,

characterized in that

the electrolyte has an electrolyte base, the electrolyte base being phosphoric acid, potassium hydroxide, water glass, deionized water or a zirconium-containing compound.

Method according to one of the preceding claims,

characterized in that

the electrical power is voltage-controlled, with the current strength being limited, or current-controlled, with the voltage being limited, or power-controlled.

Method according to one of the preceding claims,

characterized in that

the electrical power is applied at a frequency of 1 Hz to 10 kHz, in particular at a frequency of 1 Hz to 1000 Hz.

Method according to one of the preceding claims 3 or 4,

characterized in that

the voltage is applied in a range between 150 and 1500 volts, preferably in a range between 210 and 650 volts.

Method according to one of the preceding claims 3 to 5,

characterized in that

the current is applied with a current density in a range between 0.001 and 1000 A/dm ² , preferably in a range between 0.5 to 15 A/dm ² .

7. Method according to one of the preceding claims 4 to 6,

characterized in that

the applied current and/or the applied voltage is overmodulated by a higher-frequency current and/or a higher-frequency voltage.

8. Method according to one of claims 4 to 7,

characterized in that

the applied current and/or the applied voltage is regulated equally, or has the shape of a symmetrical wave, an asymmetrical wave, a rectangle or a trapezoid.

Method according to one of the preceding claims,

characterized in that

the plasma electrolytic oxidation is carried out at a temperature between 0°C and 80°C, preferably between 18°C and 50°C.

10. Method according to one of the preceding claims,

characterized in that

the electrolyte is designed as a dispersion, with one or more of the following particles being added to the electrolyte:

A1 ₂ 0 ₃ , Ti0 ₂ , Si0 ₂ , WC, Zr0 ₂ , iron oxide, graphite and/or MoS ₂ .

11. Method according to claim 10,

characterized in that

Particles with grain sizes between 0.001 and 5000 μπι, preferably between 0.1 and 100 μιη are dispersed.

12. Method according to claim 10 or 11,

characterized in that

the dispersion is generated by an ultrasonic oscillator.

13. Method according to one of the preceding claims 10 to 12,

characterized in that

Particles are polarized by adding surfactants.

14. Method according to one of the preceding claims 1 to 9,

characterized in that

Particles are deposited in the protective layer which have a relatively low or high thermal conductivity compared to a base material of the protective layer.

15. Method according to claim 14,

characterized in that the particles with relatively low thermal conductivity are provided in the entire protective layer.

16. Method according to claim 14 or 15,

characterized in that

the particles with relatively high thermal conductivity are only provided in one or more sections of the protective layer.

17. Method according to one of the preceding claims 14 to 16,

characterized in that

the particles with relatively high thermal conductivity are provided in a first sub-layer of the protective layer and the particles with relatively low thermal conductivity are provided in a second sub-layer separated by the first sub-layer.

18. Method according to one of claims 14 to 17

characterized in that

The particles with relatively low thermal conductivity zirconium oxide are used.

19. Method according to one of claims 14 to 18,

characterized in that

for the particles with relatively high thermal conductivity, copper, iron, beryllium, aluminum, silver, silicon, molybdenum, tungsten, carbon, beryllium oxide, beryllium nitrite, silicon nitrite and/or silicon carbite as well as mixtures and/or alloys thereof are used.

20. Component with a protective layer, the protective layer being produced by a method according to one of claims 1 to 19, wherein the component consists at least partially of a valve metal.

21. Component according to claim 20,

characterized in that

the component is made of aluminum, an aluminum alloy, magnesium, a magnesium alloy, titanium or a titanium alloy.

22. Component according to claim 20 or 21,

characterized in that

the protective layer has a layer thickness between 1 and 1500 μm, preferably between 25 and 600 μm.

23. Component according to one of the preceding claims 20 to 22,

characterized in that

the component is a combustion chamber, an engine block, a crankcase, a • crankcase interior, a cylinder liner, a cylinder head, an intake manifold, an exhaust manifold, a turbocharger compressor wheel, a turbocharger interior, an exhaust gas recirculation system or a cylinder piston.

24. Internal combustion engine with a component according to claim 23.

25. Motor vehicle with a component according to claim 23.