EP3084048B1 - Method for producing a protective layer on a thermally stressed component and component having such a protective layer - Google Patents

Description

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 process for producing an oxidation, wear or corrosion protection 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 motor vehicles, there is an effort to reduce the total weight of the vehicle and therefore its individual components, so as 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 that are exposed to higher temperatures, for example, about 300 ° C, 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 conduction coefficient is reduced. In particular spraying processes, for example thermal spraying or plasma spraying, are known from the state of the art. A disadvantage of this solution is that in such spray coatings the connection between the sprayed protective layer and the component is achieved by a mechanical clamping of the layer material (eg by flakes) on the substrate, ie the surface of the component, or by adhesion processes or diffusion processes. 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 into a cavity, special injection molding tool is necessary, if a spraying 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. That's how it shows DE 10 2012 002 284 A1 For example, a turbine wheel, on or in the surface of a halide from the group corridor, chlorine or bromine or is introduced and on the surface of an oxidation layer is formed by the so-called halogen effect in a heat treatment. 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. Thus, there is only a limited improvement in 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 the DE 10 2012 218 666 A1 is such a method known. Herein, a turbine wheel of a titanium alloy turbocharger is subjected to electrochemical anodization, which constructs an oxide film as a protective layer and thus protects the component from 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 in the DE 10 2012 218 666 A1 shown protective layer is that the layer has a high pore ratio 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 disadvantage is the breaking and limited formation of the protective layer in the region of edges and sharp transitions. Consequently, no adequate protection is given by a lack of training of the protective layer, especially in critical component areas.

Furthermore, from the EP 2 371 996 A1 discloses a method of electrolytic ceramic coating on metal, wherein at least one metal is used as an anode to perform anodization treatment of an anode surface in a given electrolytic solution for ceramic coating by generating glow discharge and / or arc discharge, thereby forming a ceramic coating on the metal surface wherein the average current density during the application of the positive side is in the range of 0.5 A / dm ² to 40 A / dm ² , and wherein the anodization treatment at a positive side operating time ratio (T1) of 0.02 to 0.5, an operating time ratio of the negative side (T2) of 0 to 0.5, a non-application time ratio (T3) of 0.35 to 0.95 and wherein these ratios simultaneously satisfy the following formulas: 0 ≦ T2 / T1 ≦ 10 and 0.5 ≦ T3 / (T1 + T2) ≦ 20.

In the EP 2 511 401 A2 A method for producing a coating on the surface of a light metal based substrate by plasma electrolytic oxidation is described in which the substrate is immersed as an electrode together with a counter electrode in an electrolyte liquid and a sufficient voltage for generating spark discharge on the surface of the substrate is applied, wherein the electrolyte comprises clay particles dispersed therein.

The EP 2 103 718 A1 discloses another method of applying a ceramic film to a metal which can form a dense metal-based film such as magnesium alloys. The formed ceramic film has excellent abrasion resistance, hardly attacks a corresponding material, and moreover has an excellent abrasion performance.

The CA 2 479 032 A1 discloses a coating consisting of an oxide / lubricant composite coating on light alloys based on plasma oxidation in conjunction with solid lubricant agents which are rubbed against an oxide surface during coating formation.

In the EP 2 721 270 A1 For example, there is disclosed a method of reducing emissions and / or reducing friction in an internal combustion engine, wherein a portion of the combustion chamber of aluminum and / or titanium is coated with titanium oxide, further comprising dopants in and / or on the adhered titanium oxide coating.

In the CN 102 234 828 A For example, a method is described for producing an extra strong self-lubricating ceramic coating by plasma electrolytic oxidation (PEO).

In the CN 101 429 671 A Finally, a zirconium coating on an aluminum alloy surface and a production process using a plasma Electrolytic Oxidation (PEO) for it is described. The resulting zirconium coating consists of tetragonal zirconia of monoclinic zirconia and of gamma-type alumina. The thickness of the coating is 20 μm to 300 μm. The coating is suitable for complex structures and has a strong bond with the carrier. Furthermore, it is characterized by a high hardness of 1700 HV to 1800 HV.

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. In this case, local temperature maxima are to be reduced in a combustion chamber, which is to be thermally insulated at the same time. 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 10. Advantageous developments are described in the dependent claims.

According to the invention, a method is proposed for producing a protective layer on a thermally loaded 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. Furthermore, the method is characterized in that particles are deposited in the protective layer, which relative to a base material of the protective layer, a relative have low or high thermal conductivity, wherein the particles are provided with relatively high thermal conductivity in a first sub-layer of the protective layer and the particles with relatively low thermal conductivity in a second, separated by the first sub-layer sub-layer.

A valve 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 applying an electrical power in a local plasma via spark discharge and forms an oxide ceramic or layer. The electrolyte-exposed surface is "scanned", governs electrochemically with the cleaved oxygen and / or the electrolyte to an oxide ceramic or layer (for example, Al ₂ O ₃ , spinels, mixed oxides, etc.).

A PEO process is an anodic oxidation process using a specially 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 electrochemistry, by means of which surfaces of components which are formed of so-called valve metals can be provided 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 in particular take place 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 has a defined thermal conductivity according to their ceramic character, which is well below the thermal conductivity of the substrate material, for example aluminum. Due to the smaller Wärmeleitkoeeffizienten and the low thermal conductivity of the protective layer thus higher wall temperatures are possible, so that the surface provided with the protective layer against the adjacent medium, such as hot gas, is thermally insulated.

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.

Conveniently, the electrolyte has an electrolyte base, wherein the electrolyte base is phosphoric acid (H ₃ PO ₄ ), potassium hydroxide (KOH), water glass (Na ₂ SiO ₃ ), deionized water or a zirconium-containing compound. An electrolyte base here is a substance from a variety of substances, the amount (in g / L) in addition to water and urotropin is most common in an electrolyte. Zirconium sulfate (ZrSO ₄ ) or zirconium tungstate (ZrWO ₄ ) is particularly suitable as the zirconium-containing compound. This has the advantage that with such an electrolyte composition, a component of, for example, aluminum or titanium or of the corresponding alloys can be plasma-electrochemically oxidized at all.

It is advantageous if the electrical power is voltage controlled, the current is limited, or is 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 volts and 1500 volts, preferably in a range between 210 volts and 650 volts, and if the current has a current density in a range between 0.001 A / dm ² and 1000 A / dm ² , preferably in a range between 0.5 A / dm ² 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 rectified or has the form of a symmetrical wave, an asymmetric wave, a rectangle or a trapezoid. Here, the characteristic shape with a duty cycle and an offset in the range of 0% to 100% and can thus be executed 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 be protected so safe and long-term stability against external influences, for example, from undesirable oxidation. Furthermore, with the method according to the invention components 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 , TiO ₂ , SiO ₂ , tungsten carbide (WC), ZrO ₂ , iron oxide, graphite and / or MoS ₂ . In this case, the electrolyte is subjected to an electrolyte base by the addition of said particles. The particles can 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 are incorporated as a foreign body into the protective layer or react chemically, electrochemically or physically with the substrate or the electrolyte to form a different compound.

In particular, particles of Al ₂ O ₃ , TiO ₂ , SiO ₂ , tungsten carbide (WC), ZrO ₂ , iron oxide, have a significantly reduced thermal conductivity, so that the incorporation of these particles in the protective layer further improves the insulating effect of the protective layer. In particular, zirconium oxide (ZrO ₂ ) has proved to be advantageous.

Furthermore, the addition of lubricant particles such as graphite, MoS ₂ or other corresponding particles which are incorporated into the protective layer, the coefficient of friction is reduced.

According to the invention, particles are provided in the protective layer of a material deviating from a base or matrix material of the protective layer, which have a relatively high or low thermal conductivity compared to the base or matrix material of the protective layer. Specifically, 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 distinguished 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 into the protective layer produced according to the invention, their average thermal conductivity can be further lowered and thus the thermal insulating properties can be further improved without this to a relevant extent on the further advantageous properties of the protective layer according to the invention, ie especially good durability and low surface roughness, has a negative impact.

As material for the particles with relatively low thermal conductivity, for example, Y-stabilized zirconia (Zr (Y) O ₂ ), alumina (Al ₂ O ₃ ), spinel (Al ₂ O ₃ / MgO), mullite (Al ₂ O ₃ / SiO ₂ ), zirconium corundum (Al ₂ O ₃ / ZrO ₂ ), titanium oxide (TiO ₂ ) or silicon oxide (SiO 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 particles introduced act as impurities for the propagation of the crystal oscillations (phonons). In this respect, the concretization "with relatively low thermal conductivity" according to the invention 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 is distributed. 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 particularly important for self-igniting internal combustion engines, i. diesel engines, in particular, 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 the application of the method for improving spark-ignited internal combustion engines, in particular gasoline engines.

As a material for the particles having a relatively high thermal conductivity, for example, copper, iron, beryllium, aluminum, copper, silver, silicon, molybdenum, tungsten, carbon, beryllium, beryllium nitrite, silicon nitrite and / or silicon carbide and mixtures and / or alloys thereof come into consideration.

According to the invention, both particles with relatively low thermal conductivity and particles with relatively high thermal conductivity are provided. Their distribution in the protective layer should be provided in such a way that the mean thermal conductivity of the protective layer, which is locally increased by the particles having a relatively high thermal conductivity, does not lead to a significantly higher heat transfer to the region of the coated, the combustion chamber and / or the exhaust gas guide-limiting component arranged below the protective layer leads. This is achieved in an advantageous manner 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 can be provided by the first sub-layer separate 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 having relatively low thermal conductivity has a particularly good thermal insulating effect and consequently a heat transfer from the first sub-layer reduced lying below the protective layer region of the component.

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 μm to 5000 μm, in particular in a range between 0.1 μm 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 especially cationic surfactants (e.g., ester squares) such that the polarized particles are e.g. in the cathodic part of a half-wave are pulled to the surface and in the anodic part of a half wave - in the context of the spark discharge - are integrated into 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.

Advantageously, the layer thickness of the protective layer is in a range between 1 .mu.m and 1500 .mu.m. The layer thickness is preferably in a range between 25 μm and 600 μm.

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 interior, an exhaust gas recirculation or a cylinder piston. Thus, it is advantageous if an internal combustion engine and / or a motor vehicle is provided with a component according to the invention or be.

The partial or complete generation of a protective layer in the abovementioned components with a method according to the invention, at least at 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 thus thermally less stressed, on the other hand, the temperature of the medium (for example of the hot gas) can thus be maintained over a longer path and time. Furthermore, this leads to the fact that certain aggregates of the automobile respond faster, and also the stored in the gas thermal energy can not be purely thermally dissipated but can be recovered by other aggregates or components.

In addition to the thermal insulation, the protective layers produced by the process according to the invention have a 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 bore (tribological wear due to solid, transitional, 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 attainable 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.

As a result of the method, in contrast to galvanic methods, the surface is converted by reaction of the electrolyte with the substrate. That is, there will be no locally dependent on the prevailing field lines material deposition according to the local distribution of the current density, which would be necessary, for example, in complex geometries and undercuts the use of auxiliary electrodes, but it will generate local sparking anywhere where the process-related potential is applied.

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, a good mixing of the electrolyte must be ensured in a PEO. Furthermore, by means of a suitable system technology, a 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 Anodized surfaces have protective layers produced with PEO no regularly arranged pore pattern, but a chaotic pore network, which in contrast to the anodization may also have interconnecting compounds.

Fig. 1

ein Konzept für die Erzeugung einer Schutzschicht bei einem Bauteil mittels Durchspülung der innenliegenden Kanäle;

Fig. 2

eine Anlage zur Erzeugung einer Schutzschicht mit einem erfindungsgemäßen Verfahren für einen Zylinderkolbenkopf;

Fig. 3

eine Brennkraftmaschine in einer schematischen Darstellung;

Fig. 4

einen Querschnitt durch einen Verbrennungsmotor der Brennkraftmaschine; und

Fig. 5

einen Bereich der Fig. 4 in einer vergrößerten Darstellung

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

Fig. 1

a concept for the production of a protective layer in a component by flushing the internal channels;

Fig. 2

a system for producing a protective layer with a method according to the invention for a cylinder piston head;

Fig. 3

an internal combustion engine in a schematic representation;

Fig. 4

a cross section through an internal combustion engine of the internal combustion engine; and

Fig. 5

an area of Fig. 4 in an enlarged view

In Fig. 1 a concept 1 in the form of an electrolytic cell for the production of a protective layer in a component 2 is shown. 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 immersed in 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 generates a suitable protective layer. 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 source, which, as shown, may 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.

In Fig. 2 a second embodiment is shown. Fig. 2 is made in two parts, with in Fig. 2 Part 1, the plant 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 plant with respect to the electrolyte.

As can be seen, the cylinder piston head 2 in the system 1 is charged with an electrolyte, which is fed by 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 pipe 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 the counter electrode 9 and the cylinder piston head 2, the anode 10.

The in the Fig. 3 shown internal combustion engine includes an example operating on the diesel principle internal combustion engine 110, which is formed 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 pipe 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 directly injected into the combustion chambers 120 fuel.

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 pass part of the exhaust gas mass flow past the turbine 126 in order to limit the charge pressure in the fresh gas train 112.

In the exhaust line 122 downstream of the turbine 126, an exhaust aftertreatment device is further integrated. The exhaust aftertreatment devices may include, for example, an oxidation catalyst 130 and a particulate filter 132.

The Fig. 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 guided movable 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 inlet channel 140 and at least one outlet channel 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 ports 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 exhaust of the exhaust 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, a protective layer 146 is applied by means of anodic oxidation with spark discharge, in particular on the surfaces formed by the upper sides (of main bodies) of the pistons 136. This protective layer 146 consists essentially of aluminum oxide (Al 2 O 3), which forms as part of the anodic oxidation under spark discharge at the tops of the piston 136.

The protective layer 146, which may have a layer thickness of, for example, about 200 microns, already characterized in principle due to their training of alumina by a high wear resistance and good thermal resistance, whereby their use to limit the combustion chambers 120 of the engine 110 is possible , Furthermore, the protective layer 146 is characterized by a relatively low thermal conductivity and a relatively low heat capacity compared to the aluminum alloy, from which the piston 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, for example, zirconium oxide, which have an even lower thermal conductivity compared to the aluminum oxide. As is clear from the 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 extending between the surface of the body of the corresponding Piston 136 and another, adjacent to the combustion chamber 120 (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 is characterized by a relatively high thermal conductivity compared to the matrix material serving as alumina. 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. In the Fig. 5 It is shown that the particles 150 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. The Fig. 5 also shows that also 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 zirconia). It is thus provided that in those sections of the first partial layer in which particles 150 of copper are provided, in each case a higher density of particles 150 in a central region and to the edge of the respective section decreasing density of particles 150 provide.

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

The particles 148, 150 may, for example, have a size of .ltoreq.5 .mu.m.

In addition to the upper sides of the pistons 136, individual or all other surfaces delimiting 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. The Fig. 4 shows by way of example that both the inner walls of the cylinders (at least in those sections which delimit the combustion chambers 120), the corresponding portions of the underside of the cylinder head 138 and the lower sides of the gas exchange valves 144, each having a protective layer 146 which was formed by spark discharge anodization , can be coated.

Also shows the Fig. 4 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

electrolyte cooling

6

line arrangement

7

power supply

8th

electrical cable line

9

Counter electrode / cathode

10

anode

11

intake valve

12

removal

13

cooling

110

internal combustion engine

112

Fresh gas line

114

compressor

116

Intercooler

118

suction tube

120

combustion chamber

122

exhaust gas line

124

exhaust manifold

126

turbine

128

bypass

130

oxidation catalyst

132

particulate Filter

134

cylinder housing

136

piston

138

cylinder head

140

inlet channel

142

exhaust port

144

Gas exchange valve

146

protective layer

148

Particles with relatively low thermal conductivity

150

Particles with relatively high thermal conductivity

152

piston bowl

Claims (15)

1. A method for producing a protective layer (146) on a thermally stressed component (2) that is composed at least in part of a metal whose surface can be converted into an oxide ceramic layer or an oxide layer by an electrochemical process, with the protective layer (146) being produced by an electrochemical process, and with the electrochemical process being plasma electrolytic oxidation using an electrolyte with application of electric power, characterized in that particles (148, 150) are deposited in the protective layer (146) that have a relatively low or high level of thermal conductivity in comparison to a base material of the protective layer (146), wherein the particles (150) having a relatively high level of thermal conductivity are provided in a first sublayer of the protective layer (146) and the particles (148) having a relatively low level of thermal conductivity are provided in a second sublayer, with the first sublayer being separated from the component (2) by the second sublayer.
2. The method as set forth in claim 1, characterized in that the electrolyte has an electrolyte base, with the electrolyte base being phosphoric acid, potassium hydroxide, water glass, deionized water, or a zirconium-containing compound.
3. The method as set forth in any one of the preceding claims, characterized in that the electric power is voltage-controlled, with the current intensity being limited, or current-controlled, and with the voltage being limited, or power-controlled.
4. The method as set forth in any one of the preceding claims, characterized in that the electric power is applied at a frequency of from 1 Hz to 10 kHz, particularly at a frequency of from 1 Hz to 1000 Hz.
5. The method as set forth in any one of claims 3 or 4, characterized in that the voltage is applied in a range between 150 volts and 1500 volts, preferably in a range between 210 volts and 650 volts.
6. The method as set forth in any one of the preceding claims, characterized in that the electrolyte is embodied as a dispersion, with one or more of the following particles (148, 150) being added to the electrolyte: Al₂O₃, TiO₂, SiO₂, WC, ZrO₂, iron oxide, graphite, and/or MoS₂.
7. The method as set forth in any one of claims 1 to 6, characterized in that the particles (148) having a relatively low level of thermal conductivity are applied over the entire surface of the protective layer (146) in the second sublayer.
8. The method as set forth in any one of claims 1 to 7, characterized in that the particles (150) having a relatively high level of thermal conductivity are provided only in one or more portions of the first sublayer (146).
9. The method as set forth in any one of claims 1 to 8, characterized in that copper, iron, beryllium, aluminum, silver, silicon, molybdenum, tungsten, carbon, beryllium oxide, beryllium nitrite, silicon nitrite, and/or silicon carbide and mixtures and/or alloys thereof is/are used for the particles (150) having a relatively high level of thermal conductivity.
10. A component (2) with a protective layer (146), wherein the protective layer (146) has been produced by means of a method as set forth in any one of claims 1 to 9, wherein the component (2) is composed at least in part of a metal whose surface can be converted by an electrochemical process into an oxide ceramic layer or an oxide layer, and particles (148, 150) are deposited in the protective layer (146) that have a relatively low or high level of thermal conductivity in comparison to a base material of the protective layer (146, 150), wherein the particles (150) having a relatively high level of thermal conductivity are provided in a first sublayer of the protective layer (146) and the particles (148) having a relatively low level of thermal conductivity are provided in a second sublayer, and wherein the first sublayer is separated from the component (2) by the second sublayer.
11. The component (2) as set forth in claim 10, characterized in that the component (2) is made of aluminum, an aluminum alloy, magnesium, a magnesium alloy, titanium, or a titanium alloy.
12. The component (2) as set forth in claim 10 or 11, characterized in that the protective layer (146) has a layer thickness between 1 µm and 1500 µm, preferably between 25 and 600 µm.
13. The component (2) as set forth in any one of the preceding claims 10 to 12, characterized in that the component (2) is an engine block, a crankcase, a cylinder head (138), an intake manifold, an exhaust manifold (124), a turbocharger compressor wheel, a turbocharger interior, an exhaust gas return, or a cylinder piston (136).
14. An internal combustion engine with a component (2) as set forth in claim 13.
15. A motor vehicle with a component (2) as set forth in claim 13.

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