Method for producing a surface layer on a surface of a component by means of plasma electrolytic oxidation and concrete use of such a method

Abstract

Method for producing a surface layer (46) on a surface of a component (24) by means of plasma electrolytic oxidation, in which the component (24) is in contact with an electrolyte liquid comprising an aluminate and is at least temporarily subjected to an electrical voltage as an anode, characterized in that the surface layer (46) is formed with a porosity of at least 20%.

Description

Method for producing a surface layer on a surface of a component by means of plasma-electrolytic oxidation and specific use of such a componentMethod:The invention relates to a method for producing a surface layer on a surface of a component by means of plasma-electrolytic oxidation. The invention further relates to the possibility of thermally insulating an exhaust gas guide component forming a section of an exhaust line of an internal combustion engine by means of such a surface layer and thus reducing a heat loss of the exhaust gas.In the operation of an internal combustion engine, a large portion of the thermal energy generated during the combustion of fuel and oxygen is released to the environment as waste heat. The continuing efforts to increase the efficiency of internal combustion engines therefore also start from the reduction of this waste heat.Waste heat arises in particular when the still warm exhaust gas is discharged to the environment and as a result of a transfer of thermal energy from the combustion chambers and the exhaust tract to the components of the internal combustion engine forming the combustion chambers and the exhaust tract and from these directly or indirectly to the ambient air.In modern internal combustion engines, in which the components delimiting the combustion chambers (cylinder crankcase, cylinder head, gas exchange valves and pistons) and also at least some components of the exhaust train are frequently formed from light metal and in particular aluminum for weight reasons and for achieving the smallest possible moving masses (relates in particular to the pistons), the problem of heat transfer arises to a particular extent because aluminum, in addition to its relatively low density, is also characterized in particular by a relatively high thermal conductivity.The known measure of insulating the outer surfaces of the components of an exhaust line, for example by means of a sheathing by means of insulating wool, has the disadvantage of enlarging the relevant heat-emitting surface, as a result of which the insulation effect is at least partially compensated again.Furthermore, a relevant aim in the development of an internal combustion engine is the quickest possible heating of the exhaust line in order to bring the exhaust gas aftertreatment devices integrated into the exhaust line as quickly as possible to operating temperature. For this purpose, the smallest possible thermal mass of the components of the exhaust system is advantageous. However, by insulation only on the outer side of the exhaust line, the thermal mass is not reduced.Insulation of the inner sides, i.e. the surfaces of the components of the exhaust system provided for the exhaust gas guidance, is therefore more promising from the thermodynamic point of view. However, the application of an insulation layer to the inner, exhaust-gas-conducting surfaces of an exhaust line is technically demanding. This seems to be economically implementable merely by producing a thermally insulating coating. However, the requirements for such a coating are manifold. On the one hand, a thermally stable and mechanically well adhering coating must be produced which, in addition, resists the temperature change loads occurring. On the other hand, this requires a coating method which permits the application of a coating without direct visual access and, in addition, in cavities which are in some cases of relatively small dimensions. Thermal spray layers which promising adequate adhesion and insulation effect cannot be applied or can only be applied at very great expense because of the lack of visual contact. Ceramic coatings which can be applied by slip methods, on the other hand, generally have inadequate adhesion and resistance to changes in temperature.WO 2015 / 090 267 A1 discloses providing the surfaces delimiting the combustion chambers of an internal combustion engine, including the corresponding surfaces of the pistons and also sections of the exhaust line of the internal combustion engine, with a protective layer by means of plasma electrolytic oxidation (PEO).Plasma electrolytic oxidation is a combined method from the fields of plasma technology and electrochemistry, by means of which surfaces of components formed from so-called valve metals can be provided with a surface layer made of an oxide ceramic. In this case, native barrier layer formers such as aluminum, magnesium or titanium are in particular selected as valve metals. The surface layer can be produced in particular in aqueous electrolytes. The component or components to be oxidized are immersed in the electrolyte as an electrode together with one or more other components which function as a counter electrode. When direct current is used, the component or components to be coated are subjected to anodic polarity. In the case of alternating current or bipolar pulse currents, the component or components to be coated are merely "electrodes", wherein the surface layer is always formed only in those current segments in which the component or components are anode. In the case of symmetrical pulse or alternating currents (i.e. the same proportion of current in both directions), therefore, both the components used as "electrode" and the components used as "counter electrode" can be alternately anodically connected and thus provided with a surface layer.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 passive layer of the component to be coated is locally penetrated, wherein solid-state plasma-chemical reactions, the spark discharges, are triggered. Plasma electrolytic oxidation is therefore also referred to as "anodic oxidation under spark discharge" (ANOF). The spark discharges are not generated over a large area but only locally at those locations at which the thickness of the oxide layer and thus the local electrical resistance is the smallest. Since the plasma reactions thus always take place at those points of the passive layer which locally have the smallest layer thickness and ensure layer thickness growth there, the surface is coated with a very uniform surface layer. In order to permanently break through the increasing dielectric property of the growing oxide layer with a breakdown voltage, the electrical potential applied thereto is increased until the desired layer thickness of the surface layer is reached. The local melting of the surface to be converted resulting from the spark discharge can lead to a particularly wear-resistant surface layer. At the same time, a suitable process control can be used to generate a porous surface layer which has a particularly low thermal conductivity and therefore has a good thermal insulating effect.WO 2015 / 090 267 A1 discloses the production of surface layers on components which consist of the valve metals based on aluminum, magnesium or titanium which are usually used for the application of such methods. It is also described that an application of the method is also to be possible with steel or cast iron, without, however, describing details for a possible implementation. In fact, such a conversion, i.e. a production of a protective layer by conversion of a surface of a component made of an iron metal by means of plasma electrolytic oxidation, has not been able to be realized hitherto, or at least not with satisfactory results. One reason for this can be seen in the fact that ferrous metals do not form, or only very slowly, and then do not form a sufficiently closed chemically induced passive layer, which is however considered to be required as a starting layer which is excited to grow rapidly by the applied electric voltage.A measure for bypassing this problem is described both in WO 2015 / 007 497 A1 and in EP 2 832 898 A1. There, it is provided in each case to provide a base body made of an iron metal and in particular also cast iron firstly with a coating made of a light metal and in particular aluminum or titanium, and then to convert the surface of this coating accordingly by means of a plasma electrolytic method in order to form a wear protection layer.While in modern internal combustion engines the components delimiting the combustion chambers are largely made of light metal, which can thus advantageously be thermally insulated by means of a surface layer produced according to the method disclosed in WO 2015 / 090 267 A1, the use of ferrous metals and in particular steel and cast iron for forming the exhaust lines for such internal combustion engines is still customary. One reason for this is in particular the significantly lower material costs. A further reason for the use of ferrous metals for exhaust lines of internal combustion engines is that ferrous metals fundamentally have a lower thermal conductivity than aluminum, the light metal primarily used in vehicle technology, and thus have a better thermal insulating effect. Despite this lower thermal conductivity, the heat loss from the exhaust gas via the components of the exhaust train formed from ferrous metals is considerable.DD 1 42 359 A discloses a method for treating a surface of a component, in particular consisting of iron or ferrous metals, by means of anodic spark discharge, wherein an aqueous aluminate electrolyte is used.DE 42 09 733 A1 and U.S. Pat. No. 3,293,158 A likewise each disclose a method for surface treatment of a component by means of anodic spark discharge, wherein the electrolytic bath used here can comprise aluminates.GB 1 051 665 A discloses a method for the anodic coating of metals and in particular steel sheets with the aim of corrosion protection and of colouring.DE 11 2011 102 782 B4 discloses a method for producing a porous, anodic oxidation coating on surfaces which delimit a combustion chamber of an internal combustion engine, with the aim of a thermal insulation.WO 2016 / 086 914 A2 describes a method for producing a surface layer by means of plasma electrolytic oxidation on a component of a turbo device, the component body of which consists of a titanium-aluminum alloy. An electrolyte may be used which may comprise an aluminate and in particular sodium aluminate.EP 1 774 067 B1 discloses a coating of an article made of a magnesium alloy, wherein the coating has a porosity of about 1%.U.S. Pat. No. 6,197,178 B1 describes that it may be of particular interest in the field of aerospace technology to use coatings of components made of aluminum or aluminum alloys as thermal protective layers, these coatings having a certain porosity and being able to resist thermal shock relatively well as a result.Proceeding from this prior art, the object of the invention was to specify a possibility of reducing or keeping low a heat transfer from the exhaust-gas-conducting spaces and ducts to the components of an internal combustion engine forming them in the most advantageous way possible.This object is achieved by a method according to claim 1. Advantageous embodiments and uses of the method according to the invention are the subject matter of the dependent claims and are evident from the following description of the invention.A method for producing a surface layer on a surface of a component by means of plasma electrolytic oxidation, in which the component is supplied with an electrical voltage at least temporarily as an anode in contact with an electrolyte liquid, is characterized according to the invention in that the electrolyte liquid comprises an aluminate (Al 2 O 4) and in particular sodium aluminate (NaAl(OH) 4).It has been recognized that the production of a surface layer on a surface of a metallic component can be positively influenced in principle by adding an aluminate and in particular sodium aluminate. In particular, this also makes it possible to achieve the production of such a protective layer by conversion even in the case of a surface made of an iron metal (or other heavy metal) with good results. In this case, a surface layer can be produced which is not or only partially made of iron oxide (oxidation of the base material for forming the surface layer is actually specific for plasma electrolytic oxidation on conventional valve metals), but rather to a large extent made of an aluminum oxide ceramic, which is obviously attributable to the aluminate contained in the electrolyte liquid. The addition of the aluminate to the electrolyte liquid thus avoids the problem that ferrous metals as base metals do not form a barrier layer or form a barrier layer only very slowly or only incompletely during plasma electrolytic oxidation.Accordingly, it may preferably be provided to carry out the method according to the invention with a component in which at least that surface on which the corresponding surface layer is produced is formed from an iron metal and in particular a steel or a cast iron. In particular, it can be provided that the component consists entirely of an iron metal and in particular a steel or a cast iron (e.g. iron- and nickel-based alloys, austenitic cast iron, SiMo cast iron, cast steel). It is likewise possible to use the method according to the invention in components in which at least that surface on which the surface layer is to be produced is made of any metals, in particular transition group metals (density>5 g / cm 3).The addition of at least one aluminate for the electrolyte liquid provided according to the invention can in principle have an advantageous effect on a plasma-electrolytic oxidation of a surface of a component, so that an application can also be provided if the component or at least the surface of the component to be converted consists of a classic valve metal, such as in particular aluminum (Al), magnesium (Mg), titanium (Ti) or zirconium (Zr).By using a plasma electrolytic oxidation provided according to the invention, it is possible to control relatively well how compact the material structure of the surface layer is by varying the process parameters in a known manner. A relatively compact material structure generally has a particularly advantageous effect with regard to the achievable wear resistance, while a less compact, i.e. more porous material structure can have a positive effect both with regard to achieving a low thermal conductivity and a low heat capacity because of the micro-cavities formed therein. According to the invention, the thermal insulation effect of the surface layer is focused, so that it is provided that it is formed to be relatively porous. By "relatively porous" is meant that the surface layer is produced with a porosity which lies above the minimum (i.e. material structures which are as compact as possible) within a porosity range which can be achieved by varying the process parameters. Specifically, it is provided that the porosity is at least 20% and preferably at least 30% or 40%. In particular, it can be provided that the porosity is provided as large as possible, taking into account the other requirements, in particular with regard to the stability of the surface layer and / or the anti-corrosion effect for the base material of the component.In addition to a low thermal conductivity and a relatively low heat capacity (in particular in the case of a relatively porous configuration of the material structure) and a relatively high wear resistance (in particular in the case of a relatively compact configuration of the material structure), the surface layer produced according to the invention can also be distinguished by a good thermal resistance, a low surface roughness and a good corrosion resistance. In addition, because the surface layer is not produced by a classic coating, but rather fundamentally by a conversion of a surface of the component (here, however, with the inclusion of aluminum oxide formed from the aluminate of the electrolyte liquid), a good "adhesion" of the surface layer to the base material of the component is achieved. These advantages are present in a surface layer produced according to the invention as an (at least temporarily) exhaust-gas-conducting surface of an exhaust-gas conducting component, in particular of an exhaust-gas conducting component, which forms a section of an exhaust line of an internal combustion engine, preferably of a motor vehicle. This also applies because a plasma electrolytic oxidation of an inner surface and thus possibly not directly accessible can be realized relatively easily due to the possible use in an electrolyte bath. The exhaust gas guiding component can be, for example, an exhaust manifold. Exhaust manifolds are currently still usually formed from ferrous metals and, because of the comparatively complex shape, mostly from cast iron. In addition, the heat losses from the exhaust gas in the exhaust manifold are relatively high, which is due to the arrangement of an exhaust manifold directly adjoining the outlet ducts which are formed in a cylinder head of an internal combustion engine of an internal combustion engine and the exhaust gas temperatures which are thus still very high. According to the invention, the "exhaust manifold" is understood to mean the exhaust gas guide component of an exhaust line of an internal combustion engine that is arranged (optionally with the interposition of sealing elements) directly adjoining exhaust gas ducts formed in an internal combustion engine of the internal combustion engine and that, in the case of a multi-cylinder internal combustion engine, merges partial streams of the exhaust gas from the individual exhaust gas ducts.In addition to an application in exhaust gas-conducting and in particular components which constitute a section of an exhaust line of an internal combustion engine, the method according to the invention can also be used advantageously in all components, in particular those components for which, as a result of their configuration from, for example, an iron metal, a production of a surface layer by means of plasma electrolytic oxidation has not been considered to date and in which the specific advantages of the surface layer produced, namely in particular good thermal insulation effect, high wear resistance and good mechanical, thermal and corrosion resistance have functionally advantageous effects. For example, pistons made of steel of an internal combustion engine can be thermally insulated from a combustion chamber by the configuration of a surface layer according to the invention on, in particular, the piston crown, and / or can be made more wear-resistant by the configuration of a surface layer according to the invention on, in particular, the piston skirt. By integrating particles with a relatively low coefficient of friction into the surface layer of the piston skirt, the friction of such a piston in a cylinder of the internal combustion engine can additionally be reduced and the wear behavior can thus be further optimized. In a corresponding manner, the cylinder wall of such a cylinder can of course also be provided--if appropriate only locally, for example at the reversal points of the movement of the piston guided therein--with a surface layer formed according to the invention. This also applies in particular to cylinder walls of a cylinder formed in a cylinder housing or of a so-called liner made of cast iron. During operation of an internal combustion engine, both a piston crown and a cylinder wall come temporarily into contact with the exhaust gas generated by combustion, so that this surface, too, is understood to be the "exhaust gas-carrying surface" in the sense of the invention.According to the invention, there is furthermore the advantageous possibility of forming the surface layer not on all but selectively on (only) one or more surfaces of the component by the component not being completely immersed in an electrolyte bath, but rather the selected surface(s) being / are selectively brought into contact with the electrolyte liquid within the scope of carrying out the method according to the invention. For example, in the case of an provided formation of corresponding surface layers on inner surfaces which conduct exhaust gas, it may be provided to arrange the electrolyte liquid only (in a static manner) within the channels provided for conducting exhaust gas or to allow the electrolyte liquid to flow through these channels.A thermal insulation of an exhaust line of an internal combustion engine or at least a portion thereof, which is improved by the method according to the invention, can lead in particular to an increase in efficiency for an exhaust gas aftertreatment device integrated into the exhaust line of the internal combustion engine.It has been found to be particularly advantageous in the implementation of the method according to the invention if the electrolyte liquid used, for the base of which preferably water and in particular deionized water can be provided, has a (total) concentration of the aluminate or aluminates of 1 to 60 g / l, preferably of 2 to 20 g / l. Furthermore, the electrolyte liquid advantageously used in the context of the method according to the invention can be usedone or more phosphates, in particular potassium diphosphate (K 4 P 2 O 7), in a concentration of 0 to (max.) 18 g / l;one or more silicates, in particular sodium metasilicate (Na 2 SiO 3), in a concentration of 0 to (max.) 18 g / l;potassium hydroxide (KOH) in a concentration of 0 to (max.) 30 g / l;an ethylenediamine tetraacetate (EDTA), in particular tetrasodium ethylenediamine tetraacetate (Na 4 EDTA), in a concentration of 0 to (max.) 15 g / l; and / orglycerol (C 3 H e O 3) in a concentration of 0 to (max.) 30 g / l. The specification of the lower limit value "zero" is intended to express that the addition of these additives is optional. If the individual additives are to be provided in the electrolyte liquid, it can preferably be provided that they are (in each case) provided in a minimum concentration of 0.5 g / l.In a preferred embodiment of the method according to the invention, it can be provided that the surface layer is applied using an alternating voltage or a pulsed direct voltage. As a result, the spark discharges characteristic of the method can be advantageously controlled in a known manner. However, the method according to the invention can also be implemented using a direct voltage with a constant voltage value. Alternating voltage is to be understood as meaning not only a sinusoidal voltage profile, but generally any electrical voltage whose polarity changes in regular repetition, but whose temporal mean value is zero. In particular, this can also include so-called "bipolar pulse patterns" of the voltage curve, since ultimately a sine can also be approximated by rectangles, so that modulated rectangular pulses can represent a type of basic unit. There is also the possibility of superimposing an AC voltage or a pulsed DC voltage with a DC voltage with a constant voltage value, as a result of which a voltage profile can result according to an AC voltage or according to a pulsed DC voltage with a mean value (offset) different from zero.It can furthermore preferably be provided that the electrical voltage is selected such that a current density of at least 15 A / dm 2, preferably of at least 20 A / dm 2( the density relates here to the surface to be coated) is set. Such a current density, which is higher than the current densities usually provided in the context of the production of surface layers by plasma electrolytic oxidation of conventional valve metals, can in particular positively influence the production of a corresponding surface layer on a surface made of an iron metal (or another heavy metal).On the other hand, the setting of a relatively high current density during the formation of the surface layer may lead to a generation of a relatively high process waste heat which should be sufficiently safely dissipated in order to maintain, for example, a preferred process temperature (measured in the electrolyte liquid adjacent to the surface to be converted), which may be, for example, in a temperature range of 20° C. to 80° C. For this reason, it may be expedient to provide an upper limit value for the current density to be set, which can be, for example, 200 A / dm 2. It has been found that a current density exceeding this upper limit value does not have to be provided even for the production of a sufficiently good surface layer on a surface made of an iron metal (or another transition group metal).In a further preferred embodiment of the method according to the invention, it can additionally be provided that particles made of a material differing from a base or matrix material of the surface layer are provided in the surface layer, which particles have a relatively high or low thermal conductivity compared to the base or matrix material of the surface layer. It can be provided particularly preferably that both particles are provided which have a relatively high thermal conductivity compared to the base material or matrix material of the surface layer and particles which have a relatively low thermal conductivity.This aspect of the invention is based on the finding, on the one hand, that the surface layer produced in the context of the method according to the invention can indeed represent an advantageous compromise with regard to, in particular, thermal insulation and durability, but alternative materials are present which are distinguished by an even lower thermal conductivity and thus a further improved thermal insulation effect. However, these cannot be used for the complete formation of a surface layer for various reasons. By introducing particles of one or more of these alternative materials into the surface layer produced according to the invention, the average thermal conductivity thereof can be lowered further and thus the thermally insulating properties can be improved further without this having a relevant negative effect on the further advantageous properties of the surface layer according to the invention. It can therefore be provided particularly advantageously that particles with relatively low thermal conductivity are provided in the entire surface layer (in relation to the surface and optionally also the layer thickness).Examples of materials for the particles with relatively low thermal conductivity are (pure) zirconium oxide (ZrO 2), Y-stabilized zirconium oxide (Zr(Y)O 2), aluminum oxide (Al 2 O 3), spinel (Al 2 O 3 / MgO), mullite (Al 2 O 3 / SiO 2), zirconium corundum (Al 2 O3 / ZrO2), Titanium oxide (TiO 2) or silicon oxide (SiO 2) and mixed ceramics with essential constituents of said oxides are possible.Even if the thermal conductivity of the introduced particles in their pure bulk state does not become less than that of the base material or matrix material, the thermal conductivity of the composite material of the surface layer formed from both can nevertheless be lower overall, since the introduced particles act as impurities for the propagation of the crystal oscillations (phonons). In this respect, the specifying specification "with relatively low thermal conductivity" is not restricted according to the invention exclusively to an actual material property of the particles, but rather is also intended to comprise the effect reducing thermal conductivity within the matrix.The particles with relatively high thermal conductivity, on the other hand, can be used advantageously to avoid or reduce local peaks of the wall temperature of the surface provided with the surface layer, in that a relatively high local transition of thermal energy from the exhaust gas is distributed as well as possible to a larger region of the surface layer by these particles. This makes it possible to avoid the formation of locally high wall temperatures, which may have a negative effect on the ignition delay (i.e. the time period between the injection of fuel into a combustion chamber and the ignition of the fuel in a self-igniting internal combustion engine), for example. For this purpose, it may be sufficient if the particles with relatively high thermal conductivity are provided in only one or more sections, but not in the entire surface layer (in terms of the area and preferably also the layer thickness). Such locally limited provision of particles with relatively high thermal conductivity therefore does not have to be associated with a relevant degradation of the average thermal conductivity of the entire surface layer.Suitable materials for the particles having relatively high thermal conductivity are, for example, cobalt, iron, beryllium, aluminum, copper, silver, silicon, molybdenum, tungsten, carbon, beryllium oxide, beryllium nitrite, silicon nitrite and / or silicon carbide, and mixtures and / or alloys thereof.If both particles with relatively low thermal conductivity and particles with relatively high thermal conductivity are to be provided, their distribution in the surface layer should be provided in such a way that the average thermal conductivity of the surface layer locally increased by the particles with relatively high thermal conductivity does not lead to a relevant higher heat transfer to the region of the base body of the component, which is arranged below the surface layer and delimits, for example, a combustion chamber and / or an exhaust gas duct of an internal combustion engine. This can be advantageously achieved in that, at least in sections, the particles with relatively high thermal conductivity are provided exclusively in a first sub-layer of the surface layer adjoining the combustion chamber and / or the exhaust gas duct, and the particles with relatively low thermal conductivity are provided in a second sub-layer separated from the combustion chamber and / or the exhaust gas duct by the first sub-layer. The particles with relatively high thermal conductivity can then ensure the most uniform possible distribution of the thermal energy passing into the surface layer within the first sublayer, while the second sublayer with the particles with relatively low thermal conductivity acts particularly well in a thermally insulated manner and consequently reduces a heat transfer from the first sublayer to the region of the component lying below the surface layer.Plasma electrolytic oxidation enables a targeted arrangement of particles in the surface layer in a relatively simple manner. This applies in particular when using a plasma electrolytic oxidation by means of an alternating voltage, in which either the positive or negative voltage phases can be used alternately to deposit the particles contained in the electrolyte on the growing surface layer, while the corresponding other voltage phases are used for the growing formation of the surface layer. In this case, it can also be provided that the particles, when they are deposited on the growing surface layer, are at least partially melted together, so that a material mixing phase can form in the transition between the matrix material and the particles. On the other hand, the particles can also be present essentially structurally separated in the matrix material.The indefinite articles ("a", "an", "an" and "an"), in particular in the patent claims and in the description generally explaining the patent claims, are to be understood as such and not as numerical words. Components concreted therewith are thus to be understood such that they are present at least once and can be present multiple times.The present invention is explained in more detail below with reference to exemplary embodiments shown in the drawings. The drawings show, in each case in a schematic illustration: FIG. 1 : an internal combustion engine; FIG. 2 shows a cross section through an internal combustion engine of the internal combustion engine; FIG. 3 shows a region of FIG. 2 in an enlarged illustration; and FIG. 4 : shows a plant for carrying out a method according to the invention.The internal combustion engine shown in FIG. 1 comprises an internal combustion engine 10 which operates, for example, according to the diesel principle and is designed, for example, as a four-cylinder reciprocating piston internal combustion engine. The internal combustion engine 10 is supplied with fresh gas (ambient air) via a fresh gas line 12. For this purpose, the fresh gas is compressed after being drawn in from the environment by means of a compressor 14. The compressed fresh gas is then passed through a charge air cooler 16, in which the fresh gas heated as a result of the compression is cooled until the desired temperature for entry into the internal combustion engine 10 is reached. Via an intake pipe 18, the fresh gas enters combustion chambers 20 of the internal combustion engine 10, in which the fresh gas or the oxygen contained therein is burned in a known manner with fuel injected directly into the combustion chambers 20.The exhaust gas formed during the combustion of the fresh fuel gas mixture is discharged via an exhaust line 22 of the internal combustion engine. The exhaust system 22 comprises an exhaust manifold 24, in which the exhaust gas flowing out of the individual combustion chambers 20 is combined, and a turbine 26 arranged downstream thereof. The turbine 26 forms an exhaust gas turbocharger together with the compressor 14 and is designed such that it can be bypassed by means of a controllable bypass 28 (wastegate). The bypass 28 serves to bypass a portion of the exhaust gas mass flow past the turbine 26 in certain operating states of the internal combustion engine 10 that lead to a large exhaust gas mass flow, in order to thus limit the charge pressure in the fresh gas section 12.Downstream of the turbine 26, an exhaust gas aftertreatment device is furthermore integrated into the exhaust line 22. The exhaust gas aftertreatment device may include, for example, an oxidation catalyst 30 and a particulate filter 32.FIG. 2 shows a cross section through the internal combustion engine 10 in the region of a cylinder. The internal combustion engine 10 includes a cylinder housing 34 forming the individual cylinders. In each of the cylinders, a piston 36 is guided movably up and down. A cylinder head 38 is connected above the cylinder housing 34. For example, it can be provided that the cylinder housing 34 and the pistons 36 are formed from an iron metal and in particular a cast iron (for the cylinder housing 34 and the pistons 36; for example a cast iron alloy with vermicular graphite, e.g. GJV-XSiMo4.5-0.6 with the alloy constituents [wt.-%]: C: 3.0-3.6; Si4.0-4.6; Mn: ≤ 0.6 P: ≤ 0.07; S: ≤S; Cr: ≤ 0.2; Ni ≤ 0.6; Mo: 0.4-0.6; 0.15-0.22) or a steel (for the pistons 36), while the cylinder head 38 can be formed from an aluminum alloy. Integrated into the cylinder head 38 is at least one inlet duct 40 and at least one outlet duct 42 for each cylinder. The inlet ducts 40 are part of the fresh gas train 12 of the internal combustion engine and connect the intake pipe 18 in a fluid-conducting manner to the respective cylinders. The outlet ducts 42 are part of the exhaust train 22 and connect the respective cylinders to the exhaust manifold 24. By means of gas exchange valves 44, which can be formed from steel, for example, the introduction of the fresh gas into the cylinders and the discharge of the exhaust gas from the cylinders are controlled in a known manner. In this case, the gas exchange valves 44 are actuated, for example, by means of one or more camshafts (not shown).The combustion chambers 20 formed by the individual cylinders are each bounded by a section of the inner wall of the associated cylinder, by the upper side of the associated piston 36, by a section of the lower side of the cylinder head 38 and by the lower sides of the associated gas exchange valves 44.In order to thermally isolate the combustion chambers 20 inter alia, a surface layer 46 is formed on the surfaces formed by the upper sides (of base bodies) of the pistons 36 by means of a method according to the invention.The surface layer 46, which can have a layer thickness of, for example, approximately 200 μm, is basically characterized by a high wear resistance and a good thermal resistance, as a result of which its use for delimiting the combustion chambers 20 of the internal combustion engine 46 is possible. Furthermore, the surface layer 46 is also characterized by a relatively low thermal conductivity and a relatively low heat capacity compared to the ferrous metal from which the pistons 36 are formed. This achieves the desired thermal insulation of the combustion chambers and consequently a relatively low heat transfer from gases located in the combustion chambers 20 to the pistons 36.In order to further reduce a heat transfer from the combustion chambers to the base bodies of the pistons 36, particles 48 of, for example, zirconium oxide are embedded in the surface layer 46 consisting of aluminum oxide as matrix material, which particles are distinguished by an even lower thermal conductivity compared to the material of the surface layer. As can be seen from FIG. 3, it is provided that the particles 48 of zirconium oxide are provided over the entire surface of the surface layer 46 in a (second) sub-layer which is arranged between the surface of the base body of the corresponding piston 36 and a further (first) sub-layer adjoining the combustion chamber 20.No particles 48 made of zirconium oxide are provided in the first sublayer of the surface layer 46, but locally particles 50 made of a material, for example copper, which is distinguished by a relatively high thermal conductivity compared to the matrix material of the surface layer comprising at least a large portion of aluminum oxide. It is provided that the particles 50 made of copper are provided in those regions of the first sub-layer of the surface layer 46 in which relatively high local wall temperatures can arise from experience during operation of such an internal combustion engine. The particles 50 made of copper serve to reduce such locally high wall temperatures by distributing the increased introduction of thermal energy at these points as well as possible over the entire second sub-layer. FIG. 3 shows that the particles 50 made of copper can be arranged, for example, at the edge-side transitions of a piston depression 52 and in the region of a central elevation of the piston depression 52. FIG. 3 also shows that the density of the distribution of the particles 50 made of copper, i.e. the number of particles per unit volume, can also be controlled during the formation of the surface layer 46 by means of plasma electrolytic oxidation (likewise possible for the particles 48 made of zirconium oxide). Thus, it is provided that in those sections of the first sublayer in which particles 50 made of copper are provided, a higher density of particles 50 is provided in a central region and a density of particles 50 decreasing towards the edge of the respective section is provided.The subdivision of the surface layer 46 into the first sub-layer and the second sub-layer results only from the different embedding of the different particles 48, 50 and from the different functionalities achieved therewith for the surface layer 46.The particles 48, 50 can have a size of ≤10 μm, for example.In addition to the upper sides of the pistons 36, individual surfaces or all other surfaces delimiting the combustion chambers 20 of the internal combustion engine 10 can also be provided with a corresponding surface layer 46 in order to further improve, inter alia, the thermal insulation of the combustion chambers 20 and, if appropriate, also a wear behavior. FIG. 2 shows by way of example that both the inner walls of the cylinders (at least in those sections which delimit the combustion chambers 20), the corresponding sections of the underside of the cylinder head 38 and the undersides of the gas exchange valves 44 can be provided with in each case one surface layer 46 which has been formed by plasma electrolytic oxidation.FIG. 2 also shows the possibility of providing the outlet ducts 42 of the internal combustion engine 10 serving as exhaust gas ducts with corresponding surface layers 46. Likewise, other surfaces of the exhaust tract 22 of the internal combustion engine serving for exhaust gas guidance, for example walls of an exhaust manifold 24 and / or of a turbine 26 (including a turbine impeller) of an exhaust gas turbocharger, may be provided with corresponding surface layers 46.FIG. 4 shows a system for forming a surface layer on a surface of a component according to a method according to the invention. The component 24 can be, for example, an exhaust manifold 24 of an internal combustion engine according to FIG. 1.An application of a method according to the invention is shown, in which not the entire component 24 is immersed in an electrolyte liquid for applying the plasma electrolytic oxidation, but the electrolyte liquid is flushed through the channels 54 located in the interior of the component 24, so that a surface layer is selectively produced on the inner side of the channels 24 of the component 24. For this purpose, the inner volume of the component 24 is closed by two closure elements 56, which can each have a sealing element. By means of a pump 58, the electrolyte liquid is pumped through a line arrangement 60 and the channels 54 of the component 24. In this circuit, the electrolyte liquid is cooled or tempered by means of a cooling device 62. Furthermore, the system has an electrical energy source 64, which can provide a DC voltage or an AC voltage. The component 24 and a counter electrode 68 are connected to the electrical energy source 64 via electrical lines 66. The counter electrode 68 can be fastened, for example, to one of the closure elements 56 and can thereby be positioned within the channels 54, the walls of which are to be formed with a surface layer. The counter electrode 68 constitutes the cathode when performing the plasma electrolytic oxidation, while the component 24 constitutes the anode.LIST OF REFERENCE CHARACTERS10 Internal combustion engine 12 fresh gas train 14 compressor 16 charge air cooler 18 intake pipe 20 combustion chamber 22 exhaust gas train 24 exhaust manifold / component 26 turbine 28 bypass 30 oxidation catalytic converter 32 particle filter 34 cylinder housing 36 piston 38 cylinder head 40 inlet duct 42 outlet duct 44 gas exchange valve 46 surface layer 48 particles with relatively low thermal conductivity 50 particles with relatively high thermal conductivity 52 piston depression 54 duct of the exhaust manifold / component 56 closure element 58 pump 60 line arrangement 62 cooling device 64 electrical energy source 66 electrical lines 68 counterelectrode

Claims

Method for producing a surface layer (46) on a surface of a component (24) by means of plasma electrolytic oxidation, in which an electrical voltage is applied at least temporarily to the component (24) as an anode in contact with an electrolyte liquid comprising an aluminate, characterized in that the surface layer (46) is formed with a porosity of at least 20%.The method of claim 1, characterized in that the surface layer (46) is formed on a surface of an iron metal.Method according to one of the preceding claims, characterized in that the electrolyte liquid is based on water and has the following additives: - one or more aluminates in a concentration of 1 to 60 g / l; - one or more phosphates, in particular potassium diphosphate (K 4 P 2 O 7), in a concentration of 0 to 18 g / l; - one or more silicates, in particular sodium metasilicate (Na 2 SiO 3), in a concentration of 0 to 18 g / l; - potassium hydroxide (KOH) in a concentration of 0 to 30 g / l; an ethylenediamine tetraacetate (EDTA), in particular tetrasodium ethylenediamine tetraacetate (Na 4 EDTA), in a concentration of 0 to 15 g / l; glycerol (C 3 H e O 3) in a concentration of 0 to 30 g / l;Method according to one of the preceding claims, characterized in that the surface layer (46) is formed using an electrical alternating voltage or a pulsed direct voltage.Method according to one of the preceding claims, characterized in that the electrical voltage is selected such that a current density of at least 15 A / dm 2, preferably of at least 20 A / dm 2 is established.Method according to one of the preceding claims, characterized in that the electrical voltage is selected such that a current density of at most 200 A / dm 2 is established.Method according to one of the preceding claims, characterized in that the surface layer (46) is selectively formed on one or more surfaces of the component (24).Use of a method according to one of the preceding claims for producing the surface layer (46) on an exhaust-gas-conducting surface of the component (24).Use according to Claim 8, characterized in that the component (24) delimits a cylinder and in particular a combustion chamber of an internal combustion engine of a motor vehicle or is provided as a section of an exhaust line (22) of the internal combustion engine.

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