Piston for internal combustion engine and method for manufacturing piston

Abstract

To provide a novel method upon developing coating of a piston in order to improve thermal and / or mechanical characteristics of the piston.SOLUTION: The present invention relates to a piston (1) for an internal combustion engine (10) comprising a piston crown (3) having an outer surface (2) and a heat management layer (4). The heat management layer (4) includes a functional layer (5). The functional layer (5) includes a functional layer matrix, the functional layer matrix including polysiloxane and hollow spheres (9) embedded in the functional layer matrix. Here, a thermal conductivity of the functional layer (5) is within a range of 0.2 to 2 W / (m K), and a thermal effusivity of the functional layer (5) is within a range of 400 to 1200 J / (K m2 s1 / 2).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a piston for an internal combustion engine and to a method for applying a functional layer to a piston for an internal combustion engine according to the independent claims.

[0002] To improve the heat resistance of the piston, the piston is usually provided with a coating on the outer surface of the piston crown. For example, from US 2018 / 0094603 A1, a piston for an internal combustion engine is known, in which the piston is provided with two insulating and protective layers.

[0003] The objective of the present invention is to demonstrate a novel method in the development of a piston coating to improve the thermal and / or mechanical properties of the piston.

[0004] This problem is solved according to the invention by the subject matter of the independent claims. Advantageous embodiments are the subject matter of the dependent claims.

[0005] According to the present invention, a piston for an internal combustion engine having a piston crown with an outer surface is provided with a thermal management layer, the thermal management layer including a functional layer, the functional layer including a functional layer matrix, the functional layer matrix including polysiloxane and hollow spheres embedded in the functional layer matrix, the thermal conductivity of the functional layer being in the range of 0.2 to 2 W / (m·K), and the thermal effusivity of the functional layer being in the range of 400 to 1200 J / (K·m 2 ·s 1 / 2 ) is provided.

[0006] Alternatively, according to the invention, it is also contemplated that the functional layer comprises a functional layer matrix made of polysiloxane and hollow spheres embedded in the functional layer matrix.

[0007] Within the scope of the present invention, the outer surface of the piston crown is the side facing the combustion chamber.

[0008] The piston according to the invention with a thermal management layer having the features of the independent claims and the method according to the invention for manufacturing a piston according to the invention with a thermal management layer have significant advantages over the prior art in that heat losses are reduced, disadvantages in terms of volumetric efficiency are avoided and an advantageous thermoswing effect is created, thereby increasing the efficiency of the combustion process.

[0009] This is achieved in particular by avoiding excessive heating during the intake stroke and thus performance losses due to mass losses. At the same time, heat losses during the combustion stroke are also minimized. In particular, an advantageous thermoswing effect is achieved by the fact that the functional layer according to the invention is designed to have a low thermal conductivity and a low specific heat capacity at the same time. The thermal effusivity of the functional layer according to the invention is between 400 and 1200 J / (K·m 2 ·s 1 / 2 ) range. This allows the temperature of the thermal management layer to track the temperature of the gas with a slight lag.

[0010] In the scope of the present invention, the thermal effusivity b can be defined as the square root of the product of the thermal conductivity, the density and the specific heat capacity:

number

[0011] Thermal conductivity λ has units of W / (m K) Density ρ is kg / m 3 is the unit of Specific heat capacity c p is in units of J / (kg K).

[0012] Advantageously, the thermal management layer according to the invention further prevents excessive heat absorption by the piston, which on the one hand allows to improve the heat resistance of the piston and on the other hand allows the use of higher temperatures.

[0013] For example, DE 10 2017 207 236 A1 discloses a piston for an internal combustion engine, which has a substrate, to which a porous insulating layer is applied, in which hollow spheres are embedded, and on which a cover layer is arranged. The cover layer has embedded therein a plurality of platelet-shaped inorganic particles, in particular mica, which are intended to reflect heat in a mirror-like manner. In contrast, according to the present invention, advantageous thermal properties can be obtained without the need for such an additional reflecting layer with mica particles. This also leads to an improved mechanical stability of the thermal management layer according to the present invention.

[0014] Furthermore, the thermal management layer according to the invention advantageously reduces the consumption of lubricant on the one hand and at the same time allows for better heat dissipation on the other hand, thus extending the life of the lubricant.

[0015] One beneficial consequence of less heat having to be absorbed by the pistons and dissipated by the cooling circuits is that the exhaust gas temperatures are increased, which can be advantageously used to improve the efficiency of the turbocharger, if required, or to convert heat into electrical energy (waste heat recovery).

[0016] The piston for an internal combustion engine according to the present invention having a thermal management layer comprising a functional layer and the method according to the present invention for applying a thermal management layer comprising a functional layer to a piston for an internal combustion engine are described in more detail below, whereby Figure 1 used within the scope of and in connection with the description of the piston for an internal combustion engine according to the present invention also applies mutatis mutandis to the method for applying a functional layer to a piston for an internal combustion engine and to Figures 2a and 2b used in connection therewith, and vice versa.

[0017] According to the invention, a piston for an internal combustion engine comprises a thermal management layer. The piston comprises a piston crown having an outer surface. The thermal management layer comprises at least one functional layer. The thermal management layer serves to prevent, as far as possible, excessive heat absorption by the piston due to the ingress of heat from the combustion chamber of the internal combustion engine. This reduces heat losses during the combustion stroke and protects the piston material. This is achieved by the functional layer, which has a thermal conductivity in the range of 0.2-2 W / (m·K). Furthermore, the thermal effusivity of the functional layer (5) is in the range of 400-1200 J / (K·m 2 ·s 1 / 2 ) in the range. To achieve these properties, the functional layer has as matrix material a polysiloxane in which hollow spheres are embedded. Advantageously, the thermal conductivity and thermal effusivity are measured at 25°C.

[0018] It can be understood that the invention is basically applicable to all conceivable pistons for internal combustion engines, and it can be understood that the invention also contemplates all conceivable configurations of the piston crown.

[0019] The outer surface of the piston is preferably disposed on an end surface of the piston crown, and a functional layer of the thermal management layer is in contact with the outer surface of the piston crown, wherein the functional layer includes polysiloxane and hollow spheres.

[0020] Polysiloxanes are understood to be silicon-based polymers which may, for example, correspond to the following general formula 1: [ka]

[0021] Here, different chain lengths n are contemplated, as well as different branches not shown in the exemplary formula 1. This can also be understood as a three-dimensional cross-linked cage structure, where the group R 1 , R 2 , R 3 and R 4may each be independently selected from the group consisting of hydrogen, alkyl, aryl, halogen, hydroxyl and alkoxy. 1 and R 2 may be different in different repeat units. It is also contemplated that the groups selected from the group consisting of alkyl, aryl and alkoxy are unsubstituted. It is also contemplated that these groups are also mono- or polysubstituted with further substituents. For example, it is also contemplated that such groups are also substituted with substituents from the group consisting of alkyl, aryl, halogen, hydroxyl and amino.

[0022] Alkyl groups are known to those skilled in the art. Examples of alkyl groups are the methyl group -CH3, the ethyl group -CH2-CH3 or the n-propyl group -CH2-CH2-CH3. In principle, the alkyl group can be linear or branched.

[0023] Aryl groups are known to those skilled in the art. Examples of aryl groups are phenyl or 1-naphthyl.

[0024] Alkoxy groups are also known to those skilled in the art. They are understood to be alkyl groups linked to an oxygen atom. Examples of alkoxy groups are the methoxy group -OCH3 or the ethoxy group -O-CH2-CH3.

[0025] Amino groups are also known to those skilled in the art. By primary amino group is understood here the group -NH2. Furthermore, secondary amino groups -NHR 5 and tertiary amino groups -NR 5 R 6 There is also the group R 5 and R 6 may be, for example, the same or different alkyl groups.

[0026] According to the present invention, a silicone-based resin composition containing at least one polysiloxane as a main component is used.

[0027] Usually, an air-curing polysiloxane resin paint is used. In addition to the curable polysiloxane resin, this paint also contains other components, such as at least one solvent and pigments. For example, the polysiloxane resin paint can contain 15-70 mol% p-chlorobenzotrifluoride, 20-45 mol% curable polysiloxane resin, 15-35 mol% ceramic and / or metal pigments, 0-10 mol% TiO2 and 0-2 mol% carbon black. Of course, the sum of all the components contained must be 100 mol%. Usually, TiO2, ZrO2 and WO3 are advantageously included according to the invention.

[0028] Advantageously, hollow spheres made of ceramic, glass or pure SiO2 can be used as hollow spheres. Such hollow spheres may have a shape that deviates slightly from an ideal sphere due to the manufacturing process. For example, the hollow spheres may be made of pure, amorphous, transparent SiO2. Alternatively, the hollow spheres may be made of at least one material selected from the group consisting of SiO2, ZnO2, CaO, Al2O3, Na2O, Cr2O3, aluminosilicates, MgO, BaO, TiO2, ZrO2, MnO, Fe2O3 and B2O3. For example, hollow spheres with a density of 0.46 g / cm3 or less may be used. 3 3M™ Glass Bubbles IM16K from Glass Materials Corporation can be used as hollow spheres.

[0029] It is further preferred that the piston includes a piston material having an aluminum alloy or an iron alloy, and that the thermal management layer is disposed on at least a portion of an outer surface of the piston crown.

[0030] In particular, it is preferred that the piston includes a piston material having an iron alloy and that the thermal management layer is disposed on at least a portion of an outer surface of the piston crown.

[0031] As previously mentioned, the outer surface of the piston crown is the side that faces the combustion chamber.

[0032] Furthermore, it has proven to be advantageous for the polysiloxane to exhibit heat resistance up to at least 500°C, preferably for the polysiloxane to exhibit heat resistance up to at least 650°C.

[0033] In an advantageous development of the piston, the proportion of hollow spheres in the total volume of the functional layer is 5 to 40 volume %, preferably the proportion of hollow spheres in the total volume of the functional layer is 10 to 20 volume %, more preferably the proportion of hollow spheres in the total volume of the functional layer is 11 to 16 volume %.

[0034] It has also proven to be advantageous if hollow spheres are embedded in the functional layer as fillers, which increases the insulating effect of the functional layer. The hollow spheres are preferably made of SiO2. Alternatively, the hollow spheres are preferably made of soda-lime borosilicate glass.

[0035] Furthermore, within the scope of the present invention, the hollow sphere has a diameter d 50 Preferably, the hollow sphere has a diameter d 50 More preferably, the hollow sphere has a diameter d 50 It is advantageous to have

[0036] Diameter value d 50 indicates the particle size below which 50% of all particles fall. Conversely, this also means that 50% of all particles have a particle size above this size. The diameter of such hollow spheres is usually determined using laser diffraction or automated statistical imaging methods.

[0037] Furthermore, it has proven to be advantageous if the functional layer comprises TiO2, ZrO2 and WO3. Such a functional layer has particularly good heat resistance.

[0038] Typically, functional matrices of the functional layer are suitable which are made from the commercially available Cerakote® V series, and the use of Cerakote® V-136 has proven to be particularly advantageous.

[0039] Furthermore, it has proven advantageous for the thermal conductivity of the functional layer to be in the range of 0.2 to 1.8 W / (m K), more advantageously for the thermal conductivity of the functional layer to be in the range of 0.3 to 0.4 W / (m K). As already mentioned above, the thermal conductivity is advantageously measured at 25° C.

[0040] Thermal conductivity can be measured, for example, using the laser flash method or the 3ω method. In the laser flash method, a heat pulse is applied to the sample by a laser beam and the thermal radiation of the sample on the side opposite the laser is measured by an IR detector. In the 3ω method, a heating wire sputtered onto the material is heated by a periodic current and the material's response is measured in the form of a periodic voltage.

[0041] Furthermore, the thermal effusivity of the functional layer is 500 to 900 J / (K m 2 ·s 1 / 2 ), and more preferably, the thermal effusivity of the functional layer is in the range of 600-760 J / (K m 2 ·s 1 / 2 ) As already mentioned above, the thermal effusivity is preferably measured at 25°C.

[0042] It is further provided that the specific heat capacity of the functional layer is in the range of 450 to 1000 J / (kg·K). Advantageously, the specific heat capacity is measured at 25° C.

[0043] The specific heat capacity can be measured, for example, by differential scanning calorimetry (DSC). The thermal effusivity can be calculated according to the above formula.

[0044] Due to the aforementioned thermal properties in particular an advantageous thermoswing effect is achieved.

[0045] Furthermore, it has proven advantageous for the functional layer to have a thickness, measured perpendicularly to the outer surface of the piston crown, of 50 to 250 μm, preferably 80 to 110 μm.

[0046] It has also proven to be advantageous for the functional layer to have a thickness, measured perpendicularly to the outer surface of the piston crown, of 50 to 110 μm.

[0047] Simulations have shown that for a given thermal effusivity, increasing the thickness of the functional layer to values ​​of up to about 100 μm firstly significantly improves the thermoswing effect and the temperature progression in the engine. Increasing the thickness of the functional layer to thicknesses above 110 μm did not result in any further significant improvements in the simulations. Furthermore, tests were carried out using scanning electron microscopy on various samples with different functional layer thicknesses. These tests showed that the thicker the layer, especially above 150 μm, the more likely it is to suffer from adhesion problems and delamination. Layer thicknesses of up to 110 μm proved to be significantly more robust in this regard. Thus, a functional layer thickness of 50-110 μm, especially 80-110 μm, measured perpendicular to the outer surface of the piston crown, is an advantageous range both in terms of a good thermoswing effect and good mechanical stability of the functional layer.

[0048] Typically, such layer thicknesses can be determined by making cross sections, thus producing cross-sectional preparations, which are then analyzed under a microscope. Non-destructive measurements can also be performed using XRF spectrometry. Furthermore, to quickly estimate the applied layer thickness, it has proven advantageous to determine the applied amount of material by differential weighing, relating the applied amount of coating to the coated area.

[0049] Further, the functional layer has a thickness of 50 to 110 μm when measured perpendicular to the outer surface of the piston crown, and a thermal effusivity of the functional layer is 500 to 900 J / (K·m 2 ·s 1 / 2 ) has proven to be advantageous.

[0050] Thermal effusivity is 500J / (K m 2 ·s 1 / 2It has been found that with a functional layer thickness of less than 100%, the temperature of the piston surface becomes relatively high during operation, which has a detrimental effect on the strength and thermal stability of various engine components such as the piston, valves, and cylinder head.

[0051] Regarding the embedding of hollow spheres, the selected diameter d of the hollow spheres is chosen to allow the embedding of the hollow spheres in the functional matrix reasonably. 50 However, it goes without saying that this is smaller than the selected layer thickness.

[0052] The functional layer has a thickness of 50-110 μm when measured perpendicular to the outer surface of the piston crown, and a thermal effusivity of the functional layer of 500-900 J / (K m 2 ·s 1 / 2 ) and the hollow spheres have a diameter d 50 It has proven particularly advantageous to have

[0053] Advantageously, the thermal management layer further comprises a cover layer, the cover layer being attached to the functional layer facing away from the piston crown, the cover layer comprising or consisting of polysiloxane.

[0054] Advantageously, this cover layer is heat resistant up to at least 650° C. Such a cover layer advantageously protects the functional layer against mechanical, thermal and / or chemical influences.

[0055] It has also proven to be advantageous if the cover layer has a thickness, measured perpendicularly to the outer surface of the piston crown, between 4 and 20 μm, preferably between 4.5 and 5.5 μm.

[0056] It has also proven to be particularly advantageous if the cover layer is designed to be non-porous or closed-porous, so that it covers the functional layer as airtight as possible, which allows the latter to be particularly well protected.

[0057] Advantageously, the surface roughness Ra of the thermal management layer is in the range of <2 μm, which prevents as much turbulence of the combustion mixture as possible.

[0058] By surface roughness Ra is understood the average roughness value standardized in accordance with DIN EN ISO 4287:2010.

[0059] Typically, the surface roughness Ra is determined by scanning the surface over a given measurement section and recording all height and depth differences. A given integral of the progression of this profile is then determined over the measurement section and divided by the length of the measurement section.

[0060] The present invention also provides a method for manufacturing a piston, the method comprising applying a thermal management layer, the thermal management layer including a functional layer, to a piston for an internal combustion engine, the method comprising: a) cleaning and optionally roughening the area of ​​the piston where the functional layer is to be applied; b) introducing and dispersing hollow spheres in a polysiloxane resin paint to obtain a suspension (S); c) applying the suspension (S) to the piston by atmospheric spraying; d) hardening the suspension (S) applied in step c) to obtain a functional layer; e) optionally, following step d), applying a second layer to the functional layer by atmospheric spraying of a polysiloxane resin paint; f) following step e), curing the second layer to obtain a cover layer; The present invention relates to a method comprising the steps of:

[0061] In step a), the surface to be coated is first thoroughly cleaned before the application of the suspension (S). This is usually done with acetone, tert-butyl acetate or a comparable solvent or solvent mixture. Cleaning in an ultrasonic bath is also conceivable. Optionally, the piston can then be heated to about 50-150° C., which makes it possible to remove any adhering solvent residues. Furthermore, roughening of the surface may be expedient. This can be done, for example, by sandblasting. It may also be useful to cover the areas not to be coated before the subsequent application of the coating.

[0062] Then, in step b), the desired amount of hollow spheres is introduced into the polysiloxane resin paint to obtain a suspension (S). The polysiloxane resin paint usually contains, in addition to the polysiloxane resin to be cured, further other components, such as, for example, at least one solvent and pigments. For example, the polysiloxane resin paint contains 15-70 mol % of p-chlorobenzotrifluoride, 20-45 mol % of the polysiloxane resin to be cured, 15-35 mol % of ceramic and / or metallic pigments, 0-10 mol % of TiO2 and 0-2 mol % of carbon black. Usually, according to the invention, advantageously TiO2, ZrO2 and WO3 are included. This suspension (S) is applied to the piston in step c) by atmospheric spraying. By atmospheric spraying is understood a spraying process in which liquid or molten materials are sprayed onto a substrate at atmospheric pressure, typically in air. In the present invention, spraying is usually carried out at 1-2 bar.

[0063] Following application of the suspension (S) in step c), the layer obtained is cured in step d). For this purpose, the piston is usually first left at room temperature for about 15 minutes, then heated to 150° C. and kept at 150° C. for about 1 hour. The piston is then cooled to room temperature.

[0064] After the applied suspension (S) has been cured in step d) to obtain a functional layer, it is conceivable to apply a further layer by atmospheric spraying of a polysiloxane resin paint. In producing the second layer, no hollow particles are introduced into the polysiloxane resin paint. The same applies as above for the polysiloxane resin paint. This second layer is also cured after application. For this purpose, the piston is usually first left at room temperature for about 15 minutes, then heated to 150°C and kept at 150°C for about 1 hour. The piston is then cooled to room temperature.

[0065] Further important features and advantages of the invention can be seen from the dependent claims, the drawing and the associated figure description, as well as the examples.

[0066] It goes without saying that the features mentioned above and those to be described below can be used not only in the combinations shown in each case, but also in other combinations or alone without departing from the scope of the invention. [Brief description of the drawings]

[0067] [Figure 1] FIG. 1 is a schematic diagram of an example of an internal combustion engine that can be installed in, for example, a motor vehicle. [Figure 2a] FIG. 2 is a flow diagram of a method according to the present invention. [Figure 2b] 4 is a flow diagram of an advantageous development of the method according to the invention;

[0068] FIG. 1 shows an exemplary embodiment of an internal combustion engine 10 according to the invention. The internal combustion engine comprises a cylinder 11. In the cylinder 11 of the internal combustion engine 10, a piston 1 according to the invention is illustrated diagrammatically. The piston 1 together with the cylinder 11 partially bounds the combustion chamber of the internal combustion engine 10 with an outer surface 2, for example at an end face of the piston 1. FIG. 1 shows the piston 1 with a piston crown 3 having an outer surface 2. The piston 1 further comprises a thermal management layer 4. The thermal management layer 4 is arranged in some areas on the outer surface 2 of the piston crown. FIG. 1 shows an embodiment of the invention in which the thermal management layer 4 comprises a functional layer 5 and a cover layer 6. Here, the functional layer 5 is arranged between the piston crown 3 and the cover layer 6 in contact with the outer surface 2 of the piston crown 3. Alternatively, it is conceivable according to the invention, although not shown in FIG. 1, that the thermal management layer 4 only comprises the functional layer 5 and not the cover layer 6. The functional layer 5 comprises a functional layer matrix made of polysiloxane and hollow spheres 9 embedded in the functional layer matrix.

[0069] As can also be seen in Figure 1, the functional layer 5 has a thickness 7 measured perpendicular to the outer surface 2 of the piston crown 3. According to the invention, the thickness 7 of the functional layer 5 is between 50 and 250 µm, preferably between 80 and 110 µm. Alternatively, the thickness 7 of the functional layer 5 is preferably between 50 and 110 µm.

[0070] The cover layer 6 has a thickness 8 of the cover layer 6 measured perpendicularly to the outer surface 2 of the piston crown 3. According to the invention, the thickness 8 of the cover layer 6 is between 4 and 20 μm, preferably between 4.5 and 5.5 μm, measured perpendicularly to the outer surface of the piston crown. The functional layer 5 comprises hollow spheres 9. It goes without saying that such hollow spheres may have a shape that deviates slightly from an ideal sphere due to the manufacturing process. The hollow spheres preferably comprise or consist of SiO2. Alternatively, the hollow spheres are preferably formed from soda-lime borosilicate glass. The functional layer 5 is preferably temperature stable according to the invention up to at least 500° C., more preferably up to at least 650° C. The cover layer 6, which is likewise shown in FIG. 1, is preferably formed according to the invention so as to be non-porous or closed-porous. This means that the cover layer 6 preferably has no pores or only closed pores. This makes it possible to protect the underlying functional layer 5 from the gases in the combustion chamber. It is further provided according to the invention that the cover layer 6 is preferably temperature stable up to at least 500°C, more preferably up to at least 650°C.

[0071] 1 may, for example, comprise or consist of an aluminum alloy piston material according to the invention, or may alternatively comprise or consist of an iron alloy piston material.

[0072] 2a shows in method step 200 the cleaning and optionally roughening of a region of the piston 1, which after cleaning and optional roughening is then applied with a functional layer. This is followed by method step 201, in which hollow spheres 9 are introduced and dispersed in the polysiloxane resin paint. A suspension is thus obtained. This is followed by method step 202, in which the previously obtained suspension is applied to the piston 1 by atmospheric spraying. This is followed by method step 203, in which the previously applied layer of suspension is hardened. This results in the functional layer 5.

[0073] Fig. 2b likewise first shows the method steps 200 to 203 already described in Fig. 2a. However, in an advantageous development of the invention shown in Fig. 2b, method step 203 is followed by a further method step 204. In method step 204, a second layer is applied to the obtained functional layer 5 by atmospheric spraying of a polysiloxane resin paint. This second layer is cured in the subsequent method step 205 to obtain the cover layer 6.

[0074] In the following the invention will be explained in more detail by means of examples, without limiting the scope of the invention thereto: Materials used: Cerakote® V-136 3M(trademark) Glass Bubbles IM16K Example EB1a according to the invention: 21 g of 3M™ Glass Bubbles IM16K hollow spheres are dispersed in 400 g of Cerakote® V-136, corresponding to 14% by volume. A piston made of aluminum alloy and having a previously cleaned surface to be coated is then preheated to 80° C. for 30 minutes. The suspension is applied with a spray gun at a pressure of 1 bar from a distance of 17.5 cm. A total of 10 clicks are emitted from the spray gun. The piston is then cured at 150° C. for 1 hour. A layer thickness of 80-110 μm is obtained, which is determined non-destructively by XRF spectrometry. A crack-free surface is obtained. Here, the data 80-110 μm means that the layer thickness obtained is at least 80 μm at the thinnest point and at most 110 μm at the thickest point.

[0075] Example EB1b according to the invention: A piston coated according to Example EB1a was tested in an engine test for 250 hours of continuous operation. Visual inspection of the coating before and after the engine test showed no changes and the coating was stably adhered.

[0076] Example EB1c according to the invention: To study the thermal parameters of the material, 21 g of 3M™ Glass Bubbles IM16K hollow spheres, corresponding to 14% by volume, were dispersed in 400 g of Cerakote® V-136. Aluminum foil, the surface of which had been pre-cleaned, was then preheated to 80° C. for 30 minutes. The suspension was applied with a spray gun. The material was then cured together with the aluminum foil at 150° C. for 1 hour, after which the aluminum foil was scraped off. A layer with a thickness of about 5 mm and a diameter of about 12.9 mm was obtained. These layers were then adjusted by manual polishing to a thickness of about 2.4 mm and a diameter of about 12.7 mm.

[0077] Thermal diffusivity a [mm 2 / s] was measured by the laser flash method using a NETZSCH LFA 467 Hyperflash® instrument.

[0078] Furthermore, the specific heat capacity c p [J / (g·K)] was measured by differential heat flux calorimetry using a NETZSCH-DSC 204 Phoenix® instrument.

[0079] Furthermore, the density of the sample, ρ [g / cm 3 ] was measured at 23°C by the buoyancy method using a Sartorius balance ME235 together with density measurement set YDK 01.

[0080] From this, the thermal conductivity λ [W / (m K)] was calculated according to the following formula: λ=ρ c p a Here, a[m 2 / s] ρ [kg / m 3 ] c p [J / (kg K].

[0081] No density correction for temperature was performed.

[0082] [Table 1] [Explanation of symbols]

[0083] 1: Piston 2: External surface 3: Piston crown 4: Thermal management layer 5: Functional layer 6: Cover layer 7: Thickness of the functional layer 8: Cover layer thickness 9:Hollow ball 10: Internal combustion engine 11: Cylinder 200: Cleaning 201: Preparation of suspension by introducing hollow spheres into polysiloxane resin coating and dispersing them 202: Application of suspension 203: Hardening of the applied suspension 204: Application of second layer 205: Hardening of the second layer

Claims

1. A piston (1) for an internal combustion engine (10), the piston (1) comprising: a piston crown (3) having an outer surface (2); a thermal management layer (4), - said thermal management layer (4) comprises a functional layer (5) comprising a functional layer matrix, said functional layer matrix comprising polysiloxane and hollow spheres (9) embedded in said functional layer matrix; the thermal conductivity of said functional layer (5) is in the range of 0.2 to 2 W / (m·K); The thermal effusivity of the functional layer (5) is 400 to 1200 J / (K·m ２ ・s １／２ ) range, Piston (1).

2. 2. The piston (1) of claim 1, wherein the piston (1) comprises a piston material having an aluminum alloy or an iron alloy, and the thermal management layer (4) is disposed on at least a portion of the area of ​​the outer surface (2) of the piston crown (3).

3. Piston (1) according to claim 1 or 2, wherein the polysiloxane exhibits heat resistance up to at least 500°C, preferably the polysiloxane exhibits heat resistance up to at least 650°C.

4. 4. The piston (1) according to claim 1, wherein the proportion of the hollow spheres (9) to the total volume of the functional layer (5) is 5 to 40 volume %, preferably the proportion of the hollow spheres (9) to the total volume of the functional layer (5) is 10 to 20 volume %, and more preferably the proportion of the hollow spheres (9) to the total volume of the functional layer (5) is 11 to 16 volume %.

5. The hollow sphere (9) is made of SiO ２ 5. The piston (1) according to claim 1, characterized in that it is made of a filler material embedded in the functional layer (5).

6. 5. The piston (1) according to claim 1, wherein the hollow spheres (9) consist of soda-lime borosilicate glass and are embedded in the functional layer (5) in the form of fillers.

7. The hollow sphere (9) has a diameter d ５０ and preferably the hollow sphere (9) has a diameter d ５０ More preferably, the hollow sphere (9) has a diameter d ５０ The piston (1) according to any one of the preceding claims, characterized in that it has

8. The functional layer (5) is TiO ２ , ZrO ２ and W.O. ３ The piston (1) according to any one of the preceding claims, comprising:

9. The piston (1) according to any one of claims 1 to 8, wherein the thermal conductivity of the functional layer (5) is in the range of 0.2 to 1.8 W / (m·K), preferably the thermal conductivity of the functional layer (5) is in the range of 0.3 to 0.4 W / (m·K).

10. The thermal effusivity of the functional layer (5) is 500 to 900 J / (K·m ２ ・s １／２ ), and preferably the thermal effusivity of the functional layer (5) is in the range of 600 to 760 J / (K·m ２ ・s １／２ 10. The piston (1) according to claim 1, wherein the range of

11. The piston (1) according to any one of the preceding claims, wherein the specific heat capacity of the functional layer (5) is in the range of 450 to 1000 J / (kg·K).

12. The piston (1) according to any one of the preceding claims, wherein the functional layer (5) has a thickness, measured perpendicularly to the outer surface of the piston crown, of 50 to 250 μm, preferably 80 to 110 μm.

13. Piston (1) according to any one of the preceding claims, wherein the functional layer (5) has a thickness, measured perpendicularly to the outer surface of the piston crown, of 50 to 110 μm.

14. The functional layer (5) has a thickness of 50 to 110 μm when measured perpendicular to the outer surface of the piston crown, and the thermal effusivity of the functional layer (5) is 500 to 900 J / (K·m ２ ・s １／２ 12. The piston (1) according to claim 1, wherein the range of

15. 15. The piston (1) according to any one of claims 1 to 14, wherein the thermal management layer (4) further comprises a cover layer (6), the cover layer (6) being attached to the functional layer (5) facing away from the piston crown, the cover layer (6) comprising or consisting of polysiloxane.

16. Piston (1) according to any one of the preceding claims, wherein the cover layer (6) has a thickness, measured perpendicularly to the outer surface of the piston crown, of 4 to 20 μm, preferably 4.5 to 5.5 μm.

17. 17. The piston (1) according to any one of the preceding claims, wherein the thermal management layer (4) has a surface roughness Ra in the range of <2 μm.

18. 18. A method for manufacturing a piston (1) according to any one of claims 1 to 17, comprising applying a thermal management layer (4) including a functional layer (5) to a piston (1) for an internal combustion engine, the method comprising the steps of: a) cleaning (200) and optionally roughening the area of ​​the piston (1) where the functional layer (5) is to be applied; b) introducing and dispersing (201) hollow spheres (9) into a polysiloxane resin coating to obtain a suspension (S); c) applying (202) said suspension (S) to said piston (1) by atmospheric spraying; d) hardening (203) the suspension (S) applied in step c) to obtain a functional layer (5); e) optionally following step d), applying (204) a second layer to said functional layer (5) by atmospheric spraying of a polysiloxane resin paint; f) following step e), curing (205) said second layer to obtain a cover layer (6); A method comprising:

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