Pistons for an internal combustion engine and methods for manufacturing the piston

Abstract

The invention relates to a piston (1) for an internal combustion engine (10) with a piston crown (3) having an outer surface (2) and with a heat management layer (4). The heat management layer (4) contains a functional layer (5). The functional layer (5) in turn contains a functional layer matrix, wherein the functional layer matrix comprises polysiloxane and hollow spheres (9) embedded in the functional layer matrix. The thermal conductivity of the functional layer (5) is in the range of 0.2 to 2 W / (m*K) and the thermal penetration coefficient of the functional layer (5) is in the range of 400 to 1200 J / (K*m). 2 *s 1 / 2 ).

Description

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

[0002] To improve the temperature resistance of pistons, they are typically coated on the outer surface of the piston crown. For example, US 2018 / 0094603 A1 discloses a piston for an internal combustion engine in which two thermal insulation layers and a protective layer are applied to the piston.

[0003] It is an object of the present invention to show new ways in the development of piston coatings in order to improve their thermal and / or mechanical properties.

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

[0005] According to the invention, the piston for an internal combustion engine, which comprises a piston crown having an outer surface, is provided with a heat management layer, wherein the heat management layer contains a functional layer, wherein the functional layer contains a functional layer matrix, wherein the functional layer matrix comprises polysiloxane and hollow spheres embedded in the functional layer matrix, wherein the thermal conductivity of the functional layer is in the range 0.2 to 2 W / (m*K) and wherein the heat penetration coefficient of the functional layer is in the range 400 to 1200 J / (K*m 2 *s 1 / 2 ) lies.

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

[0007] In the context 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 heat management layer and the method according to the invention for producing the piston according to the invention with a heat management layer with the features of the independent patent claims have the essential advantages over the prior art that the efficiency of the combustion process is improved because heat losses are reduced, disadvantages with regard to volumetric efficiency are avoided and an advantageous thermo-swing effect occurs.

[0009] This is achieved in particular by avoiding excessive heating during the intake stroke, thereby preventing a reduction in power due to mass losses. At the same time, heat loss during the combustion stroke is minimized. An advantageous thermo-swing effect is achieved in particular by designing the functional layer according to the invention with both low thermal conductivity and low specific heat capacity. The heat penetration coefficient of the functional layer according to the invention is in the range of 400 to 1200 J / (K*m 2 *s 1 / 2 This allows the temperature of the thermal management layer to follow that of the gas with a slight delay.

[0010] In the context of the present invention, the heat penetration coefficient b can be defined as the square root of the product of the thermal conductivity, the density and the specific heat capacity. b=λ∗ρ∗cp with the thermal conductivity λ in W / (m*K), the density ρ in kg / m 3 , and the specific heat capacity c p in J / (kg * K).

[0011] Advantageously, the heat management layer according to the invention also prevents excessive heat absorption by the piston, which on the one hand can improve the temperature resistance of the piston and on the other hand also enables the use of higher temperatures.

[0012] For example, DE10 2017 207 236 A1 discloses a piston for an internal combustion engine comprising a base body and a porous thermal insulation layer applied to this base body, in which hollow spheres are embedded, and a cover layer arranged on the thermal insulation layer. A plurality of platelet-like inorganic particles, in particular mica, are embedded in this cover layer, which are intended to reflect heat like a mirror. In contrast, according to the present invention, advantageous thermal properties can be obtained without the need for such an additional reflective layer with mica particles. This also leads to improved mechanical stability of the thermal management layer according to the invention.

[0013] Furthermore, it is advantageous that the heat management layer according to the invention, on the one hand, consumes less lubricating oil, and on the other hand, at the same time, enables better heat dissipation and extends the service life of the lubricating oil.

[0014] One beneficial effect of less heat being absorbed by the piston and having to be dissipated via the cooling circuit is that the exhaust gas temperature is increased. If necessary, this can be used to increase the efficiency of the turbocharger or to convert the heat into electrical energy (waste heat recovery).

[0015] The piston according to the invention for an internal combustion engine with a heat management layer, wherein the heat management layer contains a functional layer, as well as the method according to the invention for applying a heat management layer containing a functional layer to a piston for an internal combustion engine are explained in more detail below, wherein the methods used in the context of the explanation of the piston according to the invention for an internal combustion engine and in this context Fig. 1 also applies analogously to the method for applying a functional layer to a piston for an internal combustion engine and the methods used in this context Fig. 2a and Fig. 2b apply and vice versa.

[0016] According to the invention, the piston for an internal combustion engine is provided with a heat management layer. The piston comprises a piston crown having an outer surface. The heat management layer contains at least one functional layer. The heat management layer serves to prevent, as far as possible, excessive heat absorption by the piston due to the penetration of heat from the combustion chamber of the internal combustion engine. This reduces heat loss during the combustion cycle and protects the piston material. This is achieved by the functional layer, wherein the thermal conductivity of the functional layer lies in the range of 0.2 to 2 W / (m*K). Furthermore, the heat penetration coefficient of the functional layer (5) lies in the range of 400 to 1200 J / (K*m 2 *s 1 / 2To achieve these properties, the functional layer uses a polysiloxane matrix material in which hollow spheres are embedded. The thermal conductivity and heat penetration coefficient are preferably determined at 25°C.

[0017] It goes without saying that the invention is fundamentally applicable to all conceivable pistons for internal combustion engines. Likewise, it goes without saying that all conceivable piston crown designs are conceivable according to the invention.

[0018] The outer surface of the piston is preferably arranged at the end face of the piston crown. The functional layer of the thermal management layer is adjacent to the outer surface of the piston crown. The functional layer contains a polysiloxane and hollow spheres.

[0019] A polysiloxane is understood to be a silicon-based polymer which can, for example, correspond to the following rough formula 1:

[0020] Different chain lengths n are conceivable, as are various branches, which are not shown in the exemplary formula 1. This can also include three-dimensional networked cage structures. Here, the residues R 1 , R 2 , R 3 and R 4 are each independently selected, for example, from the group consisting of hydrogen, alkyl, aryl, halogen, hydroxyl, and alkoxy. It is also conceivable that the radicals R 1 and R 2in the different repeating units. It is also conceivable that residues selected from the group consisting of alkyl, aryl, and alkoxy are unsubstituted. It is also conceivable that these residues are in turn mono- or polysubstituted with another substituent. For example, it is conceivable that such residues are in turn substituted with a substituent from the group consisting of alkyl, aryl, halogen, hydroxy, and amino.

[0021] 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. Alkyl groups can generally be linear or branched.

[0022] Aryl groups are known to those skilled in the art. Examples of aryl groups are phenyl or 1-naphthyl.

[0023] Alkoxy groups are also known to those skilled in the art. These are defined as alkyl groups bonded to an oxygen atom. Examples of alkoxy groups are the methoxy group -OCH3 or the ethoxy group -O-CH2-CH3. Furthermore, amino groups are known to those skilled in the art. A primary amino group is understood to be the residue -NH2. Secondary -NHR also exist. 5 and tertiary amino groups -NR 5 R 6 , where the residues R 5 and R 6 which can, for example, be identical or different alkyl groups.

[0024] According to the invention, a silicone-based resin composition is used which contains at least one polysiloxane as the main component.

[0025] Typically, an air-curing polysiloxane resin varnish is used. In addition to the polysiloxane resin to be cured, this varnish contains other components, such as at least one solvent and pigments. For example, the polysiloxane resin varnish can contain 15 to 70 mol% p-chlorobenzotrifluoride, 20 to 45 mol% polysiloxane resin to be cured, 15 to 35 mol% ceramic and / or metallic pigments, 0 to 10 mol% TiO2, and 0 to 2 mol% carbon black. It goes without saying that the sum of all components contained must equal 100 mol%. Typically, TiO2, ZrO2, and WO3 are preferably present according to the invention.

[0026] Hollow spheres made of ceramic, glass, or pure SiO2 are preferably used as hollow spheres. Due to manufacturing reasons, such hollow spheres may have a geometry that differs slightly from the ideal sphere.

[0027] For example, the hollow spheres can be made of pure, amorphous, transparent SiO2. Alternatively, the hollow spheres can be formed from at least one of the materials selected from the group consisting of SiO2, ZnO2, CaO, Al2O3, Na2O, Cr2O3, aluminosilicates, MgO, BaO, TiO2, ZrO2, MnO, Fe2O3, and B2O3. For example, 3M™ Glass Bubbles IM16K with a density of 0.46 g / cm3 can be used as hollow spheres. 3 be used.

[0028] Furthermore, it is preferred that the piston comprises a piston material with an aluminum alloy or an iron alloy and that the heat management layer is arranged at least in regions on the outer surface of the piston crown.

[0029] In particular, it is preferred that the piston comprises a piston material with an iron alloy and the heat management layer is arranged at least in regions on the outer surface of the piston crown.

[0030] As previously mentioned, the outer surface of the piston crown is the side facing the combustion chamber.

[0031] Furthermore, it has proven advantageous that the polysiloxane shows a temperature resistance of at least 500°C, preferably the polysiloxane shows a temperature resistance of at least 650°C.

[0032] In an advantageous further development of the piston, the proportion of hollow spheres, based on the total volume of the functional layer, is 5 to 40 vol%, preferably the proportion of hollow spheres based on the total volume of the functional layer is 10 to 20 vol%, more preferably the proportion of hollow spheres based on the total volume of the functional layer is 11 to 16 vol%.

[0033] Furthermore, it has proven advantageous to embed hollow spheres in the functional layer as fillers. This enhances the thermal insulation 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.

[0034] Furthermore, it is advantageous within the scope of the present invention if the hollow spheres have a diameter d 50 from 5 µm to 50 µm, preferably the hollow spheres have a diameter d 50 from 15 µm to 25 µm, more preferably the hollow spheres have a diameter d 50 from 19 µm to 21 µm.

[0035] The diameter value d 50indicates the particle diameter below which 50% of all particles are found. Conversely, this also means that 50% of all particles have a larger particle diameter. The diameters of such hollow spheres are typically determined using laser diffraction or automated statistical imaging.

[0036] Furthermore, it has proven advantageous if the functional layer contains TiO2, ZrO2, and WO3. Such a functional layer exhibits particularly good thermal resistance.

[0037] Typically, functional matrices from the commercially available Cerakote® V series are suitable for the functional layer; in particular, the use of Cerakote® V-136 has proven advantageous.

[0038] 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, the thermal conductivity of the functional layer is in the range of 0.3 to 0.4 W / (m*K). As already mentioned above, the thermal conductivity is preferably determined at 25°C.

[0039] Thermal conductivity can be measured using the laser flash method or the 3-omega method, for example. With the laser flash method, a heating pulse is applied to the sample using a laser beam, and the thermal radiation on the side of the sample opposite the laser is measured using an IR detector. With the 3-omega method, a heating line sputtered onto the material is heated with a periodic current, and the material response is measured as a periodic voltage.

[0040] In addition, it is advantageous that the heat penetration coefficient of the functional layer is in the range 500 to 900 J / (K*m 2 *s 1 / 2 ), the heat penetration coefficient of the functional layer is more advantageously in the range of 600 to 760 J / (K*m 2 *s 1 / 2 ). As already mentioned above, the heat penetration coefficient is preferably determined at 25°C.

[0041] Furthermore, the specific heat capacity of the functional layer is intended to be in the range of 450 to 1000 J / (kg * K). The specific heat capacity is preferably determined at 25 °C.

[0042] The specific heat capacity can be measured, for example, using differential scanning calorimetry (DSC). The heat penetration coefficient is determined mathematically using the formula mentioned above.

[0043] The aforementioned thermal properties particularly achieve an advantageous thermo-swing effect.

[0044] Furthermore, it has proven advantageous that the functional layer has a thickness of 50 to 250 µm, preferably 80 to 110 µm, measured perpendicular to the outer surface of the piston crown.

[0045] Alternatively, it has proven advantageous for the functional layer to have a thickness of 50 to 110 µm, measured perpendicular to the outer surface of the piston crown.

[0046] Simulations have shown that, for a given heat penetration coefficient, increasing the thickness of the functional layer to values ​​up to approximately 100 µm initially results in a significant improvement in the thermal swing effect and temperature profiles in the engine. Increasing the thickness of the functional layer above 110 µm did not result in any further significant improvement in the simulation. Furthermore, scanning electron microscopy studies were conducted on various samples with different functional layer thicknesses. These studies have shown that thicker layers, particularly those greater than 150 µm, are more susceptible to adhesion problems and flaking. Layer thicknesses ≤ 110 µm have proven to be significantly more robust in this regard.Thus, a thickness of the functional layer of 50 to 110 µm, and in particular 80 to 110 µm, measured perpendicular to the outer surface of the piston crown, is an advantageous range both in terms of a good thermo-swing effect and a good mechanical stability of the functional layer.

[0047] Typically, such coating thicknesses can be determined by creating a cross-sectional section, thus preparing a cross-section, and then evaluating it under a microscope. Non-destructive measurements can also be performed using XRF spectrometry. For a quick, approximate estimate of the applied coating thickness, it has also proven advantageous to determine the amount of applied material by differential weighing and to relate the applied amount of coating to the coated area.

[0048] Furthermore, it has proven advantageous that the functional layer has a thickness of 50 to 110 µm, measured perpendicular to the outer surface of the piston crown, and the heat penetration coefficient of the functional layer is in the range of 500 to 900 J / (K*m 2 *s 1 / 2 ) lies.

[0049] For functional layers with heat penetration coefficients of less than 500 J / (K*m 2 *s 1 / 2 ) it was found that the temperature at the piston surfaces is relatively high during operation, which has a negative impact on various engine components in terms of strength and thermal stability, such as pistons, valves and cylinder head.

[0050] With regard to the embedding of hollow spheres, it goes without saying that the selected diameters d 50 for the hollow spheres are smaller than the selected layer thickness in order to enable the hollow spheres to be embedded in the functional matrix in a meaningful way.

[0051] It has proven particularly advantageous that the functional layer has a thickness of 50 to 110 µm, measured perpendicular to the outer surface of the piston crown, and the heat penetration coefficient of the functional layer is in the range of 500 to 900 J / (K*m 2 *s 1 / 2 ) and the hollow spheres have a diameter d 50 from 15 to 25 µm.

[0052] Advantageously, the thermal management layer further comprises a cover layer, wherein the cover layer is applied to the functional layer facing away from the piston crown, wherein the cover layer comprises a polysiloxane or consists of a polysiloxane.

[0053] This top layer is preferably temperature-resistant up to at least 650 °C. Such a top layer advantageously protects the functional layer from mechanical, thermal, and / or chemical influences.

[0054] It has further proven advantageous if the cover layer has a thickness of 4 to 20 µm, preferably 4.5 to 5.5 µm, measured perpendicular to the outer surface of the piston crown.

[0055] It has also proven particularly advantageous for the cover layer to be non-porous or closed-pore, so that the cover layer covers the functional layer as tightly as possible. This enables particularly good protection of the functional layer.

[0056] Advantageously, the surface roughness Ra of the thermal management layer is < 2 µm. This prevents turbulence of the combustion mixture as much as possible.

[0057] The surface roughness Ra is the mean roughness value, which is standardized according to DIN EN ISO 4287:2010.

[0058] Typically, surface roughness Ra is determined by scanning the surface along a defined measuring section and recording all height and depth differences. The definite integral of this profile along the measuring section is then determined and divided by the length of the measuring section.

[0059] The invention also relates to a method for producing a piston, wherein a heat management layer containing a functional layer is applied to a piston for an internal combustion engine, comprising the steps: a) Cleaning and optionally roughening the area of ​​the piston to which the functional layer is to be applied, b) introducing and dispersing hollow spheres into a polysiloxane resin varnish to obtain a suspension (S), c) applying the suspension (S) by atmospheric spraying onto the piston, d) curing the suspension (S) applied in step c) to obtain a functional layer, e) optionally following step d), applying a second layer by atmospheric spraying of a polysiloxane resin varnish onto the functional layer, f) following step e) curing the second layer to obtain a top layer.

[0060] In step a), the surface to be coated is first thoroughly cleaned before applying the suspension (S). This cleaning is typically carried out with acetone, tert-butyl acetate, or comparable solvents or solvent mixtures. Cleaning in an ultrasonic bath is also conceivable. Afterwards, it is optionally possible to heat the flask to approximately 50 to 150°C. This can remove any adhering solvent residues. Furthermore, roughening the surface may be useful. This can be done, for example, by sandblasting. It may also be useful to mask areas that are not to be coated before the subsequent application of the coating.

[0061] In step b), the desired amount of hollow spheres is then introduced into a polysiloxane resin varnish, resulting in a suspension (S). In addition to the polysiloxane resin to be cured, the polysiloxane resin varnish typically contains further components, such as at least one solvent and pigments. For example, the polysiloxane resin varnish contains 15 to 70 mol% p-chlorobenzotrifluoride, 20 to 45 mol% polysiloxane resin to be cured, 15 to 35 mol% ceramic and / or metallic pigments, 0 to 10 mol% TiO2, and 0 to 2 mol% carbon black. Typically, TiO2, ZrO2, and WO3 are preferably present according to the invention. This suspension (S) is applied to the piston by atmospheric spraying in step c). Atmospheric spraying is a spraying process in which a liquid or molten material is sprayed onto a substrate at atmospheric ambient pressure, typically in air.For the present invention, spraying is typically carried out at 1 to 2 bar.

[0062] Following the application of the suspension (S) in step c), the resulting layer is cured in step d). For this purpose, the flask is typically first left at room temperature for about 15 minutes, then heated to 150°C and held at 150°C for about 1 hour. The flask is then cooled to room temperature.

[0063] After the applied suspension (S) has cured to obtain a functional layer in step d), it is conceivable that a further layer is applied by atmospheric spraying of a polysiloxane resin coating. To produce the second layer, no hollow particles are introduced into the polysiloxane resin coating. The above applies to the polysiloxane resin coating. This second layer is also cured after application. For this purpose, the piston is typically first left at room temperature for about 15 minutes and then heated to 150°C and held at 150°C for about 1 hour. The piston is then cooled to room temperature.

[0064] Further important features and advantages of the invention emerge from the subclaims, from the drawing and the associated description of the figures as well as the examples.

[0065] It goes without saying that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention. Fig. 1: Rough schematic representation of an example of an internal combustion engine, which can be used for example in a motor vehicle. Fig. 2a: Flowchart of the method according to the invention Fig. 2b: Flowchart of an advantageous development of the method according to the invention

[0066] The Fig. Figure 1 shows an example of an embodiment of an internal combustion engine 10 according to the invention. The internal combustion engine comprises a cylinder 11. The piston 1 according to the invention is shown in the cylinder 11 of the internal combustion engine 10 by way of example and in a rough schematic manner. The piston 1, together with the cylinder 11, partially defines a combustion chamber of the internal combustion engine 10 with an outer surface 2, for example on the front side, of the piston 1. Fig. 1 a piston 1 with a piston crown 3, which has an outer surface 2. In addition, the piston 1 comprises a heat management layer 4. This is arranged at least in regions on the outer surface 2 of the piston crown. The Fig. 1 shows an aspect of the invention in which the heat management layer 4 comprises a functional layer 5 and a cover layer 6. The functional layer 5 is arranged on the outer surface 2 of the piston crown 3, adjacent to the piston crown 3 and the cover layer 6. According to the invention, it is alternatively also conceivable that the heat management layer 4 comprises only a functional layer 5, but no cover layer 6, which is Fig. 1 is not shown. The functional layer 5 comprises a functional layer matrix made of polysiloxane and hollow spheres 9 embedded in the functional layer matrix.

[0067] As also shown by the Fig. As can 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 from 50 to 250 µm, preferably from 80 to 110 µm. Alternatively, the thickness 7 of the functional layer 5 is preferably from 50 to 110 µm.

[0068] The cover layer 6 has a thickness 8 of the cover layer 6 measured perpendicular to the outer surface 2 of the piston crown 3. The thickness 8 of the cover layer 6 is, according to the invention, 4 to 20 µm, preferably 4.5 to 5.5 µm, measured perpendicular to the outer surface of the piston crown. The functional layer 5 comprises hollow spheres 9. It is understood that such hollow spheres may have a geometry that differs slightly from the ideal sphere due to manufacturing reasons. The hollow spheres preferably comprise or consist of SiO2. Alternatively, the hollow spheres are expediently formed from soda-lime borosilicate glass. According to the invention, the functional layer 5 is preferably temperature-stable up to at least 500°C, more preferably up to at least 650°C. The Fig. The cover layer 6 shown in Figure 1 is preferably pore-free or closed-porous according to the invention. This means that the cover layer 6 preferably has no pores or only closed pores. This enables protection of the underlying functional layer 5 from the gases in the combustion chamber. Furthermore, the invention provides that the cover layer 6 is preferably temperature-stable up to at least 500°C, more preferably up to at least 650°C.

[0069] The Fig. According to the invention, the piston crown 3 shown in Figure 1 comprises, for example, a piston material made of an aluminum alloy or consists of a piston material with such an aluminum alloy. Alternatively, the piston crown 3 comprises a piston material with an iron alloy or consists of a piston material with such an iron alloy.

[0070] The Fig. 2a shows, in process step 200, the cleaning and optional roughening of the area of ​​the piston 1 to which the functional layer is to be applied after cleaning and optional roughening. This is followed by process step 201, in which hollow spheres 9 are introduced into a polysiloxane resin lacquer and dispersed. This results in a suspension. In the subsequent process step 202, the previously obtained suspension is applied to the piston 1 by atmospheric spraying. This is followed by process step 203, in which the previously applied layer of the suspension is cured. This results in a functional layer 5.

[0071] The Fig. 2b also shows the Fig. 2a already explained procedural steps 200 to 203. However, in the Fig. In the advantageous development of the invention shown in Figure 2b, a further process step 204 follows process step 203. In process step 204, a second layer is applied to the resulting functional layer 5 by atmospheric spraying of a polysiloxane resin coating. This second layer is cured in the subsequent process step 205, thereby obtaining a cover layer 6.

[0072] In the following, the invention is described in detail using examples, without limiting the scope of the invention thereto: Materials used: Cerakote® V-136 3M™ Glass Bubbles IM16K Inventive example EB1a:

[0073] 21 g of 3M™ Glass Bubbles IM16K hollow spheres, corresponding to 14 vol%, are dispersed in 400 g of Cerakote® V-136. The piston, made of an aluminum alloy with a previously cleaned surface to be coated, is then preheated to 80°C for 30 minutes. The suspension is applied using a spray gun at a pressure of 1 bar from a distance of 17.5 cm. A total of 10 clicks are fired from the spray gun. The piston is then cured for 1 hour at 150°C. A layer thickness of 80 to 110 µm is achieved, measured non-destructively using XRF spectrometry. A crack-free surface is obtained. The specification "80 to 110 µm" means that the resulting layer thickness is at least 80 µm at the thinnest point and a maximum of 110 µm at the thickest point. Inventive example EB1b:

[0074] The pistons coated according to Example EB1a were subjected to 250 hours of continuous engine testing. Visual inspection of the coating before and after the engine test revealed no changes; the coating adhered securely. Inventive example EB1c:

[0075] To investigate the thermophysical parameters of the material, 21 g of 3M™ Glass Bubbles IM16K hollow spheres, corresponding to 14 vol%, were dispersed in 400 g of Cerakote® V-136. Aluminum foils with previously cleaned surfaces were then preheated to 80°C for 30 minutes. The suspension was applied using a spray gun. The material, including the aluminum foil, was then cured for 1 hour at 150°C and then scraped off the aluminum foil. Layers approximately 5 mm thick and approximately 12.9 mm in diameter were obtained. These layers were then manually sanded to a thickness of approximately 2.4 mm and a diameter of approximately 12.7 mm.

[0076] The thermal diffusivity a [mm 2 / s] by laser flash method using a NETZSCH LFA 467 Hyperflash® apparatus.

[0077] Furthermore, the specific heat capacity c was determined p [J / (g * K)] by heat flow differential calorimetry using a NETZSCH-DSC 204 Phoenix® apparatus.

[0078] In addition, the density ρ [g / cm 3 ] of the sample at 23°C by buoyancy method using the ME235 Sartorius balance together with the density determination set YDK 01.

[0079] From this, the thermal conductivity λ [W / (m*K)] was calculated according to the following formula: λ=ρ*cp*a with a [m 2 / s] ρ [kg / m3] c p [J / (kg * K]

[0080] A temperature-dependent density correction was not performed. Table 1: Temperatur °C 25 50 75 100 150 200 ρ g / cm 3 1,244 1,244 1,244 1,244 1,244 1,244 a mm 2 / s 0,280 0,272 0,266 0,260 0,252 0,243 c p J / (g*K) 0,831 0,880 0,926 0,969 1,047 1,110 λ W / (m*K) 0,29 0,30 0,31 0,31 0,33 0,34 Temperatur °C 250 300 350 400 450 500 ρ g / cm 3 1,244 1,244 1,244 1,244 1,244 1,244 a mm 2 / s 0,236 0,232 0,228 0,225 0,224 0,224 c p J / (g*K) 1,163 1,205 1,242 1,273 1,298 1,301 λ W / (m*K) 0,34 0,35 0,35 0,36 0,36 0,36 List of reference symbols 1 piston 2 Exterior surface 3 piston crown 4 Thermal management layer 5 Functional layer 6 Top layer 7 Thickness of the functional layer 8 Thickness of the top layer 9 hollow spheres 10 internal combustion engine 11 cylinders 200 cleaning 201 Introducing and dispersing hollow spheres into a polysiloxane resin varnish to obtain a suspension 202 Applying the suspension 203 Curing of the applied suspension 204 Applying a second layer 205 Curing of the second layer QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 2018 / 0094603 A1

[0002] DE 10 2017 207 236 A1

[0012] Cited non-patent literature

[0000] DIN EN ISO 4287:2010

[0057]

Claims

[1] Piston (1) for an internal combustion engine (10), - with a piston head (3) having an outer surface (2), - with a heat management layer (4), - wherein the thermal management layer (4) contains a functional layer (5), wherein the functional layer (5) contains a functional layer matrix, wherein the functional layer matrix comprises polysiloxane and hollow spheres (9) embedded in the functional layer matrix, - the thermal conductivity of the functional layer (5) is in the range 0.2 to 2 W / (m*K), - wherein the heat penetration coefficient of the functional layer (5) is in the range 400 to 1200 J / (K*m 2 *s 1 / 2 ) lies. [2] Piston (1) according to claim 1, wherein the piston (1) comprises a piston material with an aluminum alloy or an iron alloy and wherein the heat management layer (4) is arranged at least in regions on the outer surface (2) of the piston crown (3). [3] Piston (1) according to one of claims 1 or 2, wherein the polysiloxane shows a temperature resistance up to at least 500°C, preferably the polysiloxane shows a temperature resistance up to at least 650°C. [4] Piston (1) according to one of claims 1 to 3, wherein the proportion of hollow spheres (9) based on the total volume of the functional layer (5) is 5 to 40 vol%, preferably the proportion of hollow spheres (9) based on the total volume of the functional layer (5) is 10 to 20 vol%, more preferably the proportion of hollow spheres (9) based on the total volume of the functional layer (5) is 11 to 16 vol%. [5] Piston (1) according to one of claims 1 to 4, wherein the hollow spheres (9) consist of SiO2, which are embedded in the functional layer (5) in a filler-like manner. [6] Piston (1) according to one of claims 1 to 4, wherein the hollow spheres (9) consist of soda-lime borosilicate glass, which are embedded in the functional layer (5) in a filler-like manner. [7] Piston (1) according to one of claims 1 to 6, wherein the hollow spheres (9) have a diameter d 50 of 5 to 50 µm, preferably the hollow spheres (9) have a diameter d 50 of 15 to 25 µm, more preferably the hollow spheres (9) have a diameter d 50 from 19 to 21 µm. [8] Piston (1) according to one of claims 1 to 7, wherein the functional layer (5) contains TiO2, ZrO2 and WO3. [9] Piston (1) according to 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] Piston (1) according to one of claims 1 to 9, wherein the heat penetration coefficient of the functional layer (5) is in the range of 500 to 900 J / (K*m 2 *s 1 / 2), preferably the heat penetration coefficient of the functional layer (5) is in the range of 600 to 760 J / (K*m 2 *s 1 / 2 ). [11] Piston (1) according to one of claims 1 to 10, wherein the specific heat capacity of the functional layer (5) is in the range 450 to 1000 J / (kg * K). [12] Piston (1) according to one of claims 1 to 11, wherein the functional layer (5) has a thickness of 50 to 250 µm, preferably 80 to 110 µm, measured perpendicular to the outer surface of the piston crown. [13] Piston (1) according to one of claims 1 to 11, wherein the functional layer (5) has a thickness of 50 to 110 µm, measured perpendicular to the outer surface of the piston crown. [14] Piston (1) according to one of claims 1 to 11, wherein the functional layer (5) has a thickness of 50 to 110 µm, measured perpendicular to the outer surface of the piston crown, and the heat penetration coefficient of the functional layer (5) is in the range of 500 to 900 J / (K*m2 *s 1 / 2 ) lies. [15] Piston (1) according to one of claims 1 to 14, wherein the heat management layer (4) further comprises a cover layer (6), wherein the cover layer (6) is applied to the functional layer (5) facing away from the piston crown, wherein the cover layer (6) comprises a polysiloxane or consists of a polysiloxane. [16] Piston (1) according to one of claims 1 to 15, wherein the cover layer (6) has a thickness of 4 to 20 µm, preferably 4.5 to 5.5 µm, measured perpendicular to the outer surface of the piston crown. [17] Piston (1) according to one of claims 1 to 16, wherein the surface roughness Ra of the heat management layer (4) is in the range < 2 µm. [18] Method for producing a piston (1) according to one of claims 1 to 17, wherein a heat management layer (4) containing a functional layer (5) is applied to a piston (1) for an internal combustion engine, comprising the steps: a) cleaning (200) and optionally roughening the area of ​​the piston (1) to which the functional layer (5) is to be applied, b) introducing and dispersing (201) hollow spheres (9) into a polysiloxane resin lacquer to obtain a suspension (S), c) applying (202) the suspension (S) by atmospheric spraying onto the piston (1), d) curing (203) the suspension (S) applied in step c) to obtain a functional layer (5), e) optionally following step d), applying a second layer (204) by atmospheric spraying of a polysiloxane resin varnish onto the functional layer (5), f) following step e) curing the second layer (205) to obtain a cover layer (6).

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