Heat-protective coating, coated element, method for producing a coated element

Abstract

A thermal protection coating comprising a spongy body, wherein the spongy body has non-linear pores and a skeleton accommodating the pores, wherein the skeleton is an amorphous body comprising Al, Si, O and impurities, and has an amorphous peak specified by X-ray diffraction analysis at a position of 3.5 Å or greater as the lattice spacing, the thermal protection coating has an apparent density of 1 g / cm 3 or less, a volumetric specific heat of 1,000 kJ / m 3 ·K or less and a thermal conductivity of 2 W / m·K or less, the spongy body comprises an anodic oxide of an aluminum alloy in which Si is solidly dissolved in a supersaturated state, and the aluminum alloy contains 16 to 48 mass% of Si relative to the total alloy.

Description

[Technical field]

[0001] The present invention relates to a heat-protective coating or a heat-protective layer or a heat-insulating coating and relevant technologies. [State of the art]

[0002] To improve the thermal efficiency of internal combustion engines and similar machines, various proposals have been made for thermal barrier coatings with low thermal conductivities (high thermal protection properties) and low heat capacities (high temperature traceability) (thermal barrier coatings with excellent so-called "vibration properties"). Such thermal barrier coatings are described in the following documents. [Prior art documents][Patent documents] [Patent document 1] JP 2016 - 125 082 A [Patent document 2] JP 2016 - 216 763 A [Patent document 3] JP 2017 - 14 597 A [Patent Document 4] JP 2017 - 214 603 A [Patent document 5] US 2017 / 0 167 373 A1 [Patent Document 6] JP ​​2017 - 160 532 A [Patent Document 7] JP 2017 - 115 166 A [Summary of the invention][Technical problem]

[0003] Patent Documents 1 to 4 propose heat-shielding coatings in which micro-sized pores (voids) are distributed in anodized layers. These anodized layers themselves are composed of porous layers with linear, tubular, and narrow holes extending in one direction, and such porous layers are well-known shapes.

[0004] Patent Document 5 proposes a heat-shielding coating using a silica aerogel, but does not describe a heat-shielding coating composed of an aluminum-based oxide.

[0005] Patent Document 6 discloses a surface structure of a related-art aluminum member. Patent Document 7 discloses an anodic oxidation method for an related-art aluminum-based member.

[0006] The present invention has been made in view of these circumstances, and an object of the present invention is to provide a heat-shielding coating having a novel structure or shape different from the conventional heat-shielding coatings and relevant technologies. [Problem solving]

[0007] As a result of intensive research to solve the above problems, the present inventors have successfully achieved a heat-shielding coating with low heat capacity and low thermal conductivity, which differs from conventional anodized layers in shape and / or structure, by anodizing an aluminum alloy (simply referred to as "Al alloy") in which Si is solidly dissolved in a supersaturated state. Through this achievement, the present inventors have achieved the present invention, which is described below. «Thermal protection coating»

[0008] The present invention provides a heat-shielding coating comprising a spongy body. The spongy body has nonlinear pores and a skeleton containing the pores. The skeleton is an amorphous body comprising Al, Si, O, and impurities, and has an amorphous peak specified by X-ray diffraction analysis at a position of 3.5 Å or greater as a lattice spacing. The heat-shielding coating has an apparent density of 1 g / cm 3 or lower, a volumetric specific heat of 1,000 kJ / m 3 ·K or lower and a thermal conductivity of 2 W / m·K or lower. The spongy body comprises an anodic oxide of an aluminum alloy in which Si is solidly dissolved in a supersaturated state. The aluminum alloy contains 16 to 48 mass% of Si relative to the entire alloy.

[0009] The heat-shielding coating of the present invention comprises a spongy body having a shape obviously different from that of conventional anodized layers. The heat-shielding coating composed of the spongy body has low density, low heat capacity, and low thermal conductivity, and exhibits excellent heat-shielding properties. «Coated element»

[0010] The present invention can also be considered as a member coated with the heat-shielding coating. For example, the present invention can be considered as a coated member in which at least a portion of a main body surface is coated with the above-described heat-shielding coating. «Method for producing a coated element»

[0011] The present invention can also be regarded as a method for producing the coated member. For example, the present invention can be regarded as a method for producing a coated member, which comprises a first step of forming a base layer of an aluminum alloy on a main body surface, wherein the aluminum alloy contains 16 to 48 mass % of Si with respect to the entire alloy, and Si is solidly dissolved in a supersaturated state, and a second step of anodizing the base layer, wherein the voltage is from -5 V to 200 V, thereby obtaining the coated member described above. «More»

[0012] (1) Unless otherwise stated in the present specification, an article (member) formed by the heat-shielding coating is referred to as a "main body", and an article (for example, a portion composed of an aluminum alloy with a high Si content) which is anodized in the formation of the heat-shielding coating is referred to as a "base layer" or a "base material".

[0013] If the heat-protective coating is composed exclusively of a spongy body, the values ​​of the physical properties (density, specific heat, thermal conductivity, etc.) referred to in this description are not only the values ​​of the physical properties of the heat-protective coating, but also the values ​​of the physical properties of the spongy body.

[0014] (2) Unless otherwise specified, a numerical range “x to y” referred to in this description includes the lower limit x and the upper limit y. Any numerical value included in various numerical values ​​or numerical ranges described in this description can be selected or extracted as a new upper or lower limit, and any numerical range, such as “a to b,” can thereby be re-provided using a new upper or lower limit. Unless otherwise specified, “x to y nm” referred to in this description means x nm to y nm. The same applies to other unit systems (µm, kJ / m 3 ·K, etc.). [Short description of the drawing] Fig.1 describes a series of SEM images, etc., and a schematic diagram showing the structure of an anodized layer or coating of test sample 2, and a series of SEM images, etc., and a schematic diagram showing the structure of an anodized layer of test sample C1. Fig. 2 describes XRD profiles of anodized layers and test samples. [Embodiments for carrying out the invention]

[0015] The content described in the present specification can be applied not only to the heat-shielding coating, but also to the coated member and its manufacturing method. One or more freely selected features of the present specification can be added to the above-described features of the present invention. Features relating to a method can also be features relating to a product. Which embodiment is the best or not differs depending on the objects, the required performance, and other factors. «Spongy body»

[0016] The spongy body (spongy porous body) constituting the heat-shielding coating is composed of a skeleton and pores (voids) formed in the skeleton. Its shape is nonlinear, shapeless, irregular, or disorganized. Therefore, the shape of the spongy body is completely different from that of a conventional anodized layer (alumite layer, anodized layer, or oxide layer), which is composed of a porous layer in which linear tubular bodies are regularly arranged. While the conventional anodized layer is referred to as a tubular structure, the spongy body or anodized layer according to the present invention can be said to have a spongy structure.

[0017] (1) The skeleton of the spongy body is composed of Al, Si, O, and impurities. Si is almost absent in the skeleton, at least not as micro-sized particles (maximum length is 1 µm to 1 mm). While Si is considered to be solidly dissolved in the Al-based oxide that makes up the skeleton or to exist as an Al-Si-based oxide, the details are uncertain. However, the skeleton of the spongy body is certainly different from the alumite (Al 2 O 3 ), which constitutes a conventional anodized layer.

[0018] The skeleton of the spongy body is composed of an amorphous body. The fact that the skeleton has an amorphous structure can be concluded from the fact that the X-ray diffraction (XRD) profile (pattern) is broad. The amorphous peak on the profile is located at a different position than that of the alumite constituting a conventional anodized layer. In particular, the amorphous peak of the skeleton of the spongy body appears at a position of 3.5 Å or greater than the lattice spacing. This is shifted in the direction in which the lattice spacing increases (the direction in which the diffraction angle 2θ decreases) with respect to a lattice spacing of 3.4 Å of the alumite. This also suggests that the skeleton constituting the spongy body is composed of a compound having a structure different from that of conventional alumite.

[0019] The lattice spacing of the amorphous peak increases as the amount of Si contained in the skeleton of the spongy body increases, and is 3.6 Å or greater in one embodiment or 3.7 Å or greater in another embodiment. However, it is sufficient to say that the lattice spacing of the amorphous peak is 4.1 Å or smaller in one embodiment or 4 Å or smaller in another embodiment because Si is usually present in less than Al (less than 50 mass% or less than 50 atomic%).

[0020] (2) As described above, the pores of the spongy body are irregular in shape and not limited to closed cavities. Therefore, it is not easy to define or specify the size and other parameters. However, when the cross section along the extension direction of the spongy body (thickness direction of the heat-shielding coating) is observed with an electron microscope (such as SEM), the shape of the spongy body is completely different from that of the conventional porous layer (see Fig. 1). Clear or defined pores are observed in the spongy body on the cross-section. However, in the conventional porous layer, voids are usually only observed on the cross-section in a direction perpendicular to the layer thickness direction, and pores like those in the spongy body are not observed on the cross-section in the layer thickness direction. «Thermal protection coating»

[0021] The spongy body or the thermal protection coating composed of the spongy body has low density, low specific heat, and low thermal conductivity. This is described in sequence below. (1) Density

[0022] The apparent density (bulk density) of the thermal protection coating is, for example, 0.3 to 1 g / cm 3 in one embodiment or 0.5 to 0.8 g / cm 3 in another embodiment. On the other hand, the hard alumite (JIS H 8603 Type 3), which constitutes the conventional anodized layer, has an apparent density of approximately 1.9 g / cm 3 The thermal protection coating, which is composed of the spongy body, contains more pores and can have a lower density than the conventional anodized layer.

[0023] The apparent density (simply referred to as "density") of the heat-shielding coating (spongy body) is obtained as follows. The mass (W1) of a coated element in which the heat-shielding coating is formed on the main body is measured. Then, the heat-shielding coating is wiped off after impregnation with paraffin. The volume (V1) of the coated element, in which the pores of the spongy body have been thereby closed, is measured by the Archimedes method. After that, only the heat-shielding coating is removed using a mixed acid of sulfuric acid and chromic acid. Only the mass (W1) and volume (V1) of the remaining body are measured in the same way. From these, the mass (W1) and volume (V1) of the heat-shielding coating (spongy body) are determined, and the density (ρ) is obtained as ρ = W / V = (W1-W1) / (V1-V1).

[0024] The thickness (t) of the thermal protection coating is obtained as t = (W / ρ) / S = (V1-Val) / S using a coating area (S) of the thermal protection coating.

[0025] The thermal protection coating composed of the spongy body preferably has a porosity or porosity ratio of 70% or higher in one embodiment, 75% or higher in another embodiment, or 80% or higher in a further embodiment. The porosity is determined from the true density (ρo = 4.1 g / cm 3 ) and the apparent density (ρ) of the aluminum oxide (Al 2 O 3 ) (for example, the porosity = {1 - ρ / ρo} × 100%). The porosity of the conventional anodized layer (anodized layer) is approximately 50% to 55%. (2) Specific heat

[0026] The volumetric specific heat (volume specific heat) of the thermal protection coating is, for example, 100 to 1,000 kJ / m 3K in one embodiment or 500 to 800 kJ / m 3 ·K in another embodiment. The volumetric specific heat of the hard alumite described above is approximately 2,000 kJ / m 3 ·K.

[0027] The volumetric specific heat (simply referred to as "specific heat") of the heat-shielding coating (spongy body) is obtained using differential scanning calorimetry (DSC). There are two types of DSCs: dynamic heat flow differential calorimetry and dynamic power differential calorimetry. For example, the former may be preferentially used. The measurement sample is, for example, a substance (powder) that has been mechanically peeled off from the heat-shielding coating (spongy body) formed on the main body. The volume of a test sample is obtained from the mass of the test sample and the density described above. Thermal conductivity

[0028] The thermal conductivity of the thermal protection coating is, for example, 0.1 to 2 W / m·K in one embodiment or 0.5 to 1.5 W / m·K in another embodiment. The thermal conductivity of the hard alumites described above is approximately 2 W / m·K.

[0029] The thermal conductivity of the heat-shielding coating (spongy body) is obtained as follows. The thermal diffusivity of the main body alone and the thermal diffusivity of a coated member in which the heat-shielding coating is formed on the main body are measured using laser flash analysis. Based on these thermal diffusivities, the density and thickness of the main body, and the thickness and density of the heat-shielding coating in the coated member, the thermal diffusivity of the heat-shielding coating (spongy body) is calculated. The thickness (t) and density (p) of the heat-shielding coating are obtained using the methods described above. «Formation of the class»

[0030] The heat-shielding coating composed of the spongy body is formed, for example, using an anodizing process for an Al alloy. The Al alloy as a base material contains Si of a hypereutectic composition or higher, namely 16 to 48 mass% of Si, preferably 18 to 44 mass% of Si, with respect to the Al alloy as a whole. The Al alloy may be a binary alloy of Si and Al (containing Si and the balance of Al and impurities) or may contain other elements. Examples of such elements include Cu, Mg, P, Ti, B, Sr, Na, Sb, Zn, Fe, Mn, Ni, Pb, Sn, and Cr. (1) First step / base layer (base material) formation step

[0031] An Al alloy containing a large amount of Si may often have a metallic structure in which micro-sized Si particles (for example, the maximum length is 1 to 1,000 µm in one embodiment or 50 to 600 µm in another embodiment) (primary crystal Si particles, hypereutectic Si particles, etc.) are crystallized or precipitated. When such an Al alloy is anodized, a conventional anodized layer (tubular structure) is formed, and coarse pores (voids) appear around the Si particles that are not anodized. This means that the spongy body as in the present invention is not formed.

[0032] When forming a spongy body composed of an anodized oxide, anodizing may preferably be performed on a base material (base layer) composed of an Al alloy in which Si is solidly dissolved in a supersaturated state. Such a base material can be formed, for example, by thermal spraying, metal deposition (weld overlay), or a similar technology. Examples of thermal spraying include (high-velocity) flame spraying, arc spraying, plasma spraying, and laser spraying. Examples of metal deposition include a laser beam heat source scheme and an arc discharge scheme. Among them, laser metal deposition (LMA) using a metal powder as the raw material powder is preferred for the same reason.

[0033] The raw material powder used is preferably an atomized powder of an Al-Si-based alloy. This enables the rapid formation of the base material (base layer), in which the uneven distribution of Al and Si as the main components is suppressed. (2) Second step / anodizing step

[0034] The anodizing process of the above-described base material (base layer) can preferably be performed by an electrolytic step of applying an AC / DC superposition voltage in which an AC component and a DC component are superimposed. This enables efficient formation of the spongy body even for an Al alloy containing a large amount of Si.

[0035] In this process, it may be preferable, for example, to set the minimum voltage to -5 to 5 V in one embodiment or -2 to 2 V in another embodiment or the minimum current density to -0.4 to 0.4 A / cm 2 in one embodiment or -0.2 to 0.2 A / cm 2 in another embodiment. According to another aspect, it may be preferable, for example, to set the maximum voltage (voltage peak) to 30 to 200 V in one embodiment or 35 to 180 V in another embodiment or the maximum current density to 1 A / cm 2 or higher in one embodiment, 1.5 A / cm 2 or higher in another embodiment or 1.8 A / cm 2or higher in another embodiment. The frequency of the alternating current may, for example, preferably be set to 2 Hz to 9 kHz in one embodiment, 10 Hz to 1 kHz in another embodiment, or 20 to 200 Hz in another embodiment. The maximum voltage, the maximum current density, and other parameters may be changed (in particular, increased) during the process.

[0036] The AC waveform may be a sine wave, a square wave, a triangular wave, a sawtooth wave, a pulse wave, or any other suitable waveform. For example, the AC component may preferably have a constant frequency and a constant peak value (minimum or maximum value), and the DC component may preferably have a constant voltage or current, for example.

[0037] The electrolytic solution (solution for the anodizing process) may be, for example, an inorganic acid solution such as an aqueous sulfuric acid solution, an aqueous phosphoric acid solution, or an aqueous chromic acid solution, or may also be an organic acid solution such as an aqueous oxalic acid solution. The concentration of the aqueous sulfuric acid solution is, for example, approximately 5 to 40 mass% in one embodiment or approximately 10 to 30 mass% in another embodiment. The temperature of the electrolytic solution (bath temperature) is, for example, approximately 0°C to 40°C in one embodiment or 10°C to 30°C in another embodiment. The counter electrode is usually a platinum electrode, a graphite electrode, or a similar electrode.

[0038] After the anodizing process, one or more processes may be performed, such as pore sealing, sealing, heating, and painting. The pore sealing process is performed, for example, by treating the article with boiling water and / or high-pressure steam. The sealing process is performed, for example, by treating the surface of the thermal barrier coating (spongy body) with a suitable compound, such as polysilazane or polysiloxane, and converting the compound into silicon dioxide by combustion. «Coated element»

[0039] The material, shape, and other properties of the main body provided with the heat-shielding coating are not limited. The main body can be made of an Al alloy, an Fe-based alloy (such as iron steel or cast steel), or a ceramic, provided that the base layer can be formed as the base of the spongy body.

[0040] The main body is, for example, a member constituting a combustion chamber of an internal combustion engine. The internal combustion engine is, for example, a piston engine, a rotary engine, a gas turbine engine, a jet engine, or a similar machine. The piston engine may be any of a gasoline engine, a diesel engine, a four-stroke engine, a two-stroke engine, or the like. In the case of a piston engine, the inner wall surfaces of a combustion chamber are constituted by a piston, a cylinder head, a cylinder (shell), valves, and other necessary components, while in the case of a rotary engine, the inner wall surfaces of a combustion chamber are constituted by a rotor, a rotor housing, and other necessary components. The heat-shielding coating is preferably provided at least on the inner wall surfaces (heat-absorbing surfaces) of a combustion chamber.Specifically, the heat-shielding coating may preferably be provided, for example, on the top surface (crown surface) of a piston, the peripheral side surfaces of a top land, the combustion chamber surface of a cylinder head (including the inner surface of a sub-chamber), the top dead center side of the peripheral surface of a cylinder, the valve head back surface of a valve, or the like. The thickness of such a heat-shielding coating is, for example, 20 to 150 µm in one embodiment or 40 to 80 µm in another embodiment. Examples

[0041] A number of test samples (coated members) were prepared by coating the surfaces of the main bodies (members) with the heat-shielding coatings through the anodizing process, and the shape, structure, features, and other properties of the layer of each test sample were revealed. The present invention will be described in more detail below with reference to such specific examples. «Preparation of test samples»(1) Main body

[0042] A number of main bodies (20 × 40 × 2 mm) composed of an Al alloy for casting (JIS AC8A / Al-12%Si-1%Cu-1%Mg) were provided as objects to be coated. (2) Thermal spraying step (base layer formation step / first step)

[0043] A base layer (thermal spray layer) was formed on the surfaces of each main body by thermal spraying an alloy powder having the composition listed in Table 1. Gas-atomized powders (average diameter D 50 : 10 to 40 µm) were used as alloy powders. Thermal spraying was performed using a plasma spraying device. At this time, the current and voltage were set to 54 A and 27.5 kV, respectively.

[0044] Thus, test pieces (Test Specimens 1 and 2) were prepared, each of which had a base layer (60 µm thick) coated with a different amount of Si on one surface of the main body. For comparison, a test piece that was not thermally sprayed and remained as the main body was prepared (Test Specimen C1). (3) Anodizing step (electrolytic step / second step)

[0045] The surface to be anodized (base layer) of a test piece was immersed in an aqueous sulfuric acid solution (electrolytic solution), and a voltage was applied around the test piece as anode and to a platinum electrode as cathode.

[0046] For this process, the surfaces of the test piece other than the surfaces to be anodized were shielded to apply the voltage between the surface to be anodized and the platinum electrode. The electrolytic solution had a sulfuric acid concentration of 20 mass% and the temperature (bath temperature) was 10 °C. The voltage was applied while stirring the electrolytic solution.

[0047] An AC / DC superimposed voltage was applied to the test pieces (Test Samples 1 and 2) provided with the base layers. This AC / DC superimposed voltage was applied by voltage control, in which an AC current (frequency: 1 kHz / constant) with a sine wave voltage waveform was superimposed on a DC current with a constant voltage. During this process, the minimum voltage was set to 0 V, and the maximum voltage (peak voltage) was gradually increased. First, electrolysis was performed at an initial maximum voltage of 40 V for 5 minutes. Subsequently, only the maximum voltage was increased by 20 V, and electrolysis was performed for another 5 minutes (maximum voltage: 60 V × 5 minutes).This process was repeated at 5-minute intervals, and the electrolysis was completed at a maximum voltage of 180 V for 5 minutes. Thus, the maximum voltage was increased in eight steps, and the AC / DC superimposed electrolysis was carried out for a total of 40 minutes.

[0048] An AC / DC superimposed voltage was applied to the test sample (C1), which was provided without a primer layer. The AC / DC superimposed voltage was also applied by voltage control for 8 minutes at an average voltage of 500 V and a frequency of 20 Hz.

[0049] After completion of electrolysis, the test pieces were thoroughly washed with distilled water after being removed from the electrolytic solution. Compressed air was then blown through to remove moisture, and the test pieces were then sufficiently dried in the air. Thus, the test specimens were obtained, in which the main body surfaces were coated with various anodizing layers. «Observation / Test Sample Analysis» (1) An approximately central portion of the longitudinal section (cross-section in the thickness direction) of the anodized layer of each test sample was observed with a field emission scanning electron microscope (FE-SEM). SEM images and schematic diagrams of test sample 2 and test sample C1 are shown as examples and are included in Fig. 1 shown. (2) The crystal structure of the anodized layer (cross-section) of each test sample was analyzed using an X-ray diffractometer (available from Rigaku Corporation). The X-rays used were Cu-Kα rays, and 2θ was set between 10° and 40°. The profiles of the test samples thus obtained are shown collectively in Fig. 2. In Fig. 2, along with the diffraction angle 2θ, the grating spacing calculated by the Bragg equation was plotted along the horizontal axis.

[0050] In the range where the profile is wide, the position indicating the maximum detection intensity (arbitrary unit) is defined as the "amorphous peak position." The position of the amorphous peak of each test sample is listed in Table 1 as the grid spacing. In the Fig. The profiles shown in Figure 2 show the peaks at approximately 2θ = 28 ° and 38 ° during the preparation of the test samples mixed Si and Al. «Measuring the test samples»

[0051] The apparent density, porosity, volumetric specific heat, thermal conductivity, and thickness of the anodized layer were measured using the methods described above. «Evaluation»(1) Structure of the layer

[0052] As from Fig.As shown in Figure 1, it was found that the anodized layers (Test Samples 1 and 2) formed by the AC / DC superimposed voltage on the thermal spray layers (base layers) containing a large amount of Si formed spongy structures (spongy bodies). It was demonstrated that the anodized layers exhibiting such a shape are very different from the conventionally anodized layers (Test Sample C1), which are composed of a linear, tubular, porous layer. The absence of macro-sized Si particles in the thermal spray layers before the anodizing process was separately confirmed by SEM or similar.

[0053] As from Fig.As shown in Figure 2, it was confirmed that the anodized layer is composed of an amorphous structure (amorphous body). However, the position of the amorphous peak is different for each test sample. That is, in the anodized layers (Test Samples 1 and 2) composed of spongy bodies, the amorphous peak is shifted in the direction in which the lattice spacing increases (the direction in which the diffraction angle decreases), compared to the conventional anodized layer (Test Sample C1) composed of a linear, tubular, porous layer. (2) Layer properties

[0054] As shown in Table 1, the anodized coatings of test samples 1 and 2 have lower density, lower specific heat, and lower thermal conductivity than those of the anodized coatings of test sample C1. This means that the anodized coatings of test samples 1 and 2 were found to be thermal protection coatings with excellent vibration properties.

[0055] It was also confirmed that the anodized layers of test samples 1 and 2 can be produced with a sufficient thickness, even with a short production time.

[0056] From the above, it was found that the heat-shielding coating of the present invention has a shape and / or structure that are obviously different from those of conventional anodized layers, and exhibits excellent properties such as low heat capacity (specific heat) and low thermal conductivity. It was also found that the thickness of the heat-shielding coating can be increased in a short time, and productivity is therefore excellent. [Table 1] Test sample no. Composition of the base layer (mass %) Spongy body Layer characteristics Lattice spacing corresponding to the amorphous peak of the skeleton (Å) Apparent density (g / cm 3 ) Porosity (%) Volumetric specific heat (kJ / m 3 ·K) Thermal conductivity (W / m·K) Thickness (µm) 1 Al-20%Si 3,65 0,84 80 0,76 1,15 47 2 Al-40%Si 3,87 0,74 82 0,66 0,32 51

Claims

[1] A thermal protection coating comprising a spongy body, the spongy body having non-linear pores and a skeleton accommodating the pores, wherein the skeleton is an amorphous body comprising Al, Si, O and impurities, and has an amorphous peak specified by X-ray diffraction analysis at a position of 3.5 Å or greater as the lattice spacing, the thermal protection coating has an apparent density of 1 g / cm 3 or less, a volumetric specific heat of 1,000 kJ / m 3 ·K or less and a thermal conductivity of 2 W / m·K or less, the spongy body comprises an anodic oxide of an aluminum alloy in which Si is solidly dissolved in a supersaturated state, and the aluminum alloy contains 16 to 48 mass% of Si relative to the total alloy. [2] The heat-protective coating according to claim 1, wherein a porosity is 70% or higher. [3] A coated member, wherein at least a part of a main body surface is coated with the heat-protective coating according to claim 1 or 2. [4] Coated element according to claim 3, wherein the main body surface is an inner wall surface of a combustion chamber, and the thermal protection coating has a thickness of 20 to 150 µm. [5] A method for producing a coated element, wherein the coated element is obtained according to claim 3 or 4, comprising: a first step of forming a base layer of an aluminum alloy on a main body surface, wherein the aluminum alloy contains 16 to 48 mass% of Si with respect to the entire alloy, and Si is solidly dissolved in a supersaturated state; and a second step of anodizing the base layer, where the voltage is from -5 V to 200 V. [6] A method for producing a coated element according to claim 5, where the first step is a thermal spraying step for the aluminum alloy. [7] A method for producing a coated element according to claim 5 or 6, wherein the second step is an electrolytic step of applying an AC / DC superimposed voltage in which an AC component and a DC component are superimposed.

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