Heat shielding film and heat shielding component

Abstract

To provide a heat insulating film which, despite being provided with an inorganic compound layer, overcomes various problems that arise due to the occurrence of voids or microcracks in the inorganic compound layer.SOLUTION: A heat insulating film 10 formed on at least a portion of the surface of a component 2 to be heat-insulated is formed from: an inorganic compound layer 20 in which scaly inorganic particles 22 are dispersed in an inorganic compound 21 formed from an alkoxide; and a top coat 30 that is formed on the inorganic compound layer 20, has a thickness of 0.5-30 μm, and is formed with an organic-inorganic hybrid material comprising a mixture of a metal alkoxide and a resin. By the formation of the top coat 30, voids 23 or microcracks in the inorganic compound layer 20 are filled with the organic / inorganic hybrid material, and the surface of the inorganic compound layer 20 is covered, so that the heat insulating performance of the heat insulating film 10 is enhanced, and fuel efficiency of an engine is improved by forming the heat insulating film 10 on piston top surfaces of the engine.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a heat insulation film and a component insulated by forming the heat insulation film (referred to as a "heat insulation component" in this specification), for example, a heat insulation film formed on the top surface of a piston of an engine, and a heat insulation component such as a piston on which the heat insulation film is formed.

Background Art

[0002] For the purpose of improving fuel efficiency and the like, in order to reduce heat loss caused by heat generated in the combustion chamber of an engine being transmitted through a piston or the like and released, a heat insulation film is formed on the top surface of the piston to improve thermal efficiency.

[0003] As such a heat insulation film, Patent Document 1 described below proposes a configuration in which a heat insulation layer 71 made of a resin in which hollow particles 80 are embedded is formed on the top surface of a piston 102 as shown in FIG. 10, and an inorganic coating layer 72 (72a, 72b) made of an inorganic material such as silica, zirconia, alumina, and ceria containing hollow particles 80a, 80b is formed on the surface of the heat insulation layer 71 (see FIG. 4 of Patent Document 1).

[0004] Further, in the resin heat insulation layer 71 provided on the heat insulation film described in Patent Document 1 and the inorganic coating layer 72 (72a, 72b) containing hollow particles 80a, 80b, the heat resistance in a high-temperature environment such as the combustion chamber of an engine is insufficient, and in view of the fact that the inorganic coating layer 72 (72a, 72b) is likely to peel off due to cracks or the like, Patent Document 2 described below proposes that, as shown in FIG. 11, a heat insulation layer formed on the top surface of the piston 102 is formed by an anodized layer 241, and an inorganic compound layer 243 in which flaky inorganic particles are dispersed in an inorganic compound formed from an alkoxide is formed on this heat insulation layer (anodized layer) 241 as a protective layer, and a heat insulation film 240 composed of the aforementioned anodized layer 241 and the inorganic compound layer (protective layer) 243 is formed (see FIGS. 10 and 11 of Patent Document 2).

Prior Art Documents

Patent Documents

[0005] Patent Document 1 Japanese Patent No. 6067712 Patent Document 2 Japanese Patent No. 6339118 Summary of the Invention Problems to be Solved by the Invention

[0006] In the heat insulation film 240 described in Patent Document 2 above, the heat insulation layer 71 formed by resin and hollow particles 80 in Patent Document 1 was changed to an anodized layer 241, and the inorganic coating layer 72 (72a, 72b) formed by hollow particles 80a, 80b and an inorganic material in Patent Document 1 was changed to an inorganic compound layer (protective layer) 243 in which flaky inorganic particles were dispersed in an inorganic compound formed from an alkoxide. By doing so, the drawback of low heat resistance, which was a weakness of the heat insulation film 70 described in Patent Document 1, has been overcome.

[0007] However, the inorganic compound layer 243 formed from an alkoxide provided in the heat insulation film 240 described in Patent Document 2 above is formed by applying a liquid paint (metal alkoxide solution containing inorganic particles) that becomes the inorganic compound layer 243 to a heat insulation target part such as a piston 102 and then causing a chemical reaction by firing. Therefore, pores are formed in the inorganic compound layer 243 due to volume shrinkage during cooling after film formation, and fractures such as microcracks occur.

[0008] In addition, the occurrence of fractures such as microcracks can occur not only during the firing of the inorganic compound layer 243 but also when heating and cooling are repeated due to use in a high-temperature environment after film formation.

[0009] In this way, the pores generated in the inorganic compound layer 243 and the occurrence of microcracks become the starting points for peeling of the inorganic compound layer 243 and the like.

[0010] In addition, if the surface of the inorganic compound layer 243 becomes uneven due to the generation of microcracks and the surface area increases, the heat-receiving area increases, and the amount of heat transfer increases, making it easier for heat to be transferred to the inorganic compound layer 243, thus reducing the function as a heat-insulating film.

[0011] In particular, in the configuration where the heat-insulating film 240 of Patent Document 2 is provided on the top surface of the piston 102 of the engine, when pores or microcracks occur in the inorganic compound layer 243, a part of the fuel injected into the combustion chamber penetrates into the pores and microcracks and is retained therein.

[0012] In addition, since the flaky inorganic particles dispersed in the inorganic compound layer 243 have liquid absorbency, the fuel that has penetrated into the microcracks further penetrates into the flaky inorganic particles and is retained therein.

[0013] The fuel retained in pores, microcracks, flaky inorganic particles, etc. in this way is not burned even in the combustion stroke of the engine and is discharged outside the engine together with the exhaust gas as unburned fuel, increasing the fuel loss (hereinafter, such fuel loss due to the discharge of unburned fuel is referred to as "unburned loss").

[0014] In addition, the formation of the inorganic compound layer 243 with microcracks and an uneven surface on the top surface of the piston 102 not only reduces the heat-insulating property of the heat-insulating film 240 as described above, but also makes abnormal combustion (knocking) likely to occur on the top surface of the piston.

[0015] As a result, in the "actual engine test" in which the piston 102 with the heat-insulating film 240 described in Patent Document 2 formed on its top surface was mounted on the engine, although the heat loss was reduced by the formation of the heat-insulating film 240, sufficient improvement in fuel consumption could not be obtained.

[0016] Thus, although the heat-insulating film 240 of Patent Document 2 provided with the inorganic compound layer 243 has high heat-insulating property and heat resistance, its performance cannot be fully utilized due to the generation of pores and microcracks.

[0017] Therefore, the present invention has been made to solve the drawbacks of the above prior art, and while having an inorganic compound layer excellent in heat insulation and heat resistance as described above, it solves various problems caused by the generation of pores and microcracks in this inorganic compound layer, and aims to provide a heat insulation film and a heat insulation component on which the heat insulation film is formed.

Means for Solving the Problems

[0018] Hereinafter, the means for solving the problems will be described together with the reference numerals used in the mode for carrying out the invention. This reference numeral is for clarifying the correspondence between the description of the claims and the description of the mode for carrying out the invention, and needless to say, it is not used restrictively for interpreting the technical scope of the present invention.

[0019] In order to achieve the above object, the heat insulation film 10 of the present invention In the heat insulation film 10 formed on at least a part of the surface of the heat insulation target component 2, an inorganic compound layer 20 in which flaky inorganic particles 22 are dispersed in an inorganic compound 21 formed from an alkoxide, and a top coat 30 formed on the inorganic compound layer 20, having a thickness of 0.5 to 30 μm and formed of an organic-inorganic hybrid material composed of a mixture of a metal alkoxide and a resin (However, except for those in which inorganic particles are dispersed.) is provided (Claim 1).

[0020] The surface of the top coat 30 is preferably 0.1 to 1.5 μm in terms of the surface arithmetic mean height Sa defined in ISO 25178, or 0.01 to 1.5 in terms of the interface development area ratio Sdr (Claim 2).

[0021] An anodic oxide film layer 40 can be further provided under the inorganic compound layer 20 (Claim 3).

[0022] Further, the heat insulation component 1 of the present invention is formed by covering at least a part of the surface of the heat insulation target component 2 with the heat insulation film 10, The heat insulation film 10 comprises an inorganic compound layer 20 in which flaky inorganic particles 22 are dispersed in an inorganic compound 21 formed from an alkoxide, and a top coat 30 formed on the inorganic compound layer 20, having a thickness of 0.5 to 30 μm and formed from an organic-inorganic hybrid material composed of a mixture of a metal alkoxide and a resin (However, except for those in which inorganic particles are dispersed.) (Claim 4).

[0023] In the heat insulation component 1 having the above configuration, it is preferable that the surface of the top coat 30 has a surface arithmetic mean height Sa of 0.1 to 1.5 μm or an interface development area ratio Sdr of 0.01 to 1.5 as defined in ISO 25178 (Claim 5).

[0024] Further, an anodic oxidation coating layer 40 can be provided under the inorganic compound layer 20 (Claim 6).

[0025] The heat insulation target component 2 can be a piston for an engine, preferably a piston for an engine made of an aluminum alloy (Claim 7).

Advantages of the Invention

[0026] With the configuration of the present invention described above, in the heat insulation film 10 of the present invention and the heat insulation component 1 on which the heat insulation film 10 is formed, the following remarkable effects could be obtained.

[0027] By providing a top coat 30 formed from an organic-inorganic hybrid material, which is a mixture of a metal alkoxide and a resin, on the inorganic compound layer 20 where pores 23 and microcracks occurred during film formation, with a film thickness of 0.5 to 30 μm, the inorganic compound layer 20 could be covered with the top coat 30 in a state where the organic-inorganic hybrid material was impregnated into the pores 23 and microcracks formed in the inorganic compound layer 20.

[0028] This top coat 30 is formed of an organic-inorganic hybrid material composed of a mixture of a metal alkoxide which is an inorganic component such as silicon alkoxide, zirconium alkoxide, titanium alkoxide, etc. and a resin which is an organic component such as an alkyl silicate resin, a silicone resin, a fluororesin, etc. Therefore, it has both the properties of high hardness and high heat resistance of the metal alkoxide which is an inorganic component and the flexibility of the resin which is an organic component, and it is less likely to cause destruction such as microcracks even when used in a high-temperature environment where heating and cooling are repeatedly performed.

[0029] As a result, even when the heat insulation component 1 provided with the heat insulation film 10 of the present invention is used in a high-temperature environment, peeling of the top coat 30 is less likely to occur, and the inorganic compound layer 20 in which pores 23 and microcracks have occurred is covered with the top coat 30, and the organic-inorganic hybrid material constituting the top coat 30 penetrates into the microcracks and pores 23 and cures, so that peeling of the inorganic compound layer 20 starting from the pores 23 and microcracks can be made less likely to occur.

[0030] In addition, by forming the top coat 30, the surface of the heat insulation film 10 becomes smooth, and as a result, the surface area becomes smaller compared to the case where the surface has an uneven shape, so that heat exchange on the surface of the heat insulation film 10 is less likely to occur, and the heat insulation property of the heat insulation film 10 can be improved.

[0031] In particular, when the heat insulation film 10 of the present invention is formed on the top surface of the piston of the engine, since the surface of the inorganic compound layer 20 is covered with the top coat 30, a part of the fuel injected into the combustion chamber does not penetrate into the pores 23, microcracks, and scaly inorganic particles 22 of the inorganic compound layer 20 and is retained, so that unburned loss can be reduced.

[0032] In addition, by forming the top coat 30, the top surface of the piston becomes a smooth surface, and the occurrence of abnormal combustion (knocking) on the top surface of the piston can also be suppressed.

[0033] As a result, by forming the heat insulation film 10 of the present invention on the top surface of the piston of the engine, the fuel consumption of the engine equipped with the piston could be improved.

Brief Description of the Drawings

[0034] [[Figure 1]] Cross-sectional explanatory view showing one configuration example of the heat insulation film of the present invention. [[Figure 2]] Cross-sectional explanatory view showing another configuration example of the heat insulation film of the present invention. [[Figure 3]] Laser microscope image (×50) of the surface of the heat insulation film of Example 1. [[Figure 4]] Laser microscope image (×50) of the surface of the heat insulation film of Example 2. [[Figure 5]] Laser microscope image (×50) of the surface of the heat insulation film of Comparative Example 1. [[Figure 6]] Surface image of the heat insulation film of Comparative Example 2 (digital camera image by microscope) (×50). [[Figure 7]] Laser microscope image (×50) of the surface of the heat insulation film of Comparative Example 3. [[Figure 8]] Cross-sectional SEM image (×2000) of the heat insulation film of Example 1. [[Figure 9]] Cross-sectional SEM image (×2000) of the heat insulation film of Comparative Example 1. [[Figure 10]] Cross-sectional explanatory view of the conventional heat insulation film (corresponding to FIG. 4 of Patent Document 1). [[Figure 11]] Cross-sectional explanatory view of the conventional heat insulation film (corresponding to FIG. 10 of Patent Document 2).

Modes for Carrying Out the Invention

[0035] Next, embodiments of the present invention will be described below with reference to the accompanying drawings.

[0036] In the following description, an example will be described in which the heat insulation target component 2 is an engine piston, and the heat insulation film 10 of the present invention is formed on the top surface of this piston to form the heat insulation component 1 of the present invention. However, the heat insulation film 10 of the present invention can be formed not only on the engine piston but also on various articles that require heat insulation, such as various mechanical parts used in a high-temperature environment.

[0037] 〔Overall Structure of Heat Insulation Component and Heat Insulation Film〕 As shown in FIGS. 1 and 2, the heat insulation component 1 of the present invention is composed of a heat insulation target component 2 which is a metal component such as an engine piston, and a heat insulation film 10 formed on at least a part of the surface of this heat insulation target component 2. For example, in the case of the aforementioned piston, it is formed on its top surface.

[0038] As shown in FIGS. 1 and 2, this heat insulation film 10 is a film formed on the surface of the heat insulation target component 2 in order to block heat conduction to the heat insulation target component 2, and includes at least an inorganic compound layer 20 in which flaky inorganic particles 22 are dispersed in an inorganic compound 21 formed from an alkoxide, and a top coat 30 formed from an organic-inorganic hybrid material which is a mixture of a metal alkoxide and a resin.

[0039] The heat insulation film 10 in FIG. 1 is a two-layer heat insulation film 10 formed by the aforementioned inorganic compound layer 20 and top coat 30. FIG. 2 shows a three-layer heat insulation film 10 having an anodic oxidation coating layer 40 formed under the inorganic compound layer 20 in addition to the inorganic compound layer 20 and top coat 30.

[0040] As shown in FIG. 2, when the anodic oxidation coating layer 40 is provided under the inorganic compound layer 20, the heat insulation target component 2 is made of aluminum or an aluminum alloy, and the anodic oxidation treatment is performed on this heat insulation target component 2 in advance to form the anodic oxidation coating layer (aluminum oxide layer) 40, and the aforementioned inorganic compound layer 20 and top coat 30 may be formed on this anodic oxidation coating layer (aluminum oxide layer) 40.

[0041] Such an anodized layer 40 can be formed by anodizing aluminum using an acidic aqueous solution composed of sulfuric acid, oxalic acid, phosphoric acid, etc., and the range of its film thickness is 10 to 100 μm.

[0042] Thus, by providing the anodized layer 40 under the inorganic compound layer 20, the adhesion between the inorganic compound layer 20 and the base material of the piston made of aluminum alloy can be improved.

[0043] Note that the heat insulation film 10 of the present invention is not limited to the configuration shown in FIGS. 1 and 2. In the heat insulation film 10 having the configuration shown in FIG. 2, instead of the anodized layer 40, for example, a known heat insulation layer such as a heat insulation layer made of an inorganic material in which hollow particles are dispersed may be provided, or a known heat insulation layer such as a heat insulation layer made of an inorganic material in which the above-mentioned hollow particles are dispersed may be provided between the anodized layer 40 and the inorganic compound layer 20 in FIG. 2. Any configuration may be adopted as long as it has at least the above-mentioned inorganic compound layer 20 and the top coat 30.

[0044] 〔Inorganic compound layer〕 Among the layers constituting the heat insulation film 10 of the present invention, the aforementioned inorganic compound layer 20 has a structure in which flaky inorganic particles 22 are dispersed in the inorganic compound 21 formed from an alkoxide. As shown in FIGS. 1 and 2, the flaky inorganic particles 22 arranged with their longitudinal direction parallel to the surface of the heat insulation target part 2 are bonded with the inorganic compound 21 formed from an alkoxide as a binder.

[0045] The inorganic compound 21 constituting the inorganic compound layer 20 is composed of a metal oxide formed from an alkoxide such as silicon alkoxide, zirconium alkoxide, aluminum alkoxide, or cerium alkoxide.

[0046] In particular, zirconium alkoxide is ductile and is preferable because it easily follows the elongation of the heat insulation target part 2 which is a piston made of an aluminum alloy.

[0047] Thus, since the inorganic compound 21 of the inorganic compound layer 20 is composed of a metal oxide formed from an alkoxide, although water and alcohol are produced as by-products when the alkoxide is processed, these can be easily removed by heat treatment.

[0048] This can suppress the remaining of foreign matters, so that it is possible to improve the heat resistance.

[0049] In the inorganic compound 21, a binder having an amino group (-NH 2 ) is dispersed. As such a binder, amino-based coupling agents such as aminopropyltriethoxysilane, aminopropyltrimethoxysilane, and aminopropylmethyldimethoxysilane can be used.

[0050] The addition of such an amino-based coupling agent is carried out in an amount of 0.1 mass% to 10 mass% with respect to 100 mass% of the paint (not containing flaky inorganic particles) which is a metal alkoxide solution.

[0051] The thickness of the inorganic compound layer 20 configured as described above is 10 to 500 μm, preferably 10 to 200 μm.

[0052] Examples of the flaky inorganic particles 22 dispersed in the aforementioned inorganic compound 21 include mica, talc, and wollastonite. Any one of these may be used alone, or any two or all three of them may be mixed and used.

[0053] Mica, talc, and wollastonite do not undergo melting or the like even at a high temperature of about 1000 °C and have sufficient heat resistance.

[0054] Here, the term "flaky" refers to a shape in which the thickness is sufficiently small with respect to the length. In addition to plate-like and sheet-like ones, fibrous and needle-like ones are also included in the flaky ones here as long as the thickness is sufficiently small with respect to the length.

[0055] The size of the flaky inorganic particles 22 dispersed in the inorganic compound layer 20 is preferably about 0.1 to 100 μm, more preferably about 1 to 20 μm, in terms of the average particle diameter.

[0056] Such flaky inorganic particles 22 are dispersed in the inorganic compound 21 so that 35 to 75 vol% of the inorganic compound layer 20 becomes inorganic particles 22.

[0057] By adding such flaky inorganic particles, the peeling of the inorganic compound layer 20 can be suppressed, and high heat insulation performance can be ensured even in a high-temperature environment.

[0058] 〔Top coat〕 The top coat 30 formed on the inorganic compound layer 20 is formed from an organic-inorganic hybrid material composed of a mixture of metal alkoxides such as silicon alkoxide, zirconium alkoxide, titanium alkoxide, etc. and a resin excellent in heat resistance such as an alkyl silicate resin, a silicone resin, and a fluorine-based resin.

[0059] By forming this top coat 30 in the range of a film thickness of 0.5 to 30 μm, it is possible to cover the inorganic compound layer 20 in a state where the organic-inorganic hybrid material penetrates into the pores 23 and microcracks generated in the inorganic compound layer 20.

[0060] Thereby, the inorganic compound layer 20 in which pores 23 and microcracks are generated can be reinforced, and the heat insulation performance of the heat insulation film 10 can be improved by flattening the surface of the heat insulation film 10.

[0061] Further, when the heat insulation film 10 provided with such a top coat 30 is formed on the top surface of the piston, it is possible to prevent the fuel from penetrating into the pores 23, cracks, and flaky inorganic particles 22 of the inorganic compound layer 20, and by making the surface of the heat insulation film 10 smooth, it is possible to prevent the occurrence of abnormal combustion (knocking) of the fuel on the top surface of the piston.

[0062] As the thickness of this top coat 30 decreases, the effect of fuel penetration into the inorganic compound layer 20 becomes lower, and as the thickness increases, the internal stress increases, making it easier for cracks, peeling, etc. to occur, and it becomes difficult to maintain the top coat 30 in a good state.

[0063] Therefore, the thickness of the top coat is preferably 0.5 to 30 μm, more preferably 0.5 to 10 μm.

[0064] In addition, as described above, by forming the top coat 30, the surface of the heat insulation film 10 becomes smooth, thereby improving the heat insulation performance due to the reduction in surface area, preventing abnormal combustion (knocking) from occurring on the piston top surface, and improving the fuel consumption of the engine.

[0065] From such a viewpoint, the surface roughness of the top coat 30 is preferably 0.1 to 1.5 μm in terms of the surface arithmetic mean height Sa defined by ISO25178, or 0.01 to 1.5 in terms of the interface developed area ratio Sdr.

[0066] 〔Heat insulation target part〕 As described above, the heat insulation film 10 of the present invention described above uses the piston of the engine as the heat insulation target part and forms it on the top surface of this piston. While maintaining the effect of suppressing heat release in the combustion chamber through the piston similar to the conventional heat insulation film, it is possible to reduce the unburned loss, and the formation of the heat insulation film 10 with a smooth surface can improve the heat insulation performance compared to the conventional heat insulation film whose surface was formed unevenly, resulting in a large surface area and easy heat exchange (therefore, low heat insulation performance). In addition, it can suppress abnormal combustion (knocking) on the piston top surface and improve the fuel consumption.

[0067] Thus, the heat insulation film 10 of the present invention is suitable for forming on the top surface of the piston of the engine, particularly the piston made of an aluminum alloy with good thermal conductivity.

[0068] However, the component (heat insulation target component) 2 that performs heat insulation by forming the heat insulation film 10 of the present invention is not limited to the piston of the engine, and can be applied to mechanical components used in a high-temperature environment, for example, various mechanical components that require heat insulation, such as exhaust system components such as exhaust manifolds and EGR (exhaust gas recirculation).

Example

[0069] 1. Evaluation test The evaluation test results for the heat insulation film of the present invention will be described below. 〔Evaluation target〕 Tests and evaluations were conducted on samples (Example 1 and Example 2) with the heat insulation film of the present invention formed on test pieces made of SUS304, and samples (Comparative Example 1 to 3: However, only a test piece made of aluminum alloy was used in Comparative Example 1) with the heat insulation film of the comparative example formed.

[0070] The heat insulation films formed on the samples of Example 1 and 2 and Comparative Example 1 to 3 are as shown in Table 1 below.

[0071]

Table 1

[0072] 〔Test and observation methods〕 (1) Oil absorption test After forming the heat insulation film, each sample in its original state and each sample after applying a thermal shock were used as test subjects respectively.

[0073] As the aforementioned thermal shock, each sample was held in a heated state of 350 °C for 10 minutes and then immersed in normal-temperature water for cooling treatment.

[0074] For the evaluation of oil absorption, n-hexadecane (low-viscosity oil regarded as fuel) was dropped onto the surface of the heat insulation film and left for 3 minutes, then wiped off with a paper towel, and the weight change of each sample before the oil drop, after the oil drop, and after further wiping with the paper towel was measured as the 'oil absorption amount (mg)'.

[0075] The value obtained by dividing the measured oil absorption amount by the weight of the heat insulation film before oil dripping (×100) was determined as the "oil absorption rate (%)".

[0076] (2) Surface and cross-section observation The surface of the heat insulation film of each sample of Examples 1 and 2 and Comparative Examples 1 to 3 after applying thermal shock was observed, and the cross-sections of Examples 1 and 2 and Comparative Example 1 were observed.

[0077] The surface observation was performed with a laser microscope using an objective lens with a magnification of 50 times.

[0078] Also, the cross-section observation was performed using a scanning electron microscope (SEM) at a magnification of 2000 times.

[0079] (3) Measurement of surface roughness After forming the heat insulation film, the surface roughness of the samples of Examples 1 and 2 and Comparative Examples 1 to 3 before applying thermal shock was measured.

[0080] The measurement was performed using a laser microscope (objective lens with a magnification of 50 times), and as roughness parameters, the surface arithmetic mean height Sa defined in ISO25178 and the developed area ratio Sdr of the interface were measured respectively.

[0081] 〔Test and observation results〕 The results of the above-described oil absorption test, surface and cross-section observation, and surface roughness measurement are shown in Table 2 below.

[0082] Also, the states of the heat insulation film surfaces of each sample of Examples 1 and 2 and Comparative Examples 1 to 3 are shown in FIGS. 3 to 7, and the cross-section SEM images of Example 1 and Comparative Example 1 are shown in FIGS. 8 and 9 respectively.

[0083]

Table 2

[0084] 〔Discussion〕 The samples of Example 1 and Example 2 had a low oil absorption rate in any state before and after thermal shock, no cracks were observed, and both the surface arithmetic mean height Sa and the interface development area ratio Sdr of the surface roughness were much lower than 1.5.

[0085] Also, as shown in Fig. 8, as a result of cross-sectional observation by SEM, it was confirmed that the pores in the inorganic compound layer were filled by the penetration of the organic-inorganic hybrid material.

[0086] Fig. 8 is a cross-sectional SEM image of the sample of Example 1. Although the cross-sectional SEM image of the sample of Example 2 is not shown, the same results have been confirmed.

[0087] On the other hand, in the heat insulation film of Comparative Example 1 where no top coat was provided and the inorganic compound layer was directly exposed on the surface, the oil absorption rate was as high as 17.9% before thermal shock and 19.5% after thermal shock. Also, as a result of surface and cross-sectional observations, the generation of microcracks (see Fig. 5) and the generation of a large number of pores (see Fig. 9) were confirmed in the inorganic compound layer, and the surface roughness was also rough, with a surface arithmetic mean height Sa of 1.64 μm and an interface development area ratio Sdr of 4.26.

[0088] From the above results, the superiority of the heat insulation film of the present invention provided with a top coat on the inorganic compound layer was confirmed.

[0089] Also, in Comparative Example 2, similar to Examples 1 and 2, a top coat formed of an organic-inorganic hybrid material was provided. The oil absorption rate before thermal shock was 0.3%, which was lower than that of the heat insulation films of Examples 1 and 2. It was also confirmed that a smooth surface was obtained, with a surface arithmetic mean height Sa of 0.12 and an interface development area ratio Sdr of 0.03.

[0090] However, in the heat insulation film provided on the sample of Comparative Example 2, peeling occurred in the top coat by applying thermal shock (see Fig. 6), and it was confirmed that it could not withstand use in a high-temperature environment.

[0091] The film thickness of the top coat formed on the heat-insulating film of Comparative Example 2 was 50 μm, which was thicker than the film thicknesses of the top coats in Examples 1 and 2 (3 μm in Example 1 and 30 μm in Example 2). As a result of increasing the film thickness of the top coat in this way, the residual stress in the film increased, and it is considered that peeling occurred when a thermal shock was applied.

[0092] From the above results, the effectiveness of setting the film thickness of the top coat in the heat-insulating film of the present invention to 30 μm or less was confirmed.

[0093] The heat-insulating film of Comparative Example 3 has a film thickness of 5 μm for the top coat, which is within the range of the film thickness of the top coat of the present invention. However, it is different from the configuration of the heat-insulating film of the present invention in that the top coat is formed only of an inorganic material, alkoxide metal (Zr alkoxide).

[0094] In this configuration, the oil absorption rate before the thermal shock was already as high as 16.2%, the surface roughness was 1.30 μm in terms of the surface arithmetic mean height Sa, the interfacial developed area ratio Sdr was as high as 4.07, and furthermore, the top coat peeled off when a thermal shock was applied.

[0095] From the above results, the superiority of the heat-insulating film of the present invention that employs an organic-inorganic hybrid material as the material of the top coat was confirmed.

[0096] 2. Confirmation of the lower limit value of the film thickness of the top coat In order to determine the lower limit value of the film thickness of the top coat, the film thickness of the top coat was changed in the range of 0.5 to 2 μm, which is less than 3 μm (Example 1), the minimum value of the film thickness of the top coat in the above-mentioned test example, and the change in the appearance and the change in the oil absorption rate of the formed heat-insulating film were measured.

[0097] The measurement was performed on both the sample before applying the heat load and the sample after the heat load. For the observation of the appearance, the surface of the heat insulation film (top coat) was observed with the naked eye and a microscope. Those for which no cracks could be confirmed even when using a microscope were evaluated as "○", those for which cracks were confirmed with a microscope but not with the naked eye were evaluated as "△", and those for which cracks could be confirmed with the naked eye were evaluated as "×".

[0098] Note that the film thickness of the top coat was grasped based on cross-sectional observation.

[0099] The test results are shown in Table 3 below.

[0100]

Table 3

[0101] From the above results, it was confirmed that even when forming a relatively thin film thickness of 0.5 μm for the film thickness of the top coat in the range of 0.5 μm to 2 μm, there was no crack generation and a heat insulation film with a low oil absorption rate was obtained, and it was effective.

Explanation of Signs

[0102] 1 Heat insulation component 2 Component to be heat-insulated (piston) 10 Heat insulation film 20 Inorganic compound layer 21 Inorganic compound 22 Scaly inorganic particles 23 Void 30 Top coat 40 Anodic oxidation coating layer (aluminum oxide layer) 70 Heat insulation film 71 Heat insulation layer 72(72a, 72b) Inorganic coating layer 80, 80a, 80b Hollow particles 102 Piston 240 Heat insulation film 241 Heat insulation layer (aluminum oxide layer) 243 Inorganic compound layer (protective layer)

Claims

1. In a heat-insulating film formed on at least a part of the surface of a heat-insulating target part, an inorganic compound layer formed by dispersing flaky inorganic particles in an inorganic compound formed from an alkoxide, a top coat formed on the inorganic compound layer, having a thickness of 0.5 to 30 μm and formed of an organic-inorganic hybrid material composed of a mixture of a metal alkoxide and a resin (however, excluding those in which inorganic particles are dispersed), characterized in that the heat-insulating film is provided with the top coat.

2. The heat-insulating film according to Claim 1, wherein the surface of the top coat has an arithmetic mean height Sa of 0.1 to 1.5 μm as defined in ISO 25178, or an expanded area ratio Sdr of the interface of 0.01 to 1.

5.

3. The heat-insulating film according to Claim 1 or 2, further comprising an anodized film layer under the inorganic compound layer.

4. In a heat-insulating part formed by covering at least a part of the surface of a heat-insulating target part with a heat-insulating film, the heat-insulating film is an inorganic compound layer formed by dispersing flaky inorganic particles in an inorganic compound formed from an alkoxide, a top coat formed on the inorganic compound layer, having a thickness of 0.5 to 30 μm and formed of an organic-inorganic hybrid material composed of a mixture of a metal alkoxide and a resin, characterized in that the heat-insulating part is provided with the top coat (however, excluding those in which inorganic particles are dispersed).

5. The heat-insulating part according to Claim 4, wherein the surface of the top coat has an arithmetic mean height Sa of 0.1 to 1.5 μm as defined in ISO 25178, or an expanded area ratio Sdr of the interface of 0.01 to 1.

5.

6. The heat-insulating part according to Claim 4 or 5, further comprising an anodized film layer under the inorganic compound layer.

7. The heat-insulating part according to any one of Claims 4 to 6, wherein the heat-insulating target part is a piston for an engine.

Images