Piston for internal combustion engine, heat shielding film, and coating material for heat shielding film formation

Abstract

[Problem] To provide: a piston for an internal combustion engine, where the piston can suppress a deterioration in combustion efficiency by dissipating combustion heat during combustion in a combustion chamber of an internal combustion engine and maintains stable heat shielding properties with good strength and thermal shock resistance; and a heat shielding film or the like that is provided to the piston. [Solution] The abovementioned problem is solved by: forming a heat shielding film 4 on all or part of a top surface 2; and configuring the heat shielding film 4 to include a porous material 4a, which comprises secondary particles that are heat resistant particles, and a binder material 4b. The heat resistant particles are preferably porous silica having an average particle diameter in the range of 0.1-40 μm, the binder material is preferably a silicon compound composite material (RSiO1.5) comprising silicon dioxide (SiO2) and a silicone (R2SiO), and a fiber material is preferably further included.

Description

Pistons for internal combustion engines, thermal barrier coatings, and coating materials for forming thermal barrier coatings

[0001] The present invention relates to a piston for an internal combustion engine, a thermal barrier coating, and a coating material for forming a thermal barrier coating. More specifically, the present invention relates to a piston for an internal combustion engine that has excellent thermal shock resistance and is capable of suppressing a decrease in combustion efficiency due to the dissipation of combustion heat during combustion in the combustion chamber of an internal combustion engine, a thermal barrier coating provided on the piston, and a coating material for forming the thermal barrier coating.

[0002] The combustion chamber of an internal combustion engine, such as an automobile engine, is primarily composed of the cylinder head surface, the bore surface of the cylinder block, and the top surface of the piston. The pistons in these internal combustion engines are made of aluminum alloy and have the characteristic of extremely high thermal conductivity. Therefore, when fuel is burned in the combustion chamber, the combustion heat is conducted from the piston's top surface, which has high thermal conductivity, and then dissipated, lowering the temperature of the combustion chamber and reducing combustion efficiency. To address this issue, it is expected that providing a heat-shielding film with high thermal insulation properties on the piston's top surface will suppress heat dissipation from the piston's top surface, thereby preventing a decrease in combustion efficiency and fuel efficiency.

[0003] Regarding such heat-shielding films, the background art section of Patent Document 1 describes a conventional technique in which a low-thermal-conductivity member made of a metal material (such as titanium) with a lower thermal conductivity than aluminum is formed on the top surface of the piston to form an insulating air film between the top surface of the piston body, a conventional technique in which a heat-insulating material is formed on the top surface of the piston by ceramic spraying, and a conventional technique in which an anodized coating is formed on the inner surface of the engine combustion chamber. However, these conventional techniques have problems such as poor strength between the heat-insulating material and the bonding surface, remaining issues such as heat spots and difficulty in post-processing, and knocking, and therefore cannot be said to fully solve the problems, and have not been put into practice.

[0004] The invention described in Patent Document 1, which was proposed based on this background, aims to provide an engine and piston that can suppress knocking and contribute to improved fuel economy by having a thermal barrier coating with high thermal insulation properties and high surface smoothness. The invention is characterized in that the wall surface of a component constituting a combustion chamber is coated with a thermal barrier coating having, in that order, a thermal barrier layer and an inorganic coating layer, the thermal barrier layer comprising a resin layer containing first hollow particles, and the inorganic coating layer comprising an inorganic compound layer containing second hollow particles. This invention provides the wall surface facing the combustion chamber with high thermal insulation properties and high surface smoothness, thereby improving the thermal insulation properties of the combustion chamber and contributing to improved engine fuel economy.

[0005] The thermal barrier coating described in Patent Document 1 above is composed of a thermal barrier layer and an inorganic coating layer, both of which contain hollow particles that enhance the thermal insulation effect. Patent Documents 2 and 3 are other patent documents that enhance thermal barrier properties (thermal insulation). Patent Document 2 proposes an engine component having a thermal barrier coating with low thermal conductivity, low specific heat, and excellent heat resistance and mechanical strength. The engine component has a wall surface provided with a thermal barrier coating that includes a hot-dip galvanized layer containing silicon-based inorganic hollow particles and having a melting point of 300°C or higher and 600°C or lower. Patent Document 3 also proposes a thermal barrier coating with excellent thermal barrier properties and strength. The thermal barrier coating is composed of silica aerogel particles (secondary particles) and a silica-based binder.

[0006] WO2014 / 24494 JP 2019-60317 A JP 2020-76321 A

[0007] However, all of the conventional heat shielding films described above have insufficient thermal conductivity, and the development of a heat shielding film with lower thermal conductivity is desired. Furthermore, practical performance, such as heat resistance, thermal shock resistance, and adhesion, is still insufficient. Furthermore, the heat shielding films (thermal insulating films) proposed in Patent Documents 1 to 3 all contain hollow particles as constituent materials, but these hollow particles have poor strength and thermal shock resistance and are prone to chipping, which makes it difficult to maintain stable heat shielding (thermal insulating) properties.

[0008] The present invention has been made to solve the above-mentioned problems, and its object is to provide a piston for an internal combustion engine, which can suppress a decrease in combustion efficiency due to the radiation of combustion heat during combustion in the combustion chamber of an internal combustion engine, and which maintains stable heat insulation properties with good thermal shock resistance, a thermal barrier coating to be provided on the piston, and a coating material for forming the thermal barrier coating.

[0009] (1) The piston for an internal combustion engine according to the present invention is characterized in that a thermal barrier coating containing a porous material made of secondary particles of heat-resistant particles and a binder material is formed on the entire or part of the top surface.

[0010] According to this invention, a porous material composed of secondary particles of heat-resistant particles has voids therein. Therefore, a thermal barrier coating containing such a porous material can prevent combustion heat from radiating to the outside of the combustion chamber through the wall surface (e.g., the bore surface) during combustion in the combustion chamber of an internal combustion engine, thereby reducing combustion efficiency. As a result, the thermal barrier coating has excellent heat-shielding properties. Furthermore, a porous material composed of secondary particles contains fine voids therein, but these voids are formed by discontinuous connections between primary particles, resulting in a discontinuous skeletal structure. Compared to hollow particles (hollow fillers) with a continuous skeletal structure connected by a continuous shell, such a porous material does not transmit small impacts when subjected to small impacts, and even if cracks occur, they are stopped midway. As a result, the material acts to relieve thermal stress generated during combustion and suppress the transmission of thermal shock, resulting in excellent thermal shock resistance. Furthermore, because the thermal barrier coating contains a porous material with a discontinuous skeletal structure and an uneven surface, it is stronger and can maintain stable thermal barrier properties compared to conventional thermal barrier coatings that contain hollow particles.

[0011] In the internal combustion engine piston according to the present invention, the porous material is porous silica having an average particle size in the range of 0.1 to 40 μm. According to this invention, porous silica (secondary particles) having an average particle size in the range of 0.1 to 40 μm is used and the thermal barrier coating is formed from this porous material, so that the average size of the pores in the porous material can be set in the range of 2 to 10 nm.

[0012] In the piston for an internal combustion engine according to the present invention, the binder material is a mixture of silicon dioxide (SiO2) and silicone (R ２ SiO) and a silicon compound composite material (RSiO 1.5 This silicon compound composite material is preferred because it has a lower thermal conductivity than conventionally used inorganic compounds and crystalline compounds.

[0013] The piston for an internal combustion engine according to the present invention further includes a fiber material. According to this invention, the fiber material not only improves the dispersibility of the porous material but also acts to improve the strength of the thermal barrier coating by thickening effects and steric hindrance effects during film formation. As a result, it is possible to reduce imbalances in thermal shock resistance and strength within the thermal barrier coating and to suppress peeling and damage to the thermal barrier coating. The fiber material is preferably nanofibers having a diameter in the range of 1 to 100 nm and a length in the range of 1 to 50 μm.

[0014] In the piston for an internal combustion engine according to the present invention, the fiber material is made of one or more inorganic materials selected from aluminum oxide, titanium oxide, potassium titanate, and barium titanate, or one or more organic materials selected from cellulose, chitin, chitosan, and silk. According to this invention, a fiber material selected from these materials can be preferably used as a dispersion material for a thermal barrier coating used in a piston for an internal combustion engine.

[0015] (2) A thermal barrier coating according to the present invention is a thermal barrier coating used to form the entire top surface or a portion of the top surface of a piston for an internal combustion engine, and includes a porous material made of secondary particles of heat-resistant particles and a binder material. This invention can provide a thermal barrier coating that maintains more preferable heat insulation properties and thermal shock resistance.

[0016] (3) The coating material for forming a thermal barrier coating according to the present invention is a coating material for forming a thermal barrier coating that forms the thermal barrier coating described in (2) above.

[0017] According to the present invention, it is possible to provide a piston for an internal combustion engine that can suppress a decrease in combustion efficiency due to the heat of combustion being dissipated to the outside of the combustion chamber through the wall surface (e.g., the bore surface) of the combustion chamber during combustion in the combustion chamber, and that maintains stable heat insulation properties with good thermal shock resistance.

[0018] 1 is a schematic cross-sectional view showing an example of a piston for an internal combustion engine according to the present invention. FIG. 2 is an explanatory diagram of the structure of an internal combustion engine. (A) is a schematic diagram showing an example of the form of secondary particles (aggregates) that make up a porous material. (B) is a schematic diagram showing an example of the form of primary particles that make up the secondary particles. FIG. 3 is a schematic cross-sectional view showing an example of a thermal barrier coating. FIG. 4 is a schematic cross-sectional view showing another example of a thermal barrier coating. FIG. 5 is a graph showing the relationship between the content of porous silica in a thermal barrier coating and the specific gravity of the thermal barrier coating. (A) shows the relationship when the content of porous silica is in the range of 20 to 60%, and (B) shows the relationship when the content of porous silica is in the range of 42 to 57%.

[0019] The piston for an internal combustion engine, the thermal barrier coating, and the coating material for forming the thermal barrier coating according to the present invention will be described with reference to the drawings. Note that the present invention is not limited to the following embodiments as long as they fall within the scope of the present invention.

[0020] [Piston for Internal Combustion Engines] The piston 1 for an internal combustion engine is not particularly limited in its structural configuration. For example, as shown in FIG. 1 , the piston 1 comprises an upper top portion 11 and a lower skirt portion 12. The top surface of the top portion 11 is referred to as the top surface 2, and the back surface of the top portion 11 is referred to as the inner top surface 3. The structural configuration and dimensions of the top portion 11, including the top surface 2 and the inner top surface 3, are not limited to those shown in FIG. 1 and may be other structural configurations and dimensions. The overall shape and size of the piston are also not limited to those shown in FIG. 1 and may be other shapes and sizes. The outer peripheral surface of the top portion 11 is provided with multiple piston ring grooves. For example, a first ring groove 11a, a second ring groove 11b, and an oil ring groove 11c are formed from the top portion 11 toward the skirt portion 12, and corresponding piston rings are fitted in the respective grooves. Reference numeral 13 denotes a pin hole.

[0021] 2, a combustion chamber 21 of an internal combustion engine 20 such as an automobile engine is composed of a cylinder head 22, a bore surface 27 of a cylinder block 26, and a top surface 2 of a piston 1. The cylinder head 22 is provided with an intake valve 23, an exhaust valve 24, and an ignition plug 25. The piston 1 moves up and down via a crankshaft 28 and a connecting rod 29.

[0022] As shown in FIG. 1 , the internal combustion engine piston 1 according to the present invention has a thermal barrier coating 4 formed on all or part of the top surface 2. The thermal barrier coating 4 is characterized by including a porous material 4a composed of secondary particles of heat-resistant particles and a binder material 4b. The porous material 4a composed of secondary particles of heat-resistant particles has voids 5 therein. Therefore, the thermal barrier coating 4 containing such porous material 4a can suppress a decrease in combustion efficiency during combustion in the combustion chamber 21 of the internal combustion engine 20, which would otherwise occur if combustion heat were to be dissipated to the outside of the combustion chamber through the wall surface (e.g., bore surface 27) of the combustion chamber 21. As a result, the thermal barrier coating 4 has excellent thermal barrier properties. Furthermore, the porous material 4a composed of secondary particles contains fine voids 5 therein. However, these voids 5 are formed by discontinuous connections between primary particles, and therefore the porous material 4a has a discontinuous skeletal structure. Compared to hollow particles (hollow fillers) with a continuous skeletal structure connected by a continuous shell, this porous material 4a is less susceptible to propagation of small impacts, and even if cracks occur, they are stopped midway. As a result, it acts to alleviate the thermal stress generated during combustion and inhibit the transmission of thermal shock, resulting in excellent thermal shock resistance. Furthermore, because the thermal barrier coating 4 includes the porous material 4a with a discontinuous skeletal structure and an uneven surface, it is stronger and can maintain stable thermal barrier properties compared to conventional thermal barrier coatings containing hollow particles.

[0023] By further including the fiber material 4c in the thermal barrier coating 4, the dispersibility of the porous material 4a is improved, and the viscosity increasing effect and steric hindrance during film formation improve the strength of the thermal barrier coating 4. As a result, the imbalance in thermal shock resistance and strength within the thermal barrier coating 4 is reduced, and peeling and chipping of the thermal barrier coating 4 can be suppressed.

[0024] The components will be described in detail below. Note that a piston for an internal combustion engine will be simply referred to as a "piston."

[0025] <Piston Material> The material of the piston 1 is not particularly limited, but preferred examples include aluminum alloy materials for pistons, such as typical aluminum alloys (e.g., A4032 and AC8A). The thermal conductivity of aluminum alloys varies depending on the type. In the present invention, a thermal barrier coating 4 is provided on the top surface 2 of the piston 1 to suppress the dissipation of combustion heat. The type and thickness of the thermal barrier coating 4 can be freely selected to adjust the thermal barrier properties and heat dissipation properties. Therefore, the material of the piston 1 is not particularly limited. The piston 1 is manufactured by conventional methods, such as casting, forging, heat treatment, and machining, of such aluminum alloys for pistons. The machining is performed by conventional methods known in the art, such as drilling holes for piston pins, machining piston surfaces, machining oil ring grooves, and other processes, to finish the piston into a desired shape.

[0026] <Thermal Barrier Coating> The thermal barrier coating 4 includes a porous material 4a composed of secondary particles of heat-resistant particles and a binder material 4b. As shown in FIG. 1 , the thermal barrier coating 4 is formed on the entire or a portion of the top surface 2 of the piston 1. The top surface 2 is the surface of the top portion 11 of the piston 1, and as shown in FIGS. 1 and 2 , in an internal combustion engine 20, it is the surface located on the combustion chamber 21 side. The thermal barrier coating 4 provided on the top surface 2 includes a porous material 4a having voids 5 therein, and therefore has thermal barrier properties (also referred to as heat insulating properties). As a result, this thermal barrier property suppresses the transmission of combustion heat through the piston 1. During combustion in the combustion chamber 21, the combustion heat is dissipated to the outside of the combustion chamber through the wall surface (e.g., bore surface 27) of the combustion chamber 21, preventing an excessive drop in the combustion chamber temperature and suppressing a decrease in combustion efficiency. As a result, combustion heat can be effectively utilized to increase combustion efficiency and improve fuel economy. Furthermore, such thermal barrier properties also have the advantage of suppressing the temperature of the piston 1 from increasing, thereby suppressing cooling loss in the cylinder bore.

[0027] The thermal barrier coating 4 may be provided on a portion of the top surface 2, but is preferably provided on the entire surface as shown in Figure 1. The location of "a portion" may be any portion that effectively exhibits heat-shielding performance, and can be selected arbitrarily depending on factors such as the shape of the top surface 2 of the piston 1. By providing the thermal barrier coating 4 on the entire surface, it is possible to further suppress the transfer of combustion heat through the piston 1 and prevent the temperature of the combustion chamber 21 from dropping too much.

[0028] (Porous Material) The porous material 4a is a secondary particle of heat-resistant particles, as shown in the schematic diagram of FIG. 3 . The heat-resistant particles are primary particles, which aggregate to form secondary particles. Because the secondary particles are aggregates of primary particles, they contain fine voids 5 inside. These voids 5 form a discontinuous skeletal structure formed by discontinuous connections between primary particles. The porous material 4a, which is a secondary particle, differs from hollow particles (hollow fillers), which have a continuous skeletal structure connected by continuous shells. Therefore, compared to hollow particles, the porous material 4a does not transmit small impacts when subjected to small impacts, and even if cracks occur, they are stopped midway. Furthermore, because the secondary particles are aggregates of primary particles, the porous material 4a has an uneven surface. Because the porous material 4a, which is a secondary particle, has a discontinuous skeletal structure and an uneven surface, it is stronger and can maintain stable thermal barrier properties compared to conventional thermal barrier coatings containing hollow particles. The voids 5 here are voids surrounded by the particle walls of unconnected primary particles, which differs from the voids within hollow particles that are connected by a continuous shell. This difference leads to the above-mentioned characteristics and effects. The porous material 4a is known as a material for imparting functionality to coating films and improving durability, but is not normally used for heat insulation purposes, and is not known to be used in pistons for internal combustion engines. On the other hand, hollow particles are generally used for heat insulation purposes.

[0029] An example of the porous material 4a is porous silica, which is a secondary particle formed by agglomerating primary particles of silicon oxide. Porous silica contains the aforementioned voids 5 therein, making it a preferred porous material 4a for reducing thermal conductivity. Specifically, a preferred example of porous silica is a porous silicon dioxide structure having an average particle size in the range of 0.1 to 40 μm and voids 5 with an average pore size in the range of 2 to 10 nm. The thermal barrier properties of the thermal barrier coating 4 are achieved by the air layers contained in these voids 5. As long as the porous material 4a has the above-mentioned average pore size and average particle size ranges, materials other than porous silica may be used, and are not particularly limited, as long as they achieve the effects of the present invention. The average particle size is preferably in the range of 2 to 10 μm, so that it can be incorporated into the film.

[0030] The secondary particles of the porous material 4a are aggregates with voids 5 inside, but the binder material 4b described below has difficulty penetrating into the voids 5, allowing for effective heat insulation. The reason why the binder material 4b has difficulty penetrating into the voids 5 is not entirely clear, but it is thought that this is due to the fact that the voids 5 are extremely small and the properties of the entrances to the voids 5 (hydrophobicity, fine uneven shape, etc.). The presence of the voids 5 can be confirmed using a high-precision electron microscope.

[0031] (Binder Material) The binder material 4b is a binding material necessary to fix the porous material 4a in the thermal barrier coating 4. There are no particular limitations on the binder material 4b as long as it has properties suitable for use as a constituent material of the thermal barrier coating 4 for pistons in internal combustion engines, but a preferred example is a silicon compound. Silicon compounds are preferred because they have lower thermal conductivity than commonly used inorganic compounds and crystalline compounds, and more specifically, silicon dioxide (SiO ２ ) and silicone (R ２ SiO) and a silicon compound composite material (RSiO 1.5) (Si: silicon, O: oxygen, R: organic group). Specific examples include silicon compound composites such as silsesquioxane and polysiloxane. These silicon compound composites are materials that can withstand environments where they are exposed to high temperatures of 300°C or higher for long periods of time, and are therefore suitable for use as constituent materials for the thermal barrier coating 4 for pistons in internal combustion engines. Silicon compound composites (RSiO 1.5 ) can change its performance by changing R. For example, R=CH ３ If so, it can be preferably used as the binder material 4b constituting the present invention.

[0032] (Fiber Material) The fiber material 4c serves to improve the dispersibility of the porous material 4a. Furthermore, the fiber material 4c advantageously prevents excessive aggregation and precipitation of the porous material 4a during film formation through its thickening and steric hindrance effects, thereby improving the strength of the thermal barrier coating 4. This reduces or prevents imbalances in thermal shock resistance and strength within the thermal barrier coating, and also prevents peeling and damage to the thermal barrier coating 4. An example of the fiber material 4c is nanofiber. Nanofibers are preferred for improving the dispersion stability of the porous material 4a. Specifically, they preferably have a diameter in the range of 1 to 100 nm and a length in the range of 1 to 20,000 nm. The length of the fiber material is more preferably in the range of 2 to 10,000 nm, and even more preferably in the range of 3 to 5,000 nm, as this minimizes the impact on thermal conductivity. In the examples described below, a fiber with a length of 3 μm is used as an example. Preferred examples of nanofibers include one or more inorganic materials selected from aluminum oxide (alumina), titanium oxide (titania), potassium titanate, and barium titanate, or one or more organic materials selected from cellulose, chitin, chitosan, and silk.

[0033] (Configuration of Thermal Barrier Coating) The proportions of the constituent materials that make up the thermal barrier coating 4 may be any proportion that satisfies the desired effects of the thermal barrier coating 4 (heat insulation, thermal shock resistance, etc.), and are not particularly limited. However, it is preferable that the thermal barrier coating 4 contains at least 20 to 60% by mass of porous material 4a, with the remainder being binder material 4b, or that the thermal barrier coating 4 contains at least 20 to 60% by mass of porous material 4a and 0.05 to 5% by mass of fiber material 4c, with the remainder being binder material 4b. Such proportions may be such that the porous material 4a is porous silica, the fiber material 4c is nanofibers, and the binder material 4b is a silicon compound composite material (RSiO 1.5 ) can be exemplified as ranges in the case of, but even in cases other than these specific examples, a similar effect tends to be obtained within roughly the same range. When a constituent material that does not achieve the same effect within the same range is used, the content ratio will be outside the above range, but it is preferable to select and use a constituent material that achieves the same effect within the same range.

[0034] If the proportion (mass ratio) of the porous material 4a is within the above range, the thermal conductivity can be reduced and sufficient thermal shock resistance and strength can be easily maintained. Therefore, even when the thermal barrier coating 4 is applied to the top surface 2 of the piston 1, the thermal barrier properties can be maintained and the thermal barrier coating 4 can be stabilized. On the other hand, if the proportion of the porous material 4a is less than 20%, the proportion becomes small, which may make it difficult for the porous material 4a to fulfill its role (reducing thermal conductivity and maintaining thermal shock resistance). If the proportion of the porous material 4a exceeds 60%, the proportion becomes large and the proportion of the binder material 4b becomes relatively small, which tends to make the resulting thermal barrier coating 4 brittle. Considering the relationship between density and thermal conductivity shown in FIG. 6, the proportion of the porous material 4a is more preferably in the range of 40 to 60%, and particularly preferably in the range of 50 to 60%. In the examples described below, the mass ratio of the porous material is in the range of 42 to 56%.

[0035] If the proportion (mass ratio) of the fiber material 4c is within the above range, the dispersion stability of the porous material 4a is improved, the strength of the thermal barrier coating 4 is improved due to steric hindrance, and the effect of the porous material 4a is uniform throughout the thermal barrier coating 4. Furthermore, this range does not significantly affect the maintenance of the thermal conductivity of the thermal barrier coating 4. On the other hand, if the proportion of the fiber material 4c is less than 0.05%, the proportion is small, which can lead to insufficient dispersion of the porous material 4a and the insufficient uniformity of the effect of the porous material 4a throughout the thermal barrier coating 4. Because the fiber material 4c has a high thermal conductivity, a high content can increase the thermal conductivity of the thermal barrier coating 4, so the upper limit is preferably 5%. If the content exceeds 5%, the thermal conductivity may become too high. The mass ratio of the fiber material 4c is preferably 0.1 to 5%, with a range of 0.2 to 3% being more preferable from the standpoint of thermal conductivity and dispersibility, and a range of 0.3 to 2% being particularly preferable.

[0036] The binder material 4b constitutes the thermal barrier coating 4 as the balance of the porous material 4a and the optional fiber material 4c, but may be slightly reduced relatively when small amounts of other additives, as described below, are included. If the binder material 4b is not included in a certain proportion, the resulting thermal barrier coating 4 will be brittle. Therefore, although it depends on the content of the porous material 4a, it is preferable that the binder material 4b be included in an amount of at least about 40% by mass. The upper limit of the binder material 4b will also be affected by the proportions of the porous material 4a and the optional fiber material 4c, but can be set to, for example, about 80%.

[0037] The thermal barrier coating 4 thus obtained preferably has a thermal conductivity, as a common physical property, in the range of 0.2 W / m·K or less. A thermal barrier coating 4 having such a thermal conductivity can achieve the desired effects of the present invention.

[0038] The thickness of the thermal barrier coating 4 is not particularly limited as long as it is within a range that achieves the effects of the present invention, but is preferably within a range of 50 to 1000 μm, for example. A thermal barrier coating 4 within this thickness range may be provided over the entire top surface 2, or over a portion of it, or may be provided with thicknesses that vary within this range. Note that if the thickness of the thermal barrier coating 4 exceeds 1000 μm, it may be too thick and may peel off, and if it is less than 50 μm, it may be too thin and not provide sufficient thermal insulation.

[0039] The thermal barrier coating 4 can be formed by applying a coating material for forming a thermal barrier coating. As described above, the coating material for forming a thermal barrier coating can be a coating material containing at least 20% to 60% by mass of porous material 4a, with the remainder being binder material 4b, or a coating material containing at least 20% to 60% by mass of porous material 4a and 0.05% to 3% by mass of fiber material 4c, with the remainder being binder material 4b. The term "at least" means that the coating material for forming a thermal barrier coating may also contain additives and solvents. The coating material for forming a thermal barrier coating may optionally contain additives and solvents such as wetting and dispersing agents, anti-settling agents, leveling agents, and solvents. The inclusion of these additives as necessary can improve the stability and coatability of the paint. In the examples described below, butyl cellosolve and ethanol are used as solvents, but these are not limiting.

[0040] The thermal barrier coating 4 can be applied directly to the top surface 2 of the piston 1. The thermal barrier coating 4 of the present invention can ensure sufficient adhesion without the need for a conventional adhesion-improving film between the top surface 2 and the thermal barrier coating 4, and can therefore maintain stable thermal barrier properties (thermal insulation) without being affected by such an adhesion-improving film. Furthermore, the thermal barrier coating 4 only needs to be applied to the top surface 2, and may or may not be applied to the inner top surface 3. Furthermore, the thermal barrier coating 4 may or may not be subjected to post-processing such as polishing or cutting. The surface roughness may be utilized without post-processing, or the surface roughness may be reduced by post-processing to contribute to improved combustion performance, etc.

[0041] The present invention will be specifically explained with reference to examples.

[0042] [Investigation of density and proportion of porous material in thermal barrier coating] (Coating material for thermal barrier coating) First, the relationship between the proportion of porous material 4a contained in the thermal barrier coating 4 and the density was investigated, and the results are shown in Figure 6(A). The numbers in Figure 6(A) represent samples 1 to 4. The coating material for thermal barrier coating used to form the thermal barrier coating 4 is as follows:

[0043] As the porous material 4a, a porous silicon dioxide structure (silicon compound composite material: RSiO) having pores with a pore diameter range of 2 to 10 nm and a particle diameter range of 0.1 to 40 μm is used. 1.5 ) was used. This porous material 4a is a secondary particle obtained by agglomerating primary particles of silicon oxide particles with a particle size range of 2 to 60 μm. The pore size of the porous material 4a was measured by a gas adsorption method. The particle sizes of the silicon oxide particles (primary particles) and the porous material 4a (secondary particles) were calculated from images obtained with a scanning electron microscope. The fact that the porous material 4a is a porous silicon dioxide structure can be quantified with an elemental analyzer and identified with an X-ray diffractometer.

[0044] The content of porous material 4a after forming the thermal barrier coating 4 was 52 mass% in sample 1, 45 mass% in sample 2, 23 mass% in sample 3, and 20 mass% in sample 4.

[0045] Nanofibers with an average diameter of 4 nm and an average length of 3,000 nm were used as fiber material 4c. The content of fiber material 4c after forming into thermal barrier coating 4 was 0.5 mass% in all of Samples 1 to 4.

[0046] Silsesquioxane was used as binder material 4b. The content of binder material 4b after forming the thermal barrier coating 4 was 47 mass% for sample 1, 54 mass% for sample 2, 76 mass% for sample 3, and 79 mass% for sample 4.

[0047] The coating material containing the porous material 4a, binder material 4b, and fiber material 4c is contained in an aqueous solution containing an alcohol solvent.

[0048] (Deposition of thermal barrier coating) Using the prepared coating material for thermal barrier coating, a thermal barrier coating 4 having a thickness of 55 μm was deposited on a test piece (length 76 mm, width 52 mm, thickness 1.2 to 1.5 mm) of aluminum alloy material for piston (equivalent to AC8A).

[0049] (Density and Thermal Conductivity of Thermal Barrier Coating) Figure 6(A) shows the relationship when the content of the porous material 4a is in the range of 20 to 60%. As shown in Figure 6(A), it can be seen that the density of the thermal barrier coating 4 decreases as the content of the porous material 4a in the thermal barrier coating 4 increases. These results show that by increasing the filling amount of the porous material 4a containing the voids 5, it is possible to reduce the density and thereby achieve the effect of reducing the thermal conductivity.

[0050] Figure 6(B) shows the relationship obtained for samples in which the type of porous material 4a was changed to the following porous materials A to D and the content ratio was changed to a range of 42 to 57%. The results shown for the nanofiber content ratio are 2% by mass, which is within the appropriate range, and 5% by mass, which is excessive. "Filler" in Figure 6(B) refers to porous silica.

[0051] As the porous material A, a porous silicon dioxide structure (silicon compound composite material: RSiO) having an average pore diameter of 7.7 nm and an average particle diameter of 4 μm was used. 1.5 As the porous material B, a porous silicon dioxide structure (silicon compound composite material: RSiO) having pores with an average pore diameter of 3.4 nm and a particle size range of 0.1 to 40 μm was used. 1.5 As the porous material C, a porous silicon dioxide structure (silicon compound composite material: RSiO) having pores with a diameter of 2 to 10 nm and a particle size of 0.1 to 40 μm was used. 1.5 As the porous material D, a porous silicon dioxide structure (silicon compound composite material: RSiO) having pores with an average pore diameter of 9.7 nm and a particle size range of 0.1 to 40 μm was used. 1.5 ) was used.

[0052] From the results in Figure 6(B), the white areas containing an excess of nanofibers have a powdery surface, which is presumably due to the influence of too many nanofibers. Porous material B was able to increase the filling rate the most and also had the lowest density.

[0053] (Reliability Test) Using a sample containing porous material A and a sample containing porous material B shown in Figure 6(B), their reliability was evaluated by an adhesion test (JIS K5600 cross-cut test) and a thermal shock test (a test in which 10 minutes at 350°C and 1 minute of water immersion at 25°C were repeated five times). The results are shown in Table 1. The results in Table 1 show that both samples had excellent adhesion and thermal shock resistance.

[0054]

[0055] REFERENCE SIGNS LIST 1 internal combustion engine piston 2 top surface 3 inner top surface 4 thermal barrier coating 4a porous material 4b binder material 4c fibrous material 5 gap 11 top portion 11a first ring groove 11b second ring groove 11c oil ring groove 12 skirt portion 13 pin hole 20 internal combustion engine 21 combustion chamber 22 cylinder head 23 intake valve 24 exhaust valve 25 spark plug 26 cylinder block 27 bore surface 28 crankshaft 29 connecting rod

Claims

1. A piston for an internal combustion engine, characterized in that a heat-shielding coating containing a porous material made of secondary particles of heat-resistant particles and a binder material is formed on the entire or part of the top surface.

2. The piston for an internal combustion engine according to claim 1, wherein the heat-resistant particles are porous silica having an average particle size in the range of 0.1 to 40 μm.

3. The binder material is silicon dioxide (SiO ２ ) and silicone (R ２ SiO) and silicon compound composite material (RSiO 1.5 3. The piston for an internal combustion engine according to claim 1 or 2, wherein 4. A piston for an internal combustion engine according to claim 1 or 2, further comprising a fiber material.

5. A piston for an internal combustion engine according to claim 4, wherein the fiber material is made of one or more inorganic materials selected from aluminum oxide, titanium oxide, potassium titanate, and barium titanate, or one or more organic materials selected from cellulose, chitin, chitosan, and silk.

6. A thermal barrier coating used to form the entire or part of the top surface of a piston for an internal combustion engine, the thermal barrier coating comprising a porous material made of secondary particles of heat-resistant particles and a binder material.

7. A coating material for forming a thermal barrier coating, characterized in that it is a coating material for forming a thermal barrier coating according to claim 6.

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