Method for forming heat shield layer for engine

Abstract

To reduce internal stress of a heat shield layer while forming the stable heat shield layer by sufficiently hardening a material coat.SOLUTION: A heat shield layer formation method for an engine includes: an application process of applying a material coat including silicone-based resin to a heat shield target surface; and a burning process of forming a heat shield layer by heating and hardening the material coat applied. In the burning process, a heating temperature relative to the material coat is increased / decreased periodically, and a change rate (Rb) when the heating temperature is lowered is set smaller than a change rate (Ra) when the heating temperature is raised.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to a method for forming a heat barrier layer on a surface of an engine that is to be heat-shielded. [Background technology]

[0002] It has been proposed to cover the wall surfaces of engine parts with a heat-shielding layer for the purpose of improving the thermal efficiency of the engine, etc. For example, the following Patent Document 1 discloses forming a heat-shielding layer, in which hollow particles and nanoparticles are added to a silicone-based resin binder, on the wall surfaces of the combustion chamber, including the piston crown surface.

[0003] Specifically, Patent Document 1 discloses an example of forming a thermal barrier layer in the following procedure. First, a resin solution containing a silicone resin to which the hollow particles and nanoparticles have been added (hereinafter, referred to as a raw material coating) is prepared as a raw material for the thermal barrier layer, and the raw material coating is applied to the piston crown surface. Next, the applied raw material coating is heated, and the raw material coating hardened by heating is formed as a thermal barrier layer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2020 / 213637 Summary of the Invention [Problem to be solved by the invention]

[0005] In the above-mentioned method for forming a thermal barrier layer, in order to sufficiently harden the raw material coating to obtain a stable thermal barrier layer, it is desirable to input sufficient thermal energy when heating the raw material coating. However, according to the research of the present inventor, if the heating temperature is excessively increased in order to stabilize the thermal barrier layer, the internal stress of the thermal barrier layer may increase beyond the allowable range. One of the reasons for this is thought to be that the crosslinking reaction proceeds under conditions of very vigorous molecular motion of the silicone molecules in the raw material coating. The increase in internal stress actually weakens the adhesion between the thermal barrier layer and the surface to be thermally shielded, so measures to avoid this are necessary.

[0006] The present invention has been made in consideration of the above-mentioned circumstances, and has an object to provide a method for forming a thermal barrier layer for an engine, which is capable of sufficiently hardening a raw material coating to form a stable thermal barrier layer while reducing internal stress in the thermal barrier layer. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, the present invention provides a method for forming a heat shielding layer on a surface of an engine to be heat shielded, comprising: a coating step of applying a raw material coating containing a silicone-based resin to the surface to be heat shielded; and a baking step of forming the heat shielding layer by heating and hardening the applied raw material coating, wherein in the baking step, the heating temperature of the raw material coating is periodically increased and decreased, and the rate of change when the heating temperature is decreasing is made smaller than the rate of change when the heating temperature is increasing (Claim 1).

[0008] According to the present invention, when the raw material coating applied to the surface to be thermally shielded is heated, the heating temperature is periodically increased and decreased, so that the maximum heating temperature can be increased while maintaining the same thermal energy compared to when the heating temperature is constant. This promotes the crosslinking reaction of the silicone resin contained in the raw material coating, so that the curing rate of the raw material coating can be increased, and the hardness or strength of the thermal barrier layer made of the cured raw material coating can be improved.

[0009] However, if the rate of change when increasing or decreasing the heating temperature is constant, the internal stress of the thermal barrier layer may increase significantly. That is, as described above, the cyclic change of the heating temperature leads to an increase in the maximum value of the heating temperature, and in turn promotes the crosslinking reaction in the raw material coating. However, if the rate of change of temperature is the same when the heating temperature is increased and decreased, the period for gradually decreasing the heating temperature once increased to the maximum value is not sufficiently secured, and the temperature increase of the raw material coating is repeated frequently, which may result in an excessive increase in the internal temperature of the raw material coating. While such an excessive increase in temperature brings about the advantage of increasing the hardening rate of the raw material coating, it may also bring about the disadvantage of increasing the residual stress, i.e., the internal stress, generated inside the thermal barrier layer made of the hardened raw material coating. That is, if the temperature of the raw material coating increases excessively, the crosslinking reaction will proceed under conditions of very intense molecular motion, which may increase the internal stress of the thermal barrier layer.

[0010] In contrast, in the present invention, the rate of change of the heating temperature when it is decreasing is made smaller than the rate of change when it is increasing, so that the increase in internal stress due to the above-mentioned circumstances can be suppressed. In other words, due to the difference in the rate of change, the period during which the heating temperature decreases toward the minimum value is ensured to be longer than the period during which the heating temperature increases toward the maximum value, so that an excessive increase in the internal temperature of the raw material coating can be suppressed. As a result, while obtaining the above-mentioned effect of the cyclic change in the heating temperature (the effect of promoting the hardening of the raw material coating), the internal stress of the thermal barrier layer can be effectively reduced, and the adhesive strength between the thermal barrier layer and the surface to be thermally shielded can be increased.

[0011] Preferably, the method for forming a thermal barrier layer further includes a step of performing a surface modification treatment on the surface to be thermally shielded before the coating step (claim 2).

[0012] In this embodiment, a raw material coating is applied to the surface to be heat-shielded that has been modified by a surface modification treatment, and then fired, so that the heat shield layer made of the hardened raw material coating can be firmly adhered to the surface to be heat-shielded, thereby improving the stability of the heat shield layer.

[0013] The surface to be heat-shielded may be one surface of an aluminum alloy part. In this case, it is preferable that the surface modification treatment is anodized (claim 3).

[0014] In this embodiment, an oxide coating is formed on the surface to be heat-shielded by the anodizing treatment, and then, by heating the raw material coating that is applied to the surface to be heat-shielded, the organic groups of the silicone resin contained in the raw material coating bond with the OH groups on the surface of the oxide coating. This allows the heat shield layer made of the cured raw material coating to adhere firmly to the surface to be heat-shielded, improving the stability of the heat shield layer.

[0015] Preferably, the method for forming a thermal barrier layer further includes a step of performing a surface roughening treatment on the surface to be thermally shielded before the coating step (claim 4).

[0016] In this embodiment, the roughening treatment creates fine irregularities on the surface to be heat-shielded (increasing the surface roughness), and the raw material coating is fired in a state in which the raw material coating penetrates into the fine irregularities. As a result, the adhesion between the heat shield layer and the surface to be heat-shielded can be increased, and the stability of the heat shield layer can be improved.

[0017] Preferably, the surface to be heat-shielded is a wall surface defining a combustion chamber of the engine (claim 5).

[0018] In this embodiment, the thermal barrier layer prevents the thermal energy of the mixture burned in the combustion chamber from being released to the outside through the wall surface of the combustion chamber, thereby reducing the loss caused by the release of the thermal energy, i.e., the cooling loss, and thus effectively improving the fuel efficiency of the engine.

[0019] Preferably, in the coating step, a resin solution of a silicone resin containing hollow particles and nanoparticles is applied to the surface to be heat-shielded as the raw material coating (claim 6).

[0020] In this embodiment, the applied raw material coating is baked in a baking process to form a heat barrier layer containing a silicone-based resin binder, hollow particles, and nanoparticles. In this case, the molecular motion of the resin binder is suppressed by the nanoparticles, so that thermal deterioration of the resin binder can be suppressed and the heat resistance of the heat barrier layer can be improved. In addition, since the heat barrier layer contains hollow particles with a hollow interior, the thermal conductivity of the heat barrier layer can be further reduced and the heat barrier performance of the heat barrier layer can be improved. Effect of the Invention

[0021] As described above, according to the method of forming a thermal barrier layer for an engine of the present invention, it is possible to sufficiently harden the raw material coating to form a stable thermal barrier layer, while reducing the internal stress of the thermal barrier layer. [Brief description of the drawings]

[0022] [Figure 1] 1 is a cross-sectional view of an engine to which a thermal barrier layer forming method according to an embodiment of the present invention is applied. [Diagram 2] FIG. 2 is a cross-sectional view showing an upper structure of a piston of the engine. [Diagram 3] FIG. 2 is an enlarged cross-sectional view showing an internal structure of a heat shield layer formed on a crown surface of the piston. [Figure 4] 4 is a flowchart showing a procedure for forming the thermal barrier layer. [Diagram 5] FIG. 4 is a cross-sectional view showing a state in which a raw material coating, which is a raw material of the thermal barrier layer, is applied to a piston crown surface. [Figure 6] 4 is a graph showing the change over time in heating temperature when firing the raw material film. [Figure 7] 5 is a graph showing how the hardening of the raw material coating progresses due to heating, where (a) shows the change in heating temperature over time, and (b) shows the change in hardening rate over time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] [Overall engine configuration] Fig. 1 is a cross-sectional view of an engine 1 to which a thermal barrier layer forming method according to one embodiment of the present invention is applied. The engine 1 shown in this figure is a four-stroke gasoline direct injection engine mounted on a vehicle as a power source for running, and includes a cylinder block 3, a cylinder head 4, and a piston 5. Note that in this embodiment, the side from the cylinder block 3 toward the cylinder head 4 is treated as the top and the opposite is treated as the bottom, but this is for convenience of explanation and is not intended to limit the installation posture of the engine.

[0024] The cylinder block 3 is a hollow member having cylindrical cylinders 2 therein. The cylinder head 4 is a member that closes the cylinders 2 from above, and is fixed onto the cylinder block 3 via a gasket or the like (not shown). The pistons 5 are circular members with an outer diameter slightly smaller than the diameter (bore diameter) of the cylinders 2, and are accommodated inside the cylinders 2 so as to be capable of reciprocating motion. The engine 1 of this embodiment mounted on a vehicle is typically a multi-cylinder engine having a plurality of cylinders 2. In this case, the engine 1 has the same number of pistons 5 as the cylinders 2, but there is no particular difference in the configuration of each piston 5 and each cylinder 2, and the following description is common to all pistons 5 and cylinders 2.

[0025] A combustion chamber 6 is formed above the piston 5. The combustion chamber 6 is a space defined by the lower surface of the cylinder head 4, the side peripheral surface (cylinder liner) of the cylinder 2, and a crown surface 15 (upper surface) of the piston 5.

[0026] An injector 7 that injects fuel (fuel containing gasoline) into the combustion chamber 6 is attached to the cylinder head 4. A nozzle hole is formed at the tip of the injector 7, and the fuel injected into the combustion chamber 6 through the nozzle hole is mixed with air to form an air-fuel mixture. Although not shown, an ignition plug that ignites the air-fuel mixture in the combustion chamber 6 is attached to the cylinder head 4. When the air-fuel mixture is burned in response to ignition by this spark plug, the piston 5 reciprocates up and down due to the expansion force caused by the combustion. The reciprocating motion of the piston 5 is transmitted to the output shaft (crankshaft) of the engine 1 via a crank mechanism (not shown), causing the output shaft to rotate.

[0027] The cylinder head 4 has an intake port 8 and an exhaust port 9 that communicate with the combustion chamber 6. The intake port 8 is a port for guiding intake air introduced from an intake passage not shown in the figure to the combustion chamber 6. The exhaust port 9 is a port for guiding burnt gas (exhaust gas) generated in the combustion chamber 6 to an exhaust passage not shown in the figure. An intake valve 10 that opens and closes the opening of the intake port 8 on the combustion chamber 6 side, and an exhaust valve 11 that opens and closes the opening of the exhaust port 9 on the combustion chamber 6 side are assembled to the cylinder head 4. The bottom surfaces of the umbrella parts of the intake valve 10 and the exhaust valve 11, together with the lower surface of the cylinder head 4, define the ceiling surface of the combustion chamber 6.

[0028] [Piston structure] 2 is a cross-sectional view showing the upper structure of the piston 5. As shown in this figure, a cavity C is formed in a region including the center of the piston crown surface 15 (hereinafter also referred to as the piston crown surface 15). The cavity C is a recess formed by partially recessing the piston crown surface 15 downward (the side opposite to the cylinder head 4), and is formed at a position facing the tip of the injector 7 (FIG. 1). In other words, the piston crown surface 15 has a cavity surface 15a that defines the cavity C, and a flat base surface 15b formed on the outside of the cavity surface 15a.

[0029] A heat shield layer 20 is formed on the piston crown surface 15. The heat shield layer 20 is a thin layer having a constant thickness, and is formed so as to continuously cover both the cavity surface 15a and the base surface 15b. The thickness of the heat shield layer 20 is preferably 20 μm or more and 150 μm or less, and more preferably 40 μm or more and 100 μm or less. The piston crown surface 15 on which the heat shield layer 20 is formed corresponds to the "surface to be heat shielded" in the present invention.

[0030] The heat shield layer 20 is made of a material having a thermal conductivity significantly lower than that of the metal material (base material) constituting the piston 5. Specifically, in this embodiment, the piston 5 is made of an aluminum alloy, and the heat shield layer 20 is made of a material containing a silicone-based resin. As will be described in detail later, the heat shield layer 20 is fixed to the piston crown surface 15 by baking a paste-like raw material (raw material coating 20A) applied to the piston crown surface 15.

[0031] 3 is an enlarged cross-sectional view showing the internal structure of the thermal barrier layer 20. As shown in this figure, the thermal barrier layer 20 contains a resin binder 21, and hollow particles 22 and nanoparticles 23 held inside the resin binder 21.

[0032] The resin binder 21 is a resin material constituting the base material (matrix) of the heat shield layer 20, and is mainly composed of a heat-resistant silicone-based resin. As the silicone-based resin, a silicone resin composed of a three-dimensional polymer with a high degree of branching, such as methyl silicone resin or methyl phenyl silicone resin, is preferable. Among them, polyalkylphenyl siloxane (polyalkylphenyl silicone) is preferable.

[0033] The hollow particles 22 are inorganic particles having a cavity inside. As the hollow particles 22, it is preferable to adopt ceramic hollow particles containing at least one of an Si-based oxide component (e.g., silica) and an Al-based oxide component (e.g., alumina). For example, glass balloons, glass bubbles, fly ash balloons, shirasu balloons, silica balloons, and aluminosilicate balloons are suitable. The particle size of the hollow particles 22 is sufficiently smaller than the film thickness (20 to 150 μm) of the thermal barrier layer 20. Specifically, the average particle size of the hollow particles 22 is preferably 30 μm or less, more preferably 10 μm or less. In addition, the hollow ratio of the hollow particles 22 is preferably 60 vol. % or more, more preferably 70 vol. % or more.

[0034] As the nanoparticles 23, for example, at least one of inorganic nanoparticles made of inorganic compounds such as zirconia, alumina, silica, and titania, and metal nanoparticles made of metals such as Ti, Zr, and Al can be adopted. Among them, silica nanoparticles whose surfaces are modified with phenyl groups are preferable. The particle size of the nanoparticles 23 is sufficiently smaller than the particle size of the hollow particles 22 (average 30 μm or less). Specifically, the average particle size of the nanoparticles 23 is preferably 500 nm or less, more preferably 1 nm or more and 200 nm or less, and even more preferably 1 nm or more and 120 nm or less. The nanoparticles 23 may be hollow or solid.

[0035] The amount of hollow particles 22 is adjusted so that the proportion of hollow particles 22 in the entire heat shield layer 20 (total amount of resin binder 21, hollow particles 22, and nanoparticles 23) is 30 volume % or more and 60 volume % or less. In addition, the amount of nanoparticles 23 is adjusted so that the proportion of nanoparticles in the total amount of resin binder 21 and nanoparticles 23 is 10 volume % or more and 55 volume % or less.

[0036] Forming the heat shield layer 20 configured as described above on the piston crown surface 15 leads to a reduction in cooling loss and, in turn, an improvement in the fuel economy of the engine. That is, the heat shield layer 20 acts to suppress the thermal energy of the mixture burned in the combustion chamber 6 from being released to the outside through the piston crown surface 15, thereby reducing the loss caused by the release of the thermal energy, i.e., the cooling loss. This is expected to improve the thermal efficiency, which is the rate at which thermal energy is converted into work, in other words, to improve the fuel economy of the engine.

[0037] [Method of forming heat shield layer] Next, a method for forming the heat shield layer 20 on the piston crown surface 15 will be described in detail. The heat shield layer 20 is formed by sequentially going through steps S1 to S4 shown in Fig. 4. Note that, as a prerequisite for carrying out the method shown in Fig. 4, it is assumed that the molding and heat treatment of the piston 5 have already been completed.

[0038] 4 is started, first, a roughening step S1 is carried out to roughen the piston crown surface 15. Specifically, in this embodiment, as a process for roughening the piston crown surface 15 (roughening process), a blasting process is carried out in which minute projection materials are collided against the piston crown surface 15. As a result, dirt, oil, and the like attached to the piston crown surface 15 are removed, and minute irregularities are formed on the piston crown surface 15, increasing the surface roughness of the piston crown surface 15.

[0039] After the roughening step S1, a surface modification step S2 is carried out to modify the surface of the piston crown surface 15. Specifically, in this embodiment, as a treatment (surface modification treatment) for modifying the surface of the piston crown surface 15, an anodizing treatment (anodic oxidation treatment) is carried out by passing an electric current through the piston 5 in an electrolyte. As a result, the piston crown surface 15 is forcibly oxidized and an oxide film is formed on the piston crown surface 15.

[0040] After the surface modification step S2, a coating step S3 is performed in which the raw material coating 20A shown in FIG. 5 is applied to the piston crown surface 15. The raw material coating 20A is a paste-like coating prepared as a raw material for the thermal barrier layer 20 described above. Specifically, the raw material coating 20A includes a resin solution in which a reactive silicone resin, which is a raw material for the resin binder 21, is diluted with a solvent such as toluene or xylene, and hollow particles 22 and nanoparticles 23 are added to the resin solution. When preparing such a raw material coating 20A, the amount of solvent added is adjusted so that the viscosity of the raw material coating 20A is suitable for application to the piston crown surface 15 (application using a spray gun G described later). In addition, after the hollow particles 22 and nanoparticles 23 are added to the resin solution, the resin solution is sufficiently stirred until the hollow particles 22 and nanoparticles 23 are uniformly dispersed.

[0041] 5, a spray gun G is used to apply the raw material coating 20A to the piston crown surface 15. That is, the raw material coating 20A is sprayed from the spray gun G so as to be evenly sprayed onto the piston crown surface 15 (the cavity surface 15a and the base surface 15b). As a result, a layer of the raw material coating 20A having a substantially uniform thickness is formed on the piston crown surface 15.

[0042] After the application step S3, a baking step S4 is carried out in which the raw material coating 20A is heated and cured. Specifically, in the baking step S4, the piston 5 with the raw material coating 20A applied to the crown surface 15 is placed in an electric furnace, and the raw material coating 20A is heated by heat input from a heater provided in the electric furnace. Through this heating process, the temperature of the raw material coating 20A increases to a temperature at which crosslinking of the reactive silicone resin contained in the raw material coating 20A progresses. As a result, the paste-like raw material coating 20A is cured, and the cured raw material coating 20A is formed on the piston crown surface 15 as the thermal barrier layer 20.

[0043] Fig. 6 is a graph showing the change over time in the heating temperature in the firing step S4. The heating temperature here refers to the heat input temperature from the heat source (heater) that heats the raw material coating 20A, and is equivalent to the internal temperature of the electric furnace. As shown in this figure, in the firing step S4, a process is performed in which the raw material coating 20A is heated while the heating temperature is periodically increased and decreased. Such periodic heating is achieved by periodically increasing and decreasing the amount of electricity supplied to the heater in the electric furnace.

[0044] In Fig. 6, the time when heating starts is t1, and the time when heating ends is t2. The heating temperature rises significantly immediately after heating start time t1, and drops significantly immediately before heating end time t2. The above-mentioned periodic increase and decrease in the heating temperature is continuously performed over the actual heating period P, which is a period excluding these periods (i.e., the initial and final periods of the heating period). That is, in the firing step S4, the heater in the electric furnace is controlled so that the heating temperature periodically increases and decreases within a relatively high range over the actual heating period P, which constitutes the majority of the heating period.

[0045] Specifically, the heating temperature during the actual heating period P is controlled to increase and decrease with a constant amplitude A and cycle B. In other words, if the central temperature of the temperature increase and decrease is the reference temperature Tx, the heating temperature during the actual heating period P is repeatedly increased and decreased between a maximum value (Tx+A / 2) obtained by adding half the amplitude A to the reference temperature Tx, and a minimum value (Tx-A / 2) obtained by subtracting half the amplitude A from the reference temperature Tx. In addition, the speed at which the heating temperature is increased and decreased is set so that one cycle of temperature change (i.e., the change from the reference temperature Tx through the maximum and minimum values ​​and back to the reference temperature Tx) is completed within a constant cycle B.

[0046] In addition, the heating temperature during the actual heating period P is controlled so that the temperature change rate is different between when the temperature rises and when it falls. Specifically, assuming that the temperature change amount per unit time when the heating temperature rises is the rising change rate Ra, and the temperature change amount (absolute value) per unit time when the heating temperature falls is the falling change rate Rb, in this embodiment, the heating temperature is controlled so that the falling change rate Rb is smaller than the rising change rate Ra. In order to realize such a difference in change rate, in this embodiment, the time Δtb required for the heating temperature to fall from the maximum value (Tx+A / 2) to the minimum value (Tx-A / 2) is made longer than the time Δta required for the heating temperature to rise from the minimum value (Tx-A / 2) to the maximum value (Tx+A / 2). As a result, the falling change rate Rb defined by the value (=A / Δtb) obtained by dividing the amplitude A by Δtb is made smaller than the rising change rate Ra defined by the value (=A / Δta) obtained by dividing the amplitude A by Δta.

[0047] In the baking step S4, the raw material coating 20A is heated at a heating temperature that is increased or decreased in the manner described above, whereby a cross-linking reaction occurs in which silicone molecules contained in the raw material coating 20A bond with each other, and the raw material coating 20A is hardened. The hardened raw material coating 20A is formed as the thermal barrier layer 20 having a desired hardness (or strength) and is fixed to the piston crown surface 15.

[0048] The specific temperature conditions in the firing step S4 are as follows: The central temperature when the heating temperature is cyclically changed during the actual heating period P, i.e., the reference temperature Tx, is preferably set to 100 to 140°C. The heating temperature period B is preferably set to 4 to 5 min. The heating temperature amplitude A is preferably set to 30 to 50°C. The heating temperature rising rate of change Ra is preferably set to 40 to 60°C / min, and the heating temperature falling rate of change Rb is preferably set to 1 / 6 to 1 / 4 of the rising rate of change Ra. The rising actual heating period P is preferably set to 40 to 60 min.

[0049] [Effects] As described above, in this embodiment, the thermal barrier layer 20 is formed on the piston crown surface 15 by a method including a coating step S3 in which a paste-like raw material coating 20A containing a silicone resin is applied to the piston crown surface 15, and a baking step S4 in which the applied raw material coating 20A is heated and cured. In the baking step S4, the heating temperature is controlled so that the rate of change when the heating temperature is decreased (decrease in change rate Rb) is smaller than the rate of change when the heating temperature is increased (increase in change rate Ra). This configuration has the advantage that the raw material coating 20A can be sufficiently cured to form a stable thermal barrier layer 20, while the internal stress of the thermal barrier layer 20 can be reduced.

[0050] As a result of intensive research, the inventors of the present application have found that, under the same conditions of thermal energy input by heating the raw material coating 20A, the curing rate is easier to increase by periodically changing the heating temperature than by keeping the heating temperature constant. The curing rate here is an index showing the extent to which the cross-linking reaction of the raw material coating 20A has progressed, and the higher the curing rate, the more the cross-linking reaction has progressed. The reason why the curing rate is higher when the heating temperature is periodically changed is believed to be due to the difference in the maximum value of the heating temperature. In other words, the maximum value of the heating temperature is higher when the heating temperature is periodically changed than when the heating temperature is constant, and this is believed to promote the curing (cross-linking reaction) of the raw material coating 20A. In this embodiment, the heating temperature is periodically increased and decreased in the baking step S4, so that the curing rate of the raw material coating 20A can be increased, and the hardness or strength of the thermal barrier layer 20 made of the raw material coating 20A after curing can be improved.

[0051] FIG. 7 is a graph showing how the hardening of the raw material coating 20A progresses due to heating, where (a) shows the time change in heating temperature, and (b) shows the time change in hardening ratio. The hardening ratio in graph (b) is an estimated value estimated by analysis using a thermal analyzer. In each of graphs (a) and (b), the solid line waveform labeled "this pattern" represents the heating temperature and hardening ratio when the raw material coating 20A is heated by the method of the embodiment in which the heating temperature is periodically changed, and the dashed line waveform labeled "comparison pattern" represents the heating temperature and hardening ratio when the raw material coating 20A is heated by the method in which the heating temperature is constant. As shown in graph (a), the heating temperature in this pattern increases and decreases periodically over the actual heating period P, whereas the heating temperature in the comparison pattern is maintained at a constant value over the actual heating period P. However, the thermal energy (temperature x time) applied by the heating method of this pattern is the same as the thermal energy applied by the heating method of the comparison pattern. In other words, the heating temperature (constant value) during the actual heating period P set in the comparison pattern is the same as the reference temperature Tx shown in FIG.

[0052] As shown in FIG. 7, in the comparative pattern in which the heating temperature is constant, the hardening ratio of the raw material coating 20A increases with time from the heating start time t1, but the increase in the hardening ratio substantially stops before the heating end time t2. That is, in the comparative pattern, the hardening ratio increases to a value substantially equal to the hardening ratio Y0 at the heating end time t2 before the heating end time t2, and thereafter, the hardening ratio hardly increases. In contrast, in the present pattern in which the heating temperature is periodically changed, although the rate of increase in the hardening ratio at the beginning of heating is not particularly different from that of the comparative pattern, the hardening ratio continues to increase thereafter, and the final hardening ratio (at the heating end time t2) reaches Y1, which is greater than Y0. This suggests the superiority of the heating method according to this embodiment. That is, according to this embodiment in which the heating temperature is periodically changed, the hardening ratio of the raw material coating 20A can be increased while maintaining the same thermal energy, and a more stable thermal barrier layer 20 can be formed.

[0053] However, if the rate of change when increasing or decreasing the heating temperature is constant, the internal stress of the thermal barrier layer 20 may increase significantly. That is, as described above, the cyclic change of the heating temperature leads to an increase in the maximum value of the heating temperature, and in turn promotes the crosslinking reaction in the raw material coating 20A. However, if the rate of change of temperature is the same when the heating temperature is increased and decreased, the period for gradually decreasing the heating temperature once increased to the maximum value is not sufficiently secured, and the temperature increase of the raw material coating 20A is repeated frequently, and as a result, the internal temperature of the raw material coating 20A may increase excessively. While such an excessive temperature increase brings the advantage of increasing the curing rate of the raw material coating 20A, it may also bring the disadvantage of increasing the residual stress, i.e., the internal stress, generated inside the thermal barrier layer 20 made of the raw material coating 20A after curing. That is, if the temperature of the raw material coating 20A increases excessively, the crosslinking reaction proceeds in a state where the molecular motion is very intense, which may increase the internal stress of the thermal barrier layer 20.

[0054] In contrast, in the present embodiment, the rate of change Rb during temperature decrease, which is the rate of change when the heating temperature decreases, is made smaller than the rate of change Ra during temperature increase, which is the rate of change when the heating temperature increases. Therefore, an increase in internal stress due to the above circumstances can be suppressed.

[0055] Specifically, the fact that the rate of change Rb during temperature decrease is smaller than the rate of change Ra during temperature increase means that the period during which the heating temperature is decreasing toward the minimum value (hereinafter referred to as the temperature decrease period) is ensured to be longer than the period during which the heating temperature is increasing toward the maximum value (hereinafter referred to as the temperature increase period). Here, even if the heating temperature reaches the maximum value and then turns to decrease, the temperature of the raw material coating 20A does not immediately decrease from that point. There is a certain time lag between the decrease in the heating temperature and the decrease in the temperature of the raw material coating 20A. For this reason, if the temperature increase period and the temperature decrease period are the same, there is a risk that the heating temperature will turn to increase again before the temperature of the raw material coating 20A has sufficiently decreased, which may excessively increase the raw material coating 20A. In contrast, in the present embodiment where the rate of change of the heating temperature is set so that the temperature decrease period is longer than the temperature increase period (that is, Rb < Ra), the above situation where the temperature increases again without sufficiently decreasing can be avoided, and an excessive increase in the internal temperature of the raw material coating 20A can be suppressed. As a result, while enjoying the above-described effect (the effect of promoting the curing of the raw material coating 20A) due to the periodic change in the heating temperature, the internal stress of the heat insulating layer 20 can be effectively reduced, and the adhesion strength between the heat insulating layer 20 and the piston crown surface 15 can be increased.

[0056] In this embodiment, the surface modification step S2 of subjecting the piston crown surface 15 to anodizing (anodic oxidation) is performed before the application step S3 of applying the raw material coating 20A to the piston crown surface 15, thereby improving the adhesion strength of the heat shield layer 20 to the piston crown surface 15 made of an aluminum alloy. That is, the raw material coating 20A is applied to the piston crown surface 15 on which an oxide coating has been formed by anodizing, and the raw material coating 20A is fired in this state, so that the organic groups of the silicone-based resin contained in the raw material coating 20A bond with the OH groups on the surface of the oxide coating during the firing. This allows the heat shield layer 20 made of the hardened raw material coating 20A to be firmly adhered to the piston crown surface 15, improving the stability of the heat shield layer 20.

[0057] Furthermore, in this embodiment, the roughening step S1 of subjecting the piston crown surface 15 to a blasting treatment is performed before the surface modification step S2, which further improves the adhesion strength of the thermal barrier layer 20 to the piston crown surface 15. That is, fine irregularities are formed on the piston crown surface 15 by the blasting treatment (the surface roughness is increased), and the raw material coating 20A is fired in a state in which the raw material coating 20A penetrates into the fine irregularities. As a result, the adhesion between the thermal barrier layer 20 and the piston crown surface 15 can be increased, and the stability of the thermal barrier layer 20 can be further improved.

[0058] Furthermore, in this embodiment, the heat shield layer 20 containing the hollow particles 22 and the nanoparticles 23 is formed on the piston crown surface 15, so that the heat resistance of the heat shield layer 20 is ensured while the heat shield performance of the heat shield layer 20 is improved. That is, the molecular motion of the silicone-based resin binder 21 that is the base material of the heat shield layer 20 is suppressed by the nanoparticles, so that thermal deterioration of the resin binder 21 can be suppressed and the heat resistance of the heat shield layer 20 can be improved. Furthermore, because the heat shield layer 20 contains hollow particles 22 that are hollow inside, the thermal conductivity of the heat shield layer 20 can be further reduced and the heat shield performance of the heat shield layer 20 can be improved.

[0059] [Variations] In the above embodiment, the piston 5 with the raw material coating 20A applied to the crown surface 15 is placed in an electric furnace, and the amount of electricity supplied to the heater in the electric furnace is periodically increased and decreased to periodically increase and decrease the heating temperature of the raw material coating 20A, but the heating method for the raw material coating 20A is not limited to this. For example, a method can be considered in which the piston 5 is placed in a heating furnace having multiple stages with different heating temperatures, and the piston 5 is transported so that it passes through each stage of the heating furnace in sequence, thereby periodically increasing and decreasing the heating temperature. In addition, various heating methods can be used as long as a periodic change in the heating temperature can be obtained.

[0060] In the above embodiment, the heat insulating layer 20 is formed only on the piston crown surface 15 that defines the bottom surface of the combustion chamber 6, but a similar heat insulating layer can be applied to each part of the engine where a heat insulating effect is expected. For example, the heat insulating layer may be formed on the lower surface of the cylinder head 4 that defines the ceiling surface of the combustion chamber 6 and the bottom surface of the umbrella part of the intake and exhaust valves 10, 11, or on the inner wall surface (cylinder liner) of the cylinder block 3 that defines the side surface of the combustion chamber 6. Furthermore, the heat insulating layer may be formed on the wall surface of the flow path of the gas entering and exiting the combustion chamber 6 (for example, the intake port 8 and the exhaust port 9). The heat insulating layer on each surface can be formed by heating and firing a paste-like raw material coating that is the raw material of the heat insulating layer, as in the case of the heat insulating layer 20 in the above embodiment.

[0061] In the above embodiment, the raw material coating 20A is applied to the piston crown surface 15 using the spray gun G, but the raw material coating 20A may also be applied using a spatula, a brush, or the like.

[0062] In the above embodiment, the raw material coating 20A is directly applied to the piston crown surface 15, but an adhesive (primer) may be applied to the piston crown surface 15, and the raw material coating 20A may be applied on the applied adhesive. In other words, an adhesive may be interposed between the piston crown surface 15 and the raw material coating 20A.

[0063] In the above embodiment, a heat shielding layer 20 containing hollow particles 22 and nanoparticles 23 was formed, but the heat shielding layer 20 only needs to contain at least a silicone-based resin, and the hollow particles 22 and the nanoparticles 23 may be omitted.

[0064] In the above embodiment, the process of roughening the piston crown surface 15 (roughening step S1) involves blasting the piston crown surface 15 with fine projectiles, but the method of roughening the piston crown surface 15 is not limited to this. For example, the piston crown surface 15 may be roughened by laser processing in which a laser is irradiated onto the piston crown surface 15.

[0065] In the above embodiment, the surface modification of the piston crown surface 15 (surface modification step S2) is performed by anodizing (anodic oxidation) in which an oxide film is formed on the piston crown surface 15 by passing electricity through an electrolyte, but the method for modifying the surface of the piston crown surface 15 is not limited to this. For example, the surface of the piston crown surface 15 may be modified by plasma treatment in which the piston crown surface 15 is irradiated with plasma.

[0066] In the above embodiment, the process for roughening the piston crown surface 15 (roughening process S1) and the process for modifying the surface of the piston crown surface 15 (surface modification process S2) are performed in this order, but these two processes may be performed in the reverse order, or only one of the processes may be performed.

[0067] In the above embodiment, an example was described in which the present invention was applied to a gasoline engine that combusts gasoline fuel in a combustion chamber 6. However, engines to which the present invention can be applied are not limited to gasoline engines, and the present invention can also be applied to other types of engines, such as diesel engines. [Explanation of symbols]

[0068] 1 Engine 6 Combustion chamber 15 Piston crown surface (heat shielding surface) 20 Heat shield layer 20A raw material coating 22 Hollow particles 23 Nanoparticles S1 Roughening process S2 Surface modification process S3 Coating process S4 Firing process Ra (heating temperature) rate of change Rb (heating temperature) decreasing rate of change

Claims

1. A method for forming a heat shield layer on a surface to be heat shielded of an engine, comprising the steps of: a coating step of coating a raw material coating containing a silicone-based resin on the surface to be heat-shielded; and a firing step of heating and curing the applied raw material coating to form the thermal barrier layer, The method for forming a thermal barrier layer for an engine, wherein in the firing step, the heating temperature of the raw material coating is periodically increased and decreased, and the rate of change of the heating temperature when decreasing is smaller than the rate of change when increasing.

2. The method for forming a thermal barrier layer for an engine according to claim 1, A method for forming a thermal barrier layer for an engine, further comprising a step of performing a surface modification treatment on the surface to be thermally shielded before the coating step.

3. The method for forming a thermal barrier layer for an engine according to claim 2, The heat shielding target surface is one surface of an aluminum alloy component, The method for forming a heat shield layer for an engine, wherein the surface modification treatment is anodizing.

4. The method for forming a thermal barrier layer for an engine according to any one of claims 1 to 3, A method for forming a thermal barrier layer for an engine, further comprising a step of roughening the surface to be thermally shielded before the coating step.

5. The method for forming a thermal barrier layer for an engine according to any one of claims 1 to 4, 2. A method for forming a thermal barrier layer for an engine, wherein the surface to be thermally insulated is a wall surface defining a combustion chamber of the engine.

6. The method for forming a thermal barrier layer for an engine according to any one of claims 1 to 5, The method for forming a heat shield layer for an engine is characterized in that in the coating step, a resin solution of a silicone-based resin containing hollow particles and nanoparticles is coated onto the surface to be heat shielded as the raw material coating.

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