JP7632108B2 - Method for forming heat shield layer for engine - Google Patents

Description

The present invention relates to a method for forming a heat shield layer on the surface of an engine that needs to be heat shielded.

It has been proposed to cover the walls of engine parts with a heat-shielding layer in order to improve the thermal efficiency of the engine. For example, the following Patent Document 1 discloses the formation of a heat-shielding layer made of a silicone-based resin binder to which hollow particles and nanoparticles have been added, on the combustion chamber walls, including the piston crown surface.

Specifically, Patent Document 1 discloses an example of forming a heat shield layer in the following procedure. First, a resin solution containing a silicone resin to which the hollow particles and nanoparticles have been added (hereinafter, this will be referred to as a raw material coating) is prepared as the raw material for the heat shield 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 that has hardened by heating is formed as the heat shield layer.

International Publication No. 2020/213637

In the above-mentioned method for forming a thermal barrier layer, in order to sufficiently harden the raw material coating and 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 inventors, if the heating temperature is excessively high in order to stabilize the thermal barrier layer, the internal stress of the thermal barrier layer may increase beyond the allowable range. One cause of this is thought to be that the cross-linking reaction proceeds under conditions of very vigorous molecular motion of the silicone molecules in the raw material coating. An increase in internal stress actually weakens the adhesion between the thermal barrier layer and the surface to be thermally insulated, so measures to avoid this are necessary.

The present invention was made in consideration of the above circumstances, and aims to provide a method for forming a thermal barrier layer for an engine that can sufficiently harden the raw material coating to form a stable thermal barrier layer while reducing the internal stress of the thermal barrier layer.

In order to solve the above problem, the present invention is a method for forming a heat shielding layer on a surface of an engine to be heat shielded, comprising a coating process for coating a raw material coating containing a silicone-based resin on the surface to be heat shielded, and a baking process for forming the heat shielding layer by heating and hardening the applied raw material coating, the baking process comprising a first baking step for increasing and decreasing the heating temperature of the raw material coating in a predetermined cycle, and a second baking step for increasing and decreasing the heating temperature in a cycle longer than that of the first baking step after the first baking step (Claim 1).

According to the present invention, when the raw material coating applied to the surface to be heat-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-based 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 heat-shielding layer made of the raw material coating after curing can be improved.

However, if the cycle of increasing and decreasing the heating temperature remains constant, the internal stress of the thermal barrier layer may increase significantly. That is, as mentioned above, the cycle of changing the heating temperature increases the maximum heating temperature, and thus promotes the cross-linking reaction within the raw material coating. However, if heating is continued at the same cycle, the heating temperature will reach its maximum value more frequently, and the internal temperature of the raw material coating may rise excessively. While such an excessive temperature rise has the advantage of increasing the hardening rate of the raw material coating, it may also have the disadvantage of increasing the residual stress, i.e., internal stress, that occurs inside the thermal barrier layer made of the raw material coating after hardening. That is, if the temperature of the raw material coating rises excessively, the cross-linking reaction will proceed under conditions of very intense molecular motion, which may increase the internal stress of the thermal barrier layer.

In contrast, in the present invention, the heating temperature cycle is increased during the process of heating the raw material coating, i.e., the firing process, so that the increase in internal stress due to the above-mentioned circumstances can be suppressed. That is, the firing process is divided into a first firing step and a subsequent second firing step, and the heating temperature cycle in the second firing step is made longer than the heating temperature cycle in the first firing step, so that the frequency with which the heating temperature reaches its maximum value during the second firing step can be suppressed lower than the frequency during the first firing step. This suppresses an excessive increase in the internal temperature of the raw material coating, so that the above-mentioned effect of the periodic change in heating temperature (the effect of promoting the hardening of the raw material coating) can be enjoyed while effectively reducing the internal stress of the thermal barrier layer and increasing the adhesion strength between the thermal barrier layer and the surface to be thermally shielded.

Preferably, the method for forming a heat shielding layer further includes a step of performing a surface modification treatment on the surface to be heat shielded before the coating step (Claim 2).

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

The surface to be heat-shielded can be one surface of an aluminum alloy part. In this case, it is preferable to perform an anodizing treatment as the surface modification treatment (Claim 3).

In this embodiment, an oxide film is formed on the surface to be heat-shielded by the anodizing process, and then, by heating the raw material film applied to the surface to be heat-shielded, the organic groups of the silicone resin contained in the raw material film bond with the OH groups on the surface of the oxide film. This allows the heat-shielding layer made of the cured raw material film to be firmly adhered to the surface to be heat-shielded, improving the stability of the heat-shielding layer.

Preferably, the method for forming a heat shielding layer further includes a step of roughening the surface to be heat shielded before the coating step (claim 4).

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-shielding layer and the surface to be heat-shielded can be increased, and the stability of the heat-shielding layer can be improved.

Preferably, the surface to be heat-shielded is a wall surface that defines the combustion chamber of the engine (Claim 5).

In this embodiment, the heat shield layer prevents the thermal energy of the mixture burned in the combustion chamber from being released to the outside through the combustion chamber wall. This reduces the loss caused by the release of this thermal energy, i.e., the cooling loss, and effectively improves the fuel efficiency of the engine.

Preferably, in the coating process, a resin solution of a silicone-based resin containing hollow particles and nanoparticles is applied to the surface to be heat-shielded (claim 6).

In this embodiment, the applied raw material coating is baked in a baking process to form a heat-shielding 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-shielding layer can be improved. In addition, because the heat-shielding layer contains hollow particles with hollow interiors, the thermal conductivity of the heat-shielding layer can be further reduced and the heat-shielding performance of the heat-shielding layer can be improved.

As described above, the method for forming a thermal barrier layer for an engine of the present invention can sufficiently harden the raw material coating to form a stable thermal barrier layer while reducing the internal stress of the thermal barrier layer.

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. FIG. 2 is a cross-sectional view showing an upper structure of a piston of the engine. 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. 4 is a flowchart showing a procedure for forming the thermal barrier layer. 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. 4 is a graph showing the change over time in heating temperature when firing the raw material film. 5 is a graph showing how the hardening of the raw material coating progresses due to heating, in which (a) shows the change in heating temperature over time, and (b) shows the change in hardening rate over time.

[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 drawing 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.

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 housed inside the cylinders 2 so that they can reciprocate. The engine 1 of this embodiment mounted on a vehicle is typically a multi-cylinder engine having multiple 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.

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 surface (cylinder liner) of the cylinder 2, and the crown surface 15 (upper surface) of the piston 5.

Attached to the cylinder head 4 is an injector 7 that injects fuel (fuel containing gasoline) into the combustion chamber 6. A nozzle hole is formed at the tip of the injector 7, and the fuel injected into the combustion chamber 6 through this 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.

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 directing the intake air introduced from an intake passage not shown to the combustion chamber 6. The exhaust port 9 is a port for directing the burnt gas (exhaust gas) generated in the combustion chamber 6 to an exhaust passage not shown. The cylinder head 4 is fitted with 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. 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.

[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 outside the cavity surface 15a.

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 this invention.

The heat shield layer 20 is made of a material that has a significantly lower thermal conductivity than the metal material (base material) that constitutes 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 that contains 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) that is applied to the piston crown surface 15.

Figure 3 is an enlarged cross-sectional view showing the internal structure of the heat shield layer 20. As shown in this figure, the heat shield layer 20 includes a resin binder 21, and hollow particles 22 and nanoparticles 23 held inside the resin binder 21.

The resin binder 21 is a resin material that constitutes the base material (matrix) of the heat shield layer 20, and is mainly composed of a heat-resistant silicone-based resin. As a silicone-based resin, a silicone resin made 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.

The hollow particles 22 are inorganic particles having a cavity inside. It is preferable to use 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) as the hollow particles 22. For example, glass balloons, glass bubbles, fly ash balloons, silas balloons, silica balloons, and aluminosilicate balloons are suitable. The particle size of the hollow particles 22 is sufficiently smaller than the film thickness of the heat shield layer 20 (20 to 150 μm). 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.

For example, the nanoparticles 23 may be inorganic nanoparticles made of inorganic compounds such as zirconia, alumina, silica, titania, or metal nanoparticles made of metals such as Ti, Zr, and Al. Among these, silica nanoparticles whose surfaces are modified with phenyl groups are preferred. 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.

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. 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.

Forming the heat shield layer 20 configured as described above on the piston crown surface 15 leads to a reduction in cooling loss and thus improved fuel economy of the engine. In other words, the heat shield layer 20 prevents the thermal energy of the mixture burned in the combustion chamber 6 from being released to the outside through the piston crown surface 15, and reduces the loss caused by the release of this thermal energy, i.e., cooling loss. This is expected to improve thermal efficiency, which is the rate at which thermal energy is converted into work, or in other words, improve the fuel economy of the engine.

[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.

When the method of FIG. 4 is started, first, a roughening step S1 is carried out to roughen the piston crown surface 15. Specifically, in this embodiment, a blasting process is carried out in which minute projectiles are collided against the piston crown surface 15 as a process for roughening the piston crown surface 15 (roughening process). This removes dirt, oil, and the like adhering to the piston crown surface 15, and also forms minute irregularities on the piston crown surface 15, increasing the surface roughness of the piston crown surface 15.

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, the surface modification step (surface modification treatment) of the piston crown surface 15 is anodizing (anodic oxidation treatment) in which an electric current is passed through the piston 5 in an electrolyte. This forcibly oxidizes the piston crown surface 15, forming an oxide film on the piston crown surface 15.

After the surface modification step S2, the coating step S3 is performed to coat the raw material coating 20A shown in FIG. 5 on 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 the reactive silicone resin, which is the 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.

As shown in FIG. 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 that the raw material coating 20A is evenly sprayed onto the piston crown surface 15 (cavity surface 15a and base surface 15b). This forms a layer of the raw material coating 20A with a substantially uniform thickness on the piston crown surface 15.

After the application step S3, a baking step S4 is carried out in which the raw material coating 20A is heated and hardened. 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 in the electric furnace. Through this heating process, the temperature of the raw material coating 20A rises to a temperature at which crosslinking of the reactive silicone resin contained in the raw material coating 20A progresses. This hardens the paste-like raw material coating 20A, and the hardened raw material coating 20A is formed on the piston crown surface 15 as the heat barrier layer 20.

Figure 6 is a graph showing the change in heating temperature over time in the firing process 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 process 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.

In FIG. 6, the time when heating starts is t1, and the time when heating ends is t3. The heating temperature rises significantly immediately after heating starts t1, and drops significantly just before heating ends t3. The periodic increase and decrease in the heating temperature described above continues throughout the actual heating period P, which is the period excluding these periods (i.e., the initial and final periods of the heating period). That is, in the firing process S4, the heater in the electric furnace described above is controlled so that the heating temperature periodically increases and decreases within a relatively high range throughout the actual heating period P, which constitutes the majority of the heating period.

In addition, in the baking process S4, the heating temperature cycle is changed during the process. That is, if the time when the cycle is changed is t2, and the first period of the actual heating period P around time t2 is the first period P1, and the second period P2 is the latter period, the heating temperature cycle B2 during the second period P2 is made longer than the heating temperature cycle B1 during the first period P1. In other words, the baking process S4 includes a step of repeatedly increasing and decreasing the heating temperature at a predetermined cycle B1, and a step of repeatedly increasing and decreasing the heating temperature at a cycle B2 longer than the cycle B1. The former step corresponds to the "first baking step" in this invention, and the latter step corresponds to the "second baking step" in this invention.

The amplitude of the heating temperature is maintained constant throughout the actual heating period P (first period P1 and second period P2). That is, during both the first period P1 and the second period P2, the heating temperature is periodically increased and decreased with a constant amplitude A.

The heating temperature in the first period P1 and the second period P2 is increased or decreased around the same reference temperature Tx. That is, the heating temperature in the first period P1 and the second period P2 is repeatedly increased or 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.

However, the rate of change (speed of change) of the heating temperature is different between the first period P1 and the second period P2, which realizes the difference in the periods described above. That is, in the first period P1, the rate of change of the heating temperature is set large so that one cycle of temperature change (i.e., change from the reference temperature Tx through the maximum and minimum values and back to the reference temperature Tx) is completed in the relatively short period B1. In contrast, in the second period P2, the rate of change of the heating temperature is set small so that one cycle of temperature change is completed in the relatively long period B2.

In the baking process 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 the silicone molecules contained in the raw material coating 20A bond with each other, and the raw material coating 20A hardens. After hardening, the raw material coating 20A is formed as a heat shield layer 20 with the desired hardness (or strength) and is fixed to the piston crown surface 15.

The specific temperature conditions in the baking process S4 are as follows: The central temperature, i.e., the reference temperature Tx, when the heating temperature is changed periodically during the first period P1 and the second period P2, is preferably set to 100 to 140°C. The period B1 of the heating temperature during the first period P1 is preferably set to 70 to 90 sec, and the period B2 of the heating temperature during the second period P2 is preferably set to 5 to 7 times the period B1. The amplitude A of the heating temperature during the first period P1 and the second period P2 is preferably set to 30 to 50°C. The sum of the first period P1 and the second period P2, i.e., the actual heating period P, is preferably set to 40 to 60 min.

[Action and Effect]
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-based 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, a step of repeatedly increasing and decreasing the heating temperature for the raw material coating 20A in a cycle B1 and a step of repeatedly increasing and decreasing the heating temperature in a cycle B2 longer than the cycle B1 are carried out in this order. 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.

As a result of intensive research, the inventors of the present application have found that, under the same conditions of the thermal energy input by heating the raw material coating 20A, it is easier to increase the curing rate by periodically changing the heating temperature than by keeping the heating temperature constant. The curing rate here is an index that indicates 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 thought 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 thought 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.

7 is a graph showing the progress of the hardening of the raw material coating 20A by heating, where (a) shows the time change of the heating temperature, and (b) shows the time change of the 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 waveforms labeled "this pattern" represent 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 waveforms labeled "comparison pattern" represent 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, while the heating temperature in the comparison pattern is maintained at a constant value over the actual heating period P. However, the heat energy (temperature x time) applied by the heating method of this pattern and the heat energy applied by the heating method of the comparison pattern are assumed to be the same. 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. 6.

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 essentially stops before the heating end time t3. That is, in the comparative pattern, the hardening ratio increases to a value that is essentially the same as the hardening ratio Y0 at the heating end time t3 before the heating end time t3, and thereafter, almost no increase in the hardening ratio is observed. 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 t3) 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.

However, if the heating temperature cycle is kept constant over the period during which the heating temperature is periodically changed (actual heating period P), the internal stress of the thermal barrier layer 20 may increase significantly. That is, as described above, the periodic change of the heating temperature leads to an increase in the maximum heating temperature, and thus promotes the cross-linking reaction in the raw material coating 20A. However, if heating is continued at the same cycle, the heating temperature will reach its maximum value more frequently, and the internal temperature of the raw material coating 20A may rise excessively. While such an excessive temperature rise brings the advantage of increasing the hardening 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 hardening. That is, if the temperature of the raw material coating 20A rises excessively, the cross-linking reaction will proceed under conditions of very intense molecular motion, which may increase the internal stress of the thermal barrier layer 20.

In contrast, in this embodiment, the heating temperature cycle is increased during the actual heating period P, so that the increase in internal stress due to the above circumstances can be suppressed. That is, the actual heating period P is divided into a first period P1 and a subsequent second period P2, and the heating temperature cycle B2 in the second period P2 is made longer than the heating temperature cycle B1 in the first period P1, so that the frequency at which the heating temperature reaches its maximum value during the second period P2 can be suppressed lower than the frequency during the first period P1. This suppresses an excessive increase in the internal temperature of the raw material coating 20A, so that the internal stress of the heat shield layer 20 can be effectively reduced while enjoying the above-mentioned effect of the periodic change in the heating temperature (the effect of promoting the hardening of the raw material coating 20A), and the adhesion strength between the heat shield layer 20 and the piston crown surface 15 can be increased.

In addition, in this embodiment, before the application step S3 of applying the raw material coating 20A to the piston crown surface 15, the surface modification step S2 of applying anodizing (anodic oxidation) to the piston crown surface 15 is performed, so that the adhesion strength of the heat shield layer 20 to the aluminum alloy piston crown surface 15 can be improved. 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 baked 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 baking. 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.

Furthermore, in this embodiment, the roughening process S1 in which the piston crown surface 15 is subjected to a blasting process is carried out before the surface modification process S2, so that the adhesion strength of the heat shield layer 20 to the piston crown surface 15 can be further improved. That is, the blasting process forms fine irregularities on the piston crown surface 15 (increasing the surface roughness), 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 heat shield layer 20 and the piston crown surface 15 can be improved, and the stability of the heat shield layer 20 can be further improved.

In addition, in this embodiment, the heat shield layer 20 containing hollow particles 22 and nanoparticles 23 is formed on the piston crown surface 15, so that the heat resistance of the heat shield layer 20 can be ensured while improving the heat shield performance of the heat shield layer 20. 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 the thermal deterioration of the resin binder 21 can be suppressed and the heat resistance of the heat shield layer 20 can be improved. In addition, 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.

[Modification]
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.

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, a 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, a heat insulating layer may be formed on the wall surface of the gas flow path (e.g., the intake port 8 and the exhaust port 9) that enters and exits the combustion chamber 6. 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 heat insulating layer 20 of the above embodiment.

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

In the above embodiment, the raw material coating 20A is applied directly 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 top of the applied adhesive. In other words, an adhesive may be interposed between the piston crown surface 15 and the raw material coating 20A.

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 nanoparticles 23 may be omitted.

In the above embodiment, the process for roughening the piston crown surface 15 (roughening process S1) involves a blasting process in which minute projectiles are collided with the piston crown surface 15, but the method for 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.

In the above embodiment, the surface modification of the piston crown surface 15 (surface modification step S2) was 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 of surface modification of the piston crown surface 15 is not limited to this. For example, the piston crown surface 15 may be surface modified by plasma treatment in which the piston crown surface 15 is irradiated with plasma.

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

In the above embodiment, an example of applying the present invention to a gasoline engine that burns gasoline fuel in the combustion chamber 6 has been described, but the 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.

1 engine 6 combustion chamber 15 piston crown surface (surface to be shielded from heat)
20 Thermal barrier layer 20A Raw material coating 22 Hollow particles 23 Nanoparticles S1 Surface roughening step S2 Surface modification step S3 Coating step S4 Firing step B1 Period (in the first firing step) B2 Period (in the second firing step)

Claims (6)

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 firing step includes:
a first firing step of increasing and decreasing a heating temperature of the raw material film in a predetermined cycle;
a second baking step, which is performed after the first baking step, in which the heating temperature is increased or decreased in a longer cycle than that of the first baking step. 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. 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. 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. 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. 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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