Patent Application
20150354083
1. Field of the Invention
This invention relates to a method for forming a heat insulating film, and a structure of a heat insulating film.
2. Background Art
A method for forming a heat insulating film in an umbrella portion of an engine valve has already been disclosed in Japanese Patent Laid-Open No. 2013-014830. Specifically, the aforementioned conventional method includes a first step of forming an aluminum plating film over the entire circumference of an engine valve, a second step of, after formation of the aluminum plating film, subjecting the entire circumference of the engine valve to an anodic oxidation treatment to form an anodic oxidation coating film, and a third step of, after formation of the anodic oxidation coating film, subjecting an umbrella portion of the engine valve to a sealing treatment to form a sealing coating film. According to this conventional method, a heat insulating film can be obtained that has a structure in which a sealing coating film is formed on an anodic oxidation coating film. Further, according to the engine valve on which the above described heat insulating film is formed, in addition to improving the heat resistance and a heat insulating property of a combustion chamber of the engine, a heat radiation property can also be improved.
Other prior arts include Japanese Patent Laid-Open No. 2012-047110, Japanese Patent Laid-Open No. 2013-060620, and Japanese Patent Laid-Open No. 2012-172619.
In this connection, when performing anodic oxidation treatment of an aluminum alloy, there is the problem that because the formation of the anodic oxidation coating film is affected by inclusions that are included in the aluminum alloy, the surface of the anodic oxidation coating film that is formed is not smooth, and minute concavities and convexities arise thereon. This problem can also arise in a similar manner in the aforementioned first and second steps in a case where an aluminum alloy plating film is formed on the surface of the engine valve and the plating film is thereafter subjected to an anodic oxidation treatment.
When concavities and convexities arise on the surface of an anodic oxidation coating film, a heat transfer area thereof increases. If the heat transfer area increases, an effect of improving the heat insulating property that is obtained by the anodic oxidation coating film is weakened. If concavities and convexities have arisen on the surface of the anodic oxidation coating film, the fluidity of a flame that arises inside the combustion chamber decreases, and the combustion efficiency deteriorates. In this respect, by forming the sealing coating film in the above described third step, the surface of the heat insulating film having a structure in which the anodic oxidation coating film and the sealing coating film are formed can be made smooth to a certain extent. Ideally, it is desirable for the surface of the heat insulating film to be made as smooth as the surface of the aluminum alloy prior to the anodic oxidation treatment.
In this connection, the sealing coating film is formed by subjecting a sealing material that is the raw material of the sealing coating film to a drying and baking process. Consequently, in order to make the surface of the heat insulating film smooth by means of the sealing coating film, it is necessary to provide a large amount of the sealing material in concave portions in the surface of the anodic oxidation coating film to thereby make the sealing material thick at such concave portions. However, because the sealing material contains a solvent, the thicker that the sealing material is, the more difficult it becomes for a gas of the solvent that is generated at the time of drying and baking to escape to the outside. Therefore, there is the problem that cracks are liable to arise in the sealing coating film. Consequently, there is a trade-off relationship between thickening the sealing material to smoothen the surface of the heat insulating film, and reducing cracks in the sealing coating film, and it is difficult to achieve both a smooth surface and a reduction in the amount of cracks in a compatible manner.
The present invention has been conceived in view of the above described problem. That is, an object of the present invention is, with respect to a heat insulating film having a structure in which a sealing coating film is formed on the surface of an anodic oxidation coating film, to smooth the surface of the heat insulating film and also reduce the occurrence of cracks in the sealing coating film in a compatible manner.
A first aspect of the present invention is a method for forming a heat insulating film, including: a step of subjecting an aluminum alloy constituting a surface of a base material to an anodic oxidation treatment to form an anodic oxidation coating film having a surface in which pores are formed;
a step of coating on the surface of the anodic oxidation coating film a sealing material that includes a silicon-based polymer solution and particles of a heat insulating material that are dispersed in the silicon-based polymer solution and are particles having an average particle diameter that is larger than an average pore diameter of the pores; and
a step of drying and baking the sealing material to form a sealing coating film.
A second aspect of the present invention is in accordance with the first invention, wherein the particles may be particles that have a hollow structure.
Further, in a third aspect of the present invention, an average primary particle diameter of the particles may be greater than 30 nm.
A fourth aspect of the present invention is a structure of a heat insulating film that is formed by a formation method according to any one of the first to third inventions, may including:
an aluminum alloy constituting a surface of a base material;
an anodic oxidation coating film that is formed on a surface of the aluminum alloy, and that has a surface in which pores are formed; and
a sealing coating film that is formed so as to cover an opening portion of the pores, and that includes particles of a heat insulating material having an average particle diameter that is larger than an average pore diameter of the pores.
A fifth aspect of the present invention is in accordance with the fourth invention, wherein:
the particles may be particles that have a hollow structure; and
a porosity of the sealing coating film may be from 27.3 to 57.7%.
According to the first aspect of the present invention, a sealing treatment can be performed using a sealing material that includes a silicon-based polymer solution and particles of a heat insulating material that are dispersed in the silicon-based polymer solution and are particles having an average particle diameter that is larger than an average pore diameter of pores of an anodic oxidation coating film. In the case of using a sealing material including particles of a heat insulating material of the aforementioned size, the occurrence of cracks in a drying and baking process can be suppressed in comparison to when using a sealing material that does not include the particles. Therefore, the occurrence of cracks can be suppressed even when a sealing material is made thicker by providing a large amount thereof on concave portions of the surface of an anodic oxidation coating film. Further, the surface of the heat insulating film can be made smooth by means of a thick sealing coating film that is formed by drying and baking of the sealing material.
According to the second aspect of the present invention, since a heat insulating function of air inside particles that have a hollow structure can be utilized, a heat insulating film can be formed that has a high heat insulating property in comparison to a heat insulating film that does not include particles that have a hollow structure.
According to the third aspect of the present invention, a heat insulating film that has a high heat insulating property can be formed by using particles which have an average primary particle diameter that is greater than 30 nm.
According to the fourth aspect of the present invention, since a sealing coating film is provided that is formed so as to cover an opening portion of pores of an anodic oxidation coating film, a structure of a heat insulating film having a high heat insulating property can be provided that utilizes a heat insulating function of air inside the pores that is located at a deeper place than the opening portion.
According to the fifth aspect of the present invention, a structure of a heat insulating film having a high heat insulating property that is obtained by means of a sealing coating film in which the porosity is between 27.3 and 57.7% can be provided.
Hereunder, an embodiment of a method for forming a heat insulating film and of a structure of a heat insulating film according to the present invention will be described referring to
First, an embodiment of the method for forming a heat insulating film of the present invention will be described.
In step S1, specifically, the aforementioned base material is placed in the aforementioned treatment apparatus, and a voltage is applied between the aforementioned pair of electrodes while circulating an electrolytic solution through the aforementioned flow channel. By this means, an anodic oxidation coating film is formed on the surface of the base material. The anodic oxidation coating film is a coating film of porous alumina that has a large number of pores that open at the surface thereof (described in detail later). By means of this porous structure, the anodic oxidation coating film realizes a low thermal conductivity and a low heat capacity per unit volume (described in detail later).
However, the surface of the anodic oxidation coating film is rough in comparison to the surface of the base material before the anodic oxidation treatment.
The description of the present embodiment will now be continued referring again to
The average pore diameter of the pores of the anodic oxidation coating film is approximately 30 nm. Therefore, in the present step, hollow silica particles for which the average primary particle diameter is greater than 30 nm (preferably, 50 nm) are used. However, a target value of the surface roughness Ra of the heat insulating film that is formed by the present embodiment is approximately 1 Jam, and therefore in the present step hollow silica particles are used with respect to which an average secondary particle diameter is less than 1 μm (preferably 500 nm, more preferably 150 nm).
Here, the term “average pore diameter” refers to an arithmetic mean diameter that is determined by photographing sectional images at a plurality of magnifications using a scanning electron microscope and digitalizing the obtained images by a scanner input method, and thereafter calculating a distribution of diameters of circles having an area that is equal to the area of respective pores extracted by computer image analysis. Further, the term “average primary particle diameter” refers to an arithmetic mean diameter that is determined by photographing transparent particle images at a plurality of magnifications using a transmission electron microscope and digitalizing the obtained images by a scanner input method, and thereafter calculating a distribution of diameters of circles having an area that is equal to a projected area of respective pores extracted by computer image analysis. Furthermore, the term “average secondary particle diameter” refers to an average particle diameter (D50 value) that is obtained by a dynamic scattering method, and is a diameter that can be simply measured by a commercially available particle size analysis and measurement apparatus.
The mixing ratio of the hollow silica particles in the sealing material is appropriately adjusted in accordance with the target value (for example, a value in a range from 27.3% to 57.7%) of the porosity of the sealing coating film to be formed after drying and baking of the sealing material (after step S3).
The surface of the anodic oxidation coating film after application of the sealing material will now be described referring to
The description of the present embodiment will now be continued referring again to
After step S2, the sealing material is dried and baked to form a sealing coating film (step S3). The conditions (temperature, time period and the like) at the time of drying and baking are appropriately adjusted in accordance with the thickness of the sealing material that was coated onto the surface of the anodic oxidation coating film. A heat insulating film is formed by performing the present step.
The effects of the present embodiment will now be described referring to
As will be understood from
Next, an embodiment of the structure of a heat insulating film according to the present invention will be described. The structure of a heat insulating film of the present invention is applied to an inner wall of a combustion chamber of an engine.
A cylinder 42 of an engine 40 is formed inside a cylinder block 44. A cylinder liner 46 is provided at an inner circumferential face of the cylinder 42. Further, inside the cylinder 42, a piston 48 is slidably disposed with respect to the cylinder liner 46. A cylinder head 50 is installed at an upper portion of the cylinder block 44. An intake port 52 and an exhaust port 54 are formed in the cylinder head 50. An intake valve 56 is provided in the intake port 52, and an exhaust valve 58 is provided in the exhaust port 54.
A space that is surrounded by an inner circumferential face of the cylinder liner 46, a top face of the piston 48, a bottom face of the cylinder head 50, a bottom face of an umbrella portion of the intake valve 56 and a bottom face of an umbrella portion of the exhaust valve 58 corresponds to a combustion chamber 60. That is, an inner wall of the combustion chamber 60 is constituted by the inner circumferential face of the cylinder liner 46, the top face of the piston 48, the bottom face of the cylinder head 50, the bottom face of the umbrella portion of the intake valve 56 and the bottom face of the umbrella portion of the exhaust valve 58. The heat insulating film 22 formed by the above described method is provided on the inner wall of the combustion chamber 60.
The silica 18 has a lower thermal conductivity than the aluminum alloy, and has a lower heat capacity per unit volume (volumetric heat capacity) than the aluminum alloy. Further, the alumina 10a has a lower thermal conductivity and a lower volumetric heat capacity than not only the aluminum alloy, but also than a conventional ceramic-based heat insulation material. Therefore, by applying the structure of the heat insulating film 22, in addition to improving the heat resistance and the heat insulating property of the combustion chamber 60, a heat radiation property thereof can also be improved.
Further, according to the structure of the heat insulating film 22 including the hollow silica particles 16 of the above described size, the heat insulating property of the combustion chamber 60 can be further improved. The reason for this will now be described in detail referring to
X=Cp×ρ×α
C=Cp×ρ
Where, Cp represents specific heat capacity, ρ represents density, and α represents thermal diffusivity.
The sample containing hollow silica particles (hereunder, referred to as “sample A”) was prepared as follows. First, a base material (test piece of aluminum alloy) was subjected to an anodic oxidation treatment to form an anodic oxidation coating film. Next, hollow silica particles (hollow silica particles manufactured by GRANDEX Co., Ltd (primary particle diameter 90 to 110 nm)) were mixed in a polysilazane solution (ingredients and percentages: diethyl ether 72%, poly(perhydrosilazane) 20%, and anisole 8%) and stirred adequately using a stirrer to thereby prepare a sealing material. Thereafter, the sealing material was applied five times onto the anodic oxidation coating film using a brush, and then dried and baked for 8 hours in a constant temperature oven at 180° C. to thereby prepare the sample A. The sample that did not contain the hollow silica particles (hereunder, referred to as “sample B”) was prepared in the same manner as the sample A except that the polysilazane solution was used as the sealing material.
The measurement conditions and the like for the specific heat capacity Cp and the thermal diffusivity c were as follows.
Measurement method: DSC method
Measurement apparatus: DSC Q1000 manufactured by TA Instruments
Reference sample: Sapphire
Measurement atmosphere: N2 atmosphere
Measurement sample: After processing each sample to Φ6 mm, the base material was dissolved in hydrochloric acid to prepare samples that were constituted only by film
Measurement method: Laser flash method
Measurement apparatus: LFA 457 manufactured by NETZSCH
Temperature measurement method: Noncontact temperature measurement using InSb sensor
Surface treatment: Graphite spray
Measurement atmosphere: N2 atmosphere
Calculation technique: Base material and film were measured in an integrated state, and the thermal diffusivity of only the film was calculated by multilayer analysis including pulse width correction and heat loss.
The measurement results in
The fact that the surface roughness Ra of the sample A is small may be mentioned as another reason why the sample A has an excellent heat insulating property.
Sample A1: 27.3% (porosity: low)
Sample A2: 46.3% (porosity: medium)
Sample A3: 57.7% (porosity: high)
The surface roughness Ra was measured in accordance with JIS B601 (2001). The measurement results in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-119599 | Jun 2014 | JP | national |
