Method for forming heat insulating film, and structure of heat insulating film

Abstract

A method for forming a heat insulating film includes: 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 pores formed in the surface thereat 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.

Description

BACKGROUND[0001]1. Field of the Invention[0002]This invention relates to a method for forming a heat insulating film, and a structure of a heat insulating film.[0003]2. Background Art[0004]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 se...

AI Technical Summary

Benefits of technology

The present invention aims to smooth the surface of a heat insulating film and reduce the occurrence of cracks in the sealing coating film. To achieve this, a sealing treatment can be performed using a sealing material that includes a silicon-based polymer solution and particles of a heat insulating material dispersed in the silicon-based polymer solution. The use of particles of a heat insulating material of a specific size helps to prevent cracks during the drying and baking process. This treatment results in a smooth surface of the heat insulating film and a thick sealing coating film.

Problems solved by technology

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.

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.

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.

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