1.  Field of the Invention
This invention relates to a method for making a composite aluminum article.  This invention particularly relates to a method for making a composite aluminum article, wherein the surface of an aluminum material or an aluminum alloy material is imparted with excellent lubricating properties. 
2.  Description of the Related Art
As a method for imparting lubricating properties to oxide films of aluminum alloy materials which films are formed by anodization, methods described below have already been proposed. 
Specifically, it has been proposed to impregnate unsealed pores or unsealed cracks in an oxide film formed by an anodizing process with a lubricating oil. 
However, the aluminum alloy material obtained from such a method has drawbacks in that, for example, it cannot be used in applications where a solvent is used. 
It has also been proposed to use solid lubricants for the purpose of imparting the lubricating properties.  For example, Japanese Unexamined Patent Publication No.  56(1981)-130489 discloses a method wherein an aluminum alloy material is immersed in a solution containing a metal oxyacid salt (tetrathiomolybdate), and then treated by anodic secondary electrolysis, so as to fill pores in the oxide film formed by anodization with a metal oxide (molybdenum disulfide).  A method has also been proposed wherein an aluminum alloy material is immersed alternately in a metallic soap solution and an acid.  A method has additionally been proposed wherein an aluminum alloy material is immersed alternately in a liquid containing a metal sulfide and in an acid.  Further, a method has been proposed wherein electrolytic pores are filled with a metal soap or a metal sulfide. 
However, the proposed methods have drawbacks in that the formed film exhibits a high coefficient of wear (0.2 to 0.3), poor durability, and insufficient self-lubricating properties.  Actually, the proposed methods have not been put into practice for the purposes of imparting lubricating properties to sliding parts which requires good lubrication characteristics.  This is presumably because the degree of wear of the anodic oxide film in itself is high. 
Also, like a method typified by the method disclosed in Japanese Patent Publication No.  52(1977)-39059, a method has been known wherein pores of a hard anodic oxide film are impregnated with a polytetrafluoroethylene (hereinafter referred to as the "PTFE").  This is a composite film forming method utilizing the characteristics of a PTFE resin which exhibits a low coefficient of friction and water- and oil-repellent properties.  Specifically, in this method, a hard anodic oxide film having a thickness ranging from 20 .mu.m to 50 .mu.m is formed and is then impregnated with PTFE to a depth of approximately 10 .mu.m from the surface of the film. 
It is said that the aforesaid method utilizing PTFE can result in excellent wear-resistant properties.  However, it is doubtful that PTFE will penetrate into the pores of the hard anodic oxide film with certainty.  Also, the adhesion of PTFE is not sufficient, and improvements are rather poor in the sliding characteristics of precision parts which requires high accuracy and of compressor parts which requires high air tightness.  Additionally, sufficient resistances to abrasive wear and adhesive wear cannot be obtained.  Thus there remain problems to be solved with regard to the use of this method for parts which have to have high overall durability. 
As described above, the conventional method of treating a hard anodic oxide film for imparting the lubricating properties thereto exhibit only insufficient improvements in the sliding characteristics of precision parts which have to show high accuracy and of compressor parts which have to show high air tightness. 
SUMMARY OF THE INVENTION
A primary object of the present invention is to provide a method for making a composite aluminum article, wherein fine polytetraftuoroethylene (PTFE) particles are uniformly adhered to the surface of a hard anodic oxide film of aluminum or of an aluminum alloy, so that a composite aluminum article exhibiting excellent frictional wear characteristics and excellent anti-seizure properties can be provided. 
The present invention provides a method for making a composite aluminum article, which comprises the steps of electrochemically or chemically adsorbing and drying fine particles of a polytetrafluoroethylene to the surface of a hard anodic oxide film of aluminum or an aluminum alloy, and then rubbing the thus treated film with another member which slides on said film during actual use, so that a lubricous film is formed. 
Specifically, in the method for making a composite aluminum article in accordance with the present invention, while a hard anodic oxide film is formed on aluminum or an aluminum alloy by an ordinary anodizing process, positive or negative electrical charges are given to fine PTFE particles, and the charged fine PTFE particles are dispersed uniformly in an aqueous solution.  The aluminum or aluminum alloy which has the oxide film thereon is then immersed in the aqueous solution.  In cases where positive electrical charges have been given to the fine PTFE particles, negative electrolysis occurs in the aqueous solution, and in cases where negative electrical charges have been given to the fine PTFE particles, the PTFE fine particles can be uniformly adsorbed with an electrochemical or chemical process on the surface of the hard anodic oxide film of aluminum of aluminum alloy by simple immersion.  After drying, the surface and another member which slides along the surface of this anodic oxide film when these members are in actual use are rubbed together, so that a uniform PTFE film is formed on the surface of the hard anodic oxide film. 
With the method of the present invention, the fine PTFE particles are adsorbed onto the surface of the hard anodic oxide film of aluminum or an aluminum alloy, and crushed by, for example, the assembling contact with the opposite member.  Also, the crushed fine PTFE particles are extended into a thin film by the relative sliding motion of the two members with respect to each other, so that a lubricous film is formed. 
Therefore, when the composite aluminum article obtained in accordance with the present invention and the opposite member are assembled together for user their contact surfaces slide along with each other (i.e. they are rubbed together), so that a lubricous film is formed and the lubricating properties are obtained. 
Preferably, the composite aluminum article obtained in accordance with the present invention and the opposite member sliding along the composite aluminum article should form a pair of aluminum or aluminum alloy parts.  The composite aluminum article obtained in accordance with the present invention having the lubricous film thereon and the opposite member slide with respect to each other. 
Therefore, typical examples of the composite aluminum article obtained in accordance with the present invention and the opposite member sliding along the composite aluminum article would be a combination of a shaft and a bearing, a combination of a piston and a cylinder in a pump, a combination of a rotor and a plate in a rotary compressor, a combination of a fixed scroll and a rotary scroll in a scroll fluid machine, only to name a few. 
The composite aluminum article obtained in accordance with the present invention has a hard anodic oxide film, and a uniform PTFE film layer is overlaid on the hard anodic oxide film.  Unlike the conventional film forming process wherein the PTFE is molten at a high temperature ranging from 350.degree.  C. to 400.degree.  C., the PTFE film in the present invention is obtained from a film forming process wherein PTFE is dried at a low temperature ranging from 80.degree.  C. to 120.degree.  C. Therefore, there is no risk of degrading the strength characteristics of aluminum or an aluminum alloy.  Also, only the necessary part can be imparted with the characteristics of PTFE, i.e., its low coefficient of friction and its excellent lubricating properties. 
Specifically, with the method for making a composite aluminum article in accordance with the present invention, a composite aluminum article can be obtained which exhibits excellent frictional wear characteristics and excellent anti-seizure properties.