Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming nonelectrolytic coating after forming nonmetal...
Reexamination Certificate
2000-05-11
2002-04-16
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming nonelectrolytic coating after forming nonmetal...
C205S201000, C205S203000, C205S220000, C205S224000, C205S229000
Reexamination Certificate
active
06372115
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improvement in a process for anodizing an Si-based aluminum alloy to form an anodized film on a surface of the alloy.
2. Description of the Prior Art
In recent years, pistons of Si- or silicon-based aluminum alloy have been widely used in internal combustion engines because they are light in weight and hence can make reciprocal movements at a high speed.
A typical example of such pistons is schematically shown in
FIG. 5
hereof. As illustrated therein, a piston
100
is disposed within a cylinder
101
such that it can make reciprocal movements at a high speed between top and bottom dead centers
102
,
103
along an inner peripheral surface
104
of the cylinder
101
. The piston
100
includes piston rings
100
b
which act to normally keep the piston
100
out of contact with the cylinder
101
. However, the piston
100
occasionally inclines for certain reasons. When the piston
100
inclines more than a given amount, a skirt
100
a
of the piston
100
is brought into contact with the cylinder
101
to thereby prevent further inclination of the piston
100
. Consequently, it becomes necessary for the piston
100
, particularly the skirt
100
a
, to have excellent wear resistance.
An example of pistons which are light in weight and have excellent wear resistance may be one cast from an Si-based aluminum alloy with an anodized film formed on a surface thereof. Discussion will be made next as to a process for anodizing a surface of such a piston of Si-based aluminum alloy.
The anodization process comprises the steps of immersing the Si-based aluminum alloy piston into an electrolyte to make the piston act as an anode, charging the electrolyte with a direct current to electrolyze water therein to thereby generate oxygen, and causing the generated oxygen to react with aluminum to thereby form a film of Al
2
O
3
on a surface of the Si-based aluminum alloy piston. The film of Al
2
O
3
is passive and generally called an anodized film having good corrosion and wear resistance.
When a surface of the skirt that may be held in contact with the cylinder is rough, there is a fear that undesired scoring and burning will occur during movement of the piston. To prevent this burning, a certain proposed piston has an anodized film with a resin infiltrated into the film to reduce its friction resistance.
Discussion will be made next as to an anodized film with a resin infiltrated thereinto, with reference to
FIGS. 6A
to
6
C showing a conventional piston of Si-based aluminum alloy.
An anodized film shown as an example in
FIG. 6A
is formed by using a sulfuric acid electrolyte. An aluminum alloy piston
100
as a base material includes Si particles
111
distributed therein. Si particles
112
located closely to a surface of the piston
100
adversely affect an anodized film
113
, thereby making the anodized film uneven or rough.
In that part of the piston where an Si particle
115
is partially exposed to outside accidentally as shown in
FIG. 6B
, there is formed no anodized film but a large hollow D
1
. In that part of the piston where an Si particle
116
is positioned closely to a piston surface, there is formed an anodized film
117
but the film thickness is small compared to the surrounding film
113
. In addition, a hollow D
2
is formed above the film
117
.
Consequently, it has been found that an even or flat anodized film can not be obtained by anodizing the aluminum alloy piston in a sulfuric acid electrolyte. It has also been found that microholes
118
resulted from a sulfuric acid electrolyte generally have a small hole diameter d
1
of the order of 15 nm. In
FIG. 6C
, the microholes
118
are shown with a photosetting resin
119
in the form of a liquid, infiltrated thereinto and then photoset by applying a light thereto. Inherently, resins have small friction resistance. Thus, with the photoset resin
119
infiltrated into the anodized film portions
113
,
117
, it becomes possible to reduce friction resistance arising upon high-speed reciprocal movement of the aluminum alloy piston within the cylinder.
However, difficulty is experienced in making the anodized film
113
flat due to the hollows D
1
, D
2
formed in the film
113
as shown in FIG.
6
B. In addition, since the microholes
118
produced in the anodized film
113
have small hole diameters d
1
, it is difficult to make the film
113
contain the resin sufficiently. This leads to the fear that notwithstanding the resin
119
infiltrated into the anodized film
113
, the friction resistance of the film will not be made as small as desired.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an improved process for anodizing an Si-based aluminum alloy, which enables sufficient reduction of the friction resistance of an anodized film to be formed on the alloy.
According to an aspect of the present invention, there is provided a process for anodizing an Si-based aluminum alloy, which process comprises the steps of; subjecting the Si-based aluminum alloy to electrolysis in an electrolyte containing phosphate and fluoride to form an anodized film on the alloy; infiltrating a photosetting or thermosetting resin in liquid form into microholes in the anodized film; and radiating light or heat at the infiltrated resin to make the resin become hardened.
Phosphate causes the microholes to have large hole diameters while fluoride dissolves Si moderately and facilitates growth of the film. As a result, a large amount of the photosetting or thermosetting resin can be infiltrated into the microholes of the film, thereby making a surface of the film flat and thus reducing friction resistance of the film.
In a preferred form, the resin contains fluoride. Since fluoride has good wear- and heat-resisting properties, inclusion of fluoride makes the alloy best suited for application to pistons, which are exposed to a high temperature.
REFERENCES:
patent: 5104743 (1992-04-01), Nishikawa et la.
patent: 5775892 (1998-07-01), Miyasaka et al.
patent: 6010816 (2000-01-01), Nishio et al.
Matsukawa Haruaki
Miyasaka Hajime
Merchant & Gould P.C.
Tran Thao
Wong Edna
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