JPS6260478B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing an alumite substrate for a high-density magnetic recording material, which can prevent a decrease in current density using a chromic acid alumite method, has no black spot defects, and has excellent smoothness, heat resistance, etc. It is easy to obtain a highly precise smooth surface with excellent abrasion resistance and good abrasiveness, which is required for magnetic recording material substrates such as magnetic disks, and it is possible to easily form a thin magnetic layer on the surface. Aluminum or aluminum alloy materials that have been subjected to alumite treatment are used as materials that have the following properties. However, in this alumite substrate, impurities such as iron and silicon that are present in the aluminum or aluminum alloy material crystallize as intermetallic compounds, and these are dotted on the surface of the material, and during alumite treatment, this part becomes a very small alumite film. A so-called black spot defect occurs, which is a missing part. Initially, this missing part is very small on the submicron scale, but it expands as the alumite film grows, and in an alumite layer with a film thickness of 5 μm or more,
These are pit-like minute defects of 5 to 10 μm in size, and the larger the number, the more signal errors occur in the magnetic recording material, which is an undesirable problem. In addition, when making a high-density magnetic recording material, α-Fe ₃ O ₄ is deposited on an alumite substrate by sputtering or other methods, and then heated to 300 to 400°C to convert it into γ-Fe ₂ O ₃ . However, when such high-temperature heating is performed, there is a problem that cracks occur in the alumite film, which can easily cause product defects. Therefore, the thickness of the alumite layer is set to about 1 to 3 μm. There was also the problem that the coating had to be extremely thin, resulting in a decrease in the head crushing resistance of the alumite substrate. As a means of solving these problems,
We proposed a method for alumite treatment of aluminum or aluminum alloy materials in a chromic acid solution bath using a constant voltage method at an electrolytic voltage higher than the voltage normally used. (Patent Application No. 58-44364) However, in this method, it has been recognized that if the same bath is used for a long time, the current density gradually decreases, the electrolysis time increases, and the properties of the film tend to deteriorate. Ta. The inventors of the present invention investigated the cause of this problem and conducted repeated research to find a solution. As a result, the cause was found to be due to the fact that part of the alumite film formed during the alumite treatment was dissolved in the chromic acid bath. The amount of aluminum ions increases, which causes the current density to decrease,
The present invention was created by recognizing that this problem is caused by a prolonged electrolysis time, and that the problem can be solved by carrying out alumite treatment while maintaining the amount of aluminum ions in the chromic acid bath below a certain value.
That is, the present invention allows aluminum or aluminum alloy materials to be treated in a 1.5 to 15% by weight chromic acid solution bath while maintaining the amount of aluminum ions dissolved in the chromic acid solution bath at 1/20 or less of the chromic acid concentration. This is a method for manufacturing an alumite substrate for high-density magnetic recording materials, which is anodized using a constant voltage method at a voltage higher than 60V. As the aluminum or aluminum alloy material used in the present invention, ordinary materials can be used, but from the viewpoint of black spot defects, materials containing less iron, silicon, etc. are preferable. In addition, the concentration of the chromic acid solution used as the electrolytic bath for alumite in the present invention is 1.5 to 15% by weight.
However, the effect cannot be fully exhibited at concentrations outside this range, and the bath temperature is 35 to 50°C, which is almost the same as the conditions in the conventional constant voltage chromate alumite method called the Butssard method. However, the voltage is higher than the 40V of the Butssard method, and is 60V or more, preferably in the range of 70 to 100V. Therefore, the amount of aluminum ions dissolved in the chromic acid bath during alumite treatment is maintained at 1/20 or less of the chromic acid concentration, and when it becomes 1/20 or more, the current density decreases and the electrolysis time This increases the length of the film and deteriorates the properties of the film. In order to keep the aluminum ion concentration at 1/2 or less of the chromic acid concentration, for example, one of these can be installed in series or parallel in an electrolytic cell using the ion exchange resin method, ion dialysis membrane method, etc. to obtain the desired amount of aluminum. Ion concentration can be controlled. Next, when an aluminum alloy plate (Al-3%Mg alloy) was alumite-treated with a constant DC voltage of 80V while varying the amount of dissolved aluminum in a 3% by weight chromic acid solution bath held at 40℃. The following table shows the results of evaluating the current density and electrolysis time, as well as black spots, cracks, and hardness required for an alumite substrate for high-density magnetic recording materials. Note that the alumite film thickness was 10 μm in all cases. Evaluation of sunspots is in the microscope field (0.36 mm ² ), 〇 is 2.5 μm or less, △ is 3.5 μm or less, × is 5
Indicates that there is no more than 1 black spot of μm or less,
In addition, to evaluate the cracks, the alumite substrate after heat treatment was observed under a microscope, and 〇 indicates that there were no cracks at all, △ indicates that cracks occurred partially, and × indicates that cracks occurred all over. Furthermore, the hardness evaluation is the result of measurement using a microhardness meter, 〇 is 250Hv or more, △ is 200-250Hv, × is
Indicates that it is 200Hv or less.

[Table] As shown in the table above, when alumite treatment is performed with the amount of aluminum ions in the chromic acid solution bath being 1/20 or less of the chromic acid concentration, the decrease in current density and the extension of electrolysis time are not so large. In addition, a film that satisfies the film properties required for an alumite substrate was obtained. On the other hand, when the current density becomes 1/20 or more of the chromic acid concentration, the current density decreases rapidly, the electrolysis time increases considerably, and the film properties of the obtained alumite substrate also crack. However, it is recognized that it is unsuitable for use in high-density magnetic recording materials because the hardness also decreases. In the present invention, the amount of aluminum ions in the chromium solution bath in the chromic acid alumite method is maintained at 1/20 or less of the chromic acid concentration during electrolysis, so the electrolysis time is adjusted as the current density decreases. It is possible to produce an alumite substrate which can prevent the wafer from becoming long, has no black spot defects, and has excellent smoothness, heat resistance, etc., and excellent effects are recognized. Next, examples of the method of the present invention will be described. Example Aluminum alloy material for high-density magnetic recording material (AL
-3% Mg alloy, inner diameter 75mm, outer diameter 200mm, thickness 2
mm) was subjected to specified surface polishing, washed with a non-erosive cleaning agent, and the amount of dissolved aluminum ions in a 3% by weight chromic acid solution bath heated and maintained at 40℃ was reduced to 0.9g/mm). The aluminum alloy material was immersed and anodized at a constant DC voltage of 80V as an anode.
A milky white smooth alumite film of μm size was formed. The current density at this time was 0.45 A/dm ² and the electrolysis time was 107 minutes. The obtained film was observed under a microscope, but no black spot defects were observed. In addition, this film is further added to 350
After heating at ℃ for 2 hours, observation under a microscope showed that there were no cracks. Furthermore, the film had a Vickers hardness of 280 Hv. Comparative example: After continuous use for a long time, the amount of aluminum ions in a 3% by weight chromic acid solution bath decreased to 1.7
Alumite treatment was carried out in the same manner as in the example using a bath with a temperature of 1.5 g/g/g/g/g/dm, except that the current density was 0.31 A/dm ² and the electrolysis time was 175 minutes. The resulting alumite film had black spot defects and cracks, and had a Vickers hardness of 220 Hv.

Claims (1)

[Claims] 1.1.5 while maintaining the amount of aluminum ions dissolved in the chromic acid solution bath of aluminum or aluminum alloy material to 1/20 or less of the chromic acid concentration.
by constant voltage method in ~15 wt% chromic acid solution bath
A method for producing an alumite substrate for high-density magnetic recording materials, which is characterized by alumite treatment at a voltage higher than 60V.