JPH0685212B2 - Manufacturing method of alumite substrate for magnetic recording material - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing an alumite-treated aluminum substrate used as a substrate for a high magnetic recording density magnetic disk using a thin film as a medium.

2. Description of the Related Art As a magnetic recording medium substrate such as a magnetic disk substrate for high recording density, a so-called alumite substrate having an alumite coating layer formed on the surface of an aluminum alloy material is used.

The reason why the alumite substrate is used for magnetic recording material with high recording density is that the alumite coating is hard and rich in abrasion resistance, has good abrasiveness, and is easy to obtain a highly accurate smooth surface. This is because a thin magnetic film can be easily formed on the surface.

Previously, the present inventors have proposed to use, as such an alumite substrate, an alloy material in which high purity aluminum contains about 2 to 6% of magnesium and on which alumite chromate is formed ( JP-A-59-180832).

Conventionally, a general Al-2 to 6% Mg alloy with a sulfated alumite coating formed on it has the following drawbacks on the alumite substrate, which hinders the increase in recording density of magnetic recording materials. It was becoming.

That is, one of the black pit defects, which is a minute pit-shaped film defect, has this kind of defect in the film layer, which becomes a recessed part of the magnetic film when the magnetic film is formed, and causes a signal error during use. Therefore, it is desirable to reduce the number as much as possible and to keep the size to 3 μm or less.

Another drawback is that when obtaining a high-density magnetic recording material, α-Fe ₃ O ₄ is deposited on the alumite substrate by sputtering or another suitable method, and heated to a temperature of 300 to 400 ° C to produce γ-Fe.
_The method of converting to ₂ O ₃ is adopted, but when such high temperature heating is performed, cracks may occur in the alumite film, resulting in product failure. In order to avoid such crack generation, the alumite film layer Must be a thin film of 3 μm or less, but when an alumite substrate with such an extremely thin film is used as a magnetic disk substrate, the CSS durability deteriorates because the pressure resistance of the film deteriorates. Cause Black spot defects in the alumite film layer formed on the substrate are iron contained in the aluminum alloy,
Intermetallic compounds that crystallize based on impurities such as silicon, manganese, chromium, such as FeAl ₃ , MnAl ₆ , NiAl ₃ , β- (A
l-Fe-Si) intervenes on the surface of the alloy material, it forms a pit-like defect by forming a pit-like defect in a portion corresponding to the particle size of these inclusion particles. It is necessary to reduce the number of objects and the particle size.
On the other hand, when the chromic acid method alumite film is formed by changing the conventional alumite film layer formed on the substrate to the conventional sulfuric acid method alumite film, the occurrence of pit-like defects in the film layer is significantly suppressed and No crack is generated even by the heat treatment for forming the film.

An anodized aluminum substrate in which 2 to 6% of magnesium is added to high-purity aluminum invented based on these findings has anodized chromate, and black spot defects and cracking of the film due to heat treatment for magnetic film formation occur. No or almost none, and the thickness of the alumite coating layer is 3
By adopting a thickness of at least μm, the head crash resistance can be improved, so that it has excellent performance as a magnetic recording material substrate for high recording density.

Problems of the prior art Chromate alumite, as already studied (for example, Sato et al., Metal Surface Technology, Vol. 26, p. 456, (1975)), has fine pores of alumite formed radially. Therefore, at the interface between aluminum and alumite, a large number of micropores form a colony, giving a unique appearance to chromate alumite. Figure 2 shows the cross-sectional structure of alumite chromate. When this is observed from the alumite surface with an optical microscope, the colony structure appears to be a collection of bubbles.

It was found that in this colony structure, a remarkably large colony, that is, a large colony was sometimes formed. FIG. 3 shows a plane structure of alumite observed by an optical microscope from the alumite surface of the conventional aluminum alloy substrate, and it can be seen that coarse colonies are present. Coarse colonies are the result of preferential growth of pores. Therefore, it is considered that a large amount of current flows in the pores of the coarse colony part and the deterioration of the film progresses due to the increase in bath temperature. (Sato et al. Say that the longer the pores, the larger the pore size. (Metal surface technology 26, 456
Page (1975)) From the above, the film of the coarse colony part is slightly softer than the peripheral part, and it becomes a minute recess during polishing, and when a magnetic substance is formed on the alumite surface, it adversely affects the recording and reproducing characteristics. This results in a peak shift which causes a bit error which is a serious defect as a magnetic disk.

It has been known that if there is a minute scratch on the surface of the aluminum substrate, a large colony is likely to occur in that part, but the inventors of the present invention have noticed that a coarse colony also occurs in that part,
When the content of copper in the alloy component is kept within a certain value in the pursuit of the cause of the occurrence, the occurrence of coarse colonies is greatly reduced, and the surface of the magnetic disk substrate on which the magnetic recording medium is formed. The present invention has been completed by finding that the performance becomes uniform and stable.

Means and Actions for Solving Problems The present invention relates to 2 to 6% by weight of magnesium and 0.002 to 0.0 of copper.
Containing 3%, balance aluminum and impurities,
Aluminum alloy material whose allowable content of impurities is 0.003% of iron, 0.005% of silicon, 0.5% of zinc, 0.0005% of chromium, 0.0005% of chromium, 0.0005% of nickel, 0.0005% of titanium, and 0.001% of other impurities. The feature is that an alumite film is formed on the surface by using chromic acid having a thickness of 6 μm or more.

The reason why an alloy material containing 2 to 6% by weight of magnesium is used as the aluminum material in the alumite substrate of the present invention is that this alloy is easy to obtain the strength and smoothness required for a magnetic recording material substrate such as a magnetic disk. This is because the alumite property is good.

When the amount of magnesium is 2% or less, sufficient strength cannot be obtained as a substrate material, and when it is 6% or more, β-Al ₂ Mg ₃ is crystallized in the structure during alloy casting, and when the alloy material is processed into a substrate, it is coarse. It is likely to cause inclusions to impair the machinability of the substrate and cause black spot defects when forming the alumite coating layer.

The colony structure is peculiar to chromic acid electrolysis, and is affected by the concentration of chromic acid bath, temperature, aluminum concentration and other electrolysis conditions. In particular, it is considered that the coarse colony part was greatly affected by this and the film performance became uneven, but the addition of a specific amount of copper made the colony structure fine and unclear, and the coarse colony part The number of slabs is greatly reduced, and the surface performance of the alumite-treated magnetic disk substrate is made uniform and stable. FIG. 1 shows a plane structure of alumite seen from an alumite surface of the alumite substrate of the present invention with an optical microscope, and it is found that coarse colonies are scarcely present.

The amount of copper added is preferably 0.002 to 0.03%. 0.002%
Below, coarse colonies with a diameter of 7 μm or more, which are thought to be associated with bit errors of the magnetic disk, will be generated, and if it exceeds 0.03%, the hardness of the alumite coating will decrease and the performance required for the magnetic disk will be satisfied. It is not preferable because it disappears. FIG. 4 shows the relationship between the added amount of copper and the fine Vickers hardness of alumite chromate.

The allowable limit of impurity elements in the alloy is 0.003% for iron and 0.0 for silicon.
05%, manganese 0.0005%, chromium 0.0005%, nickel 0.
The reason for defining 0005% and titanium as 0.0005% is that these impurity elements are elements that are relatively commonly present in aluminum ingots, and the presence of these impurity elements causes the intermetallic compounds to crystallize during casting. Most of the compounds are elements that tend to cause the formation of coating defects during the formation of alumite coatings, but the presence of less than the respective limit amounts produces extremely small grain sizes of intermetallic compounds during casting. In the alumite film, the chromic acid method alumite film of the present invention does not cause pit-like defects of 3 μm or more due to film loss in the alumite film layer, and even if defects of a size smaller than that occur. This is because it is extremely small and does not affect sound magnetic film formation.

Since zinc as an impurity element has a large solid solution limit in aluminum, it does not pose a problem even if it is present in a relatively large amount, and up to 0.5% is allowed.

The method for producing an alumite substrate of the present invention is a conventional method, in which the surface of an alloy material that has been subjected to wrought processing is subjected to precision turning and polishing with a diamond tool or the like to give a mirror surface and then direct current is applied using a chromic acid electrolytic bath. To form an alumite film layer.

For the alumite treatment, the Bengow-Stuart method or the constant voltage method, which is usually used in the chromic acid method, may be used. However, in order to obtain an alumite film layer having a sufficient film thickness, it is preferable to use the constant voltage method. For adoption, it is easier to obtain good results by processing at a higher voltage than the voltage of about 40V that is generally used.

The film thickness is preferably 6 μm or more, and when it is 6 μm or less, it becomes difficult to secure sufficient CSS durability.

Examples Next, the alloy of the present invention will be described based on Examples.

Three kinds of molten aluminum-magnesium alloys having compositions A (inventive alloy) and B (comparative alloy) and C (comparative alloy) shown in Table 1 were passed through a ceramic filter according to a conventional method, and nonmetallic inclusions were used. After removing the water, a ingot with a cross section of 255 mm × 910 mm was made by a water-cooled semi-continuous casting method, heated at 490 ° C for 3 hours, homogenized and hot-rolled to a thickness of 8 mm. Cold rolled to a thickness of 2 mm at 420 ℃
Then, finish annealing was performed for 4 hours to obtain an aluminum alloy material for a substrate.

Next, the above A, B, and C types of aluminum alloy materials were roughly cut, precision-turned with a diamond bite, and then anodized with chromic acid. The thickness of the alumite coating on each substrate was 10 μm.

The alumite conditions are as follows.

Chromic acid alumite conditions Chromic acid concentration 10wt% Voltage (constant voltage method) 65V Bath temperature 40 ° C Current density 0.3A / dm ² Coarse colonies in the optical microscope field of view (0.355mm ² ) for the alumite substrate obtained under the above conditions (Diameter is 7
The number and the hardness of coatings were measured, and the results are shown in Table 2.

From Table 2, the alumite substrate (A) of the present invention had far fewer coarse colonies than the alumite substrate of the comparative alloy (B) in which the copper content was kept low. Also,
Compared with the comparative alloy (C) containing copper in an amount not less than the addition amount of the present invention, the coating hardness was high and the magnetic disk substrate showed excellent performance.

When each of the alumite substrates was heated at 350 ° C. for 2 hours, no cracks were observed in the coating and good heat resistance was exhibited.

EFFECTS OF THE INVENTION The alumite substrate of the present invention has few coarse colonies that adversely affect recording / reproducing characteristics, has sufficient surface hardness, and cracks by heat treatment at 350 ° C. even if the alumite coating thickness is 10 μm or more. It has excellent performance as an alumite substrate for high recording density magnetic recording, which does not generate heat and has excellent CSS durability.

[Brief description of drawings]

FIG. 1 is a plan view of the alumite surface of the alumite surface of the alumite substrate of the present invention when viewed with an optical microscope, FIG. 2 is a cross-sectional structure of the alumite chromate, and FIG. 3 is an optical view of the alumite surface of a conventional aluminum alloy substrate. FIG. 4 shows the plane structure of alumite as seen with a microscope, and FIG. 4 shows the relationship between the amount of copper added and the fine Vickers hardness of alumite chromate.

Claims (2)

[Claims] 1. By weight 2 to 6% magnesium and 0.002 to copper.
Containing 0.03%, balance aluminum and impurities, the allowable content of impurities is 0.003% iron, 0.005% silicon
%, Zinc 0.5%, manganese 0.0005%, chromium 0.0005%,
Magnetic recording characterized by forming an alumite film on the surface of an aluminum alloy material containing 0.0005% nickel, 0.0005% titanium, and 0.001% in total of other impurity elements and using chromic acid having a thickness of 6 μm or more on the surface thereof. Manufacturing method of alumite substrate for wood. 2. The method for producing an alumite substrate for a magnetic recording material according to claim 1, wherein the diameter of the colonies present in the coating layer is 7 μm or less.