Stock material or miscellaneous articles – Circular sheet or circular blank – Recording medium or carrier
Reexamination Certificate
2002-05-10
2004-10-26
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Circular sheet or circular blank
Recording medium or carrier
C428S066700, C428S690000
Reexamination Certificate
active
06808784
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a glass substrate for magnetic recording media and a manufacturing method thereof.
2. Description of the Related Art
Aluminum substrates have hitherto been widely used as substrates for magnetic recording media such as magnetic disks. However, as notebook/mobile personal computers have gained in popularity, there have been increased demands for magnetic disks to be made thinner, for the recording density of the recording surfaces of magnetic disks to be increased, and for magnetic disks to be made more durable to changes in usage environment. To cope with these demands, in recent years glass substrates have become widely used, since glass substrates have high shock resistance, rigidity and hardness, and high chemical durability to changes in usage environment, and also a high degree of flatness, enabling the flying height of a magnetic head to be made low, which is indispensable for increasing the recording density of the recording surfaces.
In general, in mechanical processing such as grinding to adjust the dimensions (diameters) of the inner and outer peripheral edge surfaces of a donut-shaped glass substrate to predetermined dimensions, or chamfering to form chamfered surfaces of predetermined shapes, the predetermined dimensions or shapes are realized by grinding down the glass using abrasive grains attached to grindstones.
The mechanism by which the abrasive grains grind down the glass is as follows. The abrasive grains attached to the rotating grindstones collide with the surfaces of the glass substrate, thus producing cracks in the surfaces of the glass substrate due to the impact force. The cracks then grow, whereupon a small amount of glass breaks away from the surfaces of the glass substrate.
However, if the breakaway of the small amount of glass is incomplete, then minute cracks will remain in the surfaces of the glass substrate. The glass substrate may subsequently be subjected to mechanical or thermal shock, for example thermal shock generated when films are formed on the surfaces of the glass substrate during manufacture of a magnetic disk, mechanical shock generated when the magnetic disk using the glass substrate is incorporated into a hard disk drive, or mechanical or thermal shock generated due to changes in usage environment of a notebook/mobile personal computer into which the magnetic disk has been incorporated. The minute cracks will not grow much if stress due to such mechanical or thermal shock is dispersed over the whole of the glass substrate, but will grow at quite a high rate if stress due to such mechanical or thermal shock is concentrated in one place on the glass substrate and this place coincides with a place where minute cracks remain. In such a case, the minute cracks will grow into larger cracks, and these larger cracks may cause the magnetic disk using the glass substrate to break.
Examples of causes of minute cracks remaining include the shape of the diamond abrasive grains attached to the grindstones being poor, the grain size of these abrasive grains being too large, or the grinding rate being too high.
Moreover, stress due to mechanical or thermal shock tends to be concentrated at boundary parts between the inner and outer peripheral edge surfaces and the chamfered surfaces. These boundary parts are processed by the grindstones from each side thereof, and hence minute cracks are prone to remain, which may grow into larger cracks as described above.
Furthermore, at boundary parts between the major surfaces and the chamfered surfaces, if the angle between the chamfered surface and the major surface is 135° or less, then so-called chipping in which glass breaks away from the major surface side during mechanical grinding will be prone to occur.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a glass substrate for magnetic recording media and a manufacturing method thereof, which enable damage to a magnetic recording medium using the glass substrate to be prevented.
To attain the above object, in a first aspect of the present invention, there is provided a donut-shaped glass substrate for magnetic recording media, comprising major surfaces, an outer peripheral edge surface, an inner peripheral edge surface, and chamfered surfaces, each connecting between one of the major surfaces and one of the outer peripheral edge surface and the inner peripheral edge surface, wherein each of the chamfered surfaces comprises a conical surface part and an annular curved surface part connected to one another, and for each of the chamfered surfaces, a percentage ratio of a length of an outline of the annular curved surface part to a length of an outline of the chamfered surface in a cross section of the glass substrate including a central axis of the glass substrate is not less than a predetermined value.
In the first aspect of the present invention, it is preferable that the predetermined value is 20%.
More preferably, the predetermined value is 50%.
In the first aspect of the present invention, it is also preferable that for each of the chamfered surfaces, a radius of curvature of the outline of the annular curved surface part is in a range of 0.10 to 0.50 mm.
More preferably, the radius of curvature is in a range of 0.20 to 0.35 mm.
To attain the above object, in a second aspect of the present invention, there is provided a donut-shaped glass substrate for magnetic recording media, comprising major surfaces, an outer peripheral edge surface, an inner peripheral edge surface, and chamfered surfaces, each connecting between one of the major surfaces and one of the outer peripheral edge surface and the inner peripheral edge surface, wherein for each of the chamfered surfaces, an angle between the chamfered surface and one of the major surfaces connected to the chamfered surface is an obtuse angle.
In the second aspect of the present invention, it is preferable that the angle is in a range of 136 to 165°.
More preferably, the angle is in a range of 140 to 155°.
To attain the above object, in a third aspect of the present invention, there is provided a method of manufacturing a donut-shaped glass substrate for magnetic recording media, comprising a disk processing step of forming major surfaces, an outer peripheral edge surface and an inner peripheral edge surface, and a chamfering step of chamfering angular parts, each between one of the major surfaces and one of the outer peripheral edge surface and the inner peripheral edge surface, thus forming a glass substrate for magnetic recording media according to the first or second aspect of the present invention.
Preferably, the method of manufacturing a glass substrate for magnetic recording media further comprises, after the chamfering step, a major surface polishing step of polishing the major surfaces with a polishing amount of not less than 5 &mgr;m, and an edge surface polishing step of polishing the outer peripheral edge surface and the inner peripheral edge surface with a polishing amount of not less than 5 &mgr;m.
More preferably, a mother glass of the glass substrate is a silicate glass containing at least one compound selected from the group consisting of Li
2
O and Na
2
O as an alkali oxide component, and the method further comprises, after the major surface polishing step, a chemical strengthening step of replacing alkali metal ions of the alkali oxide component in a surface layer of each of the major surfaces with alkali metal ions having a larger ionic radius than the alkali metal ions of the alkali oxide component.
According to the first aspect of the present invention, for each of the chamfered surfaces, the percentage ratio of the length of the outline of the annular curved surface part to the length of the outline of the chamfered surface in a cross section of the glass substrate including the central axis of the glass substrate is not less than a predetermined value, preferably 20%. As a result, stress arising in boundary parts, each between one of the chamfered surfaces and one of
Hashimoto Jun-ichi
Matsuno Kensuke
Watanabe Takeo
Frishauf Holtz Goodman & Chick P.C.
Hoya Corporation
Resan Stevan A.
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