Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-12-17
2004-06-01
Kiliman, Leszek B (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S426000, C428S432000, C428S433000, C428S900000, C427S128000, C427S129000, C427S130000, C427S131000, C427S132000
Reexamination Certificate
active
06743529
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a glass substrate for magnetic recording media, a process for producing the same, and a magnetic recording medium obtained with the substrate. More particularly, the invention relates to a glass substrate capable of giving a magnetic recording medium which is effective in attaining a reduced magnetic-head flying height and is so reliable that the information magnetically recorded therein is not lost or attenuated even upon long-term use at a high rotational speed. The invention further relates to a process for producing this substrate.
2. Description of the Related Art
With remarkable progress in the handling of digital information in recent years, various devices for storing such information therein have been developed. Improvements in these devices are ever-progressing, and the information recording capacity and the speed of recording/reproducing are increasing at an annual rate of tens of percents. In particular, the information recording devices which are currently used most widely are magnetic disks, and they are being improved at a higher rate than the other devices.
Under these circumstances, magnetic disks are required to be magnetic recording media capable of recording information at a higher density. For satisfying this requirement, the substrates for magnetic recording media also have come to be required to have high flatness, high smoothness, and rigidity. Because of this, glass substrates, which can be easily ground and polished, are recently coming to be employed as a substitute for the aluminum substrates, which have conventionally been used mainly.
The aluminum substrates for use in this application usually have a multilayer constitution comprising aluminum substrate
ickel layer/phosphorus layer. In the surface of the phosphorus layer, an anisotropic texture comprising concentric marks (an oriented surface roughness) is formed by a mechanical means such as grinding. A magnetic film formed on such an anisotropic texture has C-axis orientation in a circumferential direction, which is caused by an anisotropic film stress, to constitute a so-called oriented medium. This kind of substrate is disclosed in JP-A-6-231442. (The term “JP-A” as used herein means an “unexamined published Japanese patent application”.)
On the other hand, the magnetic recording media employing a glass substrate which have been practically used are ones having an isotropic surface roughness having no anisotropic texture (the term “isotropic surface roughness” is herein used also for magnetic recording media having a surface roughness formed at random). However, the recording media having an isotropic surface roughness have been found to have the following drawback. When such a recording medium is one produced so as to be capable of high-density recording, there are cases where signals once written therein are lost or rapidly attenuated, making it impossible to conduct highly reliable magnetic recording.
In JP-A-63-160010 is disclosed a glass substrate for magnetic recording media obtained by forming a texture in a circumferential direction in a smooth surface of a glass substrate by a mechanical method or chemical etching. However, the magnetic recording medium employing the substrate obtained by this process has a problem that it cannot have a fine surface roughness and, hence, it is difficult to glide a magnetic head thereover at a small height (to scan the magnetic head with a low glide height) so as to enable high-density recording. It has hence been proposed recently to use a mixture of cerium oxide, which has the high ability to abrade glasses, and a diamonds lurry (see JP-A-2000-101656). Other techniques which have been proposed include to use a slurry containing a solution having hydroxyl groups, such as an aqueous potassium hydroxide solution or aqueous sodium hydroxide solution, and thereby impart a chemical action to a mechanical processing force (see JP-A-2000-301441 and JP-A-2001-9694).
The technique of mechanically imparting a surface roughness (mechanical texture) to the multilayered film composed of a nickel layer and a phosphorus layer has a drawback that the processing for forming the roughness tends to generate foreign particles or dust particles and this not only results in a reduced yield in the production of magnetic recording media but also leads to an increased cost. It has hence been proposed in the reference cited above to use a substrate obtained by directly forming an anisotropic texture on a surface of a glass substrate. However, this technique has a drawback that since glass plates have a higher surface hardness than aluminum substrates, it is difficult to use the above-described technique of the related art to form a fine texture. Furthermore, the technique of using-cerium oxide, which has the high ability to abrade glasses, and the technique of using a slurry containing a solution having hydroxyl groups have the following drawback. Although these techniques are effective in forming a fine texture, the shape of the texture readily changes upon cleaning with a chemical due to the high fineness thereof. Because of this, there have been severe restrictions on the use of chemicals in removing diamond or other abrasive grains caught in grooves of the texture. In particular, acid aqueous solutions, which are highly effective in cleaning multicomponent glasses, cannot be used because they considerably change the texture shape. Due to such restrictions, substrates obtained through the texturing treatment often have residual abrasive grains which were contained in the slurry used.
SUMMARY OF THE INVENTION
An object of the invention is to eliminate the problems described above, and, more specifically, is to provide a glass substrate giving a magnetic recording medium in which reading/writing operations can be conducted while gliding the magnetic head at a smaller height, and to provide a process for producing the glass substrate.
According to a first aspect of the invention so as to solve the above problems, a process for producing a glass substrate for magnetic recording media is provided which comprises forming a surface roughness in the main surface of a glass plate which has been formed into a disk shape, wherein the surface roughness is formed by imparting processing marks having a permanent strain to the main surface of the glass plate in a circumferential direction thereof and then chemically etching the whole main surface of the glass plate.
The term “processing marks” as used herein means those surface and near-surface parts in a glass which have a residual permanent strain formed by a mechanical stress without or with a physical change in shape. Although it is usually difficult to directly subject a glass substrate, which has a high hardness, to texturing (surface processing) by a mechanical means, a permanent strain can be formed therein relatively easily.
In one preferred embodiment of the process of the invention for producing a glass substrate for magnetic recording media, the surface roughness is formed based on a difference in the depth of chemical etching between the areas having the processing marks and the areas not having the processing marks. This method of surface roughness formation according to the invention is based on the fact that the glass in the areas having a permanent strain differs in resistance to acids or alkalis from that in the areas not having a permanent strain. Specifically, the surface roughness formation is based on the fact that the areas having a permanent strain have higher acid resistance (lower susceptibility to chemical etching with an acid solution) than the areas not having a permanent strain, or is based on the fact that the areas having a permanent strain have higher alkali resistance (lower susceptibility to chemical etching with an alkaline solution) than the areas not having a permanent strain.
The fact described above was experimentally found by the present inventor. Based on this fact, processing marks having a permanent strain i
Ikeda Hiroshi
Kurachi Junji
Mitani Kazuishi
Saito Yasuhiro
Kiliman Leszek B
Nippon Sheet Glass Co. Ltd.
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