Abrading – Abrading process – Glass or stone abrading
Reexamination Certificate
2000-05-19
2002-04-16
Hail, III, Joseph J. (Department: 3723)
Abrading
Abrading process
Glass or stone abrading
C451S036000, C451S060000, C451S447000
Reexamination Certificate
active
06371834
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a method for preparing a glass substrate for use in making a magnetic recording medium and more particularly to a method for preparing a glass substrate for use in making a magnetic recording medium, which has a uniform surface, does not substantially have any surface defect and has a smooth surface having an extremely low surface roughness.
(b) Description of the Prior Art
There has been tried to reduce the flying height in order to improve the recording density of recording media in response to a substantial increase in the storage capacity of the magnetic disk recording unit. Thus, there has been required for the development of a substrate for use in making a magnetic recording medium, which is excellent in the surface smoothness and has a substantially small number of surface defects, in order to reduce the flying height.
Conventionally, a substrate prepared by plating an aluminum alloy with Ni—P and then subjecting the principal face thereof to a multi-stage polishing operation has mainly been employed as a substrate for use in making a magnetic recording medium, which is excellent in the surface smoothness.
However, a magnetic disk recording unit has recently been adopted even in a portable personal computer such as a notebook-sized personal computer and this accordingly requires the development of a substrate for magnetic recording media capable of withstanding any impact encountered during carrying such a portable personal computer. In addition, the substrate for magnetic recording media should be rotated at a high speed of not less than 10000 rpm to increase the response speed of the magnetic disk recording unit and this correspondingly requires the use of a substrate for magnetic recording media having high strength. Thus, a glass substrate has been used as such a substrate satisfying the foregoing various requirements.
As such a glass substrate for magnetic recording media, there have principally been used a tempered glass substrate whose strength is enhanced by surface chemical strengthening and a crystallized glass substrate obtained by maintaining a glass substrate, which is prepared by melting and molding a raw glass material, at a high temperature ranging from 600 to 800° C. over a long period of time to thus partially separate out crystalline phases therein.
The surface chemical strengthened glass substrate is a glass substrate obtained by, for instance, grinding and polishing a glass substrate capable of being chemically surface-strengthened (hereunder referred to as “chemically surface-strengthenable glass substrate”) and obtained by melting and molding a raw glass material and then immersing the chemically surface-strengthenable glass substrate in a molten salt of, for instance, sodium nitrate or potassium nitrate to thus form a compressive stress-containing layer on the surface thereof and to in turn improve the breaking strength thereof. Moreover, the crystallized glass substrate may be a glass substrate obtained by, for instance, allowing amorphous glass prepared by melting and molding a raw glass material to stand at a high temperature ranging from 600 to 800° C. over a long period of time to give a mixed phase in which 40 to 80% of a crystalline phase and 20 to 60% of an amorphous phase coexist.
If such a crystallized glass substrate or a chemically surface-strengthenable glass substrate is polished according to the well-known technique or using a soft abrasive cloth such as suede-type one along with an abrasive liquid containing a CeO
2
-base abrasive having an average particle size of about 0.5 to 2 &mgr;m in an amount of about several % by mass, in the process for grinding and polishing, in particular, final polishing the glass substrate, however, brush mark-like linear unevenness is generated due to the use of the abrasive cloth and therefore, only glass substrates having uneven surface are obtained. Moreover, the lowest possible surface roughness accomplished by this technique is on the order of 3.5 Å for the chemically surface-strengthenable glass substrate and about 4.5 Å for the crystallized glass substrate. Consequently, it has been quite difficult to achieve a surface roughness of not more than 3 Å, which is required for the glass substrate for use in making magnetic recording media.
SUMMARY OF THE INVENTION
It is thus an object of the present invention to provide a method for efficiently preparing a glass substrate for use in making a magnetic recording medium whose polished surface is quite uniform, which is substantially free of any surface defect and has a smooth surface having an extremely low surface roughness.
The inventors of this invention have conducted various studies to achieve the foregoing object, have unexpectedly found that a glass substrate for use in making a magnetic recording medium, which has a uniform surface free of any brush mark-like linear unevenness, which is substantially free of any surface defect and which has a surface roughness of not more than 3 Å can easily be prepared by polishing a raw glass substrate using an abrasive cloth, preferably a hard abrasive cloth of a foamed polyurethane in combination with an abrasive liquid which contains an ultrafine CeO
2
-base abrasive in a quite low concentration in a final polishing step, after lapping the glass substrate to a desired thickness according to the well-known method and, if necessary, polishing the substrate and have thus completed the present invention.
According to the present invention, there is thus provided a method for preparing a glass substrate for use in making a magnetic recording medium, which is characterized in that an abrasive liquid containing a CeO
2
-base abrasive having an average particle size, D
50
, of not more than 0.5 &mgr;m in an amount of not more than 1% by mass and an abrasive cloth are used in a step for final polishing the glass substrate.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will hereunder be described in more detail.
The glass substrates, to be processed, used in the method for preparing a glass substrate for use in making a magnetic recording medium are not restricted to specific ones insofar as they have sufficient strength required for the glass substrate for magnetic recording media or they can acquire sufficient strength required for the glass substrate through any surface chemical strengthening treatment. However, it is preferred to use crystallized glass substrates and chemically surface-strengthenable glass substrate while taking into consideration the present situation of the techniques.
The abrasive used in the final polishing step of the method according to the present invention comprises CeO
2
-base fine particles whose average particle size: D
50
is not more than 0.5 &mgr;m, preferably not more than 0.3 &mgr;m and more preferably not more than 0.2 &mgr;m. This is because if the glass substrate is polished using an abrasive liquid containing an abrasive having an average particle size of higher than 0.5 &mgr;m, the resulting glass substrate has an increased surface roughness. Therefore, the use of such an abrasive liquid cannot provide any desired glass substrate, which is substantially free of surface defects and has a smooth surface having an extremely low surface roughness.
As the CeO
2
-base abrasive usable in the final polishing step of the method of the present invention, there may be listed, for instance, high purity CeO
2
, mixtures of CeO
2
with oxides of rare earth elements, mixture of CeO
2
and SiO
2
, solid solutions consisting of cerium oxide and silicon oxide such as those disclosed in U.S. Pat. No. 5,766,279. Such a solid solution can be commercially available in the form of desired ultrafine particles and can particularly suitably be used in the method of the present invention because of their excellent dispersibility in water. Such a solid solution preferably consists of 100 parts by mass of cerium oxide and 0.1 to 10 parts by mass of silicon oxide.
The abrasiv
Fujimura Akio
Miura Hiroshi
Hail III Joseph J.
McDonald Shantese
Mitsui Mining and Smelting Co. Ltd.
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