Stock material or miscellaneous articles – Surface property or characteristic of web – sheet or block – Surface modified glass
Reexamination Certificate
1997-12-24
2001-02-13
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Surface property or characteristic of web, sheet or block
Surface modified glass
C428S428000, C428S690000, C428S690000, C428S900000
Reexamination Certificate
active
06187441
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a glass substrate for an information recording medium, and an information recording medium such as a magnetic recording medium having the glass substrate.
Conventionally, a glass substrate having a high strength for a magnetic recording medium is disclosed in Unexamined Japanese Patent Publication No. S64-42025. This glass contains, by weight, 65-75% of SiO
2
, 4-9% of Al
2
O
3
, 5-10% of B
2
O
3
, 5-9.5% of Na
2
O, 0-3% of K
2
O, 0-0.5% of Li
2
O, 3-6% of BaO and 0.5-3% of ZnO.
Further, a glass substrate having a chemically reinforced glass for an information recording medium is disclosed in Japanese Unexamined Patent Publication No. H5-32431. This glass contains, by weight, 62-75% of SiO
2
, 5-15% of Al
2
O
3
, 4-10% of Li
2
O, 4-12% of Na
2
O, 5.5-15% of ZrO
2
. In this event, the weight ratio Na
2
O/ZrO
2
falls within the range between 0.5 and 2.0. Further, the weight ratio of Al
2
O
3
/ZrO
3
falls within the range between 0.4 and 2.5.
In addition, a chemically reinforced glass which contains, by weight, 58-70% of SiO
2
, 13-22% of Al
2
O
3
, 5-12% of Na
2
O and 2-5% of ZrO
2
is disclosed in Japanese Unexamined Patent Publication No. H8-48537.
Recently, a flying height of a head has been lowered, and a rotating speed of a disk has become high with a miniaturization of the disk and a high recording density. Therefore, the strength and surface accuracy of the disk has been strictly required.
Under the circumstances, the above-mentioned conventional techniques have the following problems.
First, the glass substrate disclosed in the above-referenced Japanese Unexamined Patent Publication No. S64-42025 contains no ZrO
2
. Therefore, the ion exchange rate becomes slow when an ion exchange process is performed. Further, it is difficult to obtain an ion exchange layer (a compressive stress layer) having enough depth. In addition, the strength for destruction is also small.
Second, the glass substrate disclosed in the above-referenced Japanese Unexamined Patent Publication No. H5-32431 is excellent in the ion exchange performance, but contains much ZrO
2
. Consequently, it is difficult to dissolve ZrO
2
and, further, it is impossible to remove projections of undissolved substances of ZrO
2
on the glass surface even when the obtained glass is polished. As a result, the projections are often left on the surface of the glass substrate.
The obtained glass substrate may be practically applicable as a recent glass substrate for the information recording medium. However, the glass substrate may not be able to meet the demand for much higher density of the information recording medium in future. Specifically, the flying height of the magnetic head has been reduced with the increase in recording density. Consequently, the conventional glass substrate is not compatible with the magnetic head having the low flying height such as a magneto-resistive head (MR head) because the conventional glass substrate has projections on the glass surface.
Further, the conventional glass substrate contains much ZrO
2
as mentioned before. Consequently, it is difficult to sufficiently increase a specific modulus because the glass has a large specific gravity. In this event, when the specific modulus becomes enough large, the burring does not easily occur even when the disk rotates at a high speed.
Finally, the glass substrate disclosed in the above-referenced Japanese Unexamined Patent Publication No. H8-48537 has a high liquid phase temperature and a low viscosity at the liquid phase temperature. Consequently, the devitrification is easily performed when a dissolved glass is formed into a plate form by the use of the downdraw molding or the press molding.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a chemically reinforced glass substrate for an information recording medium which has an excellent ion exchange performance and which is capable of providing a deep compressive stress layer for the chemically reinforced glass.
It is another object of this invention to provide a glass substrate which is excellent in strength.
It is a still another object of this invention to provide a glass substrate which has a flat surface.
It is a yet another object of this invention to provide a glass substrate which has a high glass viscosity without devitrification during the formation of the glass substrate.
It is a further object of this invention to provide a glass substrate which has a high specific modulus so that no burring occurs during a high speed rotation of a disk.
According to this invention, the glass substrate contains at least ZrO
2
and Li
2
O. In this event, ZrO
2
has a content which falls within the range between 0.6 and 1.9 mol % while Li
2
O has a content which falls within the range between 6 and 14 mol %. In this invention, the contents of ZrO
2
and Li
2
O are restricted to a predetermined range. Consequently, the projections of the undissolved substance of ZrO
2
are not formed on the surface of the glass substrate. As a result, the glass substrate having the flat surface can be obtained. Further, the chemically reinforced glass which is obtained by the ion exchange method has the deep compressive stress layer and the high strength. Further, the viscosity in the liquid phase temperature can be lowered because the ion exchange performance is sufficiently ensured.
Further, according to this invention, the glass substrate contains at least ZrO
2
, TiO
2
and B
2
O
3
. In this event, ZrO
2
has a content which is 2.8 mol % or less while TiO
2
has a content which falls within the range between 0.2 and 3.0 mol %. Further, B
2
O
3
has a content which is 0.9 mol % or less. In this invention, Young's modulus becomes large without an excessive large specific gravity by containing TiO
2
at a predetermined content. Consequently, the specific modulus can become large.
Moreover, according to this invention, the glass substrate contains ZrO
2
. The glass substrate has projections on the substrate surface. In this case, the height of the projection is 7 &mgr;m or less. In this invention, the height of the projection on glass surface is restricted to the predetermined range. Consequently, the glass substrate can comply with the low flying height of the magnetic head.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Description will be made about a glass substrate for an information recording medium according to embodiments of this invention.
First embodiment
The glass substrate contains ZrO
2
. In this event, the content of ZrO
2
falls within the range between 0.6 and 1.9 mol %. If the content of ZrO
2
exceeds 1.9 mol %, it is difficult to completely dissolve ZrO
2
, and projections of the undissolved substance of ZrO
2
are often left on the glass surface even when the obtained glass is polished. Consequently, it is difficult to obtain enough flatness as the glass substrate for the information recording medium. Therefore, the content of ZrO
2
is restricted to 1.9 mol % or less. From the same reason, the upper limit of the preferable content of ZrO
2
is 1.7 mol %, and more preferably 1.5 mol %. On the other hand, ZrO
2
serves to improve an ion exchange performance. Consequently, the compressive stress layer becomes thick, a transverse bending strength is improved and Knoop hardness becomes large. Therefore, it is necessary that the content of ZrO
2
is 0.6 mol % or more. From the same reason, the lower limit of the content of ZrO
2
is 0.8 mol %, and more preferably 1.0 mol %.
Further, the glass substrate contains Li
2
O. In this event, the content of Li
2
O falls within the range between 6 and 14 mol %. Li
2
O is an essential component for chemically reinforcing the glass by ion-exchanging with Na ions in an ion exchanging solution. When the content of Li
2
O is less than 6 mol %, the ion exchange performance is lowered. Therefore, it is necessary that the lower limit of the content of Li
2
O is 6 mol %, preferably 8 mol %, and more preferably 9 mol %. On the other hand, the content of Li
2
O exceeds 14 mol %, the liq
Kitayama Teruki
Takeuchi Kunio
Hoya Corporation
Kiliman Leszek
Sughrue Mion Zinn Macpeak & Seas, PLLC
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