Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
1999-12-20
2002-07-02
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S070000, C501S072000, C428S065100
Reexamination Certificate
active
06413892
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a glass composition for magnetic recording medium having excellent water resistance and weather resistance, and in particular, it relates to a glass substrate for magnetic recording medium obtained by subjecting a glass blank having surface flatness and small surface warpage obtained by the float process, to tempering.
BACKGROUND ART
In general, aluminum substrates are widely used as magnetic recording medium. Thin plate is desired for the recently used substrates, and high surface flatness is desired for increasing recording density. A glass substrate is suitable as such a magnetic disk substrate, and in particular, forming this glass substrate by the float process is an advantageous method from the standpoint of the quality and cost.
If a blank with great warpage is used, the blank must be ground to correct the warpage. Because a blank formed to have high flatness by the float process does not need such correction, it is possible to produce a glass substrate for magnetic disk using a thin blank, making it possible to decrease cost for grinding.
At present, such glass substrates for magnetic recording medium are used after subjecting a glass substrate having a soda-lime glass composition, which is molded by the float process, to chemical tempering.
That is, in the glass substrates for magnetic recording medium, tempered products are mainly used., that are produced by immersing glass substrates in a molten salt containing a monovalent metal ion having an ionic radius larger than that of the monovalent metal ion contained in the glass composition to thereby exchange the metal ions in the glass for the metal ions in the molten salt.
However, problems are pointed out that if such glass substrates are subjected to a tempering treatment, weathering resistance (water resistance) deteriorates in high temperature and high humidity environment.
Further, where a glass substrate is used, thermal expansion of the glass should match that of a metal part, such as stainless steel or aluminum, which constitutes a magnetic disk drive.
U.S. Pat. No. 4,156,755 describes lithium-containing ion-exchange-reinforced glass as a tempered glass having excellent weathering resistance. This U.S. patent discloses at column 7, lines 2 to 15 a glass composition containing, in % by weight, 59-63% SiO
2
, 10-13% Na
2
O, 4-5.5% Li
2
O, 15-23% Al
2
O
3
, and 2-5% ZrO
2
, Al
2
O
3
+ZrO
2
is 19-25% by weight and Na
2
O/ZrO is 2.2-5.5.
For example, JP-A-62-187140 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a process for producing a tempered glass. This JP-A discloses on page 1, left column, lines 5 to 16 a process for producing a tempered glass containing, in % by weight, 64-70% SiO
2
, 14-20% Al
2
O
3
, 4-6% Li
2
O, 7-10% Na
2
O, 0-4% MgO and 0-1.5% ZrO
2
.
However, the glass compositions disclosed in the examples of U.S. Pat. No. 4,156,755 and JP-A-62-187140 described above require high temperature for melting and forming, and it is not easy to produce a thin sheet having less warpage by the float process.
JP-A-5-32431 describes (on page 2, left column, lines 2 to 7) another tempered glass, and discloses a tempered glass article containing, in % by weight, 62-75% SiO
2
, 4-12% Na
2
O, 4-10% Li
2
O, and 5-15% Al
2
O
3
and 5.5-15% ZrO
2
, wherein the weight ratio of Na
2
O/ZrO
2
is 0.5-2.0 and the weight ratio of Al
2
O
3
/ZrO
2
is 0.4-2.5. Because the composition disclosed in the above JP-A contains a large amount of ZrO
2
, crystals of ZrO
2
tend to precipitate in a furnace in the case of producing using a melting furnace, and there was a problem on quality. That is, when a blank in which crystals of ZrO
2
were precipitated is ground and polished to produce a magnetic disk substrate, ZrO
2
crystals that are hardly polished off remain on the surface as projections and cause a head crush.
JP-B-47-1312 (the term “JP-B” as used herein means an “examined published Japanese patent application”) discloses lithium or sodium aluminosilicate glass as a tempered glass plate suitable as a wind-screen of, for example, automobiles or aircraft and a process for producing the same. There is the disclosure on page 3, right column, lines 29-34 that particularly preferred glass is that its inner part comprises, on the weight of oxide basis, 2-6% LiO
2
, 5-10% Na
2
O, 15-25% Al
2
O
3
and 60-70% SiO
2
, with the sum of Li
2
O, Na
2
O, Al
2
O
3
and SiO
2
being at least 95% by weight of the composition. The Example fallen in the above composition range, described in 3 in Table 1 of column 3 of the above JP-B needs high temperature for melting and forming and it is difficult to produce a high quality glass by the float process.
British Patent 1,322,510 discloses METHOD OF STRENGTHENING A GLASS ARTICLE AND A STRENGTHENED GLASS ARTICLE. One object of the patent is to provide a composition that can produce a glass sheet by Froucault process, Pennvernon or Pittsburgh process, Colburn process (page 1, lines 61-75), and there is the description as conditions for a glass composition therefor that a strain point is 450-550° C., a working temperature is 980-1,150° C. and a liquidus temperature is 1,100° C. or lower. The composition described the British patent had the problems that the working temperature is high, the liquidus temperature is higher than the working temperature so that the composition is not suitable for forming into a thin plate with little warpage by the float process, and weathering resistance of the resulting tempered glass is poor.
Further, in aiming at the improvement of recording density in a magnetic disk substrate, there is a tendency to further decrease a distance between a substrate and a magnetic head, and then the cleanness of the surface of a substrate has become an issue. If any foreign matter is adhered to the surface of a substrate, the head strike against the foreign in reading and writing, causing head crush. To avoid this, the demand for cleanness of the substrate surface has been increasing. Washing with acid is an effective for the improvement of the cleanness. H
2
SO
4
, HF, NHO
3
or the like is used the acid, and HF and H
2
SO
4
are particularly effective.
If glass having poor acid resistance is washed with acid, latent scratch generates. On the other hand, glass having acid resistance more than necessary requires a concentrated acid for washing, and this results in problems on cost, on working, on environment and in waste treatment. An acid concentration suitable for washing is about 0.002-0.3% HF and about 1-20% for H
2
SO
4
.
JP-A-62-187140 supra has the description at column 3, upper right column, lines 3-6 that “Because reinforced glass having an extremely deep compression stress layer can be obtained with ease, the resulting reinforced glass can be used as a disk substrate that is required to have high reliability in strength”. Evaluation on resistance against 0.5% H
2
SO4 is made as the example. However, such is not evaluation for latent scratch by washing, but is evaluation that intends weathering resistance. There is no mention of HF resistance.
Accordingly, an object of the present invention is to provide a glass substrate for magnetic recording medium, that does not substantially contain ZrO
2
which becomes a problem in the production using a melting furnace, has good water resistance or weathering resistance after tempering treatment and comprises a composition that has a melting temperature and a working temperature suitable for forming into a thin plate with little warpage by the float process.
The present invention further has an object to provide a glass substrate for magnetic recording medium, which has a coefficient of thermal expansion that makes it possible to use the substrate in combination with a metal article. The present invention also has an object to provide a glass substrate for magnetic recording medium, which is difficult to cause latent scratch in washing with an acid, particularly in using H
2
SO
4
or HF.
DISCLOSURE OF THE INVENTION
The present inv
Koyama Akihiro
Yamamoto Nobuyuki
Nippon Sheet Glass Co. Ltd.
Sample David
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