Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
1998-12-30
2001-04-10
Evans, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S064400, C428S426000, C428S690000, C428S065100, C428S913000
Reexamination Certificate
active
06214429
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to glass disc substrates for information recording discs such as magnetic discs and optical discs. In particular, it relates to glass disc substrates for information recording disc, which exhibit high specific elastic modulus and/or Young's modulus and high heat resistance and realize excellent surface smoothness. In addition, the present invention relates to magnetic discs using the glass disc substrates of the present invention.
2. Description of the Related Art
Major components of magnetic storage devices of electronic computers and the like are a magnetic recording medium and a magnetic head for reconstruction of magnetically recorded information. Flexible discs and hard disks have been known as magnetic recording media. As substrates for hard discs (magnetic discs), for example, aluminum substrates, glass substrates, ceramic substrates, carbon substrates and the like have been known. In practical use, however, an aluminum substrate or a glass substrate is mainly used according to the intended size and the use thereof.
Recently, flying height of magnetic heads is markedly reduced as hard disc drivers for notebook personal computers are made smaller and their magnetic recording density made higher. Accordingly, extremely high precision has been demanded for the surface smoothness of magnetic disc substrates.
However, it is difficult to produce smooth surface more than a certain level of precision with an aluminum alloy. That is, even though it is polished by using highly precise abrasives and processing apparatuses, the polished surface may suffer from plastic deformation because of the low hardness of the alloy. Even if the aluminum alloy is plated with nickel-phosphorous, the surface roughness Ra cannot be made 20 Å (angstrom) or less. In addition, as hard disk drivers are made smaller and thinner, a further smaller thickness of substrates for magnetic discs is also strongly desired. However, it is difficult to produce such a thin disc with an aluminum alloy having a certain strength defined by specification of hard disk drivers because of low strength and stiffness of aluminum alloy.
Therefore, glass substrates for magnetic discs having high strength, high stiffness, high impact resistance and high surface smoothness have been developed. Because glass substrates have excellent surface smoothness and mechanical strength, they have been paid much attention as substrates for present and future use. For example, as such glass substrates, chemically tempered glass substrates whose surfaces are strengthened by the ion exchange technique, crystallized glass substrates subjected to crystallization treatment, alkali-free glass substrates which do not substantially contain alkaline substances and the like have been known well.
For example, as a chemically tempered glass substrate, Japanese Patent Unexamined Publication No. Hei 1-239036 (JP-A-239036/89) (referred to as Reference
1
hereinafter) discloses a glass substrate for magnetic recording media strengthened by subjecting to ion exchange treatment a glass material for the substrate containing, indicated in terms of % by weight, 60-70% of SiO
2
, 0.5-14% of Al
2
O
3
, 10-32% of R
2
O where R is an alkali metal, 1-15% of ZnO and 1.1-14% of B
2
O
3
.
As a crystallized glass, Japanese Patent Unexamined Publication No. Hei 7-187711 (JP-A-187711/95) (referred to as Reference
2
hereinafter) discloses a glass substrate for magnetic recording media containing, indicated in terms of % by weight, 50-65% of SiO
2
, 18-25% of CaO, 6-11% of Na
2
O, 6-12% of K
2
O, 0-2.5% of Al
2
O
3
and 5-9% of F and containing kanasite as main crystals. U.S. Pat. No. 5,391,522 (referred to as Reference
3
hereinafter) discloses a crystallized glass substrate for magnetic discs containing 65-83% of SiO
2
, 8-13% of Li
2
O, 0-7% of K
2
O, 0.5-5.5% of MgO, 0-5% of ZnO, 0-5% of PbO, (provided that MgO+ZnO+PbO is 0.5-5%), 1-4% of P
2
O
5
, 0-7% of Al
2
O
3
and 0-2% of As
2
O
3
+Sb
2
O
3
and containing microcrystalline particles of Li
2
O.2 SiO
2
as main crystals.
As an alkali-free glass, Japanese Patent Unexamined Publication No. Hei 8-169724 (JP-A-169724/96) (referred to as Reference 4 hereinafter) discloses a glass substrate for magnetic discs having a composition containing, indicated in terms of % by weight, 35-55% of SiO
2
+Al
2
O
3
, 0-10% of B
2
O
3
, 40-60% of CaO+BaO, provided that CaO≧5%, 0-10% of ZnO+SrO+MgO, 0-5% of TiO
2
, 0-5% of ZrO
2
, 0-1% of As
2
O
3
and/or Sb
2
O
3
.
Recent HDDs (hard disk drivers) have been required to have higher recording capacity to meet higher performance of personal computers, and a smaller and thinner disc substrate, smaller flying height of magnetic heads and higher revolution speed of discs have been required to meet smaller size and higher performance of personal computers. It is expected that the thickness of 2.5-inch diameter disc substrates should become thinner from the present thickness 0.635 mm to a thickness of 0.43 mm, or even 0.38 mm. In addition, for recent higher recording density of 3.5-inch hard discs for servers and higher data processing speed, requirement for stiffness of substrate materials becomes increasingly severer, and conventional aluminum substrates seem to almost reach their limits of performance. It is expected that further higher capacity and smaller size of hard discs will be sought in future. Therefore, smaller thickness, higher strength, more excellent surface smoothness, higher impact resistance and the like of substrates for magnetic recording media will be further strongly demanded.
However, as disc substrates become thinner, they become more likely to suffer deflexion and warp. On the other hand, as higher recording density is sought, lower flying height of magnetic heads and higher revolution speed of magnetic discs are further sought yet, and such deflexion and warp of substrates may cause breakdown of magnetic discs. However, if thickness of conventional glass substrates is made thinner than currently used, the problems due to the deflexion and warp mentioned above will become unacceptably marked and thus thinner discs cannot be realized.
Degree of deflexion and warp of substrates can be evaluated from specific elastic modulus (=Young's modulus/specific gravity) or Young's modulus of substrate material. Materials of higher specific elastic modulus are required for suppressing the problems of the deflexion and warp of substrates made with a smaller thickness. Further, materials of higher Young's modulus are required for suppressing the problems of the deflexion of substrates to be rotated at a high speed.
The above situation may be further explained as follows. That is, with recent smaller size, higher capacity and higher speed of HDDs, it is expected that the thickness of 3.5-inch discs currently used of 0.8 mm will be made smaller to 0.635 mm, and 0.635 mm of current 2.5-inch discs to 0.43 mm, or even to 0.38 mm. Revolution speed of substrates is also expected to be made faster from the current maximum speed of 7200 rpm to 10000 rpm, or even to 14000 rpm. As substrates for such magnetic recording media become thinner, they become more likely to suffer deflexion, undulation and warp, and it is expected that, as the revolution speed becomes higher, stress loaded on the substrates (force exerted by wind pressure caused by rotation of discs) will become larger. Based on the theory of dynamics, the deflexion W of a disc receiving load of P per unit area is represented by the following formula:
W
∝
Pa
4
h
3
E
wherein a represents an outer diameter of disc, h represents a thickness of substrate and E represents Young's modulus of disc material.
In static state, force loaded on the disc is the gravitation alone is, and the deflexion W is represented by the following formula:
W
∝
hda
4
h
3
E
=
da
4
h
2
E
=
a
4
h
2
G
wherein d represents a specific gravity of disc material a
Toratani Hisayoshi
Zou Xuelu
Burns Doane Swecker & Mathis L.L.P.
Evans Elizabeth
Hoya Corporation
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