Crystallized glass for information recording medium,...

Details Crystallized glass for information recording medium,... Crystallized glass for information recording medium,...

2003-02-25

2004-08-10

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Devitrified glass-ceramics

C501S005000, C501S009000, C428S690000

Reexamination Certificate

active

06774072

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a crystallized glass suitable for a substrate for an information recording medium such as a magnetic disc, optical disc, or optomagnetic disc, a substrate for information recording medium using such a crystallized glass substrate, and an information recording medium using such a substrate for information recording medium. More particularly, this invention relates to a crystallized glass capable of providing a glass substrate having a high Young's modulus, as well as excellent mechanical strength, surface flatness, and heat resistance and having an excellent surface smoothness upon polishing and to a glass substrate having an excellent surface smoothness using such a crystallized glass.
2. Description of Related Art
Major components of a magnetic recording apparatus in, e.g., a computer, includes a magnetic recording medium and a magnetic head for magnetic recording and reproducing. As a magnetic recording medium, known are a flexible disc and a hard disc. Aluminum alloy, among materials, has been used for the substrate material for hard disc. The floating amount of the magnetic head is significantly reduced in accordance with recent trends that hard disc drives for note type personal computer are made smaller and that the magnetic recording is made with higher density. A very high precision, according to those trends, is required for surface smoothness on the magnetic disc substrate. In the case of an aluminum alloy, however, it is difficult to manufacture a flat surface with a precision of a certain degree or higher, because the polished surface may be plastically deformed due to a low hardness even if a polishing material having a high precision and a machine tool are used for polishing. Even if nickel-phosphorus plating is made on a surface of the aluminum alloy, the surface roughness Ra cannot be controlled at five angstroms or less. According to the developments of the hard disc drives that become smaller and thinner, there are strong demands for making thinner the magnetic disc substrate. The aluminum alloy, however, has low strength and rigidity, and therefore, it is difficult to make the disc thin while the hard disc drive maintains certain strength as required from the specification for the drive.
To solve such problems, a glass substrate for magnetic disc claiming high strength, high rigidity, high impact resistance, and high surface smoothness has been developed. Chemically reinforced glass substrates whose substrate surface is reinforced with an ion exchange method and crystallized substrates subjecting to a crystallization process, inter alia, are known well.
As a glass substrate reinforced by ion exchange, e.g., a glass disclosed in Japanese Unexamined Patent Publication No. 1-239,036 has been known. This ion exchange reinforced glass substrate is made of a glass including, by percent by weight, SiO
2
of 50-65%, Al
2
O
3
of 0.5 to 14%, R
2
O (wherein R denotes alkali metal ion) of 10 to 32%, ZnO of 1 to 15%, and Ba
2
O
3
of 1.1 to 14% where the glass is reinforced by forming compression stress layers on a surface of the glass substrate by an ion exchange method with alkali ions, and the above publication discloses such a glass substrate for magnetic disc.
As a crystallized glass, e.g., there is a disclosure in Japanese Patent Publication No. 2,516,553. This crystallized glass includes, by weight, SiO
2
of 65 to 83%, Li
2
O of 8 to 13%, K
2
O of 0 to 7%, MgO of 0.5 to 5.5%, ZnO of 0 to 5%, PbO of 0 to 5% (provided that MgO+ZnO+PbO is of 0.5 to 5% by weight), P
2
O
5
of 1 to 4%, Al
2
O
3
of 0 to 7%, and As
2
O
3
+Sb
s
O
3
of 0 to 2% and is a glass for magnetic disc including fine crystal particles of LiO
2
.2SiO
2
as a primary crystal.
A crystallized glass is disclosed also in Japanese Unexamined Patent Publication No. 7-291,660. The crystallized glass is obtained by melting a glass composed of, by percent by weight, SiO
2
of 38 to 50%, Al
2
O
3
of 13 to 30%, MgO of 10 to 20% provided that, by weight ratio, Al
2
O
3
/MgO is 1.2 to 2.3, B
2
O
3
of 0 to 5%, CaO of 0 to 5%, BaO of 0 to 5%, SrO of 0 to 5%, ZnO of 0.5 to 7.5%, TiO
2
of 4 to 15%, ZrO
2
of 0 to 5%, and As
s
O
3
and/or Sb
2
O
3
of 0 to 2% and thermally treating it after molding, and is a cordierite based crystallized glass having a feature that the glass contains as a primary crystal a cordierite based crystal. The above publication also discloses a substrate for magnetic disc made of a crystallized glass.
Another crystallized glass is also disclosed in Japanese Unexamined Patent Publication No. 9-77,531. This crystallized glass is a glass ceramic product having a Young's modules in a range from about 14×10
6
to about 24×10
6
psi (96 to 165 Gpa) and a breakdown tenacity more than 1.0 Mpa·m
½. The crystallized glass is constituted of a crystal phase conglomerate mainly made of a spinel type crystal uniformly sized and evenly dispersed in the residual glass matrix phase including rich silicon. The glass is substantially made of, by percent by weight, SiO
2
of 35 to 60%, Al
2
O
3
of 20 to 35%, MgO of 0 to 25%, ZnO of 0 to 25%, TiO
2
of 0 to 20%, ZrO
2
of 0 to 10%, Li
2
O of 0 to 2%, NiO of 0 to 8%, wherein the total of MgO and ZnO is at least 10%, and may contain an arbitrary component selected up to 5% from a group constituted of BaO, CaO, PbO, SrO, P
2
O
5
, B
2
O
3
and Ga
2
O
3
, and R
2
O of 0 to 5% selected from a group constituted of Na
2
O, K
2
O, Rb
s
O, and Cs
s
O, and a transitional metal oxide of 0 to 8%. The glass may be a glass ceramic having a composition in which the total amount of TiO
2
+ZrO
2
+NiO is of 5% or more in the case where Al
2
O
3
is contained only in an amount of about 25% or less, and the above publication also discloses a substrate for magnetic disc made of a glass ceramic.
However, in accordance with recent trends that the hard discs are made smaller and thinner and that the recording is made with a higher density, flying height of the magnetic head is lower, and the disc is rotated at a higher speed, so that strength, Young's modulus, and surface smoothness of the disc substrate material are further severely needed. Particularly, the surface smoothness and surface flatness of the substrate material are strictly on demands due to trends for higher density information recording on 3.5 inch hard discs for personal computers and servers, and the disc has to be rotated at 10,000 rpm or more in corresponding to higher speed of data processing. While the rigidity of the substrate material is subject to a further strict standard, it is apparent that the conventional aluminum substrate is already limiting itself. As far as demands for disc drives having a higher capacity and higher speed are necessary from now on, a substrate material for magnetic recording medium is required doubtlessly to have high Young's modulus, high strength, excellent surface flatness, good impact resistance, and so on.
The necessity for high Young's modulus can be illustrated based on the following facts. That is, according to recent trends for HDDs (hard disc drives) which are made smaller with high capacity and high operation speed, future substrates for magnetic recording medium may have the thickness of 0.635 mm, currently 0.8 mm in the case of 3.5 inches and of 0.43 mm or 0.38 mm, currently 0.635 mm in the case of 2.5 inches, and the rotation speed of the substrate may predictably be made higher to 140,000 rpm from current 100,000 rpm as the maximum speed. Such a substrate for magnetic recording medium may tend to sustain more looseness and undulation and warp as the substrate for magnetic recording medium becomes thinner, and the stress (force exerted to the disc based on an air pressure created from rotation) that the substrate receives predictably becomes larger as the substrate spins with a higher speed. Based on a dynamics theory, flexion W of a disc which receives load P per unit area is denoted as follows:
W
⁢
 
⁢
&in

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