Compositions: ceramic – Ceramic compositions – Devitrified glass-ceramics
Reexamination Certificate
2000-07-07
2003-09-30
Group, Karl (Department: 1773)
Compositions: ceramic
Ceramic compositions
Devitrified glass-ceramics
C428S690000
Reexamination Certificate
active
06627566
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to Japanese Patent Application No. 193574/1999 filed in Japan on Jul. 7, 1999 and Japanese Patent Application No. 293003/1999 filed in Japan on Oct. 14, 1999; the entire content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to crystallized glasses suitable for substrates which are used for information recording media, such as magnetic disks, optical disks and optical magnetic disks, substrates for information recording media composed of this crystallized glass, and information recording media using said substrate for information recording medium.
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 disks and hard disks have been known as magnetic recording media. As substrates for hard disks, aluminum alloy has been mainly used. Recently, flying height of magnetic heads is markedly reduced as hard disk 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 disk 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 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 equal to or less than 5 Å (angstrom). In addition, as hard disk drivers are made smaller and thinner, a further smaller thickness of substrates for magnetic disks is also strongly desired. However, it is difficult to produce such a thin disk 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 disks of which high strength, high stiffness, high impact resistance and high surface smoothness are required have been developed. Among these, chemically reinforced glass substrates whose surfaces are strengthened by the ion exchange technique, crystallized glass substrates subjected to crystallization treatment and the like have been known well.
As a chemically reinforced glass substrate by ion-exchange, for example, a glass disclosed in Japanese Patent Unexamined Publication No. Hei. 1-239036 (JP-A-239036/89) has been known. This chemically reinforced glass substrate is such a glass substrate for magnetic disks as the glass containing, indicated in terms of % by weight, 50-65% of SiO
2
, 0.5-14% of Al
2
O
3
, 10-32% of R
2
O where R is an alkali metal ion, 1-15% of ZnO and 1.1-14% of B
2
O
3
is reinforced by forming a crushing stress layer on the glass substrate with an ion exchange method by an alkali ion.
In addition, as a crystallized glass, for example, a glass disclosed in Japanese Patent Examined Publication No. 2516553 is exemplified. This crystallized glass is such a crystallized glass for magnetic disks which contains, indicated in terms of % by weight, 65-83% of SiO
2
, 8-13% by 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 contains micro crystalline particles of Li
2
O.2SiO
2
as main crystals.
Moreover, a crystallized glass is also disclosed in Japanese Patent Unexamined Publication No. Hei. 7-291660 (JP-A-291660/95). This crystallized glass is obtained by heat treatment after fusion and forming a glass which consists of, indicated in terms of % by weight, 38-50% of SiO
2
, 18-30% of Al
2
O
3
, 10-20% of MgO, provided that having a composition containing, indicated in terms of weight ratio, 1.2-2.3 of Al
2
O
3
/MgO, 0%-5% of B
2
O
3
, 0%-5% of CaO, 0%-5% of BaO, 0%-5% of SrO, 0.5%-7.5% of ZnO, 4%-15% of TiO
2
, 0%-5% of ZrO
2
and 0%-2% of As
2
O
3
and/or Sb
2
O
3
. This glass is a cordierite based crystallized glass characterized by containing cordierite based crystals as crystals. Moreover, a substrate for magnetic disks composed of this crystallized glass is also disclosed.
In addition, a crystallized glass is also disclosed in Japanese Patent Unexamined Publication No. Hei. 9-77531 (JP-A-77531/97) (U.S. Pat. No. 5,476,821). This crystallized glass is a ceramic product having the Young's modulus in the range of from about 14×10
6
-about 24×10
6
psi (96-165 GPa) and the fracture toughness of more than 10 MPa·m
½
. In addition, this crystallized glass consists of crystalline phase laminated body which mainly consists of crystals having a spinel structure and a uniform size and dispersing uniformly in a siliceous-rich residual matrix. This is a glass ceramic which substantially consists of, indicated in terms of % by weight using oxides as a standard, 35-60% of SiO
2
, 20-35% of Al
2
O
3
, 0-25% of MgO, 0-25% of ZnO, 0-20% of TiO
2
, 0-10% of ZrO
2
, 0-2% of Li
2
O and 0-8% of NiO. This has at least about 10% of a total amount of MgO+ZnO and may contain equal to or less than 5% of an optional component selected from the group composed of BaO, CaO, PbO, SrO, P
2
O
5
, B
2
O
3
and Ga
2
O
3
, in the range of from 0 to 5% of R
2
O selected from the group consisting of Na
2
O, K
2
O, Rb
2
O and Cs
2
O, and in the range of from 0 to 8% of transition metals. In the case of containing less than about 25% of Al
2
O
3
, this is a glass ceramic having a composition in which the total amount of TiO
2
+ZrO
2
+NiO is equal to or more than 5%. In above publication, the substrate for magnetic disks consisting of this glass ceramic is disclosed.
In addition, a crystallized glass is also disclosed in U.S. Pat. No. 5,491,116. This crystallized glass is a glass ceramic product having the fracture coefficient of at least about 15,000 psi, the Knoop hardness exceeding about 760 KHN, the Young's modulus of more than about 20×10
6
psi and the fracture toughness of more than 1.0 MPa·m
½
. The main crystals of the crystallized glass are enstatite or its solid solution and spinel (spinel structure crystal), and the crystallized glass contains at least 92% of the composition substantially composed of, indicated in terms of % by weight, 35-60% of SiO
2
, 10-30% of Al
2
O
3
, 12-30% of MgO, 0-10% of ZnO, 5-20% of TiO
2
and 0-8% of NiO. Moreover, the substrate for magnetic disks composed of this crystallized glass is also disclosed. It is to be noted that the same glass as the crystallized glass disclosed in aforementioned patent is also disclosed in Journal of Non-Crystalline Solids 219(1997) 219-227.
However, along with making hard disks smaller and thinner and making recording density higher, it is rapidly developed to make flight height of magnetic heads smaller and revolution speed of disks higher. Thereby, substrate materials are more strictly required the strength, the Young's modulus, the smoothness of the surface and the like. In particular, by making the information recording density of 3.5-inch hard disks for personal computers and severs higher, the surface smoothness and the surface flatness of the substrate materials are strictly required. In addition, corresponding to the higher data processing speed, it is required to set the winding number of the disks equal to or higher than 10,000 rpm. Thus, the requirement for stiffness of substrate materials becomes increasingly severer, and the limitation of conventional aluminum substrates already becomes obvious. In future, as long as it is necessarily demanded to make the capacity of hard disks higher and to make the revolution speed of hard disks higher, it is clear that the substrate materials for magnetic recording medium is strongly required to exhibit hi
Burns Doane Swecker & Mathis L.L.P.
Group Karl
Hoya Corporation
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