Crystallized glass substrate for information recording medium

Details Crystallized glass substrate for information recording medium Crystallized glass substrate for information recording medium

2000-07-26

2003-09-30

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Devitrified glass-ceramics

C501S005000, C501S009000, C428S690000

Reexamination Certificate

active

06627565

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to crystallized glass substrates for data recording media. Particularly, the present invention relates to substrates made of a glass ceramic having high strength and high stiffness useful as various electric and electronic components such as magnetic disk substrates or ferrules.
More specifically, crystallized glasses of the present invention can be easily molded and contain crystal species precipitated by glass heat treatment, such as &agr;-quartz solid solution or quartz-based crystals, enstatite, etc. The present invention also relates to high-Young's modulus crystallized glass substrates made of a glass composition that can be easily polished and has high Young's modulus and high surface smoothness as well as an expansion coefficient adaptable to that of other stainless components incorporated into HDDs, said composition being obtained by heat-treating an MgO—Al
2
O
3
—SiO
2
glass at an appropriate temperature using TiO
2
as a nucleating agent to crystallize it.
2. Description of the Related Art
Main components of magnetic storage apparatus such as computers are magnetic recording media and magnetic recording and reproducing heads. Known magnetic recording media include flexible disks and hard disks. Aluminum alloys have been mainly used as substrate materials for hard disks. Recently, the flying height of magnetic heads has been remarkably reduced as the magnetic recording density of hard disk drives for personal computers or servers becomes higher. Thus, there is a demand for extremely high precision in surface smoothness of magnetic disk substrates. However, it is difficult to produce a flat surface from aluminum alloys to satisfy a certain precision level, because aluminum alloys have too low hardness to avoid plastic deformation even if they are polished with high-precision abrasives and machining tools. As the recording density in hard disk drives becomes higher, a demand for reducing deflection or vibration of substrates for magnetic disks during high-speed rotation has also been increasing. However, it is difficult for aluminum alloys to keep some deflection or vibration level required by the specifications of hard disk drives spinning at high speed on the order of 10000 rpm because its Young's modulus is low. Thus, glass substrates for magnetic disks with high stiffness and high surface smoothness appeared. Among those, chemically strengthened glass substrates and crystallized substrates are well known in which the substrate surface has been strengthened by ion exchange and subjected to a crystallization treatment, respectively.
For example, a glass substrate strengthened by ion exchange is disclosed in JP-A No. 239036/89, which relates to a glass substrate for magnetic disks strengthened by forming a compression stress layer by alkali ion exchange on the surface of the glass substrate comprising, expressed in weight percent, 50-65% SiO
2
, 0.5-14% Al
2
O
3
, 10-32% R
2
O wherein R represents an alkali metal ion, 1-15% ZnO and 1.1-14% B
2
O
3
.
For example, a crystallized glass is disclosed in U.S. Pat. No. 5,391,522, which relates to a crystallized glass for magnetic disks comprising, expressed in weight percent, 65-83% SiO
2
, 8-13% Li
2
O, 0-7% K
2
O, 0.5-5.5% MgO, 0-5% ZnO, 0-5% PbO, provided that MgO+ZnO+PbO=0.5-5%, 1-4% P
2
O
5
, 0-7% Al
2
O
3
, 0-2% As
2
O
3
+Sb
2
O
3
and containing fine Li
2
O.2SiO
2
crystalline grains as major crystals.
U.S. Pat. No. 5,476,821 discloses a crystallized glass for disks comprising, expressed in weight percent, oxide components such as 35-60% SiO
2
, 20-35% Al
2
O
3
, 0-25% MgO, 0-25% ZnO, provided that MgO+ZnO>10%, 0-20% TiO
2
, 0-10% ZrO
2
, 0-2% Li
2
O, 0-8% NiO, provided that TiO
2
+ZrO
2
+NiO>5% and containing spinel crystalline grains as major crystals.
U.S. Pat. No. 5,491,116also discloses a crystallized glass. This crystallized glass is a glass-ceramic article exhibiting a modulus of rupture of at least about 15,000psi, a Knoop hardness of greater than about 760 KHN, a Young's modulus of about 20×10
6
psi and a fracture toughness in excess of 1.0 Mpa.m
1/2
, wherein major crystalline phases comprise enstatite or its solid solution and spinel crystals, the article having a composition at least 92% of which consists essentially, expressed in weight percent, of 35-60% SiO
2
, 10-30% Al
2
O
3
, 12-30% MgO, 0-10% ZnO, 5-20% TiO
2
and 0-8% NiO. A substrate for magnetic disks comprising said crystallized glass is also disclosed.
However, the recent increase of magnetic recording density in hard disks accelerates the reduction of the flying height of magnetic heads and the increase of the rotation speed of disks, which imposes more strict requirements on Young's modulus or surface smoothness of substrate materials for disks. Particularly, the recent increase of data recording density in 3.5-inch hard disks for personal computers and servers strictly requires surface smoothness and flatness for substrate materials. In load/unload mode (ramp load mode), extremely flatter media with low bumps and less variation in bump height are required as compared with CCS (contact start/stop mode). Specifically, Rmax=3-10 nm or less and Ra=0.2-2.5 nm or less are needed. The requirements for the stiffness of substrate materials have also become stricter in order to provide disks with a rotation speed of 10000 rpm or more to increase data processing speed. There is also a demand for substrate materials having a high expansion coefficient of 90×10
−7
/° C. or more to suit to the thermal expansion of stainless components incorporated into hard disks. The capacity and rotation speed of hard disks are expected to be further higher in future, which should necessarily generate a great demand for substrate materials for magnetic data media having even higher Young's modulus, high expansion, excellent surface flatness, shock resistance or other properties.
Therefore, it is evident that chemically strengthened glasses as disclosed in JP-A No. 239036/89 having a Young's modulus of about 80 GPa will be insufficient for future strict requirements for hard disks. Glasses chemically strengthened by ion exchange were found to have the disadvantage that they contain a large amount of alkaline components, which are precipitated as alkali ions at pinholes in magnetic films or thin sites such as peripheries of magnetic films or the sites from which glass is exposed after long use in a high-temperature and high-humidity environment, resulting in corrosion or deterioration of the magnetic films. Since conventional glasses strengthened by ion exchange contain a large amount of alkali ions for ion exchange, most of them have too low Young's modulus (100 GPa) and too low stiffness to meet the requirements by 3.5-inch substrates with high recording density and high rotation speed. Some heat treatment is sometimes applied to improve properties such as coercive force of a magnetic layer after the magnetic layer has been provided on a glass substrate during the process for manufacturing a magnetic data medium, but conventional glasses strengthened by ion exchange as described above cannot achieve a high coercive force because they have low heat resistance as evident from their glass transition temperature of at most 500° C.
Conventional crystallized glasses as disclosed in U.S. Pat. No. 5,391,522 are a little superior to the chemically strengthened glasses in Young's modulus and heat resistance. However, they are difficult to combine with other components of hard disks and to incorporate into a precise mechanism design required for high density because of their thermal expansion coefficient of at most about 70×10
−7
/° C. Another problem is that they have low surface smoothness as shown by surface roughness>10 angstroms, which limits reduction of the flying height of magnetic heads to hinder high-density magnetic recording. Moreover, they cannot be app

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