Compositions: ceramic – Ceramic compositions – Devitrified glass-ceramics
Reexamination Certificate
2001-05-16
2004-03-09
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Devitrified glass-ceramics
C428S690000, C428S690000, C065S033800, C065S061000
Reexamination Certificate
active
06703332
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a novel material for substrate of magnetic disks widely used in various kinds of computers, in particular to a glass-ceramic material, and a process for producing the same.
BACKGROUND OF THE INVENTION
In recent years, with the development of magnetic head and magnetic recording layer technology, great improvements for high-density recording and high-speed reading/writing have been achieved, and the demands for better performances of disk substrates are becoming stronger.
In order to increase the recording density of a magnetic disk, the bit density and track density of the magnetic disk have to be increased. This makes the magnetic head become further closer to the disk surface. In this connection, an amount of the head floating from the magnetic disk is decreased to the order of 0.025 microns, so that the surface roughness of the magnetic disk should be 10 Angstroms or less and the disk surface should be very resistive to wear in order to withstand the contact with the magnetic head.
The capability of high-speed of reading/writing currently renders hard disks as a very important device widely used for the purpose of memory and the like. With evolvement of higher density of recording and faster speed for information handling of CPU and the like of a computer, there exist a requirement to correspondingly increase the reading/writing speed of a hard disk, and consequently to increase the rotary speed of the disk. At the present time, a hard disk with a rotary speed of ten thousands revolutions per minute or higher has been market available, and such a high rotary speed inevitably demand a disk substrate with better mechanical properties.
Aluminum alloys have heretofore been used for magnetic disc substrate, but the aluminum alloy substrate tends to give a surface roughness of substantially higher than 10 Angstroms by a polishing process. Furthermore, it is necessary to increase the thickness of disk substrate so as to prevent it from deformation due to the fact that aluminum alloys are soft materials. As a result, an aluminum alloy is insufficient to satisfy the recent requirements for a hard disk substrate.
In order to overcome the defects inherent in aluminum alloys, chemically strengthened glass has been proposed to give a substrate with high hardness, high elastic modulus, small deformation and excellent in surface smoothness after processing. Despite of these, chemically strengthened glass normally has certain amount of alkali components such as Na
+
in its matrix, and the alkali components may adversely affect the magnetic recording layer during the process of producing the same due to alkali migration into the magnetic recording layer. As a result a barrier layer must be applied onto the substrate before applying the magnetic recording layer, which adds to the production cost. In addition, current hard disks tend to be small-sized and thus require the substrate be thinner; however, chemically strengthened glass is instable in the strengthened layer when thinning of the disk is intended. In an attempt to provide a glass substrate for magnetic disks, U.S. Pat. No. 5,691,256 disclosed a glass composition for magnetic disk substrates, which demonstrated improvement properties after subjected to chemically strengthening by ion exchange of the glass or crystallizing the glass by heat-treatment. Although the resulting materials showed an overall improved performance, the composition still contained a substantially amount of alkali components, and the mechanical properties could be further enhanced.
Glass-ceramic has been known in the art to be used for disk substrates as to alleviate the drawbacks of the aluminum alloy substrates and the chemically strengthened glass substrates. Various glass-ceramic compositions have been proposed. For example, all the glass-ceramic systems disclosed in U.S. Pat. Nos. 5,391,522, 5,567,217 and 5,626,935 contain lithium disilicate (Li
2
O·2SiO
2
) and alpha-quartz as their main crystal phases. However, these glass-ceramics are inferior due to the fact that surface roughness (Ra) after polishing is a relative large value (i.e. 15-50 Angstroms) and doesn't satisfy the new requirement on the surface roughness of a substrate for magnetic disk (i.e. less than 10 Angstroms). Besides, the mechanical property of these glass-ceramics is not very good and needs to be further improved so as to satisfy the increasing requirements on the mechanical property of the substrate of magnetic disk.
Glass-ceramics intended for use as substrates of magnetic disks comprising a main crystal phase other than lithium disilicate (Li
2
O·2SiO
2
) are also proposed. For example, U.S. Pat. No. 5,561,089 disclosed a glass-ceramic having a main crystal phase of gahnite. The degree of surface roughness of the polished glass-ceramic is within a range of from 0.5 to 9.0 Angstroms. U.S. Pat. No. 5,726,108 disclosed a glass-ceramic containing at least one selected from the group consisting of mullite and aluminum borate as its main crystal phases. Japanese Patent application Laid-open No. 2000-169184 and 2000-169186 disclosed in their abstracts a glass-ceramic for substrate of magnetic disk having enstatite and/or beta-quartz solid solution as the predominant crystal phases. But all the glass-ceramics proposed in these prior art documents are either poor in the mechanical property or need high processing temperature (such as melting temperature, clarifying temperature, nucleation and crystallization temperature) in order to produce these glass-ceramics. There still exist a need for a glass-ceramic suitable for use as substrate of magnetic disk having higher strength and which can be fabricated by a relative economic process.
SUMMARY OF THE INVENTION
An object of the present invention is to eliminate various disadvantages involved in the above described prior art, and to provide a novel glass-ceramic with much improved bending strength that can be polished to a surface roughness of 10 Angstroms or less. Furthermore, the glass-ceramic of present invention can be fabricated with a reduced cost and is suitable for use as substrate of magnetic disks. Another object of the present invention is to provide a process for producing the glass-ceramic and a substrate of magnetic disk made of the glass-ceramic.
In one aspect, the present invention provides a glass-ceramic for substrate of magnetic disk having high strength, wherein the glass-ceramic is of Li
2
O—SiO
2
—P
2
O
5
—Y
2
O
3
system glass-ceramic and can be obtained by subjecting to heat treatment a base glass which consists essentially of in weight percent: from 65% to 80.0% of SiO
2
, from 0.5% to 5.0% of Al
2
O
3
, from 13.0% to 19.0% of Li
2
O, from 1.0% to 7.0% of P
2
O
5
, from 0.5% to 10.0% of Y
2
O
3
and from 0.2% to 2.0% of at least one of Sb
2
O
3
and As
2
O
3
, and the glass-ceramic comprises Li
2
O·2SiO
2
as the main crystal phase.
According to a preferred embodiment of the invention, the crystal phase of the glass-ceramic of the invention is composed of spherical grains or aggregated spherical grains, and the size of the spherical grains or aggregated spherical grains is controlled to the range from 0.1-1.0 micron by heat treatment.
According to a further preferred embodiment of the invention, the glass-ceramic of the invention has been subjected to a lapping and finally polishing process which produces a surface roughness (Ra) of less than 10 Angstroms.
According to another preferred embodiment of the invention, the glass-ceramic of the invention has a bending strength between 230 and 360 MPa measured in accordance with the National Standards for Testing Ceramic Materials of China (GB6569-86).
In another aspect, the present invention provide a economic process for producing the glass-ceramic for substrate of magnetic disk having high strength, comprising the steps of:
(a) Melting at a temperature within a range from 1300° C. to 1370° C. the base glass of the glass-ceramic with a composition of, in terms of weight percent of their oxides, from 65% to 8
Cao Youming
Huang Shaolin
Peng Bo
Zhan Zusheng
Group Karl
Hubei New Huanguang New Information Materials Co., Ltd.
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