Glass manufacturing – Processes – Forming product or preform from molten glass
Reexamination Certificate
2001-05-22
2003-03-18
Vincent, Sean (Department: 1731)
Glass manufacturing
Processes
Forming product or preform from molten glass
C065S105000, C428S065100
Reexamination Certificate
active
06532770
ABSTRACT:
Described are glass substrates for magnetic media, for example, hard disk drives. Also described are methods for making such substrates, methods for making a magnetic medium from such substrates and the resulting magnetic media.
BACKGROUND OF THE INVENTION
The potential for use of glass substrates for magnetic media, particularly hard disk drives, has been known for some years. Advantages of a glass substrate over the most currently used aluminum substrates include improved shock resistance and higher stability. For example, the improved shock resistance lessens the possibility of “head slap” damage to the drive which can be a problem with metal substrates and the improved stability results in disks having less flutter when driven at high speeds and/or in thinner disks which still retain adequate stability. Additionally, a glass substrate with a highly smooth surface can provide a disk which can be used with the head closer to the disk, resulting in more data storage.
However, the practical applicability of glass substrates has been hindered by the need for significant processing steps to provide a substrate with suitable surface properties from direct pressings or from glass sheets, i.e., the glass material directly from the apparatus by which it is formed into a sheet. It has been known to prepare sheets of glass by a number of processes, including downdraw, float glass, updraw and rolling methods. As for preparation of glass sheets for use as magnetic media substrates, one type of downdraw method, known as fusion-type downdraw, has been disclosed; see Kitayama, U.S. Pat. No. 5,725,625. However, glass sheets produced by this fusion downdraw method still have insufficient surface properties, i.e., the required flatness, waviness, thickness variation and surface roughness, for direct use in preparing substrates for magnetic media. Thus, as disclosed in Kitayama for example, it was necessary to subject the glass sheet to a significant flattening step. In fact, the Kitayama invention is mainly directed to methods for flattening the glass sheet prepared by the fusion downdraw process. Further, the disks cut from the flattened glass sheet in Kitayama (i.e., the substrate blanks) require significant surface treatment to provide a substrate with sufficient surface properties for use as a magnetic media substrate, particularly a hard disk drive substrate. Thus, in Example 2-1 of Kitayama, the blank cut from the flattened glass sheet still requires a course polishing, a lapping and two additional polishing steps to provide a substrate with adequate surface properties.
Sheets of glass prepared by float glass methods also do not result in an adequate combination of surface properties. Combination of surface properties, and the float glass method has the additional drawback of requiring a processing step of leaching the glass of contaminants inherent in the float glass method and/or of contaminants being left on the glass surface.
All of these additional processing steps require significant time and resources. They also require starting with a glass sheet of sufficient thickness such that, after all the surface treatments which remove material are completed, the resulting magnetic media substrate has the proper thickness. Thus, an expense is added to the method by the necessity of forming a blank of higher thickness, e.g., the expense of melting and processing a greater amount of glass material per substrate blank.
The known methods for providing glass substrates for magnetic media are deficient in failing to provide a practically applicable process wherein a substrate having the required superior surface properties can be cut directly from a glass sheet, as formed, such that little or no surface treatment is required for use in preparing magnetic media, particularly hard disk drives.
SUMMARY OF THE INVENTION
At least in part to overcome the deficiencies in the art, it is an object of this invention to provide substrates for magnetic media, particularly hard disk drives, cut from a glass sheet, as drawn, which require little or no surface treatment. Other objects include methods for preparing such substrates, methods for preparing magnetic media comprising such substrates and the resulting magnetic media products of such processes. Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
Accordingly, the invention includes:
A substrate for a magnetic medium prepared from a glass sheet or glass ribbon drawn from a slot downdraw process, particularly wherein the glass sheet has the following properties, as drawn from the slot downdraw process without any subsequent surface treatment:
flatness ≦25 microns, particularly ≦10 microns,
waviness less than 100 angstroms, particularly less than 40 angstroms,
thickness variation ±20 microns, particularly ±15 microns, and
surface roughness, less than 10 angstroms, particularly less than 5 angstroms.
A substrate for a magnetic medium wherein the substrate surface is provided with adequate surface properties solely by touch polishing and cleaning of the glass sheet as drawn from a slot downdraw process.
A substrate as described above which is for a hard disk drive, particularly wherein the substrate is in disk shape and has a magnetic layer thereon.
A method for preparing a glass magnetic media substrate which comprises:
drawing a glass sheet from a slot downdraw process, and
cutting said glass magnetic media substrate from the glass sheet.
The above method wherein the substrate has a flatness ≦25 microns, waviness less than 100 angstroms, thickness variation ±20 microns and surface roughness, less than 10 angstroms, prior to any surface treatment subsequent to drawing from the slot downdraw process.
The above method wherein the only treatment which results in removal of glass material from the substrate surface is touch polishing and cleaning, preferably where the touch polishing decreases the total thickness of the substrate by a maximum of 0.02 mm, preferably a maximum of 0.01 mm.
A magnetic medium comprising a glass substrate prepared by a method as described above and a magnetic layer, particularly wherein the magnetic medium is a hard disk drive.
A method for preparing a hard disk drive having a glass substrate which comprises:
drawing a glass sheet from a slot downdraw process,
cutting a disk from the glass sheet,
touch polishing and cleaning the disk, and
applying a magnetic layer to the disk surface.
The raw material glass used in the slot downdraw process to prepare a glass sheet for use according to the invention can be selected from a wide variety of materials. For use as a substrate for a magnetic medium, however, it is preferred that the glass have the following properties:
a. chemical temperability
b. expansion coefficient from 35 to 100×10
−7
C
−1
c. Young's modulus prior to tempering of more than 65 Mpa
d. density (g/cm
3
) of from 2.3 to 2.7
e. hardness of more than 500 HK (Knoops Hardness, as per ISO 9385 procedure)
It is also desirable that the glass possess only small temperature versus viscosity variations within the forming region in order to attain an optimum process achieving desired dimensional characteristics. This is because control of the flow of the glass through the slot during the downdraw must be uniform in order to provide a glass sheet of specified thickness and properties.
Non-limiting examples of useful glass include aluminosilicate, calcium silicate, zinc silicate and borosilicate glasses, the borosilicates being particularly preferred. Glass compositions should be formulated to provide a functionally insignificant alkali interaction between the glass substrate and the magnetic layer(s) of the magnetic media. Examples of useful glass compositions typically contain, on a mole % based on the oxide, 60-75% SiO
2
0-12% B
2
O
3
, 0-17% Al
2
O
3
, 6-13% Na
2
O, 3-8% K
2
O, 0-10% ZnO, 0-4% TiO
2
, 0-5% MgO, 0-10% CaO, and 0-1% BaO. Minor amounts of As
2
O
3
, Sb
2
Loch Horst
O'Ryan Adam
Uhlik James M.
Wegener Holger
Millen White Zelano & Branigan P.C.
Schott Glass Technologies Inc.
Vincent Sean
LandOfFree
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