Glass manufacturing – Processes – With chemically reactive treatment of glass preform
Reexamination Certificate
2001-05-24
2003-12-30
Vincent, Sean (Department: 1731)
Glass manufacturing
Processes
With chemically reactive treatment of glass preform
C065S031000, C065S033100, C065S061000, C134S002000, C134S004000, C134S007000, C134S042000
Reexamination Certificate
active
06668587
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for the production of a glass substrate for magnetic recording mediums and more particularly to a method for the production of a glass substrate for magnetic recording mediums, which can inhibit the formation of any needle-like projection containing an alkali metal carbonate on the surface of the glass substrate.
2. Description of the Prior Art
There has been desired for the reduction of the magnetic head's flying height of a magnetic disk-recording device in order to improve the recording density of a magnetic recording disk, as the storage capacity of the magnetic disk-recording device has increasingly been high. To reduce the flying height of the head, there has been desired for the development of a substrate for magnetic recording mediums, which is excellent in the surface smoothness, has only a small amount of deposits on the surface thereof and has a substantially small quantity of surface defects.
As the conventional substrates for magnetic recording mediums excellent in the surface smoothness, there have mainly been used those each prepared by plating an aluminum alloy plate with Ni—P and then polishing the plated main surface of the plate in a multiple step process.
However, the magnetic disk-recording device has recently been adopted even in the portable personal computer such as notebook-sized personal computers and the magnetic recording medium should be rotated at a high speed on the order of not less than 10,000 rpm in order to improve the response speed of the magnetic disk-recording device. For this reason, there has been desired for the development of a substrate for magnetic disk-recording mediums having high strength capable of withstanding such severe conditions. As such a substrate, which can satisfy the foregoing requirements, there has been adopted a glass substrate.
Such a mainly adopted glass substrate for magnetic recording mediums includes, for instance, a chemically strengthened glass substrate whose strength is improved by a chemical strengthening treatment or a crystallized glass substrate prepared by melting and molding glass to give a glass substrate, and maintaining the glass substrate at a high temperature ranging from 600 to 800° C. over a long period of time to thus partially separate out crystalline phases in the substrate.
The chemically strengthened glass substrate is, for instance, one obtained by melting a glass material and forming the melt into a glass substrate for chemically strengthened glass substrates, then subjecting the glass substrate to grinding and polishing treatments, and immersing it into a molten salt of, for instance, sodium nitrate or potassium nitrate to form a compression stressed layer on the surface thereof and to thus improve the breaking strength thereof and therefore, the surface layer of the resulting substrate necessarily has a high content of alkali metals. In addition, some of the crystallized glass substrates comprise alkali metals, in particular, lithium.
In general, the production of a glass substrate for magnetic recording mediums and the formation of, for instance, a magnetic recording layer and a protective layer are carried out at separate places or in separate factories. Therefore, the glass substrate for magnetic recording mediums is cleaned by a well known cleaning method such as a cleaning method comprising a combination of scrub-cleaning, which makes use of a sponge roller, and ultrasonic cleaning and then dried prior to the transportation thereof to another place or factory. Therefore, the glass substrates are temporarily stored before and/or after the transportation.
In case of the foregoing glass substrate for magnetic recording mediums consisting of a chemically strengthened glass substrate or a crystallized glass substrate containing, for instance, lithium, however, an infinite number of needle-like projections grow on the surface of the glass substrate within 24 hours after the foregoing cleaning operations. If the glass surface is observed with an atomic force microscope (AFM) while expanding the longitudinal magnification, these projections look like just spikes of spiked shoes. If a magnetic recording medium is prepared using a glass substrate for magnetic recording mediums having such needle-like projections, the resulting magnetic recording medium may come in contact with the magnetic head of a magnetic recording device during writing and/or reading due to the presence of these needle-like projections depending on the height thereof and this may accordingly interfere with the reduction of the flying height of the magnetic head. As a result of the elemental analysis, it is confirmed that the needle-like projections mainly comprise carbonates of alkali metals such as Na, K and Li, which may, by nature, be present in the glass or may be introduced into the glass substrate through the chemical strengthening. More specifically, it would be recognized that these metal ions present near the surface area of the glass substrate after the completion of the cleaning operations move towards the surface with the elapse of time, while they react with CO
2
present in the air to form carbonates and that the carbonates grow and are deposited on the surface in the form of needle-like projections.
In the conventional method for preparing a glass substrate, an acid cleaning step is added to the cleaning process to eliminate the foregoing problem. As such acids used in the acid cleaning step, there may be listed, for instance, inorganic acids such as hydrofluoric acid, hydrochloric acid, sulfuric acid and phosphoric acid; organic acids such as formic acid, oxalic acid, citric acid, tartaric acid and hydroxyacetic acid. Thus, the addition of such an acid cleaning step would permit the reduction of the alkali metal ion concentration in the proximity of the surface of the glass substrate and the reduction of the amount of such deposits of carbonates.
However, effective acid cleaning requires quite severe conditions such as a high temperature and a high acid concentration. For this reason, such an acid cleaning requires the use of an expensive apparatus having high resistance to acids and sufficient care should be taken to ensure safety.
SUMMARY OF THE INVENTION
Accordingly, there has presently been desired for the development of a simpler cleaning method, which can inhibit the growth of needle-like projections mainly comprising an alkali metal carbonate. Thus, it is an object of the present invention to provide a method for the production of a glass substrate for magnetic recording mediums, which can inhibit the formation of any needle-like projection mainly comprising an alkali metal carbonate on the surface of the glass substrate.
The inventors of this invention have conducted various studies to solve the foregoing problems associated with the conventional methods, have found that, after the cleaning step when a glass substrate is subjected to disk-processing, grinding, polishing and subsequent cleaning steps as well as a chemical strengthening step carried out between the grinding and polishing steps, or between the first and second polishing stages when the polishing step is stepwise carried out in two stages, or between the polishing and subsequent cleaning steps according to a conventionally well-known method; or after the cleaning step when the glass substrate is subjected to a chemical strengthening step between the polishing and cleaning steps; or after the cleaning step when a crystallized glass substrate is subjected to disk-processing, grinding, polishing and subsequent cleaning steps, if a glass substrate is allowed to stand in the air for 48 hours to thus sufficiently form, on the surface thereof, needle-like projections comprising an alkali metal carbonate and then subjected to scrub-cleaning using a sponge roller or the like and dip-cleaning, any fresh needle-like projection is not formed any more from the glass substrate after the cleaning step, and thus have completed the present invention.
Fujimura Akio
Kobayashi Daisaku
Mitsui Mining and Smelting Co. Ltd.
Vincent Sean
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