Abrasive tool making process – material – or composition – With inorganic material
Reexamination Certificate
1999-01-27
2001-06-19
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
With inorganic material
C051S309000, C106S003000, C510S163000, C510S180000, C510S181000, C510S397000, C451S036000, C451S041000, C451S042000
Reexamination Certificate
active
06248143
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an abrasive composition for polishing glass. More specifically, the present invention relates to an abrasive composition for polishing glass, which can form an excellent polished surface free of flaws. More particularly, the present invention relates to an abrasive composition for polishing glass such as a substrate for magnetic disks, which can polish glass to a high precision mirror face suitable for flying a magnetic head at a low glide height, and also to a method for polishing glass with the abrasive composition.
BACKGROUND OF THE INVENTION
Glass is used as a substrate for magnetic disk storage media, which disks are installed in a drive device of personal computers and the like. This is because glass substrates are advantageous in that their impact strength is high and they have increased smoothness as compared with aluminum substrates. In recent years, to cope with demands for higher recording density, the distance between the magnetic head and the magnetic disk substrate has been progressively reduced. Accordingly, the glass substrate for magnetic disks necessarily has high precision flatness, a low surface roughness and is nearly free of flaws.
For polishing various glasses, materials such as cerium oxide, zirconium oxide, iron oxide and silicon dioxide have been conventionally used. At present, abrasive compositions mainly comprising cerium oxide (hereinafter referred to as “cerium oxide-type abrasive compositions”) are predominantly used due to their high polishing efficiency.
With respect to the abrasive composition for polishing the glass substrate of a magnetic disk, the following techniques have been proposed in which additives are added to an abrasive mainly comprising cerium oxide, zirconium oxide or aluminum oxide to adjust the slurry properties and thereby improve the polishing efficiency and polishing precision.
(1) JP-A-3-146584 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) entitled “Abrasive for Polishing Glass” proposes to incorporate calcium aluminate, magnesium sulfate or magnesium chloride into an abrasive mainly comprising zirconium oxide.
(2) JP-A-3-146585 entitled “Abrasive for Polishing Glass” proposes to incorporate magnesium chloride into an abrasive mainly comprising cerium oxide.
(3) JP-A-9-109020 entitled “Composition for Polishing Magnetic Disk and Polishing Solution Using the Same” proposes to incorporate gibbsite and a dispersant into an abrasive mainly comprising aluminum oxide.
There is a great demand for glass substrates for magnetic disks having further improved surface precision. For stably achieving a low glide height as a property required for the glass substrate of a magnetic disk, conventional abrasive compositions cannot ensure the requisite low surface roughness and a clean glass surface that is free from attachments.
As described in the foregoing, in order to achieve higher magnetic disk recording density, it is essential to reduce the glide height of the magnetic head. Therefore, irregularities in the magnetic disk profile must be removed with higher precision. When a crystallized glass or a tempered glass substrate used as the glass substrate of a magnetic disk is polished using a conventional cerium oxide-type abrasive composition, a problem arises in that a low surface roughness necessary for reducing the glide height of the magnetic head to the required level cannot be obtained. Furthermore, because the cerium oxide-type abrasive compositions have high chemical reactivity with glass, attachments remain on the surface of the glass substrate of the magnetic disk. The attachments are difficult to thoroughly remove even by applying thereto mechanical energy such as ultrasonic cleaning or scrub cleaning, and the magnetic head contacts the residual attachments. Thus, the magnetic disk surface substantially fails to obtain the high precision needed for reducing glide height.
SUMMARY OF THE INVENTION
As a result of extensive investigations for an abrasive most suited to achieve a high precision polished surface for the glass substrate of a magnetic disk, the present inventors have found that by using an abrasive composition for polishing glass, comprising a basic salt of magnesium, particularly at least one salt selected from magnesium hydroxide, magnesium carbonate, basic magnesium carbonate and magnesium phosphates, a high precision polished surface which conventional cerium oxide-type abrasive compositions cannot achieve is obtained. Furthermore, when a glass polished surface obtained by polishing glass with an abrasive mainly comprising at least one of cerium oxide, zirconium oxide, iron oxide and silicon dioxide is polished using an abrasive composition containing a basic salt of magnesium, particularly at least one salt selected from magnesium hydroxide, magnesium carbonate, basic magnesium carbonate and magnesium phosphates, residual attachments such as the cerium oxide-type abrasive can be removed and at the same time, a high precision glass polished surface can be obtained. As a result, further reduction in the glide height of a magnetic head, which is difficult to realize by conventional techniques, can be achieved.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Specific examples of the basic salt of magnesium for use in the present invention include magnesium hydroxide, magnesium carbonate, basic magnesium carbonate (e.g., 3MgCO
3
.Mg(OH)
2
) and magnesium phosphates such as magnesium phosphate, magnesium hydrogenphosphate and magnesium pyrophosphate (Mg
2
P
2
O
7
). However, the present invention is by no means limited thereto.
In the present invention, the purity of these compounds is not particularly limited.
The basic salt of magnesium for use in the present invention contributes in the powder state itself to the polishing and accordingly, the grain size thereof is, in terms of an average grain size, preferably from 0.1 to 10 &mgr;m. If the average grain size exceeds 10 &mgr;m, considerable sedimentation takes place in a slurry to cause problems in working the glass substrate. Also, depending on the case, the polished surface cannot achieve the required low roughness. On the other hand, if the average grain size is less than 0.1 &mgr;m, the polishing efficiency is disadvantageously reduced.
The addition amount of the basic salt of magnesium for use in the present invention is, in terms of a solids concentration in a slurry (in the case of using two or more basic salts of magnesium, as a total amount thereof), preferably from 1 to 40 wt %, more preferably from 5 to 30 wt %. If the addition amount is less than 1 wt %, a high precision surface can hardly be obtained and surface flaws are readily apparent, whereas even if the addition amount exceeds 40 wt %, further improvement is not obtained.
In the case where one or more basic salts of magnesium are mixed, the mixing ratio thereof can be freely selected.
The basic salts of magnesium for use in the present invention may differ with respect to water of crystallization, crystal system, appearance shape or form even though they have the same chemical formula (excluding water of crystallization), and these salts are not particularly limited.
The solvent or dispersing medium for the abrasive composition of the present invention is not limited to water, however, a water system is preferred.
In order to further improve dispersibility, prevent sedimentation, improve stability or improve workability, the abrasive composition of the present invention may contain, if desired, a glycol such as ethylene glycol and polyethylene glycol, a phosphate such as tripolyphosphate and hexametaphosphate, a polymer dispersant such as polyacrylate, a cellulose such as methyl cellulose and carboxymethyl cellulose, or a water-soluble polymer such as polyvinyl alcohol. The addition amount of these additives is usually from 0.05 to 20 wt %, preferably from 0.1 to 15 wt %, more preferably from 0.1 to 10 wt %, based on the abrasive composition.
Furthermore, in order to improve the polishing efficiency,
Masuda Tomoyuki
Mizukami Hiroshi
Ohara Nobuhiko
Yamauchi Yutaka
Marcheschi Michael
Showa Denko Kabushiki Kaisha
Sughrue Mion Zinn Macpeak & Seas, PLLC
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