Abrasive tool making process – material – or composition – With inorganic material – Metal or metal oxide
Reexamination Certificate
2001-04-05
2002-11-12
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
With inorganic material
Metal or metal oxide
C106S003000, C510S167000, C510S397000, C451S036000
Reexamination Certificate
active
06478837
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an abrasive composition for substrates for magnetic recording disks, and more particularly to an abrasive composition for substrates for magnetic recording disks and a process for producing a substrate for a magnetic recording disk, which are employed to obtain a magnetic disk surface of high accuracy such that the flying-height of a magnetic head from the disk can be reduced.
BACKGROUND ART
Magnetic recording disks (memory hard disks) are widely used in the external memories of computers and word processors, as a means giving quick access. A typical example of such a magnetic recording disk is one which is produced by preparing an original substrate by subjecting an Al alloy substrate to electroless plating of NiP on the surface, surface-polishing the original substrate, and thereafter sequentially forming a Cr-alloy underlayer, a Co-alloy magnetic film, and a carbon protective film by sputtering.
When irregularities or nodules which are higher than the flying-height of a magnetic head remain on the surface of a magnetic recording disk, the magnetic head which flies over the disk at a predetermined flying-height and high speed may hit such nodules, causing damage to the head. When a substrate for a magnetic recording disk has nodules and polishing scratches, in the case of formation of a Cr-alloy underlayer or a Co-alloy magnetic film on the substrate, nodules and defects attributed to the polishing scratches arise on the surface of the film, and thus the resulting magnetic recording disk fails to have an even surface of high accuracy. Therefore, in order to produce a magnetic recording disk having a surface of high accuracy, the substrate of the disk must be accurately polished.
In view of the foregoing, there have been proposed many abrasive compositions which remove nodules or minimize the height of nodules, and rarely form polishing scratches during polishing of substrates for magnetic recording disks. For example, an abrasive composition containing alumina or an aluminum compound having a particle size of approximately 1 &mgr;m serving as an abrasive particle can be used for polishing substrates for magnetic recording disks with such accuracy that magnetic heads do not hit nodules of the magnetic recording disk at the conventional flying-height of the head. However, even when this composition is employed, high surface accuracy of a magnetic recording disk, which is required to meet the current demand for attaining high recording density, cannot be accomplished. Meanwhile, when an abrasive composition comprising colloidal silica particles having a size of several tens of nm serving as an abrasive particle is employed, high surface accuracy can be easily accomplished. However, when this composition is employed, desired productivity cannot be accomplished because of low abrasion rate. In addition, when a substrate for a magnetic recording disk is polished by use of this composition for a prolonged period of time, the circumferential edge of the substrate is excessively abraded (this phenomenon is called “roll-off”).
Japanese Patent Application-Laid-Open (kokai) No. 10-121035 discloses an abrasive composition comprising titanium oxide (titania) fine particles having a size of sub-microns serving as an abrasive particle. When this composition is employed, high surface accuracy and abrasion rate can be easily accomplished. The composition disclosed in the above publication contains rutile-type titanium oxide. The publication describes that rutile-type titanium oxide has small crystal cells, dense structure, and high hardness, and thus the composition exhibits excellent polishing efficiency, and that the percentage of rutile-type titanium oxide in overall titanium oxide (i.e., rutilation percentage) is preferably 10-80% since rutile-type titanium oxide tends to form polishing scratches.
The aforementioned publication describes that titanium oxide having a rutilation percentage of 10-80% is preferable. However, when several types of titanium oxide of different crystal structures coexist, titanium oxide is not uniformly ground or milled in formation of fine particles, and thus particle size distribution of high accuracy and sharpness is difficult to obtain. As a result, when an abrasive composition containing such titanium oxide is employed, minute defects such as micropits and microscratches may arise, though not frequently, on the substrate for a magnetic recording disk, and the yield of the substrate may be lowered.
An abrasive composition for substrates for aluminum magnetic recording disks which enable high-density magnetic recording must have sufficient quality that disk surfaces of high accuracy which facilitate reduction of the head flying-height can be accomplished.
DISCLOSURE OF INVENTION
In view of the foregoing, an object of the present invention is to provide an abrasive composition for substrates for magnetic recording disks and a process for producing a substrate for a magnetic recording disk, which enable lowering of surface roughness of a magnetic recording disk; form no irregularities or nodules, polishing scratches, and minute defects such as micropits and microscratches; enable high-density recording of the disk; and enable polishing at energy-saving speed.
The present inventors have performed extensive studies and have found that, when an abrasive compound which is constituted by titanium oxide having as close to a single type of structure as possible and not comprising a large amount of crystals having different structures is crushed or ground, a sharp particle size distribution is obtained; and that when such a compound is employed, the surface of high accuracy of a substrate for a magnetic recording disk which enables lowering of the head flying-height can be obtained, and occurrence of usual polishing scratches, particularly minute defects attributed to small polishing scratches can be prevented. The present invention has been accomplished on the basis of these findings. Titanium oxide may be of any crystal structure type so long as any one type of the crystal structures (single crystal structure alone) accounts for 90% or more of the titanium oxide.
Accordingly, the present invention provides an abrasive composition for substrates for magnetic recording disks, which comprises at least water, titanium oxide fine particles, and an abrasion promoter, characterized in that 90-100% of the titanium oxide is of a single crystal structure type.
In addition, a process for producing a substrate for a magnetic recording disk comprising the step of supplying the aforesaid abrasive composition.
The abrasive composition of the present invention is advantageously used for, for example, a substrate having a high recording density (generally, a recording density of 1 Gbit/inch
2
or more), such as a magnetic recording disk for magnetic head utilizing magnetoresistance (MR) effects. Also, the composition may effectively be used for a magnetic recording disk having a recording density lower than that of the aforementioned value, in consideration of enhancement of reliability.
BEST MODE FOR CARRYING OUT THE INVENTION
In a conventional substrate for a magnetic recording disk, polishing scratches of approximately 10 nm or more in depth cause problems. Meanwhile, in a low-flying-height-type hard disk substrate which is targeted by the present invention, a fine polishing scratch having a depth of approximately 5 nm, which has not caused problems, will induce errors in magnetic characteristics. Therefore, a substrate for a magnetic recording disk having such fine scratches is considered to fall outside the acceptable range as regards practical use.
Titanium oxide fine particles contained in the abrasive composition of the present invention as an abrasive compound comprise titanium oxide constituted by a single type of crystal structure in an amount of 90-100%. Namely, the particles comprise titanium oxide of any one type of anatase, rutile, and brookite in an amount of 90-100%. The particles may comprise titanium oxide having eit
Marcheschi Michael
Showa Denko K.K.
Sughrue & Mion, PLLC
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