Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
2001-05-22
2003-08-12
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S336000, C428S408000, C428S421000, C428S690000, C428S690000
Reexamination Certificate
active
06605335
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a magnetic recording media made by forming a under layer, a magnetic layer and an overcoat on a substrate of a non-magnetic magnetic disk, over which a liquid lubricant having a structure of perfluoropolyether is coated, particularly to a magnetic disk media having a very thin overcoat of several nano meters.
2. Description of the Related Art
Recording density of the magnetic disk devices is remarkably increasing and even a density of more than 10 G bits per square inches has been announced recently. In order to achieve such a high recording density, it is mandatory to bring the space between magnetic head and magnetic recording layer of the magnetic disk as close as possible, to the extent of less than 20 nm at present level.
Most of this space is occupied by overcoat thickness on the magnetic recording layer on the magnetic disk and flying height of magnetic head. Therefore, overcoat thickness and flying height of the magnetic head are in a trade-off relationship. For a magnetic disk, an overcoat thickness as thin as possible, but with a high wear resistance should the magnetic head touch the overcoat, is required. Judging from the current space between magnetic head and magnetic recording layer of magnetic disk, the future task is to achieve a thickness of less than 5 nm.
In the recent magnetic disk devices, load-and-unload (L/UL) method is more and more employed than the conventional contact-start-stop (CSS) method. In the L/UL method, magnetic head is unloaded parting from the magnetic disk when the disk is stopped, and when the disk starts to rotate, the magnetic head is loaded on the disk. On the other hand, in the conventional CSS method, the magnetic head stays in contact with the disk, and when the disk starts to rotate, magnetic head flys up by air flow caused by rotation. However, in the L/UL method, although requirement for wear resistance is mitigated to some extent, the disk should withstand to a shock when loading on, and a sudden contact by a mal positioning of the magnetic head which can happen even in normal operation.
With respect to overcoat, as disclosed in the U.S. Pat. No. Re. 32464, carbon based materials have been used heretofore. Many methods for enhancing hardness of the carbon based overcoat to make it thinner have been proposed. For example, a method of adding hydrogen, as disclosed in the Japanese Patent Laid-Open S 59-154641, and a method of adding nitrogen in a sputtering process, as disclosed in the Japanese Patent Laid-Open H 8-106629, to increase hardness of carbon based overcoat and make it thinner.
Attention has been riveted since many years to diamond-like-carbon (DLC) which has especially high hardness among the carbon based overcoats. Many methods for preparation of DLC have been proposed such as Japanese Patent Laid-Open S59-154641, as cited before, describing a chemical vapor deposition method (heretofore abbreviated as CVD method) in which carbon is deposited on a substrate by decomposing hydrocarbon gas by electric discharge, or ion beam deposition method (heretofore abbreviated as IBD method) in which a hydrocarbon is ionized by irradiation of thermoion which is generated by a heated filament, and carbon is deposited on a substrate by collision of the said ion beam accelerated by bias voltage applied to the substrate.
Particularly the IBD method is easy to form an overcoat having a high hardness, as described in the 24
th
column of the Japanese Patent Laid-Open 2000-105916, teaching that “by using a thermal filament, plasma can be generated under much lower pressure than in conventional CVD method, and as monomer molecules are less concentrated, decomposition of the monomer is more complete.”
On the other hand, as described in the Japanese Patent Laid-Open S61-126827, waer resistance of the carbon based overcoat is not improved merely by increasing its hardness, but it is necessary to use fluorine based lubricant such as those having a perfluoropolyether structure at the same time. Thus, optimization of combination with lubricant is an important factor in designing magnetic disk.
Recently, to improve performance of the lubricant, mixing an additive having cyclic phosphazene, as described in the U.S. Pat. No. 5,908,817, and a lubricant having cyclic phosphazene at the end of perfluoropolyether, as described in the Japanese Patent Laid-Open H6-220077, are proposed. Moreover, a technology to add a cyclic phosphazene to the end of perfluoropolyether as shown in the chemical formula (1) was developed recently by Matsumura Oil Research Corp., (the Japanese Patent Application H11-267696).
With respect to the technology of mixing an additive having a cyclic phosphazene structure with perfluoropolyether lubricant is disclosed in the Japanese Patent Laid-Open H9-305961 and the Japanese Patent Laid-Open H10-251676. Example of the prior art shows that, by forming the above mentioned film on DLC overcoat, reduction of friction force, improvement of durability, reduction in contamination of the head and suppression of lubricant decomposition are achieved.
Furthermore, it is known, as described in the Japanese Patent Laid-Open H11-328647, that material for base-coat of magnetic recording layer also affects wear resistance of magnetic disk.
SUMMARY OF THE INVENTION
Comparing to the nitrogen-added carbon overcoat, in case of the diamond-like-carbon (DLC), as described above, although the film hardness is high, easiness of heat generation in the magnetic disk device and lubricant spin-off due to air shear force by rotation of the disk have been the problems. Therefore, it is necessary to find out not only the kind of overcoat material, but also an optimal combination with lubricant in order to achieve enough wear resistance with overcoat thickness of less than 5 nm.
On the other hand, the less the roughness of surface of magnetic disk is, the closer the magnetic head can approach to the disk, resulting in an advantage of increasing recording density. However, in case a sudden contact of the magnetic head happens, more damage could be given to the overcoat because of a large friction due to a large contact surface area between the magnetic head and the magnetic disk. Therefore, the proper surface roughness should be selected.
Furthermore, in case overcoat has a large Young's modulus, deformation of the overcoat is small leading to a smaller contact area for the head, and a higher contact pressure resulting therefrom tends to promote waer. Also from this point of view, it is important to find out an optimal combination of overcoat material, lubricant and roughness of the surface.
As shown in the Japanese Patent Laid-Open H9-305961 and the Japanese Patent Laid-Open H10-251676, a lubricant coat made by mixing an additive having cyclic phosphazene structure tends to cause contamination of the head, leading to a larger friction force. This contamination is caused by phase separation of the additive which sticks to the head. As the phase separation of additive depends on the overcoat material, and an increase in friction force depends on surface shape of the magnetic disk, it is necessary to find out an optimal combination of lubricant, overcoat and surface shape.
The inventors of the present invention, after studied various combination of overcoat, overcoat thickness, lubricant and surface roughness, has found out that by using a diamond-like-carbon (DLC) prepared by IBD method and lubricant as defined by the chemical formula (1), and making surface roughness (Ra) of magnetic disk, i.e. average surface roughness of the overcoat less than 0.8 nm, an enough durability is achieved even with overcoat thickness of less than 5 nm, i.e. from 1.5 to 4.5 nm, and that above achievement is not realized if any one of the diamond-like-carbon, lubricant of the chemical formula (1) or Ra lacks.
Where the upper limit of Ra was defined because it was found that a larger Ra causes rather reduction of wear resistance.
Namely, in the magnetic recording media made by forming an underlayer,
Kozaki Tomonori
Matsumoto Hiroyuki
Nakakawaji Takayuki
Ogawa Yoko
Shyoda Mitsuhiro
Mattingly Stanger & Malur, P.C.
Resan Stevan A.
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