Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
1995-11-22
2001-02-27
Ahmad, Nasser (Department: 1772)
Stock material or miscellaneous articles
Circular sheet or circular blank
C360S135000, C428S064200, C428S065100, C428S156000, C428S172000, C428S219000, C428S220000, C428S698000, C428S704000, C428S900000, C428S928000
Reexamination Certificate
active
06194045
ABSTRACT:
TECHNICAL FIELD
The present invention relates to magnetic recording media, and in particular, to substrates which support the recording media and methods for making the same.
Typical magnetic recording utilizes a magnetic medium which magnetically stores the information. Different magnetic media are supported by different substrates. For example, a magnetic tape is supported by a flexible film. A floppy disc is supported by a similar, but slightly less flexible film. The present invention relates to rigid discs, which have traditionally been supported by aluminum and other “hard core” materials.
Each of these media, such as the magnetic tape, the floppy disc and the rigid disc, strive to maximize the amount of information stored by the medium. The amount of information stored is measured in bits stored per unit area, and is commonly referred to as areal density. The areal density is affected by both, the smoothness of the substrate surface, which underlies the magnetic medium, and the flatness of the substrate surface.
The magnetic medium is written on and read by a read-write head (also referred to as a recording head), which, in the case of rigid disc recording, ideally passes over a magnetic recording medium at either a very low and constant “flying height” or is actually in contact with the magnetic medium.
It is generally accepted that the signal received by the read-write head from the magnetic medium increases exponentially as the distance between the magnetic medium and the read-write head decreases. It is also generally accepted that the achievable areal density is directly proportional to the signal. Therefore, to optimize the areal density it is necessary to make the distance between the read-write head and the magnetic medium as small and uniform as possible.
The recording medium and the underlying substrates to date have had uneven surfaces, due to surface irregularities as herein discussed.
On a microscopic scale, disks have imperfections which generally resemble peaks and valleys. Consequently, the read-write head passes very close to, or contacts, the peaks of the magnetic medium, sometimes damaging or destroying both the head and the medium. Likewise, the read-write head passes far away from the magnetic medium at the valleys.
Another item of consideration is small holes in the surface of the substrate. Such small holes, or pits will also effectively result in an increase in distance between the read-write head and the magnetic medium.
It is desirable to make the substrate surface as smooth as possible to allow the read-write head to pass as close as possible to the magnetic medium, yet making this surface too smooth has its drawbacks. For example, when the smooth undersurface of the read-write head carrier (commonly known as a “slider”) comes to rest on the smooth substrate surface, there is no air between the two surfaces and a vacuum bond results. This phenomenon is commonly referred to as “stiction” and it inhibits the proper performance of the disc drive upon start up.
On a macroscopic scale, disc flatness (warpage) is also a critical feature. As the surface becomes less flat, a surface resembling the undulations of a potato chip may result. Current disc rotational speeds of approximately 3500 revolutions per minute and higher are the norm. At these speeds such undulations in the disc surface can cause a “ski jump” effect off of these high areas in the axial tracking of the read-write head, resulting in an increased flying height on the lee side of these areas.
Rigid discs, which use aluminum NiP (which is aluminum coated with nickel phosphorus) to support the magnetic medium, do not allow the read-write head to pass as closely as desired to the magnetic medium surface. In the course of manufacture the aluminum disc is subsequently overcoated with the NiP material to produce a harder, more durable surface.
During processing the aluminum disc, which has a relatively low melting point, is heated to high temperatures, to render the desired properties in the magnetic medium. This heating causes the aluminum NiP substrate to warp in a manner dependent on the inherent orientation of its grain structure. Once aluminum NiP substrates have warped, it is virtually impossible to flatten them.
Another material much less commonly used as a rigid disc substrate is glass. Glass offers the ability to achieve very smooth surfaces, offering advantage over aluminum NiP in this category.
An induced stress known as “clamping force” occurs when finished magnetic discs are clamped into a disk drive assembly. Due to the relatively low stiffness of both glass and aluminum NiP, they tend to warp when placed under stress. Such is the case when these materials are clamped or rotated at high speeds in a disc drive assembly.
Due to the microscopic and macroscopic uneven surfaces of conventional magnetic discs, the read-write head is forced to pass over the magnetic medium at a flying height sufficient to avoid contacting the peaks. At this height, the read-write head passes far from the valleys of the magnetic medium. Thus, the areal density is not maximized.
Certain specific materials chosen form classes such as refractory hard metals or ceramics offer the potential for significant improvement in ultimate disc substrate properties when they are selected as the hard core material. Such ideal hard core materials have a low density, a high elastic modulus, a high tensile or crossbending strength, a low coefficient of thermal expansion, a high thermal conductivity, high hardness, and excellent resistance to deformation or degradation at high temperature.
The high elastic modulus (high stiffness) and high hardness of the ideal hard core materials allow discs made from these materials to be machined much flatter and to much closer tolerances than is typically possible with more conventional materials such as aluminum NiP or glass.
When using typical lapping or grinding equipment, to machine conventional (low stiffness) magnetic disc materials such as aluminum NiP or glass, the relatively flexible nature of the materials allows a disc, which is out of flatness, to bend and thus conform to the grinding device during grinding. As long as the grinding pressure is maintained the disc remains flat, but when the pressure is removed the disc returns to its original non-flat shape. The disc will be thinner after the grinding operation but will still not be flat.
On the other hand, when the subject hard core disc materials are ground, their high stiffness minimizes their tendency to deform during the grinding and lapping operations. Thus there is no “springback” or “memory” after the grinding operation, the absence of these problems allows the achievement of much flatter and closer tolerance discs.
Unfortunately, the hard core materials are brittle because of their high elastic modulus. Since they in turn have low fracture toughness they tend to chip around the edges or to show indications of porosity or pullouts during machining.
Defects such as these which lead to a rougher than desired surface can be largely eliminated by proper selection of manufacturing technique, such as for example chemical vapor deposition or by hot pressing at relatively high pressures. However manufacturing processes such as these are not feasible to consider for fabricating hard core disc materials because of their prohibitively high costs compared to the value of such discs in the commercial marketplace.
So while one would desire, in order to increase areal density, to use a rigid, hard disc material that has been ground flat and polished to a very smooth surface, the high costs of such discs prevents their use.
It is desirable, therefore, to prepare a substrate for use in preparation of a magnetic recording rigid disc which exhibits the advantages of use of an inexpensive, high stiffness, central hard core material while providing a smooth surface which allows for a constant and low flying height by the read-write head, thus maximizing the areal density.
SUMMARY OF THE INVENTION
The present invention presents a mechani
Annacone William R.
Felis Kenneth P.
Speliotis Dennis E.
Ahmad Nasser
St. Onge Steward Johnston & Reens LLC
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