The stamping comprises a ferromagnetic substrate and a metal coating. The coating has magnetization Bs which is less than the magnetization of the substrate. The coating is harder than the substrate, preferably greater than 400 DPH hardness number. The coating is deposited with an amorphous microstructure while the substrate is crystalline. The coating has a nodular surface texture and surface oxide film which provides improved adhesive bond strength, where the medium is joined to the stamping. The coating preferably is an electroless plating deposit, comprised of nickel or cobalt base metal, with phosphorous or boron additive; most preferably it is nickel-11% phosphorous.
Mini-floppy disks are used for electromagnetically storing information, particularly in personal computers. Typically, a disk comprises a circular piece of flexible plastic medium coated with magnetic material encased in a rigid plastic housing. Adhered to the center of the flexible plastic medium is a circular cupped metal stamping. The stamping typically has one or more openings, including a square hole in the center and an off-center hole for a drive pin. In use, the stamping is engaged by mating features of a rotating drive spindle of the computer system disk drive. Thus, the disk storage medium is continuously rotated past magnetic heads of the disk drive. Such disks and drives are described Chapter 13 of the book authored by S. Mueller, 'Upgrading and Repairing PCs', 4th Ed. (1994) Que Corporation, Indianapolis, Ind., the disclosure of which is hereby incorporated by reference.
To enable mini-floppy disks to be frequently inserted and removed from the disk drive system, the drive spindle automatically advances and retracts along an axis perpendicular to the stamping and disk. Since probability dictates that the drive spindle contoured end and stamping holes usually will not align, as the drive spindle pushes against the stamping, it rotates until engagement occurs. In this process, there is metal to metal rubbing contact. The cumulative effect of such rubbing must not be such that galling or wear produce debris which would adversely affect the functioning of the finely crafted data storage surface of the disk.
To ensure positive engagement between the drive spindle and the stamping during use, and to otherwise facilitate a desired smooth rotation of the flexible disk within the disk housing, the drive spindle is permanently magnetized and the stamping is a magnetic metal. Thus, the stamping is held against the end of the drive spindle during rotation. But, the magnetic attraction between the parts has to be sufficiently weak to both enable easy disengagement of the spindle from the stamping and not interfere with the function of the data storage disk. Inasmuch as there are millions of floppy disk drives in the field, any improvement in disk or stamping construction ought be compatible with the design of existing disks and disk drives, for commercial reasons. If not, disks with a new type stamping could be either too weakly or too strongly attracted to the drive spindle, adversely affecting use of the whole floppy disk-drive system.
In addition to the above mentioned wear resistance and magnetic properties, the stamping must have corrosion and oxidation resistance. A stamping material reaction product which produces flaking or crumbling debris cannot be tolerated. And, a floppy disk stamping must have a low cost and be attractive in appearance. The cost of a stamping will depend on, among other things, the intrinsic material cost, formability, and other required processing.
The common material prevalently used in present day floppy disks is cold formed AISI Type 430 martensitic stainless steel, comprised essentially by weight percent 16 chromium, balance iron. Typically, a stamping is about 0.012-0.013 inch thick. The Type 430 steel has a desired combination of magnetic property, wear resistance, corrosion resistance and formability. In comparison to many other iron base materials, Type 430 steel has a relatively high intrinsic cost.
Thus, in the continuous search for mini-floppy disk product improvement, there is most particularly a need to reduce cost while achieving equal or better function. However, any improved disk must have a stamping which has magnetic properties compatible with the system which comprises the Type 430 stampings. The combination of performance requirements has prevented the use of what might seem to be likely engineering choices in other materials and combinations. The prevelance in the marketplace of relatively costly Type 430 steel, or like stainless steel grades, in stampings evidences this.
The mini-floppy disks described above are one type of removable information storage devices. The characteristics and requirements for mini-floppy disk stampings are shared to different degrees by other storage devices.
As will be described, the preferred embodiment of the present invention utilizes electroless nickel coatings. Such coatings have been in wide commercial use since the 1940's. They are applied from a liquid bath by means of autocatylic reaction and deposition. Electroless coatings, in particular nickel, sometimes overlaid with cobalt, have been long applied to the hard disks of computer systems. Such disks are typically made of aluminum, and they are coated with electroless nickel, typically having about 12 percent phosphorus, to protect the aluminum from corrosion and provide a suitable essentially non-magnetic surface for the information storage media.
The preferred electroless deposit coating is comprised of the base metal stamping selected from the group consisting of nickel and cobalt, and an additive selected from the group consisting of phosphorous and boron.