A MULTILAYER HARD DRIVE DISK AND METHOD TO PRODUCE SAME

Abstract: A hard drive disk substrate comprising a disk shaped core that has at least one face clad by an outer metal layer that has an average thickness of at least 10 micrometers to at most 300 micrometers. The core is comprised of a ceramic or ceramic-metal composite material. The ceramic material may be an oxide, nitride, boride, silicide, carbide or alloy thereof. The ceramic-metal composite may be the ceramic just described and a metal, such as a transition metal, beryllium, magnesium or aluminum. The disk substrate is prepared by: (a) cladding at least one face of a disk shaped core with a metal, such as aluminum, and (b) finishing the core clad with the outer metal layer to form the disk substrate. The outer metal layer may be clad by gluing a metal foil to the core or heating a metal foil in contact with a face of the core to a temperature sufficient to adhere the foil to the core. The resultant disk substrate may be flatter than the core it is made from and may be finished to make a hard drive disk capable of a faster maximum rotational speed and greater areal density than an aluminum hard drive disk of the same size.

Description: A MULTILAYER HARD DRIVE DISK AND METHOD TO PRODUCE SAME Hard drive disks typically have been prepared from aluminum coated with a thin electro-deposited nickel phosphorous (NiP) layer and a thin magnetic film such as cobalt-chromium described in U.S. Patent No. 4,069,360. As the computer industry has progressed, there has been a corresponding need for greater areal density and, in particular, faster access speed. Areal density is the amount of information that can be stored and retrieved from an area of a disk and is measured in bits per unit area of the disk. The access speed is the rate at which information can be read or stored on a disk and is measured by bits per unit time. Until recently, disks prepared from aluminum were adequate for hard drives. However, when aluminum hard drive disks are (1) clamped into the hard drive and (2) spun at high rotational speeds they tend to warp which decreases the areal density and access speed that is obtainable. Glass has recently been used as a disk substrate to avoid some of the problems of aluminum disks. However, glass disks have failed to increase the maximum rotational disk speed significantly, if at all, compared to disks prepared from aluminum. Glass also has a very low toughness (less than about 2 MPam 1 ' 2 ) and small strain to failure (that is, brittle). Because of these properties, the process of grinding and polishing of a flat thin glass disk substrate, without breaking or causing large defects, is time consuming and expensive. U.S. Patent No. 5,480,695 describes a non-oxide disk substrate, such as silicon carbide (SiC), having an amorphous SiC smoothing layer of a thickness of 1 to 50 micrometers to just cover the surface irregularities of the non-oxide disk substrate. This smoothing layer is preferably less than 20 micrometers to limit cost. U.S. Patent No. 5,487,931 describes a silicon carbide disk substrate having no unbound silicon and having an overlying smoothing layer, such as amorphous silicon. Each of these patents requires the machining and/or polishing of the ceramic disk substrate before applying a thin smoothing layer followed by a second polishing of a brittle smoothing layer. As in the case of glass, each of these steps is time consuming and expensive. Each of these patents also describes that the ceramic disks must be flat before application of the smoothing layer. Since it is known that ceramics, upon densification, can undergo a linear shrinkage of 10 percent to 20 percent, invariably these disks must undergo significant machining and grinding to achieve the required flatness. Because ceramics are brittle and hard like glass, the process of machining and polishing ceramics is also time consuming and expensive. This is exemplified by the '931 patent where disk shapes are cut from a block of ceramic (col. 10, lines 22-55). Therefore, it would be desirable to provide a disk that has a higher areal density and access speed than a comparable disk prepared from aluminum without one or more of the problems of the prior art, such as difficulty and expense of forming the hard drive disk. In a first aspect, the invention is a hard drive disk substrate comprising a disk shaped core comprised of a material selected from a ceramic or ceramic-metal composite, wherein the core has at least one face clad by an outer metal layer that has an average thickness of at least about 10 micrometers to at most about 300 micrometers. Herein, a disk substrate is understood to be a hard drive disk substrate. In a second aspect, the invention is a disk substrate comprising (a) a disk shaped core comprised of a material selected from a ceramic or ceramic-metal composite, wherein the core has at least one face clad by (b) an outer metal layer in which (c) an adhesive interlayer is disposed between the core and the outer metal layer. In a third aspect, the invention is a hard drive disk substrate comprising a disk shaped core comprised of a material selected from a ceramic or ceramic metal composite, wherein the core has at least one face clad by an outer metal layer such that the disk substrate is flatter than the core. In a fourth aspect, the invention is a method for preparing a disk substrate comprising: cladding, with a metal, at least one face of disk shaped core comprised of a material selected from a ceramic or ceramic-metal composite to form a core clad with an outer metal layer; and finishing the core clad with the outer metal layer to form the disk substrate. The invention provides a hard drive disk (finished disk having magnetic media thereon) that can have greater access speed and areal density compared to disks prepared from coated aluminum. The invention also provides a method of forming a hard drive disk that is a cost effective and elegant approach to avoid problems when producing disks prepared from glass and ceramics. Figure 1 shows one embodiment of the disk substrate of this invention. Figure 2 shows a cross-sectional view of the embodiment of Figure 1. One embodiment of the hard drive disk substrate 4 of this invention is shown in Figures 1 and 2. The disk substrate 4 comprises a disk shaped core 6 having two faces 8 that are clad by an outer metal layer 12. The core 6 and outer metal layer 12 of disk substrate 4 also define an outer edge 14 and an inner edge 16 that defines an inner hole 18. The disk substrate 4 may be any useful thickness (that is, distance between the faces 8) for preparing a hard drive disk. In general, the thickness of the disk substrate 4 is from 0.1 mm to 2 mm. The outer diameter (that is, diameter of the circle defined by the outer edge 14) of the disk substrate 4 may be any useful size for preparing a hard drive disk. The outer diameter may be, for example, from 20 mm to 1000 mm. The inner hole 18 may be any useful size for making a hard drive disk. The elastic modulus of the disk substrate 4 is desirably as great as possible, for example, to enable greater maximum rotational speeds. Generally, it is desirable for the elastic modulus of the disk substrate 4 to be at least 100 GPa. The elastic modulus is preferably at least about 125 GPa, more preferably at least about 150 GPa, and most preferably at least about 175 GPa. The modulus of the disk substrate 4 is generally limited only by the elastic modulus of the materials used to prepare it. The elastic modulus of the disk substrate 4 may be determined from the elastic stress-strain behavior of a bar cut out of the disk substrate 4. For example, the cut out bar may be subjected to a bending strain applied by a 3 point fixture using a mechanical testing apparatus, such as those available from Instron Corporation, Canton, MA. The Disk Shaped Core 6: To prepare the disk substrate 4 of the present invention, the disk shaped core 6 has the general shape of the hard drive disk substrate 4. For example, the core 6 generally has an outer diameter and thickness that is less than that desired for the disk substrate 4. The core 6 generally has an inner hole diameter that is greater than that desired for the disk substrate 4. Surprisingly, the core 6 need not be as flat as required or desired for a hard drive disk (finished disk having magnetic media thereon). The core 6 may be less flat because a core 6 clad with an outer metal layer 12 may be finished to form a flatter disk substrate 4, which is further described later. The core 6 of the hard drive disk substrate 4 is comprised of a material selected from a ceramic and ceramic-metal composite. Examples of a ceramic include an oxide, nitride, suicide, carbide, boride, mixtures of these and inorganic alloys (for example, titanium carbonitride). The ceramic may be crystalline or amorphous or combination thereof. Specific examples of the ceramic include aluminum nitride, silicon nitride, silicon carbide, siliconized silicon carbide, aluminum oxide, silicon-aluminum oxide glasses, titanium carbide- aluminum oxide composite and boron carbide. Preferably the ceramic is crystalline. It is also preferred that the ceramic is polycrystalline (that is, not a single crystal). Preferably the ceramic is a carbide, boride, nitride, combination of these or alloy of these ceramics. More preferably the ceramic is a carbide. Even more preferably the ceramic is a carbide of a metal selected from silicon, boron, aluminum, titanium, vanadium or combinations thereof. Most preferably the ceramic is boron carbide. A ceramic-metal composite is a composite of one or more of the ceramics just described and a metal in the metallic state (that is, metallically bonded). The metal should also essentially fail to react with the oxygen in air at room temperature when incorporated in the composite. Examples of a metal of the composite include transition metals, rare earth metals, beryllium, magnesium, aluminum and alloys thereof. The metal is preferably Y, Ce, Zr, La, Hf, Al, Be, Mg, a first row transition metal (that is, Sc, Ti, V, Cr, Mn, Fe, Co, Ni and Zn) and metal alloys thereof. More preferably the metal is Al, Mg, Ti, Co, Ni, Fe, Cu and metal alloys thereof. More preferably the metal is Al, Be, Mg, Ti and alloys thereof. Most preferably the metal is aluminum and aluminum alloys, such as those known in the art. The amount of metal in the composite may be determined by known techniques, such as X-ray diffraction. The ceramic or ceramic-metal composite may also contain a reinforcing component, such as inorganic fibers or whiskers (for example, SiC whiskers or fibers). A preferred ceramic-metal composite for use as the core 6 is a known aluminum-boron carbide composite described in U.S. Patents: 4,605,440; 4,702,770; 4,718,941 ; 4,961 ,778; 5,039,633; 5,298,468; 5,394,929; 5,508,120; 5,521 ,016 and 5,595,622. The amount of metal in the ceramic metal composite may vary over a large range dependent on factors, such as the maximum rotational speed desired for a hard drive disk prepared therefrom. Generally, the amount of metal may range from 0.5 percent to 90 percent by weight of the composite. Preferably the amount of metal is at most 80 percent, more preferably at most 50 percent, even more preferably at most 25 percent and most preferably at most 10 percent to preferably at least 0.75 percent, more preferably at least 1 percent, even more preferably at least 1.5 percent and most preferably at least 2.0 percent by weight of the ceramic-metal composite. The core 6 may be any elastic modulus but it is desirably as great as possible to increase the maximum rotational speed of a hard drive disk prepared from the core 6. Desirably, the elastic modulus is greater than the elastic modulus of the metal of the outer metal layer 12. Preferably the elastic modulus of the core 6 is at least about 100 GPa, more preferably at least about 150 GPa, even more preferably at least about 180 GPa, and most preferably at least about 225 GPa. The core 6 may be fully dense or porous dependent on factors, such as the maximum rotational speed, size and damping characteristics of the hard drive disk desired. For increased stiffness, the core 6 preferably is at least 90 percent, more preferably at least about 95 percent and most preferably at least about 98 percent of theoretical density. Herein, the theoretical density is the theoretical density described on page 530 of Introduction to Ceramics 2 nd Ed.. W. D. Kingery et al., John Wiley and Sons, New York, 1976. To improve the damping characteristics, the core 6, when porous, may have the pores of the core 6 filled with a polymeric material, such as a known polymer (for example,polyethylene, polycarbonate, polystyrene, epoxy resins and phenol-formaldehyde resins). The polymeric material is preferably the adhesives described herein for cladding the core 6 with the outer metal layer 12. The pores of the core 6 may be filled by known techniques, such as vacuum infiltration. If the core 6 contains a polymeric material, the polymeric material generally is present in a small amount, such as less than about 15 percent by volume of the core 6. The density of the core 6 is advantageously as low as possible, for example, to reduce the amount of energy required to spin the hard drive disk prepared therefrom. The density is preferably at most 8 g/cc, more preferably at most 6 g/cc, even more preferably at most 4 g/cc and most preferably at most 3.5 g/cc to preferably at least 0.5 g/cc. The surface roughness of the core 6 may be any roughness as long as the outer metal layer 12 adheres sufficiently to form a hard drive disk that operates in a hard drive without the outer metal layer 12 detaching from the core 6. The core 6 advantageously has a relatively rough surface to facilitate the cladding of the outer metal layer 12 to the core 6. The surface roughness (R a ) of the core 6 is preferably at least 0.01 micrometer, more preferably at least 0.1 micrometer, even more preferably at least 0.5 micrometer and most preferably at least 1.0 micrometer R a to preferably at most 32 micrometers, more preferably at most 24 micrometers and most preferably at most 16 micrometers R a , as measured, for example, by a method described by American National Standard ANSI B46.1-1985 or by interferometry techniques, such as phase-shifting interferometry for smooth surfaces and vertical-scanning interferometry for rough surfaces as employed in the WYKO Corporation (Tuscon, AZ) RST Plus Surface Measurement System. The Outer Metal Layer 12: The metal of the outer metal layer 12 may be any metal that can be adhered to the core 6 sufficiently to form the hard drive disk substrate 4 and hard drive disk formed therefrom. Exemplary metals include a transition metal, rare earth metal, beryllium, magnesium, aluminum and alloys thereof. The metal is preferably Y, Ce, Zr, La, Hf, Al, Be, Mg, a first row transition metal (that is, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) and metal alloys thereof. More preferably the metal is Al, Mg, Ti, Co, Ni, Fe, Cu and metal alloys thereof. More preferably the metal is Al, Be, Mg, Ti and alloys thereof. Most preferably the metal is aluminum and aluminum alloys, such as those known in the art. A face 8 of the core 6 is clad by the outer metal layer 12 to form the disk substrate 4. The outer metal layer 12 should cover at least one face 8 sufficiently, such that the area defined by the outer metal layer 12 is an area that is at least equal to the area where data is stored and retrieved on the finished hard drive disk. Preferably each face 8 is clad by the outer metal layer 12. The outer metal layer 12 may completely cover each face 8 as shown in Figure 1. The outer metal layer 12 may also cover the inner 16 and outer edge 14 of the core 6. The outer metal layer 12 should be sufficiently thick, so that a disk substrate 4 can be formed that is flatter than the core 6 it is produced from but not so thick that the advantages of a high elastic modulus core 6 are lost. For example, the volume of the outer metal layer 12 is desirably at most equal to the volume of the core 6. Generally, the outer metal layer 12 has an average thickness of at least 10 micrometers to at most 300 micrometers. The average outer metal layer thickness, in ascending preference, is at least 25, 30, 40, 50 and 65 micrometers to at most, in ascending preference, 200, 150 and 125 micrometers. The average thickness of the outer metal layer 12 may be determined as follows: (1 ) determine the disk substrate 4 average thickness, (2) determine the disk substrate average thickness after the outer metal layer 12 has been removed from one face 8 and then (3) subtract the thickness determined in step (2) from the thickness determined in step (1 ). The disk substrate 4 average thickness of step (1 ) may be determined by measuring the thickness at 10 places around the disk substrate 4 using a micrometer and averaging these measurements. After removing the outer metal layer 12 from one face 8, the average thickness of step (2) may be determined by the same method used to determine average thickness of step (1 ). The outer metal layer 12 may be removed from the disk substrate 4 by a technique, such as acid leaching or lapping. The flatness of the disk substrate 4 is desirably as flat as possible to increase the areal density of the hard drive disk made therefrom. Generally, the flatness of the disk substrate 4 is at most about 25 micrometers, more preferably at most about 15 micrometers, even more preferably at most about 10 micrometers and most preferably at most about 5 micrometers, where the flatness measurement is the full indicator movement (FIM) or analogous older term total indicator reading (TIR). The flatness may be measured by (1 ) placing a disk substrate 4 with one face down on a reference flat granite block, (2) positioning and zeroing the dial indicator on the remaining face (that is, up face) at the maximum material condition (that is, highest point of the disk) and subsequently (3) passing the dial indicator sufficiently over the surface of the up face to locate the full indicator movement (that is, maximum deflection of the indicator). The flatness measurement may also be made by a known optical technique, such as laser interferometry as employed in the TROPEL Corporation (Fairport, NY) FM-200 Flatness Analysis System. After removing the outer metal layer 12 from the disk substrate 4, as described above, the flatness of the core 6 may be determined by the same method used to measure the disk substrate 4. The flatness of the disk substrate 4 may be flatter than the core 6. Herein, the core 6 is flatter when the disk substrate 4 has a flatness measurement (FIM) that is less than the flatness measurement of the core 6 as measured, for example, by a method described previously. A perfectly flat disk substrate 4 would have a flatness measurement of zero. The disk substrate 4 preferably has a flatness measurement that is at least about 1 micrometer less than the flatness measurement of the core 6. More preferably the flatness measurement of the disk substrate 4 is about 5 micrometers less than the flatness measurement of the core 6. Most preferably the flatness measurement of the disk substrate 4 is about 10 micrometers less than the flatness measurement of the core 6. The outer metal layer 12 may have some porosity provided the porosity that is present fails to cause recording problems of the hard drive disk made therefrom. Since theoretical density is inversely proportional to the porosity, the porosity, as given by the theoretical density of the outer metal layer 12, is preferably at least about 98 percent of theoretical density and most preferably at least about 99 percent of theoretical density. The outer metal layer 12 desirably has a low density, for example, to lower the amount of energy required to spin the disk prepared therefrom. The density is preferably at most 8 g/cc, more preferably at most 6 g/cc, even more preferably at most 4 g/cc, and most preferably at most 3.5 g/cc to preferably at least 0.5 g/cc. In addition to the core 6 and the outer metal layer 12, the disk substrate 4 may also contain an interlayer disposed between the outer metal layer 12 and the core 6. The interlayer generally increases the strength or adhesion of the outer metal layer 12 to the core 6 compared to an outer metal layer 12 adhered to a core 6 in the absence of this interlayer. The interlayer may also provide damping of the disk substrate 4. The interlayer may be an adhesive (that is, "adhesive interlayer"). Suitable adhesives include those known in the art, such as adhesives described in Adhesives Technology Handbook. Arther H. Landrock, Noyes Publications, Park Ridge, New Jersey, 1985. Preferably the adhesive is a thermoset adhesive that is thermally cured. Examples of preferred adhesives include phenolic, polyimide, polysulfide and epoxy resins having sufficient thermal stability to withstand the processing temperatures used to make the disk substrate 4, for example, sputtering of a magnetic layer on the disk substrate. Hybrids of the preferred adhesives, such as nitrile-phenolic, neoprene-phenolic, nitrile-epoxy, nylon-epoxy and sulfide-epoxy are also preferred. The adhesive may be any form useful in adhereing the metal outer layer to the core 6, such as a liquid, paste, film or powder. The adhesive may be utilized by conventional or known methods to glue two parts together. The adhesive may contain a filler, such as inorganic particulates, that may change the damping characteristics of the disk substrate 4. Exemplary fillers include powders of the ceramics previously described. Preferably the interlayer is a material (herein interlayer material) that alloys or reacts to form a separate phase individually with one or more element, metal and compound of the core 6 and outer metal layer 12. Preferably the interlayer material is a metal. This metal may be a metal that has been described previously for the metal of the outer metal layer 12. This interlayer may be any thickness sufficient to enhance the adhesion of the outer metal layer 12 but, in general, the interlayer is at most about 10 percent of the thickness of the outer metal layer 12. Illustratively, the interlayer is typically between 0.1 to 5 micrometers thick. To aid in the formation of a very smooth wear resistant hard drive disk, the disk substrate 4 may have a hard layer disposed upon the surface of the outer metal layer 12 (that is, the hard layer is disposed on top of the outer metal layer such that the outer metal layer is sandwiched between the core and hard layer). Desirably, the hard layer is significantly harder than the metal of the outer metal layer 12. Significantly harder, herein, is a hardness that is at least about 2 times harder than the metal of outer metal layer 12. Preferably the hardness of the hard layer is at least 5, more preferably at least 10, even more preferably at least 20, and most preferably at least 30 times harder than the metal of the outer metal layer 12 to preferably at most 500 times harder than the metal of the outer metal layer 12. The hard layer may be a hard material known in the art for producing aluminum hard drive disks. The hard material may be a hard ceramic coating (for example, borides, carbides, carbon, diamond, nitrides, oxides and suicides), a nickel containing coating (for example, NiP) and silicon, each of which are described in Handbook of Tribology. Materials. Coating and Surface Treatments. B. Bhushan and B. K. Gupta, McGraw-Hill, Inc. New York. The hard material may also be boron. Preferably the hard material of the hard layer is selected from nickel, nickel phosphorous, titanium carbide, silicon carbide, silicon, boron, titanium nitride, titanium carbonitride or combinations thereof. Said hard layer may be crystalline or amorphous. A preferred embodiment is the use of a known NiP coating. Method to Form Disk substrate 4: To prepare the disk substrate 4, a core 6 must first be made. The core 6 may be prepared by a convenient or known method. For instance, the ceramic or ceramic metal composite may be formed by conventional powder metallurgical techniques and ceramic techniques. In general, powder metallalurgical or ceramic techniques involve: (1) mixing of the powder components of the body to be produced, (2) shaping a body from the mixed powders, (3) heating the shaped body to density it and (4) optionally machining or finishing the body to its final shape. Each of these steps is described in greater detail in Introduction to the Principles of Ceramic Processing. J. Reed, John Wiley and Sons, New York, 1988, and in Fundamental Principles of Powder Metallurgy. W. D. Jones, E. Arnold, London, 1960. A porous body containing a ceramic may also be infiltrated with a molten metal to form a ceramic-metal composite core 6. The infiltration of the molten metal into the porous body may be performed by a convenient or known technique. Examples of metal infiltration techniques include those described in U.S. Patent Nos. 3,864,154; 4,702,770; 4,718,941 ; 5,039,633; 5,394,929 and 5,595,622. A preferred embodiment is a porous boron carbide body that has been pressureless or vacuum infiltrated with aluminum or an alloy thereof to form an aluminum-boron carbide composite, previously described. Examples of methods to form the aluminum-boron carbide body are described in U.S. Patent Nos. 4,702,770; 4,718,941 ; 5,039,633; 5,394,929 and 5,595,622. The core 6 may be clad with a metal by a convenient or known cladding technique as long as the outer metal layer 12 that is formed remains adhered to the core 6 during subsequent processing or use (referred to herein as being adequately bonded or adequately adhered). Cladding or clad, herein, is the bonding of a metal foil to a substrate, which is further described on pages 6.9, 6.10 and pages 8.42 to 8.58 of Handbook of Tribology. Materials. Coating and Surface Treatments. B. Bhushan and B. K. Gupta, McGraw-Hill, Inc., New York, 1991. Exemplary cladding techniques include deformation cladding, such as roll cladding and extrusion cladding and electromagnetic impact cladding, diffusion bonding, braze cladding, weld cladding and laser cladding. Preferably the metal may be applied by contacting each face 8 of the core 6 with a foil and, subsequently, either gluing the foil with an adhesive previously described or heating the core 6 contacted with the foil to a temperature sufficient to bond the foil to the core 6. The core 6 may also be clad by placing the core 6 in a die and introducing a molten metal that completely envelopes and bonds to the core 6. An example of this technique is described by U.S. Patent No. 5,524,697. Before cladding the core 6 to form the metal clad disk substrate 4, the core 6 may be treated to enhance adhesion of the metal to be clad. Exemplary treatments include solvent cleaning, emulsion cleaning, alkaline cleaning, acid cleaning, pickling, salt bath descaling ultrasonic cleaning, roughening (for example, abrasive blasting, barrel finishing, polishing and buffing, chemical etching and electro-etching), as described in Chapter 7 of Handbook of Tribology. Materials. Coating and Surface Treatments. B. Bhushan and B. K. Gupta, McGraw-Hill, Inc., New York, 1991. When cladding the core 6 by contacting a face 8 of the core 6 with metal foil and then heating, the temperature of the heating may be any temperature sufficient to adequately adhere the metal. However, when heating above the melting point of the metal foil, the temperature should not be so great as to cause fracture of the core 6. In addition, if the temperature exceeds a temperature where a molten material is formed, it is preferred that the heating takes place in a die to contain any molten material that may be formed. When heating to a temperature where a molten material is not formed, a die may be used but is not preferred. Preferably the temperature ranges from 50 percent to 150 percent of the melting temperature of the metal foil that is used. More preferably the temperature ranges from 75 percent to 125 percent of the melting temperature that is used. When cladding the core 6 by contacting a face 8 of the core 6 with metal foil and then heating, the time at temperature is dependent on the metal, core 6, temperature and pressure used to bond the foil to the core 6. The time may be any time sufficient to adequately adhere or bond the metal to the core 6 for a given set of conditions. The time is preferably as short as practicable. Typical times range from a few minutes to several hours. The time is preferably at least 10 seconds, more preferably at least 2 minutes, even more preferably at least 5 minutes, most preferably at least 10 minutes to at most 15 hours, more preferably at most about 5 hours, even more preferably at most about 2 hours and most preferably at most about 1 hour. When cladding the core 6 by contacting a face 8 of the core 6 with metal foil and then heating, it is preferred that during heating a pressure is applied if the temperature of heating is less than the melting temperature of the foil. The pressure may be any pressure that helps adhere the metal foil to the substrate 4 and also maintains the dimensional thickness of the foil to a pressure that is so great that it causes the core 6 to fracture. It is preferred that the pressure is perpendicular and uniformly provided over each foil in contact with the core 6, such that each foil is adhered uniformly to the core 6. Generally, the pressure may range from 0.1 psi to 250,000 psi. The pressure preferably is at most about 1000 psi, more preferably at most about 100 psi, even more preferably the pressure is at most about 10 psi, and most preferably at most about 5 psi. During cladding, the environment should be substantially inert to the core 6 and the metal being clad, such that any reactions with the environment are insufficient to cause the metal to be inadequately adhered to the core 6. Of course, the core 6 and metal may react to form a compound that enhances the adhesion of the outer metal layer 12 to the core 6, such as those previously described. Environments may include gases, liquids or solids. Examples of gases include noble gases and nitrogen. An example of a solid is boron nitride. Examples of liquids include those common in the art, for example, to perform electroless plating and electro-plating. The cladding step may also be conducted under vacuum. After cladding, a further heat treatment may be performed to density the outer metal layer 12. Generally, the heat treatment temperature is at least a temperature sufficient to further density the outer metal layer 12 to a temperature of at most the melting temperature of the metal of the outer metal layer 12. Examples of techniques useful to further density the metal include pressureless sintering, hot isostatic pressing, hot pressing and rapid omnidirectional compaction. After cladding, but before finishing, it is desirable for the thickness of the outer metal layer 12 to be significantly thicker than the thickness of the outer metal layer 12 after finishing. It has been found that having this significantly greater thickness aids in forming a disk substrate 4 that is flatter than the core 6 it is prepared from. A significantly greater thickness corresponds to a thickness that is about 150 percent of the thickness of the outer metal layer 12 after finishing. Preferably the thickness of the outer metal layer 12 before finishing is at least about 175 percent, more preferably at least about 200 percent, most preferably at least about 250 percent to preferably at most 1000 percent, more preferably at most about 750 percent, most preferably at most about 500 percent thicker than the outer metal layer 12 after finishing. The metal clad substrate is finished to form the disk substrate 4 of the present invention. The finishing step may be a convenient technique, such as those known for finishing aluminum hard drive disks. Examples of finishing include single point machining, fixed abrasive grinding, free abrasive grind, polishing, forging and rolling. These techniques are described in Manufacturing Engineering and Technology 2 nd Ed.. S. Kalpakjian, Addison- Wesley Publishing Co., New York, 1992. The hard layer and the interlayer may be applied by a known technique, such as plasma spraying; sputtering; physical vapor deposition; chemical vapor deposition; electroless plating; electro-plating and combination thereof. Each of these techniques is described in greater detail in Handbook of Tribology. Materials. Coating and Surface Treatments. B. Bhushan and B. K. Gupta, McGraw-Hill, Inc., New York, 1991. The hard layer preferably is applied by a known technique for applying a hard layer to an aluminum hard drive disk. An example is the electroless plating of NiP. The following examples are solely for illustrative purposes and are not to be construed as limiting the scope of the present invention. EXAMPLES Example 1 A disk substrate, according to this invention, was made by first making a disk shaped core, cladding the core by adhering a metal foil and, finally, finishing the metal clad disk. Preparation of the Disk Shaped Core An aluminum-boron carbide disk shaped core was first made by the following method. One hundred parts by weight of a boron carbide powder having an average particle size of 1 to 1.5 micrometers, which is available from Elektroschmelzwerk Kempten, Germany, was mixed with 22 parts by weight methyl-ethyl-ketone, 14 parts by weight isobutyl methacrylate binder available from Rohm and Haas Company, PA and 6 parts by weight dibutyl phthalate available from Aldrich Chemical Company to form a boron carbide powder slurry. About 1 gallon of the boron carbide powder slurry was ball milled for 12 to 16 hours in a 2 gallon polypropylene jar 1/3 filled with about 5/8 ,h of an inch diameter by about 5/8 th of an inch (1.47mm) length hot pressed boron carbide right cylinder milling media. After milling, the slurry had a viscosity of about 3000 centipoise as measured by a Haake Model VT-500 viscometer at a shear rate of about 5 sec 1 . An 18 inch (457 mm) wide tape was cast from the slurry onto a polyester (MYLAR™, available from Dupont deNemours, Wilmington, DE) film coated with a silicone release agent to form a cast tape. The solvent (that is, methyl-ethyl-ketone) was removed from the cast tape by drying in air at room temperature to form a dried tape having a thickness of between 1.2 mm to 1.5 mm. The dried tape was removed from the MYLAR film and, subsequently, disk-shaped cores with an inner diameter of about 23 mm and outer diameter of about 97 mm were punched from the tape. The binder and plasticizer were removed from the punched out disk-shaped cores by heating them for 6 hours to 450°C, maintaining them at 450°C for 30 minutes under flowing nitrogen and then cooling to room temperature. These porous preform disk shaped cores were then infiltrated with liquid aluminum by contacting the disk cores with aluminum and then heating them for about 100 minutes to 1050°C, maintaining them at 1050°C for 2 hours under an argon atmosphere, such that the aluminum becomes molten and infiltrates the core to form an aluminum-boron carbide composite disk shaped core and then cooling them to room temperature. The infiltrated disk cores were lapped to a thickness of 0.6 mm using a model AL2F double-sided lapping machine available from Peter Wolters, Inc., MA. The disk cores were lapped with 320 grit boron carbide powder dispersed in kerosene at a load of 150 to 200 pounds (68.0 to 90.7 kg) on the top plate with the bottom lapping plate rotating at about 80 percent of motor speed in a counterclockwise direction. The lapped disk cores had a flatness of 25 micrometers to 70 micrometers (0.001 to 0.0027 inch) and a roughness of about 5500 angstroms R a . Cladding the Disk Core An aluminum alloy 6061 foil having a thickness of about 5 mils (about 0.13 mm) and having a pressure sensitive acrylic adhesive coating about 1 mil (about 0.025 mm) thick, available from Ideal Tape Company of Lowell, MA, was punched to form a disk-shaped foil. A disk-shape foil was then placed in contact with each face of the disk core, such that the foils covered the entire surface of the faces of the disk core. A load of 1 Kg was then applied for at least about 16 hours to the face of the foils, such that the adhesive bonded the foils to the disk core forming a core clad with an outer metal layer. Each cladded core was then pressed under a uniaxial load of 5,000 pounds (2,267 kg) using a table top hydraulic press to remove any air bubbles between the foil and disk core. The cladded cores were single-side lapped to remove excess aluminum to form the disk substrate. The cladded cores were lapped using a single-sided lapping machine having a 60 inch (152cm) diameter table, available from Speedfam Company, IL. Each face of the cladded cores was lapped at a pressure of about 0.5 pound per square inch with the table rotating counterclockwise at about 40 revolutions per minute, for a total time of about 3 minutes, using alumina grit having an average size of about 25 micrometers, available from Norton Company, MA, in a kerosene carrier medium. The lapped aluminum surface was then polished by (1 ) fixturing the disks by a vacuum table,(2) rubbing 3 micrometer diamond paste, available from Engis Corp., IL., into a cloth buffing wheel attached to a manually operated drill press, (3) rotating the buffing wheel at about 200 rpm using the drill press, (4) lowering the buffing cloth using the drill press until it contacts the surface of the disk to be polished and (5) maintaining the contact of the buffing wheel with the disk face for a suitable time to polish the disk until no lapping scratches were visible by eye. An electroless nickel plating was then applied to the surface of the polished disk using the following procedures. To clean the disks, the disks were soaked in an alkaline cleaner "8744," available from Enthone Corporation, CT, for about 1 minute, rinsed with deionized water, soaked in acid deoxidizer (ACTANE E-10™, available from Enthone Corporation, CT) for about 1 minute and rinsed in deionized water. Before Ni plating, a zinc layer was deposited by (l)immersing the disks in a zincate bath consisting of a supersaturated solution of zinc oxide in aqueous sodium hydroxide solution (that is, Fidelity "3116," available from Fidelity Corp., NJ) for about 30 seconds, (2) rinsing them in de-ionized water, (3) immersing them in an acid deoxidizer (that is, ACTANE E-10™ described above) for about 30 seconds, (4) rinsing them with deionized water, (5) immersing them again in the zincate bath for about 15 seconds and (6) thoroughly rinsing them with deionized water. The disks were then immersed in an electroless nickel bath (Bath # 4355 available from Fidelity Corp., NJ) for about 2 hours at a pH of about 4.4 and temperature of about 84°C to achieve the required plating thickness. The resulting disk substrates were about 0.032 inch thick and had a flatness (FIM) of 25 to 45 micrometers (0.001 to 0.0018 inch). Example 2 Disk shaped cores were prepared by the same method described in Example 1 , except that prior to bonding, the surface of the cores was roughened by sandblasting using a sandblaster available from Dayton Mfg. Company. The disks were sandblasted using a garnet sand. The sandblasted cores had a surface roughness of 850 to 1300 nm R a . Cladding the Disk Cores Aluminum alloy 5086 foil, 12 mils thick, was punched into disks of the same shape as the cores. Each core was sandwiched between two foils and heated under a load of about 3 kilograms to 625°C in a vacuum of 100 milliTorr for 15 hours and, subsequently, cooled at a rate of 1 to 5°C/minute to form cores cladded with outer metal. These cladded cores were lapped and finished using the same methods described in Example 1 to form disk substrates similar to those of Example 1.

Claims: 1. 1. A hard drive disk substrate comprising (a) a disk-shaped core comprised of a material selected from a ceramic or a ceramic-metal composite, wherein the core has at least one face clad by (b) an outer metal layer that has an average thickness of at least 10 micrometers to at most 300 micrometers. 2. The disk substrate of Claim 1 wherein the outer metal layer has a density of at most 6 g/cc. 3. The disk substrate of Claim 1 wherein the outer metal layer comprises a metal selected from aluminum, beryllium, titanium, magnesium or alloys thereof. 4. The disk substrate of Claim 1 wherein the metal comprises aluminum. 5. The disk substrate of Claim 1 wherein the outer metal layer has an average thickness of at least 25 micrometers. 6. The disk substrate of Claim 1 wherein the core has an elastic modulus of at least 150 GPa. 7. The disk substrate of Claim 1 wherein the core is a ceramic selected from an oxide, carbide, nitride or combination thereof or a ceramic-metal composite selected from a silicon carbide-aluminum composite or a boron carbide-aluminum composite. 8. The disk substrate of Claim 1 wherein the core is silicon carbide, aluminum nitride, silicon nitride, aluminum oxide, aluminum oxide-titanium carbide or a boron carbide aluminum composite. 9. The disk substrate of Claim 1 wherein each face of the core is clad by the outer metal layer. 10. The disk substrate of Claim 1 further comprised of a hard layer disposed upon the outer metal layer wherein the hard layer is significantly harder than the outer metal layer. 11. The disk substrate of Claim 10 wherein the hard layer is a hard material selected from NiP, titanium carbide, silicon carbide, silicon, boron, titanium nitride and titanium carbonitride and combinations thereof. 12. The disk substrate of Claim 10 wherein the disk substrate is further comprised of a magnetic medium layer upon the hard layer. 13. The disk substrate of Claim 1 further comprising an interlayer disposed between the outer metal layer and the core. 14. The disk substrate of Claim 1 wherein the interlayer is at least 0.1 micrometer to 5 micrometers thick. 15. The disk substrate of Claim 1 wherein the disk shaped core is porous. 16. The disk substrate of Claim 15 wherein the pores of the core are filled with a polymeric material. 17. A disk substrate comprising (a) a disk shaped core comprised of a material selected from a ceramic or a ceramic-metal composite wherein the core has at least one face clad by (b) an outer metal layer in which (c) an adhesive interlayer is disposed between the core and the outer metal layer. 18. The disk substrate of Claim 17 wherein the adhesive is selected from phenolic, polyimide, polysulfide or epoxy resins. 19. A hard drive substrate comprising (a) a disk shaped core comprised of a material selected from a ceramic or a ceramic metal composite wherein the core has at least one face clad by (b) an outer metal layer such that the disk substrate is flatter than the core. 20. A method for preparing a disk substrate comprising: (a) cladding, with a metal, at least one face of a disk shaped core comprised of a material selected from a ceramic or a ceramic-metal composite to form a core clad with an outer metal layer; and (b) finishing the core clad with the outer metal layer to form the disk substrate. 21. The method of Claim 20 wherein the core has a surface roughness of at least 0.1 micrometer R a . 22. The method of Claim 20 wherein the core has a surface roughness of at least 1 micrometer R a . 23. The method of Claim 20 wherein cladding of the substrate core is a method comprising: (a) contacting each face of the core with a metal foil; and (b) heating the core contacted with the metal foil to a temperature sufficient to bond the foil to each face of the core. 24. The method of Claim 23 wherein the temperature is at most the melting temperature of the metal foil. 25. The method of Claim 23 wherein a pressure is applied to the core contacted with the metal foil during heating. 26. The method of Claim 23 wherein the heating takes place under vacuum. 27. The method of Claim 20 wherein the cladding of the core comprises gluing a metal foil to the core using an adhesive. 28. The method of Claim 27 wherein the adhesive contains a filler. 29. The method of Claim 20 wherein the cladding of the core with a metal comprises placing the core in a die and introducing a molten metal that envelopes and bonds to the core to form the core clad with an outer metal layer.

Inventors: BORN DAVID W, DESHMUKH UDAY V, KAMP ARTHUR J, PYZIK ALEKSANDER J, BORN, DAVID, W., DESHMUKH, UDAY, V., KAMP, ARTHUR, J., PYZIK, ALEKSANDER, J.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1998
Main classification: G11B5
IPC codes: G11B5