Apparatus and method for near contact magnetic recording

Abstract: A rotating magnetic storage device is shown wherein the transducer (11) and media (17) are separated during operation by a thin film of low viscosity liquid lubricant that is recirculated from a reservoir (23) within the head-media enclosure. Lubricant recirculation can be effected by capillary action using a wick (22), mechanical pumping and metering or distillation techniques. As compared to devices using air bearing separation, the transducer carrying slider has very small bearing surfaces which may be in the form of narrow rails or small depending foot elements. This enables fly heights of 2 microinches using a film of one micron thickness.

Description: TECHNICAL FIELD OF THE INVENTION The present invention is directed to magnetic data storage devices and more particularly to a magnetic data storage device within a substantially sealed enclosure that has the transducer separated from the media by a thin film of recirculated, low viscosity liquid lubricant. BACKGROUND OF THE INVENTION State of the art magnetic disk drives presently, and for some time have used a transducer carrying slider which flies over the media surface and is separated therefrom by a film of air. As recording densities become greater, the head is required to fly lower and the magnetic layer of the media becomes thinner. When the spacing between head and media is reduced from 0.635 or more microns (25 or more microinches) to fewer than 0.254 microns (ten microinches) and the media magnetic layer thickness is reduced from as much as 1.27 microns (50 microinches) to fewer than 0.127 microns (5 microinches), the physical contact that occasionally occurs between head and media is of major concern. To provide an acceptable rigid disk drive lifetime, the current technology employs a thin layer of a fluorocarbon lubricant applied to the magnetic medium during manufacture of the disk. This lubricant film is critically needed by the head/disk interface (HDI) to minimize wear to these components and significantly extend the lifetime of the disk drive to an acceptable level. A problem with this technology arises from depletion of the lubricant film with use. The loss of lubricant can be attributed to hydrodynamic ejection from the HDI due to air shear phenomena, thermal volatilization, thermal decomposition followed by bonding to the medium and lubricant spinoff. These, and other wear processes at the HDI increase as the lubricant is lost and ultimately result in failure. The total wear to the HDI can be categorized into two distinct modes; first, a rapid wear rate that occurs during start/stop operations and second, a slower wear rate due to repeated high speed contacts at the HDI during normal file operations (disk at or near maximum speed). Wear associated with the start/stop process is only slightly dependent on head fly height, whereas for the latter case, a lowering of the fly height will dramatically increase the wear rate at disk drive operating speed (flyability). The increase for this latter case has been attributed to an increase in the collision frequency with disk asperities and debris (both foreign and from generated wear products). In this case, the presence of a lubricant film plays a critical role in protecting the HDI, but with the reduction in head fly height the lubricant film is also depleted more rapidly. Thus, for these reasons a reduction in flying height greatly increases the likelihood of early disk drive failure. The reason for reducing head flying height is to obtain an increase in the magnetic storage density. An additional problem encountered here is to also reduce the wear rate to the head gap. For these reasons, and because of their relationship to disk drive reliability, the current technology is limited by the achievable lowest flying height as a barrier to further increases in data storage density. A proposed technique for providing a continuous lubricant supply for a magnetic media surface is shown in US Patents 2,969,435 and 3,005,675. In both patents a supply of lubricant is sprayed onto the disk or drum surface ahead of the area of contact between transducer head and media surface. This mode of placing a layer of lubricant on the media surface would not be compatible with the system of the present invention wherein a transducer is made to adjoin the media during relative motion therebetween with a separation of but 0.025 µm or 0.05 µm (1 or 2 microinches). A later patent, US 4,633,351 attempts to overcome some of the problems associated with the earlier patented techniques by spraying water or a volatile liquid solvent ahead of the transducer to create a thin liquid film between the head and the recording media surface. In this later patent the liquid is supplied to a rotating drum surface via an applicator comprising sponge and squeegee which is connected to a water reservoir by a water supply line. Water flows down the supply line under the influence of gravity and trickles onto the surface when the sponge becomes saturated. An alternative method for maintaining lubricant on the surface of a disk is described in EP 127 444. In this document, a lubricant evaporation source is arranged in an airtight enclosure of the storage device to fill the enclosure with lubricant vapour thus suppressing the evaporation and consumption of coated lubricant layers on the disks. DISCLOSURE OF THE INVENTION The present invention provides a magnetic memory device comprising: a rotatable rigid disk having a magnetic data surface; a magnetic transducer; and means for maintaining a film of liquid lubricant on the disk data surface by recirculation of the lubricant over the surface, characterised in that said means comprises wick means providing a capillary path for transporting liquid lubricant from a reservoir of lubricant in the device to the disk data surface, the transducer being supported adjacent the disk data surface by the lubricant film during rotation of the disk. While the liquid lubricant may be similar in formulation to the permanently applied lubricants employed in conventional systems, it is preferred that the liquid lubricant is a low viscosity lubricant maintained at a substantially uniform thickness not exceeding 5 microns. In a preferred system the wick itself acts as a filter to remove particulate matter before it is deposited on the recording surface. Although the transducer head suspensions used in the present invention can be the same as used in the air bearing type systems, employed, typically, in the IBM 3340 or 3380 disk drives, the slider or transducer head structure must be modified to adapt to the use of a liquid film rather than an air film. Instead of the multiple rails that present air bearing surfaces, the modified slider can use a single rail with a very narrow bearing surface or an even more effective design that presents three minute triangular feet. In the latter design one foot adjoins the leading edge and two are at the trailing edge and terminate rearwardly at the thin film transducers. Thus a much smaller bearing surface is used with the more viscous bearing fluid. The invention will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic plan view with the cover removed showing a disk drive incorporating the present invention. Fig. 2 is a vertical section of the disk drive of Fig. 1. Fig. 3 is a side elevation of a transducer head structure used in the practice of the present invention. Fig. 4 is a bottom view of the head shown in Fig. 3. Fig. 5 shows the structure of one of the feet that present the bearing surface upon which the slider is supported. Fig. 6 is a schematic view of an alternative embodiment of a disk drive lubricant recirculating system using a pumping action integral with the hub-disk assembly. Fig. 7 is a schematic view of a further alternative embodiment of a lubricant recirculating system using a distillation technique. DETAILED DESCRIPTION OF THE INVENTION Figs. 1 and 2 respectively show a schematic plan view of a drive with the cover removed and a sectional elevation. Illustrated is a typical rigid disk drive with a linear voice coil motor (VCM) driven actuator 10. The actuator 10 drives a head-arm assembly which may include typical head suspension devices 11 such as the common Winchester or Whitney suspension designs. The head assembly is contained within an enclosure including a cast base member 12 which also serves as a liquid lubricant reservoir and a cover 13 which has a flange 14 that compresses a gasket 15 against the upper planar surface of the base 12 casting to effect a seal. The rigid disk media 17 are carried by the hub 18 that is mounted to rotate in unison with the rotor of a spindle motor positioned within an upwardly extending depression 20 in base 12. The motor shaft extends through a bearing tower to connect to the hub 18. The liquid lubricant used in the system of the present invention is required to be thermally stable, have a correct viscosity which is very low and be non-reactive. A simple, straight chain hydrocarbon with the desired low viscosity and a simple molecule that will not degrade is recommended. In operation, hexadecane has been used. Lubricants commonly used on disk surfaces, but having much reduced viscosity are satisfactory. However, with the greater quantity of lubricant used in the present technique as compared with the quantity required for the normal single application to the disk surface, the cost of the material becomes a factor in lubricant selection. A wick structure 22 is formed of porous material, such as plastic, ceramic or paper. The wick structure that carries liquid lubricant from the reservoir at the lowest point in the head disk enclosure to the disk at the inner diameter of the disk storage surface by capillary action. The liquid is gradually spun by the centrifugal force of the spinning disk to the periphery from which it leaves as minute droplets to return by gravity to the reservoir. The wick structure 22 must possess porosity to permit liquid lubricant to travel by capillary action from the reservoir 23 formed in base 12 to the media surface in minute quantities to replenish and maintain a film of lubricant on the media surfaces. The material of the wick 22, in addition to providing a capillary path, must also not shed particles and thereby become a source of particulate material within the enclosure. The film thickness should preferably be one micron and should not exceed 5 microns, since as the film thickness increases, the power required to drive the heads through the lubricant film increases. As the lubricant is spun off the disk and replaced, the wick 22 also functions as a lubricant filter to prevent the recirculation of particles captured in the liquid lubricant back to the media surfaces. To minimize wear between the wick and the location of contact at the ID of the disk, the wick should generate lift and fly' like the slider. The loading pressure of the wick on the disk can be much lighter than for the slider, because the sliding height can be much higher and less tightly controlled. Recirculation need not be accomplished using a wick structure. Two alternative methods are illustrated in Figs. 6 and 7. Lubricant can be transported with no contacting parts using the left handed spiral passage of the embodiment of Fig. 6 or the distillation technique of Fig.7. Figs. 6 and 7 show a base casting 50 that defines a liquid reservoir and is enclosed by a base cover 51. Disks 53 are mounted for rotation on a hub 55 and separated by a spacer 56. The disks 53 are compressively retained between a hub flange surface 57 and a confining surface 59 on the clamping member 60. An actuator 62 mounts the transducer heads 63 for movement between concentric tracks on the disk surfaces. Lubricant is supplied to the annular space 65 between the hub 55 and the assembly of disks 53 and spacer 56. Radial scratches or recesses in the hub surface 57, clamp surface 59 and the upper and lower spacer surfaces 66 provide a passage for the lubricant from the annular space 65 to the disk surfaces, with the size of such passages affording the limitation on the rate of fluid distribution to the respective disk surfaces. In Fig. 6, a thin plate member 70 isolates the fluid in the reservoir volume from the windage effect of the confronting rotating disk surfaces and permits the lubricant to flow radially inward toward hub 55 through an annular filter 71. The hub has a left handed spiral recess 72 formed in the inner surface that draws lubricant upward as the hub assembly rotates for delivery through the radial passages 74 to the annular space 65. The method of recirculation by distillation (Fig. 7) is most effective in the case of a hermetically sealed file with the lubricant under its own vapour pressure (no air present). The lubricant evaporates from the warmer parts of the file and condenses on the relatively cool cover 51. The cover is contoured with a conical depression 80 such that the condensate will drain to a point directly above the spinning hub assembly and drip into a hub clamp member opening 81. The passage through the clamp member opening has a conical side wall surface 82 that increases in diameter in the downward direction so that centrifugal force will not fling the liquid out of the passageway. In operation, the distillation should proceed slightly faster than the lubricant flow rate required for the disks so that the intermediate reservoir at the top of the hub will remain full. Excess lubricant overflows from the top of the hub and is flung harmlessly to the file walls. In the device of Fig. 7, the heat generated by an electronic circuit card (not shown) below the head-disk enclosure and the heat generated by the motor will be sufficient to drive this distillation process. The top cover 51 is the coolest area of the drive enclosure and localised cooling is provided at the contoured, conical surface 80 overlying the hub by the provision of cooling fins 83 at the outer surface. In this mode of lubricant recirculation a filter is not required since particulate material is retained in the reservoir and is not recirculated with the lubricant vapour. Although many of the drive system assemblies, such as the actuator 10, transducer suspensions 11 and disk assemblies, are of conventional design, the head structure is a total departure from the normal slider designs used with the more common transducers that are separated from the media surface by a film of air. Initially a three rail head was used with the rails reduced in width to reduce the fluid bearing surface to meet the needs of a system using a liquid film rather than a film of air to support a slider above the recording surface. This emulated the air bearing sliders which have been the most successful in current practice while using a reduced bearing area in response to the recognised greater supporting capability of a liquid film. A further development resulted in the slider 25 shown in Fig. 3 which is of normal length, about 4 millimetres and has a mounting grove 26 to permit attachment to the gimbal spring structure of a standard Winchester suspension; however, the bearing surfaces are wholly different. As more clearly seen in the bottom view of Fig. 4, instead of the rails or other air bearing surfaces or even very narrow rails with greatly reduced air bearing surface areas, the bearing surfaces are more optimally provided by the three very small feet 28, 29. The modified slider structure is shown in Figs. 3 through 5. The slider as illustrated has substantially the same overall dimensions as the air bearing sliders used in current product lines, a length of 4.025 millimetres, a width of 3.207 millimetres and a height of less than one millimetre. The feet 28, 29; however, are short, narrow and present very minute fluid bearing surfaces 30. The feet can be 0.145 mm (0.0057 inches) in length with a triangular form. The surface includes a 10 degree included angle at the leading end and is only 0.025 mm (0.001 inch) wide at the trailing edge. Each foot extends downwardly from the lower surface of the slider body a distance of 25 to 50 microns with the foot side walls 31 that extend to join the lower slider surface curved to blend into the slider body. This provides greater strength for the physically small foot. Because the bearing surface is so small, the slider body could be made much smaller than that shown which was based on dimensions used for air bearing type sliders. In addition to two rail and three rail sliders with reduced area rail bearing surfaces, that are suggested by the prior art air bearing sliders and the three triangular foot configuration that is shown and described, numerous other bearing surface configuration are operable. The number of triangular feet may be varied and the positioning of the feet can be changed. Also, other bearing surface shapes can be used such as rectangular feet. As seen in Fig. 5, one side 33 extends parallel to the axis of the slider and the other side 34 is at, for example, an angle of 10°. The side of the foot parallel to the slider axis is disposed toward the disk inner diameter and the inclined surface is positioned facing the outer diameter. This angled relationship of the triangular sides of the feet 28, 29 extending from the foot leading portion enable the lubricant film and any minute debris that might be included therein to be deflected toward the disk outer diameter and ultimately, off the disk. The transducer or transducers carried by the slider 25 are of the thin film type mounted on the rear surface 36 and presenting the transducing gap at the edge 35 of the foot 28. Ferrite core heads have also been made and the magnetic performance demonstrated. With such heads, the glassed gap is incorporated into one (or more) of the feet at the point where the foot width is equal to the desired magnetic track width. The depth of the lubricant film does not exceed 5 microns and preferably has a thickness of about 1 micron. The feet 28, 29 which are 25 to 50 microns in height, hold the body of the slider above the lubricant as the bearing surfaces are supported only 0.025 to 0.05 µm (1 to 2 microinches) above the media surface. The slider has been operated using a 3.5 gram load which causes a pressure at the bearing surface of less than 6894 kPa (1000 pounds per square inch). In practice, the pressure is about 5515 kPa (800 pounds per square inch) across the fluid film at the bearing surface. If a higher gram load is desired, the area of the bearing surfaces can be increased by designing the linear dimensions to be longer, and maintaining the same angles.

Claims: A magnetic memory device comprising: a rotatable rigid disk (17) having a magnetic data surface; a magnetic transducer (11); and means for maintaining a film of liquid lubricant on the disk data surface by recirculation of the lubricant over the surface, characterised in that said means comprises wick means (22) providing a capillary path for transporting liquid lubricant from a reservoir (12) of lubricant in the device to the disk data surface, the transducer being supported adjacent the disk data surface by the lubricant film during rotation of the disk. A device as claimed in claim 1 wherein the wick means acts to filter the liquid lubricant between the lubricant reservoir and the disk data surface. A device as claimed in claim 1 or claim 2, wherein the liquid lubricant is a low viscosity lubricant which is maintained on the disk data surface at a substantially uniform thickness of between 1 micron and 5 microns. A device as claimed in any preceding claim, wherein the wick structure is formed of plastic, ceramic or paper. A device as claimed in any preceding claim wherein the wick means is positioned to deposit lubricant towards the inner diameter of disk and the lubricant reservoir is positioned within the device to collect lubricant ejected by centrifugal force from the disk. A device as claimed in claim 5 further comprising a plurality of lubricant reservoirs to enable the device to operate in a plurality of attitudes. A device as claimed in any preceding claim, wherein a portion of the wick means is disposed adjacent the disk data surface to deliver lubricant thereto, said portion including an air bearing surface which flies above the disk data surface during rotation thereof. A device as claimed in any preceding claim, wherein the magnetic transducer comprises a slider (21) with a lower surface adapted to confront the rotating data surface from which extend forward and rearward depending feet (28, 29) terminating in bearing surfaces, the transducing gap being positioned at the rearward surface (30) of the slider and a rearward foot (28). A device as claimed in claim 8 wherein the slider has a triangular front foot (29) and a spaced pair of triangular rear feet (29). A device as claimed in claim 9 wherein the bearing surfaces of the feet are each bounded by a first edge (33) in the direction of the disk inner diameter parallel to the axis of the slider and a second edge (34) toward the disk outer diameter that diverges from the first edge.

Inventors: GREGORY THOMAS ALLEN, KELLER CHRISTOPHER GUILD, LARSON THOMAS SCOTT, GREGORY, THOMAS ALLEN, KELLER, CHRISTOPHER GUILD, LARSON, THOMAS SCOTT
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: G11B23
IPC codes: G11B21, G11B25, G11B23, H02J13, G11B5, G11B33, G11B17