Abstract:
A disc drive storage system including a housing having a central axis, a stationary member that is fixed with respect to the housing and coaxial with the central axis, and a rotatable member that is rotatable about the central axis with respect to the stationary member is described. A hydrodynamic bearing interconnects the stationary member and the rotatable member and includes at least one working surface comprising a wear resistant coating.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional application serial No. 60/332,490, filed Nov. 16, 2001, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of hydrodynamic motors for disc drive data storage devices and, more particularly, to a spindle motor with one or more bearing surfaces having a wear resistant coating thereon. 
     2. Description of the Related Art 
     Disc drive data storage devices, known as “Winchester” type disc drives, are well-known in the industry. In a Winchester disc drive, digital data is written to and read from a thin layer of magnetizable material on the surface of rotating discs. Write and read operations are performed through a transducer that is carried in a slider body. The slider and transducer are sometimes collectively referred to as a head, and typically a single head is associated with each disc surface. The heads are selectively moved under the control of electronic circuitry to any one of a plurality of circular, concentric data tracks on the disc surface by an actuator device. Each slider body includes a self-acting air bearing surface. As the disc rotates, the disc drags air beneath the air bearing surface, which develops a lifting force that causes the slider to lift and fly several microinches above the disc surface. 
     In the current generation of disc drive products, the most commonly used type of actuator is a rotary moving coil actuator. The discs themselves are typically mounted in a “stack” on the hub structure of a brushless DC spindle motor. The rotational speed of the spindle motor is precisely controlled by motor drive circuitry, which controls both the timing and the power of commutation signals directed to the stator windings of the motor. Typical spindle motor speeds have been in the range of 3600 RPM. Although, current technology has increased spindle motor speeds to 7200 RPM, 10,000 RPM, 15,000 RPM and above. 
     One of the principal sources of noise in disc drive data storage devices is the spindle motor. Disc drive manufacturers have recently begun looking at replacing conventional ball or roller bearings in spindle motors with “hydro” bearings, such as hydrodynamic or hydrostatic bearings. A hydrodynamic bearing relies on a fluid film which separates the bearing surfaces and is therefore much quieter and in general has lower vibrations than conventional ball bearings. A hydrodynamic bearing is a self-pumping bearing that generates a pressure internally to maintain the fluid film separation. A hydrostatic bearing requires an external pressurized fluid source to maintain the fluid separation. Relative motion between the bearing surfaces in a hydrodynamic bearing causes a shear element that occurs entirely within the fluid film such that no contact between the bearing surfaces occurs. 
     In a hydrodynamic bearing, a lubricating fluid or gas provides a bearing surface between, for example, a stationary member of the housing and a rotating member of the disc hub. Typical lubricants include oil or ferromagnetic fluids. Hydrodynamic bearings spread the bearing surface over a larger surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface decreases wobble or run-out between the rotating and fixed members. 
     Despite the presence of the lubricating fluid, in conventional hydrodynamic bearing spindle motors, the bearing surfaces are still subject to continuous wear. As a result, the gap between bearing surfaces gradually changes over the lifetime of the device, and often in a manner that is not uniform across the bearing surfaces. This results in reduced performance and eventual failure of the disk drive. Additionally, for a gas lubricated hydrodynamic bearing, low frictional properties for the bearing surfaces is also required. 
     Therefore, there exists a need in the art for a hydrodynamic fluid bearing surfaces having improved wear resistance as well as low frictional properties. 
     SUMMARY OF THE INVENTION 
     The disc drive data storage system of the present invention includes a housing having a central axis, a stationary member that is fixed with respect to the housing and coaxial with the central axis, and a rotatable member that is rotatable about the central axis with respect to the stationary member. A stator is fixed with respect to the housing. A rotor is supported by the rotatable member and is magnetically coupled to the stator. At least one data storage disc is attached to and is coaxial with the rotatable member. A hydrodynamic bearing couples the stationary member to the rotatable member. The hydrodynamic bearing includes at least one working surface with a wear resistant coating thereon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. 
     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     FIG. 1 is a top plan view of a disc drive data storage device in accordance with the present invention; 
     FIG. 2 is a sectional view of a hydrodynamic bearing spindle motor in accordance with the present invention; 
     FIG. 3 is a diagrammatic sectional view of the hydrodynamic bearing spindle motor taken along the line  3 — 3  of FIG. 2, with portions removed for clarity; 
     FIG. 4 is a close up view of FIG. 3, showing wear resistant coatings formed on one or more working surfaces of the hydrodynamic bearing; and 
     FIG. 5 is a sectional view of a hydrodynamic bearing with conical bearing surfaces. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is a disc drive data storage device having a hydrodynamic bearing spindle motor in which one or more bearing surfaces have a wear resistant coating thereon. FIG. 1 is a top plan view of a disc drive  10  in which the present invention is useful. Disc drive  10  includes a housing base  12  that is combined with top cover  14  to form a sealed environment to protect the internal components from contamination by elements from outside the sealed environment. 
     Disc drive  10  further includes a disc pack  16 , which is mounted for rotation on a spindle motor (not shown) by a disc clamp  18 . Disc pack  16  includes a plurality of individual discs, which are mounted for co-rotation about a central axis. Each disc surface has an associated head  20 , which is mounted to disc drive  10  for communicating with the disc surface. In the example shown in FIG. 1, heads  20  are supported by flexures  22 , which are in turn attached to head mounting arms  24  of an actuator body  26 . The actuator shown in FIG. 1 is of the type known as a rotary moving coil actuator and includes a voice coil motor (VCM), shown generally at  28 . Voice coil motor  28  rotates actuator body  26  with its attached heads  20  about a pivot shaft  30  to position heads  20  over a desired data track along an arcuate path. While a rotary actuator is shown in FIG. 1, the present invention is also useful in disc drives having other types of actuators, such as linear actuators. 
     FIG. 2 is a sectional view of a hydrodynamic bearing spindle motor  32  in accordance with the present invention. Spindle motor  32  includes a stationary member  34 , a hub  36  and a stator  38 . In the embodiment shown in FIG. 2, the stationary member is a shaft that is fixed and attached to base  12  through a nut  40  and a washer  42 . Hub  36  is interconnected with shaft  34  through a hydrodynamic bearing  37  for rotation about shaft  34 . Bearing  37  includes radial working surfaces  44  and  46  and axial working surfaces  48  and  50 . Shaft  34  includes fluid ports  54 ,  56  and  58  that supply lubricating fluid  60  and assist in circulating the fluid along the working surfaces of the bearing. Lubricating fluid  60  is supplied to shaft  34  by a fluid source (not shown) that is coupled to the interior of shaft  34  in a known manner. 
     Spindle motor  32  further includes a thrust bearing  45 , which forms the axial working surfaces  48  and  50  of hydrodynamic bearing  37 . A counterplate  62  bears against working surface  48  to provide axial stability for the hydrodynamic bearing and to position hub  36  within spindle motor  32 . An O-ring  64  is provided between counterplate  62  and hub  36  to seal the hydrodynamic bearing. The seal prevents hydrodynamic fluid  60  from escaping between counterplate  62  and hub  36 . 
     Hub  36  includes a central core  65  and a disc carrier member  66 , which supports disc pack  16  (shown in FIG. 1) for rotation about shaft  34 . Disc pack  16  is held on disc carrier member  66  by disc clamp  18  (also shown in FIG.  1 ). A permanent magnet  70  is attached to the outer diameter of hub  36 , which acts as a rotor for spindle motor  32 . Core  65  is formed of a magnetic material and acts as a back-iron for magnet  70 . Rotor magnet  70  can be formed as a unitary, annular ring or can be formed of a plurality of individual magnets that are spaced about the periphery of hub  36 . Rotor magnet  70  is magnetized to form one or more magnetic poles. 
     Stator  38  is attached to base  12  and includes stator laminations  72  and stator windings  74 . Stator windings  74  are attached to laminations  72 . Stator windings  74  are spaced radially from rotor magnet  70  to allow rotor magnet  70  and hub  36  to rotate about a central axis  80 . Stator  38  is attached to base  12  through a known method such as one or more C-clamps  76  which are secured to the base through bolts  78 . 
     Commutation pulses applied to stator windings  74  generate a rotating magnetic field that communicates with rotor magnet  70  and causes hub  36  to rotate about central axis  80  on bearing  37 . The commutation pulses are timed, polarization-selected DC current pulses that are directed to sequentially selected stator windings to drive the rotor magnet and control its speed. 
     In the embodiment shown in FIG. 2, spindle motor  32  is a “below-hub” type motor in which stator  38  has an axial position that is below hub  36 . Stator  38  also has a radial position that is external to hub  36 , such that stator windings  74  are secured to an inner diameter surface  82  (FIG. 3) of laminations  72 . In an alternative embodiment, the stator is positioned within the hub, as opposed to below the hub. The stator can have a radial position that is either internal to the hub or external to the hub. In addition, while FIG. 2 depicts a spindle motor with a fixed shaft, the spindle motor may have a rotating shaft. In this case, the bearing is located between the rotating shaft and an outer stationary sleeve that is coaxial with the rotating shaft. 
     FIG. 3 is a diagrammatic sectional view of hydrodynamic spindle motor  32  taken along line  3 — 3  of FIG. 2, with portions removed for clarity. Stator  38  includes laminations  72  and stator windings  74 , which are coaxial with rotor magnet  70  and central core  65 . Stator windings  74  include phase windings W 1 , V 1 , U 1 , W 2 , V 2  and U 2  that are wound around teeth in laminations  72 . The phase windings are formed of coils that have a coil axis that is normal to and intersects central axis  80 . For example, phase winding W 1  has a coil axis  83  that is normal to central axis  80 . Radial working surfaces  44  and  46  of hydrodynamic bearing  37  are formed by the outer diameter surface of shaft  34  and the inner diameter surface of central core  65 . The shaft  34  and central core  65  may be constructed of a metal such as, for example, steel or aluminum. Radial working surfaces  44  and  46  are separated by a lubrication fluid, which maintains a clearance c during normal operation. 
     FIG. 4 depicts a close-up sectional view of the hydrodynamic spindle motor  32  of FIG.  3 . Either or both radial working surfaces  44  and  46  of hydrodynamic bearing  37  are treated with a wear resistant, low frictional coatings  44   c  and  46   c . Wear resistant coatings  44   c  and  46   c  improve the wear resistance of radial working surfaces  44  and  46  by making working surfaces  44  and  46  more physically durable. Metal particle generation due to wear is reduced, resulting in much less mechanical failure of working surfaces  44  and  46 . The wear resistant and low frictional coatings  44   c  and  46   c  provide improved wear resistance and generally provide for a clearance c that remains constant throughout the lifetime of the spindle motor. 
     The wear resistant coatings  44   c  and  46   c  may comprise, for example, amorphous carbon, diamond-like carbon, or combinations thereof. The wear resistant coating may have a thickness in the range of about 100 nanometers to about 5 microns. The preferred thicknesses of wear resistant coatings  44   c  and  46   c  are dependent upon factors such as the composition of the outer diameter of shaft  34  and inner diameter of central core  65 , the magnitude of clearance c, surface roughness and loading, among others. 
     In one embodiment, wear resistant low frictional coatings  44   c  and  46   c  are deposited by physical vapor deposition (PVD), such as by a sputtering process. In another embodiment, wear resistant coatings  44   c  and  46   c  are deposited by chemical vapor deposition (CVD), such as plasma enhanced chemical vapor deposition (PECVD). In another embodiment, wear resistant coatings  44   c  and  46   c  are deposited by ion beam deposition. The wear resistant coating may also be sputtered in the presence of, for example, hydrogen (H 2 ) or nitrogen (N 2 ) to enhance the wear resistance and frictional properties thereof. 
     While FIG. 4 depicts wear resistant coatings  44   c  and  46   c  as consisting of only one layer, it is within the scope of the invention for wear resistant coatings  44   c  and  46   c  to consist of multiple coating layers. It is often desirable for wear resistant coatings  44   c  and  46   c  to consist of multiple layers in order to provide optimal adhesion, reduce crack propagation and to improve corrosion resistance of the shaft  34  and the central core  65 . In one embodiment, wear resistant coatings  44   c  and  46   c  comprise two or more layers of carbon. In one embodiment, wear resistant coatings  44   c  and  46   c  comprise a layer of silicon carbide. 
     In one embodiment, one or more adhesive layers  44   i  and  46   i  are deposited on the outer diameter of shaft  34  and inner diameter of central core  65 , respectively, prior to depositing wear resistant coatings  44   c  and  46   c . Adhesive layers  44   i  and  46   i  provide improved adhesion and mechanical properties for the wear resistant coatings  44   c  and  46   c  to outer diameter of shaft  34  and inner diameter of central core  65 . Adhesive layers may comprise, for example, chromium, silicon, titanium, zirconium, silicon carbide, and combinations thereof. 
     In another embodiment, one or more adhesion layers  44   i  and  46   i  may be used in combination with one or more wear resistant coatings  44   c  and  46   c . For example, an adhesion layer may be used in combination with a wear resistant layer and a wear resistant, low frictional layer. 
     The thickness of adhesive layers  44   i  and  46   i  may be in the range of about 1 nanometer to about 1 micron. The preferred thickness of adhesive layers  44   i  and  46   l  is dependent upon factors similar to those enumerated above for the wear resistant coatings  34   c  and  36   c . In one embodiment, either or both outer diameter surface of shaft  34  and the inner diameter surface of central core  65  are treated with a nickel or nickel phosphide plating solution prior to depositing adhesive layers  44   i  and  46   i  or wear resistant layers  44   c  and  46   c . Electroless nickel plating solutions may also be used. 
     In one embodiment, adhesive layers  44   i  and  46   i  are deposited by physical vapor deposition (PVD), such as by a sputtering process. In another embodiment, adhesive layers  44   i  and  46   i  are deposited by chemical vapor deposition (CVD), such as plasma enhanced chemical vapor deposition (PECVD). In another embodiment, adhesive layers  44   i  and  46   i  are deposited by ion beam deposition. 
     In one embodiment, the substrate is etched prior to depositing the adhesive layer and the wear resistant coating. In the case where no adhesive layer is deposited, the substrate may be etched prior to depositing the wear resistant coating. The substrate may be etched, for example, by a plasma etching process. The plasma etching process may comprise bombarding the substrate with ions of an inert gas such as, for example, argon. 
     Alternatively or in addition to wear resistant coatings  44   c  and  46   c  deposited on the outer diameter of shaft  34  and inner diameter of central core  65 , wear resistant coatings may be deposited upon other working surfaces of the spindle motor, such as, for example, axial working surface  48  on thrust bearing  45  or on lower surface  69  of counterplate  62 , shown in FIG.  2 . Optionally, adhesive layers, such as those discussed above, may be deposited prior to the deposition of the wear resistant low frictional coatings. 
     EXAMPLE 1 
     An adhesive layer was deposited on a steel substrate. The adhesive layer comprised chromium. The adhesive layer was deposited by a sputtering process, in which an inert gas sputtered material from a chromium target. An adhesive layer having a thickness of about 0.3 microns to about 0.5 microns was deposited. 
     A wear resistant low frictional coating was deposited on the chromium adhesive layer. The wear resistant coating comprised carbon. The wear resistant coating was deposited by a sputtering process, in which an inert gas sputtered material from a carbon target. A wear resistant coating having a thickness of about 1.5 microns to about 2 microns was deposited. The wear resistant coating exhibited excellent adhesion to the substrate. 
     EXAMPLE 2 
     An adhesive layer was deposited on a steel substrate. The adhesive layer comprised silicon. The adhesive layer was deposited by a sputtering process in which an inert gas sputtered material from a silicon substrate. An adhesive layer having a thickness of about 0.3 microns to about 0.5 microns was deposited. 
     A wear resistant low friction coating was deposited on the silicon adhesive layer. The wear resistant coating comprised carbon. The wear resistant coating was deposited by a sputtering process in which an inert gas sputtered material from a carbon target. A wear resistant coating having a thickness of about 1.5 microns to about 2 microns was deposited. The wear resistant coating exhibited excellent adhesion to the substrate. 
     The use of wear resistant and adhesive layers for improved wear performance is not limited to thrust bearing designs described above. Wear resistant and adhesive coatings may be used, for example, with spindle motors having bearing surfaces of other geometries known to the art. Conical and spherical bearing surfaces may be coated with the wear resistant coating of the present invention to reduce wear on the bearing surfaces. 
     Referring to FIG. 5, a hydrodynamic bearing is shown with conical bearing surfaces, which is usable to drive the discs in the disc drive  10  of FIG.  1 . The hydrodynamic bearing is shown incorporated in a spindle motor  150 . The design includes a drive rotor or hub  114  rotatably coupled to a shaft  152 . The shaft  152  includes an upper hemisphere or convex portion  154  and a lower hemisphere or convex portion  156  received in a sleeve  158  which rotates relative to the shaft. The shaft is fixedly attached to a base  160 , which may be incorporated in or supported from the housing base  12  described with respect to FIG.  1 . The sleeve  158  receives the journal  162  of shaft  152  and has upper hemisphere shaped, concave receptacle  164  and lower hemisphere shaped concave receptacle  166 . A fill hole  168  is also provided to a reservoir  159  in (as drawn, the upper end) fixed member  152 , to provide bearing fluid to the hydrodynamic bearing. The rotor  114  includes a counterplate  170 , which is used to close off one end of the hydrodynamic bearing to the atmosphere. In operation, the bearings shown in this figure comprise hydrodynamic bearings in which fluid such as oil circulates through gaps between the fixed member, which is the shaft and the rotating member, which in this case is the sleeve. One or more of these bearing surfaces may also be coated with the wear resistant layers of the present invention. 
     While foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.