Abstract:
Lubricant with non-terminal functional groups. The location of the functional groups minimizes free chain length and molecular weight of a lubricant while simultaneously maximizing evaporation temperature. The locations of the functional groups to the backbone of the molecule of the lubricant allows for shorter lengths of free backbone length between functional groups attached to a lubricated surface. The lubricant lubricates mechanical devices including hard disk drives. The lubricant can also be added to media or other mechanical parts in conjunction with other lubricants.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a divisional of a co-pending U.S. patent application Ser. No. 12/004,728, filed Dec. 21, 2007, which is a continuation-in-part of U.S. patent application Ser. No. 11/224,920, filed Sep. 12, 2005, now U.S. Pat. No. 7,683,012. The aforementioned patent and applications are herein incorporated by reference in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to lubricants including lubricants used to coat disks of hard disk drives. 
       BACKGROUND 
       [0003]    Disk lubricants use linear chain molecules made of a perfluoropolyether (PFPE) backbone, with a terminal functional group attached at both ends of the backbone. These lubricants are typically known as boundary lubricants. Boundary lubricants form a lubricating film when the functional groups of the lubricant attach to a surface being lubricated. Boundary lubricants, among other advantages, prevent solid to solid contact. If the backbone in a boundary lubricant molecule is long, the molecule tends to be heavier. A longer, heavier molecule provides the advantage of less evaporation while simultaneously providing the disadvantage of increasing free backbone length. On the other hand, if the molecule&#39;s backbone is short, the molecule tends to be lighter. A shorter, lighter molecule provides the advantage of lowering free backbone length while simultaneously providing the disadvantage of allowing greater evaporability. Lubricants, such as boundary lubricants, are used in many types of mechanical devices including disk drives and micro electronic mechanical systems. These devices typically include a moving part which is lubricated to prevent wear. The moving part moves relative to other parts of the device. 
         [0004]    One problem that potentially arises with long molecular chains tethered on both ends by terminal functional groups attached to a substrate is that they still have multiple degrees of freedom that allow the middle of the chain to lift up from the disk surface (free backbone length), creating potential head/disk clearance issues. As a result, higher molecular weight lubricants, which would be preferred in a pure evaporation (Marchon, Karis, Dai, and Pit, IEEE Trans. Magn., vol. 39(5), pp. 2447, 2003) and dewetting (Waltman, Khurshudov and Tyndall, Tribology Letters, vol. 12, pp. 163, 2002) are detrimental to the overall head disk spacing margin (Khurshudov and Waltman, Tribology Letters, vol. 11, pp. 143, 2001). Hence, lowering molecular weight (MW) to get better clearance margin is only feasible to a certain point, as evaporation from the disk surface becomes increasingly problematic. In addition to these issues, decreased flying height tends to induce more severe slider/lubricant interactions (moguls, ripples, depletion), and as a result, there is a general trend towards higher lubricant-disk surface interactions as measured by lubricant bonding. Finally, in terms of total head/disk spacing, the lubricant thickness which used to be a negligible part of the budget can no longer be ignored. There is need for a robust lubricant system with a thickness in the 0.5-1.0 nm range. Prior to the invention, lubricant thicknesses were in 1.0-1.3 nm range. 
         [0005]    Further, the article,  Tribology Challenges for Head - Disk Interface Toward  1  Tb/in 2 by Jing Gui (IEEE Transactions on Magnetics, Vol. 39, No. 2, March 2003), describes a Zdol as well as a Z lubricant. These lubricants are configured on a substrate in either a loop or train configuration respectively. However, these lubricants effectiveness are limited by use of terminal functional groups located at the ends of the lubricant backbone. 
       SUMMARY 
       [0006]    The invention is a new disk lubricant that solves the trade off problem between free backbone length and evaporation of the prior art lubricants. The lubricant of the invention is both high molecular weight and low in free backbone length.  FIG. 1(   a ) shows two short chain lubricant molecules of the prior art with a backbone  101  and functional groups  102  attached to a surface  103 . The functional groups  102  are terminal functional groups as they are at the end of the lubricant molecule. The short chain lubricant has good clearance properties since its free backbone length is low. However, the short chain lubricant also easily evaporates.  FIG. 1(   b ) shows a long chain lubricant of the prior art. Due to its molecular weight, the long chain lubricant does not easily evaporate. However, the long chain lubricant ‘˜free backbone length demands more clearance for a slider to perform’ properly.  FIG. 1(   c ) shows an exemplary lubricant molecule of the present invention. The lubricant includes a long chain molecule and therefore does not evaporate easily. Additionally, since it includes multiple functional groups, including non-terminal functional group  104 , the spacing between functional groups in the molecule may be reduced to allow for good slider clearance and less free backbone length. Additionally, the multiple functional groups in the lubricant of the present invention increase the adhesion to a disk surface. The points of surface anchoring by the functional groups are shown, for example, by functional groups  102  attaching to the surface of the disk  103  in  FIG. 1(   a ). The lubricant may also be used for other mechanical devices with movable parts, not only hard disk drives. 
         [0007]    The lubricant can also be applied to media in conjunction with another lubricant. Examples of the other lubricants include A20H and X1P. 
         [0008]    In one embodiment, a media disk includes a substrate; a magnetic recording layer and a lubricant comprising a first lubricant molecule and a second lubricant molecule. The first lubricant molecule includes at least one backbone and at least one functional group attached to the backbone, where the first functional group is a non-terminal functional group. Additionally, the first lubricant molecule is represented by: 
         [0000]      B—Z-A-CF 2 CF 2 CF 2 -A-Z—B.
 
         [0000]    Z is represented by: 
         [0000]      —CF 2 O—[(CF 2 CF 2 O) m —(CF 2 O) n ]—CF 2 —
 
         [0000]    where m and n are integers greater than zero. Each of A and B have hydrocarbon chains, where A comprises one or more non-terminal functional groups and B comprises a terminal functional group. In particular, A is represented by: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    where R are non-terminal functional groups; and B is represented by: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein T are terminal functional groups. 
         [0009]    In another embodiment, a hard disk drive includes a substrate, a magnetic recording layer above the substrate, and a lubricant comprising lubricant molecules above the magnetic recording layer, 
         [0000]    where at least 3% of the lubricant molecules include at least three functional groups attached to a backbone and at least one of the three functional groups is a non-terminal functional group. The lubricant molecules are represented by: 
         [0000]      B—Z-A-CF 2 CF 2 CF 2 -A-Z—B.
 
         [0000]    Z is represented by: 
         [0000]      —CF 2 O—[(CF 2 CF 2 O) m —(CF 2 O) n ]—CF 2 —
 
         [0000]    where m and n are integers greater than zero. Each of A and B have hydrocarbon chains, where A comprises one or more non-terminal functional groups and B comprises a terminal functional group. In particular, A is represented by: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    where R are non-terminal functional groups; and B is represented by: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein T are terminal functional groups. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1(   a ) is two short chains, low molecular weight (MW) lubricant molecules with good clearance properties and high evaporation. 
           [0011]      FIG. 1(   b ) is a high MW lubricant molecule, with low evaporation and low clearance. 
           [0012]      FIG. 1(   c ) is a lubricant molecule with low evaporation and good clearance properties. 
           [0013]      FIG. 2  is a diagram of steps to make an embodiment of a lubricant with low evaporation and good clearance properties. 
           [0014]      FIG. 3(   a ) is a diagram showing the molecular structure of Z-tetraol Multidentate (ZTMD). 
           [0015]      FIG. 3(   b ) is a diagram showing the molecular structure of Z-Tetraol. 
           [0016]      FIG. 4(   a ) is a graph of the thickness of ZTMD and Z-Tetraol 1000 on a disk after heating. 
           [0017]      FIG. 4(   b ) is a graph of the thickness of ZTMD and Z-Tetraol 2000 on a disk after heating. 
           [0018]      FIG. 5  is a chart showing sub-ambient flyability results of ZTMD, compared with Z-Tetraol 2000. 
           [0019]      FIG. 6  is a picture of a slider after flying on a disk with a lubricant of the invention. 
           [0020]      FIG. 7  is an image of a disk at 26-30 mm after a slider flying on a disk with a lubricant of the invention. 
           [0021]      FIG. 8(   a ) is a hard disk drive including lubricant 
           [0022]      FIG. 8(   b ) is an illustrative disk in a hard disk drive. 
           [0023]      FIGS. 9(   a )-( f ) are diagrams of lubricants of the present invention. 
           [0024]      FIGS. 10(   a )-( d ) are exemplary molecules. 
           [0025]      FIG. 11  is a diagram of A20H. 
           [0026]      FIG. 12  is a graph of subambient flyability of a head over various lubricants and lubricant combinations. 
           [0027]      FIG. 13  is a graph of scratch probabilities of various lubricants and lubricant combinations. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    The following description is of the best embodiments presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claimed herein. 
         [0029]    Lubricants are typically comprised of a main chain (backbone) with two attached terminal functional groups. The backbone of a lubricant is the portion of the lubricant which does not typically bind to a substrate. An example of a backbone is perfluoropolyether (PFPE). For purposes of the invention, a lubricant structure may also include two or more backbones attached at an anchor point or a plurality of anchor points. The terminal functional group attached at each end of the lubricant molecule attaches the lubricant to the surface it is lubricating. A functional group for the backbone of a lubricant provides strong interactions with a lubricated surface. Examples of functional groups are OH, piperonyl and carboxylic acid. Functional groups may be attached at the non-end portion of the backbone (non-terminal functional group) or end portions of the backbone (terminal functional group) of a lubricant molecule. A functional group is a terminal functional group if:
       1) it is attached to an end of a backbone; or   2) all carbon, oxygen, nitrogen or other linker elements of the backbone attach a functional group between it and one end of the lubricant molecule.
 
The functional groups attach to a surface of a disk by polar interactions. Functional groups generally include the properties of adhering to a surface.
       
 
         [0032]      FIG. 10(   a ) is a molecule with a single backbone and four OH functional groups. Two of the functional groups are terminal while the other two are non-terminal.  FIG. 10(   b ) is a molecule with two backbones and five functional groups. Two of the functional groups are terminal while the other three are non-terminal. 
         [0033]      FIG. 10(   c ) is a molecule with a backbone and three functional groups. Functional group  1001  is a terminal functional group since it is at the end of a backbone. Functional group  1002  is also a terminal functional group because it is attached to a carbon which is attached to a carbon that is attached to a terminal functional group  1001 . Functional group  1003  is a non-terminal functional group as the carbons  1006  that the carbon attached to functional group  1003  is attached to are not on the ends and do not attach a terminal functional group. 
         [0034]      FIG. 10(   d ) is a molecule with a backbone and three functional groups. Functional group  1001  is a terminal functional group since it is at the end of a backbone. Functional group  1004  is a non-terminal functional group because it is not on an end and the carbon it is attached to is attached to carbons (such as carbon  1005 ) which do not attach a terminal functional group. 
         [0035]    Free backbone length or free chain length is the length of the backbone between two adjacent functional groups. When free backbone lengths are shorter, the height the backbone reaches above a surface is generally less. 
         [0036]      FIG. 2  shows the steps to create an embodiment of this invention. Two lubricants, lubricant A  201  and lubricant B  202 , are put in solution with linker  203 . The lubricants may include terminal functional groups, such as OH, at each end of a molecule of the lubricant. The linker  203  may include a reactive group at each end (denoted by X in  FIG. 2 ). When put into solution, the linker  203  binds to two lubricant molecules  201 ,  202  to form lubricant molecule  204 . The lubricant  204  includes two non-terminal OH functional groups  205 . The lubricants A  201  and B  202  may both be Z-Tetraol 1000.  FIG. 3(   b ) is a diagram of the molecular structure of Z-Tetraol 1000. The linker reactant in a reaction with Z-Tetraol preferably attaches itself between two short chain Z-Tetraol 1000 lubricant molecules to form Z tetraol Multidentate (ZTMD). ZTMD includes a total of four hydroxyl non-terminal functional groups. These four non-terminal functional groups add to the two hydroxyl terminal functional groups already located on each end of the two Z-Tetraol 1000 chains incorporated into the ZTMD molecule. Therefore, the ZTMD molecule includes a total of eight functional groups (four non-terminal). The molecular weight of ZTMD is slightly greater than 2000 (two 1000 amu chain, plus the linker).  FIG. 3(   a ) is a molecular diagram of ZTMD. Of course the reaction between the linker and the lubricant may continue creating lubricants with more than two of the original lubricant components tethered together in a variety of configurations. 
         [0037]    If Z-Tetraol 1900 were used in the reaction described above, then the resulting lubricant molecule would have a molecular weight of less than 4000. The linker plus two Z-Tetraol molecules together have a molecular weight of less than 4000. One method of tailoring the weight of the resulting lubricant molecule is to change the weight of the original lubricant molecule used in the reaction that combines two lubricant molecules with a linker molecule. 
         [0038]    Other embodiments of the invention include lubricants shown in  FIGS. 9(   a )- 9 ( f ).  FIG. 9(   a ) shows an embodiment of a lubricant molecule of the invention with two functional groups. One of the functional groups is a non-terminal functional group while the other is a terminal functional group. 
         [0039]      FIG. 9(   b ) shows an embodiment of the invention with two functional groups. Both of these functional groups are non-terminal functional groups. 
         [0040]      FIG. 9(   c ) shows an embodiment of the invention with one functional group. The one functional group is a non-terminal functional group. 
         [0041]      FIG. 9(   d ) shows an embodiment of the invention ‘with three functional groups. All three of the functional groups are non-terminal functional groups. 
         [0042]      FIG. 9(   e ) shows an additional embodiment of the invention with three functional groups. Two of the three functional groups are non-terminal functional groups.’ The third functional group is a terminal functional group. 
         [0043]      FIG. 9(   f ) shows an embodiment of the invention with four functional groups. Three of the functional groups are terminal functional groups. The last functional group is a non-terminal functional group. Also, this lubricant includes backbone  901  and an attached a second backbone  902 . 
         [0044]    The embodiments of lubricants of  FIGS. 9(   a )- 9 ( f ) provide the advantages of a higher weight and less free backbone length than lubricants with only terminal functional groups. 
         [0045]      FIG. 4(   a ) compares the fractional thickness decrease of both ZTMD and Z-Tetraol 1000 on a heated disk. Identical disks were coated with one of ZTMD and Z-Tetraol 1000. The measurements of fractional decrease were taken by infrared spectroscopy. The data shows that after five hours of heating at 170° C. the disk coated with ZTMD retained over 90% of its original lubricant. On the other hand, the disk coated with the prior art Z-Tetraol 1000 retained under 60% of its original lubricant. This demonstrates the high level of bonding and low levels of evaporation supplied by ZTMD. 
         [0046]      FIG. 4(   b ) on the other hand compares fractional thickness decrease of both ZTMD and Z-Tetraol 2000 on a heated disk. Identical disks were coated with one of ZTMD and Z-Tetraol 2000. The measurements of fractional decrease were taken by infrared spectroscopy. The data shows that after five hours of heating at 170° C. both disks coated with ZTMD retained over 90% of their original lubricant. Thus, ZTMD maintains good evaporation properties while simultaneously keeping free chain lengths short. 
         [0047]      FIG. 5  shows sub-ambient flyability results of ZTMD 7 Å thickness, compared with Z-Tetraol 2000. At 7 Å, ZTMD clearly outperforms Z-Tetraol 2000, demonstrating a very robust interface. Each of bars  501  represent the results of a flyability test on a disk lubricated with Z-Tetraol 2000. Each of bars  502  represent the results of a flyability test on a disk lubricated with ZTMD. The flyability test included flying a head over the surface of a disk, at 0.25 atmospheres of pressure and 4200 rpm, lubricated with either of the lubricants. The results shown are the time to failure of the head. The results  501  showed that the four Z-Tetraol 2000 tests averaged about 4-5 hours before failure. On the other hand, results  502  showed that three of five tests with the ZTMD lubricant ended without failure in the test time frame of 24 hours. 
         [0048]      FIG. 6  is a picture of a slider after being used in conjunction with a disk lubricated with ZTMD. The slider does not include any lube pickup or smears. Thus, the lubricant is able to work in conjunction with a hard disk drive system.  FIG. 7  is a picture of a track on a used disk lubricated with ZTMD at a resolution of 26-30 mm. After use, the disk does not exhibit lube moguls or lube depletions. 
         [0049]    ZTMO may also be combined with other lubricants in a mixture to further enhance protection of a disk or other objects in a mechanical device.  FIG. 11  is a diagram of a molecule of A20H lubricant. A20H lubricant as well as X1P are two lubricants that can be combined individually or together with ZTMD. 
         [0050]      FIG. 12  is a graph of subambient flyability of a head over media with various lubricants and lubricant combinations applied to the media. The test was conducted at a pressure of a quarter of an atmosphere (0.25 atm) on identical media. Without lubricant, the disk was scratched almost immediately. An improvement to 28 minutes of flyability was seen when adding 9 Å of ZTMD lubricant. When nearly doubling the ZTMD to 16 Å, the flyability time was 97 minutes. However, adding 4 Å of A20H instead of nearly doubling the amount of ZTMD increased the flyability to 526 minutes. Further, adding X1P instead of A20H would yield similar results. 
         [0051]      FIG. 13  is a graph of scratch probabilities of a media when various lubricants and lubricant combinations applied to a media. As shown for both of media A and media B in  FIG. 13 , the scratch resistance of the media was higher when a mixture of ZTMD and A20H rather than an identical thickness of ZTMD alone was applied to the media. Further, adding X1P instead of A20H would yield similar results. 
         [0052]    The ZTMD and the other lubricant may be applied separately to the media or they may be mixed together and then applied to the media in various amounts such that the proportion of ZTMD in the lubricant is between 0-100%. It is preferred to have between 30% and 99% ZTMD in the lubricant applied to the media. It is more preferred to have between 70% and 97% ZTMD in the lubricant applied to the media. It is even more preferred to have 80% and 95% ZTMD applied to the media. The above percentages are based on the mol ratio of the lubricants. 
         [0053]    Referring now to  FIG. 8(   a ), there is shown a disk drive  800  embodying this invention. As shown in  FIG. 8(   a ), at least one rotatable magnetic disk  812  is supported on a spindle  814  and rotated by a disk drive motor  818 . The magnetic recording on each disk is in the form of annular patterns of concentric data tracks (not shown) on the magnetic disk  812 . 
         [0054]    At least one slider  813  is positioned near the magnetic disk  812 , each slider  813  supporting one or more magnetic head assemblies  821 . As the magnetic disk rotates, slider  813  moves radially in and out over the disk surface  822  so that the magnetic head assembly  821  may access different tracks of the magnetic disk where desired data are written. Each slider  813  is attached to an actuator arm  819  by way of a suspension  815 . The suspension  815  provides a slight spring force which biases slider  813  against the disk surface  822 . Each actuator arm  819  is attached to an actuator means  827 . The actuator means  827  as shown in  FIG. 1  may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the motor current signals supplied by controller  829 . 
         [0055]    During operation of the disk storage system, the rotation of the magnetic disk  812  generates an air bearing between the slider  813  and the disk surface  822  which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension  815  and supports slider  813  off and slightly above the disk surface by a small, substantially constant spacing during normal operation. 
         [0056]    The various components of the disk storage system are controlled in operation by control signals generated by control unit  829 , such as access control signals and internal clock signals. Typically, the control unit  829  comprises logic control circuits, storage means and a microprocessor. The control unit  829  generates control signals to control various system operations such as drive motor control signals on line  823  and head position and seek control signals on line  828 . The control signals on line  828  provide the desired current profiles to optimally move and position slider  813  to the desired data track on disk  812 . Write and read signals are communicated to and from write and read heads  821  by way of recording channel  825 . 
         [0057]    Disk  812  is further shown in  FIG. 8(   b ). The disk includes three portions. On the bottom is a substrate  844 . Above the substrate  844  are overcoats, magnetic recording layers, under layers and seed layers  842 . Above the overcoats, magnetic recording layers, under layers and seed layers  842  is lubricant  840 . The lubricant of the present invention may be implemented as layer the lubricant layer  840  for a hard disk drive system. 
         [0058]    While various embodiments have been described above; it should be understood that they have been’ presented by way of example only, and not limitation. Other embodiments falling within the scope of the invention may also become apparent to those skilled in the art. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.