Abstract:
This invention relates to methods for plate dressing using slurry charged with abrasives and soft polymeric or metallic micro-nano spheres additives to produce substantially uniform abrasive height. Additionally, methods for plate dressing using slurry charged with abrasives and self-assembled polymers to produce substantially uniform abrasive height are disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefits of the filing date of U.S. Provisional Patent Application Ser. No. 61/320,787, which is entitled “Lapping slurry formulations containing nanospheres additives” filed Apr. 5, 2010, which is hereby incorporated herein in their entirety by reference. 
     
    
     BACKGROUND 
       [0002]    Lapping is a well-known process of abrasion metal-removal or machining for smoothing or polishing surfaces to a high degree of refinement or accuracy using loose abrasive lapping compound embedded in soft plate referred to as lapping plates. The lapping compound is often in a liquid suspension or semi-liquid form, and is called lapping slurry. 
         [0003]    Polishing quality produced on a lapping plate is a strong function of the abrasive height distribution and abrasive density. The tighter the height distribution one can achieve the smoother the finish of the final polishing surfaces. Protruding abrasives from the mean height distribution of the diamonds produce scratches in the polishing surface. An ideal state of a uniform height distribution produces atomically smooth surfaces. 
         [0004]    A typical example of magnetic slider bars with trailing edges composed of metallic layers and ceramic layers present very severe challenges during lapping. Composite structures of hard and soft layers present differential lapping rates when lapped using conventional abrasive substrates. The variable polishing rates of the metallic and ceramic materials lead to severe recessions, sensor damage, and other problems. 
         [0005]    U.S. Pat. Nos. 7,198,533 and 6,123,612 disclose an abrasive article including a plurality of abrasive particles securely affixed to a substrate with a corrosion resistant matrix material. The matrix material includes a sintered corrosion resistant powder and a brazing alloy. The brazing alloy includes an element which reacts with and forms a chemical bond with the abrasive particles, thereby securely holding the abrasive particles in place. A method of forming the abrasive article includes arranging the abrasive particles in the matrix material, and applying sufficient heat and pressure to the mixture of abrasive particles and matrix material to cause the corrosion resistant powder to sinter, the brazing alloy flows around, react with, and forms chemical bonds with the abrasive particles, and allows the brazing alloy to flow through the interstices of the sintered corrosion resistant powder and forms an inter-metallic compound therewith. 
       SUMMARY OF THE INVENTION 
       [0006]    The nanospheres can be fabricated hollow. Full nanosphere nano particles have a wide variety industrial and biomedical uses. The manufacturing of uniform and regular nanosphere is becoming a common in an industrial setting. U.S. Pat. No. 6,720,007 B2 addresses the formation of polymeric micro-nano spheres. 
         [0007]    The abrasive particles may include diamonds, Aluminum oxides, Titanium oxides, ceria, and the like. 
         [0008]    The present invention improves the embedded abrasive height distribution in lapping plates. The height distribution of diamonds improves the surface finish and reduces the number of scratches. When lapping under ideal conditions of a very uniform abrasive height distribution a super smooth surface is attained with no scratches and surface damage. 
         [0009]    The invention proposes the addition of flexible polymeric nanospheres as an additive to existing lubricant based abrasive slurries. The sphere diameter distribution and sphere density produce a cushion balancing the applied dressing wheel preload. Manufactured nanospheres are not uniform in diameter or shape distribution (Walt et al., U.S. Pat. No. 6,700,007 B2), a Gaussian distribution can be used to characterize the incoming height distribution. The nanospheres can be coated with non-polar materials to avoid agglomeration as described in Walt et al., U.S. Pat. No. 6,700,007 B2 and Walsh et al., U.S. Pat. No. 6,207,195 B1. The nanospheres can be full sphere or hollow spheres as shown in Walt et al., U.S. Pat. No. 6,700,007 B2 and Walsh et al., U.S. Pat. No. 6,207,195 B1. 
         [0010]    The embodiments described herein relate to methods for plate dressing using slurry charged with abrasives and micro-nano sphere additives to produce substantially uniform abrasive height. In particular, soft polymeric nanospherical additives produce a constant spacing between the dressing wheel and the polishing substrate under a given load. Protruding hard abrasives are forced to embed by the dressing wheel in the softer substrate until the resistance of the uniform diameter nan micro-nano sphere balances the applied wheel load. The soft polymeric micro-nano sphere deform under the dressing wheel load to provide a uniform spacing between the lapping plate and the dressing wheel. The slurry contains both abrasives and micro-nano sphere additives. Since the abrasive have a substantially higher hardness, they will embed into the soft lapping substrate while the soft polymeric micro-nano sphere additives deform under the applied load without embedded into the plate and regain their original shape once the load is removed. The micro-nano sphere can be thought of a large number of springs resisting the applied load from the dressing wheel. Upon deforming under the applied load the nanospheres reach an equilibrium state assuring a spacing equal to the mean original height of the nanospheres minus the mean deformation of the nanospheres. 
         [0011]    In accordance with one aspect of the present invention, a lapping slurry which includes soft polymer nanospheres dispersed in a carrier fluid charged with abrasive particles. 
         [0012]    In accordance with a second aspect of the present invention, a lapping slurry includes abrasive particles and soft polymeric nanospheres dispersed in a carrier fluid. Carrier fluids are often formed from oil, water, glycerine, triethanolamine according to U.S. 2007/0135317 A1. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  depicts a diamond charged lapping plate with non uniform protruding diamonds. 
           [0014]      FIG. 2  shows the diamond height distribution of the present invention compared to prior art methods. 
           [0015]      FIG. 3  shows the dispersion of a slurry charged with abrasives and soft nanospheres on a lapping plate according to an example embodiment of the present invention. 
           [0016]      FIG. 4  shows a close up view of the charging process with a dressing wheel at a given applied load of a slurry charged with abrasives and soft nanospheres on a lapping plate according to an example embodiment of the present invention. 
           [0017]      FIG. 5  shows a full view of the charging process with a dressing wheel at a given applied load of a slurry charged with abrasives and soft nanospheres on a lapping plate according to an example embodiment of the present invention. 
           [0018]      FIG. 6  shows a charged plate where the residual nanospheres have been washed away according to an example embodiment of the present invention. 
           [0019]      FIG. 7  shows a lapping plate coated with a uniform thickness film; abrasive slurry is dispersed onto the uniform film thickness according to an example embodiment of the present invention. 
           [0020]      FIG. 8  shows a full view of the charging process with a dressing wheel at a given applied load of plate where the applied coating act as a stop layer for the diamond penetration according to an example embodiment of the present invention. 
           [0021]      FIG. 9  shows a charged plate where the residual coating has been washed away according to an example embodiment of the present invention. 
           [0022]      FIG. 10  shows an abrasive article according to an example embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]      FIG. 1  depicts a prior schematic representation of a charged lapping plate  30  with diamonds  32  on a soft substrate  34 . The non uniform height distribution of the diamonds cause a relatively rough finish with scratches experienced by the lapped surface. The high protruding diamonds cause tensile stresses at the surface of ceramic materials. Tensile stresses further cause particle release which is undesirable in many applications. 
         [0024]      FIG. 2  reflects prior art abrasive height distribution  22  and  20  versus distributions  24 ,  26 , and  28  obtained with the present invention. In prior art applications; large variations are obtained in diamond height distributions as depicted by  22  and  20 . Also mean shift from batch to batch charging operations cause mean variations as depicted by  20  and  22 . The present invention can precisely dial in a diamond height such as the examples shown in  24 ,  26 , and  28  by precisely controlling the nanospheres diameters into the slurry. For example adding nanospheres with a diameter slightly larger than 150 nm can lead to a mean abrasive height desired of 150 nm with a tight distribution. 
         [0025]      FIG. 3  depicts slurry of abrasive particles  14  and soft nanospheres  12  dispersed in a lubricant (not shown) on a substrate  16 . The dressing wheel  10  is shown for illustrative purposes with no load applied. 
         [0026]      FIGS. 4 and 5  depict slurry of abrasive particles  14  and soft nanospheres  12  dispersed in a lubricant (not shown) on a substrate  16 . The dressing wheel  10  applies a preload on the abrasive slurry charged with soft polymeric spheres or nanospheres  13 . The soft polymeric nanospheres deform under the dressing wheel preload causing them to take elliptical shapes  13 . The final protruding height  18  of the abrasive particles  14  embedded into the soft lapping plate  16  match the spacing formed between the dressing wheel  10  and the lapping plate  16 . The nanospheres can be thought of a large number of springs supporting the applied load from the dressing wheel. Upon deforming under the applied load the nanospheres reach an equilibrium state assuring that the spacing between the dressing wheel and the lapping plate equals the final deformed state of the soft polymeric nanospheres. 
         [0027]    In one example embodiment, the nanosheres are removed, such as by removal of the lubricant and nanoshperes from the substrate  16  after removing the applied load of the dressing wheel  10  (shown in  FIG. 5 ).  FIG. 6  depicts a charged lapping plate according to an example embodiment of the present invention. The final abrasive mean height  18  matches the spacing achieved between the dressing wheel  10  (shown in  FIG. 5 ) and the lapping plate or substrate  16 . The substrate  16  formed with the abrasive particles is the lapping plate. It should be noted that although a lubricant is the carrier fluid in this example embodiment, other embodiments can use different carrier fluids. 
         [0028]      FIG. 7  depicts a lapping plate  26  coated with a uniform thickness film  23  according to another example embodiment of the present invention. The thickness of the coating  23  matches the desired protruding height of the embedded abrasives  28 . Abrasive particle slurry  24  is uniformly dispersed above the coating. 
         [0029]      FIG. 8  applies a preload  29  onto a dressing wheel  20  to embed the abrasive particles  24  penetrating the soft coating  23  and the soft lapping plate  26 . The soft coating  23  deforms under the applied load  29  to provide resistance. The resistance to the displacement of the dressing wheel can be monitored to provide feedback on the penetration of the abrasive particles into the lapping plate. The coating acts as a uniformly distributed spring system supporting or opposing the applied load  29  from the dressing wheel  20 . Upon deforming the coating under the applied load  29  the coating  23  deforms and reaches an equilibrium state assuring that the spacing between the dressing wheel  20  and the lapping plate  26  equals the final deformed state of the soft coating  23 . This self limitation process causes the abrasive particles  24  to reach a height substantially uniform equally the initial thickness of the coating  23  minus the coating deformation under the load  29  applied by the dressing wheel  20 . Since the abrasive particles  24  are embedded into a soft metal layer such as Tin or Tin Bismuth, the deformation between the abrasive particles  24  and the metal layer is fully plastic presenting zero contact stiffness during an unloading operation. The contact resistance is substantially dominated by the applied coating  23  or the micro-nano spheres. 
         [0030]    Soft coatings  23  include self assembled polymers providing a substantially conformal layer throughout the lapping plate. A very thin film of gold is applied to the polishing plate to enhance the adhesion and growth of the self assembled polymer. The self assembled layer can be grown to various precise thickness  28  which is very desirable. Self assembled polymers have good tribological properties of resistance wear and erosion during the abrasive particles embedding process. The abrasive particles easily penetrate the self assembled coating. The self assembled layer can be thought of as an infinite number of springs resisting the dressing wheel applied load to provide a substantially uniform diamond protrusion height. 
         [0031]    During the application of the load  29  to embed the abrasive particles, the load will remain constant while the charging wheel  20  experience a displacement equaling the amount of abrasive particles  14 ,  24  penetration in the soft lapping plate  16 ,  26 . Once the charging wheel contacts the micro-nano spheres or the coating, the reaction due to the deformation of the micro-nano spheres  13  or the coating resists  23  the charging wheel  10 ,  20  displacement to reach equilibrium between the applied load and the micro-nano spheres or the coating deformations. The contact stiffness attained is directly proportional to the mean height of the protruding abrasive particles. The amount of deformation of the micro-nano spheres or the coating equals the height of protruded abrasive particles. 
         [0032]    Multiple layers of soft coatings can be used in combination. For example, a non light sensitive coating can be deposited first, followed by a light sensitive polymer. Desired patterns can be formed and developed onto the light sensitive material. An abrasive slurry is then dispersed. 
         [0033]      FIG. 10  shows an abrasive article  33  formed with a series of abrasive composite particles  31 . Abrasives  32  and micro-nano spheres  34  are dispersed in a binder  35  to form a regularly shaped abrasive composite  31 . The abrasive composites  31  are fabricated onto a backing material  30 . The binder  35  dissolves in the presence of a lubricant supplied during the polishing of a workpiece. Upon dissolution of the binder, the micro-nano spheres and the abrasives are supplied to the slurry formed by the lubricant, the micro-nano spheres and the abrasives. The micro-nano spheres act as spacers between the workpiece and the abrasive elements. It is desirable that the average diameter of the micro-nano spheres  34  is in the same range of height as the abrasive particles  32 . 
         [0034]    Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which these inventions belong. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present inventions, the preferred methods and materials are now described. All patents and publications mentioned herein, including those cited in the Background of the application, are hereby incorporated by reference to disclose and described the methods and/or materials in connection with which the publications are cited. 
         [0035]    The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present inventions are not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed. 
         [0036]    Other embodiments of the invention are possible. Although the description above contains much specificity, these should not be construed as limiting the scope of the invention, but as merely providing illustrations of some of the example embodiments of this invention. It is also contemplated that various combinations or sub-combinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the inventions. It should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the disclosed inventions. Thus, it is intended that the scope of at least some of the present inventions herein disclosed should not be limited by the particular disclosed embodiments described above. 
         [0037]    Thus the scope of this invention should be determined by the appended claims and their legal equivalents. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims.