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BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to drill bits for drilling into a subterranean formation, and more specifically to a thin, textured wear-resistant coating on seals used within the drill bit. 
         [0003]    2. Description of Related Art 
         [0004]    The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention that made the success of rotary drilling possible. Only soft earthen formations could be penetrated commercially with the earlier drag bit, but the two-cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 930,759, drilled the hard cap rock at the Spindletop Field, near Beaumont, Tex. with relative ease. That venerable invention, within the first decade of this century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. If the original Hughes bit drilled for hours, the modern bit drills for days. Modern bits sometimes drill for thousands of feet instead of merely a few feet. Many advances have contributed to the impressive improvement of earth-boring bits of the rolling cutter variety. 
         [0005]    In drilling boreholes in earthen formations by the rotary method, earth-boring bits typically employ at least one rolling cone cutter, rotatably mounted thereon. The bit is secured to the lower end of a drillstring that is rotated from the surface or by downhole motors. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drillstring is rotated, thereby engaging and disintegrating the formation material. The rolling cutters are provided with teeth that are forced to penetrate and gouge the bottom of the borehole by weight from the drillstring. 
         [0006]    As the cutters roll and slide along the bottom of the borehole, the cutters, and the shafts on which they are rotatably mounted, are subjected to large static loads from the weight on the bit, and large transient or shock loads encountered as the cutters roll and slide along the uneven surface of the bottom of the borehole. Thus, most earth-boring bits are provided with precision-formed journal bearings and bearing surfaces, as well as sealed lubrication systems to increase drilling life of bits. The lubrication systems typically are sealed to avoid lubricant loss and to prevent contamination of the bearings by foreign matter such as abrasive particles encountered in the borehole. A pressure compensator system minimizes pressure differential across the seal so that lubricant pressure is equal to or slightly greater than the hydrostatic pressure in the annular space between the bit and the sidewall of the borehole. 
         [0007]    Early Hughes bits had no seals or rudimentary seals with relatively short life, and, if lubricated at all, necessitated large quantities of lubricant and large lubricant reservoirs. Typically, upon exhaustion of the lubricant, journal bearing and bit failure soon followed. An advance in seal technology occurred with the “Belleville” seal, as disclosed in U.S. Pat. No. 3,075,781, to Atkinson et al. The Belleville seal minimized lubricant leakage and permitted smaller lubricant reservoirs to obtain acceptable bit life. 
         [0008]    An adequately sealed journal-bearing bit should have greater strength and load-bearing capacity than an anti-friction bearing bit. The seal disclosed by Atkinson would not seal lubricant inside a journal-bearing bit for greater than about 50-60 hours of drilling, on average. This was partially due to rapid movement of the cutter on its bearing shaft (cutter wobble), necessitated by bearing and assembly tolerances, which causes dynamic pressure surges in the lubricant, forcing lubricant past the seal, resulting in premature lubricant loss and bit failure. 
         [0009]    The O-ring, journal bearing combination disclosed in U.S. Pat. No. 3,397,928, to Galle unlocked the potential of the journal-bearing bit. Galle&#39;s O-ring-sealed, journal-bearing bit could drill one hundred hours or more in the hard, slow drilling of West Texas. The success of Galle&#39;s design was in part attributable to the ability of the O-ring design to help minimize the aforementioned dynamic pressure surges. 
         [0010]    A major advance in earth-boring bit seal technology occurred with the introduction of a successful rigid face seal. The rigid face seals used in earth-boring bits are improvements upon a seal design known as the “Duo-Cone” seal, developed by Caterpillar Tractor Co. of Peoria, Ill. Rigid face seals are known in several configurations, but typically comprise at least one rigid ring, having a precision seal face ground or lapped and polished thereon, confined in a groove near the base of the shaft on which the cutter is rotated, and an energizer member, which urges the seal face of the rigid ring into sealing engagement with a second seal face. Thus, the seal faces mate and rotate relative to each other to provide a sealing interface between the rolling cutter and the shaft on which it is mounted. 
         [0011]    The combination of the energizer member and rigid ring permits the seal assembly to move slightly to minimize pressure fluctuations in the lubricant, and to prevent extrusion of the energizer past the cutter and bearing shaft, which can result in sudden and almost total lubricant loss. U.S. Pat. No. 4,516,641, to Burr; U.S. Pat. No. 4,666,001, to Burr; U.S. Pat. No. 4,753,304, to Kelly; and U.S. Pat. No. 4,923,020 to Kelly, are examples of rigid face seals for use in earth-boring bits. Rigid face seals substantially improve the drilling life of earth-boring bits of the rolling cutter variety. Earth-boring bits with rigid face seals run with lower sliding friction relative to o-ring seals and are typically used in high speed and/or more challenge drilling applications, such as abrasive formations and high temperature wells, thus operate efficiently longer than prior-art bits. 
         [0012]    Because the seal faces of rigid face seals are in constant contact and slide relative to each other, the dominant mode of failure of the seals is wear. Eventually, the seal faces become galled due to adhesive wear and the coefficient of friction between the seal faces increases, leading to increased operating temperatures, reduction in seal efficiency, and eventual seal failure, which ultimately result in bit failure. In an effort to minimize seal wear, seal rings of prior-art rigid face seals are constructed of tool steels such as 440C stainless, or hardened alloys such as Stellite. Use of these materials in rigid face seals lengthens the drilling life of the bit, but leaves room for improvement of the drilling longevity of rigid face seals, and thus earth-boring bits. 
         [0013]    Hard coatings on the face of the seal can increase the life of the seal. The hard coatings, which often contain natural or synthetic diamonds or other alloys, can be very expensive. Some seals have employed a textured surface which can reduce friction and surface temperatures associated with the seal. Methods of creating a texture on a hard coating require a relatively thick, and thus expensive, hard coating. Furthermore, the relatively thick hard coating requires the underlying rigid face seal to be somewhat less thick than an un-coated seal. A thick coating can also change the stiffness of the seal which may not be desirable. A need exists, therefore, for a rigid face seal with a hard coating and a texture, wherein the hard coating is very thin. 
         [0014]    A need exists, therefore, for a rigid face seal for use in earth-boring bits having improved wear-resistance and reduced coefficients of sliding friction between the seal faces. 
       SUMMARY OF THE INVENTION 
       [0015]    In an exemplary embodiment of the present invention, a thin-film coating with a textured surface is formed on a seal face of a rigid sealing ring for use in an earth boring drill bit. In some embodiments, a thin-film coating with a textured surface is formed on two or more sealing rings for use in an earth boring drill bit. 
         [0016]    The textured surface may be first formed on the rigid sealing ring itself, by any technique such as mechanical techniques, chemical etching, or laser machining The textured surface may comprise pores having a diameter of, for example, 100 microns. Furthermore, the pores may have a depth of, for example, 5-7 microns. 
         [0017]    A thin-film coating may be applied on the surface of the rigid sealing ring or rings. The thin-film coating may be a hard coating, such as diamond-like carbon or AIMgB 14 . The thin-film coating may be applied by a variety of techniques, such as plasma-assist physical vapor deposition, chemical vapor deposition, or pulsed laser deposition. The thin-film coating may be very thin, with a thickness of, for example, 1-5 microns. The pore density may be 20-30 percent. Due to the thin nature of the coating, the texture on the seal face is present through the coating, thus giving the coating a textured surface. 
         [0018]    In an alternative embodiment, the thin-film coating may be applied to a smooth or a textured surface on the rigid sealing ring. A texture may be applied to the thin-film coating by, for example, using a laser to create a texture such as pores in the coating. In some embodiments, the pores may be 100 microns in diameter, 5-7 microns deep, and have a pore density of 20-30 percent. In some embodiments, the depth of the pores is greater than the thickness of the coating, thus exposing the rigid seal face through the coating. 
         [0019]    The texture promotes hydrodynamic pressure, lowers face torque and temperature, and traps wear debris and the thin hard coating protects the texture from being worn out from the asperity contact. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a fragmentary section view of a section of an earth-boring drill bit in an exemplary embodiment of the present invention. 
           [0021]      FIG. 2  is an enlarged, fragmentary section view of a seal assembly for use with the earth boring bit of  FIG. 1  according to an exemplary embodiment of the present invention. 
           [0022]      FIG. 3  is an enlarged, fragmentary section view of an alternative seal assembly contemplated for use with an exemplary embodiment of the present invention. 
           [0023]      FIG. 4  is a partial top view of a rigid ring of the earth-boring bit of  FIG. 1 . 
           [0024]      FIG. 5  is a perspective view of an exemplary embodiment of the application of texture to a rigid ring of the earth-boring bit of  FIG. 1 . 
           [0025]      FIG. 6  is a partial sectional view of a rigid ring of the earth-boring bit of  FIG. 1 . 
           [0026]      FIG. 7  is a diagrammatic view of the plasma-assisted chemical vapor deposition coating of a rigid ring of the earth-boring bit of  FIG. 1 . 
           [0027]      FIG. 8  is a partial sectional view of an alternative embodiment of the rigid ring of the earth-boring bit of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Although the following detailed description contains many specific details for purposes of illustration, one of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope and spirit of the invention. Accordingly, the exemplary embodiments of the invention described herein are set forth without any loss of generality to, and without imposing limitations thereon, the present invention. 
         [0029]      FIG. 1  depicts, in a fragmentary section view, one section of an earth-boring bit  100  according to the present invention. Earth-boring bit  100  is provided with a body  102 , which is threaded at its upper extent  104  for connection into a drillstring (not shown). 
         [0030]    Earth-boring bit  100  is provided with a pressure compensating lubrication system  106 . Pressure compensating lubrication system  106  is vacuum pressure filled with lubricant at assembly. The vacuum pressure lubrication process also ensures that the journal bearing cavity generally designated as  108  is filled with lubricant through passage  110 . Ambient borehole pressure acts through diaphragm  112  to cause lubricant pressure to be substantially the same as ambient borehole pressure. 
         [0031]    A cantilevered bearing shaft  114  depends inwardly and downwardly from body  102  of earth-boring bit  100 . A generally frusto-conical cutter  116  is rotatably mounted on cantilevered bearing shaft  114 . Cutter  116  is provided with a plurality of generally circumferential rows of inserts or teeth  118 , which engage and disintegrate formation material as earth-boring bit  100  is rotated and cutters  116  roll and slide along the bottom of the borehole. 
         [0032]    Cantilevered bearing shaft  114  is provided with a cylindrical bearing surface  120 , a thrust bearing surface  122 , and a pilot pin bearing surface  124 . These surfaces  120 ,  122 ,  124  cooperate with mating bearing surfaces on cutter  116  to form a journal bearing on cantilevered bearing shaft  114  on which cutter  116  may rotate freely. Lubricant is supplied to journal bearing through passage  110  by pressure-compensating lubricant system  106 . Cutter  116  is retained on bearing shaft  114  by means of a plurality of precision-ground ball locking members  126 . 
         [0033]    A seal assembly  128  according to the present invention is disposed proximally to a base  130  of cantilevered bearing shaft  114  and generally intermediate to cutter  116  and bearing shaft  114 . This seal assembly is provided to retain the lubricant within bearing cavity  108 , and to prevent contamination of lubricant by foreign matter from the exterior of bit  100 . The seal assembly may cooperate with pressure-compensating lubricant system  106  to minimize pressure differentials across seal  128 , which can result in rapid extrusion of and loss of the lubricant, as disclosed in U.S. Pat. No. 4,516,641, to Burr. Thus, pressure compensator  106 , with diaphragm  112 , compensates the lubricant pressure for hydrostatic pressure changes encountered by bit  100 , while seal assembly  128  compensates for dynamic pressure changes in the lubricant caused by movement of cutter  116  on shaft  114 . 
         [0034]      FIG. 2  depicts, an enlarged section view, a preferred seal configuration  128  contemplated for use with the present invention. Seal assembly  128  illustrated is known as a “dual” rigid face seal because it employs two rigid seal rings, as opposed to the single-ring configuration (not shown). Dual rigid face seal assembly  128  is disposed proximally to base  130  of bearing shaft  114  and is generally intermediate to cutter  116  and shaft  114 . Seal assembly  128  is disposed in a seal groove defined by shaft groove  132  and cutter groove  134 . Dual rigid face seal assembly  128  comprises a cutter rigid ring  136 , a cutter resilient energizer ring  138 , rigid seal ring  140 , and shaft resilient energizer ring  146 . Cutter rigid seal ring  136  and shaft rigid seal ring  140  are provided with precision-formed radial seal faces  142 ,  144 , respectively. Rigid seal rings  136  and  140  may be made of any of a variety of materials including, for example, stainless steel such as 440C. Resilient energizer rings  138 ,  146  cooperate with seal grooves  132 ,  134  and rigid seal rings  136 ,  140  to urge and maintain radial seal faces  142 ,  144  in sealing engagement. The seal interface formed by seal faces  142 ,  144  provides a barrier that prevents lubricant from exiting the journal bearing, and prevents contamination of the lubricant by foreign matter from exterior of bit  100 . 
         [0035]      FIG. 3  illustrates, in enlarged section view, an alternative seal configuration  150 . Seal assembly  150  comprises shaft seal groove  152 , cutter seal groove  154 , rigid seal ring  156 , and resilient energizer ring  158 . Rigid seal ring may be made of any of a variety of materials including, for example, stainless steel such as 440C. A precision-formed radial seal face  160  is formed on rigid seal ring  156 , and mates with a corresponding precision-formed seal face  162  carried by cutter  116 . Seal face  162  is formed on a bearing sleeve  164  interference fit in cutter  116 . Resilient energizer ring  158  cooperates with shaft seal groove  152  and rigid seal ring  156  to urge and maintain seal faces  160 ,  162  in sealing engagement. 
         [0036]    At least a portion, and preferably the entirety, of seal faces  160 ,  162  of seal assembly  150  is formed of super-hard material having a coefficient sliding friction less than that of the material of rigid seal ring  156 . Exemplary dimensions for the seal assembly depicted in  FIG. 3  may be found in U.S. Pat. No. 4,753,304 to Kelly. 
         [0037]    The seal assemblies depicted in  FIGS. 1 ,  2 , and  3  are representative of rigid face seal technology and are shown for illustrative purposes only. The utility of the present invention is not limited to the seal assemblies illustrated, but is useful in all manner of rigid face seals. Other types of rigid seal rings with dynamic engagement surfaces may be used. Different configurations of seals within various types of earth-boring bits may be used. 
         [0038]    Referring to  FIG. 4 , a textured surface  166  is created on surface  168  of rigid ring  170 . The textured surface may be created on both cutter rigid ring  136  ( FIG. 2 ) and shaft rigid ring  140  ( FIG. 2 ), or may be created on one but not the other. The following descriptions refer to texturing and coating rigid ring  170 , which may be any rigid seal ring or sealing thrust washer. The rigid ring  170  described may be cutter rigid ring  136 , shaft rigid ring  140 , rigid seal ring  156 , or a rigid ring used in other applications (not shown) requiring a rigid seal ring. 
         [0039]    Textured surface  166  may be a plurality of recesses, such as round indentations, or pores  172 , on surface  168  of rigid ring  170 . In an exemplary embodiment, each pore  172  has a generally round shape having a diameter of roughly 100 micrometers (“microns”) and a depth of roughly 5-20 microns. In some embodiments, pores  172  have a depth of roughly 5-7 microns. The pore  172  diameter may be larger or smaller, and the depth may be larger or smaller. The pores need not be uniform or homogenous. In some embodiments, pores  172  on a single rigid ring  170  may have different diameters or depths. In alternative embodiments (not shown), texturing may be other shapes such as, for example, square indentations, elliptical indentations, and the like. 
         [0040]    In embodiments using pores  172  such as round pores, the pore density may be roughly 20-30%, but any pore density may be used. Pore density refers to the percentage of the surface area of the seal face that is occupied by the pores. Thus if, for example, the pore density equals 30%, then 70% of the surface will contact a mating smooth surface. Some embodiments may use a lower pore density, such as roughly 10-20%, while other embodiments may use a higher pore density, such as roughly 30-60%. 
         [0041]    Testing has shown that a 100 micron diameter pore, with an average depth of 5 microns and a 20% pore density produced the least amount of galling on the surface of the rigid seal. Pore diameters of 50 and 100 microns were tested. Pore depths of 3, 5, and 7 microns were tested. Pore densities of 10%, 20%, and 30% were tested. The test rings having 100 micron pore diameters, 5-7 micron pore depths, and 20-30% pore density showed the least wear. The test samples with smaller diameter pores, 10% pore density, or 3 micron pore depth showed increased wear and galling. 
         [0042]    Referring to  FIG. 5 , in an exemplary embodiment, textured surface  166  is created on surface  168  of rigid ring  170  with a picosecond pulse laser having a power of 0.5 mJ/pulse in 12 picosecond. In an exemplary embodiment, a high-repetition-rate picosecond Nd:YVO 4  laser  174  is used to cold-ablate material from the surface  168  of rigid ring  170 . The short duration of the picosecond pulse is able to remove a desired amount of material without damaging surrounding material. Thus the pico-second pulse laser  174  is able to create the precise geometry required for the texture such as a pore  172 . Other types of lasers  174  may be used to create the texture pattern on the surface  168  of rigid ring  170 . Furthermore, other methods of creating recesses may be used, such as, for example, chemical etching, reactive ion etching, embossing, vibro-rolling, or vibro-mechanical texturing may be used, provided that the other methods (not shown) are able to create micro-sized recesses, such as a 100 micron diameter pore  172 , without adversely changing material properties surrounding the pore  172 . In some embodiments, post polishing may be used to remove extruded material from the edge of the pores. 
         [0043]    Referring to  FIG. 6 , thin film coating (“coating”)  176  can be applied to one or more surfaces  168  of rigid ring  170  after the pores  172  have been created. Coating  176  may be applied to all surfaces  168  of rigid ring, or just to the seal face or surfaces that will slidingly engage a mating surface. Coating  176  may be a super hard coating. 
         [0044]    Coating  176  may be a super hard coating, as defined below, applied over the textured surface  166  of  FIG. 4 . The coating has the function of protecting the textured surface from wear due to sliding contact. After application, the thin film coating  176  presents the texture of surface  166  on the exterior of coating. Thus pores  172  are present as coating pores  178 . If the coating  176  is too thick, the coating would tend to fill in the pores  172  and thus present a smooth surface rather than presenting the texture of the underlying textured surface  166 . A sufficiently thin coating  176 , with a thickness in the 1-5 micron range, and preferably less than 10 microns, is able to assume the texture of the underlying surface. 
         [0045]    Coating  176  may be harder and more lubricious than the substrate. In some embodiments, a hard coating such as diamond-like carbon (“DLC”) is applied to a textured surface on a stainless steel substrate. Such coating is described in U.S. Pat. No. 7,234,541. In other embodiments, an alloy of boronaluminum-magnesium such as, for example, AIMgB 14 , or any other super hard material can be used to form the hard coating over the textured substrate surface. Super hard materials (as the term is used herein) have micro-hardnesses in the vicinity of 5000 and upward on the Knoop scale and are to be distinguished from ceramics such as silicon carbide, aluminum oxide, or cermet such as tungsten carbide, and the like, which have micro-hardnesses of less than 3000 on the Knoop scale. The Knoop micro-hardness value should be determined according to ASTM C849, C1326 and E384 test methods. In addition to their hardness and resulting wear resistance, super-hard materials, particularly the diamond variants such as crystalline or nanocrystalline diamond coatings, have generally good-to-excellent properties in sliding friction and heat dissipation, especially acting as a friction pair. In another embodiment, ceramic or cermet material which has a hardness value greater than that of quartz is used as a protective coating for the textured surface. 
         [0046]    Referring to  FIG. 7 , coating  176  ( FIG. 6 ) may be applied to surface  168  of rigid ring  170  in any variety of ways. Coating may be applied by physical vapor deposition (“PVD”), chemical vapor deposition (“CVD”), plasma-assist chemical vapor deposition (“PACVD”), or pulsed laser deposition. In an exemplary embodiment, DLC is applied to rigid ring  170  by PACVD. 
         [0047]    As one of ordinary skill in the art will appreciate, to create a coating on rigid ring  170  using PACVD technique, rigid ring is placed in chamber  180 . Chamber  180  is pumped down by vacuum source  182  to create negative pressure within chamber  180 . Chemical vapor  184  containing chemicals for coating flows into chamber  180 . A radio frequency (“RF”) source  186  is used to strike plasma within the chamber. Plasma sheath  188  forms on the surface of rigid ring  170  during the reaction. Plasma sheath  188  assists the chemical reactions and deposition required to create coating  176  ( FIG. 6 ) on rigid ring  170 . 
         [0048]    Referring back to  FIG. 6 , in an exemplary embodiment, coating  176  is between approximately 1 micron and approximately 5 microns thick. Coating  176  may be thinner or thicker. In an exemplary embodiment, the thickness of the coating  176  does not significantly alter the dimensions of the rigid ring  170 . In other words, the coating  176  is so thin that the dimensions of rigid ring are virtually identical to the dimensions of an uncoated rigid ring (not shown). Thus the user may maintain a single type of rigid ring in inventory and have the option of having some of the single type of rigid ring coated. Furthermore, the coated and uncoated types of rigid ring may be used interchangeably in an application such as in an earth boring drill bit  100  ( FIG. 1 ). 
         [0049]    Referring to  FIG. 8 , in an alternative embodiment, thin coating  190  is a hard coating that may be created on untextured surface  192  of rigid ring  194 , and then texture such as pores  196  may be applied to thin coating  190 . In this alternative embodiment, coating  190  may be applied in any of the manners described above, including PVD, CVD, PACVD, and laser deposition. As described above, coating  190  may be roughly 1-25 microns thick, but can be thicker or thinner. 
         [0050]    In an exemplary embodiment, coating  190  may be applied to rigid ring  194  having a generally smooth surface  192 , thus causing thin coating  190  to have a generally smooth surface after deposition. Then pores  196  may be created by laser etching pores  196  into coating  190 . In this embodiment, a picosecond pulsed laser  174  ( FIG. 5 ) may be used to laser etch or laser ablated pores  196  into coating. Other techniques may be used to create pores  196 , such as chemical etching, reactive ion etching, embossing, vibro-rolling, or vibro-mechanical texturing may be used, provided that the technique used is suitable for the hardness of coating  190 . Pores  196  may be any size and shape, including, for example, a round pore having a diameter of roughly 100 microns and a depth of roughly 5-20 microns. 
         [0051]    In an exemplary embodiment, the pores  196  may extend completely through coating  190  and into the underlying surface of rigid ring  194 . Thus the rigid ring  194  material, such as 440C steel, may be exposed through each of the pores  196 . 
         [0052]    The present invention has been described with reference to several embodiments thereof. Those skilled in the art will appreciate that the invention is thus not limited, but is susceptible to variation and modification without departure from the scope and spirit thereof. As used herein, recitation of the term about and approximately with respect to a range of values should be interpreted to include both the upper and lower end of the recited range. 
         [0053]    As used in the specification and claims, the singular form “a”, “an” and “the” may include plural references, unless the context clearly dictates the singular form. 
         [0054]    Although some embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereupon without departing from the principle and scope of the invention.

Summary:
The present invention relates to a method and apparatus for forming a dynamic seal between two surfaces. More specifically, the invention relates to creating a textured surface with a thin, hard coating on a metallic annular sealing ring such as the sealing ring used in an earth boring drill bit.