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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to downhole drilling and more particularly to downhole turbine sleeves and methods for making downhole turbine sleeves. 
     2. Description of the Related Art 
     Downhole drilling environments present some of the harshest conditions on the planet. Materials able to withstand these conditions are thus critical to the performance of downhole tools. 
     Historically, the oil and gas industry has relied primarily on steel for manufacturing downhole tools. With the advent of high speed turbines, as well as harsher drilling environments, higher stresses and strains are being placed on downhole tools. Accordingly, materials that exceed the durability of steel are needed in many applications, particularly in drill bits and turbine sleeves placed adjacent to drill bits. 
     In some applications, a turbine sleeve may be placed adjacent to a downhole drill bit. A turbine sleeve is typically a substantially cylindrical structure with a series of blades running along its outside diameter and contacting the borehole. A series of channels running between the blades allow drilling fluids to pass by the sleeve. The turbine sleeve extends the gauge portion of the drill bit and is helpful to reduce lateral movement of the drill bit and prevent the hole from going undergauge. 
     The sleeve may also reduce vibration and hole-spiraling in order to provide a consistently smooth, concentric borehole. The smoothness of the borehole may be critical to placing casing and obtaining accurate logging data. The sleeve may improve rate-of-penentration (ROP) and bit life, thereby extending drilling time and decreasing tripping frequency. 
     Typical turbine sleeves may be may be made of various materials or combinations of materials. In some cases, turbine sleeves may include an internal steel structure that is coated with a matrix material, such as a tungsten carbide matrix. Nevertheless, conventional matrix-coated sleeves are known to be susceptible to blade fractures at the matrix/steel interface due to residual, mechanical, and thermal loading, thereby significantly limiting their service life. 
     In view of the foregoing, what are needed are improved matrix-coated turbine sleeves that are less susceptible to blade fractures and that can better withstand residual, mechanical, and thermal loading. Further needed are improved methods for making matrix-coated turbine sleeves. 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel turbine matrix sleeve and method for making same. The features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     In a first embodiment of the invention, a turbine matrix sleeve in accordance with the invention includes an inner cylindrical structure made up of a first material. The inner cylindrical structure may include multiple blades and multiple channels running between the blades along an outside diameter thereof. The inner cylindrical structure further includes threads, such as right-hand or left-hand threads, on an outer surface thereof. An outer layer, made up of a second material different from the first material, is integrally bonded to the threads. This outer layer may be optionally embedded with hardened inserts or buttons, such as PDC inserts, diamond inserts, TSP inserts, or the like. The threaded surface on the inner cylindrical structure significantly improves the bond between the outer layer and inner cylindrical structure and creates a mechanical lock between the outer layer and inner cylindrical structure. 
     In selected embodiments, the blades are substantially parallel to or helical with respect to an axis of the inner cylindrical structure. In certain embodiments, the inner cylindrical structure is made of steel and the outer layer is made of a matrix material. For example, the matrix material may be a tungsten carbide matrix material. Similarly, in certain embodiments, the outer layer is made of a material that is harder or more durable than the material of the inner cylindrical structure. In certain embodiments, the outer layer makes up about 5 to 95 percent of the blade height. In other embodiments, the outer layer makes up about 30 percent of the blade height. 
     In another embodiment, a method in accordance with the invention may include providing an inner cylindrical structure made up of a first material. The method may then include forming multiple blades and multiple channels running between the blades along an outside diameter of the inner cylindrical structure. The method may also include forming threads on an outer surface of the plurality of blades. The method may include forming the threads prior to or after forming the blades and channels on the inner cylindrical structure. Once the threads are formed, the method may include integrally bonding, to the threads, an outer layer made up of a second material different from the first material. Optionally, the method may include embedding buttons or inserts, such as PDC inserts, diamond inserts, TSP inserts, or the like into the outer layer. 
     In yet another embodiment, an apparatus in accordance with the invention may include an inner cylindrical structure made up of a first material. The inner cylindrical structure may have threads on an outer diameter thereof. An outer layer, made up of a second material different from the first material, may be integrally bonded to the threads. Multiple blades and channels running between the blades may be formed on an outer surface of the outer layer. The channels may extend exclusively into the outer layer or, alternatively, through the outer layer and into the inner cylindrical structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through use of the accompanying drawings, in which: 
         FIG. 1  is a perspective view of one embodiment of a turbine sleeve in accordance with the invention, connected to a drill bit; 
         FIG. 2  is a perspective view of one embodiment of a turbine sleeve in accordance with the invention; 
         FIG. 3  is an end view of the turbine sleeve illustrated in  FIG. 2 ; 
         FIG. 4  is a cross-sectional side view of the turbine sleeve illustrated in  FIG. 2 ; 
         FIG. 5  is a perspective view of one embodiment of an inner cylindrical structure (or blank) for incorporation into a turbine sleeve in accordance with the invention; 
         FIG. 6  is a perspective view of one embodiment of a mold sleeve, having hardened or durable inserts adhered to an inside diameter thereof, used for fabricating the turbine sleeve; 
         FIG. 7  is a perspective view showing the mold sleeve surrounding the inner cylindrical structure of  FIG. 5 ; 
         FIG. 8  is a cross-sectional side view of the mold sleeve and inner cylindrical structure sitting on a base fixture; 
         FIG. 9  is a cross-sectional side view of the mold sleeve and inner cylindrical structure sitting on a base fixture, along with sand formers to form channels along the turbine sleeve; and 
         FIG. 10  is a cross-sectional side view of one embodiment of an assembly for fabricating the turbine sleeve. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of apparatus and methods in accordance with the present invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of certain examples of presently contemplated embodiments in accordance with the invention. The presently described embodiments will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. 
     Referring to  FIGS. 1 and 2 , one embodiment of a downhole turbine sleeve  100  in accordance with the invention is illustrated.  FIG. 1  shows a turbine sleeve  100  attached to a drill bit  102  and  FIG. 2  shows the turbine sleeve  100  by itself. In selected embodiments, the turbine sleeve  100  may be a substantially cylindrical structure with a series of blades  104  running along an outside diameter thereof. The blades  104  may contact the borehole and extend the gauge portion (i.e., the outer diameter) of the drill bit  102 . In the illustrated embodiment, the blades  104  are substantially parallel with respect to an axis  108  of the turbine sleeve  100 . However, in other embodiments, the blades  104  may be slanted or helical with respect to the axis  108 . A series of channels  106  may run between the blades  104  to allow drilling fluids, cuttings, or other materials to flow past the turbine sleeve  100  along the borehole. 
     As previously mentioned, the turbine sleeve  100  may provide various benefits in downhole drilling applications. For example, the turbine sleeve  100  may reduce lateral movement of the drill bit  102  by providing stiffness support thereto. The turbine sleeve  100  may also reduce vibration and hole spiraling in order to provide a consistently smooth, concentric borehole. The turbine sleeve  100  may improve rate-of-penetration (ROP) and bit life. These benefits may extend drilling time and decrease tripping frequency. 
     In selected embodiments, the blades  104  and channels  106  of the turbine sleeve  100  may align with corresponding blades or channels of the drill bit  102  to provide a path for fluids and cuttings to pass by the turbine sleeve  100 . In certain embodiments, one or more blades  104  may be omitted to provide wider channels  107  along the turbine sleeve  100 , thereby provided additional space for drilling fluids or cuttings to pass by the turbine sleeve  100 . A breaker slot  110  may enable a tool or fixture to grab and apply torque to the turbine sleeve  100  when making up the drill bit  102 . In certain embodiments, one or more weld holes  112  may be provided in the turbine sleeve  100 . These weld holes  112  may be filled with a weld material to connect the sleeve  100  to an extension member  114  connecting the sleeve  100  to the drill bit  102 . The extension member  114  may include internal threads (e.g., standard API connection threads) to connect the drill bit  102  and turbine sleeve  100  to other drill tools (e.g., a motor or turbine). 
     Referring to  FIGS. 3 and 4 , in selected embodiments, a turbine sleeve  100  in accordance with the invention may include an inner cylindrical structure  300  made of a material such as steel. A more durable outer layer  302  may be adhered or attached to the outside diameter of the inner cylindrical structure  300 . For example, a matrix material such as a layer  302  of tungsten carbide matrix may be attached to the outside diameter of the inner cylindrical structure  300  to provide added hardness or durability to the turbine sleeve  100 . In another example, the matrix material may include an impreg matrix containing 10 to 40 percent diamond grit by volume. To improve the bond between the outer layer  302  and the inner cylindrical structure  300 , a matrix layer containing a transition constituent may be used. 
     In certain embodiments, the outer layer  302  maybe embedded with inserts or buttons, such as tungsten carbide buttons, polycrystalline diamond compact (PDC) buttons, diamond inserts, PDC inserts, thermally stable polycrystalline diamond inserts (TSPs), natural diamonds, or the like, to improve the hardness or durability of the outer layer  302 . The outer layer  302  may also receive durability enhancements such as impreg mix, brazed in PDC cutters on the blades  104 , and/or PDC cutters on the back angle  402  to act as upreamers. 
     In certain embodiments, the outer layer  302  may be localized to the blades  104 , meaning that the outer layer  302  may not extend to the root  304  of each blade  104 . This design may minimize residual stresses by not having the outer layer  302  fully cover the inner cylindrical structure  300 . In selected embodiments, the thickness  306  of the outer layer  302  may be about ten to eighty percent of the overall blade height  308 . In other embodiments, the thickness  306  of the outer layer  302  may be about thirty percent of the overall blade height  308 . In general, the thickness of the outer layer  302  may be chosen to avoid undercutting of the softer steel beneath the outer layer  302 . Nevertheless, in other embodiments, the outer layer  302  is not localized to the blades  104 , but rather extends to the root  304  of each blade  104  and completely covers the inner cylindrical structure  300 . 
     Referring to  FIG. 5 , as previously mentioned, conventional matrix-coated turbine sleeves are known to be susceptible to blade fractures at the matrix/steel interface  400  due to residual, mechanical, and/or thermal loading, thereby significantly limiting the turbine sleeve&#39;s service life. Thus, apparatus and methods are needed to reduce the blades&#39; susceptibility to fracture. 
     In order to address this problem, in selected embodiments, threads  500  (e.g., right-hand threads, left-hand threads) may be formed on the outside diameter of the inner cylindrical structure  300  prior to applying the outer layer  302  thereon. In this embodiment, a series of blades  104  and channels  106  are formed on the inner cylindrical structure  300  either before or after the threads  500  are formed thereon. The threads  500  may increase the surface area of the interface  400  and create a more gradual, as opposed to abrupt, transition from matrix material to steel. The threads  500  may also spread interfacial stress (due to compatibility strains, differences in coefficients of thermal expansion, etc.) over a wider area, thereby reducing the peak stresses experienced at the interface. This may significantly reduce the outer layer&#39;s tendency to separate or fracture from the underlying inner cylindrical structure  300 . This improvement has been verified in high-speed turbine applications. 
     Another advantage of the threads  500  is that they may create a mechanical lock between the outer layer  302  and the inner cylindrical structure  300 , thereby preventing separation due to tangential or thermal loading. In certain embodiments, the direction of the threads  500  may be selected based on the rotational direction of the drill bit  102 . One additional advantage of using threads  500  as opposed to other textured surfaces is the ease of forming the threads  500  on the inner cylindrical structure  300  using a lathe or other appropriate machine tool. 
     Referring to  FIG. 6 , in order to fabricate the turbine sleeve  100 , a mold sleeve  600 , such as a graphite mold sleeve  600 , may be provided. The inside diameter of the mold sleeve  600  may be designed such that it is substantially equal to a desired outside diameter of the turbine sleeve  100 . If inserts  602  or buttons  602  (e.g., PDC buttons, diamond inserts, PDC inserts, TSPs, natural diamonds, or the like) are to be embedded within the blades  104  of the turbine sleeve  100 , these buttons  602  or inserts  602  may be glued or adhered to the inside diameter of the mold sleeve  600  at locations that will align with the blades  104  of the inner cylindrical structure  300 . The mold sleeve  600  may provide a temporary form for the matrix material (i.e., the outer layer  302 ) that is deposited on the inner cylindrical structure  300 . 
     Referring to  FIG. 7 , once the buttons  602  or inserts  602  are adhered to the inside diameter of the mold sleeve  600 , the inner cylindrical structure  300  may be placed within the mold sleeve  600  such that the buttons/inserts  602  are positioned immediately over the blades  104  of the inner cylindrical structure  300 . A series of channel formers  700  (e.g., sand formers  700 ) may be placed in the channels  106  of the inner cylindrical structure  300  to form the waterways  106  or channels  106  in the turbine sleeve  100 . The remaining voids  702  may then be infiltrated with a matrix material (e.g., tungsten carbide matrix) to form the blades  104  of the turbine sleeve  100 . This process will be explained in more detail hereafter. Once the turbine sleeve  100  is fabricated, the mold sleeve  600  may be broken up and removed from the outer circumference of the turbine sleeve  100 , leaving the buttons/inserts  602  embedded within the blades  104 . 
     Referring to  FIG. 8 , in selected embodiments, a method for fabricating a turbine sleeve  100  in accordance with the invention may include initially cleaning the inner cylindrical structure  300  to ensure that corrosion, grease, and/or dirt are removed from the outside diameter thereof. The inner cylindrical structure  300  and mold sleeve  600  may then be placed on a base fixture  800 . The base fixture  800  may help keep the inner cylindrical structure  300  and the mold sleeve  600  axially centered with respect to one another. As previously mentioned, the mold sleeve  600  may be oriented such that the buttons  602  or inserts  602  that are adhered to the sleeve  600  are positioned immediately over the blades  104  of the inner cylindrical structure  300 . Ideally, the buttons  602  or inserts  602  are positioned some distance (e.g., 0.2 inches) away from the edge of the blades  104 . 
     Referring to  FIG. 9 , the channel formers  700  may then be inserted into the channels  106  of the inner cylindrical structure  300 . These channel formers  700  may create voids in the turbine sleeve  100  that will produce the waterways  106  or channels  106  along the turbine sleeve  100 . 
     Referring to  FIG. 10 , in selected embodiments, the assembly illustrated in  FIG. 9  may then be placed into a mold pot  1000 . In selected embodiments, the mold pot  1000 , as well as a funnel member  1014  and lid  1020  may be fabricated from a heavy-grade graphite material and may be re-used when producing the turbine sleeve  100 . A matrix powder, such as a tungsten carbide powder, may then be loaded into the voids  702  illustrated in  FIG. 7 . The matrix powder may be loaded to a depth (e.g., ⅛ inch) below a top surface of the channel formers  700 . If needed, the entire structure may be vibrated to compact the matrix powder. If the matrix powder compacts below the depth previously measured, additional matrix powder may be loaded into the voids  702  to bring the matrix powder up to the previous depth. At this point an upreamer ring  1004  may be added to the structure immediately above the channel formers  700  and the powder  1002 . The upreamer ring  1004  may provide a temporary form to ensure that the matrix material assumes the sloping back angle  1006 . 
     Once the upreamer ring  1004  has been positioned, additional matrix powder may be loaded into the voids  702 , such as at or near the corner  1008 . A sand stalk  1010  may then be installed into the base fixture  800  and centered with respect to the inside diameter of the inner cylindrical structure  300 . The sand stalk  1010  may keep the inside diameter of the inner cylindrical structure  300  free of powder and binder. If desired, a soft powder may be loaded into the space  1012  between the sand stalk  1010  and the inner cylindrical structure  300 . This soft powder may create a soft material that may be machined away or broken up after the turbine sleeve  100  is fabricated. 
     Once the sand stalk  1010  is installed and soft powder is loaded between the sand stalk  1010  and the inner cylindrical structure  300 , a funnel member  1014  may be attached to the top of the mold pot  1000 . In selected embodiments, the funnel member  1014  may thread onto the mold pot  1000 . The funnel member  1014  may provide a chamber  1016  where a binder material (e.g., a copper-based alloy) may be added. A lid  1020  may cover the top of the funnel member  1014 . In certain embodiments, the lid  1020  may also thread onto the funnel member  1014 . In selected embodiments, a thermocouple protection tube  1018  may extend through the lid  1020 , through a smaller sand stalk  1026 , and through the larger sand stalk  1010 . A thermocouple (not shown) may extend through the thermocouple protection tube into the assembly  1024  to measure the assembly&#39;s internal temperature when heated. A thermocouple cap  1022  may fit over the thermocouple tube  1018  and rest on the lid  1020 . 
     Once the assembly is complete, the entire assembly may be placed in a furnace and heated. For example, the assembly may be heated to temperature of about 1200° C. for about 3 hours. The heat will cause the binder in the chamber  1016  to melt and flow (in response to gravity and surface tension) into the matrix powder  1002  in the assembly  1024 . 
     At the end of the heating cycle, the assembly  1024  may be removed from the furnace and cooled. This may be accomplished by placing the assembly on a cooling table and directing a stream of water into a quench cavity  1028  on the bottom of the mold pot  1000 . By controlling the flow rate of the water stream, the cooling rate of the assembly  1024  may be controlled. As the assembly  1024  cools, the binder that has infiltrated the matrix powder  1002  will begin to solidify from the bottom up, thereby creating the solidified matrix material on the blades  104 . Solidifying the matrix material in an upward direction may ensure that liquid metal is available to fill any porosity in the matrix powder as the matrix shrinks and solidifies. 
     After the assembly  1024  cools sufficiently, the turbine sleeve  100  (which may include the inner cylindrical structure  300  and the solidified matrix material  1002 ) may be removed from the assembly  1024 . The sand stalk  1010 , mold sleeve  600 , and upreamer ring  1004  may be mechanically broken up and removed from the turbine sleeve  100 . The resulting turbine sleeve  100  may then be machined as needed to assume its final contour and shape. 
     The apparatus and methods disclosed herein may be embodied in other specific forms without departing from their spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Summary:
A turbine matrix sleeve in accordance with the invention includes an inner cylindrical structure made up of a first material. The inner cylindrical structure may include multiple blades and multiple channels running between the blades along an outside diameter thereof. The inner cylindrical structure further includes threads, such as right-hand or left-hand threads, on an outer surface thereof. An outer layer, made up of a second material different from the first material, is integrally bonded to the threads. This outer layer may be optionally embedded with hardened inserts or buttons, such as PDC inserts, diamond inserts, TSP inserts, or the like. The threaded surface on the inner cylindrical structure significantly improves the bond between the outer layer and the inner cylindrical structure and creates a mechanical lock therebetween.