Abstract:
12 A high temperature gas turbine component includes an inner core made of a monolithic ceramic material embedded within an outer CMC shell. The inner core may be formed with a through hole, blind hole, wear pads and the like. A method of making the bushing includes the steps of a) forming an inner core of silicon nitride or silicon carbide; and b) applying a ceramic matrix composite material over substantially all of the inner core.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to ceramic matrix composite components in general, and specifically, to a high temperature ceramic matrix composite component that incorporates a monolithic attachment bushing. 
     Ceramic matrix composites (CMC&#39;s) offer high material temperature capability. In the gas turbine field, however, CMC components often require attachment to, or engagement with, lower temperature metallic gas turbine components. Problems associated with the attachment of known silicon carbide ceramic matrix composites (CMC&#39;s) to metallic components include wear, oxidation (due to ionic transfer with metal), stress concentration (from clamping loads), transition to thick section fabrication, and fiber damage in creating holes in the CMC&#39;s. 
     SUMMARY OF INVENTION 
     In an exemplary embodiment of the invention, advantage is taken of the very high strength of monolithic ceramics to absorb the clamping loads of bolt and pin-type attachment means. The component in one embodiment thus includes, for example, an inner core formed as an attachment bushing or a wear pad made of either silicon carbide or silicon nitride monolithic ceramic that is embedded within the body of a CMC shell. It is understood that material selection for the inner core or attachment bushing (or wear pad) depends on specific attachment requirements, and the shape of the bushing or wear pad could be any number of shapes, several of which are disclosed herein. Preferably, the shape of the bushing or wear pad would thus be optimized to insure that it is well encased within the surrounding CMC shell fabric layers, and that the load is optimally distributed into the CMC component structure. In one exemplary embodiment, the required size of a through hole in the bushing will contribute to set the overall size of the bushing in order to preserve an appropriate surface area of monolithic ceramic within the surrounding CMC shell. 
     In other embodiments, the through hole may be eliminated in favor of a blind hole, or even a solid center with oppositely facing wear surfaces. 
     The CMC shell that incorporates the bushing or wear pad may be any gas turbine or other component, and is not limited to the shape or configuration described and/or illustrated herein. 
     Accordingly, in its broader aspects, the present invention relates to a component for use in a gas turbine comprising an inner attachment bushing comprised of a monolithic ceramic material having front and back faces and a through-hole formed therein of predetermined diameter and adapted to receive a bolt shank or pin, the attachment bushing substantially encased within an outer shell composed of CMC material. 
     In another aspect, the invention relates to a gas turbine component comprising an inner attachment bushing made of silicon nitride or silicon carbide, the attachment bushing having a first outer diameter and a through-hole formed therein having a second diameter, the first diameter being 2.5-4 times the second diameter; wherein the attachment bushing is enclosed within an outer shell of ceramic matrix composite material. 
     In another aspect, the invention relates to a gas turbine component including an attachment bushing of silicon nitride or silicon carbide material, the attachment bushing substantially encased within an outer shell composed of CMC material, wherein the attachment bushing is formed with a through hole, opposite ends of which are flush with respective opposite sides of the outer shell. 
     In still another aspect, the invention relates to a method of making a gas turbine component comprising a) forming an attachment bushing of silicon nitride or silicon carbide; and b) applying a ceramic matrix composite material over substantially all of the inner core with the exception of front and rear flat faces about a center axis of the attachment bushing. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a side elevation of a component having a monolithic ceramic core bushing encased within a ceramic matrix composite material in accordance with an exemplary embodiment of the invention; 
     FIG. 2 is a side section view of the component shown in FIG. 1; 
     FIG. 3 is a side section view of a component in accordance with an alternative embodiment of the invention; 
     FIG. 4 is a side section view of a component in accordance with another embodiment of the invention; 
     FIG. 5 is a side section view of a component in accordance with another embodiment of the invention; and 
     FIG. 6 is a side section view of a component in accordance with still another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIGS. 1 and 2, a gas turbine component  10  includes a monolithic ceramic inner core  12  formed as an attachment bushing made of either silicon nitride or silicon carbide embedded within the body of a CMC shell  24 . The silicon nitride or silicon carbide bond well with the surrounding CMC shell, while providing extremely hard and wear resistant surfaces. Monolithic ceramics also can maintain close tolerances needed for specific attachment requirements. In this embodiment, the attachment bushing  12  includes a through-hole  14  that is sized to receive another gas turbine component such as a metal bolt shank or pin. The inner core  12  also includes flat, annular portions or faces  16 ,  18  that may or may not be flush with the opposite sides  20 ,  22  of the outer CMC shell  24 , depending on the particular attachment design needed between the metallic and composite structures. 
     The bushing  12  is reduced in thickness in a radially outward direction in symmetrical fashion from the flat annular faces  16 ,  18  to an internal, maximum diameter curved edge  26  via substantially flat tapered surfaces  28 ,  30 . Edge  26  is substantially centered between the sides  20 ,  22  of the CMC shell  24 . 
     In an alternative embodiment shown in FIG. 3, where similar reference numerals for corresponding attachment elements are used, but with the prefix “1” added, the attachment bushing  112  of the component  110  reduces in thickness in a radially outward direction from the flat annular faces  116 ,  118  to an internal, maximum diameter convex curved edge  126  via concave curved surfaces  128 ,  130 . 
     The arrangements described above are intended to take advantage of the very high strength of monolithic ceramics to absorb the clamping loads parallel to the axis of the bushing exerted by bolt and pin type attachments in a CMC component. The bushings  12  and  112  allow the stress fields around attachment points to be spread out over a larger area supported by the surrounding fibers within the larger area CMC shells  24  and  124 . 
     Material selection will depend on specific applications, and the shape of the outer diameter surfaces of the bushing  12  or  112  may be varied to provide optimum load distribution into the surrounding CMC shell  24 . In addition, the diameters of the hole  14  or  114  determine in part the size of the bushing in order to preserve an approximate outer diameter surface area of the monolithic bushing within the CMC shell. Preferably, a ratio of overall monolithic bushing diameter to hole diameter of 2.5-4 is maintained, the exact ratio being determined by specific application requirements. 
     FIG. 4 illustrates another embodiment where an inner monolithic bushing  32  of silicon nitride or silicon carbide is embedded within the layers of an outer CMC shell  34 . Here, the bushing  32  is formed with radial flanges  36 ,  38  on opposite sides of the core, and a through hole  40 . The bushing  32  is substantially completely encased within the layers of the outer shell  34 , the latter having drilled (or otherwise formed) ends  42 ,  44  that are aligned with the through hole  40 . Thus, the through hole is recessed relative to the side surfaces  46 ,  48  of the component. 
     FIG. 5 illustrates yet another embodiment where the monolithic bushing  50  of silicon nitride or silicon carbide is formed substantially as a solid disk with beveled radially outer edges  52 ,  54 , with a blind hole  56  drilled (or otherwise formed) in the core. Blind hole  56  is aligned with a drilled (or otherwise formed) hole  58  in the outer CMC shell  60 . The core  50  is otherwise completely encased within the layers of the outer shell  60 . 
     FIG. 6 illustrates yet another embodiment of the invention where an inner core  62  substantially similar to inner core  50  (FIG.  5 ), but without the presence of a blind hole. In this embodiment, raised wear pads  64 ,  66  are formed on opposite sides of the inner core and substantially centered thereon. The outer shell  68  is applied such that only the wear pads  64 ,  66  are exposed, the remainder of the core encased within the layers of the outer shell  68 . 
     It will be appreciated that the through holes, blind holes or wear pads may be flush or recessed with respect to the outer CMC shell. In other instances, the inner core and/or wear pads may project beyond the outer CMC shell. It will further be appreciated that the CMC outer shell may take on any shape, as dictated by the particular gas turbine component. In other words, the CMC outer shell may be a motor casing, a bearing stand, or any other component otherwise advantageously formed of CMC composite material. 
     In each case, the manufacture of the gas turbine component is similar. Thus, after producing the silicon nitride or silicon carbide monolithic bushing or wear pad ( 12 ,  112 ,  32 ,  50  or  62 ), the outer CMC shell ( 24 ,  124 ,  34 ,  60  or  68 ) is fabricated over and/or around the bushing or wear pad. The manner in which this is done may be similar to the way in which components are embedded in fiberglass, with layer upon layer of the CMC laid up surrounding the monolithic inner core in sections or strips, until the outer dimensions of the component are achieved. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.