Abstract:
An apparatus for mounting a refractory component such as a turbine shroud ring segment ( 32 ) with a ceramic core ( 42 ) onto a combustion turbine engine structure ( 34 ). The ring segment has a ceramic matrix composite skin ( 40 ), and optionally, a thermal insulation layer ( 46 ). A pin ( 60 ) is inserted through a bore ( 48 ) in the core and through an attachment bar ( 54 ) with ends received in wells ( 50 ) in the core. The attachment bar may be attached to a backing member, or tophat ( 64 ), by a biasing device ( 76 ) that urges the refractory component snugly against the backing member to eliminate vibration. The backing member and refractory component have mating surfaces that may include angled sides ( 52 S,  70 ). The backing member is attached to the engine structure. Turbine shroud ring segments can be attached by this apparatus to a surrounding structure to form a shroud ring.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of combustion turbine engines, and more particularly to the use of ceramics and ceramic matrix composite materials in a combustion turbine engine. 
     BACKGROUND OF THE INVENTION 
     A combustion turbine engine has a rotating shaft with several circular arrays of radially oriented aerodynamic blades mounted around the circumferences of disks on the shaft. Closely surrounding these blades is a refractory shroud that contains the flow of hot combustion gasses passing through the engine. This shroud must withstand temperatures of over 1400° C. reliably over a long life span. Close spatial tolerances must be maintained in the gap between the blade tips and the shroud for engine efficiency. However, the shroud, blades, disks, and their connections are subject to wide thermal changes during variations in engine operation, including engine shutdowns and restarts. The shroud must insulate the engine case from combustion heat, and it must be durable and abrasion tolerant to withstand occasional rubbing contact with the blade tips. 
     Ceramics are known to be useful in the inner lining of shrouds to meet these requirements. A shroud is assembled from a series of adjacent rings, each ring having an inner surface typically of one or more refractory materials such as ceramics. Each ring is formed of a series of arcuate segments. Each segment is attached to a surrounding framework such as a metal ring that is attached to the interior of the engine case. However, ceramic components are difficult to attach to other components. Ceramic material cannot be welded, and it is relatively brittle and weak in tension and shear, so it cannot withstand high stress concentrations. It differs from metal in thermal conductivity and growth, making it challenging to attach ceramic parts to metal parts in a hot and varying environment. Thus, efforts are being made to advance technologies for use of ceramic components in combustion turbine engines, including technologies for reliable ceramic-to-metal connections. 
     An example of this advancement is disclosed in U.S. Pat. No. 6,758,653, which shows the use of a ceramic matrix composite (CMC) member connected to a metal support member. A CMC member using this type of connection can serve as the inner liner of a combustion turbine engine shroud. Ceramic matrix composite materials typically include layers of refractory fibers in a matrix of ceramic. Fibers provide directional tensile strength that is otherwise lacking in ceramic. CMC material has durability and longevity in hot environments, and it has lower mass density than competing metals, making it useful for combustion turbine engine components. However, it is not ideal for components with stress in areas of sharp curvature, because the fiber layers tend to separate from each other during formation and sintering, leaving voids that weaken the material at curves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a schematic sectional view of a segmented shroud ring in a combustion turbine engine taken on a plane normal to the engine shaft axis. 
         FIG. 2  is an isometric view of a refractory shroud segment. 
         FIG. 3  is a back view of the shroud segment of  FIG. 2 . 
         FIG. 4  is a sectional view of the shroud segment mounted in an engine, taken along section  4 - 4  of  FIG. 3   
         FIG. 5  is a sectional view of the shroud segment and a top hat assembly, taken along section  5 - 5  of  FIG. 3 . 
         FIG. 6  is a sectional view of the shroud segment and top hat assembly, taken along section  6 - 6  of  FIG. 3 . 
         FIG. 7  is perspective view of a prior art slotted spring bushing 
         FIG. 8  is a schematic view of pressure load boundary conditions and load paths in a shroud segment 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a schematic sectional view of a combustion turbine engine  20  taken on a plane normal to the engine shaft axis  22  through a disk  24  mounted on the shaft  26  with turbine blades  28  and an associated shroud ring  30  and a support structure. The shroud ring  30  is assembled in arcuate segments  32 . The support structure for a shroud ring  30  may comprise an intermediate support ring  34  between the shroud ring  30  and the engine casing  36 . The support ring  34  is attached to the engine casing  36 , and the segments  32  of the shroud ring are attached to the support ring  34 . The support ring  34  and the engine casing  36  may be made of metal, although this is not a requirement of the invention. Cooling air  38  flows between the shroud  34  and engine casing  36  under enough pressure to prevent combustion gases from entering this area through clearances between shroud segments  32 . Cooling air may also flow in channels within the blades (not shown). In the following description “axial” means generally parallel to the shaft axis  22 ; “circumferential” means generally along the circumference of a circle centered on the shaft axis  22  in a plane normal to the shaft axis  22 ; and “radial” means in a generally perpendicular orientation or direction relative to the shaft axis  22 . 
     A refractory shroud ring segment  32  of a combustion turbine engine is an exemplary application of the technology of the present invention. This technology can also be applied to other components of combustion turbine engines.  FIGS. 2 and 3  show an isometric and top view of a pin-loaded refractory core shroud ring segment  32 . The segment  32  is an integrated refractory component having a CMC skin  40  bonded to at least a portion of an exterior surface of a refractory ceramic core  42 . The hot side  44  of the segment  32  may be protected by a high temperature thermal insulation  46  deposited on the CMC skin  40 . 
     A wide range of ceramic matrix composites (CMCs) have been developed that combine a matrix material with a reinforcing phase of a different composition (such as mullite/silica) or of the same composition (alumina/alumina or silicon carbide/silicon carbide). The fibers may be continuous or long discontinuous fibers. The matrix may further contain whiskers, platelets or particulates. Reinforcing fibers may be disposed in the matrix material in layers, with the plies of adjacent layers being directionally oriented to achieve a desired mechanical strength. The CMC skin  40  may be a continuously wrapped structure as known in the art of fabrication of composite structures. This means that the fibers are wrapped continuously around the core  42  to avoid discontinuities that cause weak points and unevenness in the skin. 
     One or more pin bores  48  are formed through the core  42 , such as by drilling or by the removal of a fugitive material used during the casting of the ceramic core. The pin bores  48  may be oriented in the circumferential direction as shown, or in an axial direction. An axial pin orientation may facilitate insertion/assembly. Pin access wells  50  are formed into the cold side  52  of the ring segment  32 , such as by machining or through the use of a fugitive material, intersecting the pin bores  48 . Two such wells  50  intersect each pin bore in the illustrated embodiment. 
     Additional details of the pin-loaded attachment scheme are shown in  FIGS. 4-6 . The end of a respective support bar, such as U-shaped clevis bar  54 , is inserted into each of the wells  50 . The support bars are illustrated as U-shaped clevis bars  54  with each end  56  extending into a well  50  associated with a single bore  48 , although one may appreciate that other shapes of support bars are possible in other embodiments. For example, each support bar may have only one end inserted into a well. Each inserted end  56  of the clevis bar has a hole  58  that is aligned with the bores  48 . Pins  60  are inserted through the bores  48  and the holes  58  to create a clevis type attachment between the core and the clevis bar  54 . Other connection geometries between the bars and pins may be used in other embodiments, such as for example an open J-shaped hook on the bar end for receiving the pin. Support loading between the pin  60  and core  42  is distributed along the entire length of the pin  60 . The pin  60  may be sized to fit closely in the bore  48 , and the ceramic core  42  is relatively thick and rigid. This combination of geometric features limits the bowing of the pin  60 , which would otherwise create point loads rather than a distributed load. To minimize bowing of the pin, two clevis bar attachment points  58  may be spaced symmetrically at intermediate positions along the length of the pin  60 , such as 25% to 35% from each end. Since there is a combustion gas pressure drop along the axial direction, and uniform pressure in the circumferential direction, the pins  60  may be oriented in the circumferential direction in order to maintain uniform loading on the pin  60 . If the pins are oriented in the axial direction, the clevis support locations may be biased towards the high-pressure side to produce a more uniform load distribution along the length of the pin. 
     The pin bore  48  may have a compliant layer such as bushing  62  to help distribute the pin loading and to protect the metal pin  60  and ceramic core  42  from fretting and sliding wear. An example of a suitable type of bushing is a “slotted spring pin” available from Spirol International, Inc. as shown in  FIG. 7 . A close fit between the metal of the pin  60  or bushing  62  and the ceramic core  42  will maximize the contact loading area and will help to avoid point contact. An appropriately designed slotted spring bushing  62  enables a close fit tolerance at all temperature conditions by accommodating thermal expansion mismatch between the metal pin  60  and the ceramic core  42  via the spring load. The bushing  62  may be installed in three sections as shown in  FIG. 5 . This reduces differential thermal expansion along the length of the bushing  62 , and allows the holes  58  in the support bars  54  to be sized for the pin diameter. However, the bushing  62  can alternately be in two sections or the full length of the pin if desired, and thereby pass through the support bars  54  along with the pin  60 . Thus, the primary wear surface in the support contact area is between the pin  60  and the bushing  62 , which is typically a metal-to-metal contact surface. This is preferable to a ceramic-to-metal contact surface and it helps to avoid sliding and fretting wear between the metal pin  60  and the ceramic core  42  due to differential thermal growth of the metal and ceramic. 
     Such a slotted spring pin may be effective in other high temperature applications where a ceramic structure is attached to a metal structure, such as when a ceramic matrix composite material is supported by a metal support bar inserted through a bore in the CMC material. Should the metal support bar be sized for a tight fit at room temperature, the CMC material defining the bore adjacent to the bar would be crushed at high temperatures by the differential thermal expansion between the metal and the ceramic. This would cause an increase in the size of the bore, resulting in an increasingly loose fit, with subsequent high cycle wear of the CMC material against the metal bar. An intervening spring member allows the metal-to-ceramic fit to remain tight in spite of differential thermal expansion, thereby eliminating dynamic vibration between the CMC and the metal material. Such a design may still experience some localized sliding between the CMC and the metal material as the temperature cycles between room temperature and a high operating temperature, but such wear is low cycle (e.g. 10 2  cycles) when compared to the high cycle wear (e.g. 10 6  cycles) experienced by a design not including such a slotted spring pin. This concept may be applied with a pin/bore having a circular cross section, such as illustrated herein, or with a pin/bore having other shapes, such as elliptical, slotted, etc. The concept may further be applied to applications of oxide or non-oxide ceramics, and to monolithic or composite ceramics. Applications may include gas turbine engine components as well as other types of equipment experiencing operation at an elevated temperature. 
       FIG. 4  shows one embodiment of how a refractory shroud ring segment  32  can be attached to the engine casing  36  via a pin-loaded solid core  42 . The cold side  52  of the CMC ring segment  32  cooperates with a metal backing member  64 , or “tophat”, with a corresponding contoured inner surface  68 . In order to keep the CMC ring segment  32  in contact with the metal tophat  64  at all engine conditions for the life of the part, a spring load can be applied to the clevis bars  54  such that the ring segment  32  seats against in the tophat  54 . The amount of spring load may be based on the requirements of the design. The upper bound for spring load may be set such that the design of the CMC ring segment  32  has sufficient margin to carry the combined pressure and spring load. The lower bound may be set by the amount of spring load required to keep the CMC ring segment  32  from moving, either due to pressure pulses or to thermal growth mismatches that may allow the structures to separate. The tophat  64  has tabs or hooks  82  that engage with receiving portions  84  on the support ring  34 , by sliding engagement, bolts, or other known attachment mechanisms. The support ring  34  is attached to the engine casing  36  as known in the art. 
     The back surface  52  of the integrated refractory component  32  includes a central generally flat section  52 C and two radially-inwardly sloping side sections (surfaces  52 S in  FIG. 2 ). Mating sloped surfaces  70  are provided on the tophat inner surface  68 . These may be designed such that thermal growth mismatch is accommodated by sliding along these surfaces. A compliant layer (not shown) can optionally be used between the ceramic ring segment  32  and the metal tophat  64 . For example, a ceramic fabric can be used, such as Nextel® 440 fiber (aluminum oxide 70%, silicone dioxide 28%, and boron oxide 2%). 
       FIGS. 5 and 6  schematically illustrate a configuration for attaching the clevis bars  54  to the tophat  64  with a bias that pulls the ring segment  32  against the tophat  64 . In this embodiment, a clevis bar  54  is made of a flexible material such as a steel alloy, and is formed in a U-shape with a central span  66 . The clevis bars  54  pass through ports  80  in the tophat  64 , and extend out the back side  74  of the tophat  64 . A boss  72  is provided on the back surface  74  of the tophat. A retention element  76  in the boss supports the midpoint of the central span  66  of the clevis bar  54 , bowing it slightly away from the tophat  64 . The retention element  76  can be a machine screw threaded into the boss, and adjustably extended against the clevis bar by turning the screw with a wrench. The screw may have a head  78  with a shallow saddle (not shown) in which the bar rests. This locks the screw against loss of adjustment. Optionally, the wells  50  can be axially wide enough to allow the bar to pivot aside from the screw head  78  while pinned, so that the bar can be snapped on and off the saddle without readjustment. The screw/boss threads may be provided with frictional drag means as known in the art, to prevent loss of adjustment. The retention element can alternately be a compression spring filled with a damping material. The clevis bars are not limited to a “U” shape. They can be straight or other shapes. Conventional attachment means may alternately be used for attaching the bars to the supporting structure with or without a bias. For example, the bars can be attached directly to the tophat  64 , or directly to the support ring  34  if desired. 
       FIG. 8  shows a schematic view of pressure load boundary conditions and load paths for an example pin-loaded core ring segment  32  in an example engine environment. The differential pressure load is the pressure of the cooling air behind the shroud less the pressure of the engine combustion gases. The differential pressure load increases from ΔPmin (example 12.4 psi) to ΔPmax (example 58.4 psi) along the axial direction of the ring segment. As was noted previously, the pins  60  may be oriented in the circumferential direction so that they carry a uniform load along their length. The preload applied by the metal tophat  64  (not shown in FIG:  8 ) is additive to the pressure load. Bearing stress σ 1 , shear tear-out stress σ 2 , bending stress σ 3 , and corner stress σ 4  areas are indicated. At the corners there is minimal shear stress, and essentially zero tensile stress, since corners are not in a primary load path. Thus, the load path of the pin-loaded core ring segment  32  is favorable for ceramic materials. The primary paths for carrying the pressure loads are by compressive and shear loading of the ceramic core  42 . Since compressive strength of a refractory ceramic core is much greater than its tensile or shear strength, this design aligns the primary strength of the structure with the primary load path of the component. The shear area is quite large and initial calculations show that the shear strength is on the order of a factor of ten greater than the shear load resulting from the pressure stress. 
     To optimize the design, the bearing stress can be reduced by increasing the diameter of the pins  60 , thus increasing the contact area; the shear tear-out stress can be reduced by increasing the diameter of the pins, thus increasing the shear area; and/or the shear tear-out stress can be reduced by locating the bin bores  48  farther from the cold side  52  of the segment  32 , thus increasing the shear area. Bending stress on the segment  32  is reduced for two reasons. First, the loading pins  60  may be located at locations that minimize bending stress. Second, the structure is thick, and the CMC/core/CMC cross-section is quite strong in bending since the CMC  40  effectively carries the bending load as a primary membrane stress (either tensile or compressive) in the fiber direction. 
     Another advantage of the pin-loaded, CMC wrapped core structure is that it minimizes stress in areas that are particularly difficult to fabricate with CMC. CMC manufacturing development efforts have repeatedly shown that it is difficult to achieve good microstructure around a radius of curvature. The problems are related to the difficulties in compacting the fabric around a corner, and to sintering shrinkage anisotropy between the fiber and the matrix. The net result is that the as-manufactured CMC tends to have a level of delamination and void formation around the radius of curvature. This results in low interlaminar tensile and shear strength around sharp curves in a CMC structure. Prior attachment devices based on hooks, pins, or T-joints carry the pressure load as a shear load and a moment at the radius of curvature, which generate an interlaminar shear stress and an interlaminar tensile stress, respectively. In order for these attachment types to be viable, the CMC must possess sufficient interlaminar shear and interlaminar tensile strength to carry such pressure loads with sufficient margin. Even if the manufacturing difficulties were resolved, and the CMC microstructure were perfect, this is not a favorable load path for a 2D laminated CMC material 
     In comparison, the present pin-loaded core concept does not rely on CMC strength around a radius of curvature as a primary load path. First, there is minimal shear stress due to the small bending load. Second, since the core prevents an opening moment at the radius of curvature, there is essentially zero interiaminar tensile stress. Therefore, there is little driving force for delamination cracks to propagate, even if they exist in the as-manufactured CMC. An additional benefit of the pin-loaded core structure is that a continuously wrapped CMC structure may be used to minimize CMC free-edges, which reduces the likelihood of catastrophic delamination cracking, because delaminations are trapped. 
     The present segment structure is self-constrained against thermal deformations because of its large thickness and due to the complexity of the thermal gradients. There are both positive and negative aspects to the structure being self-constrained. On the negative side, it is unlikely that the structure can deform to relieve the thermal stress. Therefore, all thermal gradients manifest as a corresponding thermal stress, and sometimes these stresses can be quite high. The magnitude of the thermal stress state may be reduced to acceptable levels by the use of a lower stiffness and a highly strain tolerant core material as described in U.S. patent application publication 2004/0043889. On the positive side, it should be easier to control the gas path surface and tip clearances for a self-constrained structure. For structures that deform under a thermal gradient, the blade tip clearances must be set such that blade incursion does not occur at any temperature condition (hot or cold). Therefore, the blade tip clearance must be set according to the closest incursion point of the cycle. At other operating conditions the tip clearance would be greater than necessary. If the ring segment is self-constrained and does not deform, it is not necessary to account for deformations of the ring segment surface, and the blade-tip clearances can be decreased. It is well known that a decrease in blade tip clearance results in an increase in engine performance. 
     Another advantage of the embodiment described above is its resistance to pressure fluctuations (e.g., caused by a passing blade) and resistance to a blade strike. The resilience of this ring segment concept is related to two features. First, the large mass of the ring segment due to the solid core design will help the structure resist pressure fluctuations and/or impact events by acting as a highly damping material. Second, the ability to apply a significant preload to the structure may help the structure to resist pressure fluctuations and/or impact events. 
     The following summarizes some of the advantages of the ring segment described above.
         Optimized attachment locations and distributions reduce bending stress.   Favorable load path for a ceramic material. Pressure load carried by combination of core bearing stress and core/CMC shear stress.   Load path does not require good CMC properties around the CMC radius of curvature.   Pin support allows distributed contact load.   Pin support concept enables the use of a slotted spring pin to achieve a metal-to-metal contact surface at the pin.   Ring segment assembly is attached to the engine by metal hooks  82 ,  84 . There is a high level of confidence in using metal hooks for attachment to the engine (metal-to-metal contact surface).   Attachment hooks  82  on the tophat can be designed to match existing ring segment designs to enable retrofitting.   The metal tophat concept enables the use of a significant level of preload to the CMC ring segment to minimize high cycle fatigue effects driven by pressure fluctuations.   The self-constrained nature of the structure prevents gross deformations of the ring segment. A non-deforming structure allows reduction in the blade tip clearance, and a corresponding increase in engine performance.       

     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.