Thermal stress minimized, two component, turbine shroud seal
ORIGIN OF THE INVENTION 
     The invention described herein was made by an employee of the U.S. Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to turbomachines. The invention is particularly directed to an improved shroud seal for increasing engine thrust and efficiency. 
     The turbine shroud seal, as used in jet engines, provides an annular seal around the rotating turbine blades. An increase in insulating and sealing qualities provide more engine thrust while increasing engine efficiency. Both gains can be realized in lowering the operational cost of a typical gas turbine engine. The turbine shroud seal has evolved from an all metallic high temperature alloyed structure to a combination of the metallic alloy with ceramic materials. 
     The metallic alloy, when used alone, provided a failure proof design, but required large amounts of cooling air provided by the compressor to keep its temperature limitations from being exceeded. The cooling flow taken from the compressor then penalizes the overall engine performance. 
     The most recent advance in this type of seal has been to use ceramics and high temperature alloys in combination. Currently these materials are being used by plasma spraying the ceramic material onto the high temperature alloy base (substrate). The ceramic material is used on the combination gas flow side of the seal while the metallic alloy is used as the substrate and provides the attachment to the rest of the engine structure. This arrangement allows turbine gas temperatures to be increased and at the same time decreases the amount of cooling flow needed from the compressor. 
     The disadvantage of plasma sprayed ceramics on metallic alloy substrates is the resulting differences in the thermal expansion rates of the different materials used. These differences can cause large thermally induced stresses to exist in the plasma-sprayed ceramic layer. The thermal loading can cause the coating to spall or separate from the metallic alloy. Many attempts have been made to reduce the chance of spalling of the ceramic layer. Two of the possible methods of relieving stress is to use a strain isolation pad between the ceramic and the metallic alloy base or blending of metallic material with the ceramic as it is being applied in the plasma spray operation. 
     The strain isolation concept provides the required mechanical connection between two thermal expansion mis-matched materials. The composition of this pad is typically made from a high temperature alloy that is some percentage (typically 30%) of the totally dense alloy. The disadvantage of this particular method is that the alloy isolation pad is temperature sensitive just as the substrate alloy. The porous material has a substantially larger surface area per unit volume then dense alloy. This means that oxidation can take place at a much quicker pace than the dense alloy which leads to a shortened life of the isolation layer due to a decay in low carrying capacity. 
     The metallic graded plasma sprayed layer approach is a method whereby the metallic material is used in varying amounts in individually sprayed layers. Near the substrate the metallic weight percent is higher than the ceramic-metallic plasma sprayed material. As the layer is built up on the substrate the amount of metal added to the ceramic is reduced until the plasma-sprayed layer is all ceramic. The differences in thermal expansion are attenuated by this method, but differences in the layers themselves can still lead to a substantial amount of internal strain. 
     It is therefore the objective of the invention to provide an improved two-component seal that is thermally strained only by its own thermal gradient. 
     Another object of the invention is to provide a method for making the improved two-component seal. 
     Still another object of the invention is to provide a seal with a ceramic component and a metallic alloy component. 
     Yet another object of the invention is to provide a seal wherein the ceramic member is not constrained by rigid attachment to the substrate. 
     Another object of the invention is to provide a shroud seal that will appreciably increase engine efficiency and thrust. 
     BACKGROUND ART 
     U.S. Pat. No. 3,860,358 to Cavicchi et al is directed to a turbine blade tip seal wherein the radius of curvature of the sealing surface changes thereby minimizing blade tip clearance between cold engine conditions and design operating conditions. 
     In U.S. Pat. No. 3,986,720 to Knudson et al is directed to a turbine shroud structure composed of a material exhibiting a low coefficient of thermal expansion at low temperatures to provide large cold clearances at a high coefficient of thermal expansion at higher temperatures. 
     U.S. Pat. No. 3,690,785 to Lind is directed to a spring plate sealing system composed of members having different thermal expansion properties so as to provide sealing at high temperatures. 
     DISCLOSURE OF THE INVENTION 
     According to the present invention a two-component shroud seal has a ceramic member and a high temperature metallic alloy member. The members are independently fabricated and joined together by sliding the ceramic member onto the metallic alloy substrate. The resulting seal provides an increase in insulating and sealing qualities and correspondingly produces more engine thrust and efficiency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view illustrating a shroud seal in a turbine housing constructed in accordance with the present invention to be used in the annular seal around the turbine blades; 
     FIG. 2 is a sectional view of the two component shroud seal illustrating the ceramic and substrate components; 
     FIG. 3 is an enlarged sectional view illustrating the substrate-ceramic connection location; 
     FIG. 4 is a side view illustrating the cooling flow and hot (combustion) gas side of the shroud seal; 
     FIGS. 5a and 5b show a thermal stress contour for conventional graded layer and strain isolation seals at boundary conditions of T g  =1600° C. (3000° F.) and T c  =700° C. (1209° F.); 
     FIGS. 6a and 6b are a schematic view showing a thermal stress contour of the effect of hot gas boundary condition on the two-component seal of the present invention; and, 
     FIGS. 7a and 7b are a schematic view showing a thermal stress contour of the effect of pressure loading between the substrate and the ceramic material. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring now to the drawings there is shown in FIG. 1 a sectional view of the two component seal having a ceramic member 10, a metallic alloy substrate member 16 with a pair of L-shaped clamping rails 12 for attachment to the turbine housing 28 and a web 24 thereinbetween joining the clamping rails 12. The seal is shown in close proximity to the tips of turbine blades 30. Because there is no rigid attachment of the ceramic member 10 to the substrate member 16 the stress that the seal encounters in its operating environment is minimized. 
     Turning now to FIG. 2 there is shown a two component shroud seal having a ceramic component 10 connected to a high temperature alloy substrate component 16. The substrate 16 has a pair of clamping rails 12 for rigid connection to a turbine fan housing 28 as shown in FIG. 1. Further there is a anti-rotational pin 14 which limits the movement of the ceramic component 10 relative to the alloy substrate 16. The anti-rotational pin 14 is inserted substantially perpendicular through corresponding aligned openings 32,34 in the web 24 of the metallic alloy member 16 and the ceramic member 10 respectively. To accomodate the thermal expansion of the two component members, the annular opening 32 in the ceramic member 10 is slightly larger than the opening 34 in the metallic alloy member 16, and its depth is about 25% to 50% the ceramic member 10 thickness which may exceed 0.2 inches. An alternate embodiment could include a pin member 14 permanently attached to the substrate member 16 as by welding. 
     An enlarged view of the substrate 16--ceramic 10 connection showing the slot clearance 18 between the two components that exists at ambient conditions is shown in FIG. 3. The slot clearance 18 decreases to impose a clamping load at operating temperatures due to different thermal expansion coefficients of the two components. Accordingly the present invention does not have the constraint conditions that are imposed by rigidly fixing coatings on high temperature metallic substrates. Furthermore, the ceramic shroud segment 10 of this seal system would not be plasma sprayed but would be manufactured by an alternative ceramic forming operation. The segment 10 could be made in a near net shape that would minimize the amount of post-curing machining. Two of the possible methods that could be implemented include injection molding and hot isostatic pressing. These forming techniques and the proper control over ceramic composition facilitates the production of reliable components produced with the desired material properties. 
     The alloy substrate 16 or the structural component of this seal could be made from a high temperature alloy material such as an alloy known commercially as Hastelloy-X. At room temperature there is clearance 18 to facilitate assembling of the seal. The clearance 18 between the ceramic 10 and the high temperature alloy substrate 16 depends on the clamping load needed as well as the stress that can be tolerated. Until steady state conditions are reached the ceramic component 10 is held by a radial or antirotational pin 14 that prevents circumferential motion of the ceramic member 10. The arrangement for the two-component seal can be further simplified by having only one slot instead of two as in the preferred embodiment of this invention. 
     In FIG. 4 there is shown a side view of the shroud seal illustrating the hot gas or combustion side 20 and the cool flow side 22. During operation, hot gases flow axially across the seal surface as shown by the arrows. This then causes a thermal gradient to exist from the hot gas side 20 to the cooling flow side 22. The existence of the thermal gradient results in thermal stress in the ceramic component 10 as will be more fully described below. 
     Turning now to the thermal stress characteristics, a comparison between the finite element results of presently existing seal systems and the novel two-component seal of this invention is shown in FIGS. 5-6. In FIG. 5 is shown the thermal stress contour for graded layer and strain isolation seals at boundary conditions of T g  =1650° C. (3000° F.) and T c  =700° C. (1290° F.). Table I provides stress values (MPa) for both types of seal systems. 
     
                                           TABLE I                                 
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                Element stress values                                     
                           Contour values using nodal stress              
               &#34;x&#34;    &#34;o&#34;  Increment                                      
                                    Maximum                               
               Maximum                                                    
                     Minimun                                              
                           between contours                               
                                    tension contour &#34;o&#34;                   
Seal      Direction                                                       
               MPa (psi)                                                  
                     MPa (psi)                                            
                           MPa (psi)                                      
                                    MPa                                   
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Graded layer seal                                                         
          z    64.4  -57.6 7.2      52.7                                  
               (9340)                                                     
                     (-8360)                                              
                           (1040)   (7640)                                
          r     8.07 -27.7 2.8      6.1                                   
               (1170)                                                     
                     (-4020)                                              
                           (405)     (880)                                
Strain isolation seal                                                     
          z    27.7  -37.2 5.7      12.8                                  
               (1860)                                                     
                     -(5400)                                              
                           (825)    (4010)                                
          r     5.7   -6.5 0.7      3.9                                   
                (825)                                                     
                      (-940)                                              
                           (105)     (560)                                
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     In FIG. 6 the effect of hot gas boundary condition change is shown. The change in this boundary condition to the high temperature gas side 20 surface temperature causes the magnitude of the stress component to increase. Table II provides stress values MPa for the novel two-component seal of the present invention. 
     
                                           TABLE II                                
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             Element stress values                                        
                         Contour values using nodal stress                
             &#34;x&#34;   &#34;o&#34;   Increment                                        
                                  Maximum tension                         
Condition    Maximum                                                      
                   Minimum                                                
                         between contours                                 
                                  contour &#34;o&#34;                             
2-piece seal                                                              
        Direction                                                         
             Mpa (psi)                                                    
                   Mpa (psi)                                              
                         MPa (psi)                                        
                                  MPa (psi)                               
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T.sub.g = 1650° C.                                                 
        z    7.9     -3.96                                                
                         0.7      7.1                                     
(3000° F.)                                                         
             (1140)                                                       
                   (-575)                                                 
                         (102)    (1030)                                  
T.sub.c = 700° C.                                                  
        r    6.2   -2.1  0.57     2.9                                     
(1290° F.) (a)                                                     
             (895) (-300)                                                 
                         (83)     (420)                                   
T.sub.g = 1370° C.                                                 
        z    5.3   -3.7  0.7      4.6                                     
(2500° F.)                                                         
             (765) (-535)                                                 
                         (82)     (670)                                   
T.sub.c = 700° C.                                                  
        r    3.9   -1.5  0.36     2.3                                     
(1290° F.) (b)                                                     
             (570) (-215)                                                 
                         (52)     (330)                                   
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     The stress fields of the present invention as shown in FIG. 6a can be compared to the stress fields of the other two seal systems. Accordingly, the difference in magnitudes between the conventional seals shown in FIG. 5 and the novel two-component seal of the present invention as shown in FIG. 6 is apparent, with stresses in the two-piece seal being substantially lower than in the other two seal designs. 
     Furthermore, in FIG. 7, the effect of pressure loading between the substrate 16 and the ceramic 10 of FIG. 3 is shown. The test pressure load was applied such that the sum of the loads would not violate static equilibrium. The analysis shows that the slot&#39;s pressure increases the magnitude of the stress in the ceramic 10 of FIG. 3. In Table III, the effect of pressure loading on the two-component seal is shown. The magnitude of the stresses present due to the pressurized slot is still much lower than that from the other two seal concepts in the top ceramic surface near the high temperature side of the seal. 
     
                                           TABLE III                               
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             Element stress values                                        
                         Contour values using nodal stress                
             &#34;x&#34;   &#34;o&#34;   Increment                                        
                                  Maximum tension                         
             Maximum                                                      
                   Minimum                                                
                         between contours                                 
                                  contour &#34;o&#34;                             
Condition                                                                 
        Direction                                                         
             MPa (psi)                                                    
                   MPa (psi)                                              
                         MPa (psi)                                        
                                  MPa (psi)                               
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No pressure                                                               
        z    9.0   -4.5  1.0      8.3                                     
in slot (a)  (1305)                                                       
                   (-645)                                                 
                         (150)    (1200)                                  
        r    4.7   -1.9  0.58     1.9                                     
              (675)                                                       
                   (-280)                                                 
                          (84)     (270)                                  
Pressure                                                                  
        z    11.8  -5.1  1.05     9.5                                     
in slot (b)  (1715)                                                       
                   (-740)                                                 
                         (152)    (1380)                                  
P.sub.1 = 0.69 MPA                                                        
        r    6.4   -2.9  0.58     3.1                                     
P.sub.2 = 0.31 MPA                                                        
              (930)                                                       
                   (-420)                                                 
                          (84)     (950)                                  
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     Accordingly, the two-component seal has lower total stress due to thermal loading than the other two seal designs. This is due to the absence of rigid constraint put on the ceramic layer 10 of FIG. 3. 
     While the invention has been described with reference to certain preferred embodiments thereof, those skilled in the art will appreciate that various modifications, changes, omissions and substitutions may be made without departing from the spirit of the invention. It is intended, therefore, that the invention be limited only by the scope of the following.