Abstract:
A measuring device for measuring the temperature of a thermally loaded metallic base element, especially a gas turbine component, provided with a protective surface coating includes a sensor integrated in the base element. The sensor is arranged in a recess introduced into the base element from the outside. The protective surface coating is formed continuously over the sensor disposed in the recess.

Description:
[0001]     Priority is claimed to Swiss Patent Application No. CH 00428/06, filed on Mar. 20, 2006, the entire disclosure of which is incorporated by reference herein.  
         [0002]     The present invention relates to the field of thermal machines. It concerns a measuring device for measuring the temperature of a thermally loaded metallic base element, provided with a protective surface coating, according to the precharacterizing clause of claim  1  and a method for producing such a measuring device.  
       BACKGROUND  
       [0003]     Such a measuring device is known for example from the printed document U.S. Pat. No. B2-6,838,157.  
         [0004]     The use of integrated thermocouples for temperature measurement on gas turbine blades without a surface coating has long been known (see for example U.S. Pat. No. 3,592,061).  
         [0005]     The blades in the first stages of modern gas turbines, which are exposed to increasingly higher hot-gas temperatures to increase efficiency, are provided on the surface with a special protective surface coating (heat insulating layer or Thermal Barrier Coating TBC), which is intended to protect the underlying metal of the base element from excessively high temperatures. For normal operation (base load), the influence of the thermal barrier coating in the heat transfer models is well understood. In the part-load range, however, the effect of the thermal barrier coating decreases, since the boundary conditions for heat transfer change. Together with the lower pressures in the cooling air supply in the case of part-load, this can lead to a restriction of the service life of the blading.  
         [0006]     The existing instrumentation with thermocouples relates to blades without a thermal barrier coating, in order to eliminate the influence of the thickness and the thermal conductivity of the thermal barrier coating in the validation of the cooling in base-load operation. In order to permit validation also in part-load operation, technology with which temperature measurement under the thermal barrier coating can be realized (by means of thermocouples) is necessary.  
         [0007]     In the document mentioned at the beginning, it has already been proposed to arrange temperature sensors “buried” in gas turbine components provided with thermal barrier coatings (see for example the sensor 78 in FIG. 2 of U.S. Pat. No. B2-6,838,157), in that a trench is first made in the ready-coated component, the trench is then lined with an insulation, a conducting layer is subsequently deposited on the bottom of the trench, an insulating layer is applied thereover, and finally the trench is filled again to the surface of the thermal barrier coating ( FIG. 3 ).  
       SUMMARY OF THE INVENTION  
       [0008]     This known type of buried temperature sensor has various disadvantages:  
         [0009]     It is not suitable for thermocouples.  
         [0010]     When the trenches are created, the thermal barrier coating and the bond coat lying thereunder are locally broken through and are also not completely restored. This produces weak points, which impair the heat resistance and service life of the component.  
         [0011]     A comparatively great effort is required to make the trench penetrate through the surface coating to the metallic base element.  
         [0012]     It is an object of the invention to provide a temperature measuring device for thermally loaded components of gas turbines or the like that are provided with a thermal barrier coating which avoids one or more of the disadvantages of known forms of instrumentation and is distinguished by suitability for the use of thermocouples, simplified construction and installation, and improved long-term properties, and also to provide a method for its production.  
         [0013]     The arrangement according to the invention, in which the sensor is preferably a thermocouple, is characterized in particular in that the protective surface coating is formed continuously over the sensor located in the recess.  
         [0014]     A refinement of the invention is distinguished by the fact that the thermocouple in the recess is embedded in braze or modified wettable powder, and in that the braze or modified wettable powder fills the recess in such a way that the base element has a continuously planar surface in the region of the recess. This produces a uniform base for the surface coating lying thereover.  
         [0015]     Another refinement of the invention is characterized in that the protective surface coating comprises a metallic bond coat, preferably of MCrAlY, and in that the metallic bond coat is formed continuously over the sensor located in the recess. In particular, the metallic bond coat has a uniform thickness over the sensor located in the recess.  
         [0016]     According to a further refinement, the metallic bond coat comprises two partial layers lying one on top of the other, only the upper partial layer having a uniform thickness over the sensor located in the recess.  
         [0017]     The protective surface coating also preferably comprises a ceramic thermal barrier coating, which is arranged over the metallic bond coat and is formed continuously over the sensor located in the recess.  
         [0018]     A refinement of the one method according to the invention is characterized in that the recess is filled with braze or modified wettable powder, in that first a metallic bond coat, preferably of MCrAlY, and then a ceramic thermal barrier coating are applied as the protective surface coating. The metallic bond coat may in this case be applied in two partial layers one after the other.  
         [0019]     The metallic bond coat is preferably applied by means of atmospheric plasma spraying (APS) or low-vacuum plasma spraying (LVPS) or high-velocity oxyfuel spraying (HVOF), and the ceramic thermal barrier coating is applied by means of thermal spraying or electron-beam physical vapor deposition (EB-PVD).  
         [0020]     A refinement of the other method according to the invention is characterized in that the recess is filled with braze or modified wettable powder, and in that first a second metallic partial layer, preferably of MCrAlY, and then a ceramic thermal barrier coating are applied as the protective surface coating.  
         [0021]     The second metallic bond coat is preferably applied by means of atmospheric plasma spraying (APS) or low-vacuum plasma spraying (LVPS) or high-velocity oxyfuel spraying (HVOF), and the ceramic thermal barrier coating is preferably applied by means of thermal spraying or electron-beam physical vapor deposition (EB-PVD). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The invention is to be explained in more detail below on the basis of exemplary embodiments in conjunction with the drawings, in which:  
         [0023]      FIG. 1  shows a first exemplary embodiment of a measuring device according to the present invention in section;  
         [0024]      FIG. 2  shows a second exemplary embodiment of a measuring device according to the present invention in section;  
         [0025]      FIG. 3  shows a third exemplary embodiment of a measuring device according to the present invention in section; and  
         [0026]      FIG. 4  shows various steps in the production of a measuring device as shown in  FIG. 1  in several part-FIGS.  4 ( a ) to  4 ( d ). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     In  FIG. 1 , a first exemplary embodiment of a measuring device according to the invention is reproduced in section. The measuring device  10  is integrated in a metallic base element  11 , a detail of which is represented in the figure, and which may for example be the blade of a gas turbine. Set into the base element  11  from the outer surface is a recess  16 , which is adapted in depth and cross section to the sensor to be received, especially a thermocouple. Located in the recess  16  as the temperature sensor is a thermocouple  13 , which is surrounded on all sides by braze or modified wettable powder  12 . The braze or modified wettable powder  12  has the effect that the thermocouple  13  is thermally coupled closely to the base element  11  and at the same time fixed in the recess  16 . The braze or modified wettable powder  12  fills the recess  16  in such a way that the base element  11  has a continuously planar surface ( 17   FIG. 4   c ) in the region of the recess  16 . The planar surface of the base element is covered by a continuous metallic bond coat  14 , which preferably has the composition MCrAlY (M=Fe, Co or Ni). This bond coat  14  is suitable for imparting to a ceramic thermal barrier coating lying thereover the necessary bonding strength on the surface of the base element  11 .  
         [0028]     The production of such a measuring device  10  is performed in several steps according to  FIGS. 4   a - d.  This starts with the uncoated base element  11  ( FIG. 4   a ), into which a recess  16  is introduced ( FIG. 4   b ) from the surface (from the outside). The sensor or the thermocouple  13  is introduced into the recess  16 . The recess  16  with the sensor  13  located in it is then filled (with braze or modified wettable powder  12 ) in such a way that the base element  11  has a continuous surface  17  in the region of the recess  16  ( FIG. 4   c ). Finally, the metallic bond coat  14  is applied to the base element  11  in such a way that the region with the sensor  13  is covered ( FIG. 4   d ). The metallic bond coat  14  is in this case preferably applied by means of atmospheric plasma spraying (APS) or low-vacuum plasma spraying (LVPS) or high-velocity oxyfuel spraying (HVOF).  
         [0029]     A further exemplary embodiment of the measuring device according to the invention is reproduced in  FIG. 2 . The base element  11 , the recess  16 , its filling with braze or modified wettable powder  12 , and the thermocouple  13  of the measuring device  20  are the same as in  FIG. 1 . As a difference from  FIG. 1 , however, here a ceramic thermal barrier coating  15  is applied over the metallic bond coat  14  as a further layer. In order to permit improved bonding, the bond coat  14  is subdivided into two partial layers  14   a  and  14   b,  lying one on top of the other, the partial layer  14   a  being made smooth and the partial layer  14   b  being made rough. The production of this device can be performed in a way analogous to  FIG. 4 , the application of the thermal barrier coating  15 , preferably by means of thermal spraying or electron-beam physical vapor deposition (EB-PVD), being provided downstream as an additional step.  
         [0030]     A variant of the exemplary embodiment of  FIG. 2  is represented in  FIG. 3 . In the case of this measuring device  20 ′, the partial layer  14   a  is first applied to the uncoated base element  11 , before the recess  16  is then created (through the partial layer  14   a ). The filling of the recess  16  with braze or modified wettable powder  12  takes place up to the surface of the partial layer  14   a.  Then, the partial layer  14   b  is applied, and the thermal barrier coating  15  is applied thereover.  
         [0031]     Altogether, the invention provides a measuring device and a method for producing it that are comparatively simple, are suitable for thermocouples, and lead to an improved surface finish of the component. Furthermore, the actual temperature of the metal is sensed and there is no impairment of the service life of the component with respect to oxidation.