Abstract:
A method of forming a thermocouple ( 12 ), including: depositing a first material on a component ( 10 ) to form a first leg ( 14 ); depositing a second material through a mask ( 30 ) to form a pattern ( 50 ) on the component ( 10 ), the pattern ( 50 ) forming a plurality of discrete second leg junction ends ( 20 ) and a continuous patch ( 52 ) of the second material comprising indiscrete lead ends of the second legs ( 16 ), each second leg junction end ( 20 ) spanning from a respective junction ( 18 ) with the first leg ( 14 ) to the continuous patch ( 52 ); and laser-ablating the continuous patch ( 52 ) to form discrete lead ends ( 22 ) of the second legs ( 16 ), each lead end ( 22 ) electrically connected to a respective junction end ( 20 ), thereby forming discrete second legs ( 16 ).

Description:
[0001]    This application claims benefit of the 15 Mar. 2011 filing date of U.S. provisional patent application No. 61/452,735. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to thin film sensors on gas turbine engine components. More particularly, the invention relates to a method of forming thin film thermocouples on contoured hot gas path components. 
       BACKGROUND OF THE INVENTION 
       [0003]    Components of a gas turbine engine that are exposed to hot combustion gases are being operated closer to their design limits as gas turbine engine designs continue to increase the operating temperatures of the combustion gases. Operating closer to the design limit tends to reduce a life-span of the component when compared to a lifespan when operated in a cooler environment. Once a component reaches the end of its lifespan it must be replaced, and this necessitates a shutdown of the gas turbine engine. Such shut downs are expensive and time consuming. Without any means of determining how much of the component&#39;s lifespan may be left, component replacement often takes the form of preventative maintenance. As a result, a component may be removed prior to the end of its life. This is considered a better alternative to unintentionally leaving a component in operation beyond the end of its lifespan and risking a catastrophic failure of the component. Concurrently, there exists a desire to monitor the operating environment within the gas turbine engine to optimize turbine operation according to real time conditions. 
         [0004]    Smart components have been developed in response to the above needs. Smart components may include sensors built into the component itself, where the sensors gather information about the operating environment and transmit that information outside of the operating environment. That information may be used to enable condition based maintenance instead of preventative maintenance, and to optimize turbine operation as desired. 
         [0005]    Conventional smart component installation practice involves mounting sensors to the components, running lead wires to routers, and bringing large bundles of lead wires out of the turbine and over long distances to a monitoring station. The instrumentation process is slow, labor intensive, expensive, unreliable, and requires modification of many of the components in order to allow for the inclusion of all the lead wires. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention is explained in the following description in view of the drawings that show: 
           [0007]      FIG. 1  shows a contoured vane with a thin film thermocouple. 
           [0008]      FIG. 2  shows a mask used to apply the second material of a thermocouple. 
           [0009]      FIG. 3  shows the mask of  FIG. 2  applied to a surface of a component. 
           [0010]      FIG. 4  shows a pattern of material applied to the surface of the component of  FIG. 3 . 
           [0011]      FIG. 5  shown a plurality of thermocouples formed from the pattern of  FIG. 4 . 
           [0012]      FIG. 6  shows an alternate exemplary embodiment of a mask. 
           [0013]      FIG. 7  shows the pattern of material applied to the surface of the component including a calibration target using the mask of  FIG. 6 . 
           [0014]      FIG. 8  shows the pattern of  FIG. 7  with the calibration target electrically isolated. 
           [0015]      FIG. 9  shows an alternate exemplary embodiment of a mask. 
           [0016]      FIG. 10  shows the pattern of material applied to the surface of the component including a calibration target using the mask of  FIG. 9   
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    A recent innovation in thin film smart sensors includes thermocouples where a common first leg and a plurality of second legs, each forming a junction with the common first leg, define a thermocouple on a component having multiple junctions (which equivalently may be viewed as multiple thermocouples having a common leg). This is disclosed in a commonly assigned United States Patent Application Publication number US 2011/0222582 A1, which is incorporated by reference herein. Hot gas path components include but are not limited to blades, vanes, ring segments etc. Suitable first and second leg materials are dissimilar, able to withstand the operating conditions, and ideally include a coefficient of thermal expansion comparable to that of an adjacent surface, such as a surface onto which the materials are deposited. When on or within a thermal barrier coating of a gas turbine engine hot gas path component, materials from precious metal group can be selected. In an exemplary embodiment, metals from the platinum group metals may be selected. In an exemplary embodiment platinum and palladium may be selected as first and second materials respectively. These materials have suitable coefficients of thermal expansion, and are able to withstand the operating temperatures, which may range from 500° C. to 1500° C. 
         [0018]    The thermocouple materials may be sprayed directly onto a substrate, onto a bond coat, or onto a thermal barrier coating (TBC). When sprayed onto a substrate, the thermocouple may be covered by the bond coat. When sprayed onto the bond coat the thermocouple may be covered by the thermal barrier coating. When sprayed onto the TBC material, the thermocouple may lie on the exposed surface, or be covered with additional TBC material. Alternately, the thermocouple may be applied such that it is surrounded on one or both sides by a dielectric material. Suitable dielectrically insulating materials may include oxide materials. In an exemplary embodiment the oxide material may be aluminum oxide or yttrium aluminum garner (YAG) oxide. In an exemplary embodiment, the thermocouple may be applied at the superalloy/bond coat interface such that it is surrounded on both sides by the dielectric material. In an alternate exemplary embodiment, the thermocouple may be applied at the bond coat/TBC interface such that it is surrounded on both sides by the dielectric material. 
         [0019]    To form the thermocouple, the common first leg is applied through a first mask. To form the other legs, a devoted mask is applied per leg and material is applied through the devoted mask. For example, if there are four thermocouples, four second legs would be formed on the component, each forming a separate junction with the common first leg. Consequently, the steps of applying a devoted mask and applying the second material would need to be repeated four times to form the second legs. 
         [0020]    Gas turbine engine components are often contoured, and thus it is difficult to make rigid masks through which the material may be applied. As a result, flexible masks may be used. Further, in an exemplary embodiment the material may be applied through an air plasma spraying process (APS). Air from this process may catch edges of the mask and lift them. Such lifting may permit small amounts of material to deposit under the mask. Further, heat associated with the process may warp the thin mask, yielding the same result. In such instances, the physical characteristics of the deposited material, including width and thickness, may vary. Minor variations in the physical characteristics may yield variations in the electrical properties of the thermocouple. In particular, the resistance of a given thermocouple may vary from a target resistance as the width and thickness of the leg vary from their target dimensions. Resistance of the thermocouple influences output voltage, and that output voltage is used to estimate temperature. Consequently, using conventional manufacturing methods, these thin film smart sensors may require a certain amount of calibration. 
         [0021]    The present inventors have devised an innovative method for forming the second legs of the thermocouple that decreases manufacturing time and improves the manufacturing tolerance achievable of a manufactured thermocouple from a target resistance. Specifically, the method disclosed herein utilizes a first mask to apply a first material that forms the first leg. Then, a second mask is used to apply a pattern of second material from which all second legs are formed. Each second leg includes a junction end and a lead end. The junction end forms a junction with the first leg. The lead end is configured to be connected to an external lead. The legs are formed by a process that removes excess patch material, including but not limited to laser ablation. 
         [0022]    The pattern is configured such that discrete, junction ends of the plurality of second legs each form a junction with the first leg at respective locations, and these junction ends lead from the respective junctions to a patch of material that includes indiscrete lead ends of the second legs. The patch of material is a continuous patch from which the lead ends of the second legs will be formed. The continuous patch is then essentially a patch of material that covers a portion of the component that includes but is larger than where the lead ends of the second legs are to be. The method includes removing patch material from between where the lead ends are to be, thereby forming lead ends. Each lead end is electrically connected to a respective junction end. Since the junction ends are already discrete from each other, and the processes forms discrete lead ends connected thereto, the process forms a plurality of discrete second legs using only one mask, one application of second material, and a subsequent trimming operation. As a result it can be seen that the disclosed process is quicker. 
         [0023]    Similar to the other process, this process may use a thin, flexible mask. Consequently, when an APS process is used it may likewise lift the edges of the mask and deposit second material where none is intended to be. However, the process disclosed dispenses with any disadvantages associated with this because once the second legs are trimmed to the desired shape and size, any extraneous material left on the component is electrically isolated from the second legs, and thus its presence has no effect on the thermocouple. 
         [0024]      FIG. 1  shows an example of a contoured, gas turbine engine hot gas path vane  10  including an example thin film thermocouple  12 . The thermocouple  12  includes a common first leg  14 , a plurality of second legs  16 , and a plurality of junctions  18  where junction ends  20  the second legs  16  intersect the common first leg  14 . Lead ends  22  of the second legs  16  may be configured such that external leads (not shown) may be connected thereto. 
         [0025]      FIG. 2  shows a mask  30  with an opening  32 . The opening  32  will be configured such that it leaves a pattern of second material on the component such that the pattern covers the component at least where the second legs are to be, as well as some of the component between the second legs. 
         [0026]    As can be seen in  FIG. 3 , the pattern  30  is applied to a surface  40  of the component  10 . The common first leg  14  has already been applied to the surface  40 . The pattern  30  is positioned that tips  42  of each of the openings for the junction ends  20  expose a respective small portion  44  of the first leg  14 . The small portions  44  of the first leg  14  form part of the junctions  18 . 
         [0027]    A second material  46  is applied via a process  48 . In an exemplary embodiment the second material may be applied either in a powder or slurry form. APS deposition permits the application of powder material because APS deposition allows for tight control of the application, yet does not require a “booth” or spray “box” that may be too small for the components themselves to fit into. Other processes are limited by such booths, and therefore are unacceptable since the component often cannot fit into the restrictive booths. In APS deposition, the pattern may be built up from the application of a plurality of sublayers. Each pass of an APS process yields a sublayer of at least one micron thick. Multiple passes may build patterns to a thickness of, in an exemplary embodiment, ten microns or more. Such thickness may be compared with thicknesses of, for example, bond coat (150 micron) and thermal barrier coat (300 micron) thickness specifications. In another exemplary embodiment the pattern may be characterized by a thickness of approximately 50 to 75 microns. 
         [0028]    In  FIG. 4  the pattern  50  has been deposited and the mask  30  has been removed. The pattern includes a plurality of discrete, junction ends  20 , and a patch  52  from which the lead ends of the second legs will be formed. The first leg  14  is visible on the surface  40  of the component  10 , and the small portions  44  of the first leg  14  have been covered by tips  60  of the discrete junction ends  20 , which form junctions  18  at various respective locations on the component. These locations may be chosen so that during operation each will be in a region of temperature that is different than the temperature of other junctions, and this may provide a temperature profile for the component. 
         [0029]    The plurality of discrete junction ends  20  lead from respective junctions  18  to the common patch  52  of second material. The patch  52  leads to an interface location  54  where leads (not shown) may connect to the second legs  16 . Since there are four discrete junction ends  20  in the exemplary embodiment, in order to make four thermocouples, each with a junction end  20  and a lead end, there must be four lead ends. It can be seen that the patch  52  covers a greater area than would four lead ends, and so it is possible to trim excess patch material in order to form the four lead ends. Removal of excess material may be performed by an accurate ablation technique, such as laser ablation. In laser ablation, a laser beam  56  from a laser  58  would be directed at patch material to be removed from between desired lead ends  22  of the second legs  16 . 
         [0030]    In  FIG. 5  the laser ablation has been completed. The lead ends  22  of the second legs  16  have been formed by removing patch material from there between. Each lead end  22  joins with each junction end  20  to form a discrete thermocouple  16 . Using a laser to trim the lead ends allows for very tightly controlled edges  64  of the lead ends  22 . Any stray patch material that is unwanted may simply be removed by the laser. The laser is accurate enough to remove only the patch material if so desired, leaving the material onto which the patch is deposited unharmed by the removal process. Alternately, the laser may remove both the patch material and any material covering and/or below the patch material. This is useful in instance where the thermocouple may be located within a TBC, or between a bond coat and TBC, or directly on the substrate etc. Furthermore, the laser may be used to trim the junction ends  20 . As a result, the location of the second legs  16  on the component may be more tightly controlled, and the dimensions, in particular the width of the second legs  16 , may be more tightly controlled. 
         [0031]    More accurate positioning and width control allows for more accurate control of the electrical resistance of the thermocouple, and this greater control allows for thermocouple production to a tighter tolerance. A target loop resistance for a thermocouple, which is the resistance through the first and second legs of a given thermocouple, may be, for example, 2.2 ohms. However, conventional thermocouple manufacturing techniques may produce thermocouples with actual loop resistances of anywhere from 1.5 ohms to 3.3 ohms. This represents nearly a 30% deviation from the target loop resistance. It has been determined that using the process disclosed herein, actually loop resistance produced for the same target resistance range from approximately 1.9 ohms to 2.5 ohms. This represents a much reduced 15% deviation from the target loop resistance. In some instances the process produces actual loop resistances consistently within 10% of the target value. 
         [0032]    Another method for monitoring the temperature of the component within the gas turbine engine includes using a thermal imager with a field of view to observe the component during operation. Such a system may require a calibration target including material of a known emissivity value over the high temperature range of turbine operation. A suitable material includes, for example, palladium, and a calibration target of suitable properties may by produced by using an APS deposition process. Conventional manufacturing techniques include a separate step to form the target. However, this step can conveniently be incorporated into the method disclosed herein. 
         [0033]    As shown in  FIG. 6 , an alternate exemplary embodiment of the mask  70  includes an opening  72 . In this exemplary embodiment the opening  72  is enlarged when compared to the opening  32  of  FIG. 2 . The opening enlargement  74  is sufficient to yield a patch large enough to include both the lead ends  22  of the second legs  16  and a calibration target. In  FIG. 7  the mask  70  has been removed, the pattern has been deposited, and the lead ends  22  have been laser trimmed. Remaining are the first leg  14 , second legs  16 , and a calibration target  82  attached to a geometrically proximate second leg  84 . The calibration target  82  may remain electrically connected to the geometrically proximate second leg  84 , or it may be electrically isolated by laser trimming the calibration target  82  from the geometrically proximate second leg  84 , as shown in  FIG. 8 . 
         [0034]    In an alternate exemplary embodiment shown in  FIG. 9 , a mask  90  includes an opening  92  for the pattern and an additional opening  94  for the calibration target. In this exemplary embodiment the additional opening  94  is disposed proximate the tips  42  of each of the openings for the junction ends of the second legs where the junctions  18  will be formed on the component. In  FIG. 10  the mask  90  has been removed, the pattern has been deposited, and the lead ends  22  have been laser trimmed. Remaining are the first leg  14 , second legs  16 , junctions  18 , and a calibration target  102  geometrically proximate the junctions  18 . As shown the calibration target  102  is disposed on a first side  104  of the first leg  14  opposite a second side  106  of the first leg  14  where the second legs  16  are disposed. Being disposed proximate the junctions  18  will enable the calibration target and thermal imager to provide a second temperature reading for the component in the area proximate the junctions. This will serve to provide a more accurate temperature reading for that region of the component. Further, having temperature readings from both the thermocouples  12  and the calibration target  102  will provide redundancy, which can also be used to determine whether each system is working correctly. In all cases the calibration target is formed concurrently with the lead ends  22  of the second legs  16 . As a result the cost of the calibration target  82  approaches a negligible value as compared to when it is installed as a separate step. 
         [0035]    It has been shown that the method disclosed herein produces thin film thermocouples in a manner that reduces time and associated costs, improves the control of the thermocouple, and allows for the elimination of a separate step for forming a calibration target. 
         [0036]    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.