Abstract:
An electron beam generator having circuit interconnections between individual components that are less prone to the adverse effects of thermal cycling. The generator includes a conductor rod within a guide tube, a center conductor secured to one end of the rod, and an outer conductor secured to the adjacent end of the guide tube. An opposite end of the center conductor has an integrally-formed flange extending radially therefrom. A first tower is secured and electrically connected to the flange, while a second and adjacent tower is electrically connected to the outer conductor. A filament is mounted to and between the first and second towers. A forward leg of the filament circuit comprises the conductor rod, the center conductor, the flange and the first tower, and the return leg of the filament circuit comprises the second tower and the guide tube interconnected by the outer conductor.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to electron beam generators, particularly of the type used in electron beam physical vapor deposition apparatuses to deposit ceramic coatings. More particularly, this invention is directed to an electron beam generator that exhibits improved service life at high operating temperatures. 
     BACKGROUND OF THE INVENTION 
     Electron beam physical vapor deposition (EBPVD) is a well-known process for producing ceramic coatings, such as thermal barrier coatings (TBC) for the high-temperature components of gas turbine engines. Various ceramic materials have been used as TBC&#39;s, particularly zirconia (ZrO 2 ) stabilized by yttria (Y 2 O 3 ), magnesia (MgO) or other oxides. Advantageously, TBC&#39;s can be deposited by EBPVD to have a columnar grain structure that is able to expand with its underlying substrate without causing damaging stresses that lead to spallation, and therefore exhibits enhanced strain tolerance. 
     Processes for producing TBC by EBPVD generally entail heating a component to be coated to a temperature of about 1000° C. or more within a partially evacuated coating chamber. During coating, the component is supported above an ingot of the ceramic coating material (e.g., YSZ), and an electron beam generated by an electron beam (EB) gun is projected onto the ingot to melt the surface of the ingot and produce a vapor of the coating material. The vapor then travels upward toward the component and condenses on the component surface to form the desired coating. In order to melt ceramic materials such as YSZ, electron beam guns must be operated at a high voltage (e.g., 35 kV) and power level (e.g., 50 to 120 kW). The EB gun component that produces the electron beam is a beam generator. FIG. 1 represents a generator  110  that is commercially available from ALD Vacuum Technologies, Inc., of East Windsor Conn., USA. The generator  110  has a primary cathode (filament)  140  which produces an electron flux that heats a primary tungsten anode (block)  148  to about 2000° C. The block  148  then serves as a secondary cathode to an external secondary anode (not shown), by which the tungsten block  148  emits an electron beam due to a high voltage between it and the secondary anode. If any connection in the circuit containing the filament  140  becomes resistive due to oxidation, or mechanically opens due to thermal stress, or both, the beam generator  110  ceases to emit, stopping evaporation and terminating the coating process. 
     The filament circuit contains several bimetallic contacts in close proximity to the hottest section of the generator  110 . The two metals most widely used are copper and molybdenum, the former for its electrical and thermal conductivity and the latter for its high melting point and stability at high temperatures. For example, a conductor rod  112  that delivers current to the filament  140  is most often copper. A molybdenum ion catcher  128  has a first end  130  threaded into a bore  126  of the conductor rod  112 , by which a molybdenum spacer  124  is secured to the rod  112 . A first molybdenum filament tower  138  is then secured and electrically connected to the spacer  124  with a threaded stud  160  and copper nut  162 , which clamps a disk-shaped insulator  166  between the spacer  124  and tower  138 . A second molybdenum filament tower  139  is secured with a second stud and nut assembly  160 / 162  to the insulator  166 , between which is clamped a molybdenum mounting plate  164 . As such, both of the filament towers  138  and  139  are secured in place as a result of the spacer  124  being secured to the rod  112  with the ion catcher  128 . The rod  112 , spacer  124 , ion catcher  128  and filament tower  138  constitute a forward leg of the filament circuit. Because of their high temperature environment, the threaded connections can loosen and oxidize during operation due to differing expansion and heat conduction of the two metals. If the ion catcher  128  becomes loose and releases the spacer  124 , the filament circuit opens and the generator  110  cannot be restarted. 
     The filament tower  139 , a molybdenum cap  144 , the molybdenum mounting plate  164 , a copper fitting  142  and a copper guide tube  134  constitute the return leg of the filament circuit. The molybdenum cap  144  is threaded onto the copper fitting  142 , which is brazed or otherwise permanently attached to the copper guide tube  134  surrounding the conductor rod  112 . The cap  144  clamps the mounting plate  164  to the fitting  142  to complete the filament circuit between the tower  139  and the guide tube  134 . Consequently, if the cap  144  loosens, the filament circuit is open and the generator  110  ceases operating. The threads of the fitting  142  can distort at the high operating temperatures of the generator  110 . In addition, the threaded portion of the cap  144  may bell and crack during extended operation of the generator  110 . If the clamping action between the cap  144  and fitting  142  is lost, the filament circuit opens, again with the result that the generator  110  shuts down and cannot be restarted. 
     In view of the above, it can be appreciated that improved service life for an EB gun could be obtained if the reliability of the EB generator  110  mechanical connections could be improved. However, any change in the mechanical design of the generator  110  must be made carefully and tested with caution due to the very high operating voltages, power levels, amperage and operating temperatures involved. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is an electron beam generator of the type used in an EBPVD apparatus. A feature of the generator is that critical interconnections between individual components are made less prone to the adverse effects of thermal cycling. 
     The generator of this invention generally includes a conductor rod within a guide tube, generally as done in the prior art. However, the adjacent ends of both the conductor rod and guide tube are configured differently for purposes of the invention. The end of the conductor rod is modified to accept one end of a center conductor member. The opposite end of the center conductor member is formed to have an integrally-formed flange extending radially therefrom. An outer conductor member is secured to the adjacent end of the guide tube. A first tower is secured and electrically connected to the flange of the center conductor member, while a second tower is adjacent the first tower and electrically connected to the outer conductor member. A filament is mounted to and between the first and second towers, and a member is positioned adjacent to the filament for generating an electron beam when a sufficient current is applied to the filament via the conductor rod, the center conductor member, the flange and the first tower, which constitute a forward leg of the filament circuit. A return leg of the filament circuit includes the second tower and the guide tube, interconnected by the outer conductor member. 
     An important feature of the invention is the elimination of the discrete spacer  124  between the ion catcher  128  and the conductor rod  112  of the prior art generator  110  of FIG.  1 . Instead, the function of the spacer  124  is performed by the integral flange of the center conductor member, which serves as an intermediate connector between the first tower and the conductor rod. By eliminating the need for a discrete spacer and therefore the possibility of it loosening, the center conductor member is able to considerably reduce the possibility of an open circuit occurring between the ion catcher and the conductor rod as compared to the prior art generator  110 . 
     According to one aspect of the invention, at least one and preferably each of the center conductor member, first and second towers, outer conductor member and cap are formed of stainless steel, instead of the conventional molybdenum. As a result, the differences in coefficient of thermal expansion are less between the stainless steel components and the conventional copper components, as compared to that between the conventional molybdenum and copper components of the prior art. 
     According to another aspect of the invention, the second tower is preferably secured and electrically connected to the outer conductor member by a mounting member, which in turn is clamped to the outer conductor member. The mounting member is preferably clamped to the outer conductor member with a cap that is secured to the outer conductor member by a camming feature, instead of being secured with threads directly to the guide tube. As a result, another benefit of the invention is the reduced likelihood of an open filament circuit occurring as a result of thread distortion between the cap  144  and fitting  142  of the prior art generator  110  of FIG.  1 . 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional through an electron beam generator in accordance with the prior art. 
     FIG. 2 is a longitudinal sectional through an electron beam generator in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An electron beam generator  10  in accordance with this invention is represented in FIG.  2 . The generator  10  is of a type particularly suited for use in an EB gun used in an EBPVD apparatus to deposit a ceramic thermal barrier coating on a metal component intended for operation within a thermally hostile environment. Notable examples of such components include the high and low pressure turbine nozzles and blades, shrouds, combustor liners and augmentor hardware of gas turbine engines. While the advantages of this invention will be described with reference to depositing a ceramic coating, the teachings of this invention can be generally applied to the deposition by EBPVD of a variety of coating materials. For purposes of illustrating the invention, the generator  10  is shown in FIG. 2 as including components that are similar or identical to the components of the prior art generator  110  represented in FIG.  1 . Components common to each generator will be indicated, as well as those components unique to the present invention. 
     As with the generator  110  of FIG. 1, the generator  10  of this invention is configured to have primary and secondary cathodes. The primary cathode is a filament  40 , while the primary anode is a block  48  that also serves as a secondary cathode to an external secondary anode (not shown). A current of about 40 to 60 amps passes through the filament  40  during operation, and a potential of about 3 kV is established between the filament  40  and the block  48 . The potential between the block  48  and the external secondary anode is about 35 kV. When the block  48  is heated to a certain temperature by the electron flux from the filament  40 , the block  48  emits an electron beam of considerable power (on the order of hundreds of kilowatts) due to the potential between the block  48  and the external secondary anode. The electron beam is then contained, focused and steered by magnetic fields. 
     The electron beam generator  10  includes a conductor rod  12  disposed within an axial passage  36  of a guide tube  34 . Both the conductor rod  12  and guide tube  34  may be formed of copper, as conventional in the prior art. An important difference is that the rod  12  and tube  34  are both truncated as compared to the rod  112  and tube  134  of the prior art generator  110 . Specifically, the tube  34  does not include a permanent threaded copper fitting  142 , and the distal end  14  of the conductor rod  12  terminates well short of the end of the tube  34 . As will become apparent from the above and a comparison of FIGS. 1 and 2, an advantage of the present invention is the possibility of fabricating the conductor rod  12  and tube  34  of the generator  10  from a rod  112  and tube  134  that were salvaged from an otherwise scrapped prior art generator  110 . 
     A threaded bore  16  is formed in the distal end  14  of the rod  12 , which receives a threaded end  20  of a generally cylindrically-shaped center conductor member  18 . A suitable material for the center conductor member  18  is a stainless steel, such as Type  303 , though it is foreseeable that other high-temperature materials could be used. The end  20  of the center conductor member  18  is shown as being threaded into the bore  16  and then further secured with a pin  74 . A hole  54  is formed in the rod  12  to allow outgassing of the bore  16  as the generator  10  is evacuated and the temperature of the generator  10  increases during startup. 
     The opposite end  22  of the center conductor member  18  has an integrally-formed flange  24  that extends in a radial direction from the conductor  18 . Notably, the flange  24  eliminates the separate molybdenum spacer  124  required by the prior art generator  110  of FIG. 1. A threaded bore  26  is formed in the end  22  of the conductor  18 , into which is threaded one end  30  of an ion catcher  28 . The opposite end of the ion catcher  28  is formed to have a tapered recess  32 , which serves to capture ions returning to the generator  10  via the electron beam. Because the ion catcher  28  is separated from the copper rod  12  by the center conductor member  18 , the ion catcher  28  can be conventionally formed of molybdenum without concern for the CTE mismatch between molybdenum and copper. As with the conductor rod  12 , an outgassing hole  56  is provided in the center electrode  18  to allow outgassing between the center conductor member  18  and the ion catcher  28  during pumpdown and start up of the generator  10 . 
     A first filament tower  38  and an insulator plate  66  are secured to the flange  24  of the center conductor member  18  with a stud  60  and nut  62 . The insulator plate  66  can be seen in FIG. 2 as having a center portion disposed between the ion catcher  28  and the center conductor member  18 , and a lateral portion that extends in an opposite direction to the flange  24 . A second filament tower  39  is secured to the lateral portion of the insulator plate  66 , between which is sandwiched a mounting plate  64 . The mounting plate  64  is shown in FIG. 2 as having an opening  68  through which the ion catcher  28  and the first filament tower  38  are attached directly to the insulator plate  66  so as not to contact the mounting plate  64 . Together, the mounting plate  64  and the insulator plate  66  serve as a rigid base for the second tower  39 , while the insulator plate  66  further provides electrical insulation between the first tower  38  and the center conductor member  18 , its flange  24 , and an outer conductor member  42  (described below). A suitable material for the mounting plate  64  is molybdenum, though it is foreseeable that a stainless steel can be used. A suitable material for the insulator plate  66  is alumina (Al 2 O 3 ). The towers  38  and  39  are positioned to surround the end of the ion catcher  28  in which the tapered recess  32  is formed. As with the center conductor member  18 , a suitable material for the filament towers  38  and  39  is a stainless steel, such as Type  303 , though it is foreseeable that other high-temperature materials could be used. Furthermore, because of their critical function and their placement in the hottest part of the generator  10 , molybdenum may be preferred as the material for the towers  38  and  39  in some coating applications. The studs  60  are preferably stainless steel for compatibility with the towers  38  and  39 , while the nuts  62  may be copper or a stainless steel. 
     The filament circuit is completed with the filament  40 , each end of which is secured to one of the towers  38  and  39 . As is conventional, the filament  40  is axially aligned with the ion catcher  28 , and is preferably formed of tungsten. Aside from the copper conductor rod  12  and the tungsten filament  40 , the forward leg of the filament circuit is primarily formed by the steel center conductor member  18 , steel flange  24 , steel studs  60  and the steel filament tower  38 , such that there is a significantly reduced concern for an open circuit caused by thermally-induced stresses and distortions. Even if the molybdenum ion catcher  28  were to become loose, the generator  10  would continue to function normally. 
     The first component in the return leg of the filament circuit is the second filament tower  39 , which is electrically connected to the outer conductor member  42  through the mounting plate  64 . The mounting plate  64  is clamped to the outer conductor member. 42  with a cap  44  that surrounds the filament towers  38  and  39  and has an axial opening  46  in which a distal portion of the filament  40  is received. The outer conductor member  42  is shown as surrounding the end  22  of the center conductor member  18  with the flange  24 , and being threaded onto the guide tube  34  and secured with a pin  72 . As with the center conductor member  18 , flange  24 , studs  60  and towers  38  and  39 , suitable materials for the cap  44  and the outer conductor member  42  are stainless steels, such as Type  303 , though it is foreseeable that other high-temperature materials could be used. The return leg of the filament circuit is thus primarily formed by the second steel tower  39 , the mounting plate  64  and the steel outer conductor member  42 , against which the mounting plate  64  is clamped by the steel cap  44  such that there is a significantly reduced concern for an open circuit caused by thermally-induced stresses and distortions. 
     FIG. 2 shows the cap  44  as being secured to the outer conductor member  42  with camming features (one of which is shown), each of which includes a pin  50  that extends radially outward from the outer conductor member  42  and engages a complementary camming slot  52  formed in the cap  44 . Three sets of pins  50  and slots  52  are preferred, though it is foreseeable that fewer or more pin/slot sets could be used. The camming pins  50  and slots  52  replace the threads required by the cap  144  and fitting  142  of the prior art generator  110  shown in FIG.  1 . The end of the outer conductor member  42  opposite the cap  44  is directly secured with threads and pins  72  to the guide tube  34 . The outer conductor member  42  is preferably equipped with several vents  55  (one of which is shown in FIG. 2) for outgassing a cavity  57  defined between the outer conductor member  42  and the center conductor member  18 . A cavity present between the fitting  142  and guide tube  134  of the prior art generator  110  is eliminated with the generator  10  by nesting an insulator  70  with the end of the outer conductor member  42  threaded onto the guide tube  34 . 
     The final component of the generator  10  is the block  48  positioned adjacent the filament  40  by a suitable support frame  49 . The block  48  shown in FIG. 2 is of a conventional type formed of tungsten, and serves as the anode to the primary cathode filament  40 , as well as the secondary cathode to the external secondary anode (not shown). In the latter role, the block  48  is heated to a temperature of about 2000° C. or more by the electron flux from the filament  40 , causing the block  48  to emit an electron beam due to the potential between the block  48  and the external secondary anode. For a coating operation in which the generator  10  is used to deposit a ceramic coating on one or more parts, the EB gun in which the generator  10  is installed is used to focus the beam on an ingot of the desired coating material. Heating of the ingot forms a molten pool of the ingot material and, with further heating, vapors that deposit on the parts. The coating operation continues until the desired thickness for the coating is obtained for the particular parts in question. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.