Abstract:
An electron beam physical vapor deposition (EBPVD) apparatus and a method for using the apparatus to produce a coating material (e.g., a ceramic thermal barrier coating) on an article. The EBPVD apparatus generally includes a coating chamber that is operable at elevated temperatures and subatmospheric pressures. An electron beam gun projects an electron beam into the coating chamber and onto a coating material within the chamber, causing the coating material to melt and evaporate. An article is supported within the coating chamber so that vapors of the coating material deposit on the article. The operation of the EBPVD apparatus is enhanced by the inclusion or adaptation of one or more mechanical and/or process modifications, including those necessary or beneficial when operating the apparatus at coating pressures above 0.010 mbar.

Description:
[0001]    This application claims benefit of Provisional Patent Application No. 60/147,236, filed Aug. 4, 1999, which is hereby incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention generally relates to an electron beam physical vapor deposition coating apparatus. More particularly, this invention is directed to such a coating apparatus adapted to deposit ceramic coatings on components, such as thermal barrier coatings on superalloy components of gas turbine engines.  
         BACKGROUND OF THE INVENTION  
         [0003]    Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. While significant advances have been achieved with iron, nickel and cobalt-base superalloys, the high-temperature capabilities of these alloys alone are often inadequate for components located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBC) formed on the exposed surfaces of high temperature components have found wide use.  
           [0004]    To be effective, thermal barrier coatings must have low thermal conductivity and adhere well to the component surface. Various ceramic materials have been employed as the TBC, particularly zirconia (ZrO 2 ) stabilized by yttria (Y 2 O 3 ), magnesia (MgO) or other oxides. These particular materials are widely employed in the art because they can be readily deposited by plasma spray and vapor deposition techniques. An example of the latter is electron beam physical vapor deposition (EBPVD), which produces a thermal barrier coating having 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. Adhesion of the TBC to the component is often further enhanced by the presence of a metallic bond coat, such as a diffusion aluminide or an oxidation-resistant alloy such as MCrAlY, where M is iron, cobalt and/or nickel.  
           [0005]    Processes for producing TBC by EBPVD generally entail preheating a component to an acceptable coating temperature, and then inserting the component into a heated coating chamber maintained at a pressure of about 0.005 mbar. Higher pressures are avoided because control of the electron beam is more difficult at pressures above about 0.005 mbar, with erratic operation being reported at coating chamber pressures above 0.010 mbar. It has also been believed that the life of the electron beam gun filament would be reduced or the gun contaminated if operated at pressures above 0.005 mbar. The component is supported in proximity to an ingot of the ceramic coating material (e.g., YSZ), and an electron beam is projected onto the ingot so as to melt the surface of the ingot and produce a vapor of the coating material that deposits onto the component.  
           [0006]    The temperature range within which EBPVD processes can be performed depends in part on the compositions of the component and the coating material. A minimum process temperature is generally established to ensure the coating material will suitably evaporate and deposit on the component, while a maximum process temperature is generally established to avoid microstructural damage to the article. Throughout the deposition process, the temperature within the coating chamber continues to rise as a result of the electron beam and the presence of a molten pool of the coating material. As a result, EBPVD coating processes are often initiated near the targeted minimum process temperature and then terminated when the coating chamber nears the maximum process temperature, at which time the coating chamber is cooled and cleaned to remove coating material that has deposited on the interior walls of the coating chamber. Advanced EBPVD apparatuses permit removal of coated components from the coating chamber and replacement with preheated uncoated components without shutting down the apparatus, so that a continuous operation is achieved. The continuous operation of the apparatus during this time can be termed a “campaign,” with greater numbers of components successfully coated during the campaign corresponding to greater processing and economic efficiencies.  
           [0007]    In view of the above, there is considerable motivation to increase the number of components that can be coated within a single campaign, reduce the amount of time required to introduce and remove components from the coating chamber, and reduce the amount of time required to perform maintenance on the apparatus between campaigns. However, limitations of the prior art are often the result of the relatively narrow range of acceptable coating temperatures, the complexity of moving extremely hot components into and out of the coating chamber, and the difficulties confronted when maintaining an advanced EBPVD apparatus. Accordingly, improved EBPVD apparatuses and processes are continuously being sought for depositing coatings, and particularly ceramic coatings such as TBCs.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    The present invention is an electron beam physical vapor deposition (EBPVD) apparatus and a method for using the apparatus to produce a coating (e.g., a ceramic thermal barrier coating) on an article. The EBPVD apparatus of this invention generally includes a coating chamber that is operable at an elevated temperature (e.g., at least 800° C.) and a subatmospheric pressure (e.g., between 10 −3  mbar and 5×10 −2  mbar). An electron beam gun is used to project an electron beam into the coating chamber and onto a coating material within the chamber. The electron beam gun is operated to melt and evaporate the coating material. Also included is a device for supporting an article within the coating chamber so that vapors of the coating material can deposit on the article.  
           [0009]    According to the present invention, the operation of the EBPVD apparatus can be enhanced by the inclusion or adaptation of one or more features and/or process modifications. According to one aspect of the invention relating to process temperature control, the coating chamber contains radiation reflectors that can be moved within the coating chamber to increase and decrease the amount of reflective heating that the article receives from the molten coating material during a coating campaign. Process pressure control is also an aspect of the invention, by which processing pressures of greater than 0.010 mbar can be practiced in accordance with copending U.S. patent application Ser. No. 09/108,201 to Rigney et al. (assigned to the same assignee as the present invention) with minimal or no adverse effects on the operation and reliability of the electron beam gun, and with minimal fluctuations in process pressures. Mechanical and process improvements directed to this aspect of the invention include modifications to the electron beam gun, the coating chamber, and the manner by which gases are introduced and removed from the apparatus. Also improved by this invention is the electron beam pattern on the coating material.  
           [0010]    According to another preferred aspect of the invention, a crucible is employed to support the coating material within the coating chamber. The crucible preferably comprising at least two members, a first of which surrounds and retains a molten pool of the coating material, while the second member is secured to the first member and surrounds an unmolten portion of the coating material. The first and second members define an annular-shaped cooling passage therebetween that is closely adjacent the molten pool, so that efficient cooling of the crucible can be achieved, reducing the rate at which the process temperature increases within the coating chamber.  
           [0011]    Another preferred aspect of the invention entails a rotatable magazine that supports multiple, ingots of the coating material beneath the coating chamber. The magazine is indexed to individually align multiple stacks of one or more ingots with an aperture to the coating chamber for sequentially feeding the ingots into the coating chamber without interrupting deposition of the coating material.  
           [0012]    According to another preferred aspect of the invention, a viewport is provided for viewing the molten coating material within the coating chamber. In order to be capable of providing a view of the extremely high-temperature process occurring within the coating chamber, the viewport is fluid-cooled and has a high rotational speed stroboscopic drum and a magnetic particle seal that provides a high-temperature vacuum seal for the stroboscopic drum. Another preferred aspect is that the viewport provides a stereoscopic view of the coating chamber, by which one or more operators can simultaneously observe the coating chamber while retaining stereoscopic vision.  
           [0013]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIGS. 1 and 2 are schematic top and front views, respectively, of an electron beam physical vapor deposition apparatus used to deposit a coating material in accordance with this invention.  
         [0015]    [0015]FIGS. 3, 4 and  5  are cross-sectional views taken along section line  3 -- 3  of FIG. 1, and showing a movable platform employed in accordance with one aspect of this invention.  
         [0016]    [0016]FIGS. 6 and 7 are more detailed front and top cross-sectional views, respectively, of preferred interior components for a coating chamber of the apparatus of FIGS. 1 and 2.  
         [0017]    [0017]FIGS. 8 and 9 compare an EB gun orifice of the prior art and an orifice configured in accordance with the preferred embodiment of this invention.  
         [0018]    [0018]FIG. 10 is a cross-sectional view of a crucible housing an ingot of coating material and an electron beam projected onto the surfaces of the crucible and ingot in accordance with the preferred embodiment of this invention.  
         [0019]    [0019]FIG. 11 is a plan view of the crucible of FIG. 10 and a preferred pattern for the electron beam on the crucible and ingot.  
         [0020]    [0020]FIG. 12 depicts a preferred power intensity distribution of the electron beam pattern across the surface of the ingot and crucible of FIGS. 10 and 11.  
         [0021]    [0021]FIG. 13 shows a preferred viewport for observing the process within the coating chamber of the apparatus shown in FIGS. 1 and 2.  
         [0022]    [0022]FIG. 14 shows a control panel for monitoring and controlling the operation of the apparatus of FIGS. 1 and 2. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    An EBPVD apparatus  10  in accordance with this invention is generally depicted in FIGS. 1 and 2, with various components and features being depicted in FIGS. 3 through 14. The apparatus  10  is particularly well suited for depositing 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 parts and augmentor hardware of gas turbine engines. While the advantages of this invention will be described with reference to depositing a ceramic coating on such components, the teachings of this invention can be generally applied to a variety of coating materials and components.  
         [0024]    For purposes of illustrating the invention, the EBPVD apparatus  10  is shown in FIGS. 1 and 2 as including a coating chamber  12 , a pair of preheat chambers  14 , and two pairs of loading chambers  16  and  18 , so that the apparatus  10  has a symmetrical configuration. The front loading chambers  16  are shown as being aligned with their respective preheat chambers  14 , with parts  20  originally loaded on a rake  22  within the lefthand chamber  16  having been transferred to the preheat chamber  14  and, as depicted in FIG. 1, into the coating chamber  12 . With the symmetrical configuration of the apparatus  10 , while the parts  20  loaded through the front lefthand loading chamber  16  are being coated within the coating chamber  12 , a second batch of parts in the front righthand loading chamber  16  can be preheated in the righthand preheat chamber  14 , a third batch of parts can be loaded into the rear lefthand loading chamber  18 , and a fourth batch of parts can be unloaded from the rear righthand loading chamber  18 . Consequently, four process stages can occur simultaneously with the preferred EBPVD apparatus  10  of this invention.  
         [0025]    According to a preferred embodiment of this invention, the loading chambers  16  and  18  are mounted to low-profile movable platforms  24 , so that the loading chambers  16  and  18  can be selectively aligned with their preheat chambers  14 . For example, when the front lefthand loading chamber  16  is brought into alignment with the lefthand preheat chamber  14  to allow the parts  20  to be inserted into the coating chamber  12 , the rear lefthand loading chamber  18  is set back from the lefthand preheat chamber  14 , so that parts can be simultaneously loaded or unloaded from the rake  22  of the rear lefthand loading chamber  18 . Each platform  24  is also preferably movable to a maintenance position, in which neither of its loading chambers  16  and  18  is aligned with its preheat chamber  14 , so that the interiors of the preheat and loading chambers  14 ,  16  and  18  can be accessed for cleaning. The platforms  24  are preferably supported at least in part by roller bearings  44  mounted in the floor, though it is foreseeable that a variety of bearings could be used. Each platform  24  has a low elevational profile (projection above the floor) of not more than one inch (about 2.5 cm) with a chamfered edge (preferably 30 degrees from horizontal), which together essentially eliminate the potential for an operator tripping on the edge of the platform  24 . Stationary objects surrounding the apparatus  10  are preferably positioned away from the edges of the platforms  24  to avoid an operator being pinched by a platform  24  when it is repositioned. As alternatives to the platform configuration shown, platform systems with multiple overlapping or telescoping movable segments could be used. Furthermore, the movable segments could slip beneath a fixed elevated platform surrounding the platform assemblies. Finally, separate preheat chambers could be provided for the loading chambers  16  and  18 , so that both loading chambers  16  and  18  and their heating chambers would be surrounded by a movable platform system.  
         [0026]    As shown in FIGS. 3 through 5, a portion of the coating chamber  12  is also preferably configured to move relative to the preheat chamber  14  in order to facilitate cleaning of the interior of the chamber  12  between coating campaigns. As seen in FIG. 3, the coating chamber  12  is in its operating position with a viewport  48 , described in greater detail below, mounted to a front section of the chamber  12 . In FIG. 4, the front section of the coating chamber  12  (as well as an ingot magazine  102  associated with the coating chamber  12  and discussed below) is shown as having been moved away from the remainder of coating chamber  12  in order to access a movable work platform  50 , which is shown rotated into a working position in FIG. 5. In this position, the interior of the coating chamber  12  can be easily accessed by the work platform  50 . The platform  50  is shown as being coupled with a hinge  52  to the base of the coating chamber  12 , though it is foreseeable that other acceptable structures could be employed. The platform  50  can be configured differently from that shown in FIGS. 3 through 5, including a hinged segmented construction, and with kick plates and other safety-related accessories.  
         [0027]    The coating, preheat and loading chambers  12 ,  14 ,  16  and  18  are connected by valves (not shown) that achieve a vacuum seal between these chambers. To maximize the size and number of parts  20  that can be loaded between the chambers  12 ,  14 ,  16  and  18 , the valves preferably have a minimum dimension of about 250 mm, which is considerably larger than previously thought practical by those skilled in the art. Because the coating, preheat and loading chambers  12 ,  14 ,  16  and  18  must be pumped to varying levels of vacuum, and in some cases are required to move relative to each other as explained above, the valves must be capable of numerous cycles at relatively high pressures. Seal designs suitable for this purpose are known in the art, and therefore will not be discussed in any detail.  
         [0028]    With reference to FIGS. 6 and 7, coating is performed within the coating chamber  12  by melting and evaporating ingots  26  of ceramic material with electron beams  28  produced by electron beam (EB) guns  30  and focused on the ingots  26 . Intense heating of the ceramic material by the electron beams  28  causes the surface of each ingot  26  to melt, forming molten ceramic pools from which molecules of the ceramic material evaporate, travel upwardly, and then deposit on the surfaces of the parts  20 , producing the desired ceramic coating whose thickness will depend on the duration of the coating process. While two ingots  26  are shown in these Figures, it is within the scope of this invention that one or more ingots  26  could be present and evaporated at any given time.  
         [0029]    EBPVD coating chambers are typically capable of being maintained at a vacuum level of about 0.001 mbar (about 1×10 −3  Torr) or less. In the prior art, a vacuum of at most 0.010 mbar, and more typically about 0.005 mbar, would be drawn within the coating chamber  12  during the coating process, the reason being that higher pressures were known to cause erratic operation of the EB guns  30  and make the electron beams  28  difficult to control, with the presumption that inferior coatings would result. It has also been believed that the life of the gun filament would be reduced or the gun contaminated if operated at coating chamber pressures above 0.005 mbar. However, in accordance with copending U.S. patent application Ser. No. 09/108,201 to Rigney et al., assigned to the same assignee as this invention, the coating chamber  12  is preferably operated at higher pressures that surprisingly yield a ceramic coating with improved spallation and impact resistance, as well as promote the coating deposition rate in conjunction with higher ingot evaporation rates than that achieved in the prior art.  
         [0030]    Rough pumpdown can be performed in the coating, preheat and loading chambers  12 ,  14 ,  16  and  18  with, mechanical pumps  31 . A cryogenic pump  32  of a type known in the art is shown in FIGS. 1 and 2 as being employed to aid in the evacuation of the coating chamber  12  prior to the deposition process. Also shown in FIGS. 1, 3,  4  and  5  is a diffusion pump  34  whose operation is similar to those known in the art, but modified with a throttle valve  36  to regulate the operation of the pump  34  in accordance with this invention. More particularly, the throttle valve  36  is actuated between an open position (FIG. 3) and a closed position (FIGS. 4 and 5) as well as positions therebetween. The benefit of the throttle valve  36  is realized when the vacuum within the coating chamber  12  is maintained at the relatively high pressures employed by this invention. When the maximum operating capacity of the diffusion pump  34  is required to evacuate the coating chamber  12 , the throttle valve  36  is open as shown in FIG. 3. For processing hardware, the coating chamber  12  must be maintained at the targeted pressure (e.g., 0.015 mbar), necessitating that the throttle valve  36  is moved to a preset throttled position some distance from the fully closed position of FIGS. 4 and 5. As seen in FIG. 1, separate diffusion pumps  38  similarly equipped with throttle valves (not shown) are preferably employed to evacuate the preheat chambers  14 , again for the reason that a relatively high pressure is desired for the coating operation of this invention. The mechanical pumps  31  preferably include leak detector connections  33  to which a leak detector can be connected for detecting a system vacuum leak using helium or another gas that can be safely introduced through leaks in the chambers  12 ,  14 ,  16  and  18 , or associated equipment.  
         [0031]    With reference again to FIGS. 1 and 2, the loading chambers  16  and  18  are generally elongated in shape, and are equipped with loading doors  40  through which parts are loaded onto the rakes  22 . The loading chambers  16  and  18  are also equipped with access doors  42  to motion drives (schematically represented at  46  in FIG. 1) that control the operation of the rakes  22 . More particularly, the parts  20  supported on the rakes  22  are preferably rotated and/or oscillated within the coating chamber  12  in order to promote the desired coating distribution around the parts  20 . The access doors  42  allow the operator of the apparatus  10  to quickly adjust or change the settings of the motion drives  46  without interfering with loading and unloading of parts from the loading chambers  16  and  18 .  
         [0032]    Referring again to FIGS. 6 and 7, the interior of the coating chamber  12  will be described in more detail. In order to address the aforementioned problems concerning the control of the electron beams  28  and protection of the EB guns  30  at the higher coating pressures employed by this invention, certain improvements were made to the EB guns  30  and the coating chamber  12 . As seen in FIG. 6, oxygen and argon gases are introduced into the coating chamber  12  through an inlet  54  located near crucibles  56  that support the ingots  26  within the coating chamber  12  and retain the molten pools of ceramic material produced by the electron beams  28 . The flow rates of oxygen and argon are individually controlled based on the targeted process pressure and the targeted partial pressure of oxygen. To reduce the occurrence of pressure oscillations within the coating chamber  12 , the control loop response time for these gases was reduced by physically placing the control valves  58  for the gases immediately adjacent to the inlet  54  just outside the coating chamber  12 , as shown in FIGS. 1 and 6. Placement of the control valves  58  so close to the coating chamber  12  provided a surprisingly significant improvement in pressure control, reducing pressure fluctuations within the coating chamber  12  and reducing disturbances in the focus and position of the electron beams  28  on the ingots  26 .  
         [0033]    To further improve the electron beam focus and pattern, the EB guns  30  are relatively isolated from the higher coating pressure within the coating chamber  12  by a condensate hood  52  that catches most of the superfluous ceramic vapors that do not deposit onto the parts  20 . The hood  52  is configured according to this invention to define a coating region around the parts  20 , within which the elevated pressure desired for the coating process is specifically maintained. To facilitate cleaning between coating campaigns, the hood  52  is preferably equipped with screens  76  that can be removed and cleaned outside of the coating chamber  12 . Preferably, the screens  76  are retained by spring pins  78  instead of threaded fasteners in order to simplify removal of the screens  76  when in the condition of having been coated with a layer of the coating material by the end of a campaign. Though generally more complicated, the entire condensate hood  52  could be removed and replaced with a second clean hood  52 .  
         [0034]    Because the hood  52  surrounds the parts  20  as seen in FIG. 6, an aperture  62  is necessary for each beam  28  through the hood  52 . To promote the capability of maintaining higher pressures within the condensate hood  52  as compared to the remainder of the coating chamber  12 , including the vicinity around the EB guns  30 , the apertures  62  are preferably formed to have dimensions of not more than that necessary to allow the electron beams  28  to pass through the hood  52 . For this purpose, the apertures  62  are preferably cut with the electron beams  28  during the setup of the EBPVD apparatus  10 , so that each aperture  62  has a cross-sectional area that is approximately equal to that of its electron beam pattern at the intersection with the hood  52 .  
         [0035]    To further isolate the EB guns  30  from the elevated pressure within the condensate hood  52 , the beams  28  travel from their respective guns  30  through chambers  64  formed between the interior walls of the coating chamber  12  and the condensate hood  52 . Preferably, the diffusion pump  34  has an inlet near and pneumatically coupled to each of the chambers  64 . Because of the minimum size of the apertures  62 , the elevated pressure within the condensate hood  52  (achieved by the introduction of oxygen and argon with the inlet  54 ) bleeds into the chambers  64  at a sufficiently reduced rate to enable the diffusion pump  34  to maintain the chambers  64  at a pressure lower than that within the condensate hood  52 .  
         [0036]    [0036]FIGS. 6, 8 and  9  illustrate additional protection provided to the EB guns  30  with this invention. As is generally conventional, the EB guns  30  are equipped with vacuum pumps  66  that maintain pressures within the guns  30  at levels of about 8×10 −5  to about 8×10 −4  mbar, which is well below that existing outside the guns  30 , i.e., within the EBPVD coating chamber  12  of this invention as well as typical EBPVD coating chambers of the prior art. In order for such low pressures to be maintained, the electron beams  28  must pass through cylindrical orifices  68  to exit the guns  30 , as schematically shown in FIG. 6. FIG. 8 represents a conventional configuration for such an orifice  168 . To allow for a range of beam focussing conditions represented by focus positions A, B and C for an electron beam  128  shown in FIG. 8, the orifice  168  has a relatively large diameter and length, e.g., about 30 mm and about 120 mm, respectively. The disadvantage of the prior art is the reduced protection that such a large orifice  168  can provide to the EB guns  30  operating in the higher pressure environment of the apparatus  10  of this invention. During an investigation leading to this invention, testing evidenced that improved control of processing conditions enabled an optimum position of the beam focus point (D in FIG. 9) to be identified. A more effective orifice design was then investigated, resulting in the orifice  68  of this invention shown in FIGS. 6 and 9, which is depicted in FIG. 9 as having a smaller diameter and length than that of the prior art orifice  168  of FIG. 8. A preferred diameter and length for the orifice  68  are believed to be about 15 and 50 mm, respectively, though optimum values for these dimensions can vary depending on pressures and focus, deflection coil current, and overall geometries.  
         [0037]    As noted above, the condensate hood  52  is positioned around the parts  20  to minimize the deposition of ceramic material on the interior walls of the coating chamber  12 . According to this invention, the condensate hood  52  is also specially configured to regulate heating of the parts  20  as required to maintain an appropriate part temperature during a coating campaign. More particularly, the hood  52  is equipped with a movable reflector plate  72  that radiates heat emitted by the molten surfaces of the ingots  26  back toward the parts  20 . At the initial startup of a campaign, during which the temperature of the coating chamber  12  is relatively low, the reflector plate  72  is positioned close to the parts  22  with an actuator  74  to maximize heating of the parts  20 . As the temperature within the coating chamber  12  rises during an ongoing campaign, the reflector plate  72  is moved away from the parts  20  (as shown in phantom in FIG. 6) to reduce the amount of radiated heat reflected back onto the parts  20 . In this manner, the parts  20  can be more readily brought to a suitable deposition temperature (e.g., about 925° C.) at the start of a campaign, while attainment of the maximum allowed coating temperature (e.g., about 1140° C.) is delayed to maximize the length of the coating campaign. The hood  52  plate  72  also promote a more uniform and stable blade coating temperature, which promotes the desired columnar grain structure for the ceramic coatings on the parts  20 . To maintain the desired relatively high pressure within the condensate hood  52  while the reflector plate  72  is in the raised position, a water-cooled shroud  75  is shown that surrounds the plate  72  to inhibit gas flow between the condensate hood  52  and plate  72 , and thereby reduces pressure loss between the hood  52  and plate  72 .  
         [0038]    Shown in FIG. 7 are manipulators  77  that extend into the coating chamber  12  through a ball joint feed-through  79  in the chamber wall. The manipulators  77  are used to assist in regulating the heating of the parts  20  by moving ceramic or ceramic-coated reflectors  80  (shown as a granular material in FIG. 10) toward or away from the crucibles  56  during a coating campaign. More specifically, due to their proximity to the crucibles  56 , the reflectors  80  are at a very high temperature during the coating process, and therefore radiate heat upward toward the parts  20 . The amount of heat radiated by the reflectors  80  is generally at a maximum when the reflectors  80  are closest to the crucibles  56 , and can be reduced by moving the reflectors  80  away from the crucibles  56 . The reflectors  80  are preferably supported on a fluid-cooled plate  81  that does not appreciably radiate heat to the parts  20 . As a result, the reflectors  80  can be used in conjunction with the reflector plate  72  to regulate the temperature of parts  20  being coated within the coating chamber  12  during an ongoing campaign. At the beginning of a campaign, the reflectors  80  are originally located near the crucibles  56  to maximize heating of the parts  20 , and later moved with the manipulators  77  away from the crucibles  56  to reduce the amount of radiated heat.  
         [0039]    To survive the coating chamber environment, the portions of the manipulators  77  within the coating chamber  12  are preferably formed of a high-temperature alloy, such as a nickel-base alloy such as X-15. Instead of a granular material, the reflectors  80  could be in essentially any form and have essentially any shape. For example, one or more plates coated with a reflective material could be used. As a matter of convenience, the reflectors  80  could be relatively large pieces cut from ingots of a material similar to that being deposited, though it is apparent that other ceramic materials could be used.  
         [0040]    As noted above, the ingots  26  of ceramic material are supported within the coating chamber  12  by crucibles  56  that retain the molten pools of ceramic material produced by the electron beams  28 . One of the crucibles  56  is shown in greater detail in FIG. 10 as having a three-piece configuration. An upper member  82  with a tapered upper surface  84  is assembled with a lower member  86 , forming therebetween a coolant passage  88  through which water or another suitable coolant is flowed to maintain the temperature of the crucible  56  below the melting temperature of its material. A restriction plate  90  is also shown in FIG. 10, whose thickness can be selected to change, e.g., decrease, the cross-sectional flow area of the passage  88  between a coolant inlet  92  and outlet  94 . For reasons of thermal conductivity, a preferred material for the crucible  56  is copper or a copper alloy, necessitating that the coolant flow rates through the passage  88  must be sufficient to keep the crucible wall  96  nearest the molten portion of the ingot  26  well below the temperature of the molten ceramic. As is evident from FIG. 10, and as further discussed in reference to FIGS. 11 and 12, the electron beam  28  is preferably projected onto the tapered surface  84  as well as the ingot  26 . Consequently, in order for the exterior surface of the upper member  82  to be adequately cooled, the thickness of the wall  96  must be minimized to promote heat transfer without jeopardizing the mechanical strength of the crucible  56 . The multiple-piece crucible configuration of this invention facilitates the fabrication of an optimal configuration for the coolant passage  88 , as well as enables the thickness of the wall  96  to be produced with tight tolerances. While an optimal configuration will depend on various factors, a preferred coolant flow rate is about five to fifty gallons/minute (about twenty to two hundred liters/minute) using water at a pressure of about two to six atmospheres (about two to six bar) through a passage  88  whose cross-sectional area is about 400 mm 2 , and with a maximum wall thickness of about 10 mm adjacent the surface  84 , and about 7 mm adjacent the ingot  26 .  
         [0041]    [0041]FIGS. 11 and 12 represent a preferred pattern for the electron beams  28  on the ingots  26  to form the pools of ceramic material. As seen in FIGS. 10 and 11, the beam  28  is also projected onto that portion of the crucible surface  84  immediately surrounding the ingot  26 , with the perimeter of the beam  28  on the crucible surface  84 . The preferred power distribution  98  of the electron beam  28  is shown in FIG. 12 as having peaks located near the ingot-crucible interface, with little or no power aimed at the center of the ingot  26 . According to this invention, the benefit of directing such high beam intensities away from the center of the molten pool is a reduced tendency for spitting, which is generally when a droplet of molten ceramic is ejected from the pool during coating. Spitting is associated with defects in the coating produced on the parts  20 , and therefore is preferably avoided. Projecting the beam  28  onto the crucible  56  serves to reduce the amount of ceramic that might otherwise buildup on the crucible  56  due to spitting, and also provides a more even temperature distribution across the molten pool as determined with infrared imaging. When YSZ is used as the ingot material, suitable beam intensities at the peaks in FIG. 12 are on the order of about 0.1 kW/mm 2 , as compared to a maximum level of about 0.01 kW/mm 2  at the center of the pool.  
         [0042]    Also shown in FIG. 10 is that the electron beam  28  is incident on the surface of the ingot  26  at an oblique angle so as to establish relative to its respective EB gun  30  a proximal intersection point  100  and an oppositely-disposed distal intersection point  101  with the crucible  56  at the perimeter of the beam pattern. As shown in FIG. 11, the preferred beam pattern intensity on the ingot  26  and crucible  56  slightly diminishes, preferably by about 30% to 70% relative to the remaining perimeter of the beam pattern, at locations on the crucible  56  corresponding to the proximal and distal intersection points  100  and  101 . The purpose of reducing the intensity of the beam pattern at the proximal intersection point  100  is to reduce erosion of the crucible  56  by the beam  28 , while reducing the beam intensity at the distal intersection point  10 l has been shown to reduce waves generated by the beam  26  on the molten ceramic pool from pushing molten ceramic over the edge of the crucible  56 .  
         [0043]    Another preferred control feature of this invention for the electron beams  28  is the ability to temporarily interrupt the beam pattern on the surface of the crucibles  56  with a separate higher-intensity beam pattern  97  dedicated to achieving a faster evaporation rate over a small area in order to evaporate any ceramic that may become deposited on the crucibles  56  as a result of spitting. This feature of the invention can be performed during the coating operation with minimal or no impact on the deposition process. In a preferred embodiment, when the operator initiates an excursion of the separate pattern  97  to evaporate a buildup of ceramic on the crucible  56 , the pattern  97  is first automatically repositioned to a known position, from which the pattern  97  can then be manually moved under the direction of the operator toward the ceramic buildup. By automatically returning the pattern  97  to a known position, the likelihood of errors that could lead to damage of the crucible  56  is reduced. Alternatively, the position of the pattern  97  could be preprogrammed so that the operator can enter the location on the crucible  56  onto which the pattern  97  is to be projected. Ceramic buildup on the crucible  56  that cannot be readily removed with the pattern  97  can often be removed with the manipulator  77  shown in FIG. 7.  
         [0044]    Magazines  102  that house and feed the ingots  26  up through the floor of the coating chamber  12  and into the crucibles  56  can be seen in FIGS. 1 through 7. As most readily seen in FIGS. 2, 6 and  7 , each magazine  102  has a number of cylindrical channels  104  in which the ingots  26  are held. The magazines  102  rotate to index ingots  26  into alignment with the crucibles  56 . The magazines  102  can also move toward and away from each other (i.e., laterally relative to the coating chamber  12 ) in order to make adjustments for crucible separation and thereby optimize the coating zone over which the deviation of coating thickness is acceptable. The feed mechanisms used to grip and feed the ingots  26  into the crucibles  56  generally include clamping arms  60 , each of which is disposed at an angle from horizontal and adapted to hold the evaporating ingots  26  in place while the magazine  102  is indexed. The upper end of each arm  60  engages the evaporating ingot  26 , which facilitates feeding the ingot  26  in an upward direction with an elevator  61  without allowing the clamping arm  60  to slide downward toward a horizontal position, which was determined to cause jamming of the feed mechanism. According to the invention, each magazine  102  sequentially aligns the next ingot  26  with the lower end of the evaporating ingot  26  within the crucible  56 , and the elevator  61  feeds the next ingot  26  into the coating chamber  12  behind the evaporating ingot  26 , with no or minimal interruption of the deposition of the ceramic material on the parts  20 .  
         [0045]    The viewport  48  noted in reference to FIGS. 3 through 5 is shown in greater detail in FIG. 13. The viewport  48  is configured to permit the operator of the apparatus  10  to observe the coating operation, including the parts  20  being coated, the pools of molten ceramic, the reflectors  80  around the crucibles  56 , and the manipulators  77  used to move the reflectors  80 . As shown, the viewport  48  is generally an enclosure that includes a fluid-cooled aperture plate  106  with an optional window  108  formed of sapphire in order to withstand the high temperatures (roughly 800° C. or more) in proximity to the coating process. A shielding gas is shown as being directed toward the aperture plate  106  through a port  110  for the purpose of minimizing coating deposition on the window  108  or equipment behind the aperture plate  106 . Within the viewport  48 , a rotating stroboscopic drum  112  serves to minimize exposure of a viewing window  114  to radiant heat, light and other radiation from the coating chamber  12 . In accordance with known practice, the drum  112  has slots  116  through its wall and rotates at a high rate to eliminate visual flicker to the eye of the observer. The window  114  is preferably a multiple-pane of quartz glass, lead glass and/or colored glass. The quartz glass provides physical strength, the lead glass provides protection from x-rays, and the colored glass is useful to reduce light intensity. The viewport  48  further includes a magnetic particle seal that provides a high-temperature vacuum seal for the stroboscopic drum. Another preferred feature is that the viewport  48  provides a stereoscopic view of the interior of the coating chamber  12 , by which one or more operators can simultaneously observe the coating chamber while retaining depth perception.  
         [0046]    Shown in FIG. 14 is a preferred control panel  118  for controlling and monitoring the EBPVD apparatus  10  of this invention. The control panel  118  is shown as including a schematic of the apparatus  10  and its components, including indicia  120  for individual components (e.g., the coating chamber  12 ). Also shown are visual indicators  122  located adjacent the indicia  120  for indicating the operating status of the components, and switches  124  to change the operation of the corresponding components. The panel  118  is preferably surrounded by gauges for quantifying process parameters, such as pressures. With the panel  118 , information regarding the operating status of the EBPVD apparatus  10  can be quickly and accurately noted to allow the operator to make any appropriate adjustments to the apparatus  10  and the coating process.  
         [0047]    In operation, the apparatus  10  of this invention may initially appear as shown in FIGS. 1 and 2. As discussed previously, the parts  20  to be coated are loaded onto the rakes  22  within the loading chambers  16  and  18 . The parts  20  may be formed of any suitable material, such as a nickel-base or cobalt-base superalloy if the parts  20  are blades of a gas turbine engine. In the case of gas turbine engine blades, prior to coating with the apparatus  10 , the surfaces of the parts will typically be provided with a bond coat of known composition as discussed previously. Also prior to depositing the ceramic TBC, the surface of the bond coat is preferably grit blasted to clean the bond coat surface and produce an optimum surface finish required for depositing columnar EBPVD ceramic coatings. Also prior to depositing the ceramic coating, an alumina scale is preferably formed on the bond coat at an elevated temperature to promote adhesion of the coating. The alumina scale, often referred to as a thermally grown oxide or TGO, develops from oxidation of the aluminum-containing bond coat either through exposure to elevated temperatures prior to or during deposition of the ceramic coating, or by way of a high temperature treatment specifically performed for this purpose. According to this invention, the parts  20  are preferably preheated to about 1100° C. in an argon atmosphere. When not being used to preheat parts  20 , the preheat chamber  14  is preferably maintained at about 600° C. to minimize the temperature range to which the chamber  14  is subjected during a campaign.  
         [0048]    After preheating within the preheat chamber  14 , the rakes  22  are further extended into the coating chamber  12 . As previously noted, the apparatus  10  of this invention is particularly configured to deposit a ceramic coating under the elevated pressure conditions taught by Rigney et al. Prior to initiating the coating process, a quick vacuum check is preferably performed to track the pumpdown rate and pressure achieved within each of the coating, preheat and loading chambers  12 ,  14 ,  16  and  18  during a set time period. Doing so serves to determine the vacuum integrity of the apparatus  10 , which was previously performed with prior art EBPVD operations through an oxidation test performed on sacrificial specimens. The chambers  12 ,  14 ,  16  and  18  are evacuated with the mechanical pumps  31  from atmospheric pressure, and then a blower commenced when pressures drop to around  20  mbar. The cryogenic pump  32  is preferably started when a pressure of about 5×10 −1  mbar is reached. Thereafter, the diffusion pumps  32  and  34  are started for the coating and preheat chambers  12  and  14  when a pressure of about 5×10 −2  mbar is reached. Suitable process pressures within the loading and preheat chambers  14 ,  16  and  18  are about 10 −3  to 10 −1  mbar, with suitable coating pressures being about 10 −2  to about 5×10 −2  mbar within the coating region defined by the hood  52 . A dual-element ion gauge  55  provided with a manual shutoff valve  57  is preferably used to measure the vacuum pressure within the coating chamber  12 . By using a gauge  55  with independently operable elements, either element can be selected for use without interrupting the coating operation. Alternatively, two ion gauges separated by a valve could be provided, so that either gauge could be used or switched without interrupting the coating operation.  
         [0049]    In a preferred aspect of this invention, the cryogenic pump  32  is preferably started prior to the diffusion pump  34 , contrary to prior practice in which both pumps  32  and  34  were typically started at the same time to minimize ice buildup on the cryogenic pump  32 . Starting the cryogenic pump  32  before the diffusion pump  34  has been found to significantly reduce the amount of time required to attain the coating chamber pressures desired for this invention. While starting the cryogenic pump  32  prior to the diffusion pump  34  promotes ice buildup on the cryogenic pump  32 , this ice can be removed at the end of a coating campaign or any other convenient time.  
         [0050]    During the coating operation, the electron beams  28  are focused on the ingots  26 , thereby forming the molten pools of ceramic and vapors that deposit on the parts  20 . While various coating materials could be used, a preferred ceramic material for TBC (and therefore the ingots  26 ) is zirconia (ZrO 2 ) partially or fully stabilized by yttria (e.g., 3%-20%, preferably 4%-8% Y 2 O 3 ), though yttria stabilized with magnesia, ceria, calcia, scandia or other oxides could be used. The coating operation continues until the desired thickness for the coating on the parts  20  is obtained, after which the parts  20  are transferred through the preheat chamber  14  to the loading chamber  16 , after which the loading chamber  16  is vented to atmosphere. The vents are preferably at least 30 mm in diameter in order to increase the venting rate, but generally less than about 60 mm in diameter to avoid disturbing dust and other possible contaminants within the chambers  12 ,  14 ,  16  and  18 . For this reason, it may be desirable to initially vent with a smaller diameter valve, followed by a larger diameter valve.  
         [0051]    While our 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 our invention is to be limited only by the following claims.