Abstract:
An apparatus deposits a coating on a part. The apparatus comprises a chamber and a sting assembly for carrying the part. The sting assembly is shiftable between: an inserted condition where the sting assembly holds the part within the chamber for coating; and a retracted condition where the sting assembly holds the part outside of the chamber. The apparatus comprises a source of the coating material positioned to communicate the coating material to the part in the inserted condition. The apparatus comprises a thermal hood comprising a first member and a second member. The second member is between the first member and the part when the part is in the inserted condition. The second member is carried by the sting assembly so as to retract with the sting assembly as the sting assembly is retracted from the inserted condition to the retracted condition.

Description:
BACKGROUND 
     The disclosure relates to coating apparatus and methods. More particularly, the disclosure relates to application of thermal barrier coatings for gas turbine engines. 
     Gas turbine engine gaspath components are exposed to extreme heat and thermal gradients during various phases of engine operation. Thermal-mechanical stresses and resulting fatigue contribute to component failure. Significant efforts are made to cool such components and provide thermal barrier coatings to improve durability. 
     Exemplary thermal barrier coating systems include two-layer thermal barrier coating systems. An exemplary system includes a NiCoCrAlY bond coat (e.g., low pressure plasma sprayed (LPPS)) and a yttria-stabilized zirconia (YSZ) thermal barrier coat (TBC) (e.g., air plasma sprayed (APS)). While the TBC layer is being deposited or during an initial heating cycle, a thermally grown oxide (TGO) layer (e.g., alumina) forms atop the bond coat layer. As time-at-temperature and the number of cycles increase, this TGO interface layer grows in thickness. U.S. Pat. Nos. 4,405,659 and 6,060,177 disclose exemplary systems. 
     Exemplary TBCs are applied to thicknesses of 5-40 mils (127-1016 micrometer) and can provide in excess of 300° F. temperature reduction to the base metal. This temperature reduction translates into improved part durability, higher turbine operating temperatures, and improved turbine efficiency. 
     Examples of coating apparatus are found in US Pre-grant Publication 2010/0047474 and PCT/US10/58319 which disclose use of thermal hoods to maintain part temperature. 
     SUMMARY 
     One aspect of the disclosure involves an apparatus for depositing a coating on a part. The apparatus comprises a chamber and a sting assembly for carrying the part. The sting assembly is shiftable between: an inserted condition where the sting assembly holds the part within the chamber for coating; and a retracted condition where the sting assembly holds the part outside of the chamber. The apparatus comprises a source of the coating material positioned to communicate the coating material to the part in the inserted condition. The apparatus comprises a thermal hood comprising a first member and a second member. The second member is between the first member and the part when the part is in the inserted condition. The second member is carried by the sting assembly so as to retract with the sting assembly as the sting assembly is retracted from the inserted condition to the retracted condition. 
     In various implementations, the source may comprise an ingot and an electron beam source positioned to direct a beam to the ingot. The first member and second member may be concentric shells. The first member may be mounted to the chamber so as to remain stationary as the sting assembly is moved between the retracted condition and the inserted condition. The first member may comprise a nickel-based superalloy and the second member may comprise a nickel-based superalloy. The first member may comprise a bent sheet and the second member may comprise a perforated bent sheet. The sting assembly may comprise an inner member and an outer member. An actuator may be coupled to the inner member to move the inner member relative to the outer member. The part may be held by the inner member and the thermal hood second member may be held by the outer member. The actuator may be coupled to the inner member to rotate the inner member relative to the outer member. 
     The sting assembly may have first and second gas flowpaths respectively coupled to an oxygen source and a scattering gas source. 
     Further aspects of the disclosure involve methods for using the apparatus. An exemplary method involves coating a first said part (which may be one of a group of first said parts). After the coating, the sting assembly is retracted from the inserted condition to the retracted condition to retract the first part from the chamber. The first part is replaced with a second said part (which may be one of a group of second said parts). The thermal hood second member is replaced with a replacement thermal hood second member. The second part and the replacement thermal hood second member are inserted to the chamber. The second part is coated. 
     In various implementations, the thermal hood first member may remain in place in the chamber during the coating of the first part and the coating of the second part. The chamber may be a deposition chamber and the apparatus may further comprise: a loading chamber; and a preheat chamber between the deposition chamber and the loading chamber. The retracting may comprise retracting the first part into the loading chamber through the preheat chamber. During the coating, the part being coated may be rotated relative to the thermal hood second member. The coating may pass to the part being coated as a vapor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial schematic sectional view of a coated article. 
         FIG. 2  is a partially schematic side/cutaway view of a coating apparatus with fully inserted substrate carrier. 
         FIG. 3  is a view of the apparatus of  FIG. 2  with fully retracted carrier. 
         FIG. 4  is a view of the apparatus of  FIG. 2  with intermediate position carrier. 
         FIG. 5  is a partially schematic transverse sectional view of a deposition chamber of the apparatus of  FIG. 2 . 
         FIG. 6  is a partial, partially schematic, longitudinal cutaway/sectional view of the carrier of the apparatus of  FIG. 2 . 
         FIG. 7  is an enlarged partially schematic view of a hood mounting arrangement for the carrier of  FIG. 6 . 
         FIG. 8  is an end view of a clip of the mounting arrangement of  FIG. 7 . 
         FIG. 9  is an enlarged partially schematic view of a carrier-mounting arrangement of the system of  FIG. 2 . 
         FIG. 10  is a partially schematic transverse cutaway view of the arrangement of  FIG. 9 . 
         FIG. 11  is a partially schematic end view of an inner hood member. 
         FIG. 12  is a partial view of perforations in the inner hood member. 
         FIG. 13  is a partially schematic view of an outer hood member. 
         FIG. 14  is a partially schematic view of an alternate outer hood member. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a coating system  20  atop a superalloy substrate (the article/component/part that is coated)  22 . The system may include a bond coat  24  atop the substrate  22  and a TBC  26  atop the bond coat  24 . The exemplary bond coat  24  includes a base layer  28  and a TGO layer  30 . Exemplary substrates are of nickel- or cobalt-based superalloys used for hot gaspath components such as: turbine section blades; turbine section vanes; turbine section blade outer air seals; combustor shell pieces; combustor heat shield pieces; combustor fuel nozzles; and combustor fuel nozzle guides. Exemplary base layer thicknesses are 10-400 micrometers, more narrowly 20-200 micrometers. Exemplary TGO layer thicknesses are 0.05-1 micrometers, more narrowly 0.1-0.5 micrometers. Exemplary TBC thicknesses are 40-800 micrometers, more narrowly 100-500 micrometers. 
     An exemplary coating process includes preparing the substrate (e.g., by cleaning and surface treating). A precursor of the bond coat is applied. An exemplary application is of an MCrAlY, more particularly a NiCoCrAlY material. An exemplary application is via a spray from a powder source. An exemplary application is via a low pressure plasma-spray (LPPS) process. An exemplary application is to a thickness of 0.003-0.010 inch, (76-254 micrometers) more broadly 0.001-0.015 inch (25-381 micrometers). LPPS, VPS, EBPVD, cathodic arc, cold spray, and any other appropriate process may be used. 
     After the application, the precursor may be diffused. An exemplary diffusion is via heating (e.g., to at least 1900° F. (1038 C) for a duration of at least 4 hours) in vacuum or nonreactive (e.g., argon) atmosphere. The exemplary diffusion may create a metallurgical bond between the bond coat and the substrate. Alternatively diffusion steps may occur after applying the TBC, if at all. 
     After application of the bond coat precursor, if any, the substrate may be transferred to a coating apparatus for applying the TBC  26 . An exemplary coating apparatus is an EB-PVD coater.  FIG. 2  shows one exemplary EB-PVD coating apparatus/system (coater)  40 . The exemplary coater  40  includes a deposition or coating chamber  42 . One or more electron guns  44  are positioned at the chamber to each direct an associated electron beam  46  to a deposition material (or precursor thereof) source  48  in the chamber interior  50 . Exemplary material sources  48  comprise respective bodies (e.g., ingots)  51  of ceramic-forming material each in an associated crucible  52 . The exemplary material sources are ceramic bodies of the nominal TBC composition (e.g., a yttria-stabilized zirconia or a gadolinia-zirconia such as 7 YSZ or 59 wt. % gadolinia, respectively). The electron beams vaporize the material to form vapor clouds or plumes  54  which envelop the articles or components  22  which are held in a coating position within the chamber  50 . For introducing a reactive gas (e.g., oxygen for combining with the initially vaporated material in the vapor clouds to make up for oxygen lost from the evaporated ceramic) a gas source  55  may be provided. Exemplary gas is essentially pure oxygen. The source may be connected to an outlet (e.g., a manifold  56 ) via a gas line  57  and controlled by a gas valve  58 . As is discussed further below, the same electron guns that vaporize the deposition material may be used to heat the coating chamber (e.g., by directing their beams to a bed  59  of refractory ceramic gravel (e.g., also 7 YSZ)). This may provide a preheating of the deposition chamber (e.g., both before any coating runs and between coating runs). 
     For preheating the parts, the exemplary system  40  includes a preheat chamber (preheater)  60  (having an interior  61 ) positioned on a side of the chamber  42 . Even in the absence of preheating, such a chamber may serve merely as a transfer chamber between the deposition chamber and a loading chamber discussed below. A gate valve  62  may be positioned at a proximal end of the preheat chamber (i.e., between the preheat chamber interior and the deposition chamber interior). An additional gate valve  64  may be at a distal end of the preheat chamber. The preheat chamber is associated with a loading chamber or station  72  (load lock, having an interior  73 ). The valve  64  is thus between interiors of the preheat chamber and the loading chamber. Each of the valves  64  may, instead, be replaced by multiple valves so as to allow further isolation and allow various alternative couplings of multiple loading chambers and/or multiple preheat chambers. For example, one exemplary such coupling is shown in PCT/US10/58319. The exemplary loading station  72  may have a carrier and drive system/mechanism  74  which, when the appropriate gate valves are open, shift the carried articles into the preheat chamber  60  or all the way into the coating chamber  42 .  FIG. 2  also shows vacuum sources  76 ,  77 , and  78 , respectively, coupled to and associated with the chambers  42 ,  60 , and  72 . The exemplary vacuum sources are one or more pumps with associated conduits and valves. Various further options exist for further process gas sources (not shown). 
     An exemplary carrier and drive system/mechanism may comprise a part carrier/fixture  80  holding the parts  22  at one end of a sting assembly (sting)  82 . A drive mechanism (actuator)  84  may drive the sting assembly. The drive mechanism may have a screw drive mechanism (e.g., electric motor driven) for longitudinally shifting the sting and carrier in the associated loading station. Each carrier carries an associated group of the articles. The carrier may rotate (e.g., about a longitudinal horizontal axis  500 ). The drive mechanism may include one or more additional appropriate mechanisms (actuators) (e.g., also electric motors) for driving such rotation. For example, the sting  82  may comprise an outer member  90  and an inner member  92  partially concentrically within the outer member  90 . The drive mechanism may be mounted to the outer member to longitudinally shift the outer member (and thereby the carrier and inner member). The drive mechanism may also rotate the inner member relative to the outer member about the axis  500 . The carrier may be mounted at an inboard end of the inner member  92 . 
     A control system  100  may include an appropriately configured microcomputer, microcontroller, or other controller being configured by software and/or hardware to perform the methods described herein. The control system may be coupled to the various controllable system components as well as to sensors, input devices for receiving user input, and display devices. 
       FIG. 3  shows a maximally retracted second position of the sting assembly  74 . In this condition, the carrier and parts are in the loading chamber  72  and may be accessed via one or more doors  120 . For purposes of schematic illustration, the exemplary door  120  is shown positioned to close an opening or port  122  at the top of the chamber. However, such doors may alternatively be positioned at one or both sides of the chamber or even below. In the second position, the uncoated articles may be loaded into the loading station and the coated articles removed therefrom (e.g., through the associated port  122  (if present) or gate valve).  FIG. 4  shows an intermediate third position with the carrier and parts in the preheat chamber  60 . 
       FIG. 2  also shows a thermal hood combination  140  having an outer member (outer hood)  142  and an inner member (liner or inner hood)  144 .  FIG. 3  shows that the inner member may be mounted to the sting assembly to be retractable along with the parts. The exemplary outer member  142  is fixed in the coating chamber  42 . The thermal hood may serve to maintain the effective coating temperature surrounding the parts during their coating process. 
     In operation, a thermal hood is subject to coating by the deposition material. This may degrade performance of the thermal hood or cause coating defects due to ceramic building up and flaking off and landing on the part(s). Performance degradation can eventually necessitate removal and replacement of the thermal hood. This may be a cumbersome process with prior art hoods. The exemplary inner member  144  effectively forms a liner representing but a portion of the total thermal mass of the hood (e.g., less than half). As is discussed further below, an exemplary inner member  144  may be formed from bent, perforated sheetstock of a nickel-based superalloy. The outer member  142  may represent a thicker, relatively less perforated, piece of a similar alloy. 
       FIG. 5  shows further details of an exemplary hood configuration. The exemplary inner and outer members  144  and  142 , respectively, are shaped as sectors of circular cylindrical shells. Exemplary sectors extend approximately halfway around the axis  500  by angles θ I  for the inner shell and θ O  for the outer shell. Exemplary θ I  and θ O  are 100°-200° about the axis  500 , more narrowly, 160°-200° or 170°-190°. Exemplary hood layers/shells have respective lengths L I  and L O  ( FIG. 3 ). Exemplary shells have radii R I  and R O . Exemplary L I  and L O  are greater than R I  and R O . Exemplary R I  and R O  may be measured at inboard surfaces. Exemplary L I  and L O  are within about 10% of each other. Exemplary L I  and L O  are 0.5-0.75 m, more narrowly, 0.6-0.65 m. Exemplary R I  may be at least 75% of R O , more narrowly, at least 85%. Exemplary R I  and R O  are 0.15-0.35 m, more narrowly, 0.18-0.28 m. As is discussed further below, the exemplary hood outer member  142  may have one or more slots  145 . The slots may serve to guide gas flow out the top of the hood so as to provide desired exposure of the parts to the vapor plume.  FIG. 5  also shows a carrier gas flow  146  (e.g., inert) for collimating the plume. 
     The exemplary outer member  142  has respective inner/inboard and outer/outboard surfaces  150  and  151 , edges  152  and  153 , and ends ( FIG. 3 )  154  and  155 . Similarly, the inner member  144  has an inner/inboard surface  160 , an outer/outboard surface  161 , edges  162  and  163  and ends  164  and  165 . Respective shell thicknesses between the inboard and outboard surfaces are T O  and T I . These, as well as the other parameters, may be measured by an appropriate average (e.g., either mean, median, or modal). Exemplary T O  is 2.5-8 mm, more narrowly, 5-6.5 mm. Exemplary T I  is 2.5-8 mm, more narrowly, 3-5 mm. 
     Proximate the end  164 , the inner hood  144  is supported relative to the sting assembly via struts  180 ,  181 , and  182 . Exemplary struts  180  and  182  are respectively near the edges  162  and  163 . The exemplary outer hood  142  is supported relative to the chamber via struts  170 ,  172 . The outer hood  142  may be removable (such as via unbolting). 
       FIG. 6  shows further details of the inner hood  144  and carrier  80 .  FIG. 6  shows the outer sting shaft  90  extending to an inboard end  200  and having an inner/inboard surface  202  and an outer/outboard surface  203 . An end flange  204  may be mounted to the outer shaft  90  at the end  200  (e.g., via welding). The flange  204  carries a plurality of studs or bungs  206  (e.g., having respective axes  510  parallel to and spaced apart from the axis  500 ). In the exemplary implementation, the bungs have root portions that extend through apertures  208  in a mounting ring segment  210  (also  FIG. 11 ). The exemplary ring segment  210  joins the struts  180 ,  181 ,  182  at inboard ends thereof. For example, the inboard ends of the struts may be welded to the ring  210  at their inboard ends and welded to the hood at their outboard ends. 
     Each exemplary bung  206  ( FIG. 7 ) may have a proximal root portion  220  secured to the flange  204  (e.g., press-fit or welded into an aperture  222 ). Each exemplary bung has an intermediate portion  224  received within the aperture  208 . A distal portion  230  of the bung may protrude beyond an inboard face of the ring  210  and may bear features for engaging a fastener component to resist extraction/removal of the inner hood from the sting. In an exemplary embodiment, the portion  230  bears a groove  232  which can receive a wedge clip  234  (e.g., having a generally U-shaped body  240  ( FIG. 8 ) and a locking wire  242  having end portions  244  received in holes  246  in the legs of the U). The groove may be received in the clip and the latch closed to prevent axial removal of the hood. The exemplary clip, however, allows the user to manually disengage the latch and remove the clip whereafter the user can manually extract the hood from the outer sting. An installation of the hood may be via a reverse of this process. This allows the inner hood to be removed and replaced without use of tools. 
     Returning to  FIG. 6 , the exemplary inner sting shaft  92  extends to an inboard end  250 . A mounting flange  252  is secured to the inboard end (e.g., via welding). A receiver  254  for the carrier may be mounted to the mounting flange. For example, the receiver may have its own mounting flange  256  mounted to the flange  252  via fasteners such as bolts  258 . A receiver body  258  receives an end portion  260  of a main shaft  262  of the carrier. 
       FIG. 9  shows the receiver body  258  as having a socket or compartment  264  having a generally frustoconical taper complementary to a taper of the end portion  260 . The compartment extends and is open to an end  266  of the receiver body. With the end portion  260  in its installed position, the receiver body and end portion have circumferential chord-ways grooves  270  and  272  which are axially aligned with each other. A clip  274  having a body  276  may engage the grooves to maintain axial and circumferential positions of the shaft end portion  260  and body  258 . The clip may also have a locking wire  278  ( FIG. 10 ) as described above. The clip can allow the carrier to be installed and removed without tools. The exemplary carrier has a pair of end plates  280  and  282  mounted to the shaft  262  near the opposite ends thereof. The plate  280  is relatively near the end  260 . One or both these plates may carry fixture elements for holding the individual parts or groups thereof. In the illustrated embodiment, the parts are blades, having airfoils  300  extending from a tip  302  to a platform  304 . An attachment root  306  depends from the underside of the platform. Exemplary fixture elements comprise a firtree receptacle  310  for receiving the root  306 . A shaft  312  extends from the receptacle through an associated aperture  314  in the plate  280 . The exemplary shaft may freely rotate about an axis  520  so that as the carrier rotates about its axis  500 , the parts maintain their orientations. 
       FIG. 6  also shows the manifold  56  formed as a pair of tubular sections or conduits  320  (e.g., of a similar material to that of the sheet that forms a body of the hood) mounted to the hood (e.g., via welding) adjacent the respective edges  162  and  163 . An exemplary tubular section  320  has a closed distal end  322  and a longitudinal array of venting apertures or outlets  324 . At a proximal end, the tube may bear a fitting  330  ( FIG. 7 ) which mates with a conduit extending from the source  55  of  FIG. 2 . For example, line  57  of  FIG. 2  may discharge to the annular space between the inner and outer sting shafts. An additional line  334  may pass from this annular space into a port in one face of the flange  204  which aligns with a port in the opposite face that receives the fitting  330 . Seal  332  may block the end of the annular space between sting members. 
       FIG. 12  shows the inner hood  144  as having an array of apertures  360 . A size of the aperture is 1-5 mm (e.g., diameter for a circular aperture or average transverse dimension for others), more narrowly, 1.5-3.5 m. The exemplary apertures form an exemplary 20-70% of the surface area of the hood, more narrowly, 40-60%. The exemplary apertures may be formed as circular apertures by drilling. Alternative apertures may be formed electrical discharge machining (EDM), punching, and/or stamping. A screen material or expanded material could also be used.  FIG. 13  shows the outer hood as having a single venting slot as described above. The total area of the slot is 550-800 cm 2 , more narrowly, 300-1270 cm 2 .  FIG. 14  shows a group of slots circumferentially arrayed. Tubular features  370  at the edges of the hood in  FIGS. 13 and 14  are the gas manifolds that are fed from an external gas (e.g., oxygen) source (e.g., 55 of  FIG. 2  or a separate source) via ports (not shown) on the chamber wall. 
     The exemplary sequence of operation may start with the outer hood installed in the deposition chamber  42  and the deposition chamber pumped down to appropriate pressure conditions and heated to appropriate temperature conditions. Similarly, the preheat chamber may be heated to initial conditions. The end of the inner sting may initially be in the loading chamber without the carrier or inner hood. Parts may be pre-installed onto a carrier and several carriers may be used sequentially to speed production. The pre-loaded carrier may be installed to the inner sting shaft  92  and secured as discussed above. The inner hood may then be installed. The door  120  may be closed and the load lock pumped down to an appropriate condition whereafter the gate valve  64  is opened and the sting shaft assembly inserted to move the carrier into the preheat chamber  60 . In appropriate preheating and any further pumped down, the gate valve  62  may be opened and the carrier inserted into the deposition chamber and the coating process commenced. After completion of the coating process, the carrier may be retracted back into the preheat chamber and the gate valve  62  closed. In that extraction or after closing the gate valve  62 , the carrier may be further extracted into the loading chamber  72 . The gate valve  64  may be closed and the loading chamber  72  vented to atmosphere. Thereafter, the door  120  may be opened. In a first exemplary implementation, the hood inner member is removed at this point in every such extraction. It may further be discarded and/or cleaned or otherwise reconditioned before reuse. Thus, there may be several or many individual inner hood members sequentially used. The carrier may be removed and replaced with a fresh carrier (e.g., pre-loaded with parts). In other implementations, the inner hood member may either not be removed for several cycles or may be replaced without reconditioning for those several cycles, replacement may be at a much more frequent interval than would otherwise be done with a single fixed hood. 
     The various such replacement cycles may allow for some combination of group uniformity, reduced cycle time, and reduced cost. For example, a full replacement of a fixed hood with each cycle or few cycles would entail the cost of the substantial hood but also would create equipment downtime during cooling and venting of the deposition chamber and subsequent pump down and heating. When compared with a baseline system having a fixed hood, if the interval for replacing the inner member is substantially shorter than the interval for replacing the single fixed hood, consistency may improve because the hood inner member may become less fouled during such interval than the single hood during its interval. 
     One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, and applied as a reengineering of an existing coating apparatus and process, details of the existing apparatus and/or process (in view of the particular coating and substrate) may influence or dictate details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.