Patent Publication Number: US-6911234-B2

Title: Chemical vapor deposition apparatus and method

Description:
This is a continuation of Ser. No 09/950,013 filed Sep. 10, 2001, now U.S. Pat. No. 6,793,966. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a chemical vapor deposition (CVD) apparatus and method for applying coatings to substrates. 
     BACKGROUND OF THE INVENTION 
     Chemical vapor deposition (CVD) involves the generation of metal halide gas at low temperatures (e.g. about 100 to 600 degrees C.), introduction of the metal halide gas into a high temperature retort (e.g. 200 to 1200 degrees C. retort temperature), and reaction of the metal halide with substrates positioned in the retort to form a coating thereon. In general, a large excess of metal halide gas is used to prevent reactant starvation in the high temperature coating retort. CVD processes typically are conducted at reduced pressure (subambient pressure). CVD apparatus and method are described in Howmet U.S. Pat. Nos. 5,261,963 and 5,263,530. Howmet U.S. Pat. No. 6,143,361 described CVD apparatus and method wherein deposition of excess metal halide reactant in the coating gas exhausted from the coating retort is reduced or eliminated to reduce retort downtime required to remove deposits from the retort exhaust system. 
     The CVD process can be used to codeposit Al, Si, and one or more reactive elements such as Hf, Zr, Y, Ce, La, etc. to form protective aluminide diffusion coatings on substrates such as nickel and cobalt base superalloys commonly used to cast gas turbine engine airfoils. Copending U.S. Ser. Nos. 08/197,497 and 08/197,478 disclose CVD apparatus and method to produce protective reactive element-modified aluminide diffusion coatings. U.S. Pat. No. 5,989,733 describes a protective outwardly grown, platinum-modified aluminide diffusion coating containing Si and Hf and optionally Zr, Y, Ce, and/or La formed on a nickel or cobalt base superalloy substrate by such CVD apparatus and process. 
     There is a need to provide improved CVD apparatus and method that are capable of producing aluminide diffusion coatings modified by inclusion of one or more other coating elements, such as for example only silicon and one or more so-called reactive elements, wherein the coatings can be produced having a more uniform coating composition, microstructure, and thickness throughout the working volume (throughout multiple coating zones) of the CVD coating apparatus. It is an object of the present invention to satisfy this need. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the present invention, CVD apparatus and method are provided with an improved coating gas distribution system to provide more uniform coating gas temperature among a plurality of coating zones in a coating chamber. 
     In another embodiment of the present invention, CVD apparatus and method are provided with an improved coating gas distribution system to provide more uniform flow of coating gas among a plurality of coating zones in the coating chamber. 
     In still another embodiment of the present invention, CVD apparatus and method are provided with an improved coating gas exhaust system that reduces interaction between the inlet coating gas flow to each coating zone and exhaust gas flow from each coating zone so as provide a more uniform gas flow pattern in each coating zone. 
     The above and other objects and advantages of the present invention will become more readily apparent from the following detailed description taken with the following drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a somewhat schematic view of a CVD coating gas generator and a coating reactor chamber that is shown in a longitudinal sectional view pursuant to an embodiment of the invention. 
         FIG. 2  is an enlarged longitudinal sectional view of the coating reactor chamber and coating gas distribution system pursuant to an embodiment of the invention. 
         FIG. 3  is an enlarged longitudinal sectional view of the external coating gas generator. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     For purposes of illustration and not limitation, the present invention will be described herebelow with respect to a CVD apparatus and method for producing a protective platinum-modified aluminide diffusion coating containing Si, Hf and optionally Zr on a nickel base superalloy substrate of the type disclosed in U.S. Pat. No. 5,989,733, the teachings of which are incorporated herein by reference. Zr can be present in the coating as a result of being an impurity in the Hf pellets described below or as an intentional coating addition. The invention is not limited to making such coatings and can be practiced to form other coatings on other substrates. 
     Referring to  FIGS. 1-2 , CVD coating apparatus pursuant to an embodiment of the invention comprises a reactor or retort  12  adapted to be disposed in a refractory-lined heating furnace  14  shown schematically that is used to heat the retort  12  to an elevated CVD coating temperature. The furnace  14  can be an electrical resistance or other known type of furnace to this end. Metallic substrates SB to be coated are placed in a coating reactor chamber  20  disposed in the retort  12  and are heated by radiation from the walls of the heated retort. 
     The retort  12  includes a lid  16  to close off the upper end of the retort. To this end, the retort lid  16  is air-tight sealed on a flange  12   f  of the retort by an O-ring seal  17 . The flange  12 f includes an annular water cooling passage  12   p  through which passage water is circulated to cool the flange during operation of the retort. Lid  16  includes an annular chamber  16   a  receiving a thermal insulation block or member  16   b  therein to reduce heat losses from the retort. Components of the coating reactor chamber  20  can be supported on the lid  16  and then lowered with lid  16  into the retort  12 . The coating reactor chamber  20  includes conduits  18 ,  22  joined at connection  57 , which connection is made prior to closing the lid  16  on the retort  12 . Conduit  22  is part of the retort cover  16  as a result of being welded thereto. 
     The retort lid  16  includes central coating gas inlet conduit  22  through which reactive coating gases are supplied to the axial gas preheat and distribution conduit  18  of the reactor  20  as described below. The conduit  18  includes an inner axial gas preheat conduit  52  therein. The coating reactor chamber  20  comprises a plurality of distinct annular coating zones  24   a,    24   b,    24   c  ( FIG. 2 ) at different axial elevations in the retort and disposed about coating gas preheat and distribution pipe or conduit  18 . Referring to  FIG. 2 , substrates SB to be coated are disposed on trays  28  in the coating zones  24   a,    24   b,    24   c.  The trays  28  close off the coating zones  24   a,    24   b,    24   c.  The coating zones are shown disposed one atop another for purposes of illustration and not limitation since fewer or greater number of coating zones can be employed in practice of the invention. 
     Referring to  FIG. 1 , the coating gas inlet conduit  22  is communicated to a plurality of low temperature metal halide generators  30  of identical construction with the exception of the metal charge B therein, FIG.  3 . The metal charge B in each generator  30  is different and selected to generate a particular coating gas constituent, such as for purposes of illustration and not limitation, an aluminum or aluminum alloy pellet bed in generator # 1  to generate aluminum trichloride or other volatile aluminum halide coating gas constituent, a silicon or silicon alloy pellet bed in generator # 2  to generate silicon tetrachloride or other volatile silicon halide coating gas constituent, and a reactive element, such as Hf or an alloy thereof, in generator # 3  to generate a hafnium tetrachloride or other volatile hafnium halide coating gas constituent. Other reactive elements that can be used in lieu of, or in addition to Hf or its alloys, include Zr and its alloys, Ce and its alloys, and Ni—Mg alloys to generate a Mg-bearing coating gas. 
     The generators  30  are located externally of the retort  12  and connected to inlet conduit  22  via heated conduits  32 . The conduits  32  are heated by conventional heating devices, such as electrical resistance heated flexible tapes or electrical resistance heated rods or sticks, to prevent condensation of the metal halide coating gases therein. 
     For producing a protective platinum-modified aluminide diffusion coating containing Si, Hf and Zr on a nickel base superalloy substrate of the type disclosed in U.S. Pat. No. 5,989,733, the first metal halide generator # 1  is used to generate aluminum trichloride or other aluminum halide coating gas constituent. The generator is supplied with a gas flow F 1  comprising a mixture of an acid halide gas, such as typically HCl or other hydrogen halide gas, and a reducing or inert carrier gas, such as hydrogen, argon, helium, or mixtures thereof, via conduits  33  from suitable sources  41 ,  42 , such as respective high pressure cylinders or bulk cryogenic supplies. The acid halide gas and carrier gas are blended together in suitable proportions to provide the gas flow F 1  to the first generator. 
     Referring to  FIG. 3 , the first generator # 1  includes a bed B of aluminum metal pellets and an heating device  46 , such as an electrical resistance heater, to heat the Al pellets to a reaction temperature depending upon the acid halide gas supplied to the generator. For example only, an aluminum pellet temperature of about 200 degrees C. or higher can be used for HCl gas. The pellet temperature for other hydrogen halide gases depends on the boiling point of the aluminum halide formed in the generator. The acid halide gas/carrier gas flow F 1  is supplied to generator # 1  to flow over the Al pellets under conditions of temperature, pressure, and flow rate to form aluminum trichloride or other aluminum halide gas, depending on the hydrogen halide gas used, in the carrier gas. Typical temperature, pressure, and flow rate to form aluminum trichloride at generator # 1  are as taught in U.S. Pat. No. 5,658,614 as follows:
         Hydrogen halide/carrier gas—13 vol. % HCl; balance H 2      Pellet temperature—290 degrees C.   Flow rate—46 scfh (standard cubic feet per hour)       

     The second metal halide generator # 2  is used to generate silicon tetrachloride or other volatile silicon halide coating gas constituent. The generator is supplied with a gas flow F 2  comprising a mixture of an hydrogen halide gas, such as typically HCl gas, and a reducing or inert carrier gas, such as hydrogen, helium and argon, or mixtures thereof, from suitable sources  41 ,  42 , such as respective high pressure cylinders or bulk cryogenic supplies. The hydrogen halide gas and carrier gas are blended together in suitable proportions to provide the gas flow F 2  to the second generator. The second generator # 2  includes a bed B of silicon pellets and heating device  46 , such as an electrical resistance heater, to heat the Si pellets to a reaction temperature depending upon the acid halide gas supplied to the generator. For example only, a silicon pellet temperature of about 100 degrees C. or higher can be used for HCl gas. Pellet temperatures for other hydrogen halide gases depends on the boiling points of the silicon halide formed in the generator. Typical temperature, pressure, and flow rate to form silicon tetrachloride at generator # 2  are as follows:
         Hydrogen halide/carrier gas—2 vol. % HCl; balance H 2      Pellet temperature—290 degrees C.   Flow rate—26 scfh       

     The third metal halide generator # 3  is used to generate a reactive element chloride or other reactive element halide gas, such as hafnium tetrachloride coating gas constituent. The generator is supplied with a gas flow F 3  comprising a mixture of an acid halide gas, such as typically HCl gas, and an inert carrier gas, such as argon, helium, or mixtures thereof, from suitable sources  43 ,  44 , such as respective high pressure cylinders or bulk cryogenic supplies. The hydrogen halide gas and carrier gas are blended together in suitable proportions to provide the gas flow F 3  to the first generator. The third generator # 3  includes a bed B of hafnium pellets containing natural Zr impurities and heating device  46 , such as an electrical resistance heater, to heat the Hf pellets to a reaction temperature depending upon the acid halide gas supplied to the generator. For example only, a hafnium pellet temperature of about 430 degrees C. can be used for HCl gas. Pellet temperatures for other hydrogen halide gases depends on the boiling or sublimation points of the metal halide formed in the generator. The pellets of the bed in generator # 3  can comprise an alloy of Hf and Zr in the event Zr is to be intentionally present as an alloyant in the coating. Typical temperature, pressure, and flow rate to form hafnium tetrachloride at generator # 3  are as follows:
         Acid halide/carrier gas—3 vol. % HCl; balance Ar   Pellet temperature—430 degrees C.   Flow rate—33 scfh       

     In lieu of having three separate generators, a cogenerator can be used to cogenerate two metal halide gases. For example, aluminum trichloride and silicon tetrachloride can be cogenerated by flowing a hydrogen halide/carrier gas mixture first over a bed of Al pellets and then over a bed of Si pellets located downstream of the bed of aluminum pellets as described in copending application Ser. No. 08/197 478, the teachings of which are incorporated herein by reference, to generate a coating gas constituent that includes both AlCl 3  and SiCl 4  in proportions controlled by the flow rate of the acid halide/carrier gas over the beds. The third generator # 3  would still be used to generate the HfCl 4  coating gas constituent. Alternately, hafnium tetrachloride and silicon tetrachloride can be cogenerated by flowing a hydrogen halide/carrier gas mixture first over a bed of Hf pellets and then over a bed of Si pellets located downstream of the bed of hafnium pellets. Any combination of pellet beds where metal halide gas from the first upstream bed is more stable than a second metal halide formed in the second downstream bed can be used as a cogenerator in practice of the invention 
     The coating gas constituents from generators  30  are supplied to the inlet conduit  22  connected to the gas preheat and distribution conduit  18  at connection  57 . A suitable pump P, such as vacuum pump, is connected to the exhaust  80  of the reactor coating chamber in a manner to maintain a desired pressure, desired flow rate of the gases through the generators  30  and the coating chamber  20  and to exhaust spent coating gas from the coating chamber. 
     The metal halide generators  30  are constructed to reduce leakage of air into the generators at the inlet fitting  30   a,  outlet fitting  30   b  and flange joint  30   c  thereof. Each generator  30  is identical to the other except for the bed B of pellets therein. 
     In  FIG. 3 , generator  30  is shown including a metal (e.g. stainless steel) housing  30   h  having electrical resistance heating device  46  disposed thereabout to heat the bed B in the generator to a desired reaction temperature; for example, as described above. The housing  30   h  includes an annular, laterally extending flange region  30   f  at a lower end to rest on a generator base  35  with an O-ring seal  33  therebetween. The flange region  30   f  resting on base  35  defines joint  30   c.  The flange  30   f  includes an annular passage  30   p  through which cooling fluid (e.g. water) is flowed during operation of the generator to cool the flange and maintain its temperature in the range of about 40 to about 100 degrees C. for purposes of illustration and not limitation. Cooling of flange region  30   f  during operation of the generator  30  reduces distortion of the flange region  30   f  from the elevated temperature of the housing  30   h  during generator operation, and minimizes oxidation of the O-ring. 
     The O-ring seal  33  is compressed between the cooled flange region  30   f  and flange  35   f  of the generator base  35  to provide an air-tight seal therebetween. The O-ring seal comprises an acid resistant fluoroelastomer polymeric material that does not release carbon, sulfur or other unwanted tramp element into the generator  30  that could adversely affect the coating produced on substrates SB. A suitable O-ring  33  is commercially available as a Viton O-ring from Dupont Dow Elastomers, Wilmington, Del. More than one O-ring seal  33  can be provided between flange region  30   f  and base  35 . 
     The inlet fitting  30   a  on base  35  and outlet fitting  30   b  on housing  30   h  of the generator  30  comprise commercially available zero clearance fittings that provide knife-edge sealing surfaces (not shown) that penetrate into an annular nickel gasket (not shown) to provide an air-tight seal. Suitable zero clearance fittings  30   a,    30   b  are available as VCR metal gasket and face seal fittings from Swagelok Corporation, Solon, Ohio. 
     The bed B of pellets is disposed on a perforated gas distribution plate  37  that is positioned further downstream of the flange region  30   f;  i.e. downstream in the direction of flow of the gases in the generator, so as to reduce heat input to the flange region  30   f  and O-ring seal  33 . In the past as disclosed in U.S. Pat. Nos. 5,407,704 and 5,264,245, the plate  37  was positioned at the flange region  30   f  having a grafoil gasket that emitted carbon and sulfur into the generator. The plate  37  is heated by contact with the bed B of pellets in the generator and by proximity to the heater  46  such that the more remote positioning of the plate  37  from the flange region  30   f  reduces heat input to the flange region  30   f  and O-ring seal  33 . A typical spacing of the gas distribution plate  37  from the flange region  30   f  is 1 inch or more for purposes of illustration and not limitation. 
     Reduction of air leaks into the generators  30  at the flange joint  30   f  and fitting  30   a  reduces oxidation of the pellet charge forming bed B. Thus, the efficiency of utilization of the pellet charges is increased. For example, the efficiency of use of the hafnium pellet charge in generator # 3  was improved from less than 5% to more than 98% by prevention of air leaks into the third generator  30 . Reduction of air leaks into the coating gas conduit at fitting  30   b  prevents oxidation of the reactive element halides exiting the generator and so improves control of the coating composition. 
     Pursuant to an embodiment of the present invention, an improved coating gas distribution system is provided to provide more uniform coating gas temperature among the coating zones  24   a,    24   b,    24   c  in the coating chamber  20 . In particular, the coating gas constituents (e.g. AlCl 3 , SiCl 4 , HfCl 4  and carrier gases) are conveyed to inlet conduit  22  which defines a gas manifold  50  that is located above and upstream of the coating chamber  20  in the retort  12  and that communicates with upstanding inner coating gas preheat conduit  52  inside coating gas preheat and distribution conduit  18  such that the coating gas stream ST (comprising the coating gas constituents) entering the inlet conduit  22  flows through the manifold  50  and down the preheat conduit  52  to the lowermost coating zone  24   c  of the coating chamber  20  and back up in the annular space between the conduits  18  and  52  in a manner that the coating gas stream ST is preheated before entering the coating zones  24   a,    24   b,    24   c  via conduit  18 . The manifold  50  includes a heater device  54 , such an elongated electrical resistance heater, suspended therein such that gas stream ST flows about the heater device  54  to heat the gas stream ST. A suitable electrical resistance heater that can be placed in manifold  50  is commercially available as Firerod Cartridge from Watlow Corporation, St. Louis, Mo., although other heating devices can be used to this end. The heater device  54  can be suspended along the length of the manifold  50  by a conventional swaglock compression connection  55 . 
     The inlet conduit  22  communicates to preheat conduit  52  that resides inside coating gas preheat and distribution conduit or pipe  18 . The conduit  22  and conduits  18 ,  52  are connected by a union type pipe fitting connection  57 . 
     The conduit  52  extends axially through and along the length of the retort  12  through the coating zones  24   a,    24   b,    24   c  disposed along the length of the coating chamber  20  to the lowermost coating zone  24   c  where the conduit  52  includes a lower gas discharge opening  52   a  to discharge the coating gas stream ST into the annular space between the gas preheat and distribution conduit or pipe  18  and preheat conduit  52  for flow upwardly in the annular space to the coating zones as illustrated by the arrows. 
     For purposes of illustration and not limitation, the exemplary coating gas stream ST described above (e.g. AlCl 3 , SiCl 4 , HfCl 4  and carrier gases) can be preheated to a gas temperature of greater than 100 degrees C. by the heater device  54  in the manifold  50  and the heating provided by flowing the stream through conduits  18 ,  52  in the above described manner when the coating chamber  20  is at a temperature of 1080 degrees C. 
     In addition, radiant heat shields  70  are provided above the coating zones  24   a,    24   b,    24   c  to reduce heat losses from the top of the coating chamber  20 . The heat shields  70  comprise stainless steel plates fastened in the parallel arrangement illustrated above the coating chamber  20  to reflect radiant heat energy back toward the coating chamber  20 . The heat shields  70  include legs  70   a  spaced circumferentially about their peripheries so that the plates  70  can stacked atop one another on the upper tray  28 . Such radiant heat shields  70  can be used in lieu of the gettering screens described in U.S. Pat. No. 5,407,704. 
     Preheating of the coating steam ST using the heater device  54  in the manifold  50  and using the heating provided by flowing the stream through conduits  18 ,  52  in the above described manner as well as reduction of radiant heat losses from the coating chamber  20  by shields  70  improves uniformity of the coating gas temperature in the coating zones  24   a,    24   b,    24   c  to dramatically reduce coating thickness variations on substrates SB from one coating zone to the next. That is, the coating gas stream ST is more uniformly heated in the retort  12  to the desired coating deposition temperature prior to its being directed into the coating zones  24   a,    24   b,    24   c  by practice of this embodiment of the invention. For purposes of illustration and not limitation, a coating gas stream temperature gradient of only 50 degrees F. over the length of the coating chamber  20  can be provided as compared to a 400 degrees F. temperature gradient experienced in CVD apparatus of the type illustrated in U.S. Pat. Nos. 5,407,704 and 5,264,245. 
     Once the coating gas stream ST has reached a desired reaction or coating temperature, another embodiment of the invention provides an improved coating distribution system to provide more uniform distribution of the preheated coating gas stream among the coating zones  24   a,    24   b,    24   c  in the coating chamber  20 . 
     In particular, the preheat and distribution conduit  18  extends axially through the annular substrate support trays  28  which define therebetween the distinct annular coating zones  24   a,    24   b,    24   c  about pipe or conduit  18 . The pipe or conduit  18  includes at a mid-point of the height of each coating zone  24   a,    24   b,    24   c  a plurality of circumferentially spaced apart gas discharge holes or openings  62  to discharge the preheated coating gas stream ST to each coating zone. The number of openings  62  at each coating zone can be varied as desired. For a diameter of conduit  18  of 1½ inches and axial spacing of 6 inches between trays  28 , three or more openings  62  can be provided in conduit  18 . The area of the openings  62  (e.g. number of holes) at the coating zones  24   a,    24   b,    24   c  is systemically varied to provide equal coating gas flow from conduit  18  to each coating zone. Typically, the number of openings  62  at coating zone  24   a  are greater than those at coating zone  24   b,  and the number of holes  62  at coating zone  24   b  is greater than those at coating zone  24   c.  For example only, the number of holes at coating zone  24   a  can be 10, the number of holes at coating zone  24   b  can be 8, and the number of holes at coating zone  24   c  can be 6. 
     The conduit  52  also includes one or more bleed openings  52   b  above the lower primary coating gas discharge opening  52   a  for discharging coating gas along the length of conduit  52 . For example, one bleed opening  52   b  is located at coating zone  24   b  and one bleed opening  52   b  is located at coating zone  24   c  to assist in providing generally equal flow of coating gas among the coating zones  24   a,    24   b,    24   c.  Although one bleed opening  52   b  is shown at each coating zone  24   b  and  24   c  in the upper region of each coating zone  24   b,    24   c  to this end, more than one bleed opening can be provided at the same or different locations at coating zones  24   a,    24   b,    24   c  as needed to generally equalize the flow of coating gas among the coating zones  24   a,    24   b,    24   c.  The coating gas discharged from bleed openings  52   b  flows upwardly in conduit  18  to this end. Bleed openings  52   b  each having a diameter of 0.125 inch can be provided to this end for use with conduits  18 ,  52  having dimensions described herein. 
     The annular trays  28  are spaced axially apart proximate their inner circumference by upstanding spacer annular inner walls  64  and proximate their outer circumference by upstanding outer perforated baffles  66 . The spacer walls  64  are positioned symmetrically about pipe or conduit  18  by retaining rings  67  welded or otherwise provided on trays  28 . The trays  28  include a central hole  28   a  having inner diameter about equal to the outer diameter of pipe or conduit  18  to receive same in manner that the trays  28  are symmetrically disposed about the pipe or conduit  26 . The trays  28 , spacer walls  64 , and baffles  66  are stacked atop one another and supported on a lowermost, laterally flange  18   a  of the pipe or conduit  18 . The gas distribution pipe or conduit  18 , trays  28 , spacer walls  64  and baffles  66  thereby are arranged in fixed positions symmetrically about the central longitudinal axis of the coating chamber  20 . 
     The spacer walls  64  form an annular gas manifold  68  at each coating zone  24   a,    24   b,    24   c  between the pipe or conduit  18  and walls  64  each of which provides a manifold wall. Each spacer wall  64  opposes or faces the gas discharge openings  62  of the pipe or conduit  18  at that coating zone. Each spacer wall  64  includes first and second sets of circumferentially spaced apart gas flow opening openings  65  located an equal distance above and below the height of the openings  62  in pipe or conduit  18 . Each spacer wall  64  thereby is provided with a plurality of gas flow openings  65  that are out of alignment with the gas discharge openings  62  at each coating zone such that there is no line-of-sight gas flow path from the gas discharge openings  62  to gas flow openings  65  at each coating zone. 
     For purposes of illustration and not limitation,  48  gas flow openings  65  having a diameter of 0.25 inches can be provided in each wall  64  at each coating zone  24   a,    24   b,    24   c  when the conduit  18  includes opening  62  whose number and diameters are described above. Locating the openings  62  of the gas distribution pipe or conduit  18  midway between the sets of openings  65  prevents gas jets from flowing directly across the each coating zone. Also, deflection of the coating gas off of the inside of the wall  64  at each coating zone produces more uniform gas flow about the circumference of each coating zone  24   a,    24   b,    24   c.    
     The above gas distribution system provides a uniform and repeatable gas flow to the coating zones  24   a,    24   b,    24   c  to improve coating composition and microstructure uniformity among substrates SB on the same tray  28  and among substrates in different coating zones. 
     Once the coating gas stream ST has flowed over the substrates SB on trays  28  at each coating zone  24   a,    24   b,    24   c,  still another embodiment of the present invention provides an improved spent gas exhaust system to provide less interaction between the inlet coating gas flow to each coating zone  24   a,    24   b,    24   c  and the exhaust gas flow from each coating zone so as provide a more uniform flow pattern of coating gas in the coating zones. 
     In particular, perforated tubular baffles  66  are provided between the trays  28  at their outer circumferences as shown in  FIG. 1-2 . The tubular baffles  66  comprise IN-600 nickel base superalloy and include patterns of exhaust openings  66   a  through which spent (exhaust) gas from the coating zones  24   a,    24   b,    24   c  is exhausted. The pattern of openings  66   a  as well as their number and size (e.g. diameter) can be selected to provide more or less uniform gas flow pattern at each coating zone  24   a,    24   b,    24   c.  For purposes of illustration and not limitation, a suitable pattern of openings  66   a  is shown in  FIG. 1  wherein each baffle  66  includes  90  openings  66   a  with each opening having a diameter of 0.375 inch. Such baffles  66  can be used with the diameters and numbers of openings  62  on pipe or conduit  18  and openings  65  on spacer walls  64  described above to provide a more uniform gas flow pattern from the inner to the outer circumference of each coating zone  24   a,    24   b,    24   c  to in turn improve uniformity in the composition and microstructure of the diffusion aluminide coating (or other coating) formed on the substrates SB. 
     The spent gas exhausted through baffle openings  66   a  flows to an exhaust tube or conduit  80  that communicates to exhaust gas treatment equipment as described in U.S. Pat. No. 6,143,361, the teachings of which are incorporated herein by reference. The countercurrent flow of exhaust gas outside of inlet conduit  22  helps preheat the coating gas flowing therethrough via heat exchange between the exhaust gas and coating gas in conduit  22 . Although the invention has been described with respect to certain embodiments, those skilled in the art will appreciate that the invention is not so limited to these embodiments since changes, modifications, and the like can be made thereto within the scope of the invention as set forth in the appended claims.