Metal halide gas generator for chemical vapor deposition apparatus, has O-ring seal provided between flange region of housing and base of generator

The present invention relates to a chemical gas deposition apparatus as well as a method for applying coatings to substrates.

Gas deposition by chemical process involves the formation of a gaseous metal halide at relatively low temperatures (eg about 100 to 600 degrees C), the introduction of the gaseous metal halide into a retort at elevated temperature. (eg, a retort temperature of 200 to 1200 degrees C), and reacting the metal halide with substrates placed in the retort to form a coating on its substrates. Generally, a large excess of gaseous metal halide is used in order to avoid depletion of the reactants in the coating retort at elevated temperature. The gas phase deposition processes by chemical process are generally carried out at reduced pressure (pressure below ambient pressure). A chemical gas deposition apparatus and method is disclosed in US Patents 5,261,963 and 5,263,530 Howmet. US Patent 6,143,361 Howmet has disclosed a chemical gas deposition apparatus and method in which excess deposition of metal halide reactants in the coating gases escaping from the coating retort is reduced. or even eliminated, to decrease the retort downtime required to remove deposits from the retort exhaust system.

The chemical gas deposition process can be used to deposit together Al, Si, and one or more reactive elements such as Hf, Zr, Y, Ce, La, etc., to form diffusion protective coatings. aluminide on substrates such as superalloys based on nickel and cobalt conventionally used for casting the blades of gas turbine engines. Pending U.S. Patent Applications 08/197 497 and 08/197 478 disclose a gaseous halide generator and a useful process for making reactive modified element aluminide diffusion protective coatings. . U.S. Patent 5,989,733 discloses an outwardly growing platinum-modified aluminide diffusion protective coating which coating contains Si and Hf, as well as optionally Zr, Y, Ce and / or La, formed on a substrate of nickel or cobalt based superalloy by such chemical gas deposition apparatus and method.

There is a need for an improved type chemical gas deposition apparatus and method capable of providing aluminide diffusion coatings modified by the inclusion of one or more. other coating elements, such as, by way of example only, silicon as well as one or more so-called reactive elements, wherein the coatings can be made more efficiently using a metallic filler residing in one or more coating gas generators. The object of the present invention is to satisfy this need.

In one embodiment of the present invention, there is provided an improved type gaseous metal halide generator for forming a coating gas, and this includes features for reducing air leakage into the gas. generator so as to increase the efficiency of use of the metallic charge residing in the generator. The generator comprises for this purpose cooling sealing characteristics at a junction of the housing and the base of the generator.

According to the invention, the gaseous metal halide generator is characterized in that it comprises a base, a housing having a

metal filler for reaction with a gaseous halide to form a gaseous metal halide, said housing having an area disposed on said base, an airtight seal comprising a polymeric material disposed between said area and said base, said area having a passage fluid for its cooling, as well as a heater for heating the metallic material to a reaction temperature.

According to other advantageous characteristics of the invention:

said area comprises a flange area extending laterally from one end of said housing;

said seal is an O-ring;

said gasket comprises an acid resistant polymeric material;

an inlet fitting is provided on said base and an outlet fitting is provided on said housing, these fittings being of the zero clearance type;

said metallic material is disposed on a perforated gas distribution plate, this plate being disposed in the housing downstream of said flange in the direction of flow of the gaseous halide in said generator.

The invention extends to a method of reducing air leakage in a gaseous metal halide generator, characterized in that it comprises the steps of creating a seal comprising a polymeric material between a housing and a base for the generator, heating a metallic load in the led it housing, causing a gaseous halide to flow over said heated metallic material in order to carry out a reaction making it possible to form a gaseous metal halide, and cooling an area of ​​said housing proximate to said seal.

According to other advantageous characteristics of the invention:

- The seal comprises an O-ring disposed between said housing and said base;

- An inlet connection provided on said base is connected to a source of gaseous halide using a connection of the zero j type;

- An outlet connection provided on said housing is connected to an outlet duct using a connection of the zero j eu type;

the metal charge is placed on a perforated gas distribution plate, this plate being placed in said housing downstream of said seal.

The invention will be better understood and other objects, characteristics, details and advantages thereof will emerge more clearly during the explanatory description which follows, given with reference to the appended schematic drawings given solely by way of example illustrating one embodiment. realization of the invention and in which:

- Figure 1 is a somewhat schematic view of various chemical gas deposition coating gas generators and shows a chamber of a coating reactor which is shown in longitudinal section in accordance with one embodiment of the invention. invention taken by way of example;

FIG. 2 is an enlarged longitudinal sectional view of the chamber of the coating reactor and of the coating gas distribution system connected to the coating gas generators in accordance with one embodiment of the invention;

FIG. 3 is an enlarged longitudinal sectional view of the external coating gas generator in accordance with one embodiment of the invention.

It is by way of illustration but not of limitation that the present invention will be described below with respect to a chemical gas deposition apparatus and method for providing an aluminide to platinum diffusion protective coating. modified containing Si, Hf and, optionally, Zr on a nickel-based superalloy substrate of the type described in US Pat. No. 5,989,733, to which reference may thus be made. Zr can be present in the coating as an impurity in the Hf pellets described below or as an intentional addition. The invention is not limited to the production of such coatings and can be put into practice to form other coatings on other substrates.

Referring to Figures 1 and 2, the chemical gas deposition coating apparatus comprises, in accordance with one embodiment of the invention, a reactor or retort 12 adapted to be disposed in a heating furnace. schematically shown refractory coating 14 which is used to heat retort 12 to an elevated coating temperature by chemical gas deposition. The oven 14 may be an electric resistance oven or another oven of known type used for this purpose. Metal substrates SB to be coated are placed in a chamber 20 of the coating reactor, this chamber being disposed in the retort 12, and they are heated by radiation from the walls of the retort itself heated.

The retort 12 incorporates a cover 16 to close the upper end of the retort. To this end, the retort cover 16 is connected in an airtight manner to a flange 12f of the retort by means of an O-ring 17. The flange 12f comprises an annular cooling water passage 12p through which water is circulated to cool the flange during operation of the retort. Cover 16 incorporates an annular chamber 16a which receives a thermal insulation block or element 16b therein to reduce heat loss from the retort. The components of the chamber 20 of the coating reactor can be carried by the cover 16 and then lowered into the retort 12 at the same time as this cover. The chamber 20 of the coating reactor incorporates conduits 18, 22 joined together at a connection 57, which connection is made before the cover 16 is closed on the retort 12. The conduit 22 is part of the cover 16 because it is welded to the latter.

The retort cover 16 incorporates a central coating gas inlet conduit 22 through which coating reactive gases are supplied to the axial preheating and gas distribution conduit of the reactor 20 as described below. The duct 18 incorporates an internal axial duct 52 for preheating the gases. The chamber 20 of the coating reactor comprises a plurality of distinct annular coating zones 24a, 24b, 24c (FIG. 2) at different axial elevations in the retort and disposed around the pipe or conduit 18 for preheating and distributing the gas. coating. Referring to Figure 2, the SB substrates to be coated are arranged on trays 28 provided in the coating zones 24a, 24b, 24c. The trays 28 close the coating areas 24a, 24b, 24c. The coating areas have been shown as arranged one above the other by way of illustration but not as a limitation as less or more of the coating areas may be used in the practice. of the invention.

Referring to FIG. 1, it can be seen that a conduit 22 for entering the coating gases is made to communicate with a plurality of relatively low temperature metal halide generators 30 and of identical construction except for the example. internal metal charge B, figure 3. The metallic charge B contained in each generator 30 is different and selected to form a particular constituent element of the coating gases, such as, by way of illustration but not of limitation, a bed of aluminum or alloy pellets. aluminum formed in generator # 1 to produce aluminum trichloride or other aluminum halide coating gas component, a bed of silicon or silicon alloy pellets in generator # 2 to produce silicon tetrachloride or other volatile type silicon halide coating gas component, as well as a reactive element, such as Hf, or an alloy thereof, in generator # 3 to form hafnium tetrachloride or other volatile type hafnium halide coating gas component. Other reactive elements which can be used in place of or in addition to hafnium or its alloys include Zr and its alloys, Ce and its alloys, as well as Ni-Mg alloys to form an Mg-bearing coating gas. .

The generators 30 are placed outside the retort 12 and are connected to the inlet conduit 22 through conduits 32. The conduits 32 are heated by conventional type heating devices such as heated flexible bands. by electric resistance or also rods or sticks also heated by electric resistance, in order to prevent condensation of the metal halide coating gases in these conduits.

In order to provide a protective diffusion coating of platinum-modified aluminide containing Si, Hf and Zr on a nickel-based superalloy substrate of the type described in US Pat. No. 5,989,733, the first metal halide generator # is used. 1 to form aluminum trichloride or other component of aluminum halide coating gas. The generator is supplied by a flow of gas Fl comprising a mixture of a gaseous acid halide, such as for example HCl or another gaseous hydrogen halide, and of a reducing or inert gas, such as hydrogen , argon, helium, or mixtures of these gases, via conduits 33 from appropriate sources 41, 42 such as high pressure cylinders or bulk cryogenic supplies. The gaseous acid halide and the carrier gas are mixed together in appropriate proportions in order to provide the gas flow F1 to the first generator.

Referring to Figure 3, the first generator # 1 incorporates a bed B of metallic aluminum pellets as well as a heater 46, such as an electric resistance heater, in order to heat the Al pellets to a reaction temperature which depends on the gaseous acid halide supplied to the generator. By way of example only, it is possible to use, for the gaseous HCL, a temperature of the aluminum pellets of at least about 200 ° C. The temperature of the pellets for other gaseous hydrogen halides depends on the boiling point of the aluminum halide formed in the generator. The flow Fl of gaseous acid halide / support gas is supplied to generator # 1 to pass over the Al pellets under conditions of temperature, pressure and flow rate allowing the formation of aluminum trichloride or a another gaseous aluminum halide, depending on the hydrogen halide gas that is used, in the carrier gas. Examples of temperature, pressure and flow rate for forming aluminum trichloride at generator # 1 are shown in US Pat. No. 5,658,614 as follows:

Hydrogen halide / carrier gas - 13 vol. %

HCl; remain H2

Tablet temperature - 290 degrees C

Flow - 1.3 m³/hour.

The second metal halide generator # 2 is used to produce silicon tetrachloride or other volatile type silicon halide coating gas component. The generator is supplied by a gas flow F2 comprising a mixture of gaseous hydrogen halide, such as, for example, gaseous HCl, and a reducing or inert carrier gas, such as hydrogen, helium and argon, or mixtures of these gases, from suitable sources 41, 42, such as high pressure cylinders or cryogenic bulk feeds. The hydrogen halide gas and the support gas are mixed together in appropriate proportions in order to provide the gas flow F2 to the second generator.

The second generator # 2 incorporates a bed B of silicon wafers as well as a heater 46, such as an electric resistance heater, to heat the Si wafers to a reaction temperature which depends on the halide d. 'gaseous acid supplied

X.

to the generator. By way of example only, it is possible to use, for the gaseous HCl, a temperature of the silicon pellets of at least about 100 degrees C. The temperature of the pellets for other gaseous hydrogen halides depends on the temperature. boiling point of the silicon halide formed in the generator. Examples of temperature, pressure and flow to form silicon tetrachloride at generator # 2 are as follows:

Hydrogen halide / carrier gas - 2 vol. %

HCl; remain H₂

Tablet temperature - 290 degrees C 3

Flow - 0.74 m / hour.

The third metal halide generator # 3 is used to produce a reactive elemental chloride or other reactive elemental halide gas, such as a hafnium tetrachloride coating gas component. The generator is supplied by a gas flow F3 comprising a mixture of a gaseous acid halide, such as for example gaseous HCl, and an inert support gas such as argon, helium, or mixtures of these gases, from appropriate sources 43, 44, such as high pressure cylinders or bulk cryogenic supplies. The hydrogen halide gas and the support gas are mixed together in appropriate proportions in order to form the gas flow F3 towards the first generator.

The third generator # 3 incorporates a bed B of hafnium pellets containing natural Zr impurities and a heater 46, such as an electric resistance heater, to heat the Hf pellets to a reaction temperature which depends on The gaseous acid halide supplied to the generator. By way of example only, it is possible to use, for the gaseous HCl, a temperature of the hafnium pellets of about 430 degrees C. The temperature of the pellets for other gaseous hydrogen halides depends on the boiling point or sublimation point of the metal halide formed in the generator. The pellets of the bed formed in generator # 3 may comprise an alloy of Hf and Zr in case Zr is to be intentionally present as a component in the coating. Examples of temperature, pressure and flow rate to form hafnium tetrachloride at generator # 3 are as follows:

Acid halide / carrier gas - 3 vol. % HCl; remain Ar

Tablet temperature - 430 degrees C

Flow - 0.93 m / hour.

Instead of having three separate generators, it is possible to use a co-generator to together form two gaseous metal halides. It is possible, for example, to form aluminum trichloride and silicon tetrachloride together by passing a mixture of hydrogen halide / support gas over a bed of Al pellets, and then over a bed of aluminum pellets. If placed downstream of the bed of alumnium pellets, as described in the pending patent application filed under number 08/197 478, to which it is thus possible to refer, to produce a coating gas building block which incorporates both AICI3 and SiCl4 in proportions governed by the flow rate of the acid halide / carrier gas over the beds. The third generator # 3 would still be used to produce the HfCl4 coating gas building block. Alternatively, hafnium tetrachloride and silicon tetrachloride can be produced together by passing a mixture of¹hydrogen halide / carrier gas on a bed of Hf pellets, and then on a bed of Si pellets placed downstream of the bed of hafnium pellets. Any combination of pellet beds where the gaseous metal halide from the first upstream bed is more stable than a second metal halide formed in the second downstream bed can be used as a co-generator in the practice of the invention. .

The components of the coating gases coming from the generators 30 are brought to the inlet duct 22 connected to the duct 18 for preheating and distributing the gases at the level of a connection 57. A suitable pump P, such as a vacuum pump, is connected to the exhaust 80 of the coating chamber of the reactor so as to maintain a desired pressure and flow rate for the gases passing through the generators and the coating chamber. 20 and venting the spent coating gas out of the coating chamber.

In accordance with one embodiment of the present invention, the metal halide generators 30 are made to reduce air leakage in the generators at the inlet fitting 30a, the outlet fitting 30b and its flange junction. 30c. Each generator 30 is identical to the other with the exception of the bed B of pellets which is there.

K

In FIG. 3, a generator 30 has been shown comprising a metal casing (eg stainless steel) 30h having an electric resistance heater 46 disposed around to heat the bed B of the generator to the desired reaction temperature; for example as described above. The 3 Ohm housing includes an annular region 30f forming a laterally extending flange at a lower end to rest on a base 35 of the generator with an O-ring 33 between them. The flange 30f which rests on the base 35 defines the junction 30c. The flange 30f incorporates an annular passage 30p through which a cooling fluid (e.g. water) is passed 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 illustrative purposes but not limitation. Cooling the flange 30f during the operation of the generator 30 decreases the deformation of the flange 30f at the high temperature of the 3 Ohm housing during the operation of the generator, and decreases the oxidation of the O-ring.

O-ring 33 is compressed between the area of ​​cooled flange 30f and flange 35f of generator base 35 to create an airtight intermediate seal. The O-ring comprises an acid resistant fluoroelastomeric polymeric material which does not release into the generator 30 carbon, sulfur or other unwanted stray elements which could adversely affect the coating formed on the SB substrates. A suitable O-ring 33 is commercially available as a Viton O-ring from Dupont Dow Elastomers, Wilmington, Delaware, USA. It is possible to provide more than one O-ring 33 between the region of the flange 30f and the base 35.

The inlet fitting 30a provided on the base 35 and the outlet fitting 30b provided on the housing 30h of the generator 30 include commercially available zero clearance type fittings which provide knife edge sealing surfaces. (not shown) which penetrate a seal shown) to create a suitable fitting 30a, annular nickel (not airtight seal. Zero clearance type 30b usable in the practice of the invention are available from Swagelok Corporation, Solon, Ohio, USA as a face seal type metal gasket.

The bed B of pellets is arranged on a perforated gas distribution plate 37 which is placed further downstream of the area of ​​the flange 30f, namely downstream in the direction of gas flow in the flange. generator, thereby reducing heat input to the area of ​​flange 30f and o-ring 33. In the past, as described in US Patents 5,407,704 and 5,264,245, plate 37 was placed at the area of ​​flange 30f with a particular type of gasket which emitted carbon and sulfur in it. the generator. The plate 37 is heated by contact with the bed B of the pellets in the generator and by proximity to the radiator 46 so that the fact of placing the plate 37 further away from the flange 30f reduces heat input to the area of ​​the flange 30f and o-ring 33. An example of the spacing of the gas distribution plate 37 from the area of ​​the flange 30f is at least 2.5. cm for illustrative purposes but not limitation.

The reduction of air leaks in the generators 30 at the level of the flange 30f and the connector 30a limits the oxidation of the bed B forming a charge of pellets. Thus, the efficiency of use of the various charges of pellets is increased. For example, the efficiency of using the hafnium pellet charge in generator # 3 has been increased from less than 5% to over 98% by preventing air leaks in the third generator 30. Reducing air leakage into the coating gas conduit at connector 30b prevents oxidation of reactive elemental halides exiting the generator and thus improves control of the coating composition.

An advanced type coating gas distribution system is provided to ensure a more uniform temperature of the coating gases in coating areas 24a, 24b, 24c of coating chamber 20.

In particular, the constituent elements of the coating gases (for example AICI3, S1CI4, HfCl4 and the carrier gases) are transported to the inlet duct 22 which defines a gas manifold 50 which is placed above and upstream of the coating chamber 20 in the retort 12 and communicates with a vertical duct 52 of preheating of the coating gases inside the duct 18 for preheating and distributing the coating gases in such a way that the stream ST of the coating gases (comprising the constituent elements of the coating gases) entering the inlet duct 22 flows through manifold 50 and descends through preheating duct 52 to the lowest coating zone 24c of coating chamber 20 to return to the annular space between ducts 18 and 52 in such a way that the stream ST of the coating gases is preheated before entering the coating zones 24a, 24b, 24c via the conduit 18. The manifold 50 incorporates a heater 54, such as an elongated electric resistance heater, suspended therein such that the ST stream of gases flows around the heater 54 to heat the ST stream of gas. . A suitable electric resistance heater which can be placed in manifold 50 is commercially available as a Firerod Cartridge from Watlow Corporation, St. Louis, Missouri, USA, although other heaters may be used for this purpose. Heater 54 may be suspended the length of manifold 50 using a conventional swing-lock compression link 55.

The inlet duct 22 communicates with the preheating duct 52 which is located inside the duct or pipe 18 for preheating and distributing the coating gases. The conduit 22 and the conduits 18, 52 are connected by a coupling connection 57 of the coupling type.

The conduit 52 extends axially through and the length of the retort 12 via the coating areas 24a, 24, 24c disposed along the length of the coating chamber 20 to the lowermost coating zone 24c where the conduit 52 incorporates a lower gas discharge opening 52a to discharge the ST stream of coating gases into the annulus between the gasket. preheating and gas distribution duct or pipe 18 and preheating duct 52 for upward flow in the annular space towards the coating areas, as illustrated by the arrows.

For purposes of illustration but not limitation, the coating gas stream ST which is described above (e.g. AICI3, S1Cl4, HfCl4 and carrier gases) can be preheated to a gas temperature greater than 100 degrees C using the heating device 54 provided in the collector 50 and the heating provided by the flow of current through the conduits 18, 52 as described above when the coating chamber 20 is at a temperature of 1080 degrees C.

Radiant heat shields 70 are provided above the coating areas 24a, 24b, 24c to reduce heat loss from the top of the coating chamber 20. The heat shields 70 are made of stainless steel plates. connected in parallel, as shown above coating chamber 20, to reflect heat energy radiation back to coating chamber 20. The heat shields 70 have legs 70a spaced circumferentially around their peripheries so that the plates 70 can be stacked on top of each other on the top plate.

<td>28. He</td><td>is possible</td><td>to use</td><td>of</td><td>such</td><td>shields</td>

<td colspan="2">radiant heaters 70</td><td colspan="2">instead of</td><td>screens</td><td>absorbers</td>

<td>described</td><td>in US patent</td><td> 5 407 704.</td><td></td><td></td><td></td>

<td>The</td><td>preheating</td><td>ST current</td><td>of</td><td>gas</td><td>coating</td>

which uses the heater 54 provided in the collector 50 and which uses the heating provided by the flow of current through the conduits 18, 52 as described above as well as the reduction of heat losses by radiation from the coating chamber 20 by means of the shields 70 improve the temperature uniformity of the coating gases in the coating zones 24a, 24b, 24c so as to considerably reduce the variations in thickness of the coating on the SB substrates from one coating zone to the next. Thus, the ST stream of coating gases is heated more evenly in retort 12 to the desired coating deposition temperature before being directed into coating areas 24a, 24b, 24c. For purposes of illustration but not limitation, a temperature gradient of the coating gas stream of only 10 degrees C along the length of the coating chamber 20 can be provided which precludes the increase in temperature. 200 degree C temperature gradient experienced in the chemical process gas deposition apparatus of the type illustrated in US Patents 5,407,704 and 5,264,245.

Once the coating gas stream ST has reached a desired coating reaction or temperature, an improved type coating distribution system ensures a more even distribution of the preheated coating gas stream between the coating zones 24a, 24b. , 24c provided in the coating chamber 20.

In particular, the preheating and distribution duct 18 extends axially through the annular support plates 28 for the substrates which define between them the distinct annular coating zones 24a, 24b, 24c around the pipe or duct 18. The pipe or conduit 18 incorporates, at a midpoint of the height of each coating zone 24a, 24b, 24c, a plurality of gas discharge holes or openings 62 spaced circumferentially with respect to each other in order to discharge the gas. preheated coating gas stream ST to each coating zone. The number of openings 62 at each coating area can be changed as needed. For a duct diameter 18 of 3.75 cm and an axial spacing of 15 cm between the plates 28, it is possible to provide in the duct 18 at least three openings 62. The surface of the openings 62 (for example, the number of holes) at the coating zones 24a, 24b, 24c is systematically varied to ensure an equal flow of the coating gases from the conduit 18 to each coating zone. Typically, the number of openings 62 at the coating area 24a is greater than that at the coating area 24b, and the number of the holes 62 at the coating area 24b is greater than that. at the level of the coating zone 24c. By way of example only, the number of holes at the coating area 24a may be 10, the number of holes at the coating area 24b may be 8, and the number of holes at the level of the coating area. coating area 24c can be 6.

The conduit 52 also incorporates one or more draw-off openings 52b above the lower primary coating gas discharge opening 52a so as to discharge the coating gases along the length of the conduit 52. For example, a draw-off opening 52b is located at the coating zone 24b, and a draw-off opening 52b is located at the coating zone 24c to help ensure a generally even flow of coating gases. between the coating areas 24a, 24b, 24c.

Although a single draw-off opening 52b has been shown for this purpose at each coating zone 24b and 24c in the upper part of each coating zone 24b, 24c, it is possible to provide more than one draw-off opening. at the same or different locations of the coating area 24a, 24b, 24c, as needed, to generally equalize the flow of coating gases between the coating areas 24a, 24b, 24c.

The coating gases discharged from the draw-off openings 52b then flow upwardly into the duct 18. It is then possible to provide draw-off openings 52b each having a diameter of 0.3 cm for use with conduits 18, 52 having the dimensions indicated above.

The annular plates 28 are spaced axially from each other near their internal circumference by vertical annular internal walls forming a spacer 64 and near their external circumference by vertical external perforated deflectors 66. The walls 64 forming a spacer are placed symmetrically around the pipe or conduit 18 using retaining rings 67 welded or otherwise provided on the plates 28. The trays 28 include a central hole 28a having an internal diameter approximately equal to the external diameter of the pipe or conduit 18 for receiving the same so that the trays 28 are disposed symmetrically around the pipe or conduit 26. The trays 28, the walls forming the spacer 64 and the deflectors 66 are stacked one above the other and carried by a lower side flange 18a of the pipe or conduit 18. The gas distribution pipe or duct 18, the plates 28, the walls forming the spacer 64 and the deflectors 66 are thus arranged in symmetrical fixed positions around the central longitudinal axis of the coating chamber 20.

The spacer walls 64 define an annular gas manifold 68 at each liner zone 24a, 24b, 24c between the pipe or conduit 18 and the walls 64. Each spacer wall 64 opposes or faces the discharge openings. 62 of the pipe or conduit 18 at this coating zone. Each spacer wall 64 incorporates first and second sets of gas flow openings 65 spaced circumferentially from one another and located equidistant above and below the openings 62 provided in the pipe or conduit 18. . Each spacer wall 64 thus has a plurality of gas flow openings 65 which are out of alignment with the gas discharge openings 62 at each coating area such that there is no alignment. the gas flow path from the gas discharge openings 62 to the gas flow openings 65 at each coating zone.

For purposes of illustration but not limitation, 48 gas flow openings 65 having a diameter of 0.6 cm may be provided in each wall 64 at each coating area 24a, 24b, 24c when the conduit. 18 includes apertures 62, the number and diameters of which are as described above. Placing the openings 62 of the pipe or gas distribution conduit 18 midway between the sets of openings 65 prevents gas jets from passing directly through each liner zone. Likewise, the deflection of coating gases

<td>out</td><td>of</td><td>The interior</td><td>from the wall 64</td><td>at the level</td><td>of each</td>

<td>zoned</td><td>of</td><td>coating</td><td colspan="2">produces more flow</td><td>uniform</td>

<td>of</td><td>gas</td><td>around</td><td>the circumference</td><td>of each</td><td>zone of</td>

<td colspan="3">coating 24a, 24b,</td><td>24c.</td><td></td><td></td>

The above gas distribution system ensures a uniform and repetitive flow of gases to the coating areas 24a, 24b, 24c in order to improve the uniformity of the composition and the microstructure of the coating between the SB substrates provided on the coating. same plate 28 and between the substrates provided in different coating zones.

Once the ST stream of the coating gases has flowed over the SB substrates provided on the trays 28 at each coating zone 24a, 24b, 24c, yet another embodiment of the present invention creates a system of coating gas. 'improved type of waste gas exhaust ensuring less interaction between the flow of inlet coating gases to each coating zone 24a, 24b, 24c and the flow of exhaust gases from each coating zone so as to ensure a more uniform flow of gases from within the coating zones.

In particular, perforated tubulars 66 are provided between the plates 28 at their outer circumferences as shown in FIGS. 1 and 2. The tubular deflectors 66 are made of a nickel-based superalloy IN-600 and incorporate patterns of openings. 'exhaust 66a through which escapes the waste gas exiting the coating zones 24a, 24b, 24c. The pattern of the openings 66a as well as their number and size (for example their diameter) can be selected to ensure a more or less uniform flow pattern of the gases at each coating zone 24a, 24b, 24c. For purposes of illustration but not limitation, a suitable pattern for the openings 66a is shown in Figure 1 in which a baffle coating pattern each baffle 66 incorporates 90 openings 66a with each opening having a diameter of 0.9 cm. It is possible to use such baffles 66 with the diameters and number of the openings 62 provided on the pipe or conduit 18 and the openings 65 provided on the spacer walls 64 described above to ensure a more uniform pattern of the. gas flow from the inner circumference to the outer circumference of each coating area 24a, 24b, 24c in turn to improve the uniformity of composition and microstructure of the diffusing aluminide coating (or other coating) formed on the SB substrates.

The waste gas escaping from the openings 66a of the baffles flows to an exhaust pipe or duct 80 which communicates with the exhaust gas treatment equipment described in US Pat. No. 6,143,361, to which it is then possible to refer. The countercurrent flow of the exhaust gases out of the inlet duct 22 helps to preheat the coating gases which flow through this duct via heat exchange between the exhaust gases. and the coating gases in line 22.