Abstract:
Electrostatic deposition of high performance powdered materials onto gas turbine surfaces. The process also includes post-deposition thermal staging of the deposited powder to provide a durable coating that will satisfy the demands of turbine engine operation. The process envisions application of organic-based powdered materials, glass/ceramic powdered materials and metal-based powdered materials and combinations thereof using electrostatic techniques to components exposed to low temperature operations, such as may be found in the front section of a gas turbine engine or to the exterior portions of an aircraft engine, and metal-containing glass ceramics, glass-ceramic materials, or materials that can be transformed into glass ceramic materials, when applied to components exposed to high temperature operations, such as may be found in the turbine and exhaust sections of a gas turbine engine or the flaps of an aircraft.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is directed to a method of applying a coating to aircraft engine components, and specifically, to applying electrostatic coatings to aircraft engine components in an environmentally friendly fashion.  
       BACKGROUND OF THE INVENTION  
       [0002]     Gas turbine engine components employ coatings over base material in various applications to provide protection to the underlying structural base material. The purposes for applying the coatings are varied, and may include one or more purposes depending upon the application. For example, turbine airfoils used in the hot section or turbine section of a gas turbine engine include coatings to provide heat resistance and improved thermal capabilities. These airfoils also may be used in harsh environments, thereby additionally requiring coatings having resistance to corrosion or oxidation. Such coatings are referred to as environmental coatings.  
         [0003]     Other applications may require still other coatings. For example, the shrouds surrounding the rotating airfoils, also referred to as blades, form a tunnel through which the hot gases pass. In addition to being able to withstand the hot, corrosive gases of combustion, these shrouds must also be abradable, as the rotating airfoils (blades) expand in a radial direction and contact the shrouds. It is desirable that, as contact is made between the rotating blades and the stationary shrouds that material be abraded from the shroud without affecting the structural integrity of the shrouds.  
         [0004]     Of course, other sections of a gas turbine engine may require still other coatings. For example, in an aircraft engine, some components exposed to sand and rain may require erosion resistance. The specialized coatings in turbine engines are myriad. The coatings also may be applied to very large surface areas, such as shrouds, or to very small surface area, such as the tip regions of first stage turbine blades. The coatings also may be applied to a variety of substrate materials, such as for example superalloy materials, including nickel-based superalloys, cobalt-based superalloys, iron-based superalloys and combinations thereof, titanium and its alloys, and composites such as CMC&#39;s.  
         [0005]     Although the structures to which the coatings are applied may vary, a few time-tested techniques have been utilized for their application. The techniques include a variety of modifications that solve particular problems. However the techniques generally include physical vapor deposition techniques (PVD), chemical vapor deposition techniques, thermal spray techniques, pack cementation techniques laser deposition techniques and plating techniques. A large number of patents have issued dealing with variations of the above-mentioned techniques, and many volumes could be filled discussing the differences distinguishing these variations. These techniques, including the multiple variations, typically produce high quality coatings, as required for demanding applications such as aircraft gas turbine techniques. However, the various techniques used for these applications have differing drawbacks. Some of the above-mentioned techniques, such as PVD, deposit the coating material by a slow, expensive process. Other techniques utilize solvents or release organic effluents, many of which are undesirable. Others, such as laser processing, require very high energy sources and expensive equipment. Still other processes leave undesirable by-products, such as heavy metals, for example chromium, which must be disposed of as hazardous waste.  
         [0006]     What is needed is a process that can deposit a variety of coatings on aircraft engine parts in an economical, fast, energy efficient process that has minimal environmental impact. One method that has heretofore not been used in gas turbine components is powder coating based on electrostatic deposition of powdered materials. While this method has been used for a variety of commercial products such as home appliances, basketball poles, lawn furniture, gas grills and certain automotive applications, the methods have not heretofore been extended for demanding applications such as gas turbine components, including aircraft engine applications. It is likely that such methods have not found their way into this art because they lack a reputation for durability in such demanding applications.  
         [0007]     What is needed is a durable coating for gas turbine engines that is quick, cost effective and environmentally friendly, and which can be readily adapted for application to both large and small surface areas.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is directed to electrostatic deposition of powdered materials onto gas turbine surfaces. The process also includes post-deposition thermal staging of the deposited powder to provide a durable coating that will satisfy the demands of turbine engine operation.  
         [0009]     While the present invention is directed to applying powdered materials by electrostatic deposition, the present invention envisions application of organic-based powdered materials, glass/ceramic powdered materials and metal-based powdered materials and combinations thereof using electrostatic techniques to components exposed to low temperature operations, such as may be found in the front section of a gas turbine engine or to the exterior portions of an aircraft engine, and metal-containing glass ceramics, glass-ceramic materials, or materials that can be transformed into glass ceramic materials, to components exposed to high temperature operations, such as may be found in the turbine and exhaust sections of a gas turbine engine or the flaps of an aircraft. However the present invention may also find application in the combustor section of the engine, as well as in cooler engine sections such as the compressor section. Aircraft engine components that may be coated by the present process include, but are not limited to, shrouds, flaps, seals, liners, cowls, center bodies and combustors  
         [0010]     The coating of the present invention is applied by determining the proper coating material to provide the required properties for the intended application. Then, the material is provided as a powder in a preselected size range. The size range is selected based on required coating thickness for the intended application. If the size range has a high fraction of large particles, the coating may not have the proper density. If the size range has excessive fines, flowability may be a problem. Substrate size and configuration may also be a consideration in determining the size range of the particles. The powder particles are fed to a high voltage powder spray gun. The article to be coated is grounded and the surface is positioned facing the powder spray gun. An electrical potential is established between the article and the powder spray gun. As the powder is sprayed from the gun, an electrical charge is imparted to the powder particles, which are then drawn toward the oppositely charged surface of the article. The polarity will depend on the types of particles that are sprayed. Metal particles can be sprayed if the particles are coated, either with an oxide coating or an inorganic coating, provided that the metal powder is isolated within the coating. For the purposes of this application, such coated metal particles will be referred to as metal-based powders.  
         [0011]     After the powder is applied to the article to a preselected thickness, the article is then heat treated at an elevated temperature sufficient to form a strong bond between the substrate and the applied coating. When the applied coating applied using a metal-based powder, the heat treatment temperature should be sufficiently high so as to form a metallurgical bond between the substrate and the applied coating. Depending upon the application and powder size selected, it may be necessary to consolidate the coating prior to the final heat treatment if a high density coating is required.  
         [0012]     An advantage of the present invention is that it can be readily tailored to gas turbine applications, which may include a variety of materials and surfaces of different sizes. The equipment used in the process is readily adaptable to the different components used in gas turbine applications. Changes in flow rates and voltages are readily made.  
         [0013]     Another advantage of the present invention is that a wide variety of particle sizes can be mixed together and sprayed onto the surface of the particle. In addition, particles of different compositions can be mixed together and sprayed onto the article surface. This aspect of the invention can be utilized to apply well-known compositions or to achieve new compositions.  
         [0014]     An important advantage of the present invention is that it is environmentally friendly, providing very high yields while using no solvents and less energy. Firing can be accomplished in very short time frames. For example, firing can be accomplished to temperatures of 1500° F. in times as short as six (6) minutes. Such rapid firing can be accomplished since no binder is utilized; thus, no slow binder burnout is required. In addition, the equipment, its maintenance and operation to apply the powdered coating is inexpensive compared to other coating processes used for gas turbine and aircraft engine applications.  
         [0015]     The coating of the present invention can be tailored to yield high density or low density coatings as desired. Another advantage is that the thickness of the applied coating can also be controlled to meet existing tolerance requirements.  
         [0016]     Yet another advantage of the present invention is that the powders utilized in the spray process, but which are not incorporated onto the substrate surface, can be recovered and reused. Thus, in terms of powder usage, the yield is well above 90%, and can approach 100%, since there is very little powder loss, particularly in large volume applications. This is important as the particles themselves, when they include heavy metals such as chromium or nickel, can constitute a hazardous waste. The reusability of the powders thus eliminates a source of hazardous waste.  
         [0017]     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  depicts a powder spray gun nozzle, in cross-section, propelling a high performance powder coating at a gas turbine engine substrate.  
         [0019]      FIG. 2  depicts the powder spray gun nozzle of  FIG. 1 , in cross-section in greater detail.  
         [0020]      FIG. 3  schematically depicts a spray room operation using the powder spray nozzle of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The method of the present invention is implemented, in its simplest form, as depicted in  FIG. 1 . A powder spray gun  2 , whose partially depicted nozzle  4  includes an electrode  6 . Powder particles  8  is propelled through nozzle  4  at a surface  10  of a gas turbine engine substrate  12 . The nozzle is grounded at  14  as shown in  FIG. 1 . A voltage is set up between the electrode and the surface. As shown in  FIG. 1 , the electrode is negatively charged and the substrate is positively charged. As the particles  8  are propelled past the electrode, a charge is imparted to the particle powders, which are additionally attracted electrostatically to surface  10 . A layer  16  of powder particles  8  forms on surface  10  of the substrate  12 . It will be understood that the polarity of the electrode  6  and substrate  12  can be reversed as required. Such a reversal of polarity may be desirable based on the chemistry of the powder particles so as to improve yield. It will also be understood that the nozzle itself may be designed as an electrode, or that the electrode may be built into the nozzle, rather than being positioned in the center of the nozzle orifice as depicted in  FIG. 1 .  
         [0022]     The nozzle  4  of  FIG. 1  is shown in greater detail in  FIG. 2 . Electrode  6  is positioned at the mouth  18  of orifice  20  of nozzle  4 . The mouth  18  of nozzle  4  is flared so that the volume of the nozzle at the mouth increases. As shown, the nozzle is chamfered or beveled. However, the flaring can be achieved by any other geometric configuration, such as a mouth having a concave or parabolic opening. A deflector  22  is positioned in mouth  18 , surrounding electrode  6 . Powder particles traveling through orifice  20  of nozzle  4  are deflected outwardly by deflector  22  as they reach the nozzle, a shown in  FIG. 2 , and also are charged by electrode  6  as they pass through mouth  18 . Electrode  6 , as shown in  FIG. 2 , is connected to a power source via resistive wire  24  positioned in wall  26 . As previously noted, this is but one configuration for the nozzle of a powder spray gun. Any other configuration that imparts a charge to powder particles exiting the nozzle may also be used.  
         [0023]      FIG. 3  schematically depicts an exemplary embodiment of the equipment used for the process of the invention in greater detail in a spray room operation. A gas turbine engine substrate is depicted mounted in a chamber  30  and connected to ground at  14 . Nozzle  4  of power spray gun  2  extends through an opening  32  in a wall  34  of chamber  30 . A high voltage power pack  38  is connected to the powder gun  2 . Powder to be sprayed is loaded from a powder source, drum unloader  40  and provided to collector  42 . Here the powder can be sieved in sieve  44 , and when appropriate, run through a magnetic separator  46  to separate magnetic material from non-magnetic material. Oversized material can be removed from sieve through channel  45  and sent to a collection station (not shown.) The powder then is moved into distribution hopper  48  and then into feed hopper  50 . From here, the powder is fed into the nozzle  4  of gun  2 . The charged powder particles are then directed toward substrate  12 . Any unused powder particles can be recovered by passing them through to recycle bin  52 , where they can be redirected into collector  42 .  
         [0024]     To change powders, it is necessary to remove the drum from drum unloader  40  and purge the collector  42 , sieve  44 , magnetic separator  46  (if used) distribution hopper  48  and feed hopper  50 , as well as recycle bin  52  of powder. Sieve  44  can be removed and replaced with a different size as required and a new drum a different powder can be provided in drum unloader  40 .  
         [0025]     In accordance with the present invention, coupons representative of aircraft engine substrates were prepared. Samples were prepared both on bare substrates comprising superalloy material and on substrate to which a bond coat was applied. The bond coat was a well known NiCrAlY. Test coupons were 1″×4″ and 3″×3″. The powders sprayed were processed NiCrAl balls sized through −400 mesh. The balls were in the size range of 5-30 microns and includes an oxide scale naturally formed by exposure to the atmosphere.  
         [0026]     The powder particles were fluidized at a pressure of 5-8 psi and atomized at a pressure of 50 psi to provide a flow pressure from the nozzle of 50 psi. A voltage of 90 kv was applied between the nozzle and the substrate. The NiCrAlY powders were atomized along with a binder comprising Ferro PG-94C, commonly referred to at GROUNDCOAT™, a fused silicate glass frit available from Ferro Corporation, Frit Division, 4150 East 56 th  Street, Cleveland, Ohio 44101. After application, the coupons were fired at a temperature of 1400-1650° F. for a time of 4-6 minutes.  
       EXAMPLE 1  
       [0027]     A 3″×3″ test coupon of 0.060′ thick IN 625 was prepared. The coupon was coated with a standard NiCrAlY bond coat to a thickness of 3-12 mils. Uncoated iron-based powders and PG-94C binder was then applied to the coupon at a thickness of about 0.013″ using the procedure set forth above, including the heat treatment. After heat treatment, the outer strain of the coating was measured and found to be about 0.86%. The PG-94C, discussed above, is included in the final coating to hold other ceramic and metal particles in place.  
       EXAMPLE 2  
       [0028]     A 3″×3″ test coupon of 0.060′ thick IN 625 was prepared. Uncoated iron-based powder and PG-94C binder was then applied directly to the surface of the coupon (i.e. no bond coat) to a thickness of about 0.013″ using the procedure set forth above, including the heat treatment. After heat treatment, the outer strain of the coating was measured and found to be about 1.19%.  
       EXAMPLE 3  
       [0029]     A 3″×3″ test coupon of 0.060′ thick IN 625 was prepared. The coupon was coated with a standard NiCrAlY bond coat to a thickness of 3-12 mils. Coated iron-based powder and PG-94C binder was then applied to the coupon to a thickness of about 0.041″ using the procedure set forth above, including the heat treatment. The iron-based powder included a naturally-formed aluminum oxide coating. The oxide coating provides electrical isolation among the particles when higher metal loadings are desired. After heat treatment, the outer strain of the coating was measured and found to be about 0.70%.  
       EXAMPLE 4  
       [0030]     A 3″×3″ test coupon of 0.060′ thick IN 625 was prepared. Iron-based powder having a naturally-developed oxide coating and PG-94C binder was then applied directly to the surface of the coupon (no bond coat) to a thickness of about 0.041″ using the procedure set forth above, including the heat treatment. The coated iron-based powder comprises about 60% by weight of the sprayed coating, the sprayed coating being a mixture of coated powder and PG-94C binder. After heat treatment, the outer strain of the coating was measured and found to be about 0.98%.  
         [0031]     While the invention has been set forth in the examples and procedure set forth above, the invention is not so limited. The voltage used in the above examples was limited by the available test equipment. It is envisioned that higher voltages can be used. The only limitation on the voltage used is that the equipment and substrate not be damaged by the applied voltages, such as, for example, by arc strikes. The available voltages limited the coating thicknesses tested. It is envisioned that higher voltages can produce thicker coatings, when so desired.  
         [0032]     The above examples utilized iron-based alloy powders. These metal powders are readily charged. However, the powders used for the coating are not so limited, as any powder that can be charged can be applied by the above described process. The limitation on the powder is whether the powder can provide the required protection to the substrate.  
         [0033]     The above binder was Ferro PG-94C. This binder sets forth the current best mode of practicing the invention. However, other binders also may be acceptable. The binders must be compatible with the electrostatic powder spray procedure. The binder utilized was a glass frit that forms the ceramic matrix of the coating system. It will be recognized that other binder/matrix materials can be utilized that will become a part of the final coating. For example, a silicone may be utilized that can be converted into a glass, a glassy ceramic or a ceramic, depending upon the heat treatment applied and intended use. Whether the binder is incorporated into the final coating or is transitory depends upon the intended use of the article substrate and the component. Other binders that may be used include, but are not limited to, GE SR 350, a silicone based binder, a binder including submicron alumina and smaller particles such as ALCOA A16SG an active alumina and a high temperature glass frit, such as V212, available from Vitrifunctions, Inc., Clawson Ave., Youngwood, Pa. 15697.  
         [0034]     The above described heat treatment is effective for PG-94C and iron-based powders. It will be understood that this heat treatment will not be effective to achieve proper adherence of other types of powders, and other heat treatments will be required to develop the required properties of another and different powder or powders as a coating.  
         [0035]     The above examples do not reflect the use of additional ceramic fillers, However, ceramic fillers that can be applied by the above spray techniques may also be used as required. In addition, the size of the powders can be varied to provide required coating powders. For example, various sized powders can be applied to provide varying densities. If desired, different powders can be applied in different layers to achieve different densities. Furthermore, powders of different compositions can be applied as distinct layers to achieve different properties in different layers. These different properties may include different mechanical properties, different chemical properties, different environmental properties and different physical properties. However, care must be taken to provide the proper heat treatments to these layers to achieve the desired properties. Multiple heat treatments in the correct sequence may be required. Furthermore, different powders may be sprayed at the same time into a layer to provide novel compositions with unique properties. Once again, care must be taken to provide a proper heat treatment compatible with the powders of different compositions.  
         [0036]     The present invention can be used for application of coatings in aircraft engine turbine components that currently are applied by different processes that have disadvantages or that cannot readily be applied at all. For example, by mixing different powders that would otherwise form as a two phase material, and proper heat treatment, a substantially uniform coating can be achieved. By proper selection of powder size, for example by application of metal balls, differences in thermal expansion between the coating and substrate can be accounted for. Thus, by properly sizing metal balls, the coating can expand at a different rate than the underlying substrate to create a strain tolerant coating. Similarly, different layers can be applied with differential expansion rates so that the strain due to thermal expansion can be distributed over the various coating layers to create a strain-tolerant coating, rather than at an interface between a coating and a substrate. TBC coatings can also be applied by the present invention, as it is not limited to metallic balls. By proper selection of materials and sizing of powders, the density of TBC coatings produced by current methods can be duplicated as desired. If desired, the density of these TBC coatings can be varied to produce a strain-tolerant coating or to vary the thermal effects as desired by proper application of porosity and cooling air. Other metals can also be added, such as the family of MCrAlX, a well-known designation for an expansion matching metal coating which can also inhibit corrosion, as well as other expansion matching and/or corrosion inhibiting metal coatings, where M is an element selected from the group consisting of Fe, Ni Co and combinations thereof, while X is selected from the group consisting of Ta, Re and reactive elements, such as Y, Zr, Hf, Si, and grain boundary strengtheners consisting of B, C and combinations thereof.  
         [0037]     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.