Composition for providing a coating on an airfoil and a process for forming the coating

Abstract: The present invention relates to a composition for coating internal surfaces of an airfoil, particular high pressure temperature airfoils such as vanes and blades. The composition includes a dry composition consisting essentially of from about 2.5 wt% to about 7.0 wt% aluminum fluoride, from about 5.0 wt% to about 20 wt% of a chromium - aluminum powder, and from about 75 wt% to about 92.5 wt% Al 2 O 3 . The dry composition is mixed with water and a cellulose compound to form a slurry which is thereafter injected into the airfoil. The coating compositions of the present invention are valuable in that they are capable of forming an oxidation and corrosion resistant coating on the internal surfaces of the airfoil simultaneously with the formation of an exterior corrosion and oxidation resistant coating. A process for simultaneously forming corrosion and oxidation resistant coatings on the interior and exterior surfaces of an airfoil is described herein.

Description: The present invention relates to a particular coating composition used to form an oxidation and corrosion resistant coating on internal surfaces on an airfoil to protect it against oxidation and corrosion during operation and to a process for coating said airfoil. Aluminide coatings are applied on nickel-based superalloy turbine airfoils to protect the airfoils against oxidation and corrosion during operation in the turbine section of engines. These coatings are formed by the deposition of aluminum onto the surfaces of the airfoils. A reaction occurs between the nickel in the airfoil material and the deposited aluminum to form nickel-based aluminides. At high temperatures, in the presence of air, the aluminum in the nickel aluminide coating forms a thin, adherent aluminum oxide at the surface of the coating. This aluminum oxide provides a barrier against further oxidation and corrosion of the turbine airfoil. The external surfaces of turbine airfoils in most engines are coated with aluminides. The performance requirements of the engines determine whether aluminide coatings also are required on the internal surfaces of the airfoils. Currently, there are two processes used to internally coat airfoils. One process employs a slurry technique and is used for airfoils that operate in the low pressure turbine section of an engine. The second process uses chemical vapor deposition to coat the internal surfaces of the airfoils for the high pressure turbine section of an engine. Different coating processes are employed for the two different types of airfoils because the high pressure turbine section of an engine operates at a higher temperature and pressure than the low pressure turbine section of an engine. As a result, the coating applied to high pressure turbine airfoils must have higher temperature capacity and must be more robust than those applied to low pressure turbine airfoils. U.S. Patent No. 5,366,765 to Milaniak et al. describes a slurry technique for coating internal passages in low pressure turbine airfoils. The slurry described in this patent cannot be used to coat the internal passages of high pressure turbine airfoils for the following reasons: (1) the slurry produces a coating that is too brittle; (2) the coating is too thick to apply to the internal cooling passages of high pressure turbine airfoils; and (3) it is not compatible with the processes used to coat the external surfaces of airfoils. As previously mentioned, chemical vapor deposition processes are used to coat airfoils used in the high pressure turbine section of an engine. During the coating process, turbine airfoils 10 are placed in an upright position within a compartmentalized, large metal box or coating fixture 12, called a coat boat. The figure illustrates a typical coat box arrangement. To coat the internal passages of the airfoils 10, chemicals 14 are placed in a compartment 16 below the airfoils. The airfoils are mounted on specialized plumbing tools 18 that allow vapors to flow through the internal cooling passages of the airfoils. Argon gas is introduced into the lower compartment 16 via inlet 20 to force the coating vapors through the internal areas of the airfoils. These vapors react with the internal surfaces of the airfoil to produce an aluminide coating. At the same, chemicals 14 in an upper compartment 22 create vapors which react with the external surfaces of the airfoil to form an aluminide coating thereon. There are problems however associated with this process. The problems include the need to use a forced argon flow and the need to use specialized plumbing tools to allow the coating vapors to flow through the internal passages of the airfoil. Thus, there remains a need for a coating process which eliminates the problems associated with the chemical vapor deposition processes currently employed. There is also a need for a coating process which allows the external and internal surfaces of an airfoil to be coated during a single cycle. Accordingly, it is an object of the present invention to provide a composition for forming a corrosion and oxidation resistant coating on internal surfaces of airfoils. It is a further object of the present invention to provide a coating composition as above which is compatible with processes used to coat external surfaces and therefore allows the internal surfaces of an airfoil to be coated simultaneously with the external surfaces of the airfoil. It is yet a further object of the present invention to provide a coating process which does not need any forced argon flow and/or specialized plumbing within the coat box. The foregoing objects are attained by the coating composition and the coating process of the present invention. In accordance with the present invention, a composition for providing a corrosion and oxidation resistant coating on the internal surfaces of an airfoil includes a dry composition consisting essentially of from 2.5 wt% to 7.0 wt% aluminum fluoride, from 5.0 wt% to 20 wt% of a chromium-aluminum powder, and from 75 wt% to 92.5 wt% Al 2 O 3 . The dry composition is mixed with water and a cellulose compound to form a slurry composition which can be directly applied to the internal surfaces of an airfoil, thereby eliminating the need for an argon purge and the need for specialized plumbing. In accordance with the coating process of the present invention, the aforementioned slurry is formed by providing the aforementioned dry chemical composition and mixing the dry chemical composition with the cellulose compound and water. Thereafter, the slurry composition is placed into direct contact with the internal surfaces of the airfoil and dried. After the airfoil with the slurry composition therein has been baked so as to remove the water and to harden the solids in the slurry composition, the airfoils are placed within a coating fixture. Also placed in the coating fixture is a chemical composition for coating the exterior surfaces of the airfoil. Thereafter heat is applied for a time sufficient to form a protective coating on the exterior surfaces of the airfoil and simultaneously form a protective coating on the internal surfaces. Further details of the composition and the process of the present invention, as well as further objects and advantages, are set forth in the following detailed description and the accompanying drawings, wherein like reference numerals depict like elements. The Figure illustrates a known art coating fixture for coating internal and external surfaces of an airfoil. A first aspect of the present invention is the use of a slurry composition to coat the interior surfaces of airfoils such as high pressure turbine blades and vanes. As previously discussed, the primary purpose of the coating formed from the slurry of the present invention is to increase the oxidation and corrosion resistance of nickel alloy turbine airfoils. Compositions in accordance with the present invention for coating the internal surfaces of an airfoil include a dry composition consisting essentially of from 2.5 wt% to : 7.0 wt% aluminum fluoride, from 5.0 wt% to about 20 wt% of a chromium-aluminum powder, and from 75 wt% to 92.5 wt% Al 2 O 3 . The chromium-aluminum powder in the dry composition preferably comprises a chromium-45 aluminum powder, (that is a chromium-aluminum powder comprising 55 wt.% chromium and 45 wt% aluminum). The aluminum fluoride and the chromium-aluminum powder form the active elements in the coating compositions of the present invention. In a preferred embodiment, the dry composition consists essentially of from 2.5 wt% to about 5.0 wt% aluminum fluoride, from 15 wt% to 20 wt% of the chromium-45 aluminum powder, and from 75 wt% to 80 wt% Al 2 O 3 . In a most preferred embodiment, the Al 2 O 3 constituent is in a range of from 75 wt% to 77.5 wt%. In order to be applied to the internal surfaces of the airfoil, the dry composition must be converted into a slurry composition. This is accomplished by mixing the dry composition with non-active ingredients including water and a cellulose compound known as METHOCEL®. Suitable slurry compositions can be formed by adding water in a range of from 1900 to 2700 cc and by adding the cellulose compound in a range of from 60 to 100 grams to between 1400 to 1600 grams of the dry composition. The water may be heated to aid in dissolving the cellulose compound. Any commercially available cellulose compound is usable. A preferred compound is METHOCEL® brand cellulose compound which is distributed by The Dow Chemical Company, Midland, Michigan. A suitable coating slurry can be manufactured by adding 2300 cc of water and 80 grams of Methocel to 1505 grams of the dry compositions of the present invention. After the slurry composition has been formed, it is placed into direct contact with the internal surfaces of the airfoil. This is done by injecting the slurry into the interior of the turbine airfoils. Typically, the airfoils have internal passageways and the slurry is injected into the passageways. Any suitable means known in the art may be used to inject the slurry into the internal passages of or otherwise place it into contact with the internal surfaces of the airfoil. For example, the slurry may be placed into contact with the internal surfaces of the airfoil using the technique shown in US-A-5,366,765 to Milaniak et al.. Once the slurry composition has been injected into the interior of the airfoil, the airfoil is baked to remove water and harden the slurry solid. Typically, the baking operation consists of subjecting the airfoils with the injected slurry therein to a temperature in the range of from about 551,6° C (1025° F) to about 579° C (1075° F) for a time in the range of from about 30 minutes to about 2 hours. Any suitable means known in the art may be used to bake the airfoils. The coating compositions of the present invention are compatible with the CUD processes used to coat the external surfaces of airfoils. Thus, it is possible using the coating composition of the present invention to simultaneously form oxidation and corrosion resistant coatings on both the exterior and the interior surfaces of the airfoil. To do this, a number of airfoils containing the baked slurry compositions are placed in an upright manner in a metal coating fixture similar to that shown in the figure. The chemical composition for coating the external surfaces of the airfoil is also placed in the fixture. One suitable exterior surface coating composition which can be utilized consists essentially of 15.4 wt% aluminum fluoride and 84.6 wt% of chromium-45 aluminum. When heated to temperatures above 1079° C (1975° F), the exterior surface chemical composition vaporizes and deposits aluminum onto the exterior surfaces of the airfoils. The internal surfaces of the airfoil are simultaneously coated by direct diffusion of aluminum from the hardened slurry composition. A typical thermal cycle applied during this simultaneous coating operation comprises an initial coat cycle during which a temperature in the range of 1065,5° C (1950° F) to about 1121,1° C (2050° F) is applied for a time in the range of 4 to 10 hours and thereafter a diffusion heat cycle is performed by applying heat at a temperature in the range from about 1065,5° C (1950° F) to about 1121,1°C (2050° F) for a time in the range of 4 to 7 hours. As final steps in the process of the present invention, the airfoils are heat tint treated at a temperature of about 579° C (1075° F) for about 1 hour to determine coat quality and thereafter precipitation heat treated at a temperature of about 704° C (1300° F) to about 871° C (1600° F) for a time period in the range of 12 to 32 hours. To demonstrate the coating which can be obtained using the present invention, six slurry compositions were prepared in accordance with the present invention. The six slurry compositions contained the dry compositions set forth in Tables 1 and 2. A seventh slurry composition was prepared using the PWA 273 dry composition set forth in Tables 1 and 2. After each of the dry compositions was prepared, 1505 grams of the dry composition was mixed with 2300 grams of water and 80 grams of Methocel to form a slurry. The various slurries were then injected into the internal passages of a number of high pressure turbine airfoils. The coating trial was then completed at a coating temperature of 1079,4° C (1975° F) for a coat time of 4 hours. The objects of the test were to obtain a coating thickness between 1.5 and 2 mils and that there be no bare spots. Slurry Dry Composition Avg. Coat Thickness (m) Max. Coat Thickness (m) Min. Coat Thickness (m) 1 2.5 wt% AlF 3 (0.75 mils) (1.7 mils) 0 5 wt% Cr-45 Al 1,905 x 10 -5 4,445 x 10 -5 92.5 wt% Al 2 O 3 2 5 wt% AlF 3 (0.57 mils) (1.7 mils) (0.0003 mils) 5 wt% Cr-45Al 1,4478 x 10 -5 4,445 x 10 -5 90 wt% Al 2 O 3 0, 000762 x 10 -5 3 2.5 wt% AlF 3 (0.98 mils) (1.7 mils) (0.0005 mils) 15 wt% Cr-45Al 82.5 wt% Al 2 O 3 2,4892 x 10 -5 4,445 x 10 -5 0, 00127 x 10 -5 4 5 wt% AlF 3 (1.4 mils) (1.7 mils) (1.0 mils) 15 wt% Cr-45Al 80 wt% Al 2 O 3 3,556 x 10 -5 4,445 x 10 -5 4,54 x 10 -5 5 5 wt % AlF 3 (1.8 mils) (2.3 mils) (1.4 mils) 20 wt% Cr-45Al 75 wt% Al 2 O 3 4, 572 x 10 -5 5,842 x 10 -5 3,556 x 10 -5 6 2.5 wt% AlF 3 (1.75 mils) (2.0 mils) (1.5 mils) 20 wt% Cr-45Al 77.5 wt% Al 2 O 3 4,445 x 10 -5 5,08 x 10 -5 3,81 x 10 -5 7 (PWA 273 composition) 5 wt% AlF 3 (2.0 mils) (3.0 mils) (1.2 mils) 30 wt% Cr-45Al 65 wt% Al 2 O 3 5, 08 x 10 -5 7,62 x 10 -5 3, 048 x 10 -5 Slurry Dry Composition Avg. Coat Thickness (m) Max. Coat Thickness (m) Min. Coat Thickness (m) 1 2.5 wt% AlF 3 (0.82 mils) (1.7 mils) 0 5 wt% Cr-45Al 92.5 wt% Al 2 O 3 2,0828 x 10 -5 4,445 x 10 -5 2 5 wt% AlF 3 (0.87 mils) (1.5 mils) 0 5 wt% Cr-45Al 90 wt% Al 2 O 3 2,2098 x 10 -5 3,81 x 10 -5 3 2.5 wt% AlF 3 (1.4 mils) (2.0 mils) (1.0 mils) 15 wt% Cr-45Al 82.5 wt% Al 2 O 3 3,556 x 10 -5 5,08 x 10 -5 2,54 x 10 -5 4 5 wt% AlF 3 (1.3 mils) (1.5 mils) (1.0 mils) 15 wt% Cr-45Al 80 wt% Al 2 O 3 3,302 x 10 -5 3,81 x 10 -5 2,54 x 10 -5 5 5 wt% AlF 3 (1.8 mils) (2.1 mils) (1.5 mils) 20 wt% Cr-45Al 75 wt% Al 2 O 3 4,572 x 10 -5 5,334 x 10 -5 3,81 x 10 -5 6 2.5 wt% AlF 3 (1.8 mils) (2.2 mils) (1.5 mils) 20 wt% Cr-45Al 77.5 wt% Al 2 O 3 4,572 x 10 -5 5,588 x 10 -5 3,81 x 10 -5 7 (PWA 273 composition) 5 wt% AlF 3 (0.83 mils) (2.0 mils) 0 30 wt% Cr-45Al 65 wt% Al 2 O 3 2,1082 x 10 -5 5,08 x 10 -5 As can be seen from the data in Tables 1 and 2, the coating compositions of the present invention met the target goals. The coating compositions of the present invention yield a coating which provides oxidation and corrosion resistance under the conditions at which high pressure turbine airfoils, such as high pressure turbine blades and vanes, operate. These coatings are achieved in an economically beneficial fashion without the need for coating fixtures that have a bottom compartment connected to an argon flow pipe and without specialized plumbing therein. As can be seen from the test data reported above, the coating compositions of the present invention resulted in a uniform coating within these desired coating parameters needed for repeatable, robust coating processes.

Claims: A composition for providing an oxidation and corrosion resistant coating on internal surfaces of an airfoil, said composition including a dry composition consisting essentially of from 2.5 wt% to 7.0 wt% aluminum fluoride, from 5.0 wt% to 20 wt.% of a chromium-aluminum powder, and from 75 wt% to 92.5 wt% Al 2 O 3 . The composition of claim 1 wherein said chromium-aluminum powder comprises a chromium-45 aluminum powder comprising 55 wt.% chromium and 45 wt% aluminum, The composition of claim 1 further comprising a cellulose compound and water added to said dry composition so as to form a slurry composition to be injected into said airfoil. The composition of claim 1 wherein said dry composition consists essentially of from 75 wt% to 80 wt% of Al 2 O 3 , from 2.5 wt% to 5.0 wt% aluminum fluoride, and from 15 wt% to 20 wt% chromium-aluminum powder, wherein preferably said dry composition includes from 75 wt% to 77.5 wt% Al 2 O 3 . The coating composition of claim 3 further comprising from 60 grams to 100 grams of said cellulose compound, from 1900 cc to 2700 cc of water, and from 1400 grams to 1600 grams of the dry composition. The coating composition of claim 3 or 5 wherein said slurry composition is used to coat external surfaces of said airfoil. A process for forming an oxidation and corrosion resistant coating on selected surfaces of an airfoil comprising: forming a dry chemical composition consisting essentially of from 2.5 wt% to 7.0 wt% aluminum fluoride, from 5.0 wt% to 20 wt% of a chromium - aluminum powder, and from 75 wt% to 92.5 wt% of aluminum fluoride; mixing said dry chemical composition with a cellulose compound and water so as to form a slurry composition; placing said slurry composition into contact with internal surfaces of said airfoil; and drying the slurry. The process of claim 7 wherein said placing step comprises injecting said slurry composition into said airfoil. The process of claim 7 wherein said placing step comprises injecting said slurry composition into passageways in said airfoil. The process of one of the claims 7 to 9 further comprising: baking said airfoil with said slurry composition in contact with said internal surfaces so as to remove said water and to harden solids in said slurry composition. The process of claim 10 wherein said baking step comprises baking said airfoil with said slurry composition at a temperature of 579°C (1075° F) for up to 2 hours. The process of claim 7 further comprising: placing said airfoil with said slurry composition in a coating fixture; placing a chemical composition in said fixture for coating exterior surfaces of said airfoil; and The process of claim 12 wherein said heating step comprises applying a temperature in the range of from 1065,5° C (1950° F) to 1121,1° C (2050° F) for a time period in the range of from 4 to 17 hours. The process of claim 12 wherein said exterior surface chemical coating composition comprises a composition containing aluminum fluoride and a chromium-aluminum powder.

Inventors: BOSE KRISHNANGSHU, LAFLAMME DAVID W, MAGYAR LESTER J, PERRY TERRY T, BOSE, KRISHNANGSHU, LAFLAMME, DAVID W., MAGYAR, LESTER J., PERRY, TERRY T.
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1998
Main classification: C23C30
IPC codes: C09D5, C23C26, C23C10