Patent Application
20070134418
This invention relates to the protection of a metallic article by an aluminum-containing layer, and more particularly to improving the quality of the aluminum-containing layer by the removal of harmful constituents.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against airfoil sections of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high pressure turbine are mounted on one shaft, and the fan and low pressure turbine are mounted on a separate shaft. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust-gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1900-2150° F.
Many approaches have been used to increase the operating temperature limits of turbine blades, turbine vanes, and other hot-section components to their current levels. For example, the composition and processing of the base materials themselves have been improved, and a variety of solidification techniques have been developed to take advantage of oriented grain structures and single-crystal structures. Physical cooling techniques may also be used.
In yet another approach, coatings are applied to the surface of the substrate to inhibit the oxidation and corrosion of the substrate, thereby permitting the substrate material to be used for a longer time and at a higher temperature than would otherwise be possible. The most widely used coatings are aluminum-rich layers whose surfaces oxidize to an aluminum oxide scale to inhibit further oxidation and corrosion. The aluminum-rich layer may serve as either an environmental coating or as a bond coat under a ceramic thermal barrier coating. The ceramic thermal barrier coating, where present, insulates the surface of the substrate against the hot combustion gases.
Protective layers continue to be used to protect substrates. However, in some instances there has been a tendency for the protective layers to separate and spall away from the substrate prematurely, leaving the substrate unprotected. There is a need to understand the origin of this spallation and to correct it. The present invention fulfills this need, and further provides related advantages.
The present approach reduces the tendency toward separation and spallation of protective layers deposited by one of the commercially most-important deposition techniques. A new process step is added, but this process step modifies the chemistry of the protective coating so that spallation of the thermal barrier coating is greatly reduced.
In accordance with the invention, a method for preparing a protected article comprises the steps of furnishing an article to be protected, thereafter depositing an aluminum-containing layer onto the article from an aluminum vapor source that includes a fluoride, and thereafter heating the article with the aluminum-containing layer deposited thereon in an atmosphere comprising at least about 5 percent, more preferably at least about 50 percent, and most preferably 100 percent by volume hydrogen gas. The heating step is preferably performed for a time of from about ¼ hour to about 8 hours, and at a temperature of from about 1800° F. to about 2050° F. Preferably, a ceramic layer is deposited overlying the aluminum-containing layer after the step of heating.
The article is preferably a component of a gas turbine engine. More preferably, the article is a component of a gas turbine engine having an airfoil, and the aluminum-containing layer is deposited onto the airfoil.
The aluminum-containing layer is preferably deposited by vapor-phase aluminiding, and from an aluminum-containing vapor. The aluminum-containing vapor may be generated by an aluminum source comprising aluminum-containing donor pellets, such as aluminum or aluminum-chromium alloy pellets, and a fluoride-containing solid.
In another embodiment, a precious-metal-containing layer is deposited onto the article after the step of furnishing and prior to the step of depositing the aluminum-containing layer. The aluminum-containing layer is thereafter deposited overlying the precious-metal-containing layer. The precious-metal-containing layer may be deposited by electrodeposition, and then interdiffused with the substrate prior to the deposition of the aluminum-containing layer. The precious metal is preferably platinum.
In a preferred form, a method for preparing a protected article comprises the steps of furnishing an article to be protected, wherein the article is a component of a gas turbine engine, thereafter depositing a precious-metal-containing layer onto the article, and thereafter depositing an aluminum-containing layer by vapor-phase aluminiding onto the article overlying the precious-metal-containing layer from an aluminum vapor source that includes a fluoride. The method further includes thereafter heating the article with the aluminum-containing layer deposited thereon in an atmosphere comprising at least 5 percent, more preferably at least 50 percent, and most preferably 100 percent by volume hydrogen gas, and thereafter depositing a ceramic layer overlying the aluminum-containing layer. Other compatible features discussed herein may be used with this embodiment.
The spallation of the thermal barrier coating observed in prior processing is associated with the presence of fluorides remaining in the coated structure following the deposition of the aluminum-containing layer. These fluorides originate in the fluoride activator, such as aluminum trifluoride, that is used in the vapor-phase aluminiding process. The post-deposition hydrogen heat treatment reacts the fluorides in the coated structure with hydrogen, producing hydrogen fluoride gas that is removed. Comparative testing has shown that airfoils given the post-deposition hydrogen heat treatment have a much lower incidence of spallation than comparable airfoils that were not given the post-deposition hydrogen heat treatment. The hydrogen heat treatment also desirably interdiffuses the deposited aluminum-containing layer with the substrate and the underlying precious-metal-containing layer, if any.
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. The scope of the invention is not, however, limited to this preferred embodiment.
The entire gas turbine blade 42 is preferably made of a nickel-base superalloy. A nickel-base alloy has more nickel than any other element, and a nickel-base superalloy is a nickel-base alloy that is strengthened by the precipitation of gamma-prime phase or a related phase. An example of a nickel-base superalloy with which the present invention may be used is Rene™ N5, having a nominal composition in weight percent of about 7.5 percent cobalt, about 7.0 percent chromium, about 1.5 percent molybdenum, about 5 percent tungsten, about 3 percent rhenium, about 6.5 percent tantalum, about 6.2 percent aluminum, about 0.15 percent hafnium, about 0.05 percent carbon, about 0.004 percent boron, about 0.01 percent yttrium, balance nickel and minor elements.
As shown in the sectional view of
Optionally but preferably, the precious-metal-containing layer 60 is thereafter interdiffused with the underlying substrate of the article 40, step 24 of
An aluminum-containing layer 62 is deposited overlying and contacting the surface of the article 40, which includes the precious-metal-containing layer 60 (if present), step 26 of
The article 40 with the aluminum-containing layer 62 deposited thereon is thereafter heated, step 28 of
The deposition step 26 results in fluorides being co-deposited in the aluminum-containing layer 62. These fluorides have been identified as being a contributing cause of premature spallation of the final coating. The heating step 28 chemically reacts the deposited fluorides to remove them in gaseous form from the aluminum-containing layer 62 and thence from the protected article.
Optionally but preferably, a ceramic layer 64 is thereafter deposited overlying and contacting the aluminum-containing layer 62, step 30 of
The present invention has been reduced to practice and comparatively tested against the prior approach wherein the article having the aluminum-containing layer thereon is not post-deposition heated in hydrogen. In these studies, the heating step 28 was in 100 percent hydrogen gas for 3 hours at 1925° F. Two hundred airfoils of turbine blades were processed by the present approach. No spallation of the protective coating was observed in these parts in subsequent processing. Over 200 otherwise-identical and identically processed airfoils were maintained as a control group and were not given the hydrogen heat treatment of step 28. About 5-50 percent of the airfoils in the control group exhibited spallation failures in subsequent processing.
Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.
