Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2000-06-21
2002-09-03
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S264000, C148S283000, C427S229000
Reexamination Certificate
active
06444054
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of corrosion protection for metal substrates, and more specifically to diffusion coatings for nickel-based or cobalt-based alloy substrates.
BACKGROUND OF THE INVENTION
In a modern gas turbine engine, components such as blades, vanes, combustor cases and the like are usually made from nickel and cobalt alloys. Nickel and cobalt-based superalloys are most often used to fabricate gas turbine parts because of the high strength required for long periods of service at the high temperatures characteristic of turbine operation. These components are usually located in the “hot section” of the turbine. As such, there are special design requirements imposed upon these components due to the rigorous environment in which they operate. Turbine blades and vanes are often cast with complex hollow core passages for transporting internal cooling air. Also, the wall thickness of gas turbine hot section parts is carefully controlled to balance the need for high temperature strength with the need to minimize the weight of the component part.
The surfaces of turbine engine parts are exposed to the hot gases from the turbine combustion process. Oxidation and corrosion reactions at the surface of the component parts can cause metal wastage and loss of wall thickness. The loss of metal rapidly increases the stresses on the respective component part and can result in part failure. Protective coatings are thus applied to these component parts to protect them from degradation by oxidation and corrosion.
Diffusion aluminide coatings are a standard method for protecting the surfaces of nickel- and cobalt-alloy gas turbine hardware from oxidation and corrosion. Aluminide coatings are based on intermetallic compounds formed when nickel and cobalt react with aluminum at-the substrate's surface. An intermetallic compound is an intermediate phase in a binary metallic system, having a characteristic crystal structure enabled by a specific elemental (atomic) ratio between the binary constituents. For example, a number of such phases form in the nickel-aluminum binary system, including Ni
2
Al
3
, NiAl, or NiAl
3
. Many aluminum-based intermetallic compounds (i.e., aluminides) are resistant to high temperature degradation and therefore are preferred as protective coatings, but such coatings are more brittle than the superalloy substrates underlying the coatings. An example of one particularly useful intermetallic compound formed in nickel-based systems is NiAl.
Careful dimensional tolerances imposed on parts during manufacture must be maintained during the coating process. Uneven or excessively thick diffusion coating layers can effectively act to reduce wall thickness and hence the part's strength. Furthermore, excessively thick aluminide coatings, especially at leading and trailing edges of turbine blades where high stresses mostly occur, can result in fatigue cracking.
One method for applying a diffusion aluminide coating is via a liquid phase slurry aluminization process. Typical slurries incorporate a mixture of aluminum and/or silicon metal powders (pigments) or alloys o those elements in an inorganic binder. The slurries are directly applied to a substrate surface. Formation of the diffused aluminide is accomplished by heating the part in a non-oxidizing atmosphere or vacuum at temperatures between 1600-2000° F. for two to twenty hours. The heating melts the metal in the slurry and permits the reaction and diffusion of the aluminum and/or silicon pigments into the substrate surface. Coatings of this type have been described in U.S. Pat. No. 5,795,659.
In liquid-phase slurry aluminization, the slurry must be applied directly to the part in a controlled amount because the resulting thickness of the diffused coating is directly proportional to the amount of the slurry applied to the surface. Because of this proportional relationship between applied slurry amount and final diffused coating thickness, it is critical in this method to carefully control the application of the slurry material. The necessarily controlled application requires a great deal of operator skill and quality assurance, particularly for parts having complicated geometries such as turbine blades. This places a limit on the quantity of parts that can be coated in an economical, timely fashion.
A more common industrial method for producing aluminide coatings is by the “pack cementation” method. Pack cementation processes have been described, for example, in U.S. Pat. Nos. 3,257,230 and 3,544,348. The basic pack method requires a powder mixture including (a) a metallic source of aluminum, (b) a vaporizable halide activator, usually a metal halide, and (c) an inert filler material such as a metal oxide (i.e., Al
2
O
3
).
Parts to be coated with such a mixture are first entirely encased in the pack material and then enclosed in a sealed chamber or “retort”. The retort is purged using an inert or reducing gas and heated to a temperature between 1400-2000° F. Under these conditions, the halide activator dissociates, reacts with aluminum from the metallic source, and produces gaseous aluminum halide species. These species migrate to the substrate's surface where the aluminum-rich vapors are reduced by the nickel or cobalt alloy surface to form intermetallic coating compositions.
The amount of aluminum-rich vapors available at the surface of the part is defined by the “activity” of the process. The activity of a process is controlled in general by the amount and type of halide activator, the amount and type of aluminum source alloy, the amount of inert oxide diluent, and the temperature of the process. In some cases other metallic powders such as chromium or nickel are added to influence or “moderate” the aluminum activity in a pack.
The activity of the process influences the structure of the aluminide coating formed. “Low activity” processes produce “outwardly” diffused coatings where the coating forms predominately by the outward migration of nickel from the substrate and its subsequent reaction with aluminum at the part surface. “High activity” processes produce “inwardly” diffused coatings where the coating forms predominately by migration of aluminum into the surface of the substrate.
FIG. 1
shows an outwardly diffused coating structure produced by a low activity process. The original surface of the substrate is labeled. A limitation of outwardly diffused aluminide coatings is that oxides or contaminants present at the original surface of the part can become entrapped within the interior of the final diffused coating structure. If these oxides or contaminants are present in a somewhat continuous manner along the original substrate surface, the effectiveness of the low activity, outwardly diffused coatings is diminished under the stressful operating conditions of the turbine engine.
FIG. 2
shows an example of a higher activity, inwardly-diffused coating structure. The original surface of the substrate is labeled. The aluminum content in the outer zone is sufficient to cause precipitation of elements normally dissolved within the original superalloy substrate. Because of the inward diffusion of aluminum which predominates the coating formation process, oxides and contaminants present at the original substrate surface remain in the outer-most region of the final diffused coating structure where they are less likely to comprise the coating performance.
The pack process generally produces reliably uniform diffused aluminide surface layers on complex shapes such as those characteristic of gas turbine components. However, one major limitation of the pack cementation method is the generation of large amounts of hazardous waste. Considerably more raw material is required in a pack process than a slurry aluminization process. Although the pack mixtures can be “rejuvenated” to some extent with incremental additions of fresh powder, eventually the pack mixture must be replaced and the spent powder disposed in hazardous waste landfills. Dusts from the powder mixture
Kircher Thomas
McMordie Bruce G.
Shankar Srinivasan
Drinker Biddle & Reath LLP
Oltmans Andrew L.
Sermatech International Inc.
Sheehan John
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