Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2000-03-24
2002-03-12
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S283000, C134S041000, C134S003000, C156S345420, C216S108000, C216S109000
Reexamination Certificate
active
06355116
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to diffusion coatings formed on superalloy substrates and specifically to a novel method of renewing the diffusion coatings formed on superalloy substrates.
BACKGROUND OF THE INVENTION
The current coatings used on superalloy substrates such as airfoils exposed to the hot gases of combustion in gas turbine engines for both environmental protection and as bond coats in thermal barrier coating (TBC) systems include aluminides of nickel and platinum. These coatings are applied over superalloy substrate materials, typically nickel-base superalloys, to provide protection against oxidation and corrosion attack. These coatings are formed on the substrate in a number of different ways. For example, a nickel aluminide, NiAl, typically is grown as an outer coat on a nickel base superalloy by exposing the substrate to an aluminum rich environment at elevated temperatures. The aluminum from the outer layer diffuses into the substrate and combines with the nickel diffusing outward from the substrate to form an outer coating of NiAl. Because the formation of the coating is the result of a diffusion process, it will be recognized that there are chemical gradients of Al and Ni, as well as other elements. However, Al will have a high relative concentration at the outer surface of the article which will thermodynamically drive its diffusion into the substrate creating a diffusion zone extending into the original substrate, and this Al concentration will gradually decrease with increasing distance into the substrate. Conversely, Ni will have a higher concentration within the substrate and will diffuse through the thin layer of aluminum to form a nickel aluminide. The concentration of Ni in the diffusion zone will vary as it diffuses outward to form the NiAl. At a level below the original surface, the initial Ni composition of the substrate is maintained, but the Ni concentration in the diffusion zone will be less and will vary as a function of distance into the diffusion zone. The result is that although NiAl forms at the outer surface of the article, a gradient of varying composition of Ni and Al forms between the outer surface and the original substrate composition. The concentration gradients of Ni and other elements that diffuse outwardly from the substrate and the deposited aluminum, Al, create a diffusion zone between the outer surface of the article and that portion of the substrate having its original composition. Of course, exposure of the coated substrate to an oxidizing atmosphere typically results in the formation of an alumina layer over the nickel aluminide.
In some coating systems, a platinum aluminide (PtAl) coating is formed by electroplating a thin layer of platinum over the nickel-base substrate to a predetermined thickness. Then, exposure of the platinum to an aluminum-rich environment at elevated temperatures causes the growth of an outer layer of PtAl as the aluminum diffuses into and reacts with the platinum. At the same time, Ni diffuses outward from the substrate changing the composition of the substrate, while aluminum moves inward through the platinum into this diffusion zone of the substrate. Thus, complex structures of (Pt,Ni) Al are formed by exposing a substrate electroplated with a thin layer of Pt to an atmosphere rich in aluminum at elevated temperatures. As the aluminum diffuses inward toward the substrate and Ni diffuses in the opposite direction through the Pt creating the diffusion zone, PtAl
x
phases precipitate out of solution so that the resulting Pt—NiAl intermetallic also contains precipitates of PtAl
x
intermetallic, where x is 2 or 3. As with the nickel aluminide coating, a gradient of aluminum occurs from the aluminum rich outer surface inward toward the substrate surface, and a gradient of Ni and other elements occurs as these elements diffuse outward from the substrate into the aluminum rich additive layer. Here, as in the prior example, an aluminum rich outer layer is formed at the outer surface, which may include both platinum aluminides and nickel aluminides, while a diffusion layer below the outer layer is created. As with the nickel aluminide coating, exposure of the coated substrate to an oxidizing atmosphere typically results in the formation of an outer layer of alumina.
These aluminides are also used as bond coats in thermal barrier systems, being intermediate between the substrate and an additional applied thermally resistant ceramic coating, such as yttria-stabilized zirconia (YSZ) which is applied over the aluminide. However, the process for forming these diffusion aluminides is essentially the same, that is to say, the substrate is exposed to aluminum, usually by a pack process or a CVD process at elevated temperatures, and the resulting aluminide formed as a result of diffusion.
Over time in the hot oxidizing environment of a gas turbine engine, the coatings, whether applied as an environmental coating or as a bond coat in a thermal barrier system eventually degrade as a result of one or a combination of ongoing processes which include erosion due to the impingement of hot gases on the airfoils, corrosion due to reaction of contaminants in the products of combustion with the metallic surfaces of the airfoil, and oxidation. Products of combustion frequently build up on these outer surfaces. In addition to degradation as a consequence of exposure to the hot engine environment, airfoils may be damaged in service due to a variety of factors, and require repair after removal of damaged regions by well-known processes such as welding, cladding or the PACH process. In order to repair an airfoil after service, it is necessary to remove not only the products of combustion, the corrosion products and oxidation products resulting from routine exposure to the engine environment, but also previously applied coatings, if they haven't already been removed as a result of service.
Prior art processes for repair of coated blades chemically strip any remaining coating from the turbine blades. One of these repair methods, as set forth in U.S. Pat. No. 4,746,369 involves acid stripping. Because the coatings are grown into the substrate by a diffusion process, acid stripping attacks the diffusion zone, which includes original substrate material, as well as the nickel aluminide and any outer layer of alumina. Of course, this acid stripping procedure is further complicated because the coatings are selected due to their ability to resist chemical attacks from corrosion processes and protect the substrate airfoil. Yet, existing methods of stripping the coatings are controlled chemical attacks upon the airfoil. Unless exceptional care is maintained, the chemical solutions used to remove the coating will vigorously attack the regions underlying the protective coating. So removal of the coating will affect the outer coating layer and the diffusion layer, and may involve a direct attack on the substrate or a portion of the substrate. Because the parts are thin, a repair process that removes at least a portion of the initial substrate that was incorporated into the diffusion zone limits the number of times that the airfoils can be reused since minimum allowable wall thicknesses cannot be violated.
Other methods such as disclosed in U.S. Pat. No. 4,425,185 have as their object removal of coatings such as nickel aluminides from Hastelloy-X substrates without adversely affecting the substrate. This method may minimize the impact on a Hastelloy-X substrate, which has a low Ni content in comparison to a Ni-base superalloy, but it still removes any diffusion zone formed between the nickel aluminide and the substrate. Furthermore, while this may be an effective method for an alloy such as Hastelloy X containing only about 50% Ni, it is not effective for a Ni-base superalloy which can include Ni in excess of 80%.
Another method for removing coatings is set forth in U.S. Pat. No. 5,851,409 to Schaeffer et al. and assigned to the assignee of the present invention. This method involves mechanically impacting the environment
Chen Keng Nam
Ngiam Shih Tung
General Electric Company
Hess Andrew C.
Narciso David L.
Oltmans Andrew L.
Sheehan John
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