Coating processes – Coating by vapor – gas – or smoke – Metal coating
Reexamination Certificate
2001-09-04
2004-09-21
Chen, Bret (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Metal coating
C427S252000, C427S376100, C118S719000, C118S729000
Reexamination Certificate
active
06793968
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of coating a product with a metallic coating, in particular with a metallic anti-oxidation coating, in a vacuum plant. In the method, the product is fed into the vacuum plant and heated from room temperature to a product temperature, the metallic coating is applied to the product, and the coated product is subjected to a postheat treatment. Furthermore, the invention relates to an apparatus for coating a product with a metallic coating in a vacuum plant, the vacuum plant including a coating chamber and a postheat treatment chamber.
BACKGROUND OF THE INVENTION
Coating plants for coating gas turbine blades are known, e.g. an inline EB-PVD coating plant from Interturbine Von Ardenne GmbH (EB-PVD: Electron Beam—Physical Vapor Deposition), in which a ceramic coating is applied to the gas turbine blade by means of physical vaporization processes. Such a coating plant, for example, may be composed of chambers arranged directly one behind the other and connected to a transfer system for conveying the turbine blades. In this case, the first chamber serves as a loading chamber for turbine blades. From the loading chamber, the turbine blades are transported into a second vacuum chamber connected to the loading chamber and are preheated there. Further transport into a process chamber then takes place, in which process chamber a ceramic material, in particular an yttrium-stabilized zirconium oxide, is heated, melted and vaporized by means of electron beam vaporization. The ceramic material condenses on the turbine blades and therefore forms the ceramic coating. The turbine blades thus coated are transported further into a cooling chamber and cooled therein. The cooling is effected without monitoring, in particular in an uncontrolled manner, since the turbine blades are left on their own in the cooling chamber and consequently emit their heat to the surroundings via heat radiation until they have cooled down to room temperature.
U.S. Pat. No. 5,238,752 discloses a heat-insulating-coating system which is applied to a turbine blade. In this case, the parent material of the turbine blade consists of a nickel-base superalloy to which a metallic protective or bonding coating of the type MCrAlY or PtAl is applied. Here, M stands for nickel and/or cobalt, Cr stands for chromium, Al stands for aluminum, Y stands for yttrium and Pt stands for platinum. Forming on this metallic bonding coating is a thin coating of aluminum oxide, to which the actual ceramic heat-insulating coating of zirconium oxide stabilized with yttrium is applied. In this case, the turbine blade is coated by means of a physical vaporization process in which the ceramic material (zirconium oxide) is vaporized by being bombarded with electron beams. This coating process is effected in a vacuum chamber, the turbine blade being heated via a substrate heater by means of heat radiation to a temperature of about 1200 K to 1400 K, in particular about 1300 K.
Those coatings on turbine blades which are produced in the above-described, known methods and apparatuses are still capable of improvement with regard to their service life, in particular in the case of hot-gas admission when used in a gas turbine.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method of coating a product with a metallic coating. In this case, the fatigue strength of the metallic coating, in particular against corrosive and oxidizing attacks, is to be markedly improved. A further object of the invention is to specify an apparatus for coating a product with a metallic coating. The production of a metallic coating of high quality on the product is to be possible with the apparatus.
According to the invention, the first-mentioned object is achieved by a method of coating a product with a metallic coating, in particular with a metallic anti-oxidation coating, in a vacuum plant. In this method, the product is fed into the vacuum plant and heated from room temperature to a product temperature; the metallic coating is applied to the product; and the coated product is subjected to a postheat treatment. The postheat treatment follows the application of the coating in such a way that the temperature of the product after the application of the coating and before the postheat treatment is at least as high as a minimum temperature, the minimum temperature being relatively higher than room temperature.
In this case, the invention is based on the idea that the quality of a primary metallic coating applied to the parent material of a product is especially important. Material properties and characteristic coating properties, such as the homogeneity of the coating, the bonding to the substrate, and the structure of the boundary layer between coating and substrate for example, are important quality features. These also have an effect on the bonding and condition of further coatings which are applied to the primary coating possibly in further coating processes.
A metallic coating on a product, for example a metallic anti-oxidation coating, will therefore develop its function more effectively, for instance as a protective coating against corrosion and/or oxidation, the better the abovementioned coating properties are realized. For the service life of metallic coatings on products which appear under oxidizing or corrosive conditions, for example, in addition to the selection of the materials, in particular the bonding of the coating to the parent material of the product is decisive. This depends on the treatment of the product in all the phases of the production process. In this case, chemical and physical—in particular thermal—influences which may possibly impair the forming and bonding of the coating are to be taken into account.
Chemical influences can be largely reduced by the selection of suitable materials for all the built-in components of the equipment, which as far as possible are to be chemically inert with respect to the coating materials. Physical conditions under which the process for producing a coating takes place relate to the process control in its entirety, that is to say from the preparation of the product, via the application of the protective coating up to the further treatment of the product, normally a subsequent postheat treatment—and all possible intermediate steps.
The monitoring and configuration of the process control in all the phases of the production process is therefore very important. In this case, time-dependent and locus-dependent thermodynamic process parameters, such as pressure and temperature, to which the product is subjected in the production process are to be taken into account. For example, on account of the generally different coefficients of thermal expansion of parent material and coating material, the product temperature during the application of the coating (coating temperature) and the temperature profile up to completion of a postheat treatment of the coated product have a considerable effect on the formation of the boundary layer between product surface and coating.
Virtually steady process control with regard to the temperature in all the phases of the process for producing the metallic coating can be achieved with the method. In this case, after the application of the metallic coating to the product and before the postheat treatment, a minimum temperature of the product is ensured at all times, this minimum temperature being higher than room temperature.
In the case of products which constitute high-temperature components of gas turbines, for instance in the case of gas turbine blades or heat shield elements of combustion chambers, this minimum temperature is preferably about 500 K, in particular about 900 K to 1400 K.
The method operates on a product that is always close to a state of thermodynamic equilibrium with its surroundings. Time-dependent and spatial temperature gradients, in particular thermal shocks, are avoided. By this novel method in the process control with regard to the temperature profile, it is possible to markedly improve the bonding of t
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