Coating processes – Coating by vapor – gas – or smoke
Reexamination Certificate
1999-01-28
2001-03-20
Meeks, Timothy (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
C427S250000, C427S252000, C427S255310
Reexamination Certificate
active
06203851
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a gas phase coating process wherein one or more workpieces to be coated are arranged in a container, and whereby the container is heated and a donor substance forms a coating gas that comes into contact with the workpieces. The present is also directed to an apparatus for gas-phase coating of workpieces.
BACKGROUND OF THE INVENTION
In known apparatus and processes for gas phase coating of workpieces, for example, turbine blades, the workpieces are arranged, for example, in the upper region of a container. In its lower region, the container comprises a carrier gas inlet and a carrying grid for the acceptance of a donor metal granulate and an activator powder. The container is annealed for a certain time under argon or H
2
and a donor metal gas forms in the container in the proximity of the donor metal granulate.
The problem thereby arises that the heavy donor metal gasses that form must rise from the donor metal granulate to the parts to be coated, whereby a dilution of the donor metal gasses occurs in vertical direction according to the barometric altitude formula, thereby leading to different coat thicknesses on the parts dependent on their geodetic height relative to the donor metal. Particularly given containers with large payload spaces, a pronounced coat gradient arises in barometric dependence from bottom to top due to the heaviness of the coating gas. It is thereby of no consequence whether the donor metal is distributed at the floor of the container and/or under the cover of the container or, respectively, reactor or whether a molten donor metal is employed. When coating inside surfaces of hollow parts, the additional problem arises that the coating thickness decreases the farther the surface to be coated lies in the inside of the part.
Attempts have been made to control the gas quality and to improve it to a certain extent with a gas rinse conduit conducted into the reactor. This arrangement, however, only functions with adequately slice coat gradients given payload spaces having a low height. A disadvantageous coat gradient particularly arises given payload space heights of approximately 120 mm and more, i.e. the coated part has different coat thicknesses dependent on its respective height in the reactor during the coating.
WO 92/08821 discloses an apparatus for gas diffusion coating of hollow workpieces with a container that comprises a gas admission and a gas discharge, whereby the gas discharge follows the inside surfaces of the workpiece to be coated. In this method, the workpieces are held at a geodetically low height relative to the donor metal, whereby the donor metal is present in the form of a donor metal body that, while preserving a gap, completely envelopes the outside surfaces of the workpiece to be coated. A carrier gas conducted into the container from above is conducted past the donor metal body arrangement that is arranged above the part to be coated and around the part. Subsequently, the coating gas is conducted through the inside of the part and discharged at the underside of the container. In order to assure a high donor metal concentration in the area of the part, the gas discharge is formed as overflow or, respectively, siphon, so that a donor metal gas sump forms in the area of the part. However, this known apparatus requires an involved arrangement and fashioning of the donor metal body that, over and above this, must be exactly aligned relative to the part and must be matched to its shape. A coating of a variety of parts with different geometries requires respectively different donor metal bodies, which is involved and cost-intensive and leads to low flexibility. Over and above this, a barometric dependency of the donor metal gas concentration also exists given a siphon arrangement.
Therefore, there is a need for a gas phase coating process and an apparatus for gas phase coating which provides a more uniform coating of outside and inside surfaces of the part throughout the entire payload space of the reactor given low outlay and high flexibility.
SUMMARY OF THE INVENTION
The aforementioned needs are satisfied by a gas phase coating process according to the present invention and by an apparatus for gas phase coating according to the present invention.
In the inventive gas phase coating process, one or more workpieces to be coated, a donor substance and, preferably, an activator are arranged in a container and the container is heated, whereby the donor substance forms a coating gas that comes into contact with the workpiece, and whereby the coating gas is circulated in the container at least at times during the coating process. The circulation achieves a uniform coating gas concentration in the container, so that the formation of a coat gradient is avoided.
Advantageously, a propellant that, in particular, can be an inert propellant such as, for example, Ar, He, Kr, Ne is conducted into the container through a Venturi nozzle arranged in the container. A circulation of the coating gas is thus achieved in an especially efficient and cost-beneficial way, whereby the dilution of the coating gas is kept low. By avoiding moving parts, a circulation can also ensue at extremely high temperatures. However, the coating gas can also be mechanically circulated, preferably with a rotating disk and/or a propeller. The employment of a rotating disk in the mechanical circulation of the coating gas leads to a reduced wear even at high temperatures. H
2
is also possible as propellant, for example in order to effect the formation of HF or HCl in the coating process.
The coating gas is preferably conducted or, respectively, distributed in the container by one or more distributor diaphragms preferably arranged above and/or below the workpieces to be coated. This is especially advantageous for uniform coating of workpieces that are preferably arranged in different vertical planes of the container.
Advantageously, propellant situated in the container is separated from the coating gas by gravitation in the upper region of the container and is discharged upwardly from the container, whereas the heavier coating gas remains in the container. A high coating gas concentration is thus achieved in the circulation.
The Venturi nozzle is preferably arranged such in the lower region of the container that the coating gas and/or the propellant circulates vertically.
The coating process is preferably implemented at a temperature in the range from approximately 1000° C. through 1300° C., particularly preferably at approximately 1220° C. The donor substance can be a metallic melt or a metallic granulate such as, for example, a chromium granulate, whereby an activator substance that, in particular, can be a powdered halogen, particularly NH
4
Cl is preferably added.
The circulation advantageously ensues in a pulsed mode, whereby a circulation is repeatedly implemented at specific time intervals.
Ceramic coats can also be applied to the workpiece, whereby Zr(tmhd)
4
and/or Y(tmhd)
3
is preferably employed as donor substance. The coating process can thereby also sequence at low process temperatures.
The inventive apparatus for gas phase coating of workpieces comprises a container for the acceptance of at least one workpiece to be coated, a means for offering a donor substance in the container that forms a coating gas during coating; a heating device for heating the container; and means for circulating the coating gas in the container, so that the coating gas circulates in the container during coating.
For circulation, a Venturi nozzle can thereby be arranged at the end of a gas or, respectively, propellant pipe introduced from the outside, and a distributor diaphragm is preferably arranged, for example, in the upper region of the container parallel to the upper limitation of the container. For circulation of the coating gas, the apparatus can comprise a propeller and/or a rotatable disk, and a workpiece holder with a plurality of levels for the acceptance of a plurality of workpieces to be coated at different levels of the co
Meeks Timothy
MTU Motoren-und Turbinen-Union Munchen GmbH
Schiff & Hardin & Waite
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