EP1373593B1 - Method and device for gas phase diffusion coating of metal components - Google Patents

Description

The invention relates to a method for gas phase diffusion coating of metallic components, in particular components of gas turbines, in which a component surface to be coated with a metal halide as Beschichtungsurrgsgas to form a defined over a predetermined thickness and a predetermined coating metal content in wt .-% in the component surface diffusion layer in Contact is brought, wherein a defined coating temperature leads to a defined coating time and with a correlating thereto nominal concentration of the metal halide on the component surface.

Such diffusion layers generally serve as hot gas corrosion and oxidation protective layers or as a primer for thermal barrier coatings.

So shows EP 1 013 794 A2 a gas phase diffusion coating method in which a surface to be coated is brought into contact with a constant concentration of a metal halide as a coating gas. In this case, the coating chamber is purged with inert gas prior to introducing the halide for cleaning, wherein the inert gas pressure can be varied or pulsed in this purging process.

It is assumed that a nominal concentration of the metal halide on the component surface in a known method, which for the formation of a diffusion layer with a layer thickness in the range of 50 to 100 microns and a coating metal content of 25 to 32 wt .-% in the component surface to a defined , reproducible coating time of 14 hours leads. Alternative diffusion layers with different layer thickness ranges and / or coating metal contents can lead to coating times of, for example, 20 h. In the case of a material that is harder to coat, such as a monocrystalline solidified Ni-base alloy, a longer coating time is required under otherwise identical conditions.

The problem underlying the present invention is to provide a method of the type described above, with which diffusion layers with a defined layer thickness and a defined coating metal content in wt .-% in the component surface as economically as possible, i. while saving on coating time.

The solution to this problem is according to the invention characterized in that the metal halide over a first (coating) period above the nominal concentration, first concentration and at least one second (coating) period at least one at or below the Nominal concentration lying, second concentration is set at the component surface, wherein the first and the at least one second period are selected so that the sum of which is shorter than the coating time with nominal concentration.

In this method, it proves to be advantageous that due to the high, first concentration of the metal halide on the component surface in the first period equal to the beginning of the process, there is a large difference in concentration to the component, which generally initially little or no identical to the coating metal element, eg Al, Cr contains. Due to the high driving force, this leads to rapid introduction of a large number of coating metal atoms into the surface of the component. After the end of the first period, the component surface thus has an extremely high content of coating metal atoms, which, however, is present only over a small layer thickness. The high coating metal content at the component surface leads in the second period by diffusion processes to a higher coating metal content in the component depth and degradation on the component surface, which at the end of the second period to a diffusion layer with the desired coating metal content in wt .-% in the component surface and the desired layer thickness leads.

The high, first concentration in the first period is generated by an excess of metal halide and in the second period by dilution (supply of inert gas or hydrogen) canceled.

The metal halide may be formed by reacting a halogen or a halide with a coating metal present in a donor source, wherein the halide is powder or granular in the donor source or, alternatively, supplied by a feeder to the reaction space in which the components are disposed can be. In the latter case can the second concentration can be adjusted by reducing the supply of halogen or halide.

The metal halide may preferably contain F or Cl.

Al and / or Cr and optionally further elements such as Si, Hf, Y can be provided as the coating metal in order to protect the coated component surfaces against oxidation or corrosion.

For good effectiveness, a diffusion layer having a layer thickness of 50 to 100 μm and a coating metal content of 25 to 32% by weight is formed in the component surface.

Preferably, the first time period having the first concentration above the nominal concentration of between 5 (2) and 6 (10) hours and the at least one second time period having the below the nominal concentration, the second concentration between 3 (1) and 4 (6) Hours are set.

Due to the great driving force during the first period and the associated high introduction of coating metal atoms into the component surface, a second concentration can be set to approximately zero in a second period of time, so that the layer thickness increases as a result of diffusion of the coating metal atoms already present in the component surface.

The at least one second concentration may e.g. by introducing an inert gas, such as argon, or hydrogen into the reaction space in which the components to be coated are arranged, or by reducing the supply of supplied halogen or halide.

Before the formation of the diffusion layer, Pt can be electrodeposited on the component surface and, if necessary, heat-treated, since diffusion layers, which in addition to the coating metal still contain Pt or Pd, provide even better protection against High temperature oxidation and corrosion. For Al as a coating metal, a PtAl diffusion layer has good efficiency when the Al content in the surface is in the range of 18 to 25% by weight.

Prior to formation of the diffusion layer, other elements such as Pt, Si, Y, Hf or mixtures of the MCrAlY type (with Ni, Co as M) as a slurry or plasma sprayed layer may also be deposited on the device surface to provide specific properties of the diffusion layer, e.g. Oxidation resistance or ductility, to further improve.

The pressure of the coating gas can be changed at least temporarily in the first and / or second time period, wherein this can preferably take place intermittently. By suction from a reaction vessel receiving the components to be coated or from a retort in which at least one reaction vessel is arranged, it is possible to switch between atmospheric pressure and negative pressure. The negative pressure is preferably set to a pressure in the range of normal pressure to 100 mbar. The change in pressure causes an improved penetration of the coating metal, especially for cavities to be coated, and leads to shorter coating times. By lowering the pressure, the lower, second concentration can also be set in the second period.

Further embodiments of the invention are described in the subclaims.

Fig. 1

eine Vorrichtung zur Durchführung des erfindungsgemäßen Verfah- rens zum Gasdiffusionsbeschichten,

Fig. 2

ein Diagramm, in dem der Al-Gehalt über der Schichtdicke am Ende des ersten Zeitraums dargestellt ist, und

Fig. 3

ein Diagramm, in dem Al-Gehalt über der Schichtdicke am Ende des zweiten Zeitraums dargestellt ist.

In the following the invention with reference to an embodiment will be explained in more detail with reference to a drawing. It shows:

Fig. 1

an apparatus for carrying out the method according to the invention for gas diffusion coating,

Fig. 2

a diagram in which the Al content over the layer thickness at the end of the first period is shown, and

Fig. 3

a diagram showing Al content over the layer thickness at the end of the second period.

Fig. 1 shows a device for carrying out the method with a heatable retort 1, in which at least one reaction container 2 is arranged. Depending on the size, a plurality of reaction vessels 2 can be arranged above and / or next to one another in the retort 1. In the rotationally symmetrical design of the reaction vessel 2 in the present embodiment, a plurality of schematically illustrated components 3 of a gas turbine, such as turbine blades, are arranged with their surfaces 4 to be coated and kept suitable. The components 3 are aligned substantially radially.

The reaction vessel 2 has a centrally arranged distribution device 5 with apertures 6 shown enlarged in the drawing, which are distributed over the height thereof and around its circumference substantially uniformly. Instead of the openings 6, it is also possible to provide tubes extending radially outwards into the reaction vessel 2, each having a plurality of openings or nozzles. The reaction vessel 2 further comprises at least one semi-permeable seal 7 through which gases can escape from the reaction vessel 2. In the present case, the reaction vessel 2 is provided with a circumferential on the outer circumference 8 semipermeable seal 7.

Through a feed line 9 opening into the central distributor device 5, a halogen or halide for generating the coating gas by reaction with the coating metal and / or inert gas and / or hydrogen can be fed through the central distributor device 5 evenly into the reaction vessel 2 from its center flows and escapes through the semipermable seal 7. The retort 1 has a feed line 10 through which inert gas, such as argon, is supplied to the purge prior to the start of the process to substantially remove O ₂ to prevent oxidation.

In the present embodiment of the method, the turbine blades 3 made of a nickel or cobalt-based alloy with an aluminum diffusion layer having an Al content at the surface of 25 to 32 wt .-% and a layer thickness of 60 to 90 microns to protect against hot gas oxidation be coated. To do this, a plurality of vanes, e.g. 100 pieces, arranged in the reaction space 2 and held in a suitable manner, so that the surface 4 to be coated for the coating gas is freely accessible.

In the reaction space 2, a plurality of dispenser sources 12 are provided for the coating metal Al selected here in the form of containers which contain the powdered or granular coating metal. The donor sources 12 are arranged as close as possible to the turbine blades 3, in order to achieve the desired high, first concentration in the first period. The selected coating metal AlCr is present as granules in an amount sufficient so that several batches of turbine blades can be coated successively. Additionally, in the donor source 12 is an F-containing halide which reacts at the coating temperature with the coating metal to form a metal halide (coating gas).

Before starting the process, an inert gas, such as argon, is added to the retort 1 via the supply line 10 for rinsing in order to make the retort 1 substantially free of O ₂ and H ₂ O in order to avoid oxidation. During the subsequent heating process 1 to the coating temperature in the range of 1000 to 1100 ° C, preferably 1080 ° C, the reaction vessel 2, no gas is initially supplied via the supply line 9. From a temperature of about 700 ° C, the retort 1 via the supply line 10 and the reaction chamber 2 via the supply line 9 and the manifold 5 hydrogen (H ₂ ) is supplied. From a temperature of 1000 ° C, the hydrogen supply to the reaction space 2 is terminated.

After reaching the coating temperature of 1080 ° C, this temperature is maintained for a first period of about six hours. Under these conditions, there is a concentration of the metal halide that results in an Al content of about 38% by weight in the component surface.

Immediately thereafter, hydrogen is supplied to the reaction space 2 at the beginning of the second period via the feed line 9 and the distributor device 5, as a result of which the concentration of metal halide on the surfaces 4 of the turbine blades 3 to be coated is markedly reduced. This takes place on the one hand by the dilution in the reaction vessel 2 and on the other hand by the fact that the metal halide forming the coating gas due to the excess of hydrogen reacts to form hydrogen halides. These conditions are maintained for four hours during the second period. After completion of the second period, the retort 1 and the reaction chamber 2 are cooled by supplying 1 m ³ / h of inert gas (argon) via the supply line 10 or 9 to room temperature.

Thus, the invention requires a total of only 10 hours for the production of the diffusion layer with the desired layer parameters.

In an alternative embodiment of the method, an inert gas is supplied to the reaction space 2 via the feed line 9 and the distributor device 5 at the beginning of the second period for adjusting the second concentration of the metal halide at the component surface 4 which is below the nominal concentration.

To further improve the effectiveness of the diffusion layer against hot gas oxidation and corrosion may contain an Al diffusion layer Pt or Pd, wherein in such an embodiment, for example, first deposited Pt with a layer thickness of eg 5 microns galvanically on the component surface and optionally heat treated. Subsequently, the method according to the invention is carried out in the manner described above. Due to the high driving force of the method according to the invention due to the high Al concentration in the first coating period, Al can diffuse into the component surface through the Pt layer. In this way, a PtAl diffusion layer can be produced with a layer thickness of 70 microns, which at 5 microns depth an Al content of about 24 wt .-% and a Pt content of about 21 wt .-% and in 15 microns depth has an Al content of about 23% by weight and a Pt content of about 18% by weight and thus has an advantageous ratio between Al and Pt.

In the Fig. 2 a diagram is shown in which, by way of example for Al, the coating metal content is shown in percent by weight over the layer thickness after the end of the first period, ie the coating with the first concentration above the nominal concentration. The high driving force associated with the high concentration results in an Al content of 38% in the surface of the device which is above the desired Al content in the range of 25 to 32% by weight. The layer thickness S of the diffusion layer is only slight after the end of the first period and is far below the desired layer thickness of 50 to 100 μm.

In the in Fig. 3 The diagram shown is the Al content over the layer thickness after the end of the second period, ie applied to the end of the coating process. As a result of the diffusion of the Al atoms into the component, the desired Al content of 28% by weight is established on the component surface. The distribution of Al is significantly more uniform and leads to an increase in the layer thickness up to the desired range of 50 to 100 microns.

Claims (17)

1. Method for gas phase diffusion coating of metal components, in which a component surface to be coated is brought into contact with a metal halide as the coating gas for formation of a diffusion layer defined via a given layer thickness and a given coating metal content in wt.% in the component surface, wherein a defined coating temperature leads to a defined coating duration with a nominal concentration of the metal halide on the component surface correlating therewith, characterized in that over a first period of time a first concentration lying above the nominal concentration and over at least one second period of time at least one second concentration lying at or below the nominal concentration is established on the component surface, wherein the first and the at least one second period of time are chosen so that their sum is shorter than the coating duration with the nominal concentration.
2. Method according to claim 1, characterized in that the metal halide is produced by reaction of a halogen or a halide with a coating metal present in a donor source.
3. Method according to claim 1 or 2, characterized in that the metal halide contains F or Cl.
4. Method according to one or more of the preceding claims, characterized in that Al and/or Cr or alloys of these are provided as the coating metal.
5. Method according to claim 4, characterized in that the coating metal additionally contains one or more of the elements Si, Pt, Pd, Hf and Y.
6. Method according to one or more of the preceding claims, characterized in that a diffusion layer with a layer thickness of from 25 to 100 µm is formed.
7. Method according to one or more of the preceding claims, characterized in that a diffusion layer with a coating metal content of from 25 to 32 wt.% is formed in the component surface.
8. Method according to claim 6 and 7, characterized in that the first period of time is set at between 5 and 6 hours and the at least one second period of time is set at between 3 and 4 hours.
9. Method according to one or more of claims 1 to 7, characterized in that the first period of time is set at between 2 and 10 hours and the at least one second period of time is set at between 1 and 6 hours.
10. Method according to one or more of the preceding claims, characterized in that a coating temperature in the range of from 900 to 1,200 °C is maintained during the first and second period of time.
11. Method according to claim 10, characterized in that a coating temperature in the range of from 1,000 to 1,100 °C is maintained during the first and second period of time.
12. Method according to one or more of the preceding claims, characterized in that a second concentration is established as approximately zero in a second period of time.
13. Method according to one or more of the preceding claims, characterized in that the at least one second concentration is established by supplying an inert gas or hydrogen or by reducing the amount of halogen or halide supplied.
14. Method according to one or more of the preceding claims, characterized in that Pt is deposited by electroplating on the component surface before formation of the diffusion layer.
15. Method according to one or more of the preceding claims, characterized in that at least one element, such as Pt, Si, Y or Hf, or mixtures or alloys, such as MCrAlY (with Ni and/or Co as M) is deposited on the component surface as a slip or by plasma spraying before formation of the diffusion layer.
16. Method according to one or more of the preceding claims, characterized in that in the first and/or second period of time the pressure of the coating gas is changed at least occasionally.
17. Method according to one or more of the preceding claims, characterized in that the second concentration is established by lowering the pressure.

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