Protective coating for a thermally stressed structure

Abstract

Provided is a method for arranging a protective coating for a thermally stressed structure, having at least one layer of alpha-aluminium oxide or of element-modified alpha-aluminium oxide, and wherein the protective coating is applied by reactive cathodic arc vaporization. A protective coating produced by the method and a component having a protective coating is also provided.

Claims

1. A method for arranging a protective coating comprising at least one layer with aluminum oxide content on a thermally stressed structure by means of cathodic arc evaporation, wherein the thermally stressed structure comprises at least one thermal barrier coating made of a ceramic material, the method comprising: S1) providing the thermally stressed structure in a coating chamber; S2) providing a target as a source material for the at least one layer in the coating chamber, wherein the target comprises at least aluminium; S3) providing a controlled oxygen partial pressure in the coating chamber; and S4) igniting an arc so that material of the target is evaporated, which material is deposited on an outer side of the thermal barrier coating of the thermally stressed structure, wherein the ceramic material of the thermal barrier comprises yttrium oxide-stabilized zirconium dioxide and/or gadolinium oxide-stabilized zirconium oxide, and where the protective coating is applied as a homogeneous alpha-aluminum oxide layer, with the target additionally comprising chromium, wherein a content of chromium is measured so that the lattice constant of at least one Al—Cr—O mixed crystal which is created during the coating is adjusted within the range which lies between corundum and eskolaite.

2. The method as claimed in claim 1, wherein the target additionally comprises elements selected from the group consisting of: titanium, hafnium, silicon, and zirconium.

3. The method as claimed in claim 1, wherein a temperature of the substrate is at least 200° C.

4. The method as claimed in claim 1, wherein the substrate temperature is approximately 600° C.

5. The method as claimed in claim 1, wherein a thickness of the applied protective coating lies between 5 and 500 μm.

6. The method as claimed in claim 1, wherein a gradient is produced in the chemical composition of the protective coating.

7. The method as claimed in claim 1, further comprising applying the protective coating directly onto the at least one thermal barrier coating made of ceramic.

8. The method as claimed in claim 1, wherein an adhesive layer is located between the at least one thermal barrier coating made of ceramic and the protective coating.

9. The method as claimed in claim 1, further comprising providing a zirconium yttrium target for the adhesive layer.

Description

BRIEF DESCRIPTION

(1) Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

(2) FIG. 1 shows a schematic view of an embodiment of a component with an embodiment of a protective coating;

(3) FIG. 2 shows a schematic view of an embodiment of a component with a further embodiment of the protective coating; and

(4) FIG. 3 shows a flow diagram of an embodiment of a method.

DETAILED DESCRIPTION

(5) In the embodiment shown in FIG. 1, a layer system 1 has a base material 2 of a component. A metallic adhesive layer 3 of the MCrAlY type is arranged directly on the base material 2. M in this alloy typically stands for at least one metal from the group which includes iron, cobalt and nickel. As an alternative to yttrium (Y), another element, preferably a rare earth element, can be used in the alloy. As an alternative to the MCrAlY type, a diffusion coating such as diffusion aluminide or diffusion aluminide modified with platinum can be used as the adhesive layer 3.

(6) A thermal barrier coating 4 is arranged on the adhesive layer 3. The thermal barrier coating comprises a ceramic material, e.g. yttrium oxide-stabilized zirconium dioxide or gadolinium oxide-stabilized zirconium oxide. The thermal barrier coating 4 can also comprise both yttrium oxide-stabilized zirconium dioxide and gadolinium oxide-stabilized zirconium oxide.

(7) A protective coating 5, which in the exemplary embodiment according to FIG. 1 consists of one layer, is arranged on the thermal barrier coating 4. The protective coating 5 in one embodiment is a homogenous layer consisting of α-aluminum oxide, also referred to as a corundum structure. In a further embodiment, the layer comprises chromium. In this case, the corundum structure consists of an Al—Cr—O mixed crystal. Depending on the chromium content, chromium diffuses from a layer with high chromium content into a layer with low chromium content, wherein the corundum structure of the Al—Cr—O mixed crystal is obtained. As an example of a diffusion from the protective coating 5 into the thermal barrier coating 4, this means that a (Al.sub.0.99Cr.sub.0.01).sub.2O.sub.3 layer is formed from a (Al.sub.0.7Cr.sub.0.3).sub.2O.sub.3 layer. This transformation is with a temperature-dependent color change of the layer from an original dark gray coloring to a green and then to a red coloring. In an X-ray crystal structure analysis the transition from the mixed crystal to the so-called noble corundum (ruby) can be revealed in this case.

(8) In the exemplary embodiment according to the view of FIG. 2, the protective coating 5 consists of a first layer 5a and a second layer 5b. In addition, an adhesive layer 6 is arranged on the thermal barrier coating 4, on which is again arranged the protective coating 5. The adhesive layer 6 is formed for example by means of evaporation of a zirconium-yttrium target. Arranged on this adhesive layer is the first layer 5a, which comprises a composition of Al—Zr—O or Al—Zr (Cr)—O, i.e. with this first layer 5a between the thermal barrier coating 4 and the protective coating 5, a materially-adapted transition to the second layer 5b is created by means of the reactive CAE method, which second layer has an Al—Zr—O corundum structure or an Al—Zr—Cr—O corundum structure. In this case, the layers are not strictly delimited by each other but merge into each other, wherein they have differences with regard to the concentration of added metals such as zirconium and chromium, or in other words have a gradient. In this case, the reactive CAE method is extremely well suited to allowing the layers to merge into each other via a gradient.

(9) In a method for arranging the protective coating 5 according to the embodiment according to FIG. 1 by means of reactive CAE, in a first step S1 a thermally stressed structure is provided in a coating chamber. The thermally stressed structure is for example a blade of a gas turbine or a combustion chamber part. The coating chamber is in the main a vacuum chamber, but into which oxygen can be introduced as the reactive gas in order to deposit an oxide on the substrate.

(10) In a second step S2, a target is provided as the material source for the protective coating 5 which is to be formed, wherein the target comprises at least aluminum. Furthermore, the target preferably comprises chromium, and/or, according to choice, other elements such as zirconium, titanium, hafnium and/or silicon.

(11) In a third step S3, a controlled quantity of oxygen is introduced into the coating chamber. In this case, the person skilled in the art selects a determined, suitable partial pressure of the oxygen.

(12) In a fourth step S4, an arc is ignited so that the material of the target is evaporated. The evaporated target elements react with the oxygen forming an oxide which is deposited on the outer side of the thermal barrier coating 4 of the thermally stressed structure so that the coating being applied is an oxide layer, ideally an Al—Cr—O layer. The target, e.g. an aluminum-chromium target for producing an Al—Cr—O layer, acts as a cathode, and the wall of the coating chamber acts as an anode. In addition, a negative potential is applied to the substrate, e.g. to the thermal barrier coating 4 of the thermally stressed structure, in order to direct the ionized material vapor toward the substrate. The evaporated material condenses on the surface of the thermal barrier coating and is deposited forming a layer which forms the protective coating 5.

(13) For arranging the protective coating 5 according to the embodiment of FIG. 2, different targets are provided, from which material for the different layers is separated out. Therefore, for example a zirconium-yttrium target is provided for the adhesive layer 6, an aluminum-chromium-zirconium target is provided for the first layer 5a and an aluminum-chromium target is provided for the second layer 5b. The concentration of the elements in the targets differs depending on the desired concentration in the layers which are to be formed or of a gradient which is to be created between the layers. A gradient can also exist within one layer. In this case, the described procedure is conducted by the steps S2 to S4 for each individual layer. For this, the coating chamber can be ventilated between the applications of the individual layers so that the target can be exchanged and the coating chamber can then be pumped out again to the desired vacuum. The coating plant can, however, be designed so that it contains all the targets from the start of the coating process and these can be used in accordance with the desired coating sequence without a break in the vacuum.

(14) In addition to reactive CAE, other methods are also suitable for applying the layers of the protective coating. Possible methods, which can be used alternatively and/or in combination with the aforesaid methods, are for example sputtering processes, thermal evaporation, electron beam evaporation, laser beam evaporation or electric arc evaporation.

(15) Although the invention has been illustrated and described in greater detail with reference to the_preferred_exemplary embodiment, the invention is not limited to the examples disclosed, and further variations can be inferred by a person skilled in the art, without departing from the scope of protection of the invention.

(16) For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.