Coating for turbine blades subjected to thermal and abrasive loads

Abstract

The invention relates to a method for coating a substrate comprised by a gas turbine blade, comprising the following steps: - applying an MCrAlY matrix in a first step using a PVD method; - applying an oxide layer in a further step by a PVD method.

Description

[0001] The task of a gas turbine is to move gas (Gas) in one direction. The gas turbine comprises at least one rotor which rotates about a shaft and has a carrier On the periphery of the carrier, a plurality of radially outwardly projecting turbine blades are arranged. In order to prevent the gas from flowing counter to the desired direction to thereby achieve as high a degree of efficiency of the gas turbine as possible, a turbine lining is provided in order to ensure a gap having a minimum gap distance between the turbine blades and the turbine lining.

[0002] This is achieved by so-called run-in layers on the side of the turbine lining. These run-in layers serve to keep the gap distance between the turbine blades and the surrounding turbine lining as small as possible in order to prevent pressure losses. The run-in layers are usually porous and only weakly bound to themselves. As a result, the turbine blade tip, which still often comes into contact with the run-in layer at the beginning, will abrade it until a substantially contact-free circumferential movement is achieved at the minimum gap distance.

[0003] However, the turbine blades, for example upon thermal expansion or turbine misalignment caused by vibrations, will abrade this porous and only weakly bound to itself run-in layer in an undesirable manner and thus increase the gap spacing and reduce the efficiency.

[0004] In order to protect the blade tip from wear, blade tip coatings are used. These blade tip coatings are usually composed of abrasive particles, for example cubic boron nitride, embedded in a matrix, for example MCrAIY. Here, "M" stands for metal, wherein this is usually cobalt, nickel or a cobalt-nickel alloy. Here, "Cr" stands for chromium, "Al" stands for aluminum and "Y" stands for yttrium.

[0005] According to the prior art, such coatings are applied by complex and cost-intensive methods, for example electrolytic or electrophoretic deposition (US 5935407 A).

[0006] The disadvantage of coatings made in this way according to the prior art is poor layer adhesion. This is because, in the respective coating process, the energy input is relatively low and there is hardly any diffusion process at the interface with the substrate surface which usually ensures acceptable layer adhesion. As a result, due to the forces occurring upon rotation, a failure and delamination of the entire layer or of the abrasive particles can have occurred.

[0007] Furthermore, the abrasive particles and the matrix used in the prior art are not resistant to oxidation at high temperatures and are deactivated due to oxidation. The abrasive particles usually used have a particle size in the order of magnitude of the layer thickness and can thus reach from the surface to the interface between the coating and the substrate. If the particles are now oxidized, the blade material or the corresponding interface is attacked directly, which leads to a direct attack on the blade material or the interface between the blade material and the coating when the particles are oxidized.

[0008] There is a need to make the coatings known from the prior art more resistant to oxidation and more adhesive. The object of the present application is based on this need.

[0009] The above object is achieved according to the present application and according to claim 1, wherein for the coating deposition from the gas phase by means of a PVD method is used. The use of reactive spark evaporation is particularly preferred.

[0010] This is because by using reactive spark evaporation the adhesion of the blade tip coating can be significantly improved, since the higher energy input of the gas ions contributes to improving the layer adhesion. The production parameters can also be chosen more freely, whereby the deposition can be carried out at higher temperatures.

[0011] By using different target materials and reactive gases, adhesive layers and / or matrices and abrasive phases, such as oxides, borides, carbides or nitrides, can be deposited in a single process. These phases can be introduced as layers in a multilayer structure or as large particles in the matrix. In contrast to the conventional manufacturing method of blade tip coatings based on electrolytic or electrophoretic deposition, very small particles or thin layers can be completely embedded in the matrix here (for example comprising and preferably consisting of MCrAlY material), whereby the abrasive phases located deeper are protected by the matrix (for example MCrAlY) located above them, even if they are not resistant to oxidation (but this only applies to some cases). A protective effect of the blade tip in contrast to the running-in layer on the liner can thus be achieved, even in the case of contact or wear situations after a longer operating time compared to conventional blade tip coatings.

[0012] The layer can consist of a plurality of sub-layers, wherein the adhesive layer can be adapted to the substrate material, allowing the best adhesion to be achieved.

[0013] The abrasive phases of the blade tip coating can be adapted to the running-in layer on the turbine liner. These abrasive phases can be embedded in the layer as sub-layers or as particles.

[0014] The layer thickness can be varied in order to adapt the coating to the thermal and abrasive load profile, thus increasing the service life.

[0015] A layer with a lower heat load can be deposited on top of the blade tip coating to increase the wear resistance of the entire blade tip coating, for example, during the initial break-in period.

[0016] The invention will now be explained in detail with reference to embodiments and accompanying drawings:

[0017] Figure 1 The layer system according to the invention is schematically shown, consisting of an MCrAlY layer and an oxide layer above it.

[0018] Figure 2 The multi-sublayer system according to the present invention is illustrated schematically.

[0019] Figure 3 A schematic diagram of a turbine is shown.

[0020] Figure 4 The SEM image shows a cross-section of the multilayer system according to the present invention after being exposed to a temperature of 1200°C for 10 hours.

[0021] Figure 5 The X-ray diffraction pattern of the alumina-chromium oxide polished phase is shown.

[0022] Figure 3 The turbine shown includes at least one turbine blade 5 on a rotating disk 3, which has a blade base 7 and a blade tip 9. Figure 3 Also shown is a break-in layer 11 on the turbine liner 1 opposite to the blade tip 9, and separated from the blade tip by a gap G.

[0023] A coating consisting of MCrAlY-aluminum chromium oxide, or a multilayer layer consisting of alternating MCrAlY-aluminum chromium oxide layers, is deposited on the blade tip of a high-temperature alloy (e.g., a single crystal).

[0024] MCrAlY is deposited here by plasma-enhanced cathode spark evaporation from an MCrAlY material source (=target). The thickness of the MCrAlY layer can be 0.1-100 micrometers depending on the required oxidation resistance.

[0025] An oxide layer is now deposited on the MCrAlY adhesion and oxide protective layer. The aluminum chromium oxide sublayer is deposited by reactive cathode spark evaporation from a metallic AlCr target in an oxygen atmosphere. The oxide layer can be 0.5 to 50 micrometers thick.

[0026] To suppress harmful diffusion processes and thus improve service life, oxide layers can also be deposited as multi-sublayer layers, in which MCrAlY layers alternate with aluminum chromium oxide layers at regular or other spacings of 0.1–20 micrometers.

[0027] In this concept, the oxide coating provides a diffusion barrier and also serves as an oxidation-insensitive abrasive phase. The MCrAlY sublayers directly adhered to the substrate also provide excellent adhesion to the blade tip, and the sum of all MCrAlY sublayers in the entire blade tip coating prevents inward-oriented diffusion processes and effectively protects the substrate from oxidation.

[0028] It is generally accepted that the hardness of the entire layer system according to the invention can be set by the ratio of the abrasive phase to MCrAlY to optimally abrade the wear layer. For example, a layer with an oxide phase of 7 to 25 GPa can be configured. However, if a harder abrasive phase, such as nitride, boride, or carbide, is used, the hardness can be increased to up to 45 GPa. For example, Figure 4 The hardness of the middle layer is approximately 13 GPa.

[0029] If alumina-chromium oxide is used as the polishing phase, it forms a mixed crystal with a corundum structure during cathode spark evaporation, exhibiting a strong preferred orientation, such as... Figure 5 As shown, in the corundum structure, the mixed oxide is in its thermally stable high-temperature variant state, thus enabling high application temperatures without phase transition. This prevents phase transition-related volume changes that could lead to layer failure.

Claims

1. A method for coating a substrate comprising a gas turbine blade, comprising the following steps: - Apply the MCrAlY matrix using the PVD method in the first step - In a further step, a layer is applied by a PVD method, wherein the layer comprises at least aluminum chromium oxide, wherein the aluminum chromium oxide sublayer is deposited by a metallic AlCr target through reactive cathode spark evaporation in an oxygen atmosphere. The process involves alternately applying an MCrAlY matrix and an aluminum chromium oxide layer to deposit a multilayer of alternating MCrAlY-aluminum chromium oxide layers.

2. The method according to claim 1, characterized in that, The first step of the PVD method is the cathode spark evaporation method.

3. The method according to any one of claims 1 to 2, characterized in that, The multi-sublayer is constructed as a layer system comprising two layers, or the multi-sublayer is constructed as a layer system comprising alternating multi-sublayer systems.

4. A layer system for the leading edge of a gas turbine blade, wherein the coating comprises at least one first layer having an MCrAlY matrix, and the coating comprises at least one second layer, wherein the second layer comprises at least aluminum chromium oxide, and wherein the second layer is deposited by a metallic AlCr target via reactive cathode spark evaporation in an oxygen atmosphere. in, The coating is designed as a multi-layer system, in which the first and second layers alternate.

5. A gas turbine blade having the layer system according to claim 4.

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