Coatings for turbine blades subjected to thermal and abrasive loads.

Abstract

The present invention relates to a method for coating a substrate surrounding a gas turbine blade, in which in a first step an MCrAlY substrate is applied by PVD; in a further step an oxide layer is applied by PVD.

Description

【Background Art】 【0001】 A gas turbine has the task of moving gas in one direction. The gas turbine includes at least one rotor that rotates about an axis, and the at least one rotor has a carrier, and a plurality of turbine blades protruding radially outward are arranged on the outer circumference of the carrier. In order to prevent as much gas as possible from flowing back in the direction opposite to the desired direction, and thus to achieve the highest possible efficiency of the gas turbine, a turbine liner is provided to ensure a gap with a minimum gap distance between the turbine blades and the turbine liner. 【0002】 This is achieved by the so-called dressing layer on the turbine liner side. These dressing layers serve to keep the gap distance between the turbine blades and the surrounding turbine liner as small as possible in order to prevent pressure loss. The dressing layer is generally porous and is weakly bonded internally. The result is that the turbine blade tips, which initially still frequently contact the dressing layer, wear until a concentricity with essentially no contact is achieved at a minimum gap distance. 【0003】 However, the turbine blades may erode the porous and weakly internally bonded dressing layer in an undesirable manner, for example, during thermal expansion of the turbine or vibration-induced deflection from the center of the turbine, thus increasing the gap distance and potentially reducing the efficiency. 【0004】 Blade tip coatings are used to protect the blade tips from wear. These blade tip coatings typically consist of abrasive particles (such as cubic boron nitride) embedded in a matrix material (such as MCrAlY, etc.). "M" represents a metal, usually cobalt, nickel or a cobalt-nickel alloy. Cr represents chromium, Al represents aluminum, and Y represents yttrium. 【0005】 According to prior art, such coatings are applied by complex and cost-intensive processes such as electrophoretic or electrophoretic deposition (US5935407A). 【0006】 A drawback of coatings manufactured in this manner according to prior art is poor adhesion. In the corresponding coating process, the energy input is relatively low, and there is little diffusion at the interface with respect to the substrate surface, which usually ensures acceptable layer adhesion. As a result, the forces generated during rotation can already cause damage to the entire coating or the abrasive particles, as well as delamination. 【0007】 In addition, both the abrasive particles and the base material used in prior art are not resistant to oxidation at high temperatures and become non-functional due to oxidation. Typically used abrasive particles have a particle size approximately equal to the thickness of the layer, and therefore can reach from the surface to the interface between the coating and the substrate. When the particles are oxidized, the blade material or the corresponding interface is directly attacked, which can lead to a direct attack on the blade material or the interface between the blade material and the coating. [Overview of the project] [Problems that the invention aims to solve] 【0008】 Therefore, there is a need to improve the adhesion and resistance to oxidation of coatings known from prior art. The object of the present invention is based on this need. [Means for solving the problem] 【0009】 The above objective is achieved according to claim 1 of the present invention, in that deposition from the gas phase by a PVD process is used for coating. The use of reactive spark evaporation is particularly preferred. 【0010】 The adhesion of the blade tip coating can be significantly improved by using reactive spark evaporation, because the higher energy input from gas ions contributes to improved coating adhesion. Furthermore, manufacturing parameters can be selected more freely, meaning that deposition at higher temperatures is possible. 【0011】 By using different target materials and reactive gases, bonding layers and / or base materials, as well as 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 macroparticles in the base material. In contrast to conventional manufacturing processes for blade tip coatings based on electrolytic or electrophoretic deposition, very small particles or thin layers can be completely embedded in the base material (e.g., containing, and preferably consisting of, MCrAlY material), thereby allowing the abrasive phases located deeper to withstand oxidation, even if they are not resistant to oxidation (this is only true in part), and thus the base material above them (e.g., MCrAlY). l It is protected from Y). Therefore, even in contact or abrasion conditions after a longer service life than conventional blade tip coatings, it is possible to achieve a protective effect on the blade tip against the break-in layer on the liner. 【0012】 The coating can consist of several layers, thereby allowing the adhesive layer to be adapted to the substrate material to enable optimal adhesion. 【0013】 The abrasive phases of the blade tip coating can be adapted to the break-in layer on the turbine liner. These abrasive phases can be incorporated into the coating as layers or as particles. 【0014】 The layer thickness can be varied to adapt the coating to the thermal and abrasive stress profiles, and thus increase its service life. 【0015】 For example, to improve the overall wear resistance of the blade tip coating during the initial break-in process, a thermally less stressful layer can be deposited on the blade tip coating. The present invention will be described in detail below with reference to the drawings and examples. [Brief explanation of the drawing] 【0016】 [Figure 1] A schematic diagram of the layer system according to the present invention, consisting of an MCrAlY layer and an oxide layer on top of it, is shown. [Figure 2] A schematic representation of the multilayer coating system according to the present invention is shown. [Figure 3] A schematic diagram of the turbine is shown. [Figure 4] This shows a SEM image of a cross-section of the multilayer coating system according to the present invention after exposure to a temperature of 1200°C for 10 hours. [Figure 5] This shows the X-ray diffractogram of the polished aluminum oxide-chromium oxide phase. [Modes for carrying out the invention] 【0017】 The turbine shown in Figure 3 has 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 shows a break-in layer 11 on the turbine liner 1, which faces the blade tip 9 and is separated from the blade tip 9 by a gap G. 【0018】 A coating of the composition MCrAlY-aluminum chromium oxide, or a multilayer coating of alternating layers of MCrAlY-aluminum chromium oxide, is deposited on the tip of a blade made of a superalloy (which may be a single crystal, for example). 【0019】 MCrAlY is deposited from an MCrAlY material source (=target) by plasma-enhanced cathode spark evaporation. The MCrAlY layer can have a thickness of 0.1 to 100 micrometers, depending on the required oxidation resistance. 【0020】 Here, the oxide layer is deposited on the MCrAlY adhesion and antioxidant layer. The aluminum-chromium oxide layer is deposited from a metallic AlCr target by reactive cathode spark evaporation in an oxygen atmosphere. The oxide layer can have a thickness of 0.5 to 50 microns. 【0021】 To suppress the harmful diffusion process and thus extend the life, the oxide layer can also be deposited as a multilayer coating in which the MCrAlY layer alternates with the aluminum-chromium oxide layer at regular or other intervals of 0.1 to 20 micrometers. 【0022】 In this concept, the oxide coating provides a diffusion barrier, which also functions as a polishing phase that is not sensitive to oxidation at the same time. The MCrAlY layer directly adhered to the substrate also provides excellent adhesion to the blade tip, and the sum of all MCrAlY layers in the entire blade tip coating prevents the inwardly oriented diffusion process and efficiently protects the substrate from oxidation. 【0023】 Generally speaking, it can be said that the hardness of the entire layer system according to the present invention can be adjusted by the ratio of the polishing phase to MCrAlY in order to enable optimal removal of the dressing layer. For example, a layer having an oxide phase in the range of 7 to 25 GPa can be adjusted. However, when a harder polishing phase such as nitride, boride or carbide is used, the hardness can be increased up to 45 GPa. For example, the layer in FIG. 4 has a hardness of about 13 GPa. 【0024】 When aluminum oxide-chromium oxide is used as the polishing phase, it forms a mixed crystal of a corundum structure with a strong preferred orientation in cathode spark evaporation, as seen in FIG. 5. In the corundum structure, the mixed oxide is in its thermally stable high-temperature transformation state and can thus reach a high application temperature without undergoing a phase transition. Therefore, it is possible to prevent the volume change associated with the phase transition, which can lead to layer breakage.

Claims

[Claim 1] A method for coating a substrate surrounding a gas turbine blade, - In the first step, the application of the MCrAlY base material by the cathode spark evaporation method, - Further steps include applying a layer by cathode spark evaporation, wherein the layer is deposited in contact with the surface of the MCrAlY matrix and comprises at least one aluminum chromium oxide and / or boride and / or carbide and / or nitride. In the aforementioned MCrAlY base material, M represents cobalt, nickel, or a cobalt-nickel alloy. method. [Claim 2] The method according to claim 1, characterized in that the material source for the cathode spark evaporation method in the further step is an AlCr target, and the coating method is a reactive method in which oxygen is used. [Claim 3] The method according to claim 1 or 2, characterized in that the coating is performed as a layer system comprising two layers, or the coating is performed as a layer system comprising a multilayer alternating coating system. [Claim 4] A layer system for the tip of a gas turbine blade, wherein the coating comprises at least a first layer having an MCrAlY matrix, and the coating comprises at least a second layer, the second layer being deposited in contact with the surface of the first layer, and comprising at least one aluminum chromium oxide and / or boride and / or carbide and / or nitride, In the aforementioned MCrAlY base material, "M" represents cobalt, nickel, or a cobalt-nickel alloy, indicating a layered system. [Claim 5] The layer system according to claim 4, characterized in that the coating is a multilayer coating system in which the first layer and the second layer are alternated. [Claim 6] A gas turbine blade having the coating according to any one of claims 4 or 5.

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