The present invention relates to thermally loaded components of thermal machines, especially gas turbines. It refers to a method for applying a high-temperature stable coating layer on the surface of a component. It further refers to a component with such a coating layer.
In order to protect thermally loaded components against hot gases they are coated with various protective layers, for example a thermal barrier coating (TBC). To bond such a layer firmly to the body of the component, a bond coat may be provided between the base material of the component and the TBC. A well-known bond coat material for a component made of a Ni base superalloy or the like, is of the type MCrAlY, where M stand for a metal, e.g. Ni.
During service life, cracks might form in the bond coat and propagate into the base metal of components, which are part of gas turbine or other thermal machine, and which are exposed to high operating temperatures. Especially, low cycle fatigue (LCF)/thermo-mechanical fatigue (TMF) cracking is a limiting factor for the lifetime and the reconditionability of such components.
In the current situation, lifetime and reconditionability limits for the state of the art design and engine operation mode are specified based on calculation and experience. No solution is currently commercially available with the standard MCrAlY composition of the bond coat/overlay coat in order to extend these limits (both oxidation life and mechanical life at the same time). A self healing system would be a solution to extend them.
A different approach using nano-structured coating is presented in document U.S. Pat. No. 7,361,386 B2.
According to this document, in order to increase the efficiency of gas turbine engines, the hot-section stationary components (mainly combustors, transition pieces, and vanes) are protected with thermal barrier coatings (TBCs). In addition to providing the thermal insulation to the nickel-based superalloy components, TBCs also provide protection against high temperature oxidation and hot corrosion attack. The conventional TBCs that are used in naval (diesel) engines, in military and commercial aircraft, and in land-based gas turbine engine components, consist of a duplex structure made up of a metallic MCrAlY (M stands for either Co, Ni and/or Fe) bond coat and Yttria partially stabilized zirconia (YPSZ) ceramic top coat.
The document further asserts that the full potential of the YPSZ TBCs is yet to be realized due mainly to the cracking problem that occurs along or near the bond coat/top coat interface after a limited number of cycles of engine operation. This interfacial cracking, often leading to premature coating failure by debonding (spallation) of the top coat from the bond coat, has been amply demonstrated from microstructural evidence that was obtained from in-service degradation of deposited coatings as well as from laboratory experiments that have been conducted. The thin oxide layer that grows on top of the bond coat, at the bond coat/top coat interface, plays a critical role in the interface cracking. It is quite evident that this cracking problem negatively impacts the coating performance by reducing both the engine efficiency (because the engine operating temperature is kept below its optimum temperature) and the lifetime of the engine components. In turn, this greatly affects the reliability and the efficiency of the entire engine system.
According to document U.S. Pat. No. 7,361,386 B2, the bond coat surface, onto which the YPSZ top coat is disposed, has a thin oxide layer that consists mostly of various oxides (NiO, Ni(Cr,Al)2O4, Cr2O3, Y2O3, Al2O3). The presence of this thin oxide layer plays an important role in the adhesion (bonding) between the metallic bond coat and the ceramic top coat. However, during engine operation, another oxide layer forms in addition to the native oxide. This second layer, also mostly alumina, is commonly referred to as the thermally grown oxide (TGO) and slowly grows during exposure to elevated temperatures. Interfacial oxides, in particular the TGO layer, play a pivotal role in the cracking process. It is believed that the growth of the TGO layer leads to the build up of stresses at the interface region between the TGO layer and top coat.
To solve these problems, document U.S. Pat. No. 7,361,386 B2 proposes to modify the microstructure of the MCrAlY bond coat (in a thermal barrier coating) in a controlled way prior to exposure to high temperatures, in order to control the subsequent changes during high temperature exposure. More specifically, the structure, composition, and growth rate of the thermally grown oxide (TGO) is controlled to ultimately improve the performance of TBCs. According to U.S. Pat. No. 7,361,386 B2, a nanostructure is provided in the bond coat and, consequently, nanocrystalline dispersoids are introduced into the structure. The purpose of the dispersoids is to stabilize the nanocrystalline structure and to nucleate the desirable [alpha]-Al2O3 in the TGO.
Other prior art documents, Ajdelsztajn et al. in Surf. & Coat. Tech. 201 (2007) 9462-9467 and Funk et al. in Met. Mat. Trans. A 42 [8] (2011) 2233-2241), show that such a nano-structured bond coat has several advantages like for e.g. improved mechanical properties. Such benefit is due to the presence of ultrafine dispersoids of γ and β phases.
It is an object of the present invention to provide a method for applying an improved high-temperature stable coating layer on the surface of a component and a component being used in a high-temperature environment, which is coated with such coating layer.
This object is obtained by a method according to claim 1 and a component according to claim 14.
The method according to the invention for applying a high-temperature stable coating layer on the surface of a component, comprises the steps of:
According to an embodiment of the inventive method said powder material is applied to the surface of the component by means of a thermal spraying technique.
Especially, the thermal spraying technique used is one of High Velocity Oxygen Fuel Spraying (HVOF), Low Pressure Plasma Spraying (LPPS), Air Plasma Spraying (APS) or Suspension Plasma Spraying (SPS).
According to another embodiment of the inventive method said powder material has the form of agglomerates.
According to a further embodiment of the inventive method said powder material has the form of a suspension.
According to another embodiment of the inventive method the powder material contains powder particles of micron size and/or larger agglomerates, and that the sub-micron powder particles are in said coating layer distributed around the surface of said powder particles of micron size and/or said larger agglomerates.
According to just another embodiment of the inventive method the sub-micron powder particles are pre-oxidized before being incorporated into said coating layer.
Preferably, the pre-oxidation takes place in-flight during spraying.
Alternatively, the pre-oxidation is done by an oxidative pre heat treatment of the powder material.
According to another embodiment of the inventive method the powder material is a metallic powder.
Especially, the powder material is of the MCrAlY type with M=Ni, Co, Fe or combinations thereof.
According to just another embodiment of the inventive method the coating layer is a bond coat or an overlay coating.
According to the invention, said component having a surface, which is coated with a coating layer is characterized in that said coating layer comprises sub-micron powder particles, which are each at least partially surrounded by an oxide shell and establish with their oxide shells an at least partially interconnected sub-micron oxide network within said coating layer.
According to an embodiment of the invention, said coating layer further comprises powder particles of micron size and/or larger agglomerates.
Especially, said sub-micron powder particles are in said coating layer distributed around the surface of said powder particles of micron size and/or said larger agglomerates.
According to another embodiment of the invention the coating layer is a bond coat and the powder material is of the MCrAlY type with M=Ni, Co, Fe or combinations thereof.
The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.
The present invention discloses a specific type of sub-micron structured coating. Due to a sub-micron scale oxide network and fine grain microstructure, the invention aims to reduce the LCF/TMF cracking.
Another aspect of the invention is the retardant effect for the oxidation and the corrosion. Due to the nano-scale oxide network of the bond coat/overlay coating, the impact by oxidation and corrosion is slowed down.
In consequence, the invention should enable a longer service life and/or assure reconditionability with less scrap parts and/or decreased operation risks, such as crack formation in critical area of the component due to mechanical/thermal load, and/or oxidation/corrosion and/or FOD (Foreign Objects Damage) events.
The invention enables:
The novelty of the invention is the use of a sub-micron powder (at least to a certain percentage of the total powder mixture) and the way to process it (preparation and thermal spray application) to reach the mentioned improved coating properties. The improved coating behavior is particularly based on a reduced TMF/LCF effect of the coating with (at least partial) sub-micron structure.
The invention is based on:
Such powder is a metallic powder, preferably a MCrAlY with M=Ni, Co, Fe or combinations thereof.
During the transport in the flame 14 the sub-micron powder particles 18 undergo a reaction, as can be seen in
When the powder material is a mixture of sub-micron particles 18 and micron powder particles or agglomerates 21, as shown in
One additional embodiment of the invention is a manufacturing process for an improved thermal barrier coating system of highly thermally and especially cyclically liner segment of a gas turbine by
The result is a bondcoat/thermal barrier coating system with improved TMF and oxidation resistance with the capability of forming stable TGO scales, leading to an improved overall coating lifetime.
A further embodiment of the invention is a manufacturing process for a graded metallic overlay coating system of highly thermally and especially cyclically loaded turbine vane of a gas turbine by
The result is a graded metallic overlay coating system with improved TMF and oxidation resistance, leading to an improved overall coating lifetime.
In general, the initiation and propagation of damages within coatings exhibiting an at least partial sub-micron scale structure is retarded compared to conventional coating microstructures. The “sub-micron effect” is retained over extended lifetime periods, also due to the (at least partial) oxide network. Such aspects of the invention give to the coating a so-called self healing characteristic.
Therefore the following advantage are reached with the invention:
Longer service life and/or reduced amount of scrap parts during reconditioning and/or reduced operation risks and/or cost reduction related to crack restoration, oxidation and corrosion damage. In addition, the fine grain sized coating allows a diffusion heat treatment with a reduced number of heat treatment cycles. A nano coating as top layer improves the TMF and oxidation resistance, which results in an improved overall coating lifetime.
Number | Date | Country | Kind |
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12158129.2 | Mar 2012 | EP | regional |
This application claims priority to PCT/EP2013/054337 filed Mar. 5, 2013, which claims priority to European application 12158129.2 filed Mar. 5, 2012, both of which are hereby incorporated in their entireties.
Number | Date | Country | |
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Parent | PCT/EP2013/054337 | Mar 2013 | US |
Child | 14474564 | US |