Bilayer thermal barrier coating with improved bond between the ceramic layers

Abstract

The bonding capacity of a ceramic coating system (16) is improved by adapting the coating parameters such as size of the powder and changing of the parameters of the spraying system.

Description

[0001] Description

[0002] BILAYER THERMAL BARRIER COATING WITH IMPROVED BOND BETWEEN THE CERAMIC LAYERS

[0003] The invention relates to a component with a ceramic coating system which reveals a two layered ceramic coating system .

[0004] The ever increasing Turbine Inlet Temperatures ( TIT ) have led to the introduction of advanced Thermal Barrier Coatings ( TBC ) , that typically appear as multilayer ceramic systems . These TBC typically comprise a partially high fracture toughness partially stabili zed zirconia lower coating and one or multiple lower fracture ceramic upper coating .

[0005] A very typical ceramic coating is a fully stabili zed zirconia thermal barrier coating, which typically shows low fracture toughness , sintering resistance and phase stability at high temperatures . However, fully stabili zed coatings are characteri zed by low fracture toughness and thus low erosion resistance and are deposited on top of partially stabili zed zirconia coatings , bonded by a ceramic-ceramic interface , that can be rather weak and can lead to complete delamination of the upper coating, with adverse consequences regarding the thermal protection of the underlying component .

[0006] However, the introduction of the multilayer thermal protection ceramic coatings comes with two caveats : i ) The associated low fracture toughness of the fully stabili zed ceramic upper coating, which impacts adversely its erosion resistance , ii ) An inherited weak interface between ceramic lower coating and ceramic upper coating .

[0007] Regarding the erosion resistance of the ceramic coating system, the solution was to deposit both ceramic coatings , the lower coating partially stabili zed and the upper coating fully stabili zed, especially with a segmented microstructure . A segmented microstructure is characteri zed by low porosity (<3% ) and vertical cracks that travel along the coatings thickness , rendering it thermal compliant , which means the ceramic coating shows increased ability to absorb and withstand thermal strain . Typically, the vertical cracks will extend and continue from the lower coating to the upper coating . The dense segmented microstructure can increase the erosion resistance of the ceramic coatings at least three times .

[0008] Regarding the bonding capability of the two segmented ceramic coatings , the most important elements that influence that are surface roughness and temperature control .

[0009] Regarding roughness , a segmented lower coating is in general rather smooth and the asperities or cavities that can assist with the mechanical interlocking are small or few . This can af fect the robustness and the strength of the bond between the two ceramic coatings .

[0010] Concerning temperature , ideally a strong interface will appear when the upper stabili zed coating will be deposited on an intensely preheated lower coating . High temperature will help with the wetting and spreading of the incoming fully stabili zed particles on the partially stabili zed lower coating . The particles will spread evenly and fill almost all cavities and asperities improving in this manner the mechanical interlocking between the two coatings .

[0011] However, homogeneous preheating for bulky components with complex geometries and varying metallic wall thickness can be challenging .

[0012] Is therefore aim of the invention to improve the problem listed above .

[0013] The problem is solved by a component according to claim 1 . In the dependent claims further advantages are listed which can be arbitrarily combined with each other to yield further advantages . The figure shows an inventive ceramic coating system of a component .

[0014] A coating is usually applied by a spray torch passing over the same surface several times by applying powder in layers .

[0015] The current invention improves the strength and robustness of the interface interlocking and bonding between two ceramic coatings . The manner it does that , is through achieving higher roughness on a ceramic lower coating, without sacri ficing its microstructure .

[0016] That can be achieved as following :

[0017] Typically, the ceramic lower coating 10 is sprayed by using a finer particle powder, especially using a powder cut , in order to achieve maximum melting degree , which in turn reduces the porosity and promotes the vertical crack appearance . The reason for the smooth surface of the coating is the almost complete melting of the powder particles that resemble pancakes when they are deposited .

[0018] Usage of coarser with grain si zes particles , especially >45um will reduce their melting degree , possibly increase the inherent porosity in the coating and thus reduce the population of vertical cracks per unit of length and ultimately the thermal compliance of the coating .

[0019] The solution to that , is to adopt a " flash" coating approach . That means to deposit a rough thin coating intermediated between the lower coating and the upper coating . That can be achieved with threemethods :

[0020] 1 . Change of the spraying parameters of the lower coating 10 during the last pass ( es ) :

[0021] By adopting colder parameters during the last pass ( es ) the melting degree of the fine particles is signi ficantly reduced and the particles are deposited semimolten and not as flatten pancakes. That will increase significantly the roughness without affecting the microstructure of the lower coating.

[0022] That can be achieved by reducing the secondary high enthalphy gas flow (typically Hydrogen) by at least 25%

[0023] 2. Change of the powder for the last pass (es) of the ceramic lower coating 10. The powder can be changed and replaced by coarser particles (especially >45um) to deposit the last pass (es) of the ceramic lower coating.

[0024] 3. By tilting the angle of the torch by at least 10 degrees and not more than 45 degrees, to the perpendicular direction of spraying on the substrate. A tilted direction of spraying to the substrate, will give a saw-teeth like appearance, that increases the surface roughness. This method can be additionally combined with methods 1 and 2 above.

[0025] Essentially, the resulting ceramic coating system 16 will comprise of the following: a) partial stabilized Zirconia (PSZ) as ceramic lower coating 10 sprayed with fine powder on a substrate 4 or metallic bond coat 7 having a surface 20, b) a thin coating of partial stabilized Zirconia sprayed in the middle (ceramic intermediate coating 11) with a rougher surface 19, c) fully stabilized Zirconia (FSZ) as ceramic upper coating 13 sprayed with fine powder; the upper coating 13 represents the outer surface 23.

[0026] The present invention comes to improve the robustness of a bilayer ceramic coating system 16, which is especially segmented .

[0027] Experience has shown the importance of temperature control during the spraying of bilayer ceramic coating systems 16 and temperature control can be rather challenging during the spraying of large components. With the introduction of a rough surface 19 between the two especially segmented coatings 10, 13, the sensitivity of the ceramic coating system 16 to temperature is decreased providing an overall robuster ceramic coating system . The adoption of both approaches described above is rather easy and both can be easily incorporated in the spraying sequence of the components .

[0028] The figure shows an inventive component 1 .

[0029] The component 1 has especially a metallic substrate 4 , which is preferably a nickel or cobalt based superalloy .

[0030] On the metallic substrate 4 there is a metallic bond coat 7 , especially on a NiCoCrAl base , means NiCoCrAlY, NiCoCrAlYSi , NiCoCrAlYRe , NiCoCrAlYTa, ....

[0031] On top of the metallic bond coat 7 there is the ceramic coating system 16 as described above : a PSZ segmented lower coating 10 a PSZ segmented intermediate coating 11 a FSZ segmented upper coating 13 .

Claims

Patent claims1. Component ( 1 ) , comprising at least: a substrate ( 4 ) , especially a nickel- or cobalt-based superalloy, a metallic bond coat (7) , especially based on a NiCoCrAl composition, a ceramic coating system (16) with an lower ceramic coating (10) and an upper ceramic coating (13) , characterized by that, wherein an intermediate coating (11) is located between lower coating (10) and upper coating (13) , providing a rougher surface (19) which has a maximum thickness of 25% of the thickness of the lower coating (10) or of the upper coating (13) .

2. Coating system according to claim 1, wherein the lower coating (10) reveals a finer microstructure the upper coating (13) .

3. Method to produce a ceramic coating system according to claim 1, comprising the steps: using a torch to spray powder, spraying the lower ceramic coating (10) , especially by using a fine particles powder, in order to achieve maximum melting degree, depositing an intermediate coating (11) : either by changing of the spraying parameters of the ceramic lower (10) coating during the last pass(es) by adopting colder parameters during the last pass (es) which reduces the melting degree of the particles significantly and the particles are deposited semimolten and not as flatten pancakes ,leading to an increase significantly the roughness, or changing the powder for the last pass (es) of the ceramic lower coating (10) by replacing it by coarser particles, especially larger than 45pm, in order to deposit the last pass(es) on the intermediate coating (11) onto the ceramic lower coating (10) , and finally spraying a fully stabilized ceramic upper coating (13) sprayed with fine powder and / or tilting the angle of the torch by at least 10° and not more than 45° to the perpendicular direction of spraying on the substrate.

4. Coating or method according to any of the claims 1, 2 or 3, wherein the ceramic lower coating (10) is a partially stabilized Zirconia based coating, and is especially segmented.

5. Coating or method according to any of the claims 1, 2, 3 or 4 , wherein the ceramic intermediate coating (11) is a partially stabilized Zirconia based coating, and is especially segmented.

6. Coating or method according to any of the claims 1, 2, 3, 4 or 5, wherein the ceramic upper coating (13) is a fully stabilized Zirconia based coating, and is especially segmented.

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