BI-layered ceramic thermal barrier coatings with different porosities

Abstract

The invention relates to a layered ceramic system, which comprises at least a metallic substrate, a metallic bond coat (7) on the metallic substrate, an innermost ceramic layer (10), an outermost ceramic layer (13) and optionally an abradable layer (15), wherein the porosity of each ceramic layer is higher than 10% and wherein the average porosity of the innermost layer (10) is 17%.

Description

[0001] BI-LAYERED CERAMIC THERMAL BARRIER COATINGS WITH DIFFERENT POROSITIES

[0002] This invention relates generally to the field of thermal barrier coatings ( TBC ) and particularly to ceramic thermal barrier coatings to be used to protect metallic components in very high temperature applications such as gas turbine engines .

[0003] TBC are exposed to high temperatures during a gas turbine operation, but also to fast thermal transients during engine start-up and shutdown .

[0004] These transient thermal loads can lead to coating failure and lead to the need to replace the parts before the end of their expected li fe . In layered TBC systems , the failure can occur at an interface of bond coat and TBC, but also at the interface between the various TBC layers .

[0005] It is therefore the aim of this invention to optimi ze a layered TBC system to improve the cyclic li fe of the TBC and minimi ze the occurrence of TBC failing at the interfaces .

[0006] The problem is solved by a TBC according to claim 1 .

[0007] In the further dependent claims further advantages are listed, which can be arbitrarily combined advantageously with each other to yield further advantages .

[0008] The figures 1 , 2 show examples of the invention .

[0009] The description and the figures are only examples of the invention . We aim at solving this issue by improving the TBC porosity distribution between the layers and the thicknesses of the TBC system .

[0010] The invention solve the problem through two di f ferent strategies which are aiming at reducing the stress level at the interface between the ceramic layers but also between the bottom TBC layer and a bond coat which is onto the metallic substrate , such as a nickel or cobalt base superalloy .

[0011] The first strategy is to optimi ze the porosity of each TBC layers such to minimi ze the stresses and strains at all interfaces . To do this the porosity of each TBC layer has to be matched to the adj acent layers . The layer closest to the bond coat having a porosity lower or similar to the adj acent outer layer .

[0012] One example is a bi-layer TBC system wherein the ceramic bottom layer ( close to the bond coat ) has an average porosity of 15% and the top layer, which is in contact with the hot gas or is the outermost layer and has a porosity of 20% .

[0013] Layer means here in this context that a layer has along his thickness in vertical direction the same properties as porosity, composition, microstructure , .... It should not be mixed with the several surpasses of a coating gun to produce such a whole layer .

[0014] Some possible systems for the porosities ( average target porosity and in bracket the range of allowed porosity in the layer at any location measured at l O Ox magni fication) , wherein the layers are :

[0015] TBC layer 1 ( innermost )

[0016] TBC layer 2 ( outermost TBC layer ) TBC layer 3 / abradable layer:

[0017] • System 1: innermost: 17% (12%— 22%) - outermost: 19%

[0018] ( 14 % — 24 % )

[0019] • System 2: innermost: 17% (12%— 22%) - outermost: 22%

[0020] ( 17 % — 27 % )

[0021] • System 3: innermost: 17% (12%— 22%) - outermost: 22%

[0022] (17%— 27%) - abradable: 30% (25%-35%)

[0023] • System 4: innermost: 17% (12%— 22%) - outermost: 25%

[0024] (20%— 30%) - abradable: 35% (30%-40%)

[0025] • System 5: innermost: 17% (12%— 22%) - outermost: 19%

[0026] (14%— 24%) - abradable: 25% (20%-30%)

[0027] The porosity in TBC layer 2 (outermost) shall at least higher than 2% in any location than in TBC layer 1 (innermost) .

[0028] The porosity in TBC layer 3 (abradable) shall at least higher than 2% in any location than in TBC layer 2 (outermost)

[0029] The second strategy aims at reducing the stresses at the interface by optimizing the coating layer thicknesses.

[0030] Due to the thermal expansion mismatch between the ceramic layers (innermost - outermost - abradable) and the metallic base material, in the cold condition, the compressive stresses in the TBC are the highest at the bond coat TBC interface, while they can be tensile at the TBC surface. Based on the knowledge of the thermal mismatch between the base material and the TBC layers and the bond coat and TBC surface temperatures, the following rules are used to optimize the relative thickness of the TBC layers:

[0031] - the interface between two ceramic layers shall be in a region of compressive stress in the cold condition. If possible, the compressive strain at the interface in the external layer shall be between 0.05% and 0.2% - the temperature at the surface of a partially stabilized zirconia shall stay below the temperature at which significant amounts of monoclinic zirconia phase will be formed. For 8YSZ this limit depends on the target life of the coating and will be between 1523K (1250°C) and 1623K (1350°C) .

[0032] The thickness of the partially stabilized zirconia (PSZ) as innermost layer shall be maximized considering manufacturing tolerances .

[0033] One example is the case of a bi-layer 8YSZ / Gd2Zr2O7 (GZO) coating with a total thickness of 1.0mm with a manufacturing tolerance of + / - 100pm, a bond coat temperature of 1173K (900°C) and a TBC surface temperature of 1673K (1400°C) .

[0034] The GTE of the substrate is 16x10-6, the one of the 8YSZ layer is 11.5x10-6 and the one of the GZO is 12x10-6.

[0035] The maximum possible 8YSZ layer thickness for the 1st rule is 500pm, while for the second rule (limit of 1623K (1350°C) for the 8YSZ layer) it is 800pm.

[0036] To fulfill both rules while maximizing the 8YSZ layer thickness, the maximum 8YSZ layer thickness is defined as 500pm and the target 8YSZ layer thickness is set at 400pm.

[0037] In figure 1 a bi-layer TBC system is shown.

[0038] As a metallic substrate 4 a nickel or cobalt based superalloy is preferably used.

[0039] On to this substrate 4 a metallic bond coat 7 is applied.

[0040] The metallic bond coat 7 comprises a NiCoCrAlY alloy, such as NiCoCrAlYRe, NiCoCrAlYTa, NiCoCrAlYSi, NiCoCrAlYFeSi , NiCoCrAlYTaSi, ... .

[0041] This alloy forms during operation a chronic Alumina coating (TGO) which improves the bonding to a ceramic layer. Above the metallic bond coat 7 the innermost layer 10 is present on which the outermost layer 13 is applied on, which fulfil the requirements of thickness and porosity relationship already described above.

[0042] Figure 2 shows in comparison to figure 1 and abradable layer 15 on the outermost layer 13, though that this TBC system can be used in a seal system in combination with the stator part.

[0043] The material of the innermost layer 10 comprises especially partially stabilized Zirconia (PSZ) , very especially 8wt% Yttria stabilized Zirconia.

[0044] The material of the outermost layer 13 comprises especially fully stabilized Zirconia (FSZ) , very especially 20wt% Yttria stabilized Zirconia.

[0045] The material of the abradable is preferably a zirconia-based ceramic material.

Claims

Patent claims1. Layered ceramic system, which comprises at least: a metallic substrate (4) , especially a nickel based superalloy, a metallic bond coat (7) on the metallic substrate, which comprises especially Ni-Co-Cr-Al-Y, an innermost ceramic layer (10) , an outermost ceramic layer (13) and optionally an abradable layer (15) , wherein the porosity of each ceramic layer is higher than 10%, especially higher than 12%, and wherein the average porosity of the innermost layer (10) is 17%.

2. Layered ceramic system according to claim 1, wherein the porosity of the innermost ceramic layer (10) is between 12%-22%.

3. Layered ceramic system according to one or both of the claims 1 or 2, wherein the average porosity of the outermost ceramic layer (13) is 19%.

4. Layered ceramic system according to any of the claims 1, 2 or 3 , wherein the porosity of the outermost ceramic layer (13) is between 14%-24%.

5. Layered ceramic system according to one or both of the claims 1 or 2, wherein the average porosity of the outermost ceramic layer (13) is 22%.

6. Layered ceramic system according to any of the claims 1, 2 or 5, wherein the porosity of the outermost ceramic layer (13) is between 17%-27%.

7. Layered ceramic system according to one or both of the claims 5 or 6, wherein the average porosity of the abradable layer (15) is 30% .

8. Layered ceramic system according to any of the claims 5, 6 or 7 , wherein the porosity of the abradable layer (15) is between 25%-35%.

9. Layered ceramic system according to one or both of the claims 3 or 4, wherein the average porosity of the abradable layer (15) is 25% .

10. Layered ceramic system according to any of the claims 3, 4 or 9, wherein the porosity of the abradable layer (15) is between 20%-30%.

11. Layered ceramic system according to one or both of the claims 1, 2, wherein the average porosity of the outermost ceramic layer (15) is 25%.

12. Layered ceramic system according to any of the claims 1, 2 or 11, wherein the porosity of the outermost ceramic layer (15) is between 20%-30%.

13. Layered ceramic system according to one or both of the claims 11 or 12, wherein the average porosity of the abradable layer (15)is 35%.

14. Layered ceramic system according to any of the claims 11, 12 or 13, wherein the porosity of the abradable layer (15) is between 30%-40%.

15. Layered ceramic system according to any of the previous claims, wherein thickness of the innermost layer (10) is 400pm + / -100pm.

16. Layered ceramic system according to any of the previous claims, wherein thickness of the outermost layer (13) is 600pm + / -100pm.

17. Layered ceramic system according to any of the previous claims, wherein the innermost layer (10) comprises PSZ, especially 8wt% Yttria stabilized Zirconia.

18. Layered ceramic system according to any of the previous claims, wherein the outermost layer (13) is FSZ, especially at least 20wt% Yttria stabilized Zirconia.

19. Multilayered ceramic system according to any of the previous claims, wherein the material of the abradable is a zirconiabased ceramic material.

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