THERMAL BARRIER COATING

Abstract

The present disclosure relates to a thermal barrier coating for coating a substrate. The thermal barrier coating may comprise an inner ceramic layer (e.g. 7YSZ) having a columnar grain structure and a first outer ceramic layer (e.g. 7YSZ) having a branched grain structure. The thermal barrier coating further comprises a nucleation layer (which may comprise alumina or tantala), interposed between the inner ceramic layer and the first outer layer. The layers can be deposited by PVD using substantially contact deposition parameters because the nucleation layer induces branching in the first outer ceramic layer.

Claims

1. A thermal barrier coating for coating a substrate, the thermal barrier coating comprising: at least an inner ceramic layer having a columnar grain structure; and at least a first outer ceramic layer having a branched grain structure, wherein the thermal barrier coating further comprises a nucleation layer interposed between the inner ceramic layer and the first outer layer.

2. The thermal barrier coating according to claim 1 wherein the nucleation layer comprises one or more of: alumina (Al.sub.2O.sub.3); a rare-earth oxide or silicate; and/or a transition metal oxide.

3. The thermal barrier coating of claim 2 wherein the nucleation layer comprises alumina or tantala.

4. The thermal barrier coating of claim 1 comprising a second nucleation layer and a second outer ceramic layer having a branched grain structure overlying the first nucleation layer and first outer ceramic layer with the second nucleation layer interposed between the first and second outer ceramic layers.

5. The thermal barrier coating of claim 1 comprising greater than 50 pairs of alternating nucleation layers and outer ceramic layers having a branched grain structure.

6. The thermal barrier coating of claim 1 wherein the inner ceramic layer and the/each outer ceramic layer is independently selected from a zirconia or hafnia or titania.

7. The thermal barrier coating according to claim 6 wherein the inner ceramic layer and the/each outer ceramic layer is independently selected from a zirconia, hafnia, titania stabilised with one or more rare earth element oxide.

8. The thermal barrier coating according to claim 7 wherein the inner ceramic layer and the/each outer ceramic layer is yttria-stabilised zirconia or hafnia.

9. A component comprising a substrate at least partially coated with the thermal barrier coating according to claim 1.

10. The component according to claim 9 wherein the substrate comprises a nickel, cobalt, and/or iron based alloy, a refractory metal or an inter-metallic.

11. The component according to claim 9 wherein the component is a gas turbine engine component comprising a blade, vane nozzle, shroud, liner or deflector.

12. A method of coating a substrate with a thermal barrier coating, said method comprising: depositing an inner layer of ceramic having a columnar grain structure on the substrate; depositing a nucleation layer on the inner layer of ceramic; and depositing an outer layer of ceramic on the nucleation layer such that the nucleation layer effects nucleation of branching within the outer layer of ceramic.

13. The method of claim 12 further comprising depositing a second nucleation layer on the outer layer of ceramic and depositing a second outer layer of ceramic on the second nucleation layer such that the second nucleation layer effects nucleation of branching within the second outer layer of ceramic.

14. The method of claim 12 further comprising depositing 50 or more pairs of alternating nucleation and outer ceramic layers overlying the first/second nucleation layer and the first/second outer ceramic layer.

15. The method according to claim 12 comprising depositing the layers using physical vapour deposition (PVD).

16. The method according to claim 15 comprising depositing the layers using substantially constant deposition parameters.

17. The method of claim 16 comprising depositing the layers using a substantially constant component temperature.

18. The method according to claim 12 wherein the ceramic layers comprise yttria-stabilised zirconia or hafnia and the nucleation layer(s) comprise(s) alumina or tantala.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0063] FIG. 1a is an SEM photograph of a single layer 7YSZ coating prior to CMAS attack;

[0064] FIG. 1b is an SEM photograph of the single layer 7YSZ coating after CMAS attack;

[0065] FIG. 2a is an SEM photograph of a first embodiment of a coating having an inner columnar ceramic layer and multiple nucleation and outer branched ceramic layers prior to CMAS attack;

[0066] FIG. 2b is an enlarged section of FIG. 2a;

[0067] FIG. 2c is an SEM photograph of the first embodiment after CMAS attack;

[0068] FIG. 3a is an SEM photograph of a second embodiment of a coating having an inner columnar ceramic layer and multiple nucleation and outer branched ceramic layers prior to CMAS attack;

[0069] FIG. 3b is an enlarged section of FIG. 3a;

[0070] FIG. 3c is an SEM photograph of the second embodiment after CMAS attack; and

[0071] FIG. 3d is an enlarged section of FIG. 3c.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0072] First, a comparative sample was prepared using a metallic nickel-based substrate which was treated by grit-blasting using #220 brown alumina and subsequently coated with an aluminide bond coat layer.

[0073] A ceramic layer having a columnar grain structure, a thickness of 135 microns and comprising 7-9 wt % yttria-stabilised zirconia (7YSZ) was deposited on the substrate using electron beam-physical vapour deposition (EB-PVD) at a temperature of 980° C., a power of 36 kW and a chamber pressure of 6×10.sup.−3 mbar.

[0074] The resulting columnar ceramic layer is shown in FIG. 1a. The columnar grain structure is clearly visible with the ceramic layer comprising multiple parallel columns extending away perpendicularly to the surface of the substrate, the columns spaced by inter-columnar gaps. These gaps provide for thermal stress/strain tolerances by allowing independent movement of the columns.

[0075] Next, the CMAS resistance of the sample was tested by using an air spray gun to deposit an even film (having a loading of 15 mg/cm.sup.2) of a CMAS suspension comprising 35 mol % CaO, 10 mol % MgO, 7 mol % Al.sub.2O.sub.3 and 48 mol % SiO.sub.2 in deionized water onto the ceramic layer. The sample was then exposed to a temperature of 1300° C. in a furnace for 30 minutes to induce CMAS attack. The sample was then allowed to cool before being sectioned for assessment.

[0076] FIG. 1b shows the sample after CMAS attack. It can be seen that the sample has poor resistance to CMAS attack. The CMAS has penetrated the ceramic layer through the inter-columnar gaps to partially dissolve the ceramic material leading to degradation of the columnar grain structure as a result of sintering and merging of the columns. The degradation of the columns (and the resulting reduction in the inter-columnar gaps) reduces the stress/strain tolerance of the ceramic layer leading to premature failure and delamination of the layer.

[0077] Next, a sample having a thermal barrier coating according to a first embodiment comprising an inner columnar ceramic (7YSZ) layer and three nucleation layers comprising alumina alternating with three outer branched ceramic (7YSZ) layers was prepared and is shown in FIG. 2a.

[0078] The first embodiment sample was prepared using a metallic nickel-based substrate which was treated by grit-blasting using #220 brown alumina and subsequently coated with a bond coat layer. The EB-PVD coater was set up with 7YSZ and alumina ingot materials and the deposition parameters (e.g. temperature, chamber pressure, power) were fixed. The temperature was fixed between 930-980° C., the power at 36 kW and the chamber pressure at 6×10.sup.−3 mbar.

[0079] The 7YSZ ingot material was selected and using to deposit an inner ceramic layer having a columnar grain structure and a thickness of 57 microns. Next, without altering the deposition parameters, the ingot material selection was changed to alumina (using a moving hearth or jumping beam technology) and a first nucleation layer comprising alumina and having a thickness of 2 microns was deposited on the inner ceramic layer. The ingot material was then switched back (again without altering the deposition parameters) and a first outer layer of ceramic (7YSZ) having a thickness of 8 microns was deposited on top of the nucleation layer. The switching of ingot materials was repeated (retaining otherwise constant deposition parameters) to form a second alumina nucleation layer (2 micron thickness), second outer ceramic layer (9 micron thickness), third alumina nucleation layer (0.5 micron thickness) and third outer ceramic layer (5 micron thickness). Accordingly, the entire thermal barrier coating was formed using substantially constant deposition parameters.

[0080] It can be seen in FIG. 2a that the inner ceramic layer has a columnar grain structure with inter-columnar gaps. The enlarged section shown in FIG. 2b clearly shows the feathering within the outer ceramic layers caused as a result of side branching induced by the alumina nucleation layers.

[0081] Next, a sample having a thermal barrier coating according to a second embodiment comprising an inner columnar ceramic (7YSZ) layer (68 micron thickness) and approximately 100 nucleation layers comprising alumina (0.2 micron thickness) alternating with approximately 100 outer branched ceramic (7YSZ) layers (0.4 micron thickness) was prepared in the same manner as the first embodiment sample and is shown in FIG. 3a. FIG. 3b clearly shows the alternating alumina and branched/feathered outer ceramic layers.

[0082] Next, the CMAS resistance of the first and second embodiment thermal barrier coatings was tested by using an air spray gun to deposit an even film (having a loading of 15 mg/cm.sup.2) of a CMAS suspension comprising 35 mol % CaO, 10 mol % MgO, 7 mol % Al.sub.2O.sub.3 and 48 mol % SiO.sub.2 in deionized water onto the outermost ceramic layer. The first and second embodiment samples were then exposed to a temperature of 1300° C. in a furnace for 30 minutes to induce CMAS attack. The samples were allowed to cool before being sectioned for assessment.

[0083] FIG. 2c shows the first embodiment sample after CMAS attack. It can be seen that the first embodiment thermal barrier coating has improved resistance to CMAS attack. The branched outer ceramic layers have undergone partial dissolution by the penetrating molten CMAS but the branching has limited CMAS penetration to the inner columnar ceramic layer such that the column grain structure and inter-columnar gaps remain.

[0084] FIGS. 3c and 3d show the second embodiment sample after CMAS attack. It can be seen that the second embodiment thermal barrier coating has even greater improved resistance to CMAS attack. The numerous branched outer ceramic layers have significantly limited CMAS penetration to the inner columnar ceramic layer such that the column grain structure and inter-columnar gaps remain. FIG. 3d shows the dissolution and sintering of the outer ceramic layers which leads to a dense protective barrier limiting further penetration of molten CMAS.

[0085] Protection of the columnar inner ceramic provides a thermal barrier coating which maintains its stress/strain tolerance after CMAS attack.

[0086] It will be understood that the disclosure is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.