Development of Advanced Thermal Barrier Coatings ...

Proceedings of ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition GT2016 June 13 – 17, 2016, Seoul, South Korea

GT2016-57425

DEVELOPMENT OF ADVANCED THERMAL BARRIER COATINGS WITH IMPROVED TEMPERATURE CAPABILITY Gregoire Witz GE Power Baden, Switzerland

Markus Schaudinn GE Power Baden, Switzerland

Joerg Sopka GE Power Mannheim, Germany

Tobias Buecklers GE Power Birr, Switzerland

ABSTRACT Continuously increasing hot gas temperatures in heavy duty gas turbines lead to increased thermal loadings of the hot gas path materials. Thermal barrier coatings are used to reduce the superalloys temperature and cooling air needs. Until now 68 wt% yttria stabilized zirconia is the first choice material for such coatings, but it is slowly reaching its maximum temperature capability due to the phase transformation at high temperature and sintering. New thermal barrier coating material with increased temperature capability enable the next generation of gas turbine with >60% combined cycle efficiency. Such material solutions have been developed through a multi-stage selection process. In a first steps, critical material performance requirements for thermal barrier coating performance have been defined based on the understanding of standard TBC degradation mechanisms. Based on these requirements, more than 30 materials were a pre-selected and evaluated as potential coating materials. After carefully reviewing their properties both from literature data and laboratory test results on raw materials, five materials were selected for coating manufacturing and laboratory testing. Based on the coating manufacturing trials and laboratory test results, two materials have been selected for engine testing, in a first step in GT26 Birr Test Power Plant and afterwards in customer engines. For such tests the original coating thickness has been increased such to achieve coating surface temperature

~100K higher than with a standard thermal barrier coating. Both coatings performed as predicted in both GT26 Birr Test Power Plant and customer engines. INTRODUCTION Thermal barrier coatings (TBC) are used in cooled parts of gas turbines to reduce the metal temperature of turbine and combustor components. In order to increase the turbine efficiency, gas turbines are designed with ever increasing hot gas temperatures. This means that components in the hot gas path have to operate at the highest possible temperature and to minimize their cooling air requirements. Since the temperature capability of the superalloys used in industrial gas turbines has hardly evolved in the last 20 years, the role of the TBC has been becoming critical in this quest for increased hot gas temperatures. The standard TBC material used in gas turbines is zirconia stabilized with 6-8wt% of yttria (7YSZ) manufactured by atmospheric plasma spray (APS). This composition forms a metastable tetragonal zirconia after coating manufacturing. Since it is metastable, it decomposes at high temperature into a low yttria containing tetragonal phase and a high yttria containing cubic phase [1-4]. The low yttria tetragonal phase can transform into a monoclinic phase upon cooling. This transformation is accompanied by a volume change of ~4-5% and it has been generally considered as one of the causes of TBC failure in service. The metastable phase decomposition

Information contained in this document is indicative only. No representation or warranty is given or should be relied on that it is complete or correct or will apply to any particular project. This will depend on the technical and commercial circumstances. It is provided without liability and is subject to change without notice.

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Copyright © 2016 by ASME

stabilizing elements, partially stabilized zirconia stabilized with multiple elements, like mixtures of yttrium and tantalum or mixtures of yttrium gadolinium and ytterbium, some perovskites, magnetoplumbites and yittrium aluminum garnet (YAG). In a first step, we gathered available literature data and used them in a material selector build upon the following assumptions: - TBC materials shall be phase stable up to the highest possible temperature - Their sintering onset temperature shall be as high as possible to avoid densification and stiffening of the TBC material which can then lead to an increased stress level in the ceramic layer and at the ceramic layer-bondcoat interface. - To minimize the stress level in the ceramic layer and at the ceramic layer-bondcoat interface, the elastic modulus shall be as low as possible. - Depending on the TBC thermal conductivity, the TBC thickness shall be adjusted; this can have an impact on the stress distribution in the coating, bondcoat and substrate. - High fracture toughness is desirable, specifically at locations experiencing high stress levels, namely close to the ceramic layer-bondcoat interface. The coating architecture can depend on the materials selected for the coatings. For instance a material with high fracture toughness can be used in a single ceramic layer, while one can use a 7YSZ first inner TBC layer and another material second outer layer when the fracture toughness of this material is low. Based on such considerations, it was decided to develop a material selector based on 7 models with different weighting factors for each material property. The 7 models are aimed at evaluating the performance of the coating material for different coating designs covering the spectrum of coatings that could be used in a gas turbine: - Standard model: The temperature stability, compatibility with YSZ, sintering temperature and coefficient of thermal expansion are the most important parameters. - Low thermal conductivity model: this model is similar to the standard model, but with a weight for the thermal conductivity. - Single layer model: It is assumed that there is either no or a very thin YSZ layer, therefore, the YSZ compatibility is not considered, the thermal conductivity and fracture toughness are important parameters. - Thin layer model: The TBC material should be selected such to allow designing an as thin as possible TBC. The thermal conductivity is the most important parameter - Thick layer model: In this case, the TBC layer thickness is not limited. The thermal conductivity is not important, but the elastic modulus and the coefficient of thermal expansion are keys to avoid inducing to much stresses in the coating system.

being a thermally activated process, the higher is the service temperature experienced by the TBC, the faster the decomposition, and the earlier the appearance of the detrimental monoclinic phase are. This leads to the impossibility to use such a material for the very high TBC surface temperatures which are targeted by gas turbines developments where hot gas temperature are increasing while the amount of available cooling air is reduced. This brings the need to develop new TBC materials which are phase stables, if possible up to their melting temperature. NOMENCLATURE APS CTE LPT SHS-A

Atmospheric Plasma Spray Coefficient of Thermal Expansion Low Pressure Turbine Stator Heat Shield A (opposite to LPT Blade 1) SEV Secondary Environmental Vortex, Second combustion stage of the GT24/GT26 gas turbines TBC Thermal Barrier Coating XRD X-Ray Diffraction YAG Yttrium Aluminum Garnet YSZ Yttria Stabilized Zirconia 7YSZ 6-8 wt% Yttria Stabilized Zirconia 14 YSZ 13-15 wt% Yttria Stabilized Zirconia 2Y20CeSZ 1-3 wt% Yttria, 19-21 wt% Ceria Stabilized Zirconia 25 CeSZ 24-26 wt% Ceria Stabilized Zirconia 5CaYSZ 4-6 wt% Calcia, 1-3 wt% Yttria Stabilized Zirconia 18Ca2YSZ 17-19 wt% Calcia, 1-3 wt% Yttria Stabilized Zirconia (Gd,Yb)(Nd,Y)SZ Zirconia stabilized in either a cubic or tetragoinal form with a mixture of Gadolinia, Ytterbia, Neodinyia and Yttria forming clusters (Ti)YSZ Yttria Stabilized Zirconia with additional doping of Titania (La)YSZ Yttria Stabilized Zirconia with additional doping of Lanthana (Hf)YSZ Yttria Stabilized Zirconia with additional doping of Hafnia (Sm)YSZ Yttria Stabilized Zirconia with additional doping of Samaria (Ta)YSZ Yttria Stabilized Zirconia with additional doping of Tantala (Sc)YSZ Yttria Stabilized Zirconia with additional doping of Scandia BSAS Barium Strontium Alumino Silicate

CANDIDATE HIGH TEMPERATURE TBC MATERIALS Since more than 20 years, many materials have been proposed for application as thermal barrier coating [5-54]. Many of them have shown interesting properties, among them are: pyrochlores, fully stabilized zirconia with various

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Each materials property is used in an equation that provides a material property ranking ranging from 1 for a low performance to 5 for the highest performance. The equations used are in general linear with for instance for the coefficient of thermal expansion, the value of 5 being reached when the CTE is equal to the one of 7YSZ and the value decreasing linearly when it is higher or lower than the one of 7YSZ. This lead to a ranking in the material selector which can reach a maximum value of 5. For some materials, some data were not available. In such cases, the material data was not considered in the ranking scheme but the uncertainty on the missing data was used in the calculation of the standard deviation of the ranking. Based on literature data, 32 different materials were evaluated. Figure 1 shows the ranking result. On the left side of the figure the material marked in red is 7YSZ and is the reference material currently used in TBC’s. Materials with a ranking standard deviation above 1 are marked in orange and the top 10 ranked materials with ranking standard deviation bellow 1 are marked in green. The materials marked in blue are materials whose ranking did not lead to further consideration. The standard deviation is displayed as the error bars of each material. Some materials like SrCeO3 or BaY2O4 had good ranking values but only 3 or 4 properties available. Therefore, the uncertainty on their ranking values is significant as can be seen from their error bars. Since SrCeO3 is a perovskite with properties relatively close to the ones of SrZrO3 or CaZrO3, two materials also considered and having good rankings, there was no reason to consider this material further. BaY2O4 has only SrY2O4 as a related material, But due to the limited number of properties were available, it was decided not to consider it for the next stage.

-

Standard model with sintering allowed: It is similar to the standard model but sintering is no more considered as a critical parameter. This model could be applied to a dense vertically cracked TBC, where sintering becomes less critical. - Equal weight model: All properties have the same weight. The material selector ranks the TBC candidate materials based on their average ranking values and the standard deviation of their ranking. The following material properties were used as input parameters for the models in the material selector: - Phase stability at high temperatures: Since no standard data are available for this property, the phase stability was assessed from all type of available information about long-term high temperature stability of the candidate materials including phase diagrams. - Chemical compatibility with YSZ: Since no standard numerical data are available for this property, the chemical compatibility was assessed from all type of available information about interaction of the new TBC material with YSZ and from phase diagrams. - Sintering onset temperature: Such data can be found from dilatometer data, but one has to be careful that the sintering onset temperature is material composition and microstructure related, and very different values have been published for the same materials by different authors. - Coefficient of thermal expansion (CTE): The coefficient of thermal expansion can be determined from dilatometer data. The aim is to have a coefficient of thermal expansion which is as close as possible to the coating substrate material (which can be either the substrate, or in a duplex TBC system the underlying ceramic layer) - Thermal conductivity: Such data are in general available for coatings, but sometimes have been published only for bulk samples. It is also important to normalize all the values for a given coating porosity, since thermal conductivity is not only linked to a material chemistry but also to its microstructure. - Elastic modulus: As for thermal conductivity or sintering onset temperature, the elastic modulus is not only material but also microstructure dependent. Therefore, the elastic modulus values found in the literature have been corrected to correspond to a given microstructure. - Fracture toughness: As for thermal conductivity, elastic modulus or sintering onset temperature it is also affected by a material microstructure, therefore care was taken to avoid data that were produced on samples with very specific grain size or microstructures. When no data were available, the fracture toughness has been estimated from data available from a material having a similar structure (crystallographic and composition).

7 models average ranking

6.00 5.50 5.00

Weighted average

4.50 4.00 3.50 3.00 2.50 2.00 1.50

BSAS

La2Ce2O7

SrY2O4

SrCeO3

BaY2O4

La2Zr2O7

Y3Al5O12

La2Mo2O9

Nd2Zr2O7

Gd2Zr2O7

Sm2Zr2O7

LaMgAl11O19

LaPO4

MgAl2O4

SrZrO3

BaZrO3

CaZrO3

(Sc)YSZ

Hf(YSZ)

(Ta)YSZ

(Sm)YSZ

(La)YSZ

(Ti)YSZ

Al2O3-YSZ

Mullite-YSZ

(Gd,Yb)(Nd,Y)SZ

25CeSZ

5CaYSZ

18Ca2YSZ

2Y20CeSZ

7YSZ

14YSZ

1.00

Materials

Figure 1: Average ranking and average ranking standard deviation for 32 candidate TBC materials

The best ranked materials can be grouped in families as shown in table 1. This indicates that some of the best ranked materials can be very close in terms of possible benefits or drawbacks. For this reason it is not absolutely necessary to test all of them further. After removing materials that can be considered as duplicates or potentially blocked by intellectual

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material and the ‘New TBC 2’ and ‘New TBC 5’ materials have significantly reduced lifetime performance.

property, 5 materials were selected from this list for the next coating manufacturing and testing phase. Table 1. Top 10 ranked TBC candidate materials by grouping Family Materials

Partially stabilized zirconia Pyrochlores Perovskites

14YSZ, 2Y20CeSZ, 25CeSZ, 5CAYSZ (Ta)YSZ Gd2Zr2O7, La2Zr2O7, La2Ce2O7 CaZrO3, SrZrO3

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Normalized lifetime

Fully stabilized zirconia

1.25

0.75

0.5

0.25

COATING MANUFACTURING AND TESTING Each coating has been manufactured on 10 Hastelloy X 200 x 55 x 3 mm3 plates using process parameters compatible with industrial coating production. The coatings were manufactured by Mannheim and Birr coating shops with a proprietary APS bondcoat, a first inner 7YSZ layer and the TBC candidate material on the second outer ceramic layer, such to achieve a ~1.5 mm total TBC thickness. The coating raw powders were delivered by industrial TBC powder manufacturer such to minimize the risk of issues during transfer of the powder production to an industrial process for serial production. The process parameters were selected to achieve similar porosity levels and microstructures in both ceramic layers and the total thickness of the ceramic layers was targeted to be ~1.5 mm. The plates were then cut to provide 25x25 mm2 samples. Four to six samples were tested for cyclic lifetime performance with furnace cyclic tests at various temperatures representative of expected bondcoat service temperatures [55]. The furnace cyclic tests allow evaluating the performance of the coatings for their bondcoat-TBC interface properties driven lifetime. Four to ten samples were tested with burner rig tests at bondcoat and TBC surface temperatures exceeding expected service temperatures such to perform accelerated testing [56]. The burner rig tests allow testing the performance of the coating system within a thermal gradient close to what it will undergo in a real engine. This allows determining if any microstructure change in the outer hotter layer of the TBC can have a negative impact on the coating lifetime. For the New TBC 1, an additional set of samples was coated and tested to check if the data generated during the 1st test series were reproducible. The phase composition of free standing coatings was determined with X-Ray diffraction data after coating manufacturing and after annealing them for 100 hours at 1500°C. This temperature was selected to be much higher than the expected TBC surface temperature in service and that any sluggish phase transformation at the service temperature can be detected. Figure 2 displays furnace cyclic test results. The test results are also summarized in table 2. From the 5 materials tested, the ‘New TBC 1’ material has a lifetime comparable to the reference 7YSZ material, the ‘New TBC 3’ and ‘New TBC 4’ have lifetime values close to the one of the reference 7YS

0 7YSZ

New TBC 1

New TBC 2 New TBC 3 Material

New TBC 4

New TBC 5

Figure 2. Furnace cyclic test results for 5 different TBC candidate materials with results for a single layer 7YSZ samples as a reference.

Burner rig test were carried out with a TBC surface temperature of 1370°C, and a substrate temperature of 1100°C and 5 minutes of hot dwell time. The burner rig test results are summarized in figure 3 and table 2. As for the furnace test results, the two best performing TBC material candidates are the New TBC 1 and New TBC 4 materials.

Normalized number of cycles

4

3

2

1

0 7YSZ

NEW TBC 1

NEW TBC 2 NEW TBC 3 Material

NEW TBC 4

NEW TBC 5

Figure 3. Burner rig test results for 5 different TBC candidate materials with results for a single layer 7YSZ samples as a reference.

X-Ray diffraction spectra acquired after coating manufacturing indicates that the materials ‘New TBC 2’, ‘New TBC 3’ and ‘New TBC 5’ are not single phase. All these materials should be single phase according to their phase diagrams. It is not clear if the multiphase composition is due to inhomogeneity in the raw powder or is linked to the APS process which will lead to a decomposition of the powder. To the contrary, raw powders for 7YSZ are usually not single phase, but the after coating manufacturing, only a single

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can be seen on figures 4 and 5, on all the ‘New TBC 1’ and ‘New TBC 4’ coated SEV front panel, SEV inner liner segments and SEV outer liner segments no sign of coating damage was observed.

tetragonal YSZ phase is observed (with minor amount of monoclinic YSZ). After annealing them for 100 hours at 1500°C, only the reference 7YSZ material and the ‘New TBC 5’ undergo change in their phase composition. Table 2. Summary of the test results Material

Furnace cyclic test results (normalized lifetime)

Single phase composition after coating manufacturi ng

Change in phase composition after 100 h at 1500°C

Normalized burner rig test results

7YSZ New TBC 1 New TBC 2 New TBC 3 New TBC 4 New TBC 5

1.00 1.00

Yes Yes

Yes No

1.00 2.08

0.56

No

No

1.51

0.82

No

No

0.90

0.90

Yes

No

1.87

0.21

No

Yes

0.48

ENGINE TESTS Based on the furnace cyclic test, burner rig test results and the XRD data, the ‘New TBC material 1’ and ‘New TBC material 4’ have been selected for engine testing. The coatings have been manufactured in Birr and Mannheim coating shops. Three different engines were selected for coating testing with various parts: - GT26 Birr Test Power Plant: LPT Blade 1, SEV front panel, SEV inner liner segment and SEV outer liner segment rainbow test with ‘New TBC 1’ and ‘New TBC 4’ - Customer engine 1 (GT26): SHS-A and LPT Blade 1 rainbow test with ‘New TBC 1’ and ‘New TBC 4’ - Customer Engine 2 (GT26): SHS-A (used as an abradable coating) with ‘New TBC 1’ On the LPT Blade 1, the total ceramic layer thickness has been increased by 200 microns to increase the TBC surface temperature by ~100K when compared to original coating drawings. On the SHS-A, the coating thickness has been increased by 400 microns to increase the TBC surface temperature by at least 100K when compared to original coating drawings. The GT26 Birr Test Power Plant has been operating for a total of ~750 hours and ~200 starts. Both tested coatings have been observed to be in comparable conditions to the standard reference coating at the end of the test campaign. One part of each tested coating was cut after the test campaign to check the coating microstructure. No change in coating microstructure has been observed after the test campaign when compared to an as manufactured reference coating. Figure 4 Show a picture of a SEV outer liner coated with ‘New TBC 4’ after GT26 Birr Test Power Plant test campaign. Figure 5 shows a picture of a SEV outer liner coated with ‘New TBC 1’ also after the test campaign. In figure 5, one can also see a part of a SEV front panel coated with ‘New TBC 1’. As

Figure 4. Picture of the SEV combustor liner segment coated with ’New TBC 4’ after GT26 Birr Test Power Plant test campaign.

Figure 5. Picture of the SEV combustor liner segment coated with ’New TBC 1’ after GT26 Birr Test Power Plant test campaign.

In both customer engines, boroscopic inspections did not show any coating damage at the first A-inspection (~8000 EOH and ~100 starts). Figure 6 shows one boroscope picture of the LPT Blades 1 in Customer Engine 1 after 1 A-interval. For this engine the blade design was slightly changed leading to full coating of the blade tip, and the two different coatings tested can be distinguished by their slight difference in color, the ‘New TBC 1’ being more yellow than the ’New TBC 4’ which is more

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customer engines. After 1 A-interval the coatings were seen to be in very good conditions and perform as predicted by laboratory test results. Such coatings offer opportunities to increase the performance of gas turbines. For instance they offer the opportunity to remarkably increase the hot gas temperature in an order of magnitude of 100 K without requiring an increase of the cooling air consumption. Another possibility is to increase the TBC surface temperature without increasing the hot gas temperature. This will enable to reduce the cooling air consumption, but also will benefit to the base material lifetime by reducing its temperature gradient and improving its thermomechanical fatigue lifetime. They will have not only applications in the latest generation of gas turbines, but that they can be also be used for upgrades of existing E or F-class engines, bringing not only performance but also lifetime benefits.

creamy. One can also observe that the TBC coated SHS-A are in very good condition with no rubbing of the TBC.

ACKNOWLEGEMENT The authors acknowledge the contributions of Hans-Peter Bossmann and Camille Kunz.

Figure 6. Boroscopic picture of the LPT Blade 1 coated with ‘New TBC 1’ (yellow blades) and ’New TBC 4’ (creamy st blades) at 1 A-inspection.

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Figure 7 shows one boroscope picture of the SHS-A in Customer Engine 2 after 1 A-interval. In this engine, the SHS-A were designed such that the TBC act also as an abradable coating. The picture show that as expected, a portion of the coating on the SHS-A has been abraded by the LPT Blade 1 tip leading to a well-defined rubbing pattern without any additional coating damage.

Figure 7. Boroscopic picture of the SHS-A coated with ‘New TBC 1’. The coating was smoothly cut through by the LPT Blade 1 without bringing any coating or blade tip damages.

CONCLUSIONS Through a stepwise approach, new TBC materials have been selected, tested in laboratory conditions and introduced in

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