Dongming Zhu, Robert A. Miller, and Maria A. Kuczmarski

Glenn Research Center, Cleveland, Ohio

Design and Performance Optimizations of Advanced 

Erosion-Resistant Low Conductivity Thermal 

Barrier Coatings for Rotorcraft Engines

NASA/TM—2012-217138

May 2012
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Dongming Zhu, Robert A. Miller, and Maria A. Kuczmarski

Glenn Research Center, Cleveland, Ohio

Design and Performance Optimizations of Advanced 

Erosion-Resistant Low Conductivity Thermal 

Barrier Coatings for Rotorcraft Engines

NASA/TM—2012-217138

May 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

66th Annual Forum and Technology Display (AHS Forum 66)

sponsored by the American Helicopter Society

Phoenix, Arizona, May 11–13, 2010
Acknowledgments

The work was supported by NASA Fundamental Aeronautics Program (FAP) Subsonic Rotary Wing (SRW) project. 

The authors are grateful to Mr. Michael Perez for performing the burner rig erosion tests.

Available from

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This work was sponsored by the Fundamental Aeronautics Program 

at the NASA Glenn Research Center.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217138 1

Design and Performance Optimizations of Advanced Erosion-Resistant Low 

Conductivity Thermal Barrier Coatings for Rotorcraft Engines

Dongming Zhu, Robert A. Miller, and Maria A. Kuczmarski

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

Thermal barrier coatings will be more aggressively designed 

to protect gas turbine engine hot-section components in order 

to meet future rotorcraft engine higher fuel efficiency and 

lower emission goals. For thermal barrier coatings designed 

for rotorcraft turbine airfoil applications, further improved 

erosion and impact resistance are crucial for engine 

performance and durability, because the rotorcraft are often 

operated in the most severe sand erosive environments. 

Advanced low thermal conductivity and erosion-resistant 

thermal barrier coatings are being developed, with the current 

emphasis being placed on thermal barrier coating toughness 

improvements using multi-component alloying and processing 

optimization approaches. The performance of the advanced 

thermal barrier coatings has been evaluated in a high 

temperature erosion burner rig and a laser heat-flux rig to 

simulate engine erosion and thermal gradient environments. 

The results have shown that the coating composition and 

architecture optimizations can effectively improve the erosion 

and impact resistance of the coating systems, while 

maintaining low thermal conductivity and cyclic oxidation 

durability.

Introduction

Advanced thermal barrier coatings (TBCs) are critical for 

designing next generation rotorcraft turbine engines because 

of their ability to allow increased engine gas temperatures and 

reduced cooling requirements, thus helping to achieve 

improved engine performance and durability (Refs. 1 to 5). It 

has been shown that advanced turbine thermal barrier coatings 

can provide significant benefits in increasing engine efficiency 

(Ref. 5). Advanced hot-section material technology is 

especially critical in improving the turboshaft based rotorcraft 

engine performance, and a more nearly prime-reliant coating 

design approach is necessary to take full advantage of the 

thermal barrier coating technology in future rotorcraft engine 

systems. Turbine blade thermal barrier coatings are especially 

susceptible to engine ingested sand erosion and impact 

damage because of the high velocity flow path nature of the 

blades and complex particulate-coating interactions under high 

heat-flux combustion turbine environments. The NASA 

Rotary Wing Project has been addressing low thermal 

conductivity, high toughness and erosion resistant turbine 

blade thermal barrier coating development based on nanotetragonal phase toughening design approaches (Refs. 6 to 8). 

The main objective of this current effort is to evaluate and 

optimize the processing of advanced turbine blade thermal 

barrier coating systems with increased erosion and impact 

resistance for rotorcraft propulsion engines. The high 

temperature erosion performance of the turbine TBC systems 

will be discussed. Thermal conductivity and thermal cyclic 

durability of the coating systems have been evaluated. The 

emphasis is particularly placed on the multi-component 

alloying and processing optimization for the erosion resistance

and the combined oxidation and erosion effects, helping

understand the complex erosion mechanisms for developing 

comprehensive turbine blade TBC life prediction models. 

Expertimental Procedures

Advanced Thermal Barrier Coatings

Advanced turbine thermal barrier coatings were designed 

using a multi-component defect-clustering approach and 

processed using an Electron Beam–Physical Vapor Deposition 

(EB-PVD) technique (Refs. 1 to 3). Although the high 

temperature multi-component turbine thermal barrier coatings

have included pseudo-four component ZrO2-Y2O3-Gd2O3-

Yb2O3 (t’ ZrYGdYb) and pseudo-six component ZrO2-Y2O3-

Gd2O3-Yb2O3-TiO2-Ta2O5 (t’ ZrYGdYbTT) systems, the 

present study is primarily focused on the four component 

systems because of the less complex and better controlled EBPVD processing parameters. The EB-PVD coatings were 

processed at a vendor, and four processing conditions were 

used in the EB-PVD process by varying electron beam power, 

chamber pressure, and specimen deposition temperature. The 

advanced TBC systems, typically consisting of a 130 to 

200 μm thick ceramic top coat and a 75 to 100 μm thick PtAl 

intermediate bond coat, were electron beam-physical vapor 

deposited (EB-PVD) on to 25.4 mm diameter disks of Rene 

N5 nickel-base superalloy substrates. The EB-PVD coatings 

were all deposited using pre-fabricated evaporation ingots 

made using the designed compositions.

A Mach 0.3 to 1.0 high velocity burner rig with erosion 

capability was developed for the turbine blade thermal barrier 

coating erosion testing in a laboratory simulated turbine 

engine relevant environments (Ref. 8). Computational Fluid 

Dynamics (CFD) modeling and experimental testing were
NASA/TM—2012-217138 2

systematically studied in the high burner velocities ranging 

from Mach 0.3 to 0.9, representative to turbine engine 

conditions. In the present study, Al2O3 particles were used as 

erodent, as similarly reported in previous work by others 

(Refs. 9 and 10). The nominal particle sizes used were ranged 

from 27 to 100 μm. The coating erosion testing was primarily

conducted at 2000 °F. However, comparisons have been made

for erosion tests performed at 1800 to 2200 °F. The set of the 

process optimization coatings were also pre-cyclically 

oxidized in a furnace for 128 and 256 hr at 2025 °F in 2 hr hot 

time cycles, and then burner erosion tested at high temperature 

to investigate the pre-exposure cyclic oxidation effect. In all 

cases, the erosion rates were determined by the erodent 

amount required for the coating erosion recession and coating

breakthrough. A more sophisticated characterization of threedimensional erosion surfaces and erosion behavior of thermal 

barrier coating systems under complexity of erosion burner 

conditions will be reported at a later time. 

Thermal conductivity and cyclic durability tests were also 

conducted using the high heat flux laser test rig and cyclic 

furnaces at typical operating relevant temperatures, and 

experiment details will be discussed elsewhere (Ref. 11). The 

advanced t’ coatings were also compared with the baseline 

ZrO2-7wt%Y2O3 (7YSZ) in the performance.

Experimental Results and Discussion

Thermal Conductivity 

Figure 1 shows the thermal conductivity of four-component 

ZrO2-Y2O3-Gd2O3-Yb2O3 and six-component low conductivity

(low k) ZrO2-Y2O3-Gd2O3-Yb2O3-TiO2-Ta2O5 coatings, tested 

at 2400°F (1316°C) using a steady-state laser heat flux rig. 

The four-component turbine coatings have lower thermal 

conductivity than the baseline 7YSZ (a yttria partially 

stabilized zirconia) coating, but higher conductivity than the 

combustor application oriented cubic-phase thermal barrier 

coatings and the currently less developed six-component 

coating systems. The four-component turbine TBCs have been 

developed with toughness enhancements to improve the 

erosion resistance required by the turbine airfoil applications. 

The composition and phase optimization also have ensured the 

basic erosion resistance requirements of the coating systems.

Figure 2 shows the furnace cyclic life comparisons of the

four-component and six-component low k coatings. The 

coating cyclic life was optimized for a highly stable tetragonal 

t’ phase among the different coating systems. It can be seen 

that the four-component coatings have an optimum durability 

with lower total Rare Earth (RE) oxide dopant (RE2O3) 

concentrations similar to the baseline 7YSZ, with the cyclic 

life of 300 to 400 hr at 2125 °F (1163 °C), as compared to the 

six-component coating systems having additional Ti and Ta 

additions. The Ti and Ta in the six component systems have 

the advantages of allowing increased rare earth dopants while 

still maintaining t’ phase and lower thermal conductivities. 

However, the lower dopant concentration and less complexity 

of the four-component systems helped the initial EB-PVD 

processing optimizations where the homogenization and 

precise control of the coating compositions are highly critical.

Figure 1.—Thermal conductivity of four-component ZrO2-Y2O3-

Gd2O3-Yb2O3 and six-component low conductivity ZrO2-Y2O3-

Gd2O3-Yb2O3-TiO2-Ta2O5 coatings, tested at 2400 °F 

(1316 °C) using a steady-state laser heat flux rig. The fourcomponent turbine coatings have lower thermal conductivity 

than the baseline 7YSZ coating, but higher conductivity than 

the combustor cubic phase thermal barrier coatings.

Figure 2.—Furnace cyclic life comparisons of four-component 

ZrO2-Y2O3-Gd2O3-Yb2O3 and six-component low conductivity 

ZrO2-Y2O3-Gd2O3-Yb2O3-TiO2-Ta2O5 coatings. The cyclic life 

is optimized for a highly stable tetragonal t’ phase for the 

different coating systems. The four-component coatings have 

an optimum life of 300 to 400 hr at 2125 °F (1163 °C), with 

lower total rare earth oxide (RE2O3) dopants as compared to 

the six-component coating systems with Ti and Ta additions.

Baseline 7YSZ

Composite t’ coating

Composite t’ coating

Cubic phase coating

Normalized thermal conductivity

0

100

200

300

400

500

2 4 6 8 10 12 14

Four-component low k

7YSZ

Cycles to failure, hr

Totoal RE2O3 dopant concentration

2125°F (1163°C)

Six-componnet low k (with TT addtions)

Total TiO2 + Ta2O5 (TT) concentration
NASA/TM—2012-217138 3

Erosion Resistance

Figure 3 shows erosion behavior of a ZrO2-2mol%Y2O3-

1.5mol%Gd2O3-1.5mol%Yb2O3 thermal barrier coating at 

high temperature using the EB-PVD process optimization

experiment. The coatings were tested in the erosion burner rig 

under Mach 0.5 conditions at the specimen temperature of 

2000 °F. The Process 1 was the baseline process condition 

which is the same as the standard process for the 7YSZ 

coating and optimized for the baseline coating. The Process 2 

and 3 resulted in somewhat decreased erosion resistance of the 

coating systems due to the less optimized coating columnar 

architectures and higher porosity levels. The Process 4, 

however, showed quite significant, approximately 30 percent

erosion resistance improvements as compared to the baseline 

coating. 

The TBC surface morphologies of process optimization 

specimens were examined using a Field Emission GunScanning Electron Microscope (FEG-SEM), as shown in 

Figure 4. It can be seen that the specimens with the standard 

processing (Process 1) had relatively large and uniform 

columnar grains (~10 to 20 μm in size). However, the TBC 

specimens with best optimized processing (Process 4) had 

finer columnar structures (~ 10 μm in size), and with 

additional much finer smaller columns (2 to 5 μm in size). The 

smaller column sized coatings would expect to offer higher 

strength and significantly improved coating strain tolerance 

for improved erosion/impact resistance based on the previous 

erosion modeling work (Ref. 8), and also potentially providing 

improved furnace cyclic life durability. 

Figure 3.—High temperature erosion behavior of an 

optimization runs of low conductivity thermal barrier coatings. 

The optimized process 4 of the low conductivity thermal 

barrier coating showed improvements in erosion resistance 

due to better optimized columnar microstructures. 

(a)

(b)

Figure 4.—The FEG-SEM surface morphologies of turbine EB-PVD thermal barrier coatings under 

different process conditions. (a) Standard Process 1 condition; (b) Optimized Process 2 condition.

0 5 10 15 20 25

Erosion resistance, grams erodent/25 micrometer coating recession

Mach 0.5

7YSZ 2000°F

Zr2Y1.5Gd1.5Yb Process 4

Zr2Y1.5Gd1.5Yb Process 3

Zr2Y1.5Gd1.5Yb Process 2

Zr2Y1.5Gd1.5Yb Process 1
NASA/TM—2012-217138 4

Cyclic Oxidation Pre-Exposure on Erosion 

Resistance

Figure 5 shows the pre-cyclic oxidation effect on the 

erosion resistance of the turbine EB-PVD thermal barrier 

coatings. It can be seen that in most cases, the pre-cyclic 

oxidation under the moderate temperature (2025°F) and times 

(below 300 hr) seemed to increase the erosion resistance as 

compared to the as processed conditions. It is speculated that 

the cyclic exposure has moderate sintering effects which may 

strengthen the coating systems. Also the pre-cyclic exposure

may induce certain degrees of the interface roughening as 

often observed, which can further strengthen the TBC/metal 

bond coat interface and reduce the coating spallation under the 

high velocity erosion/impact testing. More systematic studies 

are underway to further understand the oxidation and erosion 

behavior and to help the development of coating life 

prediction models. 

Figure 6 shows the erosion and oxidation based failure map 

under turbine testing conditions. The effect of temperature on 

the turbine coating erosion and oxidation life is also clearly 

illustrated. It can be seen that in the oxidation-based failure, 

the coating life is exponentially reduced with increasing 

temperature (ln life in hours vs. 1/T), because interface 

damage effects are significantly increased from the accelerated 

oxide scale growth and increased cyclic stress-temperature 

amplitudes during the cycling. In contrast, the erosion-based 

coating failure life is generally increased with temperature 

because the coating toughness and plasticity increase with 

temperature. Therefore, one can expect that in the lower 

temperature regime, when coating erosion is the life limiting 

factor, the coating would generally fail by an erosion 

mechanism given sufficient erodent is present, as is frequently 

the case in rotorcraft operating environments. On the other 

hand, in the higher temperature regime, when coating 

oxidation is the life-limiting factor, the coating would 

predominantly fail by an oxidation mechanism. Under very 

high-temperature combined erosion and oxidation conditions

(2100 to 2200 °F), significantly reduced coating life was

observed in the burner erosion rig tests, and has been 

characterized as an oxidation-erosion mechanism as shown in 

Figure 6. In addition to the oxidation erosion interactions, the 

significant softening of the baseline 7YSZ thermal barrier 

coating may also have contributed to the accelerated erosion 

failure. The coating strain-based approaches are also being 

developed for turbine thermal barrier coating life predictions

and will reported in future publications.

Concluding Remarks

Advanced t’ phase rare earth oxide (Gd2O3 and Yb2O3 codoped)- and TiO2/Ta2O5-alloyed, ZrO2-based thermal barrier 

coatings were designed and processed. The turbine thermal 

barrier coatings were developed for rotorcraft turbine engine

applications, aimed at combined low conductivity and high

Figure 5.—The pre-cyclic oxidation effect on erosion resistance 

of EB-PVD thermal barrier coatings. The coatings were 

compared with the as processed, 128 hr oxidation, and 

256 hr oxidation and at 2025 °F. The erosion was tested at 

2000 °F. The results showed that the pre-cyclic oxidation 

seemed to increase the coating resistance to erosion.

Figure 6.—The erosion and oxidation based failure map under 

turbine testing conditions.

toughness for improved thermal barrier performance and 

erosion resistance. The coating thermal conductivity, cyclic 

durability, and erosion resistance have been optimized from 

composition, phase, and processing as well as microstructure 

optimizations. Combined erosion and oxidation life prediction 

approaches based on the failure maps were also illustrated.

0.5

1

1.5

2

2.5

0 50 100 150 200 250 300

Zr-2Y-1.5Gd-1.5Yb Process 1

Zr-2Y-1.5Gd-1.5Yb Process 2

Zr-2Y-1.5Gd-1.5Yb Porcess 3

Zr-2Y-1.5Gd-1.5Yb Porcess 4

Relative erosion resistance change

Pre-cyclic oxidion time, hours

4

5

6

7

8

9

10

1

2

3

4

5

0.00068 0.00072 0.00076 0.00080 0.00084

Zr2.5Y0.75Gd0.75Yb

Zr2.0Y1.5Gd1.5Yb

Zr1.6Y1.2Gd1.2Yb

Zr2.5Y0.75Gd0.75Yb

Zr2.0Y1.5Gd1.5Yb

Zr1.6Y1.2Gd1.2Yb

Zr2.5Y0.75Gd0.75Yb

Zr2.0Y1.5Gd1.5Yb

Zr1.6Y1.2Gd1.2Yb

7YSZ 

7YSZ 

7YSZ

7YSZ

7YSZ Laser heat flux

7YSZ burner heat flux rig

Mach 0.3

Oxidation-erosion

ln (Oxidation cycle time to failure), hours

Erosion resistance, grams erodent required 

per 25 micrometer coating erosion

1/T, 1/K

Surface heat flux cyclic

Furnace cyclic

Erosion
NASA/TM—2012-217138 5

References 

1. Dongming Zhu, and Robert A. Miller, “Low 

Conductivity and Sintering Resistant Thermal Barrier 

Coatings,” U.S. Patent No. 6,812,176, 2004; U.S. Patent 

No. 7,001,859, 2006; U.S. Patent No. 7,186,466, 2007.

2. Dongming Zhu, and Robert A. Miller, “Thermal 

Conductivity and Sintering Behavior of Advanced 

Thermal Barrier Coatings,” Ceram. Eng. Sci. Proc., Vol. 

23 (2002), pp. 457–468.

3. Dongming Zhu and Robert A. Miller, “Development of 

Advanced Low Conductivity Thermal Barrier Coatings,”

International Journal of Applied Ceramic Technology, 

Vol. 1 (2004), pp. 86–94.

4. Dongming Zhu et al, “Thermal and Environmental 

Barrier Coating Development for Advanced Propulsion 

Engine Systems,” AIAA–2007–2130, 48th 

AIAA/ASME/ASCE/ AHS/ASC Structures, Structural 

Dynamics, and Materials Conference, 23–26 Apr. 2007, 

Honolulu, HI.

5. Michael T. Tong, Scott M. Jones and Philip C. Arcara, 

Jr., “A Probabilistic Assessment of NASA UltraEfficient Engine Technologies for a Large Subsonic 

Transport,” Proceedings of Turbo Expo 2004, GT2004–

53485.

6. Dongming Zhu and Robert A. Miller, “Low conductivity 

and high toughness tetragonal phase structured ceramic 

thermal barrier coatings,” U.S. Provisional Patent 

Application No. 60/712,613, 2005. U.S. Patent No. 

7,700,508, 2010.

7. Dongming Zhu, Maria A. Kuczmarski, Robert A. Miller, 

and Michael D. Cuy, “Evaluation of Erosion Resistance

of Advanced Turbine Thermal Barrier Coatings,” The 

31st International Cocoa Beach Conference & 

Exposition on Advanced Ceramics & Composites, 

Daytona Beach, FL, Jan. 22–26, 2007.

8. Dongming Zhu, Robert A. Miller, and Maria A. 

Kuczmarski, “Development and Life Prediction of 

Erosion Resistant Turbine Low Conductivity Thermal 

Barrier Coatings,” 65th AHS International Annual 

Forum Proceedings, v 2, p 1088–1094, 2009.

9. Robert F. Handschuh, “High Temperature Erosion of 

Plasma-Sprayed Yttria-Stabilized Zirconia in a

Simulated Turbine Environment,” NASA TP–2406; 

AVSCOM TR–84–C–17, Dec. 1984. Also AIAA–85–

1219.

10. Robert W. Bruce, et al., “Development of 1232°C 

(2250°F) erosion and impact tests for thermal barrier 

coatings,” Tribology Transactions, Vol. 44 (1998), 

3990410.

11. Dongming Zhu, Robert A. Miller, and James L. Smialek, 

“Ionic Transport and Durability of Advanced Thermal 

Barrier Coatings,” 8th Spring Meeting of the 

International Society of Electrochemistry Advances in 

Corrosion Science for Lifetime Prediction and 

Sustainability: A Celebration of the 100th Birthday of 

Mars Fontana, May 2–5, 2010, Columbus, OH.
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Design and Performance Optimizations of Advanced Erosion-Resistant Low Conductivity 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

Thermal barrier coatings will be more aggressively designed to protect gas turbine engine hot-section components in order to meet future 

rotorcraft engine higher fuel efficiency and lower emission goals. For thermal barrier coatings designed for rotorcraft turbine airfoil 

applications, further improved erosion and impact resistance are crucial for engine performance and durability, because the rotorcraft are 

often operated in the most severe sand erosive environments. Advanced low thermal conductivity and erosion-resistant thermal barrier 

coatings are being developed, with the current emphasis being placed on thermal barrier coating toughness improvements using multicomponent alloying and processing optimization approaches. The performance of the advanced thermal barrier coatings has been evaluated 

in a high temperature erosion burner rig and a laser heat-flux rig to simulate engine erosion and thermal gradient environments. The results 

have shown that the coating composition and architecture optimizations can effectively improve the erosion and impact resistance of the 

coating systems, while maintaining low thermal conductivity and cyclic oxidation durability.

15. SUBJECT TERMS

High toughness thermal barrier coatings; Erosion resistance; Turbine engines life prediction; Thermal conductivity; Cyclic durability 

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