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at TOKYO ELECTRON House of Creativity 3F,
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Evaluation of degradation of high temperature materials
including coatings in a reducing environment
Xie Yang, Jiyuan Cui, Hiroki Saito, Kazuhiro Ogawa
November 9, 2023

Fracture and Reliability Research Institute,Tohoku University
Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, Sendai, JAPANIntroduction
Higher performance andefficiency of gas turbines
Thermal Barrier Coating (TBC)

A diagram of a gas turbine engine
https://car.motor-fan.jp/tech/10014243

Protect hot-section components from oxidationand corrosion under high temperature① Top coat: Y2O3 stabilized ZrO2 (YSZ)
・Role: Thermal barrier② Bond coat: MCrAlY (M means Ni and/or Co)
・Role: Improvement of bonding strength andinhibition of oxidation and hot corrosion③ Substrate: Ni-based superalloy④ Thermally grown oxide (TGO): formed withprolonged oxidation at high temperature
between top coat and bond coat
Photograph of a turbine blade with thermal barrier coating (TBC)

Padture, Nitin P., Maurice Gell, and Eric H. Jordan. Science 296.5566 (2002): 280-284.

2

Traditional failure mechanisms
Thermal effects Mechanical effects Force and chemical effects

main form

Al2O3

Two characteristic morphology of TGO:
dense Al2O3 and porous mixed oxide layer

The growth of TGO will cause interfacialdelamination and accelerate the spallation
Cross-sectional SEM image of TBC agedfor 3000 hours at 1000°C
K.Ogawa, T.Shoji, H.Aoki, N.Fujita, T.Torigoe. JSME int. J, 44A, 4(2001), 507-513

Wang Tiejun. Chinese Journal of Solid Mechanics, 2016, 37(6):413Research status
To achieve the carbon
neutrality goal by 2050, usinghydrogen fuel instead of fossil
fuel in gas turbines cansignificantly reduce carbon
dioxide emissions.
https://newswitch.jp/p/36219

Hydrogen・Ammonia
gas turbines

Fossil fuel gas turbines
High temperature oxidation
Will hydrogen and ammonia gas turbines
eliminate degradation due to high temperatureoxidation compared to conventional ones?

Oxidation?
Or Nitriding?
Reducing reaction?

4ETN Global, Hydrogen Gas Turbines – The Path Towards a Zero-Carbon Gas

Turbine Stefan, et al. International Materials Reviews 67.5 (2022): 461-486.

Research purpose
In ammonia gas turbines, two types of degradation are possible: nitriding
and reduction. In this study, as a first step, we investigated the reduction
reaction in a hydrogen environment.

New challenge: If the hydrogen fuel is not completely combusted,
the oxidizing environment will be replaced by areducing environment.

Further discussion is needed on how hydrogenaffects the new degradation mechanism of thethermal barrier coating systems.
In particular, it is not known what kind of material degradation occurs inTBCs, which use oxide ceramics, in a reducing environment. In this study,
material degradation in a hydrogen environment was first evaluated.

5Experimental
Substrate: Inconel 738LC
Bond Coat: CoNiCrAlY
Topcoat: 8YSZ
12 mm

Bond Coat

CoNiCrAlY

CoNiCrAlY

Bal.

3222

80.5

Bond Coat Spray Method

Thickness

High Pressure Cold Spray

100 μmTopcoat

3 mm

Bond coat

Topcoat Material

Spray Method

Thickness

8 wt.% Yttria Stabilized Zirconia

Atmospheric Plasma Spray

300 μmSubstrate(a) Muffle Furnace(b) Tube Furnace

Heat treatment conditions:Temperature: 1100°C
Time: 0 h, 10 h, 50 h, 100 h, and 500 h
Gas Type: (a) Air(b) 5% H2 + 95% pure Ar gas

6Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University

Interface degradation behavior at the interface
between TC and BC

7

Microstructural changes of TBCs samples
after heat treatment at 1100°C in air
SEM Results(a) 0 h(c) 50 h(b) 10 h
YSZ

YSZ

YSZ

YSZ

β phaseCoNiCrAlY
CoNiCrAlY50 μm
YSZ(d) 100 h

CoNiCrAlY(e) 500 h

Mixed Oxides

TGO50 μm50 μm

Resin

Mixed Oxides

CracksCoNiCrAlY

CoNiCrAlY

50 μm

TGOCracks

CoNiCrAlY

50 μm

Topcoat separation occurred
after heat treatment at 1100℃
for 500 h in air.

7Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University

Microstructural changes of TBCs samples
after heat treatment at 1100°C in 5% H2 gas
SEM Results(c) 50 h(b) 10 h(a) 0 h
YSZ

YSZ

YSZ

TGO

β phaseYSZ
β phaseTGO

CoNiCrAlY

CoNiCrAlY

50 μm

CoNiCrAlY

50 μm(e) 500 h(d)
100 h
YSZ
YSZ

CoNiCrAlY

YSZ50 μm

TGOYSZTGO
CoNiCrAlY
CoNiCrAlY

50 μm

CoNiCrAlY

CoNiCrAlY
50 μm

10 μm

No separation occurred at TC-TGO interface after heat treatment at 1100 ℃ in 5% H2 for 500 h.

8Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University

TGO thickness with time variation
16in air -500 hat 1100C in airat 1100C in 5% H2 gas

Thickness of TGO (mm)14YSZ

1210CoNiCrAlY

8400 μm

64in 5% H2 -500 h

2YSZ

00100200300
Time (h)400500

CoNiCrAlY400 μm

1100°C-10 hIn Air
2.19±0.24 μm

In 5% H20.89±0.20 μm

1100°C-50 h

4.73±1.31 μm

1.33±0.26 μm

1100°C-100 h

8.48±2.25 μm

1.77±0.41 μm

1100°C-500 h

9.51±3.19 μm

3.04±0.58 μm

TBC systems exposed in air and 5% H2 gas enabled the formation of oxide scales at different growth rates.
Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University

9Elements composition of the oxide scales
In Air -1100°C-500 h

TGO

In 5% H2 -1100°C-500 h

TGOOAlYZrCrCoNi

136.3138.15

22.470.00

0.57

1.12

1.38

2

32.16

30.69

33.28

0.00

0.76

1.16

1.49

3

39.14

41.83

13.65

2.11

0.41

1.13

1.40

321

CoNiCrAlYYttrium

10 μm

CoNiCrAlY
10 μm

OAlCrO

AlCrCo

Zr

YCo

Zr

Y

10
Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University

Schematic illustration of TGO formation
In Air O2O2-

O2O2-

YSZ

O2O2-

YSZ

Al3+

Al3+

O2-

YSZ

H+

YSZ 2-

O2-O

CoNiCrAlY
O2O2-

YSZ

YSZ

Y3+

O2-

H+
Y3+ 2O

H+Alumina
scale

Mixed oxide
Al2O3

H+Al3+Al3+

CoNiCrAlY
O2-H+H+ In 5% H2

CoNiCrAlY

CoNiCrAlY
O2O2-H+Cr3+
Al3+Al3+

Al3+

CoNiCrAlY

Co3+

Al3+

CoNiCrAlY

Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University

Y-pinning

11Phase transition of topcoat surface
In 5% H2 Reducing environment

Oxidizing environment¨¨¨

Intensity (A.U.)¨¨¨ t'-ZrO2

Impurity PeakOxidized-1100C-50 h¨¨Oxidized-1100C-500 h
Oxidized-1100C-100 h¨ t'-ZrO2¨

Intensity (A.U.)

In AirReduced-1100C-500 h

Reduced-1100C-100 h
Reduced-1100C-50 h

Oxidized-1100C-10 h

Reduced-1100C-10 h

As-sprayed
20304050602q/degree7080As-sprayed
90

2030

4050602q/degree

70

80

90

XRD results show that after heat treatment at 1100°C for 0~500 h, the topcoat surface ofTBCs samples consist of t′-ZrO2, which means there is no any phase transformation of thetopcoat surface under both environments.

12Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku UniversityMicro-Vickers Indentation Test……
10 times……

……

8 positions

YSZ

Micro-Vickers
indentation test
conditions:
Load: 100 mN/15 s……

CoNiCrAlY
The microhardness testing system

400 μmFISCHERSCOPE HM 2000

In Air10060
4060

20000.2

0.4

0.6

Displacement, h (mm)0.81.0

Typical load-displacement
curves of the cross-section ofthe YSZ topcoat.

40

200.0

As-received
Reduced-10 h
Reduced-50 h
Reduced-100 h
Reduced-500 h

80

Load, P (mN)

Load, P (mN)

100

As-received
Oxidized-10 h
Oxidized-50 h
Oxidized-100 h80

Schematic diagram of mechanical properties
measurement.

In 5% H20.00.2

0.4

0.6

0.8

1.0

Displacement, h (mm)Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University

13Hardness and Young’s Modulus
240
oxidized
reduced

14Sinteringoxidized
reduced200

Youngs modulus (GPa)Vicker microhadrness (GPa)161210864160
12080
40

2000

1050100
Heat treatment time (h)

0500

1050
100
Heat treatment time (h)500Hardness (GPa)Oxidized

ReducedYoung’s modulus (GPa)Oxidized

ReducedAs-received

6.11±2.30

6.11±2.30

As-received106.96±20.59

106.96±20.59

10 h

9.05±2.66

8.62±2.37

10 h

160.43+28.12

138.35+25.64

50 h

10.45+3.28

9.11+2.77

50 h

173.09+34.82

157.74+27.43

100 h

10.96±2.99

9.05±2.02

100 h174.61+27.53

154.24±24.88

500 h

-

8.62±2.24

500 h

-

138.82±24.78(peeled off)

decrease(peeled off)

The TBCs lifetime is extended in 5% H2 gas, which might be attributed
to the slower growth rate of the alumina scale.
Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University14Sintering behavior of top coatAs-sprayed

O-500hR-500hsurfacecross-section

1. healing of micro-cracks and micro-pores due to sintering effects
2. sintering under reducing environments shows a lower degree compared tooxidizing environments* The reduced sintering rate may be due to the reaction of hydrogen with the pre-existing oxygen in the holes created by theAPS spraying process, and the formation of local water vapor at high temperatures may further prevent the healing of the cracks.
It may be accurately proven in the future pure Ar atmosphere experiment.

16Sintering behavior of top coat14As-sprayedoxidizing
reducing

Porosity (%)

12O-500h1086010h

50h

100h

500h

Heat treatment time (h)larger-sized voids are observed➡ The statistical results of porosity
show no significant decrease.
R-500hSintering characteristics:
Micro-cracks heal and original large
size holes become larger
Cross-section morphology of top coat in a large range

17

Crack propagation by indentation test
Top coat

Load : 2N / 15s

as-sprayed

Cracks generate and propagate
at the location of pre-existing
microcracks.

O-500hno obvious cracks are caused bythe indentation test

intergranular crack① Cracks occur at the pre-existing
microcracks area.
② Cracks show obvious intergranularpropagation.

R-500h➡ intergranular embrittlement
the same area before and after the indentation test18

Interface degradation behavior at the interface
between substrate and BC

19

Initial BC/substrate interface

Al

As-sprayed sample

initial oxide film of powders

NiOCo

Cr

dense bonding

no delamination and pores between coating and substrate20

BC/substrate interface
Al
Oxidizing-1100℃ 100h

ONi

Kirkendall voids

AlOxidizing-1100℃ 500h

Kirkendall voids

ONiunequal element diffusion
between substrate and coating

21

BC/substrate interface
Reducing-1100℃ 500h

Reducing-1100℃ 1000h

AlO

TiCrAlO

Ti

Cr

Interdiffusion area

Interdiffusion area
Although metallurgical bonding is enhanced, the segregation of elements at the interface
will lead to a decrease in bond strength between bond coat and substrate duringisothermal exposure, proving a new failure position in the reducing environment.

22

Conclusions
1. TBC systems exposed in air and 5% H2 gas enabled the formation ofoxide scales at different growth rates. The TBCs lifetime is extended in
5% H2 gas, which might be attributed to the slower growth rate of thealumina scale.
2. After heat treatment in 5% H2, TGO mainly composed of Al and O,with a large amounts of Yttrium.
3. The presently available results indicate that H2 might promote thediffusion of oxygen in YSZ topcoat to the BC-TC interface, whichreduces the stability of YSZ. The decrease in mechanical properties of
the cross-section of YSZ after heat treatment in H2 is a strong evidence.
4. The presence of a large amount of hydrogen will destroy thehomogeneity of the alumina layer, and the ability of Al2O3 to preventthe diffusion of elements is decreased.
5. The bond strength between the coating and substrate tends to be
decreased under the reducing environment. This conclusion will befurther investigated in the future.
25
Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University

Thank you for your kind attention!

Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University