The durability of thermal barrier coatings (TBCs) plays an important role in the service reliability and maintainability of hot-section components in advanced turbine engines for aerospace and utility applications. Photostimulated luminescence spectroscopy (PSLS) and electrochemical impedance spectroscopy (EIS) are being concurrently developed as complimentary nondestructive evaluation (NDE) techniques for quality control and liferemain assessment of TBCs. This paper discusses recent achievements in understanding the residual stress, phase constituents, and electrochemical resistance (or capacitance) of TBC constituents—with an emphasis on the thermally grown oxide. Results from NDE by PSLS and EIS are correlated to the nano- and microstructural development of TBCs

Byeon, J. W.

Franke, B.

Jayaraj, B.

Karlsson, Anette M

Laxman, S.

Sohn, Y. H.

English

EngagedScholarship @ Cleveland State University

Cleveland State University
EngagedScholarship@CSU
Mechanical Engineering Faculty Publications Mechanical Engineering Department
10-2004
The Non-Destructive and Nano-Microstructural
Characterization of Thermal-Barrier Coatings
Y. H. Sohn
University of Central Florida, ysohn@mail.ucf.edu
B. Jayaraj
University of Central Florida
S. Laxman
University of Central Florida
B. Franke
University of Central Florida
J. W. Byeon
University of Central Florida
See next page for additional authors
Follow this and additional works at: https://engagedscholarship.csuohio.edu/enme_facpub
Part of the Mechanical Engineering Commons
How does access to this work benefit you? Let us know!
Publisher's Statement
The final publication is available at Springer via http://dx.doi.org/10.1007/s11837-004-0292-2
This Article is brought to you for free and open access by the Mechanical Engineering Department at EngagedScholarship@CSU. It has been accepted
for inclusion in Mechanical Engineering Faculty Publications by an authorized administrator of EngagedScholarship@CSU. For more information,
please contact library.es@csuohio.edu.
Original Citation
Sohn, Y. H., Jayaraj, B., Laxman, S., 2004, "The Non-Destructive and Nano-Microstructural Characterization of Thermal-Barrier
Coatings," JOM, 56(10) pp. 53-56.
Authors
Y. H. Sohn, B. Jayaraj, S. Laxman, B. Franke, J. W. Byeon, and Anette M. Karlsson
This article is available at EngagedScholarship@CSU: https://engagedscholarship.csuohio.edu/enme_facpub/140
Authors' Note: More information on the authors’ research and 
education activities can be obtained from mmae.ucf.edu/~ysohn 
and me.udel.edu/karlsson.
 The durability of thermal barrier 
coatings (TBCs) plays an important role 
in the service reliability and maintain-
ability of hot-section components in 
advanced turbine engines for aerospace 
and utility applications. Photostimulated 
luminescence spectroscopy (PSLS) 
and electrochemical impedance spec-
troscopy (EIS) are being concurrently 
developed as complimentary non-
destructive evaluation (NDE) tech-
niques for quality control and life-
remain assessment of TBCs. This 
paper discusses recent achievements 
in understanding the residual stress, 
phase constituents, and electrochemical 
resistance (or capacitance) of TBC 
constituents—with an emphasis on the 
thermally grown oxide. Results from 
NDE by PSLS and EIS are correlated to 
the nano- and microstructural develop-
ment of TBCs. 
INTRODUCTION
 Thermal-barrier coatings (TBCs) 
consisting of 7–8 wt.% Y2O3 stabilized 
ZrO2 (YSZ), applied by air-plasma spray 
(APS) (Figure 1a), or electron-beam 
physical vapor deposition (EB-PVD) 
(Figure 1b), play a critical role in 
the performance and reliability of 
modern gas turbine engines.1–4 The 
insulating YSZ coatings are deposited 
on internally-cooled hot-section com-
ponents and provide a thermal gradient 
between the hot exhaust gas and the 
metallic surface. Oxygen diffuses 
through the ceramic coating and oxidizes 
the alumina-forming MCrAlY (M = Ni 
and/or Co) or (Ni,Pt)Al bond coats, 
which is an intermediate coating layer 
between the YSZ and the superalloy. 
As a result of oxidation, the thermally 
The Non-Destructive and 
Nano-Microstructural Characterization 
of Thermal-Barrier Coatings
Y.H. Sohn, B. Jayaraj, S. Laxman, B. Franke, J.W. Byeon, and A.M. Karlsson 
grown oxide (TGO), consisting primar-
ily of α-Al2O3, forms and grows with 
associated growth stress. 
 In addition to mechanical load 
associated with the use-pattern of a gas 
turbine, TBCs are subjected to thermo-
mechanical load during thermal cycling 
due to the thermal expansion mismatch 
between the metallic substrate and the 
ceramic coatings. The effect of the 
thermal mismatch alone can give rise 
to stresses as high as 4 GPa in the TGO 
at room temperature.5–7 Thus, failures 
frequently occur during cooling or once 
ambient temperature is reached. The 
damage evolution up to the final failure 
is often related to a complex evolution 
of the microstructure, where damage 
during thermo-mechanical cycling 
initiates at multiple interfacial flaws in 
the vicinity of the TGO. With continued 
cycling the flaws eventually link up and 
the TBCs fail by the spallation of the 
TGO and/or YSZ coating. After TBC 
spallation, the bond coat is directly 
exposed to high-temperature gas, leading 
to accelerated degradation of the bond 
coat and superalloy components.1–4,8,9
 Since TBC failure frequently involves 
component failure and unscheduled 
maintenance of gas turbine engines, 
it is important that the integrity of 
TBCs be nondestructively evaluated by 
examining the characteristics of TBCs 
including residual stress, thickness and 
the phase constituents of the TGO, as 
well as the presence of any insipient 
damage. Non-destructive evaluation 
(NDE) of these characteristics can 
provide critical information regarding 
the damage/failure modes of TBCs and 
can be a part of parameters monitored 
for lifetime management of TBCs and 
engine operation.
 The development of TBCs with 
enhanced durability and reliability along 
with a mechanism-based life prediction 
model require refined understanding of 
failure mechanisms based on detailed 
analysis at the nano- and microscopic 
scale, coupled with mechanics-based 
failure evaluation. The focus in this 
paper will be on novel tools for nano- 
and microstructural analysis. Some 
investigations on microstructural devel-
opment using transmission-electron 
microscopy (TEM) and scanning TEM 
(STEM) have been reported,10–14 but 
specimen preparation of thermally 
cycled TBCs has been difficult due to the 
extremely fragile nature of specimens 
with multiple interfaces. 
Figure 1. Backscattered electron micro-
graphs of the (a) APS and (b) EB-PVD 
TBCs. 
PSLS AND EIS: 
COMPLIMENTARY 
NDE TECHNIQUES 
 Critical factors typically associated 
with the spallation failure include the 
thickness of the TGO, the residual stress 
of the TGO, and the phase constituents, 
especially the polymorphic transforma-
tion of the Al2O3 in the TGO scale.
1–4,15 
These three characteristics of TBCs can 
be examined non-destructively by using 
both photostimulated spectroscopy 
(PSLS) and electrochemical impedance 
spectroscopy (EIS). However, failure 
mechanisms are unique for each TBC 
system. There is, for example, no unique 
value for maximum TGO thickness.16 
Nor is there one critical TGO stress that 
can be used for all systems.5–7 Instead, 
TGO stress has to be selected for the 
particular system used and must be 
based on structural evaluation linked to 
mechanisms-based modeling.  
 The use of PSLS as an NDE technique 
for TBCs was pioneered by Clarke and 
refined by several researchers.5,6,17–26 The 
governing principles and the applicabil-
ity of PSLS has been extensively 
reported,5,6,17–26 particularly for the 
determination of residual stress of 
the α-Al2O3 in the TGO scale, and 
polymorphic phase constituents of Al2O3 
(e.g., γ, θ, and α phases). 
 Figure 2 represents a typical evolution 
Figure 2. The typical photostimulated 
luminescence spectra as a function of 1 
h thermal cycling for EB-PVD TBCs with 
(Ni,Pt)Al bond coats. The TBC spallation 
occurred after 420 cycles while the 
luminescence from stress-relieved TGO 
scale (marked by arrows) was observed 
starting 350 cycles. 
of photoluminescence spectra as a 
function of 1 h thermal cycling at 
1,121°C (10 min. heat-up, 40 min. 
dwell, and 10 min. forced-air-quench) 
for an EB-PVD TBC with (Ni,Pt)Al 
bond coat and CMSX-4 superalloy 
substrate. While the failure of the TBC 
specimen occurred after 420 cycles, 
a detectable increase in luminescence 
associated with the stress relief of the 
TGO scale emerged starting at 350 
cycles27 underneath the YSZ topcoat. 
After 400 cycles, more than half of 
20 randomly selected non-destructive 
measurements detected the localized 
stress relief associated with sub-critical 
damage in the TGO scale.27 A cor-
responding microstructural analysis 
reveals that there is significant damage 
in the TGO scales prior to the final 
failure. The damage modes observed 
included ratcheting and/or cracking 
from out-of-plane tensile stress in the 
vicinity of asperities in (Ni,Pt)Al grain 
boundary ridges.2,3,28
 Electrochemical impedance spectros-
copy is an NDE technique that measures 
electrochemical impedance of materials 
with an applied voltage with varying 
frequency. The electrochemical imped-
ance characteristics of TBCs may depend 
on the thickness and microstructure of 
the YSZ coating, the thickness and 
microstructure of the TGO, and poten-
tially, the damage at critical interfaces in 
the vicinity of the TGO.29–31 Figure 3 
represents a typical evolution in the 
electrochemical resistance (R) and 
capacitance (C) of the YSZ topcoat and 
the TGO scale. In this study, EB-PVD 
TBCs with NiCoCrAlY bond coats and 
IN738 superalloy substrates were tested 
using the 1 h thermal cycle at 1,121°C. 
The failure of the TBC specimen 
occurred after 419 cycles.29 There was 
an initial increase in the RYSZ and an 
initial decrease in the CYSZ with thermal 
cycling. The RTGO and CTGO also exhibited 
an initial increase and decrease, respec-
tively. Experimental results based on a 
sintering study of monolithic YSZ with 
open pores and isothermal/cyclic 
oxidation study of Al2O3-forming alloys 
have demonstrated that the initial 
variation in the RYSZ, CYSZ, RTGO, and 
CTGO with high-temperature exposure 
Figure 3. The typical variation in the resistance and capacitance of the YSZ and TGO as a 
function of 1-hour thermal cycling in air at 1,121°C for EB-PVD TBCs. 
ASD 1,00010010
Thermal Cycles (Cycles)
ASD 1,00010010
Thermal Cycles (Cycles)
1
10
100
Re
sis
ta
nc
e 
of
 Y
SZ
 (k
Ω
)
0
10
18
Re
sis
ta
nc
e 
of
 T
G
O
 (k
Ω
)
a b
ASD 1,00010010
Thermal Cycles (Cycles)
ASD 1,00010010
Thermal Cycles (Cycles)
c d
1e7
10
1e4
1e5
1e6
100
1e3
1
1e5
10
1e4
100
1e3
1
Ca
pa
cit
an
ac
e 
of
 Y
SZ
 (µ
F)
Ca
pa
cit
an
ce
 o
f T
G
O
 (n
F)
2
12
14
16
4
6
8
can be attributed to the sintering of the 
YSZ and the growth of the TGO scale.31,32 
In fact, the thickness of the TGO scale 
and the CTGO were inversely related, as 
the theory predicts. With further thermal 
cycling, gradual decreases in RYSZ and 
RTGO were observed as well as abrupt 
increases in the CYSZ and CTGO. These 
changes correlated to the damage within 
the TBC, through which the conductive 
electrolyte can penetrate and short-
circuit with a conductive metallic 
substrate. This correlation between 
damage and electrochemical impedance 
was also demonstrated using a sinter-
and-quench study of monolithic YSZ 
with open pores, and isothermal/cyclic 
oxidation study of Al2O3-forming 
alloys.31,32 In addition, EIS has demon-
strated its potential to examine the 
processing-dependent changes in the 
microstructure of TBCs, including 
thickness, chemistry, and porosity.30 
NANO-MICROSCALE 
CHARACTERIZATION BY 
TEM/STEM VIA FIB-INLO 
 Most microstructural analysis of 
TBCs has relied on qualitative correla-
tion based on resolution-limited x-ray 
diffraction, and SEM with EDS. How-
ever, recent developments in nano- and 
microstructural evaluation techniques 
have opened up new ventures. For 
example, a TEM/STEM analysis carried 
out using focused ion beam–in-situ 
lift-out (FIB-INLO) for the specimen 
preparation is particularly useful for 
fragile specimens such as TBCs.33–35
 In preparing the site-specific TEM 
specimen of TBCs using FIB-INLO, the 
cross-sectional samples are embedded 
in transparent cold-setting epoxy, and 
ground using 600-grit SiC paper, 
followed by polishing with diamond 
paste down to 1/4 µm. The polished 
surface of the specimens is then sputter 
coated with Au-Pd to create adequate 
conduction during FIB imaging and 
milling operations. First, a wedge-
shaped, site-specific specimen is cut free 
from the bulk material by means of FIB 
milling. The sample is then attached to 
the in-situ tungsten probe using ion-beam 
assisted platinum chemical vapor 
deposition (CVD).  Next, the ion-beam 
assisted platinum CVD is used to attach 
the sample to a 2 mm × 1 mm copper 
TEM grid that had been previously cut 
to accommodate access to the grid with 
 20 µm  20 µm 
 20 µm  20 µm 
Figure 4. Secondary electron FIB images showing the sequence of the FIB INLO TEM 
specimen preparation process for a representative TBC specimen.33
Figure 5. A bright-field TEM 
micrograph of an intact EB-PVD 
TBC specimen after 645 10-h 
thermal cycles at 1,038°C. 
Extensive decohesion at or 
near the YSZ/TGO (α-Al2O3) 
interface was observed. The 
FFT diffractogram (inset) from 
the bond coat corresponds to 
that of  γ′-Ni3Al solid-solution 
phase.34
 2 µm 
the in-situ tungsten probe.  Once it is 
secured to the TEM grid, the sample is 
milled free from the in-situ tungsten 
probe and then milled to electron 
transparency. The sequential images of 
the specimen preparation via FIB-INLO 
are presented in Figure 4. 
 Once the TEM specimen is prepared 
using FIB-INLO, the nanostructure and 
microstructure of the TBCs can be 
analyzed using TEM and STEM tech-
niques. In this study, a Philips/FEI Tecnai 
F30 300KeV TEM, equipped with 
high-angle annular dark-field (HAADF) 
and x-ray energy-dispersive spectros-
copy (XEDS) was employed to examine 
the microstructure of the as-coated and 
thermally cycled TBCs.33–35 In these 
studies, HAADF (i.e., analogous to 
backscatter-electron imaging), electron 
diffraction, and fast-Fourier transformed 
(FFT) diffractograms were employed to 
identify phase constituents, particularly 
the polymorphic make-up of the Al2O3 
in the TGO scale, and were correlated to 
the non-destructive phase identification 
via PSLS.33
 Figure 5 shows the bright-field image 
of thermally cycled EB-PVD TBC with 
grit-blasted (Ni,Pt)Al bond coat, a thick 
TGO (nearly 8 µm) scale, and the 
development of extensive decohesion at 
the YSZ/TGO interface after 645 10-h 
thermal cycles at 1,038°C.27 No apparent 
damage was observed at the TGO-to-
bond-coat interface as illustrated by the 
high-resolution image presented in the 
inset of Figure 5. The formation of 
voids/decohesion at the YSZ/TGO 
interface has been observed frequently 
via TEM/STEM examination as a 
function of thermal exposure.33 For the 
material system under consideration, the 
constrained volumetric changes associ-
ated with the polymorphic transforma-
tion of Al2O3 and morphological change 
of TGO scale (i.e., rumpling and/or 
ratcheting) may cause this damage at the 
YSZ/TGO interface. 
 Observation of decohesion at the 
YSZ/TGO interface in Figure 5 warrants 
a further discussion since this set of 
samples displayed an interesting failure 
evolution not documented previously. 
Delamination buckling is a fairly 
complex failure mechanism, where a 
crack reaching a critical size allows the 
coating to release its strain energy by 
buckling. Once the coating has buckled, 
the configuration becomes unstable, 
resulting in continued crack and buckle 
propagation,36,37 often leading to spall-
ation. A thicker coating is more resistant 
to buckling, but it can store more energy 
for potentially driving a crack once the 
instability is reached. 
 The presence of a YSZ topcoat 
suppresses the buckling of the TGO 
scale, provided that the YSZ/TGO 
interface remains adhered, as illustrated 
in Figure 6. In other words, the critical 
flaw size required for TBC (YSZ + TGO) 
is larger than that for TGO only. With 
deterioration of the YSZ/TGO interface, 
as observed by TEM/STEM, the buck-
ling failure of TBC requires a smaller 
flaw (or a crack) size (e.g., critical enough 
just for the TGO) with the addition of 
more thermal cycling (i.e., increasing 
strain energy release rate) as illustrated 
in Figure 6. Thus, if the amount of voids 
increases to a critical level, the YSZ is 
not suppressing the TGO and the TGO is 
free to buckle. The results can be 
dramatic large-scale buckling and 
spallation.  
ACKNOWLEDGEMENTS 
 Yongho Sohn would like to thank the 
University Turbine Systems Research 
(No. 02-01-SR103) of the U.S. Depart-
ment of Energy (DE-FC26-02NT41431) 
and the Faculty Early Career Develop-
ment (CAREER) Program of the 
Na t iona l Sc i ence Founda t ion 
(DMR0238356) for financial support. 
Anette M. Karlsson would like to thank 
the Office of Naval Research (N00014-
04-1-0498) for its financial support via 
the Young Investigator Award. 
References
1. R.A. Miller, J. Thermal Spray Technol., 6 (1997), p. 
35. 
2. A.G. Evans et al., Prog. Mater. Sci., 46 (2001), p. 505. 
3. N.P. Padture, E.H. Jordan, and M. Gell, Science, 296 
(2002), p. 280. 
4. P.K. Wright, Current Opinions in Solid State Mater. 
Sci., 4 (1999), p. 255. 
5. V.K. Tolpygo, J.R. Dryden, and D.R. Clarke, Acta 
Mater., 46 (1998), p. 927. 
6. T. Tomimatsu, S. Zhu, and Y. Kagawa, Acta Mater., 51 
(2003), p. 2397. 
7. J.A. Nuchka et al., Surf. Coat. Technol., 163/164 
(2003), p. 87. 
8. G.M. Kim et al., Scr. Mater., 46 (2002), p. 489. 
9. U. Schulz et al., J. Eng. Gas Turbines Power, 124 
(2002), p. 229. 
10. O. Unal, T.E. Mitchell, and A.H. Heuer, J. Am. 
Ceram. Soc., 77 (1994), p. 984. 
11. M.R. Brickey and J.L. Lee, Oxi. Met., 54 (2000), p. 
237. 
12. A.H. Carim et al., Mater. Sci. Eng. A, A334 
(2002), p. 65. 
13. U. Schulz et al., Surf. Coat. Technol., 146/147 
(2001), p. 117. 
14. K.S. Murphy, K.L. More, and M.J. Lance, Surf. 
Coat. Technol., 146/147 (2001), p. 152. 
15. C. Mennicke, D.R. Mumm, and D.R. Clarke, Z. 
Metallkd., 90 (1999), p. 12. 
16. D.R. Mumm et al., Acta Mater., 52 (2004), p. 
1123. 
17. Q. Ma and D.R. Clarke, J. Am. Ceram. Soc., 
76 (1993), p. 1443. 
18. R.J. Christensen et al., Appl. Phys. Lett., 69 
(1996), p. 3754. 
19. D.R. Clarke, J.R. Christensen, and V.K. Tolpygo, 
Surf. Coat. Technol., 94/95 (1997), p. 89. 
20. Y.H. Sohn et al., Surf. Coat. Technol., 146/147 
(2001), p. 102. 
21. Y.H. Sohn et al., Metall. Mater. Trans. A, 31A 
(2000), p. 2388. 
22. K.W. Schlichting et al., Mater. Sci. Eng. A, A291 
(2000), p. 68. 
23. A. Atkinson, A. Selcuk, and S.J. Webb, Oxi. Met., 
54 (2000), p. 371. 
24. A. Selcuk and Atkinson, Mater. Sci. Eng. A, A335 
(2002), p. 147. 
25. A. Selcuk and A. Atkinson, Acta Mater., 51 
(2003), p. 535. 
26. V.K. Tolpygo and D.R. Clarke, Mater. High Temp., 
17 (2000), p. 59. 
27. B. Franke et al., unpublished research (2004). 
28. A.M. Karlssen, J.W. Hutchinson, and A.G. Evans, 
Mater. Sci. Eng. A, A351 (2003), p. 244. 
29. B. Jayaraj et al., Surf. Coat. Technol., 177/178 
(2004), p. 140. 
30. S. Vishweswaraiah et al., Thermal Spray 2003: 
Advancing the Science and Applying the Technology, 
ed. B.R. Marple and C. Moreau (Materials Park, OH: 
ASM International, 2003), pp. 1487. 
31. B. Jayaraj et al., Mater. Sci. Eng. A, A372 (2004), 
p. 278. 
32. S. Vishweswaraiah et al., unpublished research 
(2004). 
33. S. Laxman et al., Surf. Coat. Technol., 177/178 
(2004), p. 121. 
34. B. Kempshall et al., Thin Solid Films, (2004) 
in press. 
35. A.J. Burns et al., Surf. Coat. Technol., 177/178 
(2004), p. 89. 
36. A.G. Evan, J.W. Hutchinson, and M.Y. He, Acta 
Mater., 47 (1999), p. 1413. 
37. S.R. Choi, J.W. Hutchinson, and A.G. Evans, Mech. 
Mater., 31 (1999), p. 431. 
Figure 6. The evolution of 
critical flaw size required for 
the buckling failure of TBCs 
with and without the dete-
rioration of the YSZ/TGO 
interface.


The Non-Destructive and Nano-Microstructural Characterization of Thermal-Barrier Coatings

https://engagedscholarship.csuohio.edu/cgi/viewcontent.cgi?article=1139&amp;context=enme_facpub

The Non-Destructive and Nano-Microstructural Characterization of Thermal-Barrier Coatings

Abstract

Similar works

Full text

Available Versions

EngagedScholarship @ Cleveland State University