Recession of an EB-PVD YSZ coated turbine blade by CaSO4 and Fe, Ti -Rich CMAS-Type deposits

Article abstract of DOI:10.1111/j.1551-2916.2011.04747.x

An in-service high-pressure turbine blade with a columnar, Y ₂O₃-stabilized ZrO₂ (YSZ) thermal barrier coating (TBC) fabricated by electron-beam physical vapor deposition was investigated to access the TBC hot corrosion mechanisms during turbine operation. The TBC exhibits a through-thickness pore filling with anhydrite-type CaSO₄. Chemical analysis of the CMAS-type particle deposits reveals relatively low SiO₂ but high CaO contents and substantial amounts of Fe₂O₃ and TiO₂. The hot corrosion scenario observed at the YSZ column tips involves newly formed CaZrO₃ and the garnet-type phase Ca₃(Zr,Mg,Ti)₂(Fe,Al,Si) ₃O₁₂, also known as the mineral kimzeyite. The phase relationships were confirmed in laboratory experiments. CaSO₄ as well as the particle deposits prove to be effective solvents for YSZ introducing distinct solid-state reactions. The results support the idea of a dual YSZ hot corrosion process. A first stage controlled by a SiO₂-free Ca-source, most likely primary CaSO₄ produces a thin CaZrO₃ layer. A second, CMAS-type stage providing high concentrations of Fe₂O ₃, TiO₂, and SiO₂ favors the formation of kimzeyite. The melting temperature of kimzeyite presumably defines a thermal operation limit for YSZ-based TBCs.

Full text of DOI:10.1111/j.1551-2916.2011.04747.x

J. Am. Ceram. Soc., 94 [12] 4483–4489 (2011)
DOI: 10.1111/j.1551-2916.2011.04747.x
©
2011 The American Ceramic Society
ournal
J
Recession of an EB-PVD YSZ Coated Turbine Blade by CaSO and
4
Fe, Ti-Rich CMAS-Type Deposits
†
Wolfgang Braue and Peter Mechnich
German Aerospace Center (DLR), Institute of Materials Research, 51170, Cologne, Germany
An in-service high-pressure turbine blade with a columnar,
-stabilized ZrO (YSZ) thermal barrier coating (TBC)
compositions equivalent to congruently melting eutectics in
the quaternary system CaO–MgO–Al –SiO (“CMAS”).
Y
O
2 3
2
2
O
3
2
fabricated by electron-beam physical vapor deposition was
investigated to access the TBC hot corrosion mechanisms
Above the melting temperature (approx. 1230°C) this model
CMAS is able to fully penetrate a 200 lm EB-PVD YSZ
TBC in a few hours.
during turbine operation. The TBC exhibits
thickness pore filling with anhydrite-type CaSO
a
through-
. Chemical
4
The blades and vanes of the first stage of high-pressure
turbines (HPT) are directly exposed to combustion gases and
analysis of the CMAS-type particle deposits reveals relatively
low SiO2 but high CaO contents and substantial amounts of
1
0
to impingement of airborne particles, and consequently,
corrosive attack is severe. This research is focusing on the
validation of phase relationships and reaction sequences
found on the pressure surface and the leading edge of an
in-service EB-PVD YSZ coated first stage HPT blade. The
present work benchmarks the corrosive effects of a multi-
phase, essentially crystalline particle deposit with incongruent
melting characteristics. Phase relationships are corroborated
by powder syntheses and annealing experiments.
Fe
YSZ column tips involves newly formed CaZrO
net-type phase Ca (Zr,Mg,Ti) (Fe,Al,Si) 12, also known as
the mineral kimzeyite. The phase relationships were confirmed
in laboratory experiments. CaSO as well as the particle depos-
O
2 3
2
and TiO . The hot corrosion scenario observed at the
3
and the gar-
3
2
3
O
4
its prove to be effective solvents for YSZ introducing distinct
solid-state reactions. The results support the idea of a dual YSZ
hot corrosion process. A first stage controlled by a SiO -free
2
4 3
Ca-source, most likely primary CaSO produces a thin CaZrO
layer. A second, CMAS-type stage providing high concentra-
tions of Fe O , TiO , and SiO favors the formation of kimzey-
2
3
2
2
II. Experimental Procedure
ite. The melting temperature of kimzeyite presumably defines a
thermal operation limit for YSZ-based TBCs.
The first stage HPT blade was provided by Lufthansa
Technik AG, Power Plant Services-Engine Parts Repair
(
Hamburg, Germany). The accumulated in-service time was
I. Introduction
17 000 h which is equivalent to approximately 3000 cycles of
the engine in different operating regions. Similar HPT blades
are employed for example in Pratt & Whitney 4000-type
aero-engines. The HPT blade comprises a γ/γ′ PWA 1484
Ni-base alloy, a two-phase γ-Ni/ß-NiAl low pressure plasma
sprayed MCrAlY bond coat with Hf, Y, and Si added and a
HE state-of-the art material for thermal barrier coatings
(TBC) of turbine engine airfoils is tetragonal Y O
2 3
partially stabilized ZrO (YSZ). YSZ TBCs are usually fabri-
2
cated by electron-beam physical vapor deposition (EB-PVD).
EB-PVD produces a characteristic columnar and highly
porous microstructure, showing beneficial strain-tolerance
T
1
1
7
wt% (4 mol%) EB-PVD YSZ top coat.
The microstructure of the blade’s TBC system was sur-
1
and a favorable low thermal conductivity. Ingestion of
veyed in a scanning electron microscope (SEM) (Ultra 55;
Carl Zeiss NTS, Wetzlar, Germany) equipped with an Inca
energy-dispersive spectroscopy (EDS) system (Oxford Instru-
ments, Abingdon, UK). Cross-section specimens from
selected reactive interfaces were prepared for analytical trans-
mission electron microscopy (TEM) via focused-ion-beam
(FIB) sectioning through the lift-out technique, employing a
single source FIB (Strata 205; FEI Inc., Eindhoven, The
Netherlands). A 300 kV Tecnai F30 transmission electron
microscope (FEI Inc.) with a field emission gun was utilized
for analysis of FIB lamellae. Besides conventional TEM
work, dark-field imaging in scanning transmission (STEM)
mode employing a high-angle annular dark field (HAADF)
detector was used for Z-contrast imaging in conjunction with
energy-dispersive nanospectroscopy (EDS), selected area dif-
fraction (SAD), and convergent-beam electron diffraction
(CBED) for nanosized particles.
airborne mineral dust during in-flight operation defines a
serious issue for aero-engine performance and service life
which has stimulated considerable research effort in industry
2
–6
and academia.
Upon heating in the combustor section
molten or partially molten mineral particles impinge on hot
surfaces. Besides possible reductions in air flow via clogging
of turbine cooling air passages the covering and infiltration
7
of EB-PVD YSZ TBCs of turbine blades by molten inor-
ganic particles is considered a major threat to the lifetime of
turbine engines. The spallation of the melt-infiltrated TBC
upon thermal cycling has been interpreted as a cold shock
effect and is generally recognized as a key failure mechanism
of YSZ-based TBCs in the 100 to some 1000 h lifetime
8
range.
Synthetic inorganic materials have been successfully
employed in model infiltration experiments to approximate
the particle-induced TBC recession. Most prominent are
9
Starting materials for powder syntheses include Al
NO ) ·9H O, Ca(NO ) ·4H O Mg(NO ) ·6H O Fe(NO ) ·
(
3 3 2 3 2 2 3 2 2 3 3
9
TiO
H
2
O, ZrO(NO
3
)
2
·xH
2
O, (NH
4
)
2
SO
4
, nanosized SiO
(Merck, Darmstadt, Germany). A 4 mol% Y O -stabi-
2
, and
2
2 3
D. J. Green—contributing editor
lized ZrO2 (TZ-4Y; Tosoh Chemicals, Tokyo, Japan) was
used as reference powder. X-ray diffraction (XRD) phase
analyses were performed in a D 5000 diffractometer utilizing
CuKa radiation with secondary graphite monochromator
(Bruker Analytical X-Ray Systems, Karlsruhe, Germany).
Manuscript No. 29492. Received April 06, 2011; approved June 15, 2011.
Author to whom correspondence should be addressed. e-mail: peter.mechnich@
dlr.de.
†
4483

4484
Journal of the American Ceramic Society—Braue and Mechnich
Vol. 94, No. 12
(
a)
4
Fig. 2. Completely CaSO -infiltrated EB-PVD YSZ top coat (SE
image, trailing edge). Inset is shown in Fig. 4 at higher magnifi-
cation.
(
b)
Fig. 3. (a) Anhydrite CaSO
section adjacent to the TGO layer (STEM HAADF image, leading
4
pore fillings from lower coating
Fig. 1. The in-service HPT blade investigated (SE images). (a)
Rough pressure surface holding numerous deposits at the trailing
and fewer at the leading edge. (b) Smooth suction surface segment
devoid of macroscopic deposits.
edge). (b) SAD pattern taken from larger CaSO
4
pocket in (a).
III. Results
(
1) Microstructural Analysis of the In-Service HPT Blade
In general, the investigated turbine blade does not show criti-
cally damaged TBC areas. As depicted in Fig. 1(a) surface
roughness of the pressure surface of the blade is high close to
the trailing edge. Particle deposits are most abundant in this
area and on the leading edge as well. The suction surface is
tarnished [Fig. 1(b)], but appears macroscopically smoother
holding, with few deposits only.
4
In this particular turbine blade CaSO is a primary con-
stituent at both, the pressure, and the suction surfaces as well
as the leading edge. CaSO infiltrates the intercolumnar pores
4
of the coating and most of the thin feather arms through the
full coating thickness (Fig. 2, thin section at the leading
edge). Figure 3(a) displays a STEM dark field image from
ꢀ
the YSZ/TGO interface. Figure 3(b) shows a [1 0 3] zone axis
pattern collected from the larger CaSO
emphasizing that CaSO is a fully crystalline anhydrite.
Indexing of the electron diffraction pattern is based on the
cell parameters given in Ref. (12). Small probe microanalysis
4
pocket in Fig. 3(a),
Fig. 4. Freestanding (unpolished) CaSO
marked location in Fig. 2 characterized by nanoscale columnar
growth.
4
microcrystals from
4
(
EDS) shows that anhydrite composition is stoichiometric
with neither dissolved impurities nor Y or Zr from the coat-
ing. The overall anhydrite growth morphology is character-
ized by intergrowth of nanoscale columns to form distinct
layers and platelet-type structures which have nucleated at
free surfaces inside the pore channels (Fig. 4)
derived via EDS area microanalysis and is compared in
Table I with frequently referred CMAS literature data. The
Al O concentration in the present deposit is about the same
2 3
and the MgO concentration is higher. In contrast to the
model CMAS, the present deposit contains significant
9
Figure 5 shows a SEM image of the top coat at the pres-
sure surface close to the blade’s trailing edge. The particle
deposit appears granular and exhibits a patchy porosity. The
average chemical bulk composition of the deposit was
amounts of Fe
deposit composition is now referred to as Fe-Ti-CMAS
(“FTCMAS”). Between the CaSO -filled YSZ columns and
the FTCMAS deposit, a pronounced reactive interface with
2 3 2
O and TiO . To recognize this fact, the
4

December 2011
Recession of an EB-PVD YSZ Coated Turbine Blade
4485
that this phase contains considerable amounts of ZrO
Fe , TiO , SiO , and Al
A detailed examination of the phase relationships at the
2
, CaO,
2
O
3
2
2
2 3
O .
reactive interface was achieved through a combination of
electron diffraction and EDS microanalysis. A STEM dark
field image of a reactive interface is displayed in Fig. 7(a).
The blunted YSZ column tips are covered by the continuous
CaZrO layer. A SAD pattern of CaZrO oriented parallel to
3
3
ꢀ
[012] is shown in Fig. 7(c), with indexing according to ICDD
PDF-file no. 35-0790. Only Fe traces are detected in CaZrO
3
,
while small amounts of Fe, Ti, and Ca are found close to
YSZ columns. The marked change from the facetted YSZ tip
morphology in the as-coated condition to blunted tip
morphology in the corroded state obviously is due to partial
zirconia dissolution. The YSZ tips were co-stabilized by CaO
2 3
and Y O which changed for the standard YSZ composition
when analyzed further away from the interface. The phase in
between CaZrO3 and the FTCMAS deposit is unambigu-
ously identified as the mineral kimzeyite with the empirical
Fig. 5. FTCMAS deposited on YSZ top coat fully infiltrated with
CaSO (SE image, pressure surface close to trailing edge). Boxed
4
region is displayed at higher magnification in Fig. 6.
formula Ca
3-0130]. Small garnet microcrystals from the reactive
interface are conveniently investigated in CBED mode, a
3 2 3
(Zr,Ti,Fe,) (Al,Fe,Si) O12 [ICDD PDF-file no.
1
Table I. Chemical Compositions of CMAS-Type Materials
ꢀ
[
011] zone axis composite diffraction pattern with the zero
Laue zone (ZOLZ) pattern superimposed on the first- und
second order Laue (FOLZ, SOLZ) zone ring structure is dis-
played in Fig. 7(b). Note the anhydrite phase sealing the
Deposit on
HPT-Airfoil
(this work)
Model
Model
Fe,Ti-CMAS
(this work)
9
[
mol%]
CMAS
3
pore channel just below the CaZrO /kimzeyite layer.
CaO
MgO
35.3
9.6
6.9
48.2
39.3
16.1
6.5
24.4
2.5
7.0
0.7
2.6
0.4
41.0
16.8
6.8
25.5
2.6
Despite minor morphological differences the major loca-
tions of the turbine blade share the same phase assemblage
at the FTCMAS/YSZ interface. Figure 8 shows an example
of an FTCMAS/YSZ reaction zone from the leading edge. In
this case, CaZrO and kimzeyite-type garnet form an inter-
3
penetrating interface without dispersion of Ca-FSZ particles.
Al
SiO
TiO2
Fe
NiO
SO
P2O5
ZrO
2
O
3
2
2
O
3
7.3
3
(2) Examination of Phase Equilibria by Annealing of
Synthetic Powder Mixtures
2
0.5
The complex microstructure of the in-service TBC raises the
question whether or not the microscopically observed phase
relationships can be accessed in a simplified laboratory
approach. Basically, three major reactive interfaces can be
distinguished in the TBC:
1.
2.
3.
Anhydrite is found as through-thickness inter- and
intra-columnar pore filling of EB-PVD YSZ TBC.
3
A zone consisting of CaZrO and cubic FSZ exists
close to the YSZ columns tips.
A kimzeyite layer separates the CaZrO
from the FTCMAS deposit.
3
/FSZ zone
It is anticipated that this sequence reflects the availability
of reactive species as well as the temperature gradient across
the TBC system during service. To examine these phase rela-
tionships, annealing experiments using powder mixtures were
carried out.
(
A) The CaSO /4 mol% Y O -Stabilized ZrO Reaction
4 2 3 2
Fig. 6. FTCMAS/YSZ reactive interface (close-up from boxed
region in Fig. 5) including garnet (kimzeyite) layer followed by
CaZrO -layer with some dispersed Ca-FSZ particles. Note blunting
3
Couple: A single-phase anhydrite powder (CaSO ) was pre-
pared by co-precipitation of aqueous solutions of Ca
4
of YSZ column tips (BSE image).
(NO
precipitates at 1000°C. To study the phase stability, a 50/
0 wt% powder mixture of CaSO and tetragonal 4 mol%
3 2 2 4 2 4
) ·4H O and (NH ) SO followed by annealing of the
5
4
an average thickness of approximately 5 lm is clearly visible.
Figure 6 depicts a close-up of the marked area in Fig. 5.
Blunted YSZ column tips are covered by a continuous Ca-
YSZ (t-YSZ) was annealed at 1100°C, 1150°C and 1200°C
for 1 h, respectively. The results of the XRD analyses are
given in Fig. 9. After annealing at 1100°C, the powder mix-
ZrO
3
layer. On the right hand part of the image, the CaZrO
layer additionally exhibits dispersed Ca-stabilized, cubic
3
ture still consists of the starting materials CaSO and t-YSZ.
After annealing at 1150°C, additional XRD reflections appear
4
ZrO2 grains (fully stabilized ZrO , FSZ). The CaO concen-
2
e.g., at approximately 22°, 45°, and 57° 2h, respectively. The
same effect is observed to a much larger extent upon anneal-
ing at 1200°C. The newly formed phase is identified unambig-
3
uously as being CaZrO . A second, significant effect is
observed for t-YSZ. The tetragonal crystal structure is still
tration of the Ca-FSZ grains was of the order of 12 wt%,
some grains were co-stabilized with an additional 1.5 wt%
Y O . Between the CaZrO layer and the FTCMAS, a sec-
2
3
3
ond newly formed phase is stabilized. EDS analysis reveals

4486
Journal of the American Ceramic Society—Braue and Mechnich
Vol. 94, No. 12
(
b)
(
a)
(
c)
Fig. 7. Validation of CaZrO
magnification overview of garnet/CaZrO
b) composite SAD/CBED pattern of garnet phase including the zero order Laue zone (ZOLZ) and the rings of the first and second order Laue
zone (FOLZ, SOLZ), respectively. (c) CaZrO SAD pattern.
3
and Ca-Zr-garnet (kimzeyite) from the reactive FTCMAS/YSZ interface via electron diffraction. (a) Low
3
reaction layer (STEM, HAADF). Diffraction patterns in (b) and (c) collected from positions indicated.
(
3
valid at 1100°C as indicated by successful indexing in the
crystallographic space-group P4 /nmc (no. 137). As represen-
tative examples the XRD-reflection pairs (002)/(110) and
103)/(211) are highlighted in Fig. 9. However, at 1150° there
2
(
is a significant change of position, shape, and intensity of
these XRD peaks. After annealing at 1200°C, a discrimina-
tion of the tetragonal (002)/(110) and (103)/(211) reflections is
not possible, instead the reflections can be indexed as (200)
and (311) in the cubic space group Fm3 m. Therefore, fully
stabilized, cubic ZrO is now the prevalent YSZ polymorph.
2
Two mechanisms may be derived from this effect. First, the
formation of CaZrO
enrichment of stabilizing Y
3
at the expense of ZrO
O
3
2
may lead to an
in the residual YSZ. Second,
2
excess Ca not consumed during the formation of CaZrO3
may diffuse into YSZ providing stabilization of the cubic
phase. From the annealing experiments it can be concluded
Fig. 8. FTCMAS/YSZ reaction zone from the leading edge
displaying the CaZrO /kimzeyite double layer (BSE image).
that the onset temperature for the formation of CaZrO
fully stabilized, cubic ZrO2 at the expense of t-ZrO2 and
CaSO is approximately 1150°C.
B) The CaZrO /FTCMAS Reaction Couple: To
3
and
3
4
(
3
access the FTCMAS-deposit retrieved from the turbine
blade, an FTCMAS powder was synthesized by thermal
decomposition of metal nitrates along with nanosized SiO
and TiO . For this purpose the original blade FTCMAS
composition was normalized to the Fe O –TiO –CaO–MgO–
2
2
2
3
2
Al O –SiO -system (Table I). Upon annealing for 1 h at
2
3
2
1
250°C, synthetic FTCMAS consists of a melilite-type phase,
a spinel-type ferrite, merwinite, and a perowskite-type cal-
cium-titanate (XRD analysis in Fig. 10). Melilite refers to
tetragonal solid solutions between the end members gehlenite
(
3 2 7 2 2 7
Ca Al SiO ) and akermanite (Ca MgSi O ). The SEM
image [Fig. 11(a)] shows a polished cross section of synthetic
FTCMAS after annealing at 1250°C. All phases exhibit
characteristic microstructural features. Melilite forms the
dominant phase exhibiting only weak grain orientation con-
trasts. Quantitative EDX-analysis yields a molar chemical
composition of 44% CaO, 10% MgO, 10% Al O , 33%
2
3
2 2 3 2 3 2 3
SiO , and 3% Fe O . Assuming Fe O substituting Al O in
the gehlenite component, the present melilite can be assigned
to a nearly 1:1 solid solution of gehlenite and akermanite.
Spinel-type ferrite Mg(Al,Fe) O forms high-relief prismatic
2 4
grains. Quantitative EDX-analysis of this phase confirms an
Fig. 9. XRD profiles of
stabilized ZrO reaction pair upon a 1 h annealing at 1100°C, 1150°C,
and 1200°C indicating a threshold temperature of 1150°C for CaZrO
formation. Symbols include CaSO (open circles), t-ZrO (open
diamonds), c-ZrO (full squares), and CaZrO (full triangles).
4 2 3
a 50:50 wt% CaSO /4 mol% Y O
2
3
4
2
2
3

December 2011
Recession of an EB-PVD YSZ Coated Turbine Blade
4487
Fig. 10. XRD profile of synthetic FTCMAS batch upon annealing
at 1250°C/1 h revealing a four-phase assemblage including gehlenite-
Fig. 12. XRD profile of a 50:50 wt% CaZrO /FTCMAS powder
3
mixture upon an 1 h annealing at 1150°C, 1200°C, and 1250°C,
respectively, revealing a substantial amount of newly formed
kimzeyite at 1250°C. Due to excess CaZrO3 in the starting mixture
CaZrO3 still prevails upon kimzeyite formation. Symbols include
rich melilite solid solution (ss) Ca
spinel ss Mg(Fe,Al) (open hexagons), merwinite Ca
crosses), and perowskite CaTiO (rectangles), respectively.
2
(Mg,Al)(Si,Al)SiO
7
(stars), Fe-rich
2
O
4
3
MgSi
O
2 8
(
3
kimzeyite (open crosses), CaZrO
3
2 4
(full triangles), Mg(Fe,Al) O spinel
(open hexagons), melilite (stars), merwinite (crosses), perowskite
CaTiO
3
(rectangles), and bredigite Ca14Mg
2
Si
8
O32 (full rhomboids).
(
a)
A
single phase CaZrO
3
powder was prepared by
co-decomposition of Ca(NO ) ·4H O and ZrO(NO ) ·xH O
3
2
2
3 2
2
followed by annealing at 1200°C. Consistent with the
CaSO /t-YSZ annealing experiments, a 50/50 wt% powder
4
mixture of CaZrO3 and synthetic FTCMAS was annealed.
Figure 12 shows the XRD analyses after 1 h annealing at
1
150°C, 1200°C and 1250°C, respectively. After the annealing
at 1150°C, the powder mixture still consists of the starting
materials, i.e., CaZrO and the crystalline FTCMAS phases.
At 1200°C there seems to be no significant reaction between
3
3
CaZrO and FTCMAS at first glance. However, the XRD
peak intensities of both merwinite and melilite reflections
decrease. After annealing at 1250°C new XRD peaks appear,
unambiguously belonging to the kimzeyite-type garnet phase.
A broad, probably multiple XRD reflection at approximately
3
digite Ca Mg Si O . From these annealing experiments it is
3° 2h may be associated with the orthorhombic phase bre-
1
4
2
8
32
concluded that kimzeyite forms at the expense of CaZrO
and FTCMAS as soon as the temperatures exceed 1200°C.
3
IV. Discussion
The YSZ TBC of the investigated in-service turbine blade
exhibits a characteristic corrosion scenario representing a
cumulative response to the specific engine operation range. In
4
the present case a CaSO pore filling and a FTCMAS-type
particle deposit are the two dominating impurity sources.
Along with these reactive species, a characteristic in-service
temperature gradient across the YSZ top coat is evident.
Anhydrite growth morphology is characterized by inter-
growth of nanoscale columns extending into a platelet-type
morphology (Fig. 4). The complete filling of even very small
pores, the grain morphology and the remarkable high chemi-
(
b)
Fig. 11. Matching (a) synthetic FTCMAS and (b) real FTCMAS
deposit from HPT blade (pressure surface) (SE images). Phase
assemblages consist of melilite solid solution (ss), Ca
Si,Al)SiO (stars), spinel ss Mg(Fe,Al) (hexagons) along with
MgSi (crosses). CaTiO is identified only in synthetic
2
(Mg,Al)
4
cal purity cannot reasonably be explained with CaSO crys-
(
7
2 4
O
tallized from a liquid phase. On the other hand, the striking
microstructural similarities with anhydrite built via magmatic
merwinite Ca
sample (a).
3
2
O
8
3
1
3
vapor condensation during volcanic eruptions suggest that
anhydrite deposition on the YSZ pore walls from a Ca- and
S-rich gas phase may be a likely formation mechanism during
iron-rich solid solution Mg(Fe0.83Al0.17
)
2
O
Ca MgSi O grains typically exhibit a low relief along with a
4
.
Merwinite
turbine operation. The thermal compatibility of CaSO
YSZ, however, was found to be limited to temperatures in
the 1100°C range. Above 1100°C the stabilization of CaZrO
4
and
3
2
8
3
textured surface. Perowskite-type CaTiO crystals are small
and form isolated clusters. EDS analyses reveal that both
merwinite and perowskite crystals are essentially stoichiome-
tric. SEM images confirm the very good agreement between
the synthetic FTCMAS and the genuine FTCMAS retrieved
from the turbine blade [Fig. 11(b)].
3
and a co-existing FSZ phase was observed in lab-scale
annealing experiments. The microstructural features from the
blade’s reactive interfaces support the idea that at tempera-
tures above the 1100°C threshold a CaZrO /FSZ layer is
3

4488
Journal of the American Ceramic Society—Braue and Mechnich
Vol. 94, No. 12
formed at the expense of the YSZ column tips and CaSO
At lower temperatures CaSO
walls co-exist.
The second major source contributing to TBC recession is
related to the airborne mineral particles ingested by the tur-
bine and accumulated on the YSZ surface. It should be kept
in mind that deposit bulk-compositions are case-specific,
reflecting the engine operation range. In contrast to the four-
component CMAS model systems the present FTCMAS
4
.
2 3
expected from phase relationships in the ternary CaO–Y O –
1
6,19
4
pore filling and YSZ pore
ZrO
garnet was formed upon annealing of a FTCMAS/CaZrO
powder mixture at 1250°C. This confirms the thermodynamic
instability of CaZrO as soon as the corrosion environment
2
.
On the other hand, substantial kimzeyite-type
3
3
becomes Fe-, Ti, and Si-bearing. The microstructure of the
blade’s reactive interface as well as phase compatibilities
derived from the supplementary annealing experiments sup-
port the idea of a dual corrosion process:
deposits are crystalline and exhibit a much lower SiO
tent. The substantial Fe O and TiO concentrations in these
CMAS-type deposits emphasize the need to introduce an
expanded “FTCMAS” system (Table I). This extended chem-
2
con-
1
.
A first stage controlled by a Si-free Ca-source, most
likely primary gas-phase derived CaSO producing a
4
thin CaZrO layer on the top coat
3
2
3
2
2
.
A second stage with high Fe, Ti, and Si-activity unfa-
vorable for CaZrO thus stabilizing kimzeyite instead.
2 3 2
ical composition is plausible since both Fe O and TiO are
3
main constituents of the lithosphere, i.e., of rock-forming
minerals which are a major source of airborne inorganic par-
ticles. Whereas the congruent-type melting model CMAS
gives rise to ZrO2 solution/precipitation in the root section
It is anticipated that both corrosion mechanisms are
cumulative and may also proceed simultaneously. Under the
present HPT operation conditions, the FTCMAS deposit did
not provide substantial melt phase which is able to seal the
YSZ TBC. Hence even an extensive FTCMAS particle cover-
age of the turbine blade will not prevent gas-phase condensa-
tion of CaSO
investigation did not reveal YSZ/kimzeyite interfaces (i.e.,
without CaZrO interlayer), there is evidence that the Si-free
9
of the melt-infiltrated top coat, chemical reactions between
YSZ surface and the crystalline FTCMAS particle deposit
occur predominantly in solid state.
From the FTCMAS composition rather complex phase
4
in YSZ pore channels. However, since the
3
relationships may be expected at the FTCMAS/CaZrO reac-
tive interface. Instead, a single phase layer of kimzeyite is
formed. Kimzeyite, the only natural Ca-garnet containing Zr
as a major constituent, is a rare accessory mineral from
3
corrosion stage favoring CaZrO formation is primary.
3
In terms of the different threshold temperatures derived
for CaZrO and kimzeyite formation from our short-term
3
14
carbonatite rocks and has triggered interest as a potential
1
5
nuclear waste storage material. Although ternary Ca-Zr-
silicates may also occur in specific regions of turbine
sintering experiments, a turbine blade surface temperature in
the 1200°C range appears reasonable (Fig. 13) and is in
agreement with modeled surface temperatures of TBC-coated
1
1,16
blades,
it was not before this work that the significance
of the garnet phase has been fully recognized in the context
of turbine airfoil recession. Owing to its crystal chemistry,
kimzeyite represents an almost ideal sink at the FTCMAS/
20
3
turbine blades. Thus, the presence of a CaZrO /kimzeyite
double layer may be used as a gauge for estimating effective
in-service temperatures at the turbine blade’s YSZ TBC.
With respect to long-term TBC integrity, both corrosion
mechanisms exhibit beneficial features which may compen-
sate the disadvantageous destabilization of YSZ. At the hot
[
8]
[6] [4]
YSZ interface. In the general garnet structure X
3 2 3 12
Y Z O
3
+
3+
4+
the ZO
4
tetrahedra (Z = Al , Fe , Si ) are linked by Y-cat-
ion octahedra (Y = Zr , Y , Ti , Fe ), while the larger
divalent X-cations (X = Ca , Mg ) are located on the
dodecahedral X-site in the center of the island silicate
framework. As expressed by the so-called kimzeyite-type substi-
tution mechanism Zr (Y) + Me (Z) ↔ Si (Z) + Me (Y)
(Me = Al, Fe), a high Zr -level garnet requires cation
distribution over the octahedrally coordinated Y-site and the
2+ 3+ 4+ 2+
2
+
2+
3
coating surface, the thin CaZrO /kimzeyite double layer
provides an effective diffusion barrier for CMAS constituents.
Further mitigation of reactive species may be provided by
CaSO filling of the bulk coating porosity. The low mismatch
stresses introduced by similar coefficients of thermal expansion
4
of 7-YSZ (a ~ 11 ppm/K) and CaSO (a ~ 11 ppm/K)
17
4+
3+
4+
3+
3
+
4+
4
21
22
tetrahedral Z-site. A similar cation substitution mechanism
involves the Ti
are seemingly favorable for long-term thermocyclic
operation. On the other hand, TBC strain tolerance will
4+
sites. Therefore kimzeyite-type garnets can
23
comfortably accommodate chemical gradients across the
FTCMAS/YSZ interface. The molar chemical bulk composi-
tion of kimzeyite was determined using EDS microanalysis to
be approximately 40% CaO, 18% ZrO , 16% SiO , 10%
presumably be deteriorated by the thin CaZrO
4
surface layer and, in particular, the CaSO filling of bulk
3
/kimzeyite
2
2
Fe
2
O
3
, 9% TiO
3
, 4% Al
2
O
3
, and 2% MgO. Y
2
O
3
is detected,
but the concentration is too low for accurate EDS quanti-
fication. The re-calculation of the kimzeyite mineral formula
(
(
based on 12 oxygen atoms per formula unit) yields
12. Following
on the Z-site. In
Ca0.95Mg0.05
)
3
(Zr0.67Ti0.33
)
2
(Al0.18Fe0.45Si0.37
3+
3
) O
1
5
ref. ( ), Fe is positioned solely as Fe
addition to Zr and Mg, trace Y will most likely also enter the
X-position.
The blade results clearly indicate that the preferred forma-
tion of kimzeyite over CaZrO is controlled by the abun-
3
dance of Fe, Ti, and Si at the interface. Due to the limited
homogeneity range of the perowskite-type structure a single
CaZrO layer is solely found at pure CaSO /YSZ interfaces.
3 4
A supplementary series of annealing experiments was con-
ducted to highlight the interplay of FTCMAS with CaZrO3
3
producing the notable CaZrO /kimzeyite double layer which
is observed at most turbine blade surfaces holding FTCMAS
particles. The key argument for rationalizing this double
layer is related to the fact that even for Ca-and Zr-rich envi-
Fig. 13. Schematic of the reactive interfaces from HPT blade with
fully infiltrated YSZ TBC in the light of the (i) experimentally
derived formation temperatures for the interfacial compounds
3
ronments CaZrO is stable under zero or very low Si-activity
18
3
conditions only. We have shown that CaZrO easily forms
3
CaZrO and garnet phase (kimzeyite) and (ii) essential environmental
at the expense of CaSO4 and YSZ (Fig. 8) above 1100°C.
The Ca gradient at the reactive interface then gives rise to
constraints (low vs. high Si-activity) during hot-corrosion. Note that
CaSO4 may deposit simultaneously to FTCMAS particles once the
the observed sequence CaZrO
3
?Ca/Y-FSZ?YSZ, as
4
top coat infiltration by CaSO is completed.

December 2011
Recession of an EB-PVD YSZ Coated Turbine Blade
4489
YSZ intercolumnar porosity. It is difficult to rate these effects
from post-service fractographical analysis where also prepa-
ration-induced artifacts are not unlikely. Nonetheless, the
investigated airfoil emphasizes the moderate impact of
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Acknowledgments
CaO-MgO-Al
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O
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2
We thank C. W. Siry, O. Schubert and T. Fischer of Lufthansa Technik AG,
Power Plant Services-Engine Parts Repair, Hamburg, Germany for providing
the turbine blade.
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h