Journal
J. Am. Ceram. Soc., 94 [8] 2671–2679 (2011)
DOI: 10.1111/j.1551-2916.2011.04427.x
r 2011 The American Ceramic Society
The Vapor Deposition and Oxidation of Platinum- and Yttria-Stabilized
Zirconia Multilayers
Zhuo Yu,z Hengbei Zhao,w,z and Haydn N. G. Wadley
Department of Materials Sciences and Engineering, University of Virginia, Charlottesville, Virginia 22904
of the strain energy arises from the mismatch in expansion
coefficient of the TGO layer and the other components of the
system.7 The growth rate of the TGO layer therefore plays a
significant role in determining the thermal cyclic life of the coating system.6–8 The lowest oxide growth rates occur when a pure,
large grain size a-phase aluminum oxide is formed directly on
the bond coat surface with no intermediate metastable oxide
precursor phase.9
Yttria-stabilized zirconia containing 6–8 wt% Y2O3 (7YSZ)
for stabilization of the tetragonal phase is typically used as the
thermal barrier layer. 7YSZ is chemically inert with the combustion environment inside a gas turbine engine10 and is also
thermochemically stable with the a-phase of alumina that forms
on the bond coat.1 However, oxygen is easily transported
through 7YSZ coatings due to the high ionic diffusivity of
YSZ at the operational temperature and the presence of interconnected pores from the outer to inner surface of the coating.11
In pore-free single crystals of ZrO2–4.9%Y2O3, the measured
oxygen ion diffusion coefficient is about 1011 m2/s at 10001C.
At this temperature, Fox and Clyne12 predict an oxygen ion flux
of B2 104 mol (m2 s)1 can be transported through a
250-mm-thick 5YSZ coating. This transported by permeation
through its pores is estimated to be an order of magnitude
greater (B2 103 mol (m2 s)1). If such a combined flux
could be immediately converted to a-alumina, it is sufficient to
support an oxidation growth rate of 4100 mm/h.
Fortunately, real TGO thickening rates are much slower
because the kinetics of bond coat oxidation are controlled by
diffusion processes within the TGO layer.9 While the very early
stages of oxidation sometimes exhibit linear oxidation kinetics,
the oxide thickening quickly becomes parabolic with a rate
constant that increases exponentially upon temperature.7
Because TGO growth is thermally activated, the only role of
the ceramic layer in current TBC systems is to reduce the bond
coats surface temperature. This temperature then establishes
the oxygen and aluminum (and other reactive metal) mass transport rates through the oxide and thus the overall rate of the
oxidation reaction.13
In aero gas turbines where the cyclic thermal loading can be
severe, failure of the TBC system is driven by the TGO layer
residual stress-induced buckling or edge delamination.6,14–17
Failure under prolonged thermal cycling then occurs either
near the TGO/YSZ or at the TGO/bond coat interface.3,6
Recent experiments indicate that the failure mode and cyclic
life depend upon the bond coat composition and its surface
morphology,6,18 the structure and mechanical properties of the
ceramic coating,6 and the toughness of the interfaces.15 While
the high-temperature cyclic life clearly depends upon many
factors, experiments and simulations indicate that the rate of
TGO formation plays a critical role in establishing the thermal
cyclic life.7,8,19,20 Reducing the oxidation rate of a bond
coat surface is therefore likely to have beneficial consequences
for the TBC system durability when thermomechanical failure
modes dominate.
One approach to slow the oxidation rate is to reduce the bond
coat surface temperature by development of new ceramic layer
compositions with lower intrinsic thermal conductivities.1,21,22
However, these efforts have been complicated by the introduc-
Attempts to increase the gas temperature within gas turbine engines are driving the development of thermal barrier coatings that
reduce superalloy component oxidation rates by disrupting thermal
transport processes. Novel metal–ceramic multilayer’s combining
thin metal layers with low thermal conductivity oxide ceramics
offer a potential approach for impeding both the radiative and
conductive transport of heat to a component surface. A gas jetassisted vapor deposition technique has been modified and used to
experimentally explore the deposition model thermal protection
system consisting of platinum- and yttria-stabilized zirconia (YSZ)
multilayers. Coatings containing one, three and four platinum layers in 7YSZ have been deposited on NiCoCrAlY bond-coated
Hastelloy X substrates and compared with conventional 7YSZ
monolayers deposited on the same bond-coated substrates. The
multilayer samples have been thermally cycled to 11001C and
found to be less susceptible to delamination failure than the conventional coatings. Their bond coat oxidation rate at 11001C was
also measured and discovered to decrease as the number of platinum layers was increased. The observations are consistent with a
retarded inward diffusion of oxygen by the platinum layers.
I. Introduction
T
HE remarkable increase in efficiency of gas turbine engines
over the last 60 years has been achieved in significant measure by elevation of the engine gas-operating temperature. This
was enabled by (i) the development of superalloys that were increasingly resistant to creep, hot corrosion and oxidation, (ii) the
invention of novel blade cooling techniques integrated into
single crystal airfoil fabrication processes, and (iii) the emergence of oxidation and hot corrosion-resistant metallic bond
coats and thermal barrier coatings (TBCs) that reduced their
temperature.1–3 Improved TBC systems for hot section air foils
are of critical importance as the turbine inlet gas temperature
continue to rise.2,4
Conventional TBC systems consist of a trilayer made up of a
100–200-mm-thick low thermal conductivity ceramic outer layer
(the top coat), a 10–20-mm-thick, aluminum-rich metallic
bond coat applied directly to the superalloy air foil, and a thermally grown oxide (TGO) layer at the interface between the
bond coat and the outer ceramic layer.5 During high-temperature service, aluminum, chromium, and yttrium in the bond coat
react with ambient oxygen to form this TGO layer. Delamination of the coatings during cyclic thermal exposure occurs when
the TGO layer reaches a critical thickness of about 5 mm whereupon residual strain energies are able to drive delamination
fracture at the lowest toughness interface.6 The primary source
N. Padture—contributing editor
Manuscript No. 28488. Received August 26, 2010; approved January 3, 2011.
This work was supported by Office of Naval Research (Dr. David Shifler, program
manager) through ONR Contract No. N00014-03-1-0297.
w
Author to whom correspondence should be addressed. e-mail: [email protected]
z
Joint First Authors.
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Journal of the American Ceramic Society—Yu et al.
II. Multilayer Coating Deposition
Fig. 1. An example of a notional multilayer coating in which refractory
metal layers are distributed within the ceramic layer.
tion of new failure modes. For example, some of the most
promising materials systems are thermochemically incompatible
with the TGO phase requiring the use of intervening diffusion barriers.22,23 Continued increases in the inlet gas
temperature also increase the role of radiative rather than conductive thermal transport processes because the oxide ceramic
layer is often semitransparent at infrared wavelengths.24 This is
leading to interest in optically opaque TBC materials. However,
efforts to create these novel TBC systems are constrained by
the significant limitations of current coating application
processes.25,26
The other approach is a conceptual design of multilayered
TBC. A conceptual multilayered TBC system, shown in Fig. 1,
offers promise for addressing several failure modes caused
by ever increasing temperature, intense thermal cycling, impact
by ingested particles of various diameters, and attack by
calcium–magnesium–alumina–silicate (CMAS). The system
consists of periodically stacked ceramic/metal layers. If multisource vapor deposition tools are used, it could be synthesized
from a variety of ceramics and metals with each layer designed
for some specific combination of functionalities. For example,
the metal layers might allow reflection of radiatively transported
heat, or act as a barrier to the inward diffusion of oxygen27 or to
inhibit infiltration of CMAS, or to provide toughening to reduce
the effects of foreign object impacts. The chemistry of each
ceramic layer might also be varied. For example, the layer next
to the primary bond coat (where TGO formation occurs) could
be based upon 7YSZ, which exhibits good thermochemical capability with alumina. At this location, its operating temperature could be maintained below that where deleterious phase
changes occur.28 The other ceramic layers could then be selected
for greater high-temperature phase stability, lower thermal conductivity, higher impact toughness, or for CMAS resistance
consistent with thermochemical compatibility with their bonding metallic layers.
The metal layer of a multilayer system could be a platinum
group metal (PGM), because they all have good oxidation resistance (though all their oxide species are volatile).29,30 To begin an
experimental investigation of such coating concepts, we have
selected elemental platinum as the metallic layer in a model
Pt/7YSZ multilayer structure. Platinum is thermochemically compatible with 7YSZ forming no reaction products at the temperatures of relevance.1,3 It has a high melting point (17691C) and the
formation rate of its volatile oxide is low at the temperatures
of interest (the metal is reported to be removed at a rate of
6 104 mg (cm2 h)1 at 11001C in flowing air).29 Platinum
also has a very low oxygen diffusivity (3.7 1017 m2/s at
10001C) compared with that of 5YSZ (1011 m2/s at 10001C).12,31
Here, we describe a dual-source, electron beam-directed vapor deposition method and its use to synthesize novel multilayer
coatings that incorporate thin refractory metal layers in a 7YSZ
coating. We demonstrate the growth of TBC multilayers with
one, three, and four platinum interlayers, and show a reduction
in their TGO layer growth rate that appears to be consistent
with retarded oxygen diffusion by the metal layers.
A directed vapor deposition (DVD) technique has been adapted
for multilayer synthesis. The detailed DVD technique has been
described elsewhere.32,33 The multilayer coatings were deposited
by first evaporating some of the 7YSZ. The resulting ceramic
vapor plume was transported to a substrate located 15 cm above
the source using supersonic helium—10 mol% oxygen gas jets
created by gas expansion through the nozzle. The pressure upstream of the nozzle was 58 Pa, while that in the chamber was
maintained at 11 Pa (the oxygen partial pressure in the chamber
was, therefore, 1 Pa). The upstream to downstream pressure ratio influences the jet speed. For the pressure ratio of 5.3 used
here, the estimated gas jet speed was 1400 m/s at the annular
nozzle opening. The total gas flow rate through the annular
nozzle was 7 slm and resulted in a 7YSZ deposition rate of 7 mm/
min. The substrate temperature was maintained between 10001
and 10201C during ceramic layer deposition.
The metal layer was deposited at a temperature of 10501C.
The platinum vapor plume was entrained in a pure (nonoxygendoped) helium gas jet for transport to the substrate. The
gas pressures and flow rates were otherwise identical to those
used for the deposition of the 7YSZ layers. The platinum
deposition rate under these conditions was 1.5 mm/min. This
sequence was then repeated to create the desired multilayered
coating architecture.
The coatings were applied to commercial purity, 25.4-mm-diameter Hastelloy X coupons (provided by GE Aircraft Engines,
Cincinnati, OH). They had been overlay coated with a 100–
200-mm-thick Ni0.36Co0.18Cr0.16Al0.29Y0.005 bond coat. The bondcoated surfaces were polished using SiC grinding paper to a 800
grit finish followed by ultrasonic cleaning in acetone and then
methanol solutions. During deposition, a flat-plate heater was used
to heat the substrate from the backside. The substrates were first
preheated to 4501C for 1 h to clean their surface, and then heated
to 10001C for 7YSZ deposition at a deposition rate of 7 mm/min.
For the multilayer samples, platinum deposition required the removal of the residual oxygen from the deposition chamber. This
resulted in an approximately half-hour hold of the sample at
10001C while the deposition chamber was purged with helium.
The substrate temperature was then increased to 10501C for about
30 min while the platinum layer was deposited. The substrate temperature was then reduced to 10001C for deposition of the next
YSZ layer and the thermal sequence repeated. Table I summarizes
the thermal cycles experienced for each of the samples. The substrates were not rotated during deposition. After deposition, the
samples were cooled to ambient within the deposition chamber
under helium at a pressure of 3 Pa. A set of reference 7YSZ
monolayer coatings were also grown using the same thermal conditions used to grow the four metal layer sample. About a half of
the 7YSZ coating was first deposited at 10001C, the sample was
then held in vacuum at 10501C for 2 h followed by the deposition
of the remaining YSZ at 10001C.
Cyclic oxidation experiments were conducted to explore the
growth of the multilayer coatings. The cycle consisted of a
60-min isothermal holding period in air at 11001C followed by
forced air cooling for 10 min to the ambient temperature. The
samples were removed, cross-sectioned, polished, and examined
Table I. Processing Duration for the Growth of Multilayer
Coatings
Deposition duration (h)
Sample
YSZ layer
One metal layer
Three metal layer
Four metal layer
w
No Pt deposited.
Heating time
to 10001C (h)
7 YSZ at
10001C
Pt at
10501C
Helium
purging time
at 10001C (h)
2
2
2
2
0.5
0.5
0.5
0.5
2w
0.5
1.5
2
0
0.5
1.5
2
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Vapor Deposition and Oxidation of Pt/YSZ Multilayers
by scanning electron microscopy (SEM) after 50, 200, 380, and
800 cycles. The TGO thickness was measured in the polished
SEM images of as-deposited and thermally cycled samples.
III. Experimental Results
(1) Metal Deposition on Columnar Ceramic Structures
For the deposition temperature used here, 7YSZ forms a columnar microstructure with 1-mm-diameter columns and very
narrow intercolumnar pores. The surface of the coating has a
significant surface roughness and occasional wide, deeply penetrating intercolumnar voids. The surface roughness and pore
width of a columnar coating can be controlled by selection of
the upstream to downstream pressure ratio (which effects the
angular incidence distribution of the incident vapor and therefore the degree of shadowing that is responsible for both coating
morphology features).24,34,35 We selected deposition conditions
that reduce the intercolumn gap width in the 7YSZ layers. The
effectiveness of platinum layer sealing of these intercolumnar
gaps also depended upon the process conditions through their
effect upon the angular incidence distribution of the incident
flux.36 Incident fluxes with broad angle of incidence distributions promote gap coverage and the relatively high chamber
pressures used here enhanced this effect.37
To explore the feasibility of depositing a coating that bridged
the surface intercolumnar gaps, a 7YSZ/Pt/7YSZ trilayer structure was made using a platinum layer thickness of 7 mm and a
pressure ratio of 5.2. The metal layer coverage over the ceramic
coatings largest intercolumn gaps is shown in Fig. 2. The coating
cross sections indicate that the width of the vertical gaps determines the continuity of the metal layer. For the chamber pressure, jet flow and substrate temperature conditions used here,
intergrowth column gaps with widths Wg43 mm could not be
fully bridged by a 7-mm-thick metallic layer (Fig. 2(a)). When the
gap width was reduced to 2 mm (Fig 2(b)), a thin ‘‘funnel-like’’
void was present in the metallic layer. As the width of the
intercolumnar gap was decreased to 1.5 mm, the metallic layer
was able to fully cover the gap although a funnel-like top metal
surface was still created (Fig. 2(c)). Thinner voids were fully
covered with no funnel-like surface features (Fig. 2(d)). It is also
evident in Fig. 2 that new intercolumn gaps were only formed in
a second 7YSZ layer deposited upon the platinum when the first
layer gap coverage was incomplete (Wg41.5 mm).
A series of experiments were then conducted to identify
the deposition conditions that ensured intercolumnar gaps of
o1.5 mm. For a substrate temperature of 10001C, this constraint
could be achieved using a pressure ratio below 5.5. Under these
conditions, a large volume fraction of fine pores was formed in
the 7YSZ layer, while the fraction of intercolumn pores with
diameters 41.5 mm was limited.34
(2) As-Deposited Morphology
Multilayer TBC samples were made using the DVD conditions
identified in Section III above. Reference 7YSZ monolayer layer
samples containing no platinum layers were also made under the
same conditions to investigate the effect of the platinum layers
upon the oxidization rate. Because the oxidation rate is sensitive
to the bond coat thermal history during deposition of the coating, we deposited the 7YSZ samples using a similar thermal sequence to that used for the multilayers (Table I).
The cross-section morphologies of multilayer samples are
shown in Figs. 3(a) and 4(a). The total thickness of the multilayer coatings was about 120710 mm. The thickness of each
metallic layer was 3–7 mm, while that of ceramic layers was in
the range of 25–35 mm. Even though quite wide columnar gaps
were formed in the porous ceramic layers, they were almost
(b) Wg ~ 2μm
(a) Wg > 3μm
Pt
7YSZ
2μm
2μm
(c) Wg ~ 1.5μm
(d) Wg ~ 1μm
Growth
Column
Facets
2μm
2μm
Fig. 2. Micrographs showing platinum coverage of vertical gaps (intercolumn pores) of width Wg in the TBC. (a) Wg43 mm, as grown metallic layer
does not completely cover wide columnar gaps. Intercolumnar porosity is renucleated in the next deposited ceramic layer. (b) WgB2 mm, crack-like void
extends through the metallic layer. A columnar gap is again formed at this location in the next deposited ceramic layer. (c) WgB1.5 mm, funnel-like void
formed but does not fully penetrate metallic layer. A new columnar gap was nucleated at the metal depression. (d) Wgo1 mm, complete metal coverage
with no surface imperfection. No columnar gap was created at this location in the next ceramic layer.
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Journal of the American Ceramic Society—Yu et al.
(a) As deposited
Vol. 94, No. 8
(b) 50 cycles
7YSZ
Pt
7YSZ
TGO
Bond Coat
20μm
20μm
(d) 800 cycles
(c) 380 cycles
20μm
20μm
Fig. 3. Cross-section morphologies of thermally cycled one metal layer samples. (a) As-grown, (b) after 50, (c) after 380, and (d) after 800 thermal cycles
(from ambient to 11001C and back to ambient).
always fully bridged by a metal layer. Very occasionally, a much
wider column gap was randomly formed but even these were
eventually bridged by either the second or the third metallic
layer. Careful examination of the micrographs in Fig. 2 reveal
that the platinum layers have a much rougher interface at the
metal on ceramic (lower) interface than vice versa. This roughness at the lower interface had a short wavelength of about 1 mm
and corresponds to the facets formed on top of the growth
(b) 50 cycles
(a) As deposited
20μm
20μm
(d) 800 cycles
(c) 380 cycles
20μm
20μm
Fig. 4. Cross-section morphologies of thermally cycled three metal layer samples. (a) As-grown, (b) after 50, (c) 380, and (d) 800 thermal cycles.
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Vapor Deposition and Oxidation of Pt/YSZ Multilayers
columns of the first ceramic layer. The much higher adatom
mobility during deposition of the metal layer has eliminated this
short length scale roughness and only long wavelength variations in metal height are evident.
The TGO layer that formed on the bond coat during deposition of the single metal layer sample had a thickness of 0.3 mm.
This was a little thinner than that of YSZ sample (Figs. 5(a) and
(b)), because of the shorter holding period at 10501C (as residual
oxygen was pumped from the deposition chamber) (Table I).
The multilayer sample with three metal layers had a thicker
(0.4 mm) initial TGO layer (Fig. 5(c)), about equal in thickness
to that of the YSZ coating.
(a) 7YSZ top coat
7YSZ
TGO
β-NiAl phase
(3) Thermal Cycling
The single YSZ layer sample (with no platinum interlayer) had
almost completely spalled after 720 cycles. The delamination
crack had formed at the TGO/bond coat interface consistent
with prior observations in this system.6 However, none of the
multilayer samples showed spallation failure even after 800 thermal cycles. All the coatings exhibited significant ceramic layer
sintering, eventual platinum layer fracture, and slow TGO layer
growth (Figs. 3 and 4). Ceramic layer sintering during the thermal cycling widened the preexisting columnar gaps and as the
high-temperature exposure time increased, these gradually extended through the coating. At first these layers were bridged by
the metallic layers but the metal layers eventually failed in tension at the location of the large vertical voids in the ceramic
layer (Fig. 6). The failure of the platinum layers was sometimes
accompanied by shear fracture in the platinum layer near the
platinum/YSZ interface (Fig. 6(c)).
(a) Initial stage
γ-Ni phase
Bond coat
1 μm
(b) One metal layer
10 μm
(b) Middle stage
1 μm
(c) Three metal layers
10 μm
(c) Final stage
10 μm
1 μm
Fig. 5. Coating cross-sections of the TGO layer for three different as
deposited samples. (a) YSZ sample (no metal layer), (b) one metal layer
sample, and (c) three metal layer sample.
Fig. 6. Intercolumnar pore and metallic layer morphology during thermal cycling. (a) Initial (as-deposited) structure showing complete gap
coverage by the metal layer. (b) Mid-stage showing widened intercolumnar gap bridged by a metal layer. Note the local thinning of the metal
near the site of the bridge. (c) Eventual tensile fracture of metallic layer
with a shear delamination crack extending along the interface.
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Journal of the American Ceramic Society—Yu et al.
YSZ layer
kp = 0.015μm2 / hr
1 metal layer
kp = 0.009 μm2 / hr
Initial stage
YSZ
3 metal layers
kp = 0.008 μm2 / hr
4 metal layers
kp = 0.008 μm2 / hr
4 metal
3 metal
1 metal
layer
Vol. 94, No. 8
TGO/bond coat interface disappeared as the aluminum content
in this region of the alloy was consumed by formation of the
alumina layer. Figure 8 clearly shows the gradual thickening of
the b-depleted region as the thermal exposure was increased.
Y–Al oxide (yttrium aluminate) pegs in the TGO layer near the
interface between TGO and primary bond coat were also
observed as early as 50 cycles in the YSZ sample (Fig. 8(a)).
The one and three metal layer samples contained similar pegs
after 200 cycles. Figure 8 also shows high-magnification views of
the TGO region after 380 thermal cycles. A significant delamination crack at the TGO/BC interface of the YSZ coating
occurs after 720 thermal cycles. The voids that eventually lead to
coating spallation in YSZ coatings6 are not evident in the
samples with intermediate metal layers. The bond coat oxidation characteristics are clearly dependent on the architecture
of the multilayer coating as well as the cyclic thermal exposure
conditions.41,42
IV. Discussion
Fig. 7. The TGO layer thickness versus number of 1-h thermal cycles
for a YSZ TBC and three multilayer structures containing one, three,
and four metal layers. The error bars correspond to the standard deviation in the thickness measurements. The initial stage of thermal cycles
was amplified in the insertion.
(1) Platinum Oxidation
The equilibrium relationships between the PGM, M, and their
volatile oxides, MxOy can be express as:
Figure 7 shows the change in the bond coat TGO layer thickness during thermal cyclic oxidation. In order to analyze the statistical significance of the test results, over 100 TGO thickness
measurements were made on every sample and the mean and
standard deviation of the TGO thickness were determined. The
TGO thickness differences between the as-deposited samples arose
from the different times used to fabricate the coatings (Table I).
The data indicate that the initial growth rate of the TGO during
cyclic oxidation decreased as the number of metal layers was increased. However, even after B720 cycles, the YSZ layer sample
had a B5.1-mm-thick TGO layer, while that of the one, three, and
four metal layer samples were 3.9, 3.7, and 3.6 mm, respectively.
The TGO thickness data could be fitted to a parabolic oxide
thickening law of the form:
Platinum’s oxidation behavior has been carefully investigated.30,43 It loses mass during high-temperature oxidation due
to the formation of a volatile platinum oxide and no oxide scales
are observed. Experiments indicate that platinum has a linear oxidation rate that is dependent upon the local environment above
the oxidizing surface.30 Conditions that promote gas phase mixing
and removal of the oxide vapor lead to the fastest rate of oxidation. The oxidation rates reported by different groups exhibit wide
scatter due to the different experimental setups.29,44 The data of
Krier and Jaffee29 appears representative and they report a mass
loss rate of 6 104 mg (cm2 h)1 at 11001C in flowing air.
To verify the oxidation rate of platinum under our experimental conditions, a 40-mm-thick platinum foil was used for a
mass loss test at 11001C. The foil was placed in the thermal cycling furnace and subjected to 230 h of oxidation at 11001C. The
resulting mass loss rate, Dm 5 1.06 103 mg (cm2 h)1. The
corresponding thickness recession rate
ðx xo Þ2 ¼ kp t
where x and xo are the oxide layer thickness at time t and at the
completion of the deposition process and kp is a parabolic rate
constant. The YSZ layer sample had the highest parabolic rate
constant of 0.015 mm2/h. This was similar to the 0.014 mm2/h
value measured previously for samples coated at 10001C.6
The one, three, and four metal layer samples had smaller
parabolic rate constants of 0.009, 0.008, and 0.008 mm2/h,
respectively (Fig. 7).
Nickel- and cobalt-based bond coats containing high aluminum atomic fractions form metastable g and y alumina polymorphs during low temperature (8001–11001C) oxidation.20,38,39
These metastable phases eventually convert to the a phase as
the oxidation temperature increases above 11001C.40 The transient phases grow faster than a alumina,20 and so a potentially
contributing factor to the reduction in TGO growth rate in
the platinum-containing TBC is a transformation of the asdeposited metastable TGO to a-alumina during the deposition
process. However, the YSZ-coated samples were subjected to
the same deposition thermal cycle as the three metal layer samples. It therefore appears unlikely that differences in transient
oxide transformation could have accounted for the lower
oxidation rates of the multilayer samples.
In all samples, the growth of the TGO layer during thermal
cycling was accompanied by a change in the g-Ni and b-NiAl
phase volume fractions in the NiCoCrAlY bond coat adjacent
to the TGO layer (Fig. 5). As the high-temperature exposure
time increased, the aluminum-rich phase adjacent to the
y
xMðsÞ þ O2ðgÞ , Mx OyðgÞ
2
Dh_ ¼
Dm_
1:06 103 mg=cm2 h
¼
¼ 4:9 104 mm=h
r
21:45 103 mg=cm3
where r is the metal density. This result is a factor of 2 greater
than that reported by Krier and colleagues. The difference may
lie in the vertical design of the furnace and the use of a fan for air
ventilation in our experiment. This permitted a continuous airflow over the oxidizing surface, replenishment of oxygen at the
metal surface and removal of the oxide reaction product.
The initial thickness of the platinum layer and the kinetics of
its evaporative oxidation determine an upper bound lifetime for
a coating containing platinum layers. For a 7-mm-thick platinum
layer tested under the conditions used here, the predicted life
would be between 14 000 h when ideally oxidized from one side
of the platinum layer and 7000 h for double side oxidation. This
is comparable with that required of a TBC system applied to an
aircraft engine airfoil.44
(2) Oxygen Diffusion Kinetics
The TGO growth rate depends on diffusion of aluminum and
oxygen to and through the TGO. Platinum has a very low oxygen diffusion coefficient. Grain-boundary oxygen diffusion is
expected to be the dominating transport mechanism through the
platinum layer.12 Velho and Bartlett’s31 approach can be used to
estimate the inward rate of diffusion through a hermetic platinum layer over an experimental temperature range of 14351–
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Vapor Deposition and Oxidation of Pt/YSZ Multilayers
(d) YSZ layer, 400 cycles
(a) YSZ layer, 50 cycles
1 μm
1 μm
(e) 1 metal layer, 380 cycles
(b) 1 metal layer, 50 cycles
1 μm
1 μm
(f) 3 metal layers, 380 cycles
(c) 3 metal layers, 50 cycles
1 μm
1 μm
Fig. 8. Morphology of the TGO layers of samples with a YSZ layer (a and d), a single metal layer (b and e) and three metal layers sample (c and f) after
50 (a, b, and c) and 380 1-h thermal cycles (d, e, and f). Note the emergence of a delamination crack at the bond coat –TGO interface in the sample
containing no platinum layers (d).
15041C. Under steady-state conditions, the diffusion coefficient,
D, of oxygen in pure platinum can be expressed as:
78 000 25 000
D ¼ ð9:3 1:8Þ exp ðcm2 =sÞ
RT
The equilibrium solubility of oxygen in pure platinum, Cso, is
proportional to the square root of the oxygen partial pres1=2
sure, PO2 :
S
CO
12
¼ ð0:27 0:13Þ 10
1 17 000 34 000 1=2
PO2 ðmol=cm3 Þ
exp RT
To extrapolate to our test temperature of 11001C, we note
that the oxygen flux, J, permeating a platinum metal layer is
given by the approximation to Fick’s law:
J¼
S
DCO
h
where h is the platinum layer thickness. For ambient air, PO2 is
S
about 0.21. The Arrhenius expression for DCO
is given by31
S
DCO
1 95 000 59 000
¼ð2:4 0:7Þ 10 exp RT
pffiffiffiffiffiffiffiffiffi
1
0:21 ðmol ðcm sÞ Þ
12
The oxygen flux penetrating a platinum layer can therefore be
written as
J¼
S
DCO
1012
1 95 000 59 000
¼ ð2:4 0:7Þ exp RT
h
h
pffiffiffiffiffiffiffiffiffi
2
1
0:21 ðmol ðcm sÞ Þ
For our multilayer samples, a typical platinum layer thickness
is 7 mm and the oxidation temperature was 11001C. The calculated oxygen flux that can penetrate such a layer is then about
1.4 1016 mol (cm2 s)1. This is sufficient to support the for-
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Journal of the American Ceramic Society—Yu et al.
mation of about 108 mm of alumina per hour. It is clear that
alumina growth would be almost arrested under a 7 mm thick,
perfectly hermetic platinum layer.
The model assumes a uniform thickness, fully dense,
uncracked (hermetic) platinum layer. In reality, as deposited
platinum layers contain small isolated pores and regions that are
locally thinner that reduce the distance of oxygen diffusion in
the metal. Localized volatile oxide evaporation at regions of
highest oxygen permeation through the YSZ (at intersections
with intercolumn voids) can also lead to local thinning of the
metal layers. The metal layers are also eventually fractured by
shrinkage of the porous ceramic layer. Once cracks or locally
thin regions are created in the outer metallic layer, oxygen
quickly penetrates to the next metal layer through the vertical
intercolumn separations. Because the unbroken layers might still
remain protective, the multilayer structure is likely to gradually
lose its protective function as the layers progressively fail by
volatile oxidation or microfracture.
If the growth rate of the TGO is indicative of the rate at
which oxygen diffuses through the multilayer coating to the
bond coat, it is apparent from Fig. 7 that oxygen transport to
the TGO layer was significantly retarded by the platinum layers,
and the retardation appears to be greatest for samples containing the most metal layers. As the samples were thermally cycled
to 400 cycles and beyond, the metal layers began to locally thin
and to crack at the large intercolumnar gaps in the structure.
This coincides with an increased rate of oxidation consistent
with the presence of a thinner TGO layer in the multilayer. Even
so, after 800 thermal cycles the multilayer’s TGO thicknesses
were in the 3.6–3.9 mm range, while the 7YSZ samples spalled at
the TGO thickness of 5.1 mm.
When platinum’s evaporative oxidation is not life limiting,
other modes of coating failure become of interest. The thermal
cycling experiments conducted here indicate that even after 800
thermal cycles of heating to 11001C, no delamination failure of
the multilayer samples had occurred, whereas spallation in the
YSZ coating had happened after B720 cycles. This is consistent
with recent work of Zhao and colleagues who thermally cycled
EB-DVD 7YSZ coatings on the same substrate—bond coat system. They found delamination started at around 800 cycles.6
They experimentally confirmed that in a conventional TBC/
TGO bilayer system, 7YSZ coatings spall when the TGO layer
reached a critical thickness that was governed by the steadystate energy release rate Gss. The thermal cyclic lifetime could be
predicted from the thickness, residual strain, and Young’s modulus of both the TGO and TBC layers and the toughness of the
plane upon which spallation occurred. If it is assumed that for
the metal/ceramic multilayer samples synthesized here, the platinum layer(s) do not significantly contribute to strain energy
storage (they have very low strengths), the critical thickness of
the TGO layer in a 120-mm-thick 7YSZ multilayer coating with
an elastic modulus of 30 GPa is B5.7 mm. Extrapolation of the
TGO thickness–time relations for the metal multilayer data
(Fig. 7) to this thickness predicts a 1600 cycle life for a one
metal layer system and a lifetime of more than 1900 cycles for
multilayers with three or four metal layers.
V. Conclusions
Multilayer YSZ/Pt coatings have been made using a dual-source
EB-DVD approach. By switching the electron beam between the
metallic and ceramic source materials, YSZ/Pt multilayers with
one to four Pt layers were deposited on overlay-coated Hastelloy
X substrates. By utilizing high-pressure deposition conditions, it
has been possible to fully cover the intercolumnar pores in the
YSZ layers creating structures and therefore impede the subsequent transport of oxygen through the coating while these layers
remained uncracked. The presence of the metal layers was correlated with a reduction in the oxidation rate of the underlying
bond coat with potentially significant implications for the durability of thermal protection coating concepts.
Acknowledgments
We would like to thank Professors Tony Evans, Carlos Levi, and David Clarke
of the University of California, Santa Barbara, and Dr. Derek Hass of Directed
Vapor Deposition Technology’s International for helpful discussions. We are also
grateful to Dr. David Wortman of GE Aviation for kindly providing substrates.
References
1
C. G. Levi, ‘‘Emerging Materials and Processes for Thermal Barrier Systems,’’
Curr. Opin. Solid State Mater. Sci., 8 [1] 77–91 (2004).
2
National Research Council (U.S.), Committee on Coatings for High-Temperature Structural Materials: Trends and Opportunities. National Materials Advisory
Board, Washington, DC, 1996.
3
N. P. Padture, M. Gell, and E. H. Jordan, ‘‘Thermal Barrier Coatings for GasTurbine Engine Applications,’’ Science, 296 [5566] 280–4 (2002).
4
G. W. Goward, ‘‘Progress in Coatings for Gas Turbine Airfoils,’’ Surf. Coat.
Technol., 108–109 [1–3] 73–9 (1998).
5
B. J. Gill and R. C. Tucker, ‘‘Plasma Spray Coating Processes,’’ Mater. Sci.
Technol., 2, 207–13 (1986).
6
H. Zhao, Z. Yu, and H. N. G. Wadley, ‘‘The Influence of Coating Compliance
on the Delamination of Thermal Barrier Coatings,’’ Surf. Coat. Technol., 204 [15]
2432–41 (2010).
7
A. G. Evans, D. R. Mumm, J. W. Hutchinson, G. H. Meier, and F. S. Pettit,
‘‘Mechanisms Controlling the Durability of Thermal Barrier Coatings,’’ Progress
Mater. Sci., 46 [5] 505–53 (2001).
8
D. S. Balint and J. W. Hutchinson, ‘‘An Analytical Model of Rumpling in
Thermal Barrier Coatings,’’ J. Mech. Phys. Solids, 53 [4] 949–73 (2005).
9
A. H. Heuer, ‘‘Oxygen and Aluminum Diffusion in [Alpha]-Al2O3: How Much
do We really Understand,’’ J. Eur. Ceram. Soc., 28 [7] 1495–507 (2008).
10
J. W. Vogan, L. Hsu, and A. R. Stetson, ‘‘Thermal Barrier Coatings for
Thermal Insulation and Corrosion Resistance in Industrial Gas Turbine Engines,’’
Thin Solid Films, 84 [1] 75–87 (1981).
11
B. A. Pint, I. G. Wright, W. Y. Lee, Y. Zhang, K. PrüXner, and K. B. Alexander, ‘‘Substrate and Bond Coat Compositions: Factors Affecting Alumina
Scale Adhesion,’’ Mater. Sci. Eng. A, 245 [2] 201–11 (1998).
12
A. C. Fox and T. W. Clyne, ‘‘Oxygen Transport by Gas Permeation through
the Zirconia Layer in Plasma Sprayed Thermal Barrier Coatings,’’ Surf. Coat.
Technol., 184 [2–3] 311–21 (2004).
13
F. H. Stott and G. C. Wood, ‘‘Growth and Adhesion of Oxide Scales on
Al2O3-Forming Alloys and Coatings,’’ Mater. Sci. Eng., 87, 267–74 (1987).
14
S. R. Choi, J. W. Hutchinson, and A. G. Evans, ‘‘Delamination of Multilayer
Thermal Barrier Coatings,’’ Mech. Mater., 31 [7] 431–47 (1999).
15
V. K. Tolpygo, D. R. Clarke, and K. S. Murphy, ‘‘Oxidation-Induced Failure of
EB-PVD Thermal Barrier Coatings,’’ Surf. Coat. Technol., 146–147, 124–31 (2001).
16
R. A. Miller, ‘‘Oxidation-Based Model for Thermal Barrier Coating Life,’’
J. Am. Ceram. Soc., 67 [8] 517–21 (1984).
17
S. M. Meier, D. M. Nissley, K. D. Sheffler, and T. A. Cruse, ‘‘Thermal Barrier
Coating Life Prediction Model Development,’’ J. Eng. Gas Turbines Power, 114 [2]
258–63 (1992).
18
N. M. Yanar, G. H. Meier, and S. Pettit, ‘‘The Influence of Platinum on the
Failure of EBPVD YSZ TBCs on NiCoCrAlY Bond Coats,’’ Scripta Mater., 46 [4]
325–30 (2002).
19
V. Tolpygo, D. Clarke, and K. Murphy, ‘‘The Effect of Grit Blasting on the
Oxidation Behavior of a Platinum-Modified Nickel–Aluminide Coating,’’ Metall.
Mater. Trans. A, 32 [6] 1467–78 (2001).
20
V. K. Tolpygo and D. R. Clarke, ‘‘Microstructural Study of the Theta–Alpha
Transformation in Alumina Scales Formed on Nickel–Aluminides,’’ Mater. High
Temp., 17 [1] 59–70 (2000).
21
D. R. Clarke and S. R. Phillpot, ‘‘Thermal Barrier Coating Materials,’’ Mater.
Today, 8 [6] 22–9 (2005).
22
!H. Zhao, C. G. Levi, and H. N. G. Wadley, ‘‘Vapor Deposited Samarium
Zirconate Thermal Barrier Coatings,’’ Surf. Coat. Technol., 203 [20–21] 3157–67
(2009).
23
S. Krämer, J. Yang, and C. G. Levi, ‘‘Infiltration-Inhibiting Reaction of Gadolinium Zirconate Thermal Barrier Coatings with CMAS Melts,’’ J. Am. Ceram.
Soc., 91 [2] 576–83 (2008).
24
T. J. Lu, C. G. Levi, H. N. G. Wadley, and A. G. Evans, ‘‘Distributed Porosity
as a Control Parameter for Oxide Thermal Barriers Made by Physical Vapor
Deposition,’’ J. Am. Ceram. Soc., 84 [12] 2937–46 (2001).
25
R. A. Miller, ‘‘Current Status of Thermal Barrier Coatings – An Overview,’’
Surf. Coat. Technol., 30 [1] 1–11 (1987).
26
D. J. Wortman, B. A. Nagaraj, and E. C. Duderstadt, ‘‘Thermal Barrier
Coatings for Gas Turbine Use,’’ Mater. Sci. Eng. A, 120–121 [Part 2] 433–40
(1989).
27
G. Kim, W. Lee, J. A. Haynes, and T. R. Watkins, ‘‘Morphological Evolution
during the Early Stages of Aluminide Coating Growth on a Single-Crystal Nickel
Superalloy Surface,’’ Metall. Mater. Trans. A, 32 [3] 615–24 (2001).
28
O. Fabrichnaya and F. Aldinger, ‘‘Assessment of the Thermodynamic
Parameters in the System ZrO2–Y2O3–Al2O3,’’ Z. Metallkd., 95 [1] 27–39 (2004).
29
C. A. Krier and R. I. Jaffee, ‘‘Oxidation of the Platinum-Group Metals,’’
J. Less Common Metals, 5 [5] 411–31 (1963).
30
H. Jehn, ‘‘High Temperature Behaviour of Platinum Group Metals in Oxidizing Atmospheres,’’ J. Less Common Metals, 100, 321–39 (1984).
31
L. R. Velho and R. W. Bartlett, ‘‘Diffusivity and Solubility of Oxygen in
Platinum and Pt–Ni Alloys,’’ Metall. Trans., 3, 65–71 (1972).
32
H. Zhao, F. Yu, T. D. Bennett, and H. N. G. Wadley, ‘‘Morphology and
Thermal Conductivity of Yttria-Stabilized Zirconia Coatings,’’ Acta Mater., 54
[19] 5195–207 (2006).
August 2011
33
Vapor Deposition and Oxidation of Pt/YSZ Multilayers
Z. Yu, K. P. Dharmasena, D. D. Hass, and H. N. G. Wadley, ‘‘Vapor Deposition of Platinum Alloyed Nickel Aluminide Coatings,’’ Surf. Coat. Technol.,
201 [6] 2326–34 (2006).
34
Y. G. Yang, D. D. Hass, and H. N. G. Wadley, ‘‘Porosity Control in Zig-Zag
Vapor-Deposited Films,’’ Thin Solid Films, 471 [1–2] 1–11 (2005).
35
D. D. Hass, A. J. Slifka, and H. N. G. Wadley, ‘‘Low Thermal Conductivity
Vapor Deposited Zirconia Microstructures,’’ Acta Mater., 49 [6] 973–83 (2001).
36
Y. G. Yang, X. W. Zhou, R. A. Johnson, and H. N. G. Wadley, ‘‘Atomistic
Simulations of Deep Submicron Interconnect Metallization,’’ J. Vacuum Sci.
Technol. B: Microelectron. Nanometer Struct., 20 [2] 622–30 (2002).
37
D. D. Hass and H. N. G. Wadley, ‘‘Gas Jet Assisted Vapor Deposition of
Yttria Stabilized Zirconia,’’ J. Vacuum Sci. Technol. A: Vacuum, Surf. Films, 27 [2]
404–14 (2009).
38
K. S. Murphy, K. L. More, and M. J. Lance, ‘‘As-Deposited Mixed Zone in
Thermally Grown Oxide beneath a Thermal Barrier Coating,’’ Surf. Coat. Technol., 146–147, 152–61 (2001).
39
2679
H. Svensson, J. Angenete, and K. Stiller, ‘‘Microstructure of Oxide Scales on
Aluminide Diffusion Coatings after Short Time Oxidation at 10501C,’’ Surf. Coat.
Technol., 177–178, 152–7 (2004).
40
S. Laxman, B. Franke, B. W. Kempshall, Y. H. Sohn, L. A. Giannuzzi, and K.
S. Murphy, ‘‘Phase Transformations of Thermally Grown Oxide on (Ni,Pt)Al
Bondcoat During Electron Beam Physical Vapor Deposition and Subsequent
Oxidation,’’ Surf. Coat. Technol., 177–178, 121–30 (2004).
41
V. K. Tolpygo and D. R. Clarke, ‘‘Surface Rumpling of a (Ni, Pt)Al Bond
Coat Induced by Cyclic Oxidation,’’ Acta Mater., 48 [13] 3283–93 (2000).
42
P. Kofstad, High Temperature Corrosion. Elsevier Applied Science, London,
1988.
43
E. Raub and W. Plate, ‘‘About the Behavior of Noble Metals and their Alloys
with Oxygen at High Temperatures in Solid State,’’ Z. Metallkd., 48, 529–39
(1957).
44
W. A. Nelson and R. M. Orenstein, ‘‘TBC Experience in Land-Based Gas
Turbines,’’ J. Therm. Spray Technol., 6 [2] 176–80 (1997).
&