ICMCTF 2012! - CD-Lab Application Oriented Coating Development

Coatings for Use at High Temperature
Room: Sunrise - Session A2-2
Tuesday Afternoon, April 24, 2012
Thermal and Environmental Barrier Coatings
Moderator: R. Wellman, Cranfield University, UK, D.
Litton, Pratt & Whitney, US, R. Trice, Purdue University,
US
1:50pm A2-2-1 Process and Equipment for Advanced Thermal Barrier
Coating Systems, A. Feuerstein (albert_feuerstein@praxair.com), C.
Petorak, L. Li, T.A. Taylor, Praxair Surface Technologies, Inc., US
INVITED
Hot section components in aero and power generation engines utilize
advanced thermal barrier coating systems for life extension and better
efficiency. Thermally-sprayed ceramic / bondcoat systems are extensively
used for combustors and power generation blades and vanes whereas
EBPVD TBC on Pt modified diffusion aluminide coating is the coating of
choice for highly stressed airfoils in aero engines. New technologies such as
the suspension plasma spray process (SPS) are finding more and more
interest for applying TBC’s. In addition, challenges such as the trend to low
thermal conductivity and CMAS resistant coatings require new
compositions, and respective processing technology. The process and
coating characteristics of 7wt% YSZ based APS low density and dense
vertically cracked (DVC) Zircoat TBC as well as EBPVD coatings are
described, highlighting recent advances with ultra pure Zirconia for
improved sintering resistance. New coating compositions for low thermal
conductivity TBC’s and CMAS resistant TBC’s are also addressed. Lastly,
the properties of new coating processes such as SPS are compared with
conventional coating processes.
2:30pm A2-2-3 Calcium-Magnesium-Alumino-Silicate (CMAS)
degradation of EB-PVD thermal barrier coatings: solubility of different
oxides from ZrO2-Y2O3 and ZrO2-Nd2O3 systems in the molten
model CMAS, N. Chellah, M.H. Vidal-Sétif (Marie-Helene.Vidal-
Setif@onera.fr), Thermal and Environmental Barrier Coatings, France
Thermal barrier coatings (TBCs) are used to protect blades and vanes in the
hot sections of gas turbines. They consist of a series of layers: the Ni-based
superalloy substrate is coated with an alumina forming metallic bond coat
onto which a porous ceramic top coat of yttria partially stabilized zirconia
(YPSZ) is deposited. The TBC system allows higher gas temperatures,
resulting in enhanced engine efficiency and performance. However, in
service, engines ingest dust, sand and ash particles which melt on the hot
TBC surface at these high operating temperatures and form calciummagnesium-aluminosilicate
(CMAS) glass deposits. Previous investigations
on blades removed from service showed that the molten CMAS penetrates
into the open porosity of the top coat. Firstly, upon cooling, the molten
CMAS solidifies and the infiltrated TBC becomes rigid. Thus, delamination
cracks can develop in the coating leading to progressive TBC spallation
during in-service thermal cycling. Secondly, a chemical interaction can take
place between the molten CMAS and the TBC leading to the dissolution of
the YPSZ TBC in the molten CMAS.
This paper investigates the chemical degradation of an electron beam
physical vapor deposited YPSZ TBC by a synthetic model CMAS. The
chosen CMAS was the tridymite-pseudowollastonite-anorthite eutectic
composition in the ternary (CaO-Al2O3-SiO2) system, melting at 1170°C.
The model system has given very good replication of the CMAS corrosion
observed on ex-service blades in terms of thermochemical interaction: TBC
infiltration, TBC dissolution in the CMAS melt and formation of new
crystalline phases.
In order to understand the mechanisms of degradation by dissolution of the
YPSZ TBC in the molten CMAS, a method was used which was designed
initially to measure the solubility of oxides in a glass. First the solubility of
different compositions of the ZrO2-Y2O3 system in the glassy model
CMAS was studied at several temperatures, in order to access the
dissolution kinetics. The solubility limits and thermodynamic equilibrium
constants were determined and possible new crystalline phases were
identified. The results enabled to improve the understanding of the
mechanisms of the chemical degradation of the YPSZ TBC by the CMAS.
In a second part, solubility tests of different compositions of the ZrO2-
Nd2O3 system in the same model glass were performed. These allowed
selecting a new composition to substitute YPSZ for mitigating CMAS
attack. The selected new oxide was tested as a dense ceramic and actually it
showed restricted chemical degradation by the model CMAS.
2:50pm A2-2-4 Bond Coat Cavitation under CMAS-Infiltrated TBCs,
K. Wessels (kwessels@engineering.ucsb.edu), University of California,
Santa Barbara, US, D. Konitzer, GE Aviation, US, C.G. Levi, University of
California, Santa Barbara, US
Turbine airfoils in advanced aircraft turbine engines are protected from the
aggressive environment by thermal barrier coating systems (TBCs)
comprising an insulating oxide and a metallic bond coat. Environmental
contaminants ingested with the intake air form deposits generically known
as CMAS (calcium-magnesium alumino-silicates) on the protective
coatings. As the deposits melt during the engine cycle a silicate glass forms
that infiltrates the porous coating and crystallizes under the imposed
thermal gradient, stiffening the TBC and compromising its strain tolerance.
Delamination failures have been documented in the past. A new failure
mechanism that involves the formation of cavities within the bond coat
under regions of the TBC penetrated by CMAS has been identified recently.
Examination suggests that cavity formation occurs in regions subject to
lateral thermal gradients; channel cracking and scalloping of the TBC are
also observed above the bond coat cavities. Once the voids grow large
enough to compromise the bond coat, the TBC delaminates and eventually
spalls, leaving behind a thermally unprotected airfoil with a residual bond
coat. This presentation will discuss the characteristics of this failure mode,
and the possible underlying mechanisms.
3:10pm A2-2-5 Assessing the Delamination Behavior of CMAS
Infiltrated TBCs under a Thermal Gradient, R.W. Jackson
(rwesleyjackson@engineering.ucsb.edu), E. Zaleski, C.G. Levi, University
of California, Santa Barbara, US
With rising operating temperature, the prevalence of calcium magnesium
alumino-silicate (CMAS) deposits melting on the surface of thermal barrier
coatings (TBCs) used in gas turbines has increased. These molten CMAS
deposits infiltrate and crystallize within the pores of the structure, stiffening
the penetrated layer and leading to a loss of strain tolerance. The loss of
compliance promotes coating delamination when the strain energy
generated from the thermal expansion mismatch during thermal cycling
reaches a critical level. A laser thermal gradient test (LGT), in which the
thermal gradient and cooling rate can be controlled, was used to assess the
TBC durability by imposing a range of thermal stresses. Both 7YSZ and
gadolinium zirconate (GZO) TBCs, with and without CMAS deposits, were
subjected to the LGT. In the absence of CMAS, no microstructural
degradation was observed for either composition. When loaded with
CMAS, TBC degradation was found to increase with increased cooling rate,
and was generally higher for GZO than for 7YSZ, both materials processed
by EB-PVD. The CMAS penetration, phase evolution and crack
morphology of the thermally cycled TBCs have been characterized as a
function of the thermal history and will be analyzed in the context of current
delamination models.
This investigation was sponsored by the Office of Naval Research under
grant N00014-08-1-0522, monitored by Dr. David Shifler
3:30pm A2-2-6 CMAS infiltration of YSZ thermal barrier coatings
and potential protection measures, V. Kuchenreuther
(veronica.kuchenreuther@ict.fraunhofer.de), V. Kolarik, M. Juez Lorenzo,
Fraunhofer ICT, Germany, W. Stamm, Siemens Power Generation,
Germany, H. Fietzek, Fraunhofer ICT, Germany
Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) are widely
used to protect the components in the hot area of power generation turbines.
One identified cause of TBC failure is the degradation by molten deposits,
mostly calcium-magnesium-alumina-silicates (CMAS), which enter the
turbine from the environment. It infiltrates the pores and cracks, reacts with
the YSZ and leads to its destabilization. The main purpose of the current
research is to investigate to which extent the attack by molten CMAS can be
reduced by coating the TBC with alumina.
A model CMAS, composed of 38 mol% CaO, 6 mol% MgO, 5 mol%Al2O3,
50 mol% SiO2 and 1 mol% Fe2O3, ultra-milled, molten two times for 4 h at
1400°C and milled again, was deposited on the surface of a free standing
sample from a commercial APS TBC. The samples were exposed to 1100°C
and 1240°C for 50, 100 and 200 hours in air and were analyzed by X-ray
diffraction with micro-focus (µ-XRD) and by field emission SEM. Surface
scans by µ-XRD stepwise from the unaffected area to the CMAS infiltrated
surface area show at 1100°C considerable portions of the monoclinic phase
from the first exposure time of 50h. The micrographs however reveal only
superficial infiltration. At 1240°C again the phase decomposition is
detected already after 50 h and a deep infiltration is observed in the
micrographs, almost across the whole TBC.
39 Tuesday Afternoon, April 24, 2012