ICMCTF 2012! - CD-Lab Application Oriented Coating Development

Coatings for Use at High Temperature
Room: Sunrise - Session A2-1
Thermal and Environmental Barrier Coatings
Moderator: R. Wellman, Cranfield University, UK, D.
Litton, Pratt & Whitney, US, R. Trice, Purdue University,
US
Tuesday Morning, April 24, 2012
8:00am A2-1-1 Progress in Measuring and Understanding the
Delamination Toughness of Zirconia Coatings, E. Donohue
(erin_donohue@engineering.ucsb.edu), N. Philips, M. Begley, C.G. Levi,
University of California, Santa Barbara, US
Failure mechanisms in thermal barrier coatings (TBCs) often involve the
propagation of delamination cracks through the ceramic layer. Mode I
toughness measurements on air plasma-sprayed, dense, vertically cracked
(DVC) 8YSZ TBCs using a double cantilever beam (DCB) test revealed Rcurve
behavior and steady state toughness values of ΓIc ~ 320±30 J/m 2 . This
unexpectedly high value has motivated an analysis of the test itself and the
possible mechanisms responsible for the toughness elevation. Examination
of local displacement data along the entire length of the cantilevers,
including at the load point, reveals the location of the crack front. In
analyzing the experiment, the compliant foundation of the cantilever (due to
the presence of the TBC) and shear effects at the crack tip must be
incorporated. Finite element analysis of the DCB specimen includes a layer
with reduced stiffness between the beams that simulates the behavior of the
compliant foundation. The model produces results that are consistent with
those from experiment; thus it is possible to calculate an energy release rate
solely with measured parameters, known material properties, and the
displacement data. Additional experiments on a variety of air plasmasprayed
coatings show the evolution of the toughness and the possible
contributions of multiple toughening mechanisms, including ferroelastic
domain switching, crack bridging and pull-out.
8:20am A2-1-2 Monitoring Delamination of Thermal Barrier Coatings
by Combined Photoluminescence Piezospectroscopy Imaging and
Upconversion Luminescence Imaging Techniques, J.I. Eldridge
(jeffrey.i.eldridge@nasa.gov), NASA Glenn Research Center, US, B. Heeg,
Lumium, Netherlands
Previous work has demonstrated that delamination progression of thermal
barrier coatings (TBC) composed of yttria-stabilized zirconia (YSZ) can be
monitored by photoluminescence piezospectroscopy (PLPS) and more
recently by upconversion luminescence imaging of TBCs composed of YSZ
incorporating a thin base layer co-doped with erbium and ytterbium
(YSZ:Er,Yb). The recent development of imaging mode PLPS using a
tunable filter now allows the comparison of both techniques by direct
imaging of the same specimens. In this study, both PLPS imaging and
upconversion luminescence imaging were performed to monitor the
delamination progression of electron-beam physical vapor deposited (EB-
PVD) TBCs at different stages of interrupted furnace cycling to 1163 ° C.
In addition, the extent of mechanically induced delamination produced by
Rockwell indentation at selected stages of TBC cyclic life was evaluated by
both techniques. The TBC damage associated with the imaging results was
verified by post-imaging SEM inspection of the specimen cross-sections.
While each technique has its own strengths and weaknesses, it is shown that
the information provided by both techniques is complementary and provides
a better identification of the location/depth at which delamination cracks
occur than by either technique alone. The complementary nature of these
techniques can be attributed to their very different mechanisms for
achieving the contrast between different stages of delamination. In
particular, the delamination contrast for PLPS imaging relies on the
reduction in stress in the thermally grown oxide (TGO), while upconversion
luminescence imaging relies on the total internal reflection that occurs at
cracks within or below the YSZ:Er,Yb layer. Therefore, PLPS imaging is
more sensitive to damage to the TGO or to the TGO/bond coat interface,
whereas upconversion luminescence imaging is more sensitive to damage at
the TGO/TBC interface or above.
8:40am A2-1-3 The influence of transient thermal gradients and
substrate constraint on the delamination of thermal barrier coatings,
Hutchinson (jhutchin@fas.harvard.edu), School of Engineering and
Applied Sciences, Harvard University, US INVITED
The influence of steep thermal gradients combined with rapid hot surface
cooling on delamination of thermal barrier coatings is investigated.
Transient thermal gradients induce stress gradients through the coating and
substrate which, in turn, produce overall bending if the substrate is not very
thick and if it is not constrained. Substrate thickness and constraint are
important aspects of the mechanics of delamination due to transient thermal
loading of coating-substrate systems. These aspects must be considered
when laboratory tests are designed, and they must be considered for lifetime
assessment under in-service conditions.
9:20am A2-1-5 Raman Spectroscopy and Neutron scattering of
Ferroelastic Switching in Ceria Stabilized Zirconia, A. Bolon, M.
Gentleman (mgentleman@tamu.edu), Texas A&M University,US
Ferroelastic switching has been identified as a method for increasing
the toughness of thermal barrier materials. Here we present the
results of observations of ferroelastic switching by Raman
spectroscopy and neutron scatter as a function of time and temperature
to understand the effectiveness of the ferroelastic process in
toughening ceria stabilized zirconia. Results will demonstrate the
effect of temperature on domain motion as well as the resulting bulk
surfaces of a highly switched material.
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9:40am A2-1-6 Thermo-mechanical properties of lanthanide added
zirconia film deposited by EB PVD, Y.S. Oh (ysoh30@kicet.re.kr), K.H.
Kwak, H.T. Kim, S.W. Kim, S.M. Lee, Korea Institute of Ceramic
Engineering and Technology, Republic of Korea, B.K. Jang, National
Institute for Materials Science, Japan
For its excellent thermo-mechanical stability in high temperature, Electron
Beam (EB) PVD method has been developed to replace the conventional
plasma spray method to fabricate the thermal barrier coating of gas turbine.
However, industrial application is limited for its low cost efficiency and
higher thermal conductivity compared to plasma sprayed film despite the
excellent mechanical properties.
To lower the thermal conductivity, we have used the mixture of lanthanide
and zirconia as a film source. The pore distribution and column structures
were developed to sustain mechanical properties in high temperature
condition. Deposition rate was increased up to ~3 ㎛/min by control the EB
scanning condition of ingot during the evaporation. Thermal conductivity of
thick film, over 300㎛, was measured by laser flash method. Hardness and
adhesion of film was measured by Vickers hardness tester and tensile test.
The phase of lanthanide doped composite film was revealed as pyrochlore
from X-ray diffraction and also the thermal conductivity was calculated as
under 1.5 W/m·K. Adhesion force of composite film was found to 3 times
higher than YSZ film by conventional plasma spray coating.
10:00am A2-1-7 Effect of post heat treatment on thermal durability of
thermal barrier coatings in thermal fatigue tests, S. Myoung, H. Kim, M.
Kim, S. Lee, Y. Jung (jungyg@changwon.ac.kr), Changwon National
University, Republic of Korea, S. Jung, T. Woo, Sung Il Co., Ltd. (SIM),
Republic of Korea
The hot-section stationary components of gas turbine are protected by
thermal barrier coatings (TBCs), normally deposited by the air plasma spray
(APS) and electron bean physical vapor deposition (EB-PVD) processes.
The APS is more commercial method, because of less expense and lower
thermal conductivity than the EB-PVD, even though there are lots of
defects such as pores, microcracks, and unmelted powders. However, the
TBC prepared by the APS shows a less thermal stability due to the low
strain tolerance. Therefore, in this study, the effects of post heat treatment
and its sequence on the microstructural evolution and oxidation behavior at
the interface between the bond and top coats have been investigated in a
specially designed apparatus—one side of the sample is exposed by flame
and the other side air cooled. The TBC system with the thicknesses of 2000
and 200 mm in the top and bond coats, respectively, were prepared with the
APS system using 9MB gun using ZrO2–8wt% Y2O3 (METCO 204 C-NS)
for the top coat and Ni-based metallic powder (AMDRY 962) for the bond
coat. The post heating was performed in two ways — one is after the bond
coat deposition and the other after the top coat deposition. T he flame
thermal fatigue tests were performed at a surface temperature of 1100 °C
with a temperature difference of 800 °C between the surface and bottom of
sample, with a dwell time of 10 min. for 860 cycles (18000 EOH;
Equivalent Operating Hour ). The TBC after the post heat treatment is more
efficient in improving thermal durability than that without the treatment,
and the post heat treatment on the TBC after the top coat deposition show a
higher adhesive strength and a better thermal durability than that after the
bond coat deposition. Results indicate that the post heat treatment is to
propose the efficient process in improving lifetime performance of TBC at
high temperature environments. The influences of thermal fatigue condition
on the microstructural evolution and thermal durability of TBC are
discussed.
23 Tuesday Morning, April 24, 2012