Impermeable Thin Al2O3 Overlay for TBC Protection From Sulfate and Vanadate Attack in Gas Turbines Quarterly Report Page: 3 of 9
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TABLE OF CONTENTS
1. Introduction
2. Executive summary
3. Experimental
4. Results and discussion
5. Plans for the next reporting period
6. Conclusion
7. References
LIST OF GRAPHICAL MATERIALS
Table 1 Materials properties in TBCs system used for FEM analysis.
Fig.1 TBCs finite element model. (a) two dimension and (b) three dimension.
Fig.2 Stress in X axis direction in the TBC system without A1203 overlay.
Fig.3 Stress in X axis direction in the TBC system with A1203 overlay
1. INTRODUCTION
Plasma sprayed thermal barrier coatings (TBCs) are widely used in gas turbine hot
section components such as burners, transition ducts, shrouds, blades and vans. The most
common TBC materials is Y203 (8wt%)-stabilized ZrO2 type (YSZ) which has been developed
over many years because of its high temperature stability, low thermal diffusivity and high
coefficient of thermal expansion (CTE) [1,2]. However, when exposed to acidic molten salt,
stabilizer yttria will be leached out from the zirconia solid solution, resulting in destabilization of
the zirconia from tetragonal to the monoclinic phase and destruction of the coating.
The major failure mechanism that causes TBC spallation in gas turbine is bond coat
oxidation and the growth of the thermally grown oxide (TGO), while hot corrosion of TBC will
dominate coating failure in diesel engines which are usually operated with low quality fuels
containing lots of impurities such as sulfur and vanadium [2].
Molten sodium salts of vanadium and sulfur oxides condense on to the TBCs at the
temperature of 600-1000 C [3, 4]. Although zirconia itself shows good resistance to the molten
sulfate or vanadate compounds arising from fuel impurities, yttria is leached out of the zirconia
by the reaction with V205 or NaVO3 to form YVO4, causing structural destabilization of ZrO2
(i.e., transformation of the zirconia from the tetragonal and/or cubic to monoclinic phase upon
cooling, which is accompanied by a large destructive volume change) [5-10]. Stresses resulting
from destabilization of the zirconia eventually cause the delamination and spalling of the coating.
Thus, extension of the benefits of TBCs to such impurity-containing environments
requires the development of hot corrosion resistant coating. Based on Lewis acid-base concept,
zirconias stabilized with india (In203) [11, 12], scandia (Sc203) [13] and ceria (CeO2) [8,14] as
well as Ta205 [6,15] and YTaO4 [15] have been evaluated for their hot corrosion resistance. On
the other hand, over the years there have been, and still continue to be, effects to close the
surface of zirconia TBCs by laser-glazing and arc lamp [16-18] or various "seal coats" [18-25] to
prevent penetration of molten deposits into the porous YSZ coating.
Alumina has a high melting point and stability without showing phase transition at high
temperature like the ZrO2 ceramics. A1203 has a small solubility particularly in molten salts and3
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Mao, Scott X. Impermeable Thin Al2O3 Overlay for TBC Protection From Sulfate and Vanadate Attack in Gas Turbines Quarterly Report, report, March 31, 2004; United States. (https://digital.library.unt.edu/ark:/67531/metadc785666/m1/3/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.