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NON-DESTRUCTIVE MICROSTRUCTURAL EVALUATION OF YTTRIA STABILIZED ZIRCONIA, NICKEL ALUMINIDES AND THERMAL BARRIER COATINGS USING ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY
Title
NON-DESTRUCTIVE MICROSTRUCTURAL EVALUATION OF YTTRIA STABILIZED ZIRCONIA, NICKEL ALUMINIDES AND THERMAL BARRIER COATINGS USING ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY.
Creator
Vishweswaraiah, Srinivas, Sohn, Yongho, University of Central Florida
Abstract / Description
There has been an urge for increasing the efficiency in advanced gas turbine engines. To fulfill these needs the inlet gas temperatures should be increased in the gas turbine engines, thermal barrier coatings (TBCs) have gained significant applications in increasing the gas inlet temperatures. Insulating characteristics of ceramic TBCs allow the operation at up to 150~250 ˚C higher gas temperatures. Because of the severe turbine engine operating conditions that include high...
Show moreThere has been an urge for increasing the efficiency in advanced gas turbine engines. To fulfill these needs the inlet gas temperatures should be increased in the gas turbine engines, thermal barrier coatings (TBCs) have gained significant applications in increasing the gas inlet temperatures. Insulating characteristics of ceramic TBCs allow the operation at up to 150~250 ˚C higher gas temperatures. Because of the severe turbine engine operating conditions that include high temperature, steep temperature gradient, thermal cycling, oxidation and hot-corrosion, TBCs can fail by spallation at the interface between the metal and ceramic. The lack of understanding in failure mechanisms and their prediction warrant a development of non-destructive evaluation technique that can monitor the quality and degradation of TBCs. In addition, the development of NDE technique must be based on a robust correlation to the characteristics of TBC failure.The objective of this study is to develop electrochemical impedance spectroscopy (EIS) as a Non-destructive evaluation (NDE) technology for application to TBCs. To have a better understanding of the multilayer TBCs using EIS they were divided into individual layers and EIS were performed on them. The individual layers included polycrystalline ZrO2-7~8 wt.%Y2O3 (YSZ) (topcoat) of two different densities were subjected to sintering by varying the sintering temperature and holding time for three different thickness and hot extruded NiAl alloy buttons which were subjected to isothermal oxidation with varying temperature and time. NiAl is as similar to the available commercial bondcoats used in TBCs. Then degradation monitoring with electrolyte penetration was carried out on electron beam physical vapor deposited (EB-PVD) TBCs as a function of isothermal exposure. Quality control for air plasma sprayed TBCs were carried out as a function of density, thickness and microstructure. Dense vertically cracked TBCs were tested as a function of vertical crack density and thickness.Electrochemical impedance response was acquired from all specimens at room temperature and analyzed with an AC equivalent circuit based on the impedance response as well as multi-layered structure and micro-constituents of specimens. Physical and microstructural features of these specimens were also examined by optical and electron microscopy. The EIS measurement was carried out in a three-electrode system using a standard Flat Cell (K0235) from Princeton Applied Research and IM6e BAS ZAHNER frequency response analyzer. The electrolyte employed in this investigation was 0.01M (molar) potassium Ferri/Ferro Cyanide {(K3Fe(CN)6/K4Fe(CN)6•3H2O)}.The thickness and density were directly related to the resistance and capacitance of the polycrystalline YSZ with varying thickness and open pores. As the effective thickness of the YSZ increased with sintering time and temperature, the resistance of the YSZ (RYSZ) increased proportionally. The variation in capacitance of YSZ (CYSZ) with respect to the change in porosity/density and thickness was clearly detected by EIS. The samples with high porosity (less dense) exhibited large capacitance, CYSZ, compared to those with less porosity (high density), given similar thickness. Cracking in the YSZ monoliths resulted in decrease of resistance and increase in capacitance and this was related to the electrolyte penetration.Growth and spallation of TGO scale on NiAl alloys during isothermal oxidation at various temperatures and holding time was also correlated with resistance and capacitance of the TGO scale. With an increase in the TGO thickness, the resistance of the TGO (RTGO) increased and capacitance of the TGO (CTGO) decreased. This trend in the resistance and capacitance of the TGO changed after prolonged heat treatment. This is because of the spallation of the TGO scale from the metal surface. The parabolic growth of TGO
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Date Issued
2004
Identifier
CFE0000041, ucf:52855
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0000041
Characterization of SLM-Manufactured Turbine Blade Microfeatures from Superalloy Powders
Title
Characterization of SLM-Manufactured Turbine Blade Microfeatures from Superalloy Powders.
Creator
Ealy, Brandon, Kapat, Jayanta, Ahmed, Kareem, Vasu Sumathi, Subith, University of Central Florida
Abstract / Description
The limits of gas turbine technology are heavily influenced by materials and manufacturing capabilities. Inconel remains the material of choice for most hot gas path (HGP) components in gas turbines, however recent increases in turbine inlet temperature (TIT) are associated with the development of advanced convective cooling methods and ceramic thermal barrier coatings. Increasing cycle efficiency and cycle specific work are the primary drivers for increasing TIT. Lately, incremental...
Show moreThe limits of gas turbine technology are heavily influenced by materials and manufacturing capabilities. Inconel remains the material of choice for most hot gas path (HGP) components in gas turbines, however recent increases in turbine inlet temperature (TIT) are associated with the development of advanced convective cooling methods and ceramic thermal barrier coatings. Increasing cycle efficiency and cycle specific work are the primary drivers for increasing TIT. Lately, incremental performance gains responsible for increasing the allowable TIT have been made mainly through innovations in cooling technology, specifically convective cooling schemes. An emerging manufacturing technology may further facilitate the increase of allowable maximum TIT, thereby impacting cycle efficiencies. Laser Additive Manufacturing (LAM) is a promising manufacturing technology that uses lasers to selectively melt powders of metal in a layer-by-layer process to directly manufacture components, paving the way to produce designs that are not possible with conventional casting methods. This study investigates manufacturing qualities seen in LAM methods and its ability to successfully produce complex microfeatures in a mock turbine blade leading edge. Various cooling features are incorporated in design, consisting of internal impingement cooling, internal lattice structures, and external showerhead cooling. The internal structure is designed as a lattice of intersecting cylinders in order to mimic that of a porous material. Through a non-destructive approach, the presented design is analyzed against the departure of the design by utilizing X-ray computed tomography (CT). Employing this non-destructive testing (NDT) method, a more thorough analysis of the quality of manufacture is established by revealing the internal structures of the porous region and internal impingement array. Variance distribution between the design and manufactured test article are carried out for both internal impingement and external transpiration hole diameters from CT data. Flow testing is performed to characterize the uniformity of porous regions and flow behavior across the entire article for various pressure ratios. Discharge coefficients of internal impingement arrays and porous structures are quantified. A numerical model of fluid flow through the exact CAD geometry is analyzed over the range of experimental flowrates. By comparison of experimental and numerical data, performance discrepancies associated with manufacturing quality are observed. Simplifying assumptions to the domain are evaluated to compare predictions of CFD using the exact geometry. This study yields quantitative data on the build quality of the LAM process, providing more insight as to whether it is a viable option for manufacture of micro-features in current turbine blade production.
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Date Issued
2016
Identifier
CFE0006452, ucf:51428
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006452