Gas turbine engines remain an integral part of providing the world’s propulsion and power generation needs. The continued use of gas turbines requires increased temperature operation to reach higher efficiencies and the implementation of alternative fuels for a lower net-carbon footprint. This necessitates evaluation of the material coatings used to shield the hot section components of gas turbines in these new extreme environments in order to understand how material degradation mechanisms change. Recently, the US Navy has sought to reduce its use of fossil fuels by implementing a blended hydroprocessed renewable diesel (HRD) derived from algae in its fleet. To evaluate the material degradation in this alternative environment, metal alloys are exposed in a simulated combustion environment using this blended fuel or the traditional diesel-like fuel. Evaluation of the metal alloys showed the development of thick, porous scales with a large depletion of aluminum for the blend fuel test. A mechanism linking an increased solubility of the scale to the blend fuel test environment will be discussed.

For power generation applications, Integrated Gasification Combined Cycle (IGCC) power plants can provide electricity with 45% efficiency and full carbon capture by using a synthetic gas (syngas) derived from coal, biomass, or another carbon feedstock. However, the combustion of syngas is known to cause high water vapor content levels in the exhaust stream with unknown material consequences. To evaluate the effect of increased humidity, air-plasma sprayed (APS), yttria-stabilized zirconia (YSZ) is thermally aged in an environment with and without humidity. An enhanced destabilization of the parent phase by humid aging is revealed by x-ray diffraction (XRD) and Raman spectroscopy. Microstructural analysis by transmission electron microscopy (TEM) and scanning-TEM (STEM) indicate an enhanced coarsening of the domain structure of the YSZ in the humid environment. The enhanced destabilization and coarsening in the humid aging environment is explained mechanistically by water-derived species being incorporated into the YSZ structure and altering the anion sublattice. The characterization of the metal alloy and ceramic coatings exposed in these alternative environments allows for a deeper understanding of the mechanisms behind the material evolution in these environments.

Hot section abradable coatings play an integral role in reaching optimal efficiency of gas turbine engines by preventing unnecessary gas leakage through clearance control. As turbine material technologies advance, there is a push for the development of abradable coatings that can withstand more severe operating conditions and retain the optimum balance of abradability and durability. However, current understanding of abradable coating properties that promote a controlled removal of these highly brittle materials is missing. In this work, a theoretical analysis is first presented for expected abradable mechanisms detailing key properties and microstructural features that enable each mechanism. Next, experimental studies are completed to evaluate coating evolution in representative turbine operating conditions and to determine key factors affecting abradable behavior. Two current technology abradable coatings processed by air plasma spraying, dysprosia-stabilized zirconia (DySZ) with hexagonal boron nitride (hBN) and yttria-stabilized zirconia (YSZ) with nanozone features, have been studied using scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and optical vibrometry. A challenge in evaluating hot section abradable materials is that of testing in engine relevant conditions but in a significantly scaled-down, more controlled lab test. Appropriate testing methodologies were first established. Both as-processed and aged coatings were then tested in a representative macroscratch test to investigate the influence of different defects, their evolution with aging, and observed damage behavior. Results for YSZ-based coatings show highly brittle fracture that penetrates deep within the coating and also changes with aging. The DySZ, hBN coating showed a change in phase of the weakening phase, hexagonal BN, after processing and a significant increase in hardness with aging. Yet, these coatings showed the ability to maintain a shallow damage zone even after aging with little damage propagation further into the remaining coating material. This suggests that the presence of a secondary phase plays a significant role in maintaining a more controlled and consistent damage mechanism. The results of this study, in combination with a study of deformation seen in engine hardware, can be used to build further predictive models of abradable damage accommodation behavior.

Investigating the mechanisms of phase transformation in t’-YSZ using first principle calculations

By

Seyed Amirhossein Saeidi

Doctor of Philosophy in Materials and Manufacturing technology

University of California, Irvine, 2019

Professor Daniel R. Mumm, Chair

Power generation is one of the driving forces of economic growth. Gas turbine engines are one of the main power generation approaches that is in use stationary powerplants and propulsion systems in aviation and naval vessels. A basic design principle for gas turbine engines is that efficiency increases with increasing temperatures; as such, designs and materials allowing higher gas combustion temperatures are desired. To protect the metallic parts, ceramics with low thermal conductivity such as yttria stabilized zirconia (YSZ) are used as thermal barrier coatings. Use of new alternative fuels, with higher hydrogen content than conventional fuels, increase both the temperature and water production during combustion. This new extreme condition has shown to result in faster degradation of YSZ coatings. To understand the mechanisms behind this accelerated degradation, an understanding of the material behavior at the atomic scale under such exposure conditions is necessary, which is the focus of this work. First principles density functional theory (DFT) calculations are employed to study point defects and their interactions in the bulk of YSZ systems in different phase fractions and dopant concentrations. Cation vacancies and anion vacancy-cation vacancy pairs are investigated to determine the rate controlling defects in material aging processes. Equations derived from the grand canonical approach are used to determine the formation energy of defects. The effect of water vapor pressure, temperature and electronic structure of YSZ on these defects are studied. The change in formation energy of cationic defects and their concentrations as a function of the Fermi energy and water vapor pressure are reported. In addition, the possibility of changes in the main defect species responsible for the predominant cation diffusion at varying Fermi energies is presented. The results are used to propose a mechanism that can explain a series of experimental observations showing an acceleration of aging degradation of YSZ coatings under elevated water vapor exposures.

The insights offered by this work advance our knowledge and predictive understanding of the aging process in t’-YSZ and give directions for future experimental and theoretical efforts directed at the design of more degradation resistant thermal barrier coatings.

Air plasma sprayed (APS) abradable coatings are used in the turbine hot section to reduce the stator-rotor gap, minimizing gas leakage. These coatings are designed to exhibit controlled removal of material in thin layers when the turbine blades sweep through the coating, which protects the mechanical integrity of the turbine blade. In an effort to lower CO2 emissions, high H2 content fuel is being explored. This change in chemical composition of the fuel may affect the microstructure, abradability and durability of the coatings at turbine operational temperatures.

The presence of high water vapor in the combustion chamber leads to accelerated degradation of the sacrificial coating materials. In this work, zirconia based composite materials with a machinable phase and varied porosity have been used to study microstructural evolution, thermal and chemical stability of the phases and abradable characteristics of baseline coating systems in both humid and dry environments. Investigation of the mechanisms that control the removal of materials and performance of abradable coatings through thermo-mechanical tests will be discussed.

With utilization of gas turbine engines in power generation, aerospace and marine propulsion applications, the materials that enable those gas turbine technologies are exposed to a wide range of service temperatures and material exposure environments resulting in application dependent degradation modes. The most severe types of degradation are seen in the hottest section of the turbine with its combined interaction of external contaminants and high temperatures. Although specialized coatings have been developed to try to alleviate the degradation experienced, hot corrosion continues to be a concerning, life-limiting factor, particularly in the case of marine turbines, and it is, therefore, the focus of this study. This work presents the evaluation of new candidate materials for improved marine turbine performance at higher operating temperatures. Three different areas of work are discussed. First, the current methodology for the evaluation of hot corrosion attack in pin-shaped samples, typical of burner rig testing, is presented, and its shortcomings are discussed. A new sample assessment protocol based on image analysis was established and validated. Next, a new nickel-based superalloy and three doped variations, intended to replace current blade and vane substrates, were evaluated under type I hot corrosion conditions in a low-velocity burner rig (LVBR). The tests included both long-term and short-term exposures as well as pre-oxidized and bare materials. Scanning electron microscopy and energy dispersive spectroscopy were used to study the attack mechanisms as a function of doping material and concentration.

It was found that different dopants affected the hot corrosion resistance by promoting the incorporation of certain elements, which changed the types of sulfides and oxides, protective or non-protective, that formed. Silicon was found to be an effective dopant at increasing hot corrosion resistance through two mechanisms: a) by promoting chromia formation and suppressing the activity of titanium, resulting in a more protective oxide able to slow down internal sulfidation, and b) by promoting a different coarsening behavior of the internal sulfides. Co-doping with hafnium and silicon had a synergistic effect where the presence of hafnium enhanced the effects of silicon, and the overall hot corrosion resistance was significantly improved, even though hafnium doping, by itself, had poor performance.

The third area of work is focused on the performance, compatibility, and hot corrosion resistance of substrate-coating material pairs evaluated in a LVBR. The coatings that were evaluated included several commercially available diffusion coatings, and both commercially available and new developmental candidate overlay coatings. In the case of diffusion coatings, it was observed that the formation of topologically closed pack (TCP) phases and elemental segregation along the interdiffusion zone (IDZ) are crucial, limiting factors determining the lifespan of the coating. In the case of overlay coatings, initial observations provided evidence for substrate-dependent performance. However, upon closer inspection, it was revealed that this dependence was a function of differing initial microstructures most likely originating in processing variations. The best performing coatings, evaluated on multiple substrates, were comprised of a modified NiCrAlY and a platinum modified CoCrAlY. As a direct result of this work, new substrate and coating materials with enhanced performance were selected for implementation in the next generation of marine turbine engines, and the testing and sample evaluation framework developed will continue to guide future material selection efforts. The study of commercially processed substrates and coatings led to key findings pointing to the importance of coating microstructure control and prevention of unwanted phase formation and elemental segregation.

Increasing demands for energy storage and conversion has fueled research and development of next-generation electrochemical devices, including batteries, supercapacitors, and catalysts. Viability of technology beyond the proof-of-concept is dependent on the morphology and topology of the electrode, which should be carefully designed to maximize material utilization during operation. Despite the close relationship between microstructure and operational efficiency, there does not currently exist a single configuration that can be broadly applied across electrochemically active materials. The ideal microstructure is expected to have co-continuous and interpenetrating domains that have high interfacial area and present minimal resistance to ionic and electronic transport.

In the following dissertation, I present a technique to create such a structure through bicontinuous interfacially jammed emulsion gels (bijels), which are generated via spinodal decomposition and therefore confer characteristic microstructural qualities to derivative materials. Domain size distribution, interfacial curvature, tortuosity, and self-similarity are discussed in detail and compared quantitatively to alternative microstructures that have been proposed for electrochemical devices. These qualities are shown to influence material utilization in two specific applications, energy storage in zinc electrodes, and electrocatalytic water splitting for hydrogen generation.

Maintaining electronic conductivity in electrodes has been shown previously to delay capacity loss during repeated charge-discharge cycling. In the case of zinc anodes, converting the metallic material to the semiconducting zinc oxide creates heterogeneities in current distribution that inspire material reconfiguration and premature cell failure. Spinodal-like electrodes mitigate these effects by improving electronic accessibility of the active material and maintaining conduction pathways to a high degree of discharge compared to electrodes built with randomly sized features.

Homogeneous activity and co-continuity in an electrode are also advantageous in the electrocatalytic separation of water into constituent hydrogen and oxygen gases. Product desorption from the electrode is necessary to continue the reaction, but microstructural impediments to efficient removal result in the underutilization of active surfaces. Bijel templated electrodes improve gas transport through the same microstructural qualities discussed above and are shown to reduce energy losses associated with this inefficiency.