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.