Processing and Thermo-Mechanical Considerations for Rare Earth Disilicates as Environmental Barrier Coatings

Silicon carbide based ceramic matrix composites (CMCs) are state of the art materials for shrouds in high temperature gas turbine engines. In this application, CMCs react in the presence of water vapor to produce a gaseous silicon hydroxide species. The CMCs require adequate protection through the use of a thermal/environmental barrier coating (EBC) to avoid premature degradation. The current technology at the forefront of EBCs are rare earth disilicates (REDS) due to a similar coefficient of thermal expansion (CTE) relative to CMCs, high melting temperatures, and low silicon activity relative to pure silica. This thesis involves several key studies to develop new knowledge on the properties of REDS: 1) the synthesis of single cation, multi-cation, and high-entropy REDSs through the sol-gel process; 2) the characterization of REDS through in situ hot stage diffraction to identify phase stability; and 3) the characterization of anisotropic CTE behavior of REDSs using X-ray and neutron diffraction and scattering.
The sol-gel synthesis was shown to be highly dependent on water as well as acid or base concentrations that served as a catalyst for the hydrolysis reaction necessary to produce silica from its precursor, tetraethyl orthosilicate. Additionally, multi- and high-entropy REDS sol-gels were successfully synthesized and stabilized into the monoclinic C 2/m phase following a rule of mixtures. In situ hot-stage diffraction was used to identify and determine phase stability from room temperature to temperatures up to 1200 °C. Synchrotron based diffraction and pair distribution function (PDF) analysis data was used to develop a tensor derived model for the anisotropic CTE behavior. The data showed that the directions of minimum and maximum CTE were temperature dependent for REDS containing lutetium, ytterbium, yttrium, and thulium. A series of experiments investigated the many possible properties that could be related to CTE anisotropy. electronic configuration of REDS through the band gap, calculating differences of up to 1 eV for different REDS presumably due to the rare earth cation. Despite the results, band gap did not seem to be directly correlated to CTE anisotropy. Next, the optical phonon behavior was investigated through Raman spectroscopy and FTIR. The optical phonon behavior showed no correlation to CTE anisotropy as all REDS have near identical spectra from room temperature to 1200 °C. The acoustic phonons were investigated, and phonon dispersion were provided through computational work performed by collaborators. Differences in the phonon dispersion curves of REDS did show distinctions that may be related to CTE anisotropy but require a deeper look and more experiments to prove or disprove. The differences in CTE behavior are attributed to the expansion and rotation of the rare earth-oxygen octahedron induced by local distortions. This thesis outlines the mechanisms through which REDSs can be further tailored to produce single phase homogeneous powders, stabilize rare earth cations in atypical but desired crystalline phases, and develop a model for anisotropic CTE that can inform the stresses that develop as a function of heating for EBCs with specific attention on the short-range and long-range crystal symmetries and their evolution with temperature.