Suitability of single layer yb2sio5 environmental barrier coating for gas turbine applications
File(s)
Author(s)
Kassem Hussein, Reem
Type
Thesis
Abstract
Environmental barrier coatings (EBCs) are employed to shield silicon carbide (SiC) elements in gas-turbine engines from reacting with the water vapour produced during hydrocarbon combustion. Under high-temperature operational conditions, oxidising species like H2O and O2 react with SiC, forming of gaseous CO and a solid SiO2 layer. However, the protective silica layer further reacts with water vapour, producing gaseous Si(OH)4 and causing volatilisation of the SiO2 layer. The volatilisation rate increases with water vapour pressure, surface flow rate flow, and temperature. Because modern gas-turbine engines operate at high temperatures, resulting in high water vapour pressures and flows, the use of SiC-based ceramics is limited. Consequently, for EBCs to prevent corrosive species from reaching the underlying substrate, structural integrity must be maintained. One promising coating system is the single-layer ytterbium mono-silicate (Yb2SiO5) EBC.
The mechanical and thermo-mechanical properties of dense Yb2SiO5 were evaluated using a combination of experimental techniques, including a thorough analysis of the reverse indentation size effect. Following the determination of the room-temperature properties, the effect of high-humidity on the coating microstructure and its related properties was examined. It was observed that the exposure to humidity caused significant pitting and cracking owing to phase transformations, thereby reducing the damage tolerance of the coating.
The energy release rate (ERR) for delamination of a single-layer Yb2SiO5 coating on a reaction-bonded silicon carbide-boron carbide substrate, exposed to a steady thermal gradient and mechanical load, was theoretically investigated. ERR calculations for edge and isolated cracks showed that the edge crack had significantly higher ERR, making them more prone to propagation and failure. This highlights the importance of edge crack mitigation in component design. Understanding this behaviour enables improved coating designs and strategies to prevent delamination. Finally, the development of thicker and denser Yb2SiO5 was demonstrated using a novel contactless flash sintering method.
The mechanical and thermo-mechanical properties of dense Yb2SiO5 were evaluated using a combination of experimental techniques, including a thorough analysis of the reverse indentation size effect. Following the determination of the room-temperature properties, the effect of high-humidity on the coating microstructure and its related properties was examined. It was observed that the exposure to humidity caused significant pitting and cracking owing to phase transformations, thereby reducing the damage tolerance of the coating.
The energy release rate (ERR) for delamination of a single-layer Yb2SiO5 coating on a reaction-bonded silicon carbide-boron carbide substrate, exposed to a steady thermal gradient and mechanical load, was theoretically investigated. ERR calculations for edge and isolated cracks showed that the edge crack had significantly higher ERR, making them more prone to propagation and failure. This highlights the importance of edge crack mitigation in component design. Understanding this behaviour enables improved coating designs and strategies to prevent delamination. Finally, the development of thicker and denser Yb2SiO5 was demonstrated using a novel contactless flash sintering method.
Version
Open Access
Date Issued
2024-07-28
Date Awarded
01/03/2025
License URL
Advisor
Nowell, David
Publisher Department
Department of Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)