Oxidation and mechanical behaviour of reaction-bonded silicon carbide-boron carbide and the corrosion of dip-coated rare-earth silicate environmental barrier coatings under CMAS exposure
File(s)
Author(s)
Abdelhadi, Kareem
Type
Thesis
Abstract
The oxidation of reaction-bonded SiC–B4C (RBSBC) in air was studied at 1250–1350°C for 5–96 hours. SEM showed a 3–5 μm uniform oxide layer after 48 h. TEM, Raman, and SIMS analyses revealed that the oxidation of 7 wt% B4C with (SiC+ Si)/B4C ratio of ⁓13 produced an amorphous Si-rich borosilicate layer. Oxidation followed a parabolic rate controlled by ionic oxygen transport, and with an activation energy of 353 kJ mol–1. RBSBC with 7 wt% B4C offers a cost-effective and lower-density alternative to SiC for oxidation-resistant applications.
The CMAS corrosion of three different (RE)-based (Yb, Lu and Er) EBCs was studied at 1300 °C for 1–48 h. The EBCs were applied on the RBSBC substrate using dip coating. At the CMAS–EBC interface, Yb and Lu formed protective garnet, while Er-silicate formed non-protective stoichiometric apatite. SEM and EDS showed that Yb and Lu had lower diffusion coefficients compared to Er-silicate, indicating higher CMAS resistance. XPS revealed that the Ca–Yb–Si oxyapatite phase was formed by substitutional diffusion that is controlled by vacancy formation in the RE2O3 lattice. Higher Ca content in the structure suppresses vacancy formation, which decreases Ca diffusion through the coat, thereby enhancing CMAS resistance.
DIC using high-speed camera was used to study the fracture behaviour of RBSBC. RBSBC failed transgranularly, where randomly-dispersed coarse B4C grains deflects the crack path. R-curve for the stress intensity at the crack tip K_I was determined experimentally using the displacement values at the crack tip using virtual extensometers on the DIC images, and using the LEFM SENB theory based on the fracture load and crack length. The experimental K_I has higher values than the quasistatic because it reflects the actual conditions at the crack tip. Increasing B4C content enhances crack resistance compared to SiC ceramics.
The CMAS corrosion of three different (RE)-based (Yb, Lu and Er) EBCs was studied at 1300 °C for 1–48 h. The EBCs were applied on the RBSBC substrate using dip coating. At the CMAS–EBC interface, Yb and Lu formed protective garnet, while Er-silicate formed non-protective stoichiometric apatite. SEM and EDS showed that Yb and Lu had lower diffusion coefficients compared to Er-silicate, indicating higher CMAS resistance. XPS revealed that the Ca–Yb–Si oxyapatite phase was formed by substitutional diffusion that is controlled by vacancy formation in the RE2O3 lattice. Higher Ca content in the structure suppresses vacancy formation, which decreases Ca diffusion through the coat, thereby enhancing CMAS resistance.
DIC using high-speed camera was used to study the fracture behaviour of RBSBC. RBSBC failed transgranularly, where randomly-dispersed coarse B4C grains deflects the crack path. R-curve for the stress intensity at the crack tip K_I was determined experimentally using the displacement values at the crack tip using virtual extensometers on the DIC images, and using the LEFM SENB theory based on the fracture load and crack length. The experimental K_I has higher values than the quasistatic because it reflects the actual conditions at the crack tip. Increasing B4C content enhances crack resistance compared to SiC ceramics.
Version
Open Access
Date Issued
2024-04
Date Awarded
2024-09
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Nowell, David
Al Nasiri, Nasrin
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)