In-Situ Investigations of Calcium-Magnesium-Aluminosilicates (CMAS) Infiltration Effects on Thermal Barrier Coatings Under Extreme Environments Replicating Jet Engines

<p>Calcium-magnesium-aluminosilicate (CMAS) particulates, such as sand or volcanic ash, are ingested by gas turbine jet engines during operation. When operating within high abundance regions, these particulates negatively interact and degrade the high temperature thermal barrier coatings (TBC) protecting underlying superalloy turbine blades vital to engine operation. These CMAS particulates melt within the engine and infiltrate into the ceramic coatings. In electron-beam physical vapor deposited (EB-PVD) TBCs, the CMAS within the intercolumnar gaps stiffens the coatings and causes thermomechanical induced high stress concentrations, risking crack formation. While molten, the CMAS thermochemically alters and destabilizes the coating, also risking coating failure. Premature, localized spallation failure of these ceramic thermal barrier coatings (TBCs) exposes underlying metallic components to the extreme temperatures of the hot gas streams exiting the combustor. Therefore, it is paramount to dynamically capture and characterize the evolution of CMAS infiltration and its reaction kinetics under replicated service conditions and engine environments during operation. In this research, in-situ synchrotron X-ray diffraction measurements were leveraged for the high temporal and spatial resolution it offers, to enable the first known detailed capture of complex, transient micro-mechanisms governing active CMAS infiltration processes of infiltration, interaction, and degradation in a thermal gradient simulated engine environment. The results were analyzed to reveal new knowledge, experimentally showing rapid infiltration of CMAS in less than 2 minutes. The ensuing interaction was characterized and quantified by immediate changes in strain that reveal the thermomechanical effects of stiffening. Meanwhile, thermochemical destabilization of the coating, responsible for enabling the tetragonal to monoclinic phase transformation upon cooling, produced a discernible strain response after a time lag of over 20 minutes and showed most of its effects within the 60 minutes. The accompanying volume change and strain was tracked to show this correlation. Microscale spatial mapping revealed the strong coupling between thermochemical and thermomechanical degradation mechanisms, with the most extensive coating degradation occurred during cooling between 200 - 400 ◦C following infiltration. Upon cooling, the high tetragonal to monoclinic transformation rates induced compressive coating responses up to 2.36x larger than those measured prior to CMAS exposure. Capturing these phenomena elucidates CMAS infiltration wetting behaviors, crack initiation, and pore evolution in coatings exposed to CMAS during realistic flight-representative conditions. These quantitative findings further elucidate the key developmental stages of CMAS-induced degradation, from its time and temperature dependent nucleation to growth stages, and highlights the transient evolution of various reaction-kinetic mechanisms driving degradation throughout a single thermal cycle. These insights will be instrumental in guiding the development of CMAS-resistant coatings and in advancing passive and active CMAS mitigation strategies. In turn, these advancements extend the service life of coating and engine operating in CMAS-rich environments.</p>

Name thesis:degree_name

Doctor of Philosophy in Aerospace Engineering

Level thesis:degree_level

Dissertation - Open Access

Discipline thesis:degree_discipline

Aerospace Engineering

Year

2025

Author dc:creator

• Stein, Zachary

• CMAS infiltration
• in-situ synchrotron XRD
• thermal gradient environment
• flight-representative conditions
• high temporal resolution
• high spatial resolution
• thermal barrier coatings (TBCs)
• high-temperature coatings
• electron-beam physical vapor deposition (EB-PVD)
• 7YSZ
• thermochemical degradation
• thermomechanical degradation
• infiltration rate
• strain mapping
• phase transformation kinetics
• residual stress evolution
• coating failure mechanisms
• CMAS mitigation strategies
• Aerospace Engineering
• Ceramic Materials
• Engineering
• Engineering Science and Materials
• Materials Science and Engineering
• Mechanics of Materials
• Other Materials Science and Engineering
• Structures and Materials

Repository record dc:identifier

https://commons.erau.edu/edt/945

OAI identifier oai:identifier

oai:commons.erau.edu:edt-1986