Laser Diagnostics of CMAS Infiltration into Environmental Barrier Coatings Through Spectroscopic Ellipsometry

Jet turbine engines, during use cycles, are susceptible to intaking environmental particles such as dust and ash, which often contain calcium-magnesium-aluminosilicate (CMAS). CMAS infiltration becomes problematic once the particles progress past the combustion chamber, where they interact with the hot, ceramic matrix composite (CMC) turbine blades. As the engine climbs in temperature, CMAS will form a molten, corrosive species that degrades the barrier coatings meant to protect turbine blades from the extreme, high temperature environment. The spallation of thermal barrier coatings can shorten overall engine performance and lifespan. Environmental barrier coatings (EBCs) aim to serve as an additional layer of protection to defend against CMAS infiltration.
This study focuses on non-destructive detection of CMAS infiltration into EBCs using spectroscopic ellipsometry. Spectroscopic ellipsometry measures the change in polarization of light upon reflection off a sample surface, enabling the extraction of several material characteristics such as absorption spectra and the dielectric function. This method is an appealing approach for this application due to its potential to be more easily industrialized than alternative research methods that are more time consuming and require disfigurement of the turbine blades. In this work, we examine three YbMS/YbDS EBC coatings exposed to CMAS for 4, 24, and 96 hours. Through preliminary testing, spectroscopic ellipsometry displayed detectable differences in the range of 17– 33 µm, correlating with increasing infiltration depths. These findings demonstrate the feasibility of spectroscopic ellipsometry for rapid, non-destructive CMAS detection in turbine blade coatings.