Rebekah Webster, Ph.D.

Postdoctoral Scholar/Instructor, University of Virginia
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Presented: December 10 and 11, 2020

“Mixed Phase Ytterbium Silicate Environmental-Barrier Coatings for Improved Calcium–Magnesium–Alumino-Silicate Resistance”

Aircraft operating temperature, and thus efficiency, is expected to increase with the implementation of ceramic matrix composites (CMCs) as engine materials to replace some traditional nickel-base superalloys. With increasing temperature, the threat of ingested siliceous debris, collectively referred to as calcium magnesium alumino-silicates or CMAS, to engine longevity becomes a critical challenge. CMAS attack is a pressing issue in the development of environmental barrier coatings (EBCs) for CMCs. CMAS originates as siliceous debris such as sand or volcanic ash, which becomes molten at temperatures greater than  ̴1200°C and can penetrate EBC materials, causing premature coating failure.

The effect of EBC composition and microstructure on CMAS infiltration behavior was explored in model ytterbium silicate materials containing controlled amounts and dispersions of Yb2SiO5 (YbMS) within a Yb2Si2O7 (YbDS) matrix. YbMS was introduced as coarse granules to model an air plasma spray (APS)-deposited EBC. Samples were exposed to CMAS deposits for up to 96 h at 1300°C. The addition of YbMS to YbDS improved the overall material resistance to CMAS infiltration. It was determined that including ≥ 20 vol% YbMS was beneficial in reducing glass penetration, as compared to phase pure YbDS. Model APS coatings exhibited a combination of grain boundary attack of the YbDS matrix and reactive crystallization of YbMS granules (most notably to form apatite, Ca2Yb8(SiO4)6O2). The formation of apatite slowed the incoming CMAS front.

Preliminary results have indicated that our findings can be translated to actual APS ytterbium silicate coatings. APS YbDS coatings containing various amounts of YbMS were prepared at the Commonwealth Center for Advanced Manufacturing (Disputanta, Virginia) and exposed to CMAS at 1300°C. Coatings containing 25-30% YbMS were more resistant to CMAS infiltration than coatings containing  0-10% YbMS through their improved ability to form apatite. Our results indicate that APS EBC composition and microstructure can be optimized for CMAS resistance.

Rebekah Webster, Ph.D., Postdoctoral Scholar/Instructor, University of Virginia

Rebekah Webster has been a postdoctoral scholar in materials science and engineering at the University of Virginia since June of 2019 and is also developing and instructing an undergraduate course on advanced ceramics (Fall 2020). From 2014 to 2019, she was a Ph.D. student in the Materials Science and Engineering Deptartment at the University of Virginia, studying the interaction between ceramic coatings and molten siliceous deposits. Prior to earning her Ph.D. in 2019, she received a B.S. in chemistry from Radford University in 2014. She has held internships at both NASA Glenn Research Center (2017) and the Metropolitan Museum of Art (2013) studying ceramic materials. She has been involved with her department’s Graduate Student Board and collaborated with other early-career materials scientists on the development of programming for international conferences, such as the International Conference and Expo on Advanced Ceramics and Composites (ICACC), and served as co-chair for ICACC’s 9th Global Young Investigator Forum on Advanced and Nanostructured Materials (2020).