Surface Protection for Enhanced Materials Performance: Science, Technology, and Application: CMAS Degradation and Mitigation / Environmental Barrier Coatings
Program Organizers: Kang Lee, NASA Glenn Research Center; Jun Song, McGill University; Yutaka Kagawa, University of Tokyo; Rodney Trice, Purdue University; Daniel Mumm, University of California, Irvine; Mitchell Dorfman, Oerlikon Metco (US) Inc.; Christian Moreau, Concordia University; Emmanuel Boakye, UES Inc.; Edward Gorzkowski, Naval Research Laboratory; Scooter Johnson, Naval Research Laboratory; Stephen Yue, McGill University; Richard Chromik, McGill University

Tuesday 2:00 PM
October 1, 2019
Room: D137
Location: Oregon Convention Center

Session Chair: Kang Lee, NASA Glenn Research Center; Valerie Wiesner, NASA Glenn Research Center

2:00 PM  Invited
Understanding and Mitigating the CMAS Problem in Gas Turbine Coatings: Carlos Levi1; David Poerschke2; Collin Holgate1; William Summers1; Frank Zok1; 1University of California, Santa Barbara; 2University of Minnesota
    CMAS degradation of T/EBCs represents a fundamental barrier to progress in gas turbine technology by limiting the operating temperature of the components owing to melting of the deposits. The mechanisms are multiple and complex, but broadly the failures are thermo-mechanical, induced by the CTE mismatch between the CMAS-modified layer and the substrate. Loss of strain tolerance in TBCs arises from melt infiltration to an extent determined by the interplay between the melt flow dynamics, the dissolution of the TBC and the crystallization of product phases. There is nominally no infiltration issue in EBCs but the reactions can be rapid and the melt can sometime penetrate the grain boundaries. Because of the stochastic nature of the siliceous deposits a mitigation strategy must involve understanding of the effect of their chemical composition on the thermo-chemical interaction with the coating. This presentation will discuss the progress and challenges to implement these mitigation strategies.

2:40 PM  
Some Implications to CMAS Infiltration and Arrest Kinetics of Gradual Application: Eric Jordan1; Byung Jun1; 1University of Connecticut
    In laboratory testing CMAS is generally applied to a fatal dose from the beginning of the furnace test. Actual engines will acquire CMAS over time. In most cases the time to accumulate a fatal dose is much longer than the time associated with melt infiltration and with arrest by apatite formation. Consideration of this aspect of kinetics involves multiple possible cases depending on the accumulation rate, the coverage behavior associated with CMAS application, the homogeneity of the CMAS and the spread in melting points of arriving materials and materials made from multiple arriving species. Several of the limiting cases will be discussed and further illuminated by Monte Carlo simulations.

3:00 PM  
Investigation of Environmental Barrier Coating Degradation by Molten Calcium-Magnesium-Aluminosilicate (CMAS) at Low Concentrations: Valerie Wiesner1; Gustavo Costa2; John Setlock3; Kang Lee1; Bryan Harder1; 1NASA Glenn Research Center; 2Vantage Partners, LLC; 3University of Toledo
    Particulates, such as sand and volcanic ash, are ingested by air-breathing turbine engines during routine operation. At operating temperatures above 1200°C, these particulates become molten and reduce the durability of engine materials, particularly environmental barrier coatings (EBCs) designed to protect ceramic matrix composites (CMCs). Depending on flight conditions, the concentration of ingested particles, comprised of calcium-magnesium-aluminosilicate (CMAS) and other metal oxides, can vary from ppm to thousands of mg/m3. This variation in loading complicates the development of CMAS-resistant EBCs, as no standard CMAS loading has been established in the research community. Loadings often vary in literature from 10mg/cm2 to above 60mg/cm2. In this study, a CMAS glass was applied to the surface of candidate environmental barrier coatings at low loadings (<10mg/cm2) and heated to temperatures above 1200°C. Specimen cross-sections were characterized after CMAS exposure to evaluate the resulting microstructure and phase compositions at the CMAS/EBC interface.

3:20 PM  Invited
Manufacture of Environmental Barrier Coatings by Thermal Spray Techniques: Robert Vassen1; Emine Bakan1; Seongwong Kim2; Daniel Mack1; Olivier Guillon3; 1Forschungszentrum Jülich GmbH; 2Forschungszentrum Jülich GmbH; Korea Institute of Ceramic Engineering and Technology (KICET); 3Forschungszentrum Jülich GmbH; Jülich Aachen Research Alliance
    Environmental barrier coatings (EBCs) are essential to protect ceramic matrix composites against water vapor recession in typical gas turbine environments. The most often used thermal spray techniques for the deposition of EBCs is atmospheric plasma spraying (APS). This technique with its major problems as limited crystallinity, crack formation or loss of constituents will be addressed. In addition, also results on more advanced thermal spray processes as high velocity oxygen fuel (HVOF), suspension plasma spraying (SPS) or very low pressure plasma spraying (VLPPS) will be described. Especially the last method appears suitable to deposit crystalline, dense coatings for example made of YB2Si2O7. Finally, also results of the performance of the different coating systems with respect to thermal cycling, water vapor recession and partially CMAS attack will be presented.

3:50 PM  Invited
Role of Process Induced Chemical Composition Changes and Rapid Solidification on the Characterestics of Plasma Sprayed Yb2Si2O7 Environmental Barrier Coatings: Sanjay Sampath1; Eugenio Garcia1; 1Stony Brook University
    Environmental barrier coatings-EBC are the envisioned solution to protect ceramic matrix composites components in forthcoming power generation and aircraft/spacecraft turbine engines from Si volatilization caused by the moisture laden combustion atmosphere. The most promising candidates for this application are atmospheric plasma sprayed (APS) rare earth silicates and among them Yb2Si2O7 has gained consideration in the last few years. APS rare earth silicates present two main drawbacks; the volatilization of SiO2 which shift the original feedstock composition and favors the development of Yb2O3 enriched phases and the amorphous nature of the as deposited coatings. The present work aims to understand the role of the amorphous character of the as sprayed coatings and Yb2SiO5, and their subsequent crystallization after annealing treatments, on the microstructure and properties of APS Yb2Si2O7 coatings, assessing the crystalline phases (XRD, DTA, CTE), microstructure (SEM) and chemical composition of the obtained coatings before and after post-spraying treatments.

4:20 PM  
High-temperature, High-velocity Water Vapor Studies of Environmental Barrier Coating Candidates: Mackenzie Ridley1; Elizabeth Opila1; 1University of Virginia
    Environmental Barrier Coatings (EBCs) are required to protect SiC-based ceramic matrix composites from high temperature water vapor corrosion in combustion environments. While EBC degradation from silica volatility is already known to occur, greater understanding of microstructural changes is needed to identify optimum coatings for various environments. EBC candidate materials HfSiO4, Yb2Si2O7, and BSAS (Ba/SrAl2Si2O8) are studied. Microstructure evolution of bulk EBCs and their product oxides are characterized in a steamjet environment, where high velocity water vapor impinges on the material surface within a controlled tube furnace environment to rapidly progress EBC corrosion. Silica depletion and weight change measurements are used to inform lifetime prediction models for each material. A quantitative understanding of the rate limiting factors for silica depletion is determined through analysis of microstructural changes with testing times from 24 to 250 h, temperatures between 1200 and 1400°C, and water vapor velocities up to 250 m/s.