Coatings to Protect Materials from Extreme Environments: Environmental and Thermal Barrier Coatings
Sponsored by: ACerS Engineering Ceramics Division
Program Organizers: Kang Lee, NASA Glenn Research Center; Yutaka Kagawa, The University of Tokyo; Daniel Mumm, University of California, Irvine; Rodney Trice, Purdue University; Emmanuel Boakye, UES Inc.; Valerie Wiesner, NASA Langley Research Center; Edward Gorzkowski, Naval Research Laboratory; Scooter Johnson, Naval Research Laboratory

Monday 8:00 AM
October 18, 2021
Room: A222
Location: Greater Columbus Convention Center

Session Chair: Rodney Trice, Purdue University; Kang Lee, NASA Glenn Research Center

8:00 AM  Cancelled
Break-away Oxidation in Ytterbium Silicate Environmental Barrier Coatings: Kenneth Kane¹; Eugenio Garcia²; Michael Lance¹; Cory Parker¹; Sanjay Sampath²; Bruce Pint¹; ¹Oak Ridge National Laboratory; ²Stony Brook University
    The failure mechanisms of ytterbium silicate based EBCs are still the subject of much investigation. In the current study, 1350°C cyclic oxidation of multilayer Silicon/Ytterbium disilicate (Si/YbDS) and single layer ytterbium disilicate (YbDS) EBCs in 90vol%H2O/10vol% air environments up to ≥ 1000 h is presented. Both coating architectures were deposited utilizing atmospheric plasma spray. The single layer YbDS coating, which previously showed poor adhesion when deposited onto grit-blasted SiC, was deposited onto SiC with enhanced roughness to facilitate better adhesion. Single layer coatings were found to remain well adhered up to at least 1500 h. For both the multilayer and single layer architectures, a transition from parabolic growth to break-away linear recession kinetics was observed, and is related to both EBCs not mitigating H2O diffusion rates to their respective oxidizing interfaces. This research was sponsored by the U. S. Department of Energy, Office of Fossil Energy, Advanced Turbine Program.

8:20 AM
Effects of Topcoat Modifications on Bond Coat Oxidation, Internal Stresses, and Interface Toughness in Multilayer Si/Yb₂Si₂O₇ Environmental Barrier Coatings: Benjamin Herren¹; Chihpin Chuang²; Jonathan Almer²; Kang Lee³; Katherine Faber¹; ¹California Institute of Technology; ²Argonne National Laboratory, Advanced Photon Source; ³NASA Glenn Research Center
    In Environmental Barrier Coating (EBC) systems exposed to high-temperature combustion environments, progressive bondcoat oxidation results in mismatched material properties, internal strains, and ultimately topcoat spallation. Limiting bondcoat oxide growth is thus expected to increase EBC reliability and lifespan. This study examines the effects of modifying topcoat composition on oxide growth and internal stresses in a current (Si/Yb₂Si₂O₇) EBC. Steam cycling simulated the humid, high-temperature engine exposures and was used to assess the compositional effects on thermal oxide thickness, microstructure, and chemistry. Nanoindentation was used to evaluate interface toughness changes with composition and exposure. Synchrotron X-ray scattering and imaging permitted observation of internal strains and damage. By comparing internal strains, associated stresses, microstructure, and interface toughness between modified and baseline EBCs, the influence of topcoat composition and thickness modifications on EBC longevity may be discerned.

8:40 AM
Polymorph Stability and Thermal Expansion Tensors of Mixed and High Entropy Rare Earth Disilicates: Alejandro Salanova¹; Rachel Guarriello¹; Mackenzie Ridley¹; Cormac Toher²; Stefano Curtarolo²; Elizabeth Opila¹; Jon Ihlefeld¹; ¹University of Virginia; ²Duke University
    Entropic stabilization at high temperatures is a relatively new approach for developing robust thermal/environmental barrier coatings (T/EBCs). A challenge facing the realization of high entropy EBCs is achieving the necessary atomic scale mixing to achieve the multiple configurations required for large entropic contributions to free energy. In this work, multiple rare earth (RE) and high entropy alloy (HEA) disilicate powders are synthesized via sol-gel methods. The addition of multiple REs results in the stabilization of phases not readily formed for individual compounds. Combining REs leads to an “averaging” effect where phases not possible for single RE disilicates can be stabilized. The coefficient of thermal expansion second-rank tensor was determined for RE disilicates from temperature dependent X-ray diffraction. Knowledge of this tensor is vital for designing T/EBCs that are thermo-mechanically matched to the underlying material and can be used to design microstructures that minimize thermal stresses.

9:00 AM
Evaluation of YbPO₄ as an Environmental Barrier Coating Candidate: Mackenzie Ridley¹; Bohuslava McFarland²; Cameron Miller¹; Elizabeth Opila¹; ¹University of Virginia; ²Pratt & Whitney
    Application of SiC-base ceramic matrix composites (CMCs) as turbine hot section components requires environmental barrier coatings (EBCs) for protection against thermochemical reactions with the combustion environment. Current generation EBCs are ytterbium disilicates, yet durability issues remain prevalent. Ytterbium phosphate, YbPO₄, was analyzed as a novel environmental barrier coating material candidate. Thermal expansion, high-temperature high-velocity steam resistance, CaO-MgO-Al₂O₃-SiO₂ (CMAS) resistance, and thermochemical stability with SiO₂ were addressed. YbPO₄ displayed enhanced stability against high-velocity steam and CMAS degradation compared to state-of-the-art EBC material Yb₂Si₂O₇. Initial thermomechanical and thermochemical analysis indicate that YbPO₄ should be further explored as a viable material candidate for next-generation EBC systems.

9:20 AM
Protocol for Selecting Exemplary Silicate Deposit Compositions for Evaluation of Thermal and Environmental Barrier Coatings: Andrew Ericks¹; Frank Zok¹; David Poerschke²; Carlos Levi¹; ¹University of California, Santa Barbara; ²University of Minnesota
    Molten silicate deposits pose a significant threat to the durability of protective oxide coatings in aeroengines. But the spread in deposit properties remains largely unexplored, hindering critical evaluation of candidate coating materials. To address this deficiency, a comprehensive dataset of over 100 compositions was compiled from various sources, and the thermophysical properties of each was computed using established analytical or computational models. Classes of deposits sharing similar compositional and property profiles are identified using principal component analysis and k-means clustering. The structure of the dataset is reduced to four distinct compositional classes. More importantly, the study represents a conceptual advancement in selection of exemplary compositions from a large disparate population in order to guide experimental and computational assessments of coating durability in service-relevant environments.

9:40 AM
High Temperature Stability and Decomposition of Mixed Oxide and Sulfate CMFAS-type Deposits and Implications for Coating Degradation: Eeshani Paresh Godbole¹; Atharva Chikhalikar¹; David Poerschke¹; ¹University of Minnesota
    Deposit-induced degradation of alloys, bond coats, and thermal/environmental barrier coatings is a critical performance- and life-limiting challenge. Understanding the intrinsic behavior of ingested deposits is key to designing robust next generation coatings. Although the behavior of oxide-based deposits (CMFAS) have been studied extensively, that of mixed anion, (oxide/sulfate/chloride) deposits is less well understood. This study advances the understanding of multi-cation sulfate-oxide deposits. Deposit compositions, chosen to maximize experimental insights while leveraging existing knowledge, were synthesized and heat treated over a range of temperatures relevant to hot corrosion and coating degradation. The heat-treated samples were then analyzed to understand the sulfate decomposition and other structural changes. The results show that the presence of oxides significantly accelerates the decomposition of sulfates. Computational thermodynamics models were used to further understand these effects. These results provide insights into the influence of multiple anions on the stability and reactivity of complex deposits.

10:00 AM Break

10:20 AM
Na₂SO₄ Interactions with Rare Earth Silicate Environmental Barrier Coatings: Kristyn Ardrey¹; Elizabeth Opila¹; ¹University of Virginia
    Rare earth (RE) silicate environmental barrier coatings (EBCs) are presently being investigated for application on SiC-based ceramic matrix composites in aircraft turbine engines. RE silicates primary function is to mitigate the volatility of SiO₂ and limit the thermally grown oxide (TGO) on the silicon bond coat during operation. In marine combustion environments, Na₂SO₄ can form molten deposits at temperatures around 884^oC, resulting in EBC degradation at potentially lower temperatures than the maximum operating conditions (1300-1400^oC). Currently, there is minimal research and understanding of EBC hot corrosion in the presence of molten Na₂SO₄. This research addresses this knowledge gap by exploring the reactions between Na₂SO₄ and free-standing air plasma sprayed RE silicate EBC materials, Y₂Si₂O₇, Y₂SiO₅, Yb₂Si₂O₇, and Yb₂SiO₅, at 825^oC for times up to 250hr where reactions are observed even below the melting temperature of Na₂SO₄.

10:40 AM
Protection of Yttria Stabilized Zirconia (YSZ) Surface from CMAS Attack by Sacrificial Layer of Si3N4: Said Bakkar¹; Elora Zucah¹; Tim Hossain²; Jacob Moldenhauer¹; Ellen Steinmiller¹; Will Flanagan¹; ¹University of Dallas; ²Ceriumlabs
    Single crystal (110) of Yttria-stabilized zirconia (YSZ) Y₂O₃ 8 mol% ZrO₂ was covered with 10 μm of silicon nitride Si3N4 as a sacrificial layer using chemical vapor deposition (CVD). The Si₃N₄ layer was used to seal the YSZ surface to mitigate (calcium–magnesium–aluminum–silicon oxide) CMAS attack. CMAS testing was carried out on the covered and uncovered surfaces by melting 0.1g of the CMAS powder on the surface in a furnace at 1250 °C for 10 h. The conformal surface reaction of the sealed layer confirmed that no cracking and delamination at high temperatures (1250 °C). Scanning electron microscopy (SEM) micrographs confirmed that the surface of YSZ surface was successfully sealed. The newly coating of Si₃N₄ was shown to be a viable technique to significantly reduce CMAS infiltration in porous thermal barrier coatings.

11:00 AM
Understanding Modes of Mixed Deposit-Induced Degradation on Advanced Alloys and Bond Coat Systems: Atharva Chikhalikar¹; Eeshani Godbole¹; David Poerschke¹; ¹University of Minnesota, Twin Cities
    Understanding the corrosive effects of ingested debris on alloys and protective coatings is critical for developing next-generation turbine materials. Although intermediate-temperature degradation of alloy components has been studied extensively in the presence of simple sulfate deposits (e.g., Na2SO4), there is limited knowledge of degradation processes in the presence of deposits containing multiple cations and mixed anions. This work expands the understanding of deposit-induced alloy degradation under mixed, multi-cation oxide and sulfate deposits. A systematically defined set of deposit compositions were used to understand the influence of specific anions and cations on the oxidation of alumina-forming high-temperature alloys. The oxidized samples were characterized to identify phase transformations in the thermally grown oxide and observe the corresponding corrosion morphology. Complementary computational thermodynamics models were used to understand the deposit stability. The results provide an enhanced understanding of the effect of anions on the degradation of alloys and implications for developing new materials.