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Cyclic Durability of Thermal Barrier Coatings Subject to CMAS Attack Alan Harris University of Connecticut - Storrs, [email protected]

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Cyclic Durability of Thermal Barrier Coatings Subject to CMAS Attack Alan Harris University of Connecticut - Storrs, Alan.Harris@Uconn.Edu University of Connecticut OpenCommons@UConn Doctoral Dissertations University of Connecticut Graduate School 4-20-2018 Cyclic Durability of Thermal Barrier Coatings Subject to CMAS Attack Alan Harris University of Connecticut - Storrs, [email protected] Follow this and additional works at: https://opencommons.uconn.edu/dissertations Recommended Citation Harris, Alan, "Cyclic Durability of Thermal Barrier Coatings Subject to CMAS Attack" (2018). Doctoral Dissertations. 1732. https://opencommons.uconn.edu/dissertations/1732 Cyclic Durability of Thermal Barrier Coatings Subject to CMAS Attack Alan Barkley Harris, PhD University of Connecticut, 2018 Thermal barrier coatings (TBCs) are critical in modern gas turbine engines, increasing operating temperature, efficiency, and component life. Engines have reached temperatures at which ingested debris (CMAS) forms silicate melts that chemically and mechanically attack TBCs, leading to premature failure. New, CMAS-resistant coatings must be validated under conditions that recreate real-world TBC-CMAS interactions. No standardized testing to perform these analyses currently exists. A cyclic thermal gradient rig with incremental CMAS deposition was developed based on modified literature designs. Tests performed using literature-based parameters showed TBC- CMAS interactions and failure morphology deemed not real-world representative by an engine manufacturer. The results were rationalized with the understanding that, in an engine, “cooling air” is relatively hot (~400 °C). Minimizing transient thermal gradients across the TBC coupon resulted in more representative test outcomes. A second generation thermal gradient rig was developed with higher heat flux and sample throughput. This rig incrementally deposits CMAS powder, a feature not found on existing rigs. Heterogeneous CMAS was deposited onto an EBPVD YSZ TBC coupon. The CMAS layer had Alan Barkley Harris – University of Connecticut, 2018 intriguing non-uniformities in accumulation and chemical heterogeneity. This has implications for CMAS materials used for testing. Engine manufacturers need to model TBC life reduction from CMAS attack for different engine parameters and CMAS environments. Preliminary experiments were performed that provided insight for such models. First, EBPVD YSZ coupons were cycled with varying CMAS dose rates. Over the range investigated, the lightest CMAS dose rate used 80% less CMAS to cause failure compared to the heaviest rate, disproving the concept of a “critical CMAS dose” for failure. Rather, failure is a mix of cycling and CMAS damage. As hot time and the number of thermal cycles increase, TGO growth and accumulated damage effectively reduce the toughness of the coating, making it more susceptible to spallation with less CMAS penetration. Second, differences between testing with homogeneous and heterogeneous CMAS were investigated on APS YSZ TBCs. While failures were similar, partial-life microstructures revealed differences in melting kinetics, which may have implications on how reactive TBC compositions interact with CMAS. Cyclic Durability of Thermal Barrier Coatings Subject to CMAS Attack Alan Barkley Harris B.S., University of Maryland, Baltimore County, 2009 A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy at the University of Connecticut 2018 Copyright by Alan Barkley Harris 2018 This Document has been Publicly Released ii APPROVAL PAGE Doctor of Philosophy Dissertation Cyclic Durability of Thermal Barrier Coatings Subject to CMAS Attack Presented by Alan Harris, B.S. Major Advisor__________________________________________________________________ Eric H. Jordan Associate Advisor_______________________________________________________________ Maurice Gell Associate Advisor_______________________________________________________________ Mark Aindow University of Connecticut 2018 iii DEDICATION In memory of George Barkley, mein Opa iv ACKNOWLEDGEMENTS First, I would like to thank Professor Eric Jordan for his years of support as my advisor, with constant ideas, encouragement, and a healthy balance of carrot and stick. Thanks also to Professors Maurice Gell and Mark Aindow for their feedback in developing this thesis. I would also like to thank my lab mates through the years for their shared experience, knowledge, skills, and a helping hand when I just needed to hold these hoses upright, bend this strut to the left, and tighten this bolt just a little more… Mario Bochiechio, Chen Jiang, Rishi Kumar, Byung Jun, and Amadeusz Nasuta all contributed to the success of this research. Thank you to the wonderful staff of IMS and C2E2. Shari Masinda and Nancy Kellerann were ever ready to deal with the next dozen purchase orders with a smile and just the right amount of sarcasm. Thank you to Mark Biron and Peter Menard for the Sisyphean effort of keeping the SEM running. Peter was also instrumental in the total demolition and renovation of our lab space, which was a personal education unto itself. Special thanks to Jeffrey Roth for his boundless energy, which could brighten even the most discouraging day. A special acknowledgement is required for John Fikiet, who, with Amadeusz, constructed our test rigs. Without you, none of this would have been possible. Finally, I’d like to thank my family for their love and support through this long process. I’d especially like to thank my wife, Maureen Harris, for her steadfast support, as well as her reminders that the only person making me do this was me. v TABLE OF CONTENTS 1 Introduction .................................................................................................................. 1 1.1 Gas Turbine Engines and TBCs ............................................................................ 1 1.2 The Challenge of CMAS ....................................................................................... 3 1.3 Research Goals ...................................................................................................... 4 2 Background and Motivation ......................................................................................... 7 2.1 Thermal Barrier Coatings ...................................................................................... 7 2.1.1 Anatomy of a TBC .......................................................................................... 7 2.1.2 TBC Materials ................................................................................................. 9 2.1.3 TBC Depositions Methods ............................................................................ 10 2.1.4 TBC Failure Mechanisms.............................................................................. 12 2.2 CMAS.................................................................................................................. 15 2.3 CMAS – TBC Interactions .................................................................................. 16 2.3.1 CMAS and TBC Chemistry .......................................................................... 16 2.3.2 Thermomechanical Damage .......................................................................... 17 2.4 Mitigation Strategies ........................................................................................... 20 2.5 CMAS Durability Testing of TBCs .................................................................... 22 2.6 Research Motivation ........................................................................................... 25 3 Methods ...................................................................................................................... 26 3.1 Materials Characterization Techniques ............................................................... 26 3.1.1 Microstructural and Elemental Composition Evaluation .............................. 26 3.1.2 Phase Identification ....................................................................................... 26 3.1.3 Calorimetry.................................................................................................... 26 3.2 Test Materials ...................................................................................................... 27 3.2.1 TBC Coupons ................................................................................................ 27 3.2.2 CMAS Compositions and Precursor Development ....................................... 28 3.3 CMAS Characterization and Comparison with Precursors ................................. 30 3.3.1 Calorimetry.................................................................................................... 30 3.3.2 Phase Evolution ............................................................................................. 31 3.4 Isothermal Furnace Testing ................................................................................. 31 vi 3.5 Modeling of Transient Temperature Distributions.............................................. 33 3.6 Development of Test Rigs and Operational Parameters ..................................... 36 4 Test Materials Characterization and Isothermal Testing Results ............................... 37 4.1 Analysis of As-sprayed TBC Coupons ............................................................... 37 4.2 Characterization and Comparison of CMAS Compositions and Precursors....... 41 4.2.1 Microstructure of AFRL-03 Powder ............................................................. 41 4.2.2 Calorimetry Results, CMAS-1 and AFRL-03 ..............................................
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