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Next-Generation Thermal/Environmental Barrier Coatings for Ceramic-Matrix Composites

Dissertation

Presented in Partial Fulfillment of the Requirements of the Degree Doctor of Philosophy in the Graduate School of Brown University

Laura Ruth Turcer, M.S.

Graduate Program: Engineering

Brown University

2020

Dissertation Committee:

Dr. Nitin P. Padture (Advisor)

Dr. Reid F. Cooper

Dr. Brian W. Sheldon

This dissertation by Laura R. Turcer is accepted in its present form by the School of Engineering

as satisfying the dissertation requirement of Doctor of Philosophy.

Date ______

Nitin P. Padture, Advisor

Recommended to the Graduate Council

Date ______

Reid F. Cooper, Reader

Date ______

Brian W. Sheldon, Reader

Approved by the Graduate Council

Date ______

Andrew G. Campbell, Dean of the Graduate School

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CURRICULUM VITAE

2015 to present…...……..…………………Graduate Research Associate, School of Engineering,

Brown University

2017…..………………………..M.S. Materials Science and Engineering, School of Engineering,

Brown University

2014…………..……………………………...…….…….B.S. Materials Science and Engineering,

The Ohio State University

2010……………….…….……………………………………………..Dublin Scioto High School

1992…..………………………………………………………………….Born, Youngstown, Ohio

PUBLICATIONS

1. L.R. Turcer, N.P. Padture, “Rare-earth solid-solution environmental-barrier coating ceramics for Resistance Against Attack by Molten Calcia-Magnesia-Aluminosilicate (CMAS) Glass,” Journal of Materials Research. {Invited, Submitted}

2. L.R. Turcer, N.P. Padture, “Towards thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramics,” Scripta Materialia, 154, 111-117 (2018). {Invited Viewpoint Article}

3. L.R. Turcer*, A.R. Krause*, H.F. Garces, L. Zhang, and N.P. Padture, “Environmental- Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia- Aluminosilicate (CMAS) Glass: Part I, YAlO3 and γ-Y2Si2O7, Journal of the European Ceramic Society, 38, 3905-3913 (2018).

4. L.R. Turcer*, A.R. Krause*, H.F. Garces, L. Zhang, and N.P. Padture, “Environmental- Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia- Aluminosilicate (CMAS) Glass: Part II, β-Yb2Si2O7 and β-Sc2Si2O7, Journal of the European Ceramic Society, 38, 3914-3924 (2018).

*These authors contributed equally

DEDICATION

Dedicated to my family

ACKNOWLEDGEMENTS

I would like to thank Professor Nitin Padture, my advisor, for his support and supervision.

His mentorship has helped me grow as a researcher and as an individual. I really appreciate how much he cares about his graduate students. He not only focuses on supporting my research goals, but has supported me through my experiments’ successes and failures, papers and presentations.

Thank you to Professor Reid Cooper for his support and guidance. I really enjoyed our discussions and I am grateful for his encouragement. I appreciate Professor Brian Sheldon’s support and advice. Both Professors Cooper and Sheldon are wonderful teachers and I am so grateful I was able to take their classes and that they made time for my defense.

My lab mates were also supportive. I would first like to thank Professor Amanda (Mandie)

Krause. When I first started at Brown University, she was concluding work on her PhD. Mandie mentored me in many ways. She trained me on how to use lab equipment: furnaces, CMAS testing,

FIB lift-out, TEM, etc. She helped me conceptualize and organize my research. She also helped me select classes to achieve my research goals. Overall, Mandie made my transition into grad school a smooth one. Hector Garces was also very helpful as I began graduate work. He taught me ceramic processing and XRD and has continued to help me when equipment isn’t functioning. I would like to thank Mollie Koval, Connor Watts, Hadas Sternlicht, Anh Tran, and Arundhati

Sengupta who all contributed significantly to this project. My lab mates Dr. Lin Zhang, Dr.

Yuanyuan Zhou, Qizhong Wang, Min Chen, Srinivas Yadavalli and Zhenghong Dai, Dr. Christos

Athanasiou and Dr. Cristina Ramírez have been supportive. I would like to give a special thanks to Qizhong Wang who helped me talk through problems and checked my math. I would like to thank Yoojin Kim, Helena Liu, Steven Ahn, Selda Büyüköztürk, Juny Cho, Nupur Jain, Sayan

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Samanta, Gali Alon Tzenzana, Ana Oliveira, Ally MacInnis, and Cintia J. B. de Castilho for their support and friendship.

I would like to thank Tony McCormick for his help. He taught me how to use the characterization tools necessary for most of this work and was always friendly and willing to help.

I appreciate Indrek Kulaots and Zack Saleeba for their help in DTA analysis. I would also like to thank John Shilko and Brian Corkum for their assistance. Much thanks to Peggy Mercurio, Cathy

McElroy, and Diane Felber for their friendly assistance and administrative expertise. Although my defense will now be held on Zoom, I would like to thank Kathy Diorio, Beth James, Amy Simmons and Paul Waltz for their assistance navigating arrangements and helping me find a room for my defense.

All of this work would not have been completed without the contributions of Professor

Sanjay Sampath and Dr. Eugenio Garcia at the State University of New York at Stony Brook

University. I am grateful for their collaboration and ability to produce APS coatings. Thanks to

Dr. Gopal Dwivedi at Oerlikon Metco for providing materials. I would also like to thank Professor

Martin Harmer at Lehigh University for allowing me use of his SPS while ours was down. Thanks to Professor Elizabeth Opila of the University of Virginia and her students, Dr. Bekah Webster and Mackenzie Ridley, for their help with water vapor corrosion studies.

Last, but not least, I would like to thank my family and friends for their support and love.

A special thanks to my parents Joe and Catherine. I really grateful for my mom, my Aunt Elizabeth

(Zee) Enke and my friend Ally MacInnis. They took time out of busy schedules to review my thesis. They sent care packages and listened to my whining.

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TABLE OF CONTENTS

TITLE PAGE ...... i COPYRIGHT PAGE ...... ii SIGNATURE PAGE ...... iii CURRICULUM VITAE ...... iv PUBLICATIONS ...... v DEDICATION...... vi ACKNOWLEDGEMENTS ...... vii TABLE OF CONTENTS ...... ix TABLE OF TABLES ...... xiii TABLE OF FIGURES ...... xv CHAPTER 1: INTRODUCTION ...... 1 1.1 Gas-Turbine Engine Materials ...... 1 1.2 Environmental Barrier Coatings ...... 3 1.2.1 EBC Requirements ...... 4 1.2.2 EBC Materials and Processing ...... 5 1.2.3 EBC Failure ...... 7 1.3 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits ...... 8 1.3.1 CMAS Induced Failure ...... 10 1.3.2 Approaches for CMAS Mitigation ...... 12 1.4 Approach ...... 13 1.4.1 Materials Selection/Optical Basicity ...... 13 1.4.2 Objectives ...... 16 CHAPTER 2: Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN CMAS ...... 18 2.1 Introduction ...... 18 2.2 Experimental Procedure ...... 19 2.2.1 Processing ...... 19 2.2.2 CMAS interactions ...... 20 2.2.3 Characterization ...... 21 2.3 Results ...... 22 2.3.1 Polycrystalline Pellets ...... 22

2.3.2 YAlO3-CMAS Interactions ...... 24

2.3.3 Y2Si2O7-CMAS Interactions ...... 30 2.4 Discussion ...... 34 2.5 Summary ...... 36 CHAPTER 3: Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN CMAS ...... 38 3.1 Introduction ...... 38 3.2 Experimental Procedure ...... 40 3.2.1 Processing ...... 40 3.2.2 CMAS Interactions ...... 41 3.2.3 Characterization ...... 41 3.3 Results ...... 42 3.3.1 Polycrystalline Pellets ...... 42

3.3.2 Yb2Si2O7-CMAs Interactions ...... 44

3.3.3 Sc2Si2O7-CMAS Interactions ...... 51

3.3.4 Lu2Si2O7-CMAS Interactions ...... 55 3.4 Discussion ...... 60 3.5 Summary ...... 65 CHAPTER 4: RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS ...... 67 4.1 Introduction ...... 67 4.2 Experimental Procedures ...... 69 4.2.1 Powders...... 69 4.2.2 CMAS Interaction ...... 70 4.2.3 Characterization ...... 70 4.3 Results ...... 71 4.3.1 Powder and Polycrystalline Pellets ...... 71 4.3.2 NAVAIR CMAS Interactions ...... 75 4.3.3 NASA CMAS Interactions ...... 78 4.3.4 Icelandic Volcanic Ash CMAS Interactions ...... 80 4.4 Discussion ...... 82 4.5 Summary ...... 84

CHAPTER 5: THERMAL CONDUCTIVITY ...... 85 5.1 Introduction ...... 85 5.1.1 Coefficient of Thermal Expansion ...... 86 5.1.2 Phase Stability ...... 87 5.1.3 Solid solutions ...... 88 5.2 Calculated Thermal Conductivity of Binary Solid-Solutions ...... 89 5.2.1 Experimental Procedure ...... 89

5.2.2 Pure RE2Si2O7 (RE = Yb, Y, Lu, Sc) Thermal Conductivity ...... 90 5.2.3 Thermal Conductivity Calculations for Binary Solid-Solutions ...... 91

5.3 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity ...... 96 5.3.1 Experimental Procedure ...... 96 5.3.2 Comparison of Experimental and Calculated Thermal Conductivity...... 97 5.4 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution ...... 100 5.4.1 Introduction to High-Entropy Ceramics ...... 100 5.4.2 Experimental Procedure ...... 101 5.4.3 Solid Solution Confirmation ...... 103 5.4.4 Experimental Thermal Conductivity Results ...... 106 5.5 Summary ...... 107 CHAPTER 6: RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS ...... 109 6.1 Introduction ...... 109 6.2 Experimental Procedures ...... 111 6.2.1 Air Plasma Sprayed Coatings ...... 111 6.2.2 Heat Treatments ...... 111 6.2.3 CMAS Interactions ...... 111 6.2.4 Characterization ...... 112 6.3 Results ...... 113 6.3.1 As-sprayed and Heat-Treated Coatings...... 113 6.3.2 NAVAIR CMAS Interactions ...... 117 6.4 Discussion ...... 122 6.5 Future Work ...... 124 6.6 Summary ...... 124

CHAPTER 7: CONCLUSIONS AND FUTURE WORK ...... 126 7.1 Summary and Conclusions ...... 126 7.2 Future Work ...... 129 REFERENCES ...... 132

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TABLE OF TABLES

Table 1: Optical Basicities of relevant single cation oxides for EBCs. Based off Ref. [78]...... 14

Table 2: Calculated Optical Basicities of various potential EBC compositions that have been tested with CMASs. Based off Ref. [78] ...... 15

Table 4: Average EDS elemental composition (at%; cation basis) from the regions indicated in the TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 °C for 1 h...... 26

Table 5: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 °C for 24 h...... 29

Table 6: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 °C for 1 h...... 31

Table 7: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 °C for 24 h...... 33

Table 10: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 °C for 1 h. . 52

Table 11: Average EDS elemental composition (at%; cation basis) from the regions indicated in the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 °C for 24 h. 55

Table 12: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 °C for 1 h. . 57

Table 13: Original CMAS compositions used in this study (mol%) and the Ca/Si (at.) ratio for each...... 69

Table 14: Average EDS elemental composition (at%; cation basis) from the regions numbered in the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic. The ideal composition is also included...... 75

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Table 18: Properties and parameters for pure β-RE-pyrosilicates...... 93

Table 20: Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and rule-of-mixture calculations...... 99

Table 21: Average EDS elemental composition (at%; cation basis) from the regions numbered in the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 EBC ceramic pellet...... 106

Table 22: Density measurements, relative density and open porosity for the as-sprayed and heat- treated (HT, 1300 °C, 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings...... 116

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TABLE OF FIGURES

Figure 1: Schematic illustration of a TBC-coated turbine blade. The blue line is the relative thermal gradient through the TBC layers. From Ref. [1]...... 1

Figure 2: Operational temperatures of gas-turbine engines over the past five decades, redrawn from Ref. [3]. The orange region denotes the temperature at which calcia-magnesia-aluminosilicate (CMAS) deposits melt, interact, and degrade coatings...... 2

Figure 3: A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B) volatilization of the SiO2 layer in the presence of water vapor, leading to recession of the SiC- based CMC material [12]...... 4

Figure 4: (A) Cross-sectional SEM image of BSAS/BSAS+mullite/Si on melt-infiltrated (MI) CMC [18]. (B) Cross-sectional SEM image of a later generation T/EBC [13]...... 5

Figure 5: Schematic illustrations of key EBC failure modes: (A) Recession by water vapor, (B) Steam oxidation, (C) Thermo-mechanical fatigue, (D) CMAS ingestion, (E) Erosion, and (F) Foreign object damage [51]...... 8

Figure 6: Compositions of major components of three different classes of CMAS (mineral sources, engine deposits, and simulated CMAS sand) from the literature (name of author/company on the x-axis), and their calculated optical basicities (OB or Λ, discussed in Section 1.4.1), modified from References [59,78]. Mineral sources: Ford/Average Earth’s Crust [62,79], Smialek/Saudi Sand [61], PTI/Airport Runway Sand [80], Taylor/Mt. St. Helen’s Volcanic Ash [81], Drexler/Eyjafjallajökull Volcanic Ash [71], Chesner/Subbituminous Fly Ash [82], Chesner/Bituminous Fly Ash [82], and Krause/Lignite Fly Ash [78]. Engine deposits: Smialek [61], Borom [62], Bacos [83], and Braue [84]. Simulated CMAS Sand: Steinke [85], Aygun [70,86], Krämer [65], Wu [87], and Rai [88]...... 9

Figure 7: (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat EBC/Mullite/Si bond-coat/SiC-CMC after just 1 minute at 1300 °C [33]. (B-C) Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3, lighter streaks) EBCs that have interacted with CMAS at 1300 °C for (B) 4 h and (C) 24 h [36]...... 11

Figure 8: Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3, lighter streaks) EBCs that have interacted with CMAS at 1300 °C for (A) 100 h and (B) 200 h [36]...... 11

Figure 9: (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are present)...... 23

Figure 10: (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed XRD pattern showing phase-pure γ-Y2Si2O7...... 23

Figure 11: Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at 1500 °C in air for (A) 1 min and (B) 1 h. The circled numbers correspond to locations where elemental compositions were obtained using EDS, and they are reported in Table 3. The dashed boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB. .... 24

Figure 12: Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 °C, 1 h) from regions within the interaction zone similar to those indicated in Figure 11B: (A) near-top and (B) near-bottom. Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers, and their elemental compositions (EDS) are reported in Table 4. The inset in Figure 12A is indexed SAEDP from a Y-Ca-Si apatite (ss) grain. Transmitted beam and zone axis are denoted by ‘T’ and ‘B,’ respectively...... 26

Figure 13: Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 °C, 24 h) at (A) low and (B) high magnification. Corresponding EDS elemental maps: (C) Ca and (D) Si. The dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14 were collected...... 28

Figure 14: Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3 pellet (1500 °C, 24 h) from regions within the interaction zone similar to those indicated in Figure 13B: (A) near-top and (B) near-bottom. The circled numbers correspond to locations where elemental compositions were obtained using EDS, and they are reported in Table 5...... 29

Figure 15: Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at 1500 °C for 24 h, showing the presence of Y-Ca-Si apatite (ss), Y3Al5O12 (YAG), and Y4Al2O9 (YAM), in addition to unreacted YAlO3...... 30

Figure 16: Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at 1500 °C for 1 h. The circled numbers correspond to regions where the elemental compositions were measured by EDS, and they are reported in Table 6...... 31

Figure 17: Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 °C, 24 h): (A) low and (B) high magnification. Corresponding EDS elemental maps: (C) Ca and (D) Si. The dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18 were collected...... 32

Figure 18: Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7 pellet (1500 °C, 24 h) from regions within the interaction zone similar to those indicated in Figure 17B: (A) near-top and (B) near-bottom. The circled numbers correspond to locations where elemental compositions were obtained using EDS, and they are reported in Table 7...... 33

Figure 19: Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at 1500 °C for 24 h, showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7...... 34

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Figure 20: (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B) indexed XRD pattern showing phase-pure β-Yb2Si2O7...... 42

Figure 21: (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed XRD pattern showing phase-pure β-Sc2Si2O7...... 43

Figure 22: (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B) indexed XRD pattern showing phase-pure β-Lu2Si2O7...... 44

Figure 23: Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 °C, 1 h) at: (A) low and (B) high magnifications. (C) EDS elemental Ca map corresponding to (B). The dashed box in (A) indicates the region from where higher-magnification SEM image in (B) was collected. The circled numbers correspond to locations where elemental compositions were obtained using EDS, and they are reported in Table 8. The dashed boxes in (B) indicate the regions from where the TEM specimens were extracted using the FIB...... 45

Figure 24: Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7 pellet (1500 °C, 1 h) from regions within the interaction zone similar to those indicated in Figure 23B: (A) near-top and (B) middle. Yb-Ca-Si apatite (ss) grain, β-Yb2Si2O7 grain, and CMAS glass are marked. Transmitted beam and zone axis are denoted by ‘T’ and ‘B,’ respectively...... 46

Figure 25: Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 °C, 24 h): (A) low (whole pellet) and (B,D) high magnification. The dashed boxes in (A) indicate regions from where higher-magnification SEM images in (B) and (D) were collected. The circled numbers in (B) correspond to locations where elemental compositions were obtained using EDS, and they are reported in Table 9. The dashed box in (B) indicates the region from where the TEM specimen was extracted using the FIB...... 48

Figure 26: Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated at 1500 °C for 24 h, showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7...... 49

Figure 27: Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 °C, 24 h) from regions within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS glass are marked. The circled number corresponds to a location where elemental composition was obtained using EDS, and it is reported in Table 9...... 49

Figure 28: Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have interacted with CMAS at 1500 °C for: (A) 1 h, (B) 3 h, (C) 12 h, (D) 24 h, and (E) 24 h. The pellets in (A)-(D) are ~2 mm thick, and the pellet in (E) is ~1 mm thick. The region between the arrows is where the CMAS was applied. The gray contrast in the ‘blister’ cracks in some of the micrographs is epoxy from the sample mounting...... 50

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Figure 29: SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm thickness) that has interacted with CMAS at 1500 °C for 6 h: (A) top region and (B) bottom region...... 51

Figure 30: (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 °C, 1 h) and (B) corresponding EDS elemental Ca map. The circled numbers in (A) correspond to locations where elemental compositions were obtained using EDS, and they are reported in Table 10. .... 52

Figure 31: Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 °C, 24 h) at: (A) low (whole pellet) and (B,C) high magnifications. The dashed boxes in (A) indicate regions from where higher-magnification SEM images in (B) and (C) were collected, and the region from where the TEM specimen was extracted using the FIB...... 53

Figure 32: (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 °C, 24 h) from region within the interaction zone similar to that indicated in Figure 31A. Indexed SAEDP is from the grain marked β-Sc2Si2O7. (B) Higher-magnification bright-field TEM image from region indicated by the dashed box in (A). (C) EDS elemental Ca map corresponding to (B). Transmitted beam and zone axis are denoted by ‘T’ and ‘B,’ respectively. The circled numbers in (B) correspond to locations where elemental compositions were obtained using EDS, and they are reported in Table 11...... 54

Figure 33: Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at 1500 °C for 24 h, showing only the presence of unreacted β-Sc2Si2O7...... 55

Figure 34: Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 °C, 1 h) at: (A) low (entire CMAS-interacted zone), (B) low (whole pellet thickness), and (C) higher magnification. The dashed boxes in (A) indicate regions from where higher-magnification images in (B) and (C) were collected. (D, E) Higher magnification images represented in (C) as dashed boxes, and (F, G) their corresponding EDS Ca maps, respectively. The circled numbers in (D, F) correspond to locations where elemental compositions were obtained using EDS, and they are reported in Table 12...... 56

Figure 35: Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 °C, 24 h) at: (A) low (whole pellet thickness) and (B) high magnifications. The dashed box in (A) indicates the region from where (B) was collected. (C) EDS elemental Ca map corresponding to (B)...... 58

Figure 36: Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 °C, 24 h) at: (A) low and (B, C) higher magnifications. (B) was obtained from a region near the bottom of the CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction zone, close to the edge of the pellet...... 59

Figure 37: Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated at 1500 °C for 24 h, showing only the presence of unreacted β-Lu2Si2O7...... 59

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Figure 38: (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain boundaries causing a dilatation gradient, (B) resulting in a ‘blister’ crack due to buckling of the top dilated layer...... 61

Figure 39: (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β- Yb2Si2O7/1 vol% CMAS pellet and (B) corresponding EDS elemental Ca map...... 62

Figure 40: (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O7/1 vol% CMAS pellet that have interacted with CMAS at 1500 °C for 24 h. The region between the arrows is where the CMAS was applied. (B) Cross-sectional SEM image of the whole pellet from the region marked by the dashed box in (A). (C) Higher-magnification cross-sectional SEM image of the region marked by the dashed box in (B), and (D) corresponding EDS elemental Ca map...... 63

Figure 41:Cross-section SEM images of dense, polycrystalline RE2Si2O7 pyrosilicate ceramic pellets that have interacted with the CMAS glass under identical conditions (1500 °C, 24 h): (A) Y2Si2O7, (B) Yb2Si2O7, (C) Sc2Si2O7, and (D) Lu2Si2O7...... 65

Figure 42: (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs. Redrawn and adapted from Ref. [37]. (B) Binary phase diagram showing complete solid-solubility of the Yb(2-x)YbxSi2O7 system, with different polymorphs. The dashed lines represent the compositions chosen in this chapter. Adapted from Ref. [38]...... 68

Figure 43: SEM images of powders: (A) Yb1.8Y0.2Si2O7 and (B) Yb1Y1Si2O7. Cross-sectional SEM images of the thermally-etched EBC ceramics: (C) Yb1.8Y0.2Si2O7 and (D) Yb1Y1Si2O7. (E) XRD pattern of Yb2Si2O7, Yb1.8Y0.2Si2O7, and Yb1Y1Si2O7 EBC ceramics showing β-phase. (F) Higher resolution XRD patterns...... 72

Figure 44: (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic. (B) Higher magnification bright-field TEM image of the region marked in (A). The circled numbers correspond to regions from where EDS elemental compositions are obtained (see Table 14). (C) High-magnification bright-field TEM image showing a grain boundary. (D) EDS line scan along L-R in (C)...... 74

Figure 45: Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C, 24 h) EBC ceramics: (A) Yb2Si2O7, (B) Yb1.8Y0.2Si2O7, (C) Yb1Y1Si2O7, and (D) Y2Si2O7. Dashed boxes indicate from where the corresponding higher-magnification SEM images are collected: (E) Yb1.8Y0.2Si2O7 and (G) Yb1Y1Si2O7. Corresponding EDS Ca elemental maps: (F) Yb1.8Y0.2Si2O7 and (H) Yb1Y1Si2O7. The circled numbers in (E) and (G) correspond to regions from where EDS elemental compositions are obtained (see Table 15). (A) and (D) adapted from Refs. [117] and [116], respectively...... 77

xix elemental maps: (I) Yb2Si2O7, (J) Yb1.8Y0.2Si2O7, (K) Yb1Y1Si2O7, and (L) Y2Si2O7. The circled numbers in (E) through (G) correspond to regions from where EDS elemental compositions are obtained (see Table 16)...... 79

Figure 47: Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C, 24 h) EBC ceramics: (A) Yb2Si2O7, (B) Yb1.8Y0.2Si2O7, (C) Yb1Y1Si2O7, and (D) Y2Si2O7. Dashed boxes indicate from where the corresponding higher-magnification SEM images are collected: (E) Yb2Si2O7, (F) Yb1.8Y0.2Si2O7, (G) Yb1Y1Si2O7, and (H) Y2Si2O7. Corresponding EDS Ca elemental maps: (I) Yb2Si2O7, (J) Yb1.8Y0.2Si2O7, (K) Yb1Y1Si2O7, and (L) Y2Si2O7. The circled numbers in (E) through (G) correspond to regions from where EDS elemental compositions are obtained (see Table 17)...... 81

Figure 48: (A) Cross-sectional SEM micrograph of a T/EBC on a CMC [13]. (B) Schematic illustration of the T/EBC concept adapted from [4]. (C) Schematic Illustration of the TEBC concept...... 85

Figure 49: (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs, which is adapted from Ref. [137]. The horizontal band represents the CTE of SiC-based CMCs. (B) Stability diagram of the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref. [37]...... 87

Figure 50: Thermal conductivities of dense, polycrystalline RE2Si2O7 pyrosilicate ceramic pellets as a function of temperature. The data for Lu2Si2O7 is from Ref [142]...... 91

Figure 51: The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions at 27, 200, 400, 600, 800, and 1000 °C: (A) YxYb(2-x)Si2O7, (B) YxLu(2-x)Si2O7, (C) YxSc(2-x)Si2O7, (D) YbxSc(2-x)Si2O7, (E) LuxSc(2-x)Si2O7, and (F) LuxYb(2-x)Si2O7. The thermal conductivities of the pure, dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes. The dashed lines represent 1 W·m-1·K-1...... 94

Figure 52: Thermal conductivities of dense, polycrystalline Yb2Si2O7, Y2Si2O7, Y0.2Yb1.8Si2O7, and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature. The dashed line represents 1 W·m-1·K-1...... 97

Figure 53: The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27, 200, 400, 600, 800, and 1000 °C (solid lines) compared to the experimentally collected thermal conductivities, which can also be found in Figure 52 (circles). The dashed line represents 1 W·m-1·K-1...... 98

Figure 54: Indexed XRD pattern from an as-SPSed β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 pellet, compared to β-Yb2Si2O7, β-Yb1.8Y0.2Si2O7, and β-Yb1Y1Si2O7 pellets...... 103

Figure 55: Cross-sectional SEM image of an as-SPSed β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 pellet and the corresponding EDS elemental maps: Y, Yb, Lu, Sc, Gd, and Si...... 104

Figure 56: (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β- (Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 pellet. β-RE2Si2O7 grains are found. Transmitted beam and zone

xx axis are denoted by ‘T’ and ‘B,’ respectively. (B, C) Two higher magnification regions showing grain boundaries and the corresponding EDS elemental maps: Y, Yb, Lu, Sc, Gd, and Si. The circled regions are where EDS elemental compositions were obtained and can be found in Table 21...... 105

Figure 57: Thermal conductivities of dense, polycrystalline Yb2Si2O7, Y2Si2O7, Y0.2Yb1.8Si2O7, Y1Yb1Si2O7, and β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 pyrosilicate ceramic pellets as a function of temperature. The dashed line represents 1 W·m-1·K-1...... 107

Figure 58: Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at: (A) low and (B) high magnification. The lighter gray regions in these images contain less silica. (C) Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides, showing a mostly amorphous coating...... 113

Figure 59: Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at: (A) low and (B) high magnification. The lighter gray regions in these images contain less silica. (C) Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides, showing a mostly amorphous coating...... 114

Figure 60: In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from room temperature (25 °C). XRD patterns were collected also collected at 800, 900, 1000, 1100, 1200, 1300, and 1350 °C. The circle markers and the solid lines index the Yb1Y1Si2O7 phase and the square markers and dashed line index the Yb1Y1SiO5 phase...... 115

Figure 61: Cross-sectional SEM micrographs of the heat-treated (1300 °C, 4 h) Yb2Si2O7 APS coating at: (A) low and (B) high magnification. The lighter gray regions in these images are Yb2SiO5, the darker gray regions are Yb2Si2O7 and the black regions are pores. (C) Indexed XRD patterns from the heat-treated (1300 °C, 4 h) Yb2Si2O7 APS coating on the top and bottom sides, showing both Yb2Si2O7 and Yb2SiO5 are present...... 116

Figure 62: Cross-sectional SEM micrographs of the heat-treated (1300 °C, 4 h) Yb1Y1Si2O7 APS coating at: (A) low and (B) high magnification. The lighter gray regions in these images are Yb1Y1SiO5, the darker gray regions are Yb1Y1Si2O7 and the black regions are pores. (C) Indexed XRD patterns from the heat-treated (1300 °C, 4 h) Yb1Y1Si2O7 APS coating on the top and bottom sides, showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present...... 117

Figure 63: (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 °C, 24 h) Yb2Si2O7 APS Coating. The dashed line indicates the depth of the CMAS interaction zone. The dashed box indicates the region where (B) was collected. (B) A higher magnification image and its corresponding Si, Ca, and Yb elemental EDS maps...... 118

Figure 64: (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted (1500 °C, 24 h) Yb2Si2O7 APS coating. The dashed boxes in (A) indicate where higher magnification images were obtained. (B, D) The higher magnification SEM micrographs and (C, E) their corresponding elemental Ca EDS maps, respectively. The circled numbers in (B, D)

xxi correspond to locations where elemental compositions were obtained using EDS, and they are reported in Table 23...... 119

Figure 65: (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 °C, 24 h) Yb1Y1Si2O7 APS Coating. The dashed line indicates the depth of the CMAS interaction zone. The dashed box indicates the region where (B) was collected. (B) A higher magnification image and its corresponding Si, Ca, Y and Yb elemental EDS maps...... 120

Figure 66: (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted (1500 °C, 24 h) Yb1Y1Si2O7 APS coating. The dashed boxes in (A) indicate where higher magnification images were obtained. (B, D) The higher magnification SEM micrographs and (C, E) their corresponding elemental Ca EDS maps, respectively. The circled numbers in (B, D) correspond to locations where elemental compositions were obtained using EDS, and they are reported in Table 24...... 121

Figure 67: (A-D) Plan view SEM images of the impingement site for Yb2Si2O7, Yb1.8Y0.2Si2O7, Yb1Y1Si2O7, and Yb2Si2O7, respectively. (E-H) Cross-sectional SEM images of the impingement zone for Yb2Si2O7, Yb1.8Y0.2Si2O7, Yb1Y1Si2O7, and Yb2Si2O7, respectively. (I-L) The corresponding Si elemental EDS maps to (E-H), respectively...... 130

xxii

CHAPTER 1: INTRODUCTION

1.1 Gas-Turbine Engine Materials

The use of ceramic thermal barrier coatings (TBCs) on Ni-based superalloy components,

in conjunction with air-cooling, has resulted in the hot-section of gas-turbine engines ability to

operate at maximum temperatures above 1500 °C [1–4]. Figure 1 is a schematic illustration of a

TBC-coated turbine blade allowing for higher operating temperatures and the relative thermal

gradient through the TBC layers. This has resulted in outstanding power and efficiency gains in

gas-turbine engines used for aircraft propulsion and land-based power generation.

Figure 1: Schematic illustration of a TBC-coated turbine blade. The blue line is the relative thermal gradient through the TBC layers. From Ref. [1].

TBC microstructures usually contain cracks and pores, which are deliberate, to reduce TBC

thermal conductivity and to provide strain-tolerance against residual stresses that buildup due to

the thermal expansion coefficient (CTE) mismatch with the base metal substrate. TBCs with even

1 higher temperature capabilities and lower thermal conductivities are being developed [3–5]. Figure

2 shows the progress over decades for the temperature capabilities of Ni-based superalloys, TBCs, and Ceramic-Matrix Composites (CMCs) along with the allowable gas temperature in a gas- turbine engine. However, TBC developments have outpaced those of the Ni-based superalloys, which has led to more aggressive cooling requirements. Unfortunately, this results in an increase of inefficiency losses, or the difference in ideal and actual specific core power for a gas-inlet temperature [4,6].

Therefore, hot-section materials with inherently higher temperature capabilities are needed. In this context, CMCs, typically comprising of silicon carbide (SiC) fibers in a SiC matrix, are showing promise to replace Ni-based superalloys in the engine hot-section [4,6–8]. CMCs have already replaced some Ni-based superalloy hot-section, stationary components in gas-turbine engines that are in-service commercially, both for aircraft propulsion and power generation.

1.2 Environmental Barrier Coatings

CMCs for gas-turbine applications, both aerospace and power generation, are primarily

SiC-based, continuous SiC fibers in a SiC matrix. SiC-based CMCs are lightweight, damage tolerant, resistant to thermal shock and impact, and display better resistance to high temperatures and aggressive environments than metals [9]. SiC-based CMCs have excellent high temperature capabilities; they maintain mechanical properties at temperatures up to 3000 °C [10].

Unfortunately, SiC-based CMCs undergo active oxidation and recession in the high-velocity hot- gas stream, containing both oxygen and water vapor [4,11–13]. In the presence of oxygen, SiC forms a passive SiO2 layer on the surface, using the chemical reaction below [14], and shown as a schematic illustration in Figure 3A.

3 푆푖퐶 + 푂 (푔) = 푆푖푂 + 퐶푂 (푔) (Equation 1) 2 2 2 However, in the gas-turbine engine combustion environment, ~ 10% water vapor is also present.

This leads to the volatilization of the SiO2 layer and active recession of the base layer according to the reaction below [15], which can also be seen as a schematic illustration in Figure 3B:

푆푖푂2 + 2퐻2푂 (푔) = 푆푖(푂퐻)4 (푔) (Equation 2)

A B

Therefore, SiC-based CMCs need to be protected by ceramic environmental barrier coatings (EBCs) [4,7,13,16,17].

1.2.1 EBC Requirements

Along with the need to protect SiC-based CMCs from oxygen and water vapor due to active oxidation and recession, there are many other requirements on EBCs. EBCs should have low permeability of oxygen and water vapor. Therefore, they should also be dense and crack-free to prevent recession of the SiC-based CMC. Consequently, they must have a good coefficient of thermal expansion (CTE) match with the SiC-based CMCs. [7,8] EBCs must also have low silica activity/volatility, so that they do not show major recession, like the SiC-based CMCs. EBCs will be operating at temperatures around 1500 °C, so they should have high-temperature capability, phase stability and robust mechanical properties. They need to have chemical compatibility with the bond-coat material. And lastly, they must be resistant to molten calcia-magnesia- aluminosilicate (CMAS) deposits, which will be discussed in more detail is Section 1.3.

1.2.2 EBC Materials and Processing

In the late 1990s, EBCs comprised of a silicon bond-coat on a CMC, an interlayer of barium strontium aluminum silicate (BSAS, (1 - x)BaO·xSrO·Al2O3·2SiO2 with 0 < x < 1) and mullite

(3Al2O3·2SiO2) mixture and a top coat of BSAS, called Gen I, were early, successful EBC architectures [7,13,18]. This Gen I EBC system is shown in Figure 4A. All layers were deposited by thermal spray [18]. The Si bond-coat enhances the adherence between the CMC and the mullite layer and promotes the formation of a dense and protective SiO2 thermally grown oxide (TGO), which adds additional protection to the CMC [13,17,18]. Mullite was promising due to its low

CTE. Unfortunately, crystalline mullite coatings experience silica volatility and phase instability in water vapor environments [17,19]. An Al2O3 layer remains, but it is porous and brittle. Adding a topcoat of BSAS, which has a lower silica activity than mullite and a CTE of ~4.3 x 10-6 °C-1 in the celsian phase, closely matching that of SiC (~4.5 x 10-6 °C-1), has been found to provide adequate high-pressure protection at temperatures below 1300 °C [18].

A B

Figure 4: (A) Cross-sectional SEM image of BSAS/BSAS+mullite/Si on melt-infiltrated (MI) CMC [18]. (B) Cross-sectional SEM image of a later generation T/EBC [13].

The next generation EBCs, or Gen II to VI, were developed for higher temperature applications. These are based on rare earth (RE) silicates, with several variations such as the

5 additions of oxides (i.e. HfO2, mullite, etc.) [13] The most studied EBCs have been Y-silicates

(Y2SiO5 [20–22] and Y2Si2O7 [22–27]) and Yb-silicates (Yb2SiO5 [28–32] and Yb2Si2O7

[23,25,26,33–36]). The monosilicates, Y2SiO5 and Yb2SiO5, have low silica activity and high melting points, but they have higher CTEs than SiC. The disilicates, Y2Si2O7 and Yb2Si2O7, have a better CTE match to SiC, but a higher silica activity. [7] However, EBCs tend to fail mechanically, therefore, disilicate EBCs are being used. Yb2Si2O7 has been a focus due to its phase stability, as it does not experience a phase transition up to 1700 °C [37,38].

Bond coat replacements are also being studied due to the low melting point of Si (1410 °C)

[13]. Oxide bond-coats containing rare earths (i.e. Hf, Zr, Y) could improve oxidation resistance and thermal cycling durability [13]. EBC systems that also include thermal barrier coatings (TBCs) on top of the EBC system described, called T/EBC, have also been studied. The TBC has a lower thermal conductivity to help with high temperatures experienced in a gas-turbine engine. However, the CTE difference of the TBC (9-10 x 10-6 °C-1) and the EBC (4-5 x 10-6 °C-1) in T/EBC systems is large, which means a graded CTE interlayer is needed between the two coatings to alleviate stress concentrations that occur at interfaces [4,13]. An example of this T/EBC system can be seen in Figure 4B.

EBC deposition is still a significant challenge [39,40]. Conventional air plasma spray

(APS) is preferred, but the EBCs typically deposit as an amorphous coating [41]. Many have performed APS inside a box furnace so that the substate is heated to temperatures around 1000 °C so that the coating can crystalize during spraying [17,33,36,42,43], but this is difficult in a manufacturing setting. Post-deposition heat treatment has also been done on APS Yb2Si2O7 EBC coatings [41], however, crystallization has a significant volume change, which leads to porous coatings and undesirable phases can form during crystallization. Other methods being studied are

6 plasma spray physical vapor deposition (PS-PVD) [39], high-velocity oxygen fuel spraying

(HVOF) [40], slurry dipping [44,45], electron beam physical vapor deposition (EB-PVD) [46,47], chemical vapor deposition (CVD) [48], magnetron sputtering [49], and sol-gel nanoparticle application [50].

1.2.3 EBC Failure

EBCs are subjected to hostile operating conditions in the hot-section of gas-turbine engines. The typical environment is ~10 atm of pressure with a ~300 m.s-1 velocity of gas-stream that contains a water vapor partial pressure of ~0.1 atm and an oxygen partial pressure of ~0.2 atm

[9]. Below, in Figure 5, Lee [51] shows schematic illustrations of the different failure mechanisms

EBCs face. As seen earlier in Section 1.2.1, SiC volatilization occurs in the presence of water vapor. Like CMCs, EBCs usually contain Si (i.e. RE2SiO5 or RE2Si2O7), therefore, they have a non-zero silica activity [52,53] (less than that of SiO2) which will lead to recession of the EBC, which is shown schematically in Figure 5A [51]. Figure 5B shows a schematic illustration of steam oxidation. This occurs when water vapor permeates through the EBC and reacts with the Si bond coat, forming a SiO2 scale, or thermally grown oxide (TGO) [17,42,54]. As the Si bond-coat becomes the SiO2 TGO, many factors increase the stresses in the EBC system including: (i) ~2.2- fold volume expansion as the SiO2 TGO forms [42], (ii) phase transformation (β → α cristobalite)

-6 -1 of SiO2 [55], and (iii) mismatch in the CTE between the α cristobalite SiO2 (10.3 x 10 °C [56])

-6 -1 and the EBC (4-5 x 10 °C [17,57]). As the thickness of the SiO2 TGO increases, stresses build up and once a critical thickness is reached, spallation of the EBC occurs [51,58].

EBCs must also withstand thermo-mechanical cycling (up to 1700 °C) (see Figure 5C) and degradation due to molten calcia-magnesia-aluminosilicate (CMAS, discussed further is Section

1.3) at high temperatures, above 1200 °C (see Figure 5D). Particle damage can occur by erosion

(see Figure 5E) or foreign object damage (FOD) (see Figure 5F), which decreases EBC lifetimes significantly. [51] And in the case of rotating parts, they will need to carry loads that may cause creep and rupture. EBCs are expected to be ‘prime reliant’ or last for the lifetime of the components, which can be several 10,000s of hours of operation [9].

A D

B E

C F

1.3 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits

As the coating-surface temperatures in gas-turbine engines reached 1200 °C, a new damage mechanism has become important: the degradation of TBCs [59–68] and EBCs [23,25–

27,33,34,36,69] from the melting and adhesion of calcia-magnesia-aluminosilicate (CMAS)

8 deposits. In aircraft engines, CMAS is introduced in the form of ingested airborne sand [61–

65,69,70] or volcanic ash [24,60,67,71–73]. In power-generation engines, CMAS is introduced in the form of ‘fly ash’, an impurity in alternative fuels such as syngas [68,74–77]. Figure 6 shows the composition of various CMASs including mineral sources, like volcanic ash, deposits found in engines, and synthetic CMASs used in laboratory experiments. The compositional differences lead to differences in the melt temperature, viscosity, and wetting of the CMAS, which all play a role in how the CMAS will interact with EBCs.

1.3.1 CMAS Induced Failure

The most prevalent failure mode in EBCs is caused by the CTE mismatch between the

CMAS glass and the EBC. CMAS has a CTE of 9-10 x 10-6 °C-1 [89] while most potential EBCs have CTEs of ~4-5 x 10-6 °C-1 [17,57]. Upon cooling to room temperature this can lead to through cracks, which originate in the glass and travel all the way to the bond coat [33]. Stolzenburg et al.

[33] showed an example with a multi-layer EBC system: substrate, Si bond-coat, mullite and

Yb2Si2O7 as the top-coat EBC. After just one minute at 1300 °C, the stresses in the coating caused cracking through the coating, which can be seen in Figure 7A. In Figures 7B and 7C, Zhao et al.

[36] also saw similar cracking. The coatings in this study were majority Yb2Si2O7 with Yb2SiO5 and Yb2O3 impurities. These tests were also conducted at 1300 °C, but for longer times of (B) 4 h and (C) 24 h. Sharp cracks are observed coming from the surface of the CMAS and through the apatite (Ca2RE8(SiO4)6O2) layer. Once the cracks hit the Yb2Si2O7, a lower CTE material, they seem to deflect or turn left or right. This cracking mechanism has also been seen in TBCs that have interacted with CMAS. In TBCs and EBCS, during cooling, vertically aligned or ‘channel’ cracks form near the surface. Delamination, between ‘channel’ cracks can occur leading to spallation of the coating due to crack propagation and coalescence [64].

If spallation occurs, the base materials are exposed, and silica volatilization will proceed.

If spallation does not occur, these cracks are still fast channels to the CMC for oxygen and water vapor or molten CMAS. Lee [51] has showed that even without cracks, the Si bond-coat forms a

TGO and after a critical thickness, EBC spallation can occur. If cracks are present, the Si bond- coat has a direct path for oxygen and water vapor, so localized silica volatilization can occur, leading to premature spallation of the coatings.

A B – 4 h

C – 24 h

Another CMAS-induced failure mechanism observed in EBCs has been the formation of a reaction-crystallization product, apatite (Ca2RE8(SiO4)6O2), which can be seen in Figure 8. Zhao et al. [36] found that after 200 h at 1300 °C almost half of the coating thickness has either been incorporated into the CMAS melt or has formed an apatite reaction phase. It has been seen that apatite formation in Y-containing materials is faster than ytterbium silicates [24,27].

A – 100 h

B – 200 h

1.3.2 Approaches for CMAS Mitigation

CMAS-attack of EBCs is a relatively new issue, and there is a paucity of approaches for

CMAS mitigation. EBCs that react heavily with CMAS have been shown to lose coating thickness and have additional reaction products form [33,36]. The CTE of potential reaction products are unknown. If they have a CTE mismatch with the EBC, through-cracks can occur (more detail can be found in 1.3.1). An example of a reaction product with a mismatched CTE can be seen in

Figures 7 and 8. Due to EBC requirements of dense and crack-free coatings, the concept of optical basicity (OB, see Section 1.4.1 for more detail) has been used. Briefly, OB quantifies the chemical reactivity of oxides and glasses. OB was used to select potential EBC ceramics that would not react heavily with CMAS [78]. Materials selection, of EBCs with low reactivity with CMAS, is a major focus because dissolution of the EBC would be stopped after the solubility limit of the EBC in CMAS was reached.

Coating systems for gas-turbine engines tend to include a porous TBC top-coat on the EBC system. Significant amount of research has gone into improving TBC resistance to CMAS.

Sacrificial, non-wetting and impermeable layers have been applied to the surface of TBCs to stop

CMAS penetration or sticking [90,91]. These coatings increase the CMAS melt temperature or viscosity upon dissolution [90,92,93]. However, once consumed, CMAS can then attack the coating system. Therefore, TBCs that react heavily with CMAS, so that CMAS is consumed by the formation of a reaction-crystallization product, have been shown to provide better protection

[78,94]. Crystallization of reaction products of unknown CTEs works with the TBC because TBCs are porous. However, TBCs are not the focus of this study.

1.4 Approach

First, the concept of optical basicity (OB, Λ) was used as a first order screening for potential

EBCs (see Section 1.4.1 for more details). Then the selected materials were made through powder processing and spark plasma sintering (SPS) to obtain dense, polycrystalline ‘model’ EBC ceramic pellets for ‘model’ CMAS experiments. Their high-temperature interactions were studied (see

Section 1.4.2 for more details).

1.4.1 Materials Selection/Optical Basicity

As a first order screening, optical basicity (OB, Λ) was used to determine potential EBC materials. EBC must be dense, impervious and crack-free, therefore, a limited reaction with CMAS is desired so that the EBC is not consumed by the CMAS or a reaction-crystallization product with unknown or different CTEs. Duffy et al. [95] first used the concept of OB to quantify the chemical activity of oxides and glasses. The OB concept is based on the Lewis acid-base theory, which defines acids as electron acceptors and bases as electron donors. OB of a single metal oxide is defined as the measure of the oxygen anion’s ability to donate electrons, which depends on the polarizability of the metal cation [95,96].

Cations with high polarizability draw the electrons away from the oxygen, which does not allow the oxygen to donate electrons to other cations, which is more ‘acidic’ or a low OB value.

On the other end of the scale, the ‘basic’ or high OB values, oxygen can donate electrons to other cations due to the low polarizability of the cation. [97] OBs of relevant single cation oxides for

EBCs are seen below in Table 1. Ultraviolet spectroscopy [96,98,99], X-ray photoelectron spectroscopy [97], and mathematical relationships between refractivity and electronegativity

[100–102] have been used to measure or estimate the OBs for single cation oxides.

Table 1: Optical Basicities of relevant single cation oxides for EBCs. Based off Ref. [78]. Single Cation Oxide Λ Ref. CaO 1.00 [103] MgO 0.78 [103] * Al2O3 0.60 [103,104] SiO2 0.48 [103] Gd2O3 1.18 [105] Y2O3 1.00 [100] Yb2O3 0.94 [105] La2O3 1.18 [105] Sc2O3 0.89 [100] Lu2O3 0.886 [106] *Based on Al3+ CN = 4. For CN = 6, OB = 0.40.

Duffy [96] found that the OB (Λ) for an oxide or glass composed of several single cation oxides can be calculated using the equation below:

Λ푀푢푙푡푖−푐푎푡푖표푛 푂푥푖푑푒/퐺푙푎푠푠 = 푋퐴 × Λ퐴 + 푋퐵 × Λ퐵 + 푋퐶 × Λ퐶 + ⋯ (Equation 3) where ΛA, ΛB, and ΛC are the OB values of the single cation components, and XA, XB, and XC are the fraction of oxygen ions each single cation oxide donates. Although this model was used to determine the chemical reactivity of glasses, it has also been used to access crystalline materials as well [104,107]. However, for crystalline materials, coordination states need to be considered.

OB values change based on the coordination number (CN) in glasses with an intermediate oxide,

Al2O3 [104].

The difference in OB values of products in a reaction tend to be less than that of the reactants, i.e. there is a ‘smooth[ing] out’ the overall electron density of the oxygen atoms [96].

Therefore, the reactivity is proportional to the change in OB:

푅푒푎푐푡푖푣푖푡푦 ∝ ΔΛ (= Λ푇퐵퐶/퐸퐵퐶 − Λ퐶푀퐴푆) (Equation 4)

This has been used to describe high-temperature reactivity in metallurgical slags [108,109], glasses

[100,105] and oxide catalysts [110]. Acidity, a variation of the OB concept, has also been to

14 explain the hot corrosion behavior of TBCs interaction with sodium vanadates [111]. They found that TBCs (basic OB values) readily react with corrosive agents (acidic OB values). Krause et al.

[78] showed that OB difference calculations are a quantitative chemical basis for screening

CMAS-resistant TBC and EBC compositions. TBC are porous and a reaction is desired (i.e. high reactivity with CMAS) so that the CMAS is consumed by a reaction-crystallization product which will stop the progression of CMAS into the base material. The OBs of a wide range of CMAS compositions, which can be seen in Figure 6, fall within a narrow OB range of 0.49 to 0.75, which is acidic. Unlike TBCs, EBCs need to be dense, so a limited reaction with CMAS is desired [78].

Below is a table of EBC ceramics that have been studied to determine their resistance to CMAS

(Table 2). There is a column in Table 2 that is the change in OB (ΔΛ) between a common CMAS sand with an OB of 0.64 and the chosen EBC ceramics.

Table 2: Calculated Optical Basicities of various potential EBC compositions that have been tested with CMASs. Based off Ref. [78] ΔΛ w.r.t. Sand Multi-Cation Oxide Ref. Λ (Λ = 0.64) * Gd4Al2O9 [112] 0.99 0.35 * Y4Al2O9 [112] 0.87 0.23 ** GdAlO3 [112] 0.79 0.15 ** LaAlO3 [112] 0.79 0.15 Y2SiO5 [69,113] 0.79 0.15 Yb2SiO5 [114] 0.76 0.12 ** YAlO3 [115] 0.70 0.06 Y2Si2O7 [25,69] 0.70 0.06 Yb2Si2O7 [25,114] 0.68 0.04 Sc2Si2O7 [25] 0.66 0.02 Lu2Si2O7 [25] 0.66 0.02 Yb1.8Y0.2Si2O7 -- 0.69 0.05 Yb1Y1Si2O7 -- 0.68 0.04 Based off Krause et al. [78]. For Al3+: *CN = 4, **CN = 6

As stated earlier, the focus of EBCs has been primarily on RE2Si2O7, which can be seen to have small OB difference with CMAS glass. There have been a few experiments conducted with these ceramics and their interactions with CMAS glass [23,25,26,33–36]. However, a systematic study and understanding of CMAS interactions at 1500 °C with dense EBC ceramics had yet to be done. The preliminary ‘model’ EBCs chosen for this study are Yb2Si2O7, Y2Si2O7, Sc2Si2O7 and

Lu2Si2O7. YAlO3 was also chosen because it is Si-free and has been included in a patent as a potential EBC ceramic [115].

1.4.2 Objectives

This work is focused on exploring potential EBC ceramics. First ‘model’ CMAS interaction studies at 1500 °C for varying amounts of time were conducted on ‘model’ EBC ceramics or dense, polycrystalline spark plasma sintered (SPSed) pellets. This was done with the overall goal of providing insights into the chemo-thermal-mechanical mechanisms of these interactions, and to use this understanding to guide the design and development of CMAS-resistant

EBCs. A comparison between Y-containing EBC ceramics, viz YAlO3 and Y2Si2O7, and Y-free

EBC ceramics, viz Yb2Si2O7, Sc2Si2O7 and Lu2Si2O7, and their high-temperature interactions with

CMAS are seen in Chapter 2 and 3, respectively. [116,117]

Chapter 4 uses the insights learned in Chapters 2 and 3 to explore ‘model’ EBC ceramics of solid-solutions of Yb2Si2O7 and Y2Si2O7, or Yb(2-x)YxSi2O7. Two solid solutions, Yb1.8Y0.2Si2O7 and Yb1Y1Si2O7, and their pure end components, Yb2Si2O7 and Y2Si2O7, have been chosen to explore their high temperature interactions with CMAS. In this section, three different CMAS compositions are chosen with varying amounts of Ca and Si (Ca/Si of 0.76, 0.44 and 0.10) to determine how different compositions change the interaction with the same EBC ceramics. The

16 thermal conductivity of these solid solution ceramics and the concept of low-thermal conductivity thermal environmental barrier coatings (TEBCs) are explored in Chapter 5. [118,119]

After completing ‘model’ experiments on dense, polycrystalline EBC ceramic pellets, a few ceramics were air plasma sprayed (APS) as EBC coatings. These APS EBCs were made at

Stony Brook University in collaboration with Professor Sanjay Sampath’s group. In Chapter 6, the focus will be on the coating interactions with CMAS and understanding the effect of the APS coating microstructure (i.e. grain size, porosity, and splat boundaries).

CHAPTER 2: Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN CMAS

This chapter was reproduced from a previously published article: L.R. Turcer, A.R. Krause, H.F. Garces, L. Zhang, and N.P. Padture, “Environmental-barrier coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass: Part I, YAlO3 and γ- Y2Si2O7,” Journal of the European Ceramic Society, 38, 3095-3913 (2018). [116]

2.1 Introduction

Based on the optical basicity (OB) concept (for more detail see Section 1.4.1), YAlO3, γ-

Y2Si2O7, β-Yb2Si2O7, β-Sc2Si2O7, and β-Lu2Si2O7 have been identified as promising CMAS- resistant EBC ceramics [78]. It should be emphasized that the OB-difference analysis provides a rough screening criterion based on purely chemical considerations, and that the actual reactivity will depend on various other factors including the nature of the cations in the EBC ceramics and the CMAS composition. Interactions of these five promising ‘model’ EBC ceramics (dense, polycrystalline ceramic pellets) with a ‘model’ CMAS at 1500 °C are studied in some detail. The overall goal is to provide insights into the chemo-thermo-mechanical mechanisms of these interactions, and to use this understanding to guide the design and development of CMAS-resistant

EBCs. It is found that the Y-containing group of EBC ceramics, viz YAlO3 and γ-Y2Si2O7, show distinctly different behavior compared to the Y-free group of EBC ceramics, viz β-Yb2Si2O7, β-

Sc2Si2O7, and β-Lu2Si2O7.

Briefly, Y-containing EBC ceramics show extensive reaction-crystallization and no grain- boundary penetration of the CMAS glass. In contrast, the Y-free EBC ceramics show little to no reaction-crystallization and extensive grain-boundary penetration resulting in a dilatation gradient and a new type of ‘blister’ cracking damage. The former group of EBC ceramics are presented in this chapter, and the latter group is presented in the next chapter.

YAlO3 (yttrium aluminate perovskite or YAP) is a line compound of orthorhombic crystal structure [120], with no phase transformation from room temperature up to its congruent melting point of 1913 °C [121]. Its average CTE is 6-7 x 10-6 °C-1 [120,122], Young’s modulus is 316 GPa

-3 [123], and density is 5.35 Mg.m [122]. Although the YAlO3 CTE is on the high side compared to the CTE of SiC (4.7 x 10-6 °C-1) [16], the major CMC material, its most attractive feature for

EBC application is that it is Si-free. YAlO3 has been included in a patent as a potential EBC ceramic [115], but there has been no significant research reported in the open literature on this ceramic in the context of EBCs.

In the case of γ-Y2Si2O7-based EBCs, there have been limited studies on their high- temperature interaction with CMAS [25,69]. Y2Si2O7 has five polymorphs [37], but the γ-Y2Si2O7 monoclinic phase is the most desirable for EBC application. It has a melting point of 1775 °C

[124], average CTE of 3.9 x 10-6 °C-1 [125], Young’s modulus of 155 GPa [125], and a density of

-3 3.96 Mg.m [125]. While achieving the γ-Y2Si2O7 polymorph in the deposition of EBCs is a challenge and its temperature capability is relatively low, γ-Y2Si2O7 has an excellent CTE-match with SiC, and it is also relatively lightweight.

2.2 Experimental Procedure

2.2.1 Processing

The YAlO3 powder was prepared in-house by combining stochiometric amounts of Al2O3

(Nanophase Technologies Corporation, Romeoville, IL) and Y2O3 (Nanocerox, Ann Arbor, MI).

LiCl was added to this mixture in a 2:1 ratio of LiCl:Al2O3+Y2O3 to reduce the temperature required to form the YAlO3 powder [126]. The mixture was then ball-milled, using ZrO2 media in ethanol for 48 h. The mixed slurry was then dried at 90 °C while being stirred. The dry powder

19 mixture was placed in a Pt crucible and calcined at 1400 °C in air for 4 h in a box furnace (CM

Furnaces Inc, Bloomfield, NJ) to complete the solid-state reaction between Al2O3 and Y2O3. The reacted mixture was washed at least four times with hot deuterium-depleted water and filtered to remove the LiCl from the mixture. The YAlO3 powder was then dried and crushed.

The γ-Y2Si2O7 powder was also prepared in-house by combining stochiometric amounts of Y2O3 (Nanocerox, Ann Arbor, MI) and SiO2 (Atlantic Equipment Engineers, Bergenfield, NJ), respectively [127]. This mixture was then ball-milled and dried using the same procedure described above. The dried powder mixture was placed in a Pt crucible for calcination at 1600 °C in air for 4 h in the box furnace. The resulting γ-Y2Si2O7 powder was then ball-milled for an additional 24 h, dried, and crushed.

The powders were then loaded into graphite dies (20mm diameter) lined with graphfoil and densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC, Santa Rosa, CA) in an argon atmosphere. The SPS conditions were: 75 MPa applied pressure, 50 °C.min-1 heating rate, 1600 °C hold temperature, 5 min hold time, and 100 °C.min-1 cooling rate. The surfaces of the resulting dense pellets (∼2mm thickness) were ground to remove the graphfoil, and the pellets were heat-treated at 1500 °C in air for 1 h (10 °C.min-1 heating and cooling rates) in the box furnace. The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic polishing techniques for CMAS-interaction testing. Some pellets were cut using a low-speed diamond saw, and the cross-sections were polished to a 1-μm finish.

2.2.2 CMAS interactions

The composition of the CMAS used is (mol%) 51.5 SiO2, 39.2 CaO, 4.1 Al2O3, and 5.2

MgO, which is from a previous study [128], and it is close to the composition of the AFRL-03

20 standard CMAS (desert sand). Powder of this CMAS glass composition was prepared using a procedure described elsewhere [70,86]. CMAS interaction studies were performed by applying the

CMAS powder paste (in ethanol) uniformly over the center of the polished surfaces of the YAlO3

-2 and the γ-Y2Si2O7 pellets at ∼15 mg cm loading. The specimens were then placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box furnace at 1500 °C in air for different durations (10 °C min-1 heating and cooling rates). The CMAS-interacted pellets were then cut using a low-speed diamond saw, and the cross-sections were polished to a 1-μm finish.

In separate experiments, the CMAS powder and the YAlO3 powder or the γ-Y2Si2O7 powder were mixed in 1:1 ratio by weight and ball-milled for 24 h using the procedure described in Section 2.2.1. The resulting dry powder-mixtures were placed in Pt crucibles, heat-treated in the box furnace for 1500 °C in air for 24 h, and crushed into fine powders.

2.2.3 Characterization

The as-prepared YAlO3 and γ-Y2Si2O7 powders were characterized using an X-ray diffractometer (XRD; D8 Advance, Bruker AXS, Karlsruhe, Germany) to check for phase purity.

The heat-treated mixtures of YAlO3-CMAS and γ-Y2Si2O7-CMAS powders were also characterized using XRD. The phases present in the reaction products were identified using the

PDF2 database.

The densities of the as-SPSed pellets were measured using the Archimedes principle, with distilled water as the immersion medium. The polished cross-sections of the as-SPSed pellets were thermally-etched at 1500 °C for 1 min (10 °C min-1 heating and cooling rates).

The cross-sections of the as-SPSed and CMAS-interacted pellets were observed in a scanning electron microscope (SEM; LEO 1530VP, Carl Zeiss, Munich, Germany or Helios 600,

FEI, Hillsboro, Oregon, USA), equipped with energy-dispersive spectroscopy (EDS) systems

(Inca, Oxford Instruments, Oxfordshire, UK), operated at 20 kV accelerating voltage. EDS elemental maps, particularly Ca and Si, were also collected and used to determine CMAS penetration into the pellets. Cross-sectional SEM micrographs (3–4 per material) were used to measure the average grain sizes (linear-intercept method) of the as-SPSed pellets.

Transmission electron microscopy (TEM) specimens from specific locations within the polished cross-sections of the CMAS-interacted pellets were prepared using focused ion beam

(FIB; Helios 600, FEI, Hillsboro, Oregon, USA) and in situ lift-out. These samples were then examined using a TEM (2100 F, JEOL, Peabody, MA), equipped with an EDS system (Inca,

Oxford Instruments, Oxfordshire, UK), operated at 200 kV accelerating voltage. Selected-area electron diffraction patterns (SAEDPs) from various phases in the TEM micrographs were recorded and indexed using standard procedures.

2.3 Results

2.3.1 Polycrystalline Pellets

Figures 9A and 9B show a SEM micrograph and a XRD pattern of SPSed YAlO3 pellet, respectively. The density of the pellet is 5.22 M.gm−3 (∼97%), and the average grain size is ∼8

μm. The indexed XRD pattern shows the presence of some Y3Al5O12 (yttrium aluminum garnet or

YAG) and Y4Al2O9 (yttrium aluminum monoclinic or YAM) in the pellet. It is not unusual to have

YAG or YAM impurities in YAlO3 (YAP) ceramics due to slight shifts in the stoichiometry during processing. Also, it is difficult to obtain phase pure YAlO3 powders using conventional ceramic- powder processing.

A B

Figure 9: (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are present).

Figures 10A and 10B are a SEM micrograph and a XRD pattern of a SPSed γ-Y2Si2O7 pellet, respectively. The density of the pellet is 3.94 Mg.m−3 (∼99%), and the average grain size is ∼31 μm. Some cracking is observed in these pellets. The indexed XRD pattern shows phase- pure γ-Y2Si2O7.

A B

Figure 10: (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed XRD pattern showing phase-pure γ-Y2Si2O7.

2.3.2 YAlO3-CMAS Interactions

Figures 11A and 11B are cross-sectional SEM micrographs showing interaction between the YAlO3 ceramic and CMAS at 1500 °C for 1 min and 1 h, respectively, and the corresponding

EDS elemental compositions of the marked regions are presented in Table 3. YAlO3 appears to have reacted with the CMAS within 1 min, forming two reaction layers (∼30 μm total thickness).

The top layer (region #2) consists of vertically-aligned needle-shaped grains containing Y, Ca, Si, and O primarily, and the composition roughly corresponds to Y8Ca2(SiO4)6O2 apatite with some

Al in solid solution (Y-Ca-Si apatite (ss)). Some CMAS glass is also observed in that layer, although it appears to contain excess Y and Al (region #1). The second layer (region #3) contains

‘blocky’ grains and they have a composition presented in Table 3. It is assumed to be a YAG (ss) phase, with Ca and Si in solid solution. The base YAlO3 pellet (region #4) has a Y-rich composition.

A B

Figure 12A

Figure 12B

The total thickness of the reaction zone increases up to ∼40 μm after 1-h heat-treatment at

1500 °C (Figure 11B), and it appears to have three layers. The top layer (region #5) still consists of needle-shaped Y-Ca-Si apatite (ss) phase, which is confirmed using SAEDP in the TEM (Figure

12A). The second layer (region #6) still contains the YAG (ss) phase, whereas the third layer

(region #7) is Si-free, and it also is assumed to be a YAG (ss) phase. The base YAlO3 pellet

(regions #8 and #11) is still Y-rich composition, while the minor ‘gray’ inclusions (regions #9 and

#10) appear to be a Y-rich YAG phase (see XRD in Figure 9B).

Table 3: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 °C for 1 min and 1 h. The ideal compositions of the three main phases and CMAS are also included. Region # Y Al Ca Si Mg Phase 1 18 23 23 31 5 CMAS Glass 2 47 2 15 36 - Y-Ca-Si Apatite (ss) 3 34 45 8 11 2 Y-Al-Ca YAG (ss) 4 54 46 - - - Y-rich YAP (Base) 5 50 1 13 36 - Y-Ca-Si Apatite (ss) 6 36 43 7 12 2 Y-Al-Ca YAG (ss) 7 46 43 11 - - Y-Al-Ca YAG (ss) 8 55 45 - - - Y-rich YAP (Base) 9 55 45 - - - Y-rich YAG (Base) 10 46 54 - - - Y-rich YAG (Base) 11 45 55 - - - Y-rich YAP (Base) Ideal Compositions: 50.0 50.0 - - - YAlO3 (YAP) 50.0 - - 50.0 - γ-Y2Si2O7 50.0 - 12.5 37.5 - Y8Ca2(SiO4)6O2 Apatite 37.5 62.5 - - - Y3Al5O12 (YAG) - 7.9 37.6 49.5 5.0 Original CMAS Glass

Figures 12A and 12B are TEM micrographs from top and bottom regions, as indicated in

Figure 11B, and Table 4 includes the EDS elemental compositions of the marked regions. The indexed SAEDP (Figure 12A inset) confirms that the region #1 is Y-Ca-Si apatite (ss) phase. While

25 region #2 has significant amounts of Ca and Si, regions #3-#7 have near-ideal Y/Al ratio of YAG with some Ca in solid solution. Thus, the SEM and the TEM characterization results are consistent.

Table 4: Average EDS elemental composition (at%; cation basis) from the regions indicated in the TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 °C for 1 h. Region # Y Al Ca Si Mg Phase 1 46 - 12 42 - Y-Ca-Si Apatite (ss) 2 27 53 7 11 2 Y-Al-Ca YAG (ss) 3 33 61 4 - 2 Y-Al-Ca YAG (ss) 4 33 62 3 - 2 Y-Al-Ca YAG (ss) 5 30 62 3 - 2 Y-Al-Ca YAG (ss) 6 31 63 6 - - Y-Al-Ca YAG (ss) 7 32 63 5 - - Y-Al-Ca YAG (ss)

Upon further interaction of YAlO3 with CMAS glass for 24 h at 1500 °C, the reaction- layer thickness has doubled (∼80 μm). Figure 13A is a SEM micrograph of the entire YAlO3 pellet showing no evidence of ‘blistering’ cracking that is typically observed in Y-free (β-Yb2Si2O7, β-

Sc2Si2O7, and β-Lu2Si2O7) EBC ceramics in Chapter 3 [117,119]. Figure 13B is a higher- magnification SEM image of the reaction zone, and Figures 13C and 13D are corresponding Ca and Si elemental EDS maps, respectively.

A B

Figure 13B Figure 14A

Figure 14B

The chemical composition of the different regions in the higher-magnification SEM images in Figures 14A and 14B from the top and bottom (marked in Figure 13B), respectively, are given in Table 5. From these results, the remnants of the three reaction layers can be seen, with the top

Si-rich layer being mostly Y-Ca-Si apatite (ss), the middle Ca-lean layer being mostly YAG (ss), and the bottom layer being a mixture of Y-Ca-Si apatite (ss) and YAG (ss). The boundary between the bottom reaction layer and the base YAlO3 is still sharp. It also appears that all the CMAS glass has been consumed during its reaction with YAlO3 as no obvious CMAS pockets are found.

A B

Table 5: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 °C for 24 h. Region # Y Al Ca Si Mg Phase 1 51 - 13 36 - Y-Ca-Si Apatite (ss) 2 50 11 16 23 - Y-Ca-Si Apatite (ss) 3 37 48 5 9 1 Y-Al-Ca YAG (ss) 4 49 13 16 22 - Y-Ca-Si Apatite (ss) 5 37 48 5 9 1 Y-Al-Ca YAG (ss) 6 53 47 - - - Y-rich YAP (Base)

Figure 15 presents a XRD pattern of the YAlO3-CMAS powder mixture heat-treated at

1500 °C for 24 h. The XRD results confirm the presence of the Y-Ca-Si apatite (ss) and YAG phases, along with some unreacted YAlO3 and YAM phases.

2.3.3 Y2Si2O7-CMAS Interactions

Figure 16 is a cross-sectional SEM micrograph showing interaction between γ-Y2Si2O7

EBC ceramic and CMAS at 1500 °C for 1 h, and the EDS elemental compositions of the marked regions are presented in Table 6. The γ-Y2Si2O7 appears to have reacted with CMAS glass to a depth of ∼400 μm from the top, which is about an order-of-magnitude deeper than in the YAlO3 case under the same conditions. The reaction zone has two layers. The top layer contains only needle-shaped Y-Ca-Si apatite (ss) and CMAS glass. In contrast to the YAlO3 case, a significant amount of CMAS glass remains on top, which is Y-enriched and Ca-depleted. The second layer

(∼150 μm) comprises Y-Ca-Si apatite (ss) grains primarily, with some CMAS glass pockets.

Table 6: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 °C for 1 h. Region # Y Al Ca Si Mg Phase 1 8 8 19 61 4 CMAS Glass 2 51 - 12 37 - Y-Ca-Si Apatite (ss) 3 9 6 16 65 4 CMAS Glass 4 49 13 16 22 - Y-Ca-Si Apatite (ss)

Figure 17A shows cross-section SEM micrograph of the entire γ-Y2Si2O7 pellet after

CMAS interaction at 1500 °C for 24 h. Similar to the YAlO3 case, no ‘blistering’ cracks are observed. The higher magnification SEM image (Figure 17B) shows that the total reaction layer thickness is ∼300 μm, and the amount of CMAS glass remaining at the top has decreased compared with the 1-h case. The thickness of the bottom Y-Ca-Si apatite (ss) layer has increased to ∼200

μm, indicating the consumption of the CMAS glass and the growth of the Y-Ca-Si apatite (ss) layer.

A B

Figure 18A Figure 17B

Figure 18B

Figures 18A and 18B shows the top and the bottom area, respectively, of the reaction zone at a higher magnification. The compositions of the Y-Ca-Si apatite (ss) and the CMAS glass (Table

7) appear to be very similar to the ones in the 1-h case (Table 6).

A B

Table 7: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 °C for 24 h. Region # Y Al Ca Si Mg Phase 1 8 7 14 68 3 CMAS Glass 2 51 - 12 37 - Y-Ca-Si Apatite (ss) 3 6 8 14 68 4 CMAS Glass 4 51 - 12 37 - Y-Ca-Si Apatite (ss)

Figure 19 presents a XRD pattern of the γ-Y2Si2O7-CMAS powder mixture heat-treated at

1500 °C for 24 h, confirming the presence of the Y-Ca-Si apatite (ss) phase, along with some unreacted γ-Y2Si2O7.

Figure 19: Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at 1500 °C for 24 h, showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7.

2.4 Discussion

The results from this study show that the ‘model’ Y-bearing YAlO3 and γ-Y2Si2O7 EBC ceramics react with the ‘model’ CMAS glass despite the fact that their OBs are quite similar, resulting in extensive reaction-crystallization, but no ‘blister’ cracking. The reaction- crystallization propensity is attributed to the strong affinity between Y in the EBC ceramics and the Ca in the CMAS, highlighting the limitation of the use of the OBs-difference screening criterion.

In the case of the YAlO3 EBC ceramic, it reacts with the CMAS glass very rapidly. It appears that the first reaction product is vertically-aligned, needle-shaped Y-Ca-Si apatite (ss).

Similar Y-Ca-Si apatite (ss) formation has been observed in the cases of 2ZrO2∙Y2O3 [94,129,130] and rare-earth zirconate [71,128,131–133] TBCs interacting with CMASs of wide range of compositions. This typically occurs by the dissolution of the ceramic in the CMAS glass, supersaturation, and reaction-crystallization of needle-shaped grains of Y-Ca-Si apatite (ss). This

34 same mechanism is likely to be responsible in the case of YAlO3: dissolution of YAlO3 in the

CMAS glass and reaction-crystallization of Y-Ca-Si apatite (ss) from the supersaturated CMAS glass melt. The formation of the YAG (ss) layer containing Ca and Si in solid solution appears to be related to inadequate access to the CMAS glass, precluding further Y-Ca-Si apatite (ss) formation, but Y-depletion can still occur. Solid solutions of YAG, Y(3-x)CaxAl(5-x)SixO12, are also known to exist, where Ca2+ and Si4+ co-substitute for Y3+ and Al3+ in the octahedral and tetrahedral sites, respectively [134]. Further down in the third layer the YAG (ss) phase is devoid of Si, which could be the result of no access to the CMAS glass. In this context, YAG (ss) is known to have appreciable solubility for Ca, where Ca2+ occupies Y3+ sites according to the following defect reaction [135]:

′ ∙∙ 2퐶푎푂 ⇆ 2퐶푎푌 + 푉푂 (Equation 5)

Rapid reaction with the CMAS and the formation of a relatively thin protective reaction layer could be advantageous in YAlO3 EBCs for CMAS resistance. Also, the silica activity of

YAlO3 is zero, which is also a big advantage over Si-containing EBC ceramics from the standpoint of high-temperature, high-velocity water-vapor corrosion. Finally, the very high temperature- capability and the potential low-cost of YAlO3 makes it an attractive EBC ceramic. However, the moderate CTE mismatch of YAlO3 with SiC-based CMCs is a disadvantage, but CTE-mismatch- induced cracking at sharp interfaces can be mitigated by including a CTE-graded bond-coat between the CMC and the YAlO3 EBC.

γ-Y2Si2O7 EBC ceramic also reacts with the chosen CMAS, but the nature of the reaction is quite different from that observed in the case of YAlO3. The reaction zone is almost an order- of-magnitude thicker in the case of γ-Y2Si2O7 compared to that in YAlO3, and there is significant amount of CMAS remaining after 24 h heat-treatment (at 1500 °C) in the former. This is primarily

35 because YAlO3 is Si-free, resulting in more rapid consumption of the CMAS. The mechanism of reaction-crystallization of the needle-shaped Y-Ca-Si apatite (ss) in γ-Y2Si2O7 appears to be similar to that in YAlO3, and also in Zr-containing ceramics. However, unlike YAlO3, where YAG

(ss) phases form underneath the Y-Ca-Si apatite (ss) layer, no other phases form in the case of γ-

Y2Si2O7. This is consistent with what has been observed by others [25,69].

While the CTE match with SiC is very good and it is relatively lightweight, the formation of the significantly thicker reaction layer in γ-Y2Si2O7 is a concern, making this EBC ceramic less effective against high-temperature CMAS attack. Also, the deposition of phase-pure γ-Y2Si2O7

EBCs will be a significant challenge because Y2Si2O7 can exist as four other undesirable polymorphs. Furthermore, the temperature capability of γ-Y2Si2O7 is limited to ∼1700 °C, and its silica activity is very high. Considering all these drawbacks, overall, γ-Y2Si2O7 may not be an attractive candidate ceramic for EBCs.

2.5 Summary

Here we have systematically studied the high-temperature (1500 °C) interactions between two promising dense, polycrystalline EBC ceramics, YAlO3 (YAP) and γ-Y2Si2O7, and a CMAS glass. Despite the small differences in the OBs of the two EBC ceramics and that of the CMAS, they both react with the CMAS. In the case of the Si-free YAlO3, the reaction zone is small and it comprises three regions of reaction-crystallization products: (i) needle-like Y-Ca-Si apatite (ss) grains, (ii) blocky grains of YAG (ss), and (iii) a mixture of Y-Ca-Si apatite (ss) and YAG (ss) blocky grains. The YAG (ss) is found to contain Ca, Al, and Si in solid solution. In contrast, only

Y-Ca-Si apatite (ss) needle-like grains form in the case of Si-containing γ-Y2Si2O7, and the reaction zone is an order-of magnitude thicker. These CMAS interactions are analyzed in detail

36 and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7,

β-Sc2Si2O7 and β-Lu2Si2O7) in Chapter 3 [117,119]. This is attributed to the presence of the Y in the YAlO3 and γ-Y2Si2O7 EBC ceramics.

CHAPTER 3: Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN CMAS

This chapter was modified from previously published articles along with unpublished data: L.R. Turcer, A.R. Krause, H.F. Garces, L. Zhang, and N.P. Padture, “Environmental-barrier coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass: Part II, β-Yb2Si2O7 and β-Sc2Si2O7,” Journal of the European Ceramic Society, 38, 3914- 3924 (2018). [117] and L.R. Turcer and N.P. Padture, “Towards multifunctional thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramics,” Scripta Materialia, 154, 111-117 (2018). [119]

3.1 Introduction

In Chapter 2, it is found that the Y-containing group of EBC ceramics, viz YAlO3 and γ-

Y2Si2O7, show distinctly different behavior compared to the Y-free group of EBC ceramics, viz β-

Yb2Si2O7, β-Sc2Si2O7, and β-Lu2Si2O7. Briefly, Y-containing EBC ceramics show extensive reaction-crystallization and no grain-boundary penetration of the CMAS glass [116]. In contrast, the Y-free EBC ceramics show little to no reaction-crystallization and extensive grain-boundary penetration resulting in a dilatation gradient, and a new type of ‘blister’ cracking damage.

-6 -1 β-Yb2Si2O7 has a melting point of 1850 °C [136], average CTE of 4.0 x 10 °C [137],

Young’s modulus of 205 GPa [33], density of 6.13 Mg.m-3 [34]. High-temperature interactions between Yb2Si2O7 (pellets or powders or coatings) and CMAS have been studied by others [25,33–

36,69]. Stolzenburg et al. [33] and Liu et al. [25] have shown limited reaction between Yb2Si2O7

(pellets and/or powders) and CMAS. However, The testing temperature used by Stolzenburg et al.

[33] is limited to 1300 °C, and the density of the β-Yb2Si2O7 pellet is not specified. Interestingly, the same authors report extensive CMAS infiltration and reaction with porous air-plasma sprayed

(APS) Yb2Si2O7 EBC at 1300 °C [34]. Liu et al. [25] conducted their tests on Yb2Si2O7 pellets that are ∼25% porous, at 1400 °C in water vapor environment, complicating the interpretation of the results. Ahlborg et al. [69] reported extensive reaction between Yb2Si2O7 pellets and CMAS at

1500 °C. However, the density of the pellets is not reported, and their microstructures appear to be heterogeneous. Zhao et al. [36] reported reaction between dense Yb2Si2O7 APS EBC and

CMAS at a lower temperature of 1300 °C. However, the APS Yb2Si2O7 EBC contains appreciable quantities of Yb2SiO5, making these EBCs two-phase, thus, complicating the issue. Finally,

Poerschke et al. [35] have studied the interaction between Yb2Si2O7 EBC deposited using electron- beam directed-vapor deposition (EB-DVD) and CMAS at 1300 °C and 1500 °C. However, in their experiments, the EBC is buried under a Yb4Hf3O12 TBC or a bi-layer Yb4Hf3O12/Yb2SiO5 T/EBC, making these interactions indirect and strongly influenced by the TBC or the T/EBC [35].

-6 -1 β-Sc2Si2O7 has a melting point of 1860 °C [138], average CTE of 5.4 x 10 ° C [137],

Young’s modulus of 200 GPa [139], and density of 3.40 Mg.m-3 [138]. There has been only one report in the open literature on the high-temperature interaction between Sc2Si2O7 and CMAS. Liu et al. [25] conducted their tests on a ∼19% porous Sc2Si2O7 pellet at 1400 °C in water vapor environment. They showed penetration of the molten CMAS in the porous pellet, and some reaction resulting in the formation of Ca3Sc2Si3O12. However, the highly porous nature of the pellet precludes proper understanding of the high-temperature interactions of Sc2Si2O7 with CMAS.

-6 -1 β-Lu2Si2O7 has a melting point of 2000 °C [140], average CTE of 3.8-3.9 x 10 °C

[137,141], Young’s modulus of 178 GPa [142], and density of 6.25 Mg.m-3 [143]. Liu et al. [25] is the only report in the open literature on the high-temperature interaction between Lu2Si2O7 and

CMAS. They showed penetration of the molten CMAS in the porous pellet and a limited reaction between Lu2Si2O7 pellets and CMAS. However, the tests were conducted on a ∼25% porous

Lu2Si2O7 pellet at 1400 °C in water vapor environment, which complicates the interpretation of the results [25].

Thus, the objective of this study is to use fully dense, phase-pure β-Yb2Si2O7, β-Sc2Si2O7, and β-Lu2Si2O7 ‘model’ EBC ceramic pellets, and to investigate their interaction with a ‘model’

CMAS at 1500 °C in air. The overall goal is to provide insights into the thermo-chemo-mechanical mechanisms of these interactions, and to use this understanding to guide the design and development of future CMAS-resistant EBCs.

3.2 Experimental Procedure

3.2.1 Processing

The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073,

Oerlikon Metco, Westbury, NY).

The β-Sc2Si2O7 powder was prepared in-house by combining stochiometric amounts of

Sc2O3 (Reade Advanced Materials, Riverside, RI) and SiO2 (Atlantic Equipment Engineers,

Bergenfield, NJ) powders [144]. The β-Lu2Si2O7 powder was prepared in-house by combining stochiometric amounts of Lu2O3 (Sigma Aldrich, St. Louis, MO) and SiO2 (Atlantic Equipment

Engineers, Bergenfield, NJ) powders. The powder mixtures were then ball-milled, using ZrO2 balls media in ethanol for 48 h. The mixed slurries were then dried while being stirred. The dried powder-mixtures were placed in Pt crucibles for calcination at 1600 °C for 4 h in air in a box furnace (CM Furnaces Inc, Bloomfield, NJ). The resulting β-Sc2Si2O7 powder and β-Lu2Si2O7 powder were then ball-milled for an additional 24 h and dried.

The powders were then densified into 20 mm diameter polycrystalline pellets using spark plasma sintering (SPS) like the Y-containing EBC ceramics from the previous chapter. More details can be found in Section 2.2.1.

In addition, the β-Yb2Si2O7 powder was mixed with 1 vol% CMAS powder, and ball-milled for 48 h. The powder mixture was then dried and dry-pressed into pellets (25mm diameter), followed by cold isostatic pressing (AIP, Columbus, OH) at 275 MPa. The pellets were pressureless sintered at 1500 °C in air for 4 h in the box furnace. The thickness of the sintered pellets was ∼2.5 mm.

The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic polishing techniques for CMAS-interaction testing. Some pellets were cut through the center using a low-speed diamond saw, and the cross-sections were polished to a 1-μm finish. In some instances, the polished cross-sections were etched using dilute HF for 10 min.

3.2.2 CMAS Interactions

CMAS interaction experiments were preformed like the CMAS interaction with Y- containing EBC ceramics in Chapter 2. Briefly, CMAS (51.5 SiO2, 39.2 CaO, 4.1 Al2O3, and 5.2

MgO, in mol%) [128] was applied uniformly over the center of the polished surfaces of pellets (β-

-2 Yb2Si2O7, β-Sc2Si2O7, β-Lu2Si2O7, and β-Yb2Si2O7 + 1 vol% CMAS) at 15 mg.cm loading. The specimens were then heat-treated in the box furnace at 1500 °C in air for different durations (10

°C.min-1 heating and cooling rates) and then cross-sectioned to observe the interaction zone.

CMAS powder and Y-free EBC ceramic powders (β-Yb2Si2O7, β-Sc2Si2O7 and β-Lu2Si2O7) were mixed in 1:1 ratio by weight, ball-milled, heat-treated for 24 h in air at 1500 °C, and crushed into fine powders. Please see Section 2.2.2 for more details.

3.2.3 Characterization

The characterization for these experiments is similar to the Y-containing EBC ceramics found in Chapter 2. Please refer to Section 2.2.3 for more detail. Briefly, X-ray diffraction (XRD)

41 was conducted on the as-received β-Yb2Si2O7 powder, the as-prepared β-Sc2Si2O7 and β-Lu2Si2O7 powders, and the heat-treated mixtures. Densities of the as-SPSed and pressureless-sintered pellets were measured using the Archimedes principle (immersion medium = distilled water).

Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to observe the cross-sections of the as-SPSed, as-pressureless-sintered and CMAS-interacted pellets. Transmission electron microscopy (TEM) equipped with an EDS system was used to observe specific locations within the cross-sections of the CMAS-interacted pellets. These samples were prepared using focused ion beam and in-situ lift-out.

3.3 Results

3.3.1 Polycrystalline Pellets

Figures 20A and 20B show a SEM micrograph and a XRD pattern of SPSed β-Yb2Si2O7 pellet, respectively. The density of the pellet is 6.08 Mg.m-3 (99%), and the average grain size is

∼10 μm. The indexed XRD pattern shows phase-pure β-Yb2Si2O7.

A B

Figure 20: (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B) indexed XRD pattern showing phase-pure β-Yb2Si2O7.

Figures 21A and 21B show a SEM micrograph and a XRD pattern of SPSed β-Sc2Si2O7 pellet, respectively. The density of the pellet is 3.34 Mg.m-3 (99%), and the average grain size is

∼8 μm. The indexed XRD pattern shows phase-pure β-Sc2Si2O7.

A B

Figure 21: (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed XRD pattern showing phase-pure β-Sc2Si2O7.

Figures 22A and 22B show a SEM micrograph and a XRD pattern of SPSed β-Lu2Si2O7 pellet, respectively. The density of the pellet is 6.15 Mg.m-3 (98%), and the average grain size is

∼8 μm. The indexed XRD pattern shows phase-pure β-Lu2Si2O7.

A B

Figure 22: (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B) indexed XRD pattern showing phase-pure β-Lu2Si2O7.

3.3.2 Yb2Si2O7-CMAs Interactions

Figure 23A is a cross-sectional SEM image of a β-Yb2Si2O7 pellet that has interacted with

CMAS at 1500 °C for 1 h. A thick CMAS layer on top is observed, and its interaction with the β-

Yb2Si2O7 pellet appears to be limited. The latter is confirmed in Figures 23B and 23C, which are higher magnification SEM image and corresponding Ca elemental EDS map, respectively, of the interaction zone. The EDS elemental compositions of regions #1 to #4 are reported in Table 8. The amount of Yb in the CMAS glass (region #1) is ∼8 at%, which is similar to what has been observed for Y in the case of YAlO3 and γ-Y2Si2O7 EBC ceramics [116], despite the somewhat higher solubility of Y3+ in the CMAS glass. Region #2 has a composition similar to that of Yb-Ca-Si apatite solid solution (ss) phase, which is confirmed using the indexed SAEDP (Figure 24A). The distribution of Yb-Ca-Si apatite (ss) phase (Ca-containing grains) is clearly seen in Figure 23C, which does not appear to form a continuous layer. Thus, the amount of Yb-Ca-Si apatite (ss) formed is significantly less than that in the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) in

Chapter 2. Region #3 appears to be reprecipitated Ca-containing β-Yb2Si2O7, while region #4 is

44 base β-Yb2Si2O7. Also, CMAS glass can be found in pockets in the base β-Yb2Si2O7 below the

Yb-Ca-Si apatite (ss) in Figure 24B, which is typically not the case in Y-containing EBC ceramics

[116].

Figure 23B

B C Figure 24A

Figure 24B

Table 8: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 °C for 1 h. The ideal compositions of the two main phases and the CMAS are also included. Region # Yb Al Ca Si Mg Phase 1 8 5 27 57 3 CMAS Glass 2 47 - 13 41 - Yb-Ca-Si Apatite (ss) 3 46 - 1 53 - β-Yb2Si2O7 (Re-precipitated) 4 46 - - 54 - β-Yb2Si2O7 (Base) Ideal Compositions: 50.0 - 12.5 37.5 - Yb8Ca2(SiO4)6O2 Apatite 50.0 - - 50.0 - β-Yb2Si2O7 (Base) - 7.9 37.6 49.5 5.0 Original CMAS Glass

A B

Upon further interaction between β-Yb2Si2O7 and CMAS glass at 1500 °C for 24 h, ‘blister’ cracks form under the CMAS deposit (Figure 25A), but the occurrence of Yb-Ca-Si apatite (ss) phase is rare (see Figures 25B and 25C, and Table 9). The latter is confirmed by XRD results in

Figure 26 from β-Yb2Si2O7-CMAS powder mixture heat-treated at 1500 °C for 24 h. Also, no

CMAS glass is found on top, which is the opposite of the γ-Y2Si2O7 case [116]. Throughout the pellet, small Ca EDS signal is detected (Figure 25C) and CMAS glass pockets are found (Figure

27), with the latter containing ∼10 at.% Yb (Table 9). This indicates that there is reaction between

β-Yb2Si2O7 and the CMAS glass, but there is little reprecipitation of β-Yb2Si2O7 or reaction- crystallization of Yb-Ca-Si apatite (ss). The Yb-saturated CMAS glass appears to have penetrated throughout the pellet, most likely via the grain-boundary network as the pellet is fully dense. The higher-magnification SEM image of the ‘blister’ cracks in Figure 25D shows that the cracks are wide and blunt, reminiscent of typical high-temperature cracking observed in ceramics [145]. This indicates that the ‘blister’ cracks formed at a high temperature, and not during cooling.

A B

Figure 27 Figure 25B

Figure 25D

Table 9: Average EDS elemental composition (at%; cation basis) from the regions indicated in SEM and TEM micrographs in Figures 25 and 27, respectively, of β-Yb2Si2O7 interaction with CMAS at 1500 °C for 24 h. Region # Yb Al Ca Si Mg Phase 1 46 - 12 42 - Yb-Ca-Si Apatite (ss) 2 46 - - 54 - β-Yb2Si2O7 (Base) 3 10 11 21 53 5 CMAS Glass

Figure 26: Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated at 1500 °C for 24 h, showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7.

Figures 28A–28D show the evolution of the ‘blister’ cracking in β-Yb2Si2O7 pellets (∼2 mm thickness) after interaction with CMAS glass at 1500 °C. At 1-h heat-treatment, no significant damage is visible in the optical micrograph collage of the whole pellet (Figure 28A), and same is the case at 2 h (not shown here). At 3 h (Figure 28B), ‘blister’ cracks start to appear beneath the interaction zone. At 6 h (Figure 28C), the ‘blister’ cracks are fully formed, and remain at 24 h

(Figure 28D). Similar ‘blister’ cracks are also observed in thinner pellets (∼1 mm thickness) in

Figure 28E.

Figures 29A and 29B are SEM micrographs of β-Yb2Si2O7 pellet (∼2 mm thickness) after interaction with the CMAS glass at 1500 °C for 6 h from the top and the bottom regions of the

50 pellet, respectively. The HF-etching reveals gradient in the CMAS glass, where there is large amount of CMAS near the top of the pellet, and hardly any CMAS glass near the bottom.

A B

Figure 29: SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm thickness) that has interacted with CMAS at 1500 °C for 6 h: (A) top region and (B) bottom region.

3.3.3 Sc2Si2O7-CMAS Interactions

Figures 30A and 30B are cross-sectional SEM micrograph and corresponding Ca elemental

EDS map, respectively, of β-Sc2Si2O7 pellet that has interacted with CMAS glass at 1500 °C for 1 h. Region #1 is CMAS glass with ∼9 at.% Sc (Table 10), regions #2 and #3 are reprecipitated β-

Sc2Si2O7 grains containing a small amount of Ca, and region #4 is base β-Sc2Si2O7. No Sc-Ca-Si apatite (ss) could be detected. This is in contrast with the β-Yb2Si2O7 case, where some reaction- crystallized Yb-Ca-Si apatite (ss) is found.

A B

Table 10: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 °C for 1 h. Region # Sc Al Ca Si Mg Phase 1 9 6 31 50 4 CMAS Glass 2 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated) 3 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated) 4 51 - - 49 - β-Sc2Si2O7 (Base)

After 24-h interaction between β-Sc2Si2O7 pellet and CMAS glass at 1500 °C, there is no

CMAS glass remaining on top, but ‘blister’ cracks are observed (Figure 31A), similar to those in

β-Yb2Si2O7. Once again, no reaction-crystallized Sc-Ca-Si apatite (ss) is detected (Figures 31B and 31C).

A Figure 31B B

Figure 32A

Figure 31C

TEM/SAEDP (Figure 32A) and XRD (Figure 33) results confirm that β-Sc2Si2O7 is the only crystalline phase, and there are Sc-bearing CMAS glass pockets in the interior of the pellet

(Figures 32B and 32C). Similar to the β-Yb2Si2O7 case, the Sc-saturated CMAS glass appears to have penetrated throughout the pellet. Once again, this is most likely via the grain-boundary network as the β-Sc2Si2O7 pellet is also fully dense.

Figure 32B B

Table 11: Average EDS elemental composition (at%; cation basis) from the regions indicated in the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 °C for 24 h. Region # Sc Al Ca Si Mg Phase 1 11 12 13 62 2 CMAS Glass 2 47 - - 53 - β-Sc2Si2O7 (Base)

Figure 33: Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at 1500 °C for 24 h, showing only the presence of unreacted β-Sc2Si2O7.

3.3.4 Lu2Si2O7-CMAS Interactions

Figure 34A is a cross-sectional SEM micrograph of the entire CMAS-interacted zone in the β-Lu2Si2O7 pellet at 1500 °C for 1 h. A cross-sectional SEM micrograph of the pellet thickness in the CMAS-interacted zone can be seen in Figure 34B. Figures 34D and 34F are cross-sectional

SEM micrographs, and Figures 34E and 34G are their corresponding Ca elemental EDS maps, respectively. CMAS glass is not found on the surface of the β-Lu2Si2O7 pellet after 1 h at 1500 °C.

Instead, pockets of CMAS are found in-between grains and in triple junctions, which can be seen in regions #3 – #6 (Table 12), and ‘blister’ cracks are observed near the surface of the pellet. No

Lu-Ca-Si apatite (ss) could be detected. This is similar to the β-Sc2Si2O7 case and in contrast with the β-Yb2Si2O7 case, where some reaction-crystallized Yb-Ca-Si apatite (ss) is found.

A Figure 34C Figure 34B

B C Figure 34D

Figure 34F

D E

F G

Table 12: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 °C for 1 h.

Region # Lu Al Ca Si Mg Phase 1 55 - - 45 - β-Lu2Si2O7 2 55 - - 45 - β-Lu2Si2O7 3 11 7 24 55 3 CMAS Glass 4 10 7 26 54 3 CMAS Glass 5 6 9 32 50 4 CMAS Glass 6 16 9 24 49 3 CMAS Glass 7 55 - - 45 - β-Lu2Si2O7 8 55 - - 45 - β-Lu2Si2O7

After 24 h at 1500 °C, the ‘blister’ cracks are more prevalent, which can be seen in Figure

35A. These ‘blister’ cracks can be seen throughout the thickness of the pellet. A noticeable change in porosity is seen from the top to the bottom of the β-Lu2Si2O7 pellet. This change in porosity can also be seen in Figure 36 from the CMAS-interacted region (left) to the edge of the pellet (right).

Figures 36B and 36C are cross-sectional images taken from regions in the CMAS-interacted zone

(close to the bottom of the pellet) and away from the CMAS-interacted zone (close to the edge of the pellet), respectively.

Like in the β-Sc2Si2O7, Lu-Ca-Si apatite (ss) was not found in the β-Lu2Si2O7 pellets. XRD

(Figure 36) confirms that β-Lu2Si2O7 is the only crystalline phase. Similar to both β-Yb2Si2O7 and

β-Sc2Si2O7, the CMAS glass appears to have penetrated through the pellet. Once again, this is most likely via the grain-boundary network as the β-Lu2Si2O7 pellet is also fully dense.

A B

Figure 35B

B C

Figure 37: Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated at 1500 °C for 24 h, showing only the presence of unreacted β-Lu2Si2O7.

3.4 Discussion

In stark contrast with the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) [116], the reaction-recrystallization of apatite (ss) is minimal in β-Yb2Si2O7 and non-existent in β-Sc2Si2O7

3+ and β-Lu2Si2O7. This is consistent with the fact that Y (0.900 Å), with its larger ionic radius than those of Sc3+ (0.745 Å), Lu3+ (0.861 Å) and Yb3+ (0.868 Å), has stronger propensity for Ca and provides a higher driving force for the reaction-crystallization of apatite (ss) [128,146,147]. Instead of reaction-crystallization, the CMAS glass appears to penetrate the grain boundaries of the dense

β-Yb2Si2O7, β-Sc2Si2O7, and β-Lu2Si2O7 EBC ceramic pellets. Assuming the glass is in chemical equilibrium with the crystal, the driving force for penetration of molten glass into grain boundaries in ceramics is reduction in the total energy of the system due to the formation of two glass/ceramic interfaces from one ceramic/ceramic interface, typically a high-angle grain boundary [148–150]:

훾퐺퐵 > 2훾퐼 (Equation 6) where γGB is the grain-boundary energy and γI is the ceramic/glass interface energy. The ‘stuffing’ of the grain boundaries by CMAS glass results in the dilatation of the ceramic. However, unlike porous ceramics (e.g. TBCs), where penetration of molten CMAS glass is very rapid (within minutes at 1500 °C), its grain boundary penetration in dense ceramics is a very slow process.

Therefore, the top region has more CMAS than the bottom region, as confirmed in Figure 29. This results in a dilatation gradient, where the top region wants to expand compared to the bottom unaffected region, as depicted schematically in Figure 38A. But the constraint provided by the unpenetrated (undilated) base material creates effective compression in the top dilated layer. This compression is likely to build up as the top dilated layer thickens, albeit some relaxation due to creep. When the top dilated layer is sufficiently thick with increasing heat-treatment duration (e.g.

3 h at 1500 °C for β-Yb2Si2O7 (Figure 28)), the built-up compressive strain in that layer appears

60 to cause the ‘blister’ cracking, perhaps by a mechanism akin to buckling of compressed films

(Figure 38B). [151] The wide and blunt nature of the ‘blister’ cracks confirms that the cracking occurred at high temperature as hypothesized, and not during cooling to room temperature.

It appears that the genesis of this new type of ‘blister’ cracking damage mode in EBC ceramics subjected to CMAS attack is the slow buildup of the dilatation gradient, and possibly inadequate creep relaxation of the built-up compressive strain. While full understanding of this phenomenon is lacking at this time, in order to address this issue and mitigate the ‘blister’ cracking damage, a new approach is explored — add a small amount of CMAS glass to the EBC ceramic powders before sintering. This CMAS glass is expected to segregate at grain boundaries in the sintered EBC ceramics, and its ‘soft’ nature at high temperatures will accomplish two goals: (i) facilitate relatively rapid penetration of the deposited CMAS glass along grain boundaries, thereby reducing the severity of the dilatation gradient and (ii) facilitate rapid creep relaxation of the compression. To that end, 1 vol% CMAS glass powder was mixed in with the β-Yb2Si2O7 powder before sintering as a case study. Figures 39A and 39B are the SEM micrograph and corresponding

Ca elemental EDS map, respectively, of the β-Yb2Si2O7/1 vol% CMAS pellet (polished and etched cross-section), showing a near-full density (5.88 Mgm−3 or ∼96%), equiaxed microstructure

(average grain size ∼20 μm). Somewhat uniform distribution of CMAS glass can also be seen in

Figure 39B.

A B

Figure 39: (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β- Yb2Si2O7/1 vol% CMAS pellet and (B) corresponding EDS elemental Ca map.

Figure 40A is an optical-micrograph collage of the whole pellet after its interaction with

CMAS glass deposit on top at 1500 °C for 24 h, where no evidence of ‘blister’ cracks can be found.

Figure 40B is a SEM micrograph of the region marked in Figure 40A, once again, showing no

‘blister’ cracks. Figures 40C and 40D are a higher magnification SEM image and its corresponding

Ca elemental EDS map, showing some Yb-Ca-Si apatite (ss) formation and minor cracks (sharp, narrow) during cooling due to CTE mismatch at the surface.

Figure 40B

B C Figure 40C

These results clearly demonstrate the success of this approach in mitigating the ‘blister’ cracking damage mode in β-Yb2Si2O7 EBC ceramics, and it is likely to work in β-Sc2Si2O7, β-

Lu2Si2O7, and other EBC ceramics as well. Most importantly, the amount of CMAS glass additive needed is very small (1 vol%), which is unlikely to affect other properties of EBC ceramic significantly. Thus, for EBC ceramics where reaction-crystallization upon interaction with CMAS glass does not occur, the mitigation of the ‘blister’ cracking damage using this approach is very attractive.

In the case of β-Yb2Si2O7, its good CTE match with SiC and high-temperature capability are advantages. However, its high silica activity is a disadvantage. Also, APS deposition of phase- pure β-Yb2Si2O7 can be a challenge, where the substrate needs to be held at ∼1000 °C in a furnace during APS deposition [43]. In the case of β-Sc2Si2O7, it is lightweight, in addition to having good

CTE match with SiC and high temperature capability. β-Lu2Si2O7 also has a good CTE match and high temperature capabilities. But the high silica activity and high cost are disadvantages for both

β-Sc2Si2O7 and β-Lu2Si2O7, and the challenges associated with the APS deposition of phase-pure

β-Sc2Si2O7 and β-Lu2Si2O7 are not known.

Finally, while the new damage mode of ‘blister’ cracking is seen in EBC ceramic pellets in this study, it is likely to persist in actual EBCs on CMCs. This is because the CMC substrate, with its very high stiffness, is likely to provide similar, if not greater, constraint as the unpenetrated

(undilated) bottom part of the ceramic pellet. Thus, the ‘blister’ cracking damage mode is likely to be important in actual EBCs on CMCs. Furthermore, the approach demonstrated here for the mitigation of ‘blister’ cracking in pellets should also work in actual EBCs on CMCs, but that remains to be demonstrated.

3.5 Summary

Here we have systematically studied the high-temperature (1500 °C) interactions of three promising dense, polycrystalline EBC ceramics, β-Yb2Si2O7, β-Sc2Si2O7 and β-Lu2Si2O7, with a

CMAS glass. Unlike Y-containing YAlO3 and γ-Y2Si2O7 in Chapter 2 [116], little or no reaction is found between the Y-free EBC ceramics and the CMAS.

A B

C D

In the case of β-Yb2Si2O7, a small amount of reaction-crystallization product Yb-Ca-Si apatite (ss) is detected, whereas none is detected in the cases of β-Sc2Si2O7 and β-Lu2Si2O7.

Instead, the CMAS glass is found to penetrate the grain boundaries of β-Yb2Si2O7, β-Sc2Si2O7 and

β-Lu2Si2O7 EBC ceramics, and they all suffer from a new type of ‘blister’ cracking damage comprising large and wide cracks. This is attributed to the through-thickness dilatation-gradient caused by the slow penetration of the CMAS glass into the grain boundaries. Based on this understanding, a ‘blistering’-damage-mitigation approach is devised and successfully demonstrated, where 1 vol% CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering.

The resulting EBC ceramic does not show the ‘blister’ cracking damage, as the presence of the

CMAS-glass phase at the grain boundaries appears to promote rapid CMAS-glass penetration, thereby avoiding the dilatation-gradient.

CHAPTER 4: RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS

This chapter was modified from a submitted (February 20, 2020) article: L.R. Turcer and N.P. Padture, “Rare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass,” Journal of Materials Research, submitted for focus issue: sand-phobic thermal/environmental barrier coatings for gas turbine engines (2020).

4.1 Introduction

In Chapter 3, it was shown that, while Yb2Si2O7 EBC ceramic has minimal reaction with a

CMAS at 1500 ˚C, large ‘blister’ cracks form as a result of the dilatation gradient set up due to the progressive penetration of CMAS glass into the Yb2Si2O7 ceramic grain boundaries [117]. In contrast, Y2Si2O7 is found to react with the CMAS to form a Y-Ca-Si apatite (ss), preventing the

CMAS from penetrating the grain boundaries and forming ‘blister’ cracks (Chapter 2) [116]. This raises the interesting possibility of tempering these extreme CMAS-interaction behaviors by forming Yb(2 x)YxSi2O7 solid-solution EBC ceramics. Furthermore, the thermal conductivities of substitutional solid-solutions with large atomic-number contrast (ZYb=70, ZY=39) are expected to be low for potential thermal-environmental barrier coating (TEBC) applications [119], which will be discussed further in Chapter 5.

In this context, although there have been several studies focused on the interactions between RE-pyrosilicates and CMAS [23–27,33–36,69,146,152], there is little known about

CMAS interactions with pyrosilicate solid-solutions. Figure 42A shows the polymorphism of several RE2Si2O7 [37]. It is seen that Yb2Si2O7 does not undergo polymorphic transformation, and remains as β-phase from room temperature up to its melting point. In contrast, Y2Si2O7 shows several polymorphic transformations in that temperature range. In this context, it has been shown

67 that the β-phase can be stabilized in Yb(2-x)YxSi2O7 solid-solutions, where x < 1.1 (Figure 42B)

[38,153]. A B

Here we have studied the interactions at 1500 °C of two solid-solution ‘model’ EBC ceramics (dense, polycrystalline ceramic pellets) of compositions Yb1.8Y0.2Si2O7 (x = 0.2) and

Yb1Y1Si2O7 (x= 1) with three ‘model’ CMAS compositions with different Ca/Si ratios: (i) Naval

Air Systems Command (NAVAIR) CMAS (Ca/Si = 0.76) [116,117,128], (ii) National Aeronautics and Space Administration (NASA) CMAS (Ca/Si = 0.44) [61] and (iii) Icelandic volcanic ash

(IVA) CMAS (Ca/Si = 0.10) [71]. The chemical compositions of these CMASs are reported in

Table 13. Interactions of these CMASs with pure RE-pyrosilicates (Y2Si2O7 (x = 2) and Yb2Si2O7

(x = 0)) are also studied for comparison. This is with the overall goal of providing insights into the chemo-thermo-mechanical mechanisms of these interactions, and to use this understanding to guide the design and development of future CMAS-resistant, low thermal-conductivity TEBCs.

Table 13: Original CMAS compositions used in this study (mol%) and the Ca/Si (at.) ratio for each. Phase CaO MgO AlO1.5 SiO2 Ca/Si NAVAIR CMAS [116,117,128] 37.6 5.0 7.9 49.5 0.76 NASA CMAS [61] 26.6 5.0 7.9 60.5 0.44 Icelandic Volcanic Ash [71] 7.9 5.0 7.9 79.2 0.10

4.2 Experimental Procedures

4.2.1 Powders

Experimental procedures for making γ-Y2Si2O7 powder have already been reported and can be found in Section 2.2.1. The β-Yb2Si2O7 powders were obtained commercially from

Oerlikon Metco (AE 11073, Oerlikon Metco, Westbury, NY). β-Yb1.8Y0.2Si2O7 and β-Yb1Y1Si2O7 solid-solution powders were prepared in-house by combining stoichiometric amounts of β-

Yb2Si2O7 and γ-Y2Si2O7 powders. The mixture was then ball-milled and dried using the same procedure described in Section 2.2.1. The dried powders were placed in Pt crucibles for calcination at 1600 ˚C in air for 24 h in the box furnace. The resulting powders were then crushed, ball-milled for an additional 24 h, and dried.

These ceramic powders followed the same procedure as stated for YAlO3, Y2Si2O7,

Yb2Si2O7, Sc2Si2O7 and Lu2Si2O7, which can be found in Section 2.2.1 for more detail. Briefly, pellets (~2 mm thick, 20 mm in diameter) were made using spark plasma sintering (SPS: 75 MPa applied pressure, 50 °C.min-1 heating rate, 1500 °C hold temperature, 5 min hold time, and 100

°C.min-1 cooling rate). The pellets were ground, heat-treated (1500 °C, 1 h) and polished for

CMAS-interaction testing.

4.2.2 CMAS Interaction

Three different simulated CMASs were used in this study: NAVAIR CMAS (Ca/Si = 0.76),

NASA CMAS (Ca/Si = 0.44), and IVA CMAS (Ca/Si = 0.10). The chemical compositions of these

CMASs are reported in Table 13, and they have been chosen to study the effect of CMAS Ca/Si ratio on the interaction of the CMAS with RE2Si2O7 (RE = Yb, Y, Yb/Y). NAVIAR CMAS is from Chapters 2 and 3, and a previous study [116,117,128], and it is close to the composition of the AFRL-03 standard CMAS (desert sand). The NASA CMAS [61] and the IVA CMAS [71] compositions are based on literature, where the Ca/Si ratio is changed, while maintaining the same amounts of MgO and AlO1.5.

Powders of the CMAS glasses of these compositions were prepared using a procedure described elsewhere [70,86]. CMAS interaction studies were performed by applying the CMAS powder paste (in ethanol) uniformly over the center of the polished surfaces of the Yb2Si2O7,

-2 Yb1.8Y0.2Si2O7, Yb1Y1Si2O7, and Y2Si2O7 pellets at ∼15 mg.cm loading. The specimens were then placed on a Pt sheet with the CMAS-coated surface facing up, and heat-treated in the box furnace at 1500 °C in air for 24 h (10 °C.min-1 heating and cooling rates). The CMAS-interacted pellets were then cut using a low-speed diamond saw, and the cross-sections were polished to a 1-

μm finish.

4.2.3 Characterization

The characterization for these experiments is similar to the EBC ceramics found in

Chapters 2 and 3. Please refer to Section 2.2.3 for more detail. Briefly, X-ray diffraction (XRD) was conducted on the as-prepared β-Yb1.8Y0.2Si2O7 and β-Yb1Y1Si2O7 powders, and the heat-

70 treated pellets. Densities of the as-SPSed pellets were measured using the Archimedes principle

(immersion medium = distilled water).

Scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy

(EDS) was used to observe the cross-sections of the as-SPSed and CMAS-interacted pellets.

Transmission electron microscopy (TEM) equipped with an EDS system was used to observe the

β-Yb1Y1Si2O7 as-SPSed sample. The sample was prepared using focused ion beam and in-situ lift- out.

4.3 Results

4.3.1 Powder and Polycrystalline Pellets

Figures 43A and 43B are SEM micrographs of as-processed Yb1.8Y0.2Si2O7 and

Yb1Y1Si2O7 powders, respectively. Figures 43C and 43D are cross-sectional SEM micrographs of

Yb1.8Y0.2Si2O7 and Yb1Y1Si2O7 thermally-etched SPSed pellets, respectively. The density of the

-3 Yb1.8Y0.2Si2O7 pellet is found to be 5.93 Mg.m (~99% dense), and the average grain size is ~14

μm. The density of the Yb1Y1Si2O7 pellet is found to be 5.03 Mg.m-3 (~99% dense), and the average grain size is ~15 μm. Figure 43E presents indexed XRD patterns of the Yb1.8Y0.2Si2O7 and

Yb1Y1Si2O7 pellets, along with that of the Yb2Si2O7 pellet. The progressive peak-shift with increasing x from 0 to 1, as evident in the higher-resolution XRD pattern in Figure 43F, indicates single-phase (β) solid solutions.

Figure 44A is a bright-field TEM micrograph of the as-SPSed Yb1Y1Si2O7 pellet, with

Figure 44B showing a higher magnification image from the area marked in Figure 44A. The EDS composition (at.% cation basis) corresponding to the points marked (encircled numbers) in Figure

44B are presented in Table 14, which appear to be uniform. Also, there is no visible contrast within the grains. Figure 44C is another high-magnification bright-field TEM image showing no phase contrast within the grains, and a grain boundary; Figure 44D presents EDS line scans (Si, Yb, Y) along the line marked L-R. The Y/Yb ratios along the entire line are within the EDS detection limit, indicating compositional homogeneity, i.e. no evidence of nanoscale phase separation. Thus, the XRD data in Figures 43E and 43F, coupled with the TEM and EDS data in Figure 44 and Table

14 unambiguously confirm that the as-SPSed Yb1Y1Si2O7 pellet is a RE-pyrosilicate ceramic solid- solution. Although Yb1Y1Si2O7 was the focus of this TEM analysis, Yb1.8Y0.2Si2O7 is expected to form a complete solid-solution without phase separation as well.

Figure 44B

Region # Yb Y Si 1 30 25 45 2 30 23 47 3 & 4 28 23 49 Ideal Composition 25 25 50

4.3.2 NAVAIR CMAS Interactions

Figures 45A, 45B, 45C, and 45D are cross-sectional SEM micrographs of Yb2Si2O7,

Yb1.8Y0.2Si2O7, Yb1Y1Si2O7, and Y2Si2O7 pellets, respectively, that have interacted with the

NAVAIR CMAS (Ca/Si = 0.76) at 1500 ˚C for 24 h. Figure 45A is from Chapter 3 [117] and

Figure 45D is from Chapter 2 [116]. As mentioned earlier, Y2Si2O7 has extensive reaction with

NAVAIR CMAS, resulting in the formation of a needle-like Y-Ca-Si apatite reaction product. In contrast, Yb2Si2O7 does not form Yb-Ca-Si-apatite readily, and instead large ‘blister’ cracks

(horizontal) are observed in the pellet. Figures 45B and 45C clearly show the tempering of these extreme behaviors in the Yb1.8Y0.2Si2O7 and Yb1Y1Si2O7 solid-solutions, respectively. In the

Yb1.8Y0.2Si2O7 pellet no ‘blister’ cracks are seen, and the higher magnification SEM image in

Figure 45E shows some formation of Yb-Y-Ca-Si apatite (region #1 in Table 15). See also the corresponding EDS elemental Ca map in Figure 45F. Thus, with the addition of 10 at% Y (x = 0.2) to Yb2Si2O7, the ‘blister’ cracks are eliminated in exchange for a slightly higher propensity for reaction with the CMAS. However, the small amount of Yb-Y-Ca-Si apatite does not appear to arrest the penetration of the NAVAIR CMAS into the grain boundaries; CMAS pockets can be found (regions #3 and #6 in Table 15). Figure 45G is a higher magnification SEM image of the

Yb1Y1Si2O7 pellet, and the corresponding EDS Ca elemental map is presented in Figure 45H. With

3+ the higher amount of Y in Yb1Y1Si2O7, it appears to react with NAVAIR CMAS in a manner similar to that of the Y2Si2O7 pellet (Figure 45D). There are two reaction layers, a CMAS-rich zone on the top of the sample and an Yb-Y-Ca-Si apatite zone at the interface. The Yb-Y-Ca-Si apatite layer is 80-100 μm thick, which is approximately half the thickness of the Y-Ca-Si apatite layer found in the Y2Si2O7 pellet (Figure 45D). Once again, no ‘blister’ cracks are observed in

Figure 45C.

Figure 45E Figure 45G

Table 15: Average EDS elemental composition (at%; cation basis) from the regions numbered in the SEM micrographs in Figures 45E and 45G of interaction of Yb1.8Y0.2Si2O7 and Yb1Y1Si2O7, respectively, EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h. The ideal compositions are also included. Region # Yb Y Ca Mg Al Si Phase 1 & 2 39 5 12 - - 44 Yb-Y-Ca-Si Apatite 3 & 4 4 1 28 4 8 55 CMAS Glass 5 41 4 - - - 55 Yb1.8Y0.2Si2O7 6 3 1 28 5 8 55 CMAS Glass 7 & 8 39 5 - - - 56 Yb1.8Y0.2Si2O7 9 20 20 13 - - 47 Y-Y-Ca-Si Apatite 10 & 11 4 4 22 3 5 62 CMAS Glass 12 4 3 21 3 5 64 CMAS Glass 13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite 14 2 3 24 4 6 61 CMAS Glass 15 & 16 23 18 - - - 59 Yb1Y1Si2O7 Ideal Compositions 45 5 12.5 - - 37.5 Yb7.2Y0.8Ca2(SiO4)6O2 Apatite 25 25 12.5 - - 37.5 Yb4Y4Ca2(SiO4)6O2 Apatite 45 5 - - - 50 Yb1.8Y0.2Si2O7 25 25 - - - 50 Yb1Y1Si2O7

4.3.3 NASA CMAS Interactions

Figures 46A–46D are cross-sectional SEM micrographs of Yb2Si2O7, Yb1.8Y0.2Si2O7,

Yb1Y1Si2O7, and Y2Si2O7 pellets, respectively, that have interacted with NASA CMAS (Ca/Si =

0.44) at 1500 ˚C for 24 h. Unlike the NAVAIR CMAS case, the Yb2Si2O7 pellet does not show

‘blister’ cracks in Figure 46A. The higher magnification SEM image in Figure 46E, the EDS Ca elemental map (Figure 46I), and the EDS compositions in Table 16 of the regions marked in Figure

46E all confirm that there is no Yb-Ca-Si apatite present. Similarly, ‘blister’ cracks and apatite are absent in Yb1.8Y0.2Si2O7 (Figures 46B, 46F, and 46J, and Table 16) and Yb1Y1Si2O7 (Figures 46C,

46G, and 46K, and Table 16) pellets that have interacted with the NASA CMAS. Pockets of NASA

CMAS can be seen in triple junctions in the Yb2Si2O7, Yb1.8Y0.2Si2O7, Yb1Y1Si2O7 pellets. Y-Ca-

Si apatite formation is found in the Y2Si2O7 pellets that has interacted with the NASA CMAS

(regions #13 and #14 in Figure 46H and Table 16), but the apatite layer is much thinner (~50 μm thickness) and NASA CMAS is also found in pockets between Y2Si2O7 grains (region #15 in

Figure 46H and Table 16). The porosity in the Y2Si2O7 pellet also appears to be affected after

NASA-CMAS interaction, where in Figure 46D, larger pores can be seen near the top of the sample as compared to the middle of the sample (toward the bottom of the micrograph).

Figure 46E Figure 46F Figure 46H Figure 46G

Figure 46: Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C, 24 h) EBC ceramics: (A) Yb2Si2O7, (B) Yb1.8Y0.2Si2O7, (C) Yb1Y1Si2O7, and (D) Y2Si2O7. Dashed boxes indicate from where the corresponding higher-magnification SEM images are collected: (E) Yb2Si2O7, (F) Yb1.8Y0.2Si2O7, (G) Yb1Y1Si2O7, and (H) Y2Si2O7. Corresponding EDS Ca elemental maps: (I) Yb2Si2O7, (J) Yb1.8Y0.2Si2O7, (K) Yb1Y1Si2O7, and (L) Y2Si2O7. The circled numbers in (E) through (G) correspond to regions from where EDS elemental compositions are obtained (see Table 16).

Table 16: Average EDS elemental composition (at%; cation basis) from the regions numbered in the SEM micrographs in Figures 46E, 46F, 46G, and 46H of interactions of Yb2Si2O7, Yb1.8Y0.2Si2O7, Yb1Y1Si2O7, and Y2Si2O7 EBC ceramics, respectively, with NASA CMAS at 1500 ˚C for 24 h. Region # Yb Y Ca Mg Al Si Phase 1 44 - - - - 56 Yb2Si2O7 2 18 - 15 3 3 61 CMAS Glass* 3 25 - 10 3 1 61 CMAS Glass* 4 44 - - - - 56 Yb2Si2O7 5 40 4 - - - 56 Yb1.8Y0.2Si2O7 6 3 1 26 4 6 60 CMAS Glass 7 40 4 - - - 56 Yb1.8Y0.2Si2O7 8 5 1 23 3 6 63 CMAS Glass 9 23 18 - - - 59 Yb1Y1Si2O7 10 3 2 24 4 6 61 CMAS Glass 11 22 18 - - - 59 Yb1Y1Si2O7 12 3 2 24 4 5 62 CMAS Glass 13 & 14 - 42 14 - - 44 Y-Ca-Si Apatite 15 - 15 15 4 6 60 CMAS Glass* 16 - 45 - - - 55 Y2Si2O7 *Includes signal from surrounding material

4.3.4 Icelandic Volcanic Ash CMAS Interactions

Figures 47A, 47B, 47C, and 47D are cross-sectional SEM micrographs of Yb2Si2O7,

Yb1.8Y0.2Si2O7, Yb1Y1Si2O7, and Y2Si2O7 pellets, respectively, that have interacted with IVA

CMAS (Ca/Si = 0.10) at 1500 ˚C for 24 h. The corresponding higher magnification SEM images and EDS Ca elemental maps are presented in Figures 47E-47H and Figures 47I-47L, respectively.

This low Ca/Si-ratio CMAS shows the most unusual behavior, where crystallization of pure SiO2

(α-cristobalite phase) grains is observed within the CMAS. Neither ‘blister’ cracks nor apatite formation is detected in any of these pellets. Only slight penetration of the IVA CMAS is observed in the Y2Si2O7 pellet (Figures 47H and 47L). In Yb2Si2O7, Yb1.8Y0.2Si2O7, and Yb1Y1Si2O7 pellets, reprecipitated phases can be seen in the CMAS pool at the top of the sample. Their chemical compositions are reported in Table 17 (regions #3, #7 and #10).

Figure 47E Figure 47G Figure 47F Figure 47H

Table 17: Average EDS elemental composition (at%; cation basis) from the regions numbered in the SEM micrographs in Figures 47E, 47F, 47G, and 47H of interactions of Yb2Si2O7, Yb1.8Y0.2Si2O7, Yb1Y1Si2O7, and Y2Si2O7 EBC ceramics, respectively, with Icelandic Volcanic Ash CMAS at 1500 ˚C for 24 h. Region # Yb Y Ca Mg Al Si Phase 1 - - - - - 100 SiO2 2 4 - 17 7 11 61 CMAS Glass 3 36 - 2 - - 62 Re-precipitated Yb2Si2O7 4 44 - - - - 56 Yb2Si2O7 5 3 1 16 7 12 61 CMAS Glass 6 - - - - - 100 SiO2 7 32 4 2 - - 62 Re-precipitated Yb1.8Y0.2Si2O7 8 38 5 - - - 57 Yb1.8Y0.2Si2O7 9 2 3 17 7 11 60 CMAS Glass 10 20 18 1 - - 61 Re-precipitated Yb1Y1Si2O7 11 - - - - - 100 SiO2 12 17 25 - - - 58 Yb1Y1Si2O7 13 - - - - - 100 SiO2 14 - 5 12 5 10 68 CMAS Glass 15 & 16 - 45 - - - 55 Y2Si2O7

4.4 Discussion

The results from this study show systematically that the Ca/Si ratio in the CMAS can influence profoundly its interaction with Yb(2-x)YxSi2O7 EBC ceramics, which also depends critically on the x value. First consider the propensity for the formation of the apatite reaction product. Y-Ca-Si apatite is significantly more stable compared to Yb-Ca-Si apatite as the ionic radius of Y3+ is closer to that of Ca2+ than is Yb3+ to Ca2+. This is the driving force for apatite formation [128,146,147]. Thus, the combination of CMAS with the highest Ca content (Ca/Si =

0.76, NAVAIR) and EBC ceramic with the highest Y content (x = 2, Y2Si2O7) shows the greatest propensity for apatite formation. Apatite formation is a ‘double edged sword.’ On the one hand, formation of apatite consumes the CMAS and arrests its further penetration into the EBC (pores and/or grain boundaries). On the other hand, extensive formation of apatite is detrimental as this reaction-product layer does not have the desirable thermal (CTE) and mechanical properties of the

3+ EBC itself. As expected, a reduction in the Y content (x value) in the Yb(2-x)YxSi2O7 EBC ceramic, for the same high Ca-content CMAS (NAVAIR), reduces the propensity for apatite formation. Next consider the ‘blister’ cracks formation. This occurs when Y3+ is completely eliminated (x = 0) in Yb2Si2O7, where the lack of apatite formation allows the CMAS glass to penetrate into Yb2Si2O7 grain boundaries. This sets up a dilatation gradient, which is the driving force for ‘blister’ cracking. Thus, the benefit of solid-solution EBCs is clearly demonstrated in this study, where the CMAS-interaction behavior is tuned to prevent ‘blister’ crack formation and to reduce apatite formation.

As the Ca/Si ratio decreases in the NASA CMAS (Ca/Si = 0.44), the overall propensity for apatite formation decreases. This is expected due to insufficient Ca2+ availability in the NASA

CMAS. But surprisingly, ‘blister’ cracking is also suppressed in Yb2Si2O7 despite the grain- boundary penetration of the NASA CMAS. The reason for this is not clear at this time, but it could be related to the relatively facile grain-boundary penetration of NASA CMAS, which may preclude the formation of a dilatation gradient.

With further decrease in the Ca/Si ratio to 0.10 in IVA CMAS, the propensity for apatite formation decreases further. The amount of molten CMAS that can react or interact with the pellets decreases due to the crystallization of pure SiO2 cristobalite. However, this increases the Ca/Si ratio in the remaining CMAS, complicating the issue. Nonetheless, the Ca/Si ratio in the remaining

CMAS is still less than 0.44 that is in NASA CMAS (Table 16), resulting in virtually no apatite formation and the suppression of ‘blister’ cracks.

This first systematic report on CMAS interactions with Yb(2-x)YxSi2O7 EBC ceramics clearly shows the benefit of solid-solutions. This allows tuning of the CMAS interaction by

83 reducing the amount of apatite formation and suppressing ‘blister’ cracking, while maintaining polymorphic β-phase stability and the desirable CTE match with SiC-based CMCs.

4.5 Summary

Here a systematic study of the high-temperature (1500 °C) interactions between promising dense, polycrystalline EBC ceramic pellets, Yb2Si2O7, Yb1.8Y0.2Si2O7, Yb1Y1Si2O7, and Y2Si2O7, and three CMAS glasses, NAVAIR (Ca/Si = 0.76), NASA (Ca/Si = 0.44), Icelandic Volcanic Ash

(Ca/Si = 0.10) was performed. Yb(2-x)YxSi2O7 solid solutions are confirmed to be pure β-phase.

NAVAIR CMAS with its highest Ca/Si ratio, shows a tempering effect between the extensive reaction-crystallization (apatite formation) in Y2Si2O7 and the ‘blister’ crack formation in

Yb2Si2O7 EBC ceramics. The Yb1.8Y0.2Si2O7 and Yb1Y1Si2O7 solid-solution EBC ceramics do not show any ‘blister’ cracks. There is some apatite formation, but it is not as extensive as in the case of Y2Si2O7 EBC ceramics. The NASA CMAS when reacted with the EBC ceramics does not show

‘blister’ cracks, although CMAS still penetrates the grain boundaries. In the Yb2Si2O7,

Yb1.8Y0.2Si2O7, and Yb1Y1Si2O7 EBC ceramics, no reaction products are observed. In the case of

Y2Si2O7 EBC ceramic there is an apatite reaction zone, but it is much smaller compared to the

NAVAIR CMAS (Ca/Si = 0.76) case. Penetration of the NASA CMAS into grain boundaries and pores are also observed in the Y2Si2O7 EBC ceramics. The IVA CMAS, with its lowest Ca/Si ratio, does not show apatite formation in any of the EBC ceramics studied. There is some crystallization of pure SiO2 (α-cristobalite) in the CMAS melt. No ‘blister’ cracks are observed in any of the EBC ceramics. This study highlights the interplay between the CMAS and the EBC ceramic compositions in determining the nature of the high-temperature interaction, and suggests a way to tune that interaction in rare-earth pyrosilicate solid-solutions.

CHAPTER 5: THERMAL CONDUCTIVITY

This chapter was modified from a previously published article along with unpublished data, that may be used in future publications: L.R. Turcer and N.P. Padture, “Towards multifunctional thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramics,” Scripta Materialia, 154, 111-117 (2018).

5.1 Introduction

EBC-coated CMC components need to be attached to the lower-temperature metallic hardware within the engine, which invariably results in temperature gradients. It is therefore imperative that EBCs have enhanced thermal-insulation properties. There is also an increasing demand for thermal protection of CMCs for even higher temperature applications [4,13,35,154].

Furthermore, thin-shelled, hollow CMCs are being developed using the integral ceramic textile structure (ICTS) approach which can be actively cooled [4,155,156]. In all of these cases, an additional thermally-insulating TBC top-coat capable of withstanding higher temperatures (>1700

°C) is needed – the concept of T/EBC (Figures 48A and 48B) [4,13,146,154,157].

B A

Figure 48: (A) Cross-sectional SEM micrograph of a T/EBC on a CMC [13]. (B) Schematic illustration of the T/EBC concept adapted from [4]. (C) Schematic Illustration of the TEBC concept.

The TBC top-coat is typically made of low thermal-conductivity, refractory oxides such as a RE-zirconate or RE-hafanate. However, the CTEs of Si-free TBC oxides (~10×10−6 °C) are typically significantly higher than that of SiC (~4.5×10−6 °C). While the cracks and pores in TBC

85 top-coats can provide strain-tolerance, exposure of the TBC top-coat to temperatures approaching

1700 °C can result in their sintering. This leads to a reduction in the strain-tolerance and increases the thermal conductivity of the TBC top-coat. The introduction of an intermediate layer or gradation between the TBC top-coat and the underlying EBC can mitigate the CTE-mismatch problems to some extent. However, the options of available high-temperature materials for this additional layer or gradation that satisfy the various onerous requirements is vanishingly small: intermediate CTE, high-temperature capability, phase stability, chemical compatibility with both

TBC and EBC, robust mechanical properties, etc. Thus, at operating temperatures approaching

1700 °C, deleterious reactions between the different layers, and homogenization of any gradations, are inevitable over time. Also, any additional interfaces can become sources of failure during in- service thermal cycling/excursions.

In order to avoid these shortcomings of the current T/EBCs, it is highly desirable to replace the EBC, the intermediate layer/gradation, and the TBC top-coat with a single layer of one material that can perform both the thermal- and environmental-barrier functions (Figure 48C) – the TEBC concept. Thus, the four most important properties, among several other requirements, this single material must possess are: (i) good CTE match with SiC, (ii) high-temperature phase stability, (iii) inherently low thermal conductivity in its dense state, and (iv) resistance to CMAS attack. This chapter proposes that solid-solutions of some RE-pyrosilicates (or RE-disilicates – RE2Si2O7) may satisfy these key requirements for TEBC applications.

5.1.1 Coefficient of Thermal Expansion

As previously stated, individual RE-pyrosilicate ceramics are showing promise for EBC application as they have good CTE match with SiC. Figure 49A shows the measured average CTEs

86 of several RE2Si2O7 polymorphs [137,158]. The β polymorph of RE2Si2O7 (RE = Sc, Lu, Yb, Er,

Y) and γ polymorph of RE2Si2O7 (RE = Y, Ho) have average CTEs that are close to that of SiC

[137]. Both β (space groups C2/m, C2, Cm) and γ (space group P21/a) polymorphs have the monoclinic crystal structure, and, therefore, their CTEs are anisotropic [137,158]. (Note that the polymorphs β, γ, δ, and α correspond to C, D, E, and B, respectively, in the original notation by

Felsche [37].)

5.1.2 Phase Stability

While CTEs of the above RE-pyrosilicate polymorphs are acceptable for EBC application, some of them undergo polymorphic phase transformation in the temperature range 25–1700 °C.

Figure 49B presents the phase-stability diagram for the different RE-pyrosilicates (excluding RE

= Sc and Y) showing that, except for Yb2Si2O7 (MP 1850 °C [136]) and Lu2Si2O7 (MP 2000 °C

[140]), all RE-pyrosilicates undergo phase transformation(s) [37]. While Er2Si2O7 and Ho2Si2O7, have a good CTE match with SiC, they may not be suitable for EBC application as both undergo phase transformations. Y2Si2O7 (MP 1775 °C [124]) may also seem unsuitable for EBC application

87 as Y3+ has an ionic radius very close to that of Ho3+, and it also undergoes phase transformation,

3+ δ→γ→β→α, during cooling [159]. On the other hand, Sc2Si2O7, with its very small Sc ionic radius (0.745 Å, coordination number 6), has only one polymorph, β, up to its melting point (1860

°C [138]) [144]. This narrows the list of RE pyrosilicate ceramics suitable for EBCs to β-Yb2Si2O7,

β-Sc2Si2O7, and β-Lu2Si2O7. (Note that some of the polymorphic transformations in RE- pyrosilicates can be sluggish, and, therefore, the high temperature polymorphs can be kinetically stabilized at lower temperatures. Also, the volume change associated with some of the polymorphic transformations can be small, making them relatively benign for high-temperature structural applications, but the CTEs of the product phases may be undesirable (Figure 49A).)

5.1.3 Solid solutions

Phase equilibria in Y2Si2O7-Yb2Si2O7 [38,160], Y2Si2O7-Lu2Si2O7 [160,161], and Y2Si2O7-

Sc2Si2O7 [144] have been studied, and are all shown to form complete solid-solutions. While

Yb2Si2O7, Lu2Si2O7, and Sc2Si2O7 all exist only as the β phase, their respective solid solutions with

Y2Si2O7 exist as β, γ, or δ phase depending on the Y content and the temperature: the trend follows

β→γ→δ with increasing Y-content and temperature [38]. For example, the β phase is stable up to

1700 °C for x < 1.1 for both YxYb(2-x)Si2O7 and YxLu(2-x)Si2O7 and x < 1.7 for YxSc(2-x)Si2O7. Since these solid-solutions are isomorphous without any low-melting eutectics, they are expected to have higher MPs compared to pure Y2Si2O7, which has the lowest MP among the four RE-pyrosilicates considered here [38]. Thus, Y2Si2O7, when alloyed with higher-melting Yb2Si2O7, Lu2Si2O7, or

Sc2Si2O7, becomes a viable ceramic for EBC application. The Sc2Si2O7-Lu2Si2O7 system is shown to form complete β-phase solid-solution [162]. While phase equilibria studies in the Sc2Si2O7-

Yb2Si2O7 and the Lu2Si2O7-Yb2Si2O7 systems have not been reported in the open literature, it is likely that they also form complete solid-solutions considering that these RE-pyrosilicates are

88 isostructural and that the ionic radius of Yb3+ is only slightly larger than that of Lu3+ (Figure 49B).

Thus, in addition to individual β-phase RE-pyrosilicates Yb2Si2O7, Lu2Si2O7, and Sc2Si2O7, the list of potential candidates for TEBC application includes the following β-phase RE-pyrosilicate solid-solutions: (i) YxYb(2-x)Si2O7 (x < 1.1), (ii) YxLu(2-x)Si2O7 (x < 1.1), (iii) YxSc(2-x)Si2O7 (x <

1.7), (iv) YbxSc(2-x)Si2O7, (v) LuxSc(2-x)Si2O7, and (vi) LuxYb(2-x)Si2O7. While the CTEs of these solid-solutions are likely to follow rule-of-mixtures behavior, their thermal conductivities may be depressed significantly relative to the rule-of-mixtures behavior, and is discussed in the next section.

5.2 Calculated Thermal Conductivity of Binary Solid-Solutions

5.2.1 Experimental Procedure

I II In order to calculate the thermal conductivity of solid-solutions (RE푥RE(2−푥)Si2O7), experimentally collected data on the pure RE2Si2O7 ceramics were needed including thermal conductivity and Young’s modulus.

Dense, polycrystalline ceramic pellets (~2 mm thickness) of γ-Y2Si2O7, β-Yb2Si2O7, and

β-Sc2Si2O7, from previous studies, were used to measure their thermal diffusivity. They were sent to NETZSCH Instruments North America, LLC (Burlington, MA) for thermal diffusivity (κ) measurements. They machined the pellets to fit their testing apparatus, and followed the ASTM

E1461-13, “Standard Test Method for Thermal Diffusivity by the Flash Method.” Using the flash diffusivity method on a NETZSCH LFA 467 HT HyperFlash® instrument the thermal diffusivities at 27, 200, 400, 600, 800, and 1000 °C were measured. Using the Neumann-Kopp rule for oxides,

[163] the specific heat capacities for the RE2Si2O7 (RE = Y, Yb, and Sc) were calculated by the specific heat capacities (CP) of the present constituent oxides, Yb2O3, Y2O3, Sc2O3, and SiO2 [164].

The thermal conductivity (k) at each temperature was then calculated using 푘 = 휅휌퐶푃, where ρ is the measured room-temperature density.

The Young’s modulus of Sc2Si2O7 was obtained by nanoindentation on random grains using the TI950 Triboindenter (Hysitron, Minneapolis, MN). The Berkovich diamond tip was used to estimate the E values, with a maximum load of 25 mN and a rate of 277.78 µN.s-1. The load- displacement curves were then used to determine the E using the Oliver-Pharr analysis [165]. Nine indentations were made, and the average E of Sc2Si2O7 was found to be 202 GPa, with a minimum of 153 GPa and a maximum of 323 GPa. This large scatter is attributed to the anisotropic E of monoclinic β-Sc2Si2O7.

5.2.2 Pure RE2Si2O7 (RE = Yb, Y, Lu, Sc) Thermal Conductivity

Among the four β-RE-pyrosilicates considered here, the high temperature thermal conductivities of Y2Si2O7 [142], Yb2Si2O7 [123,142], and Lu2Si2O7 [142] have been measured experimentally. However, the pellets used were not completely dense and instead thermal

conductivity data was extrapolated. Dense, polycrystalline Yb2Si2O7 and Y2Si2O7 pellets, similar to those used in Chapters 2 and 3 were measured experimentally by NETZSCH. These results are plotted in Figure 50 along with the Lu2Si2O7 data from literature. The thermal conductivities of

−1 −1 the Y2Si2O7 and Lu2Si2O7 RE-pyrosilicates are low, and they are in the range of 1.5–2 W·m ·K

(at 1000 °C). To the best of our knowledge, the thermal conductivity of Sc2Si2O7 has not been reported in the open literature. In order to address this paucity, the thermal conductivities of a fully dense, phase-pure Sc2Si2O7 ceramic pellet in the temperature range 27–1000 °C were measured.

These are reported in Figure 50. It is seen that Sc2Si2O7 has a significantly higher thermal conductivity, 3.2 W·m-1·K-1 (at 1000 °C), compared to other RE-pyrosilicates.

Figure 50: Thermal conductivities of dense, polycrystalline RE2Si2O7 pyrosilicate ceramic pellets as a function of temperature. The data for Lu2Si2O7 is from Ref [142].

5.2.3 Thermal Conductivity Calculations for Binary Solid-Solutions

None of the thermal conductivities of the RE-pyrosilicate solid-solutions have been reported in literature. In this context, there is a tantalizing possibility of obtaining even lower thermal conductivities in dense RE-pyrosilicate solid-solutions, where the substitutional-solute point defects can be used as effective phonon scatterers, especially where the atomic number (ZRE) contrast between the host and the solute RE-ions is large. To that end, analytical calculations have been performed to estimate the thermal conductivities of RE-pyrosilicate solid-solutions in six systems YxYb(2-x)Si2O7, YxLu(2-x)Si2O7, YxSc(2-x)Si2O7, YbxSc(2-x)Si2O7, LuxSc(2-x)Si2O7, and

LuxYb(2-x)Si2O7, with ZSc = 21, ZY = 39, ZYb = 70, and ZLu = 71.

The thermal conductivity of a solid-solution, in relation with its pure host material, as a function of temperature is given by: [166]

휔표 −1 휔푀 푘푠푠 = 푘푃푢푟푒 ( ) tan ( ) (Equation 7) 휔푀 휔표 where

−1 휔 2 4휓훾2푚푘 Δ푀 2 표 퐵 (Equation 8) ( ) = 푓(푇) ( 3 ) 푇 [푐 ( ) ] . 휔푀 3휋휇푎 푀

Here, ωo is the phonon frequency at which the mean free paths due to point-defect scattering and intrinsic scattering are equal, and ωM is the phonon frequency corresponding to the

-1/3 maximum of the acoustic branch of the phonon spectrum. The latter is given by ωDm , where m

2 3 1/3 is the number atoms per molecular unit and ωD is the Debye frequency, given by (6π v /a) . Here,

3 a is the atomic volume (a = MW/mNA, where MW is the molecular weight and NA is Avagadro's number) and v is the transverse phonon velocity (v = (μ/ρ)1/2, where ρ is the density and μ is the

2 shear modulus). Also, γ is the Grüneisen anharmoncity parameter, kB is the Boltzmann constant, c is the concentration of the solute differing in mass from the host atom of mass M by ΔM (for a simple substitutional solid-solution), and ψ is an adjustable parameter included to obtain an empirical fit between the theory and experiment at room temperature (298 K), and it is set to unity in this case. The function f(T) takes into account the ‘minimum thermal conductivity,’ and it is given empirically by: [167]

300 × 푘 |300 푃푢푟푒 (Equation 9) 푓(푇) = 푇 . 푇 × 푘푃푢푟푒|

Using the available values for all the parameters (listed in Table 18) [34,125,138,142,143], the thermal conductivities, kss, of the six RE-pyrosilicate solid-solutions are plotted in Figure 51.

Note that E of Sc2Si2O7 coating is mentioned to be 200 GPa in the literature [25]. Here it was confirmed that the average E is 202 GPa, using nanoindentation of different individual grains in a

92 dense, polycrystalline Sc2Si2O7 ceramic pellet (see Section 5.2.1 for experimental details).

However, the E appears to be highly anisotropic, ranging from 153 to 323 GPa for individual grains. The Poisson's ratio is assumed to be 0.31. The experimental data points from Figure 50 are included on the y-axes in Figure 51.

Table 18: Properties and parameters for pure β-RE-pyrosilicates. β-Sc2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 β-Lu2Si2O7 -3 # † ‡ § ρ (Mg·m ) 3.40 3.93 6.13 6.25 v 0.31¶ 0.32 0.31 0.32 Ave. μ (GPa) 77 65 62 68 Ave. E (GPa) 202* 170 162 178 a3 (x 10-29 m2) 1.15 1.33 1.27 1.27 m (#) 11 11 11 11 γ 3.373¶ 3.491 3.477 3.487 v (m·s-1) 4762 4067 3180 3322 Min. E (GPa) 153* 102 102 114 MW (g·mol-1) 258.2 346.0 514.2 518.2 -1 -1 * kMin (W·m ·K ) 1.59 1.09 0.90 0.95 *This work. ¶Fitted value. #Ref. [138]. †Ref. [125]. ‡Ref. [34]. §Ref. [143]. All other values are from Ref. [142].

As expected, the largest Z-contrast solid-solutions, YxYb(2-x)Si2O7, YxLu(2-x)Si2O7, YbxSc(2- x)Si2O7, and LuxSc(2-x)Si2O7, show the largest decrease in thermal conductivities due to alloying.

Whereas the solid-solutions with the smallest Z-contrast, YxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7, show the smallest decrease. LuxYb(2-x)Si2O7 shows a rule-of-mixtures behavior since Yb and Lu are next to each other in the periodic table and both have high Z. All but the last two, of the dense solid- solutions of RE-pyrosilicates can have thermal conductivities below 1 W·m-1·K-1 at 1000 °C. This is unprecedented even for TBC ceramics [168], making dense RE-pyrosilicate solid-solutions good candidates for the new single-material TEBCs discussed earlier. So far, only binary solid-solutions have been considered, but phonon scattering in ternary solid-solutions with high Z-contrast REs, e.g. Sc(2-x-y)YxLuySi2O7, could prove to be even more effective.

In this context, the ‘minimum thermal conductivity’ (kMin), where the phonon mean free path approaches interatomic spacing [169], may limit how low the thermal conductivity of RE- pyrosilicate solid-solutions can be depressed. For pure RE-pyrosilicates, the ‘minimum thermal conductivity’ (kMin) is estimated using the following relation: [170]

푚2/3휌1/6퐸1/2 푘 → 0.87푘 푁2/3 , (Equation 10) 푀푖푛 퐵 퐴 (푀푊)2/3 where E is the Young's modulus (minimum value if anisotropic), and the corresponding properties

(see Table 18). The properties in Equation 10 for isomorphous solid-solutions are not known, but are expected to follow rule-of-mixture behavior. In Figure 51, where the x values display the lowest thermal conductivity, the rule-of-mixture properties of the solid-solutions are estimated. They are listed in Table 19. Substituting these property values into Equation 10, the kMin for the six solid- solutions are calculated, and are also reported in Table 19. It should be noted that Equation 10 is derived based on approximations and provides a rough estimate for the ‘minimum thermal conductivity.’ Thus, it remains to be seen if high-temperature thermal conductivities below 1 W·m-

1·K-1 can, in fact, be achieved experimentally in dense RE-pyrosilicate solid-solution (binary or ternary) ceramics.

Table 19: Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the calculated thermal conductivities in Figure 51 are the lowest; kMin calculated using Equation 10. ρ Min. E MW kMin x (Mg·m-3) (Gpa) (g·mol-1) (W·m-1·K-1) YxYb(2-x)Si2O7 1.04 5.00 102 426.6 0.99 YxLu(2-x)Si2O7 0.79 5.34 109 450.5 1.00 YxSc(2-x)Si2O7 1.72 3.88 109 333.7 1.07 YbxSc(2-x)Si2O7 1.34 5.23 119 429.4 1.15 LuxSc(2-x)Si2O7 1.67 5.78 120 475.6 1.02 LuxYb(2-x)Si2O7 2.00 6.25 114 518.1 0.99

5.3 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity

5.3.1 Experimental Procedure

Dense, polycrystalline ceramic pellets (~2 mm thickness) of β-Yb1.8Y0.2Si2O7 and β-

Yb1Y1Si2O7, from the previous study in Chapter 4¸ were used to measure their thermal diffusivity.

They were sent to NETZSCH Instruments North America, LLC (Burlington, MA) for thermal diffusivity (κ) measurements, like the pure RE2Si2O7 ceramics. For more details on this process please refer to Section 5.2.1. Using the flash diffusivity method on a NETZSCH LFA 467 HT

HyperFlash® instrument the thermal diffusivities at 27, 200, 400, 600, 800, and 1000 °C were measured following ASTM E1461-13. Using the Neumann-Kopp rule for oxides, [163] specific heat capacities for the RE2Si2O7 (RE = Yb1.8Y0.2 and Yb1Y1) were calculated by the specific heat capacities (CP) of the constituent oxides, Yb2O3, Y2O3, and SiO2 [164]. The thermal conductivity

(k) at each temperature was then calculated using 푘 = 휅휌퐶푃, where ρ is the measured room- temperature density.

Other experimental data including density, Young’s modulus, etc., were obtained by using rule-of-mixture calculations.

5.3.2 Comparison of Experimental and Calculated Thermal Conductivity

Figure 52 shows the thermal conductivity measurements for Yb2Si2O7, Y2Si2O7, Yb1.8Y-

0.2Si2O7, and Yb1Y1Si2O7. At room temperature (27 °C) the thermal conductivity of Yb1Y1Si2O7 is the lowest. For the rest of the thermal conductivity measurements, the solid-solutions,

Yb1.8Y0.2Si2O7 and Yb1Y1Si2O7, fall in the range of the thermal conductivity values of the pure components, Yb2Si2O7 and Y2Si2O7.

To more easily compare this data, the experimental data points are plotted against the calculated values from Section 5.2.3, which can be seen in Figure 53. The experimental data does not have as significant a decrease in thermal conductivity as expected from the analytical calculations. From room temperature to 600 °C, the data shows a decrease in thermal conductivity lower than the rule-of-mixtures prediction. This comparison can also be seen in Table 20. From

600 to 1000 °C, the solid-solution thermal conductivities seem to follow a rule-of-mixtures estimate.

Table 20: Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and rule-of-mixture calculations. Thermal Conductivities (W·m-1·K-1) Temperature Yb1.8Y0.2Si2O7 Yb1Y1Si2O7 (°C) Experimental Rule-of-Mixture Experimental Rule-of-Mixture 27 4.20 5.07 3.61 4.47 200 3.51 4.05 3.02 3.42 400 3.04 3.35 2.64 2.76 600 2.63 2.80 2.31 2.29 800 2.47 2.58 2.16 2.10 1000 2.47 2.52 2.12 2.09

Similarly, Tian et al. [171] have measured the thermal conductivities of RE2SiO5 solid- solutions hot-pressed ceramics, (YxYb1-x)2SiO5, as a function of x (0 to 1) and temperature (27 to

1000 °C) for possible TEBCs. They did not observe the expected ‘dip’ in the thermal conductivities, which could be attributed to the “minimum conductivity” limit [171]. However, they observed lower than expected thermal conductivity in a Yb-rich RE2SiO5 composition (x =

0.17) [171]. They attributed this to the presence of oxygen vacancies created by some reduction of

Yb3+ to Yb2+ in the ceramic fabricated using hot-pressing [171], which invariably has a reducing atmosphere. While such oxygen vacancies are unlikely to exist in equilibrium ceramics in an oxidizing environment of a gas-turbine engine, equilibrium oxygen vacancies can be formed by

2+ 2+ alloying them with group IIA aliovalent substitutional cations, such as Mg (ZMg = 12), Ca (ZCa

2+ 2+ = 20), Sr (ZSr = 38), or Ba (ZBa = 56).

It is known that point defects such as oxygen vacancies are potent phonon scatterers in

RE2O3-ZrO2 solid-solutions and compounds [5,167,168,172]. Thus, for example, alloying a RE- pyrosilicate, such as Yb2Si2O7, with a group IIA oxide, such as MgO, will result in high Z-contrast

′ ∙∙ cation substitution and oxygen vacancies: 2푀푔푂 ⟷ 2푀푔푌푏 + 2푂푂 + 푉푂. This effect could be further enhanced in ternary, or even quaternary, solid-solutions of RE-pyrosilicates and group IIA oxides, notwithstanding the ‘minimum thermal conductivity’ limit. Unfortunately, phase equilibria

99 studies in these systems have not been reported in the open literature, and, therefore, the relative solid-solubilities are not known. Also, there is the danger of forming low-melting eutectics and/or glasses in such multicomponent silicate systems, which may limit their utility in high-temperature

TEBC applications.

Another possible way to decrease the thermal conductivity in RE-pyrosilicates would be to use equiatomic solid-solution mixtures, like high-entropy ceramics. This will be discussed further in the following section.

5.4 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution

5.4.1 Introduction to High-Entropy Ceramics

High-entropy alloys were first studied in 2004 [173]. These were made by mixing equimolar amounts of metallic elements, which creates a disordered solid-solution. This increases the entropy of the system, which causes a decrease in the energy of the system. Since then many studies have focused on high-entropy ceramic materials to enhance certain properties. High- entropy oxides [174–176], borides [177], carbides [178–180], nitrides [181], sulfides [182], and silicides [183,184] have all been studied. They have demonstrated phase stability and have been shown to have adjustable and enhanced properties [185].

In 2019, high-entropy ceramics of RE2Si2O7 [186] and RE2SiO5 [187,188] were first studied. Chen et al. [187] synthesized a homogenous (Yb0.25Y0.25Lu0.25Er0.25)2SiO5 ceramic, which was confirmed by EDS mapping on a SEM and high temperature XRD. Ridley et al. [188] studied the thermal conductivity and coefficient of thermal expansion for (Sc0.2Y0.2Dy0.2Er0.2Yb0.2)2SiO5 compared to pure RE2SiO5 ceramics. Again, only EDS mapping on a SEM and XRD confirmed solid-solution high-entropy ceramics. To the best of my knowledge, the only high-entropy

100

RE2Si2O7 found in literature is β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 [186]. Dong et al. [186] confirms a phase pure homogenous solid-solution through XRD, TEM and SAEDP. However, the ‘high- entropy’ nature of this system has not been confirmed.

For the focus of this project, the thermal conductivity of a 5-compontent equiatomic solid- solution, or β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 was studied. Here it will not be referred to as ‘high- entropy’ due to insufficient evidence. However, it has been shown to form a phase pure solid- solution and due to the difference in Z-contrast (ZSc = 21, ZY = 39, ZGd = 64 ZYb = 70, and ZLu =

71) and the randomly distributed RE cations in a β-RE2Si2O7 structure, it is believed that the thermal conductivity will decrease. The overall goal is to provide insights into the thermal conductivity of the 5-component equiatomic β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7, and to use this understanding to guide the design and development of future low thermal-conductivity TEBCs.

5.4.2 Experimental Procedure

The β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 powder was prepared in-house by combining stochiometric amounts of Y2O3 (Nanocerox, Ann Arbor, MI), Yb2O3 (Sigma Aldrich, St. Louis,

MO), Lu2O3 (Sigma Aldrich, St. Louis, MO), Sc2O3 (Reade Advanced Materials, Riverside, RI),

Gd2O3 (Alfa AESAR, Ward Hill, MA), and SiO2 (Atlantic Equipment Engineers, Bergenfield, NJ).

This mixture was then ball-milled and dried while stirring. The dried powder mixture was placed in a Pt crucible for calcination at 1600 °C in air for 4 h in the box furnace. The resulting β-(Y0.2Y-

0.2Lu0.2Sc0.2Gd0.2)2Si2O7 powder was then ball-milled for an additional 24 h, dried, and crushed.

101 rate, 1500 °C hold temperature, 5 min hold time, and 100 °C.min-1 cooling rate. The surfaces of the resulting dense pellets (∼2 mm thickness) were ground to remove the graphfoil, and the pellets were heat-treated at 1500 °C in air for 1 h (10 °C.min-1 heating and cooling rates) in the box furnace. The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic polishing techniques. Some pellets were cut using a low-speed diamond saw, and the cross-sections were polished to a 1-μm finish.

The as-prepared powder was characterized using an X-ray diffractometer (XRD; D8

Advance, Bruker AXS, Karlsruhe, Germany) to check for phase purity. The phase present was identified using the PDF2 database. The densities of the as-SPSed pellets were measured using the

Archimedes principle, with distilled water as the immersion medium.

The cross-sections of the as-SPSed pellet was observed in a SEM (LEO 1530VP, Carl

Zeiss, Munich, Germany or Helios 600, FEI, Hillsboro, Oregon, USA), equipped with EDS (Inca,

Oxford Instruments, Oxfordshire, UK), operated at 20 kV accelerating voltage. EDS elemental maps were also collected and used to determine homogeneity in the pellets.

A transmission electron microscopy (TEM) specimen from a location within the polished cross-section of the as-SPSed pellet was prepared using focused ion beam (FIB; Helios 600, FEI,

Hillsboro, Oregon, USA) and in situ lift-out. The sample was then examined using a TEM (2100

F, JEOL, Peabody, MA), equipped with an EDS system (Inca, Oxford Instruments, Oxfordshire,

UK), operated at 200 kV accelerating voltage. Selected-area electron diffraction patterns

(SAEDPs) from various phases in the TEM micrographs were recorded and indexed using standard procedures.

102

5.4.3 Solid Solution Confirmation

Although the material was confirmed to be solid-solution by Dong et al. [186], they made samples using a sol-gel process. Here, the samples were made by mixing oxide constituents and calcinating the powders. Therefore, due to the difference in materials processing, a confirmation of the solid-solubility of β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 is needed.

Figure 54 shows an XRD pattern of the β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 pellet, compared to Yb2Si2O7 and the solid-solution mixtures, Yb1.8Y0.2Si2O7 and Yb1Y1Si2O7 (from Chapter 4 and

Section 5.3 in this chapter). The indexed XRD pattern shows a β-phase pure material. The density

-3 of the β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 pellet is 5.08 Mg.m (~98% dense, compared to the theoretical density obtained by reitveld analysis).

Figure 54: Indexed XRD pattern from an as-SPSed β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 pellet, compared to β-Yb2Si2O7, β-Yb1.8Y0.2Si2O7, and β-Yb1Y1Si2O7 pellets.

Figure 55 shows a SEM micrograph of the as-SPSed β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 pellet and its corresponding elemental EDS maps: Y, Yb, Lu, Sc, Gd and Si. The elemental EDS

103 maps show a homogenous dispersion of the 5 RE components and Si. EDS elemental compositions were also collected in different grains across this sample and were Y7-Yb9-Lu9-Sc10-Gd9-Si56 (at%; cation basis), which is similar to the ideal composition of Y10-Yb10-Lu10-Sc10-Gd10-Si50 (at%; cation basis).

Figure 55: Cross-sectional SEM image of an as-SPSed β-(Y0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 pellet and the corresponding EDS elemental maps: Y, Yb, Lu, Sc, Gd, and Si.

Figure 56A shows a TEM sample collected from the as-SPSed β-(Y0.2Y0.2Lu-

0.2Sc0.2Gd0.2)2Si2O7 pellet. An indexed SAEDP confirms β-phase. Figures 56B and 56C are two higher magnification TEM micrographs of regions marked in Figure 56A. Elemental EDS maps for Y, Yb, Lu, Sc, Gd and Si are also shown. Within the grain and along grain boundaries, the EDS maps are showing a homogenous material. EDS elemental compositions were collected (circled numbers) and can be found in Table 21.

104

Figure 56B

Figure 56C

Figure 56: (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-(Y0.2Y0.2Lu- 0.2Sc0.2Gd0.2)2Si2O7 pellet. β-RE2Si2O7 grains are found. Transmitted beam and zone axis are denoted by ‘T’ and ‘B,’ respectively. (B, C) Two higher magnification regions showing grain boundaries and the corresponding EDS elemental maps: Y, Yb, Lu, Sc, Gd, and Si. The circled regions are where EDS elemental compositions were obtained and can be found in Table 21.

105

TEM/SAEDP (Figure 56 and Table 21) and XRD (Figure 54) results confirm that β-

(Y0.2Yb0.2Lu0.2Sc0.2Gd0.2)2Si2O7 is the only crystalline phase and that there does not appear to be nano-scale phase separation in this material, i.e. the material is confirmed to be a solid-solution of

β-(Y0.2Yb0.2Lu0.2Sc0.2Gd0.2)2Si2O7.

5.4.4 Experimental Thermal Conductivity Results

Thermal conductivity β-(Y0.2Yb0.2Lu0.2Sc0.2Gd0.2)2Si2O7 was measured by NETZSCH and can be seen below, in Figure 57. Room temperature thermal conductivity of the β-

-1 -1 (Y0.2Yb0.2Lu0.2Sc0.2Gd0.2)2Si2O7 is 2.15 W·m ·K , which is much lower than the thermal conductivities of Yb2Si2O7, Y2Si2O7, Yb1.8Y0.2Si2O7 and Yb1Y1Si2O7. However, as temperature is

- increased, the thermal conductivity starts to align with that of the Y2Si2O7 sample (~1.51 W·m

1·K-1 at 800 and 1000 °C).

106

Interestingly, this shows a similar relationship to the Yb(2-x)YxSi2O7 solid-solutions. The 5- component equiatomic RE2Si2O7 shows much lower thermal conductivities up to 600 °C. The solid-solutions saw a greater decrease than the rule-of-mixtures up to 600 °C. From 600 to 1000

°C, β-(Y0.2Yb0.2Lu0.2Sc0.2Gd0.2)2Si2O7 follows the thermal conductivity of Y2Si2O7. In the same temperature range, the thermal conductivity of the Yb(2-x)YxSi2O7 solid-solutions did not show a decrease in thermal conductivity compared to the rule-of-mixtures calculations. At the higher temperatures (> 600 °C), the lack of the expected decrease in thermal conductivity could be attributed to the “minimum conductivity” limit [171].

5.5 Summary

Analytical calculations of the thermal conductivities for six systems, YxYb(2-x)Si2O7,

YxLu(2-x)Si2O7, YxSc(2-x)Si2O7, YbxSc(2-x)Si2O7, LuxSc(2-x)Si2O7, and LuxYb(2-x)Si2O7, were

107 performed. Substitutional-solute point defects are an effective way to scatter phonons and decrease thermal conductivity, especially when the Z-contrast is high. As expected, the largest Z-contrast solid-solutions, YxYb(2-x)Si2O7, YxLu(2-x)Si2O7, YbxSc(2-x)Si2O7, and LuxSc(2-x)Si2O7, show the largest decrease in thermal conductivities due to alloying.

Solid-solutions of Yb(2-x)YxSi2O7 were studied in more detail and experimental thermal conductivity data was obtained for Yb1.8Y0.2Si2O7 and Yb1Y1Si2O7. The experimental data does not have as significant a decrease in thermal conductivity as expected by the analytical calculations.

A 5-component equiatomic β-(Y0.2Yb0.2Lu0.2Sc0.2Gd0.2)2Si2O7 was also studied. XRD and

TEM/SAEDP were used to confirm powder processing by mixing oxide constituents results in a single phase homogeneous solid-solution. β-(Y0.2Yb0.2Lu0.2Sc0.2Gd0.2)2Si2O7 has a much lower room temperature thermal conductivity than the previous RE2Si2O7 (pure and Yb-Y pyrosilicate solid-solutions). However, as the temperature increases, the thermal conductivity plateaus at ~1.51

W·m-1·K-1. At higher temperatures (> 600 °C), the lack of the expected decrease in thermal conductivity could be attributed to the “minimum conductivity” limit [171].

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CHAPTER 6: RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS

This chapter is unpublished data that may be used in a future publication.

6.1 Introduction

In Chapters 2 and 3, how potential RE2Si2O7 (Y, Yb, Lu, Sc) EBC ceramics interact with a ‘model’ CMAS (NAVAIR, Ca/Si = 0.76) was demonstrated. In Chapter 4, Yb2Si2O7, Y2Si2O7 and their solid-solution (Yb1.8Y0.2Si2O7 and Yb1Y1Si2O7) EBC ceramics were also analyzed with

CMAS. They were tested with 3 different CMAS compositions (with different Ca/Si ratios). It was shown that in some cases solid-solutions can temper the failure mechanisms of the pure components, like in the NAVAIR CMAS, while also lowering the thermal conductivity of the EBC

(Chapter 5). It has been shown that dense, polycrystalline pellets can be used as ‘model’ experiments to determine the reaction between EBC materials and CMAS glass. However, the microstructure of coatings is different to that of polycrystalline pellets. Therefore, the next step was to determine how air plasma sprayed (APS) EBCs would interact with CMAS.

Unfortunately, EBC deposition is still a significant challenge [39,40]. Conventional air plasma spray (APS) is preferred, due to its efficiency and relative low cost. However, the EBCs typically deposit as an amorphous coating [41]. To crystallize the coating during spraying, many researchers have performed APS inside a box furnace where the substrate is heated to temperatures above 1000 °C [17,33,36,42,43], but this is difficult in a manufacturing setting. Garcia et al. [41] has studied the microstructural evolution when a post-deposition heat treatment is performed on

APS Yb2Si2O7 EBC coatings with different spray conditions. Crystallization has a significant volume change which can lead to porous coatings. Also, undesirable phases may form during

109 crystallization. However, it was determined that a more amorphous coating included less porosity initially and fewer SiO2 inclusions.

In this context, there are only a few studies on Yb2Si2O7 EBC coatings and their interactions with CMAS [33,35,36]. Stolzenburg et al. [33] and Zhao et al. [36] both used APS coatings.

Stolzenburg et al. [33] obtained and studied coatings produced by Rolls Royce, however, the APS processing parameters were not disclosed. Zhao et al. [36] sprayed coatings into a furnace at 1200

°C to produce a crystalline coating. Poerschke et al. [35] used electron-beam-directed vapor deposition (EB-DVD) to produce coatings. Poerschke et al. [35] applied a TBC on top of the Yb- silicate EBC, which makes the interactions indirect and strongly influenced by the TBC.

Zhao et al. [36] and Stolzenburg et al. [33] used the same CMAS composition (a high Ca/Si ratio (= 0.73)), but found differing results. Zhao et al. [36] showed Yb-Ca-Si apatite (ss) formation in APS coatings when interacted with CMAS, whereas Stolzenburg et al. [33] showed little reaction between the Yb2Si2O7 EBC and the CMAS. This could be due to Yb2SiO5 areas found in the Yb2Si2O7 coatings used by Zhao et al. [36].

There is little known about the interaction between CMAS and solid-solution, i.e.

Yb1Y1Si2O7, APS coatings.

Here, the interactions at 1500 °C of two APS EBCs of compositions Yb2Si2O7 and

Yb1Y1Si2O7 with a ‘model’ CMAS, Naval Air Systems Command (NAVAIR) CMAS (Ca/Si =

0.76) have been studied [116,117,128]. The objective is to provide insights into the chemo-thermo- mechanical mechanisms of these interactions, and to use this understanding to guide the design and development of future CMAS-resistant, low thermal-conductivity TEBCs.

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6.2 Experimental Procedures

6.2.1 Air Plasma Sprayed Coatings

The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073,

Oerlikon Metco, Westbury, NY). The β-Yb1Y1Si2O7 powders were also obtained from Oerlikon

Metco, in collaboration with Dr. Gopal Dwivedi, as an experimental R&D powder.

The coatings were sprayed by our colleagues at Stony Brook University, Professor Sanjay

Sampath and Dr. Eugenio Garcia. The coatings, Yb2Si2O7 and Yb1Y1Si2O7, were air plasma sprayed using a F4MB-XL plasma gun (Oerlikon Metco, Westbury, NY) controlled by a 9MC console (Oerlikon-Metco, Westbury, NY). The spray parameters used for both powders were: as- plasma forming gas, Ar, with a flow rate of 47.5 standard liters per minute (slpm), a secondary gas, H2, with a flow rate of 9 slpm, and a current of 550 A. These conditions reported a voltage of

71.2 V or a power of 39.2 kW. The stand-of distance was maintained at 150 mm. The raster speed was 500 mm.s-1. A mass rate of 12 g.min-1 was used for both powders.

6.2.2 Heat Treatments

Some as-sprayed β-Yb2Si2O7 and β-Yb1Y1Si2O7 coatings were analyzed as arrived, which will be described below in Section 6.2.4. Some of the as-sprayed coatings were placed on Pt sheets for a heat treatment at 1300 °C for 4 h in air in a box furnace (CM Furnaces Inc, Bloomfield, NJ).

6.2.3 CMAS Interactions

The composition of the CMAS used is (mol%) 51.5 SiO2, 39.2 CaO, 4.1 Al2O3, and 5.2

MgO, which is from a previous study [128] and in Chapters 2-4, and it is close to the composition of the AFRL-03 standard CMAS (desert sand). Powder of this CMAS glass composition was

111 prepared using a procedure described elsewhere [70,86]. CMAS interaction studies were performed by applying the CMAS powder paste (in ethanol) uniformly over the center of the heat-

-2 treated Yb2Si2O7 and Yb1Y1Si2O7 APS coatings at ∼15 mg.cm loading. The specimens were then placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box furnace at 1500 °C in air for 24 h (10 °C.min-1 heating and cooling rates). The CMAS-interacted coatings were then cut using a low-speed diamond saw, and the cross-sections were polished to a 1-μm finish.

6.2.4 Characterization

The as-sprayed and heat-treated APS coatings were characterized using an X-ray diffractometer (XRD; D8 Advance, Bruker AXS, Karlsruhe, Germany) to check for phase purity.

The phases present were identified using the PDF2 database. In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating at 25, 800, 900, 1000, 1100, 1200, 1300 and 1350 °C were conducted to determine the temperature needed for the coatings to crystallize. A ramping rate of

10 °C.min-1 was used and the temperatures were held for 10 minutes before the XRD scan was performed.

The densities of the as-sprayed and heat-treated coatings were measured using the

Archimedes principle, with distilled water as the immersion medium.

Cross-sections of the as-sprayed, heat-treated, and CMAS-interacted APS coatings were observed in a scanning electron microscope (SEM; LEO 1530VP, Carl Zeiss, Munich, Germany or Helios 600, FEI, Hillsboro, Oregon, USA), equipped with energy-dispersive spectroscopy

(EDS; Inca, Oxford Instruments, Oxfordshire, UK), operated at 20 kV accelerating voltage. EDS

112 elemental maps, particularly Ca and Si, were also collected and used to determine CMAS penetration into the pellets.

6.3 Results

6.3.1 As-sprayed and Heat-Treated Coatings

As-received, as-sprayed Yb2Si2O7 APS coatings were cross-sectioned and SEM micrographs can be found in Figures 58A and 58B. The Yb2Si2O7 coating is ~1 mm thick and some porosity is observed. There are lighter and darker gray regions in this microstructure, indicating a change in silica concentration. Lighter regions have lower amounts of silica, which was confirmed using EDS. Figure 58C shows the indexed XRD patterns for the Yb2Si2O7 APS coating. XRD was collected on both the top and bottom of the coating. Slight differences can be seen between the top to bottom of the coating, but both confirm that the coating is mostly amorphous with small amounts of un-melted particles.

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Figures 59A and 59B show SEM micrographs of the as-received, as-sprayed Yb1Y1Si2O7

APS coating. Like the Yb2Si2O7 coating, porosity is observed and there are lighter (less silica) and darker (more silica) gray regions in this microstructure. The Yb1Y1Si2O7 coating is ~1.5 mm thick.

Figure 59C shows the indexed XRD pattern for the Yb1Y1Si2O7 APS coating. Again, XRD patterns were collected on both the top and bottom of the coating. The bottom of the coating is almost purely amorphous. The top of the coating shows more peaks, indicating it contains more un-melted

Yb1Y1Si2O7 particles. Both show a mostly amorphous coating.

To determine the heat treatment needed to crystallize the coatings, in-situ high-temperature

XRD on the Yb1Y1Si2O7 APS coating was conducted and can be found in Figure 60. Between 25 and 900 °C, the coating remains amorphous. At 1000 °C, crystalline peaks begin to emerge. The coating at 1100 and 1200 °C seems to be forming Yb1Y1SiO5 over β-Yb1Y1Si2O7. At 1300 °C, the coating is crystalline and contains more β-Yb1Y1Si2O7 than Yb1Y1SiO5. At 1350 °C, the XRD remains the same as the 1300 °C XRD pattern. Therefore, 1300 °C was selected as the heat treatment temperature for the APS coatings.

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Heat treatments at 1300 °C for 4 hours were performed on both coatings. Figures 61A and

61B show SEM micrographs of the heat-treated, crystalline Yb2Si2O7 APS coating. The density of all the coatings can be found in Table 22. The density of the Yb2Si2O7 coating after heat treatment

-3 is 6.12 Mg.m . When compared to the theoretical density of Yb2Si2O7, the relative density is 99%.

However, as seen in the micrographs and the XRD (Figure 61C), there is also Yb2SiO5 present, which has a higher density of 6.92 Mg.m-3 [189]. This would increase the coatings relative density compared to pure Yb2Si2O7.

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Table 22: Density measurements, relative density and open porosity for the as-sprayed and heat- treated (HT, 1300 °C, 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings. Density Theoretical Relative Open Coatings (Mg.m-3) Density (Mg.m-3) Density Porosity Yb2Si2O7 As-sprayed 6.39 6.15 104% 4% Yb2Si2O7 HT (1300 °C, 4 h) 6.12 6.15 99% 5% Yb1Y1Si2O7 As-sprayed 4.92 5.045 98% 4% Yb1Y1Si2O7 HT (1300 °C, 4 h) 4.81 5.045 95% 3%

Figures 62A and 62B show SEM micrographs of the heat-treated (1300 °C, 4 h), crystalline

Yb1Y1Si2O7 APS coating. Porosity is observed along with Yb1Y1Si2O7 and Yb1Y1SiO5. This is also confirmed by XRD in Figure 62C. Based on the peak height ratio of the XRD patterns, the

Yb1Y1Si2O7 APS coating contains less RE2SiO5 than the Yb2Si2O7 APS coating, which is also confirmed in the SEM micrographs. The density of the heat-treated (1300°C, 4 h) Yb1Y1Si2O7

-3 APS coating is 4.81 Mg.m , which is ~95% dense relative to pure Yb1Y1Si2O7 (calculated by rule- of-mixtures from Yb2Si2O7 and Y2Si2O7). As stated above, the relative density could be skewed due the presence of Yb1Y1SiO5. The theoretical density of Yb1Y1SiO5, calculated by rule-of-

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-3 -3 mixtures of Yb2SiO5 and Y2SiO5 (4.44 Mg.m [190]), is 5.68 Mg.m , which is higher than that of the pure Yb1Y1Si2O7.

6.3.2 NAVAIR CMAS Interactions

All CMAS interactions were performed on the crystalline or heat-treated (1300 °C, 4 h)

APS coatings.

Figure 63A is a cross-sectional SEM micrograph of a Yb2Si2O7 APS coating that has interacted with CMAS at 1500 °C for 24 h. Figure 63B is a higher magnification image of the region indicated in Figure 63A and its corresponding Si, Ca and Yb elemental EDS maps. No

CMAS glass is observed on the top of the coating. The dashed line indicates the approximate

CMAS penetration.

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Figures 64A, 64B and 64D are higher magnification cross-sectional SEM images of a

Yb2Si2O7 APS coating that has interacted with CMAS at 1500 °C for 24 h. Figures 64C and 64E are Ca elemental EDS maps corresponding to Figures 64B and 64D, respectively. The EDS elemental compositions of regions #1 to #7 are reported in Table 23. The top of the coating has a thin Yb-Ca-Si apatite (ss) layer (region #1). Further into the coating, more Yb-Ca-Si apatite (ss) can be found (region #2). In the region containing the Yb-Ca-Si apatite phase (ss), Yb2Si2O7 is also present. However, there is no Yb2SiO5 present in that region (~40 μm in depth). Even further into the coating, Yb2Si2O7 (regions #4 and #6) and Yb2SiO5 (regions #3, #5, and #7) can be found.

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Table 23: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with CMAS at 1500 °C for 24 h. Region # Yb Ca Si Phase 1 45 12 43 Yb-Ca-Si Apatite (ss) 2 47 10 43 Yb-Ca-Si Apatite (ss) 3 62 - 38 Yb2SiO5 4 44 - 56 Yb2Si2O7 5 61 - 39 Yb2SiO5 6 45 - 55 Yb2Si2O7 7 61 - 39 Yb2SiO5 Ideal Compositions 50.0 12.5 37.5 Yb8Ca2(SiO4)6O2 Apatite 50.0 - 50.0 Yb2Si2O7 66.7 - 33.3 Yb2SiO5

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Figure 65A is a cross-sectional SEM micrograph of a Yb1Y1Si2O7 APS coating that has interacted with CMAS at 1500 °C for 24 h. Figure 65B is a higher magnification image of the region indicated in Figure 65A and its corresponding Si, Ca and Yb elemental EDS maps. No

CMAS glass is observed on the top of the coating. The dashed line indicates the approximate

CMAS penetration.

Figures 66A, 66B and 66D are higher magnification cross-sectional SEM images of a

Yb1Y1Si2O7 APS coating that has interacted with CMAS at 1500 °C for 24 h. Figures 66C and

66E are Ca elemental EDS maps corresponding to Figures 66B and 66D, respectively. The EDS elemental compositions of regions #1 to #8 are reported in Table 24. The top of the coating has a layer of Yb-Y-Ca-Si apatite (ss) (region #1). Further into the coating, more Yb-Y-Ca-Si apatite

(ss) can be found (region #3 and Figure 66C). In the region containing the Yb-Y-Ca-Si apatite phase (ss), Yb1Y1Si2O7 is also present (regions #2 and #4). However, there is no Yb1Y1SiO5 present in that region (~150 μm in depth). This is clearly observed in the Si elemental EDS map

120 in Figure 65. Even further into the coating (Figure 66D), Yb2Si2O7 (regions #5 and #7) and

Yb2SiO5 (regions #6 and #8) can be found.

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Table 24: Average EDS elemental composition (at%; cation basis) from the regions indicated in the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with CMAS at 1500 °C for 24 h. Region # Yb Y Ca Si Phase 1 21 21 12 46 Yb-Y-Ca-Si Apatite (ss) 2 24 18 - 58 Yb1Y1Si2O7 3 22 20 10 48 Yb-Y-Ca-Si Apatite (ss) 4 24 18 - 58 Yb1Y1Si2O7 5 22 20 - 58 Yb1Y1Si2O7 6 33 25 - 42 Yb1Y1SiO5 7 22 20 - 58 Yb1Y1Si2O7 8 30 27 - 43 Yb1Y1SiO5 Ideal Compositions 25.0 25.0 12.5 37.5 Yb4Y4Ca2(SiO4)6O2 Apatite 25.0 25.0 - 50.0 Yb1Y1Si2O7 33.3 33.3 - 33.4 Yb1Y1SiO5

6.4 Discussion

Both APS coatings, Yb2Si2O7 and Yb1Y1Si2O7, showed apatite (ss) formation. In Chapter

3, it was demonstrated that Yb2Si2O7 when in contact with the same CMAS (NAVAIR, Ca/Si ratio

= 0.76) can form Yb-Ca-Si apatite (ss). However, it did not form as readily as the Yb1Y1Si2O7 pellet seen in Chapter 4. There is higher propensity to form apatite (ss) in Y3+ containing materials than in the Yb3+ due to the ionic radii size. This can also be seen in the APS coatings. More apatite formation is found in the Yb1Y1Si2O7 APS coating.

Another explanation for the formation of apatite (ss) can be the RE2SiO5 phase found in the APS coatings. It has an enhanced effect on the formation of apatite (ss) [36,72]. Zhao et al.

[36] compared Yb2Si2O7 and Yb2SiO5 APS coatings and their interactions with CMAS (Ca/Si ratio

= 0.73). Yb2SiO5 was shown to react more readily with CMAS to form Yb-Ca-Si apatite (ss) [36].

Jang et al. [72] also observed Yb-Ca-Si apatite (ss) forms as a continuous layer on dense, sintered polycrystalline Yb2SiO5 pellets.

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In both the Yb2Si2O7 and Yb1Y1Si2O7 APS coatings, a nearly continuous layer of apatite

(ss) is found on the surface of the coating. No pockets of CMAS glass were found. Below the surface there are grains of apatite (ss), which can be seen in Figures 64 and 66 for Yb2Si2O7 and

Yb1Y1Si2O7, respectively. The formation of apatite (ss) could be due to the RE2SiO5 (RE = Yb,

YbY) present. The depth of CMAS penetration in the Yb2Si2O7 APS coating, based on the elemental Ca map, is ~40 μm, which is relatively small compared to that of the Yb1Y1Si2O7 (~150

μm). This could be due to the placement of the cross-section (slightly off center of the CMAS interaction zone) or the amount of Yb2SiO5 in the Yb2Si2O7 coating. The more RE2SiO5 (RE = Yb,

YbY) in the coating, the faster the CMAS is consumed. This is due to the reaction between the

RE2SiO5 (RE = Yb, YbY) and the CMAS melt. CaO and SiO2 are needed to form apatite (ss). The example reaction for the pure Yb system is shown:

4Yb2SiO5 + 2CaO (melt) + 2SiO2(melt) → Ca2Yb8(SiO4)6O2. (Equation 11)

Yb2Si2O7 contains the required amount of SiO2 to form apatite (ss), so only CaO is removed from the melt:

4Yb2Si2O7 + 2CaO (melt) → Ca2Yb8(SiO4)6O2 + 2SiO2(melt). (Equation 12)

In fact, excess SiO2 from the Yb2Si2O7 is added into the melt.

In the pellets of pure Yb2Si2O7 and Yb1Y1Si2O7, the CMAS remained either in grain boundaries, or on the surface of the pellet, respectively. However, in the APS coatings, RE2SiO5

(RE = Yb, YbY) is present and another reaction with the CMAS can occur:

Yb2SiO5 + 2SiO2(melt) → Yb2Si2O7. (Equation 13)

This is observed in both coatings, but it is more apparent in the Yb1Y1Si2O7 APS coating in the Si elemental EDS map in Figure 65. The top region shows only apatite (ss) and Yb1Y1Si2O7, which have approximately the same Si concentration; this is the CMAS interaction zone. Below that, in

123 the bottom region, there are areas of lower Si concentration, or Yb1Y1SiO5. Due to these reactions, the CMAS is almost completely consumed by the formation of apatite (ss) and RE2Si2O7 (RE =

Yb, YbY) in these APS coatings.

The ‘blistering’ damage mechanism was not observed in the either APS coating. This could be due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb, YbY) from the

RE2SiO5 (RE = Yb, YbY) and the melt or the porosity seen in these coatings, which stops the formation of a dilatation gradient.

6.5 Future Work

There is ongoing work for the APS coatings and CMAS interaction studies. Currently, a post-doctoral fellow, Dr. Hadas Sternlicht, is focusing on the crystallization of these coatings. She is also working on confirming solid-solutions of the Yb1Y1Si2O7 coating using TEM.

The quantitative amounts of RE2Si2O7 and RE2SiO5 in the APS coatings will also be determined through high-resolution XRD and rietveld analysis.

CMAS interaction studies (1500 °C, 24 h) of these APS coatings with the CMASs used in

Chapter 4 (NASA CMAS and Icelandic Volcanic Ash (IVA) CMAS) should be done to complete a systematic study. However, it is believed that the other CMASs, with lower Ca/Si ratios (NASA

= 0.44 and IVA = 0.10) would mostly show RE2Si2O7 formation and limited or no apatite (ss) formation.

6.6 Summary

Here amorphous as-sprayed APS coatings of Yb2Si2O7 and Yb1Y1Si2O7 were studied. A heat treatment of 4 h at 1300 °C was performed to obtain crystalline coatings. The crystalline

124 coatings were found to contain both β-RE2Si2O7 and RE2SiO5 (RE = Yb, YbY). Based on XRD and cross-sectional SEM micrographs, the Yb2Si2O7 APS coating has a higher RE2SiO5 to β-

RE2Si2O7 ratio than the Yb1Y1Si2O7 APS coatings.

The high-temperature (1500 °C, 24 h) interactions of the two promising APS EBCs,

Yb2Si2O7 and Yb1Y1Si2O7, with a CMAS glass (NAVAIR, Ca/Si ratio = 0.76) were studied.

CMAS glass was consumed by the formation of apatite (ss) and RE2Si2O7 (RE = Yb, YbY) due to the presence of RE2SiO5 (RE = Yb, YbY) in the APS coatings and CaO and SiO2 in the CMAS melt. Therefore, no remaining CMAS glass was observed in either coatings.

The ‘blistering’ damage mechanism was not observed in the APS coatings. This could be due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb, YbY) from the

RE2SiO5 (RE = Yb, YbY) and the melt or the porosity seen in these coatings, which stops the formation of a dilatation gradient.

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CHAPTER 7: CONCLUSIONS AND FUTURE WORK

7.1 Summary and Conclusions

Ceramic-matrix-composites (CMCs), typically comprising of a SiC-based matrix and fibers, are showing great promise in the engine’s hot-section due to their inherently high temperature capabilities [4,6–8]. However the oxygen and steam present in the high-velocity hot- gas stream in the engine causes the SiC-based CMCs to undergo active oxidation and recession

[4,11–13]. Thus, SiC-based CMCs need to be protected by ceramic environmental barrier coatings

(EBCs) [4,9,13,16,17]. EBCs must also have low SiO2 activity, among other requirements

[13,16,17].

Gas-turbine engines can ingest silicates, collectively referred to as calcia-magnesia- aluminosilicate (CMAS) [3,4,59,146]. CMAS can be in the form of airborne sand, runway debris, or volcanic ash in aircraft engines, and ambient dust and/or fly ash in power-generation engines.

Since the surface temperatures of EBCs are expected to be well above the melting point of most

CMAS, the ingested CMAS will melt, adhere to the EBC surface, and attack the EBC. The CMAS attack of EBCs is expected to be severe due to the high operating temperatures, and the fact that all the relevant processes (diffusion, reaction, viscosity, etc.) are thermally-activated [4,146].

Since EBCs need to be dense, it is preferred that they have low reactivity with the CMAS to retain the EBC’s integrity. Optical-basicity (OB or Λ) is introduced as a screening criterion for choosing CMAS-resistant EBC ceramics. In this context, a small OB difference between CMAS and potential EBC ceramics is desired [78]. Therefore, rare-earth pyrosilicates (RE = rare earth,

RE2Si2O7) such as γ-Y2Si2O7 and β-Yb2Si2O7 have been identified as promising CMAS-resistant

EBC ceramics [78]. It should be emphasized that the OB-difference analysis provides a rough screening criterion based purely on chemical considerations. The actual reactivity will depend on

126 many other factors including the nature of the cations in the EBC ceramics, the CMAS composition, and the relative stability of the reaction products.

In Chapter 2, the high-temperature (1500 ˚C) interactions of two promising dense, polycrystalline EBC ceramics, YAlO3 (YAP) and -Y2Si2O7, with a CMAS (NAVAIR, Ca/Si ratio

= 0.76) glass have been explored as part of a model study. Despite the fact that the optical basicities of both the Y-containing EBC ceramics and the CMAS are similar, reactions with the CMAS occur. In the case of the Si-free YAlO3, the reaction zone is small and it comprises three regions of reaction-crystallization products, including Y-Ca-Si apatite solid-solution (ss) and Y3Al5O12

(YAG (ss)). In contrast, only Y-Ca-Si apatite (ss) forms in the case of Si-containing -Y2Si2O7, and the reaction zone is an order-of-magnitude thicker. This is attributed to the presence of the Y in the YAlO3 and γ-Y2Si2O7 EBC ceramics. These CMAS interactions are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7, β-Sc2Si2O7, and β-Lu2Si2O7) in Chapter 3.

Little or no reaction is found between the Y-free EBC ceramics (β-Yb2Si2O7, β-Sc2Si2O7, and β-Lu2Si2O7) and the CMAS in Chapter 3. In the case of β-Yb2Si2O7, a small amount of reaction-crystallization product Yb-Ca-Si apatite (ss) forms, whereas none is detected in the cases of β-Sc2Si2O7 and β-Lu2Si2O7. The CMAS glass penetrates the grain boundaries of the Y-free EBC ceramics, and they suffer from a new damage mechanism: ‘blister’ cracking. This is attributed to the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the

CMAS glass. The success of a ‘blistering’-damage-mitigation approach is demonstrated, where 1 vol% CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering. The CMAS-glassy phase at the grain boundaries promotes rapid CMAS glass penetration, thereby eliminating the dilatation-gradient.

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Based on the interactions with CMAS in Chapters 2 and 3, an interesting possibility of tempering these extreme CMAS-interaction behaviors by forming binary solid-solution EBC ceramics was proposed and studied in Chapter 4. High-temperature (1500 °C) interactions of environmental-barrier coating (EBC) ceramics in the rare-earth pyrosilicates system, Yb(2- x)YxSi2O7 (x=0, 0.2, 1, or 2), with three different CMAS glass compositions are explored. Only the

Ca/Si ratio is varied in the CMAS: 0.76 (NAVAIR), 0.44 (NASA), or 0.10 (Icelandic Volcanic

Ash). Interaction between the highest-Ca/Si CMAS and the EBC ceramic with the lowest x (= 0,

Yb2Si2O7) promotes no reaction and formation of ‘blister’ cracks. In contrast, the highest x (= 2,

Y2Si2O7) promotes formation of an apatite (ss) reaction product, but no ‘blister’ cracks.

Observationally it is found that a decrease in the CMAS Ca/Si ratio (0.76 to 0.10) and a decrease in Y-content or x (2 to 0) decreases the propensity for the reaction-crystallization (apatite formation) and ‘blister’ cracks. These observations are rationalized based on the ionic radii size.

Y3+ is closer to that of Ca2+ than is Yb3+, which is the driving force for apatite (ss) formation. This suggests a way to tune the CMAS interactions in rare-earth pyrosilicate solid-solutions.

Chapter 5 introduces a new concept based on the formation of solid-solutions: thermal environmental barrier coatings (TEBCs), or a coating that has the ability to act as both an EBC and a TBC. The thermal conductivities of six binary solid-solutions were analytically calculated.

The thermal conductivities of Yb(2-x)YxSi2O7 (x = 0.2 and 1) were obtained experimentally and compared to calculated data. A 5-component equiatomic β-(Y0.2Yb0.2Lu0.2Sc0.2Gd0.2)2Si2O7 was also studied. Between room temperature and 600 °C, a large decrease in thermal conductivity compared to the other materials studied in this chapter was observed. However, at higher temperatures, the thermal conductivity plateaued. The lack of the expected decrease in thermal

128 conductivity of the Yb(2-x)YxSi2O7 (x = 0.2 and 1) solid-solutions and β-

(Y0.2Yb0.2Lu0.2Sc0.2Gd0.2)2Si2O7 could be attributed to the “minimum conductivity” limit.

Based on interactions with CMAS in the previous chapters (2–4), two potential EBC ceramics, Yb2Si2O7 and Yb1Y1Si2O7, were chosen to be deposited as coatings using air plasma spray (APS). In Chapter 6, the high-temperature (1500 ˚C) interactions of two promising APS coatings, Yb2Si2O7 and Yb1Y1Si2O7, with a CMAS (NAVAIR, Ca/Si ratio = 0.76) glass have been explored as part of a model study. Before CMAS testing could occur, the APS coatings needed to be heat-treated (1300 °C, 4 h) to obtain a crystalline structure. The coatings contained RE2SiO5 as well as the desired β-RE2Si2O7. The high-temperature (1500 °C, 24 h) CMAS interactions found the presence of apatite (ss) near the surface of the coatings, while no CMAS glass was observed.

Instead, the CMAS glass has interacted with the APS coatings to not only form apatite (ss), but also RE2Si2O7 (RE = Yb, YbY). This is due to the presence of RE2SiO5 (RE = Yb, YbY) in the

APS coatings and SiO2 in the CMAS melt. The ‘blistering’ damage mechanism found in the pellets was not observed in the APS coatings, which could be due to the depletion of CMAS or the porosity in the coatings.

7.2 Future Work

Although we have gained insight into potential coatings used as EBCs on hot-section components in gas-turbine engines, there is more that needs to be researched. In the context of dense, polycrystalline pellets, the interaction with NASA CMAS (Ca/Si ratio = 0.44) should be studied in more detail. The results obtained show no ‘blistering’ cracks and full penetration of

CMAS into grain boundaries, which is not the case for the NAVAIR CMAS. The reason behind this is not known and should be investigated further.

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Another area of focus will be water vapor corrosion studies on the dense, polycrystalline solid-solution pellets, Yb1.8Y0.2Si2O7 and Yb1Y1Si2O7, and their pure components, Yb2Si2O7 and

Y2Si2O7. Most of this testing has already been conducted by our colleagues at the University of

Virginia Professor Elizabeth Opila, Dr. Rebekah Webster and Mr. Mackenzie Ridley. These data are still in the process of being analyzed to determine the recession of the pellet and the reaction products. The impingement site can be seen in Figures 67A–67D. Cross-sectional SEM micrographs of the impingement zone can be seen in Figures 67E–67H. Their corresponding Si elemental EDS maps can be seen in Figures 67I–67L, respectively.

The equiatomic solid-solution RE2Si2O7 mixtures should be a major subject of interest moving forward. So far, β-(Y0.2Yb0.2Lu0.2Sc0.2Gd0.2)2Si2O7 has been studied, confirmed to be a homogeneous solid-solution, and showed a decrease in thermal conductivity compared to pure

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RE2Si2O7 ceramics. However, the CMAS resistance and water-vapor corrosion has not yet been studied.

Another investigation exploring other potential 4 or 5 equiatomic RE2Si2O7 using combinations of known RE2Si2O7 (RE = Y, Yb, Sc, Lu, Gd, Nb, Ho, etc.) should be conducted.

As mentioned in Chapter 6, there is ongoing work on the crystallization, porosity and solid- solution homogeneity of the APS Yb2Si2O7 and Yb1Y1Si2O7 coatings. Quantitative analysis should also be explored through high-resolution XRD and Rietveld analysis. Finally, CMAS interaction studies (1500 °C, 24 h) of these APS coatings with the other two CMASs used in Chapter 4 will be done to complete this systematic study.*

* These tests have been conducted, but the data have not been analyzed yet due to a lab/microscopy facility shutdown.

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