Progress Thermal Barrier Coatings · 2013. 7. 24. · Contents . Introduction . APPLICATIONS ....

Progress in Thermal Barrier Coatings

A John Wiley & Sons, Inc., Publication

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progress in Thermal Barrier Coatings


A John Wiley & Sons, Inc., Publication

Copyright 0 2009 by The American Ceramic Society. All rights reserved.

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Contents

Introduction

APPLICATIONS

Corrosion Resistant Thermal Barrier Coating Materials for Industrial Gas Turbine Applications Michael D. Hill, Davin P. Phelps, and Douglas E. Wolfe CESR Vol. 29, IS. 4, 123-132, 2008

Industrial Sensor TBCs: Studies on Temperature Detection and Durability X. Chen, 2. Mutasim, and J. Price, J. P. Feist, A. L. Heyes and S. Seefeldt Int. J. of Appl. Ceram. Technol., Vol. 2, No. 5, p. 41 4-421, 2005

Industrial TBCs A. Kulkarni and H. Herman Am. &?ram. SOC. Bull., Vol. 83, No. 6, p. 9801-9804, 2004

Low Thermal Conductivity Ceramics for Turbine Blade Thermal Barrier Coating Application U. Schulz, 6. Saint-Ramond, 0. Lavigne, P. Moretto, A. vanlieshout, and A. Borger CESe VOI. 25, NO. 4, p. 375-380, 2004

Thermal and Environmental Barrier Coatings for SiC/SiC CMCs in Aircraft Engine Applications I. Spitsberg and J. Steivel Int. J. Appl. Ceram. Technol., Vol. 1, No. 4, P. 291-301, 2004

Review on Advanced EB-PVD Ceramic Topcoats for TBC Applications U. Schulz, B. Saruhan, K. Fritscher, and C. Leyens Int. J. Appl. Ceram. Techno/., Vol. 1, N0.4, p. 302-314, 2004

MATERIAL IMPROVEMENTS AND NOVEL COMPOSITIONS

Corrosion Behavior of New Thermal Barrier Coatings R. VaOen, D. Sebold, and D. Stover CESF: Vol. 28, NO. 3, p. 27-38, 2007

13

21

25

31

43

59

Thermal Conductivity of Plasma-Sprayed Aluminum Oxide-Multiwalled Carbon Nanotube Composites 71 Srinivas R. Bakshi, Kantesh Balani, Arvind Agarwal J. Am. Ceram. SOC. Vol. 91, No. 3, 942-947, 2008

Infiltration-Inhibiting Reaction of Gadolinium Zirconate Thermal Barrier Coatings with CMAS Melts 77 S. Kramer, J. Yang, and C. Levi J. Am. Ceram. SOC., Vol. 91, No. 2, p. 576-583, 2008

Segmentation Cracks in Plasma Sprayed Thin Thermal Barrier Coatings H. Guo, H. Murakami, and S. Kuroda CESF: Vol. 27, NO. 3, p. 17-27, 2007

85

Contents V

Design of Alternative Multilayer Thick Thermal Barrier Coatings H. Samadi and T. Coy1 CESP, VOl. 27, No. 3, p. 29-35, 2007

Lanthanum-Lithium Hexaaluminate-A New Material for Thermal Barrier Coatings in Magnetoplumbite Structure-Material and Process Development

G. Pracht, R. VaOen and D. Stover CESR VOl. 27, NO. 3, p. 87-99, 2007

Thermal Barrier Coatings Design with Increased Reflectivity and Lower Thermal Conductivity for High- Temperature Turbine Applications

M. Kelly, D. Wolfe, J. Singh, J. Eldridge, D-M Zhu, and R. Miller Int. J. Appl. Ceram. Techno/., Vol. 3, No. 2, p. 81-93, 2006

Delamination-Indicating Thermal Barrier Coatings Using YSZ:Eu Sublayers J. Eldridge, T. Bencic, C. Spuckler, J. Singh, and D. Wolfe J. Am. Ceram. SOC., Vol. 89, No. 10, p. 3246-3251,2006

Erosion-Indicating Thermal Barrier Coatings Using Luminescent Sublayers J. Eldridge, J. Singh, and D. Wolfe J. Am. Ceram. SOC., Vol. 89, No. 10, p. 3252-3254, 2006

Rare-Earth Zirconate Ceramics with Fluorite Structure for Thermal Barrier Coatings Q. Xu, W. Pan, J. Wang, C. Wan, L. Qi, H. Miao, K. Mori, and T. Torigoe J. Am. Ceram. SOC., Vol. 89, No. 1, p. 340-342, 2006.

Co-Doping of Air Plasma-Sprayed Yttria- and Ceria-Stabilized Zirconia for Thermal Barrier Applications Z. Chen, R. Trice, H. Wang, W. Porter, J. Howe, M. Besser and D. Sordelet J. Am. Ceram. SOC., Vol. 88, No. 6, p. 1584-1590, 2005

Ta,O,/Nb,O, and Y,O, Co-doped Zirconias for Thermal Barrier Coatings S. Raghavan, H. Wang, R. Dinwiddie, W. Porter, R. Vassen, D. Stover, and M. Mayo J. An Ceram. SOC., Vol. 87, No. 3, p. 431-37, 2004

New Thermal Barrier Coatings Based on PyrochloreNSZ Double-Layer Systems R. VaOen, F. Traeger, and D. Stover Int. J. Appl. Ceram. Techno/., Vol. 1, No. 4, p. 351-361, 2004

Development of Advanced Low Conductivity Thermal Barrier Coatings D. Zhu and R. Miller Int. J. Appl. Ceram. Techno/., Vol. 1, No. 1, p. 86-94, 2004

DEVELOPMENTS IN PROCESSING

Process and Equipment for Advanced Thermal Barrier Coatings Albert Feuerstein, Neil Hitchman, Thomas A. Taylor, and Don Lemen CESR VOl. 29, IS. 4, 107-122, 2008

Influence of Porosity on Thermal Conductivity and Sintering in Suspension Plasma Sprayed Thermal Barrier Coatings

H. KaOner, A. Stuke, M. Rodig, R. VaOen, and D. Stover CESP, VOl. 29, IS. 4, 147-1 58, 2008

Thermal and Mechanical Properties of ZirconidMonazite-Type LaPO, Nanocomposites Fabricated by PECS

S-H Kim, T. Sekino, T. Kusunose, and A. Hirvonen CESP, VOl. 28, IS. 3, p. 19-26, 2007

Dense Alumina-Zirconia Coatings Using the Solution Precursor Plasma Spray Process D. Chen, E. Jordan, M. Gell, and X. Ma J. Am. Ceram. SOC., Vol. 91, No. 2, p. 359-365, 2008

97

105

119

133

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155

163

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203

21 5

223

vi Progress in Thermal Barrier Coatings

Thermal Stability of Air Plasma Spray and Solution Precursor Plasma Spray Thermal Barrier Coatings 231 D. Chen, M. Gell, E. Jordan, E. Cao, and X. Ma J. Am. Ceram. SOC., Vol. 90, No. 10, p. 31 60-31 66, 2007

Mechanical Design for Accommodating Thermal Expansion Mismatch in Multilayer Coatings for Environmental Protection at Ultrahigh Temperatures

Jie Bai, Kurt Maute, Sandeep R. Shah and Rishi Raj J. Am. Ceram. SOC., Vol. 90, No. 1, p. 170-1 76, 2007

239

Grain-Boundary Grooving of Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings K. Erk, C. Deschaseaux, and R. Trice J. Am. Ceram. SOC., Vol. 89, No. 5, p. 1673-1678, 2006

Novel Deposition of Columnar Y,AI,O,, Coatings by Electrostatic Spray-Assisted Vapor Deposition Y. Wu, J. Du and K-L Choy J. Am. Ceram. SOC., Vol. 89, No. 1, p. 385-387, 2006

TESTING AND CHARACTERIZATION

Monitoring the Phase Evolution of Yttria Stabilized Zirconia in Thermal Barrier Coatings Using the Rietveld Method

G. Witz, V. Shklover, W. Steure, S. Bachegowda, and H.-P. Bossmann CESe Vol. 28, No. 3, p. 41-51, 2007

Thermal Imaging Characterization of Thermal Barrier Coatings J. Sun CESR Vol. 28, NO. 3, p. 53-60, 2008

247

253

259

271

Examination on Microstructural Change of a Bond Coat in a Thermal Barrier Coating for Temperature Estimation and Aluminum-Content Prediction

279

M. Okada, T. Hisamatsu, and T. Kitarnura CESF: VOl. 28, NO. 3, p. 61-69, 2008

Quantitative Microstructural Analysis of Thermal Barrier Coatings Produced by Electron Beam Physical Vapor Deposition

289

M. Kelly, J. Singh, J. Todd, S. Copley, and D. Wolfe CESP, VOl. 28, NO. 3, p. 71 -80, 2008

Investigation of Damage Prediction of Thermal Barrier Coating Y. Ohtake CESR Vol. 28, NO. 3, p. 81-84, 2008

Corrosion Rig Testing of Thermal Barrier Coating Systems R. VaOen, D. Sebold, G. Pracht, and D. Stover CESF: VOl. 27, NO. 3, p. 47-59, 2007

299

303

Oxidation Behavior and Main Causes for Accelerated Oxidation in Plasma Sprayed Thermal Barrier Coatings

31 7

H. Arikawa, Y. Kojima, M. Okada, T. Hoshioka, and T. Hisarnatsu CESF: VOl. 27, NO. 3, p. 69-80, 2007

Crack Growth and Delamination of Air Plasma-Sprayed Y,O,-ZrO, TBC After Formation of TGO Layer 329 M. Hasegawa, Y-F Liu, and Y. Kagawa CESF: Vol. 27, No. 3, p. 81 -85, 2007

Characterization of Cracks in Thermal Barrier Coatings Using Impedance Spectroscopy L. Deng, X. Zhao, and P. Xiao CESF: VOl. 27, NO. 3, p. 191-206, 2007

Nondestructive Evaluation Methods for High Temperature Ceramic Coatings W. Ellingson, R. Lipanovich, S. Hopson, and R. Visher CESF: Vol. 27, No. 3, p. 207-214, 2007

335

351

Contents vii

Phase Evolution in Yttria-Stabilized Zirconia Thermal Barrier Coatings Studied by Rietveld Refinement of X-Ray Powder Diffraction Patterns

G. Witz, V. Shklover, W. Steurer, S. Bachegowda, H-P Bossmann J. Am. Ceram. SOC., Vol. 90, No. 9, p. 2935-2940, 2007

Characterization of Chemical Vapor-Deposited (CVD) Mullite+CVD Alumina+Plasma-Sprayed Tantalum Oxide Coatings on Silicon Nitride Vanes After an Industrial Gas Turbine Engine Field Test

J. A. Haynes, S. M. Zernskova, H. T. Lin, M. K. Ferber and W. Westphal J. Am. Ceram. SOC., Vol. 89, No. 11, p. 3560-3563, 2006

Monitoring Delamination Progression in Thermal Barrier Coatings by Mid-Infrared Reflectance Imaging J. Eldridge, C. Spuckler, and R. Martin Int. J. Appl. Ceram. Technol., Vol. 3, No. 2, p. 94-104, 2006

Noncontact Methods for Measuring Thermal Barrier Coating Temperatures M. Gentleman, V. Lughi, J. Nychka, and D. Clarke Int. J. ofAppl. Ceram. Technol., Vol. 3, No. 2, p. 105-112, 2006

Modeling the Influence of Reactive Elements on the Work of Adhesion between Oxides and Metal Alloys J. Bennett, J. M. Kranenburg and W. G. Sloof J. Am. Ceram. SOC., Vol. 88, No. 8, p. 2209-2216, 2005

Hot Corrosion Mechanism of Composite AluminaNttria-Stabilized Zirconia Coating in Molten Sulfate- Vanadate Salt

N. Wu, Z. Chen, and S. Mao J. Am. Ceram. SOC., Vol. 88, No. 3, p. 675-682, 2005

Microstructure-Property Correlations in Industrial Thermal Barrier Coatings A. Kulkarni, A. Goland, Herbert Herman, A. Allen, J. Ilavsky, G. Long, C. Johnson, and J. Ruud J. Am. Ceram. SOC., Vol. 87, No. 7, p. 1294-1300, 2004

TBC Integrity J. Eldridge, C. Spuckler, J. Nesbiit, and K. Street Am. Ceram. SOC. Bull. Online, Vol. 83, No. 6, p. 9801-9804, 2004

Photoluminescence Piezospectroscopy: A Multi-Purpose Quality Control and NDI Technique for Thermal Barrier Coatings

M. Gell, S. Sridharan, M. Wen, and E. Jordan Int. J. Appl. Ceram. Technol., Vol. 1, No. 4, p. 316-319, 2004

MECHANICAL PROPERTIES

Elastic and Inelastic Deformation Properties of Free Standing Ceramic EB-PVD Coatings M. Bartsch, U. Fuchs, and J. Xu CESR VO. 28, NO. 3, p . 11-18, 2007

Creep Behavior of Plasma Sprayed Thermal Barrier Coatings R. Soltani, T. Coyle, and J. Mostaghimi CESe Vol. 27, No. 3, p. 37-46, 2007

Simulation of Stress Development and Crack Formation in APS-TBCS for Cyclic Oxidation Loading and Comparison with Experimental Observations

R. Herzog, P. Bednarz, E. Trunova, V. Shernet, R. Steinbrech, F. Schubert, and L. Singheiser CESP, VOl. 27, NO. 3, p.103-114, 2007

Numerical Simulation of Crack Growth Mechanisms Occurring Near the Bondcoat Surface in Air Plasma Sprayed Thermal Barrier Coatings

Casu, J.-L. Marques, R. Vassen, and D. Stover CESP, VOl. 27, NO. 3, p. 11 5-126, 2007

359

365

369

381

389

397

405

41 3

41 7

433

441

451

463

Damage Prediction of Thermal Barrier Coating Y. Ohtake CESe VOl. 27, NO. 3, p. 139-146, 2007

viii

475


Creep Behavior of Plasma-Sprayed Zirconia Thermal Barrier Coatings R. Soltani, T. Coyle, and J. Mostaghimi J. Am. Ceram. SOC., Vol. 90, No. 9, p. 2873-2878, 2007

Application of Hertzian Tests to Measure Stress-Strain Characteristics of Ceramics at Elevated Tem peratures

E. Sanchez-Gonzalez, J. Melendez-Martinez, A. Pajares, P. Miranda, F. Guiberteau and B. Lawn J. Am. Ceram. SOC., Vol. 90, No. 1, p. 149-153, 2007

483

489

Effect of Sintering on Mechanical Properties of Plasma-Sprayed Zirconia-Based Thermal Barrier Coatings 495 S. Choi, D. Zhu and R. Miller J. Am. Ceram. SOC., Vol. 88, No. 10, p. 2859-2867, 2005

The Measurement of Residual Strains within Thermal Barrier Coatings Using High-Energy X-Ray Diffraction 505 J. Thornton, S. Slater, and J. Almer J. Am. Ceram. Soc., Vol. 88, No. 10, p. 2817-2825,2005

Stress Relaxation of Compression Loaded Plasma-Sprayed 7 Wt% Y,O,-ZrO, Stand-Alone Coatings 51 5 G. Dickinson, C. Petorak, K. Bowman, and R. Trice J. Am. Ceram. SOC., Vol. 88, No. 8, p. 2202-2208, 2005

Mechanical Properties/Database of Plasma-Sprayed Zr02-8wt% Y,O, Thermal Barrier Coatings 523 S. Choi, D. Zhu, and R. Miller Int. J. Appl. Ceram. Techno/., Vol. 1, No. 4, p. 330-342, 2004

THERMAL PROPERTIES

Thermal and Mechanical Properties of Zirconia Coatings Produced by Electrophoretic Deposition Baufeld, 0. van der Beist, and H-J Ratzer-Scheibe CESR Vol. 28, No. 3, p. 3-10, 2008

Effect of angpaque Reflecting Layer on the Thermal Behavior of a Thermal Barrier Coating C. Spuckler CESR VOI. 28, No. 3, p. 87-98, 2008

Optimizing of the Reflectivity of Air Plasma Sprayed Ceramic Thermal Barrier Coatings A. Stuke, R. Carius, J.-L. Marques, G. Mauer, M. Schulte, D. Sebold, R. VaOen, and D. Stover CESR Vol. 28, No. 3, p. 99-1 13, 2008

Thermal Conductivity of Nanoporous YSZ Thermal Barrier Coatings Fabricated by EB-PVD B-K Jang and H. Matsubara CESR Vol. 28, NO. 3, p. 115-123, 2008

Comparison of the Radiative Two-Flux and Diffusion Approximations C. Spuckler CESe Vol. 27, NO. 3, p.127-137, 2007

Relation of Thermal Conductivity with Process Induced Anisotropic Void Systems in EB-PVD PYSZ Thermal Barrier Coatings

A. Flores Renteria, B. Saruhan, and J. llavsky CESR Vol. 27, NO. 3, p. 3-15, 2007

Thermal Properties of Nanoporous YSZ Coatings Fabricated by EB-PVD B-K Jang, N. Yamaguchi, and H. Matsubara CESR Vol. 27, NO. 3, p. 61-67, 2007

539

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559

575

585

597

61 1

Thermochemical Interaction of Thermal Barrier Coatings with Molten CaO-MgO-AI,O,-SiO, (CMAS) Deposits

61 9

S. Kramer, J. Yang, C. Levi, and C. Johnson J. Am. Ceram. SOC., Vol. 89, No. 10, p. 3167-3175, 2006

Contents ix

Introduction

Ceramics are used to coat other materials, usually metals, to protect them from high temperatures, moisture, oxygen, wear, corrosive fluids and body fluids. Thermal barrier coatings (TBCs) have their greatest application in protecting metal parts used in heat engines. The metal parts have the strength required for heat engine operation; however, they cannot withstand the high temperatures necessary for efficient and clean operation of the heat engine. TBCs provide this protection.

This edition of Progress in Ceramic Technology series is a compilation of articles published on TBCs by The American Ce- ramic Society (ACerS). These publications include the American Ceratnic Society Bulletin, Journal of the American Ceramic Society, International Journal ofApplied Ceramic Technology, Ceramic Engineering and Science Proceedings (CESP) and Ce- ramic Transactions (CT).

Papers in this edition are divided into five categories: Applications, Material Improvements and Novel Compositions, De- velopments in Processing, Testing and Characterization, Mechanical Properties, and Thermal Properties. The publication cita- tions are included after each title in the table of contents.

Other articles on thermal barrier coatings can be located by searching the Society’s website at www.ceramics.org.

Introduction xi

Applications

CORROSION RESISTANT THERMAL BARRIER COATING MATERIALS FOR INDUSTRIAL GAS TURBINE APPLICATIONS

Michael D. Hill and Davin P. Phelps. Trans-Tech Inc. Adamstown, MD 2 17 10 USA

Douglas E. Wolfe. Assist Professor, Materials Science and Engineering Department The Pennsylvania State University University Park, Pa 16802 USA

ABSTRACT

aircraft engines or industrial gas turbines which allow these engines to operate at higher temperatures. These coatings protect the underlying metal superalloy from creep, oxidation and/or localized melting by serving as an insulating barrier to protect the metal from the hot gases in the engine core. While for aircraft engines, pure refined fuels are used, it is desirable for industrial gas turbine applications that expensive refining operations be minimized. However, acidic impurities such as sulfur and vanadium are commo\in these “dirty” fuels and will attack the thermal barrier coating causing reduced coating lifetimes and in the worse case catastrophic failure due to spallation of the coating. The industry standard coating material is stabilized zirconia with seven weight percent yttria stabilized zirconia being the most common. When used in industrial gas turbines, the vanadium oxide impurities react with the tetragonal zirconia phase causing undesirable phase transformations. Among these transformations is that from tetragonal to monoclinic zirconia. This transformation is accompanied by a volume expansion which serves to tear apart the coating reducing the coating lifetime. Indium oxide is an alternative stabilizing agent which does not react readily with vanadium oxide. Unfortunately, indium oxide is very volatile and does not readily stabilize zirconia, making it difficult to incorporate the indium into the coating. However, by pre-reacting the indium oxide with samarium oxide or gadolinium oxide to form a stable perovskite (GdIn03 or SmInO3) the indium oxide volatilization is prevented allowing the indium oxide incorporation into the coating. Comparison of EDX data from evaporated coatings containing solely indium oxide and those containing GdIn03 are presented and show that the indium is present in greater quantities in those coatings containing the additional stabilizer. Corrosion tests by reaction with vanadium pentoxide were performed to determine the reaction sequence and to optimize the chemical composition of the coating material. Lastly, select x- ray diffraction phase analysis will be presented.

Thermal Barrier Coatings are ceramic materials that are deposited on metal turbine blades in

INTRODUCTION

aircraft engines or industrial gas turbines which allow these engines to operate at higher temperatures. These coatings protect the underlying metal superalloy from creep, oxidation and/or localized melting by serving as an insulating barrier to protect the metal from the hot gases in the engine core.

Thermal Barrier Coatings are ceramic materials that are deposited on metal turbine blades in

Applications 3

Several impurities common in fuels have been identified and associated with corrosion in EB- PVD coatings. These impurities include sodium, sulfur, phosphorus and especially vanadium. impurities react with conventional YSZ turbine blade coatings, severely limiting the coating lifetime. Therefore, it is of great interest to develop alternative materials that react less readily with fuel contaminants and therefore increase the operating lifetime of the coating.

Standard 8YSZ EB-PVD coatings contain 8-weight percent yttria and crystallize in the metastable t‘ phase that is derived from a martensitic distortion of the “stabilized” cubic fluorite structure of zirconia. This rapidly cooled t‘ structure is the most desirable of all of the possible polymorphs in the yttria-zirconia system for TBC applications. Jones’ described several mechanisms of chemical attack on 8YSZ coatings. These include chemical reaction, mineralization, bond coat corrosion and physical damage due to molten salt penetration. Of the four, only the first two mechanisms will be featured in this discussion.

phase, destabilizing the Y203-Zr02 by extraction of the Y203. Of these, V2O5 has been determined to be the worst offender. Hamilton’ and Susnitsky’ have studied the reaction mechanism in detail. The reaction:

These

Acidic species such as SO3 and V2O5 have been shown to react with the yttria stabilizing the t‘

is especially deleterious to the TBC integrity. The vanadium has been shown to leach the yttria out of the zirconia leaving the yttria deficient monoclinic phase of zirconia remaining. The large volume expansion (7%) caused by this transformation leads to the TBC spalling therefore exposing the bond coat to further chemical attack.

this caso, the t! phase) is broken into its stable phase assemblages by a catalyst or mineralizer. For example, ceria stabilized zirconia was investigated as a corrosion resistant coating due to the fact that ceria does not react with vanadium pentoxide.

Mineralization, on the other hand, describes a catalytic process by which a metastable phase (in

~ r l - ~ ~ e ~ 0 2 - . S ~ (t‘) + yv205 3 (1 -x)~r02 (monoclinic) + x ~ e 0 2 + yv205

However, vanadium does act as a mineralizer, destabilizing the t‘ phase without reacting to form the vanadate.

Alternate stabilizers for zirconia: A large number of cationic species act to stabilize the cubic and t phases of zirconia. Therefore, one strategy toward finding corrosion resistant coatings was to find a stabilizer that is resistant to chemical attack by vanadium pentoxide. As mentioned above, ceria was investigated but found to be subject to a mineralization reaction4. Previous work at NRL’ focused on studying acidic stabilizers to zirconia since basic stabilizers such as MgO and Y2O3 were especially susceptible to chemical attack by acidic vanadium pentoxide. Scandia (Sc2O3) and india (111203) in particular were examined in detail (Jones et. al.’ Sheu et. a1.6). Of these, india was found to be the most resistant to chemical attack by vanadium pentoxide.

India stabilized Zirconia as a TBC coating: Although india stabilized zirconia shows promise due to its relative inertness in vanadia containing atmospheres, there are still significant drawbacks in its use,as a TBC material. First, india volatilizes at a lower temperature than zirconia. This resultin significant challenges for applyingplasma sprayed TBC’ S’ . Although india stabilized zirconia coatings have been made in the t! phase (Sheu6), concerns about the volatility of indium oxide raise questions

4 Progress in Thermal Barrier Coatings

about the ability of india stabilized zirconia to form a homogenous coatings.

In20 sublimes at 600 "C In203 sublimes at 850 "C

loe4 torr at 650°C ton at 850 "C

Jones, Reidy and Mess' were able to co-stabilize zirconia with yttrium oxide and indium oxide using a sol gel process. However, no attempt was made to provide ingot feedstock of this composition for EB- PVD testing. Furthermore, the high cost (> $300/kg) of In203 has also been a barrier for further research and development efforts.

would make the indium oxide less volatile, therefore minimizing incidents of spitting, pressure fluctuations, and increase coating homogeneity while still providing enhanced corrosion resistant coating solely consisting of the t' phase. The strategy was to pre-react the indium oxide with a lanthanide oxide which forms either the LnIn03 perovskite (La, Nd or Sm) or the hexagonal LnInO 3 (Gd or Dy). If the ingot contains zirconia and the LnIn03 or just partially stabilized zirconia without free indium oxide, it was believed that a more homogeneous corrosion resistant coating could be deposited by electron beam physical vapor deposition (EB-PVD).

Therefore, a logical approach was to incorporate the indium oxide into the ingot in a form that

Advantages of Indate pre-cursor:

1) Perovskite indates (LnIn03) are refractory compounds. The electropositive lanthanide ion (also stabilizers of the t'phase) stabilizes the In3+ state. It is the reduction to In" that leads to the volatilization of In.

2) Multiple stabilizing ions reduce thermal conductivity. The work of R. Miller ' showed that TBC thermal conductivity decreases when numerous ions of different ionic sizes, valence and ionic weights are simultaneously incorporated into the zirconia as stabilizing agents. These are often referred to as oxide dopant clusters.

Lanthanide Selection: There are numerous factors that will determine the selection of the lanthanide ion accompanying the indium oxide.

1)

2)

3)

4)

Applications

Range of metastable t' phase field. Ideally one would like the largest rangepossible. Sasaki* found the t' phase between 15 and 20-mol% In203 when quenched from temperatures above 1500°C. Ideally this phase region would accompany the In mol% alone as well as the entire range up to the (Ln + In) mole percentage. Melting temperature of LnInO3 compound. The more refractory the compound, the better is the performance Acidityhasicity of lanthanide ion. If La is used, this is likely to be strongly attacked by vanadium because of its basicity. As we progress through the heavier lanthanides (left to right on periodic table), the basicity decreases. Ionic size and weight. Y is of the ideal atomic size for decreasing the monoclinic-tetragonal transformation temperature in Zr02. (Sasaki * 1993). As we move to smaller ions or larger ions this change in the transformation temperature is decreased. In addition, the greater the difference in ionic size and ionic weight between the In3+ and the Ln3+ ions, the lower the thermal conductivity (Miller72004).

5

Phase Diagram Information: Only one ternary phase diagram exists containing any Ln203-In203-ZrOz ternary systems. That one is for Ln=Pr and it was produced by Bates 'et.al in 1989. The compatibility relationships expressed in this diagram suggest that PrIn03 perovskite would react with zirconia to form the Pr2Zr207 pyrochlore and free indium oxide, the exact situation one should avoid. In addition, it has been shown'' that the larger lanthanide ions (La-Gd) in zirconate pyrochlores react with the thermally grown oxide to form undersirable lanthanide aluminate phases. Therefore, the authors investigatedLn ions that formed stable binary oxides of the perovskite structure with In203 but did not form the pyrochlore structure or formed the pyrochlore structure sluggishly. Like the formation of the indate perovskites, the stability of the pyrochlore phase decreases as we proceed from the light to heavy lanthanides. The lanthanides of greatest interest are therefore Sm, Gd and Dy.

Sm203 Forms Sm2Zr207 pyrochlore Forms SmInOj perovskite Stable to 1800°C (Yokakawa 1992) (Schneider, Roth and Waring l 2 196 1)

Gd203 Forms Gd2Zr207 pyrochlore Forms hexagonal GdInO3 Stable to 1575°C (Yokakawa I ' 1992) (Schneider, Roth and Waring 121961)

Dy203 Does not form Dy~Zr207 pyrochlore Forms hexagonal DyIn03 (Pascual and DuranI3 1980) Stable to 1600 C

(Schneider, Roth and Waring "1961)

Lanthnides heavier than Dy do not form either the p y r ~ c h l o r e ' ~ or binary indate phasesI2. The samarium series is of interest because the indiate perovskite forms and since Sm is the most electropositive ion of the lanthanide series (to prevent In" formation and volatilization); however, Sm also forms the most stable pyrochlore which is undesirable. Conversely, the dysprosium series is of interest because it does not form the pyrochlore zirconate or the perovskite structure. The hexagonal compound that does form is unstable above 1600°C. Therefore the challenge is to find a compound indium oxide precursor that will prevent indium volatilization but will not react with zirconia to form a pyrochlore and thus liberate free (and volatile) In203.

In 2007, Mohan et. ai.l4 reported that in addition to forming the zircon Y v o 4 phase that YSZ will react with vanadate salts below 747°C to form the zirconium pyrovanadate (ZrV207) phase. The role this phase plays in the mechanical properties of YSZ coatings containing vanadium warrants further study.

EXPERIMENTAL

oxides (loss on ignition determined at 1300°C for all starting oxides) with indium oxide in a ball-mill with yttria-stabilized zirconia (YSZ) media at 55% solids loading without dispersants for 4h. slurry was pan dried and calcined at 1300°C for 8h. X-ray diffraction was used to evaluate the phase purity of the material by comparing with the appropriate JCPDS cards. If the reaction was incomplete, the milling and calcinations were repeated. The fully-reacted lanthanide indate compositions were then ball-milled with YSZ media until the median particle size was 2 microns or less.

LnIn03 materials were synthesized by blending yttrium, samarium, gadolinium or dysprosium

The


Table I. - Physical and Chemical Properties of the Fired Ingot Material

Ingot Material Fired

6 mole% SmIn03 4.81 g/cc Density

Phase Content Evaporation

t-ZrOz, m-ZrO2 Poor - Spitting Quality

6 mole% GdIn03 I 4.85 g/cc I t-ZrOz, m-ZrO2 I Poor - Spitting I 6 mole% DyIn03

+ LnIn03 4.80 g/cc t-ZrOz, m-ZrOz Extremely Poor

6 mole% SmIn03 +3 mole% Y203 6 mole% GdIn03 +3 mole% YzO3

The indate precursors were then blended with zirconia to the desired composition and formed by cold isostatic pressing into the EB-PVD ingots. The materials were heat treated between 1430 “C and 1530°C for 10h to achieve a theoretical density between 60 and 70%. Table I shows the fired densities, the phase content and the evapoaration quality of the ingot material as a function of the chemical composition. XRD revealed the fluorite structure along with residual monoclinic zirconia and the indate perovskites as listed in Table I.

The ingots were evaporated onto platinum aluminide coated MAR-M-247 nickel based alloy one inch diameter buttons in an industrial prototype EB-PVD coating system at Penn State University. XRD and SEM microstructures were prepared for each coating, with selectEDX presented for semi- quantitative coating chemistry analysis.

Corrosion reactivity tests were performed by reacting the coated coupons with a thin coating of vanadium pentoxide and heated to temperatures between 400 - 650°C for 4 - 6 hours. X-ray diffraction was performed on the pre-reacted and as-reacted coating to identify any phases forming due to the reaction with vanadium pentoxide.

4.59 g/cc t-ZrO2, m-ZrOz Poor -Spitting

4.63 g/cc t-ZrO2, m-ZrOz Poor - Spitting + LnIn03

+ LnIn03

RESULTS 1) Evaporation: In general, the ingots evaporated poorly in the industrial scale EB-PVD coating unit. The material showed “spitting” and extensive cracking during evaporation. The spitting is most likely due to the difference in the vapor pressure between zirconium oxide and indium oxide containing phases in the ingot, but can also be the result of localized differences in ingot densities and degree of connected porosity. Cracking can also occur if the ingot density is too high or the ingot does not have sufficient thermal shock resistance. Despite the difficulties during ingot evaporation, coatings were obtained for each material studied. However, it should be noted that some “spits” or coating defects were observed on the surface of the coated coupons. Lastly, yttrium oxide was added into the composition as an evaporation aid during powder formulation and ingot fabrication, but it did not appear to substantially improve ingot evaporability.

2) Coating Properties: XRD revealed that all of the coatings were single phase with the desired t’ structure. The coating microstructure as observed by scanning electron microscopy revealed a

Applications 7

columnar microstructure typical of those applied by the EB-PVD process. Figure 1 shows an SEM micrograph of the 6 mol% GdIn03 stabilized zirconia coating surface morphology. In addition, EDX was performed on the coating surface to determine semi quantitative compositional information regarding traces of rare earth and indium oxide compositions. These results are listed in Table 11.

The first measure of success was to obtain a coating which contained the acidic stabilizer InzO3. Table I1 compares the ease of evaporability and the relative amount of india within the coating for the various compositions studied. The two compositions containing samarium indate showed the highest amounts of residual indium followed by the sample containing both gadolinium and indium oxide. The ingot starting with 6 mole % indium oxide showed moderate amounts of indium remaining in the EDX trace although considerably less than either samarium containing composition despite starting with double the amount of indium oxide in the ingot.

Figure 1 : SEM image of surface morphology of the EB-PVD coating obtained by evaporation of the 6 mol% GdIn03-3 mol% Y203doped zirconia ingot composition. The coatings were applied on a platinum aluminide coated nickel base alloy. The top images show a lower magnification than the bottom images

3.) Reactivity Tests: Table I11 shows the results of the vanadium pentoxide reactivity tests. X-ray diffraction was performed on the various coatings before and after the reactivity tests in order to determine whether the coatings reactive with vanadium oxide. If any reactions occurred, the phases were identified. The sample containing samarium indate showed only the tetragonal prime phase until

a Progress in Thermal Barrier Coatings

500°C at 16h. Traces of the LnV04 phase with the zircon structure were observed in the samples containing the gadolinium and dysprosium indate at 400°C at 4 hours. Upon further testing at 500°C when exposed to vanadium oxide, traces of monoclinic zirconia and the ZrV207 phase appearedfor DyInO3 containing samples. With the exception of the coating that contained 611101% GdIn03, the t’ phase completely disappeared at 650°C suggesting that these coatings reacted with the vanadium oxide to destabilize the yttria stabilized zirconia. The 6mo1% GdIn03 composition showed the most promising results with regards to resistance against vanadium oxide attack.

Poor (60 and

ingots) Poor (60% TC

1 ingot)

70% TD

Table 11: A comparison of properties for ingots of various compositions studied. Ease of evaporation, EDX In203 content, SEM microstructure and the phase content.

4mount of In in

coating

Composition ~~

Microstructure

i mol% In203

most

most

6 mol% SmIn03

Columnar

TBD 6 mol% SmIn03

3 mol% Y2O3 6 mol% GdIn03

little

most 6 mol% GdIn03

3 mol% Y2O3 6 mol% DyIn03

Columnar - not homogenous

TBD

Ease of EB- PVD

Evaporation (TD:

theoretical density .)

Poor (62 % TD ingot)

Poor (60 and 70% TD

Poor (60% TC ingot)

Poor (70% TC ingot)

some I TBD

some I Poorlyformed columns

Coating Phasc

t’

t’

t’

t’

t’

t’

DISCUSSION All of the india containing compositions were difficult to evaporate as an ingot. This makes it

unlikely that these materials would be useful for EB-PVD applications. EB-PVD is typically used for aircraft engine coatings. This application would typically use clean fuels devoid of acidic corrosive impurities. The material may be more useful as a plasma sprayed powder, which is a more typical TBC form for the industrial gas turbine industry with increased probability of being exposed to vanadium.

determined by EDX. Sm is the most electropositive of the lanthanide co-stabilizers and is less likely to form pyrochlores than the lighter lanthanides like La or Nd. The Sm and Gd containing materials

The samarium containing compounds showed the most residual india in the coating as

Applications 9

formed the typical columnar microstructure while the Dy containing sample showed poorly formed columns. It is not clear whether this was the result of processing difficulties caused by the ingot composition or phase stability.

Composition

6 mol% SmIn03 *

6 mol% GdIn03

6 mol% DyIn03

The reactivity test showed that of all of the lanthanide co-stabilizers, the SmIn03 containing composition showed the highest onset temperature before LnIn03 formation. This is at least partly a result of the higher india content in the evaporated coating. Along with the formation of the LnIn03 and the expected monoclinic zirconia, the ZrV207 phase appeared as well. There is no evidence of any influence of the stabilizing agent on the formation of this phase. It is uncertain whether this phase has a role on the mechanical durability or lifetime of the TBC. In the Gd and Dy containing coatings (which showed lessincorporation by EDX) the onset temperature for the appearance of LnIn03 was the same as that for the formation of the ZrV207 phase. The appearance of the zircon structure vanadate either at lower temperatures than or concurrently with the monoclinic zirconia suggests that mineralization reactions are not taking place.

Reaction Reaction Reaction Reaction Reaction Reaction Reaction with with with with with with with vanadia vanadia vanadia vanadia vanadia vanadia vanadia a t at400C/ a t 500 at500 at600 at600 at650 400C/6h 16h C/8h C/16h C/8h C/16h C/16h t’ t’ t’ t’ + t’ + t’ + SmV04

SmV04 SmV04 SmV04 + mono +t race + mono + mono ( Z r 0 2 ) + ZrV207 (Zr02) + (Zr02) + ZrV207

ZrV207 ZrV207

t’ t’ + t’ + t’ + t’ + t’ + t’ + trace trace trace GdV04 GdV04 GdV04 GdV04 GdV04 GdV04 + mono + mono + mono

(Zr02) + (Zr02) + (Zr02) + ZrV207 ZrV207 ZrV207

t’ + t’ + t’ + t’ + t’ + t’ + DyV04 trace trace trace DyV04 DyV04 DyV04 + mono DyV04 DyV04 DyV04 + t r ace + mono + mono ( Z r 0 2 ) +

(Zr02) + (Zr02) + ZrV207 mono (Zr02) ZrV207 ZrV207

Table 111. Table listing the reaction temperatures and phases observed when exposed to vanadium oxide at elevated temperatures.


CONCLUSIONS

react readily with vanadium oxide. A processing technique has been developed to incorporate increased amounts of indium oxide by using rare earth oxides by pre-reacting the indium oxide with samarium oxide or gadolinium oxide to form a stable perovskite (GdIn03 or SmIn03). This resulted in reduced volatilization of the indium oxide and thus increased volume fractions of indium oxide being incorporated into the coating. Comparison of EDX data from evaporated coatings to the coatings produced after electron beam evaporation containing solely indium oxide and those containing GdIn03 showed increased indium content present in greater quantities for those coatings containing the additional stabilizer. The primary findings of the presented work are summarized below:

It has been shown that indium oxide is an alternative stabilizing agent to yttria which does not

1) That the addition of a lanthanide co-stabilizer (i.e.,Sm) will assist india incorporation into a EB- PVD thermal barrier coating. EDX revealed a greater india concentration in the 3 mol% coating as SmIn03 than with 6 mol% In203, 2) The indate materials investigated in this effort do not appear to be ideal for EB-PVD coatings. This material combination is more likely to be better suited for plasma spraying. 3) Samples containing samarium indate showed the most resistance to reaction with vanadium pentoxide 4) The appearance of the LnV04 phase at temperatures below or concurrently with the monoclinic zirconia contra-indicates a mineralization reaction.

Continued efforts are suggested to further optimize the LnIn03 content, to explore hot corrosion tests mimicking service conditions and to understand the role of the ZrV2O.l phase. In addition, additional efforts to prepare and field test plasma sprayed coatings of the india co-stabilized zirconia will be investigated. The materials described within are subject to a pending US patent.

REFERENCES 1) R.L. Jones J Thermal Spray Technology 6 [ 11 1997 pp77-84 2) Hamilton and Nagelberg JACerS 67 [ 101 1984 pp 686-690 3) Susnitzky, Hertl and Carter JACerS 71 [ 111 1988 pp 992-1004 4) Jones and Mess JACerS 75 [7] 1992 pp 18 18-2 1 5) Jones, Reidy and Mess JACerS 76 [ 101 1993 pp2660-2662 6) Sheu, Xu and Tien JACerS 76 [8] 1993 pp2027-2032 7) Miller, Int. J. of Applied Ceramic Tech 1 [2] 2004 8) Sasaki, Bohac and Gaukler JACerS 76[3] 1993 pp 689-698 9) Bates, Weber and Gatkin Proc. - Electrochem. SOC. [Proc. Int. Symp. Solid Oxide Fuel

Cells, lSt] 1989 pp 141-156 10) C.G. Levi Current Opinion in Solid State and Materials Science 8 2004 pp77-91 11) Yokakawa, Sakai, Kaweda and Dokiya Sci. Technol. Zirconia V [Int. Conf. 5th] 1993 pp

12) Schneider, Roth and Waring J. Res. Nat. Bur. Stds. Section A 65 [4] 1961 pp345-374 13) Pascual and Duran J. MaterSci. 15 [7] 1980 pp 170 1 - 1708 14) Mohan, Yuan, Patterson, Desai and Sohn JACerS In Press

59-68

Applications 11

Ceramic Product Development and Commercialization

Industrial Sensor TBCs: Studies on Temperature Detection and Durability

X Chen, Z. Mutasim, and J. Price

Solar Turbines Inc., San Diego, CA

J. P. Feist

Southside Thermal Sciences Ltd., London, U.K.

A. L. Heyes* and S. Seefeldt

Deparment of Mechanical Engineering, Imperial College, London, U.K.

This article describes recent developments of the thermal barrier sensor concept for non-destructive evaluation (NDE) of thermal barrier coatings (TBCs) and on-line condition monitoring in gas turbines. Increases in turbine entry temperature in pursuit of higher efficiency will make it necessary improve or upgrade current thermal protection systems in gas turbines. As these become critical to safe operation it will also be necessary to devise techniques for on-line conditions monitoring and NDE. Thermal barrier sensor coatings, which consist of a ceramic doped with rare-earth activator to provide luminescence, may be a possible solution. The thermo-luminescent response of such materials has been shown to be suitable for surface and sub-surface temperature measurement and possibly for material phase determination. Herein we describe a number of steps in the development of the sensor coating technology. For the first time sensor coatings have been successfully produced using a production standard air plasma spray (APS) process. Microscopic analysis of the coatings showed them to be similar to standard TBCs and thermal cycle testing of the coatings to destruction showed them to exhibit durability similar to that of standard TBCs suggesting that the addition of rare earth dopants to produce sensor coatings does not change the material structure or the longevity of coatings. Calibration of the coatings using the lifeteime decay response mode showed them to have a dynamic range for temperature measurement extending to just under 1000°C. However, it should be noted that newer compositions have been shown to respond up to 1300°C. Finally, a study of surface temperatures and film cooling has been conducted in a research combustor using APS sensor coatings and some preliminary results are presented.

‘a,heyes@imperial,ac.uk 2005 The American Ceramic Sociery

Introduction

The efficiency of gas turbines is linked to the temperature at the entry to the turbine so that there is a motivation to find ways to increase this temperature.

Applications 13

New materials would allow the turbine entry temperature to be increased but for the moment nickel-based super alloys are likely to remain the materials of choice and a better bet for short-term gains would seem to be offered by the development of improved cooling and insulation systems. Drawing air from the main gas path for cooling purposes reduces efficiency and can lead to an increase in noxious emissions. Hence, efficiency gains would seem to be best sought, in the short term at least, by improving and fully utilising the insulating properties of ceramic thermal barrier coatings (TBCs).

TBCs

Current TBCs consist of an intermetallic bond coat to provide oxidation resistance overlaid with a ceramic, which is typically yttria-stablized zirconia (YSZ). There are a number of features of the composition and structure that have made this the coating of choice but de- ficiencies, which may limit the maximum temperature at which it can be used. The durability of YSZ is temperature dependent and may be called into question if surface temperatures in excess of 1200°C are to be expected. If operated above this temperature on a long- term basis the material phase stability is undermined and a destabilizing phase transformation can occur. At these elevated temperatures sintering also becomes a problem. Furthermore, although the mechanism is not completely understood, failure of TBCs may occur via delamination at the TGO/bond coat interface leading to spallation.2 Failure is correlated with the thickness of the TGO with the latter’s growth rate a function of temperature.

In previous generations of gas turbines, failure of the TBC would result in increases in the temperature of the underlying metal that were still within design limits. However, in future designs, if the performance of every component is fully exploited, and gas stream temperature raised, then a similar failure would result metal temperatures beyond design limits. The inherent phase instability of YSZ above 12OO”C, may make it necessary to look for new materials and indeed research is already underway Clarke and L e ~ i . ~ Given the potentially severe consequences of coating failure, even if these become available and lifetime characteristics and failure modes become better understood, it will be necessary to mon- itor coatings to ensure temperature limits are not ex- ceeded and to provide early detection of degradation

and prediction of remaining life. Hence new non-destructive evaluation (NDE) technologies are required. These may be used in development testing or in service and ideally should enable temperature to be measured at critical regions such as the surface and bond coat/TGO interface. In addition, measurement of degradation on or off line, by erosion, changes in phase composition or hot corrosion, for example, to enable remaining life to be estimated would be of considerable value.

Thermal Barrier Sensor Coatings

The thermal barrier sensor coating (TBSC) concept proposed by Choy et al.‘ will, it is hoped, meet these requirements. Sensor coatings are made by modifying the composition of a TBC to include a small amount of a rare-earth element. These are the activators used in phosphors and hence imbue the TBC with phosphorescent properties. Phosphor thermometry is a well-known technique by which temperature may be deduced from the phosphorescent response of certain “thermographic phosphors.” The method is thoroughly reviewed by Allison and Gillies* and in HeyessV6 and, for the sake of brevity will not be described herein.

Previous Work

The authors and colleagues have already demonstrated the viability of creating thermal barrier sensor coatings. The first step involved production of a thermographic phosphor based on YSZ as the host material.’” Samples of 8YSZ (zirconia stabilized with approximately 8% by weight of yttria) doped with approximately 1 .A% by weight of Eu203 were prepared as powders and coatings were laid down using a vapor deposition technique. With these samples temperatures could be measured up to around 830°C with both powder samples and coatings. Measurements were made at the substrate/TBC interface using a sample consisting of a layer of YSZ:Eu approximately 10 pm thick overlaid with about 50 pm of undoped YSZ. Hence the viability of sub-surface temperature measurements was demonstrated. Finally, differences in the emission spectra of samples differently processed were noted. These are thought to have been attributable to different phase compositions (and hence different phonon spectra) so that as suggested by Dexpert-Ghys, et the lanthanide


activator may act as a structural micro-probe to detect phase composition changes.

In the next phase, a YSZ:Dy coating was produced using electron beam physical vapor deposition (EBPVD) in an industry standard coating facility at Cranfield University." The concentration of dysprosium in the coating was not exactly known due to the method of manufacture but was believed to be 5-10% by weight. The coating was shown to be suitable for temperature measurements up to at least 950K. No changes to the emission spectra were noted and it was concluded that the phase stability of the coating re- mained intact at least to the temperatures tested.

In a subsequent investigation," the effect of the concentration of dysprosium in yttria-stabilised zirconia on luminescent response and temperature dynamic range was studied. Concentration quenching was shown to be an important factor in the performance ofYSZ:Dy with the brightest output obtained from a sample containing only 0.005 mol% of Dy. The temperature dynamic range at this concentration was shown to extend to 800°C. This is well below the target for gas turbine applications. However, new proprietary compositions devised by STS have also been tested and show a dynamic range extending to at least 1300°C.

Current Objectives

The previous experiments described above have shown that thermal barrier sensor coatings can be man- ufactured based on the current standard material used for TBCs, i.e., YSZ and using the EBPVD industrial coating manufacturing method. Surface and sub-surface temperature measurements have been demonstrated as has the potential for phase change detection. On-going research is directed toward improving TBSCs to the point where they can be applied in an industrial setting on rotating or stationary components in a full-scale gas turbine. The technique is equally applicable in aero- and land-based gas turbines but the latter are regarded as the appropriate setting for a first application of the technology due to lower regulatory hurdles and the absence of weight restrictions. New TBC materials are under development but their composition is commercially sensitive and, at this stage, it is not clear what material if any will supersede YSZ. Our research is therefore currently concentrated on YSZ as the host material. Nevertheless, the basic technology remains the same and

when new compositions become known the technique will be adapted to suite. The research objectives associated with bringing the technology to point of industrial application are outlined below.

Optimize Composition

Initial sensor materials and coatings based on YSZ showed a temperature dynamic range limited to around 800°C. More recently, compositions allowing temperatures of up to 1300°C to be measured have been es- tablished. However, further composition optimization is required with a dynamic range extending to at least 1400°C is the target. Material composition affects the dynamic range primarily as a result of the temperature dependent phonon spectra of the host lattice. However, other variables include activator concentration and the inclusion of sensitising agents. Concentration quenching is known to diminish luminescent output so that an optimum activator concentration should exist. Sensitiz- ing agents may enhance performance by absorbing exciting radiation and pumping the activator by energy transfer.

Co n . r m Durability

TBC coating failure may occur as a result of phase instability, sintering or delamination at the bond coat/ TGO interface. The durability of coatings has been shown to be a function of operating temperature, ceramic and bond coat composition, substrate composition and a number of variables related to the coating process. Hence any change in coating composition must be validated to ensure no degradation in durability. This involves testing by long-term exposure at typical operating temperatures and temperature gradients and thermal cycling between ambient and operating temperatures. In addition it may be necessary to re-define coating control parameters such as the substrate temperature to retain the desired morphology.

Instrumentation Development

On-line application of the TBSC technology will require a means to deliver exciting radiation to components inside the engine and to collect and return the luminescence emission. This represents a significant technical challenge since any probe must survive the harsh environment inside hot sections of an engine whilst continuing to perform for thousands of hours.

Applications 15

For commercial applications it would also be necessary to use a reliable and preferably low cost light source for excitation rather than high cost relatively temperamental pulsed laser sources used in laboratory experiments. T o achieve this it will be necessary to characterize the excitation spectra of proposed TBSC compositions. This will enable the excitation wavelength to be optimized, for sub-surface measurements for example, and an appropriate light source selected.

Herein we review the manufacture and testing of the first sensor coatings to have been produced using a commercial plasma spraying process. A number of samples have been produced including coatings of various thicknesses and dual layer coatings with the doped layer underneath to enable depth selective temperature sens- ing. These coatings have been calibrated and a sample of the results is presented. A number of coatings have also been subjected to long term isothermal cyclic testing to establish their longevity relative to standard TBCs and some initial results are presented. The removable wall section of a development combustor at Imperial College has also been coated and a study of wall film cooling conducted. Some initial results of this study are also presented.

Coating Manufacture and Endurance Testing

The sensor TBC material was applied to 2.54cm diameter by 0.635 cm thick Haynes 230 buttons for calibration and thermal cyclic testing and to two 23 cm by 11 cm nimonic plates for combustion rig testing. Both substrates were made from nickel-based superalloy. The coating system consisted of a NiCrAlY bond coating, 0.05 mm Dysprosium (Dy)-doped YSZ intermediate coating following by a 0.55 mm overcoat of commercial 6-8 wt% YSZ. All coatings were applied by the air plasma spray process. Thermal cyclic tests were conducted on the sensor TBC in a CM Rapid Temp Furnace (CM Furnaces, Inc., Bloomfield, NJ) at temperatures between 25°C and 1148°C. Holding time at the peak temperature (1148°C) was 1Oh for each cycle. TBC failure was identified as having occurred when 20% of the area of the TBC or more had cracked and spalled from the substrate. Metallurgical evaluation was conducted using an optical microscope and a scanning electron microscope (SEM) with an energy-dispersive X-ray spectroscope (EDS) . The coating

application, analysis and furnace test were conducted at Solar Turbines Incorporated.

Figure 1 shows a micrograph of the sensor TBC in as-coated condition and reveals that the microstructure was very similar to that of a conventional TBC. Figures 2 and 3 show the EDS spectra taken from Dy-doped YSZ intermediate layer and a conventional YSZ outer layer, respectively. The EDS analysis confirms the pres- ence of Dy in the intermediate 1ayer.The sensor TBC failed after an average of 122 cycles. Figure 4 shows a micrograph of the sensor TBC after failure. The sensor TBC failed at the interface between TBC and bond coat. The failure mode and the coating life were very similar to those of conventional TBCs.

Temperature Response Calibration

The experimental arrangement was as shown in Fig. 5. A pulsed YAG:Nd laser (Newport Corporation, Spec- tra-Physics, Lasers Division, Model GCR-20 1, Moun- tain View, CA) was used to excite samples housed in a furnace (Lenton Furnace Ltd., Hope Valley, England) capable of reaching temperatures of 1600°C and spe- cially modified to provide optical access. The coated coupons were placed onto a solid ceramic stand providing good thermal connection with the furnace. The temperature controller displayed the temperature in 1 " steps. The laser was operated either at 355 or 266nm (Q-switch-mode), a repetition rate of 16 Hz and mon- itored continuously using the power meter shown in the

Fig. I . coated condition.

Microstructure o f the sensor thermal barrier coating in as-


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