Papers

ICMCTF2008 Session A3-1: Thermal Barrier Coatings

Wednesday, April 30, 2008 8:00 AM in Room Sunrise

Wednesday Morning

Start	Invited?	Item
8:00 AM		A3-1-1 Considerations in Developing a Design Strategy for Novel TBCs C.G. Levi (University of California, Santa Barbara) Zirconia partially stabilized with 7±1wt%Y₂O₃ (7YSZ) has been the standard material for TBCs since their commercial insertion. The demands for increased engine performance and fuel flexibility in advanced gas turbines translate into higher temperatures and more aggressive operating environments for TBCs, motivating the search for alternate materials often retaining ZrO₂ as the base oxide. Because a key element in the durability of current TBCs is their non-transformable tetragonal (t’) structure, which is metastable, one must tread carefully in the design space to achieve desired targets in functionality such as lower thermal conductivity or increased corrosion resistance, while maintaining adequate toughness, phase stability, compatibility with the underlying thermally grown oxide (TGO) and morphological stability of the strain-tolerant microstructure. These challenges are discussed in this presentation in light of current work on alternate TBCs driven by desired improvements in resistance to erosion, cyclic life and attack by molten siliceous deposits that are commonly encountered in aircraft engines. This presentation is based on contributions from current and former members of the speaker’s research group, notably T.A. Schadler, R.M. Leckie, F.M. Pitek. N.R. Rebollo, S.G. Terry, S. Krämer and A.S. Gandhi, The research was performed under programs sponsored by the National Science Foundation and the Office of Naval Research.
8:40 AM		A3-1-3 Mechanisms of Lifetime Improvement in Thermal Barrier Coatings with Hf and/or Y Modified Superalloy Substrates J. Liu (University of Central Florida); K. Murphy (Howmet Castings); Y. Sohn (University of Central Florida) Mechanisms related to superior lifetime of thermal barrier coatings (TBCs) with (Ni,Pt)Al bond coat and Hf- and/or Y-modified CMSX-4 superalloy substrates were examined. TBC lifetime was observed to improve from 600 cycles to over 3200 cycles with appropriate Hf- and/or Y-alloying of CMSX-4 superalloys, where each thermal cycle consisted of 10-minute heat-up, 50-minute dwell at 1135°C, and 10-minute forced-air-quench. Mechanisms of degradation and failure for TBCs were examined by scanning and transmission electron microscopy (SEM and TEM/STEM) and secondary ion mass spectroscopy (SIMS) with emphasis on rumpling, ratcheting, interfacial segregation, TGO growth, TGO phase constituents, TGO grain size, and martensitic transformations within the (Ni,Pt)Al bond coat. While rumpling and racheting of the TGO/bond coat interface were observed to be the main damage mechanisms for TBCs on baseline CMSX-4, the same interface remained relatively flat for durable TBCs with Hf- and/or Y-modified CMSX-4. Hf and/or Y were observed to segregate to the TGO/bond coat interface. Monoclinic HfO₂ was observed to be embedded along the grain boundaries of alpha-Al₂O₃ TGO scale. The parabolic growth constant of the TGO scale was slightly lower and columnar diameter of the TGO scale was observed to be smaller for TBCs with modified CMSX-4. Martensitic transformation within the bond coat did not vary with compositional modification of CMSX-4. Results are presented and discussed with respect to improved furnace thermal cyclic lifetime of TBCs.
9:00 AM		A3-1-4 Thermo-Mechanical Properties and Delamination Mechanism of CMAS Penetrated Thermal Barrier Coatings S Faulhaber, A.G. Evans (University of California, Santa Barbara) Delamination of thermal barrier coatings infiltrated with CMAS (calcium-magnesium-alumino-silicate) is gaining importance as a failure mechanism with increasing engine operating temperatures. Thermo-mechanical stresses are based on cooling scenarios as well as changes in mechanical properties making a thorough understanding of both parameters paramount to assessing the failure probability. Mechanical properties of YSZ-coatings deposited by atmospheric plasma spray (APS) and isothermally infiltrated with CMAS have been measured by 4-point bend test and compared with values obtained by indentation tests. Thermal conductivity was determined by laser measurement and the CTE (coefficient of thermal expansion) extracted from high-temperature images. The microstructure of the infiltrated coating has been characterized by SEM, (scanning electron microscopy), EDS (energy-dispersive x-ray analysis) and TEM (transmission electron microscopy). The measured properties have been used to recalculate the mechanism map proposed by Hutchinson and Evans. The boundaries can be probed by carefully designed thermal gradient experiments. A laser setup equipped with various temperature measurement devices was used for the experiments; cooling scenarios have been achieved by shut-off and forced-air. Samples submitted to these experimental conditions have been evaluated with regards to delamination, changes in properties and microstructure.
9:20 AM		A3-1-5 Oxidation Behavior of Air Plasma Sprayed NiCoCrAlY Bond Coats for Air Plasma Sprayed Thermal Barrier Coatings T. Patterson, A. Leon, B. Jayaraj, J. Liu, Y. Sohn (University of Central Florida) Furnace thermal cycling lifetime and microstructural degradation are reported for thermal barrier coatings (TBCs) with air plasma sprayed (APS) ZrO2-8wt.%Y₂O₃ (YSZ) and APS NiCoCrAlY bond coat. TBCs examined in this study consisted of 600 µm-thick APS YSZ, 175 µm-thick APS NiCoCrAlY and 5mm-thick Haynes 230 superalloy substrate. Thermal cycling was carried out using 1-, 10- and 50 hour thermal cycles that consisted of 10-minute heat-up to 1121°C, dwell at 1121°C and followed by 10-minute forced-air-quench. Despite the significant internal oxidation of APS NiCoCrAlY bond coat, these TBCs exhibited excellent thermal cycling lifetime when compared with APS YSZs with conventionally processed NiCoCrAlY bond coats. Microstructural evolution of TBCs with an emphasis on the development of thermally grown oxide including internal oxidation was examined by scanning electron microscopy, transmission electron microscopy (TEM) and scanning TEM. Quantitative image analysis of internal oxidation suggests that the internal oxidation is dominated by rapid oxidation of supersaturated oxygen within the APS bond coat. Site-specific TEM specimen was prepared by using Focus Ion Beam In-situ Lift-out technique. Extensive internal oxidation was observed including formation of spinels. Oxides such as Al₂O₃, (Al,Cr)₂O₃ and (Ni,Co)(Al,Cr)₂O₄ within the TGO were identified by electron diffraction. Superior performance of these TBCs with APS NiCoCrAlY bond coats is discussed in terms of reduced thermal expansion mismatch.
10:00 AM		A3-1-7 Advanced Low Conductivity Thermal Barrier Coatings: Performance and Future Directions D. Zhu, R.A. Miller (NASA Glenn Research Center) Thermal barrier coatings will be more aggressively designed to protect gas turbine engine hot-section components in order to meet future engine higher fuel efficiency and lower emission goals. In this presentation, thermal barrier coating development considerations and performance will be emphasized. Advanced thermal barrier coatings have been developed using a multi-component defect clustering approach, and shown to have improved thermal stability and lower conductivity. The coating systems have been demonstrated for high temperature combustor applications. For thermal barrier coatings designed for turbine airfoil applications, further improved erosion and impact resistance are crucial for engine performance and durability. Erosion resistant thermal barrier coatings are being developed, with a current emphasis on the toughness improvements using a combined rare earth- and transition metal-oxide doping approach. The performance of the toughened thermal barrier coatings has been evaluated in burner rig and laser heat-flux rig simulated engine erosion and thermal gradient environments. The results have shown that the coating composition optimizations can effectively improve the erosion and impact resistance of the coating systems, while maintaining low thermal conductivity and cyclic durability. The erosion, impact and high heat-flux damage mechanisms of the thermal barrier coatings will also be described.
10:40 AM		A3-1-9 Yttria-Stabilized Zirconia Thick Coatings Deposited from Aqueous Solution in a Low Pressure Plasma Reactor F. Rousseau (Ecole nationale Supérieure de Chimie de Paris-Université Pierre et Marie Curie, France); S. Awamat, D. Morvan (Laboratoire de Génie des Procédés Plasmas et Traitements de Surfaces, France); R. Mevrel (ONERA, France) A low pressure plasma process has been developed which enables to deposit oxide layers from microdroplets introduced in a reactor. With this process, up to 100 µm thick porous yttria-stabilized zirconia (YSZ) coatings have been deposited. Zirconyl and yttrium nitrates (ZrO(NO₃)₂ and Y(NO₃)₃) dissolved in water were used as precursors. These precursors were introduced as micrometric droplets into a plasma discharge. After reacting with the oxidant agents into the plasma, the droplets deposited onto the substrate to form the YSZ coating. The structure and composition of the coatings have been characterized by optical and scanning electron microscopy (+EDX), XRD, and their thermal diffusivity measured by the laser flash technique. The results obtained show that this deposition technique can be used to deposit the ceramic layer of thermal barrier coatings. In particular, its flexibility may help to explore rapidly a wide range of new ceramic compositions. In addition, it offers the possibility of preparing easily multi-layers by varying the nature, the concentration or the flow rate of the precursors injected.
11:00 AM		A3-1-10 Oxide and TBC Spallation in ß-NiAl Coated Systems Under Mechanical Loading M. Harvey, C. Courcier (SNECMA SAFRAN Group & Mines Paris, France); V. Maurel, L. Remy (Ecole Nationale Supérieure des Mines de Paris - Paristech, France) Protections such as aluminide and thermal barrier coatings have been introduced in turbine blade technology to reduce oxidation damage and surface temperature. Recent studies have brought information about degradation kinetics and mechanisms under cyclic oxidation, but little data is available for coupled thermal and mechanical loads, closer to in service conditions. Coating lifetime models should be able to take into account the complex coupling effects. In this work, damage localization has been studied under thermo-mechanical solicitations on single crystal aluminide coated, with and without TBC. Strong temperature dependencies have been observed for both systems. As expected, oxide spallation and TBC fracture seem to depend on the bond coat ability to accommodate strain mismatch between substrate and oxide layers. Substrate strain localization triggers surface damage at low temperatures, whereas for high temperatures, bond coat relaxation slows the substrate strain and the oxide growth strain impact on damage in the oxide layer, and modifies the rupture interface in TBC systems. A cinematic multi-layer model has been proposed to take the statements into account through an elastoviscoplastic bond coat behavior. The substrate is assumed to impose mechanical strain mechanical strain to the other layers. Thermal expansion coefficient mismatch and oxide growth strain have also been taken into account. Parameters have been calibrated on simple tests, and the model was validated for TMF solicitations approximating real flight conditions. This work was undertaken by turbine manufacturer SNECMA (SAFRAN) and Mines Paris in order to develop a new industrial lifetime model.