Papers

ICMCTF2009 Session A3-1: Thermal Barrier Coatings

Thursday, April 30, 2009 1:30 PM in Room Royal Palm 1-3

Thursday Afternoon

Start	Invited?	Item
1:30 PM		A3-1-1 Toughening Mechanisms in 7YSZ Thermal Barrier Coatings N.R. Philips, E.M. Donohue (University of California, Santa Barbara); D. Kohl (Universität Stuttgart, Germany / summer internship at University of California, Santa Barbara); C.G. Levi, A.G. Evans (University of California, Santa Barbara) The cyclic durability of thermal barrier coatings (TBCs) based on yttria-stabilized zirconia (YSZ) is greatest for compositions in the tetragonal (t') phase field, with the maximum occurring at 7 mole % (7YSZ). This trend in durability has been ascribed to a relationship between toughness and tetragonality, enabled by ferroelastic domain switching. While the existence of this mechanism has been demonstrated on dense, polycrystalline 7YSZ (toughness, Γ=45Jm^-2), it has yet to be demonstrated on coatings. This investigation probes the mode I crack propagation resistance of air plasma sprayed (APS) 7YSZ coatings incorporating dense, vertical-cracks (DVCs). High fidelity has been enabled by using a wedge-loaded double cantilever beam method that extends the cracks in a stable manner. The toughness Γ=250Jm^-2 is substantially higher than that for the dense material, indicating unexpectedly large effects of the APS microstructure and of the DVCs. Various x-ray and electron probes, as well as nanoindentation, have been used to assess the associated mechanisms.
1:50 PM		A3-1-2 Nondestructive Microstructural Characterization of Thermal Barrier Coatings by Laser Flash Technique F. Cernuschi (Cesi Ricerca, Italy); P. Bison (CNR-ITC, Italy); A. Moscatelli (Politecnico di Milano, Italy) In the case of materials with open porosity such as thermal barrier coatings, the environment the thermal diffusivity measurement is carried out (i.e. vacuum or gases) significantly affects the thermal diffusivity value. In fact, most of the more common gases have higher thermal diffusivity than that of YPSZ even if gas thermal diffusivity within the pores is lower than in large volumes ("bulk" value), when the pore dimension is comparable to the mean free path of gas molecules (Knudsen effect). The contribution of the measurement atmosphere to the thermal diffusivity strictly depends on the specific microstructural features of the porous sample under investigation such as porosity content, orientation and morphology. Thus, if thermal diffusivity measurements would be performed by varying gases a non destructive estimation of microstructural parameters would be possible. This approach can be used also to characterise sintering phenomena typically consisting in the healing of very fine crack-like pores which are difficult to be properly detected by conventional image analysis techniques. In this work an inversion procedure has been developed to obtain microstructural parameters describing the porosity morphology of porous thermal barrier coatings (TBC) from the thermal diffusivity measured in different environments by a Laser Flash technique. A simplified approach in the inversion procedure has been proposed, and the reliability has been checked by simulating different microstructures within the TBC. The inversion procedure has been also applied to experimental thermal diffusivity values of a TBC which were obtained by filling pores with He, N2, Ar and in vacuo. This work has been financed by the Research Fund for the Italian Electrical System under the Contract Agreement between CESI RICERCA and the Ministry of Economic Development - General Directorate for Energy and Mining Resources stipulated on June 21, 2007 in compliance with the Decree n.73 of June 18, 2007.
2:10 PM		A3-1-3 Recent Results on Advanced Thermal Barrier Coatings R. Vaßen, O. Jarligo, H. Kassner, Y. Zhang, D. Mack, G. Mauer, D. Stöver (Institute of Energy Research (IEF-1), Germany) The performance of thermal barrier coating (TBC) systems is largely influenced by the microstructure of the ceramic topcoat. This has been observed for standard TBC systems made of yttria partially stabilized zirconia (YSZ), and also for those made of new TBC materials. One of the major processing routes for the manufacture of TBCs is the atmospheric plasma spraying (APS). The microstructure of atmospheric plasma-sprayed coatings with conventionally micro-cracked, highly porous, segmented and other advanced microstructures and their processing conditions will be outlined. This overview will also include suspension plasma spraying in which a liquid feedstock instead of particulates is used. A comparison of the properties of these different kinds of coatings will be given. Although the standard TBC material YSZ shows unique properties with respect to a TBC application it has a limited temperature capability of about 1200°C in long-term applications. Therefore potential new TBC materials are being evaluated as pyrochlores, perovskites, and aluminates. The typical relevant physical properties of the bulk materials will be shown. In addition, specific processing issues for the different materials will be discussed. Finally, the performance of these materials as TBC systems will be presented.
2:50 PM		A3-1-5 The Relation Between Morphology, Phase and Thermal Conductivity Changes in the Thermally Loaded EB-PVD TBCs R. Ochrombel (German Aerospace Center, Germany); V. Ryukhtin (Technical University of Berlin, Germany); B. Saruhan (German Aerospace Center, Germany) Increase in combustion temperature for improved turbine efficiency causes greater thermal loading at the thermal barrier coatings (TBCs) of the stationary and aircraft turbine blades. At these elevated temperatures (> 1300°C), the state of the art TBC-material, partially yttria stabilized zirconia (PYSZ), suffers phase instability and heavy sintering-induced morphological changes resulting in an increase at their thermal conductivity. Previously, it was demonstrated that fully stabilized zirconia (FYSZ) containing 14 wt. % Ytrria yields intrinsically lower thermal conductivity. It is also possible to optimize the TBC properties by tailoring its microstructure, since an important relationship between thermal conductivity and processing conditions of EB-PVD TBCs exists. To optimize the performence of TBCs, a parameter study is necessary. In this work, the results obtained on EB-PVD deposited PYSZ and FYSZ samples varying the process parameters and applying X-Ray powder d iffraction (XRD), Scanning Electron Microscopy (SEM), Laser Flash Analysis (LFA) and in-situ high temperature Small Angle Neutron Scattering (SANS) will be presented and compared with each other. The influence of the resulting texture, the resulting phase changes/phase mixture, morphology, especially thermal induced changes in pore shape and size will be correlated with the thermal conductivity and discussed in terms of process parameter and phase constituency influences.
3:10 PM		A3-1-6 The Behavior of High-Purity, Low-Density Air Plasma Sprayed Thermal Barrier Coatings G.H. Meier, M.A. Helminiak, N.M. Yanar, F.S. Pettit (University of Pittsburgh); T.A. Taylor (Praxair Surface Technologies) This paper describes research on the behavior of high-purity, low-density (85%) air plasma sprayed (APS) thermal barrier coatings (TBC) with NiCoCrAlY bond coats deposited by argon-shrouded plasma spraying. The microstructure of the APS topcoats was controlled to maximize the coating thicknesses that can be applied without spallation and to minimize the thermal conductivity of the TBC. The specimens are being evaluated using cyclic oxidation and thermal shock tests, and important properties of the TBCs, such as resistance to sintering and phase transformation, thermal conductivity, and fracture toughness are being determined. The high purity resulted in top coats which are highly resistant to sintering and transformation from the metastable tetragonal phase to the equilibrium mixture of monoclinic and cubic phases. The porous topcoat microstructure also resulted in significant durability during thermal cycling. A 750 µm thick APS coating was found to have a cyclic life that was at least as long as that of a standard 100 µm thick electron beam physical vapor deposition (EBPVD) coating, as measured in a furnace cycle test. The actual failure mechanisms of the APS coatings were found to depend on topcoat thickness and the nature of the thermal exposure.
3:30 PM		A3-1-7 Thermo-Mechanical Properties and Gradient Testing of Thermal Barrier Coatings Subject to Spallation Due to CMAS (Calcium-Magnesium-Alumino-Silicate) Penetration S. Faulhaber, A.G. Evans (University of California Santa Barbara) Spallation in Thermal Barrier Systems can occur as a consequence of changes in the mechanical and thermal properties of the coating due to infiltration with molten deposits. Investigation of delaminations and microstructural changes by microscopy and related techniques has allowed insights into the thermal conditions experienced by the material leading to a basic understanding of the delamination mechanism. To refine delamination maps that help predict conditions under which spallation occurs the mechanical properties, Young’s modulus and fracture toughness, and the thermal properties, diffusivity and thermal expansion coefficient, have been measured. Thermal gradient experiments with well-defined thermal conditions were conducted to confirm the thermomechanics of spallation.
3:50 PM		A3-1-8 Effect of Low Level CMAS Attack on EB PVD TBCs R.G. Wellman, G. Whitman, J.R. Nicholls (Cranfield University, United Kingdom) When debris deposits onto thermal barrier coatings (TBCs) in gas turbine engines at high temperatures, it can melt to form Calcium-Magnesium-Aluminosilicates (CMAS). This molten CMAS attacks the TBC by infiltrating the columnar structure, which has a detrimental effect on the TBCs morphology and microstructure. This paper discusses the early effects of CMAS attack at low concentrations to understand the evolution of the degradation. By depositing small amounts of CMAS onto TBCs, a minimum safe level was established under which CMAS degradation does not occur. Both pre-reacted CMAS and CMAS as constituent powder were used in this study, since it is unknown as to whether the CMAS forms before or after deposition onto the turbine blade. It was found that at least 4.8mg/cm² of CMAS powder was required to form a uniform level of attack after 4hrs at 1300°C. This minimum level reduced significantly by increasing the exposure time to 12 hours, but variations in temperature was found to have much less of an effect. Increasing the temperature did not reduce the level required to cause the transformation of the TBC. Preliminary studies into mechanical properties of CMAS degraded samples showed that 4.8mg/cm² for 4 hours at 1300°C was sufficient to cause an increase in room temperature erosion rate by factor of five. This evidence suggests that even very low levels of CMAS infiltration is sufficient to cause severe degradation of the TBC, leading to significantly reduced service lives of TBC coated components. This paper looks at the differences between using pre-reacted CMAS as opposed to the constituent powders on the degradation of EB PVD TBCs at very low levels as well as investigating how the low levels of CMAS infiltration affect the erosion performance and erosion mechanism of the EB PVD TBCs.
4:10 PM		A3-1-9 The Role of Crystallization in Arresting CMAS Infiltration into TBCs E.M. Vogel, S Krämer, C.G. Levi (University of California, Santa Barbara) Thermal barrier coatings (TBCs) in engines subject to siliceous debris ingestion with the intake air experience infiltration by calcium magnesium alumino-silicate (CMAS) melts with detrimental effects on the durability of the coating. Field samples reveal that CMAS penetration in 7YSZ TBCs is arrested by crystallization at temperatures within the coating well below the melting isotherm. When CMAS interacts with zirconate TBCs, penetration is arrested by crystallization much closer to the surface, even in the absence of a thermal gradient. The implication is that the dynamic crystallization behavior is influenced not only by the thermal history, but also by the chemical interaction with the coating. The present investigation aims at understanding these phenomena and underlying mechanisms by a combination of differential scanning calorimetry (DSC) and direct visualization of the infiltration behavior in a tube furnace set up, followed by extensive microstructural analysis. DSC of a model crystalline CMAS with average composition 35CaO-10MgO-7Al₂O₃-48SiO₂ revealed a sharp melting endotherm at T_m~1240°C, with subsequent crystallization on cooling marked by a broad exotherm peaking at ~1150°C. Amorphous CMAS of the same composition showed also crystallization over a broad exotherm with a maximum at ~1100°C. Visualization experiments reveal remarkably different behaviors of CMAS on the TBC depending on whether the “deposit” is crystalline or amorphous, the temperature ramp up rate, the hold temperature and the nature of the TBC. These issues and the implications to the potential mitigation of CMAS penetration are discussed in the context of recent research.
4:30 PM		A3-1-10 Electrophoretically Deposited Alumina as Protective Overlay for Thermal Barrier Coatings Against CMAS Degradation P. Mohan, T. Patterson, Y.H. Sohn (University of Central Florida) Thermal barrier coatings (TBCs) can be highly susceptible to degradation due to air ingested CMAS (calcium-magnesium alumino silicate) sand deposits during operation in a dust-laden environment. At high temperature, CMAS deposits melt and degrade the TBC system via repeated freeze-thaw action and to a certain extent, direct chemical reaction with TBC constituents (e.g., destabilization of yttria stabilized zirconia, YSZ coatings). In order to protect TBCs from CMAS attack, a dense crack-free overlay of alumina was fabricated by electrophoretic deposition (EPD) on YSZ coatings. For EPD, a colloidal suspension of α-Al₂O₃ powders in acetone/ethanol mixture was used. At an applied DC voltage of 25V, positively charged alumina powders in the colloidal suspension were drifted and deposited on YSZ for up to 10 minutes. The deposited powder compact by EPD was carefully dried and densified by sintering at various temperatures up to 1400°C for 5 h. EPD process can deposit dense , crack-free alumina overlay of uniform thickness up to 20 μm. Effect of alumina as protective overlay against CMAS melt ingression was examined by exposing the modified YSZ coatings to a laboratory-synthesized CMAS deposit at temperature up to 1350°C. The tested specimens were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). Preliminary results show that CMAS melt ingression into the YSZ was completely suppressed with dense EPD alumina overlay. Attributed mechanisms include complete crystallization of CMAS glass due to a shift in glass composition with an increase in Al content resulting in compounds such as anorthite (CaAl₂Si₂O₈), which can further act as an overlay constituent to suppress CMAS ingression. Improved performance of modified YSZ coatings against CMAS attack will be presented in detail along with processing strategies adoptable for coatings industry.
4:50 PM		A3-1-11 Dependence of Microstructure and Mechanical Properties with Starting Powder Morphology in Zirconia-Based Thermal Barrier Coatings Y.G. Jung, S.I. Jung, J.Y. Kwon, Y.S. Sim, U. Paik (Changwon national University, Korea); K.S. Lee (Kookmin University, Korea) The effects of powder morphology in air plasma-sprayed coatings on microstructure and mechanical properties of thermal barrier coatings (TBCs) were investigated under thermal exposure. Two kinds of powders were prepared with different processing parameters, especially solvent, in spray drying process, showing a deformed hollow type and a filled spherical type, and then heat-treated at 1,250°C for 1 h. The coating thickness prepared by the hollow type is thicker than that by the spherical type. All peaks in phase analysis are tetragonal and cubic phases without any monoclinic phase in the developed powders. The relatively porous microstructure could be obtained in the hollow type, while the thickness of thermally grown oxide (TGO) layer in the hollow type is thicker than the spherical type with a saturated point at 100 h thermal exposure. The hardness values on sectional plane of TBCs are slightly higher than those on surface plane, whereas the spherical type show s higher values on both planes in all temperatures tested. However, the toughness values on surface plane are definitely higher than those on sectional plane, without an effect of power morphology. The hardness values obtained by nanoindentation in each component—substrate, bond coat, and top coat are not greatly affected by thermal exposure with a large scatter in the bond and top coats, whereas the values of elastic modulus in the bond coat are dominantly affected by thermal exposure.