ICMCTF2007 Session A3-1: Thermal Barrier Coatings

Wednesday, April 25, 2007 8:00 AM in Room Sunrise

Wednesday Morning

Start	Invited?	Item
8:00 AM	Invited	A3-1-1 Thermal Barrier Coatings for Gas Turbine Engines: Lessons Learned and Challenges Ahead R. Darolia (GE Aviation) Thermal barrier coatings (TBC) are extensively used as insulating coatings in various components of gas turbine propulsion systems. Higher temperature capability and life extension are key reasons for increasing use of TBC. A variety of lessons learned based on field observations and laboratory results of development activities will be described. Various modes of failure and their mitigation approaches will be discussed. The presentation will also discuss future development needs and challenges
8:40 AM	Invited	A3-1-3 Mechanisms Affecting the Durability of Thermal Barrier Systems A. Evans (University of California, Santa Barbara) The most recent advances in turbine airfoil technology have been enabled by multilayer coatings that impart thermal and oxidation protection. The former is provided by an oxide with exceptionally low thermal conductivity. The latter is achieved using alloy coatings that form alumina. For continued performance enhancement a systems-level methodology governing durability is needed, based on an understanding of the modes of spalling and delamination that occur in actual engines. A total of five different mechanism categories has been identified. Each category is described briefly and the progress toward identifying the material properties that dictate the delamination resistance discussed. Emphasis is placed on mechanisms that arise in the presence of a thermal gradient, subject to particle impact and the ingestion of CMAS.
9:20 AM		A3-1-5 Adhesion Energy of a YPSZ EBPVD Layer in Two Thermal Barrier Coating Systems P.-Y. Théry, M. Poulain (ONERA, France); M. Dupeux, M. Braccini (LTPCM, France) In order to better understand the degradation of thermal barrier coating (TBC) systems, we determined the adhesion energy (G_c) between the bond coat and the top coat and its evolution during cyclic oxidation. This energy was evaluated by means of a modified four-point bending test¹ for two thermal barrier systems. The specimens include a stiffening layer bonded to the top coat in order to increase the stored energy in the ceramic layer and therefore the driving force for delamination. A notch was machined through the stiffening material and the top coat and a symmetric interfacial precrack was introduced. The adhesion energy can be derived from the plateau force corresponding to the stable propagation of the interfacial crack. The systems tested are constituted of a YPSZ EBPVD ceramic topcoat deposited either on a β-(Ni,Pt)Al bond coat or a newly developed Zr-doped β-NiAl bond coat². In both cases, the substrate is an AM1 superalloy. The results show a sharp drop of the adhesion energy after the 50 first one-hour cycles at 1100°C, from 110 J/m² for the as-deposited state down to 50 J/m², indicating that the degradations of the interface occurred at the beginning of the lifetime of both systems. These results are discussed in relation with the microstructural evolution observed on oxidised samples and the aspect of the fracture surfaces. ¹Y. Yamazaki, A. Schmidt, A. Sholz, The Determination of the Delamination Resistance in Thermal Barrier Coating System by Four-Point Bending Tests, Surface and Coating Technology, Vol. 201, 744-754, 2006. ²S. Navéos, G. Oberlaender, Y. Cadoret, P. Josso, M.-P. Bacos, Zirconium Modified Aluminide by Vapour Pack Cementation Process for Thermal Barrier Applications : Formation Mechanisms and Properties, Materials Science Forum, Vol. 461-464, 375-382, 2004.
10:00 AM		A3-1-7 Numerical Performance Assessment of TBC Microstructures Subject to Foreign Object Damage M.W. Crowell, A.G. Evans, R.M. McMeeking (University of California, Santa Barbara) Thermal barrier coatings (TBCs) applied to high pressure turbine (HPT) blades in jet aircraft engines are subject to a number of different damage mechanisms including thermal spallation, small particle erosion, and large particle impacts or "foreign object" damage (FOD). In particular FOD has proven very difficult to study experimentally due to the complexities of the HPT blade/engine environment and the unknown impacting particle sizes, materials, speeds, and trajectories. In response to these difficulties a performance metric for TBCs under FOD conditions has been developed which uses the results of finite element simulations. Stress intensity factors are calculated for cracks at various locations within the TBCs using the stress distributions obtained from the simulations. An extensive parameter space of TBC microstructures and impact conditions can quickly and accurately be probed within the simulations and the stress intensity factors used to quantitatively compare the TBC performance.
10:20 AM		A3-1-8 Investigation of the Role of Ferroelasticity on the Toughness of Tetragonal (t') Yttria-Stablized Zirconia C. Mercer, J.R. Williams, D.R. Clarke, A.G. Evans (University of California, Santa Barbara) The toughness of yttria-stabilized zirconia (YSZ) with a tetragonal (t') crystal structure has been investigated. The material was found to be significantly tougher than cubic YSZ, but less tough than compositions that undergo transformation toughnening. Based on prior literature, a ferroelastic toughening mechanism is hypothesized. This assertion is explored by examining the material in the wake of an indentation-induced crack by using optical interferometry and transmission electron microscopy. The assessment has revealed a process zone about 3 microns in width, containing a high density of nano-scale ferroelastic domains (separated by 90° {110} twin boundaries), with approximately equal proportions of all three orthogonal variants. Outside the process zone, the material contains a single parent variant (no domains). Consequently, the toughening mechanism is controlled by the nucleation of ferroelastic domains, rather than the motion of pre-existing domain boundaries (switching). The viability of the mechanism is assessed by using a process zone model that relates the toughening to the stress/strain hysteresis accompanying domain formation. Based on the measured process zone size, the tetragonality of t'-7YSZ and the increase in toughness relative to cubic YSZ, consistency is demonstrated for a reasonable value of the coercive stress.
10:40 AM		A3-1-9 Thermo-Mechanical Properties of Thermal Barrier Coatings Subject to CMAS (Calcium-Magnesium-Alumino-Silicate) Penetration S. Faulhaber, S. Kraemer, V. Lughi, D.R. Clarke, C.G. Levi (University of California, Santa Barbara); J.W. Hutchinson (Harvard University); A.G. Evans (University of California, Santa Barbara) Delaminations in Thermal Barrier Coatings have been observed in engine hardware infiltrated by molten deposits (CMAS, calcium-magnesium-alumino-silicate) both for APS coatings (shroud) as well as EB-PVD material (airfoil). Cross-sectioning and SEM observation complemented by TEM studies were employed to identify the penetration depth, location of delaminations and changes in the morphology of the coating. To ascertain the details of the failure mechanism the determination of the mechanical properties, in particular the toughness of the infiltrated part of the coating is necessary. Establishing the room-temperature stress-gradients in the coating in combination with the determination of the melting and crystallization behavior of the deposits will allow an assessment of the relevant thermal scenarios. Thermal gradient experiments on the systems under investigation will help verify the conclusions drawn from the determination of the thermo-mechanical properties.
11:00 AM		A3-1-10 Mechanism Governing Inhibition of CMAS Melt Infiltration Into Gadolinium Zirconate TBCs S. Kraemer, R. Leckie, J. Yang, C.G. Levi (University of California, Santa Barbara) Gadolinium zirconate (GZO) is a viable alternative to standard 7YSZ as material for thermal barrier coatings (TBCs). This work analyses the interaction of GZO with calcium-magnesium alumino-silicate melts (CMAS) which form from ingested siliceous debris if temperatures exceed ~1200°C. A model CMAS with the composition 35CaO-10MgO-7Al₂O₃-48SiO₂ (T_m~1240°C) was applied to the TBC surface in pelletized form. After isothermal heat treatments at 1300°C changes in morphology and chemistry were characterized by SEM, and FIB+TEM. The reaction was governed by dissolution of GZO in the CMAS melt and consecutive crystallization of CaGd₄Si₃O₁₃, an apatite-type phase with some Zr in solid solution, and precipitation of cubic ZrO₂ with Gd and Ca in solid solution. Two time domains characterize the microstructure evolution. During the early stages solid reaction products fill the inter-columnar gaps, stopping the infiltration ~15μm below the original TBC surface and leaving the rest of the porosity unaffected. In a subsequent stage chemical attack continues at the column tips leading after 4h to a 6μm thick polycrystalline conglomerate of the aforementioned reaction products. The amount of consumed TBC material is significantly reduced compared to 7YSZ under identical conditions, and the CMAS penetration is suppressed even in the absence of a thermal gradient through the coating. The underlying kinetics as well as limitations of the infiltration protection will be discussed.
11:20 AM		A3-1-11 CMAS Degradation of Environmental Barrier Coatings K.M. Grant, S. Krämer, J.P.A. Löfvander, C.G. Levi (University of California, Santa Barbara) Environmental barrier coatings (EBCs) based on Ba_{1-x√sub x}Al₂Si₂O₈ (BSAS) have demonstrated potential for the protection of Si-based ceramic matrix composites (CMSs) against moisture-induced degradation in gas turbines. Siliceous materials (dust, sand, runway debris) ingested with the engine intake air deposit on coated component surfaces, yielding glassy melts of Calcium-Magnesium Alumino-Silicate (CMAS). BSAS EBCs are susceptible to high temperature attack by CMAS. The mechanism involves the dissolution of BSAS into CMAS and re-precipitation as a modified Celsian phase incorporating Ca, as well as secondary crystalline phases that may be more prone to volatilization. The process appears to be aggravated by grain boundary penetration of the polycrystalline BSAS. The mechanisms and their potential implications for durability are discussed.