ICMCTF2005 Session A3-1: Thermal Barrier Coatings

Tuesday, May 3, 2005 8:30 AM in Room Sunrise

Tuesday Morning

Start	Invited?	Item
8:30 AM	Invited	A3-1-1 Intriguing Prospects for Using Pt-Modified γ'- Ni₃ Al Bondcoats +γ-Ni Bondcoats in Next-Generation TBC Systems B. Gleeson, D.J. Sordelet (Iowa State University) Many high-temperature coatings rely on the formation of a continuous and adherent thermally grown oxide (TGO) scale of α-Al₂O₃ for extended resistance to degradation. For instance, the durability and reliability of current thermal barrier coating (TBC) systems in gas turbines are critically linked to the oxidation behavior and stability of the alumina-forming β-NiAl-based bond coat. This presentation will discuss the development of unique Pt-modified γ'-Ni₃Al+γ-Ni coating compositions that form highly adherent, slow-growing TGO scales of Al₂O₃' during both isothermal and cyclic oxidation at high temperature. Moreover, the thermodynamic activity of aluminum in these compositions is below that in Ni-base superalloys used for areo-engine turbine applications. As a consequence, and in complete contrast to the typically used β-based coating compositions, aluminum diffuses from the substrate alloys to the γ'+γ coatings. Thus, the novel γ'+γ coating compositions offer the advantages of (1) forming a slow-growing and adherent TGO scale of α-Al₂O₃, (2) not depleting in aluminum due to coating/substrate interdiffusion during service, and (3) being compatible with superalloy substrate in terms of phase constitution and, hence, coefficient of thermal expansion. Recent findings on the oxidation and interdiffusion behaviors of Pt-modified γ'+γ alloys and coatings will be summarized.
9:10 AM	Invited	A3-1-3 NASA's Experience with Low Conductivity/Sintering Resistant Combustor and Turbine Section Thermal Barrier Coatings R.A. Miller, D. Zhu (NASA-Glenn Research Center) Low conductivity/sintering resistant thermal barrier coatings -- consisting of zirconia, yttria, and certain combinations of two or more additional rare earth oxides -- have been developed under the NASA Ultra Efficient Engine Project. The total level of dopant oxides were adjusted to produce coatings with compositions in both the cubic and tetragonal' phase fields. The cubic compositions were found to have the lowest conductivity and the tetragonal' structure had the best properties in terms of durability. Both the plasma spray and EB-PVD processes were used to deposit the coatings -- with the former approach generally being more applicable for combustor coatings and the later being more for turbine blade coatings. A high heat flux laser rig test has played a central role in the development of these new coatings. These coatings and their performance in the laser rig and other tests will be discussed.
9:50 AM		A3-1-5 Improving the Phase Stability of TBCs with Yb Additions J.M. Cairney (University of New South Wales, Australia); N.R. Rebollo (University of California, Santa Barbara); A. Catanoiu, M. Rühle (Max Planck Institut fuer Metallforschung, Germany); C.G. Levi (University of California, Santa Barbara) The maximum operating temperature of conventional thermal barrier coatings based on 7YSZ is ultimately limited by the effect of ageing on the phase stability of YSZ. As temperature increases, the metastable non-transformable tetragonal (t') phase produced during TBC deposition partitions into the equilibrium assemblage of cubic and tetragonal (c+t) phases, wherein the latter becomes susceptible to the detrimental transformation to the monoclinic(m) phase upon cooling. Preliminary investigations into alternative rare earth (RE) dopants indicated that Yb-doped zirconia (YbSZ) shows significant improvement in stability over YSZ at the same level of dopant addition. In comparison, the more commonly advocated ScSZ is inferior to YSZ at the same level of cation substitution. Specimens of YSZ and YbSZ with concentrations of 7.6 and 11.4mol%REO_1.5 were synthesized by precursor methods and compacted into pellets. These were subsequently annealed isothermally at 1450°C for various times and compared at various stages of decomposition. X-ray diffraction reveals that replacing Y with Yb substantially improves the stability of the tetragonal phase upon annealing. Monoclinic transformation was observed in the 7YSZ specimens after 32h whereas it took 128h to destabilize the 7YbSZ. Transmission electron microscopy has been used to investigate the microstructural evolution upon annealing and to compare the sequence of phase transformations occurring between samples containing Y and Yb.
10:10 AM		A3-1-6 Microstructural and Mechanical Property Characterization of TBC Bond Coats K.J. Hemker, R.J. Thompson, P. Lillehoj, D. Butler (Johns Hopkins University) Durability of a TBC is directly tied to its ability to resist spallation, and nickel aluminide bond coats play an important role in determining TBC life. Elevated temperature bond coat plasticity and the occurrence of a martensitic phase transformation during thermal cycling in some commercial bond coats have both been shown to play an important role in promoting of out-of-plane displacements of the TGO. Efforts to characterize the microstructural evolution and elevated temperature strength in a variety of platinum modified nickel aluminide bond coats are ongoing. A series of microsample tensile experiments have been used to determining the effect of bond coat chemistry on elevated temperature strength. The results of these experiments will be presented and compared with observations and microprobe measurements of the underlying microstructure. The degree to which bond coat strength can be modified will be discussed. The support of the National Science Foundation (Grant No. DMR9986752) is gratefully acknowledged.}
10:30 AM		A3-1-7 High Temperature and Room Temperature Erosion Testing of Gadolinia Doped EB PVD TBCs R.G. Wellman, R. Steenbakker, J.R. Nicholls (Cranfield University, United Kingdom) Thermal barrier coatings using 8wt% yttria partially stabilised zirconia as the top coat have been used in gas turbine engines for a number of decades and is an accepted technology. In recent years there has been a drive towards increasing the efficiency of engines which has resulted in the drive towards higher turbine entry temperatures, thus pushing the current materials to their limits, hence the need to develop coatings with lower thermal conductivities and greater temperature capabilities. One proposed solution is the addition of RE oxides to lower the thermal conductivity of TBCs. Gadolinia additions have been shown to significantly reduce the thermal conductivity of EB PVD TBCs, but there are concerns on how such additions affect their impact resistance. The aim of this paper is to further the understanding on the effects of dopents on the erosion resistance of EB PVD TBCs by studying the effects of 2wt% gadolinia additions on the RT and HT erosion resistance of as received and aged EB PVD TBCs. It has been reported that gadolinia additions increased the erosion rate of EB PVD TBCs, this is indeed the case for RT erosion, however under HT erosion conditions this is not the case and the doped TBCs have a slightly lower erosion rate than the standard YSZ EB PVD TBCs. This has been attributed to a change in the erosion mechanisms that operate at the different temperatures. This change in mechanism was not expected under the impact conditions used and has been attributed to a change in the column diameter, and how this influences the dynamics of particle impaction. It is proposed that the important factor in determining the predominant erosion mechanism is the ratio between the contact diameter of the impacting particle and the diameter of the columns of the EB PVD coating. This ratio can also be related to the observed reduction in the measured nano hardness of EB PVD TBCs as the load increases and the number of columns interacting with the indenter increases.
10:50 AM		A3-1-8 A Platinum-Enriched γ+γ' Two-Phase Bond Coat on Ni-base Superalloys B.A. Pint (Oak Ridge National Laboratory); Y. Zhang (Tennessee Technological University); J.A. Haynes, L.D. Chitwood, I.G. Wright (Oak Ridge National Laboratory) The constant demand for increased operating temperatures in gas turbine engines has been the driving force for development of more reliable thermal barrier coating (TBC) systems. Recently, it has been recognized that compared to the commercial single-phase β-(Ni,Pt)Al bond coat, a Pt-enriched γ+γ' two-phase coating could offer some potential advantages such as higher creep strength, better compatibility between coating and superalloy substrate, improved metallurgical stability, and reduced manufacturing cost. In this study, the Pt-enriched γ+γ' two-phase coatings were applied to both single-crystal and directionally-solidified Ni-base superalloys. The coating was prepared by electroplating a thin layer of Pt (~7µm) on the superalloy substrate followed by a diffusion treatment in vacuum at 1150°C. Isothermal and cyclic oxidation tests were conducted on the coated specimens in the temperature range of 1100-1150°C. The oxidation performance of the g+g' coating was compared with that of the industrial standard β-(Ni,Pt)Al bond coat. In addition, the effects of alloying elements in the superalloy substrates, particularly the refractory elements, on the coating oxidation behavior were examined.
11:10 AM		A3-1-9 Processing and Mechanical Properties of Ultra-Thick Thermal Barrier Coatings Deposited using the Solution-Precursor Plasma-Spray Method A. Jadhav, N.P. Padture, E.H. Jordan, M. Gell (University of Connecticut) The next generation of high-efficiency, durable gas-turbine engines used for energy production and aircraft propulsion will operate at higher temperatures and in harsher environmental conditions. Ultra-thick ceramic thermal barrier coatings (TBCs) made of 7 wt%Y₂O₃ stabilized ZrO₂ (YSZ) have the potential of mitigating the effects of these operating condition on the base metal alloys. Ultra-thick TBCs deposited using the cost-effective method of air plasma spray (APS) spall spontaneously. Graded interfaces can prevent spontaneous spallation, but they are expensive and suffer from extensive internal oxidation. In this context, we have deposited TBCs of a maximum thickness of 4 mm, for the first time, using the new solution precursor plasma spray (SPPS) method. Since the deposition mechanisms in SPPS are fundamentally different from those in APS, the SPPS TBCs are replete with strain-relieving vertical cracks. We have also determined the thermo-mechanical fatigue performance of the ultra-thick SPPS TBCs. The ultra-thick nature of the SPPS coatings also allows us to characterize the mechanical and thermal properties of the ceramic in some detail. To that end, we have characterized and compared toughness, compression behavior, and thermal conductivity of free-standing SPPS and APS TBCs. These results will be presented and discussed with reference to microstructural effects.
11:30 AM		A3-1-10 Luminescence Sensing of Temperature in Pyrochlore Zirconate Materials for Thermal Barrier Coatings M.M Gentleman, D.R. Clarke (University of California, Santa Barbara); J.I. Eldridge (Glenn Research Center, NASA) A new approach to non-destructive evaluation of thermal barrier coatings as well as monitoring their local temperatures is to utilize the luminescence from rare-earth dopants incorporated within the crystal structure of the coating material. This is quite distinct from the traditional use of extrinsic sensors in which a sensor is physically attached or embedded to a component. In recent work europium has been identified as a particularly promising rare-earth ion for luminescence sensing of both yttria-stabilized zirconia (YSZ) and gadolinium zirconate (GZO) coating materials. However, in order to produce a viable, practical sensor without compromising the long-term integrity of the TBC, the phase stability, luminescence intensity, and luminescence lifetime as a function of temperature must be examined for candidate materials and concentrations. Studies have been carried out on the effect of the concentration of the dopant as well as the total amount of stabilizer on the luminescence of europia-doped materials. Concentrations ranging from 0.1% to 5.0% EuO1.5 in Zr0.93(Y1-yEuy)0.07O1.93 and (Zr1-xEux)0.93Y0.07O1.93-x for yttria stabilized zirconia (YSZ) and (Gd1-xEux)2Zr2O7 for gadolinium zirconate (GZO) of both bulk ceramic and EB-PVD coatings have been made and examined in terms of the conditions listed above. We will present the result of these studies and discuss the choice of optimum concentration of dopant for several excitation frequencies as well the use of those materials for temperature measurements up to 1100°C .
11:50 AM		A3-1-11 DVD Processing of Sm₂Zr₂O₇ with/out Substrate Rotation H. Zhao, H. Wadley (University of Virginia) A new ceramic material, pyrochlore-structure based zirconate Sm₂Zr₂O₇, has been investigated for application for new thermal barrier coatings at high operating temperature. This work describes the fabrication of Sm₂Zr₂O₇ using electron beam directed vapor deposition (EB-DVD) technology with stationary and rotationary substrates. Sm₂Zr₂O₇ has a lower thermal conductivity and low-sintering activity. The microstructure of the coating has been analyzed by SEM, EDS and XRD. The changes of the morphology and texture of the coating have been influenced by the substrate rotation.
12:10 PM		A3-1-12 Microstructure and Properties of Direct Current Magnetron Sputtered NiAl Coatings Containing up to 1 at% Hf addition B. Ning, M. Shamsuzzoha, M. Weaver (The University of Alabama) In this investigation, NiAl coatings containing different Hf addition (up to 1 at%) have been deposited on CMS_X-4 super alloy substrates via DC magnetron sputtering. Microstructural analysis using transmission electron microscopy (TEM) showed that the sputtering parameters and post heat treatment conditions had a great influence on the distribution of Hf in the sputtered NiAl coatings which resulted in different mechanical properties and oxidation performance. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) techniques were used to examine the microstructual and chemical changes of the NiAl-Hf coatings after isothermal and cyclic oxidation.