ICMCTF2013 Session A2-1: Thermal and Environmental Barrier Coatings

Tuesday, April 30, 2013 8:00 AM in San Diego

Tuesday Morning

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8:00 AM A2-1-1 Columnar Thermal Barrier Coatings (TBCs) by PS-PVD
Robert Vassen, Georg Mauer, Stefan Rezanka (Forschungszentrum Jülich GmbH, Germany)
During the last years the Plasma Spray – Physical Vapor Deposition Process (PS-PVD) attracted a lot of interest due to its unique features. In this process plasma spraying is performed at reduced chamber pressures of 100 to 200 Pa. In combination with a high torch power a partial or complete vaporization of the used powderous feedstocks can be achieved. Correspondingly, the deposition mechanisms and the resulting coating microstructures differ from conventionally sprayed coatings. Thin and dense ceramic coatings can be obtained as well as columnar structured strain tolerant coatings with low thermal conductivity.

The paper will focus on strain tolerant zirconia based thermal barrier coatings. In contrast to the high roughness of bond coats used for atmospherically plasma sprayed TBCs the bond coating for the PS-PVD process has to be smooth to achieve sufficient bonding. In addition, the growth of the thermally grown oxide (TGO) on the bond coat as well as the growth rate of the TBC itself has to be adopted. For the optimized coatings excellent thermal cycling performance in burner rig facilities could be demonstrated. The coatings showed lifetime more than two times better than conventional APS TBCs. The reason for this excellent behavior will be discussed in detail.

Finally, an outlook for future applications of the flexible and powerful PS-PVD process especially in energy systems will be given.

8:40 AM A2-1-3 PS-PVD - Deposition of Thermal Barrier Coatings
Marek Goral, Sławomir Kotowski, Jan Sieniawski (Rzeszów University of Technology, Poland)
In the article the results of ceramic coating deposition by PS-PVD method will be presented. Plasma Spray Physical Vapour Deposition enables to obtain columnar structure of ceramic coatings (EB-PVD like). The Rene 80 nickel superallowy was used as a base material. The Zr-modified aluminide coating deposited by CVD method as well as MeCrAlY coating deposited by APS method were used as a bond coat. The LPPS-Thin Film system produced by Sulzer Metco on Research and Development Laboratory for Aerospace Materials was used for YSZ coting deposition. The Metco 6700 yttria-stabilized zirconia powder was used as a coating material. The 03CP plasma gun was used in research for ceramic powder evaporation. The power current, plasma gas flow and composition, pressure in process chamber, rotation speed of sample and powder feed rate were changed during experimental processes. The microstructural observation and chemical composition analysis was conducted using scanning electron microscope with EDS analyzer. The XRD phase analysis was conducted as well. The research showed that power current, pressure and powder feed rate had the stron influence on thickness, structure and composition of ceramic layer. The PS-PVD method is a alternative process for APS and EB-PVD technologies.
9:00 AM A2-1-4 Development of Porous TBC Systems with Enhanced Durability using TriplexPro 210 Technology
Mitchell Dorfman, Chris Dambra, Juan Medrano, Dianying Chen, Montia Nestler (Sulzer Metco (US) Inc.)
Over the last few years, there has been a great deal of R&D looking at alternative chemistries to traditional 7-8 wt% Yttria Stabilized Zirconium oxide (YSZ) coating systems. The main reason for this is that engine temperatures have been increasing and coating surface temperatures greater than 1200C for prolonged periods of time will result in: 1) accelerated phase destabilization, 2) increased surface sintering, and 3) cracking and spallation. However, many existing engines will see service temperatures that do not exceed 1200C. For these engines, legacy YSZ materials will still be used for cost reasons. This program has evaluated several different YSZ materials in developing a cost efficient, mechanically durable YSZ system using TriplexPro technology. Key areas studied include the effects of powder properties and spray paramters on coating microstructure and performance. This program also reviews application cost verses performance. Key testing included: 1) burner rig testing, 2) Furnace Cyclic Testing (FCT), 2) thermal Conductivity and 4) Metallographic analysis. Technical results showed that high purity agglomerated and sintered and/or HOSP YSZ powders can be manufactured and sprayed to produce poroous coatings with improved thermal cyclic performance.
9:20 AM A2-1-5 Investigating CeO2, TiO2 Stabilized ZrO2 for Application in Thermal Barrier Coatings (TBCs)
Chandra Macauley (University of California, Santa Barbara); Don Lipkin (General Electric (Global Research Center), US); Carlos Levi (University of California, Santa Barbara, US)

As gas turbine operating temperatures are raised to reap the environmental and economic benefits of increased efficiency, the failure mechanisms that shorten the working life of TBCs are exacerbated and expanded. The current industry-standard TBC composition, yittria stabilized zirconia (YSZ), is inherently limited to reach the prospective goals of technology motivating the search for new compositions with improved capabilities. Zirconia co-doped with CeO2 and TiO2 has shown promise as an alternative ‘next generation’ TBC. Previous work showed a relatively large, non-transformable, single-phase tetragonal field stabilized at 1350°C . Compositions within this field have been manufactured into coatings by air-plasma spray. Phase and microstructural evolution of thermally sprayed freestanding TBCs upon isothermal aging are investigated as a function of time at 1316° C and 1427° C. Coating performance during thermal gradient tests and possible explanations for observed behavior will also be discussed.

9:40 AM A2-1-6 Thermal Barrier Effect of Topcoats from Sintered Micro-sized Hollow Spherical Alumina Particles
Raquel Roussel, Vladislav Kolarik, MariadelMar Juez-Lorenzo, Harald Fietzek (Fraunhofer ICT, Germany)
Micro-sized spherical Al particles in the range of 1 to 20 µm, deposited according to the PARTICOAT concept ( www.particoat.eu ) as slurry by brushing or spraying on the surface of a Ni- or Fe-based alloy, oxidize at high temperatures to a topcoat from sintered hollow alumina spheres while forming an aluminized diffusion zone in the substrate. The topcoat effectuates as a thermal barrier by gas phase insulation and the diffusion zone forms a protective alumina layer. For investigating the thermal barrier effect of the topcoat, an experimental set-up using a radiation heater was designed, which allows to heat the sample from one side while being cooled by airflow on the backside. The temperature is measured by thermocouples on both sides as a function of the time. Free standing samples of topcoats from sintered hollow alumina spheres, produced with Al particles in a size range of 1 to 20 µm, were prepared with various thicknesses. Samples from an industrial APS YSZ thermal barrier coating with the same thickness were used for comparison. Exposing 2 mm thick coatings to the heat radiation, the temperature of 850°C was measured on the exposed side. Without cooling, 300°C were measured on the backside for the hollow alumina particle based topcoat and 350°C for the commercial TBC. On both sides the temperatures remain without change until the end of the experiment after 3 h providing a stable temperature difference of 550°C and 500°C respectively. With airflow cooling, the backside temperature drops to 100°C with an exposed side temperature of 850°C for the hollow alumina particle based coating and to 120°C with an exposed side temperature of 880°C for the commercial YSZ. With a 300 µm thick alumina particle based topcoat a temperature difference of 356°C without and of 550°C with backside cooling was observed. The topcoat from sintered hollow alumina particles achieves a thermal barrier effect comparable to that of commercial YSZ and is capable to protect materials against temperature at low costs. Results from exposure experiments with coated IN738 and Alloy 321 confirm the impact of this novel coating system obtained by a heat treatment in one production step.
10:00 AM A2-1-7 Multilayer Thermal Barrier Coatings: Interplay among coating design, processing and properties
Sanjay Sampath (Stony Brook University); Gopal Dwivedi (Stony Brook University, US); Vaishak Vishwanathan (Stony Brook University); Yikai Chen (Stony Brook University, US)

The continued need for increments in gas turbine operating temperatures has necessitated developments in new thermal barrier materials and their processing. Of particular interest in recent years is the potential for Gadolinium Zirconate as a candidate TBCs to replace yttria stabilized zirconia. Gd2Zr2O7 in particular provide lower conductivity and resistant to environmental damage from ingested sand particles (categorized as CMAS for calcium magnesium alumino-silicate). Zirconates however, have several challenges including low fracture toughness, incompatibility with the thermally grown alumina and low erosion resistance. Multilayer concepts based on combinations of zirconia and zirconate have been developed to address the multifunctional requiremetns. This paper seeks advances in processing science and control for layer-by-layer optimization of coating microstructure and properties so as to meet multifunctional obligations. The paper will demonstrate the advantages of plasma spray for such layered coating system along with identification of critical challenges.

10:40 AM A2-1-9 Influence of Temperature on Phase Stability and Thermal Conductivity of Single- and Double-Ceramic-Layer EB-PVD TBC Top Coats consisting of 7YSZ, Gd2Zr2O7 and La2Zr2O7
Kirsten Bobzin, Nazlim Bagcivan, Tobias Brögelmann, Baycan Yildirim (Surface Engineering Institute - RWTH Aachen University, Germany)

More than 650 million tons per year of CO -emission is generated by air traffic. In order to meet the future demands like reducing the CO -emission and the fuel consumption of gas turbines, the increase of turbine inlet temperature (TET) is necessary. However, state-of-the-art coated superalloy materials might not stand a further increase of TET. Therefore the development of new top coats is necessary.

Yttria stabilized Zirconia (YSZ) is usually used as ceramic top coat for gas turbine parts of the first and the second stage of the gas turbine. Investigations have shown the accelerated phase transformation and the intensified sinter effects of the YSZ top coat at temperatures between 1,200 °C and 1,300 °C, leading to changes of microstructure. Such modifications of the microstructure lead to higher thermal stresses and consequently reduce the lifetime. Furthermore, thermal conductivity λ of the top coat increases. For this reason the pyrochlore zirconates lanthanum zirconate (La2Zr2O7) and gadolinium zirconate (Gd2Zr2O7) as top coat get into focus because of their promising material properties like both the phase stability up to their melting points and the lower thermal conductivity compared to YSZ.

Within this work single- (SCL) and double-ceramic-layer (DCL) top coats consisting of 7 wt. % yttria stabilized zirconia (7YSZ), La2Zr2O7 or Gd2Zr2O7 are deposited by means of Electron Beam-Physical Vapor Deposition (EB-PVD). Aim of this work is on the one hand the investigation of temperature-dependent phase behavior and change of thermal conductivity of SCL and DCL top coats. On the other hand the influence of different top coat materials and architectures on the growth of thermally grown oxide (TGO) between top coat and substrate is of key interest. In a first step morphology and coating thickness are determined using scanning electron microscopy (SEM). The SCL and DCL systems show a columnar microstructure with a coating thickness of about 150 µm. In a second step thermal conductivity of SCL and DCL systems is measured between 800 °C and 1,300 °C by means of laser flash technique. The third step is high-temperature X-ray diffraction measurements of SCL and DCL systems during heating and cooling between 800 °C and 1,300 °C at atmosphere. Finally, the TGO at the interface between top coat system and substrate is analyzed. Thickness of the TGO is determined by means of SEM, composition using chemical analysis and phases using X-ray diffraction. Thus, a correlation between morphology, architecture, coating material and the thickness of the TGO can give information about oxygen diffusion processes.

11:00 AM A2-1-10 Experimental Determination of Mode II Fracture Toughness of TBC's
Binwei Zhang, Simon Lockyer-Bratton, Jaafar ElAwady, Kevin Hemker (Johns Hopkins University, US)
Modern thermal and environmental protection systems have multiple layers and functionalities, and important phenomena governing the life of these systems occur in each layer and especially at the interfaces between the layers. Mechanical characterization of the ceramic topcoat is complicated by its brittle nature. Micro-scale bending experiments provide direct routes for characterizing the elastic response of EBPVD 7YSZ topcoats and for quantifying delamination toughness . A new experimental technique for measuring mode II delamination toughness, the compression edge delamination test, will also be presented, and results for commercial thermal barrier coatings for turbine engines will be presented. The insight gained in these experiments will be interpreted in context of the need for hierarchical models of layered protections systems.
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