Papers

ICMCTF2007 Session A3-4: Thermal Barrier Coatings

Thursday, April 26, 2007 1:30 PM in Room Sunrise

Thursday Afternoon

Start	Invited?	Item
1:30 PM		A3-4-1 New Hybrid Processes to Deposit Advanced Gas Phase Thermal Barrier and Bond Coatings for Turbine Applications W. Beele (Sulzer Metco Coatings BV, Netherlands); A. Refke (Sulzer Metco (AG), Switzerland) Two new coating processes, High Speed PVD (HS-PVD) and Low Pressure Plasma Spray–Thin Film (LPPS-TF), are offering process properties that are partially thermal spray and partially PVD-like. They are used to combine gas phase coating synthesis with high deposition rates, respectively with a high degree of chemical alloying freedom. HS-PVD is synthesizing the coating from its metal starting compounds and reactively deposits it to form a stochiometric ceramic oxide coating. The process has been demonstrated for high purity YSZ thermal barriers including the appropriate formation of the TGO on classic as well as HS-PVD processed bond coats. LPPS-TF, on the other hand, is using ceramic powders in custom cut fractions and allows to full evaporation of the materials so that the coating is frozen out like an EB-PVD columnar TBC coating, too. Furthermore, both processes have practical relevance due to there commercial attractiveness: HS-PVD is a low investment PVD type process that is ideally suited for event-type coating needs like in repair stations, while LPPS-TF is taking the deposition rates to 2000µm/h with all economy advancements related to accelerated throughput times. The authors are highlighting the recent achievements regarding the controlled deposition of EB-PVD like, columnar structured TBC´s, the TGO-formation and the gas phase synthesis of MCrAlY and intermetallic bond coats. The complete coating system can be coated over existing film-cooling patterns without facing cooling hole closure problems. The paper is also reporting out on the initial coating system performance tests that have been carried out in a benchmark against the state-of-the-art coating system properties for EB-PVD systems based on alumindes and LPPS-MCrAlY bond coatings.
2:10 PM		A3-4-3 The Deposition of Thermal Barrier Coating Systems onto Gas Turbine Engine Components by Directed Vapor Deposition D. Hass, B. Musynski, B. Slawski, C. Elzey (Vapor Technologies International) Alternative coatings and coating approaches are required to improve the performance of thermal barrier coatings (TBC) desired for use on gas turbine engine components. These coatings can be used to increase the durability of hot-section engine components, to improve their time "on-wing" and to improve the safety and readiness of gas turbine engines. The continual elevation of engine operation temperatures and the need to improve the reliability of these coatings has led to increased interest in TBC systems having better thermo-mechanical durability, reduced top coat thermal conductivity and sintering rate and an increased resistance to high temperature corrosive environments. Directed Vapor Technologies International (DVTI), is currently investigating the use of an advanced electron beam vapor deposition approach, Directed Vapor Deposition (DVD), as a method for applying high quality thermal barrier coatings at high rates onto engine components. The DVD process operates in a novel processing environment that employs a supersonic gas jet to "direct" vapor atoms onto components resulting in highly efficient deposition of complex coating structures and compositions. Here, the use of this approach to deposit advanced TBC compositions and microstructures to achieve a comprehensive thermal barrier coating system that provides improved resistance to spallation and corrosion will be discussed.
2:30 PM		A3-4-4 Zircoat-HP^TM : A New High Purity Segmented YSZ Coating T.A. Taylor, N. Hitchman, A. Feuerstein, A. Bolcavage (Praxair Surface Technologies) A new YSZ coating for thermal barrier applications has been developed using ultra-high purity starting powder. The coating has superior thermal shock resistance and higher tetragonal phase stability than the current nominal purity Zircoat^TM. Both have the vertical crack-segmentation structure developed by Praxair. Several mechanical and thermal property results are discussed.
2:50 PM		A3-4-5 The Effectiveness of Oxidation Barriers for the Application of Lanthanum Zirconate as EB-PVD TBC on Laves-Phase Strengthened NiAl K. Bobzin, E. Lugscheider, R. Nickel, N. Bagcivan (RWTH Aachen University, Germany) A continuous rise over the last decades in the efficiency of land based gas turbines can be observed. A further increase of the efficiency can be realized increasing the temperatures introducing innovative material solutions and sophisticated cooling techniques. An increase of temperatures results in higher thermal loads of the hot section elements of the gas turbine and the combustion chamber. To withstand higher temperatures Laves-phase strengthened intermetallic NiAl alloy in combination with pyrochloric Lanthanum Zirconate is introduced. This research focuses on the thermal cycling behavior of pyrochloric Lanthanum Zirconate as EB-PVD TBC on Laves-phase strengthened NiAl alloy at 1300°C. Since the used substrate material tends to increased oxidation Al₂O₃, TiAlON and CrAlON PVD coatings are used as oxidation barrier. The effectiveness of the different barriers is analyzed. TGO growth during the thermal cycling test is examined by SEM. Phase structures of TBCs are analyzed by XRD-Analysis. EDS is applied to investigate the thermally induced chemical changes at the interfaces TBC/oxidation barrier and oxidation barrier/substrate. Cycling is stopped when a visible spallation of TBCs occurs.
3:30 PM		A3-4-7 Pulsed Electron Beam Treatment of MCrAlY Bondcoats for EB PVD TBC Systems Part 1 of 2: Coating Production A. Weisenburger (Forschungszentrum Karlsruhe GmbH, Germany); G. Rizzi, A. Scrivani (Turbocoating, Italy); G. Mueller (Forschungszentrum Karlsruhe GmbH, Germany); J.R. Nicholls (Cranfield University, United Kingdom) This is the first part of a two part paper discussing the influence of pulsed electron beam (PEB) treatment of thermal sprayed MCrAlY bondcoats on the cyclic life of thermal barrier coatings (TBCs). Part one describes the production of Standard MCrAlY bondcoats via HVOF, VPS and LVPS thermal spray methods. Some of these coatings were then modified in using a pulsed electron beam treatment before all the samples were coated with an EB PVD 7-8wt% yttria partially stabilized zirconina topcoat. Part two describes the results obtained under cyclic oxidation conditions at 1150°C with 1h at temperature and 15min cooling. The MCrAlY bondcoats manufactured with the three thermal spray methods show differences in microstructure, pore evaluation and surface roughness, these factors will be discussed along with details of the PEB treatment method in this paper. After PEB treatment the two phase structure at the micron scale was no longer visible at the bond coat surface. The surface roughness was reduced by the treatment to an RZ below 4µm and the porosity in the modified layer was completely removed. Some YAl-oxides were found at the surface of the PEB treated layer. Their influence on the cyclic oxidation performance is further discussed in part 2 of this study.
3:50 PM		A3-4-8 Pulsed Electron Beam Treatment of MCrAlY Bondcoats for EB PVD TBC Systems Part 2 of 2: Cyclic Oxidation of the Coatings R.G. Wellman (Cranfield University, United Kingdom); G. Rizzi (Turbocoating, Italy); A. Weisenburger (Forschungszentrum Karlsruhe GmbH, Germany); F.H. Tenailleau, J.R. Nicholls (Cranfield University, United Kingdom) This paper discusses the effect of pulsed electron beam (PEB) treatment of thermal sprayed MCrAlY bondcoats on the cyclic life of thermal barrier coatings (TBCs). Standard MCrAlY bondcoats were produced via HVOF, VPS and LVPS thermal spray methods. Some of the HVOF and VPS coatings were then given a PEB treatment before all the samples were coated with an EB PVD 7-8wt% yttria partially stabilized zirconina topcoat. The samples were all tested under cyclic oxidation conditions at 1150°C with 1hr at temperature and 15min cooling, samples were removed after 20% coating spallation and prepared for cross sectional analyses. Cyclic testing revealed that although the PEB treatment had no measurable effect on the VPS sprayed samples the HVOF coatings showed a significant increase in the cyclic life after the PEB treatment. The effect of the PEB treatment on the various samples is discussed as well as its effect on TGO growth morphology. PEB treatment of the HVOF bondcoat was found to reduce the rate of alumina growth and to suppress the formation of oxide pegs resulting in a smoother bondcoat interface.
4:10 PM		A3-4-9 Low Conductivity Plasma Sprayed Thermal Barrier Coating using Hollow PSZ Spheres Correlation Between Thermophysical Properties and Microstructure G. Bertrand, P. Bertrand, P. Roy (LERMPS, France); C. Rio (ONERA, France) Life and thermal properties of plasma sprayed TBC "widely used in gas turbine engines " are closely related to the microstructure of the ceramic top coating. In particular, the thermal behaviour of this coating is strongly linked with the void shapes and networks which are in turn determined by both the spraying conditions and the feedstock material. A specific hollow yttria partially stabilised zirconia powder was produced in a one-step process by spray-drying and an experimental statistical design study was conducted to investigate the influence of spraying variables on structure and properties of resulting plasma sprayed coatings. A limited number of key spraying parameters were selected, concerning the plasma (primary and secondary gas flow rates, arc current), the cooling and the deposition kinetic (spraying distance, spraying angle, traverse speed) and their influence was evaluated. The coatings were characterised with respect to deposition efficiency, roughness, porosity and thermal conductivity. A reduction of 25% of the thermal conductivity was achieved by improving the spray and powder parameters. The porous structure was quantified using image analysis of polished cross sections and a procedure was defined to separate coarse pores from cracks, based on their size and shape. Through statistical calculation, the parameters that have a relevant influence on the coating porous structure were identified, and their relative importance was determined. An attempt was made to identify morphological criteria of the porous network (coarse/crack ratio, cracks total length, cracks orientation) to relate these results with the thermal conductivity.
4:30 PM		A3-4-10 Deposition of Thick Oxide Layers from Solutions in a Low Pressure Plasma Reactor F. Rousseau, S. Awamat, D. Morvan, J. Amouroux (Universite Pierre et Marie Curie, France); R. Mevrel (ONERA, France) The ceramic layer of a thermal barrier coating is usually deposited by plasma spraying or by EBPVD. To explore the potential of new oxide compositions in view of thermal barrier coating applications, these deposition techniques may be difficult to employ. For example, oxide powders may not be available for plasma spraying, or appropriate evaporation conditions for complex oxides may not be easy to determine in EBPVD. A flexible technique has been developed, which enables to deposit a large range of oxide material compositions with minimal investment. This technique is based on the injection of micrometric droplets of solution containing the elements to deposit, in a plasma reactor equipped with a convergent nozzle. Along their trajectory through the plasma and towards an alumina substrate, the elements in the droplets react with oxidant species. Deposition rates of 1 µm per minute can be achieved and the temperature of gases can be as low as 360K. Examples of thick porous oxide layers of yttria-stabilized zirconia and of a high temperature complex perovskite will be given, with microstructural and chemical characterization (SEM, EDS, XRD, IRTF).
4:50 PM		A3-4-11 Thermal Stability of Solution Precursor Plasma Spray and Air Plasma Spray Thermal Barrier Coatings M. Gell, E.H. Jordan (University of Connecticut); E. Cao (Gillette Corporation); D. Chen (University of Connecticut) The thermal stability of thermal barrier coatings (TBCs) has a strong influence on the thermal conductivity and the spallation life of TBCs during engine service. In this study, the thermal stability of air plasma spray (APS) and solution precursor plasma spray(SPPS)yttria stabilized zirconia coatings are evaluated at temperatures between 1200 to 1400°C. The size and distribution of porosity and the hardness for both TBCs are characterized in the as-coated and thermally exposed conditions. SPPS TBCs exhibit finer porosity, much of it on the nanometer scale, that is more resistant to coarsening than APS TBCs. In addition, SPPS TBCs are more resistant to phase destabilization and exhibit no monoclinic phase after 1400oC exposure.
5:10 PM		A3-4-12 Improving the Erosion Resistance of Plasma-Sprayed Zirconia Thermal Barrier Coatings by Laser Glazing P.-C. Tsai, J.-H. Lee (National Formosa University, Taiwan); C.-L. Chang (Mingdao University, Taiwan) In this study, the substrates of 100mm×25mm×2mm SUS 420 stainless steel coupons were first sprayed with a Ni-22Cr-10Al-1Y bond coat and then with a 19.5 wt. % yttria-stabilized zirconia (YSZ) top coat. After that, the plasma-sprayed yttria-stabilized zirconia thermal barrier coatings (TBCs) were glazed using a pulsed CO₂ laser. The effects of laser glazing on the microstructure and erosion behavior of these coatings have been evaluated. The erosion tests were conducted at room temperature using 50 μm silica erodent particles with impact velocity of 50 m sec-1. The microstructures of both the as-processed and the tested TBCs were investigated by scanning electron microscopy (SEM). The phase of the coatings was measured with X-ray diffractometry (XRD). The results of this investigation showed that the laser glazing process increased the microhardness from about 550Hv for the as-sprayed layer to about 1550Hv for the as-glazed layer. The erosion rate increased with impingement angle for both plasma-sprayed and laser glazed TBCs. Laser glazing enhanced the erosion resistance of plasma-sprayed TBCs by about 1.5 to 3 times with the impingement angle between 30^O-75^O, while the erosion resistance did not significantly improve at a 90^O impingement angle. The erosion morphology analysis shows that the erosion of the plasma-sprayed TBCs was deemed to be the erosion of the protrusions and the sprayed splats. The erosion of the laser-glazed TBCs was proved to be the spallation of the glazed layer. The spallation was occurred in the laser-glazed layer/plasma-sprayed splats interface.