ICMCTF2013 Session A2-2: Thermal and Environmental Barrier Coating

Tuesday, April 30, 2013 2:10 PM in Room San Diego
Tuesday Afternoon

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Start Invited? Item
2:10 PM A2-2-1 Thermal Barrier Coating Lifetimes for High Temperature, Low Density Superalloys
James Nesbitt, Rebecca MacKay, Kayleigh Reamy (NASA Glenn Research Center, US)
Recent studies at the Glenn Research Center have shown that select low density, single-crystal (LDS) superalloys possess an ideal combination of high creep strength and high oxidation resistance with lower density than today’s state of the art superalloys. There is a significant payoff in the use of these LDS alloys since a reduction in the turbine blade weight has a cascading effect throughout the entire rotor (e.g., disk, hub, and shaft) as well as to non-rotating support structures. As with traditional superalloys, taking full advantage of these LDS alloys in the hottest sections of the turbine requires environmental coatings (oxidation and thermal barrier coatings (TBC’s)). The purpose of this work was to compare the TBC lifetime of an LDS alloy with that of the commercial superalloy CMSX-4. The LDS alloy was evaluated both with and without small additions of Hf (0.15 wt.%). A conventional Pt-modified aluminide bondcoat was used with these alloys. The effect of bond coat was also examined by comparing the TBC lifetime for the Pt-modified aluminide bondcoat with a Pt-only bondcoat on the LDS alloy. Conventional electron beam-physical vapor deposited (EB-PVD) top coats of ZrO2-7wt.%Y2O3 were used with all alloys.

The TBC lifetimes were evaluated by testing triplicate samples for one-hour cycles at 1135°C. The small Hf addition to the LDS alloy significantly improved the TBC lifetime. The lifetime of the Pt-only bondcoat was intermediate to that of the LDS alloy with and without Hf but with the Pt-modified aluminide bondcoat. The lifetimes of the Hf-containing LDS alloy were similar to that of the CMSX-4 alloy, both with the same Pt-modified bondcoat. It is suspected that the Hf within the CMSX-4 alloy and in the Hf containing LDS alloy contributed to the high TBC lifetimes. This work showed that LDS alloys, when containing small Hf additions, have TBC lifetimes similar to current commercial superalloys.

2:30 PM A2-2-2 Ultra-Low Thermal Conductivity Yttria Stabilized Zirconia Thermal Barrier Coatings Using the Solution Precursor Plasma Spray Process
Maurice Gell, Eric Jordan, Jeffrey Roth, Chen Jiang (University of Connecticut)

The Solution Precursor Plasma Spray (SPPS) process has the potential of providing more durable and low thermal conductivity thermal barrier coatings (TBCs). The increased durability derives from a highly strain-tolerant microstructure consisting of fine, through-coating-thickness cracks and an increased inter-splat crack resistance associated with ultra-fine splats (<2 microns). Low thermal conductivity SPPS TBCs are associated with unique planar arrays of nano- and micro-porosity that are referred to as inter-pass boundaries (IPBs). Success in this effort will extend the use of YSZ TBCs and minimize the use of rare-earth elements required in most alternate low thermal conductivity TBCs. An extensive series of plasma trials were conducted to produce inter-pass boundaries in the standard, porous SPPS microstructure containing vertical cracks. The key variables influencing the formation of IPBs were determined. A Taguchi Design of Experiments was conducted to optimize the IPB microstructure. The key metric employed throughout this effort was the thermal conductivity of the individual coating, determined initially by using the Object-Oriented-Finite (OOF) Element method and then later, with more accuracy, the Laser Flash Analysis (LFA) method. This presentation will present the results of these trials and show how YSZ thermal conductivity is affected by SPPS processing variables. It will be shown that the thermal conductivity of SPPS YSZ TBCs can be reduced by more than 50% to value of 0.5 watt/meter/oK.

2:50 PM Invited A2-2-3 Observations of Ferroelastic Switching by Raman Spectroscopy
Molly Gentleman (SUNY - Stony Brook, US)

Raman spectroscopy has been shown to be a successful tool in the characterization of ferroelastic switching in zirconia based thermal barrier materials. In this study, Raman spectroscopy is used to examine damage induced by erosion testing on EB-PVD thermal barrier coatings. Mapping of thermal barrier coatings has revealed varying degrees of switching depending on erodent angle and temperature. Other studies have also shown the relationship between composition and ferroelastic parameters.

3:30 PM A2-2-5 Impact of Superalloy Composition and Bond Coat Roughness on Plasma-Sprayed TBCs with HVOF NiCoCrAlX Bond Coatings
James Haynes, Kinga Unocic, Bruce Pint (Oak Ridge National Laboratory, US)

Globally, there is increasing demand for land-based power generation gas turbines that can burn fuels with increasing ranges of impurity levels. Such turbine systems may require different structural materials strategies for the hot section components, as compared to engines designed to burn higher purity fuels. Modifying superalloy compositions to be more resistant to a wider variety of corrosion mechanisms may also impact the performance of standard protective coating systems, particularly over the longer-duration cycles and lifetimes of land-based turbine components. This study investigated the influence of superalloy substrate composition, bond coat surface roughness, cycle length, temperature and water vapor on the furnace thermal cycle lifetime of plasma-sprayed thermal barrier coatings (TBCs). The TBC systems evaluated consisted of air plasma-sprayed (APS), yttria-stabilized zirconia (YSZ) top coatings with high velocity oxy fuel (HVOF)-deposited NiCoCrAlYHfSi bond coatings. Bond coat roughness was increased in selected specimens by overlaying with a larger MCrAlX powder size. The TBCs were deposited on two superalloy compositions, Alloy X4 and Alloy 1483, the latter of which has higher Cr and lower Al content for improved hot corrosion resistance. Furnace cycle experiments were conducted in dry O2 and air with 10% water vapor at 1100°C and at 1150oC, with cycle lengths of up to 100h.

* corresponding author

KEYWORDS: Coating, oxidation, TBC, alumina, bond coat

Notice: The United States Government retains, and by accepting the article for publication, the publisher acknowledges that the United States Government retains, a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States Government purposes

3:50 PM A2-2-6 A New Approach to Protect Thermal Barrier Coatings Against CMAS Corrosion using Sol-gel Process
Guillaume Pujol, Florence Ansart, Jean-Pierre Bonino (CIRIMAT, France); André Malié, Sarah Hamadi (Snecma, SAFRAN Group, France)

Thermal Barrier Coatings (TBCs) are widely used to protect critical metallic parts in hot sections of gas turbine engines. TBCs are designed to improve the durability of alloy components and the engine efficiency by increasing operating temperature.

Currently, the ceramic layer which provides thermal insulation is industrially deposited by either Air Plasma Spraying (APS) or Electron Beam Vapor Deposition (EBPVD) resulting in lamellar or columnar microstructures, respectively.

In working conditions, TBCs are subject to various kinds of degradation (erosion, F.O.D, oxidation…) which deteriorate integrity and mechanical properties of the system. Moreover, with the aim to increase the turbine inlet temperature, a new type of damage has been highlighted: corrosion by molten Calcium-Magnesium-Alumino-Silicates, better known as CMAS. These particles come from siliceous debris (sand, dust, volcanic ashes…) ingested with the intake air, and form deposits on airfoil surfaces. At the operating temperature, the glassy deposit can melt and infiltrate the ceramic TBC. In fact, its wetting characteristics and its low viscosity allow CMAS to penetrate porosity of the ceramic top-coat. This is particularly true for EBPVD coatings where the vertically oriented microstructure eases the infiltration.

Here, we propose to use sol-gel route, to synthesize new coatings to protect TBC systems from CMAS damage. This soft chemical process has already shown a real potential to make high purity nanocrystalline materials with a controlled morphology. Associated with dip-coating or spray-coating technique, this process allows to produce either thin or thick ceramic coatings with a non-oriented microstructure[1] (in opposition to EBPVD or APS coatings). Our aim, in this study, is to use sol-gel process to realize a protective layer deposited on the conventional EBPVD TBC in order to provide CMAS resistant capability and therefore improve their lifetime. This outer layer could be sacrificial, tight or non-wetting. In this work, several routes have been investigated. Various materials known to be good candidates[2] against CMAS attack have been synthesized and their efficiency in correlation with their microstructure and chemical formulation have been studied.

[1] J. Fenech, M. Dalbin, A. Barnabe, J.P. Bonino, F. Ansart, Sol–gel processing and characterization of (RE-Y)-zirconia powders for thermal barrier coatings, Powder Technology. 208 (2011) 480-487.

[2] S. Krämer, J. Yang, C.G. Levi, Infiltration‐Inhibiting Reaction of Gadolinium Zirconate Thermal Barrier Coatings with CMAS Melts, Journal of the American Ceramic Society. 91 (2008) 576-583.

4:10 PM A2-2-7 Interaction of CMAS with MOCVD Coatings in the System Y2O3-Al2O3
NadineKarin Eils, Peter Mechnich, Wolfgang Braue (German Aerospace Center (DLR), Germany)
Calcium-magnesium-alumina-silicate (CMAS) particles deposit on protective ceramic coatings in gas turbine engines. High gas temperatures lead to partial or even complete melting of CMAS deposits and subsequent damage of the coatings due to CMAS infiltration, phase formation and spallation. Therefore the development of new coating materials, showing a high resistance against CMAS degradation, is aspired.

The interaction of a model CMAS powder with MOCVD coatings in the binary system Y2O3-Al2O3 was investigated. Heat treatments at 1200 °C and 1250 °C were carried out on coatings with binary compositions of Y3Al5O12, YAlO3, Y4Al2O9, as well as on pure Y2O3 and Al2O3 coatings, respectively. The reaction behavior of these coatings in contact to CMAS powder was compared to each other. The investigations were focused on phase relationships and CMAS infiltration behavior. X-ray diffraction was used for phase identification; imaging as well as elemental analyses were carried out by means of scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS).

In the reaction zones of each sample, crystallization processes were observed. Formation of anorthite and Mg,Al-spinel were found in aluminum rich coatings, while Ca,Y-oxyapatite, melilite and a new Ca,Y-garnet phase were found in yttrium rich coatings. The pure Y2O3 coating forms a continuous layer of oxyapatite in contact with CMAS and shows the best resistance against CMAS infiltration. Concerning the yttrium aluminates the coating with the highest yttrium content (Y4Al2O9) reveals less CMAS infiltration and degradation in comparison to the binary Y3Al5O12 and YAlO3 coatings.

4:30 PM A2-2-8 Examination of CMAS-induced TBC Failure in Typical Service Conditions
Vladimir Tolpygo (Honeywell Aerospace, US)
A new form of CMAS-induced degradation of thermal barrier coatings is described. The principal feature of this process is chemical reaction of some of CMAS constituents with the TGO that bonds TBC to metallic substrate. Such reaction may cause local delamination along TBC-TGO interface and eventually lead to TBC spallation. This mechanism has been observed to reduce TBC lives well below the limit set by oxidation kinetics at a given service temperature. The reaction between CMAS and TGO occurs simultaneously with other commonly accepted degradation mechanisms, such as liquid-phase infiltration and chemical reactions at the TBC top surface. Several examples of CMAS-TGO interaction during burner rig testing of 7YSZ TBC at 1150oC, as well as CMAS-induced TBC failure on engine airfoils are shown. Possible mechanisms of CMAS penetration through TBC and mitigation methods are discussed.
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