ICMCTF2013 Session A2-2: Thermal and Environmental Barrier Coating

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.

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.

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.

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 1150^oC, with cycle lengths of up to 100h.

* corresponding author

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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.

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.