ICMCTF2015 Session A2-1: Thermal and Environmental Barrier Coatings

Thursday, April 23, 2015 8:00 AM in Room Royal Palm 4-6
Thursday Morning

Time Period ThM Sessions | Abstract Timeline | Topic A Sessions | Time Periods | Topics | ICMCTF2015 Schedule

Start Invited? Item
8:00 AM A2-1-1 Elucidating the Mechanism of Bond Coat Cavitation under CMAS-infiltrated TBCs through Modeling and Experimentation
Kaylan Wessels, R.Wesley Jackson (University of California, Santa Barbara, USA); Douglas Konitzer (GE Aviation, USA); Matthew Begley, Tresa Pollock, Carlos Levi (University of California, Santa Barbara, USA)

Thermal barrier coatings (TBCs) are essential in advanced gas turbines that require higher operating temperatures for increased efficiency and performance. As operating temperatures rise TBCs are vulnerable to degradation by ingested siliceous contaminants; the deposits generally referred to as CMAS (calcium-magnesium-alumino-silicates) melt during engine operation and infiltrate the porous ceramic top coat, compromising its strain tolerance. The ensuing stiffening can lead to delaminations through the top coat and at the TBC/TGO interface. A recently identified mechanism that involves bond coat cavitation can also lead to spallation of the TBC with the delamination occurring entirely within the bond coat. While bond coat void formation is not a new phenomenon, the correlation between CMAS infiltration of the TBC and cavitation has not been studied previously. Examination of specimens displaying signs of this damage suggests that creep deformation of the bond coat material motivated by stresses associated with the stiffening of the TBC can lead to void nucleation, growth, and coalescence. Numerical modeling, complemented by laser gradient experiments, is used to understand the mechanism behind the degradation and the evolution of the stresses during thermal cycling and high temperature dwells.

8:20 AM Invited A2-1-2 Pt Effect on MCrAlY Coatings for TBC System Applications
Aurélie Rouaix-Vande Put, Marie-Christine Lafont, Lydia Laffont (Université de Toulouse, Institut Carnot CIRIMAT, France); Eve Péré (Université de Pau et des Pays de l’Adour, IPREM, France); Djar Oquab (Université de Toulouse, Institut Carnot CIRIMAT, France); Aymeric Raffaitin (Turbomeca, France); Daniel Monceau (Université de Toulouse, Institut Carnot CIRIMAT, France)

Because of their high Cr- and Al-contents and the presence of the reactive element Y, MCrAlY coatings exhibit a great resistance to high temperature corrosion and oxidation. However, they are generally not perfect alumino-formers. This constitutes a key issue for thermal barrier coating system applications for which Pt-modified β-NiAl or Pt-rich γ-Ni+γ’-Ni3Al bond-coatings are often preferred. In the aim of overperforming these laters, NiCoCrAlYTa coatings were modified by a Pt overlayer and the effect of such Pt addition was investigated. This included the study of the influence of Pt on the coating microstructure, the impact on the oxidation resistance of the bond-coating as well as on the lifetime of thermal barrier coating systems. In addition, the influence of the manufacturing (NiCoCrAlYTa and Pt deposition processes, surface preparations) was analyzed after thermal cycling tests and degradation mechanisms were proposed. Due to an extensive Al diffusion toward the external Pt-rich zone during the bond-coating heat treatment, enrichment in Al occurred in the sub-surface and martensite formed. This favored Al selective oxidation, increased the oxide scale adherence, lead to a decrease in the oxidation rate and finally resulted in longer thermal barrier coating system lifetimes. Thermal cycling tests highlighted the relevance of manufacturing dense, well interdiffused and oxide-free NiCoCrAlYTa coatings, as well as suitably smoothing NiCoCrAlYTa surfaces before and after the modification by Pt.

9:00 AM A2-1-4 Understanding the Presence of CASO4 Within CMAS and its Effect on the Infiltration Behaviour in EB-PVD 7YSZ
Ravisankar Naraparaju, Uwe Schulz, Peter Mechnich, MondragonGuillermo Rodriguez (DLR Institute of Materials Research, Germany)

Infiltration of CaO-MgO-Al2O3-SiO2 (CMAS) in TBCs is an issue of concern for the aeronautical industry. From the existing research it is known that as soon as CMAS melts it infiltrates the TBC structure, chemically attacks the TBC, changes its phase composition and ultimately leads to the spallation of the TBC. Few investigations have shown the presence of anhydrite (CaSO4) within CMAS deposits. These investigations could not provide a complete understanding of the presence and the effect of CaSO4 on CMAS induced TBCs damage in this study. A systematic approach in understanding the effect of CaSO4 in CMAS has been taken. Five different CMAS compositions with and without CaSO4 were synthesized in the laboratory and their infiltration behaviours were investigated by depositing them on EB-PVD 7YSZ samples and subsequent heating at 1225 and1250°C. In addition, mass spectroscopy was applied on CaSO4 containing CMAS and the vaporization behaviour of sulphur was studied at high temperature. XRD was applied on the molten CMAS compositions and differences in phase formation due to the presence of CaSO4 were analysed. Based on this information, it is proved that sulphur which was present in the form of anhydrite evaporates as SO3 during the high temperature heating and extra CaO adds to the CMAS composition. This CaO-enriched CMAS is found to be more destructive than the initial CMAS. Hence it can be emphasised that CaSO4 presence in CMAS has no direct effect on the infiltration, but rather an indirect effect in changing the CMAS composition.

9:20 AM A2-1-5 Effect of Yttria Content on the Spallation Resistance of Plasma Sprayed YSZ in the Presence of Volcanic Ash
Catalina Taltavull, William Clyne (Cambridge University, UK)

This study concerns the effect of volcanic ash content on the microstructure, stiffness and spallation lifetime of plasma sprayed YSZ coatings. These were produced using two different YSZ powders, containing 8 mol% and 38 mol% of Y2O3, with the latter formulation being claimed to produce coatings more resistant to CMAS-induced degradation. Following previous work on the ingestion [1] of Volcanic Ash (VA) into aeroengines, and on their tendency to promote [2, 3] spallation of Thermal Barrier Coatings (TBCs), two types of Icelandic VA, from Laki and Hekla volcanoes, were deposited onto plasma sprayed YSZ, at different dosage levels. Microstructural interactions between the YSZ and the penetrating VA were studied using XRD and EDX element mapping, after heat treatment at 1500˚C for periods up to 60 h. Free-standing coatings were used to measure the Young modulus and coatings attached to alumina substrates were employed to measure spallation lifetimes (in a periodic quenching furnace). Penetration of the VA into YSZ samples accelerates the sintering and associated stiffening of these coatings, and hence promotes spallation by raising the strain energy release rate during quenching. It is clear from microstructural examination that this is the main mechanism responsible for the reduced spallation lifetime, rather than the VA reaching the interface and impairing its toughness. It was found that the yttria level did have an effect on the microstructural changes, and on the associated sintering effects, and the details of this are explained.

References

[1] Shinozaki, M. Roberts, K.A., van de Goor, B and T.W. Clyne, Deposition of ingested volcanic ash on surfaces in the turbine of a small jet engine. Adv. Eng. Mats., 2013. 15: p. 986-994.

[2] Shinozaki, M. and T.W. Clyne, A methodology, based on sintering-induced stiffening, for prediction of the spallation lifetime of plasma sprayed coatings. Acta Mater., 2013. 61: p.579-588.

[3] Shinozaki, M. and T.W. Clyne, The effect of vermiculite on the degradation and spallation of plasma sprayed thermal barrier coatings. Surf. & Coating Techn., 2013. 216: p.172-177.

9:40 AM A2-1-6 Calcium-Magnesium-Aluminosilicate (CMAS) Interactions with Ytterbium-Silicate Environmental Barrier Coatings
Fabian Stolzenburg (Northwestern University, USA); Kang Lee (Rolls Royce, USA); Katherine Faber (Northwestern University, USA)

Although silicon-based ceramics (i.e., SiC, Si3N4) have been identified as some of the most promising materials systems for high-temperature structural applications in engine environments, a passivating SiO2 surface layer, which forms on SiC and Si3N4 in oxygen-containing environments, becomes unstable in combustion environments, resulting in the volatilization of the silica layer and component recession. Environmental barrier coatings (EBCs) have been identified as protection from these harsh turbine environments. In this study, the interaction of two candidate EBC topcoats, Yb2Si2O7 and Yb2SiO5, with CMAS was investigated in detail using quantitative laboratory-based X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and selected area electron diffraction. Of particular interest was the formation of a new phase, an ytterbium-silicate oxyapatite (Ca2Yb8(SiO4)6O2) in both ytterbium silicates.

10:00 AM Invited A2-1-7 Analysis of Ex-service Parts for the Development of TBC Lifetime Prediction Models
Gregoire Witz, Daniel Renusch, Markus Schaudinn, Bettina Bordenet, Hans-Peter Bossmann (Alstom ltd., Switzerland)
Lifetime prediction of thermal barrier coatings (TBCs) is usually performed based on laboratory tests. Due to the multilayer structure of the TBC, standard fatigue tests are not well suited and some specific tests are used: like furnace cyclic tests, burner rig tests or thermoshock tests. While furnace cyclic tests can bring a good understanding of the behavior of the bondcoat TBC interface, they do not allow studying the effect of the thermal gradient on the coating lifetime. Burner rig tests allow to apply a thermal gradient in the system, but are usually carried out with very harsh condition such to bring a coating failure within a reasonable time. This leads to results that bring some qualitative information, but cannot be used to derive quantitative lifetime prediction models. Additionally, some models have been derived based on the coating field experience, but due to the limited information of the coating load during engine operation, it is difficult to generalize them. In Alstom, we have developed a TBC temperature mapping tool, which allows to gain some understanding of the thermal history of the coating during engine operation. Combining it with other tests performed on ex-service parts, some insight on the TBC degradation mechanisms can be gained, leading to the possibility to develop new TBC lifetime prediction models. Some examples will be provided showing how these methodologies have been used in Alstom. This ensures that the lifetime prediction models cover the various degradation mechanisms occurring in the gas turbine, allowing to design parts with a minimized risk of TBC spallation.
10:40 AM A2-1-9 Fracture Behavior and Lifetime Performance of Thermal Barrier Coatings in Thermally Graded Mechanical Fatigue Environments
Zhe Lu, Sang-Won Myoung, Yeon-Gil Jung (Changwon National University, Republic of Korea)
The effects of bond coating species on the fracture behavior and lifetime performance of thermal barrier coating (TBC) systems were investigated through thermally graded mechanical fatigue (TGMF) tests. Two types of process, air plasma spray (APS) and high-velocity oxy-fuel (HVOF), were employed to prepare the bond coats of about 300 μm thickness, and then the top coat of about 600 μm thickness was coated on the bond coats by APS process. The TGMF tests with two tensile loads of 100 and 150 MPa were performed at a surface temperature of 850 and 1100 °C for a dwell time of 10 min, till 900 cycles. When the tensile load of 100 MPa was applied at 850°C in TGMF tests, the TBC with APS bond coat showed delamination phenomena at the interface between the top and bond coats and small cracks on the surface after about 250 cycles, while the TBC with HVOF bond coat showed a long crack at the interface without delamination phenomena after 900 cycles. As the tensile load in TGMF tests was increased to 150 MPa at 850°C, delamination and/or cracks were created at about 1 30 and 280 cycles for the TBCs of the APS and HVOF bond coats, respectively. When the surface temperature was increased to 1100°C at the tensile load of 100 MPa, the TBCs of the APS and HVOF bond coats showed delamination and/or cracks at about 50 and 120 cycles, respectively. These evidences indicate that the TBC with HVOF bond coat is more efficient in improving lifetime performance than that with APS bond coat, when the thermal and mechanical stresses are applied simultaneously, indicating that the thermal stress gives a more severe effect on the thermal durability of TBC that the mechanical stress.
11:00 AM A2-1-10 Analysis of Interfacial Crack Growth in Pre-cracked Ceramic Coating Systems under Biaxial Loading
Hélène Sapardanis, Vincent Maurel, Alain Köster, Vincent Guipont, Steve Duvinage (Ecole des Mines, France)

Ceramic coatings processed by thermal spraying are involved in many industrial applications such as thermal barrier coatings (TBCs), electrical insulating and anti-corrosive systems. Due to the multilayered structure of TBCs, the mismatch of coefficients of thermal expansion (CTE) between each layer generates an equi-biaxial compressive stress state during thermal cycling. Usually, service loading conditions result in both thermal and mechanical loadings which conducts to the ceramic layer failure. The equi-biaxial stress state due to thermal cycling could be modified by mechanical loading varying the biaxiality stress ratio underwent by the TBC. Hence, to clarify the mechanisms of failure, the influence of bi-axial loading on crack growth is investigated. To that end, pre-cracked ceramic coatings were prepared by applying a laser shock debonding process: LASAT technique (Laser Shock Adhesion Test). The advantage of this contactless method is to easily control the size of a crack at the metal/ceramic interface. For this study, a polycristalline cobalt base alloy substrate and an alumina coating deposited by Air Plasma Spraying (APS) are considered. A coplanar biaxial fatigue machine is employed to prescribe various biaxiality stress ratios to a cruciform specimen. Top surface observation enables crack growth measurement under mechanical loading. In order to discuss the influence of biaxial loading, both uniaxial and biaxial tensile/compression tests results are compared. Finally, analysis of observed interfacial cracking enables to discuss the relevance of existing FE models.

Keywords: ceramic coating, biaxial loading, crack growth, interface

11:20 AM A2-1-11 Stability of Rare Earth Silicates in High-Temperature High-Velocity Water Vapor
Elizabeth Opila, Robert Golden, Cory Parker (University of Virginia, USA)
Both dense and air-plasma sprayed yttrium- and ytterbium-mono- and di-silicates were exposed in a steam-jet furnace to evaluate the thermochemical stability of these EBC candidate materials in a simulated turbine engine environment. The steam-jet environment is controlled at 1 atm water vapor pressure, temperatures between 1200 and 1400°C, and gas velocities on the order of 170 m/sec. In the steam-jet furnace, a one millimeter diameter area of the test specimen surface experiences the high velocity condition. The rare earth disilicates show surface depletion of silica to form rare earth monosilicates. Key microstructural changes include porosity development, grain refinement, and grain fallout near the steam jet impingement site. Downstream of the steam impingement, non-uniform faceting is observed, most likely due to silica depletion that varies as a function of rare earth disilicate crystallographic orientation. Cross-sectional microstructural characterization reveals that the silica depletion depth of rare earth disilicates varies, likely due to surface defects, crystallographic orientation effects, and grain fallout. The time, temperature, and initial porosity dependence of rare earth disilicate depletion will be described. Strategies for thermochemical life prediction of rare earth disilicate EBCs based on these results will be discussed. Finally, early results for rare earth monosilicate stability in the steam-jet furnace environment will also be presented.
11:40 AM A2-1-12 Hybrid EBC/TBC Coatings for Si-Based Ceramics in Corrosive Environments
Satish Dixit (Plasma Technology Inc., USA); Soumendra Basu, JiaPeng Xu, Vinod Sarin (Boston University, USA)

SiC/SiC ceramic matrix composites (CMCs) are being used increasingly in the hot-sections of gas turbines, especially for aerospace applications. These CMCs are subject to recession of their surface if exposed to a flow of high-velocity water vapor, and to hot-corrosion when exposed to molten alkali salts. This research involves developing a hybrid system containing an environmental barrier coating (EBC) for protection of the CMC from chemical attack and a thermal barrier coating (TBC) that allows a steep temperature gradient across it to lower the temperature of the CMC for increased lifetimes. The EBC coating is a functionally graded mullite (3Al2O3*2SiO2) deposited by chemical vapor deposition (CVD), the TBC layer is yttria-stabilized zirconia (YSZ) deposited by air plasma spray (APS). The stability of this system is investigated, which includes the adhesion between the two coating layers and the substrate, the physical and chemical stability of each layer at high temperature, and the performance under severe thermal shock and during exposure to hot corrosion. The effect of vertical cracks in the TBC on the EBC layer below it is also examined.

12:00 PM A2-1-13 Fracture Mechanics Based Lifetime Assessment of Bi-Layer Thermal Barrier Coatings
Mario Rudolphi, Mathias Galetz, Michael Schütze (DECHEMA-Forschungsinstitut, Germany); Martin Frommherz, Alfred Scholz, Matthias Oechsner (IfW, Technische Universität Darmstadt, Germany); Emine Bakan, Robert Vaßen (Research Centre Jülich, Germany); Werner Stamm (Siemens Energy, Germany)

Bi-Layer thermal barrier systems made of a bottom layer of standard yttria stabilized zirconia (YSZ) and a top layer of gadolinium zirconate (GZO) offer the potential to increase the operating temperature of gas turbines or aero engines beyond the temperature limit of single layer YSZ systems. This is due to the exceeding phase stability of gadolinium zirconate at temperatures above 1200°C compared with yttria stabilized zirconia.

In this work, the mechanical stability of such novel thermal barrier coating systems prepared by atmospheric plasma spraying (APS) was investigated after isothermal oxidation at 1050°C. 4-point bend testing with in-situ acoustic emission measurement served for determining the critical strain to failure of the coating, while the use of the acoustic emission technique enabled distinction of individual failure modes in the bi-layer system, such as, shear failure of the top GZO-layer, delamination along the GZO/YSZ interface, or shear failure of the bottom YSZ-layer.

The measured critical strain values in combination with careful metallographic inspection were used to establish a fracture mechanics based lifetime model in form of mechanical stability diagrams. With the use of these diagrams, the failure of each individual layer can be assessed with respect to the externally applied load. The developed lifetime model enables a lifetime assessment by comparing the critical strain to failure with component loading cycles.

Key Words: Bi-layer thermal barrier coatings, gadolinium zirconate, 4-point bend testing, acoustic emission.

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