ICMCTF2007 Session A3-2: Thermal Barrier Coatings

Wednesday, April 25, 2007 1:30 PM in Room Sunrise

Wednesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	A3-2-1 Damage Mechanisms, Life Prediction, and Development of TBCs for Turbine Airfoil T. Strangman, D. Raybould, A. Jameel, W. Baker (Honeywell Corporation) TBCs must successfully resist damage from a variety of environmental and mechanical mechanisms to be viable on turbine airfoils. This paper reviews engine-operative TBC damage mechanisms and the requirements for life methods that enable a designer and engine operator to achieve acceptable TBCed component lives. Understanding TBC damage mechanisms facilitates development of advanced TBCs. The following damage mechanisms will be discussed: (i) Growth and cracking of the bond coating’s thermally grown oxide and its interfaces, (ii) Molten dust deposit (Calcia-Magnesia-Alumina-Silicate) wicking into the TBC, (iii) Sulfate salt deposit wicking into the TBC, (iv) Particle impact, (v) Bond coating creep.
2:10 PM	Invited	A3-2-3 Cracking and Delamination Behavior in EB-PVD Thermal Barrier Coating Under Thermo-Mechanical Fatigue Condition Y. Kagawa (University of Tokyo, Japan) Understanding of TBC degradation under service condition is important subject of researches. Thermo-mechanical fatigue tests on EB-PVD Y₂O₃-ZrO₂ thermal barrier coating system have been carried out to examine macro- and micro-damage evolution behaviors. A plate-shaped coating system was subjected to in-phase thermo-mechanical test in ambient air under temperature gradient condition: surface temperature 1150°C and substrate temperature 1000°C, applied stress level was from 20 to 140 MPa. Transverse segmentation cracking of TBC layer and creep behavior of substrate were major macroscopic damages, these damages are related to creep and fatigue damage behaviors of TGO layer, bond coat layer and substrate, which strongly depend on applied stress levels. TGO stress measurement was carried out using piezospectroscopy and the stress was compared with TGO growth behavior and segmentation cracking of TBC layer. Damage model was developed based on experimental observations and discussions were made on the difference from isothermal fatigue tests and isothermal creep tests.
3:10 PM		A3-2-7 Design and Manufacturing of a Novel Bondcoat System for TBC, Oxidation and Thermal Cycling Behaviour M. Carlin (Cranfield University, United Kingdom); F. Bourlier (Snecma, France); J.R. Nicholls (Cranfield University, United Kingdom) This paper discusses a novel manufacturing route to produce overlay α-NiPt₂Al coatings on single crystal superalloy AM1, by the sputtering of the three pure metals as a multilayer coating. The metal layers are annealed under argon to form the resulting intermetallic. Typical systems have over 100 layers for a total thickness of only 5µm. These bondcoats are very thin due to the high platinum content of the NiPt₂Al compound compared to typical β-(Ni,Pt)Al. The control of the manufacturing process and the range of deposition parameters will be discussed. The isothermal oxidation behaviour of these coatings at 1100°C in air is detailed, with oxide scale analysis and microstructure evolution of the coating/superalloy system (XRD, SEM, TEM and FIB picturing) after 1h and 50h exposure. Complete TBCs with these bondcoats and standard EB-PVD deposited YSZ topcoat were successfully manufactured. Thermal cycling behaviour at 1100°C in air of these coatings (novel bondcoat + ceramic topcoat) is described. The relationship between parameters such as layer periodicity, surface finish and aluminium content with thermal cycling lifetime and behaviour is discussed.
3:30 PM		A3-2-8 Effect of Composition and Pre-Treatment Procedures on Oxidation of Vacuum Plasma Sprayed MCrAlY Bondcoats for Thermal Barrier Coatings M. Subanovic, E. Wessel, L. Niewolak, D. Naumenko, L. Singheiser, W.J. Quadakkers (Forschungszentrum Jülich GmbH, Germany) The oxidation behaviour of vacuum plasma sprayed MCrAlY overlay coatings and bondcoats for TBC-applications crucially depends on the coating composition and manufacturing parameters, such as surface and vacuum pre-treatment procedures. The latter include e.g. the time, temperature and vacuum quality during the heat-treatment as well as surface finishing processing prior to EB-PVD coating deposition. In the present work microstructures of the thermally grown oxide (TGO) scales on different MCrAlY bondcoats were studied. For this purpose coating microstructures in the near surface region were evaluated after various heat-treatment steps on free-standing coatings and bondcoats on superalloy substrates. Detailed characterization was preformed using secondary neutrals mass spectrometry, SEM with cathodoluminescence and laser Raman/fluorescence spectroscopy. These coatings microstructures after heat-treatment were correlated with the TGO growth and adherence during subsequent oxidation of free-standing coatings and bondcoats in EB-PVD-TBC systems. The results are discussed in terms of mechanisms or reactive element (RE) incorporation and RE-reservoir depletion at different stages of the high temperature oxidation tests.
3:50 PM		A3-2-9 Cyclic Thermogravimetry of TBC System A. Vande-Put, D. Oquab, D. Monceau (University of Toulouse - CIRIMAT Laboratory, France) The previously developed cyclic thermogravimetry analysis (CTGA) [Monceau D, Poquillon D. Oxid Met 2004;61:143,163] method is applied to the cyclic oxidation at 1100 C of a ZrO₂-Y₂O₃/NiPtAl/single-crystal nickel-based superalloy AM3. With such a TBC system, cyclic thermogravimetry with fast heating and cooling and high accuracy in mass measurement, should allow to follow both the kinetics of oxidation of the bondcoat (thermally grown oxide TGO) and also to detect the occurrence of the zirconia topcoat cracking and partial spalling. The resulting data can be used for time of life modeling of TBC systems. Potentialities and limits of the technique are demonstrated in this work.
4:10 PM		A3-2-10 A High Temperature Instrumented Microindentation Probe to Investigate the Mechanical Behaviour of (Ni,Pt)Al Alloys B. Passilly, A. Villemiane, P. Kanoute, R. Mevrel (ONERA, France) The integrity of thermal barrier coating systems is strongly influenced by the mechanical properties of the different layers constituting them, in particular the metallic bondcoat (NiAl-based or MCrAlY alloy). At high temperature, interdiffusion and oxidation phenomena cause changes in the composition of the bondcoats and as a result their mechanical behaviour is likely to change also. To determine the mechanical behaviour of such alloys in particular, we have designed and developed a high temperature instrumented microindenter capable of functioning up to 1000°C. Several technological solutions have been implemented to control the position of indents (within a range of a few micrometers) and the loads applied (max. 1N), to ensure the thermal homogeneity within the testing zone, and to limit oxidation effects. In order to exploit the information derived from high temperature microindentation, a simulation of this test, based on FEM calculations, has been developed to solve the inverse problem. Examples of mechanical behaviour of various Ni(Pt)Al alloys thus determined will be presented.
4:30 PM		A3-2-11 Compatibility of Mixed Zone Constituents (YAG, YAP,YCrO₃) with a Chromia-Enriched TGO Phase During the Late Stage of TBC Lifetime W. Braue, P. Mechnich, K. Fritscher (German Aerospace Center (DLR), Germany) In EB-PVD 7YSZ TBCs employing a NiCoCrAlY bond coat, the thermally grown oxide (TGO) scale comprises a columnar alumina zone followed by a fine-grained, alumina-zirconia mixed zone adjacent to the TBC top coat. The defect distribution in the TGO scale and along the mixed zone/TBC interface as well as the stability of mixed zone microstructures are considered critical features which may enhance spallation. This paper shows that the mixed zone is characterized by a triple-stage growth pattern over TBC lifetime: (i) during processing it adds thickness through outward aluminum diffusion, (ii) remains constant during thickening of the columnar alumina zone via oxygen inward diffusion, and (iii) again resumes growth at the end of lifetime due to outward growth of a (Al, Cr)₂O₃ mixed oxide phase and dissolving of the bottom Y-PSZ layer along the mixed zone/TBC reaction front. Chromium outward diffusion into the TGO is a late-stage event in TBC life which is activated upon progressive Al-depletion of the bond coat. Typically nanoscale Y-aluminate and zirconia microcrystals bound to (Al, Cr)₂O₃ grains and/or grain boundaries are left behind the mixed zone/ TBC reaction front. For interpretation of this microstructural evolution and possible zirconia destabilization mechanisms in the bottom TBC layer, the compatibility of the other mixed zone constituents YAG, YAP and YCrO₃ with the chromia-enriched TGO phase is of interest. Therefore, the 1100°C isothermal section of the ternary system alumina-chromia-yttria was synthesized from compacted powder mixtures which have been processed via co-decomposition of nitric salt solutions followed by annealing at 1100°C/ 50 hours. Phase relationships derived are put into perspective with the prevalent literature and compared with TEM data from thermally cycled late stage TBC specimens.
4:50 PM		A3-2-12 Oxidation Behaviour of Gamma Titanium Aluminides with EB-PVD Thermal Barrier Coatings Exposed to Air at 900^oC R. Braun, M. Fröhlich, W. Braue (German Aerospace Center (DLR), Germany); C. Leyens (Technical University of Brandenburg at Cottbus, Germany) The oxidation behaviour of two γ-TiAl alloys (02G and 98G received from UES, Dayton) coated with thermal barrier coatings (TBCs) was investigated under isothermal oxidation conditions at 900^oC for up to 1000 h. Samples bare or with a 4 µm thick Ti-Al-Cr protective layer were coated with yttria-stabilized zirconia using electron-beam physical vapour deposition. Prior to TBC deposition, the specimens were pre-oxidized in air or oxygen at low pressure. Mass change of the specimens thermally exposed to laboratory air was measured once every week. Post-oxidation analysis of the coating systems was performed using SEM with EDX detector. Furthermore, the interface between TBC and thermally grown oxide scale was investigated by transmission electron microscopy. Thermal barrier coatings on samples of alloy 98G did not spall off during the maximum exposure time of 1000 h, whereas failure occurred with alloy 02G specimens pre-oxidized in air and oxygen. Microstructural examinations revealed an outer oxide scale with a columnar structure beneath the TBC. Below this columnar oxide layer, a broad porous zone consisting predominantly of titania was observed. At the transition region between oxide scale and substrate, a discontinuous nitride layer was found. Spallation of the TBC was associated with cracking in the porous titania-rich layer. The zirconia top coat layer exhibited an excellent adherence on the oxide scale. Pre-exposure in oxygen at low pressure did not result in an improvement of the oxidation resistance. Post-oxidation analysis of specimens with Ti-Al-Cr layer revealed that the protective coating was nearly completely oxidized. The degradation of the Ti-Al-Cr coating was associated with dissolution of the Laves phase due to chromium depletion and transformation of the γ phase into the α₂-Ti₃Al phase. On the oxidized Ti-Al-Cr layer, an outer oxide scale with columnar structure formed again beneath the TBC.
5:10 PM		A3-2-13 Measuring and Modeling the Bond Coat Aluminum Depletion Kinetics D. Renusch, M. Schuetze (Karl-Winnacker-Institut der DECHEMA e.V., Germany) Bond coat oxidation as well as bond coat depletion of Al are still believed to be a major degradation mechanism with respect to the lifetime of thermal barrier coating (TBC) systems. In this study the top coat lifetime is described as being limited by both bond coat depletion of Al and mechanical failure of the top coat. The empirical results are introduced by considering three spallation cases, namely, Al depletion failure, thermal fatigue failure, and thermal ageing failure. Aluminum depletion failure occurs when the Al content within the bond coat reaches a critical value. In this paper bond coat depletion of Al is modeled by considering the diffusion of Al into both the thermally grown oxide (TGO) and substrate. The diffusion model results are compared to Al concentration profiles measured with an electron beam micro probe. These measured results are from oxidized air plasma sprayed thermal barrier coating systems (APS-TBC) with vacuum plasma sprayed (VPS) bond coats. The exposures temperatures are 950, 1000, 1050 and 1100°C. The current study is scheduled for exposures time up to 10,000hr.