Development of strain tolerant aluminia scasles on MCr-AL-Basis high temperatur alloys and coatings
Final Report Abstract
MCrAlY-type (M=Fe, Ni, Co) alloys and coatings are commonly used in many high-temperature applications where oxidation resistance is required, such as resistance heating-elements, gas-turbine components etc. The high-temperature oxidation resistance of MCrAlY-materials relies upon the formation of slow growing and adherent alumina (Al2O3) scales. For the scale formation Al is consumed from the metal substrate, whereas the Al consumption is accelerated in the case of scale spallation during temperature cycling. Upon critical Al-depletion the alloy and/or coating can no longer re-form the alumina scale. In this case the component corrodes at a much faster rate since the growth rates of Cr, Ni and especially Fe-based oxides are significantly higher than that of Al2O3. Further increase in the operating temperature of high-temperature devices for increased efficiency and reduction of environmental pollution limits the applicability of MCrAlY-materials. This is because the critical Al-depletion in the components due to alumina scale growth and spallation is expected to occur within the required service life. In the critical application of MCrAlY-materials as bondcoats for primereliant ceramic thermal barrier coatings (TBCs) on internally cooled turbine blades the alumina scale spallation is unacceptable since it results in failure of the ceramic top-coat and as a consequence in component failure due to overheating. The scale and TBC spallation has been shown to occur at the increased temperatures, such as 1000°C before the critical Al-consumption, however, at much shorter times than the required service life of the gas-turbine components (25000 h). Therefore, the motivation of the present work was to develop the material composition and/or material pre-treatment procedures, which allow the alumina scale to be "strain-tolerant", i.e. less susceptible to spallation driven by thermal mismatch with the underlying substrate. In the present project the microstructures and growth mechanism of the alumina scales were studied as a function of the alloy composition and annealing/mechanical pre-treatments. The oxide scale microstructure was correlated with its growth rate and resistance to spallation (strain-tolerance). The approaches for development of strain-tolerant scales were derived on single-phase Fe20Cr5AlYbased alloys and the results were later on transferred to multi-phase Ni20Co20Cr10AlY-coatings for gas-turbine applications. It was found that the scale formed on the alloys containing combined minor less than 1% Reactive Element (RE= Y, Zr, Hf) additions Y+Zr, Y+Hf exhibit in general higher resistance to spallation compared to alloys doped only with Y. This could be explained on one side by Zr and Hf gettering the carbon impurity, which otherwise was shown to form brittle Cr-carbide phases at the scale/metal interface thus compromising the scale adherence. More importantly, Zr and Hf minor alloy additions modified the microstructure of the alumina scale by reducing the alumina grain size, forming zirconia (hafnia) precipitates and microporosity. Such scale microstructure appeared to be "strain-tolerant", however, in the case of Zr-addition it also led to a higher scale growth rate. An important finding of the present project was that the effect of Reactive Elements (RE = Y, Zr, Hf etc.) on the microstructure of alumina scales is determined by the RE-reservoir, i.e. not only by their chemical contents but also by other parameters, such as the coating (alloy) thickness and type and amount of impurities. It has been shown that the depletion of the RE-reservoir leads to a change in the oxide scale microstructure and decreases the scale resistance to spallation. Preliminary results suggest that the exhaustion of the RE-reservoirs in the commercially prevailing NiCoCrAlY-coatings will occur within the lifetime requirement of the advanced gas turbine components. On the other hand increase in the RE-reservoir leads in addition to the positive effect on the scale adherence to a negative effect, i.e. an enhanced oxidation rate due to "overdoping" of the scale. The results of the present project indicate that for a given MCrAl-alloy (coating) an optimum REreservoir exists, which can provide maximum possible strain-tolerance of the alumina scale without having the adverse effect on its growth rate. This optimum RE-reservoir should, therefore, lead to the longest alloy (coating) lifetime. Considering the mechanism of RE-incorporation into the alumina scale by an internal oxidation process the optimum RE-reservoir is also expected to depend on the experimental (service) parameters. The latter parameters, such as temperature and composition of the oxidising environment, determine to a large extent the morphology and distribution of the RE-oxide precipitates and, consequently, the alumina scale microstructure and adherence. The obtained results will be directly transferred to the development of coatings for gas-turbine components in the advanced power plants with CO2-capture in a project funded by the DFG in the frame of the Emmy-Noether programme. In these new power plants the H2O/CO2-rich turbine atmospheres are expected to have an adverse effect on the corrosion resistance in particular deteriorating the alumina scale adhesion. Therefore, the depletion of the coating RE-reservoir found in the present work is expected to be one of the major parameters affecting the scale resistance to spallation becomes even a more critical issue. In general, the results derived in the present project are of great importance for metallurgists in respect to optimisation of the minor alloy composition as well as manufacturing procedure of the MCrAl-alloys and coatings. Such an optimisation should be performed considering the component thickness and the fact that the RE-reservoir necessary to maintain the adhesion of the protective alumina scale tends to be depleted during service. For MCrAlY-coatings for gas-turbine applications the results indicated a high sensitivity of the REreservoir to the manufacturing parameters, such as oxygen content during coating spraying and heattreatment procedure. This, in turn, can in many cases explain the commonly observed large variations in the oxidation behaviour and lifetime of the MCrAlY and TBC coatings. Coating manufacture steps therefore have to be defined and controlled in a different manner as done until now. The effect of processing could be demonstrated in the present project by application of advanced analytical techniques to studies of the RE-distribution in the MCrAlY-coatings. Using the new findings in the project, some of these techniques such as Cathodoluminescence SEM-imaging and Raman spectroscopy have potential to be transferred to quality control methods in the complex coating processing by industry.
Publications
- D. Naumenko, V. Kochubey, A. Galerie and W.J. Quadakkers, Nucleation and growth of metastable alumina on FeCrAl-alloys and its inhibition by a gas-annealing treatment, International Conference on High Temperature Oxidation and Corrosion, ISHOC-2005, 29.11-02.12.2005, Nara, Japan
- D. Naumenko, V.Kochubey, H. Al-Badairy et. al., Effect of Carbon Content on the Oxidation Behaviour of FeCrAlY Alloys in the Temperature Range 1200 - 1300°C, International Conference on Microscopy of Oxidation 6, University of Birmingham UK, 4-6 April, 2005
- D. Naumenko, J. Toscano, R. Vaßen, L. Singheiser, W.J. Quadakkers, Geometrical Factors Affecting TGO Growth and Adherence on MCrAlY-Bondcoats for TBC's, 1st German-Japanese Workshop on Properties and Performance of Thermal Barrier Coating Systems and Factors Affecting It, 13-15.09.2006, Institute for Materials Technology, Darmstadt University of Technology, Darmstadt, Germany
- J. Toscano, R. Vaßen, A. Gil, D. Naumenko, L. Singheiser, W.J. Quadakkers, Parameters Affecting TGO Growth and Adherence on MCrAlY-Bond Coats for TBC's, Surface and Coating Technology, (2006) 3906-3910
- A. Gil, V. Shemet, R. Vassen, M. Subanovic, J. Toscano, D. Naumenko, L. Singheiser, W.J. Quadakkers The Effect of Surface Condition on the Oxidation Behaviour of MCrAlY coatings, Surface and Coating Technology, 201 (2006) 3824¿3828
- D. Naumenko, A. Gil, V. Shemet, R. Vassen, M. Subanovic, J. Toscano, Effect of Surface Condition on the Oxidation Behaviour of MCrAlY-coatings, International Conference on Metallurgical Coatings and Thin Films, ICMCTF-2006, 29.04-05.05.2006, San-Diego, USA
- D. Naumenko, B. Gleeson, E. Wessel, L. Singheiser and W.J. Quadakkers, Correlation between the Microstructure, Growth Mechanism and Growth Kinetics of Alumina Scales on an FeCrAlY-Alloy, Metallurgical and Materials Transactions 38A (2007) 2974-2983
- D. Naumenko, Effect of Minor Elements on the microstructure and growth kinetics of alumina scales on FeCrAl-alloys, invited presentation at the Gordon Research Conference on High Temperature Corrosion, 24-29 July 2005, New London, New Hampshire, USA
- D. Naumenko, Growth Mechanisms of Alumina and Chromia Scales in H2O-containing, low pO2-environments, invited presentation at the Gordon Research Conference on High Temperature Corrosion, 1 August 2007, New London, New Hampshire, USA
- D. Naumenko, J. Toscano, M. Subanovic, L. Singheiser and W.J. Quadakkers, Effects of Reactive Elements on the Alumina Scale Microstructure and Associated Growth Kinetics, Workshop on Advanced Oxidation Resistant Superalloys, 22-24.08.2006, Fairborn, Ohio, USA
- D. Naumenko, M. Subanovic, M. Kamruddin, E. Wessel, L. Niewolak, L. Singheiser, W.J. Quadakkers, Effect of Manufacturing Parameters on the Oxidation Resistance of MCrAlY-Coatings and Bondcoats in Power Generating Gas Turbines , Proc. 7th Int. Charles Parsons Turbine Conference, 11-13 Sept. 2007 Glasgow, UK, Edt. A.Strang, G.M. McColvin, W.M. Banks, R.W. Vanstone, J.E. Oakey, IOM Communications Ltd, London, UK, ISBN 1-86125-168-8 (2007), 193-204
- D. Naumenko, Quadakkers W.J., Korrosion und Komponentenlebensdauer, DGM-Seminar Hochtemperaturkorrosion, 13. Nov. 2007, Forschungszentrum Jülich, Jülich, Germany
- D. Naumenko, Quadakkers W.J., Korrosion und Komponentenlebensdauer, DGM-Seminar Hochtemperaturkorrosion, 24-26 Oct. 2005, Forschungszentrum Jülich, Jülich, Germany
- D. Naumenko, Quadakkers W.J., Korrosion und Komponentenlebensdauer, DGM-Seminar Hochtemperaturkorrosion, 7. Nov. 2006, Forschungszentrum Jülich, Jülich, Germany
- D. Naumenko, V. Kochubey, L. Niewolak, A. Dymiati, J. Mayer, L. Singheiser and W.J. Quadakkers Modification of alumina scale formation on FeCrAlY-alloys by minor additions of group IVa elements, submitted to the Journal of Materials Science, Dec (2007)
- D. Naumenko, W.J. Quadakkers, L. Singheiser, Korrosionsschutz von Hochtemperaturkomponenten, Korrosionsschutz von Metallischen Oberflächen durch Beschichtungen, OTTI e.V. - Seminar, 15-16.03.2006, Regensburg, Germany
- E. Wessel, V. Kochubey, D. Naumenko, L. Niewolak, L. Singheiser, W. J. Quadakkers, Effect of Zr-addition on the microstructure of the alumina scales on FeCrAlY-alloys, Scripta Materialia 51(10), (2004) 987-992
- J. Toscano, A. Gil, T. Hüttel, E. Wessel, D. Naumenko, L. Singheiser, W.J. Quadakkers, Temperature dependence of phase relationships in different types of MCrAlY-coatings, Surface and coatings technology, 202 (2007) 603-607
- L. Niewolak, D. Naumenko, E. Wessel, L. Singheiser, W.J. Quadakkers, Optical Fluorescence Spectroscopy for Identification of Minor Oxide Phases in Alumina Scales Grown on High Temperature Alloys, Materials Characterization, 55 (2005) 320-321
- V. Kochubey, D. Naumenko, E. Wessel, J. Le Coze, L. Singheiser, H. Al-Badairy, G. J. Tatlock and W. J. Quadakkers, Evidence for Cr-carbide Formation at the Scale/Metal Interface during Oxidation of FeCrAl Alloys, Materials Letters, 60 (2006) 1654-1658
- V. Kochubey, H. Al-Badairy, G. Tatlock, J. Le-Coze, D. Naumenko and W.J. Quadakkers, Effects of minor additions and impurities on oxidation behaviour of FeCrAl-alloys. Development of novel surface coatings compositions, Materials and Corrosion 56, No. 12, p. 848-853 (2005)
