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Oxidation behaviour of MCrAlX coatings: effect of surface treatment and an Al-activity based life criterion
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

MCrAlY coatings (M=Ni and/or Co) have been widely used for the protection of superalloy components against oxidation and hot corrosion in the hot sections of gas turbines. The drive to improve engine combustion efficiency while reducing emissions by increasing the operation temperature brings a big challenge for coating design. As a result, the need for improvement of MCrAlY coatings for better oxidation resistance is essential.

Formation of a stable, dense, continuous, and slow-growing α-Al2O3 layer, on the MCrAlY coating surface, is the key to oxidation protection, since the protective α-Al2O3 scale offers superior oxidation resistance due to its lower oxygen-diffusion rate as compared with other oxides. The ability of a MCrAlY coating to form and maintain such a protective scale depends on the coating composition and microstructure, and can be improved through optimization of deposition parameters, modification of coating surface conditions, and so on. Part of this thesis work focuses on studying the effect of post-deposition surface treatments on the oxidation behavior of MCrAlX coatings (X can be yttrium and/or other minor alloying elements). The aim is to gain fundamental understanding of alumina scale evolution during oxidation which is important for achieving improved oxidation resistance of MCrAlX coatings. Oxide scale formed on coatings at initial oxidation stage and the effect of surface treatment were investigated by a multi-approach study combining photo-stimulated luminescence, microstructural observation and weight gain. Results showed that both mechanically polished and shot-peened coatings exhibited superior performance due to rapid formation of α-Al2O3 fully covering the coating and suppressing growth of transient alumina, assisted by the high density of α-Al2O3 nuclei on surface treatment induced defects. The early development of a two-layer alumina scale, consisting of an inward-grown inner α-Al2O3 layer and an outer layer transformed from outward-grown transient alumina, resulted in a higher oxide growth rate of the as-sprayed coating. The positive effect of the surface treatments on retarding oxide scale growth and suppressing formation of spinel was also observed in oxidation test up to 1000 hrs.

As the oxidation proceeds to the close-to-end stage, a reliable criterion to estimate the capability of coating to form α-Al2O3 is of great importance to accurately evaluate coating lifetime, which is the aim of the other part of the thesis work. Survey of published results on a number of binary Ni-Al and ternary Ni-Cr-Al, Ni-Al-Si systems shows that the empirical Al-concentration based criterion is inadequate to properly predict the formation of a continuous α-Al2O3 scale. On the other hand, correlating the corresponding Al-activity data, calculated from measured chemical compositions using the Thermo-Calc software, to the experimental oxidation results has revealed a temperature dependent, critical Al-activity value for forming continuous α-Al2O3 scale. To validate the criterion, long-term oxidation tests were performed on five MCrAlX coatings with varying compositions and the implementation of the Al-activity based criterion on these coatings successfully predicted α-Al2O3 formation, showing a good agreement with experiment results.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. , p. 60
Series
Linköping Studies in Science and Technology. Licentiate Thesis, ISSN 0280-7971 ; 1799
National Category
Manufacturing, Surface and Joining Technology
Identifiers
URN: urn:nbn:se:liu:diva-145095DOI: 10.3384/lic.diva-145095ISBN: 9789176853818 (print)OAI: oai:DiVA.org:liu-145095DiVA, id: diva2:1181845
Presentation
2018-02-23, ACAS, A-huset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2018-02-09 Created: 2018-02-09 Last updated: 2018-02-13Bibliographically approved
List of papers
1. Long-term oxidation of MCrAlY coatings at 1000 degrees C and an Al-activity based coating life criterion
Open this publication in new window or tab >>Long-term oxidation of MCrAlY coatings at 1000 degrees C and an Al-activity based coating life criterion
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2017 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 332, p. 12-21Article in journal (Refereed) Published
Abstract [en]

MCrAlY type (M = Ni and/or Co) coatings are widely used for the protection of components in the hot sections of gas turbines at high service temperatures by forming a continuous alpha-alumina. A reliable criterion to estimate the capability of coating to form alpha-alumina is of great importance to accurately evaluate coating lifetime. However, some coatings retain the ability to form a continuous alpha-alumina scale when the concentration of Al in coatings decreases to a critical level, therefore, the empirical Al-concentration based criterion is inadequate to properly predict the formation of a continuous alpha-alumina. Thus, a new life criterion, namely the critical Al-activity criterion, is proposed. In this work, the critical Al-activity to form a continuous a-alumina was validated by Al-activity calculation using Thermo-Calc software based on survey of research results of critical Al-concentration to form alpha-alumina on binary Ni-Al and ternary Ni-Cr-Al systems. Long-term oxidation tests were performed to support the criterion: three different MCrAlY coatings coated on IN-792 superalloy substrates were oxidized at 1000 degrees C for various periods of time up to 10,000 h. The microstructural evolution of MCrAlY coatings was investigated using Scanning Electron Microscope. The near-surface Al concentration and interdiffusion behaviour between substrate and coating were measured using Energy Dispersive X-ray Spectroscopy. The new critical Al-activity criterion has been successfully adopted in alpha-alumina formation prediction, showing a good agreement with experiment results. Therefore, it can be concluded that the extrapolation of new criterion from binary and ternary systems to multi-alloyed MCrAlY system is reasonable. Furthermore, the partial pressure of oxygen (P-O2) in atmosphere has been taken into consideration by combination with Al-activity to calculate the critical chemical reaction constant (K) of formation of a-alumina. The potential applicability of the methodology to predict MCrAlY life is also discussed.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2017
Keywords
MCrAIY; Al-activity; alpha-Alumina; Life criterion; Long-term oxidation
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:liu:diva-144267 (URN)10.1016/j.surfcoat.2017.09.086 (DOI)000418968100003 ()
Conference
44th International Conference on Metallurgical Coatings and Thin Films (ICMCTF)
Note

Funding Agencies|Siemens Industrial Turbomachinery AB (Finspang, Sweden); Swedish Energy Agency through KME consortium [KME-703]; research school of Agora Materials and AFM Strategic Faculty Grant SFO-MAT-LiU in Linkoping University [2009-00971]

Available from: 2018-01-12 Created: 2018-01-12 Last updated: 2018-02-09
2. Isothermal oxidation behavior of HVAF-sprayed NiCoCrAlY coatings: Effect of surface treatment
Open this publication in new window or tab >>Isothermal oxidation behavior of HVAF-sprayed NiCoCrAlY coatings: Effect of surface treatment
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2017 (English)In: Proceedings of International Thermal Sprayed Conference (ITSC), May 7-9,Germany, 456-461 (2017), 2017, p. 456-461Conference paper, Oral presentation only (Other academic)
Abstract [en]

NiCoCrAlY coatings are widely used as bond coats for ceramic thermal barrier coatings (TBCs) andoxidation and corrosion protective overlay coatings in industrial gas turbines. High temperature oxidation behaviour of NiCoCrAlYs has a great influence on the coating performance and lifetime of TBCs. A promising route to decrease the oxidation rate of such coatings is post-coating surface modification which can facilitate formation of a uniform alumina scale with a considerably slower growth rate compared to the as-sprayed coatings. In this work, the effect of surface treatment by means of shot peening and laser surface melting (LSM) on the oxidation resistance of high velocity air-fuel (HVAF) sprayed NiCoCrAlY coatings was studied. Isothermal oxidation was carried out at 1000⁰C for 1000h. Results showed that the rough surface of as-sprayed HVAF sprayed coatings was significantly changed after shot peening and LSM treatment, with a compact and smooth appearance. After the exposure, the oxide scales formed on surface-treated NiCoCrAlY coatings showed different morphology and growth rate compared to those formed on as-sprayed coating surface. The oxidation behaviour of surface treated HVAF-sprayed NiCoCrAlY coatings were revealed and discussed.

Keywords
Surface treatment, shot peening, Laser Surface Melting (LSM), High Velocity Air-Fuel (HVAF), NiCoCrAlY coating
National Category
Manufacturing, Surface and Joining Technology Ceramics
Identifiers
urn:nbn:se:liu:diva-145094 (URN)
Conference
International Thermal Spray Conference & Exposition, ITSC 2017, Düsseldorf, Germany, June 7-9, 2017
Available from: 2017-12-28 Created: 2018-02-09 Last updated: 2018-02-09

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