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A study of damage evolution in high purity nano TBCs during thermal cycling: A fracture mechanics based modelling approach
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
Beijing Gen Research Institute Min and Met, Peoples R China.
Siemens Ind Turbomachineiy AB, Sweden.
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2017 (English)In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 37, no 8, p. 2889-2899Article in journal (Refereed) Published
Abstract [en]

This work concerns the study of damage evolution in a newly developed high purity nano 8YSZ thermal barrier coating during thermal cyclic fatigue tests (TCF). TCF tests were conducted between 100 degrees C-1100 degrees C with a hold time of 1 hat 1100 degrees C, first till failure and later for interrupted tests. Cross section analysis along the diameter of the interrupted test samples revealed a mixed-type failure and that the most of the damage occurred towards the end of the coatings life. To understand the most likely crack growth mechanism leading to failure, different crack growth paths have been modelled using finite element analysis. Crack growing from an existing defect in the top coat towards the top coat/TGO interface has been identified as the most likely mechanism. Estimated damage by the model could predict the rapid increase in the damage towards the end of the coatings life. (C) 2017 Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
ELSEVIER SCI LTD , 2017. Vol. 37, no 8, p. 2889-2899
Keywords [en]
Thermal cyclic fatigue; High purity nano YSZ; Crack growth modelling; Damage evolution
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:liu:diva-137827DOI: 10.1016/j.jeurceramsoc.2017.02.054ISI: 000400531500015OAI: oai:DiVA.org:liu-137827DiVA, id: diva2:1105256
Note

Funding Agencies|Vinnova in Sweden

Available from: 2017-06-02 Created: 2017-06-02 Last updated: 2019-02-26
In thesis
1. Thermal Barrier Coatings: Failure Mechanisms and Life Prediction
Open this publication in new window or tab >>Thermal Barrier Coatings: Failure Mechanisms and Life Prediction
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thermal barrier coatings (TBCs) use in the hot sections of gas turbine engine enables them to run at higher temperatures, and as a consequence, achieve higher thermal efficiency. For full operational exploitation of TBCs, understanding their failure and knowing the service life is essential. The broad objective of the current research is to study the failure mechanisms of new TBC materials and deposition techniques during corrosion and thermal cycling and to develop life models capable of predicting the final failure during thermal cycling.

Yttria-stabilized zirconia (YSZ) has constraints such as limited operation temperature, despite being the current industry standard. Pyrochlores of A2B2O7 type have been suggested as a potential replacement for YSZ and were studied in this work. Additionally, improvements to the conventional YSZ in the form of nanostructured YSZ were also explored. The requirement for the new deposition process comes from the fact that the existing low-cost deposition processes, like atmospheric plasma spray (APS), generally exhibit lower strain tolerance. A relatively new technique, suspension plasma spray (SPS), known to be promising with better strain tolerance, has been studied in this work.

At the gas turbine operating conditions, TBCs degrade and eventually fail. Common failure observed in gas turbines can be due to corrosion, thermal mismatch between the ceramic and the metallic layers, and bond coat oxidation during thermal cycling. SPS and APS TBCs were subjected to different test conditions to understand their corrosion behavior. A study on the multi-layered SPS TBCs in the presence of V2O5+Na2SO4 showed that YSZ based SPS coatings were less susceptible to corrosion damage compared to Gd2Zr2O7 SPS TBCs. A study on the influence of a sealing layer in multi-layered SPS TBCs in the presence of Na2SO4+NaCl showed that the sealing layer is ineffective if the material used for sealing is inert to the molten salts. A new study on the influence of corrosion, caused by a mixed-gas atmosphere, on the thermal cycling fatigue life of SPS TBCs was conducted. Results showed that corrosive products grew inside the top coat close to the bond coat/top coat interface along with accelerated growth of alumina. These, together, reduced the TCF life of corrosion exposed samples significantly. Finally, a study on the influence of salt concentration and temperature on a thin (dense) and a thick (porous) coating showed that thick and porous coatings have lower corrosion resistance than the thin and dense coatings. Additionally, a combination of low temperature and high salt concentration was observed to cause more damage.

Thermal cycling studies were done with the objective of understanding the failure mechanisms and developing a life model. A life model based on fracture mechanics approach has been developed by taking into account different crack growth paths during thermal cycling, sintering of the top coat, oxidation of the bond coat and the thermal mismatch stresses. Validation of such a life model by comparing to the experimental results showed that the model could predict the TCF life reasonably well at temperatures of 1100 °C or below. At higher temperatures, the accuracy of the model became worse. As a further development, a simplified crack growth model was established. This simplified model was shown to be capable of predicting the TCF life as well as the effect of hold times with good accuracy.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2019. p. 57
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1975
National Category
Manufacturing, Surface and Joining Technology Corrosion Engineering Materials Chemistry Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:liu:diva-154777 (URN)10.3384/diss.diva-154777 (DOI)9789176851388 (ISBN)
Public defence
2019-03-13, C3, C-huset, Campus Valla, Linköping, 10:15 (English)
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Available from: 2019-02-26 Created: 2019-02-26 Last updated: 2019-03-04Bibliographically approved

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