A Study of Failure Development in Thick Thermal Barrier Coatings. Karin Carlsson

Transcription

1 A Study of Failure Development in Thick Thermal Barrier Coatings Karin Carlsson LITH-IEI-TEK--07/ SE Examensarbete Institutionen för ekonomisk och industriell utveckling

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3 Examensarbete LITH-IEI-TEK--07/ SE A Study of Failure Development in Thick Thermal Barrier Coatings Karin Carlsson Handledare: Examinator: Håkan Brodin SIEMENS Industrial Turbomachinery AB Sten Johansson IEI, Linköping University Linköping, 6 December, 2007

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5 Avdelning, Institution Division, Department Division of Engineering Materials Department of Management and Engineering Linköpings universitet SE Linköping, Sweden Datum Date Språk Language Svenska/Swedish Engelska/English Rapporttyp Report category Licentiatavhandling Examensarbete C-uppsats D-uppsats Övrig rapport ISBN ISRN LITH-IEI-TEK--07/ SE Serietitel och serienummer Title of series, numbering ISSN URL för elektronisk version Titel Title A Study of Failure Development in Thick Thermal Barrier Coatings Studie av skadeutvecklingen hos tjockt termiskt barriärskikt Författare Author Karin Carlsson Sammanfattning Abstract Thermal barrier coatings (TBC) are used for reduction of component temperatures in gas turbines. The service temperature for turbines can be as high as 1100 o C and the components are exposed to thermal cycling and gases that will cause the component to oxidize and corrode. The coatings are designed to protect the substrate material from this, but eventually it will lead to failure of the TBC. It is important to have knowledge about when this failure is expected, since it is detrimental for the gas turbine. The scope of this thesis has been to see if an existing life model for thin TBC also is valid for thick TBC. In order to do so, a thermal cycling fatigue test, a tensile test and finite element calculation have been performed. The thermal cycling fatigue test and finite element calculation were done to find correlations between the damage due to thermal cycling, the number of thermal cycles and the energy release rate. The tensile test was preformed to find the amount accumulated strain until damage. The thermal cycling lead to failure of the TBC at the bond coat/top coat interface. The measurment of damage, porosity and thickness of thermally grown oxide were unsatisfying due to problems with the specimen preparation. However, a tendency for the damage development were seen. The finite element calculations gave values for the energy release rate the stress intensity factors in mode I and mode II that can be used in the life model. The tensile test showed that the failure mechanism is dependent of the coating thickness and it gave a rough value of the maximum strain acceptable. Nyckelord Keywords Thick Thermal Barrier Coatings, Thermal Cycling Fatigue, Tenile Test with Acoustic Emission, Failure Development

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7 Abstract Thermal barrier coatings (TBC) are used for reduction of component temperatures in gas turbines. The service temperature for turbines can be as high as 1100 o C and the components are exposed to thermal cycling and gases that will cause the component to oxidize and corrode. The coatings are designed to protect the substrate material from this, but eventually it will lead to failure of the TBC. It is important to have knowledge about when this failure is expected, since it is detrimental for the gas turbine. The scope of this thesis has been to see if an existing life model for thin TBC also is valid for thick TBC. In order to do so, a thermal cycling fatigue test, a tensile test and finite element calculation have been performed. The thermal cycling fatigue test and finite element calculation were done to find correlations between the damage due to thermal cycling, the number of thermal cycles and the energy release rate. The tensile test was preformed to find the amount accumulated strain until damage. The thermal cycling lead to failure of the TBC at the bond coat/top coat interface. The measurment of damage, porosity and thickness of thermally grown oxide were unsatisfying due to problems with the specimen preparation. However, a tendency for the damage development were seen. The finite element calculations gave values for the energy release rate the stress intensity factors in mode I and mode II that can be used in the life model. The tensile test showed that the failure mechanism is dependent of the coating thickness and it gave a rough value of the maximum strain acceptable. v

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9 A.3 Tensile Test with Acoustic Emission 53 Figure A µm test 1. Figure A µm test 2.

10 54 A.4 Appendix Chemical Analyses Figure A.17, A.18, A.19, A.20, A.21 and A.22 are line scans. Figure A.23 and A.24 are mappings. Figure A.17. K0. Analysis started in the top coat. Figure A.18. The result from the line scan. Spectrum (x) represent the point where the measurement was done.

11 A.4 Chemical Analyses 55 Figure A.19. K300. Analysis started at the top of the bond coat. Figure A.20. The result from the line scan. Spectrum (x) represent the point where the measurement was done.

12 56 Appendix Figure A.21. K450. Analysis started at the top of the bond coat. Figure A.22. The result from the line scan. Spectrum (x) represent the point where the measurement was done.

13 A.4 Chemical Analyses 57 Figure A.23. A internal bond coat oxide from K450. Figure A.24. The results from the line scan. The different graphs represent different materials oxygen, silicon, chromium, cobalt, nickel, yttrium and aluminum.

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