In-Situ Observation of Crack Behavior in Plasma-Sprayed 7 wt% Yttria-Stabilized Zirconia

1. In-Situ Observation of Crack Behavior in Plasma-Sprayed 7 wt% Yttria-Stabilized Zirconia Jonathan Levin Purdue University Advisor: Prof. Trice

2. Introduction Background on Material • The material used in this experiment was 7 wt% yttria-stabilized zirconia (YSZ). • Yttria-stabilized zirconia is a commonly used thermal barrier coating (TBC) in applications such as coating turbine blades in gas engines. • Thermally coating the turbine blades lets engines run hotter and therefore more fuel efficiently.1 1. http://www.nist.gov.sigmaxi.Posters02/Dobbins-abst.html

3. Introduction Thermally Coated Turbine Blade www.kp.dlr.de/WB-WF/schaufel.jpg

4. Introduction Plasma Spraying • For this experiment, thin, stand-alone YSZ tubes with cylindrical geometry were used. • These tubes were created using the method of plasma spraying. www.swunited.com/plasma/pages/placap.htm

5. Introduction Plasma Spray Gun

6. Introduction Microstructure • The molten YSZ droplets from the plasma gun solidify as layered lamellae on the substrate. • This method results in many defects in the microstructure in the form of microcracks and pores in and between the lamellae. Intralamellar Crack Intralamellar Pore Interlamellar Pore

7. Introduction Background on Experiment • Previous experiments have shown an increase in modulus as the stress on the tube increases. • In this experiment, a compressive stress was put on the tube to see if this increase in modulus was due to cracks perpendicular to the applied load closing in, increasing the density of the material.

8. Introduction Background on Experiment • How cracks parallel to the applied stress open and propagate was also examined in this experiment • A compressive stress was used because it approximates the compressive thermal stresses experienced in service.

9. Introduction Surface Microstructure

10. Procedure Sample Preparation  Cut   Dissolve Al in HCl Removing alumina core

11. Procedure ESEM with Load Frame • This experiment will be performed using an environmental scanning electron microscope (ESEM) equipped with a load frame. Load Frame ESEM

12. Procedure Specimen in Load Frame YSZ Tube

13. Procedure Strain Gauges • Two strain gauges were attached to opposite sides of the tube in the axial direction. • The tube was then placed into the load frame and balanced by putting a small load on the tube and inserting thin metal shims on either side until the strains on each side were as close to equal as possible. Strain Gauge

14. Procedure Experimental Procedure • Once the sample was calibrated and the Load frame was inserted in to the ESEM, a region was found which exhibited cracks oriented in various directions. • The sample was then incrementally loaded, with strain data and micrographs taken at each stress increment. • The behavior of the cracks was observed at each increment.

15. Results and Discussion Stress-Strain Curve • The stress-strain curve obtained from the compression test of the tube showed two distinct regions with differing modulus. • The modulus also increased, probably due to cracks closing.

16. Results and Discussion Micrographs 0 MPa 129.1 MPa

17. Results and Discussion Crack Behavior • The Micrographs show the vertical cracks opening and the horizontal cracks closing. • This graph shows crack width vs. stress for five crack locations.

18. Results and Discussion Stress and Strain in Sample • Poisson’s Effect causes the tube to have a positive strain in the directions perpendicular to the direction of the applied stress. This is because as the tube is compressed, it is resisting a change in volume by changing shape instead. • This causes vertical cracks to open, horizontal cracks to close, and both to propagate.

19. Results and Discussion Crack Propagation • These images show a crack propagating from an existing angled crack. • This starts occurring around 58.3 which is about half of the failure stress. This agrees with previous work done on the failure of brittle solids.2 33.3 MPa 58.3 MPa 83.3 MPa 129.1 MPa 2. Ashby, M. F. and Hallam, S. D. “The Failure of Brittle Solids Containging Small Cracks Under Compressive Stress States.” Acta Metall., 34, pp. 497-510 (1986)

20. Conclusions • Thin YSZ tubes were tested in compression to observe crack behavior in the material. • The stress-strain curve for the YSZ tube showed and increasing modulus with two distinct regions of differing modulus. • Micrographs taken using an ESEM showed that cracks oriented parallel to the applied stress opened and cracks oriented perpendicularly to the applied stress closed.

21. Conclusions • One region exhibited a crack initiating from a angled crack and propagating roughly in the direction of the applied stress. • The crack initiated at roughly half of the failure stress which agrees with previous work done on the failure of brittle solids.

22. Acknowledgements I would like to thank the financial support of the National Science Foundation grant #DMR0134286. I would also like to thank Prof. Rodney Trice for his support and guidance and Jan Eberle for his technical support with the ESEM.

23. Questions?