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Failure of Thick, Low Density Air Plasma Sprayed Thermal Barrier Coatings

Helminiak, Michael (2013) Failure of Thick, Low Density Air Plasma Sprayed Thermal Barrier Coatings. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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This research was directed at developing fundamental understandings of the variables that influence the performance of air plasma sprayed (APS) yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBC). Focus was placed on understanding how and why each variable influenced the performance of the TBC system along with how the individual variables interacted with one another.
It includes research on the effect of surface roughness of NiCoCrAlY bond coats deposited by argon-shrouded plasma spraying, the interdiffusion behavior of bond coats coupled to commercial superalloys, and the microstructural and compositional control of APS topcoats to maximize the coating thicknesses that can be applied without spallation. The specimens used for this research were prepared by Praxair Surface Technologies and have been evaluated using cyclic oxidation and thermal shock tests.
TBC performance was sensitive to bond coat roughness with the rougher bond coats having improved cyclic performance than the smoother bond coats. The explanation being the rough bond coat surface hindered the propagation of the delamination cracks.
The failure mechanisms of the APS coatings were found to depend on a combination of the topcoat thickness, topcoat microstructure and the coefficient of thermal expansion (CTE) mismatch between the superalloy and topcoat. Thinner topcoats tended to fail at the topcoat/TGO interface due to bond coat oxidation whereas thicker topcoats failed within the topcoat due to the strain energy release rate of the thicker coating exceeding the fracture strength of the topcoat.
Properties of free-standing high and conventional purity YSZ topcoats of both a low-density (LD) and dense-vertically fissure (DVF) microstructures were evaluated. The densification rate and phase evolution were sensitive to the YSZ purity and the starting microstructure. Increasing the impurity content resulted in enhanced sintering and phase decomposition rates, with the exception of the conventional-purity DVF which exhibited a density decrease during sintering. A combination of the DVF and LD topcoat microstructures (dual TBC) resulted in significant increase in cyclic durability. A 1275 μm thick dual TBC coating was found to have a comparable furnace cyclic life to that of a 100 μm LD TBC.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Helminiak, Michaelmah82@pitt.eduMAH82
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairMeier, Geraldghmeier@pitt.eduGHMEIER
Committee MemberPettit, Fredrickpettitfs@pitt.eduPETTITFS
Committee MemberNettleship, Iannettles@pitt.eduNETTLES
Committee MemberYanar, Naziknmy4@pitt.eduNMY4
Committee MemberTaylor,
Committee MemberBeuth,
Date: 28 June 2013
Date Type: Publication
Defense Date: 28 February 2013
Approval Date: 28 June 2013
Submission Date: 11 March 2013
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 274
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Materials Science and Engineering
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: TBC, Bond Coat, YSZ, APS, Topcoat Thickness, roughness, purity, microstructure
Date Deposited: 28 Jun 2013 20:09
Last Modified: 19 Jul 2024 18:39


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