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Degradation of TBC Systems in Environments Relevant to Advanced Gas Turbines For IGCC Systems

Bohna, Nathaniel (2017) Degradation of TBC Systems in Environments Relevant to Advanced Gas Turbines For IGCC Systems. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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Plasma sprayed (PS) thermal barrier coatings (TBCs) are used to provide thermal insulation for the hottest components in gas turbines. Zirconia stabilized with 7wt% yttria (7YSZ) is the most common ceramic top coat used for turbine blades. The 7YSZ coating can be degraded by the buildup of fly-ash deposits which can arise from the fuel source (coal/biomass) used in the combustion process in gas turbines. Fly-ash from the integrated gasification combined cycle (IGCC) process can result from coal-based syngas and also from ambient air which passes through the system. TBCs are also exposed to harsh gas environments containing CO2, SO2, and steam. As presented in this thesis, degradation from the combined effects of fly-ash and harsh gas atmosphere can severely limit TBC lifetimes.

A key focus of this study is to assess the mode and extent of TBC degradation at 1100°C in cases when some amount of liquid forms owing to the presence of K2SO4 as a minor ash constituent. Two types of liquid infiltrations are observed depending on the principal oxide (i.e., CaO or SiO2) in the deposit. The degradation is primarily the result of mechanical damage, which results from infiltration caused by the interaction of liquid K2SO4 with either the CaO or SiO2. The TBCs used in this work are representative of commonly used coatings used in the hottest sections of land-based gas turbines. The specimens consist of 7YSZ top coats deposited on superalloy (Rene’ N5 and PWA 1484) substrates that had been coated with NiCoCrAlY bond coats. Two different top coats are studied: conventional low-density 7YSZ, and dense vertically cracked coatings.

The specific mechanisms of liquid infiltration resulting from CaO and SiO2 are studied by conducting isothermal exposures followed by detailed characterizations. The resulting consequences on cyclic lifetimes are also determined. Further, the cyclic lifetimes are studied in several gas atmospheres to examine the combined effect of deposit and gas atmosphere on TBC lifetime.

This work identifies a TBC degradation mechanism which had previously not been considered. It will be clearly shown that deposit-induced attack of TBCs can be highly detrimental at an intermediate temperature like 1100°C.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Bohna, Nathanielnab85@pitt.edunab85
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Thesis AdvisorGleeson,
Committee MemberMeier,
Committee MemberNettleship,
Committee MemberLi,
Date: 14 June 2017
Date Type: Publication
Defense Date: 14 March 2017
Approval Date: 14 June 2017
Submission Date: 20 March 2017
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 206
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: Thermal Barrier Coatings, Gas Turbines, Deposit-Induced Degradation
Date Deposited: 14 Jun 2017 19:07
Last Modified: 14 Jun 2017 19:07


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