Date of Completion

5-3-2018

Embargo Period

4-26-2018

Keywords

thermal barrier coatings, yttrium aluminum garnet, low thermal conductivity, inter pass boundaries, CMAS resistance, viscosity

Major Advisor

Eric H. Jordan

Associate Advisor

Maurice Gell

Associate Advisor

Puxian Gao

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Thermal barrier coatings (TBCs) are insulating coatings used in gas turbine engines to improve energy efficiency. The current choice of TBC material i.e. yttria stabilized zirconia (YSZ), is limited to temperatures of less than 1200°C because of (a) undesirable phase transformations and (b) prone to the attacks from calcium-magnesium-aluminum-silicate (CMAS) deposits.

In this research, the solution precursor plasma spray (SPPS) was employed for the further development of yttrium aluminum garnet (YAG) coatings previously developed at UConn. Thermal conductivity of SPPS YAG was reduced (0.58 W/mK at 1300 °C) by process modifications to generate microstructures with layered porosity termed inter pass boundaries (IPBs). Improvement in the SPPS process for YAG coatings was achieved by enhancing deposition efficiency and deposition rate (DR) through optimizing spraying parameters and precursor concentration. A highest DR value of 209 g/hour was attained thus cutting the cost by 4X over previously deposited SPPS YAG. A 58% increase in standoff distance was also achieved by employing a cascaded high energy gun.

The reactivity of YAG with CMAS was evaluated for the first time using systematic heat treatment of composite powder pellets. Experimental results along with optical basicity theory demonstrate that YAG is less reactive to CMAS than YSZ. Simultaneously, resistance of SPPS YAG TBCs was evaluated through CMAS interaction tests, which demonstrated that YAG performed 2X and 8X better than air plasma spray (APS) YSZ in different tests. The performance of YAG TBCs was enhanced drastically (15X higher than previously tested SPPS YAG) by high prominence of IPBs in the microstructure. It was concluded that IPBs act as secondary channels for CMAS infiltration thereby limiting the infiltration depth and prolonging the life. This is proposed as a novel and alternate CMAS mitigation strategy with relies only on microstructural features. The influence of microstructure on CMAS infiltration was also studied on the highly CMAS resistant gadolinium zirconate (GZO) TBCs deposited by both APS and SPPS process. A strong microstructural influence was observed where APS outperformed SPPS GZO by 10X, arresting CMAS at a depth of 25 microns. In SPPS GZO, crack width of arrest.

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