UBC Theses and Dissertations
Process development for advanced functionally graded thermal barrier coatings Yaghtin, Maryam
The need for advanced thermal barrier coatings (TBCs) with submicron or nano-sized morphologies, improved thermal and mechanical properties as compared to the conventional TBCs, has recently attracted interest toward suspension plasma spray (SPS) and solution precursor plasma spray (SPPS) techniques, in which replacing the typically used alcoholic solvent with water would reduce the manufacturing costs and enhance the process safety. A major challenge of SPS, addressed in this work, is development of industrially scalable well-dispersed and stable aqueous suspensions. This research was broadly aimed at advancing knowledge of processing of aqueous suspensions and solutions and subsequent plasma spraying of yttria stabilized zirconia (YSZ) and lanthanum zirconate (LZ). YSZ suspension and LZ solution optimization/deposition were studied in Phase 1 and Phase 2, respectively. In Phase 3, a functionally graded coating (FGC) made of SPS–YSZ and SPPS–LZ was designed to retain the desirable properties of both YSZ and LZ while avoiding their limitations. The coatings were deposited by Mettech Axial III plasma spray system. Phases 1 and 2 of this work evaluated the rheology, stability, aging, injection and atomization of aqueous suspensions and solutions. The studies of aqueous YSZ suspensions revealed that a proper combination of dispersants with pH adjustment led to a maximum stability and low viscosity. Minimizing the viscosity and surface tension of the liquid feedstocks was in favor of a successful injection, and optimum atomized droplet size, which in turn contributed to coatings with desirable microstructural features for TBC application. TBCs with columnar microstructure, and ~18 vol.% porosity, were obtained for aqueous SPS–YSZ suspensions stabilized with 0.1 wt.% α-Terpineol dispersant, at pH=2.5. The SPPS coatings deposited from dilute lanthanum nitrate–zirconium acetate aqueous precursor solutions produced columnar morphology with ~23 vol.% porosity. Composition of the five-layered YSZ–LZ FGCs varied gradually from 100% YSZ to 100% LZ. We achieved feathery columnar morphology for these coatings with total porosity of 19-25 vol.%. Overall, this work contributes both to the advancement of fundamental science of liquid plasma spray processes and to the control and optimization of the microstructure and, therefore, thermal and lifetime properties of aqueous TBCs.
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