PhD Oral Exam - Hamideh Vakilifard, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Advanced thermal barrier coatings (TBCs) are critical for enabling higher operating temperatures and improving the efficiency of gas turbines. Although yttria-stabilized zirconia (YSZ) remains the industrial benchmark, its susceptibility to phase instability and sintering at elevated temperatures motivates the development of alternative topcoat materials. High entropy zirconates (HEZ) have recently emerged as promising candidates, owing to their reported high-temperature phase stability and sluggish diffusion behavior. In this work, two equiatomic HEZ compositions, (Y₀.₂Nd₀.₂Gd₀.₂Sm₀.₂Dy₀.₂)₂Zr₂O₇ and (Y₀.₂Nd₀.₂Gd₀.₂La₀.₂Eu₀.₂)₂Zr₂O₇, were synthesized via the solid-state reaction method and systematically evaluated. Both compositions exhibited a pyrochlore structure and remained single-phase after heat treatment at 1500 °C for 100 h. Upon thermal spraying the first composition, it transformed into a fluorite structure and retained a single-phase state in the as-sprayed condition. This composition was selected for coating development and, for the first time, deposited using axial suspension plasma spray (SPS). By tailoring spray parameters, dense vertically cracked (~11 cracks/mm) and columnar (~19 columns/mm) microstructures were deposited. HEZ coatings were deposited on a 50 μm YSZ interlayer, and YSZ coatings with comparable microstructures were prepared as reference TBC systems. Isothermal oxidation testing at 1000°C and 1150°C showed that HEZ coatings developed thinner thermally grown oxide (TGO) layers and exhibited reduced sintering and densification compared to YSZ. The DVC HEZ structure demonstrated the most favorable performance. Under CMAS exposure, HEZ coatings exhibited an interaction mechanism fundamentally different from that of YSZ, dominated by near-surface chemical immobilization rather than deep melt infiltration. In both columnar and DVC HEZ microstructures, a dense Zr-rich reaction band with an underlying multicomponent apatite phase formed rapidly and stabilized, effectively arresting CMAS penetration for exposure times up to 96 hours at 1350 °C. In contrast, YSZ did not develop any reaction layer, and CMAS transport was governed by grain-boundary and inter-splat infiltration accompanied by thermal coarsening. Overall, this thesis provides the first comprehensive assessment of SPS-deposited HEZ topcoats, demonstrating their oxidation and CMAS resistance and offering new insights into the role of chemistry-microstructure coupling in the design of next-generation TBC systems.