PhD Oral Exam - Farzam Arhami, 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

Environmental barrier coatings (EBCs) are essential for enhancing the efficiency and durability of SiC-based ceramic matrix composites (CMCs) in gas turbines, enabling higher operating temperatures and reduced emissions. This study focuses on addressing the current challenges of deposition of Yb2Si2O7 EBCs via atmospheric plasma spraying (APS), including optimizing the microstructure by balancing high density and minimizing cracking, controlling silicon evaporation during spraying, tailoring the distribution of secondary phases, addressing the amorphous phase formation, and understanding the crystallization mechanisms. Highly crystalline (>85%) Yb2Si2O7 coatings were successfully fabricated without heat treatment or a vacuum chamber, demonstrating an innovative approach to in-situ crystallization during spraying. Approximately 30 wt.% unmelted particles served as nucleation sites, enabling heterogeneous crystallization and reducing amorphous content. The process parameters were systematically studied, leading to the optimization of the coating microstructure, resulting in an almost crack-free structure with ~10% porosity and only ~20 wt.% Yb2SiO5 secondary phase. Injection methods, torch power, and plasma gas composition were modified, producing coatings ranging from dense lamellar (~90% amorphous) to highly crystalline (>85%) granular structures. Post-deposition heat treatment at 1300°C revealed variable crystallization pathways, resulting in diverse microstructures, such as eutectic-like, lamellar and granular morphologies. The study investigated the Calcium Magnesium Alumino Silicate (CMAS) interaction and infiltration resistance in the Yb2Si2O7 coatings. Radially injected EBCs with a dense lamellar structure and ~65 wt.% Yb2SiO5 exhibited the highest CMAS resistance. This was attributed to the self-limiting effect of the continuous Ca-bearing apatite phase formed at the coating/CMAS interface, acting as a barrier against further infiltration and increased CMAS viscosity. In contrast, highly crystalline Yb2Si2O7-rich coatings (~80 wt.%) with a granular microstructure were more susceptible to CMAS infiltration due to their extensive grain boundary networks and inherently low reactivity with CMAS, which hindered apatite formation and reduced CMAS sequestration. The valuable insights gained in this study, including tailored process parameters, crystallization mechanisms, and process-microstructure-property relationships, are not limited to Yb2Si2O7 coatings but can be extended to other potential EBC materials, contributing to the development of advanced EBCs for next-generation gas turbines and even hydrogen-powered turbines.