PhD Oral Exam - Hossein Shahbazi, 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

Thermal barrier coatings (TBCs) serve as essential thermal protection systems in harsh environments, consisting of a ceramic top coat and a metallic bond coat. The bond coat plays a crucial role in enhancing adhesion between the top coat and the substrate while also providing oxidation and corrosion resistance at elevated temperatures. Traditionally, MCrAlX alloys have been utilized as bond coat materials, where M represents metals or alloys (such as Ni, Co, or NiCo), and X represents reactive elements like Y, Hf, Ta, and Si.

As gas engines and turbine components are increasingly exposed to extreme operating temperatures and aggressive environments, conventional bond coats encounter limitations such as phase instability, accelerated oxidation, and susceptibility to thermal cycling degradation. To address these challenges, high-entropy alloys (HEAs) have emerged as advanced alternatives, offering superior thermal and chemical stability. The multi-principal-element composition of HEAs contributes to improved oxidation resistance, enhanced hot corrosion resistance, and optimized diffusion behavior, mitigating degradation mechanisms commonly observed in traditional MCrAlX coatings.

The exceptional properties of HEAs facilitate the formation of a dense, continuous, and stable thermally grown oxide (TGO) layer, reducing spallation risks and improving the overall durability of the bond coat. Additionally, HEAs exhibit reduced oxygen permeability, the formation of stable α-Al₂O₃, and the inhibition of detrimental phase segregation, further extending service life in high-temperature applications.

This study investigates the influence of HEA compositions, both with and without reactive elements, as well as the effects of lower-temperature spray techniques, including HVOF and HVAF (M3, i7 guns). Additionally, it examines one of the most critical yet less-studied processes, vacuum heat treatment of bond coats. A comparative analysis with conventional MCrAlX coatings is conducted to highlight the advantages, challenges, and potential applications of HEAs in next-generation TBC systems.