Date of Award

January 2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Surojit Gupta

Abstract

Binary borides are well known for their exceptional mechanical and thermal properties, including high strength, high melting points, outstanding hardness, and excellent chemical stability, making them suitable for high-temperature structural applications such as components in hypersonic vehicles, jet engines, nose cones, and the leading edges of aircraft. Despite these advantages, they are inherently brittle and suffer from low fracture toughness, limiting their ability to absorb impact or resist crack propagation.

To this end, attention shifted to ternary transition metal borides which are materials composed of a transition metal, boron, and a third element. These ternary compounds exhibit a broader range of crystal structures, allowing for greater tunability of mechanical and functional properties. Several studies have reported that ternary borides demonstrate improved fracture toughness compared to their binary counterparts. Among these, MoAlB has gained much attention for its unique combination of oxidation resistance at elevated temperatures, crack healing capabilities, and abrasion resistance.

In today’s technology-driven world, silicon remains a foundational material across numerous industries due to its abundance, versatility, and cost-effectiveness. Its integration into ternary boride systems presents a compelling route to engineer new materials with enhanced high-temperature performance. Motivated by this potential, this dissertation focuses on the synthesis and characterization of a silicon-containing ternary transition metal boride—Ni₆Si₂B.

In this study, bulk polycrystalline Ni₆Si₂B was successfully synthesized and characterized. High-temperature tribological testing revealed a decreasing coefficient of friction with increasing temperature, vital for energy saving in applications where friction reduction is a major concern. Additionally, oxidation studies showed that Ni₆Si₂B remains stable up to 800 °C, with its oxidation behavior following a parabolic kinetic model. This performance is attributed to the formation of a protective silicon oxide layer on the surface, which effectively reduces atomic diffusion and inhibits further oxidation. The oxide layer was also responsible for improved compressive strength after long term oxidation at 800 °C and 900 °C.

In summary, this work contributes to the expanding body of knowledge on ternary transition metal borides by exploring the structural, mechanical, and environmental stability of a silicon-based composition. The findings present the promise of Ni₆Si₂B as a multifunctional material for extreme service environments.

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