Doctoral Dissertations

Keywords and Phrases

Extreme Environments; Hafnium Carbide; Hafnium Nitride; Oxidation; Oxygen Bomb Calorimetry; UHTC

Abstract

"Materials capable of withstanding oxidative environments at temperatures >2000°C are necessary for hypersonic applications. Hafnium carbide (HfC) shows a combination of properties that lends itself toward these applications; however, its oxidation behavior is poorly understood. At 1300°C, HfC was found to oxidize via the following mechanism: 1) Initial oxidation of both C and Hf present in the system, 2) Formation of an interconnected pore network, whose size is related to the volume of gases being generated during the oxidation process, 3) Growth of the oxide scale to become sufficiently protective, 4) Formation of an initially amorphous “HfO2C” at the interface between the protective scale and the parent carbide material, on the order of nanometers, 5) subsequent decomposition of “HfO2C” into nanocrystalline HfO2 and turbostratic C, leading to nanoscale microstructures that are responsible for the observed gas phase diffusion controlled oxidation kinetics. The oxidation of HfC at T > 1800°C developed oxide scales containing only HfO2; oxidation behavior of this material appeared to change between 1600°C-1800°C. Microstructural observations of the oxide scales formed indicated a change in oxidation regime from gas-phase-controlled to solid-state diffusion controlled. This transition was found to correlate with the max range of the thermodynamically predicted HfO2 + C interlayer, which was affected by carbon sub stoichiometry of the system. The maximum predicted HfO2 + C stability was calculated to be 2070°C for highly sub stoichiometric HfC, providing a theoretical mechanism for extending gas-phase diffusion limited oxidation control to higher temperatures. Investigations of the enthalpies of formation of HfC1-x and HfN1-y showed good agreement with theoretical predictions" -- Abstract, p. iv

Advisor(s)

Lipke, David W.

Committee Member(s)

Fahrenholtz, William
O'Malley, Ronald J.
Miller, F. Scott, 1956-
He, Xiaoqing

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Materials Science and Engineering

Publisher

Missouri University of Science and Technology

Publication Date

Summer 2024

Pagination

xi, 118 pages

Note about bibliography

Includes_bibliographical_references_(pages 49, 70, 87 & 115-117)

Rights

©2024 Jonathan Allen Scott , All Rights Reserved

Document Type

Dissertation - Open Access

File Type

text

Language

English

Thesis Number

T 12409

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