Doctoral Dissertations

Keywords and Phrases

High Entropy Carbide; Mechanical Properties; Tantalum Carbide; Thermal Properties; Ultra-High Temperature Ceramics; Zeta Phase


"This research focuses on studying the thermal and mechanical properties of novel carbide materials at room and elevated temperatures. The novel carbide materials investigated include zeta phase tantalum carbide (ζ-Ta4C3-x) and a high entropy (Ta,Hf,Nb,Zr)C carbide. The electrical resistivity and thermal conductivity of zeta phase tantalum carbide (~96 wt.%) were measured as 160 ± 4.2 μΩ-cm and 9.6 W/m•K. These are higher and lower (respectively) than for cubic tantalum carbide, most likely due to planes of carbon vacancies present in the ζ-Ta4C3-x crystal structure. The thermal conductivity of (Ta,Hf,Nb,Zr)C was lower than any of its carbide constituents with values ranging from 10.7 W/m•K at room temperature to 39.9 W/m•K at 2000°C. The electrical resistivity was higher than any of the carbide constituents ranging from 80.9 μΩ•cm at room temperature to 114.1 μΩ•cm at 800 °C. The electron contribution to thermal conductivity increased with temperature, and the phonon contribution was unaffected by temperature. The strength and fracture toughness of tantalum carbide ceramics (~75 wt.% ζ-Ta4C3-x) were 710 ± 36.5 MPa and 10.2 ± 0.4 MPa· m1/2 at room temperature and decreased to 180 ± 19.7 MPa and 4.7 ± 0.1 MPa· m1/2 at 1600°C. The high aspect ratio grains (along the long axis) appeared to be the critical flaw. A brittle to ductile transition was observed between 1400°C and 1600°C. The ductility was attributed to deformation via kinking and delamination observed in the microstructure at room temperature. ζ-Ta4C3-x was observed to be machinable with traditional tools such as a hacksaw, drill bit, and end mill. The machinability is attributed to cleaving of grains along weakly bonded planes and prevention of crack propagation into the bulk ceramic"--Abstract, p. iv


Hilmas, Greg

Committee Member(s)

Fahrenholtz, William W.
Watts, Jeremy Lee, 1980-
Lipke, David W.
Mochalin, Vadym


Materials Science and Engineering

Degree Name

Ph. D. in Ceramic Engineering


Missouri University of Science and Technology

Publication Date

Summer 2021


xii, 131 pages

Note about bibliography

Includes_bibliographical_references_(pages 119-130)


© 2021 Evan Charles Schwind, All Rights Reserved

Document Type

Dissertation - Open Access

File Type




Thesis Number

T 12147