Doctoral Dissertations

Alec Murchie

Keywords and Phrases

Ceramic; UHTC

Abstract

“This research focuses on the elevated mechanical properties of diboride based materials at room and elevated temperatures. These materials include a ZrB2 ceramic, a ZrB2-WC-SiC ceramic, and a (Hf,Mo,Nb,Ta,W,Zr)B2 high entropy boride (HEB). The room temperature flexural strength for the ZrB2 ceramic was 447 MPa and the fracture toughness was 2.3 MPa·m1/2. The strength decreased to 196 MPa at 1800°C, and then increased to 360 MPa at 2300°C. The fracture toughness increased to 3.1 MPa·m1/2 at 2200°C. The strength up to 1800°C was likely controlled by a combination of effects: surface damage from oxidation of the specimens, stress relaxation, and decreases in the elastic modulus, while the largest grain size plus pore to be the critical flaw size for strengths above 1800°C. The room temperature strength of the ZrB2-WC-SiC ceramic was measured to be 510 MPa and stayed consistent with increasing temperature, still maintaining 520 MPa at 2100°C. The small WC and SiC addition are believed to aid in the strength retention by the removal of oxide impurities from the microstructure, and in the case of SiC provide some amount of oxidation protection.

The room temperature mechanical properties of the (Hf,Mo,Nb,Ta,W,Zr)B2 were measured; flexural strength was 528 ± 53 MPa, Young’s modulus was 520 ± 12 GPa, fracture toughness was 3.9 ± 1.2 MPa·m1/2 and hardness was 33.1 ± 1.1 GPa. Strength decreased from ∼530 MPa at room temperature to ∼180 MPa at 1000°C. Strength then increased to ∼570 MPa at 1200°C and kept gradually increasing to ∼800 MPa at 1600°C. Fracture toughness decreased from ∼3.9MPa·m1/2 at room temperature to ∼3.1MPa·m1/2 at 1200°C before increasing to ∼6.5 MPa·m1/2 at 1600°C. A Griffith type analysis determined the strength-limiting flaw of the HEB material to be the largest grains in the microstructure for all temperatures except 1000°C”--Abstract, page ivi.

Advisor(s)

Hilmas, Greg

Committee Member(s)

Fahrenholtz, William
Watts, Jeremy Lee, 1980-
O'Malley, Ronald J.
Samaranayake, V. A.

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Ceramic Engineering

Comments

This work was financially supported by the Office of Naval Research on contract N00014-16-1-2303.

The format has been revised for the document downloaded by the button above: landscape-oriented tables, graphs and/or illustrations have been rotated to allow easier online reading. The supplemental file below is the original report format, for printing.

Publisher

Missouri University of Science and Technology

Publication Date

Spring 2022

Journal article titles appearing in thesis/dissertation

  • Mechanical Properties of Borothermally Synthesized Zirconium Diboride at Elevated Temperatures
  • Elevated Temperature Mechanical Properties of a ZrB₂-WC-SiC Ceramic
  • Room Temperature Mechanical Properties of a High-Entropy Diboride
  • Elevated Temperature Mechanical Properties of a High-Entropy Diboride

Pagination

xiii, 109 pages

Note about bibliography

Includes bibliographic references.

Rights

© 2022 Alec Christopher Murchie, All rights reserved.

Document Type

Dissertation - Open Access

File Type

text

Language

English

Thesis Number

T 12126

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