Doctoral Dissertations

Keywords and Phrases

High-Temperature Ceramics; Mechanical Properties; Microstructure; Molybdenum Disilicide; Processing; Zirconium Diboride


"This research had two objectives: characterization of processing-microstructure-mechanical property relationships of conventional ZrB2-MoSi2 ceramics at room temperature (RT) and 1500⁰C in air, and fabrication of ZrB2-MoSi2 dual composite architectures (DCAs) for use near 1500⁰C. Elastic moduli, fracture toughness, and flexure strength were measured at RT and 1500⁰C for 15 ZrB2-MoSi2 ceramics hot pressed using fine, medium, or coarse ZrB2 starting powder with 5-70 vol.% MoSi2, referred to as FX, MX, and CX respectively where X is the nominal MoSi2 content. MoSi2 decomposed during sintering, resulting in microstructures with ZrB2 cores and (Zr1-xMox)B2 shells via surface and grain boundary diffusion. Flexure strength at RT (700-800 MPa for FX, 560-720 MPa for MX, and 440-590 MPa for CX) was controlled by the maximum ZrB2 grain size, and toughness (2.7-3.9 MPa·m1/2) did not trend with MoSi2 content. At 1500⁰C toughness increased with MoSi2 content and ZrB2 grain size, and strength of FX and MX was controlled by oxidation damage at 1500⁰C. Strength of CX followed the opposite trend, with C10 exhibiting a strength of ~600 MPa.

Four ZrB2-MoSi2 DCAs were fabricated by dispersing granules of selected ZrB2-MoSi2 compositions in matrices of different ZrB2-MoSi2 compositions. Strength limitation at 1500⁰C by differential oxidation of granules and matrix was resolved by compositional adjustment, but microcracking due to granule-matrix CTE mismatch limited strength to ~140 MPa at RT and ~360 MPa at 1500⁰C. The granule-matrix interface did not deflect cracks, and the toughness at 1500⁰C was 6.1-6.9 MPa·m1/2, similar to that of conventional ZrB2-MoSi2 ceramics. CTE matching via addition of a third phase and use of a weak granule-matrix interface are recommended areas of focus for future development of high-temperature DCAs"--Abstract, page iv.


Hilmas, Greg

Committee Member(s)

Fahrenholtz, William
Smith, Jeffrey D.
Van Aken, David C.
Dharani, Lokeswarappa R.


Materials Science and Engineering

Degree Name

Ph. D. in Materials Science and Engineering


National Science Foundation (U.S.)
Italian National Council of Research (CNR).


Partial funding for this project was provided by the National Science Foundation’s Materials World Network program grant DMR-1209262.


Missouri University of Science and Technology

Publication Date

Spring 2017

Journal article titles appearing in thesis/dissertation

  • ZrB2-MoSi2 ceramics with varying MoSi2 content: Part 1. Processing and microstructure with varying ZrB2 powder particle size
  • ZrB2-MoSi2 ceramics with varying MoSi2 content: Part 2. Mechanical properties for medium ZrB2 particle size
  • Densification processes and formation of solid solution shell in ZrB2-MoSi2 ceramics
  • ZrB2-MoSi2 ceramics with varying MoSi2 content: Part 3. Mechanical properties with varying ZrB2 particle size
  • Processing of ZrB2-MoSi2 dual composite architectures


xviii, 261 pages

Note about bibliography

Includes bibliographic references.


© 2017 Ryan Joseph Grohsmeyer, All rights reserved.

Document Type

Dissertation - Open Access

File Type




Thesis Number

T 11093

Electronic OCLC #