Glen E. Pry

Date of Award

3-23-2018

Document Type

Thesis

Degree Name

Master of Science in Materials Science

Department

Department of Aeronautics and Astronautics

First Advisor

Marina B. Ruggles-Wrenn, PhD.

Abstract

Refractory metal borides, commonly referred to as Ultra High Temperature Ceramics (UHTCs), exhibit a number of unique properties, such as extremely high melting temperature and hardness, chemical stability, high electrical and thermal conductivity and corrosion resistance. It has been demonstrated that the addition of SiC improves the oxidation resistance of ZrB2- and HfB2-based UHTCs above 1200°C by modifying the composition of the oxide scale. Addition of SiC retards the oxidation rate of ZrB2 and HfB2 by forming a protective layer of borosilicate glass. Creep deformation is one of the critical criterion for structural application of ceramics at elevated temperatures. Compression creep of HfB2-20 vol% SiC was studied at 1500°C in air at stresses ranging from -50 to -200 MPa. Primary and secondary creep regimes were observed in all tests. Minimum creep rates were measured. Post-test examination of material microstructure with SEM was employed to characterize the evolution of oxide scale with time. Additional analysis using EDS and XRD revealed the elemental and crystallographic makeup of the oxide scale and provided evidence that supports the assumed creep mechanism, grain boundary sliding. Comparison of the results of this effort with those from prior work indicate that sustained compressive stress has little effect on the growth rate of oxide scale. Likewise, oxidation appears to have negligible influence on steady-state creep rates.

AFIT Designator

AFIT-ENY-MS-18-M-286

DTIC Accession Number

AD1056619

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