Text

theses

Bending – the basis of the bow and arrow, leaf springs, and one of the most common modes of failure – has been studied for hundreds of years. This thesis brought mechanical characterization of micron-scale fibers and observation of the change in position of Raman peaks with stress together under the umbrella of bending. Both utilized bending to a large degree – one to determine its mechanical properties and the other to examine in-situ behavior of ceramics under stress.

Silicon carbide fibers were grown by Free Form Fibers LLC through a unique method called laser-induced CVD. This method is capable of growing fully dense fibers 30-60 microns in diameter and meters long. Characterization methods included nanoindentation, helium-ion microscopy, Raman spectroscopy, and the development of a new large-deflection cantilever bend testing technique. The fibers consist of an inner core of crystalline SiC and an outer layer of amorphous carbon-rich silicon. Nanoindentation of the outside of the fibers and over the cross-section show that the outer layer has a reduced elastic modulus of 160 GPa and a hardness of 18 GPa, while the inner core has a reduced elastic modulus of 330 GPa and a hardness of 36 GPa. Large-deflection cantilever bend testing showed that the elastic modulus can vary between batches and that the overall elastic modulus of the fibers is 300 GPa. Fractography using the helium-ion microscope indicates that the fibers failed at ~2.2 GPa. These values are consistent with those obtained for third generation commercially available fibers.

Raman spectra, arising from chemical bonds, are dependent on stress in solid materials. If a material is stretched, some bonds will stretch, and if a material is compressed the bonds will likewise compress, affecting the location of different Raman peaks. This concept was applied to determine experimentally what the relation between peak shift and stress was for several materials. This was done by designing, building, and validating a four-point bend apparatus for in-situ micro-Raman testing. Single crystal silicon was used as a control material to validate the machine and the relation obtained as C=0.0022 cm-1/MPa, the expected value for this coefficient. Silicon carbide, quartz, YAG, and two types of feldspar were tested and all were observed to have multiple peaks shift with stress.

ETD_9204

Ph.D.

Includes bibliographical references

by Shawn H. Ward

doi:10.7282/t3-epk7-yq27

ETD doctoral

The author owns the copyright to this work.