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theses

Boron carbide is a material well known for its favorable properties such as high hardness, low specific gravity, and chemical inertness. These properties make boron carbide a seemingly ideal material for extreme and dynamic environments, such as those encountered by ballistic armors. However, boron carbide suffers a loss in shear strength under high nonhydrostatic stresses, which exceed the Hugoniot elastic limit, due to nanoscale amorphization. This loss in sheer strength will often result in catastrophic failure in the material. Density functional theory (DFT) simulations indicate silicon doping of the 3-atom linear chains of boron carbide might suppress this amorphization.1 With the amorphization mitigated, boron carbide will theoretically have a much lower chance for failure. In this work, multiple routes to incorporate silicon into boron carbide were attempted with the greatest insight stemming from formation of diffusion couples with boron carbide and boron silicides. The boron carbide used in this study was manufactured using rapid carbothermal reduction to minimize free carbon present and prevent unwanted reactions during coupling with the boron silicides. These powders along with silicon hexaboride powders were both individually sintered into 3g disks using a spark plasma sintering furnace then processed to ensure clean contact surfaces for diffusion. These disks were layered and heat treated under pressure for extended time frames to allow diffusion to occur. The diffusion zones in these couples were microstructurally, chemically, and mechanically characterized to investigate the possible formation of silicon-doped boron carbide. Upon characterization the diffusion zones were noted as microstructurally and chemically distinct, consisting primarily of silicon-doped boron carbide with widths up to 1050μm depending on temperature, time, and heating rate used. Further characterization of mechanical properties showed measured nanohardness values for silicon-doped boron carbide were lower than high-purity boron carbide, but Raman spectra from indents showed a significant decrease in amorphization peak intensities which was in agreement with the DFT simulations. While the DFT simulations made the initial assumption of 14 atomic percent silicon in boron carbide, our experiments resulted in a maximum of only 2.2 atomic percent silicon present in the boron carbide obtained through Scanning Transmission Electron Spectroscopy - Energy Dispersive Spectroscopy (STEM-EDS) analysis. Despite the stark contrast in assumed and actual dopant quantities, the resultant mitigation of stress-induced amorphization confirmed predictions.

ETD_8691

Ph.D.

Includes bibliographical references

by Anthony Maxwell Etzold

doi:10.7282/T3PC35SM

ETD doctoral

The author owns the copyright to this work.