Silicon doped boron carbide for armour

Abstract

Boron carbide is a popular candidate armour ceramic. During high velocity impact, however, amorphous bands form, leading to the collapse of the structure, and reducing the usefulness of boron carbide in such applications. Recent experimental data from the literature suggests that silicon (Si) doping of boron carbide nanowires reduces this amorphisation. This project focuses on creating a process for Si doping of boron carbide that could potentially be up-scaled to commercial quantities. As shown in this thesis, as long as free carbon, or, in fact, carbon-rich boron carbide is present, Si additions react with carbon to form silicon carbide. Therefore, three methods for reducing the carbon content in boron carbide powders were investigated: plasma cleaning, oxidation/reduction, and annealing in the presence of amorphous boron (B). This resulted in a range of boron carbide powders with various carbon contents covering a wide range of the phase diagram. The effect on the structure of the powder will be discussed. Si was mixed with these boron carbides, and evidence for Si-doped boron carbide phase B12(C,Si,B)3 was found in many of the powders produced, as well as with additional phases. An enhancement of the doping correlated with a reduction in initial carbon content for a comparable concentration of Si. A promising result of the reduction of the amorphisation on the doped powder was confirmed for one condition after high pressure diamond anvil testing. Similarly, the reduction of amorphisation was also confirmed by indentation at the interface of a diffusion couple formed from a wafer of Si annealed at 1400 °C between two pieces of boron carbide. The boron carbide at the interface exhibited Raman features similar to the Si-doped powder. These results of powder and interface suggest that a new type of lightweight armour material could be produced that overcomes one of the biggest challenges of this ceramic: the amorphisation.