Body armour given added resilience with silicon

The study by researchers at Texas A&M University has been published in Science Advances.

"For the past 12 years, researchers have been looking for ways to reduce the damage caused by the impact of high-speed bullets on armour made with boron carbide," said Dr Kelvin Xie, assistant professor in the Department of Materials Science and Engineering. "Our work finally addresses this unmet need and is a step forward in designing superior body armour that will safeguard against even more powerful firearms during combat."

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Dubbed ‘black diamond,’ boron carbide ranks second below cubic boron nitride for hardness. Unlike cubic boron nitride, boron carbide is said to be easier to produce on a large scale and is harder and lighter than other armour materials like silicon carbide, making it well-suited for ballistic vests and other items of armour.

According to Texas A&M University, boron carbide's main shortfall is that it can damage very quickly upon high-velocity impact.

"Boron carbide is really good at stopping bullets traveling below 900 metres per second, and so it can block bullets from most handguns quite effectively," Xie said in a statement. "But above this critical speed, boron carbide suddenly loses its ballistic performance and is not as effective."

Scientists know high-speed jolts cause boron carbide to have phase transformations, where a material changes its internal structure and assumes two or more physical states simultaneously. The bullet's impact converts boron carbide from a crystalline state where atoms are systematically ordered to a glass-like state where atoms are haphazardly arranged. This glass-like state weakens the material's integrity at the site of contact between the bullet and boron carbide.

"When boron carbide undergoes phase transformation, the glassy phase creates a highway for cracks to propagate," said Xie. "So, any local damage caused by the impact of a bullet easily travels throughout the material and causes progressively more damage."

Previous work using computer simulations predicted that adding a small quantity of another element, such as silicon, had the potential to make boron carbide less brittle. Xie and his group investigated if adding a tiny quantity of silicon also reduced phase transformation.

To simulate the initial impact of a high-speed bullet, the researchers made well-controlled dents on boron carbide samples with a diamond tip. They then looked at the microscopic damage that was formed from the blows with a high-powered electron microscope.

Xie and his collaborators found that even with tiny quantities of silicon, the extent of phase transformation went down by 30 per cent, noticeably reducing the damage from the indentation.

Although silicon serves well to enhance boron carbide's properties, Xie said that more experiments need to be done to know if other elements, like lithium and aluminium, could also improve boron carbide's performance.