Boron carbide - the third hardest material on earth - has been built into the fabric of cotton T-shirts, dramatically increasing its toughness. The process is a novel way to make nanocomposites that are both strong and flexible, and is a step towards creating effective new materials for body armour.

Modern military forces use plates of boron carbide (B 4 C) as ceramic inserts for bulletproof clothing but these can restrict mobility, so the design of a nanocomposite - where B 4 C is used to reinforce another material - could provide the perfect balance of strength and flexibility.

Now, a breakthrough has been made by a collaboration of researchers from China, Switzerland and the US, who found that the durable and porous nature of woven cotton fibres were ideal for the task.

’We were able to make large quantities of high-strength B 4 C nanowires using cotton T-shirts as both the template and carbon source,’ says Xiaodong Li, who led the group from the University of South Carolina, US. By using cotton as a template, the team overcame a previous problem of the nanowires clumping together.

In the research, squares were cut from 100 per cent cotton T-shirts and soaked in a solution of boron powder and a nickel-based catalyst, before being heated to around 1100?C for four hours under a flow of argon (to stop the material burning). ’Cotton fibres have lots of small pores which can be used to trap the powder,’ Li explains. ’During the process the cotton fibres change to carbon fibres - which react with the boron powder, producing B 4 C.’

After the reaction, the T-shirts change from white to black, but remain lightweight and pliable. But despite the dramatic change in their properties, this type of ’armoured cotton’ is not yet ready to replace conventional bulletproof materials, such as Kevlar.

’Although it is unlikely that these threads will replace others that are comparable in strength to Kevlar, this is an interesting development in the technology of advanced textiles modified with nanoscale materials,’ says Nicholas Kotov, an expert in composite materials at the University of Michigan, US.

Boris Yakobson, a materials scientist at Rice University, US, agrees. ’It is clever to use cotton as a natural template to both prevent aggregation of emerging boron-rich nanowires and act as a carbon source. Although the performance of the final composite is not very high, the approach looks very promising.’

Lewis Brindley