The inside of today’s rocket engines can reach a blistering 1600 degrees Celsius – hot enough to melt steel – and tomorrow’s engines will need to be even more scorching.
Hotter engines are more fuel-efficient, produce more thrust and can carry larger loads – all key for Mars-bound spacecraft and advanced aircraft.
In the quest for rocket materials that can tolerate more heat, engineers have been trying to devise tough, lightweight composites made of silicon carbide fibres, a small fraction of the width of a human hair, embedded in a ceramic material, reports Scientific American.
Silicon carbide can withstand 2000 degrees C – the temperature of the hoped-for hotter engines. Today’s composites are made by layering woven mats of silicon carbide fibres and filling the space between them with a porous ceramic.
But existing composites can crack under the high pressures that occur in engines because the fibres slip against one another and pull out of the ceramic.
In a possible breakthrough, scientists at Rice University and the NASA Glenn Research Centre have developed “fuzzy” silicon carbide fibres whose surfaces resemble a microscopic version of Velcro.
The fibres, described recently in Applied Materials & Interfaces, would be less likely to slip or pull out of a surrounding ceramic medium because their fuzzy tangles lock them together.
To make these threads, the researchers first grew curly carbon nanotubes that stick out from the silicon carbide surface like ringlets of hair. Then they dipped the fibres in an ultrafine silicon powder and heated them, which converts the carbon nanotubes into silicon carbide fibres.
The team tested the fuzzy fibres’ strength by embedding them in a transparent, rubbery polymer – and found these composites to be four times as strong as those made with smooth threads.
NASA research engineer and study co-author Janet Hurst says the team now wants to test the new, curly fibres in a ceramic medium. They also want to make fibres with a fuzzy boron nitride nanotube coating because it is strong and shields the fibres from damaging oxygen exposure.
Silicon carbide fibres are strong along their length but can snap across their width under high pressure.
Yet the new fibres should resist breakage because their soft fuzz helps to dissipate the strain by distributing it, says Steven Suib, director of the Institute of Materials Science at the University of Connecticut, who was not involved in the new research.