The insides of today's rocket engines can reach a blistering 1,600 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 fibers, a small fraction of the width of a human hair, embedded in a ceramic material. Silicon carbide can withstand 2,000 degrees C—the temperature of the hoped-for hotter engines. Today's composites are made by layering woven mats of silicon carbide fibers 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 fibers slip against one another and pull out of the ceramic.
In a possible breakthrough, scientists at Rice University and the NASA Glenn Research Center have developed “fuzzy” silicon carbide fibers whose surfaces resemble a microscopic version of Velcro. The fibers, 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 fibers in an ultrafine silicon powder and heated them, which converts the carbon nanotubes into silicon carbide fibers. The team tested the fuzzy fibers' 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 fibers in a ceramic medium. They also want to make fibers with a fuzzy boron nitride nanotube coating because it is strong and shields the fibers from damaging oxygen exposure.
Silicon carbide fibers are strong along their length but can snap across their width under high pressure. Yet the new fibers 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.