How do you make the superstrong composites used in airplanes even stronger? Just add carbon. Researchers have discovered that coating the carbon fibers in composites with more carbon--in this case microscopic carbon nanotubes--increases their toughness substantially. If commercialized, the process could render critical aircraft components and other machinery nearly unbreakable.
In terms of strength and hardness, carbon is king. In its crystalline form, better known as a diamond, element number six on the periodic table can resist pressures that would crush iron and titanium. Fabricated into filaments thinner than a human hair, carbon fiber outclasses even the strongest steel.
But when it comes to making more rigid structures such as airplane parts, carbon fiber has a weakness. Engineers must sandwich alternating layers of carbon fiber and epoxy resin, and this resin becomes the weak link in the overall laminate. Sufficient stress can break the layers apart, a condition called interlaminar fracture, which can be disastrous for an aircraft in flight.
Now a team from the Massachusetts Institute of Technology in Cambridge may have found an elegantly simple improvement. The researchers developed a chemical process in which carbon fibers, heated to 750° C, sprout nanotube whiskers. They then wove those stubbly fibers into a fabric, which they injected with epoxy. The nanotubes tied the layers together and created a Velcro-like effect.
As the team will report in an upcoming issue of the Journal of Composite Materials, the new hybrid material can be as much as 10 times tougher than existing composites.
Aerospace engineer and team leader Brian Wardle says he expects further refinements will increase the new material's strength. But the process has advanced enough that the team expects to form a company later this year to develop its commercial potential.
"The work is of potentially great significance," says materials engineer S. Mark Spearing of the University of Southampton in the United Kingdom. Interlaminar fracture has been a major concern for laminated composites since their inception in the late 1960s, he says, and nanostitching could be the way to strengthen the material with fewer detrimental effects than with other approaches.