3D-printing ultrastrong and ductile ceramic metamaterials
Ceramics are stiff, strong, lightweight, and have a very high melting point. These properties make them perfect for cookware, engines, and spacecraft parts, for example. Yet they come with a big “if.” “Ceramics would be the best engineering material,” says Lorenzo Valdevit, a mechanical and aerospace engineering professor at the University of California, Irvine, “if their brittleness could be overcome.”
Now by three-dimensional (3D) printing custom-designed structures with nanoscale features, Valdevit and his colleagues at HRL Laboratories have made ultrastrong yet ductile ceramics that do not crack easily. The ceramic samples consist of pillars that are 20 µm wide, which is larger than other similar materials made so far.
Ceramics are typically made by heating and pressing together inorganic powders to fuse them. The process results in the incorporation of microscopic cracks that can quickly grow under stress. To overcome the brittleness of ceramics, manufacturers make composites of ceramic fibers embedded in a ceramic matrix.
Another approach is to look at the nanoscale. Ceramic materials consisting of grains that have dimensions ranging from tens of nanometers to a few micrometers are ductile. That’s because atomic bonds are weaker at the boundaries between grains, so the atoms can move around more easily at those interfaces, helping to dissipate energy from mechanical stress. Some researchers are trying to translate that ductility to a larger scale by creating engineered metamaterials, which consist of repeated 3D lattices with nanoscale dimensions.
Valdevit and his research team made their nanoceramics using a preceramic resin (polymer) and two-photon polymerization direct laser writing, a technique that creates the smallest structures possible so far. The process involves zapping light-sensitive monomers with intense femtosecond laser pulses, which polymerizes them. Scanning the laser beam through the sample gives detailed 3D structures.
The researchers start with a mix of two UV light-sensitive siloxane resins: (mercaptopropyl)methylsiloxane and vinylmethoxysiloxane. They use the laser to polymerize the resin and create 3D lattices with struts that are 200–600 nm wide and 500–1720 nm high. After rinsing the sample, the researchers heat it at 1000°C for one hour. This leaves behind a lattice of silicon oxicarbide (SiOC), an amorphous glassy ceramic made of silicon, oxygen, and carbon.
The team 3D-printed SiOC pillars ranging from 500 nm to 20 µm in diameter. At all dimensions, the pillars were incredibly strong and could withstand pressures of up to 7 GPa before they shattered. “That is enormous for a ceramic,” Valdevit says, and 20 times more than 3D-printed SiOC materials made so far. The pillars can also be deformed by up to 25% of their size, “a plastic strength that is rare for ceramics at room temperature.”
The new architected materials show remarkable specific strength, says Haydn Wadley, professor of materials science and engineering at the University of Virginia. “The result is a lighter-than-water material with a compressive strength exceeding that of the strongest light metals.” But scaling the approach to create practical materials on the macroscale could take a long time, he says.
Creating nanometer-resolution structures with two-photon polymerization is indeed a slow process, Valdevit agrees, and the laboratory equipment limits the size of the samples. For use in aerospace and automotive parts or other structural applications, the materials would have to be much larger. Improvements in laser writing techniques would help, he says, but also “we might try to use self-assembly or some other clever way of fabricating things.”
Read the article in Matter.