Consider your humble ceramic coffee mug. This marvel of materials science is stiff, lightweight, and resistant to heat or corrosive chemicals. But a swift impact on a hard tile floor soon exposes its Achilles heel: ceramics are notoriously brittle.
To overcome this weakness, researchers have now used 3-D printing to create ceramics that are both strong and ductile (Matter 2019, DOI: 10.1016/j.matt.2019.09.009). Such materials could eventually be used in engineering applications that require a combination of strength, fracture resistance, and high heat tolerance, such as in airplane engine components. “We would love to operate those components at much higher temperatures, because energy efficiency increases with temperature,” explains Katherine T. Faber at Caltech, who works on silicon oxycarbide ceramics and was not involved in the new study. “But it’s not likely to happen with metals, so there is a push towards high-temperature ceramics.”
Traditional methods for making ceramic objects use pressure and heat to fuse inorganic powders together, but these processes tend to create tiny pores and cracks that quickly turn into larger breaks when the material is stressed.
3-D printing offers a way to build ceramics without those fatal flaws, says Lorenzo Valdevit at the University of California, Irvine. He and his team used a commercially available technique called two-photon polymerization direct laser writing to build silicon oxycarbide structures that could withstand up to 7 GPa of pressure before breaking apart. That’s more pressure than high-strength steel can endure before it breaks, Valdevit says.
The structures were also remarkably ductile, with some compressing by up to 25% before finally giving way. “What’s unique is that together with this high strength, you also have really high ductility, which you almost never see in ceramics,” Valdevit says.
To make the ceramics, the researchers mixed (mercaptopropyl)methylsiloxane and vinylmethoxysiloxane with a phosphine oxide reagent. Exposed to pulses of infrared laser light, this reagent forms a radical that triggers the siloxanes to polymerize, creating a solid. Crucially, the radical forms only when and where two photons of infrared light arrive simultaneously, which confines the polymerization to the focal point of the laser, where the intensity of the light is greatest. Moving the focal point around enabled the researchers to trace and precisely print different 3-D shapes.
After washing away unreacted siloxane monomers, the researchers heated their polymer structures at 1,000 °C for one hour to force out any organic components, leaving only silicon, oxygen, and carbon atoms. The remaining silicon oxycarbide ceramic has an amorphous structure that is similar to silica glass, forming a network of tetrahedral SiO4 units in which some oxygen atoms have been replaced by carbon. “They end up with an amorphous material that has very few flaws, and strengths approaching the maximum predicted by theory,” Faber says.
The team used this approach to create ceramic pillars up to 20 μm wide and complex lattices—known as architected materials—with struts up to 600 nm wide. Researchers from HRL Laboratories, who were involved in this study, had previously used other 3-D printing methods to make similar silicon oxycarbide lattices. But the greater precision of two-photon laser writing has helped to improve the material’s physical properties, such that the new lattices are the stiffest and strongest architected materials ever made, the researchers contend.
“Being able to print high strength ceramics with good ductility is impressive,” says Minh-Son Pham at Imperial College London, who works on architected materials. For practical applications, though, 20 μm is still quite tiny, he adds.
Scaling up might require a different 3-D printing technique—two-photon polymerization is slow, expensive, and not suited to building bigger structures. So Valdevit’s team is exploring alternative 3-D printing processes that could produce larger ceramic objects without sacrificing strength and ductility. If it were possible to craft 500-μm-wide struts from the same flawless silicon oxycarbide, “then you could print structures that are many centimeters across,” Valdevit says.