THE LATEST RESEARCH AND DEVELOPMENT NEWS IN MANUFACTURING AND TECHNOLOGYTECH FRONT Smashing Silver Microcubes Toughens Up Materials S
cientists at Rice University (Houston) are smashing tiny silver cubes into a hard target in order to make these metallic microcubes ultrastrong and tough by rear- ranging their nanostructures upon impact. The Rice team reported in Science magazine that fi ring a
tiny, nearly perfect cube of silver onto a hard target turns its single-crystal microstructure into a gradient-nano-grained (GNG) structure. The experiment aimed to learn more about how materials deform under overwhelming stress, as might be experienced by a bulletproof vest or a spacecraft that encounters micrometeorites.
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Rice University researchers (from left) Olawale Lawal, Ramathasan Thevama- ran, Edwin Thomas and Sadegh Yazdi hold clay models of deformed cubes that represent the results of their microscale experiments. The researchers smashed silver microcubes at near supersonic speeds to see how deforming their crystalline structures could make them stronger and tougher.
Creating a gradient nanostructure in materials by way of deformation will make them more ductile, the researchers believe, and therefore less likely to fail catastrophically when subsequently stressed. Ultimately, they want to develop nano-grained metals that are tougher and stronger than anything available today.
Led by materials scientist Edwin Thomas, the William and Stephanie Sick Dean of Rice’s George R. Brown School of Engineering, the team used its advanced laser-induced
projectile impact test (LIPIT) rig to shoot microcubes onto a silicon target. The mechanism allowed them to be sure the cube hit the target squarely. The Thomas lab developed the LIPIT technique several years ago to fi re microbullets to test the strength of polymer and graphene fi lm materials. This time the researchers were essentially testing the bullet itself. “The high-velocity impact generates very high pressure that far exceeds the material’s strength,” Thomas said. “This leads to high plasticity at the impact side of the cube while the top region retains its initial structure, ultimately creating a grain-size gradient along its height.” The original projectiles needed to be as perfect as possible. That required a custom fabrication method, Thomas said. The cubes for the study were synthesized as single crystals via bottom-up seed growth to about 1.4 microns per side, about 50 times smaller than the width of a human hair. LIPIT transformed laser power into the mechanical energy that propelled the cubes toward a target at supersonic velocity. The cubes were placed on top of a thin polymer fi lm that thermally isolated them and prevented the laser itself from deforming them. When a laser pulse hit an absorbing thin-fi lm gold layer underneath the polymer, the laser energy caused it to vaporize. That expanded the poly- mer fi lm, which launched the microcubes. The distance covered was small—about
500 micrometers—but the effect was large. While the experiments were carried out at room temperature, the cube’s temperature rose by about 350ºF (177ºC) upon impact at 400 m/sec and allowed dynamic recrystallization. The one-step Rice process makes such structures with a range of grains from about 10 to 500 nanometers over a distance of 500 nanometers. That produces a gradient at least 10 times higher than the other techniques, the researchers reported. For more information, an abstract of the paper, published in the Oct. 21 issue of Science, is available at http://science.
sciencemag.org/content/354/6310/312.
January 2017 |
AdvancedManufacturing.org 33
Photo courtesy Jeff Fitlow/Rice University
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