Multi-layer graphene (MLG) is a promising functional material for mechanical applications due to its exceptionally anisotropic nano-structure comprised of the strongest carbon monolayers, graphene. Because most mechanical characterizations of graphene and MLG have been carried out under static or quasi-static conditions, the dynamic mechanical behavior of MLG largely unexplored. For deformation at a high strain rate (HSR) over 103/s, various effects including dynamic stress localization become important, and the static mechanical characteristics cannot be extrapolated to the high strain rates. However, the lack of HSR evaluation techniques for nanomaterials including MLG has hindered the quantitative investigation of their intrinsic HSR characteristics. To address this challenge, we developed a micro-ballistic technique, so called advanced laser-induced projectile impact test (α-LIPIT). In this approach, a single 3.72 µm diameter silica sphere is propelled as a micro-bullet via laser ablation of gold, and impacts onto a micro-sample at a supersonic speed with a high aiming accuracy less than 1.1 degree deflection.
We present the dynamic responses of MLG membranes, in a range of thicknesses from 10 to 100 nm, observed from the high-speed penetration of the MLG membrane in α-LIPIT. The MLG membranes showed characteristic crack propagation, correlating to the two preferential directions, the zigzag and armchair directions. The dark-field TEM study of a thin MLG membrane (~10 nm thick) demonstrated fine-scale moiré fringes, resulting from extensive folding near the impact origin, due to rapid elastic snapback after penetration. By measuring the speed of a micro-bullet before and after penetration, we explicitly determined the energy absorbed during MLG penetration. In order to compare the α-LIPIT results with macroscopic ballistic results of steel, aluminum, and Plexiglas, we normalized the penetration energy by the MLG mass under the projected area of a projectile, which is the specific penetration energy. For the net specific penetration energy, excluding the kinetic energy transfer to the debris, MLG membrane could dissipate about 10 times more energy per mass than what steel does at a range of impact speeds (600-900 m/s). Our results revealed that MLG’s superior speed of sound (~22 km/s) and the exceptional structural anisotropy enables MLG to exhibit excellent impact energy delocalization, which means material far beyond the direct impact region can also take kinetic energy from the projectile. We also confirmed the correspondence of our α-LIPIT results to macroscopic ballistic test results on typical isotropic materials: amorphous PMMA and polycrystalline gold.