Shock loading on AlN ceramics: A large scale molecular dynamics study

The nanoscale structure of shock waves in high strength AlN ceramics is studied using molecular-dynamics simulations. Impacts using four independent crystallographic directions, with particle velocities in the range 0.8–4.0 km/s, reveal the generation of a well-defined two-wave structure, consisting of an elastic precursor followed by a wurtzite-to-rocksalt structural transformation wave (STW). Impacts with particle velocities below 0.8 km/s or above 4.0 km/s generate single elastic or single overdriven shock waves, respectively. The latter propagates faster than the longitudinal sound velocity along the impact direction. The structure of the STW exhibits a strong anisotropy with respect to the crystallographic directions. Results show distinct shock front features from a steady, sharp and well defined STW front for impact at the basal plane to a rough STW front with the formation of an intermediate metastable fivefold coordinated phase or the generation and propagation of dislocations from the shock front for impact at other directions. These results indicate that shock-induced damage in brittle materials can be highly intricate even in defect-free systems. Simulations show no deformation twinning or other well defined plastic wave as found in experiments on polycrystalline AlN suggesting that interfaces and defects strongly affect the materials response to shock loading.