Origin of the elastic anisotropy of silica particles: Insights from first-principles calculations and nanoindentation molecular dynamic simulations

To better elucidate and comprehend the origin of the elastic anisotropy observed in silica sand, this study investigates the nanoscale mechanical properties of α-quartz (the main component of silica sand) using the first-principles calculations (density functional theory, DFT) and nanoindentation molecular dynamics (MD) simulations. The cell parameters and elastic constants (Cij) of α-quartz are obtained by DFT with PBESOL functional. The electron density difference of the most commonly exposed crystal surfaces {100}, {001}, and {101}, exhibits significant distinction, which is considered as the underlying cause of α-quartz's elastic anisotropy. This study reveals a significant difference in Young's modulus E, Poisson's ratio v, and hardness H along different directions, which are also visualized in three dimensional graphics. Notably, E{100} = 81.576 GPa < E{001} = 101.198 GPa < E{101} = 113.323 GPa, and H{001} = 9.068 GPa < H{100} = 11.258 GPa < H{101} = 19.870 GPa. The change in E calculated by MD simulations is consistent with DFT calculations. Additionally, this study observed distinct initiation of yielding during nanoindentation for different crystal surfaces, with the corresponding indentation yielding depth following the trend D{100} > D{001} > D{101}. Furthermore, both DFT and MD simulation results indicate that the negative v effect on E is negligible in α-quartz.