Atomistic-scale modelling of nanoindentation into optical coatings

Simulations of nanoindentation into a typical optical-coatings stack employed in energy efficient glazing have been performed using classical molecular dynamics (MD) and a coupled finite element/MD methodology. The coatings stack consists of a low-emissivity material, Ag, sandwiched between two layers of a transparent conducting oxide (TCO), ZnO. Simulations into both the ZnO and the coatings stack show a strong interaction between the tip symmetry and crystal symmetry in the observed displacement field. A large amount of elastic recovery is observed for both the ZnO system and the coatings stack, but with an impression left on the surface that looks like a crack but extends no further than the tip imprint at maximum depth. The full stack is observed to have a lower hardness once there is a significant penetration of the displacement field into the Ag, when compared to the pure ZnO system. A comparison between the coupled finite element/MD methodology and the fixed boundary MD-only model shows that the boundary conditions have little influence on the calculated results.