Amorphization Toughening Induced by Microcracks in SiO₂ Thin Films

Fused silica, a widely used facing material of transparent armor, was found to transform into stishovite under heavy shock compression. The ballistic-resistant performance of the partially transformed fused silica, which is highly correlated to the crack initiation and propagation processes, is important for multi-hit possibility. The ultra-high hardness and strength of stishovite are beneficial for the reduction of crack initiation. However, how the pre-existing cracks in stishovite propagate is an open question. Here, by combining molecular dynamics simulations and density functional theory calculations, we investigate the fracture behavior of crystalline stishovite with pre-existing cracks at room temperature. It is found that the crystalline stishovite phase transforms into an amorphous phase via a deformed phase under tensile loading, leading to a ductile fracture. Amorphization is localized on crack tips because of the strain concentration. Amorphization helps to inhibit crack propagation by volume expansion, increasing the final fracture strain. The amorphization mechanism and crack propagation path are robust when the shape of cracks changes. These results provide a reference for application of fused silica-based transparent armor systems.