Densification and Devitrification of Fused Silica Induced by Ballistic Impact: A Computational Investigation

Mica Grujicic,Bryan Cheeseman,Jennifer Snipes,Ramin Yavari,S Ramaswami

doi:10.1155/2015/650625

Abstract

A molecular-level computational investigation is carried out to determine the dynamic response and material topology changes of fused silica subjected to ballistic impact by a hard projectile. The analysis was focused on the investigation of specific aspects of the dynamic response and of the topological changes such as the deformation of highly sheared and densified regions, and the conversion of amorphous fused silica to SiO2crystalline polymorphs (in particular,α-quartz and stishovite). The topological changes in question were determined by carrying out a postprocessing atom-coordination procedure. This procedure suggested the formation of stishovite (and perhapsα-quartz) within fused silica during ballistic impact. To rationalize the findings obtained, the all-atom molecular-level computational analysis is complemented by a series of quantum-mechanics density functional theory (DFT) computations. The latter computations enable determination of the relative potential energies of the fused silica,α-quartz and stishovite, under ambient pressure (i.e., under their natural densities) as well as under imposed (as high as 50 GPa) pressures (i.e., under higher densities) and shear strains. In addition, the transition states associated with various fused-silica devitrification processes were identified. The results obtained are found to be in good agreement with their respective experimental counterparts.

Highlights

The present work deals with the problem of formation of crystalline phases ( α-quartz and stishovite) within a fused-silica target during a ballistic impact
To compute the material mass density, the partial radial distribution functions, and atomic coordination combinations, NPT equilibrium molecular dynamics simulations were run at the room temperature and the ambient pressure
Based on the results obtained in the present work, the following summary remarks and main conclusions can be drawn

Summary

Introduction

The present work deals with the problem of formation of crystalline phases ( α-quartz and stishovite) within a fused-silica (ceramic glass containing a high purity SiO2) target during a ballistic impact. Despite the absence of a crystalline structure, the topology of ceramic glasses is not completely random Rather, it involves different extents of short- and intermediate-range order spanning over a range of length-scales (from the quantummechanical to the continuum-level). To describe the structure of ceramic glasses as determined using various experimental techniques (e.g., neutron-diffraction, nuclear magnetic resonance, and small angle X-ray scattering (SAXS)), the so-called “random network model” [1] is typically employed. Such a model represents an amorphous material as a three-dimensional linked network of polyhedra. Since silicon has a tendency to form a continuous network with (bridging) oxygen atoms, SiO2 is commonly referred to as a “network former.”

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